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\title{A New Spectral Classification System for the Earliest O Stars:
Definition of Type O2}

\author{Nolan R.\ Walborn,\altaffilmark{1,2,3}
Ian D.\ Howarth,\altaffilmark{4,5}
Daniel J.\ Lennon,\altaffilmark{6,7}}
%\authoremail{djl@ing.iac.es}

\author{Philip Massey,\altaffilmark{2,3,8}
M.~S.\ Oey,\altaffilmark{2,8}
Anthony F.~J.\ Moffat\/\altaffilmark{2,9}} 

\author{Gwen Skalkowski,\altaffilmark{9}
Nidia I.\ Morrell,\altaffilmark{2,10,11}}

\author{Laurent Drissen,\altaffilmark{3,12} and
Joel Wm.\ Parker\/\altaffilmark{2,3,13}}
\vspace{1in}
\centerline{\titlelarge To be published in the May 2002 issue of the \emph{Astronomical Journal}}
\received{18 September 2001}
\accepted{22 January 2002}
\sluginfo
\vfill
\altaffiltext{1}{Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, 
MD 21218.}
\altaffiltext{2}{Visiting or Staff Astronomer, Cerro 
Tololo Inter-American Observatory and/or Kitt Peak National Observatory, 
National Optical Astronomy Observatory, operated by the Association of 
Universities for Research in Astronomy, Inc., under a cooperative
agreement with the NSF.}
\altaffiltext{3}{General Observer, Hubble Space
Telescope, Space Telescope Science Institute, operated by the
Association of Universities for Research in Astronomy, Inc., under NASA
contract NAS5-26555.}
\altaffiltext{4}{Department of Physics and Astronomy, University College London,
Gower Street, London WC1E 6BT, UK.}
\altaffiltext{5}{Visiting Observer, Isaac Newton Group, La
Palma and Anglo-Australian Observatory, Siding Spring.}
\altaffiltext{6}{Isaac Newton Group, Apartado 321, 38700 Santa Cruz de La Palma,
Canary Islands, Spain.}
\altaffiltext{7}{Visiting Observer, European Southern
Observatory, La Silla.}
\altaffiltext{8}{Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001.}
\altaffiltext{9}{D\'epartement de Physique, Universit\'e de Montreal, C.P.\ 6128,
Succ.\ Centre-Ville, Montreal, QC H3C 3J7, Canada.}
\altaffiltext{10}{Facultad de Ciencias Astron\'omicas y Geof\'{\i}sicas, Universidad
Nacional de La Plata, Paseo del Bosque, 1900 La Plata, Argentina.}
\altaffiltext{11}{Visiting Astronomer, Complejo
Astron\'omico El Leoncito, operated under agreement of the Universidad
Nacional de La Plata, the Universidad Nacional de C\'ordoba, the Universidad
Nacional de San Juan, and the Consejo Nacional de Investicaciones
Cient\'{\i}ficas y T\'ecnicas de la Rep\'ublica Argentina.  Member of the
Carrera del Investigador Cient\'{\i}fico, CONICET, Argentina.}
\altaffiltext{12}{D\'epartement de Physique, Universit\'e Laval, Ste-Foy, 
QC G1K 7P4, Canada.}
\altaffiltext{13}{Southwest Research Institute, 1050 Walnut Street, Suite 426,
Boulder, CO 80302.}


\begin{abstract}
High-quality, blue-violet spectroscopic data are collected for 24 stars 
that have been classified as type O3 and that display the hallmark N~IV 
and N~V lines.  A new member of the class is presented; it is the 
second known in the Cygnus~OB2 association, and only the second in the 
Northern Hemisphere.  New digital data are also presented for several of 
the other stars.  Although the data are inhomogeneous, the uniform 
plots by subcategory reveal some interesting new relationships. 
Several issues concerning the classification of the hottest O-type spectra 
are discussed, and new digital data are presented for the five original 
O3 dwarfs in the Carina Nebula, in which the N~IV, N~V features are very 
weak or absent.  New spectral types O2 and O3.5 are introduced here as 
steps toward resolving these issues.  The relationship between the derived 
absolute visual magnitudes and the spectroscopic luminosity classes of the 
O2-O3 stars shows more scatter than at later O~types, at least partly 
because some overluminous dwarfs are unresolved multiple systems, and some 
close binary systems of relatively low luminosity and mass emulate O3 
supergiant spectra.  However, it also appears that the behavior of He~II 
$\lambda4686$, the primary luminosity criterion at later O types, responds 
to other phenomena in addition to luminosity at spectral types O2-O3.  There 
is evidence that these spectral types may correspond to an immediate pre-WN
phase, with a correspondingly large range of luminosities and masses. 
A complete census of spectra classified into the original O3 subcategories 
considered here (not including intermediate O3/WN types or O3 dwarfs without 
N~IV, N~V features) totals 45 stars; 34 of them belong to the Large 
Magellanic Cloud and 20 of the latter to 30~Doradus.   
\end{abstract}

%\keywords{stars: early-type---stars: fundamental parameters}

\section{Introduction}

Spectral class O3 was introduced by Walborn (1971a) to describe four stars
in the Carina Nebula with earlier spectral types than the earliest (O4)
MK standards.  The initial spectroscopic criterion for the new class was
the absence of He~I $\lambda$4471 in moderate-resolution (1~\AA), widened 
photographic (IIa-O) spectrograms, whereas it is a well-defined, albeit
weak, absorption line in O4 spectra.  Three of the original O3 stars are 
of luminosity class V according to the criteria of Walborn (1971b, 1973a),
with strong He~II $\lambda$4686 absorption; but one of them, HD~93129A,
is the prototype of a qualitatively new kind of O-type spectra,
displaying not only $\lambda$4686 emission as in the Of supergiants, but
also narrow N~IV $\lambda$4058 emission \textit{stronger} than the normal Of
N~III $\lambda\lambda$4634, 4640-4642 (denoted as f*), and strong N~V 
$\lambda\lambda$4604, 4620 absorption lines.  As also shown in the
initial O3 paper, these N~IV and N~V features are weakly present in O4~If
spectra, in particular with the N~IV emission weaker than the N~III; thus
a correlation between the absorption-line (He~II/He~I) and selective 
emission-line (Walborn 2001) ionization ratios in these spectra was 
demonstrated.  Two more O3 dwarfs in the Carina Nebula were added by
Walborn (1973b), and HD~93129A was classified as O3~If*.

Subsequent quantitative analyses have demonstrated that the O3 stars are
indeed the hottest representatives of Population~I, and the most massive 
(except for peculiar objects such as Eta~Carinae and certain WNL stars, 
which are associated with the O3 stars in the Carina Nebula and are likely 
evolutionary descendants from them); furthermore, they have the most 
extreme stellar winds in terms of both ionization and terminal velocities
(Kudritzki 1980; Simon et~al.\ 1983; Kudritzki et~al.\ 1992; Puls et~al.\ 1996; 
Taresch et~al.\ 1997; Haser et~al.\ 1998; de~Koter, Heap, \& Hubeny 1998; 
Herrero, Puls, \& Villamariz 2000; Herrero et~al.\ 2001).

On the empirical side, the membership of the O3 class has grown
steadily.  An initial review by Walborn (1982) listed ten members, all
classified by the author, and presented the first members of
subcategories O3~V((f*)), O3~III(f*), and O3~If*/WN6-A.  By the time of a
second review (Walborn 1994), the class had grown to 27 definite and 43
possible members, through the work of a number of investigators.  Since
then, many additional O3 spectra have been found; they are scattered
throughout the literature, so that a further review is now in order.
Here we concentrate on a compilation of the pure O3 spectra that display 
the N~IV and N~V features, although the ``adjacent'' subcategories will
also be discussed.  In addition to a census, the objectives are to
review and possibly resolve some outstanding issues in the spectral 
classification of these extreme stars, as well as their derived absolute 
magnitudes and masses.  

\vspace{-.15in}
\section{Data}

The spectrograms displayed here are characterized by their resolution
and signal-to-noise in the tenth and eleventh columns of Table~1,
respectively.  Many of them have been published previously, and further
information can be found in the references listed in the last column of
the table.  The remainder are presented here for the first time, and some
further details about them are given below.  It should be emphasized that
all but one of the latter O3 stars have been previously classified as
such in the literature, usually on the basis of earlier data; the 
corresponding references are given in the notes to the table.

The new O3 star announced here is Cygnus~OB2-22A.  Cyg~OB2-22 was
classified O4~III(f) by Massey \& Thompson (1991; their No.~417), but as
further discussed later, this kind of spectrum can be produced by a
composite of an O3 and a later OB type.  Such has now been determined to
be the case for Cyg~OB2-22.  Red spectroscopy was obtained by IDH using the 
Intermediate-Dispersion Spectrograph (IDS) at the 2.5~m Isaac Newton Telescope 
(INT) on 2000 July~20 and 21; it was immediately evident from the acquisition 
camera that the object is a visual double star.  Follow-up blue-violet 
spectroscopy was obtained on 2000 August~11 with the Intermediate-dispersion
Spectroscopic and Imaging System (ISIS) spectrograph at the 4.2~m
William Herschel Telescope (WHT), through the Isaac Newton Group service
program.  For all observations, the spectrograph slit was aligned with
the double-star components (the ``B'' component is estimated to be at PA
$150^{\circ}{\pm}10^{\circ}$), and the spectrograms of the components were 
separately extracted by modeling the profile across the dispersion as the sum 
of two gaussians of equal, but wavelength-dependent, width (an excellent
approximation for these images).  In this way the separation of the
components was measured as $1\farcs55$ (with an uncertainty $<0\farcs1$); for
comparison, the spatial scale of the ISIS data is $0\farcs20$~pixel$^{-1}$ with 
a PSF FWHM of $\sim{0\farcs9}$, while the corresponding figures for the IDS 
data are $0\farcs33$~pixel$^{-1}$ and $\sim{1\arcsec}$.  In both the blue and 
red data, the resolutions are $\sim{0.9}$~\AA\ and the S/N 150-200 per
wavelength point.  The data yield relative magnitudes of $\Delta{B}=0.33$ and 
$\Delta{R}=0.40$; the difference is somewhat larger than the estimated 
uncertainties, but insufficient to provide compelling evidence for any 
difference in $B-R$ color between the components.  We subsequently discovered 
that Pigulski \& Ko{\l}aczkowski (1998) found that Cyg~OB2-22 ``appeared to 
be a close double'', with $\Delta{I}=0.37$.  The composite blue-violet 
spectrogram of Cygnus~OB2-22AB shown later is derived from the spatially 
resolved WHT observation described above. 

HD~64568 in NGC~2467/Puppis~OB2 is the prototype of the O3~V((f*))
category (Walborn 1982), but no digital observation of its spectrum has 
been published.  Such was kindly obtained for this project at the request 
of IDH by Dr.\ K.~C.\ Freeman at the Mount Stromlo/Siding Spring 2.3~m 
telescope on 2000 December~9-10.  The total exposure time was 20~minutes.  
The resolution is 1.25~\AA\ and the S/N is $\sim200$ per wavelength point.

The sources of the other new data listed in Table~1 are as follows.
Observations of stars in the Carina Nebula and N11/LH10 in the LMC
were made by DJL with the EMMI instrument at the New Technology Telescope
of the European Southern Observatory, La Silla during 1992 December~2--6; 
the resolution is 0.9~\AA\ and the typical S/N is 100--200.  The observation 
of LH114-7 was obtained and discussed, but not reproduced, by Oey (1996; 
her designation D301SW-3); the instrument was the Argus multifiber
spectrograph at the 4~m telescope of the Cerro Tololo Inter-American
Observatory.  BI253 was reobserved by PM with the CTIO 4~m RC
spectrograph in 1999 January (see Massey et~al.\ 1995, 2000).  
The spectrogram of Sk~$-71^{\circ}~51$ was obtained by NRW
with the two-dimensional photon-counting system at the CTIO 1~m telescope 
on 1989 December~26--27; the observational parameters were as in Walborn \& 
Fitzpatrick (1990), and a total exposure time of 3.3~hours yielded a
maximum of 18,000~counts in the continuum, but substantial detector
fixed-pattern noise remains in this spectrogram despite efforts to reduce
it during the observations.  The new data for Parker (1993) 871, 
1163, and 1311 in 30~Doradus are from an Argus program led by AFJM, with
three runs during 1993--1996 involving multiple exposures to detect
spectroscopic binaries, currently being analyzed; the spectrograms shown
here are sums of the available exposures (no velocity shifts being
detected in these cases), with a resolution of $\sim2$~\AA\
and typical S/N of 150--200.  Finally, additional, high-resolution
observations of the Carina Nebula dwarfs were obtained by NIM and NRW
with the Bench-Mounted Echelle spectrograph at the CTIO 1.5~m telescope
during 1992 March~19--22; the 2-pixel resolution of these data is
0.26~\AA\ and the S/N ranges between 100 and 150.  


\section{Results}

\subsection{Cygnus~OB2-22}

Figures~1 and 2 present the blue and red, respectively, spatially resolved 
spectra of Cyg~OB2-22A and B, compared with the spectrum of Cyg~OB2-7, O3~If*, 
as discussed by Walborn \& Howarth (2000).  As already mentioned, the
combined light of No.~22AB was reasonably classified as O4~III(f) by Massey 
\& Thompson (1991), and it was analyzed as such by Herrero et~al.\ (1999).  
However, the spatially resolved spectrum of No.~22A shows no trace of He~I
lines, so it is of pure O3 type, here also classified as O3~If* for
reasons that will become apparent later, although He~II $\lambda4686$
emission is somewhat weaker and the line has a clearer P~Cygni profile
than in No.~7.  The He~I lines in the combined light of No.~22 originate in 
the B~component, which has a well defined spectral type of O6~V((f)).  Some
other cases of O3 spectra with strong He~I lines that have been resolved
into multiple systems will be noted below, and this general
classification issue will be discussed further.  As shown in Figure~2, the
peculiar H$\alpha$ profiles of Nos.~7 and 22A are remarkably similar.  Thus,
Cyg~OB2-22A becomes the second O3 supergiant in that association, and
only the second O3 star of any kind known in the Northern Hemisphere.

\subsection{The (Former) O3 Class}

Observational and derived data for the stars to be discussed are
listed in Table~1, in the order in which their spectra are presented in
Figures~3 through 8.  The emphasis is on normal supergiants and giants
with the best available data, with the addition of dwarfs showing the
N~IV, N~V lines, and the original Carina Nebula O3 dwarfs in which those
lines are weak or absent.  Successive columns of Table~1 give the stellar 
designation; coordinates for equinox 2000; $V$ and $B-V$\footnote{The
magnitude of Cyg~OB2-22A has been calculated from the combined light with
the assumptions of the same color for both components and 
$\Delta$$V=0.4$.}; stellar or nebular association; distance modulus 
$V_{0}-M_{V}$; absolute visual magnitude\footnote{Derived from the photometry 
and distance modulus, with the assumptions of $(B-V)_{0}=-0.32$ and $R=3.0$ 
(4.0 for NGC~2244 only); except that in cases with ${B-V}>1.00$, $E_{B-V}$ 
has been calculated from the 50,000~K, log~g~=~5.0 model of Lejeune, 
Cuisinier, \& Buser (1997) to account for bandpass effects in the BV filters, 
kindly done by Dr.\ Jes\'us Ma\'{\i}z-Apell\'aniz.  The calculated 
$E_{B-V}$ values are 1.94 for Cyg~OB2-7 and Pismis~24-1, 2.70 for
Cyg~OB2-22, 1.99 for Pismis~24-17, and 1.54 for the NGC~3603 stars.}; mass 
in $M_{\sun}$\footnote{The first value is from the evolutionary calibration 
of Vacca, Garmany, \& Shull (1996) for the corresponding O3-O5 luminosity 
class, scaled linearly by visual luminosity as may be appropriate for stars 
with the same $T_{\mathrm{eff}}$ and $g$.  The second value has been either
interpolated from the models of Schaller et~al.\ (1992) or Schaerer et~al.\
(1993) for the appropriate galaxy, with log~$T_{\mathrm{eff}}=4.705$ and $BC=-4.5$ 
for spectral types O2-O3.5~III-I and 4.680, $-$4.3 for O3.5-O5~V; or 
extrapolated by second-order fits to ZAMS mass$-M_{\mathrm{bol}}$ relations from the 
same models for $M>120$~M$_{\sun}$.}; the original resolution in \AA; the S/N 
per resolution element; and the reference to the spectroscopic data displayed 
here.  Other key references, including the original sources of the O3 
classifications, are given in notes to the table.

It will be noted that new spectral types have been introduced, and a number of 
the former O3 stars reclassified into them, in Table~1 and Figures~3 through~8. 
These developments have resulted from the detailed consideration of the spectra 
in this section, and they are described in the following section.  

To support a census, all additional stars classified as O3 giants or
supergiants in the literature known to us are listed in Table~2.  Some of
these cases involve data of lower quality or not readily available,
spectral peculiarities, or possible membership in ``adjacent'' subcategories.
These last comprise the intermediate O3~If*/WN-A subclass, O3~V or
O3~V((f)) types (although the original Carina Nebula representatives are
included in Table~1 for reference in the discussion of classification issues),
and O4 giants or supergiants in which weaker N~IV, N~V features are seen
in high-quality data.  In general, these subcategories will not be
considered explicitly here, although some specific cases will be
referenced in the context of the discussion.  

\subsubsection{Supergiant Spectra, Figure 3}

The six spectra shown in Figure~3 are placed in the supergiant class
because of their strong He~II $\lambda4686$ emission features.  The strong, 
narrow N~IV $\lambda4058$ emission and N~V $\lambda\lambda4604$, 4620
absorption lines are further attributes of this spectral type.  

\noindent {\it HD~93129A} in Trumpler~14, one of the ionizing clusters of the 
Carina Nebula (NGC~3372), is the prototype of the subcategory, originally 
described as such by Walborn (1971a); the relationship of this new kind of 
spectrum to O4~If+ and WN-A (or WNL) spectra was discussed there and by Walborn 
(1973b, 1974, 1982) and Walborn et~al.\ (1992).  

\noindent {\it Cygnus~OB2$-$7} was the second O3 supergiant to 
be found (Walborn 1973c), while {\it Cyg~OB2$-$22A} is announced as such here 
(Section~3.1).  These two spectra are of somewhat lower ionization than that 
of HD~93129A, in terms of their N~IV/N~III emission-line ratios, an important 
point to the subsequent discussion.  

\noindent {\it Pismis~24$-$1}, belonging to the cluster 
associated with the Galactic nebula NGC~6357, was recently classified by 
Massey et~al.\ (2001); it has an even smaller N~IV/N~III ratio, barely 
qualifying for the f* designation, and a weak but definite He~I $\lambda4471$
absorption line is detected, yet the ionization ratios are too high and the
N~V absorption lines too strong for type O4 (Walborn et~al.\ 1995; 
Walborn \& Howarth 2000).  This kind of spectrum is well described by a new 
O3.5 subclass, as will be further discussed later.  

\noindent {\it MACHO 05:34:41.3$-$69:31:39} (Ostrov 2001) is a short-period
eclipsing binary some distance to the southwest of 30~Doradus.

\noindent {\it MH36} (Massey \& Hunter 1998) is only 0.4~pc in projection 
from the center of R136.

All of these spectra have heretofore received the same classification, but 
their diversity in detail is also noteworthy and likely of physical 
significance.  Ionization differences have already been noted, but the range 
of $\lambda4686$ profiles is also remarkable: none of the others is exactly 
like HD~93129A!  Cyg~OB2-22A and Pismis~24$-$1 have quite similar, well defined
P~Cygni profiles in that line (which rarely occur in Of and WN spectra)
and Cyg~OB2-7 has a related profile.  On the other hand, the greater width
of $\lambda4686$ in the MACHO object and MH36 indicates that they are
further along toward the O3~If*/WN-A state (Walborn 1982; Melnick 1985;
Walborn et~al.\ 1992; Walborn \& Blades 1997; Massey \& Hunter 1998).
This point may be related to the surprisingly low luminosities and masses
of these last two objects, as will also be further discussed later. 

\subsubsection{Giant Spectra, Figures 4 and 5}

Three subcategories of O2-O3 giant spectra are distinguished in Figures~4
and~5.  An important result of this study is the recognition that the
majority of O2-O3~III(f*) spectra display incipient P~Cyg profiles at He~II 
$\lambda4686$ (Fig.~4).  Ironically, the prototype and the second known
member of the class (HDE~269810 in the LMC [Walborn 1982] and NGC~346-3 
in the SMC [Walborn \& Blades 1986], respectively; top of Fig.~5) have pure 
absorption in that line, at least in the data shown here, although the
line appears asymmetric in the sense expected from a wind.  LH10$-$3061 in N11 
also has an He~II $\lambda4686$ absorption line in the present observation, 
but the lower-resolution spectrogram of Parker et~al.\ (1992) has an apparent,
incipient P-Cyg profile there.  Moreover, an even higher quality digital
observation of HDE~269810 by DJL (unpublished) shows a definite incipient
P-Cyg profile at $\lambda4686$; and Walborn (1977) originally classified
this spectrum photographically as O3~If* for the identical reason, but 
Walborn (1982) found pure absorption at that line in a higher quality
spectrogram, then preferring the giant luminosity class.  Evidence for
variable emission features in HD~93129A was cited by Walborn (1971a). 
Thus, line-profile variability in these types of spectra may not be 
uncommon and should be investigated systematically.  Comparing Figures~3 
through 5, one sees that O2-O3 giant and supergiant spectra show a
\textit{continuum} of He~II $\lambda4686$ profiles from pure (asymmetric) 
absorption, through P~Cyg profiles of different degrees of development, to 
pure emission.  This fact may be related to the result that $\lambda4686$
emission correlates poorly with luminosity in O2-O3 spectra, unlike at later
O~types where P~Cyg profiles are very seldom encountered in this line.
It appears that at spectral types O2-O3, He~II $\lambda4686$ may be providing
primarily information about wind development and/or evolutionary stage, 
rather than luminosity.  The evidence for variability is also suggestive;
these stars are likely to be relatively near to the WN stage.  

\noindent \textit{Sk~$-$68$^{\circ}$~137} was classified O3-4~III by Conti, 
Garmany, \& Massey (1986) and the earlier type was confirmed by Garmany \& 
Walborn (1987); note that the emission line at $\lambda4631$ is due to Si~IV 
(with possible contributions from N~IV, O~IV, and S~IV) and {\it not} N~III, 
while C~IV $\lambda4658$ is very strong, blended with Si~IV $\lambda4654$.  
These same emission lines are present in the spectrum of HDE~269810 (Fig.~5).  
Sk~$-68^{\circ}~137$ is located at the northern edge of the 30~Doradus 
Nebula, about 200~pc from R136 in projection, and one wonders if it might 
be a runaway star from the ionizing cluster; a transverse velocity of 
100~km~s$^{-1}$ would carry it that far in 2~Myr.  

\noindent \textit{LH64$-$16} was found to be of spectral type O3 by Massey et~al.\ 
(2000); although partly a resolution effect, the N~IV and N~V lines are 
remarkably strong in this spectrum, and there is no trace of He~I 
$\lambda4471$ at very high S/N.  

\noindent \textit{LH114$-$7} was classified as O3 by Massey et~al.\ (1995; their 
No.~lmc2-675) and Oey (1996; her No.~D301SW-3); there are weak He~I lines in 
this spectrum, more clearly seen in the data of Massey et~al., that likely
arise in a companion.  

\noindent \textit{Pismis~24$-$17} was first classified as O3 by Massey 
et~al.\ (2001); it shares a small N~IV/N~III emission ratio with Pismis~24$-$1 
(Fig.~3), but no He~I $\lambda4471$ is detected (there is an artifact 
just longward of that wavelength), and the N~V absorption is far too 
strong for type O4.  

\noindent The last three spectrograms in Fig.~4 have relatively low 
spectral resolution by the Faint Object Spectrograph on
the Hubble Space Telescope, but the high spatial resolution of HST was
essential to isolate them within their very compact clusters (see
references in Table~1 and further discussion of LH10$-$3209 below).  NGC~3603 
is the most luminous, optically visible H~II region in the Galaxy, with 
strong similarities to 30~Doradus, as well as between the NGC~3603 central 
cluster and the 30~Dor cluster core, R136 (Walborn 1973d; Moffat et~al.\ 
1994); the NGC~3603 objects shown here established a close connection 
between O3~III(f*) and high-luminosity WNL stars (Drissen et~al.\ 1995).  

The three spectra at the bottom of Figure~5 combine strong O2-O3 signatures
of N~IV, N~V with moderately strong He~I lines, which suggests a
composite O2-O3 plus later OB-type system.  

\noindent The composite nature of {\it Cygnus~OB2-22} 
has been demonstrated here (Section~3.1 and Figures~1, 2).
Note that the composite spectrum has been given a luminosity class of 
III, but the resolved spectrum of No.~22A a class of~I, because of 
the dilution of the O3 features in the former.  

\noindent A composite classification of \textit{LH10$-$3209} in N11B was given by 
Parker et~al.\ (1992) and Walborn \& Parker (1992).  This interpretation was
subsequently verified directly by HST images and spatially resolved
spectroscopy, which showed that the He~I originates in several close
companions to the O3 star (Walborn et~al.\ 1999); the uncontaminated O2-O3
spectrum of LH10$-$3209A is reproduced here in Figure~4.  

\noindent The spectrum of {\it Sk~$-$66$^{\circ}$~172} was discussed by Walborn 
et~al.\ (1995); a faint companion was detected in HST images by Kudritzki et~al.\ 
(1996), and there is evidence from current analysis of scans with an HST Fine
Guidance Sensor interferometer for further structure in one of the
components (E.~Nelan, private communication).  

Two other cases of O2-O3 spectra with weaker He~I lines are included in the 
previous figures (Pismis~24$-$1 and LH114$-$7), and some further examples will be 
shown in the subsequent figures.  However, in the case of \textit{weak} He~I 
lines detected in high-quality data, a composite interpretation may not be
obvious.  This general issue will be further discussed later.

\subsubsection{Dwarf Spectra, Figure 6}

The spectrum of HD~64568 in NGC~2467/Puppis~OB2 was the first to be
classified as O3~V((f*)), that is, as a dwarf from the strong He~II
$\lambda4686$ absorption, but with (weak) N~IV $\lambda4058$ emission
stronger than N~III $\lambda\lambda4634$, 4640--4642 (Walborn 1982).  
Commensurate N~V $\lambda\lambda4604$, 4620 absorption lines are also 
present.  The spectrum of LH10$-$3058 in N11B is very similar and was the 
second to be so classified (Parker et~al.\ 1992; Walborn \& Parker 1992).   
Both of these spectra show weak but definite He~I lines in the
present high-quality data (although some uncertainty is introduced by the
nebular emission-line subtraction in LH10$-$3058).  The presence of a later-type 
companion cannot be excluded, but neither can it be assumed from the available 
information, especially in view of the absolute visual magnitudes of these 
stars, which are at the faint end of the range for class~V (Table~1).  As 
reviewed in the Introduction, the original definition of spectral class O3 was 
the absence of He~I in the moderate-resolution photographic data then in use.
Clearly, the criteria must evolve for data of the quality shown here; these 
two spectra cannot be classified into any other existing category.  Again, 
the general problem of He~I in O2-O3 spectra will be further discussed below.  

\noindent In contrast, the spectrum of \textit{BI253}, first classified as
O3 by Massey et~al.\ (1995), shows no hint of He~I; note that unlike the 
previous two, it has no trace of N~III emission, either, but does have
well marked C~IV $\lambda4658$ emission, indicating a hotter spectrum, 
a point that will be taken up later.  This star is located at the
northwestern edge of the 30~Doradus Nebula (Brunet et~al.\ 1975), about
110~pc from R136 in projection and not far from Sk~$-68^{\circ}~137$ 
discussed in the previous section, again suggesting the possibility of a 
runaway star from the ionizing cluster; in this case a transverse
velocity of 55~km~s$^{-1}$ would be required for a lifetime of 2~Myr.

\noindent \textit{Sk~$-$71$^{\circ}$~51} is the brightest star in the central 
cluster of the small, isolated H~II region NGC~2103 within Henize N214 
south of 30~Doradus (Garmany \& Walborn 1987; Walborn 1994); in this case 
the excessively bright absolute magnitude is clearly due to the aperture 
photometry of Isserstedt (1975) having included the entire cluster core.
There is a strange, semi-stellar object brighter than the O3 star at the
northern edge of this H~II region, which may be related to an early epoch
of massive star formation; a detailed study of this region will likely
prove worthwhile.  

\noindent The last three stars in Figure~6 belong to the
30~Doradus ionizing cluster and were originally classified as O3 by 
Melnick (1985) or Walborn \& Blades (1997); the stellar designations are
from Parker (1993).  They are strongly affected by nebular emission lines, 
but the O2-O3 signatures of N~IV, N~V are clear.  Note that the spectrum 
of {\it P871} has no N~III emission, while the other two do; in these spectra 
it is unclear whether $\lambda4658$ is due to C~IV or to nebular [Fe~III].

\subsubsection{Carina Nebula Dwarf Spectra, Figures 7 and 8}

Although spectra without the f* (N~IV~$>$~N~III emission) qualifier are not 
within the purview of this study per se, new, high-quality digital observations 
of the original O3~V stars in the Carina Nebula (Walborn 1971a, 1973b) are 
displayed in Figures~7 and~8, for comparison with the spectra in Figure~6 
and to support the discussion of classification issues in the next section.  
The O5~V((f)) spectrum of HD~93204 in the same region is also shown in
Figure~7, and those of the primary classification standards HD~46223,
O4~V((f+)) and HD~46150, O5~V((f)) in NGC~2244 in Figure~8; these comparison 
stars were observed during the same runs and with the same equipment as the 
other stars in their respective figures.  All of these spectra now receive 
the ((f)) or ((f+)) designation, since N~III emission is detected; the ``+'' 
qualifier denotes the additional presence of Si~IV $\lambda\lambda4089$, 4116 
emission lines.  Weak N~IV $\lambda4058$ emission also appears to be detected 
in HD~93128, HD~93129B, and HD~93250, but in no case is it as strong as
the N~III; the N~V absorption lines are also clear in these spectra.  The 
strengths of the He~I $\lambda4471$ absorption lines in these data are 
particularly significant, and the montages are ordered by that feature.  
It is clearly detected in all of them, but very weakly in HD~93128, HD~93129B, 
HD~93205, and HD~93250, consistent with the original O3 classifications.  
(HD~93205 is a double-lined spectroscopic binary with both components 
visible here, the stronger He~I line corresponding to the later-type 
companion--Conti \& Walborn 1976; Morrell et~al.\ 2001.)  However, the 
He~I~$\lambda4471$/He~II~$\lambda4541$ ratio in HDE~303308 requires a
reclassification to O4~V((f+)), as clearly shown by the comparison with 
HD~46223.  On the assumption that the spectrum has not changed, the
likely explanation is that contamination by the He~I nebular emission
line affected the original photographic spectrograms of HDE~303308 to a
greater extent than was appreciated at the time.  In any event, a
comparison of HD~93128, HD~93129B, HD~93205, and HD~93250 with HD~64568 
and LH10$-$3058 in Figure~6 immediately shows another classification issue: 
while both groups have weak He~I lines in data of the present quality, the
latter two stars have much larger N~IV/N~III emission-line ratios,
rendering them uncomfortable spectral classmates of the Carina Nebula dwarfs. 
Finally, note the somewhat weakened He~II $\lambda4686$ absorption line
in HD~93250 relative to the other spectra; this effect is in the sense
expected for its anomalously bright absolute magnitude (Table~1).


\subsubsection{Census}

Table~2 lists additional stars that have been reliably classified as O3 
giants or supergiants in the literature, or as O3~V((f*)) or related
subcategories.  There are 23 stars in Table~2, but two have been
reclassified as O4~III(f+) in the present study, leaving 21 stars to be
added to the 24 listed in Table~1 (again, not including the Carina Nebula
dwarfs).  Thus, a total of 45 stars have been classified to date into the 
O3 subcategories primarily addressed by this study.

Several stars in Table~2 warrant special mention.  High-quality data for
the two reclassified as O4~III(f+), W28-23 in LH81 and ST2-22 in LH90,
are illustrated by Massey et~al.\ (2000); these are the best published
examples of this spectral type as defined in the present analysis.  Two
other objects in Table~2, ST5$-$31 in LH101 also shown by Massey et~al.\
(2000) and MH16 in 30~Doradus shown by Massey \& Hunter (1998), define a
new peculiar category in which all line ratios are consistent with type
O3~If*, but all lines, both absorption and emission, are anomalously
weak.  Finally, a composite O3~V((f*))~+~OB interpretation of the
interesting spectrum of ST5-52 in LH101, also well illustrated by Massey
et~al.\ (2000), is preferred in the present analysis.

The locations of these 45 stars are interesting.  As previously
mentioned, only two of them, the pair in Cygnus~OB2, are in the Northern
Hemisphere.  Just one, NGC~346$-$3 (Walborn \& Blades 1986; Niemela, Marraco, 
\& Cabanne 1986), is in the Small Magellanic Cloud; NGC~346 is the largest
H~II region of the SMC and contains over half the O~stars known in that
galaxy (Massey, Parker, \& Garmany 1989, in which this star is No.~355). 
No fewer than 34 of these stars are located in the Large Magellanic
Cloud, and 20 of those are associated with the 30~Doradus ionizing
cluster (including the two at the edge of the Nebula, Sk~$-68^{\circ}~137$ 
and BI253, suggested as possible runaways above).  These statistics
re-emphasize the importance of the LMC, and of 30~Dor in particular, to the
study of the evolution of the most massive stars (Massey \& Hunter 1998).
The remaining 8 stars are located in the Southern Milky Way.


\section{Discussion}

\subsection{Spectral Classification Issues}

In the detailed consideration of the spectra above, some issues
regarding their classification became apparent.  Particularly, ranges in
the criteria within a given subcategory, and the presence of weak He~I
lines in some spectra, require further discussion.  Although some of
these issues were recognized previously (Walborn 1994), the higher quality 
and larger sample of present spectroscopic data for the former O3 class
now support some revisions and extensions of the classification system.

\subsubsection{New Spectral Types O2 and O3.5}

The original primary criterion for spectral type O3 was the absence of
He~I $\lambda4471$ in moderate-dispersion, photographic spectrograms.
Thus, the class was bounded on the low-temperature side only, since the
He~I/He~II classification ratios were no longer available.  It might be 
expected that with improved observational material, a range of (small)
He~I/He~II ratios would be discernible, allowing some lifting of the 
degeneracy.  However, as further discussed in the next section, the presence 
of weak He~I lines in spectra earlier than O4 raises several questions that
cannot necessarily be answered by the classification material itself.
Thus, it now appears preferable to shift the primary classification
criterion to the N~IV/N~III emission-line ratio in the earliest O-type
spectra, which in general correlates well with the He~I/He~II
absorption-line ratio, but is not subject to the complications affecting
the latter.  Following this approach, it has been found useful to
introduce new spectral types O2 and O3.5 to describe more consistently
the spectra considered here.  The criteria and representative stars for
the O2-O4 types at the three luminosity classes in use are summarized in
Table~3.

Considering first the class~V stars in Figures~6 through~8, one sees very
weak He~I lines in both the Carina Nebula dwarfs and HD~64568/LH10$-$3058, 
but considerably larger N~IV/N~III ratios in the latter two spectra.  The
Carina stars HD~93128, 93129B, 93205, and 93250 are hereby reclassified as
O3.5~V((f+)); as previously discussed, HDE~303308 is reclassified to
O4~V((f+)).  HD~64568 and LH10$-$3058 now redefine spectral type O3~V((f*)).
Then BI253, with no trace of N~III or He~I in its spectrum, becomes the 
primary standard for the new type O2~V((f*)).  Although the data quality
is lower, Sk~$-71^{\circ}~51$ remains at O3~V((f*)) for the time being,
while P1163 and P1311 definitely belong there as well on the basis of
their N~III emission-line strengths, despite the strong nebular emission
(demonstrating one advantage of the N emission ratio over the He absorption
as a classification criterion for these stars).  On the other hand, P871
has very weak or no N~III emission in its spectrum, thus joining BI253 at
O2~V((f*)).

The O2-O4 N~IV/N~III scale at luminosity classes~I and III is somewhat shifted 
relative to that at class V, tending toward similar values along diagonal lines 
in the two-dimensional (spectral type--luminosity class) grid, i.e., entirely 
analogously to the Of effects at later types (Walborn 1971b).  One objective 
in the reformulation is to minimize revisions to well-established existing
classifications.  Thus, the small N~IV/N~III ratios in O4~If+ spectra remain as
defined by HD~190429A (Walborn \& Howarth 2000) and Sk~$-67^{\circ}~166/167$
(Walborn et~al.\ 1995).  The high-quality data for ST2$-$22 and W28$-$23
illustrated by Massey et~al.\ (2000) provide an excellent definition of
type O4~III(f+) in the present system.  Now the spectra of Pismis~24$-$1 and 
24$-$17 (Figures~3 and~4), with N~IV~$\sim$~N~III, define spectral types
O3.5~If* and O3.5~III(f*), respectively.  HDE~228766 (Walborn 1973e; see
Walborn 1971a for a photographic comparison with HD~190429A), NGC~2044W$-$9A 
(Heydari-Malayeri et~al.\ 1993; Walborn et~al.\ 1999; Massey et~al.\ 2000), and 
MH14 (Massey \& Hunter 1998) are further examples of type O3.5~If*.  The 
spectra of Cygnus~OB2-7 and 22A (Figure~3), with N~IV~$>$~N~III, remain to 
define spectral type O3~If*.  All of the previous O3~III(f*) spectra shown 
here (Figures~4 and~5) have N~IV~$\gg$~N~III and so move to O2~III(f*), 
except for those with FOS data (LH10-3209A and the NGC~3603 stars), which are 
inadequate to decide and must remain as indeterminate O2-O3 for the present; 
presumably some O3~III(f*) spectra with N~IV/N~III ratios similar to those of 
the Cygnus~OB2 stars will be found eventually.  Finally, HD~93129A (Figure~3), 
the original O3~If* spectrum, also has N~IV~$\gg$~N~III and so assumes a new
prototype role as the sole current representative of type O2~If*; MH36 (FOS 
data) and the MACHO star receive the indeterminate O2-O3 designation for now.  

Two methodological or philosophical points should be noted here.  
(1)~The use of an emission-line ratio as a horizontal classification
criterion in O-type spectra is a qualitative departure from previous
practice, although the strength of the N~III emission has been used
successfully as a vertical criterion for some time and has even received
theoretical support as such (Mihalas, Hummer, \& Conti 1972; Mihalas \&
Hummer 1973).  It should be recalled that these are \textit{selective}
emission lines (Walborn 2001), which have narrow, symmetrical, and
undisplaced profiles quite similar to those of the photospheric
absorption lines, and for which there is ample evidence of correlation
with the stellar atmospheric parameters.  In particular, the correlation
of the N~IV/N~III ratios with the previous O3f*-O4f classifications has 
been emphasized since the outset and in fact suggested the new
system.  The full range of these emission-line strengths may depend on
stellar-wind effects, but it has also been shown that O-type stellar
winds display an excellent correlation with the optical spectral types
(Walborn, Nichols-Bohlin, \& Panek 1985).  Any new classification system
remains hypothetical until tested by calibration, application, and
modeling.  It will be of considerable interest to have detailed
investigations of the line-formation mechanisms for the N~IV and other
selective emission lines, comparable to that available for the N~III.
The effects of metallicity on the N~IV/N~III should also be taken into
account, in view of the results of Crowther (2000).  For the moment,
there is no indication of a correlation between the new spectral types 
and the host galaxy, and there are good empirical reasons to propose 
the emission-line criterion as discussed above.
(2)~The quality of many of the data discussed here exceeds that of
typical classification material, and some developments depend on it, e.g.\
the detection of Si~IV emission in the Carina Nebula dwarfs, as denoted
by the ((f+)) notation.  The present philosophy is to include all
available information in the spectral types, even though some of it may
not be reproducible with lower observational quality.  

\subsubsection{He~I in O2-O3 Spectra}

The presence of \textit{strong} He~I lines in a spectrum with O2-O3
signatures immediately suggests the presence of a later-type companion,
and this inference has been borne out in three cases discussed in 
Section~3.2.2.  ST5$-$52 illustrated by Massey et~al.\ (2000) is hypothesized here
to be a further case at O3~V((f*))~+~OB.  However, the interpretation of 
\textit{weak} He~I lines in such spectra is not so straightforward, since
they may be either intrinsic or from a companion.  In the first place,
their detection is, of course, a function of observational quality.
Kudritzki (1980) and Simon et al. (1983) first detected weak He~I lines in 
several of the Carina O3 prototypes in high-S/N, high-resolution photographic
(IIIa-J) spectrograms, using them to derive physical parameters; Kudritzki 
et~al.\ (1992) later measured them in high-resolution digital data, and several 
more cases are clear in the high-S/N data displayed here, as discussed above.  
In view of the high incidence of spectroscopic binaries and multiple systems
among massive stars, the presence of a companion cannot be
entirely excluded without intensive radial-velocity monitoring and
high-resolution imaging.  As discussed by Walborn et~al.\ (1999), however,
closing the gap between these two techniques in the Magellanic Clouds
will require future generations of imaging capabilities.  Even the
spectroscopic observations are problematic, since they may require very
long baselines, large telescopes, and/or high spatial resolution in the
case of compact clusters; and, there is always a possibility of an  
unfavorable inclination.  A further problem is He~I emission-line
contamination from the inhomogeneous nebulae within which many of the
hottest stars are found; subtraction uncertainties at moderate resolution
can easily produce under- or overestimates of the stellar He~I
absorption-line strengths. 

In several of the spectra discussed here, the weak He~I lines detected
correlate well with the N~IV/N~III emission-line ratios and can reasonably 
be inferred to be intrinsic.  Examples are the Carina Nebula dwarfs and
Pismis~24$-$1 at O3.5.  Conversely, the absence of He~I at very high S/N 
supports the O2 reclassification of, e.g.\ BI253, HDE~269810, and LH64$-$16
among others.  But several other cases are less clear or ambiguous.  For
example, there is a weak He~I $\lambda4471$ line in HD~93129A, but it appears 
to have a shallow P~Cyg profile and may be formed at least partly in the 
stellar wind (see also Kudritzki et~al.\ 1992); $\lambda4471$ wind profiles are 
often present in WN-A spectra, to which that of HD~93129A is related (Walborn 
1971a, 1974; Walborn et~al.\ 1992; Walborn \& Fitzpatrick 2000).  Therefore, 
given the available information, it has been judged preferable here to shift 
primary reliance to the N~IV/N~III emission-line ratio for the horizontal 
spectral classification of these stars with the present data quality, as 
discussed in the previous section. 

\subsubsection{Other Wavelength Ranges}

Given the paucity of classification criteria for the hottest spectra in 
the traditional wavelength range, it is reasonable to hope for additional 
constraints from other ranges.  Such an investigation must, of course,
proceed as any classification project involving new parameters, with the 
observation of a significant sample, charting of the criteria, definition 
of standards, and a determination of the consistency (or otherwise) with 
the results from the blue-violet domain.  For example, Drissen et~al.\ (1995) 
found that several N~IV and O~IV absorption features near 3400~\AA\
discriminated sharply between spectral types~O3 and~O4.  While CNO
features must always be viewed with caution as classification criteria,
because of the good evidence for mixing of processed material in massive
stars (e.g., Walborn et~al.\ 2000 and references therein), a systematic
study of the behavior of these near-UV features in as many as possible of
the stars discussed here appears well worthwhile.  Such a program is
being planned by NIM and NRW.

In the case of hot stars the far UV is likely to provide valuable
information, and indeed in an Atlas of IUE high-resolution data Walborn     
et~al.\ (1985) found several features that appear to be
potential discriminants at the earliest O~types.  They include O~V
$\lambda1371$, in which the presence of a stellar-wind profile is a
unique signature of O2-O3 giant and supergiant spectra (see also Walborn 
et~al.\ 1995; Taresch et~al.\ 1997; Haser et~al.\ 1998; de~Koter, Heap, 
\& Hubeny 1998; Herrero et~al.\ 2001); it may also be a useful criterion for 
dwarf spectra, in ratio to O~IV $\lambda\lambda1339$, 1343 (Morrell et~al.\ 
2001).  N~III $\lambda\lambda1748$, 1752 appear to strengthen significantly
between types O3 and O4 (references cited).  Far-UV observations are not
yet available for all of the stars discussed here, but a systematic
analysis of all existing data and further observations will be worthwhile.
Current analyses of the growing FUSE database may also provide valuable
criteria.

\subsection{Absolute Visual Magnitudes}

While not a ``fundamental'' parameter, the derived absolute visual
magnitudes are a critical observable, since given a spectral type and 
an effective-temperature calibration, they determine the bolometric
luminosities and the masses of the stars.  Thus the $M{_V}$'s are of
vital interest to spectral classifiers, galactic-structure specialists,
and stellar evolutionists alike.  Unfortunately, they are notoriously
difficult quantities to obtain for the early-type stars, because of
uncertainties in their distances, reddening laws, and multiplicities.
The first two problems are the most serious in the Galaxy, although the
third can be a factor even for quite nearby objects and it dominates the
other two in the Magellanic Clouds (e.g., Walborn et~al.\ 1999).  Other
sources of scatter in the luminosity calibration are extraneous physical
phenomena that vitiate the normal interpretation of the classification
criteria, e.g., in interacting close binaries; and the recently
recognized effects of rotation on massive stellar evolution (Heger \&
Langer 2000; Meynet \& Maeder 2000; Maeder \& Meynet 2000).  

From an analysis of the ten O3 stars then known, Walborn (1982) concluded 
that the correlation between the absolute magnitudes and the spectroscopic
luminosity classes was considerably poorer than for the later-type O~stars
(e.g., Walborn 1972, 1973a), with large individual discrepancies in both
directions and no distinction between classes~III and~I.  The present
results extend and confirm the earlier analysis.  Inspection of the
individual $M{_V}$'s in Table~1 shows considerable overlap among the
luminosity classes.  The formal average values are $-6.35\pm0.4$ (n~=~6)
for O2-O3.5~If*; $-6.0\pm0.1$ (n~=~10) for O2-O3.5~III(f*); and
$-5.7\pm0.2$ (n~=~7) for O2-O3~V((f*)), where standard deviations of the 
means are quoted.  There is a marginal trend in the expected sense, but the
differences among the means are within their errors.

This is a disappointing result from the viewpoint of spectral
classification, but an important one to know for subsequent astrophysical
interpretation of the hottest Population~I stars.  Several further
remarks can be made about some individual cases.  The MACHO star
05:34:41.3-69:31:39 is a 1.4~d eclipsing binary system (Ostrov 2001), a 
fact that may well be related to its spectroscopic characteristics.  
Its anomalously faint $M{_V}$, as well as that of MH36 (Massey \& Hunter
1998), is reminiscent of the original O3~If*/WN6-A stars Sk~$-67^{\circ}~22$ 
in the LMC and AB2 in the SMC (Walborn 1982, 1986).  A possible hypothesis     
for further investigation is that all of these faint ``supergiant'' and
``transition'' objects may be immediate pre-WN interacting binary systems.
Alternatively, some of them may be pre-WN single stars at the low
end of the progenitor mass range, as further discussed in the next section.

In the case of $M{_V}$'s that are too bright, there is always the
possibility of an unresolved multiple system.  As already noted above,
Sk~$-71^{\circ}~51$ is a demonstrable case in point, the available
aperture photometry having included the entire cluster core of NGC~2103.
A digital imaging photometric study is required to better resolve the O3 
star in this system.

Omitting the three stars just discussed, the average $M{_V}$'s for the
three O2-O3 luminosity classes become $-6.95~\pm~0.2$ (n~=~4),
$-6.0~\pm~0.1$ (no change), and $-5.6\pm0.2$ (n~=~6), a somewhat more
satisfying result.  The main difference from Walborn (1982) is that the
class III mean $M{_V}$ is now closer that of class~V instead of class~I; 
there was only one class III object in the earlier study (HDE~269810), 
which is at the bright end of the corresponding $M{_V}$ range found for 
ten stars here.

Of course, there may be other unknown interacting binaries or multiple
systems among the remaining discrepant cases in the present sample, but
it appears unlikely that such anomalies are the entire explanation of the
$M{_V}$ results.  Rather, as already mentioned above, the behavior of the
primary luminosity criterion He~II $\lambda4686$ is qualitatively
different at spectral types O2-O3 from that at later types, suggesting
that it may well be sensitive to other competitive effects in addition to 
luminosity.  In particular, the $\lambda4686$ absorption remains quite
strong in the O2-O3 giants, the distinction from the dwarfs being based
more on the much greater strengths of the N~IV emission and N~V
absorption features in the giants.  Moreover, $\lambda4686$ tends to
develop a P~Cygni profile in both giants and supergiants at types O2-O3,
which is seldom, or by definition never, observed in \textit{normal} (i.e.,
typical) O~stars at types O4 and later.  This distinction in the behavior
of He~II $\lambda4686$ at spectral types O2-O3 will be a crucial
constraint to be reproduced and explained by astrophysical modeling and
interpretation of these extreme stars; it may well turn out to be an
evolutionary (and/or rotational?) effect among stars of similar initial
(high) masses.

\subsection{Masses and Evolutionary Status}

There is current controversy regarding the effective temperatures of the
O3 stars, and, of course, the new spectral types introduced here remain to
be calibrated.  Values derived by different authors range from 40,000~K
(de~Koter et~al.\ 1998) to 60,000~K (Puls et~al.\ 1996), with correspondingly 
large effects on the derived stellar masses (see also Massey \& Hunter 1998).  
Recent line-blanketed models favor lower values (Martins, Schaerer, \& Hillier 
2001).  Here we have adopted the intermediate temperatures near 50,000~K of 
Vacca et~al.\ (1996) and made estimates of the evolutionary masses from their 
calibration; alternatively, we have also made independent estimates from the 
evolutionary tracks of Schaller et~al.\ (1992) or Schaerer et~al.\ (1993), as 
explained in footnote~16.  These mass estimates are in reasonable 
agreement with results from detailed evolutionary analyses in 
the literature (e.g., Puls et~al.\ 1996), except for the very high values for 
Cyg~OB2$-$22A and Pismis~24$-$1, which, however, depend sensitively on large model 
reddening corrections and large extrapolations of the mass calibrations.  It 
should also be noted that some of the few available orbital mass determinations
for the hottest stars tend to give lower values than the evolutionary
models (e.g., Ostrov 2001; Morrell et~al.\ 2001), suggesting the possibility 
of evolutionary mass ``inflation'', although Massey, Penny, \& Vukovich (2002) 
find good agreement.  The present estimates must suffice for our purposes.
Inspection of Table~1 shows a full range from 38 to 291~M$_{\sun}$ from the 
linear scaling, or a somewhat smaller range of 53 to 210~M$_{\sun}$ from the 
evolutionary models, with reasonable agreement inside of the extremes.  

How can this large range of masses producing similar spectra be
understood?  From the outset, it has been clear that the O3 stars are
very closely related to type WN, on the basis of both spectral morphology
and spatial association.  These relationships were clear from the Carina
Nebula prototypes (Walborn 1971a, 1973b, 1974), and then in 30~Doradus  
(Melnick 1985; Walborn \& Blades 1997; Massey \& Hunter 1998) and in
NGC~3603 (Drissen et~al.\ 1995).  A further striking piece of evidence is
the very large N/C abundance ratio in the stellar wind of NGC~346-3 
(Walborn et~al.\ 1995, 2000; Haser et~al.\ 1998); it is quite possible that
all O2-O3 giants and supergiants have similar large N/C ratios in their
winds, but it is obvious only in the SMC object because the low initial
systemic abundance allows the very sensitive C~IV profile to desaturate.  
But there are at least three distinct channels leading to the WN stage: very 
massive single stars in giant H~II regions, post-red supergiants at lower 
masses, and mass-transfer binaries.  A reasonable hypothesis from the present 
empirical information is that all of these diverse WN phenomena may be 
immediately preceded by an O2-O3 state, leading directly to the large 
range of masses observed for the latter.

A final observational point that may be either a curious coincidence or a 
vital clue to the interpretation of the O2-O3 stars is the pairing of
very similar spectra in Cygnus~OB2 and Pismis~24.  These two pairs of
spectra have details in common (within each pair) that are not shared
with any others in the present sample; i.e., these pairs comprise the
only giant/supergiant spectra currently classified as O3 or O3.5,
respectively, as described above.  This circumstance suggests the
possibilities of cluster metallicity effects and/or extreme evolutionary
synchronization determining the spectroscopic details.  It is also reminiscent 
of the spatial twinning of very rare peculiar objects in the ``OB Zoo'' 
reported by Walborn \& Fitzpatrick (2000).  The derived luminosities and 
masses within each of the present pairs are quite different, although as noted 
above the very large values for Cyg~OB2$-$22A and Pismis~24$-$1 are uncertain.
Time will tell the significance or otherwise of this particular point.


\section{Summary}

We have performed a careful descriptive review of high-quality digital
spectroscopic data for a substantial sample of stars previously 
classified as O3~If*, O3~III(f*), or O3~V((f*)), as well as the O3~V((f))
stars in the Carina Nebula that originally defined the type.  A new
member of the O3~If* category has been presented, namely Cyg~OB2$-$22A,
which is the second O3 star known in that association and only the 
second in the Northern Hemisphere.  The principal result of this study is
that the previous O3 spectral type has been subdivided into the three new 
types O2, O3, and O3.5, to accommodate the range in the classification
criteria revealed by these data.  Thus, a new earliest spectral type has
been defined.  Because of various extraneous effects on the detection and
interpretation of the very weak or absent He~I absorption lines in these
spectra, the principal classification criterion as been shifted to the 
N~IV/N~III selective emission-line ratio, which was already known to
correlate well with the He~I/He~II absorption-line definition of giant
and supergiant types O3 vs. O4.  All of the Carina Nebula dwarfs have 
been reclassified as O3.5~V((f+)), except for one, HDE~303308, which 
has been revised to O4~V((f+)) on the basis of the higher quality data.  
A census of all previous, pure O3 spectra assigned the f* parameter
(signifying N~IV emission stronger than N~III) totals 45 stars, of which 
34 reside in the LMC and 20 of those in 30~Doradus.

Subject to uncertainties in the distances and reddening corrections of
some Galactic stars, and pending the physical calibration of the new
spectral types as well as the availability of line-blanketed atmospheric/wind 
models and (rotating) evolutionary models for masses greater than
120~M$_{\sun}$, the absolute visual magnitudes and masses of the O2-O3.5
stars have been reviewed.  A major result is that the individual spectral
types/luminosity classes show substantially greater ranges in these
parameters than is characteristic of the later O classifications, or than 
is reasonably expected from the uncertainties just enumerated.  This
result may be related to the qualitatively different behavior of the
primary luminosity criterion He~II $\lambda4686$ in the O2-O3.5 spectra,
namely the development of P~Cygni profiles in that line, that are not seen 
at the later types.  A speculative interpretation, supported by both spectral
and spatial morphological evidence, is that O2-O3.5 stars may represent 
an immediate pre-WN phase, so that they mimic the wide luminosity and
mass ranges of the latter, corresponding to different formation channels.


\acknowledgments
We are grateful to Ken Freeman for kindly obtaining the important observation 
of HD~64568 for this discussion, and to Jes\'us Ma\'{\i}z-Apell\'aniz for
the model reddening calculations.


\begin{references}
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\end{references}

\newpage
\begin{landscape}
\begin{figure}%1
\centering
\includegraphics[width=.9\textwidth,angle=-90]{fig1.eps}
\caption{
Rectified linear-intensity, blue-violet spectrograms of the spatially
resolved components of Cygnus~OB2$-$22, with Cygnus~OB2$-$7 for comparison.
The stellar features identified above the spectrum of No.~7 are 
N~IV $\lambda$4058; Si~IV $\lambda\lambda$4089-4116, 4654;
N~III $\lambda\lambda$4634-4640-4642; C~IV $\lambda$4658; and 
He~II $\lambda$4686; the wavelengths of the interstellar lines and 
diffuse interstellar bands identified below are given by Walborn \&
Howarth 2000.  The lines identified in Cyg~OB2-22A are
N~IV $\lambda\lambda$4478-4496-4530 and N~V $\lambda\lambda$4604-4620; 
in Cyg~OB2-22B they are He~I $\lambda\lambda$4026 (+ II), 4387, 4471;
H$\delta$~$\lambda$4102, H$\gamma$~$\lambda$4340; He~II $\lambda\lambda$4200, 
4541, 4686; and (above), the C~III $\lambda$4650 blend.
\label{fig1}}
\end{figure}
\end{landscape}

\begin{landscape}
\begin{figure}%2
\centering
\includegraphics[width=.9\textwidth,angle=-90]{fig2.eps}
\caption{
Red spectrograms of the Cygnus~OB2-22 components and Cygnus~OB2-7. 
The features identified in the spectrum of No.~7 are H$\alpha$~$\lambda$6563;
N~V $\lambda\lambda$6716-6718; and C~III $\lambda\lambda$6721, 6727-6731.
In Cyg~OB2-22A, the lines identified are N~IV $\lambda$6381 and
He~II $\lambda\lambda$6406-6527-6560-6683.  The wavelengths of the
diffuse interstellar bands marked in Cyg~OB2-22B are given by Walborn \&
Howarth 2000.
\label{fig2}}
\end{figure}
\end{landscape}


\begin{figure}%3
\centering
\includegraphics[width=.9\textwidth]{fig3.eps}
\caption{
Rectified linear-intensity, blue-violet spectrograms of O2-O3~If* stars.
In Figures~3--8, the spectrograms are separated by 0.3 continuum units.
The lines identified in the spectrum of HD~93129A are N~IV $\lambda$4058;
Si~IV $\lambda\lambda$4089-4116; N~III $\lambda\lambda$4634-4640-4642;
and He~II $\lambda$4686.  In addition, Si~IV $\lambda$4654 is identified
in Cyg~OB2$-$7 and C~IV $\lambda$4658 in Cyg~OB2$-$22A.  The lines identified
in MH36 are He~II $\lambda\lambda$4026, 4200, 4541; He~I $\lambda$4471;
and N~V $\lambda\lambda$4604-4620.
\label{fig3}}
\end{figure}

\begin{figure}%4
\centering
\includegraphics[width=\textwidth]{fig4.eps}
\caption{
Blue-violet spectrograms of O2-O3.5~III(f*) stars with incipient P~Cygni
profiles at He~II $\lambda$4686.  The line identifications are as in
Figure~3, with the addition of Si~IV $\lambda$4631 in Sk~$-68^{\circ}~137$.
\label{fig4}}
\end{figure}

\begin{figure}%5
\centering
\includegraphics[width=\textwidth]{fig5.eps}
\caption{
Blue-violet spectrograms of three O2~III(f*) stars with pure absorption
at He~II $\lambda$4686 (top) and three O2-O3~III(f*)~+~OB composite systems
(bottom).  The lines identified are as in the preceding figures, with the
additions of He~I $\lambda\lambda$4026, 4387 in LH10$-$3209.
\label{fig5}}
\end{figure}

\begin{figure}%6
\centering
\includegraphics[width=\textwidth]{fig6.eps}
\caption{
The blue-violet spectra of O2-O3~V((f*)) stars.  The wavelengths of the
lines identified are given in the captions of the preceding figures.  A
number of nebular emission lines have been truncated in the three
30~Doradus objects at the bottom.
\label{fig6}}
\end{figure}

\begin{figure}%7
\centering
\includegraphics[width=\textwidth]{fig7.eps}
\caption{
ESO NTT/EMMI spectrograms of four original O3~V stars in the Carina
Nebula; revised spectral types are shown and the spectrum of an associated 
O5~V((f)) star is included.  The wavelengths of the lines identified are 
given in the captions of the preceding figures. 
\label{fig7}}
\end{figure}

\begin{figure}%8
\centering
\includegraphics[width=.9\textwidth]{fig8.eps}
\caption{
CTIO 1.5~m/BME spectrograms of four original O3~V stars in the Carina
Nebula; revised spectral types are shown and the spectra of the primary
classification standards HD~46223 and HD~46150 in NGC~2244 are included.  
Some instrumental signatures of these echelle data remain in the profiles 
of broad or complex features such as interstellar $\lambda4430$ and the 
$\lambda\lambda4630-4650$ region, respectively.  The wavelengths of the
lines identified are given in the captions of the preceding figures, with
the addition of C~IV $\lambda4441$ in HD~93128.
\label{fig8}}
\end{figure}

\begin{landscape}
%\centering
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%\textwidth=10.1in
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\def\en{\enspace}
%\begin{document}
\begin{deluxetable}{lrrrrcclrcrl}
\tablewidth{9in}
%\tablenum{3}
\tablecaption{Stellar and Observational Data for the Spectra Illustrated}
\tablehead{
\colhead{Star}&\colhead{RA (2000)}&\colhead{Dec (2000)}&\colhead{$V$}
&\colhead{$B-V$}&\colhead{Association}&\colhead{$V_0-M_V$}
&\colhead{$M_V$}&\colhead{$\frac{M}{M_{\odot}}$} &\colhead{Res.\ [\AA]} &\colhead{$\frac{S}{N}$}&\colhead{Source}
}
\startdata
\sidehead{O2$-$O3.5 If$^{*}$ (Figure 3)}
HD 93129A &10 43 57.5 &$-$59 32 53 &7.3\en &0.22 &Carina Neb. &12.2
&$-$6.5 &127/127 &0\rlap{.9} &200 &this paper\\
Cyg OB2$-$7 &20 33 14.2 &41 20 19 &10.55 &1.45 &VI Cygni &11.3 &$-$6.6
&139/135 &1\rlap{.9} &200 &Walborn \& Howarth 2000\\
Cyg OB2$-$22A &20 33 08.9 &41 13 15 &12.12 &2.04 &VI Cygni &11.3 &$-$7.3
&266/200 &0\rlap{.9} &200 &this paper\\
Pismis 24$-$1 &17 24 43.4 &$-$34 11 56 &10.43 &1.45 &NGC 6357 &12.0 &$-$7.4 &291/210 
&1\rlap{.8} &80 &Massey et al.\ 2001\\
05:34$-$69:31 &5 34 41.3 &$-$69 31 39 &13.69 &$-$0.06 &LMC &18.6 &$-$5.7
&61/82 &3\rlap{.6} &100 &Ostrov 2001\\
MH36 &5 38 42.8 &$-$69 06 03 &14.49 &0.06 &30 Doradus &18.6 &$-$5.2
&38/66 &3 &60 &Massey \& Hunter 1998\\
\sidehead{O2$-$O3.5 III(f$^{*}$) with He II $\lambda$4686 P Cygni profile (Figure 4)}
Sk $-$68$^{\circ}$ 137 &5 38 24.8 &$-$68 52 33 &13.26 &$-$0.07 &30
Doradus? &18.6 &$-$6.1 &101/99 &0\rlap{.5} &70 &Walborn et al.\ 1995\\
LH64$-$16 &5 28 47.0 &$-$68 47 48 &13.62 &$-$0.17 &W16$-$8 &18.6 &$-$5.4
&53/72 &1 &160 &Massey et al.\ 2000\\
LH114$-$7 &5 43 12.9 &$-$67 51 16 &13.66 &$-$0.25 &N70 &18.6 &$-$5.2
&44/66 &2\rlap{.2} &80 &this paper\\
Pismis 24$-$17 &17 24 44.7 &$-$34 12 02 &11.84 &1.49 &NGC 6357 &12.0
&$-$6.1 &101/101 
&1\rlap{.8} &80 &Massey et al.\ 2001\\
LH10$-$3209A &4 56 59.0 &$-$66 24 38 &12.73 &$-$0.07 &N11B &18.6 &$-$6.6
&160/140 &3 &60 &Walborn et al.\ 1999\\
HST$-$42 &11 15 07.2 &$-$61 15 39 &12.86 &1.12 &NGC 3603 &14.2 &$-$6.0
&92/95 &3 &60 &Drissen et al.\ 1995\\
HST$-$A3 &11 15 07.5 &$-$61 15 38 &12.95 &1.12 &NGC 3603 &14.2 &$-$5.9
&84/91 &3 &60 &Drissen et al.\ 1995\\
\sidehead{O2 III(f$^{*}$) without He II $\lambda$4686 P Cygni profile (Figure 5)}
HDE 269810 &5 35 13.9 &$-$67 33 27 &12.28 &$-$0.23 &NGC 2029 &18.6
&$-$6.6 &160/140 
&0\rlap{.5} &100 &Walborn et al.\ 1995\\
LH10$-$3061 &4 56 42.5 &$-$66 25 18 &13.68 &$-$0.01 &N11B &18.6 &$-$5.8
&77/85 &0\rlap{.9} &100 &this paper\\
NGC 346$-$3 &0 59 01.1 &$-$72 10 28 &13.50 &$-$0.23 &NGC 346 &19.1
&$-$5.9 &84/81 
&0\rlap{.16} &140 &Walborn et al.\ 2000\\
\sidehead{O2$-$O3 III(f$^{*}$) + OB (Figure 5)}
Sk $-$66$^{\circ}$ 172 &5 37 05.6 &$-$66 21 36 &13.13 &$-$0.12 &N64 &18.6
&$-$6.1 &101/94 &0\rlap{.5} &50 &Walborn et al.\ 1995\\
Cyg OB2$-$22AB &20 33 08.9 &41 13 15 &11.55 &2.04 &VI Cygni &11.3 &$-$7.8 &\nodata 
&0\rlap{.9} &200 &this paper\\
LH10$-$3209 &4 56 59.0 &$-$66 24 38 &12.66 &$-$0.11 &N11B &18.6 &$-$6.6 &\nodata 
&3\rlap{.4} &90 &Parker et al.\ 1992\\
\sidehead{O2$-$O3 V((f$^{*}$)) (Figure 6)}
BI253 &5 37 34.3 &$-$69 01 11 &13.88 &$-$0.13 &30 Doradus? &18.6 &$-$5.3
&56/69 &1 &160 &this paper\\
HD 64568 &7 53 38.2 &$-$26 13 57 &9.39 &0.11 &NGC 2467 &13.2 &$-$5.1
&46/63 &1\rlap{.25} &200 &this paper\\
LH10$-$3058 &4 56 42.2 &$-$66 24 54 &14.24 &$-$0.06 &N11B &18.6 &$-$5.1
&46/63 &0\rlap{.9} &100 &this paper\\
Sk $-71^{\circ}$ 51 &5 41 39.6 &$-$71 19 56 &12.71 &$-$0.09 &NGC 2103 &18.6 &$-$6.6 &\nodata &1\rlap{.5} &80 &this paper\\
P871 &5 38 41.6 &$-$69 05 19 &13.24 &0.05 &30 Doradus &18.6 &$-$6.5
&168/130 &2\rlap{.2} &180 &this paper\\
P1163 &5 38 44.7 &$-$69 04 50 &13.96 &$-$0.04 &30 Doradus &18.6 &$-$5.5
&67/75 &2\rlap{.2} &180 &this paper\\
P1311 &5 38 46.2 &$-$69 06 16 &13.80 &0.08 &30 Doradus &18.6 &$-$6.0
&106/94 &2\rlap{.2} &180 &this paper\\
\\
\\
\\
\\
\sidehead{O3.5$-$O5 V((f)) (Figures 7 and 8)}
HD 93128 &10 43 54.4 &$-$59 32 58 &8.77 &0.24 &Carina Neb. &12.2 &$-$5.1
&46/56 &0\rlap{.9} &200 &this paper\\*
          &           &          &    &     &            &     &     &   &0\rlap{.26} &130 &this paper\\
HD 93129B &10 43 57.8 &$-$59 32 55 &8.9\en &0.22 &Carina Neb. &12.2
&$-$4.9 &38/53 
&0\rlap{.26} &130 &this paper\\
HD 93205 &10 44 33.8 &$-$59 44 16 &7.75 &0.05 &Carina Neb. &12.2 &$-$5.6
&73/69 &0\rlap{.9} &200 &this paper\\
HD 93250 &10 44 45.1 &$-$59 33 55 &7.38 &0.15 &Carina Neb. &12.2 &$-$6.2
&127/91 &0\rlap{.9} &200 &this paper\\
         &           &          &     &     &            &     &     &    &0\rlap{.26} &130 &this paper\\
HDE 303308 &10 45 06.0 &$-$59 40 06 &8.17 &0.13 &Carina Neb. &12.2
&$-$5.4 &60/63 &0\rlap{.9} &200 &this paper\\
          &           &          &     &     &            &     &     &   &0\rlap{.26} &130 &this paper\\
HD 93204 &10 44 32.4 &$-$59 44 31 &8.42 &0.10 &Carina Neb. &12.2 &$-$5.0
&43/55 &0\rlap{.9} &100 &this paper\\
HD 46223 &6 32 09.3 &4 49 24 &7.25 &0.22 &NGC 2244 &10.5 &$-$5.4 &60/63 &0\rlap{.26} &130 &this paper\\
HD 46150 &6 31 55.5 &4 56 34 &6.72\rlap{v} &0.13 &NGC 2244 &10.5 &$-$5.6
&75/69 &0\rlap{.26} &130 &this paper\\
\enddata
\vspace{-.3in}
\tablerefs{HD~93129A: ``Walborn's Star'' 1971a, 1973b, 1974; Conti et~al.\ 1979;
   Simon et~al.\ 1983; Taresch et~al.\ 1997. 
Cyg~OB2-7: Walborn 1973c; Herrero et~al.\ 2000, 2001.
Pismis~24: Lortet et~al.\ 1984; Vijapurkar \& Drilling 1993 (LSS~4142).
Sk~$-68^{\circ}~137$: Garmany \& Walborn 1987; Haser et~al.\ 1998.
LH114$-$7: Massey et~al.\ 1995 (lmc2$-$675); Oey 1996 (D301SW$-$3).
HDE~269810: Walborn 1977, 1982; Puls et~al.\ 1996.
LH10: Parker et~al.\ 1992; Walborn \& Parker 1992.
NGC~346$-$3: Walborn \& Blades 1986; Niemela et~al.\ 1986; Kudritzki et~al.\
   1989; Walborn et~al.\ 1995; Haser et~al.\ 1998.
BI253: Massey et~al.\ 1995. 
HD~64568: Walborn 1982.
Sk~$-71^{\circ}~51$: Garmany \& Walborn 1987; Walborn 1994.
P871, 1163, 1311: Melnick 1985; Parker 1993; Walborn \& Blades 1997.
Carina dwarfs: Walborn 1971a, 1973b, 1995; Conti \& Walborn 1976; 
   Kudritzki 1980; Simon et~al.\ 1983; Puls et~al.\ 1996; Morrell et~al.\ 2001.}
\end{deluxetable}
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\tablewidth{0pt}
\tablenum{2}
\tablecaption{Other Spectra Classified as or Related to O3\textrm{f}$^*$}
\tablehead{
\colhead{Stars} &\colhead{Spectral Types} &\colhead{References} &\colhead{Comments} 
}
\startdata
LH81/W28$-$23 &O3 V((f)) &Massey et~al.\ (2000) &Here O4 III(f+)\\[4pt]
LH90/ST2$-$22 &O3 III(f*), O3 V((f)) &Schild \& Testor (1992), Testor et~al.\ 
(1993),\hfill\break Massey et~al.\ (2000) &Here O4 III(f+)\\[4pt]
LH99/ST1$-$71 &O3 III(f) &Schild \& Testor (1992), Moffat et~al.\ (in prep.) &Possibly O3 V((f*)),\hfill\break better data needed\\[4pt]
LH101/ST5$-$31 &O3 If, O3 If* &Testor \& Niemela (1998), Massey et~al.\ (2000) &Peculiar, weak lines\\[4pt]
LH101/ST5$-$52 &O3 V, ON5.5 V((f)) &Testor \& Niemela (1998), Massey et~al.\ (2000) &Here O3 V((f*))+OB\\[4pt]
LH117$-$11/214 &O3$-$O4 III &Conti et~al.\ (1986), Garmany \& Walborn (1987), Massey et~al.\ (1989)\\[4pt]
LH117$-$43A/140 &O3 III(f*), O3$-$O4(f*) &Garmany \& Walborn\hfill\break (1987), Massey et~al.\hfill\break (1989)\\[4pt]
P1423 &O3$-$O4 III(f) &Walborn \& Blades (1997) &Better data needed\\[4pt]
P1350/H96$-$28 &O3 III(f*) &Massey \& Hunter (1998)\\[4pt]
MH16 &O3 If* &Massey \& Hunter (1998) &Peculiar, weak lines\\[4pt]
MH18, 23, 24, 37, 38, 41, 47, 48, 57, 58, 59 &O3 III(f*) &Massey \& Hunter (1998)\\
\noalign{\smallskip}
MJ257 &O3$-$O4 If &Massey \& Johnson (1993)\\[4pt]
NGC 3603/HST$-$22 &O3 V((f*))? &Moffat et~al.\ (in prep.) &Possibly, better data needed\\
\enddata
\end{deluxetable}
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\begin{table}
\begin{center}
\begin{tabular}{cccc}
\multicolumn{4}{c}{Table 3. Horizontal Criteria and Standard [Example] Stars for Spectral Types O2--O4}\\
\\
\hline\hline
\noalign{\vspace{3pt}}
O2 If* &O3 If* &O3.5 If* &O4 If+\\
\hline
\noalign{\vspace{3pt}}
N IV $\lambda$4058 $\gg$ N III $\lambda$4640 &N IV $>$ N III &N IV $\approx$ N III &N IV $<$ N III\\
No or v.\ wk.\ He I $\lambda$4771 &V.\ wk.\ or no He I &V.\ wk.\ He I &Wk.\ He I\\
HD 93129A &Cyg OB2$-$7 &Pismis 24$-$1 &HD 190429A\\
&Cyg OB2$-$22A &[HDE 228766] &Sk $-67^{\circ}$ 166\\
&&[NGC 2044W$-$9A] &Sk $-67^{\circ}$ 167\\
&&[MH14]\\[6pt]
\hline
\noalign{\vspace{3pt}}
O2 III(f*) &O3 III(f*) &O3.5 III(f*) &O4 III(f+)\\
\hline
\noalign{\vspace{3pt}}
\multicolumn{4}{c}{Criteria same as at class I}\\
HDE 269810 &\nodata &Pismis 24$-$17 &ST2$-$22\\
Sk $-68^{\circ}$ 137 &&&W28$-$23\\
LH10$-$3061\\
LH64$-$16\\
LH114$-$7\\
NGC 346$-$3\\
\mbox{[Sk $-66^{\circ}$ 172 (+OB)]}\\[6pt]
\hline
\noalign{\vspace{3pt}}
O2 V((f*)) &O3 V((f*)) &O3.5 V((f+)) &O4 V((f+))\\
\hline
\noalign{\vspace{3pt}}
N IV $\gg$ N III &N IV $>$ or $\approx$ N III &N IV $<$ N III &No N IV\\
No He I &V.\ wk.\ He I &V.\ wk.\ He I &Wk.\ He I\\
BI253 &HD 64568 &HD 93128 &HD 46223\\
\mbox{[P871]} &LH10$-$3058 &HD 93129B &HDE 303308\\
&[Sk $-71^{\circ}$ 51] &[HD 93205] &W28$-$5\\
&\mbox{[P1163]} &[HD 93250]\\
&\mbox{[P1311]} \\
&[ST5$-$52 (+OB)]\\[3pt]
\hline
\end{tabular}
\vspace{3pt}
\begin{tabular}{p{5.9in}}
\hspace{.25in} References---HDE~228766: Walborn 1973e.
NGC~2044W$-$9A: Heydari-Malayeri et~al.\ 1993; Walborn et~al.\ 1999.
MH14: Massey \& Hunter 1998.
HD~190429A: Walborn 1971a; Walborn \& Howarth 2000.
Sk~$-67^{\circ}~166/167$: Walborn et~al.\ 1995.
W28$-$5: Massey et~al.\ 2000.
\end{tabular}
\end{center}
\end{table}
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