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\title{Far Ultraviolet Spectroscopic Explorer Atlas}
\title{of OB Stars in the Magellanic Clouds\altaffilmark{1}}

\author{Nolan~R.~Walborn,\altaffilmark{2}
        Alex~W.~Fullerton,\altaffilmark{3,4}
        Paul~A.~Crowther,\altaffilmark{5,6}}
\author{Luciana~Bianchi,\altaffilmark{4,7}
        John~B.~Hutchings,\altaffilmark{8}
        Anne~Pellerin,\altaffilmark{9}}
\author{George~Sonneborn,\altaffilmark{10}
    and Allan~J.~Willis\altaffilmark{5} }
\vspace{1in}
\centerline{\titlelarge To be published in the August 2002 issue of \emph{ApJ Supplements}, \textbf{141}}
\received{22 February 2002}
\accepted{7 March 2002}
\sluginfo
\vfill

\altaffiltext{1} {Based on observations made with the
NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated
for NASA by The Johns Hopkins University under NASA contract NAS5-32985.
Also based in part on observations collected at the European Southern
Observatory in program 67.D-0238, and at the Australian National
University/Siding Spring Observatory.}                                        
\altaffiltext{2}{Space Telescope Science Institute, 3700 San Martin Drive,
Baltimore, MD 21218; walborn@stsci.edu. ST~ScI is operated by AURA, Inc., under NASA contract NAS5-26555.  }

\altaffiltext{3}{Department of Physics \& Astronomy, University of Victoria, 
P.O. Box 3055, Victoria, BC, V8W 3P6, Canada}

\altaffiltext{4}{Department of Physics \& Astronomy, The Johns Hopkins 
University, 3400 N.\ Charles Street, Baltimore, MD 21218; awf@pha.jhu.edu, bianchi@pha.jhu.edu }
                              
\altaffiltext{5}{Department of Physics \& Astronomy, University College London, 
Gower Street, London, WC1E 6BT, UK; pac@star.ucl.ac.uk, ajw@star.ucl.ac.uk} 

\altaffiltext{6}{Visiting Observer, European Southern Observatory and 
Australian National University/Siding Spring Observatory.}

\altaffiltext{7}{On leave from the Astronomical Observatory of Torino.}
                                                    
\altaffiltext{8}{Herzberg Institute of Astrophysics, National Research Council 
of Canada, 5071 West Saanich Road, Victoria, BC, V8X 4M6, Canada; john.hutchings@hia.nrc.ca} 
                                                                    
\altaffiltext{9}{D\'epartement de Physique, de G\'enie Physique, et
d'Optique, Universit\'e Laval, Qu\'ebec, QC, G1K 7P4, Canada; apelleri@phy.ulaval.ca} 

\altaffiltext{10}{Laboratory for Astronomy \& Solar Physics, NASA Goddard 
Space Flight Center, Code 681, Greenbelt, MD 20771; sonneborn@stars.gsfc.nasa.gov} 
                                                   

\begin{abstract}
An atlas of the 900--1200~\AA\ region in the spectra of 47 OB stars in 
the Large and Small Magellanic Clouds, observed at high resolution by the 
{\it Far Ultraviolet Spectroscopic Explorer\/}, is presented and discussed.  
The systematic trends in the numerous stellar-wind features in this region, 
some from species (and ionizations) not represented at longer wavelengths, 
are charted as a function of the optical spectral types.  The \textit{FUSE} 
sample is by far the most powerful to date for that purpose.  A special effort 
has been made to verify the spectral types of all stars included in the atlas, 
in a number of cases with new optical observations that are also illustrated, 
to avoid uncertainties from that source in the stellar-wind trends.  A new O2 
star has been found in the process.  Most of these stars have been previously
observed at longer ultraviolet wavelengths by the {\it Hubble Space
Telescope}, and in the optical from the ground with high-resolution, 
digital instruments; thus very comprehensive physical modeling of
these OB atmospheres and winds now becomes possible.  This atlas will serve
as a guide to the \textit{FUSE} Magellanic Cloud OB database for that purpose.
The Magellanic Cloud sample provides a very important complement to the
\textit{FUSE} database of Galactic OB counterparts (Pellerin et~al.\ 2002),
both because the lower extinction and interstellar H$_2$ absorption
toward the Cloud stars allow a much clearer view of the stellar spectra
below 1100~\AA, and because of the metallicity differences among the three 
galaxies.  In particular, most wind features in the SMC spectra are seen to 
be significantly weaker than those in the LMC, at the same spectral types.     
\end{abstract}

\keywords{atlases --- Magellanic Clouds --- stars: early-type --- 
stars: winds --- ultraviolet: stars}


\section{Introduction}

While accurate spectral classifications and atmospheric parameters of OB
stars can be derived from optical spectroscopy, the space ultraviolet
wavelengths are essential for complete physical models.  The UV regime
covers or approaches the maxima of OB energy distributions (1000~\AA\ at
30,000~K), and it correspondingly contains a high density of strong
metallic transitions, including those that reveal the ubiquitous stellar
winds of normal stars.  For some time, the wavelengths longer than about
1200~\AA\ have been well covered, with extensive samples at good
resolution provided by the {\it International Ultraviolet Explorer\/} ({\it
IUE\/}), launched in 1978 (e.g., Walborn, Nichols-Bohlin, \& Panek 1985;
Howarth \& Prinja 1989; Rountree \& Sonneborn 1993; Snow et~al.\ 1994; 
Walborn, Parker, \& Nichols 1995); and the spectrographs aboard the 
{\it Hubble Space Telescope} ({\it HST\/}), launched in 1990 and 1997
(e.g., Walborn et~al.\ 1995a, 2000; Puls et~al.\ 1996).  In contrast,
coverage of the 900--1200~\AA\ region has been far more limited, although 
it offers even more features sensitive to stellar winds, including some
of higher ionizations, than those available at longer UV wavelengths.
The first satellite dedicated to UV stellar spectroscopy, {\it Copernicus\/}, 
launched in 1972, did cover the shorter wavelengths, but its sensitivity
was restricted to bright stars (e.g., Snow \& Morton 1976; Snow \& Jenkins
1977; Lamers, Gathier, \& Snow 1982; Walborn \& Bohlin 1996).  The {\it
Hopkins Ultraviolet Telescope\/} ({\it HUT\/}) was more sensitive, but the
spectral resolution was low and it was operated only during two Shuttle
missions in 1990 and 1995 (e.g., Walborn et~al.\ 1995b; Buss, Kruk, \&
Ferguson 1995).  This situation has been amply remedied, and the coverage 
of the 900--1200~\AA\ region brought to the level of that at longer UV
wavelengths, by the {\it Far Ultraviolet Spectroscopic Explorer\/} {(\it FUSE\/}), 
launched in 1999 (Moos et~al.\ 2000).  The \textit{FUSE} archive now contains
high-quality data covering those wavelengths in the spectra of hundreds
of OB stars in the Galaxy and the Magellanic Clouds.

An atlas of \textit{FUSE} observations of Galactic OB spectra has been
prepared by Pellerin et~al.\ (2002).  In this paper, we present an
analogous guide to the \textit{FUSE} data for OB stars in the Magellanic
Clouds.  The Magellanic Cloud observations provide an important
complement to the Galactic data for two reasons.  First, the very strong
far-UV extinction and interstellar H$_2$ absorption seriously obscure the
stellar spectra below 1100~\AA\ even for mild amounts of reddening, which
affects essentially all the Galactic O~stars accessible to \textit{FUSE}.
The reddenings of the brighter OB stars in the Magellanic Clouds are
sufficiently small that this problem is substantially alleviated.
Second, the metallicities of the Magellanic Clouds are smaller than that
of the solar neighborhood and different from each other, which permits
the investigation of systematic differences among the spectra of OB stars
in the three galaxies from that cause.

The earlier {\it Copernicus\/}, {\it IUE\/}, and {\it HST\/} studies
established detailed correlations between the trends among the UV
stellar-wind profiles and the optical spectral classifications of most OB
stars.  Since the latter are calibrated in terms of fundamental stellar
parameters, this result sets an important constraint on physical models
of the winds.  A primary objective of the present work is to comparatively 
explore the trends among the rich array of stellar-wind features in the
900--1200~\AA\ wavelength range, by means of the unprecedented database
provided by \textit{FUSE}.  Many of the \textit{FUSE} targets in the Magellanic
Clouds were specifically selected because of prior {\it HST\/} coverage, so
that very detailed comparisons and eventually comprehensive modeling are
indeed possible (Crowther et~al.\ 2002).  


\section{Observations}

\subsection{{\it FUSE}}
Stellar and exposure data for the 47 \textit{FUSE} targets included in the
present Atlas are listed in Table~1, in the order that they appear in the
spectral montages, with the LMC stars first, followed by those in the SMC.
(Stars that appear more than once in the figures are listed redundantly
in the table for convenience of reference.) 

The \textit{FUSE} instrument and its initial performance are described
in detail by Moos et~al.\ (2000) and Sahnow et~al.\ (2000a), respectively.
Briefly, \textit{FUSE} consists of four coaligned telescopes, each of which
feeds a Rowland circle spectrograph.
These four ``channels'' provide redundant coverage of the region between
905 and 1187~{\AA} with spectral resolving power of $\sim$20,000.
The optics for two of the channels are made from silicon carbide (SiC),
which gives good throughput for wavelengths below $\sim$1020~{\AA}.
The optical elements for the other channels are overcoated
with lithium fluoride (LiF) to maximize efficiency at longer wavelengths.
Spectral data from pairs of (SiC, LiF) channels are recorded by separate
large-format, photon-counting detectors, which are
themselves further divided into two segments (A~and~B).
Thus, a \textit{FUSE} observation consists of 8~separate datasets,
one from each combination of detector segment (1A, 1B, 2A, 2B)
and spectrograph channel (SiC, LiF).

The observations presented here were obtained between 1999 December~15
and 2001 August~26.
With a few exceptions, the data were obtained as parts of the various
programs undertaken by the \textit{FUSE} Principal Investigator Team.
All data were obtained in time-tag mode, with the total exposure times
indicated in Table~1.  Most of these observations consisted of several
exposures obtained during consecutive orbital viewing periods.
Except for the stars in NGC~346 in the SMC, the observations were made
through the LWRS ($30\arcsec \times 30\arcsec$) aperture.
The targets in NGC~346 were observed through the narrower MDRS
($20\arcsec \times 4\arcsec$) aperture, suitably oriented to 
minimize contamination from nearby sources.  The \textit{FUSE} aperture
placements on the sky for all of the present targets are illustrated in
the atlas of Danforth et~al.\ (2002).

\subsection{Optical}
Optical exposure data for 8~stars, 4~in each Magellanic Cloud, whose 
spectral types have been confirmed or revised from new digital observations 
for this work, are listed in Table~2.  The new spectrograms are shown in
Figures~1 (LMC) and~2 (SMC); they will be discussed together with the
corresponding \textit{FUSE} data below. 

The majority of our newly presented optical datasets were obtained at the 
European Southern Observatory (ESO) 8.1~m Very Large Telescope (VLT) 
in Paranal, Chile, or at the 2.3~m Australian National University (ANU)
telescope in Siding Spring, Australia. 

The VLT observations were obtained during 2001 September~24--28
with the UV-Visual Echelle Spectrograph (UVES) mounted on Kueyen
(UT2).  The data were recorded on a $2\times4$~K EEV CCD
($15\times15 \mu$m pixels) via the blue arm of UVES, covering
3731--4999~\AA. A 1$''$ wide slit was used in variable seeing conditions
(0.8--2$''$), providing a 2~pixel spectral resolution of 0.09~\AA.

The ANU observations were taken with the Double Beam Spectrograph 
(DBS) on 2000 December~15.  The 1200B lines\,mm$^{-1}$ grating
was used with the blue arm of DBS plus a $1752\times532$ SITE CCD
($15\times15 \mu$m pixels), covering a spectral range of 3969--4967~\AA. 
A 1.5$''$ wide slit was used in moderate seeing conditions 
($\sim$1.2$''$) to achieve a 2~pixel spectral resolution of 1.2~\AA.

Both datasets were cleaned of cosmic rays, bias-corrected,
flat-fielded, and optimally extracted in {\sc iraf}.\footnote{{\sc iraf} 
is distributed by the National Optical Astronomy Observatory, which is 
operated by the Association of Universities for Research in Astronomy, 
Inc., under cooperative agreement with the National Science Foundation.} 
Subsequent reductions (wavelength correction and merging of echelle 
orders) were carried out with {\sc figaro} and related packages.  UVES 
observations of AV\,70 were kindly provided by Lex Kaper, and they 
were reduced with the standard pipeline.

We also include ESO 3.6~m Cassegrain echelle spectrograph (CASPEC) 
observations of AV\,423 obtained in 1997 November, courtesy of 
Danny Lennon.  Reduction details are described in Walborn et~al.\ (2000).

For classification and display purposes, the echelle data have been
Gaussian-smoothed and rebinned to a 2~pixel resolution of 1~\AA\ FWHM.
The ANU data have also been Gaussian-smoothed with a 1.2~\AA\ FWHM.
The measured S/N per (rebinned) pixel in the continuum near 
He~I $\lambda$4471 ranges from 105--240 in the UVES data to 
35--45 in the ANU data.


\section{\emph{FUSE} Data Processing}

The \textit{FUSE} data included in the atlas were reduced uniformly in 
a customized, three-step process.

The first step consisted of standard processing with the latest
version of the \textit{FUSE} calibration software, 
{\tt CALFUSE v2.0.5}.\footnote{The {\it CALFUSE Pipeline Reference Guide}, 
which is available at 
{\tt http://fuse.pha.jhu.edu/analysis/pipeline\_ref\-erence.html},
provides a detailed description of this package.}
Before running {\tt CALFUSE}, the photon lists associated with the
separate exposures of a given observation were concatenated, so only
a single file required processing.  This version of the {\tt CALFUSE} 
software corrects the data for ``gain sag'' caused by 
depletion of electrons in the microchannel plates, and it removes data
attributable to ``event bursts'' (Sahnow et~al.\ 2000a).
It also shifts the positions of detected photons to correct for 
the motion of the satellite; thermally-induced motions of 
the diffraction gratings; small, thermally-induced drifts in 
the read-out electronics of the detector; fixed geometric distortions
in the detector electronics; and residual astigmatism in 
the images produced by the Rowland circle spectrograph.
Application of these shifts results in a two-dimensional, 
distortion-corrected image of each detector segment.
Next, a background image is constructed for each detector 
segment based on the total length of the observation, 
the fraction of the observation obtained during orbital night, 
and templates for the day/night distribution of scattered and stray 
light across the particular detector segment.
The background image is subtracted from the distortion-corrected image,
and one-dimensional spectral data are produced by optimal extraction.
Finally, corrections for detector dead-time (which are small for targets
in the Magellanic Clouds) are applied, along with the most
recent wavelength and effective-area calibrations, to produce
8 flux-calibrated spectrograms in the heliocentric reference frame.

The second step repeated the {\tt CALFUSE} processing 
with an initial screening of the input photon list so that only data
collected during orbital night were processed.  This procedure produced 
calibrated ``night only'' data for the targets, which are characterized 
by substantially reduced contamination from airglow lines, usually at 
the cost of an overall reduction of the signal-to-noise ratio.
Both these characteristics depend on the fraction of the observation
that was obtained during orbital night, which is listed in Table~1.
This step was skipped for the 3~cases (AV~83, AV~232, and AV~235) where 
less than 10\% of the total exposure was obtained during orbital night.

The third processing step merged the 8~separate spectrograms
associated with each observation.  To start, small ($\pm0.4$~\AA) regions 
of the ``day~$+$~night'' data in the vicinity of strong airglow lines were 
replaced by the same regions extracted from ``night-only'' versions of the 
data.  These substitutions effectively suppressed the distortions due to 
airglow contamination, though a small amount of residual emission in
Ly~{$\beta$} had to be excised by linear interpolation across the base 
of the airglow line.  Linear interpolation was also used to remove the 
strongest airglow contamination from spectrograms of the 3~objects that were 
mostly observed during orbital day.  Following airglow mitigation, the 
spectrograms were aligned in wavelength with shifts determined by 
cross-correlation.  Next, the flux levels in the 8~spectrograms were 
balanced via a comparison of the median fluxes in overlap regions, and  
small multiplicative correction factors as required.  At the same time, 
the broad depression in LiF1B data known as ``the worm'' (Sahnow et~al.\ 
2000b) was corrected approximately by applying multiplicative factors
determined from a comparison of the afflicted regions of LiF1B 
spectrograms with the same sections of LiF2A data, which are not affected.
The spectrograms were subsequently merged\footnote{Merging data 
from different channels is not recommended by the \textit{FUSE} Project for 
applications requiring high spectral resolution.}
by weighted coaddition, where the weights were inversely proportional to 
the square of the propagated statistical errors.  Localized regions of the 
spectrograms with flat-field or other defects were excluded from the sum. 

The spectrograms were then normalized consistently by means of cubic spline
functions to define a local pseudo-continuum.  Even though internally
consistent criteria were adopted, the continuum placement shortward of
$\sim$1000~{\AA} is very uncertain due to interstellar absorption,
particularly from interstellar H$_2$, and the confluence of the Lyman
series of interstellar H~I.  Finally, the spectrograms were rebinned to a 
constant wavelength step of 0.25~{\AA}, which corresponds to a spectral 
resolving power of $\sim$4,000, and the stellar velocities listed in 
Table~1 were used to transform heliocentric wavelengths to the laboratory 
rest frame.  The fully processed spectrograms have signal-to-noise
ratios between 80 and 120 per 0.25~{\AA} bin.


\section{The Atlas}

The Atlas consists of 14 spectral montages, 7~for each Magellanic Cloud,
each containing normalized \textit{FUSE} data for 4~stars.  The first
montage for each Cloud, Figure~3 for the LMC and Figure~10 for the SMC, 
provides an overview of early- to late-O giants and supergiants, to
facilitate appreciation of the temperature dependence in the principal
wind features.  The full samples are displayed in Figures~4--9 (24~stars) 
for the LMC and Figures~11--16 (23~stars; AV~423 is shown twice) for 
the SMC, with restricted spectral-type ranges in each.  Spectral types
O2--B1.5 (B1 in the SMC) are covered, fairly completely for O-type giants
and supergiants, but only a few early-O dwarfs and B-type supergiants are
included.  (It is likely that some relatively bright Magellanic Cloud stars 
classified as mid- or late-O dwarfs are either more luminous or unresolved 
multiple systems.  Comments on resolved multiplicity of present targets 
within the \textit{FUSE} apertures are made below, based on the convenient
atlas of Danforth et~al.\ 2002.)  A key to the strong interstellar lines is 
provided in Figure~17.  For reference, the ions and transitions discussed 
here in the stellar spectra are listed in Table~3.

\subsection{LMC Overview (Figure 3)}
A giant/supergiant sequence spanning the O~subtypes in the LMC is shown 
in Figure~3, to illustrate the full horizontal trends in the strong
stellar-wind profiles within that spectral class.  The sequence is
illuminating, because it contains features with maxima at the opposite
ends, as well as others that peak within the range.  As might be
expected, the highest ionization lines, S~VI $\lambda\lambda$933, 944 and 
O~VI $\lambda\lambda$1032, 1038, have maximum strength at the earliest
spectral type (O2; Walborn et~al.\ 2002) and decline toward later types;
while some relatively low-ionization lines (N~II $\lambda$1085; Si~III 
$\lambda\lambda$1108, 1110, 1113; and C~III $\lambda$1176) display
exactly the inverse behavior.  Other prominent wind features have maxima
toward the middle of the spectral-type range:  N~IV $\lambda$955; C~III 
$\lambda$979; N~III $\lambda$991; S~IV $\lambda\lambda$1063, 1073; and 
P~V $\lambda\lambda$1118, 1128.  P~IV $\lambda$951 and Si~IV 
$\lambda\lambda$1122, 1128 may peak near type O8~I, not represented here 
(Walborn \& Bohlin 1996). 

The relative component strengths of the S~VI doublet present a puzzle:
the longward member is much the stronger in the O2 spectrum, but the
opposite is true at O4.  Although the normalization is uncertain at those
wavelengths because of the rapidly declining sensitivity, this effect is
not an artifact of the \textit{FUSE} data or its processing, since it is
also present in the {\it HUT\/} data (Walborn et~al.\ 1995b).  An analogous
trend is present in some models (Kudritzki et~al.\ 1992), but it is far
smaller than observed.  It is possible that these effects are due to
interference by N~IV $\lambda$923 with the shortward component of S~VI in
the hotter spectra, and by P~IV $\lambda$951 with the longward component 
of S~VI at the later types; spectral synthesis studies should be able to
elucidate this problem.

\subsection{LMC Early-O Stars (Figures 4 and 5)}
Only the highest ionization resonance lines (S~VI and O~VI) have 
stellar-wind profiles in the O2 spectra (Figure~4; note that
Sk~$-67^{\circ}$~211~= HDE~269810).  The present spectral
classification of Sanduleak (Sk)~$-70^{\circ}$~91 is an important result
of our optical verification program (Section~2.2 and Figure~1).  This
star had been classified as O6.5~V by Conti, Garmany, \& Massey (1986),
which is a fair description of its He~II/He~I line ratios.  However,
the presence of strong N~IV emission and N~V absorption lines in our
digital observation reveals a composite spectrum with an O2 component.
Indeed, the optical spectrum is very similar to that of Sk~$-66^{\circ}$~172 
(Walborn et~al.\ 2002), including the characteristic incipient P~Cygni 
profile at He~II $\lambda$4686; and the far-UV spectra of the two stars are
virtually identical, even to the details of the O~VI profiles.  Note that
the absorptions in the latter are not black (the portions extending to zero
intensity are due to interstellar Ly$\beta$ $\lambda$1026), as is the case 
for the N~V and C~IV profiles in the HST observation of Sk~$-66^{\circ}$~172
(Walborn et~al.\ 1995a), probably due to the presence of the companions. 
The images of both stars are extended in direct photographs.  

In the O4 spectra (Figure~5), lower ionization lines develop wind profiles.  
Most of them increase significantly in strength with luminosity (cf.\ also 
the O5~V spectrum of Sk~$-70^{\circ}$~69 in Fig.~4), in contrast to 
the O~VI and S~VI in which any such effect is quite small.  This behavior
of the highest ionization lines at early-O types is entirely analogous to
that of the N~V and C~IV wind profiles at longer UV wavelengths (Walborn
et~al.\ 1985) and is likely due to saturation of these lines in the
hottest winds.  On the other hand, the P~V doublet displays a
particularly nice luminosity effect at types O4-O5, probably because of
its lower abundance relative to the other high ions.  The wind profiles
of P~V, S~IV, and C~III $\lambda$1176 are of intermediate strength in the
O4f supergiants, similarly to that of the Si~IV in the IUE/HST range
(Walborn et~al.\ 1985, 1995a).

The spectra of the two giants in Fig.~5 are very similar, despite the fact 
that Sk~$-71^{\circ}$~45 (=~HDE~269676) is a multiple system (Walborn 1977; 
Heydari-Malayeri \& Hutsem\'ekers 1991).  The extreme dichotomy
between the relative strengths of the CNO lines in the two supergiants
was found in the {\it HUT\/} data (Walborn et~al.\ 1995b) and is seen even
more clearly in the \textit{FUSE} observations.  Ironically, the slower
rotator Sk~$-67^{\circ}$~166 (=~HDE~269698) has the stronger N~lines,
possibly indicating a larger fraction of processed material in its
wind, contrary to theoretical expectations.       

\subsection{LMC Mid-O Stars (Figure 6)}
A sequence of O6 giant/supergiant spectra is shown in Figure~6.  The
positive luminosity effects in S~IV and P~V have reached their full
dynamic range by this spectral type, with saturated S~IV profiles in the
most luminous star (Sk~$-65^{\circ}$~22 = HDE~270952); see also Walborn
\& Bohlin (1996) and Pellerin et~al.\ (2002), where far-UV spectra of lower 
luminosity classes at and near this spectral type are shown, albeit for
Galactic stars.  The C, N, and Si lines also display luminosity effects at 
this spectral type.  On the other hand, the relative weakness of the S~VI and 
O~VI features in the spectrum of Sk~$-65^{\circ}$~22 is surprising.  It would
appear that there is a sharp threshold for the onset of these high-ion wind
profiles, possibly as a function of wind density and temperature, and/or
of the terminal velocity and shock activity (Pauldrach, Hoffmann, \& 
Lennon 2001; Bianchi et~al.\ 2002).  A higher luminosity hot
star will have a somewhat lower effective temperature than a lower
luminosity star of the same spectral type, because the lower gravity,
hence atmospheric pressure, of the former contributes to the ionization.
The more luminous star will also have a denser wind and lower terminal
velocity.  Similar behavior of the high ions will be seen in other very 
luminous stars in subsequent montages.

Preliminary analyses of the \textit{FUSE} observations of Sk~$-67^{\circ}$~111
were made by Fullerton et~al.\ (2000), Massa et~al.\ (2000), and Bianchi et~al.\ (2000), in comparison with the SMC O7f~supergiant AV~232 (shown later
here).  Sk~$-67^{\circ}$~111 had been classified as O7f by Conti et~al.\
(1986) and as O7~Ib(f) by Fitzpatrick (1988).  The former type implies
that He~II $\lambda$4686 is an emission line, but the latter that the line 
is neutralized, i.e.\ neither emission nor absorption is detected in the
observational material.  The present digital observations differ from
both previous classifications (Fig.~1):  the He~II/He~I absorption-line
ratios correspond to a type of O6, and $\lambda$4686 is strongly in
emission, but with an absorption reversal.  The latter peculiarity, seen
in some rapid rotators, corresponds to the Onfp category distinguished by
Walborn (1973), also called Oef by Conti \& Leep (1974).  We estimate a 
$v \sin i$ of 180~km~s$^{-1}$ for Sk~$-67^{\circ}$~111.  Moreover, in a
second digital spectrogram of this star (not shown here), the $\lambda$4686
emission is single-peaked, with no absorption reversal, leading to the
``var'' qualifier on the present spectral type.  Some wind variations
were found by Massa et~al.\ (2000).  Further optical observations are
required to determine whether the range of variations is sufficient to
encompass the previous classification discrepancies.  However, it appears
that some of the differences from AV~232 found in the \textit{FUSE} analyses
may be due to the earlier spectral type of Sk~$-67^{\circ}$~111.    

\subsection{LMC Late-O Stars (Figures 7 and 8)}
Late-O bright giants and Ib supergiants are shown in Figure~7.  Wind
effects are relatively less prominent at these LMC spectral types, 
although C~III, N~III, and even O~VI wind profiles are present.  C~III 
$\lambda$1176 distinguishes particularly well between the two luminosity
classes.  The spectral classification of Sk~$-69^{\circ}$~124 is from
Conti et~al.\ (1986), and the far-UV spectrum is consistent.  It appears
possible from the latter that this star may be somewhat N-deficient; a
high-quality digital optical observation will be of interest to
investigate that possibility.  New optical observations of
Sk~$-67^{\circ}$~101 and BI~170, from which the present spectral 
classifications are derived, are shown in Fig.~1.

More luminous late-O supergiants are shown in Figure~8, in which the wind
effects are correspondingly stronger.  Sk~$-67^{\circ}$~5 = HDE~268605
was classified by Walborn (1977), where the possible effect of LMC metal
deficiency on the luminosity classes at late-O types was discussed.  The
C~III $\lambda$1176 and other wind profiles in its far-UV spectrum are
certainly more consistent with a Iab class than with Ib, while the
absolute visual magnitude (Table~1) is brighter than that of a normal Ia,
suggesting the possibility of a multiple system; in fact, the direct
image of this star is composite (Danforth et~al.\ 2002).  The Ia+ luminosity
classes of Sk~$-68^{\circ}$~135~= HDE~269896 and Sk~$-66^{\circ}$~169 are
based on the unusual presence of He~II $\lambda$4686 emission lines at
this late spectral type (Walborn 1977 and Fitzpatrick 1991, respectively).
Both of these spectra show weaker O~VI profiles than the less luminous
supergiants at the same spectral type, analogously to Sk~$-65^{\circ}$~22
discussed above; the wind terminal velocities of the Ia+ stars also
appear smaller than those of the other supergiants.  Sk~$-68^{\circ}$~135 
is relatively heavily reddened and suffers from moderately strong H$_2$ 
contamination; nevertheless, its optical N enhancement is well reflected 
in the relatively stronger far-UV N~III and N~II wind profiles.  

The prominent absorption line at 1099 \AA\ in this spectral-type range
remained unidentified in previous studies (Walborn \& Bohlin 1996).  It
has recently been identified as an S~IV transition by Werner \& Rauch (2001)
and Crowther et~al.\ (2002).

\subsection{LMC Early-B Supergiants (Figure 9)}
Primarily to bound the O-type sequence, some early-B supergiants are 
displayed in Figure~9.  Their wind profiles continue the smooth decline 
from the late-O supergiants, and no significant emission components are 
present in this wavelength range by type B1.  In fact, oppositely to the
O stars, early-B supergiants have richer wind spectra at longer UV 
wavelengths (Walborn, Parker, \& Nichols 1995).  
 
\subsection{SMC Overview (Figure 10)}
The trends of the stellar-wind profiles in the SMC giant/supergiant
sequence of Figure~10 parallel those in the LMC counterparts (Fig.~3),
but most of their intensities and terminal velocities are substantially
smaller due to the lower SMC metallicity, as previously found at longer
UV wavelengths (e.g., Walborn et~al.\ 1995a).  Exceptions to that
generalization are that the strengths of the C~III and N~III resonance
lines in the mid/late-O supergiants are similar in the two galaxies---likely 
a saturation effect despite lower abundances in the SMC; and that
the relatively strong O~VI profile in AV~372 appears to break the trend in 
that feature.  However, the latter behavior is likely due to the ``negative''
(inverse) luminosity effect of O~VI discussed above for the most luminous
LMC supergiants; the CNO profiles of AV~372 are in fact similar to those
of the LMC Ib/Iab supergiants in Figures~7--8.  The ``w'' in the spectral
type of AV~372 refers to weakness of the metal lines in the optical
(Walborn 1977, 1983), whence the classification is derived.   

\subsection{SMC Early-O Stars (Figure 11)}
NGC~346 contains over half of the known O stars in the SMC (Massey, Parker,
\& Garmany 1989), including most of those of the earliest types, so that
their UV spectra are essential to any survey.  Because of the compactness
of the cluster, a smaller \textit{FUSE} entrance slot was used for the
observations as discussed above, to reduce cross-contamination among the
brighter members.  The far-UV spectra of four of them are shown in
Figure~11.  The stronger O~VI and weaker C~III in the O2 spectrum of
No.~3 demonstrate that contamination was substantially avoided (as do
the aperture images in Danforth et~al.\ 2002).  {\it HST\/} and digital optical 
spectroscopy of these stars (as well as many others in the present Atlas) 
is presented by Walborn et~al.\ (1995a, 2000).  As also found at the longer 
UV wavelengths, stellar-wind features are few and weak in SMC dwarfs and 
giants at all O spectral subtypes.  At the early subtypes, only those of 
O~VI and S~VI are present, significantly weaker than in the LMC counterparts.  
The O5~III(f+) spectrum of AV~75 is included in Figure~12.  

\subsection{SMC Mid-O Stars (Figures 12 and 13)}
The SMC O6--O7 dwarf and giants in Figure~12 show essentially no wind
emission above the continuum in the \textit{FUSE} wavelength range, a 
probable asymmetrical absorption feature in O~VI being the only prominent 
wind diagnostic in these spectra.  All of these stars have well developed 
C~IV and (except the nitrogen-deficient AV~69) N~V wind profiles at longer 
UV wavelengths (Walborn 1995a, 2000).  The wind spectra of the SMC mid-O
supergiants in Figure~13 are more interesting, displaying all of the features
seen in the LMC counterparts, albeit more weakly except for the C~III and
N~III resonance lines.  The strong spectroscopic resemblance between
AV~232 (=~Sk~80) and AV~83, despite their large luminosity difference 
(Table~1), is maintained in the \textit{FUSE} regime, although O~VI may be
somewhat stronger in the less luminous star.  AV~220 belongs to the small 
Of?p category defined by Walborn (1973), for which there is ample evidence 
of a non-supergiant nature, further confirmed by the \textit{FUSE} data here 
(see also Walborn et~al.\ 2000). 

\subsection{SMC Late-O Stars (Figures 14 and 15)}
The \textit{FUSE} observations of seven SMC late-O giants and supergiants
are shown in Figures~14--15; new digital optical observations of four of
them, from which they have been classified here, are presented in Figure~2.  
Only the O8.5 bright giant AV~469 displays some prominent stellar-wind
profiles in Fig.~14; its peculiar C~III $\lambda$1176 profile is remarkably 
similar to those of the two similarly classified LMC stars in Fig.~7.  
However, all of these spectra may have asymmetrical O~VI absorption. 
The optical spectra of AV~70 (Fig.~2) and AV~327 (Walborn et~al.\ 2000)
are similar, as are their far-UV spectra (Fig.~15).  AV~327 has a companion 
within the \textit{FUSE} aperture.  As already discussed above, the wind 
spectrum of AV~372 is rather similar to those of the LMC counterparts.

\subsection{SMC Early-B Supergiants (Figure 16)}
The early-B supergiants in Figure 16 ``close out'' the SMC O-type
sequence.  As in the LMC, fully developed wind profiles are absent after
type B0.5 Ia.  As found for the O-type supergiants, the C~III and N~III
resonance lines are not weaker than in the LMC counterparts, but the S~IV
and non-resonance C~III $\lambda$1176 are significantly weaker at a given
spectral type in the SMC.  {\it HST\/} and optical observations of these B0.5 
supergiants in both Clouds are presented by Walborn et~al.\ (1995a); note 
that the SMC object is substantially more luminous than the LMC one (Table~1) 
and probably somewhat hotter, which may compensate the lower metallicity 
of the former in the comparative strengths of some spectral features.  


\section{Conclusions}

The morphological trends of twelve stellar-wind features have been
charted in the 900--1200~\AA\ spectra of 47 OB stars, equally divided
between the two Magellanic Clouds (one fewer in the SMC), by means of
data from the {\it Far Ultraviolet Spectroscopic Explorer\/}.  Strong
correlations have been found as functions of the optical spectral types
and luminosity classes, as well as the different metallicities of the two
Clouds, expanding and confirming previous investigations at longer UV
wavelengths.  These results have two strong implications:  the stellar
winds of most stars are governed by the fundamental stellar parameters 
(in terms of which the optical spectral types are calibrated), and any 
temporal variations in the stellar-wind features of normal stars must be 
small relative to the morphological variations, which otherwise would not 
be observed.  It has been shown how accurate optical spectral types are 
essential to such studies of the UV characteristics, with several critical 
examples; in particular, a new member of the O2 class has been recognized
in the composite spectrum of Sk~$-70^{\circ}$~91, which is vital to the
interpretation of its UV wind spectrum.  Physical modeling of these 
trends will provide valuable diagnostics of the parameters determining the 
atmospheric and wind properties of these stars.

Some specific principal conclusions are as follows.

$\bullet$ All of the stellar-wind features show strong temperature
dependences, with maxima ranging from the hottest to the coolest O-type
spectra, depending on their ionizations and other line-formation
properties.  Modeling of these trends, and the ratios of different
features in a given spectrum, will provide very sensitive determinations
of the wind parameters. 

$\bullet$ S~IV $\lambda\lambda$1063, 1073; P~V $\lambda\lambda$1118, 1128;
and C~III $\lambda$1176 display very sensitive positive luminosity
effects, which usefully complement the Si IV effect at longer UV
wavelengths.  In the cases of S~IV, C~III, and Si~IV it is the similar,
relatively low ionization potentials, and in the case of P~V the low
abundance, that inhibit wind features in these species at the lower
luminosities/wind densities, and thus prevent saturation of the profiles,
allowing them to respond to luminosity differences.  The P~V does not
saturate even in the most luminous normal O~stars.

$\bullet$ The very high ionization O~VI $\lambda\lambda$1032, 1038
profiles are saturated in the early LMC O~stars and thus do not display a
luminosity dependence, similarly to the N~V and C~IV features at longer
UV wavelengths.  However, a surprising {\it weakening\/} of the O~VI in the
spectra of the most luminous mid/late-O supergiants has been described 
here for the first time.  This phenomenon is possibly due to the lower
temperatures, higher densities, and/or lower terminal velocities in the
winds of the most luminous stars.  Physical modeling of this effect
will be of considerable interest; a sharp threshold for O~VI formation as
a function of one or more of those parameters may exist.

$\bullet$ The SMC wind spectra mimic the LMC trends, but with weaker
features overall due to the smaller metallicity; wind profiles are
especially deficient in the SMC dwarfs and giants.  An exception is the
strength of the C~III $\lambda$979 and N~III $\lambda$991 resonance
lines in the SMC supergiants, which saturate despite the lower abundances
and hence do not appear weaker than in the LMC counterparts.  The N~V and
C~IV profiles at longer UV wavelengths behave similarly.    

A further comparison can be made with the \textit{FUSE} observations of
Galactic OB stars in the solar neighborhood displayed by Pellerin et~al.\
(2002), supplemented by the {\it Copernicus} presentation of Walborn \&
Bohlin (1996).  Straightforward comparison is limited to wavelengths longer 
than 1100~\AA\ by the interstellar H$_2$ dis-aster (literally!) in the Galactic 
spectra shortward of that point, i.e.\ to P~V, Si~IV, and C~III $\lambda$1176
insofar as wind features are concerned.  Despite the fairly extensive 
\textit{FUSE} samples, their intercomparison is also limited by the dearths
of dwarfs in the Magellanic Cloud data and of the most luminous supergiants 
in the Galactic data; the {\it Copernicus\/} data alleviate the latter point 
somewhat.  As expected from the relative metallicities, the comparison 
between the LMC and the Galaxy is more relevant, since the SMC wind features 
have already been shown above to be generally weaker than those in the LMC.    
In general, the differences between the LMC and Galactic wind features
are small or nil.  The P~V features are quite similar at a given spectral
type and luminosity class.  C~III $\lambda$1176 shows some effects in the
expected sense, but care must be taken to compare exact spectral-type matches, 
because of its steep two-dimensional (temperature, luminosity) dependences.
E.g., the Galactic O5~III spectrum has an intermediate wind profile in
this line while the two LMC O4~III stars have none, but the similarly
behaved Si~IV at longer wavelengths increases substantially in strength
between types O4 and O5 (Walborn et~al.\ 1985).  The (CNO-normal) LMC and
Galactic O4--O6 supergiants have similar $\lambda$1176 profiles.  On the 
other hand, the $\lambda$1176 profiles of the Galactic O6 III stars are 
clearly more similar to the LMC O6~Ib than the O6~II, which is likely a 
metallicity effect.  Again, the LMC O6~Ib cannot be compared with the
Galactic O7~Ib, because of the substantial strengthening of Si~IV in the
{\it IUE\/} range between those spectral types.  But $\lambda$1176 in the 
two LMC O8~II spectra is consistent with the Galactic O7--O9~II spectra.
The same is true of the comparison between the O9--B0 supergiants in the
two galaxies.  In summary, the only significant difference found among
the OB wind features in this spectral range between the two galaxies is
in the C~III $\lambda$1176 profiles at the O6 intermediate luminosity
classes.  As always, these morphological descriptions serve to formulate
the phenomena that need to be quantified and explained by the subsequent
astrophysical analyses.


\acknowledgments
We thank Danny Lennon for providing the optical data for AV~423, Lex
Kaper for AV~70, Chris Evans for reducing the other VLT/UVES data, and
Stephan McCandliss for pointing out the possible effect of N~IV
$\lambda$923 on the S~VI profiles.  PAC also thanks Stephane Brillant for 
conducting his VLT observations when circumstances prevented him from doing 
so; and the Royal Society for financial support.  LB acknowledges support 
from NASA LTSA grant NAG5-9219 (NRA99-01-LTSA-029).  US members of the 
\textit{FUSE} team have been supported by NASA through contract NAS5-32985. 
This work has also been partially supported by NASA through grant
GO-7437.04-96 to NRW from the Space Telescope Science Institute, which is
operated by AURA, Inc., under contract NAS5-26555.


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\newpage
\begin{landscape}
\begin{figure}%1
\centering
\includegraphics[width=5.5in,angle=-90]{fig1.eps}
\caption{
Rectified VLT/UVES or ANU/DBS blue-violet spectrograms of four LMC stars.
In all the figures, the axes are labeled with wavelengths in \AA\ and 
continuum flux units.  The spectral lines identified in BI~170 are, from left 
to right by ion, He~I $\lambda\lambda4026$, 4121, 4144, 4387, 4471, 4713; 
He~II $\lambda\lambda4026$, 4200, 4541, 4686; Si~IV $\lambda\lambda4089$, 4116;
N~III $\lambda4097$; and C~III $\lambda\lambda4647$, 4650--52.  In
Sk~$-67^{\circ}$~111, N~III $\lambda\lambda4634$, 4640--42 emission, and in 
Sk~$-70^{\circ}$~91, N~IV $\lambda4058$ emission and N~V $\lambda\lambda4604$, 
4620 absorption, are also identified.  The Balmer lines H$\epsilon$ 
$\lambda3970$, H$\delta$ $\lambda4101$, and H$\gamma$ $\lambda4340$ are not 
identified in the figures.
\label{fig1}}
\end{figure}
%\end{landscape}

\begin{figure}%2
\centering
\includegraphics[width=5.5in,angle=-90]{fig2.eps}
\caption{
Rectified VLT/UVES or ESO/CASPEC blue-violet spectrograms of four SMC stars.
See the Fig.~1 caption for the wavelengths of the identified spectral lines,
with the addition of He~I $\lambda4009$ here.
\label{fig2}}
\end{figure}

\begin{figure}%3
\centering
\includegraphics[width=5.5in,angle=90]{fig3.eps}
\caption{
Rectified \textit{FUSE} far-ultraviolet spectrograms of an LMC giant/supergiant 
sequence spanning the O~spectral class.  The stellar-wind features 
identified at the tops of the UV figures are, from left to right, 
S~VI $\lambda\lambda$933, 944; P~IV $\lambda$951; N~IV $\lambda$955; 
C~III $\lambda$979; N~III $\lambda$991; O~VI $\lambda\lambda$1032, 1038; 
S~IV $\lambda\lambda$1063, 1073; N~II $\lambda$1085; 
Si~III $\lambda\lambda$1108, 1110, 1113; P~V $\lambda\lambda$1118, 1128; 
Si~IV $\lambda\lambda$1122, 1128; and C~III $\lambda$1176.
Note the opposite, monotonic trends in the O~VI and C~III $\lambda$1176,
while some other features peak within this spectral-type range.
\label{fig3}}
\end{figure}

\begin{figure}%4
\centering
\includegraphics[width=5.5in,angle=90]{fig4.eps}
\caption{
Far-UV spectra of LMC early-O stars.  The comb in one panel of each montage 
marks the H~I Lyman series beginning with Ly$\beta$ $\lambda$1026, which is 
primarily interstellar in these spectra.  Only the highest ions display
stellar-wind profiles at these spectral types.  The two stars classified 
identically in the optical have closely similar UV spectra.
\label{fig4}}
\end{figure}

\begin{figure}%5
\centering
\includegraphics[width=5.5in,angle=90]{fig5.eps}
\caption{
Far-UV spectra of LMC early-O stars.  P~V displays a marked positive
luminosity effect.  The N vs.\ C, O~wind features are in extreme contrast
between these two similarly classified supergiants.
\label{fig5}}
\end{figure}

\begin{figure}%6
\centering
\includegraphics[width=5.5in,angle=90]{fig6.eps}
\caption{
Far-UV spectra of LMC mid-O stars.  Most wind features display
positive luminosity effects, but in contrast, the weakening of the O~VI in
the most luminous star is surprising.
\label{fig6}}
\end{figure}

\begin{figure}%7
\centering
\includegraphics[width=5.5in,angle=90]{fig7.eps}
\caption{
Far-UV spectra of LMC late-O stars.  The C~III $\lambda$1176 wind
profiles are well correlated with the optical luminosity classes.     
\label{fig7}}
\end{figure}

\begin{figure}%8
\centering
\includegraphics[width=5.5in,angle=90]{fig8.eps}
\caption{
Far-UV spectra of LMC late-O supergiants.  The two Ia+ spectra show
lower wind terminal velocities and weaker O~VI profiles than the others.
\label{fig8}}
\end{figure}

\begin{figure}%9
\centering
\includegraphics[width=5.5in,angle=90]{fig9.eps}
\caption{
Far-UV spectra of LMC early-B supergiants.  P~Cygni profiles 
disappear within this spectral-type range at these wavelengths.
\label{fig9}}
\end{figure}

\begin{figure}%10
\centering
\includegraphics[width=5.5in,angle=90]{fig10.eps}
\caption{
A sequence of SMC giant/supergiant far-UV spectra spanning the O~spectral 
class.  By comparison with Fig.~3 for the LMC, analogous trends in the
stellar-wind features can be seen here, but most of the lines are weaker
in the SMC stars at a given spectral type.  The saturated C~III and N~III 
resonance lines in the mid/late-O supergiants are an exception, appearing 
similar in both galaxies.
\label{fig10}}
\end{figure}

\begin{figure}%11
\centering
\includegraphics[width=5.5in,angle=90]{fig11.eps}
\caption{
Far-UV spectra of SMC early-O stars in NGC~346, the largest H~II region
in that galaxy.
\label{fig11}}
\end{figure}

\begin{figure}%12
\centering
\includegraphics[width=5.5in,angle=90]{fig12.eps}
\caption{
Far-UV spectra of SMC early/mid-O stars.  All stellar-wind features in
this wavelength range are very weak or absent at these spectral types 
in the SMC.
\label{fig12}}
\end{figure}

\begin{figure}%13
\centering
\includegraphics[width=5.5in,angle=90]{fig13.eps}
\caption{
Far-UV spectra of SMC mid-O stars.  The supergiants display relatively
prominent stellar-wind profiles, although most of them are still weaker
than in the LMC counterparts.
\label{fig13}}
\end{figure}

\begin{figure}%14
\centering
\includegraphics[width=5.5in,angle=90]{fig14.eps}
\caption{
Far-UV spectra of SMC late-O giants.  
\label{fig14}}
\end{figure}

\begin{figure}%15
\centering
\includegraphics[width=5.5in,angle=90]{fig15.eps}
\caption{
Far-UV spectra of SMC late-O bright giants/supergiants.
\label{fig15}}
\end{figure}

\begin{figure}%16
\centering
\includegraphics[width=5.5in,angle=90]{fig16.eps}
\caption{
Far-UV spectra of SMC early-B supergiants.  
\label{fig16}}
\end{figure}

\begin{figure}%17

\centering
\includegraphics[width=5.5in,angle=90]{fig17.eps}
\caption{
Identification of the principal far-UV interstellar features present in 
the Atlas spectra.  {\it Upper Panel.}  Metal lines in synthetic spectra 
computed for a mixture of cool ($\la$ 2000~K) and hot ($\sim$300,000~K) 
gas with LMC abundances.  Both components are assumed to be at the nominal 
velocity of the LMC (250~km~s$^{-1}$).  Prominent lines are identified 
above the spectrum, while the Lyman series of H~I is indicated by an 
inverted comb below. {\it Middle Panels.}  Synthetic spectra showing the 
location and strength of the Lyman and Werner bands of H$_2$ over a range of 
column densities spanning the values typically seen in the LMC. The Lyman 
series of atomic H is also indicated.  The models were computed for a single 
cool ($\sim$250~K) component in the LMC.  {\it Lower Panel.}  The spectrum 
of Sk~$-68^{\circ}$~135, an ON9.7~Ia+ star in the LMC that exhibits 
substantial interstellar contamination, including the highest absorbing 
column of H$_2$ of any star in the Atlas.  Note that the models are 
illustrative only and are not meant to provide a fit to this line of sight.
\label{fig17}}
\end{figure}
\end{landscape}

\clearpage
\input{tab1.tex}
\input{tab2.tex}
\input{tab3.tex}
\end{document}

