\documentstyle[paasms4,pptwocol,pub]{preprint}

\begin{document}


\title{SOME CHARACTERISTICS OF CURRENT STAR FORMATION IN THE 30~DORADUS 
NEBULA REVEALED BY HST/NICMOS\/\thanks{This paper is dedicated 
to W.W. Morgan, who taught me the power of morphology to uncover new phenomena 
in astronomy--NRW.}\morethanks{Based on observations with the NASA/ESA Hubble 
Space Telescope obtained at the Space Telescope Science Institute, which is 
operated by the Association of Universities for Research in Astronomy, Inc., 
under NASA contract NAS5-26555.}}

\author{Nolan R.\ Walborn, Rodolfo H.\ Barb\'a\\
\\
Space Telescope Science Institute, 3700 San Martin Drive, 
Baltimore, Maryland 21218\\
\and
Wolfgang Brandner\\
\\
Jet Propulsion Laboratory/Infrared Processing and Analysis Center,\\
\\
California Institute of Technology, 770 South Wilson Avenue, Pasadena, 
California 91125\\
\and
M\'onica Rubio\\
\\
Departamento de Astronom\'{\i}a, Universidad de Chile, Casilla 36-D, 
Santiago, Chile\\
\and
Eva K.\ Grebel\\
\\
Lick Observatory, University of California, Santa Cruz, California
95064\\
\and
Ronald G.\ Probst\\
\\
Cerro Tololo Inter-American Observatory, National Optical Astronomy 
Observatories,\\
\\
Casilla 603, La Serena, Chile}

\tobe{January 1999}{Astronomical Journal}
\recacc{25 August 1998}{15 September 1998}
\url{1288}
\maketitle
\abstract{
The extensive ``second generation'' of star formation within the 30~Doradus 
Nebula, evidently triggered by the R136 central cluster around its 
periphery, has been imaged with HST/NICMOS.  Many new IR sources, including 
multiple systems, clusters, and nebular structures, are found in these 
images.  Six of the NICMOS fields are described here, in comparison 
with the WFPC2 images of the same fields.  Knots 1-3 of Walborn \& Blades 
(early O~stars embedded in dense nebular knots) are all found to be compact 
multiple systems.  Knot~1 is shown to reside at the top of a massive dust 
pillar oriented directly toward R136, whose summit has just been removed, 
exposing the newborn stellar system.  Knots~1 and~3 are also near the 
brightest IR sources in the region, while parsec-scale jet structures 
are discovered in association with Knots~2 and~3.  The Knot~2 structures 
consist of detached, nonstellar IR sources aligned on either side of 
the stellar system, which are interpreted as impact points of a highly 
collimated, possibly rotating bipolar jet on the surrounding dark clouds; 
the H$_2$O maser found by Whiteoak et~al.\ is also in this field.  These 
outflows from young massive stars in 30~Dor are the first extragalactic 
examples of the phenomenon.  In the field of the pillars south of R136, 
recently discussed in comparison with the M16 pillars by Scowen et~al., 
a new luminous stellar IR source has been discovered.  These results 
establish the 30~Doradus Nebula as a prime region in which to investigate 
the formation and very early evolution of massive stars and multiple systems.  
The theme of triggered formation within the heads of extensive dust pillars 
oriented toward R136 is strong.  In addition, these results provide further 
insights into the global structure and evolution of 30~Doradus, which are 
significant in view of its status as the best resolved extragalactic starburst.}
\base

\section{INTRODUCTION}
Until recently, the 30~Doradus Nebula was regarded essentially as an 
evolved H~{\sc ii} region.  Attention has been focused on the spectacular 
central cluster, with its compact core of very massive stars, R136 
(Massey \& Hunter 1998 and references therein).  The most luminous 
stars in this cluster have reached the WN stage, indicating an age of 
$2-3~\times~10^6$ yrs (Maeder \& Meynet 1988, Maeder 1990).  In 
addition, there are numerous high-velocity shells within the nebula, 
related to its striking filamentary structure (Chu \& Kennicutt 1994).
In an earlier review, Walborn (1984) wrote, ``...the 30~Doradus 
Nebula...appears to be a well evolved H~{\sc ii} region.  No compact 
far-IR sources were found there...and only some weak CO emission was 
detected...  On the other hand, it harbors large internal motions...''
However, that picture can now be understood as largely determined by 
the instrumental limitations of the time.  In fact, the most prominent 
filaments in 30~Doradus, which dominate the nebular emission, can now 
be recognized as highly active ``star-formation fronts,'' or interfaces 
between the energetic outflows from the central cluster and the 
surrounding dark clouds, in which an extensive new stellar generation 
is being triggered. 

The first observational indications of this radically revised view of 
30~Doradus consisted of an H$_2$O maser (Whiteoak et~al.\ 1983, Whiteoak 
\& Gardner 1986), three early O~stars embedded in dense nebular knots 
(Walborn \& Blades 1987, Walborn 1991), and four luminous IR protostars 
(Hyland et~al.\ 1992).  All of these objects were found in or just beyond 
the brightest nebular filaments west and northeast of R136.  With an 
improved IR detector, Rubio, Roth, \& Garcia (1992) found 17~IR 
sources in the same regions, most of those to the west aligned along the 
curved nebular filament there like beads on a chain.  With still better 
groundbased IR technology, Rubio et~al.\ (1998) discuss 84~IR sources 
in these regions.  Meanwhile, HST/WFPC2 images of 30~Doradus have 
revealed numerous nebular microstructures in these fields characteristic 
of active star formation, such as bright and dark compact dust  
globules and extended pillars (Hunter et~al.\ 1995, Walborn \& Barb\'a 
1998, Scowen et~al.\ 1998, Rubio et~al.\ 1998).  Contemporaneously,
improved CO (Johansson et~al.\ 1998) and H$_2$ (Probst \& Rubio 1998, 
Rubio et~al.\ 1998) data have shown substantial molecular 
concentrations in the dark clouds beyond (with respect to R136) 
the west and northeast nebular filaments.

Walborn \& Blades (1997) conducted an extensive optical spectral-classification 
study of the stellar populations within the 30~Doradus Nebula, identifying 
five spatially and/or temporally distinct groups.  Three of them are older 
than the central (R136) cluster, but one, again located predominantly to the 
west and northeast of R136, shows spectroscopic and environmental evidence of 
extreme youth.  In particular, the class of early O~stars embedded in dense 
nebular knots, classified O3-6~V, was increased to eleven in this study.  As 
shown by Rubio et~al.\ (1998), these objects are closely associated spatially 
with the IR sources.

The above observations suggested the regions west and northeast of R136 as 
promising targets for the Near-Infrared Camera and Multi-Object Spectrometer 
(NICMOS) on the Hubble Space Telescope (Thompson et~al.\ 1998).  A proposal 
was submitted and accepted for the supplementary call following the 
discovery of the reduced cryogen lifetime.  Some of the results, which 
because of their 0$\farcs$1 resolution provide a qualitative advance in 
knowledge of the new stellar generation in 30 Doradus, are described below.

\section{OBSERVATIONS}
The NICMOS observations were obtained under HST General Observer 
Program ID No.~7819, during February and March 1998.  Thirteen HST 
orbits were requested and allocated to observe 17 selected fields in 
and near the 30~Dor filaments with NICMOS Camera~2 (NIC2), which has a 
19\arcsec\ square field of view.  Images with broadband filters F110W, F160W, 
F205W (approximately $J, H, K$, respectively, with FWHM 0.4--0.6~$\mu$m; the 
numerical designations of the filters correspond to the central wavelengths 
in hundredths of $\mu$m), were specified for 16 fields.  Narrowband H$_2$, 
continuum, and Br$\gamma$ images (F212N, F215N, F216N filters, 
respectively; FWHM 0.02~$\mu$m) were specified for 3~fields, 2~of which 
are in common with the broadband ones.  In addition, the NIC1 and 
NIC3 cameras were operated in parallel with some of these and other 
filters during all exposures, providing interesting augmentations 
to the dataset.  The broadband observations were conducted during 
8~orbits at 2~fields/orbit, while the narrowband observations consumed an 
orbit each.  The remaining 2~orbits were used for broadband and 
narrowband thermal background observations of relatively blank fields, 
to correct the 2~$\mu$m data.  All of the observations were successful 
except for one broadband orbit, during which NICMOS suspended due to a 
particle hit, resulting in the loss of the $K$~image for one field and 
of all images for the other. 

Every observation consisted of two dithered exposures, each of duration 
160~sec for F110W, 192~sec for F160W, 224~sec for F205W, 640~sec for F212N 
and F215N, and 128~sec for F216N.  These exposures were expected to reach 
$J, H, K$ limiting magnitudes of 22, 20.5, and 20, respectively, with 
S/N of 9 for an isolated point source.  The narrowband exposures were 
expected to reach S/N of 15 for typical nebular features in 30~Dor.  
The broadband expectations were well realized, while the NIC2 
narrowband exposures are rather weak, although they yield some useful 
information as discussed below.

A complete photometric analysis of all the images will be presented in 
subsequent publications.  Detailed calibration work will be required to 
extract the full information for the fainter stars and diffuse structures.  
In this paper, morphological descriptions of several of the more remarkable 
structures found in these images will be presented.  Preliminary $H, K$ 
aperture photometry of the NICMOS images for a few new objects is given 
in the text below.  Most of the NICMOS images shown here have been processed 
through the STScI data pipeline; cosmic rays and bad pixels have been 
corrected, and the dithered exposures combined.  In two cases as specified 
below, the images were reprocessed with updated, orbital dark and flat-field 
calibration files; they were then $2 \times 2$ oversampled and deconvolved 
with a TinyTim PSF, which had itself first been deconvolved with a 
$0\farcs1$ FWHM Gaussian PSF.  Deconvolution suppresses the extended wings 
of the NICMOS PSF and improves the contrast of faint, diffuse nebulosity.

For the first time, IR images of these fields can be compared with the 
HST Wide Field-Planetary Camera~2 (WFPC2) optical images at the same 
resolution.  We shall make extensive use of archival WFPC2 images of 
30 Doradus from Programs No.~5114 (PI J.~Westphal) and 5589 (PI J.~Trauger) 
for comparison with the NICMOS results.  The former covers the region 
northeast of R136 with F336W, F555W, and F814W broadband filters only, 
while the latter covers the west and south regions with F502N ([O~{\sc 
iii}]), F656N (H$\alpha$), and F673N ([S~{\sc ii}]) narrowband filters 
in addition to the broadband ones.

\section{RESULTS}
Every one of the NICMOS broadband images reveals new IR point sources, 
multiple systems, clusters, and/or diffuse structures in 30 Doradus.  
We shall discuss here six of the more remarkable fields, containing 
interesting optical stars and nebular structures known previously, 
which can now be related in detail to the IR objects.

\subsection{Knot 1}
The patch of nebulosity at the base of the northeast filament, designated 
as Knot~1 by Walborn \& Blades (1987), is the brightest in 30~Doradus; 
they detected early-O dwarf absorption features within it and suggested 
that this object might be very young, representing a new stellar generation.  
Figure~1 is a false-color WFPC2 F336W, F555W, F814W (approximately $U, V, I$, 
respectively) composite of Knot~1 and the surrounding field, particularly 
to the north and east (including Knot~2 which is discussed separately 
in the next section).  Figure~2 is a montage of four NIC2 fields covering 
the same region of the sky, presented as a false-color composite of the 
$J, H, K$ exposures.  The variety and complexity of the stellar and nebular 
structures in these images are almost overwhelming; we shall describe 
some of the more salient ones here.

The stellar content of Knot~1 is seen to consist of a compact cluster with 
three bright and numerous fainter members resolved (but recall that at 
the LMC, $0\farcs1$ subtends 5000~AU).  The two brightest, separated by 
1\arcsec, are (Parker 1993) P1222, $V=14.72$ and P1201, $V=15.44$; or 
(Rubio et~al.\ 1998) IRSN-101, $Ks=13.62$ and IRSN-96, $Ks=13.77$, 
respectively.  (Actually, IRSN-96 includes the third brightest component 
as well, which appears redder than the others.)  Walborn \& Blades (1997) 
classified P1222 as O3-6~V and derived $M_V=-4.9$, for $V_0-M_V=18.6$ and 
$R=3$, half a magnitude fainter than the normal calibration value for the 
spectral type, indicating subluminosity and/or peculiar extinction, either 
of which would be consistent with extreme youth.  This object was also
discussed in deconvolved WF/PC~I images by Hunter et~al.\ (1995), who 
described some [S~{\sc ii}] structures as shells surrounding the stars.
However, the WFPC2 images reveal these structures as the remnants of the 
summit of a massive dust pillar, within which the stellar system has formed.  
The pillar is oriented directly toward R136; evidently the pressure 
from the outflows of the latter triggered the formation of the new 
stars near the apex of the pillar, which they in turn have blown away. 
The appearance is that of a post-eruptive volcano, or less violently, 
of eggs in a nest.  Rubio et~al.\ (1998) show a strong H$_2$ concentration 
just behind Knot~1 (with respect to R136), and a large molecular plume 
extending beyond that.

Just 10\arcsec--12\arcsec\ to the northeast of Knot~1, or $\sim3$~pc in
projection, are Rubio et~al.\ (1998) IRSN-122, $Ks=13.00$ and IRSN-126, 
$Ks=11.61$, two of the brightest and reddest IR sources in 30~Dor.  They
have no counterparts in the WFPC2 $V$~images down to magnitude $\sim22$, 
but they are just visible in $I$ at magnitudes 20.4 and 21.2, respectively.
However, a bright nebular arc roughly centered on IRSN-122, whose axis again 
points in the direction of R136, is prominent in the optical as well as the 
IR images.  In fact, this arc appears to be the summit of another massive 
pillar, which is still intact.  IRSN-122 and 126 are also within the extensive 
H$_2$ plume that includes Knot~1 (Rubio et~al.\ 1998).  Three fainter, 
very red IR stars are near IRSN-122.

To the north of Knot~1 is an ``inverted Y'' configuration of four
compact, nearly circular dark globules, which are very prominent in 
projection against the bright nebulosity in the WFPC2 image.  In the NICMOS 
image, a faint, extremely red IR star ($H=19.7$, $K=17.0$) is seen exactly 
within the globule at the eastern vertex of the~Y.  This object may be 
a rare example of a single protostar formed within an isolated globule. 

\subsection{Knot 2}
The field of Knot~2 (Walborn \& Blades 1987) is included in Figures~1 and~2, 
but it is shown separately in Figure~3 at a larger scale to preserve 
the details of the remarkable structures it contains.  The left panel of 
Figure~3 is the WFPC2 $U, V, I$ composite, while the right panel presents 
the NICMOS $J, H, K$ composite of deconvolved images in this case.

Again, the principal optical stellar object is revealed as a compact 
multiple system (enlarged in an outset to the right panel of Figure~3) 
within a rich cluster of fainter stars.  The unresolved system is 
(Parker 1993) P1429, $V=15.39$, also classified O3-6~V by Walborn \& Blades 
(1997) with a derived $M_V=-5.0$, again nearly half a magnitude fainter 
than the normal calibration value for the spectral type.  However, the 
F555W magnitudes of the brightest optical stellar components in Knot~2 are 
15.83 and 17.94, so that the individual absolute magnitudes are proportionally 
fainter still.  Note that the third, faintest component of Knot~2 is much 
redder than the others, just as in Knot~1.  Knot~2 is also Rubio et~al.\ (1998) 
IRSN-135, $Ks=13.85$.  The fainter optical star (F555W magnitude 17.66, 
F814W 15.67) in a smaller bright nebulosity 3\arcsec\ southeast of 
Knot~2 is IRSN-138, which becomes much brighter than the former in the 
IR, $Ks=12.78$.  There are three more bright (and numerous faint) IR 
sources in this field:  IRSN-125, 133, and 152 ($Ks=16.91, 14.27, 14.53$, 
respectively).

IRSN-125 and 152 provide the most spectacular result in this field.  
They are shown enlarged in outsets to the right panel of Figure~3.  
The NICMOS resolution clearly reveals them as extended, nonstellar 
objects, with opposite color gradients along their lengths, located in 
dark clouds at the edges of the bright nebulosity.  Moreover, they are 
exactly aligned across Knot~2, at separations and PAs of $5\farcs3$ 
(1.3~pc in projection), $290^\circ$ and $6\farcs0$ (1.5~pc), $110^\circ$, 
respectively.  This configuration immediately suggests an interpretation 
of these sources as impact points of a highly collimated bipolar jet from 
Knot~2 on the surrounding dust clouds.  The opposite gradients along the 
extended sources, which are also perpendicular to the presumed jet, suggest 
that the jet may be rotating counterclockwise.  Tantalizingly, the H$_2$O 
maser of Whiteoak et~al.\ (1983), Whiteoak \& Gardner (1986) is located 
in this field, but the current accuracy of its position does not allow 
a determination of which if any of the IR sources it may be associated 
with.  Whiteoak (private communication) is currently deriving an 
improved position for the maser, which will be of some interest.

\subsection{Knot 3}
This object in the bright nebular filament west of R136 was first 
identified by Walborn (1991).  It is shown together with the other 
remarkable structures in its immediate vicinity in Figure~4, the left 
panel of which is a WFPC2 H$\alpha$, [S~{\sc ii}] composite and the right 
panel the deconvolved NICMOS $J, H, K$ composite.  The complexity of both 
the stellar and nebular structures within and around Knot~3 requires two 
additional, enlarged presentations: Figure~5 shows the stellar multiple system 
in both WFPC2 and NICMOS continuum images, while Figure~6 presents the 
intricate nebular details around the knot as shown by the WFPC2 as well as 
the IR images, including several apparent jet and shock structures. 
Finally, Figure~7 displays the full NICMOS H$_2$ image of this field.

Yet again, the stellar content of Knot~3 turns out to be a compact multiple 
system, albeit without an associated cluster of fainter stars in this case.  
The unresolved system is (Parker 1993) P409, $V=15.64$, again classified 
O3-6~V by Walborn \& Blades (1997), with a derived $M_V=-4.55$, nearly
a full magnitude fainter than the normal calibration value for the spectral 
type!  It is also (Rubio et~al.\ 1998) IRSW-29, with $Ks=14.29$ and quite 
incomprehensible IR colors.  The latter conundrum is nicely explained by the 
HST images (Figure~5): the object consists of two optical components with 
F555W magnitudes 16.70, 16.97 and $K=15.5$ for each, separated by $0\farcs16$; 
and a third IR component with F555W magnitude 20.43, F814W 18.75, and $K=15.2$, 
$0\farcs36$ from the brighter optical one!  So the individual components are 
even more subluminous, and it also appears that Parker's photometry of this 
object must include a substantial nebular contribution.  

The nebular structures associated with Knot~3 are equally remarkable.  
Figure 4 shows a very bright patch of nebulosity adjacent to the 
overexposed region containing the stellar triple system, immediately to 
the northeast of the latter, and other structured nebulosities in the 
same direction as well as toward the south.  Rubio et~al.\ (1998) showed 
an extended H$_2$ feature coinciding with the nebulosities to the northeast 
of Knot~3, suggesting a molecular jet.  Figure~6 shows the intricate 
structures within the bright nebular patch in both the WFPC2 and IR 
images; the latter also reveal several fainter IR stars in the 
vicinity which may well be the sources of some of the nebular structures.
Four compact nebulosities are identified, which are bright in 
different combinations of filters: n1 in [O~{\sc iii}] and H$\alpha$, 
n2 in H$\alpha$ and $K$, n3 in [S~{\sc ii}] and $K$ (although the former
image is displaced $0\farcs2$ NE of the latter in this case), and n4 in 
[S~{\sc ii}] only.  Nebulosities n1-3 are within the bright patch and appear 
to reside on concentric parabolic arcs which may be bow shocks.  There are 
striking morphological similarities to the structures of the HH~1 and HH~111 
bow shocks discussed by Hester, Stapelfeldt, \& Scowen (1998) and by 
Reipurth et~al.\ (1997), respectively, although the 30~Dor object is an 
order of magnitude larger, perhaps because of more massive stellar sources.  
Nebulosity n4 may be the base of a jet from Knot~3 extending toward the south 
(see also Figure~4).  Three fainter IR stars, s1-3, are identified in the 
$H$~image in Figure~6.  Star~s2 ($H=20.3$, $K\sim20$) is within the bright 
patch and the parabolic arcs; s1 ($H=18.9$, $K=18.1$) is outside but also 
below the arcs.  One or both of these stars may well be responsible for the 
nebular structures of the bright patch.  The longest linear feature in the 
complex nebulosities further to the northeast of Knot~3 points directly 
toward star~s3 ($H=20.0$, $K=19.3$), which may well be its source; this 
linear feature and the detached nebulosity off its other end are very 
prominent in the NICMOS H$_2$~image and likely correspond to a separate jet 
structure associated with s3.  In summary, the region adjacent to Knot~3 
is exceedingly complex and will be a rewarding subject for future 
quantitative analysis.

Finally, we discuss the other outstanding IR stellar sources and extended 
structures in the field of Knot~3 as shown in Figure~4.  They include Rubio 
et~al. (1998) IRSW-26, 30, and 46.  IRSW-30 ($Ks=11.63$) is one of the two 
brightest IR sources in the fields considered; it has a faint optical 
counterpart with F555W magnitude 20.26 and F814W 19.03, located exactly in the 
head of a bright-rimmed pillar oriented toward R136, which was also discussed 
by Scowen et~al.\ (1998).  The optical stellar image appears asymmetrical, 
indicating a multiple system; although very faint, it will likely yield 
important information about the IR source in the future.  (The very bright 
optical star to the east of this structure is the unrelated A-type supergiant 
R138, $V=11.87$.)  Rubio et~al.\ (1998) showed that the adjacent dark pillar to 
the south is a strong H$_2$ source without IR continuum emission, which is 
nicely corroborated by the NICMOS H$_2$ image in Figure~7.  This image also 
shows that the bright-rimmed pillar containing IRSW-30 is a strong H$_2$ 
source, and that there is extensive diffuse H$_2$ emission from other 
dark clouds immediately beyond the ionized regions.  Note that 
IRSW-26 ($Ks=14.89$) and 46 ($Ks=12.96$) also have optical counterparts; 
that of the latter is (Parker 1993) P454, with $V=17.29$.  

\subsection{The South Pillars}
This remarkable active region $\sim1$\arcmin south of R136 is shown in 
Figure~8, the left panel of which is a WFPC2 [O~{\sc iii}], H$\alpha$,
[S~{\sc ii}] composite and the right panel the NICMOS $J, H, K$ composite.
Scowen et~al.\ (1998) presented a detailed astrophysical analysis of 
the most prominent, western pillar structure, showing that it is very 
similar to those in M16 discussed by Hester et~al.\ (1996).  Rubio et~al.\ 
(1998) show a strong H$_2$ source coincident with this pillar.

This region has not been covered by previous IR continuum surveys, and our 
new contribution is the discovery of a very luminous stellar source 
($H=14.48$, $K=14.04$) in the NICMOS images.  It is located at the base of a 
secondary pillar to the east of the one analyzed by Scowen et~al.; the latter 
contains only two very faint IR stars, located at the apexes of internal dust 
substructures.  Again, both of these bright pillars are oriented toward R136.  
The new luminous IR source has an optical counterpart in the WFPC2 images, 
with F555W magnitude 18.93 and F814W 16.81.  This result emphasizes a basic 
difference between 30 Doradus and M16, namely that in the former {\it
massive\/} star formation is currently being triggered, as revealed by the IR 
images.

\section{DISCUSSION}
This paper presents a morphological description of several of the most 
interesting fields in our NICMOS program.  The remaining fields likewise
contain numerous new IR stars and multiple systems in or near the 30~Doradus 
nebular filaments.  To give but three of many further examples, Rubio et~al.\ 
(1998) IRSW-11 and 44 are resolved into luminous, compact IR double and triple 
stellar systems, respectively, the latter centered in a nebular bubble; while  
the luminous IRSN-134/137 complex is revealed as an IR cluster consisting 
of one dominant and a multitude of faint stars.  In the near future, all 
of these images will be measured and analyzed photometrically, so that we 
can obtain estimates of the luminosity and mass functions, as well as their 
integrated values, in the current, triggered star formation in 30~Doradus. 

These results establish the 30~Doradus Nebula as a prime region in 
which to investigate the formation and very early evolution of massive 
stars and multiple (Trapezium) systems.  Various, likely sequential phases 
of this process can be observed with less extinction than in the Galaxy, and 
indeed, some of the structures described above are probably unprecedented.  
Very few bipolar outflows from {\it massive\/} young stars are known in the 
Galaxy; our discovery of the first extragalactic examples increases 
the available sample and suggests that disk-mediated accretion is 
essential to star formation at very high masses, just as it has been 
established to be at low stellar masses in the solar neighborhood.
The theme of massive multiple systems formed within the heads of dust 
pillars in 30~Dor oriented toward R136 is strong and obviously significant.
One can hypothesize a sequence of stages observable in the IR and/or optical, 
from luminous embedded sources with very faint or no optical counterparts, 
through emerging subluminous optical systems such as Knots~1--3 associated 
in some cases with parsec-scale jets, through perhaps the Vz (``ZAMS'') 
spectroscopic category of Walborn \& Parker (1992) and Walborn \& Blades 
(1997), to the ``normal'' or typical main-sequence O-type dwarfs.  Clearly 
there is a rich future for quantitative, comparative spectroscopy and modeling 
of these classes of objects, all of which are represented in 30~Doradus.

These results also improve our understanding of the global structure 
and evolution of 30~Doradus, and hence of more distant unresolved 
starbursts.  The NICMOS data permit IR comparisons with the nebular 
microstructures seen in WFPC2 images at the same resolution.  We 
strongly concur with the important conclusion of Scowen et~al.\ (1998), 
that giant H~{\sc ii} regions like 30~Doradus are not uniformly filled 
volumes of ionized gas, but rather they are dominated by emission from 
the narrow, filamentary interfaces between essentially evacuated regions 
and the surrounding dust clouds.  The equally important ingredient which 
we add to this picture from the IR images is that those filaments are in 
fact ``star-formation fronts'' moving into the surrounding molecular clouds, 
in which a second stellar generation is being triggered by the energetic 
outflows from the original, centrally condensed massive cluster.

The next stage of this global evolution has already been observed,
namely in N11 of the LMC (Walborn \& Parker 1992, Parker et~al.\ 1992) 
and in NGC 604 of M33 (unpublished HST/WFPC2 images).  These are the 
giant shell H~{\sc ii} regions, in which the central cavity is completely 
evacuated of gas and dust, and all of the nebular emission comes from 
younger regions around the periphery.  The stellar populations of N11 have 
been discussed in detail in the references cited, and it has been shown 
that the age {\it difference\/} of $\sim2$ million years between the core 
and peripheral clusters is identical to that in 30~Doradus, but that the 
{\it absolute\/} ages of both are a few million years greater in N11.  In 
30~Doradus, we are witnessing the birth of the second, peripheral generation  
of very massive stars, triggered by the first generation which still 
exists.  In a few million years, the most massive stars will have 
disappeared from its core, and 30~Doradus will be a giant shell 
H~{\sc ii} region like N11 and NGC~604, with its most massive stars 
around the periphery.  The energetics of this process in N11 has been 
discussed by Meaburn et~al.\ (1989) and Rosado et~al.\ (1996).  
The term {\it two-stage starbursts\/} has been proposed to describe it by 
Walborn \& Parker (1992).

\acknow
We would like to thank all of those responsible for designing and 
implementing the supplemental call for NICMOS proposals, without which 
these results would not exist for some time to come.  We also thank 
the corresponding HST Interstellar Medium review panel chaired by Dr.\ 
H.~Dinerstein (ST~ScI Newsletter, Vol.~15, No.~1, 1998) for recognizing the 
potential of our proposal.  Support for this work was provided by NASA 
through grants GO-7819.01-96A and AR-7545.01-96A from the Space 
Telescope Science Institute, which is operated by AURA, Inc., under NASA 
contract NAS 5-26555.  M.R.\ acknowledges support from FONDECYT (Chile) 
through grant 1960926.

\begin{references}
\reference{ch94} Chu, Y.-H., \& Kennicutt, R.~C., Jr. 1994, \apj, 425, 720
\reference{hy92} Hyland, A.~R., Straw, S., Jones, T.~J., \& Gatley, I. 1992, 
   \mnras, 257, 391
\raggedbottom\newpage
\reference{he96} Hester, J.~J. et~al.\ 1996, \aj, 111, 2349
\reference{he98} Hester, J.~J., Stapelfeldt, K.~R., and Scowen, P.~A. 1998,
   \aj, 116, 372
\reference{hu95} Hunter, D.~A., Shaya, E.~J., Scowen, P., Hester, J.~J., 
   Groth, E.~J., Lynds, R., \& O'Neil, E.~J., Jr. 1995, \apj, 444, 758
\reference{jo98} Johansson, L.~E.~B. et~al.\ 1998, \aap, 331, 857
\reference{md90} Maeder, A. 1990, \aaps, 84, 139
\reference{md88} Maeder, A., \& Meynet, G. 1988, \aaps, 76, 411
\reference{ma98} Massey, P., \& Hunter, D.~A. 1998, \apj, 493, 180
\reference{me89} Meaburn, J., Solomos, N., Laspias, V., \& Goudis, C. 
   1989, \aap, 225, 497
\reference{pa93} Parker, J.~Wm. 1993, \aj, 106, 560
\reference{pa92} Parker, J.~Wm., Garmany, C.~D., Massey, P., \& Walborn, N.~R. 
   1992, \aj, 103, 1205
\reference{pr98} Probst, R., \& Rubio, M. 1998, in preparation
\reference{re97} Reipurth, B., Hartigan, P., Heathcote, S., Morse, J.~A.,
   \& Bally, J. 1997, \aj, 114, 757
\reference{ro96} Rosado, M. et~al.\ 1996, \aap, 308, 588
\reference{ru98} Rubio, M., Barb\'a, R.~H., Walborn, N.~R., Probst, R.~G., 
   Garc\'{\i}a, J., \& Roth, M.~R. 1998, \aj, 116, 1708
\reference{ru92} Rubio, M., Roth, M., \& Garc\'{\i}a, J. 1992,
   \aap, 261, L29
\reference{sc98} Scowen, P.~A. et~al., 1998, \aj, 116, 163
\reference{th98} Thompson, R.~I., Rieke, M., Schneider, G., Hines, D.~C., \&
   Corbin, M.~R. 1998, \apj, 492, L95
\reference{wa84} Walborn, N.~R. 1984, in IAU Symp.~108, Structure and Evolution 
   of the Magellanic Clouds, ed.\ S.~van den Bergh \& K.~S. de~Boer
   (Dordrecht: Reidel), 243
\reference{wa91} Walborn, N.~R. 1991, in ST~ScI Symp.\ Ser.~5,
   Massive Stars in Starbursts, ed.\ C.~Leitherer, N.~R.\ Walborn, 
   T.~M.\ Heckman, \& C.~A.\ Norman (Cambridge: Cambridge Univ.\ Press), 145
\reference{wa98} Walborn, N.~R., \& Barb\'a, R.~H. 1998, in ASP Conf.\
   Ser.~131, Boulder-Munich~II: Properties of Hot, Luminous Stars, 
   ed.\ I.~D.\ Howarth (San Francisco: ASP), 336
\reference{wa87} Walborn, N.~R., \& Blades, J.~C. 1987, \apj, 323, L65
\reference{wa97} Walborn, N.~R., \& Blades, J.~C. 1997, \apjs, 112, 457
\reference{wa92} Walborn, N.~R., \& Parker, J.~Wm. 1992, \apj, 399, L87
\reference{wh86} Whiteoak, J.~B., \& Gardner, F.~F. 1986, \mnras, 222, 513
\reference{wh83} Whiteoak, J.~B., Wellington, K.~J., Jauncey, D.~L., 
   Gardner, F.~F., Forster, J.~R., Caswell, J.~L., \& Batchelor, R.~A. 1983,
   \mnras, 205, 275
\end{references}

\onecolumn
\begin{figure}
\caption {WFPC2 F336W, F555W, F814W (blue, green, red, respectively) composite 
of the field of Knots~1 and~2 in 30~Doradus.  The scale bars indicate 
N, E and are 4\arcsec\ = 1~pc in length.  Knot~1 is the triple stellar 
system to the right of center and Knot~2 is the composite image toward the
upper left.  Note the location of Knot~1 at the apex of a large dust 
pillar, whose summit has been blown off; the pillar is oriented toward 
R136, outside the field $\sim1$\arcmin SW of Knot~1.  Note also the 
``inverted Y'' configuration of four prominent dark globules north of Knot~1.
\label{fig1}}
\end{figure}

\begin{figure}
\caption{NICMOS $J, H, K$ (blue, green, red, respectively) composite of the 
field of Knots~1 and~2, with the same orientation and scale as Figure~1.
A montage of four NIC2 fields is shown.  The two very bright IR sources 
below center are Rubio et~al.\ (1998) IRSN-122 and 126; they are just barely  
visible as very faint red ($\sim$$I$) stars in Figure~1.  Note also the 
faint, very red object within the easternmost of the four dark globules 
north of Knot~1.  The field of Knot~2 (at the left) is enlarged in Figure~3.
\label{fig2}}
\end{figure}

\begin{figure}
\caption{Enlargements of the field centered on Knot~2 in WFPC2 F336W, F555W, F814W 
(left panel) and deconvolved NICMOS $J, H, K$ (right panel) composites. 
The scale bars again denote N, E and are 4\arcsec\ = 1~pc in length.  
The outsets to the right panel show $3\times$ enlargements of the Knot~2 
stellar system and of the nonstellar sources Rubio et al. (1998) IRSN-125
and 152; the boxes are 2\arcsec\ square and those of the nonstellar 
sources are vertically aligned with their images in the large panel.  
Note the linear alignment of the nonstellar sources across Knot~2 and their 
opposite color gradients, suggesting impact points of a rotating jet.  The 
bright stellar IR sources 5\arcsec\ NW and 3\arcsec\ SE of Knot~2 are IRSN-133 
and 138, respectively; their colors are saturated in the NICMOS composite 
to preserve those of the fainter sources.  Note also the rich clustering of 
faint IR stars.  The deep red extended feature at upper right is an 
artefact due to the NIC2 coronographic hole.
\label{fig3}}
\end{figure}

\begin{figure}
\caption{WFPC2 H$\alpha$, [S~{\sc ii}] (green, red, respectively; left panel) and 
deconvolved NICMOS $J, H, K$ (blue, green, red, respectively; right panel)
composites of the field of Knot~3.  The scale bars indicate N, E and are 
4\arcsec\ = 1~pc in length.  Knot~3 is the overexposed, asymmetrical image
below and to the left of center; its internal and adjacent stellar and 
nebular structures are shown in Figures~5 and~6, respectively.  
The very bright stellar IR source above and to the right of center is Rubio 
et~al.\ (1998) IRSW-30; it has a faint optical counterpart and is located in 
the head of a bright-rimmed pillar; the brightest part of the rim directly
faces R136, outside the field $\sim40\arcsec$ to the east.  Two other 
bright stellar IR sources with faint optical counterparts in this field are 
IRSW-26, 2\arcsec\ SW of IRSW-30, and IRSW-46, $\sim5\arcsec$ SE of Knot~3.  
The bright optical star at top left is the unrelated A-type supergiant R138.
\label{fig4}}
\end{figure}

\begin{figure}
\caption{Enlarged images of the stellar system within Knot~3.  North is up and 
east to the left in each panel, and the scale bars are 1\arcsec\ = 0.25~pc 
in length.  The left panel is the WFPC2 F336W, F555W, F814W composite, 
the center panel the deconvolved NIC2 $J, H, K$ composite, and the right 
panel a NIC1 $H$~image.  These images show that the stellar content of 
Knot~3 is a triple system consisting of two optical components separated 
by $0\farcs16$ SW-NE, and an IR component located $0\farcs36$ NW of 
the brighter optical one.
\label{fig5}}
\end{figure}

\begin{figure}
\caption{Structural details near Knot~3 (the brightest image just below center) as 
shown by three WFPC2 (top panels) and three NICMOS (bottom panels) passbands.
The scale bars indicate N, E and are 2\arcsec\ = 0.5~pc in length.
Four compact nebulosities (n1-4), which are bright in different combinations 
of filters, and three faint IR stars (s1-3) are identified here and discussed
in the text.  Several jet and shock structures with different stellar sources
are probably present in this field.  The H$_2$ image has been continuum 
subtracted and 3~pixel median filtered; the full H$_2$ image is shown 
in Figure~7.  The northward linear streaks in the $K$~image are diffraction 
spikes from IRSW-30, outside the field of view (see Figure~4).
\label{fig6}}
\end{figure}

\begin{figure}
\caption{The morphology of H$_2$ emission in the field of Knot~3.  The left 
panel shows the NICMOS H$_2$ contours superposed on the same WFPC2 
H$\alpha$, [S~{\sc ii}] composite as in the left panel of Figure~4, 
although the field coverage and orientation are slightly different.  
The right panel presents the H$_2$ image in grayscale.  The scale bars 
indicate N, E and are 4\arcsec\ = 1~pc in length.  The H$_2$ images 
have been continuum subtracted and 3~pixel median filtered.  There is 
a small, dark continuum-subtraction artefact due to the very bright stellar 
source IRSW-30 in the head of the bright-rimmed pillar at upper right.  The 
extensive H$_2$ emission is strikingly correlated with dust features just 
beyond ionized interfaces, including both the bright-rimmed pillar and the 
adjacent dark one.  The H$_2$ jet emission at upper left is shown enlarged
in Figure~6.
\label{fig7}}
\end{figure}

\begin{figure}
\caption{WFPC2 [O~{\sc iii}], H$\alpha$, [S~{\sc ii}] (left panel) and NICMOS $J, H, K$ 
(right panel) blue, green, red composites of an active region $\sim1$\arcmin 
south of R136.  The scale bars indicate N, E and are 4\arcsec\ = 1~pc in 
length.  Two very faint IR stars are seen near dust structures within the more 
prominent pillar toward the west, while a luminous IR star (at the NICMOS 
image center, with a faint optical counterpart) is found near the ``base'' of 
the narrower pillar toward the east.  Both pillars are oriented toward R136.
\label{fig8}}
\end{figure}
\begin{figure}
\vspace{5.5in}
\end{figure}

\end{document}
