J. R. Walsh
Space Telescope European Coordinating Facility, European Southern
Observatory, Germany
B. Lasker
Space Telescope Science Institute, USA
D. R. Garnett
Astronomy Department, University of Minnesota, USA
Y.-H. Chu
Astronomy Department, University of Illinois, USA
J. Meaburn
Department of Astronomy, University of Manchester, UK
A. J. Holloway
Department of Astronomy, University of Manchester, UK
G. Meylan
European Southern Observatory, Germany
M. Azzopardi
Observatoire de Marseille, France
H. Heydari-Malayeri
Institut d'Astrophysique, Paris, France
J. Lequeux
Observatoire de Paris Meudon, France
N. Meysonnier
Observatoire de Marseille
Two sets of Cycle 5 images have so far been obtained---one field
in H
and [O III], R and V, and a second in H-alpha and R,
in the vicinity of SN 1987A. A wealth of nebular structures are
seen in the first field, with many fine filaments down to widths
of 0.3
or less. A small wind-blown bubble of diameter 21
has
been found around one of the brighter stars. The second field
shows less evidence for shell-like structures and more for dust
features, suggesting on-going star formation. A small dust
globule of diameter 0.9
(0.17pc) has been discovered in this
field, giving the first indication that this channel for star
formation may also occur in the lower metallicity environment of
the Magellanic Clouds. A color-magnitude diagram of stars in one
of the fields is presented and shows evidence for an old and a young
population.
Since Cycle 2 we have obtained coordinated parallel WFPC and WFPC-2 images of the Large and Small Magellanic Clouds. The primary aim is to study the morphology of various interstellar medium processes, taking advantage of the well determined distance and low extinction to the Clouds. We aim to explore many nebular phenomena in a Population I system in a galactic context, at a spatial resolution down to 0.025pc. The Magellanic Clouds also offer the opportunity to observe phenomena rare or unknown in our Galaxy (on account of the high extinction in the Galactic plane and the differences in metallicity and Hubble types).
The classes of object selected cover many nebular structures already known in the Magellanic Clouds as well as others predicted by extension of Galactic Studies: proto-stellar regions, including Herbig Haro (HH) objects; Very Low Excitation (VLE) nebulae; compact H II regions; ejecta around massive stars---e.g., Ofpe/WN9, LBV's & WR stars; H II complexes; supernova remnants; 30 Doradus and shell nebulae; and planetary nebulae.
In practice WFPC images are requested when a primary pointing
allows the annulus traced out by the WFC to sample interesting
extended structure or small nebular objects. A range of acceptable
roll angles is supplied within
which images of interest are detailed.
Emission line filters for H
+[N II] (F653N) and [O III]5007Å
(F502N) are considered primary (depending on the targets), with
supplemental broad band
images (R band - F675W and V band - F547M) to allow distinction
between stars and
compact nebulae. The photometry from these images provides further
astrophysical information which can be exploited.
Some images were taken in Cycles 2--4 with WFPC. However, on account of the severe difficulties of scheduling coordinated parallels, few images were taken and suffered from spherical aberration. The improvements in HST scheduling now allow parallels to be specified by crafting rules and this new system has resulted in a great improvement in efficiency for our programme. Already in Cycle 5, there have been two sets of images taken and further sets are scheduled. The improvements resulting from the elimination of spherical aberration in WFPC-2 have vindicated the promise of this programme and demonstrated that interesting science can emerge. We, here, present examples of initial analysis of these first WFPC-2 fields. It is intended to continue this program to form an archive of images of nebular phenomena in the Magellanic Clouds.
Figure 1 shows the two regions in the LMC where parallel images have been
obtained in Cycle 5, superposed on the Digital Sky Survey image
centered to the south-west of 30 Doradus. The primary pointing was SN1987A.
A total of 16 images in F656N, F675W, F502N and F547M filters have been
obtained in the north-east field (called Field 1) and
four images (F656N and F675W) in the south-west field (Field 2).
Figure 2 shows the mosaiced F656N image of Field 2. This shows weak emission
on sky survey plates but is seen to have a wealth of filamentary
emission. The narrowest filaments detectable
on these images have widths of 
0.3
, thus are resolved by HST.
Figure: Location of the two parallel fields in the LMC. The 30
Doradus nebula is to the north-east and the position of SN1987A is
in the center of the field.
Figure: H
+[N II] image of Parallel Field 2 near SN1987A
(location shown in Fig. 1).
Figure 3 shows a section of an [O III] image with a fine example of an
oval-shaped wind-blown bubble with a major axis diameter of about 21
.
The central star is saturated on the images but is about 14th mag. in R.
The rim to the NW is very
sharp indicating a wind-blown shock and the bubble is partly filled by
emission on both the H
+[N II] and [O III] images.
Figure: [O III] image of a wind-blown shell nebula. The x dimension
of the image is 38
.
Figure 4 shows an unusual H II region found in Field 1 to the
north. This resembles a hanging basket. The central nebula appears
very complex with evidence of dust obscuration to the east,
somewhat reminiscent of M17. No
obvious ionizing stars are seen, but there is a faint star (R
24)
situated in the center of the dust complex which could be an obscured
ionizing star. The system of arcs and radial spokes to the south suggests a
blow-out of material.
Figure: H
+[N II] image of `Hanging Basket' nebula.The image
area is 38
on a side.
More surprisingly, compact dust structures have been detected--see
Figure 5 taken from the F656N image of Field 1. The dust condensation
arrowed has full-width at half maximum absorption of
0.87
1.10
(N-S
E-W) (
0.21
0.27pc),
so would be identified as a Bok globule. In the Galaxy, low mass star
formation proceeds in Bok globules (e.g., Reipurth & Gee 1986).
This is the first example
of a compact dust globule found in the Magellanic Clouds; if associated
with star formation it would demonstrate that low mass star formation proceeds
by a similar channel in the lower metallicity environment of the
Magellanic Clouds. IR mapping of these dust
condensations could reveal direct evidence for star formation.
Figure: H
+[N II] image showing compact dust globules
in absorption against the emission background. The x dimension of
the image is 20
and the globule referred to in the text is
arrowed.
Photometry of the non-saturated star images on the F547M and F675W frames in
Field 2 was obtained using DAOPHOT2 and Figure 6 shows a m(547M) 
.
m(547M)-m(675W) color-magnitude (C-M) diagram for the three Wide Field
chips. There is evidence here for two stellar populations--one young
extending to m(547M)
16 (m(547M) 
V) ; the other old and metal
rich similar to 47 Tuc, for example. The clump of stars at about m(547M) =
19.5 and m(547M)-m(675W) = -0.1 (V-R 
0.5, see Holtzman et al.
(1995) for WFPC-2 color equations) is the horizontal branch, indicating high
metallicity, again in a way similar to 47 Tuc. The horizontal branch gives
the magnitude of the RR Lyraes, whose absolute magnitude is V
0.6,
giving a distance modulus
of 
18.9.
Figure: m(547M) 
. m(547M)-m(675W) color magnitude
diagram for stars in Parallel Field 2. Overlaid are
isochrones for 15Gyr (bold line) and 10Myr (dashed line).
Superposed on the instrumental magnitudes C-M diagram are two isochrones from the compilation of Bertelli et al. (1994) for Z=0.004 (Y=0.24) and Z=0.001 (Y=0.23). Bertelli et al. (1994) only compute Johnson and Cousins bands; the correction from V-R to m(547M)-m(675W) color was assumed to be a constant independent of color, although it probably has a small color-dependent term. The bold line shows the isochrone for 15Gyr and Z=0.004 and the dotted line the isochrone for Z=0.001 and 10Myr, giving a fair match to the metal rich old population and the young lower metallicity main sequence, respectively.
We should like to thank the scheduling team at STScI, and in particular Doug van Orsow, for their dedication in the scheduling of our parallel HST observations.
Bertelli, G., Bressan, A., Chiosi, C., Fagotto, F., & Nasi, E., 1994, A&AS, 106, 275
Holtzman, J., Hester, J. J., Casertano, C., Trauger, J. T., Watson, A. M., Ballester, G. E., Burrows, C. J., Clarke, J. T., Crisp, D., Evans, R. W., Gallagher, J. S., Griffiths, R. E., Hoessel, J. G., Matthews, L. D., Mould, J. R., Scowen, P. A., Stapelfeldt, K. R., & Westphal, J. A. 1995, PASP, 107, 156
Reipurth, B. & Gee, G. 1986, A&A, 166, 148