J. R. Walsh
Space Telescope European Coordinating Facility, European Southern Observatory, Germany
Space Telescope Science Institute, USA
D. R. Garnett
Astronomy Department, University of Minnesota, USA
Astronomy Department, University of Illinois, USA
Department of Astronomy, University of Manchester, UK
A. J. Holloway
Department of Astronomy, University of Manchester, UK
European Southern Observatory, Germany
Observatoire de Marseille, France
Institut d'Astrophysique, Paris, France
Observatoire de Paris Meudon, France
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 (R24) 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 0.871.10 (N-SE-W) ( 0.210.27pc), 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 V0.6, 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.
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