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OBSERVERS with the VLT VLT/ISAAC and HST/WFPC2 Observations of NGC 3603

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OBSERVERS with the VLT VLT/ISAAC and HST/WFPC2 Observations of NGC 3603 OBSERVERS WITH THE VLT VLT/ISAAC and HST/WFPC2 Observations of NGC 3603 B. BRANDL1, W. BRANDNER2, E.K. GREBEL3 AND H. ZINNECKER 4* 1Cornell University, Ithaca; 2University of Hawaii, Honolulu; 3University of Washington at Seattle; 4Astrophysikalisches Institut Potsdam 1. Abstract spectral signatures of Wolf-Rayet (W-R) side (Churchwell et al. 1987; O’Dell et al. stars contribute more than 2000 M0 to 1993). FUV photons (13.6 eV > hν > 6 We have studied NGC 3603, the the cluster mass. Normally, W-R stars are eV) heat up the inside of the proplyd en- most massive visible HII region in the evolved supergiants that have long left velope and lead to the dissociation of mol- Galaxy, with VLT/ISAAC in the near-in- the main sequence and have ages of 3–5 ecules in the outer layers of the circum- frared (NIR) Js, H, and Ks-bands and Myr. In NGC 3603, however, the W-R stellar disk (Johnstone et al. 1998). The HST/WFPC2 at Hα and [N II] wave- stars also show hydrogen absorption resulting evaporation flow provides a lengths. In this Messenger article we lines in addition to the typical W-R fea- steady supply of neutral atoms to the ion- describe the data analysis and some tures. It is believed that these stars are isation front and leads to the development first results from both our complemen- still main-sequence, core hydrogen- of a cometary tail (McCullough et al. 1995; tary observations. burning stars that are so massive and so Störzer & Hollenbach 1999). Until re- Our HST/WFPC2 gave us an un- close to the Eddington limit that they are cently, only one other proplyd had been precedented high-resolution view of loosing their outer envelopes through fast found outside the Orion nebula. It is lo- the interstellar medium in the giant HII winds, and thus resemble evolved W-R cated in the vicinity of the O7V star Her- region and its ionisation structure. stars. In comparison to the Orion Trapez- schel 36 in the Lagoon Nebula (M8, Among the findings are two gigantic ium system, NGC 3603 with its more than Stecklum et al. 1998). gas columns (similar to the famous 50 O and W-R stars producing a Lyman “elephant trunks” in M16) and three pro- continuum flux of 1051 s–1 (Kennicutt 3. VLT/ISAAC Observations plyd-like structures. The emission neb- 1984; Drissen et al. 1995) has about 100 ulae are clearly resolved; all three neb- times the ionising power of the Trapezi- We observed NGC 3603 through the ulae are tadpole shaped, with the bright um cluster. Js = 1.16–1.32 µm, H = 1.50–1.80 µm, 7 ionisation front at the head facing the With a bolometric luminosity Lbol > 10 and Ks = 2.03–2.30 µm broadband filters central cluster and a fainter ionisation L0, NGC 3603 has about 10% of the lu- using the NIR camera ISAAC on ANTU, front around the tail pointing away from minosity of 30 Doradus and looks in many the first VLT unit telescope. The obser- the cluster. Typical sizes are 6000 A.U. respects very similar to its stellar core vations were made during the 4 nights of × 20,000 A.U. The nebulae share the R136 (Brandl et al. 1996). In fact, it has April 4–6 and 9, 1999, in service mode overall morphology of the proplyds been called a Galactic clone of R 136 but when the optical seeing was equal to, (“PROto PLanetarY DiskS”) in Orion, without the massive surrounding cluster or better than, 0.4″ in 1-minute expo- but are 20 to 30 times larger in size. halo (Moffat, Drissen & Shara 1994). In sures. Such seeing was essential for The VLT observations are the most many ways NGC 3603 and R136 can be accurate photometry in the crowded sensitive near-infrared observations regarded as representative building cluster and increased our sensitivity made to date of a dense starburst blocks of more distant and luminous star- to the faintest stars. The majority of region, allowing us to investigate with un- burst galaxies (Brandl, Brandner & Zin- our data were taken under photometric precedented quality its low-mass stellar necker 1999, and references therein). So conditions. population. Our sensitivity limit to stars the best way to study collective star for- Our observing strategy was to use detected in all three bands corresponds mation in a violent environment is by ex- the shortest possible frame times of to 0.1 M0 for a pre-main sequence star amining these regions on a star-by-star 1.77 seconds to keep the number of of age 0.7 Myr. Our observations clear- basis. Until recently, observations of the saturated stars to a minimum. However, ly show that sub-solar-mass stars down entire stellar populations were limited to due to the system’s excellent sensi- to at least 0.1 M0 do form in massive star- the massive stars by sensitivity or wave- tivity, about two dozen of the brightest bursts. length restrictions. The formation of low- stars ended up being saturated. Never- mass stars in starburst regions is of par- theless, this does not impose a prob- 2. Introduction ticular interest. Fundamental questions lem to our study of the low-mass stars. that arise in this context are: Does the Thirty-four short exposures were co- NGC 3603 is located in the Carina spi- slope of the IMF vary on small scales? added to an effective one-minute expo- ral arm (RA = 11h, DEC = –61°) at a dis- Do low-mass stars in a starburst event sure, the minimum time per pointing tance of 6–7 kpc. It is the only massive, form together with the most massive stars required to stabilise the telescope’s ac- Galactic HII region whose ionising cen- or do they form at different times or on tive optics control system. Between the tral cluster can be studied at optical wave- different timescales? And finally, one 1-minute pointings we moved the tele- lengths, due to only moderate (mainly might even ask if low-mass stars form at scope by up to 20″ offsets in a random foreground) extinction of AV = 4–5 mag all in such environments? pattern. This approach has several ad- (Moffat 1983; Melnick et al. 1989). The In addition, NGC 3603 is an ideal lab- vantages: OB stars (> 10 M0) and stars with the oratory to study individual (!) star forma- • Enlargement of the observed field of tion. Some young stars in the vicinity of view (FOV) with maximum signal-to-noise massive stars are surrounded by par- (S/N) in the cluster centre. *Our collaborators on the project described in this tially ionised circumstellar clouds with • Reduction of residual images and article are Y.-H. Chu, H. Dottori, F. Eisenhauer, A.F.J. Moffat, F. Palla, S. Richling, H.W. Yorke and S. D. cometary shape, so-called protoplanetary other array artefacts, using the median Points. disks (“Proplyds”), ionised from the out- filtering technique. 46 • Derivation of the “sky” from the target exposures using the median filtering technique. No additional time for “blank” sky frames outside the cluster was re- quired. The sky frames have been comput- ed using between 15 and 37 subse- quent exposures per waveband and night, and careful eye-inspection showed that all sources have been efficiently removed using our modified median filtering technique which re- turns the lower 1/3 instead of the mean (1/2) value. We subtracted the sky background and flat-fielded each exposure using the twilight flat-fields provided by ESO. The relative position offsets were derived from cross-correlat- ing the images; the exposures were co- aligned on a 0.5 × 0.5-pixel sub-grid for better spatial resolution, and then added together us- Figure 1: Three-colour image of NGC 3603 composed from Js (blue), H (green), and Ks-band (red) images. Intensi- ing the median fil- ties are scaled in logarithmic units. The field of view is 3.′4 × 3.′4. North is up, East to the left. The insert to the lower tering technique. right is a blow-up of the central parsec2. The resulting im- ages are 3.′4 × 3.′4 in size with pixels of 0.″074. The effective details). About 1′ south of the central observations and the systematic pho- exposure times of the final broadband im- cluster, we detect the brightest mem- tometric errors are dominated by un- ages in the central 2.′5 × 2.′5 are 37, 45, bers of the deeply embedded pro- certainties in the aperture offsets. (A and 48 minutes in Js, H, and Ks, re- tocluster IRS 9. detailed error analysis will be part spectively. In order to derive the photometric of a subsequent paper). Comparing Figure 1 shows the impressive 3-colour fluxes of the stars we used the IRAF our photometric fluxes of numerous composite image that has recently been implementation of DAOPHOT (Stetson sources with the fluxes derived by the subject of an ESO press release. 1987). We first ran DAOFIND to detect Eisenhauer et al. (1998) yields a m (http://www.eso.org/outreach/press-rel/pr- the individual sources, leading to systematic offset of 0.1 in Js and m 1999/pr-16-99.html). The brightest star in P 20,000 peaks in each waveband. 0.05 in Ks. the FOV (80″ northeast of the core) is the Many of these may be noise or red supergiant IRS 4 (Frogel, Persson, peaks in the nebular background and & Aaronson 1977). To the south of the appear only in one waveband.
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