The LMC as a Massive Star Formation Laboratory: The Case of N44

1. The LMC as a Massive Star Formation Laboratory: The Case of N44 C.-H. R. Chen1, J. P. Seale2, L. W. Looney2, T. Wong2, Y.-H. Chu2, R. A. Gruendl2, J. Ott3 (1U of Virginia, 2U of Illinois, 3NRAO) Recent Spitzer mid-IR observations have revealed a large number of individually resolved massive young stellar objects (YSOs) in the Magellanic Clouds, providing an excellent opportunity to study massive star formation with metallicity and galactic environment different from the Milky Way. Using Spitzer and complementary high-resolution ground-based data, we have identified 60 YSOs in the LMC HII complex N44 (Fig 1), modeled their spectral energy distributions (SEDs), and found that these YSOs span a range of mass from a few to ~45M, as well as a range of evolutionary stage from highly embedded to retaining only remnant circumstellar material (Chen et al. 2009a) . We present new HCN & HCO+ (1-0) observations of the molecular core N44BC (Chin et al. 1997; Fukui et al. 2001) made with Australia Telescope Compact Array, to investigate where the dense molecular gas is and whether & how the energy feedback from massive YSOs shapes the dense gas distribution. Identification,Classification, & Properties of Massive YSOs 1. YSO Identification. Color-magnitude criteria for initial selection; further examination of images & SEDs using multi-wavelength datasets from optical to 24 µm. Our method has a > 95% success rate, confirmed by our Spitzer IRS observations of ~ 300 YSOs (Gruendl & Chu 2009; Seale et al. 2009a). 60 YSOs identified in N44 (Chen et al. 2009a); Fig 2 shows those in N44BC. 2. YSO Classification. Empirical classification from observed & modeled SEDs (Whitney et al. 2004; Robitaille et al. 2006; Chen et al. 2009a). Analog to Class I/II/III: -- Type I: rising from near-IR to 24 µm and beyond; -- Type II: a low peak at optical & a high peak at 8-24 µm; -- Type III: bright stellar & modest dust emissions.• Complications: 65% (39/60) YSOs are resolved into multiples or appear extended in high-resolution HST, CTIO 4m Mosaic & ISPI images. Comparisons of YSOs with IRS spectra show Type I & III YSOs in general have spectral features consistent with their evolutionary stages (Chen et al. 2009b). 3. YSO Properties. SEDs of YSOs are modeled for those that appear single or are dominant within IRAC PSFs. The results show that 93% YSOs with [8.0] ≤9.0 in this sample are indeed massive with M ≥ 8M, a few even up to 30-45M. Dense Gas Environment around Massive YSOs Our ATCA HCO+ & HCN observations of the molecular core in N44BC reveal: 1. Young YSOs often associated with dense gas clumps. Fig 2 shows that Type I & half of Type II YSOs are within HCO+ contours, while Type III are not. Further, older YSOs tend to offset more from HCO+ peak, indicating that massive YSOs effectively disperse dense gas soon after their most embedded phase.2. Lower HCN/HCO+ ratios found near older YSOs. Fig 3 show that HCN emission generally follows HCO+ emission. Most of HCN/HCO+ ratios are ≤ 0.5, but higher ratios are found in very young YSO #1 or where no mid-IR YSOs are detected. As HCN traces denser gas (Young Owl et al. 2000), the decrease of HCN/HCO+ ratios along evolutionary stages of YSOs may result from energy feedback.References: Chen et al. 2009a, ApJ 695, 511; Chen et al. 2009b, in prep.; Chin et al. 1997, A&A 317,548; Fukui et al. 2001, PASJ 53, 41; Gruendl & Chu 2009, ApJS, submitted; Robitaille et al. 2006, ApJS 167, 256; Seale et al. 2009a, ApJ, accepted; Seale et al. 2009b, in prep; Whitney et al. 2004, ApJ 617, 1177; Young Owl et al. 2000, ApJ 540, 886. Fig 1. Color composite of 3.6, 8.0 & 24 µm images of N44. Diffuse features show PAH/ dust emission. Dust shrouded objects, e.g., YSOs & AGBs, appear red. The box shows the FOV of N44BC. Fig 2. (Upper) YSOs in N44BC in circles: large--O, medium-- early B, small--mid B. Circles in red, green , blue indicate Types I, II, & III. YSOs w/o mass estimates in pluses. YSOs w/ IRS spectra marked w/ additional squares & numbers. Contours from ATCA HCO+ map in 0.25n Jy/bmkm/s (n=1,2,…). Dashed lines indicate Fig 3. HCN/ HCO+ ratio map of N44BC molecular core. Contours from ATCA HCO+ map in 0.25n Jy/bmkm/s (n=1,2…). Dashed lines indicate 50% sensitivity contour of the HCO+ mosaic. Poster Contact: rchen@virginia.edu ; seale@illinois.edu 50% sensitivity contours of the 36-pointing mosaic. (Lower) Spitzer IRS spectra of 6 YSOs in N44BC. Source 1 shows young (deep silicate absorption) while others exhibit older (e.g. PAH emission) spectral features.