A VLA Study of Ultracompact and Hypercompact H II Regions from 0.7 to 3.6 cm

We report multi-frequency Very Large Array observations of three massive star formation regions (MSFRs) containing radio continuum components that were identified as broad radio recombination line (RRL) sources and hypercompact (HC) H II region candidates in our previous H92alpha and H76alpha study: G10.96+0.01 (component W), G28.20-0.04 (N), and G34.26+0.15 (B). An additional HC H II region candidate, G45.07+0.13, known to have broad H66alpha and H76alpha lines, small size, high electron density and emission measure, was also included. We observed with high spatial resolution (0.9" to 2.3") the H53alpha, H66alpha, H76alpha, and H92alpha RRLs and the radio continuum at the corresponding wavelengths (0.7 to 3.6 cm). The motivation for these observations was to obtain RRLs over a range of principal quantum states to look for signatures of pressure broadening and macroscopic velocity structure. We find that pressure broadening contributes significantly to the line widths, but it is not the sole cause of the broad lines. We compare radio continuum and dust emission distributions and find a good correspondence. We also discuss maser emission and multi-wavelength observations reported in the literature for these MSFRs.Comment: Accepted for publication in ApJ; 55 pages, 10 tables, 12 figure

An H2CO 6cm Maser Pinpointing a Possible Circumstellar Torus in IRAS18566+0408

We report observations of 6cm, 3.6cm, 1.3cm, and 7mm radio continuum, conducted with the Very Large Array towards IRAS18566+0408, one of the few sources known to harbor H2CO 6cm maser emission. Our observations reveal that the emission is dominated by an ionized jet at cm wavelengths. Spitzer/IRAC images from GLIMPSE support this interpretation, given the presence of 4.5um excess emission at approximately the same orientation as the cm continuum. The 7mm emission is dominated by thermal dust from a flattened structure almost perpendicular to the ionized jet, thus, the 7mm emission appears to trace a torus associated with a young massive stellar object. The H2CO 6cm maser is coincident with the center of the torus-like structure. Our observations rule out radiative pumping via radio continuum as the excitation mechanism for the H2CO 6cm maser in IRAS18566+0408.Comment: 20 pages, 4 figures, ApJ (in press

Integral field spectroscopy of massive young stellar objects in the N113 H II region in the Large Magellanic Cloud

The Spitzer Surveying the Agents of Galaxy Evolution (SAGE) survey has allowed the identification and analysis of significant samples of Young Stellar Object (YSO) candidates in the Large Magellanic Cloud (LMC). However, the angular resolution of Spitzer is relatively poor meaning that at the distance of the LMC, it is likely that many of the Spitzer YSO candidates in fact contain multiple components. We present high-resolution K-band integral field spectroscopic observations of the three most prominent massive YSO candidates in the N113 H?II region using Very Large Telescope/Spectrograph for INtegral Field Observations in the Near Infrared (VLT/SINFONI). We have identified six K-band continuum sources within the three Spitzer sources and we have mapped the morphology and velocity fields of extended line emission around these sources. Br ?, He?I and H2 emission is found at the position of all six K-band sources; we discuss whether the emission is associated with the continuum sources or whether it is ambient emission. H2 emission appears to be mostly ambient emission and no evidence of CO emission arising in the discs of YSOs has been found. We have mapped the centroid velocities of extended Br ? emission and He?I emission and found evidence of two expanding compact H?II regions. One source shows compact and strong H2 emission suggestive of a molecular outflow. The diversity of spectroscopic properties observed is interpreted in the context of a range of evolutionary stages associated with massive star formation

The dust properties and physical conditions of the interstellar medium in the LMC massive star forming complex N11

We combine Spitzer and Herschel data of the star-forming region N11 in the Large Magellanic Cloud to produce detailed maps of the dust properties in the complex and study their variations with the ISM conditions. We also compare APEX/LABOCA 870um observations with our model predictions in order to decompose the 870um emission into dust and non-dust (free-free emission and CO(3-2) line) contributions. We find that in N11, the 870um can be fully accounted for by these 3 components. The dust surface density map of N11 is combined with HI and CO observations to study local variations in the gas-to-dust mass ratios. Our analysis leads to values lower than those expected from the LMC low-metallicity as well as to a decrease of the gas-to-dust mass ratio with the dust surface density. We explore potential hypotheses that could explain the low observed gas-to-dust mass ratios (variations in the XCO factor, presence of CO-dark gas or of optically thick HI or variations in the dust abundance in the dense regions). We finally decompose the local SEDs using a Principal Component Analysis (i.e. with no a priori assumption on the dust composition in the complex). Our results lead to a promising decomposition of the local SEDs in various dust components (hot, warm, cold) coherent with that expected for the region. Further analysis on a larger sample of galaxies will follow in order to understand how unique this decomposition is or how it evolves from one environment to another.Comment: 24 pages, 16 figures, accepted for publication in MNRA

Spitzer View of Massive Star Formation in the Tidally Stripped Magellanic Bridge

The Magellanic Bridge is the nearest low-metallicity, tidally stripped environment, offering a unique high-resolution view of physical conditions in merging and forming galaxies. In this paper we present analysis of candidate massive young stellar objects (YSOs), i.e., {\it in situ, current} massive star formation (MSF) in the Bridge using {\it Spitzer} mid-IR and complementary optical and near-IR photometry. While we definitely find YSOs in the Bridge, the most massive are $\sim10 M_\odot$, $\ll45 M_\odot$ found in the Large Magellanic Cloud (LMC). The intensity of MSF in the Bridge also appears decreasing, as the most massive YSOs are less massive than those formed in the past. To investigate environmental effects on MSF, we have compared properties of massive YSOs in the Bridge to those in the LMC. First, YSOs in the Bridge are apparently less embedded than in the LMC: 81% of Bridge YSOs show optical counterparts, compared to only 56% of LMC sources with the same range of mass, circumstellar dust mass, and line-of-sight extinction. Circumstellar envelopes are evidently more porous or clumpy in the Bridge's low-metallicity environment. Second, we have used whole samples of YSOs in the LMC and the Bridge to estimate the probability of finding YSOs at a given \hi\ column density, N(HI). We found that the LMC has $\sim3\times$ higher probability than the Bridge for N(HI) $>10\times10^{20}$ cm$^{-2}$, but the trend reverses at lower N(HI). Investigating whether this lower efficiency relative to HI is due to less efficient molecular cloud formation, or less efficient cloud collapse, or both, will require sensitive molecular gas observations.Comment: 41 pages, 20 figures, 6 tables; accepted for publication in ApJ; several figures are in low resolution due to the size limit here and a high resolution version can be downloaded via http://www.astro.virginia.edu/~cc5ye/ms_bridge20140215.pd

Internal Dynamics of the Hypercompact H II Region G28.20-0.04N

High resolution (0.15") Very Large Array observations of 7 mm continuum and H53a line emission toward the hypercompact H II region G28.20-0.04N reveal the presence of large-scale ordered motions. We find a velocity gradient of 1000 km/s/pc along the minor axis of the continuum source. Lower resolution (1.0"-2.3") radio recombination line observations indicate a systematic increase of line width from H30alpha to H92alpha. Under the assumption that the H30alpha line does not suffer significant pressure broadening, we have deconvolved the contributions of statistical broadening (thermal, turbulent, and pressure) from large-scale motions. The pressure broadening of the H53alpha, H76alpha, and H92alpha lines implies an electron density of 6.9E+06, 8.5E+05, and 2.8E+05 cm^(-3), respectively.Comment: 12 pages, 2 figures, 1 table, Accepted for publication in Ap

Herschel Observations of a Newly Discovered UX Ori Star in the Large Magellanic Cloud

The LMC star, SSTISAGE1C J050756.44-703453.9, was first noticed during a survey of EROS-2 lightcurves for stars with large irregular brightness variations typical of the R Coronae Borealis (RCB) class. However, the visible spectrum showing emission lines including the Balmer and Paschen series as well as many Fe II lines is emphatically not that of an RCB star. This star has all of the characteristics of a typical UX Ori star. It has a spectral type of approximately A2 and has excited an H II region in its vicinity. However, if it is an LMC member, then it is very luminous for a Herbig Ae/Be star. It shows irregular drops in brightness of up to 2 mag, and displays the reddening and "blueing" typical of this class of stars. Its spectrum, showing a combination of emission and absorption lines, is typical of a UX Ori star that is in a decline caused by obscuration from the circumstellar dust. SSTISAGE1C J050756.44-703453.9 has a strong IR excess and significant emission is present out to 500 micron. Monte Carlo radiative transfer modeling of the SED requires that SSTISAGE1C J050756.44-703453.9 has both a dusty disk as well as a large extended diffuse envelope to fit both the mid- and far-IR dust emission. This star is a new member of the UX Ori subclass of the Herbig Ae/Be stars and only the second such star to be discovered in the LMC.Comment: ApJ, in press. 9 pages, 5 figure

The Detection of Hot Cores and Complex Organic Molecules in the Large Magellanic Cloud

We report the first extragalactic detection of the complex organic molecules (COMs) dimethyl ether (CH3OCH3) and methyl formate (CH3OCHO) with the Atacama Large Millimeter/submillimeter Array (ALMA). These COMs, together with their parent species methanol (CH3OH), were detected toward two 1.3 mm continuum sources in the N 113 star-forming region in the low-metallicity Large Magellanic Cloud (LMC). Rotational temperatures (${T}_{\mathrm{rot}}\sim 130$ K) and total column densities (${N}_{\mathrm{rot}}\sim {10}^{16}$ cm−2) have been calculated for each source based on multiple transitions of CH3OH. We present the ALMA molecular emission maps for COMs and measured abundances for all detected species. The physical and chemical properties of two sources with COMs detection, and the association with H2O and OH maser emission, indicate that they are hot cores. The fractional abundances of COMs scaled by a factor of 2.5 to account for the lower metallicity in the LMC are comparable to those found at the lower end of the range in Galactic hot cores. Our results have important implications for studies of organic chemistry at higher redshift

The youngest massive protostars in the Large Magellanic Cloud

We demonstrate the unique capabilities of Herschel to study very young luminous extragalactic young stellar objects (YSOs) by analyzing a central strip of the Large Magellanic Cloud obtained through the HERITAGE Science Demonstration Program. We combine PACS 100 and 160, and SPIRE 250, 350, and 500 microns photometry with 2MASS (1.25-2.17 microns) and Spitzer IRAC and MIPS (3.6-70 microns) to construct complete spectral energy distributions (SEDs) of compact sources. From these, we identify 207 candidate embedded YSOs in the observed region, ~40% never-before identified. We discuss their position in far-infrared color-magnitude space, comparing with previously studied, spectroscopically confirmed YSOs and maser emission. All have red colors indicating massive cool envelopes and great youth. We analyze four example YSOs, determining their physical properties by fitting their SEDs with radiative transfer models. Fitting full SEDs including the Herschel data requires us to increase the size and mass of envelopes included in the models. This implies higher accretion rates (greater than or equal to 0.0001 M_sun/yr), in agreement with previous outflow studies of high-mass protostars. Our results show that Herschel provides reliable longwave SEDs of large samples of high-mass YSOs; discovers the youngest YSOs whose SEDs peak in Herschel bands; and constrains the physical properties and evolutionary stages of YSOs more precisely than was previously possible.Comment: Main text: 4 pages, 3 figures, 1 table; Online material: 3 figures, 1 table; to appear in the A&A Herschel Special Issu