Formation of High-Mass stars in an isolated environment in the Large Magellanic Cloud

The aim of this study is to characterize the distribution and basic properties of the natal gas associated with high-mass young stellar objects (YSOs) in isolated environments in the Large Magellanic Cloud (LMC). High-mass stars usually form in Giant Molecular Clouds (GMCs) as part of a young stellar cluster, but some OB stars are observed far from GMCs. By examining the spatial coincidence between the high-mass YSOs and 12CO (J = 1-0) emission detected by NANTEN and Mopra observations, we selected ten high-mass YSOs that are located away from any of the NANTEN clouds but are detected by the Mopra pointed observations. The ALMA observations revealed that a compact molecular cloud whose mass is a few thousand solar masses or smaller is associated with the high-mass YSOs, which indicates that these compact clouds are the sites of high-mass star formation. The high-density and high-temperature throughout the clouds are explained by the severe photodissociation of CO due to the lower metallicity than in the Galaxy. The star formation efficiency ranges from several to as high as ~ 40%, indicating efficient star formation in these environments. The enhanced turbulence may be a cause of the efficient star formation therein, as judged from the gas velocity information and the association with the lower density gas.Comment: 31 pages, 14 figures, PASJ accepted for publicatio

Relations Between Molecular Cloud Structure Sizes and Line Widths in the Large Magellanic Cloud

We present a comparative study of the size-line width relation for substructures within six molecular clouds in the Large Magellanic Cloud (LMC) mapped with the Atacama Large Millimeter/submillimeter Array (ALMA). Our sample extends our previous study, which compared a Planck detected cold cloud in the outskirts of the LMC with the 30 Doradus molecular cloud and found the typical line width for 1 pc radius structures to be 5 times larger in 30 Doradus. By observing clouds with intermediate levels of star formation activity, we find evidence that line width at a given size increases with increasing local and cloud-scale 8${\mu}$m intensity. At the same time, line width at a given size appears to independently correlate with measures of mass surface density. Our results suggest that both virial-like motions due to gravity and local energy injection by star formation feedback play important roles in determining intracloud dynamics.Comment: 20 pages, to appear in ApJ. Data presented in this paper can be found at https://mmwave.astro.illinois.edu/almalmc

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

Identifying Young Stellar Objects in the Outer Galaxy: l = 224 deg Region in Canis Major

We study a very young star-forming region in the outer Galaxy that is the most concentrated source of outflows in the Spitzer Space Telescope GLIMPSE360 survey. This region, dubbed CMa-l224, is located in the Canis Major OB1 association. CMa-l224 is relatively faint in the mid-infrared, but it shines brightly at the far-infrared wavelengths as revealed by the Herschel Space Observatory data from the Hi-GAL survey. Using the 3.6 and 4.5 $\mu$m data from the Spitzer/GLIMPSE360 survey, combined with the JHK$_s$ 2MASS and the 70-500 $\mu$m Herschel/Hi-GAL data, we develop a young stellar object (YSO) selection criteria based on color-color cuts and fitting of the YSO candidates' spectral energy distributions with YSO 2D radiative transfer models. We identify 293 YSO candidates and estimate physical parameters for 210 sources well-fit with YSO models. We select an additional 47 sources with GLIMPSE360-only photometry as `possible YSO candidates'. The vast majority of these sources are associated with high H$_2$ column density regions and are good targets for follow-up studies. The distribution of YSO candidates at different evolutionary stages with respect to Herschel filaments supports the idea that stars are formed in the filaments and become more dispersed with time. Both the supernova-induced and spontaneous star formation scenarios are plausible in the environmental context of CMa-l224. However, our results indicate that a spontaneous gravitational collapse of filaments is a more likely scenario. The methods developed for CMa-l224 can be used for larger regions in the Galactic plane where the same set of photometry is available.Comment: Accepted for publication in the Astrophysical Journal Supplement Series; 54 pages including appendice

ALMA Observations of a Quiescent Molecular Cloud in the Large Magellanic Cloud

We present high-resolution (sub-parsec) observations of a giant molecular cloud in the nearest star-forming galaxy, the Large Magellanic Cloud. ALMA Band 6 observations trace the bulk of the molecular gas in $^{12}$CO(2-1) and high column density regions in $^{13}$CO(2-1). Our target is a quiescent cloud (PGCC G282.98-32.40, which we refer to as the "Planck cold cloud" or PCC) in the southern outskirts of the galaxy where star-formation activity is very low and largely confined to one location. We decompose the cloud into structures using a dendrogram and apply an identical analysis to matched-resolution cubes of the 30 Doradus molecular cloud (located near intense star formation) for comparison. Structures in the PCC exhibit roughly 10 times lower surface density and 5 times lower velocity dispersion than comparably sized structures in 30 Dor, underscoring the non-universality of molecular cloud properties. In both clouds, structures with relatively higher surface density lie closer to simple virial equilibrium, whereas lower surface density structures tend to exhibit super-virial line widths. In the PCC, relatively high line widths are found in the vicinity of an infrared source whose properties are consistent with a luminous young stellar object. More generally, we find that the smallest resolved structures ("leaves") of the dendrogram span close to the full range of line widths observed across all scales. As a result, while the bulk of the kinetic energy is found on the largest scales, the small-scale energetics tend to be dominated by only a few structures, leading to substantial scatter in observed size-linewidth relationships.Comment: Accepted by ApJ; 21 pages in AASTeX two-column styl

Investigating the Impact of Metallicity on Star Formation in the Outer Galaxy. I. VLT/KMOS Survey of Young Stellar Objects in Canis Major

The effects of metallicity on the evolution of protoplanetary disks may be studied in the outer Galaxy where the metallicity is lower than in the solar neighbourhood. We present the VLT/KMOS integral field spectroscopy in the near-infrared of $\sim$120 candidate young stellar objects (YSOs) in the CMa-$\ell$224 star-forming region located at a Galactocentric distance of 9.1 kpc. We characterise the YSO accretion luminosities and accretion rates using the hydrogen Br$\gamma$ emission and find the median accretion luminosity of $\log{(L_{\rm acc})} = -0.82^{+0.80}_{-0.82} L_\odot$. Based on the measured accretion luminosities, we investigate the hypothesis of star formation history in the CMa-$\ell$224. Their median values suggest that Cluster C, where most of YSO candidates have been identified, might be the most evolved part of the region. The accretion luminosities are similar to those observed toward low-mass YSOs in the Perseus and Orion molecular clouds, and do not reveal the impact of lower metallicity. Similar studies in other outer Galaxy clouds covering a wide range of metallicities are critical to gain a complete picture of star formation in the Galaxy.Comment: Accepted for publication in APJS, 51 pages, 37 figures, 6 table

The Detection of Higher-Order Millimeter Hydrogen Recombination Lines in the Large Magellanic Cloud

We report the first extragalactic detection of the higher-order millimeter hydrogen recombination lines ($\Delta n>2$). The $\gamma$-, $\epsilon$-, and $\eta$-transitions have been detected toward the millimeter continuum source N105-1A in the star-forming region N105 in the Large Magellanic Cloud (LMC) with the Atacama Large Millimeter/submillimeter Array (ALMA). We use the H40$\alpha$ line, the brightest of the detected recombination lines (H40$\alpha$, H36$\beta$, H50$\beta$, H41$\gamma$, H57$\gamma$, H49$\epsilon$, H53$\eta$, and H54$\eta$), and/or the 3 mm free-free continuum emission to determine the physical parameters of N105-1A (the electron temperature, emission measure, electron density, and size) and study ionized gas kinematics. We compare the physical properties of N105-1A to a large sample of Galactic compact and ultracompact (UC) H II regions and conclude that N105-1A is similar to the most luminous ($L>10^5$ $L_{\odot}$) UC H II regions in the Galaxy. N105-1A is ionized by an O5.5 V star, it is deeply embedded in its natal molecular clump, and likely associated with a (proto)cluster. We incorporate high-resolution molecular line data including CS, SO, SO$_2$, and CH$_3$OH ($\sim$0.12 pc), and HCO$^{+}$ and CO ($\sim$0.087 pc) to explore the molecular environment of N105-1A. Based on the CO data, we find evidence for a cloud-cloud collision that likely triggered star formation in the region. We find no clear outflow signatures, but the presence of filaments and streamers indicates on-going accretion onto the clump hosting the UC H II region. Sulfur chemistry in N105-1A is consistent with the accretion shock model predictions.Comment: 51 pages, 30 figures, 2 tables (including appendices); accepted for publication in The Astrophysical Journal (ApJ