Youngung Lee

A Neutral Hydrogen Survey of the Large Magellanic Cloud: Aperture Synthesis and Multibeam Data Combined

Recent H I surveys of the Large Magellanic Cloud (LMC) with the Australia Telescope Compact Array and the Parkes multibeam receiver have focused, respectively, on the small-scale ( 1°) structure of the galaxy. Using a Fourier-plane technique, we have merged both data sets, providing an accurate set of images of the LMC sensitive to structure on scales of 15 pc upward. The spatial dynamic range (2.8 orders of magnitude), velocity resolution (1.649 km s-1), brightness temperature sensitivity (2.4 K), and column density sensitivity (7.2 × 1018 cm-2 per 1.649 km s-1 channel) allow for studies of phenomena ranging from the galaxy-wide interaction of the LMC with its close neighbors to the small-scale injection of energy from supernovae and stellar associations into the ISM of the LMC. This paper presents the merged data and size spectrum of H I clouds, which is similar to the typical size spectrum of the holes and shells in the H I distribution. The H I clouds in the LMC have been identified by defining a cloud to be an object composed of all pixels in right ascension, declination, and velocity that are simply connected and that lie above the threshold brightness temperature.

Probing Inward Motions in Starless Cores Using the HCN(J = 1-0) Hyperfine Transitions: A Pointing Survey toward Central Regions

We have carried out a survey toward the central regions of 85 starless cores in HCN(J = 1-0) to study inward motions in the cores. Sixty-four cores were detected with HCN lines. The infall asymmetry in the HCN spectra is found to be more prevalent, and more prominent than in any other previously used infall tracers such as CS(J = 2-1), DCO+(J = 2-1), and N2H+(J = 1-0). We have found a close relation between the intensities of the HCN and N2H+ lines. This implies that the HCN is not very depleted in the central regions of the cores. In some cores, the HCN spectra show different signs of asymmetry than other molecular lines. A few cores show various signs of asymmetry in individual HCN hyperfine lines. The distribution of the velocity shift δV of the HCN profiles with respect to the systemic velocity of the optically thin tracer is found to be more shifted toward the bluer side than those of other infall tracers, indicating that the HCN traces inward motions more frequently. The δV distribution of each HCN hyperfine line for all sources is similar. Moreover, the δV values obtained from different HCN hyperfine lines for each source are nearly similar. These may mean that most starless cores are in similar kinematic states across the layers of the cores. We identify 17 infall candidates using all available indicators such as the velocity shift δV and the blue-to-red peak intensity ratio of double-peaked profiles for HCN(J = 1-0), CS(J = 2-1), CS(J = 3-2), DCO+(J = 2-1), and N2H+(J = 1-0). Four of them, L63, L492, L694-2, and L1197, are found to show a higher blue-to-red ratio in the HCN hyperfine line along the lower opacity, suggesting that infall speed becomes higher toward the center.

EMBEDDED YOUNG STELLAR OBJECT CANDIDATES IN THE ACTIVE STAR-FORMING COMPLEX W51: MASS FUNCTION AND SPATIAL DISTRIBUTION

We present 737 candidate young stellar objects (YSOs) near the W51 giant molecular cloud over an area of 125 × 100 selected from Spitzer Space Telescope data. We use spectral energy distribution fits to identify YSOs and distinguish them from main-sequence (MS) or red giant stars, asymptotic giant branch stars, and background galaxies. Based on extinction of each YSO, we separate a total of 437 YSOs associated with the W51 region from the possible foreground sources. We identify 69 highly embedded Stage 0/I candidate YSOs in our field with masses >5 M ☉ (corresponding to mid- to early-B MS spectral types), 46 of which are located in the central active star-forming regions of W51A and W51B. From the YSOs associated with W51, we find evidence for mass segregation showing that the most massive YSOs are concentrated on the W51 H II region complex. We find a variation in the spatial distribution of the mass function (MF) of YSOs in the mass range between 5 M ☉ and 18 M ☉. The derived slopes of the MF are –1.26 and –2.36 in the active star-forming region and the outer region, respectively. The variation of the MF for YSOs embedded in the molecular cloud implies that the distribution of stellar masses in clusters depends on the local conditions in the parent molecular cloud.

A Molecular Line Survey of W3(OH) and W3 IRS 5 from 84.7 to 115.6 GHz: Observational Data and Analyses

We have carried out observations toward the W3 complex and G34.3+0.15 using the TRAO 14 m radio telescope to examine in detail the chemical variations occurring while molecular clouds evolve from the prestellar to the H II region phase. Observations include spectral surveys of these objects between 84.7 and 115.6 GHz; mapping observations toward W3(OH) with the emissions of CS (2-1), HCN (1-0), HNC (1-0), and HCO+ (1-0); and mapping of CS (2-1) emission toward W3 IRS 5. Chemical model calculations are used to estimate the age of W3(OH) by comparing with the fractional abundances of detected molecules. We found that G34.3+0.15 and W3(OH) are at a similar evolutionary stage, although large differences in the fractional abundances are found in CH3CN and HC3N. Overall, the properties of the detected species and abundances in three regions support the view that chemistry varies as molecular clouds evolve from a cold, collapsing phase to a high-temperature phase, such as the hot core and H II phase. Chemical model calculations for W3(OH) indicate that the evolutionary age of the cloud is 104-105 yr with temperature in the range 10-60 K.

Analysis of (C-12)O and (C-13)O emission in a 3 square degree region of the Galactic plane between L = 23 deg and 25 deg

A 3-sq deg section of the inner Galactic plane centered at l = 24 deg, b = 0 deg in both (C-13)O and CS. The observations were supplemented by preexisting CO data from the Massachusetts-Stony Brook Galactic plane survey. It was possible to identify 47 molecular clouds in the region surveyed. Distances to the clouds were assigned using a size-line width relation which was derived for a set of calibrator clouds at known distances. By combining the CO and (C-13)O data, it was possible to calculate LTE masses for the clouds and to compare them with masses estimated from the virial theorem. Masses estimated from the two methods are tightly correlated, suggesting that the clouds observed in this study are close to virial equilibrium. The masses derived using the two methods are in reasonable agreement. Little CS emission was detected in this section or the Galactic plane; averaged over the region of the survey, the CO/CS integrated intensity ratio is found to be 660 + or - 160, considerably larger than the ratio in the Galactic center. 26 refs.

MASS MEASUREMENTS of ISOLATED OBJECTS from SPACE-BASED MICROLENSING

We report on the mass and distance measurements of two single-lens events from the 2015 Spitzer microlensing campaign. With both finite-source effect and microlens parallax measurements, we find that the lens of OGLE-2015-BLG-1268 is very likely a brown dwarf (BD). Assuming that the source star lies behind the same amount of dust as the Bulge red clump, we find the lens is a 45 ± 7 M_J BD at 5.9 ± 1.0 kpc. The lens of of the second event, OGLE-2015-BLG-0763, is a 0.50 ± 0.04 M_M☉ star at 6.9 ± 1.0 kpc. We show that the probability to definitively measure the mass of isolated microlenses is dramatically increased once simultaneous ground- and space-based observations are conducted.

TRIGGERED STAR FORMATION IN A DOUBLE SHELL NEAR W51A

We present Heinrich Hertz Telescope CO observations of the shell structure near the active star-forming complex W51A to investigate the process of star formation triggered by the expansion of an H II region. The CO observations confirm that dense molecular material has been collected along the shell detected in Spitzer IRAC images. The CO distribution shows that the shell is blown out toward a lower density region to the northwest. Total hydrogen column density around the shell is high enough to form new stars. We find two CO condensations with the same central velocity of 59 km s–1 to the east and north along the edge of the IRAC shell. We identify two young stellar objects in early evolutionary stages (Stage 0/I) within the densest molecular condensation. From the CO kinematics, we find that the H II region is currently expanding with a velocity of 3.4 km s–1, implying that the shells expansion age is ~1 Myr. This timescale is in good agreement with numerical simulations of the expansion of the H II region (Hosokawa & Inutsuka). We conclude that the star formation on the border of the shell is triggered by the expansion of the H II region.

The Scale Height of Giant Molecular Clouds Is Less than That of Smaller Clouds

An antenna temperature thresholding algorithm is used on the Bell Laboratories 13CO J = 1 → 0 Milky Way Survey to create a catalog of 1400 molecular clouds. Of these, 281 clouds are selected for having well-determined kinematic distances. The scale height, luminosity, internal velocity dispersion, and size of the cloud sample are analyzed to show that clouds smaller than ~105.5 M☉ have a scale height which is about 35 pc, roughly independent of cloud mass, while larger clouds, the giant molecular clouds, have a reduced scale height which declines with increasing cloud mass.

A CO LINE AND INFRARED CONTINUUM STUDY OF THE ACTIVE STAR-FORMING COMPLEX W51

We present the results of an extensive observational study of the active star-forming complex W51 that was observed in the J = 2 – 1 transition of the 12CO and 13CO molecules over a 125 × 100 region with the University of Arizona Heinrich Hertz Submillimeter Telescope. We use a statistical equilibrium code to estimate physical properties of the molecular gas. We compare the molecular cloud morphology with the distribution of infrared (IR) and radio continuum sources and find associations between molecular clouds and young stellar objects (YSOs) listed in Spitzer IR catalogs. The ratios of CO lines associated with H II regions are different from the ratios outside the active star-forming regions. We present evidence of star formation triggered by the expansion of the H II regions and by cloud-cloud collisions. We estimate that about 1% of the cloud mass is currently in YSOs.

Spitzer Observations of OGLE-2015-BLG-1212 Reveal a New Path toward Breaking Strong Microlens Degeneracies

Spitzer microlensing parallax observations of OGLE-2015-BLG-1212 decisively break a degeneracy between planetary and binary solutions that is somewhat ambiguous when only ground-based data are considered. Only eight viable models survive out of an initial set of 32 local minima in the parameter space. These models clearly indicate that the lens is a stellar binary system possibly located within the bulge of our Galaxy, ruling out the planetary alternative. We argue that several types of discrete degeneracies can be broken via such space-based parallax observations.