Abstract
We present low-resolution optical spectra and classifications of 92 blue objects with mid-infrared excesses in the Large Magellanic Cloud. The majority of these objects were selected with ...the criteria of
U
−
B
< 0 and
V
< 17 from the potential young stellar object (YSO) candidates in Gruendl & Chu (GC09), which were identified based on Spitzer Infrared Array Camera and Multiband Imaging Photometer for Spitzer observations in conjunction with optical photometry from the Magellanic Clouds Photometric Survey. Many of the sample objects have ambiguous classifications. We examined the properties of these 92 objects using low-resolution optical spectra obtained with the SOAR 4.1 m Telescope at Cerro Pachon and the Blanco 4 m Telescope at Cerro Tololo Inter-American Observatory, supplemented by available photometric and imaging observations. We estimated the spectral types, temperatures, and luminosities of these objects from the optical to near-IR spectral energy distributions based on the photometric data, and further examined stellar absorption line features in the optical spectra to verify the spectral types. The interstellar/circumstellar environments, assessed from nebular line imaging observations and nebular lines detected in the stellar spectra, further helped constrain the nature of stars. Among these 92 objects, we confirm 42 stars as YSOs, and the remaining 50 objects as protoplanetary nebulae, post-AGB/RGB stars, blue evolved massive stars, stars with dust in vicinity, or uncertain classifications. Our results show that the photometric criteria in GC09 are generally effective in the initial selection of YSO candidates, and the low-resolution spectroscopy combined with environment assessment can be useful to better constrain the classifications and ameliorate most ambiguities.
ABSTRACT Observations suggest that there is a significant fraction of O stars in the field of the Milky Way that appear to have formed in isolation or in low-mass clusters (<100 ). The existence of ...these high-mass stars that apparently formed in the field challenges the generally accepted paradigm, which requires star formation to occur in clustered environments. In order to understand the physical conditions for the formation of these stars, it is necessary to observe isolated high-mass stars while they are still forming. With the Hubble Space Telescope, we observe the seven most isolated massive (>8 ) young stellar objects (MYSOs) in the Large Magellanic Cloud. The observations show that while these MYSOs are remote from other MYSOs, OB associations, and even known giant molecular clouds, they are actually not isolated at all. Imaging reveals ∼100 to several hundred pre-main-sequence (PMS) stars in the vicinity of each MYSO. These previously undetected PMS stars form prominent compact clusters around the MYSOs, and in most cases they are also distributed sparsely across the observed regions. Contrary to what previous high-mass field star studies show, these observations suggest that high-mass stars may not be able to form in clusters with masses less than 100 . If these MYSOs are indeed the best candidates for isolated high-mass star formation, then the lack of isolation is at odds with random sampling of the initial mass function. Moreover, while isolated MYSOs may not exist, we find evidence that isolated clusters containing O stars can exist, which in itself is rare.
Archival Spitzer Infrared Array Camera (IRAC) and MIPS observations of the Large Magellanic Cloud (LMC) have been used to search for young stellar objects (YSOs). We have carried out independent ...aperture photometry of these data and merged the results from different passbands to produce a photometric catalog. To verify our methodology we have also analyzed the data from the SAGE and SWIRE Legacy programs; our photometric measurements are in general agreement with the photometry released by these programs. A detailed completeness analysis for our photometric catalog of the LMC shows that the 90% completeness limits are, on average, 16.0, 15.0, 14.3, 13.1, and 9.2 mag at 3.6, 4.5, 5.8, 8.0, and 24 Delta *mm, respectively. Using our mid-infrared photometric catalogs and two simple selection criteria, 4.5-8.0>2.0 to exclude normal and evolved stars and 8.0>14-(4.5-8.0) to exclude background galaxies, we have identified a sample of 2910 sources in the LMC that could potentially be YSOs. We then used the Spitzer observations complemented by optical and near-infrared data to carefully assess the nature of each source. To do so we simultaneously considered multiwavelength images and photometry to assess the source morphology, spectral energy distribution (SED) from the optical through the mid-infrared wavelengths, and the surrounding interstellar environment to determine the most likely nature of each source. From this examination of the initial sample, we suggest that 1172 sources are most likely YSOs. We have also identified 1075 probable background galaxies, consistent with the expected number estimated from the SWIRE survey. Spitzer IRS observations of 269 of the brightest YSOs from our sample have confirmed that 95% are indeed YSOs. An examination of color-color and color-magnitude diagrams shows no simple criteria in color-magnitude space that can unambiguously separate the LMC YSOs from all asymptotic giant branch (AGB)/post-AGB stars, planetary nebulae, and background galaxies. A comprehensive search for YSOs in the LMC has also been carried out by the SAGE team and reported by Whitney et al. There are three major differences between these two searches. (1) In the common region of color-magnitude space, ~850 of our 1172 probable YSOs are missed in the SAGE YSO catalog because their conservative point-source identification criteria have excluded YSOs superposed on complex stellar and interstellar environments. (2) About 20%-30% of the YSOs identified by the SAGE team are sources we classify as background galaxies. (3) The SAGE YSO catalog identifies YSO in parts of color-magnitude space that we excluded and thus contains more evolved or fainter YSOs missed by our analysis. The shortcomings and strengths of both these YSO catalogs should be considered prior to statistical studies of star formation in the LMC. Finally, the mid-infrared luminosity functions in the IRAC bands of our most likely YSO candidates in the LMC can be well described by N(L) L -1, which is consistent with the Salpeter initial mass function if a mass-luminosity relation of L M 2.4 is adopted.
We have used two methods to search for surviving companions of Type Ia supernova progenitors in three Balmer-dominated supernova remnants in the Large Magellanic Cloud: 0519-69.0, 0505-67.9 (DEM ...L71), and 0548-70.4. In the first method, we use the Hubble Space Telescope photometric measurements of stars to construct color-magnitude diagrams (CMDs) and compare positions of stars in the CMDs with those expected from theoretical post-impact evolution of surviving main-sequence or helium star companions. No obvious candidates of surviving companions are identified in this photometric search. Future models for surviving red giant companions or with different explosion mechanisms are needed for thorough comparisons with these observations in order to make more definitive conclusions. In the second method, we use Multi Unit Spectroscopic Explorer observations of 0519-69.0 and DEM L71 to carry out spectroscopic analyses of stars in order to use large peculiar radial velocities as diagnostics of surviving companions. We find a star in 0519-69.0 and a star in DEM L71 moving at radial velocities of 182 0 km s−1 and 213 0 km s−1, respectively, more than 2.5 from the mean radial velocity of the underlying stellar population, 264 and 270 km s−1, respectively. These stars need higher-quality spectra to investigate their abundances and rotation velocities to determine whether they are indeed surviving companions of the supernova progenitors.
We present the properties of an extensive sample of molecular clouds in the Large Magellanic Cloud (LMC) mapped at 11 pc resolution in the CO(1-0) line. Targets were chosen based on a limiting CO ...flux and peak brightness as measured by the NANTEN survey. The observations were conducted with the ATNF Mopra Telescope as part of the Magellanic Mopra Assessment. We identify clouds as regions of connected CO emission and find that the distributions of cloud sizes, fluxes, and masses are sensitive to the choice of decomposition parameters. In all cases, however, the luminosity function of CO clouds is steeper than dN/dLL --2, suggesting that a substantial fraction of mass is in low-mass clouds. A correlation between size and linewidth, while apparent for the largest emission structures, breaks down when those structures are decomposed into smaller structures. We argue that the correlation between virial mass and CO luminosity is the result of comparing two covariant quantities, with the correlation appearing tighter on larger scales where a size-linewidth relation holds. The virial parameter (the ratio of a cloud's kinetic to self-gravitational energy) shows a wide range of values and exhibits no clear trends with the CO luminosity or the likelihood of hosting young stellar object (YSO) candidates, casting further doubt on the assumption of virialization for molecular clouds in the LMC. Higher CO luminosity increases the likelihood of a cloud harboring a YSO candidate, and more luminous YSOs are more likely to be coincident with detectable CO emission, confirming the close link between giant molecular clouds and massive star formation.
We examine a diffuse emission region near the center of SNR 0509−67.5 to determine its nature. Within this diffuse region we observe a point-like source that is bright in the near-IR, but is not ...visible in the B and V bands. We consider an emission line observed at 6766 and the possibilities that it is Ly , H , and O ii λ3727. We examine the spectral energy distribution (SED) of the source, comprised of Hubble Space Telescope B, V, I, J, and H bands in addition to Spitzer/IRAC 3.6, 4.5, 5.8, and 8 m bands. The peak of the SED is consistent with a background galaxy at z 0.8 0.2 and a possible Balmer jump places the galaxy at z 0.9 0.3. These SED considerations support the emission line's identification as O ii λ3727. We conclude that the diffuse source in SNR 0509−67.5 is a background galaxy at z 0.82. Furthermore, we identify the point-like source superposed near the center of the galaxy as its central bulge. Finally, we find no evidence for a surviving companion star, indicating a double-degenerate origin for SNR 0509−67.5.
N103B is a Type Ia supernova remnant (SNR) projected in the outskirts of the superbubble around the rich cluster NGC 1850 in the Large Magellanic Cloud (LMC). We have obtained H and continuum images ...of N103B with the Hubble Space Telescope (HST) and high-dispersion spectra with the 4 m and 1.5 m telescopes at Cerro Tololo Inter-American Observatory. The HST H image exhibits a complex system of nebular knots inside an incomplete filamentary elliptical shell that opens to the east, where X-ray and radio emission extends farther out. Electron densities of the nebular knots, determined from the S ii doublet, reach 5300 cm−3, indicating an origin of circumstellar medium, rather than interstellar medium. The high-dispersion spectra reveal three kinematic components in N103B: (1) a narrow component with N ii λ6583/H ∼ 0.14 from the ionized interstellar gas associated with the superbubble of NGC 1850 in the background, (2) a broader H component with no N ii counterpart from the SNR's collisionless shocks into a mostly neutral ambient medium, and (3) a broad component, ∼ 500 km s−1, in both H and N ii lines from shocked material in the nebular knots. The Balmer-dominated filaments can be fitted by an ellipse, and we adopt its center as the site of SN explosion. We find that the star closest to this explosion center has colors and luminosity consistent with a 1 surviving subgiant companion as modeled by Podsiadlowski. Follow-up spectroscopic observations are needed to confirm this star as the SN's surviving companion.
The Large Magellanic Cloud (LMC) has ∼60 confirmed supernova remnants (SNRs). Because of the known distance, 50 kpc, the SNRs' angular sizes can be converted to linear sizes, and their X-ray ...observations can be used to assess X-ray luminosities (LX). We have critically examined the LMC SNRs' sizes reported in the literature to determine the most plausible sizes. These sizes and the LX determined from XMM-Newton observations are used to investigate their relationship to explore the environmental and evolutionary effects on the X-ray properties of SNRs. Our research provides the following three results. (1) Small LMC SNRs, a few to 10 pc in size, are all Type Ia with LX > 1036 erg s−1. The scarcity of small core-collapse (CC) SNRs is a result of CC SNe exploding in the low-density interiors of interstellar bubbles blown by their massive progenitors during their main-sequence phase. (2) Medium-sized (10-30 pc) CC SNRs show bifurcation in LX, with the X-ray-bright SNRs either in an environment associated with molecular clouds or containing pulsars and pulsar-wind nebulae and the X-ray-faint SNRs being located in low-density interstellar environments. (3) Large (size > 30 pc) SNRs show a trend of LX fading with size, although the scatter is large. The observed relationship between LX and sizes can help constrain models of SNR evolution.
Abstract
We measure the projected number density profiles of galaxies and the splashback feature in clusters selected by the Sunyaev–Zel’dovich effect from the Advanced Atacama Cosmology Telescope ...(AdvACT) survey using galaxies observed by the Dark Energy Survey (DES). The splashback radius is consistent with CDM-only simulations and is located at
2.4
−
0.4
+
0.3
Mpc
h
−
1
. We split the galaxies on color and find significant differences in their profile shapes. Red and green-valley galaxies show a splashback-like minimum in their slope profile consistent with theory, while the bluest galaxies show a weak feature at a smaller radius. We develop a mapping of galaxies to subhalos in simulations and assign colors based on infall time onto their hosts. We find that the shift in location of the steepest slope and different profile shapes can be mapped to the average time of infall of galaxies of different colors. The steepest slope traces a discontinuity in the phase space of dark matter halos. By relating spatial profiles to infall time, we can use splashback as a clock to understand galaxy quenching. We find that red galaxies have on average been in clusters over 3.2 Gyr, green galaxies about 2.2 Gyr, while blue galaxies have been accreted most recently and have not reached apocenter. Using the full radial profiles, we fit a simple quenching model and find that the onset of galaxy quenching occurs after a delay of about a gigayear and that galaxies quench rapidly thereafter with an exponential timescale of 0.6 Gyr.
ABSTRACT We present the discovery of a new dwarf galaxy, Hydra II, found serendipitously within the data from the ongoing Survey of the Magellanic Stellar History conducted with the Dark Energy ...Camera on the Blanco 4 m Telescope. The new satellite is compact ( 11 pc) and faint ( 0.3), but well within the realm of dwarf galaxies. The stellar distribution of Hydra II in the color-magnitude diagram is well-described by a metal-poor ( ) and old (13 Gyr) isochrone and shows a distinct blue horizontal branch, some possible red clump stars, and faint stars that are suggestive of blue stragglers. At a heliocentric distance of 134 10 kpc, Hydra II is located in a region of the Galactic halo that models have suggested may host material from the leading arm of the Magellanic Stream. A comparison with N-body simulations hints that the new dwarf galaxy could be or could have been a satellite of the Magellanic Clouds.
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