1,402 research outputs found
The Dependence of the Soft X-ray Properties of LMXBs on the Metallicity of Their Environment
We determine the X-ray spectral properties of a sample of low-mass X-ray
binaries (LMXBs) which reside in globular clusters of M31, as well as five
LMXBs in Galactic globular clusters and in the Large Magellanic Cloud using the
ROSAT PSPC. We find a trend in the X-ray properties of the LMXBs as a function
of globular cluster metallicity. The spectra of LMXBs become progressively
softer as the metallicity of its environment increases. The one M31 globular
cluster LMXB in our sample which has a metallicity greater than solar has
spectral properties similar to those of LMXBs in the bulge of M31, but markedly
different from those which reside in low metallicity globular clusters, both in
M31 and the Galaxy. The spectral properties of this high metallicity LMXB is
also similar to those of X-ray faint early-type galaxies. This lends support to
the claim that a majority of the X-ray emission from these X-ray faint
early-type galaxies results from LMXBs and not hot gas, as is the case in their
X-ray bright counterparts.Comment: 5 pages, 2 embedded Postscript figures, uses emulateapj.sty,
Astrophysical Journal Letters, in pres
A Reanalysis of theUltraviolet Extinction from Interstellar Dust in the Large Magellanic Cloud
We have reanalyzed the Large Magellanic Cloud's (LMC) ultraviolet (UV)
extinction using data from the IUE final archive. Our new analysis takes
advantage of the improved signal--to--noise of the IUE NEWSIPS reduction, the
exclusion of stars with very low reddening, the careful selection of well
matched comparison stars, and an analysis of the effects of Galactic foreground
dust. Differences between the average extinction curves of the 30 Dor region
and the rest of the LMC are reduced compared to previous studies. We find that
there is a group of stars with very weak 2175 Ang. bumps that lie in or near
the region occupied by the supergiant shell, LMC 2, on the southeast side of 30
Dor. The average extinction curves inside and outside LMC 2 show a very
significant difference in 2175 Ang. bump strength, but their far--UV
extinctions are similar. While it is unclear whether or not the extinction
outside the LMC 2 region can be fit with the relation of Cardelli, Clayton and
Mathis (CCM), sightlines near LMC 2 cannot be fit with CCM due to their weak
2175 Ang. bumps. While the extinction properties seen in the LMC lie within the
range of properties seen in the Galaxy, the correlations of UV extinction
properties with environment seen in the Galaxy do not appear to hold in the
LMC.Comment: 29 pages, 10 figures, to be published in Ap
The Distance of the First Overtone RR Lyrae Variables in the MACHO LMC Database: A New Method to Correct for the Effects of Crowding
Previous studies have indicated that many of the RR Lyrae variables in the
LMC have properties similar to the ones in the Galactic globular cluster M3.
Assuming that the M3 RR Lyrae variables follow the same relationships among
period, temperature, amplitude and Fourier phase parameter phi31 as their LMC
counterparts, we have used the M3 phi31-logP relation to identify the M3-like
unevolved first overtone RR Lyrae variables in 16 fields near the LMC bar. The
temperatures of these variables were calculated from the M3 logP-logTe relation
so that the extinction could be derived for each star separately. Since blended
stars have lower amplitudes for a given period, the period amplitude relation
should be a useful tool for identifying which stars are affected by crowding.
We find that the low amplitude stars are brighter. We remove them from the
sample and derive an LMC distance modulus 18.49+/-0.11.Comment: 30 pages, 7 figures, accepted for publication in the Astronomical
Journa
Low-Mass Pre-Main Sequence Stars in the Large Magellanic Cloud - III: Accretion Rates from HST-WFPC2 Observations
We have measured the present accretion rate of roughly 800 low-mass (~1-1.4
Mo) pre-Main Sequence stars in the field of Supernova 1987A in the Large
Magellanic Cloud (LMC, Z~0.3 Zo). It is the first time that this fundamental
parameter for star formation is determined for low-mass stars outside our
Galaxy. The Balmer continuum emission used to derive the accretion rate
positively correlates with the Halpha excess. Both these phenomena are believed
to originate from accretion from a circumstellar disk so that their
simultaneous detection provides an important confirmation of the pre-Main
Sequence nature of the Halpha and UV excess objects, which are likely to be the
LMC equivalent of Galactic Classical TTauri stars. The stars with statistically
significant excesses are measured to have accretion rates larger than
1.5x10^{-8}Mo/yr at an age of 12-16 Myrs. For comparison, the time scale for
disk dissipation observed in the Galaxy is of the order of 6 Myrs. Moreover,
the oldest Classical TTauri star known in the Milky Way (TW Hydrae, with 10
Myrs of age) has a measured accretion rate of only 5x10^{-10} Mo/yr, ie 30
times less than what we measure for stars at a comparable age in the LMC. Our
findings indicate that metallicity plays a major role in regulating the
formation of low-mass stars.Comment: Accepted for publication in the Astrophysical Journal (10 June 2004),
28 pages, 9 figures. Typo corrected in the abstract on 21 February 200
Ultraviolet Imaging Polarimetry of the Large Magellanic Cloud. II. Models
Motivated by new sounding-rocket wide-field polarimetric images of the Large
Magellanic Cloud, we have used a three-dimensional Monte Carlo radiation
transfer code to investigate the escape of near-ultraviolet photons from young
stellar associations embedded within a disk of dusty material (i.e. a galaxy).
As photons propagate through the disk, they may be scattered or absorbed by
dust. Scattered photons are polarized and tracked until they escape to be
observed; absorbed photons heat the dust, which radiates isotropically in the
far-infrared, where the galaxy is optically thin. The code produces four output
images: near- UV and far-IR flux, and near-UV images in the linear Stokes
parameters Q and U. From these images we construct simulated UV polarization
maps of the LMC. We use these maps to place constraints on the star + dust
geometry of the LMC and the optical properties of its dust grains. By tuning
the model input parameters to produce maps that match the observed polarization
maps, we derive information about the inclination of the LMC disk to the plane
of the sky, and about the scattering phase function g. We compute a grid of
models with i = 28 deg., 36 deg., and 45 deg., and g = 0.64, 0.70, 0.77, 0.83,
and 0.90. The model which best reproduces the observed polarization maps has i
= 36 +2/-5 degrees and g ~0.7. Because of the low signal-to-noise in the data,
we cannot place firm constraints on the value of g. The highly inclined models
do not match the observed centro-symmetric polarization patterns around bright
OB associations, or the distribution of polarization values. Our models
approximately reproduce the observed ultraviolet photopolarimetry of the
western side of the LMC; however, the output images depend on many input
parameters and are nonunique.Comment: Accepted to AJ. 20 pages, 7 figure
Detection of Cold Atomic Clouds in the Magellanic Bridge
We report a detection of cold atomic hydrogen in the Magellanic Bridge using
21-cm absorption spectroscopy toward the radio source B0312-770. With a column
density of N_HI=1.2E20 cm^-2, a maximum absorption optical depth of tau=0.10
and a maximum 21-cm emission brightness temperature of 1.4 K, this line of
sight yields a spin temperature, T_s, between 20 K and 40 K. H I 21-cm
absorption and emission spectroscopy toward 7 other low column density
sightlines on the periphery of the LMC and SMC reveal absorption toward one
additional background radio source behind the SMC with tau=0.03. The data have
typical sensitivities of sigma_tau=0.005 to 0.070 in absorption and
sigma_{T_B}=0.03 K in emission. These data demonstrate the presence of a cold
atomic phase which is probably accompanied by molecular condensations in the
tenuous interstellar medium of the Bridge region. Young OB stars observed in
the Magellanic Bridge could form "in situ" from these cold condensations rather
than migrate from regions of active star formation in the main body of the SMC.
The existence of cold condensations and star formation in the Magellanic Bridge
might be understood as a small scale version of the mechanism that produces
star formation in the tidal tails of interacting galaxies.Comment: 25 pages, uses AASTeX and psfig; Accepted for Publication in the
Astronomical Journa
MSX, 2MASS, and the LMC: A Combined Near and Mid Infrared View
The Large Magellanic Cloud (LMC) has been observed by both the Midcourse
Space Experiment (MSX) in the mid-infrared and the Two Micron All Sky Survey
(2MASS) in the near-infrared. We have performed a cross-correlation of the 1806
MSX catalog sources and nearly 1.4 million 2MASS catalogued point and extended
sources and find 1664 matches. Using the available color information, we
identify a number of stellar populations and nebulae, including main sequence
stars, giant stars, red supergiants, carbon- and oxygen-rich asymptotic giant
branch (AGB) stars, planetary nebulae, H II regions, and other dusty objects
likely associated with early-type stars. 731 of these sources have no previous
identification. We compile a listing of all objects, which includes photometry
and astrometry. The 8.3 micron MSX sensitivity is the limiting factor for
object detection: only the brighter red objects, specifically the red
supergiants, AGB stars, planetary nebulae and HII regions, are detected in the
LMC. The remaining objects are likely in the Galactic foreground. The spatial
distribution of the infrared LMC sources may contribute to understanding
stellar formation and evolution and the overall galactic evolution. We
demonstrate that a combined mid- and near-infrared photometric baseline
provides a powerful means of identifying new objects in the LMC for future
ground-based and space-based follow-up observations.Comment: 23 pages, 10 figures, to appear in the AJ (2001 Oct issue). N.B:
Tables 2 & 3 corrected and available as html file
Supernova Remnants in the Magellanic Clouds. VI. The DEML316 Supernova Remnants
The DEML316 system contains two shells, both with the characteristic
signatures of supernova remnants (SNRs). We analyze Chandra and XMM-Newton data
for DEML316, investigating its spatial and spectral X-ray features. Our Chandra
observations resolve the structure of the northeastern SNR (Shell A) as a
bright inner ring and a set of "arcs" surrounded by fainter diffuse emission.
The spectrum is well fit by a thermal plasma model with temperature ~1.4 keV;
we do not find significant spectral differences for different regions of this
SNR. The southwestern SNR (Shell B) exhibits an irregular X-ray outline, with a
brighter interior ring of emission including a bright knot of emission. Overall
the emission of the SNR is well described by a thermal plasma of temperature
~0.6 keV. The Bright Knot, however, is spectrally distinct from the rest of the
SNR, requiring the addition of a high-energy spectral component consistent with
a power-law spectrum of photon index 1.6--1.8.
We confirm the findings of Nishiuchi et al. (2001) that the spectra of these
shells are notably different, with Shell A requiring a high iron abundance for
a good spectral fit, implying a Type Ia origin. We further explicitly compare
abundance ratios to model predictions for Type Ia and Type II supernovae. The
low ratios for Shell A (O/Fe of 1.5 and Ne/Fe of 0.2) and the high ratios for
Shell B (O/Fe of 30--130 and Ne/Fe of 8--16) are consistent with Type Ia and
Type II origins, respectively. The difference between the SNR progenitor types
casts some doubt on the suggestion that these SNRs are interacting with one
another.Comment: Accepted for ApJ v. 635 (December issue
MACHO 96-LMC-2: Lensing of a Binary Source in the LMC and Constraints on the Lensing Object
We present photometry and analysis of the microlensing alert MACHO 96-LMC-2.
The ~3% photometry provided by the Global Microlensing Alert Network follow--up
effort reveals a periodic modulation in the lightcurve. We attribute this to
binarity of the lensed source. Microlensing fits to a rotating binary source
magnified by a single lens converge on two minima, separated by delta chi^2 ~
1. The most significant fit X1 predicts a primary which contributes ~100% of
the light, a dark secondary, and an orbital period (T) of 9.2 days. The second
fit X2 yields a binary source with two stars of roughly equal mass and
luminosity, and T = 21.2 days.
The lensed object appears to lie on the upper LMC main sequence. We estimate
the mass of the primary component of the binary system, M ~2 M_sun. For the
preferred model X1, we explore the range of dark companions by assuming 0.1
M_sun and 1.4 M_sun objects in models X1a and X1b, respectively. We find lens
velocities projected to the LMC in these models of v^hat_X1a = 18.3 +/- 3.1
km/s and v^hat_X1b = 188 +/- 32 k/ms. In both these cases, a likelihood
analysis suggests an LMC lens is preferred over a Galactic halo lens, although
only marginally so in model X1b. We also find v^hat_X2 = 39.6 +/- 6.1 k/ms,
where the likelihood for the lens location is strongly dominated by the LMC
disk. In all cases, the lens mass is consistent with that of an M-dwarf. The
LMC self-lensing rate contributed by 96-LMC-2 is consistent with model
self-lensing rates. (Abridged)Comment: 23 pages, including 3 tables and 6 figures; Accepted for publication
in The Astrophysical Journa
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