3,282 research outputs found
2dF-AAOmega spectroscopy of massive stars in the Magellanic Clouds: The north-eastern region of the Large Magellanic Cloud
We present spectral classifications from optical spectroscopy of 263 massive
stars in the north-eastern region of the Large Magellanic Cloud. The observed
two-degree field includes the massive 30 Doradus star-forming region, the
environs of SN1987A, and a number of star-forming complexes to the south of 30
Dor. These are the first classifications for the majority (203) of the stars
and include eleven double-lined spectroscopic binaries. The sample also
includes the first examples of early OC-type spectra (AAOmega 30 Dor 248 and
280), distinguished by the weakness of their nitrogen spectra and by C IV 4658
emission. We propose that these stars have relatively unprocessed CNO
abundances compared to morphologically normal O-type stars, indicative of an
earlier evolutionary phase. From analysis of observations obtained on two
consecutive nights, we present radial-velocity estimates for 233 stars, finding
one apparent single-lined binary and nine (>3sigma) outliers compared to the
systemic velocity; the latter objects could be runaway stars or large-amplitude
binary systems and further spectroscopy is required to investigate their
nature.Comment: Accepted by A&
Ice chemistry in massive Young Stellar Objects: the role of metallicity
We present the comparison of the three most important ice constituents
(water, CO and CO2) in the envelopes of massive Young Stellar Objects (YSOs),
in environments of different metallicities: the Galaxy, the Large Magellanic
Cloud (LMC) and, for the first time, the Small Magellanic Cloud (SMC). We
present observations of water, CO and CO2 ice in 4 SMC and 3 LMC YSOs (obtained
with Spitzer-IRS and VLT/ISAAC). While water and CO2 ice are detected in all
Magellanic YSOs, CO ice is not detected in the SMC objects. Both CO and CO2 ice
abundances are enhanced in the LMC when compared to high-luminosity Galactic
YSOs. Based on the fact that both species appear to be enhanced in a consistent
way, this effect is unlikely to be the result of enhanced CO2 production in
hotter YSO envelopes as previously thought. Instead we propose that this
results from a reduced water column density in the envelopes of LMC YSOs, a
direct consequence of both the stronger UV radiation field and the reduced
dust-to-gas ratio at lower metallicity. In the SMC the environmental conditions
are harsher, and we observe a reduction in CO2 column density. Furthermore, the
low gas-phase CO density and higher dust temperature in YSO envelopes in the
SMC seem to inhibit CO freeze-out. The scenario we propose can be tested with
further observations.Comment: accepted by MNRAS Letters; 5 pages, 3 figures, 1 tabl
A Spitzer IRAC Census of the Asymptotic Giant Branch Populations in Local Group Dwarfs. II. IC 1613
We present Spitzer Space Telescope IRAC photometry of the Local Group dwarf
irregular galaxy IC 1613. We compare our 3.6, 4.5, 5.8, and 8.0 micron
photometry with broadband optical photometry and find that the optical data do
not detect 43% and misidentify an additional 11% of the total AGB population,
likely because of extinction caused by circumstellar material. Further, we find
that a narrowband optical carbon star study of IC 1613 detects 50% of the total
AGB population and only considers 18% of this population in calculating the
carbon to M-type AGB ratio. We derive an integrated mass-loss rate from the AGB
stars of 0.2-1.0 x 10^(-3) solar masses per year and find that the distribution
of bolometric luminosities and mass-loss rates are consistent with those for
other nearby metal-poor galaxies. Both the optical completeness fractions and
mass-loss rates in IC 1613 are very similar to those in the Local Group dwarf
irregular, WLM, which is expected given their similar characteristics and
evolutionary histories.Comment: Accepted by ApJ, 26 pages, 10 figures, version with high-resolution
figures available at: http://webusers.astro.umn.edu/~djackson
Plasmons in strongly correlated systems: spectral weight transfer and renormalized dispersion
We study the charge-density dynamics within the two-dimensional extended
Hubbard model in the presence of long-range Coulomb interaction across the
metal-insulator transition point. To take into account strong correlations we
start from self-consistent extended dynamical mean-field theory and include
non-local dynamical vertex corrections through a ladder approximation to the
polarization operator. This is necessary to fulfill charge conservation and to
describe plasmons in the correlated state. The calculated plasmon spectra are
qualitatively different from those in the random-phase approximation: they
exhibit a spectral density transfer and a renormalized dispersion with enhanced
deviation from the canonical -behavior. Both features are reminiscent
of interaction induced changes found in single-electron spectra of strongly
correlated systems.Comment: 5 pages, 5 figures + appendix (3 pages, 1 figure
Spitzer SAGE-SMC Infrared Photometry of Massive Stars in the Small Magellanic Cloud
We present a catalog of 5324 massive stars in the Small Magellanic Cloud
(SMC), with accurate spectral types compiled from the literature, and a
photometric catalog for a subset of 3654 of these stars, with the goal of
exploring their infrared properties. The photometric catalog consists of stars
with infrared counterparts in the Spitzer, SAGE-SMC survey database, for which
we present uniform photometry from 0.3-24 um in the UBVIJHKs+IRAC+MIPS24 bands.
We compare the color magnitude diagrams and color-color diagrams to those of
the Large Magellanic Cloud (LMC), finding that the brightest infrared sources
in the SMC are also the red supergiants, supergiant B[e] (sgB[e]) stars,
luminous blue variables, and Wolf-Rayet stars, with the latter exhibiting less
infrared excess, the red supergiants being less dusty and the sgB[e] stars
being on average less luminous. Among the objects detected at 24 um are a few
very luminous hypergiants, 4 B-type stars with peculiar, flat spectral energy
distributions, and all 3 known luminous blue variables. We detect a distinct Be
star sequence, displaced to the red, and suggest a novel method of confirming
Be star candidates photometrically. We find a higher fraction of Oe and Be
stars among O and early-B stars in the SMC, respectively, when compared to the
LMC, and that the SMC Be stars occur at higher luminosities. We estimate
mass-loss rates for the red supergiants, confirming the correlation with
luminosity even at the metallicity of the SMC. Finally, we confirm the new
class of stars displaying composite A & F type spectra, the sgB[e] nature of
2dFS1804 and find the F0 supergiant 2dFS3528 to be a candidate luminous blue
variable with cold dust.Comment: 23 pages, 17 figures, 5 tables, accepted for publication in the
Astronomical Journa
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