304 research outputs found
A non-LTE abundance analysis of the post-AGB star ROA 5701
An analysis of high-resolution Anglo-Australian Telescope (AAT)/ University
College London Echelle Spectrograph (UCLES) optical spectra for the ultraviolet
(UV)-bright star ROA 5701 in the globular cluster omega Cen (NGC 5139) is
performed, using non-local thermodynamic equilibrium (non-LTE) model
atmospheres to estimate stellar atmospheric parameters and chemical
composition. Abundances are derived for C, N, O, Mg, Si and S, and compared
with those found previously by Moehler et al. We find a general metal
underabundance relative to young B-type stars, consistent with the average
metallicity of the cluster. Our results indicate that ROA 5701 has not
undergone a gas-dust separation scenario as previously suggested. However, its
abundance pattern does imply that ROA 5701 has evolved off the AGB prior to the
onset of the third dredge-up.Comment: 9 pages, 2 figures. Accepted for publication in MNRAS (Online Early
Earnings announcement promotions: a Yahoo Finance field experiment
This study presents a field experiment in which media articles for a random sample of firms with earnings announcements are promoted to a one percent subset of Yahoo Finance users. Promoted firms have higher abnormal returns and some evidence of lower bid-ask spreads on the day of the earnings announcement. These results are more pronounced for less visible firms, negative earnings news, and on days with fewer promoted firms. These findings suggest that investor attention affects the pricing of earnings and that retail investors buy stocks that catch their attention, in a setting where attention is randomly assigned
The VLT-FLAMES survey of massive stars: rotation and nitrogen enrichment as the key to understanding massive star evolution
Rotation has become an important element in evolutionary models of massive
stars, specifically via the prediction of rotational mixing. Here, we study a
sample of stars, including rapid rotators, to constrain such models and use
nitrogen enrichments as a probe of the mixing process. Chemical compositions
(C, N, O, Mg and Si) have been estimated for 135 early B-type stars in the
Large Magellanic Cloud with projected rotational velocities up to ~300km/s
using a non-LTE TLUSTY model atmosphere grid. Evolutionary models, including
rotational mixing, have been generated attempting to reproduce these
observations by adjusting the overshooting and rotational mixing parameters and
produce reasonable agreement with 60% of our core hydrogen burning sample. We
find (excluding known binaries) a significant population of highly nitrogen
enriched intrinsic slow rotators vsini less than 50km/s incompatible with our
models ~20% of the sample). Furthermore, while we find fast rotators with
enrichments in agreement with the models, the observation of evolved (log g
less than 3.7dex) fast rotators that are relatively unenriched (a further ~20%
of the sample) challenges the concept of rotational mixing. We also find that
70% of our blue supergiant sample cannot have evolved directly from the
hydrogen burning main-sequence. We are left with a picture where invoking
binarity and perhaps fossil magnetic fields are required to understand the
surface properties of a population of massive main sequence stars.Comment: ApJL. 10 pages, 1 figure. Updated to match accepted versio
Iron abundances from optical Fe III absorption lines in B-type stellar spectra
The role of optical Fe III absorption lines in B-type stars as iron abundance
diagnostics is considered. To date, ultraviolet Fe lines have been widely used
in B-type stars, although line blending can severely hinder their diagnostic
power. Using optical spectra, covering a wavelength range ~ 3560 - 9200 A, a
sample of Galactic B-type main-sequence and supergiant stars of spectral types
B0.5 to B7 are investigated. A comparison of the observed Fe III spectra of
supergiants, and those predicted from the model atmosphere codes TLUSTY
(plane-parallel, non-LTE), with spectra generated using SYNSPEC (LTE), and
CMFGEN (spherical, non-LTE), reveal that non-LTE effects appear small. In
addition, a sample of main-sequence and supergiant objects, observed with
FEROS, reveal LTE abundance estimates consistent with the Galactic environment
and previous optical studies. Based on the present study, we list a number of
Fe III transitions which we recommend for estimating the iron abundance from
early B-type stellar spectra.Comment: 3 figures and 8 tables. Table 3 is to be published online only
(included here on last page). Accepted for publication in MNRA
Iron abundances of B-type post-Asymptotic Giant Branch stars in globular clusters: Barnard 29 in M 13 and ROA 5701 in omega Cen
High resolution optical and ultraviolet spectra of two B-type post-Asymptotic
Giant Branch (post-AGB) stars in globular clusters, Barnard 29 in M 13 and ROA
5701 in omega Cen, have been analysed using model atmosphere techniques. The
optical spectra have been obtained with FEROS on the ESO 2.2-m telescope and
the 2d-Coud\'e spectrograph on the 2.7-m McDonald telescope, while the
ultraviolet observations are from the GHRS on the HST. Abundances of light
elements (C, N, O, Mg, Al and S) plus Fe have been determined from the optical
spectra, while the ultraviolet data provide additional Fe abundance estimates
from Fe III absorption lines in the 1875-1900 {\AA} wavelength region. A
general metal underabundance relative to young B-type stars is found for both
Barnard 29 and ROA 5701. These results are consistent with the metallicities of
the respective clusters, as well as with previous studies of the objects. The
derived abundance patterns suggest that the stars have not undergone a gas-dust
separation, contrary to previous suggestions, although they may have evolved
from the AGB before the onset of the third dredge-up. However, the Fe
abundances derived from the HST spectra are lower than those expected from the
metallicities of the respective clusters, by 0.5 dex for Barnard 29 and 0.8 dex
for ROA 5701. A similar systematic underabundance is also found for other
B-type stars in environments of known metallicity, such as the Magellanic
Clouds. These results indicate that the Fe III ultraviolet lines may yield
abundance values which are systematically too low by typically 0.6 dex and
hence such estimates should be treated with caution.Comment: 15 pages, 3 figures. Accepted for publication in MNRA
Detection of Voigt Spectral Line Profiles of Hydrogen Radio Recombination Lines toward Sagittarius B2(N)
We report the detection of Voigt spectral line profiles of radio
recombination lines (RRLs) toward Sagittarius B2(N) with the 100-m Green Bank
Telescope (GBT). At radio wavelengths, astronomical spectra are highly
populated with RRLs, which serve as ideal probes of the physical conditions in
molecular cloud complexes. An analysis of the Hn(alpha) lines presented herein
shows that RRLs of higher principal quantum number (n>90) are generally
divergent from their expected Gaussian profiles and, moreover, are well
described by their respective Voigt profiles. This is in agreement with the
theory that spectral lines experience pressure broadening as a result of
electron collisions at lower radio frequencies. Given the inherent technical
difficulties regarding the detection and profiling of true RRL wing spans and
shapes, it is crucial that the observing instrumentation produce flat baselines
as well as high sensitivity, high resolution data. The GBT has demonstrated its
capabilities regarding all of these aspects, and we believe that future
observations of RRL emission via the GBT will be crucial towards advancing our
knowledge of the larger-scale extended structures of ionized gas in the
interstellar medium (ISM)
Current Status of the SuperWASP Project
We present the current status of the SuperWASP project, a Wide Angle Search
for Planets. SuperWASP consists of up to 8 individual cameras using ultra-wide
field lenses backed by high-quality passively cooled CCDs. Each camera covers
7.8 x 7.8 sq degrees of sky, for nearly 500 sq degrees of sky coverage.
SuperWASP I, located in LaPalma, is currently operational with 5 cameras and is
conducting a photometric survey of a large numbers of stars in the magnitude
range ~7 to 15. The collaboration has developed a custom-built reduction
pipeline and aims to achieve better than 1 percent photometric precision. The
pipeline will also produce well sampled light curves for all the stars in each
field which will be used to detect: planetary transits, optical transients, and
track Near-Earth Objects. Status of current observations, and expected rates of
extrasolar planetary detections will be presented. The consortium members,
institutions, and further details can be found on the web site at:
http://www.superwasp.org.Comment: 3 pages, 2 figures, submitted to the Proceedings of the 13th Cool
Stars Workshop, Ed. F. Favata, ESA-S
Soft X-ray emission lines of Fe XV in solar flare observations and the Chandra spectrum of Capella
Recent calculations of atomic data for Fe XV have been used to generate
theoretical line ratios involving n = 3-4 transitions in the soft X-ray
spectral region (52-83 A), for a wide range of electron temperatures and
densities applicable to solar and stellar coronal plasmas. A comparison of
these with solar flare observations from a rocket-borne spectrograph (XSST)
reveals generally good agreement between theory and experiment. In particular,
the 82.76 A emission line in the XSST spectrum is identified, for the first
time to our knowledge in an astrophysical source. Most of the Fe XV transitions
which are blended have had the species responsible clearly identified, although
there remain a few instances where this has not been possible. The line ratio
calculations are also compared with a co-added spectrum of Capella obtained
with the Chandra satellite, which is probably the highest signal-to-noise
observation achieved for a stellar source in the 25-175 A soft X-ray region.
Good agreement is found between theory and experiment, indicating that the Fe
XV lines are reliably detected in Chandra spectra, and hence may be employed as
diagnostics to determine the temperature and/or density of the emitting plasma.
However the line blending in the Chandra data is such that individual emission
lines are difficult to measure accurately, and fluxes may only be reliably
determined via detailed profile fitting of the observations. The co-added
Capella spectrum is made available to hopefully encourage further exploration
of the soft X-ray region in astronomical sources.Comment: 27 pages, 10 figures, Astrophysical Journal, in pres
Low Metallicity Indicates that the Hypervelocity Star HE 0437-5439 was Ejected from the LMC
We measure the metallicity of the unusual hypervelocity star HE 0437-5439
from high resolution spectroscopy to be half-solar. We determine a spectral
type of B2 IV-III for the star and derive an effective temperature Teff= 21,500
+/- 1,000 K and a surface gravity log(g) = 3.7 +/- 0.2 (cgs). We also present
BV time series photometry and find the star to be non-variable at the 0.02 mag
level. We refine the magnitude of the hypervelocity star to V=16.36+/-0.04 mag,
with a color B-V=-0.23+/-0.03 mag, confirming its early-type nature. Our
metallicity result establishes the origin of HE 0437-5439 in the Large
Magellanic Cloud and implies the existence of a massive black hole somewhere in
this galaxy.Comment: 12 pages, 3 figures, 1 table; accepted to ApJL. Added 2 coauthors &
full abundance analysis. Results are strengthene
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