1,227 research outputs found
Chemical Abundances from the Continuum
The calculation of solar absolute fluxes in the near-UV is revisited,
discussing in some detail recent updates in theoretical calculations of
bound-free opacity from metals. Modest changes in the abundances of elements
such as Mg and the iron-peak elements have a significant impact on the
atmospheric structure, and therefore self-consistent calculations are
necessary. With small adjustments to the solar photospheric composition, we are
able to reproduce fairly well the observed solar fluxes between 200 and 270 nm,
and between 300 and 420 nm, but find too much absorption in the 270-290 nm
window. A comparison between our reference 1D model and a 3D time-dependent
hydrodynamical simulation indicates that the continuum flux is only weakly
sensitive to 3D effects, with corrections reaching <10% in the near-UV, and <2%
in the optical.Comment: 10 pages, 5 figures, to appear in the proceedings of the conference A
Stellar Journey, a symposium in celebration of Bengt Gustafsson's 65th
birthday, June 23-27, 2008, Uppsal
The Lowest Mass White Dwarf
Extremely low mass white dwarfs are very rare objects likely formed in
compact binary systems. We present MMT optical spectroscopy of 42 low mass
white dwarf candidates serendipitously discovered in a survey for hypervelocity
B-type stars. One of these objects, SDSS J0917+46, has Teff= 11,288 \pm 72 K
and log g = 5.48 \pm 0.03; with an estimated mass of 0.17 M_sun, it is the
lowest gravity/mass white dwarf currently known. However, 40 of the low mass
candidates are normal DA white dwarfs with apparently inaccurate SDSS g
magnitudes. We revisit the identification of low mass white dwarf candidates
previously found in the SDSS, and conclude that four objects have M < 0.2
M_sun. None of these white dwarfs show excess emission from a binary companion,
and radial velocity searches will be necessary to constrain the nature of the
unseen companions.Comment: ApJ, accepted versio
HS 1857+5144 : a hot and young pre-cataclysmic variable
Aims. We report the discovery of a new white dwarf/M dwarf binary, HS 1857+5144, identified in the Hamburg Quasar Survey (HQS).
Methods. Time-resolved optical spectroscopy and photometry were carried out to determine the properties of this new cataclysmic variable progenitor (pre-CV).
Results. The light curves of HS 1857+5144 display a sinusoidal variation with a period of Porb = 383.52 min and peak-to-peak amplitudes of 0.7 mag and 1.1mag in the B-band and R-band, respectively. The large amplitude of the brightness variation results from a reflection effect on the heated inner hemisphere of the companion star, suggesting a very high temperature of the white
dwarf. Our radial velocity study confirms the photometric period as the orbital period of the system. A model atmosphere fit to the spectrum of the white dwarf obtained at minimum light provides limits to its mass and temperature of Mwd 0.6â1.0 M and Twd 70 000â100 000 K, respectively. The detection of He II λ4686 absorption classifies the primary star of HS 1857+5144 as a
DAO white dwarf. Combining the results from our spectroscopy and photometry, we estimate the mass of the companion star and the binary inclination to be Msec 0.15â0.30 M and i 45âŠâ55âŠ, respectively.
Conclusions. We classify HS 1857+5144 as one of the youngest pre-CV known to date. The cooling age of the white dwarf suggests that the present system has just emerged from a common envelope phase âŒ105 yr ago. HS 1857+5144 will start mass transfer within or below the 2â3 h period gap
The Discovery of a Companion to the Lowest Mass White Dwarf
We report the detection of a radial velocity companion to SDSS
J091709.55+463821.8, the lowest mass white dwarf currently known with
M~0.17Msun. The radial velocity of the white dwarf shows variations with a
semi-amplitude of 148.8 km/s and a period of 7.5936 hours, which implies a
companion mass of M > 0.28Msun. The lack of evidence of a companion in the
optical photometry forces any main-sequence companion to be smaller than
0.1Msun, hence a low mass main sequence star companion is ruled out for this
system. The companion is most likely another white dwarf, and we present
tentative evidence for an evolutionary scenario which could have produced it.
However, a neutron star companion cannot be ruled out and follow-up radio
observations are required to search for a pulsar companion.Comment: ApJ, in press. See the Press Release at
http://www.cfa.harvard.edu/press/2007/pr200708.htm
sMILES SSPs: A Library of Semi-Empirical MILES Stellar Population Models with Variable [/Fe] Abundances
We present a new library of semi-empirical stellar population models that are
based on the empirical MILES and semi-empirical sMILES stellar libraries. The
models span a large range of age and metallicity, in addition to an
[/Fe] coverage from 0.2 to 0.6 dex, at MILES resolution
(FWHM=2.5) and wavelength coverage (3540.5-7409.6). These models are aimed at exploring abundance ratios
in the integrated light from stellar populations in star clusters and galaxies.
Our approach is to build SSPs from semi-empirical stars at particular
[/Fe] values, thus producing new SSPs at a range of [/Fe]
values from sub-solar to super-solar. We compare these new SSPs with previously
published and well-used models and find similar abundance pattern predictions,
but with some differences in age indicators. We illustrate a potential
application of our new SSPs, by fitting them to the high signal-to-noise data
of stacked SDSS galaxy spectra. Age, metallicity and [/Fe] trends were
measured for galaxy stacks with different stellar velocity dispersions and show
systematic changes, in agreement with previous analyses of subsets of those
data. These new SSPs are made publicly available.Comment: 21 pages, 18 figures plus Supplementary Material. Accepted for
publication in MNRA
A Study of the Near-Ultraviolet Spectrum of Vega
UV, optical, and near-IR spectra of Vega have been combined to test our
understanding of stellar atmospheric opacities, and to examine the possibility
of constraining chemical abundances from low-resolution UV fluxes. We have
carried out a detailed analysis assuming Local Thermodynamic Equilibrium (LTE)
to identify the most important contributors to the UV continuous opacity: H,
H, C I, and Si II. Our analysis also assumes that Vega is spherically
symmetric and its atmosphere is well described with the plane parallel
approximation. Comparing observations and computed fluxes we have been able to
discriminate between two different flux scales that have been proposed, the
IUE-INES and the HST scales, favoring the latter. The effective temperature and
angular diameter derived from the analysis of observed optical and near-UV
spectra are in very good agreement with previous determinations based on
different techniques. The silicon abundance is poorly constrained by the UV
observations of the continuum and strong lines, but the situation is more
favorable for carbon and the abundances inferred from the UV continuum and
optical absorption lines are in good agreement. Some spectral intervals in the
UV spectrum of Vega that the calculations do not reproduce well are likely
affected by deviations from LTE, but we conclude that our understanding of UV
atmospheric opacities is fairly complete for early A-type stars.Comment: 13 pages, 9 figures, to be published in Ap
Surface abundances of light elements for a large sample of early B-type stars - IV. The magnesium abundance in 52 stars - a test of metallicity
From high-resolution spectra a non-LTE analysis of the MgII 4481.2 A feature
is implemented for 52 early and medium local B stars on the main sequence (MS).
The influence of the neighbouring line AlIII 4479.9 A is considered. The
magnesium abundance is determined; it is found that log e(Mg) = 7.67 +- 0.21 on
average. It is shown that uncertainties in the microturbulent parameter Vt are
the main source of errors in log e(Mg). When using 36 stars with the most
reliable Vt values derived from OII and NII lines, we obtain the mean abundance
log e(Mg) = 7.59 +- 0.15. The latter value is precisely confirmed for several
hot B stars from an analysis of the MgII 7877 A weak line. The derived
abundance log e(Mg) = 7.59 +- 0.15 is in excellent agreement with the solar
magnesium abundance log e_sun(Mg) = 7.55 +- 0.02, as well as with the proto-Sun
abundance log e_ps(Mg) = 7.62 +- 0.02. Thus, it is confirmed that the Sun and
the B-type MS stars in our neighbourhood have the same metallicity.Comment: 9 pages, 6 figures. Has been accepted for publication at MNRA
The X-ray emission from Young Stellar Objects in the rho Ophiuchi cloud core as seen by XMM-Newton
We observed the main core F of the rho Ophiuchi cloud, an active star-forming
region located at ~140 pc, using XMM-Newton with an exposure of 33 ks. We
detect 87 X-ray sources within the 30' diameter field-of-view of the it EPIC
imaging detector array. We cross-correlate the positions of XMM-Newton X-ray
sources with previous X-ray and infrared (IR) catalogs: 25 previously unknown
X-ray sources are found from our observation; 43 X-ray sources are detected by
both XMM-Newton and Chandra; 68 XMM-Newton X-ray sources have 2MASS near-IR
counterparts. We show that XMM-Newton and Chandra have comparable sensitivity
for point source detection when the exposure time is set to ~30 ks for both. We
detect X-ray emission from 7 Class I sources, 26 Class II sources, and 17 Class
III sources. The X-ray detection rate of Class I sources is very high (64 %),
which is consistent with previous Chandra observations in this area. We propose
that 15 X-ray sources are new class III candidates, which doubles the number of
known Class III sources, and helps to complete the census of YSOs in this area.
We also detect X-ray emission from two young bona fide brown dwarfs, GY310 and
GY141, out of three known in the field of view. GY141 appears brighter by
nearly two orders of magnitude than in the Chandra observation. We extract
X-ray light curves and spectra from these YSOs, and find some of them showed
weak X-ray flares. We observed an X-ray flare from the bona fide brown dwarf
GY310. We find as in the previous Chandra observation of this region that Class
I sources tend to have higher temperatures and heavier X-ray absorptions than
Class II and III sources.Comment: 17 pages, 13 figures, 4 tables, accepted by A&
An equatorial ultra iron-poor star identified in BOSS
We report the discovery of SDSS J131326.89-001941.4, an ultra iron-poor red
giant star ([Fe/H] ~ -4.3) with a very high carbon abundance ([C/Fe]~ +2.5).
This object is the fifth star in this rare class, and the combination of a
fairly low effective temperature (Teff ~ 5300 K), which enhances line
absorption, with its brightness (g=16.9), makes it possible to measure the
abundances of calcium, carbon and iron using a low-resolution spectrum from the
Sloan Digital Sky Survey. We examine the carbon and iron abundance ratios in
this star and other similar objects in the light of predicted yields from
metal-free massive stars, and conclude that they are consistent. By way of
comparison, stars with similarly low iron abundances but lower carbon-to-iron
ratios deviate from the theoretical predictions.Comment: 6 pages, 4 figures, accepted for publication in A&
The HERMES Solar Atlas and the spectroscopic analysis of the seismic solar analogue KIC3241581
Solar-analog stars provide an excellent opportunity to study the Sun's
evolution, i.e. the changes with time in stellar structure, activity, or
rotation for solar-like stars. The unparalleled photometric data from the NASA
space telescope Kepler allows us to study and characterise solar-like stars
through asteroseismology. We aim to spectroscopically investigate the
fundamental parameter and chromospheric activity of solar analogues and twins,
based on observations obtained with the HERMES spectrograph and combine them
with asteroseismology. Therefore, we need to build a solar atlas for the
spectrograph, to provide accurate calibrations of the spectroscopically
determined abundances of solar and late type stars observed with this
instrument and thus perform differential spectral comparisons. We acquire
high-resolution and high signal-to-noise spectroscopy to construct three solar
reference spectra by observing the reflected light of Vesta and Victoria
asteroids and Europa (100<S/N<450) with the \Hermes spectrograph. We then
observe the Kepler solar analog KIC3241581 (S/N~170). We constructed three
solar spectrum atlases from 385 to 900 nm obtained with the Hermes spectrograph
from observations of two bright asteroids and Europa. A comparison between our
solar spectra atlas to the Kurucz and HARPS solar spectrum shows an excellent
agreement. KIC3241581 was found to be a long-periodic binary system. The
fundamental parameter for the stellar primary component are Teff=5689+/-11K,
logg=4.385+/-0.005, [Fe/H]=+0.22+/-0.01, being in agreement with the published
global seismic values confirming its status of solar analogue. KIC 3241581 is a
metal rich solar analogue with a solar-like activity level in a binary system
of unknown period. The chromospheric activity level is compatible to the solar
magnetic activity.Comment: 12 pages, 8 figures, accepted for publication in A&
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