398 research outputs found
Abundances in Przybylski's star
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74704/1/j.1365-8711.2000.03578.x.pd
MOST discovers a multimode delta Scuti star in a triple system: HD 61199
A field star, HD 61199 (V ~ 8), simultaneously observed with Procyon by the
MOST (Microvariability & Oscillations of STars) satellite in continuous runs of
34, 17, and 34 days in 2004, 2005, and 2007, was found to pulsate in 11
frequencies in the delta Scuti range with amplitudes from 1.7 down to 0.09
mmag. The photometry also showed variations with a period of about four days.
To investigate the nature of the longer period, 45 days of time-resolved
spectroscopy was obtained at the Thueringer Landessternwarte Tautenburg in
2004. The radial velocity measurements indicate that HD 61199 is a triple
system. A delta Scuti pulsator with a rich eigenspectrum in a multiple system
is promising for asteroseismology. Our objectives were to identify which of the
stars in the system is the delta Scuti variable and to obtain the orbital
elements of the system and the fundamental parameters of the individual
components, which are constrained by the pulsation frequencies of the delta
Scuti star. Classical Fourier techniques and least-squares multi-sinusoidal
fits were applied to the MOST photometry to identify the pulsation frequencies.
The groundbased spectroscopy was analysed with least-squares-deconvolution
(LSD) techniques, and the orbital elements derived with the KOREL and ORBITX
routines. Asteroseismic models were also generated. The photometric and
spectroscopic data are compatible with a triple system consisting of a close
binary with an orbital period of 3.57 days and a delta Scuti companion (HD
61199,A) as the most luminous component. The delta Scuti star is a rapid
rotator with about vsin i = 130 km/s and an upper mass limit of about 2.1 Msun.
For the close binary components, we find they are of nearly equal mass, with
lower mass limits of about 0.7 Msun.Comment: 11 pages, 14 figures, accepted by A&
The Carina Project. V. The impact of NLTE effects on the iron content
We have performed accurate iron abundance measurements for 44 red giants
(RGs) in the Carina dwarf spheroidal (dSph) galaxy. We used archival,
high-resolution spectra (R~38,000) collected with UVES at ESO/VLT either in
slit mode (5) or in fiber mode (39, FLAMES/GIRAFFE-UVES). The sample is more
than a factor of four larger than any previous spectroscopic investigation of
stars in dSphs based on high-resolution (R>38,000) spectra. We did not impose
the ionization equilibrium between neutral and singly-ionized iron lines. The
effective temperatures and the surface gravities were estimated by fitting
stellar isochrones in the V, B-V color-magnitude diagram. To measure the iron
abundance of individual lines we applied the LTE spectrum synthesis fitting
method using MARCS model atmospheres of appropriate metallicity. We found
evidence of NLTE effects between neutral and singly-ionized iron abundances.
Assuming that the FeII abundances are minimally affected by NLTE effects, we
corrected the FeI stellar abundances using a linear fit between FeI and FeII
stellar abundance determinations.
We found that the Carina metallicity distribution based on the corrected FeI
abundances (44 RGs) has a weighted mean metallicity of [Fe/H]=-1.80 and a
weighted standard deviation of sigma=0.24 dex. The Carina metallicity
distribution based on the FeII abundances (27 RGs) gives similar estimates
([Fe/H]=-1.72, sigma=0.24 dex). The current weighted mean metallicities are
slightly more metal poor when compared with similar estimates available in the
literature. Furthermore, if we restrict our analysis to stars with the most
accurate iron abundances, ~20 FeI and at least three FeII measurements (15
stars), we found that the range in iron abundances covered by Carina RGs (~1
dex) agrees quite well with similar estimates based on high-resolution spectra.Comment: Accepted for publication in PASP, 16 pages, 7 figures, 3 tables, 1 MR
table Note: the electronic version of Table1 is included, but commented, in
the tex fil
Re II and Other Exotic Spectra in HD 65949
Powerful astronomical spectra reveal an urgent need for additional work on
atomic lines, levels, and oscillator strengths. The star HD 65949 provides some
excellent examples of species rarely identified in stellar spectra. For
example, the Re II spectrum is well developed, with 17 lines between 3731 and
4904 [A], attributed wholly or partially to Re II. Classifications and
oscillator strengths are lacking for a number of these lines. The spectrum of
Os II is well identified. Of 14 lines attributed wholly or partially to Os II,
only one has an entry in the VALD database. We find strong evidence that Te II
is present. There are NO Te II lines in the VALD database. Ru II is clearly
present, but oscillator strengths for lines in the visual are lacking. There is
excellent to marginal evidence for a number of less commonly identified
species, including Kr II, Nb II, Sb II, Xe II, Pr III, Ho III, Au II, and Pt II
(probably Pt-198), to be present in the spectrum of HD 65949. The line Hg II at
3984 [A] is of outstanding strength, and all three lines of Multiplet 1 of Hg I
are present, even though the surface temperature of HD 65949 is relatively
high. Finally, we present the case of an unidentified, 24 [mA], line at 3859.63
[A], which could be the same feature seen in magnetic CP stars. It is typically
blended with a putative U II line used in cosmochronology.Comment: ASOS9 Poster (Lund, Sweden, August 2007), to be published in Journal
of Physics: Conference Series (JPCS), 6 pages 1 figur
Discovery of the first tau Sco analogues: HD 66665 and HD 63425
The B0.2 V magnetic star tau Sco stands out from the larger population of
massive OB stars due to its high X-ray activity, peculiar wind diagnostics and
highly complex magnetic field. This paper presents the discovery of the first
two tau Sco analogues - HD 66665 and HD 63425, identified by the striking
similarity of their UV spectra to that of tau Sco. ESPaDOnS spectropolarimetric
observations were secured by the Magnetism in Massive Stars CFHT Large Program,
in order to characterize the stellar and magnetic properties of these stars.
CMFGEN modelling of optical ESPaDOnS spectra and archived IUE UV spectra showed
that these stars have stellar parameters similar to those of tau Sco. A
magnetic field of similar surface strength is found on both stars, reinforcing
the connection between the presence of a magnetic field and wind peculiarities.
However, additional phase-resolved observations will be required in order to
assess the potential complexity of the magnetic fields, and verify if the wind
anomalies are linked to this property.Comment: 6 pages, 2 tables, 3 figures. Accepted for publication in MNRAS. The
definitive version will be available at www.blackwel-synergy.co
HE 0557-4840 - Ultra-Metal-Poor and Carbon-Rich
We report the discovery and high-resolution, high S/N, spectroscopic analysis
of the ultra-metal-poor red giant HE 0557-4840, which is the third most
heavy-element deficient star currently known. Its atmospheric parameters are
T_eff = 4900 K, log g = 2.2, and [Fe/H]= -4.75. This brings the number of stars
with [Fe/H] < -4.0 to three, and the discovery of HE 0557-4840 suggests that
the metallicity distribution function of the Galactic halo does not have a
"gap" between [Fe/H] = -4.0, where several stars are known, and the two most
metal-poor stars, at [Fe/H] ~ -5.3. HE 0557-4840 is carbon rich - [C/Fe] = +1.6
- a property shared by all three objects with [Fe/H] < -4.0, suggesting that
the well-known increase of carbon relative to iron with decreasing [Fe/H]
reaches its logical conclusion - ubiquitous carbon richness - at lowest
abundance. We also present abundances (nine) and limits (nine) for a further 18
elements. For species having well-measured abundances or strong upper limits,
HE 0557-4840 is "normal" in comparison with the bulk of the stellar population
at [Fe/H] ~ -4.0 - with the possible exception of Co. We discuss the
implications of these results for chemical enrichment at the earliest times, in
the context of single ("mixing and fallback") and two-component enrichment
models. While neither offers a clear solution, the latter appears closer to the
mark. Further data are required to determine the oxygen abundance and improve
that of Co, and hence more strongly constrain the origin of this object.Comment: Submitted to Astrophysical Journal. 52 pages (41 text, 11 figures
Evolutionary stage of the spectral variable BD+48 1220=IRAS 05040+4820
Based on high-resolution observations (R=60000 and 75000), we have studied
the optical spectral variability of the star BD+48 1220 (IRAS05040+4820). We
have measured the equivalent widths of numerous absorption lines of neutral
atoms and ions at wavelengths from 4500 to 6760 AA, as well as the
corresponding radial velocities. We use model atmospheres to determine
Teff=7900K, log g=0.0, microturbulence velocity xi_t=6.0, and the abundances
for 16 elements. The star's metallicity differs little from the solar value:
[Fe/H]=-0.10 dex. The main peculiarity of the chemical composition of the star
is a large He-excess, derived from the HeI 5876 A absorption, [He/H]=+1.04, and
the equally large O-excess, [O/Fe]=+0.72 dex. The C-excess is small,
[C/Fe]=+0.09 dex, and the ratio [C/O] 1. We obtained a revised relation
for the light-metal abundances: [Na/Fe]=+0.87 with [Mg/Fe]=-0.31. The barium
abundance is low, [Ba/Fe]=-0.84. The radial velocity of the star measured from
photospheric absorption lines over three years of observations varies in the
interval V_sun = -(7 - 15) km/s. Time variable differential line shifts have
been revealed. The entire set of available data (the luminosity Mv~-5m,
velocity V_lsr~-20 km/s, metallicity [Fe/H]=-0.10, and peculiarities of the
optical spectrum and chemical composition) confirms the status of BD+48 1220 as
a post-AGB star with He- and O-excesses belonging to the Galactic disk.Comment: 42 pages, 7 figures, 6 tables, accepted by Astronomy Report
Vertical abundance stratification in the blue horizontal branch star HD135485
It is commonly believed that the observed overabundances of many chemical
species relative to the expected cluster metallicity in blue horizontal branch
(BHB) stars appear as a result of atomic diffusion in the photosphere. The slow
rotation of BHB stars (with T_eff > 11,500K), typically v sin{i} < 10 km/s, is
consistent with this idea. In this work we search for observational evidence of
vertical chemical stratification in the atmosphere of HD135485. If this
evidence exists, it will demonstrate the importance of atomic diffusion
processes in the atmospheres of BHB stars. We undertake an extensive abundance
stratification analysis of the atmosphere of HD135485, based on recently
acquired high resolution and S/N CFHT ESPaDOnS spectra and a McDonald-CE
spectrum. Our numerical simulations show that nitrogen and sulfur reveal
signatures of vertical abundance stratification in the stellar atmosphere. It
appears that the abundances of these elements increase toward the upper
atmosphere. This fact cannot be explained by the influence of microturbulent
velocity, because oxygen, carbon, neon, argon, titanium and chromium do not
show similar behavior and their abundances remain constant throughout the
atmosphere. It seems that the iron abundance may increase marginally toward the
lower atmosphere. This is the first demonstration of vertical abundance
stratification of metals in a BHB star.Comment: 8 pages, 5 figures, accepted to A&
A Search for Nitrogen-Enhanced Metal-Poor Stars
Theoretical models of very metal-poor intermediate-mass Asymptotic Giant
Branch (AGB) stars predict a large overabundance of primary nitrogen. The very
metal-poor, carbon-enhanced, s-process-rich stars, which are thought to be the
polluted companions of now-extinct AGB stars, provide direct tests of the
predictions of these models. Recent studies of the carbon and nitrogen
abundances in metal-poor stars have focused on the most carbon-rich stars,
leading to a potential selection bias against stars that have been polluted by
AGB stars that produced large amounts of nitrogen, and hence have small [C/N]
ratios. We call these stars Nitrogen-Enhanced Metal-Poor (NEMP) stars, and
define them as having [N/Fe] > +0.5 and [C/N] < -0.5. In this paper, we report
on the [C/N] abundances of a sample of 21 carbon-enhanced stars, all but three
of which have [C/Fe] < +2.0. If NEMP stars were made as easily as
Carbon-Enhanced Metal-Poor (CEMP) stars, then we expected to find between two
and seven NEMP stars. Instead, we found no NEMP stars in our sample. Therefore,
this observational bias is not an important contributor to the apparent dearth
of N-rich stars. Our [C/N] values are in the same range as values reported
previously in the literature (-0.5 to +2.0), and all stars are in disagreement
with the predicted [C/N] ratios for both low-mass and high-mass AGB stars. We
suggest that the decrease in [C/N] from the low-mass AGB models is due to
enhanced extra-mixing, while the lack of NEMP stars may be caused by
unfavorable mass ratios in binaries or the difficulty of mass transfer in
binary systems with large mass ratios.Comment: 14 pages, 7 figures, to be published in Ap
A new code for automatic determination of equivalent widths: Automatic Routine for line Equivalent widths in stellar Spectra (ARES)
We present a new automatic code (ARES) for determining equivalent widths of
the absorption lines present in stellar spectra. We also describe its use for
determining fundamental spectroscopic stellar parameters.
The code is written in C++ based on the standard method of determining EWs
and is available for the community. The code automates the manual procedure
that the users normally carry out when using interactive routines such as the
splot routine implemented in IRAF.
We test the code using both simulated and real spectra with different levels
of resolution and noise and comparing its measurements to the manual ones
obtained in the standard way. The results shows a small systematic difference,
always below 1.5m\AA. This can be explained by errors in the manual
measurements caused by subjective continuum determination. The code works
better and faster than others tested before.Comment: 10 pages, 9 figures, Accepted for publication on A&
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