335 research outputs found
Rotations and Abundances of Blue Horizontal-Branch Stars in Globular Cluster M15
High-resolution optical spectra of eighteen blue horizontal-branch (BHB)
stars in the globular cluster M15 indicate that their stellar rotation rates
and photospheric compositions vary strongly as a function of effective
temperature. Among the cooler stars in the sample, at Teff ~ 8500 K, metal
abundances are in rough agreement with the canonical cluster metallicity, and
the v sin i rotations appear to have a bimodal distribution, with eight stars
at v sin i < 15 km/s and two stars at v sin i ~ 35 km/s. Most of the stars at
Teff > 10000 K, however, are slowly rotating, v sin i < 7 km/s, and their iron
and titanium are enhanced by a factor of 300 to solar abundance levels.
Magnesium maintains a nearly constant abundance over the entire range of Teff,
and helium is depleted by factors of 10 to 30 in three of the hotter stars.
Diffusion effects in the stellar atmospheres are the most likely explanation
for these large differences in composition. Our results are qualitatively very
similar to those previously reported for M13 and NGC 6752, but with even larger
enhancement amplitudes, presumably due to the increased efficiency of radiative
levitation at lower intrinsic [Fe/H]. We also see evidence for faster stellar
rotation explicitly preventing the onset of the diffusion mechanisms among a
subset of the hotter stars.Comment: 11 pages, 1 figure, 1 table, accepted to ApJ
Rocket- and aircraft-borne trace gas measurements in the winter polar stratosphere
In January and February 1987 stratospheric rocket- and aircraft-borne trace gas measurements were done in the North Polar region using ACIMS (Active Chemical Ionization Mass Spectrometry) and PACIMS (PAssive Chemical Ionization Mass Spectrometry) instruments. The rocket was launched at ESRANGE (European Sounding Rocket Launching Range) (68 N, 21 E, Northern Sweden) and the twin-jet research aircraft operated by the DFVLR (Deutsche Forschungs- und Versuchs-anstalt fuer Luft- und Raumfahrt), and equipped with a mass spectrometer laboratory was stationed at Kiruna airport. Various stratospheric trace gases were measured including nitric acid, sulfuric acid, non-methane hydrocarbons (acetone, hydrogen cyanide, acetonitrile, methanol etc.), and ambient cluster ions. The experimental data is presented and possible implications for polar stratospheric ozone discussed
The virtual observatory service TheoSSA: Establishing a database of synthetic stellar flux standards. II. NLTE spectral analysis of the OB-type subdwarf Feige 110
In the framework of the Virtual Observatory (VO), the German Astrophysical
Virtual Observatory (GAVO) developed the registered service TheoSSA
(Theoretical Stellar Spectra Access). It provides easy access to stellar
spectral energy distributions (SEDs) and is intended to ingest SEDs calculated
by any model-atmosphere code, generally for all effective temperature, surface
gravities, and elemental compositions. We will establish a database of SEDs of
flux standards that are easily accessible via TheoSSA's web interface.
The OB-type subdwarf Feige 110 is a standard star for flux calibration.
State-of-the-art non-local thermodynamic equilibrium (NLTE) stellar-atmosphere
models that consider opacities of species up to trans-iron elements will be
used to provide a reliable synthetic spectrum to compare with observations.
In case of Feige 110, we demonstrate that the model reproduces not only its
overall continuum shape from the far-ultraviolet (FUV) to the optical
wavelength range but also the numerous metal lines exhibited in its FUV
spectrum.
We present a state-of-the-art spectral analysis of Feige 110. We determined
, and
the abundances of He, N, P, S, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, and Ge. Ti, V,
Mn, Co, Zn, and Ge were identified for the first time in this star. Upper
abundance limits were derived for C, O, Si, Ca, and Sc.
The TheoSSA database of theoretical SEDs of stellar flux standards guarantees
that the flux calibration of astronomical data and cross-calibration between
different instruments can be based on models and SEDs calculated with
state-of-the-art model-atmosphere codes.Comment: 19 pages, 7 figure
SHOTGLAS I: The ultimate spectroscopic census of extreme horizontal branch stars in Centauri
The presence of extreme horizontal branch (EHB) and blue hook stars in some
Galactic globular clusters (GGCs) constitutes one of the remaining mysteries of
stellar evolution. In this paper, we focus on Cen, a peculiar, massive
GGC that hosts multiple stellar populations. We use non-LTE model atmospheres
to derive atmospheric parameters (Teff, log g and N(He)/N(H)) and spectroscopic
masses for 152 EHB stars in the cluster. This constitutes the largest
spectroscopic sample of EHB stars ever analyzed in a GGC and represents ~20% of
the EHB population of Cen. We also search for close binaries among
these stars based on radial velocity variations. Our results show that the EHB
population of Cen is divided into three spectroscopic groups that are
very distinct in the Teff - helium abundance plane. The majority of our sample
consists of sdOB stars that have roughly solar or super-solar atmospheric
helium abundances. It is these objects that constitute the blue hook at
18.5 mag in the Cen color-magnitude diagram. Interestingly, the
helium-enriched sdOBs do not have a significant counterpart population in the
Galactic field. Another major difference between the EHB stars in Cen
and the field is the fraction of close binaries. From our radial velocity
survey we identify two binary candidates and estimate an EHB close binary
fraction of ~5% in Cen. This low fraction is in line with findings for
other GGCs, but in sharp contrast to the situation in the field, where around
50% of the sdB stars reside in close binaries. Finally, the mass distribution
derived is very similar for all three spectroscopic groups, however the average
mass (0.38 Msun) is lower than that expected from stellar evolution theory.
While this mass conundrum has previously been noted for EHB stars in
Cen, it so far appears to be unique to that cluster.Comment: 25 pages, Accepted for publication in A&
The Age Of Globular Clusters In Light Of Hipparcos: Resolving the Age Problem?
We review five independent techniques which are used to set the distance
scale to globular clusters, including subdwarf main sequence fitting utilizing
the recent Hipparcos parallax catalogue. These data together all indicate that
globular clusters are farther away than previously believed, implying a
reduction in age estimates. This new distance scale estimate is combined with a
detailed numerical Monte Carlo study designed to assess the uncertainty
associated with the theoretical age-turnoff luminosity relationship in order to
estimate both the absolute age and uncertainty in age of the oldest globular
clusters. Our best estimate for the mean age of the oldest globular clusters is
now Gyr, with a one-sided, 95% confidence level lower limit of
9.5 Gyr. This represents a systematic shift of over 2 compared to our
earlier estimate, due completely to the new distance scale---which we emphasize
is not just due to the Hipparcos data. This now provides a lower limit on the
age of the universe which is consistent with either an open universe, or a
flat, matter dominated universe (the latter requiring H_0 \le 67 \kmsmpc).
Our new study also explicitly quantifies how remaining uncertainties in the
distance scale and stellar evolution models translate into uncertainties in the
derived globular cluster ages. Simple formulae are provided which can be used
to update our age estimate as improved determinations for various quantities
become available.Comment: 41 pages, including 10 eps figs, uses aaspp4.sty and flushrt.sty,
submitted to Ap.J., revised to incorporate FULL Hipparcos catalogue dat
Modeling He-rich subdwarfs through the hot-flasher Scenario
We present 1D numerical simulations aimed at studying the hot-flasher
scenario for the formation of He-rich subdwarf stars. Sequences were calculated
for a wide range of metallicities and physical assumptions, such as the stellar
mass at the moment of the helium core flash. This allows us to study the two
previously proposed flavors of the hot-flasher scenario ("deep" and "shallow"
mixing cases) and to identify a third transition type. Our sequences are
calculated by solving simultaneously the mixing and burning equations within a
diffusive convection picture, and in the context of standard mixing length
theory. We are able to follow chemical evolution during deep-mixing events in
which hydrogen is burned violently, and therefore able to present a homogeneous
set of abundances for different metallicities and varieties of hot-flashers. We
extend the scope of our work by analyzing the effects of non-standard
assumptions, such as the effect of chemical gradients, extra-mixing at
convective boundaries, possible reduction in convective velocities, or the
interplay between difussion and mass loss. Particular emphasis is placed on the
predicted surface properties of the models.
We find that the hot-flasher scenario is a viable explanation for the
formation and surface properties of He-sdO stars. Our results also show that,
during the early He-core burning stage, element diffusion may produce the
transformation of (post hot-flasher) He-rich atmospheres into He-deficient
ones. If this is so, then we find that He-sdO stars should be the progenitors
of some of the hottest sdB stars.Comment: 13 pages, including 8 figures and 6 tables. Accepted for publication
in A&A. Replaced to match the final version, including a note added in proof
regarding PG 1544+48
Variable stars in the field of the old open cluster Melotte 66
We report the results of photometric monitoring of the Melotte 66 field in
BVI filters. Ten variables were identified with nine being new discoveries. The
sample includes eight eclipsing binaries of which four are W UMa type stars,
one star is a candidate blue straggler. All four contact binaries are likely
members of the cluster based on their estimated distances. Ten blue stars with
U-B<-0.3 were detected inside a 14.8 x 22.8 arcmin^2 field centred on the
cluster. Time series photometry for 7 of them showed no evidence for any
variability. The brightest object in the sample of blue stars is a promising
candidate for a hot subdwarf belonging to the cluster. We show that the
anomalously wide main sequence of the cluster, reported in some earlier
studies, results from a combination of two effects: variable reddening occuring
across the cluster field and the presence of a rich population of binary stars
in the cluster itself. The density profile of the cluster field is derived and
the total number of member stars with 16<V<21 or 2.8<M_{V}<7.8 is estimated
conservatively at about 1100.Comment: 8 pages, 12 figures, accepted to MNRAS - 29 June 200
Spectral analysis of the sdO K 648, the exciting star of the planetary nebula Ps 1 in the globular cluster M 15 (NGC 7078)
We present a spectral analysis of the sdO central star K 648 based on
medium-resolution optical and high-resolution UV spectra. The photospheric
parameters are determined by means of state-of-the-art NLTE model atmosphere
techniques.
We found Teff = 39 +/- 2 kK and log g = 3.9 +/- 0.2. The helium (He/H=0.08)
and oxygen (O/H=0.001) abundances are about solar while carbon is enriched by a
factor of 2.5 (C/H=0.001). Nitrogen (N/H = 10**(-6), [N/H] = -2.0) appears at a
sub-solar value. However, these metal abundances are much higher than the
cluster's metallicity M 15: [Fe/H] = -2.25).
The surface composition appears to be a mixture of the original hydrogen-rich
material and products of helium burning (3 alpha process) which have been mixed
up to the surface. The abundances of He, C, and N are consistent with the
nebular abundance, while O is considerably more abundant in the photosphere
than in the nebula.
From a comparison of its position in the log Teff - log g plane with
evolutionary calculations a mass of 0.57 (+0.02, -0.01) Msun and a luminosity
of 3810 +/- 1200 Lsun are deduced.
Our spectroscopic distance d = 11.1 (+2.4, -2.9) kpc is in agreement with the
distance of M 15 as determined by Alves et al. (2000).
From the GHRS spectra we measure a radial velocity of vrad = -130 km/sec.Comment: 8 pages, 13 figure
Hysteresis of atmospheric parameters of 12 RR Lyrae stars based on multichannel simultaneous Stroemgren photometry
RR Lyrae stars have been observed to improve the insight into the processes
at work in their atmospheres. Simultaneous Stroemgren-photometry allows to
obtain a rapid sequence of measurements in which photometric indices are
unaffected by non-optimum observing conditions. The indices y, b-y, and c_1 are
used with an established calibration to derive T_eff and to derive the gravity,
log g_BJ from the Balmer jump, throughout the pulsation cycle. By employing the
equations for stellar structure, additional parameters can be derived.
Stroemgren photometry and its calibration in terms of T_eff and log g can be
used to determine the run of R and the atmosphere pulsation velocity. We find
that the Balmer-line strengths are correlated with T_eff and that the strength
of the Ca_ii K line correlates well with the radius of the star and thus the
pulsation-dependent density of the atmosphere. The density in the stellar
atmosphere fluctuates as indicated by the changes in the gravity log g_BJ,
derived from c_1, between 2.3 and 4.5 dex. Also the Stroemgren metal index,
m_1, fluctuates. We find a disagreement between log g(T,L,M), the gravity
calculated from T_eff, L, and the mass M,and the gravity log g_BJ. This can be
used to reassess the mass and the absolute magnitude of an individual star.The
curves derived for the pulsational velocity V_pul differ from curves obtained
from spectra needed to apply the Baade-Wesselink method; we think these
differences are due to phase dependent differences in the optical depth levels
sampled in continuum photometry and in spectroscopy. We find an atmospheric
oscillation in these fundamental mode RR Lyrae stars of periodicity P/7.Comment: 12 pages, 11 figure
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