172 research outputs found
Detached white dwarf main-sequence star binaries
We considered the formation of detached white dwarf main-sequence star (WDMS)
binaries through seven evolutionary channels subdivided according to the
evolutionary process that gives rise to the formation of the white dwarf or its
helium-star progenitor: dynamically stable Roche-lobe overflow (Algol-type
evolution), dynamically unstable Roche-lobe overflow (common-envelope
evolution), or stellar winds (single star evolution). We examine the
sensitivity of the population to changes in the amount of mass lost from the
system during stable Roche-lobe overflow, the common-envelope ejection
efficiency, and the initial mass ratio or initial secondary mass distribution.
In the case of a flat initial mass ratio distribution, the local space density
of WDMS binaries is of the order of 10^{-3}/pc^3. This number decreases to
10^{-4}/pc^3 when the initial mass ratio distribution is approximately
proportional to the inverse of the initial mass ratio. More than 75% of the
WDMS binary population stems from wide systems in which both components evolve
as if they were single stars. The remaining part of the population is dominated
by systems in which the white dwarf is formed in a common-envelope phase. The
birthrate of WDMS binaries forming through a common-envelope phase is about 10
times larger than the birthrate of WDMS binaries forming through a stable
Roche-lobe overflow phase. The ratio of the number of helium white dwarf
systems to the number of carbon/oxygen or oxygen/neon/magnesium white dwarf
systems derived from large samples of observed WDMS binaries by, e.g., future
planet-search missions such as SuperWASP, COROT, and Kepler may furthermore
constrain the common-envelope ejection efficiency.Comment: 22 pages, accepted for publication in A&
Constraints on the origin of the massive, hot, and rapidly rotating magnetic white dwarf RE J 0317-853 from an HST parallax measurement
We use the parallax measurements of RE J 0317-853 to determine its mass,
radius, and cooling age and thereby constrain its evolutionary origins. We
observed RE J 0317-853 with the the Hubble Space Telescope's Fine Guidance
System to measure the parallax of RE J 0317-853 and its binary companion, the
non-magnetic white dwarf LB 9802. In addition, we acquired spectra of
comparison stars with the Boller & Chivens spectrograph of the SMARTS telescope
to correct the parallax zero point. For the corrected parallax, we determine
the radius, mass, and the cooling age with the help of evolutionary models from
the literature. The properties of RE J 0317-853 are constrained using the
parallax information. We discuss the different cases of the core composition
and the uncertain effective temperature. We confirm that RE J 0317-853 is close
to the Chandrasekhar's mass limit in all cases and almost as old as its
companion LB 9802. The precise evolutionary history of RE J 0317-853 depends on
our knowledge of its effective temperature. It is possible that it had a single
star progenitor possible if we assume that the effective temperature is at the
cooler end of the possible range from 30000 to 50000; if T_eff is instead at
the hotter end, a binary-merger scenario for RE J 0317-853 becomes more
plausible.Comment: 11 pages, 6 figures; language revised edition with added references.
Accepted for publication in Astronomy and Astrophysic
UV and X-Ray Monitoring of AG Draconis During the 1994/1995 Outbursts
The recent 1994-1995 active phase of AG Draconis has given us for the first
time the opportunity to follow the full X-ray behaviour of a symbiotic star
during two successive outbursts and to compare with its quiescence X-ray
emission. With \ros observations we have discovered a remarkable decrease of
the X-ray flux during both optical maxima, followed by a gradual recovering to
the pre-outburst flux. In the UV the events were characterized by a large
increase of the emission line and continuum fluxes, comparable to the behaviour
of AG Dra during the 1980-81 active phase. The anticorrelation of X-ray/UV flux
and optical brightness evolution is shown to very likely be due to a
temperature decrease of the hot component. Such a temperature decrease could be
produced by an increased mass transfer to the burning compact object, causing
it to slowly expand to about twice its original size.Comment: 12 pages postscript incl. figures, Proc. of Workshop on Supersoft
X-Ray Sources, to appear in Lecture Notes in Physics vol. 472 (1996
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
M31-RV evolution and its alleged multi-outburst pattern
The photometric evolution of M31-RV has been investigated on 1447 plates of
the Andromeda galaxy obtained over half a century with the Asiago telescopes.
M31-RV is a gigantic stellar explosion that occurred during 1988 in the Bulge
of M31 and that was characterized by the appearance for a few months of an M
supergiant reaching M_bol=-10. The 1988 outburst has been positively detected
on Asiago plates, and it has been the only such event recorded over the period
covered by the plates (1942-1993). In particular, an alleged previous outburst
in 1967 (Sharov 1990, SvAL, 16, 199) is excluded by the more numerous and
deeper Asiago plates, with relevant implication for the interpretative models
of this unique event. We outline a close analogy in spectral and photometric
evolution with those of V838 Mon which exploded in our Galaxy in 2002. The
analogy is found to extend also to the closely similar absolute magnitude at
the time of the sudden drop in photospheric temperature that both M31-RV and
V838 Mon exhibited. These similarities, in spite of the greatly differing
metallicity, age and mass of the two objects, suggest that the same, universal
and not yet identified process was at work in both cases.Comment: A&A in press. Table 3 available only in electronic form from CDS or
http://ulisse.pd.astro.it/M31-RV.htm
HE0107-5240, A Chemically Ancient Star.I. A Detailed Abundance Analysis
We report a detailed abundance analysis for HE0107-5240, a halo giant with
[Fe/H]_NLTE=-5.3. This star was discovered in the course of follow-up
medium-resolution spectroscopy of extremely metal-poor candidates selected from
the digitized Hamburg/ESO objective-prism survey. On the basis of
high-resolution VLT/UVES spectra, we derive abundances for 8 elements (C, N,
Na, Mg, Ca, Ti, Fe, and Ni), and upper limits for another 12 elements. A
plane-parallel LTE model atmosphere has been specifically tailored for the
chemical composition of {\he}. Scenarios for the origin of the abundance
pattern observed in the star are discussed. We argue that HE0107-5240 is most
likely not a post-AGB star, and that the extremely low abundances of the
iron-peak, and other elements, are not due to selective dust depletion. The
abundance pattern of HE0107-5240 can be explained by pre-enrichment from a
zero-metallicity type-II supernova of 20-25M_Sun, plus either self-enrichment
with C and N, or production of these elements in the AGB phase of a formerly
more massive companion, which is now a white dwarf. However, significant radial
velocity variations have not been detected within the 52 days covered by our
moderate-and high-resolution spectra. Alternatively, the abundance pattern can
be explained by enrichment of the gas cloud from which HE0107-5240 formed by a
25M_Sun first-generation star exploding as a subluminous SNII, as proposed by
Umeda & Nomoto (2003). We discuss consequences of the existence of HE0107-5240
for low-mass star formation in extremely metal-poor environments, and for
currently ongoing and future searches for the most metal-poor stars in the
Galaxy.Comment: 60 pages, 16 figures. Accepted for publication in Ap
Cluster-based density-functional approach to quantum transport through molecular and atomic contacts
We present a cluster-based density-functional approach to model charge
transport through molecular and atomic contacts. The electronic structure of
the contacts is determined in the framework of density functional theory, and
the parameters needed to describe transport are extracted from finite clusters.
A similar procedure, restricted to nearest-neighbor interactions in the
electrodes, has been presented by Damle et al. [Chem. Phys. 281, 171 (2002)].
Here, we show how to systematically improve the description of the electrodes
by extracting bulk parameters from sufficiently large metal clusters. In this
way we avoid problems arising from the use of nonorthogonal basis functions.
For demonstration we apply our method to electron transport through Au contacts
with various atomic-chain configurations and to a single-atom contact of Al.Comment: 18 pages, 13 figure
Chemical Inhomogeneities in the Milky Way Stellar Halo
We have compiled a sample of 699 stars from the recent literature with
detailed chemical abundance information (spanning -4.2 < [Fe/H] < +0.3), and we
compute their space velocities and Galactic orbital parameters. We identify
members of the inner and outer stellar halo populations in our sample based
only on their kinematic properties and then compare the abundance ratios of
these populations as a function of [Fe/H]. In the metallicity range where the
two populations overlap (-2.5 < [Fe/H] < -1.5), the mean [Mg/Fe] of the outer
halo is lower than the inner halo by ~0.1 dex. For [Ni/Fe] and [Ba/Fe], the
star-to-star abundance scatter of the inner halo is consistently smaller than
in the outer halo. The [Na/Fe], [Y/Fe], [Ca/Fe], and [Ti/Fe] ratios of both
populations show similar means and levels of scatter. Our inner halo population
is chemically homogeneous, suggesting that a significant fraction of the Milky
Way stellar halo originated from a well-mixed ISM. In contrast, our outer halo
population is chemically diverse, suggesting that another significant fraction
of the Milky Way stellar halo formed in remote regions where chemical
enrichment was dominated by local supernova events. We find no abundance trends
with maximum radial distance from the Galactic center or maximum vertical
distance from the Galactic disk. We also find no common kinematic signature for
groups of metal-poor stars with peculiar abundance patters, such as the
alpha-poor stars or stars showing unique neutron-capture enrichment patterns.
Several stars and dSph systems with unique abundance patterns spend the
majority of their time in the distant regions of the Milky Way stellar halo,
suggesting that the true outer halo of the Galaxy may have little resemblance
to the local stellar halo.Comment: Accepted for publication in AJ. Full tables available upon reques
Detection of a companion to the pulsating sdB Feige 48
We present the discovery of a binary companion to the pulsating sdB Feige 48.
Using HST/STIS and archival FUSE spectra, we measure a period of 0.376+/-0.003
d and a velocity semi-amplitude of 28.0+/-0.2 km/s. This implies that the
companion star must either be of very low mass, or the orbit is at high
inclination. Combining 2MASS fluxes, the lack of a reflection effect, results
from asteroseismology and a measurement of the rotation velocity of Feige 48,
we show that the orbital inclination must be <= 11.4 degrees and that the
unseen companion is a white dwarf with mass >= 0.46 Msun. The implications of
this discovery, and of binarity amongst sdB pulsators, is then discussed in the
context of recent theoretical work on sdB formation. In particular we suggest
that radial velocity studies focus on sdB pulsators with no known companion,
and that asteroseismological studies of sdBs investigate a larger mass range
than previously considered in order to test formation models.Comment: 7 pages, 5 figures, to appear in A&
Evolution of the number of accreting white dwarfs with shell nuclear burning and of occurrence rate of SN Ia
We analyze temporal evolution of the number of accreting white dwarfs with
shell hydrogen burning in semidetached and detached binaries. We consider a
stellar system in which star formation lasts for 10 Gyr with a constant rate,
as well as a system in which the same amount of stars is formed in a single
burst lasting for 1 Gyr. Evolution of the number of white dwarfs is confronted
to the evolution of occurrence rate of events that usually are identified with
SN Ia or accretion-induced collapses, i.e. with accumulation of Chandrasekhar
mass by a white dwarf or a merger of a pair of CO white dwarfs with total mass
not lower than the Chandrasekhar one. In the systems with a burst of star
formation, at 10 Gyr observed supersoft X-ray sources, most probably, are
not precursors of SN Ia. The same is true for an overwhelming majority of the
sources in the systems with constant star formation rate. In the systems of
both kinds mergers of white dwarfs is the dominant SN Ia scenario. In symbiotic
binaries, accreting CO-dwarfs do not accumulate enough mass for SN Ia
explosion, while ONeMg-dwarfs finish their evolution by an accretion-induced
collapse with formation of a neutron star.Comment: 11 pages, 2 figures, accepted by Astronomy Letter
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