1,071 research outputs found
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
A new HW Vir binary from the Palomar Transient Factory: PTF1 J072455.75+125300.3 - An eclipsing subdwarf B binary with a M-star companion
We report the discovery of an eclipsing binary -- PTF1 J072456125301--
composed of a subdwarf B (sdB) star () with a faint companion.
Subdwarf B stars are core helium-burning stars, which can be found on the
extreme horizontal branch. About half of them reside in close binary systems,
but few are known to be eclipsing, for which fundamental stellar parameters can
be derived.\newline We conducted an analysis of photometric data and spectra
from the Palomar 60'' and the 200" Hale telescope respectively. A quantitative
spectral analysis found an effective temperature of
\,K, log g = and
log(, typical for an sdB star. The
companion does not contribute to the optical light of the system, except
through a distinct reflection effect. From the light curve an orbital period of
0.09980(25)\,d and a system inclination of 83.56\pm0.30\,^{\circ} were
derived. The radial velocity curve yielded an orbital semi-amplitude of
K_1=95.8\pm 8.1\,\text{km s^{-1}}. The mass for the M-type dwarf companion
is . PTF1\,J072456125301 has similar atmospheric
parameters to those of pulsating sdB stars (V346 Hya stars). Therefore it could
be a high-priority object for asteroseismology, if pulsations were detected
such as in the enigmatic case of NY Vir.Comment: Accepted to A&A, 7pages, 4 figure
Candidate hypervelocity stars of spectral type G and K revisited
Hypervelocity stars (HVS) move so fast that they are unbound to the Galaxy.
When they were first discovered in 2005, dynamical ejection from the
supermassive black hole (SMBH) in the Galactic Centre (GC) was suggested as
their origin. The two dozen HVSs known today are young massive B stars, mostly
of 3-4 solar masses. Recently, 20 HVS candidates of low mass were discovered in
the Segue G and K dwarf sample, but none of them originates from the GC. We
embarked on a kinematic analysis of the Segue HVS candidate sample using the
full 6D phase space information based on new proper motion measurements. Their
orbital properties can then be derived by tracing back their trajectories in
different mass models of our Galaxy. We present the results for 14 candidate
HVSs, for which proper motion measurements were possible. Significantly lower
proper motions than found in the previous study were derived. Considering three
different Galactic mass models we find that all stars are bound to the Galaxy.
We confirm that the stars do not originate from the GC. The distribution of
their proper motions and radial velocities is consistent with predictions for
runaway stars ejected from the Galactic disk by the binary supernova mechanism.
However, their kinematics are also consistent with old disk membership.
Moreover, most stars have rather low metallicities and strong -element
enrichment as typical for thick disk and halo stars, whereas the metallicity of
the three most metal-rich stars could possibly indicate that they are runaway
stars from the thin disk. One star shows halo kinematics.Comment: A&A letter accepte
Subdwarf B Stars from the ESO Supernova Ia Progenitor Survey -- Observation versus Theory
Original paper can be found at: http://www.astrosociety.org/pubs/cs/328.html--Copyright Astronomical Society of the PacificWe present the analysis of a high-quality sample of optical spectra for 76 sdB stars from the ESO Supernova Ia Progenitor Survey. Effective temperature, surface gravity, and photospheric helium abundance were derived from line profile fits. We demonstrate that our subsample of 52 single-lined sdB stars is a useful tool to compare observation and theory. The predictions of population synthesis models for close binary evolution are compared to our data. We show that the simulations cover the observed parameter range of sdBs, but fail to reproduce the observed distribution in detail
Hot subdwarf stars in close-up view. I. Rotational properties of subdwarf B stars in close binary systems and nature of their unseen companions
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO)The origin of hot subdwarf B stars (sdBs) is still unclear. About half of the known sdBs are in close binary systems for which common envelope ejection is the most likely formation channel. Little is known about this dynamic phase of binary evolution. Since most of the known sdB systems are single-lined spectroscopic binaries, it is difficult to derive masses and unravel the companions' nature, which is the aim of this paper. Due to the tidal influence of the companion in close binary systems, the rotation of the primary becomes synchronised to its orbital motion. In this case it is possible to constrain the mass of the companion, if the primary mass, its projected rotational velocity as well as its surface gravity are known. For the first time we measured the projected rotational velocities of a large sdB binary sample from high resolution spectra. We analysed a sample of 51 sdB stars in close binaries, 40 of which have known orbital parameters comprising half of all such systems known today. Synchronisation in sdB binaries is discussed both from the theoretical and the observational point of view. The masses and the nature of the unseen companions could be constrained in 31 cases. We found orbital synchronisation most likely to be established in binaries with orbital periods shorter than . Only in five cases it was impossible to decide whether the sdB's companion is a white dwarf or an M dwarf. The companions to seven sdBs could be clearly identified as late M stars. One binary may have a brown dwarf companion. The unseen companions of nine sdBs are white dwarfs with typical masses. The mass of one white dwarf companion is very low. In eight cases (including the well known system KPD1930+2752) the companion mass exceeds , four of which even exceed the Chandrasekhar limit indicating that they may be neutron stars. Even stellar mass black holes are possible for the most massive companions. The distribution of the inclinations of the systems with low mass companions appears to be consistent with expectations, whereas a lack of high inclinations becomes obvious for the massive systems. We show that the formation of such systems can be explained with common envelope evolution and present an appropriate formation channel including two phases of unstable mass transfer and one supernova explosion. The sample also contains a candidate post-RGB star, which rotates fast despite its long orbital period. The post-RGB stars are expected to spin-up caused by their ongoing contraction. The age of the sdB is another important factor. If the EHB star is too young, the synchronisation process might not be finished yet. Estimating the ages of the target stars from their positions on the EHB band, we found PG 2345+318, which is known not to be synchronised, to lie near the zero-age extreme horizontal branch as are the massive candidates PG 1232-136, PG 1432+159 and PG 1101+249. These star may possibly be too young to have reached synchronisation. The derived large fraction of putative massive sdB binary systems in low inclination orbits is inconsistent with theoretical predictions. Even if we dismiss three candidates because they may be too young and assume that the other sdB primaries are of low mass, PG 1743+477 and, in particular, HE 0532-4503 remain as candidates whose companions may have masses close to or above the Chandrasekhar limit. X-ray observations and accurate photometry are suggested to clarify their nature. As high inclination systems must also exist, an appropriate survey has already been launched to find such binaries.Peer reviewe
D'atri spaces of type k and related classes of geometries concerning jacobi operators
In this article we continue the study of the geometry of -D'Atri spaces,
( denotes the dimension of the manifold) began by
the second author. It is known that -D'Atri spaces, are related
to properties of Jacobi operators along geodesics, since she has shown
that , are invariant
under the geodesic flow for any unit tangent vector . Here, assuming that
the Riemannian manifold is a D'Atri space, we prove in our main result that
is also invariant under the geodesic flow if . In addition, other properties of Jacobi operators related to the
Ledger conditions are obtained and they are used to give applications to
Iwasawa type spaces. In the class of D'Atri spaces of Iwasawa type, we show two
different characterizations of the symmetric spaces of noncompact type: they
are exactly the -spaces and on the other hand they are -D'Atri
spaces for some In the last case, they are -D'Atri for all
as well. In particular, Damek-Ricci spaces that are -D'Atri
for some are symmetric.
Finally, we characterize -D'Atri spaces for all as the -spaces (geodesic symmetries preserve the principal curvatures of
small geodesic spheres). Moreover, applying this result in the case of 4%
-dimensional homogeneous spaces we prove that the properties of being a D'Atri
(1-D'Atri) space, or a 3-D'Atri space, are equivalent to the property of being
a -D'Atri space for all .Comment: 19 pages. This paper substitute the previous one where one Theorem
has been deleted and one section has been adde
- …