233 research outputs found
Detection of accretion X-rays from QS Vir: cataclysmic or a lot of hot air?
An XMM-Newton observation of the nearby "pre-cataclysmic" short-period (P_orb
= 3.62 hr) binary QS Vir (EC 13471-1258) revealed regular narrow X-ray eclipses
when the white dwarf passed behind its M2-4 dwarf companion. The X-ray emission
provides a clear signature of mass transfer and accretion onto the white dwarf.
The low-resolution XMM-Newton EPIC spectra are consistent with a cooling flow
model and indicate an accretion rate of Mdot= 1.7\times10^-13M\odot/yr. At 48
pc distant, QS Vir is then the second nearest accreting cataclysmic variable
known, with one of the lowest accretion rates found to date for a non-magnetic
system. To feed this accretion through a wind would require a wind mass loss
rate of Mdot ~ 2 \times 10^-12M\odot/yr if the accretion efficiency is of the
order of 10%. Consideration of likely mass loss rates for M dwarfs suggests
this is improbably high and pure wind accretion unlikely. A lack of accretion
disk signatures also presents some difficulties for direct Roche lobe overflow.
We speculate that QS Vir is on the verge of Roche lobe overflow, and that the
observed mass transfer could be supplemented by upward chromospheric flows on
the M dwarf, analogous to spicules and mottles on the Sun, that escape the
Roche surface to be subsequently swept up into the white dwarf Roche lobe. If
so, QS Vir would be in a rare evolutionary phase lasting only a million years.
The X-ray luminosity of the M dwarf estimated during primary eclipse is L_X = 3
\times 10^28 erg/s, which is consistent with that of rapidly rotating
"saturated" K and M dwarfs.Comment: ApJ in pres
The masses of hot subdwarfs
Masses are a fundamental parameter, but they are not well known for most hot
subdwarfs. In general, the mass of a hot subdwarf is derived with
asteroseismology or dynamical methods, for which it is often difficult to
obtain the necessary data from observations. We intend to find an approach to
deriving the masses of hot subdwarfs from observational data in the literature.
We presented full evolutionary calculations for hot subdwarfs in a wide mass
range (0.33 to 1.4 ) for a Population I metallicity of
=0.02, and obtained a relation between and , where , , and are the most probable
mass, effective temperature, and gravity. This relation is used to study the
masses of some observed hot subdwarfs. We proposed a method of determining the
masses of hot subdwarfs. Using this method, we studied the masses of hot
subdwarfs from the ESO supernova Ia progenitor survey and Hamburg quasar
survey. The study shows that most of subdwarf B stars have masses between 0.42
and 0.54 , whilst most sdO stars are in the range 0.40 0.55
. Comparing our study to the theoretical mass distributions of Han et
al. (2003), we found that sdO stars with mass less than 0.5
may evolve from sdB stars, whilst most high-mass( 0.5 ) sdO stars
result from mergers directly.Comment: 5 pages, 6 figures, accepted for publication in A&A Letter
A photometric and spectroscopic study of NSVS 14256825: the second sdOB+dM eclipsing binary
We present an analysis of UBVRIJH photometry and
phase-resolved optical spectroscopy of NSVS 14256825, an HW Vir type binary.
The members of this class consist of a hot subdwarf and a main-sequence
low-mass star in a close orbit ( d). Using the
primary-eclipse timings, we refine the ephemeris for the system, which has an
orbital period of 0.11037 d. From the spectroscopic data analysis, we derive
the effective temperature, K, the surface gravity, , and the helium abundance, , for the hot component. Simultaneously modelling the
photometric and spectroscopic data using the Wilson-Devinney code, we obtain
the geometrical and physical parameters of NSVS 14256825. Using the fitted
orbital inclination and mass ratio (i = 82\fdg5\pm0\fdg3 and , respectively), the components of the system have , , , and . From its spectral
characteristics, the hot star is classified as an sdOB star.Comment: 8 pages, 7 figures, accepted for publication in MNRA
Connecting planets around horizontal branch stars with known exoplanets
We study the distribution of exoplanets around main sequence (MS) stars and
apply our results to the binary model for the formation of extreme horizontal
branch (EHB; sdO; sdB; hot subdwarfs) stars. By Binary model we refer both to
stellar and substellar companions that enhance the mass loss rate, where
substellar companions stand for both massive planets and brown dwarfs. We
conclude that sdB (EHB) stars are prime targets for planet searches. We reach
this conclusion by noticing that the bimodal distribution of planets around
stars with respect to the parameter M_p*a^2, is most prominent for stars in the
mass range 1Mo < M < 1.5Mo; 'a' is the orbital separation, 'M' is the stellar
mass and 'M_p' the planet mass. This is also the mass range of the progenitors
of EHB stars that are formed through the interaction of their progenitors with
planets (assuming the EHB formation mechanism is the binary model). In the
binary model for the formation of EHB stars interaction with a binary companion
or a substellar object (a planet or a brown dwarf), causes the progenitor to
lose most of its envelope mass during its red giant branch (RGB) phase. As a
result of that the descendant HB star is hot, i.e., an EHB (sdB) star. The
bimodal distribution suggests that even if the close-in planet that formed the
EHB star did not survive its RGB common envelope evolution, one planet or more
might survive at a>1AU. Also, if a planet or more are observed at a>1AU, it is
possible that a closer massive planet did survive the common envelope phase,
and it is orbiting the EHB with an orbital period of hours to days.Comment: MNRAS, in pres
Observations and asteroseismic analysis of the rapidly pulsating hot B subdwarf PG 0911+456
The principal aim of this project is to determine the structural parameters
of the rapidly pulsating subdwarf B star PG 0911+456 from asteroseismology. Our
work forms part of an ongoing programme to constrain the internal
characteristics of hot B subdwarfs with the long-term goal of differentiating
between the various formation scenarios proposed for these objects. First
comparisons of asteroseismic values with evolutionary theory look promising,
however it is clear that more targets are needed for meaningful statistics to
be derived. The observational pulsation periods of PG 0911+456 were extracted
from rapid time-series photometry using standard Fourier analysis techniques.
Supplemented by spectroscopic estimates of the star's mean atmospheric
parameters, they were used as a basis for the "forward modelling" approach in
asteroseismology. The latter culminates in the identification of one or more
"optimal" models that can accurately reproduce the observed period spectrum.
This naturally leads to an identification of the oscillations detected in terms
of degree l and radial order k, and infers the structural parameters of the
target. From the photometry it was possible to extract 7 independent pulsation
periods in the 150-200 s range with amplitudes between 0.05 and 0.8 % of the
star's mean brightness. An asteroseismic search of parameter space identified
several models that matched the observed properties of PG 0911+456 well, one of
which was isolated as the "optimal" model on the basis of spectroscopic and
mode identification considerations. All the observed pulsations are identified
with low-order acoustic modes with degree indices l=0,1,2 and 4, and match the
computed periods with a dispersion of only ~0.26 %.Comment: accepted for publication in A&A, 14 pages, 13 figure
The binary properties of the pulsating subdwarf B eclipsing binary PG 1336-018 (NY Virginis)
Aims. We present an unbiased orbit solution and mass determination of the components of the eclipsing binary PG1336−018 as a critical test for the formation scenarios of subdwarf B stars.
Methods. We obtained high-resolution time series VLT/UVES spectra and high-speed multicolour VLT/ULTRACAM photometric
observations of PG1336−018, a rapidly pulsating subdwarf B star in a short period eclipsing binary.
Results. Combining the radial velocity curve obtained from the VLT/UVES spectra with the VLT/ULTRACAM multicolour
lightcurves, we determined numerical orbital solutions for this eclipsing binary. Due to the large number of free parameters and their strong correlations, no unique solution could be found, only families of solutions. We present three solutions of equal statistical
significance, two of which are compatible with the primary having gone through a core He-flash and a common-envelope phase described by the α-formalism. These two models have an sdB primary of 0.466 M and 0.389 M, respectively. Finally, we report the detection of the Rossiter-McLaughlin effect for PG1336−018
An evolutionary study of the pulsating subdwarf B eclipsing binary PG1336-018 (NY Vir)
The formation of subdwarf B (sdB) stars is not well understood within the
current framework of stellar single and binary evolution. In this study, we
focus on the formation and evolution of the pulsating sdB star in the very
short-period eclipsing binary PG1336-018. We aim at refining the formation
scenario of this unique system, so that it can be confronted with observations.
We probe the stellar structure of the progenitors of sdB stars in short-period
binaries using detailed stellar evolution calculations. Applying this to
PG1336-018 we reconstruct the common-envelope phase during which the sdB star
was formed. The results are interpreted in terms of the standard
common-envelope formalism (the alpha-formalism) based on the energy equation,
and an alternative description (the gamma-formalism) using the angular momentum
equation. We find that if the common-envelope evolution is described by the
alpha-formalism, the sdB progenitor most likely experienced a helium flash. We
then expect the sdB mass to be between 0.39 and 0.48 Msun, and the sdB
progenitor initial mass to be below ~2 Msun. However, the results for the
gamma-formalism are less restrictive, and a broader sdB mass range (0.3 - 0.8
Msun) is possible in this case. Future seismic mass determination will give
strong constraints on the formation of PG1336-018 and, in particular, on the CE
phase.Comment: 9 pages, 7 figures, 2 tables, accepted for publication in A&
Spectral analyses of eighteen hot H-deficient (pre-) white dwarfs from the Sloan Digital Sky Survey Data Release 4
Context: The Sloan Digital Sky Survey Data Release 4 has provided spectra of
several new PG 1159 stars and DO white dwarfs. This increase in known hot
H-deficient compact objects significantly improves the statistics and helps to
investigate late stages of stellar evolution. Aims: From the optical SDSS
spectra, effective temperatures and surface gravities are derived in order to
place the observed objects in an evolutionary context. Especially the
connection between PG 1159 stars and DO white dwarfs shall be investigated.
Method: Using our non-LTE model atmospheres and applying chi^2-fitting
techniques, we determine stellar parameters and their errors. We derive total
stellar masses for the DO white dwarfs using model evolutionary tracks.
Results: We confirm three PG 1159 stars, with one showing ultra-high excitation
ion features, and one sdO which we originally classified as a PG 1159 star.
Additionally, we re-analysed the known PG 1159 star, PG 1424+535, with our new
models. Furthermore, we present the first spectral analyses of thirteen DO
white dwarfs, three of which show M-star features in their spectra, while two
display ultra-high excitation ion features.Comment: 9 pages, 6 figures, accepted for publication in A&
Maximum-likelihood estimation of lithospheric flexural rigidity, initial-loading fraction, and load correlation, under isotropy
Topography and gravity are geophysical fields whose joint statistical
structure derives from interface-loading processes modulated by the underlying
mechanics of isostatic and flexural compensation in the shallow lithosphere.
Under this dual statistical-mechanistic viewpoint an estimation problem can be
formulated where the knowns are topography and gravity and the principal
unknown the elastic flexural rigidity of the lithosphere. In the guise of an
equivalent "effective elastic thickness", this important, geographically
varying, structural parameter has been the subject of many interpretative
studies, but precisely how well it is known or how best it can be found from
the data, abundant nonetheless, has remained contentious and unresolved
throughout the last few decades of dedicated study. The popular methods whereby
admittance or coherence, both spectral measures of the relation between gravity
and topography, are inverted for the flexural rigidity, have revealed
themselves to have insufficient power to independently constrain both it and
the additional unknown initial-loading fraction and load-correlation fac- tors,
respectively. Solving this extremely ill-posed inversion problem leads to
non-uniqueness and is further complicated by practical considerations such as
the choice of regularizing data tapers to render the analysis sufficiently
selective both in the spatial and spectral domains. Here, we rewrite the
problem in a form amenable to maximum-likelihood estimation theory, which we
show yields unbiased, minimum-variance estimates of flexural rigidity,
initial-loading frac- tion and load correlation, each of those separably
resolved with little a posteriori correlation between their estimates. We are
also able to separately characterize the isotropic spectral shape of the
initial loading processes.Comment: 41 pages, 13 figures, accepted for publication by Geophysical Journal
Internationa
A binary model for the UV-upturn of elliptical galaxies (MNRAS version)
The discovery of a flux excess in the far-ultraviolet (UV) spectrum of
elliptical galaxies was a major surprise in 1969. While it is now clear that
this UV excess is caused by an old population of hot helium-burning stars
without large hydrogen-rich envelopes, rather than young stars, their origin
has remained a mystery. Here we show that these stars most likely lost their
envelopes because of binary interactions, similar to the hot subdwarf
population in our own Galaxy. We have developed an evolutionary population
synthesis model for the far-UV excess of elliptical galaxies based on the
binary model developed by Han et al (2002, 2003) for the formation of hot
subdwarfs in our Galaxy. Despite its simplicity, it successfully reproduces
most of the properties of elliptical galaxies with a UV excess: the range of
observed UV excesses, both in and , and their evolution
with redshift. We also present colour-colour diagrams for use as diagnostic
tools in the study of elliptical galaxies. The model has major implications for
understanding the evolution of the UV excess and of elliptical galaxies in
general. In particular, it implies that the UV excess is not a sign of age, as
had been postulated previously, and predicts that it should not be strongly
dependent on the metallicity of the population, but exists universally from
dwarf ellipticals to giant ellipticals.Comment: accepted for publication in MNRAS, 24 pages, 15 figures, 2 table
- …