469 research outputs found
Observations and asteroseismological analysis of the rapid subdwarf B pulsator EC 09582-1137
We made photometric and spectroscopic observations of the rapidly pulsating
subdwarf B star EC 09582-1137 with the aim of determining the target's
fundamental structural parameters from asteroseismology. The new data comprise
~ 30 hours of fast time-series photometry obtained with SUSI2 at the NTT on La
Silla, Chile, as well as 1 hour of low-resolution spectroscopy gathered with
EMMI, also mounted on the NTT. From the photometry we detected 5 independent
harmonic oscillations in the 135-170 s period range with amplitudes up to 0.5%
of the mean brightness of the star. In addition, we extracted two periodicities
interpreted as components of a rotationally split multiplet that indicate a
rotation period of the order of 2-5 days. We also recovered the first harmonic
of the dominant pulsation, albeit at an amplitude below the imposed 4-sigma
detection threshold. The spectroscopic observations led to the following
estimates of the atmospheric parameters of EC 09582-1137: Teff = 34,806+-233 K,
log g = 5.80+-0.04, and log[N(He)/N(H)] = - 1.68+-0.06.
Using the observed oscillations as input, we searched in model parameter
space for unique solutions that present a good fit to the data. Under the
assumption that the two dominant observed periodicities correspond to radial or
dipole modes, we were able to isolate a well-constrained optimal model that
agrees with the atmospheric parameters derived from spectroscopy. The inferred
structural parameters of EC 09582-1137 are Teff = 34,806 K (from spectroscopy),
log g = 5.788+-0.004, M = 0.485+-0.011 M_solar, log(M_env/M_star) = -
4.39+-0.10, R = 0.147+-0.002 R_solar, and L = 28.6+-1.7 L_solar. We
additionally derive the absolute magnitude M_V = 4.44+-0.05 and the distance d
= 1460+-66 pc.Comment: accepted for publication in Astronomy and Astrophysic
PG 1613+426: a new sdB pulsator
We report the detection of short period oscillations in the hot subdwarf B
(sdB) star PG 1613+426 from time-series photometry carried out with the 91-cm
Cassegrain telescope of the Catania Astrophysical Observatory. This star, which
is brighter than the average of the presently known sdB pulsators, with B =
14.14 mag, has and , its position
is near the hot end of the sdB instability strip, and it is a pulsator with a
well observed peak in the power spectrum at . This star
seems to be well suited for high precision measurements, which could detect a
possible multi-mode pulsation behaviourComment: 3 pages, 4 figures. to appear on A&
Structure and evolution of pulsating hot subdwarfs
Hot subdwarfs are evolved low--mass stars that have survived core helium
ignition and are now in (or recently finished with) the core helium burning
stage. At the hot end of the Horizontal Branch (HB), many of these stars are
multiperiodic pulsators. These pulsations have revealed details of their global
and internal structure, and provide important constraints on the origin of hot
HB stars. While many features of their structure deduced from seismic fits have
confirmed what we expected from evolutionary considerations, there have been
some surprises as well.Comment: 6 pages, invited review from Fourth HELAS International Conference:
Seismological Challenges for Stellar Structure (February 2010
Mode identification from monochromatic amplitude and phase variations for the rapidly pulsating subdwarf B star EC 20338-1925
We obtain time-series spectrophotometry observations at the VLT with the aim
of partially identifying the dominant oscillation modes in the rapidly
pulsating subdwarf B star EC 20338-1925 on the basis of monochromatic amplitude
and phase variations. From the data gathered, we detect four previously known
pulsations with periods near 147, 168, 126 and 140 s and amplitudes between 0.2
and 2.3 % of the star's mean brightness. We also determine the atmospheric
parameters of EC 20338-1925 by fitting our non-LTE model atmospheres to an
averaged combined spectrum. The inferred parameters are Teff = 34,153+-94 K,
log g =5.966+-0.017 and log[N(He)/N(H)] = - 1.642+-0.022, where the uncertainty
estimates quoted refer to the formal fitting errors. Finally, we calculate the
observed monochromatic amplitudes and phases for the periodicities extracted
using least-squares fitting to the light curves obtained for each wavelength
bin. These observed quantities are then compared to the corresponding
theoretical values computed on the basis of dedicated model atmosphere codes
and also taking into account non-adiabatic effects. We find that the quality of
the data is sufficient to identify the dominant pulsation at 146.9 s as a
radial mode, while two of the lower amplitude periodicities must be low-degree
modes with l=0-2. This is the first time that monochromatic amplitudes and
phases have been used for mode identification in a subdwarf B star, and the
results are highly encouraging.Comment: 11 pages. Accepted for publication in Astronomy & Astrophysic
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
Study of sdO models. Pulsation Analysis
We have explored the possibility of driving pulsation modes in models of sdO
stars in which the effects of element diffusion, gravitational settling and
radiative levitation have been neglected so that the distribution of iron-peak
elements remains uniform throughout the evolution. The stability of these
models was determined using a non-adiabatic oscillations code. We analysed 27
sdO models from 16 different evolutionary sequences and discovered the first
ever sdO models capable of driving high-radial order g-modes. In one model, the
driving is by a classical kappa-mechanism due to the opacity bump from
iron-peak elements at temperature ~200,000 K. In a second model, the driving
result from the combined action of kappa-mechanisms operating in three distinct
regions of the star: (i) a carbon-oxygen partial ionization zone at temperature
~2 10^6 K, (ii) a deeper region at temperature ~2 10^7 K, which we attribute to
ionization of argon, and (iii) at the transition from radiative to conductive
opacity in the core of the star.Comment: 13 pages, 19 figures, accepted for publication in MNRAS, 2009
September 1
First Kepler results on compact pulsators II: KIC 010139564, a new pulsating subdwarf B (V361 Hya) star with an additional low-frequency mode
We present the discovery of nonradial pulsations in a hot subdwarf B star
based on 30.5 days of nearly continuous time-series photometry using the
\emph{Kepler} spacecraft. KIC 010139564 is found to be a short-period pulsator
of the V361 Hya (EC 14026) class with more than 10 independent pulsation modes
whose periods range from 130 to 190 seconds. It also shows one periodicity at a
period of 3165 seconds. If this periodicity is a high order g-mode, then this
star may be the hottest member of the hybrid DW Lyn stars. In addition to the
resolved pulsation frequencies, additional periodic variations in the light
curve suggest that a significant number of additional pulsation frequencies may
be present. The long duration of the run, the extremely high duty cycle, and
the well-behaved noise properties allow us to explore the stability of the
periodic variations, and to place strong constraints on how many of them are
independent stellar oscillation modes. We find that most of the identified
periodicities are indeed stable in phase and amplitude, suggesting a rotation
period of 2-3 weeks for this star, but further observations are needed to
confirm this suspicion.Comment: 10 pages, accepted for publication in MNRA
Horizontal Branch evolution, metallicity and sdB stars
Context. Abundance anomalies have been observed in field sdB stars and in
nearly all Horizontal Branch (HB) stars of globular clusters with Teff > 11
000K whatever be the cluster metallicity. Aims. The aim is to determine the
abundance variations to be expected in sdB stars and in HB stars of
metallicities Z \geq 0.0001 and what observed abundances teach us about
hydrodynamical processes competing with atomic diffusion. Methods. Complete
stellar evolution models, including the effects of atomic diffusion and
radiative acceleration, have been computed from the zero age main-sequence for
metallicities of Z0 = 0.0001, 0.001, 0.004 and 0.02. On the HB the masses were
selected to cover the Teff interval from 7000 to 37000K. Some 60 evolutionary
HB models were calculated. The calculations of surface abundance anomalies
during the horizontal branch depend on one parameter, the surface mixed mass.
Results. For sdB stars with Teff 11 000K
in all observed clusters, independent of metallicity, it was found that most
observed abundance anomalies (even up to ~ x 200) were compatible, within error
bars, with expected abundances. A mixed mass of ~1.E-7 M\odot was determined by
comparison with observations. Conclusions. Observations of globular cluster HB
stars with Teff > 11 000K and of sdB stars with Teff < 37 000K suggest that
most observed abundance anomalies can be explained by element separation driven
by radiative acceleration occuring at a mass fraction of ~1.E-7 M\odot. Mass
loss or turbulence appear to limit the separation between 1.E-7 M\odot and the
surface.Comment: Accepted for publication by A&
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