335 research outputs found
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
Pre-main sequence stars in the Cepheus flare region
We present results of optical spectroscopic and BVR_CI_C photometric
observations of 77 pre-main sequence (PMS) stars in the Cepheus flare region. A
total of 64 of these are newly confirmed PMS stars, originally selected from
various published candidate lists. We estimate effective temperatures and
luminosities for the PMS stars, and comparing the results with pre-main
sequence evolutionary models we estimate stellar masses of 0.2-2.4M_sun and
stellar ages of 0.1-15 Myr. Among the PMS stars, we identify 15 visual binaries
with separations of 2-10 arcsec. From archival IRAS, 2MASS, and Spitzer data,
we construct their spectral energy distributions and classify 5% of the stars
as Class I, 10% as Flat SED, 60% as Class II, and 3% as Class III young stellar
objects (YSOs). We identify 12 CTTS and 2 WTTS as members of NGC 7023, with
mean age of 1.6 Myr. The 13 PMS stars associated with L1228 belong to three
small aggregates: RNO 129, L1228A, and L1228S. The age distribution of the 17
PMS stars associated with L1251 suggests that star formation has propagated
with the expansion of the Cepheus flare shell. We detect sparse aggregates of
6-7 Myr old PMS stars around the dark clouds L1177 and L1219, at a distance of
400 pc. Three T Tauri stars appear to be associated with the Herbig Ae star SV
Cep at a distance of 600 pc. Our results confirm that the molecular complex in
the Cepheus flare region contains clouds of various distances and star forming
histories.Comment: 61 pages, 27 figures, 8 tables; accepted for publication by ApJ
The sdB pulsating star V391 Peg and its putative giant planet revisited after 13 years of time-series photometric data
V391 Peg (alias HS 2201+2610) is a subdwarf B (sdB) pulsating star that shows both p- and g-modes. By studying the arrival times
of the p-mode maxima and minima through the O–C method, in a previous article the presence of a planet was inferred with an
orbital period of 3.2 years and a minimum mass of 3.2 MJup. Here we present an updated O–C analysis using a larger data set of
1066 h of photometric time series (∼2.5× larger in terms of the number of data points), which covers the period between 1999 and 2012
(compared with 1999–2006 of the previous analysis). Up to the end of 2008, the new O–C diagram of the main pulsation frequency (f1)
is compatible with (and improves) the previous two-component solution representing the long-term variation of the pulsation period
(parabolic component) and the giant planet (sine wave component). Since 2009, the O–C trend of f1 changes, and the time derivative
of the pulsation period (p˙) passes from positive to negative; the reason of this change of regime is not clear and could be related to
nonlinear interactions between different pulsation modes. With the new data, the O–C diagram of the secondary pulsation frequency
(f2) continues to show two components (parabola and sine wave), like in the previous analysis. Various solutions are proposed to fit
the O–C diagrams of f1 and f2, but in all of them, the sinusoidal components of f1 and f2 differ or at least agree less well than before.
The nice agreement found previously was a coincidence due to various small effects that are carefully analyzed. Now, with a larger
dataset, the presence of a planet is more uncertain and would require confirmation with an independent method. The new data allow
us to improve the measurement of p˙ for f1 and f2: using only the data up to the end of 2008, we obtain p˙ 1 = (1.34 ± 0.04) × 10−12 and
p˙ 2 = (1.62 ± 0.22) × 10−12. The long-term variation of the two main pulsation periods (and the change of sign of p˙ 1) is visible also in
direct measurements made over several years. The absence of peaks near f1 in the Fourier transform and the secondary peak close to
f2 confirm a previous identification as l = 0 and l = 1, respectively, and suggest a stellar rotation period of about 40 days. The new data
allow constraining the main g-mode pulsation periods of the star
Time-resolved spectroscopy of the planet-hosting sdB pulsator V391 Pegasi
The subdwarf B (sdB) star V391 Peg oscillates in short-period p modes and
long-period g modes, making it one of the three known hybrids among sdBs. As a
by-product of the effort to measure secular period changes in the p modes due
to evolutionary effects on a time scale of almost a decade, the O-C diagram has
revealed an additional sinusoidal component attributed to a periodic shift in
the light travel time caused by a planetary-mass companion around the sdB star
in a 3.2 yr orbit. In order to derive the mass of the companion object, it is
necessary to determine the orbital inclination. One promising possibility to do
this is to use the stellar inclination as a primer for the orbital orientation.
The stellar inclination can refer to the rotational or the pulsational axis,
which are assumed to be aligned, and can in turn then be derived by combining
measurements of v_(rot) and v_(rot)sin i. The former is in principle accessible
through rotational splitting in the photometric frequency spectrum (which has
however not been found for V391 Peg yet), while the projected rotational
velocity can be measured from the rotational broadening of spectral lines. The
latter must be deconvolved from the additional pulsational broadening caused by
the surface radial velocity variation in high S/N phase averaged spectra. This
work gives limits on pulsational radial velocities from a series of phase
resolved spectra. Phase averaged and phase resolved high resolution echelle
spectra were obtained in May and September 2007 with the 9m-class Hobby-Eberly
Telescope (HET), and one phase averaged spectrum in May 2008 with the 10m-Keck
1 telescope.Comment: 3 pages, JENAM 2008 proceedings, to be published in 'Communications
in Asteroseismology', 15
RAT J0455+1305: A rare hybrid pulsating subdwarf B star
We present results on the second-faintest pulsating subdwarf B (sdB) star
known, RAT J0455+1305, derived from photometric data obtained in 2009. It shows
both short and long periods oscillations, theoretically assigned as pressure
and gravity modes. We identify six short-period frequencies (with one being a
combination) and six long-period frequencies. This star is the fourth hybrid
sdB star discovered so far which makes it of special interest as each type of
mode probes a different part of the star. This star is similar to the sdB
hybrid pulsator Balloon 090100001 in that it exhibits short-period mode
groupings, which can be used to identify pulsation parameters and constrain
theoretical models.Comment: published in MNRA
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