388 research outputs found
Corrected Weight Functions for Stellar Oscillation Eigenfrequencies
Kawaler et al. (1985) present a variational expression for the
eigenfrequencies associated with stellar oscillations. We highlight and correct
a typographical error in the weight functions appearing in these expressions,
and validate the correction numerically.Comment: 3 pages, 1 figure, submitted to RNAA
Evolutionary Timescale of the DAV G117-B15A: The Most Stable Optical Clock Known
We observe G117-B15A, the most precise optical clock known, to measure the
rate of change of the main pulsation period of this blue-edge DAV white dwarf.
Even though the obtained value is only within 1 sigma, Pdot = (2.3 +/- 1.4) x
10^{-15} s/s, it is already constraining the evolutionary timescale of this
cooling white dwarf star.Comment: Accepted for publication in ApJ
Internal rotation of subdwarf B stars: limiting cases and asteroseismological consequences
Observations of the rotation rates of horizontal branch (HB) stars show
puzzling systematics. In particular, cooler HB stars often show rapid rotation
(with velocities in excess of 10 km/s), while hotter HB stars typically show
much smaller rotation velocities. Simple models of angular momentum evolution
of stars from the main sequence through the red giant branch fail to explain
these effects. In general, evolutionary models in all cases preserve a rapidly
rotating core. The observed angular velocities of HB stars require that some of
the angular momentum stored in the core reaches the surface.
To test the idea that HB stars contain such a core, one can appeal to
detailed computations of trace element abundences and rotational mixing.
However, a more direct probe is available to test these limiting cases of
angular momentum evolution. Some of the hottest horizontal branch stars are
members of the pulsating sdB class. They frequently show rich pulsation spectra
characteristic of nonradially pulsating stars. Thus their pulsations probe the
internal rotation of these stars, and should show the effects of rapid rotation
in their cores. Using models of sdB stars that include angular momentum
evolution, we explore this possibility and show that some of the sdB pulsators
may indeed have rapidly rotating cores.Comment: accepted for publication in The Astrophysical Journa
First Kepler results on compact pulsators VIII: Mode identifications via period spacings in mode pulsating Subdwarf B stars
We investigate the possibility of nearly-equally spaced periods in 13 hot
subdwarf B (sdB) stars observed with the Kepler spacecraft and one observed
with CoRoT. Asymptotic limits for gravity (g-)mode pulsations provide
relationships between equal period spacings of modes with differing degrees and
relationships between periods of the same radial order but differing degrees.
Period transforms, Kolmogorov-Smirnov tests, and linear least-squares fits have
been used to detect and determine the significance of equal period spacings. We
have also used Monte Carlo simulations to estimate the likelihood that the
detected spacings could be produced randomly.
Period transforms for nine of the Kepler stars indicate ell=1 period
spacings, with five also showing peaks for ell=2 modes. 12 stars indicate ell=1
modes using the Kolmogorov-Smirnov test while another shows solely ell=2 modes.
Monte Carlo results indicate that equal period spacings are significant in 10
stars above 99% confidence and 13 of the 14 are above 94% confidence. For 12
stars, the various methods find consistent regular period spacing values to
within the errors, two others show some inconsistencies, likely caused by
binarity, and the last has significant detections but the mode assignment
disagrees between methods.
We find a common ell=1 period spacing spanning a range from 231 to 272 s
allowing us to correlate pulsation modes with 222 periodicities and that the
ell=2 period spacings are related to the ell=1 spacings by the asymptotic
relationship . We briefly discuss the impact of equal period
spacings which indicate low-degree modes with a lack of significant mode
trappings.Comment: 27 pages, 4 figures, 17 tables. Accepted for publication in Monthly
Notices of the Royal Astronomical Societ
Seismological constraints on the high-gravity DOV stars PG2131+066 and PG 1707+427
A seismological study of the pulsating PG1159 stars PG2131+066 and PG
1707+427 is presented. We perform extensive adiabatic computations of g-mode
pulsation periods of PG1159 evolutionary models with stellar masses ranging
from 0.530 to 0.741 Msun. We constrain the stellar mass of PG2131+066 and PG
1707+427 by comparing the observed period spacing of each star with the
theoretical asymptotic period spacings and with the average of the computed
period spacings. We also employ the individual observed periods to find
representative seismological models for both stars.Comment: Proceedings, 16th European White Dwarf Workshop, Barcelona, 200
Probing the internal rotation of pre-white dwarf stars with asteroseismology: the case of PG 122+200
We put asteroseismological constraints on the internal rotation profile of
the GW Vir (PG1159-type) star PG 0122+200. To this end we employ a
state-of-the-art asteroseismological model for this star and we assess the
expected frequency splittings induced by rotation adopting a forward approach
in which we compare the theoretical frequency separations with the observed
ones assuming different types of plausible internal rotation profiles. We also
employ two asteroseismological inversion methods for the inversion of the
rotation profile of PG 0122+200. We find evidence for differential rotation in
this star. We demonstrate that the frequency splittings of the rotational
multiplets exhibited by PG 0122+200 are compatible with a rotation profile in
which the central regions are spinning about 2.4 times faster than the stellar
surface.Comment: 8 pages, 6 figures, 2 tables. To be published in MNRA
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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