3,260 research outputs found
Asteroseismology and evolution of EHB stars
The properties of the Extreme Horizontal Branch stars are quite well
understood, but much uncertainty surrounds the many paths that bring a star to
this peculiar configuration. Asteroseismology of pulsating EHB stars has been
performed on a number of objects, bringing us to the stage where comparisons of
the inferred properties with evolutionary models becomes feasible. In this
review I outline our current understanding of the formation and evolution of
these stars, with emphasis on recent progress. The aim is to show how the
physical parameters derived by asteroseismology can enable the discrimination
between different evolutionary models.Comment: 13 pages, 6 figures, invited review to appear in Communications in
Asteroseismology vol.159, "Proceedings of the JENAM 2008 Symposium No 4:
Asteroseismology and Stellar Evolution
Indications of a Large Fraction of Spectroscopic Binaries Among Nuclei of Planetary Nebulae
Previous work indicates that about 10% of planetary-nebula nuclei (PNNi) are
photometrically variable short-period binaries with periods of hours to a few
days. These systems have most likely descended from common-envelope (CE)
interactions in initially much wider binaries. Population-synthesis studies
suggest that these very close pairs could be the short-period tail of a much
larger post-CE binary population with periods of up to a few months. We have
initiated a radial-velocity (RV) survey of PNNi with the WIYN 3.5-m telescope
and Hydra spectrograph, which is aimed at discovering these intermediate-period
binaries. We present initial results showing that 10 out of 11 well-observed
PNNi have variable RVs, suggesting that a significant binary population may be
present. However, further observations are required because we have as yet been
unable to fit our sparse measurements with definite orbital periods, and
because some of the RV variability might be due to variations in the stellar
winds of some of our PNNi.Comment: 11 pages, 1 table, no figures. Accepted by the Astrophysical Journal
Letter
Search for sdB/WD pulsators in the Kepler FOV
In this article we present the preliminary results of an observational search
for subdwarf B and white dwarf pulsators in the Kepler field of view, performed
using the DOLORES camera attached to the 3.6m Telescopio Nazionale Galileo
(TNG).Comment: Communications in Asteroseismology, in press; 2 pages, 1 figur
The Hvar survey for roAp stars: II. Final results (Research Note)
The 60 known rapidly oscillating Ap (roAp) stars are excellent laboratories
to test pulsation models in the presence of stellar magnetic fields. Our survey
is dedicated to search for new group members in the Northern Hemisphere. We
attempt to increase the number of known chemically peculiar stars that are
known to be pulsationally unstable. About 40 h of new CCD photometric data of
21 roAp candidates, observed at the 1m Austrian-Croatian Telescope (Hvar
Observatory) are presented. We carefully analysed these to search for
pulsations in the frequency range of up to 10mHz. No new roAp star was detected
among the observed targets. The distribution of the upper limits for roAp-like
variations is similar to that of previoius similar efforts using
photomultipliers and comparable telescope sizes. In addition to photometric
observations, we need to consolidate spectroscopic information to select
suitable targets.Comment: 8 pages, 5 figures, accepted by Astronomy & Astrophysic
The role of turbulent pressure as a coherent pulsational driving mechanism: the case of the delta Scuti star HD 187547
HD 187547 was the first candidate that led to the suggestion that solar-like
oscillations are present in delta Scuti stars. Longer observations, however,
show that the modes interpreted as solar-like oscillations have either very
long mode lifetimes, longer than 960 days, or are coherent. These results are
incompatible with the nature of `pure' stochastic excitation as observed in
solar-like stars. Nonetheless, one point is certain: the opacity mechanism
alone cannot explain the oscillation spectrum of HD 187547. Here we present new
theoretical investigations showing that convection dynamics can intrinsically
excite coherent pulsations in the chemically peculiar delta Scuti star HD
187547. More precisely, it is the perturbations of the mean Reynold stresses
(turbulent pressure) that drives the pulsations and the excitation takes place
predominantly in the hydrogen ionization zone.Comment: 8 pages, 4 figures, accepted to Ap
Searching for solar-like oscillations in the delta Scuti star rho Puppis
Despite the shallow convective envelopes of delta Scuti pulsators, solar-like
oscillations are theoretically predicted to be excited in those stars as well.
To search for such stochastic oscillations we organised a spectroscopic
multi-site campaign for the bright, metal-rich delta Sct star rho Puppis. We
obtained a total of 2763 high-resolution spectra using four telescopes. We
discuss the reduction and analysis with the iodine cell technique, developed
for searching for low-amplitude radial velocity variations, in the presence of
high-amplitude variability. Furthermore, we have determined the angular
diameter of rho Puppis to be 1.68 \pm 0.03 mas, translating into a radius of
3.52 \pm 0.07Rsun. Using this value, the frequency of maximum power of possible
solar-like oscillations, is expected at ~43 \pm 2 c/d (498 \pm 23 muHz). The
dominant delta Scuti-type pulsation mode of rho Puppis is known to be the
radial fundamental mode which allows us to determine the mean density of the
star, and therefore an expected large frequency separation of 2.73 c/d (31.6
muHz). We conclude that 1) the radial velocity amplitudes of the delta Scuti
pulsations are different for different spectral lines; 2) we can exclude
solar-like oscillations to be present in rho Puppis with an amplitude per
radial mode larger than 0.5 m/s.Comment: 14 pages, 12 figure, accepted for MNRA
The unusual pulsation spectrum of the cool ZZ Ceti star HS 0507+0434B
We present the analysis of one week of single-site high-speed CCD photometric
observations of the cool ZZ Ceti star HS 0507+0434B. Ten independent
frequencies are detected in the star's light variations: one singlet and three
nearly-equally spaced triplets. We argue that these triplets are due to
rotationally split modes of spherical degree l=1. This is the first detection
of consistent multiplet structure in the amplitude spectrum of a cool ZZ Ceti
star and it allows us to determine the star's rotation period: 1.70 +/- 0.11 d.
We report exactly equal frequency, not period, spacings between the detected
mode groups. In addition, certain pairs of modes from the four principal groups
have frequency ratios which are very close to 3:4 or 4:5; while these ratios
are nearly exact (within one part in 10^4), they still lie outside the computed
error bars. We speculate that these relationships between different frequencies
could be caused by resonances. One of the three triplets may not be constant in
amplitude and/or frequency.
We compare our frequency solution for the combination frequencies (of which
we detected 38) to Wu's (1998, 2001) model thereof. We obtain consistent
results when trying to infer the star's convective thermal time and the
inclination angle of its rotational axis. Theoretical combination-frequency
amplitude spectra also resemble those of the observations well, and direct
theoretical predictions of the observed second-order light-curve distortions
were also reasonably successful assuming the three triplets are due to l=1
modes. Attempts to reproduce the observed combination frequencies adopting all
possible l=2 identifications for the triplets did not provide similarly
consistent results, supporting their identification with l=1.Comment: Accepted for publication in MNRAS; 12 pages, 8 figure
HD 24355 observed by the Kepler K2 mission: a rapidly oscillating Ap star pulsating in a distorted quadrupole mode
We present an analysis of the first Kepler K2 mission observations of a rapidly oscillating Ap (roAp) star, HD 24355 (V = 9.65). The star was discovered in SuperWASP broad-band photometry with a frequency of 224.31 d−1 (2596.18 μHz; P = 6.4 min) and an amplitude of
1.51 mmag, with later spectroscopic analysis of low-resolution spectra showing HD 24355 to
be an A5 Vp SrEu star. The high-precision K2 data allow us to identify 13 rotationally split sidelobes to the main pulsation frequency of HD 24355. This number of sidelobes combined with an unusual rotational phase variation show this star to be the most distorted quadrupole roAp pulsator yet observed. In modelling this star, we are able to reproduce well the amplitude modulation of the pulsation, and find a close match to the unusual phase variations. We show this star to have a pulsation frequency higher than the critical cut-off frequency. This is currently the only roAp star observed with the Kepler spacecraft in short cadence mode that has a photometric amplitude detectable from the ground, thus allowing comparison between the mmag amplitude ground-based targets and the μmag spaced-based discoveries. No further pulsation modes are identified in the K2 data, showing this star to be a single-mode pulsator
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