1,568 research outputs found
Masses of subgiant stars from asteroseismology using the coupling strengths of mixed modes
Since few decades, asteroseismology, the study of stellar oscillations,
enables us to probe the interiors of stars with great precision. It allows
stringent tests of stellar models and can provide accurate radii, masses and
ages for individual stars. Of particular interest are the mixed modes that
occur in subgiant solar-like stars since they can place very strong constraints
on stellar ages. Here we measure the characteristics of the mixed modes,
particularly the coupling strength, using a grid of stellar models for stars
with masses between 0.9 and 1.5 M_{\odot}. We show that the coupling strength
of the mixed modes is predominantly a function of stellar mass and
appears to be independent of metallicity. This should allow an accurate mass
evaluation, further increasing the usefulness of mixed modes in subgiants as
asteroseismic tools.Comment: 7 pages, 4 figures, 1 table, Accepted (ApJL
The Orbit of the Binary Star Delta Scorpii
Although delta Scorpii is a bright and well-studied star, the details of its
multiplicity have remained unclear. Here we present the first
diffraction-limited image of this 0.12 arcsec binary star, made using optical
interferometry, and resolve the confusion that has existed in the literature
over its multiplicity.
Examining published speckle measurements, together with the present result,
reveals a periodicity of 10.5 yr and allows calculation of the orbital
parameters. The orbit has a high eccentricity (e=0.82) and large inclination
(i=70 degrees), making it a favourable target for radial velocity measurements
during the next periastron (in 2000).Comment: (LaTex 8 pages. Send email to '[email protected]' for 4 postscript
figures). To appear in the Astronomical Journal, August, 1993. ESO-TR
Correcting stellar oscillation frequencies for near-surface effects
In helioseismology, there is a well-known offset between observed and
computed oscillation frequencies. This offset is known to arise from improper
modeling of the near-surface layers of the Sun, and a similar effect must occur
for models of other stars. Such an effect impedes progress in asteroseismology,
which involves comparing observed oscillation frequencies with those calculated
from theoretical models. Here, we use data for the Sun to derive an empirical
correction for the near-surface offset, which we then apply three other stars
(alpha Cen A, alpha Cen B and beta Hyi). The method appears to give good
results, in particular providing an accurate estimate of the mean density of
each star.Comment: accepted for publication in ApJ Letter
Red variables in the OGLE-II database. I. Pulsations and period-luminosity relations below the tip of the Red Giant Branch of the LMC
We present period-luminosity relations for more than 23,000 red giants in the
Large Magellanic Cloud observed by the OGLE-II microlensing project. The OGLE
period values were combined with the 2MASS single-epoch JHK photometric data.
For the brighter stars we find agreement with previous results (four different
sequences corresponding to different modes of pulsation in AGB stars). We also
discovered two distinct and well-separated sequences below the tip of the Red
Giant Branch. They consist of almost 10,000 short-period (15 d <P< 50 d),
low-amplitude (A_I<0.04 mag) red variable stars, for which we propose that a
significant fraction is likely to be on the Red Giant Branch, showing radial
pulsations in the second and third overtone modes. The excitation mechanism
could be either Mira-like pulsation or solar-like oscillations driven by
convection.Comment: 5 pages, 4 figures; accepted for publication in MNRAS (Pink Pages);
proof corrections adde
Measurements of Stellar Properties through Asteroseismology: A Tool for Planet Transit Studies
Oscillations occur in stars of most masses and essentially all stages of
evolution. Asteroseismology is the study of the frequencies and other
properties of stellar oscillations, from which we can extract fundamental
parameters such as density, mass, radius, age and rotation period. We present
an overview of asteroseismic analysis methods, focusing on how this technique
may be used as a tool to measure stellar properties relevant to planet transit
studies. We also discuss details of the Kepler Asteroseismic Investigation --
the use of asteroseismology on the Kepler mission in order to measure basic
stellar parameters. We estimate that applying asteroseismology to stars
observed by Kepler will allow the determination of stellar mean densities to an
accuracy of 1%, radii to 2-3%, masses to 5%, and ages to 5-10% of the
main-sequence lifetime. For rotating stars, the angle of inclination can also
be determined.Comment: To appear in the Proceedings of the 253rd IAU Symposium: "Transiting
Planets", May 2008, Cambridge, M
Solar-like oscillations in the metal-poor subgiant nu Indi: II. Acoustic spectrum and mode lifetime
Convection in stars excites resonant acoustic waves which depend on the sound
speed inside the star, which in turn depends on properties of the stellar
interior. Therefore, asteroseismology is an unrivaled method to probe the
internal structure of a star. We made a seismic study of the metal-poor
subgiant star nu Indi with the goal of constraining its interior structure. Our
study is based on a time series of 1201 radial velocity measurements spread
over 14 nights obtained from two sites, Siding Spring Observatory in Australia
and ESO La Silla Observatory in Chile. The power spectrum of the high precision
velocity time series clearly presents several identifiable peaks between 200
and 500 uHz showing regularity with a large and small spacing of 25.14 +- 0.09
uHz and 2.96 +- 0.22 uHz at 330 uHz. Thirteen individual modes have been
identified with amplitudes in the range 53 to 173 cm/s. The mode damping time
is estimated to be about 16 days (1-sigma range between 9 and 50 days),
substantially longer than in other stars like the Sun, the alpha Cen system or
the giant xi Hya.Comment: 5 pages, 7 figures, A&A accepte
- …