115 research outputs found
Spatial incoherence of solar granulation: a global analysis using BiSON 2B data
A poor understanding of the impact of convective turbulence in the outer
layers of the Sun and Sun-like stars challenges the advance towards an improved
understanding of their internal structure and dynamics. Assessing and
calibrating these effects is therefore of great importance. Here we study the
spatial coherence of granulation noise and oscillation modes in the Sun, with
the aim of exploiting any incoherence to beat-down observed granulation noise,
hence improving the detection of low-frequency p-modes. Using data from the
BiSON 2B instrument, we assess the coherence between different atmospheric
heights and between different surface regions. We find that granulation noise
from the different atmospheric heights probed is largely incoherent; frequency
regions dominated by oscillations are almost fully coherent. We find a
randomised phase difference for the granulation noise, and a near zero
difference for the evanescent oscillations. A reduction of the incoherent
granulation noise is shown by application of the cross-spectrum.Comment: 8 pages, 7 figures, MNRAS in pres
Testing asteroseismology with Gaia DR2: Hierarchical models of the Red Clump
Asteroseismology provides fundamental stellar parameters independent of
distance, but subject to systematics under calibration. Gaia DR2 has provided
parallaxes for a billion stars, which are offset by a parallax zero-point. Red
Clump (RC) stars have a narrow spread in luminosity, thus functioning as
standard candles to calibrate these systematics. This work measures how the
magnitude and spread of the RC in the Kepler field are affected by changes to
temperature and scaling relations for seismology, and changes to the parallax
zero-point for Gaia. We use a sample of 5576 RC stars classified through
asteroseismology. We apply hierarchical Bayesian latent variable models,
finding the population level properties of the RC with seismology, and use
those as priors on Gaia parallaxes to find the parallax zero-point offset. We
then find the position of the RC using published values for the zero-point. We
find a seismic temperature insensitive spread of the RC of ~0.03 mag in the
2MASS K band and a larger and slightly temperature-dependent spread of ~0.13
mag in the Gaia G band. This intrinsic dispersion in the K band provides a
distance precision of ~1% for RC stars. Using Gaia data alone, we find a mean
zero-point of -41 10 as. This offset yields RC absolute magnitudes
of -1.634 0.018 in K and 0.546 0.016 in G. Obtaining these same
values through seismology would require a global temperature shift of ~-70 K,
which is compatible with known systematics in spectroscopy.Comment: Accepted for publication in MNRA
Are short-term variations in solar oscillation frequencies the signature of a second solar dynamo?
In addition to the well-known 11-year solar cycle, the Sun's magnetic
activity also shows significant variation on shorter time scales, e.g. between
one and two years. We observe a quasi-biennial (2-year) signal in the solar
p-mode oscillation frequencies, which are sensitive probes of the solar
interior. The signal is visible in Sun-as-a-star data observed by different
instruments and here we describe the results obtained using BiSON, GOLF, and
VIRGO data. Our results imply that the 2-year signal is susceptible to the
influence of the main 11-year solar cycle. However, the source of the signal
appears to be separate from that of the 11-year cycle. We speculate as to
whether it might be the signature of a second dynamo, located in the region of
near-surface rotational shear.Comment: 6 pages, 2 figures, proceedings for SOHO-24/GONG 2010 conference, to
be published in JPC
Investigating Gaia EDR3 parallax systematics using asteroseismology of Cool Giant Stars observed by Kepler, K2, and TESS I. Asteroseismic distances to 12,500 red-giant stars
Gaia EDR3 has provided unprecedented data that generate a lot of interest in
the astrophysical community, despite the fact that systematics affect the
reported parallaxes at the level of ~ 10 muas. Independent distance
measurements are available from asteroseismology of red-giant stars with
measurable parallaxes, whose magnitude and colour ranges more closely reflect
those of other stars of interest. In this paper, we determine distances to
nearly 12,500 red-giant branch and red clump stars observed by Kepler, K2, and
TESS. This is done via a grid-based modelling method, where global
asteroseismic observables, constraints on the photospheric chemical
composition, and on the unreddened photometry are used as observational inputs.
This large catalogue of asteroseismic distances allows us to provide a first
comparison with Gaia EDR3 parallaxes. Offset values estimated with
asteroseismology show no clear trend with ecliptic latitude or magnitude, and
the trend whereby they increase (in absolute terms) as we move towards redder
colours is dominated by the brightest stars. The correction model proposed by
Lindegren et al. (2021) is not suitable for all the fields considered in this
study. We find a good agreement between asteroseismic results and model
predictions of the red clump magnitude. We discuss possible trends with the
Gaia scan law statistics, and show that two magnitude regimes exist where
either asteroseismology or Gaia provides the best precision in parallax.Comment: 11 pages, 8 figures, Accepted for publication in A&
Investigating the properties of granulation in the red giants observed by Kepler
More than 1000 red giants have been observed by NASA/Kepler mission during a
nearly continuous period of ~ 13 months. The resulting high-frequency
resolution (< 0.03 muHz) allows us to study the granulation parameters of these
stars. The granulation pattern results from the convection motions leading to
upward flows of hot plasma and downward flows of cooler plasma. We fitted
Harvey-like functions to the power spectra, to retrieve the timescale and
amplitude of granulation. We show that there is an anti-correlation between
both of these parameters and the position of maximum power of acoustic modes,
while we also find a correlation with the radius, which agrees with the theory.
We finally compare our results with 3D models of the convection.Comment: 4 pages, 1 figure. To appear in the ASP proceedings of "The 61st
Fujihara seminar: Progress in solar/stellar physics with helio- and
asteroseismology", 13th-17th March 2011, Hakone, Japa
Testing the Asteroseismic Mass Scale Using Metal-Poor Stars Characterized with APOGEE and Kepler
Fundamental stellar properties, such as mass, radius, and age, can be
inferred using asteroseismology. Cool stars with convective envelopes have
turbulent motions that can stochastically drive and damp pulsations. The
properties of the oscillation frequency power spectrum can be tied to mass and
radius through solar-scaled asteroseismic relations. Stellar properties derived
using these scaling relations need verification over a range of metallicities.
Because the age and mass of halo stars are well-constrained by astrophysical
priors, they provide an independent, empirical check on asteroseismic mass
estimates in the low-metallicity regime. We identify nine metal-poor red giants
(including six stars that are kinematically associated with the halo) from a
sample observed by both the Kepler space telescope and the Sloan Digital Sky
Survey-III APOGEE spectroscopic survey. We compare masses inferred using
asteroseismology to those expected for halo and thick-disk stars. Although our
sample is small, standard scaling relations, combined with asteroseismic
parameters from the APOKASC Catalog, produce masses that are systematically
higher (=0.17+/-0.05 Msun) than astrophysical expectations. The
magnitude of the mass discrepancy is reduced by known theoretical corrections
to the measured large frequency separation scaling relationship. Using
alternative methods for measuring asteroseismic parameters induces systematic
shifts at the 0.04 Msun level. We also compare published asteroseismic analyses
with scaling relationship masses to examine the impact of using the frequency
of maximum power as a constraint. Upcoming APOKASC observations will provide a
larger sample of ~100 metal-poor stars, important for detailed asteroseismic
characterization of Galactic stellar populations.Comment: 4 figures; 1 table. Accepted to ApJ
Calibrating Convective properties of Solar-like Stars in the Kepler Field of View
Stellar models generally use simple parametrizations to treat convection. The
most widely used parametrization is the so-called "Mixing Length Theory" where
the convective eddy sizes are described using a single number, \alpha, the
mixing-length parameter. This is a free parameter, and the general practice is
to calibrate \alpha using the known properties of the Sun and apply that to all
stars. Using data from NASA's Kepler mission we show that using the
solar-calibrated \alpha is not always appropriate, and that in many cases it
would lead to estimates of initial helium abundances that are lower than the
primordial helium abundance. Kepler data allow us to calibrate \alpha for many
other stars and we show that for the sample of stars we have studied, the
mixing-length parameter is generally lower than the solar value. We studied the
correlation between \alpha and stellar properties, and we find that \alpha
increases with metallicity. We therefore conclude that results obtained by
fitting stellar models or by using population-synthesis models constructed with
solar values of \alpha are likely to have large systematic errors. Our results
also confirm theoretical expectations that the mixing-length parameter should
vary with stellar properties.Comment: 16 pages, 4 figures, accepted for publication in ApJ
Solving the mode identification problem in asteroseismology of F stars observed with Kepler
Asteroseismology of F-type stars has been hindered by an ambiguity in
identification of their oscillation modes. The regular mode pattern that makes
this task trivial in cooler stars is masked by increased linewidths. The
absolute mode frequencies, encapsulated in the asteroseismic variable epsilon,
can help solve this impasse because the values of epsilon implied by the two
possible mode identifications are distinct. We find that the correct epsilon
can be deduced from the effective temperature and the linewidths and we apply
these methods to a sample of solar-like oscillators observed with Kepler.Comment: 7 pages, 4 figures, 1 table, accepted for publication in The
Astrophysical Journal Letter
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