25 research outputs found
Isochrones of M67 with an Expanded Set of Parameters
We create isochrones of M67 using the Yale Rotating Stellar Evolution Code.
In addition to metallicity, parameters that are traditionally held fixed, such
as the mixing length parameter and initial helium abundance, also vary. The
amount of convective overshoot is also changed in different sets of isochrones.
Models are constructed both with and without diffusion. From the resulting
isochrones that fit the cluster, the age range is between 3.6 and 4.8 Gyr and
the distance is between 755 and 868 pc. We also confirm Michaud et al. (2004)
claim that M67 can be fit without overshoot if diffusion is included.Comment: 4 pages, 3 figures, to appear in the proceedings of the joint
TASC2/KASC9/SPACEINN/HELAS8 conference "Seismology of the Sun and the Distant
Stars 2016
Changing the Scaling Relation: The Need For a Mean Molecular Weight Term
The scaling relations that relate the average asteroseismic parameters
and to the global properties of stars are used quite
extensively to determine stellar properties. While the scaling
relation has been examined carefully and the deviations from the relation have
been well documented, the scaling relation has not been examined
as extensively. In this paper we examine the scaling relation
using a set of stellar models constructed to have a wide range of mass,
metallicity, and age. We find that as with , does not
follow the simple scaling relation. The most visible deviation is because of a
mean molecular weight term and a term that are commonly ignored. The
remaining deviation is more difficult to address. We find that the influence of
the scaling relation errors on asteroseismically derived values of are
well within uncertainties. The influence of the errors on mass and radius
estimates is small for main sequence and subgiants, but can be quite large for
red giants.Comment: 15 pages, 14 figures, accepted for publication in Ap
Investigating the Metallicity-Mixing Length Relation
Stellar models typically use the mixing length approximation as a way to
implement convection in a simplified manner. While conventionally the value of
the mixing length parameter, , used is the solar calibrated value, many
studies have shown that other values of are needed to properly model
stars. This uncertainty in the value of the mixing length parameter is a major
source of error in stellar models and isochrones. Using asteroseismic data, we
determine the value of the mixing length parameter required to properly model a
set of about 450 stars ranging in , , and
. The relationship between the value of required and
the properties of the star is then investigated. For Eddington atmosphere,
non-diffusion models, we find that the value of can be approximated by
a linear model, in the form of . This process is
repeated using a variety of model physics as well as compared to previous
studies and results from 3D convective simulations.Comment: 20 pages, 17 figures, accepted for publication in Ap
Asteroseismology of the Hyades with K2: first detection of main-sequence solar-like oscillations in an open cluster
The Hyades open cluster was targeted during Campaign 4 (C4) of the NASA K2 mission, and short-cadence data were collected on a number of cool main-sequence stars. Here, we report results on two F-type stars that show detectable oscillations of a quality that allows asteroseismic analyses to be performed. These are the first ever detections of solar-like oscillations in main-sequence stars in an open cluster
Asteroseismology of the Hyades with K2: first detection of main-sequence solar-like oscillations in an open cluster
The Hyades open cluster was targeted during Campaign 4 (C4) of the NASA K2
mission, and short-cadence data were collected on a number of cool
main-sequence stars. Here, we report results on two F-type stars that show
detectable oscillations of a quality that allows asteroseismic analyses to be
performed. These are the first ever detections of solar-like oscillations in
main-sequence stars in an open cluster.Comment: 12 pages, 8 figures, 2 tables; accepted for publication in MNRA
Seismic Constraints on Helium Abundances from the TESS Southern CVZ
Poster for Cool Stars 21
Stellar helium abundances strongly determine their structure and evolution. However, since helium cannot be detected directly in the photospheres of cool stars, helium abundances are one of the most poorly-constrained inputs to stellar models. It is therefore typical to assume a relationship with the initial abundances of other heavy elements, typically of linear form described by a gradient ΔY/ΔZ. Attempts to determine from globular-cluster stellar populations and Galactic H-II regions have so far not yielded any consensus about empirically reasonable values of ΔY/ΔZ, or, for that matter, even whether such a linear relation is observationally justifiable. Separately, asteroseismology permits the inference of stellar helium abundances, either directly through acoustic-glitch measurements, or indirectly through the forward modelling of stellar oscillation mode frequencies. Using constraints on the initial helium abundance derived from ensemble asteroseismology and stellar forward modelling against individual mode frequencies of a collection of field stars in the TESS, Kepler, and K2 fields, we characterise the helium-metallicity relation of the brightest cool stars in the solar neighbourhood. We find a large spread of seismic initial helium abundances for any given metallicity, rather than a single well-defined linear enrichment law
Determining the Best Method of Calculating the Large Frequency Separation For Stellar Models
Asteroseismology of solar-like oscillators often relies on the comparisons
between stellar models and stellar observations in order to determine the
properties of stars. The values of the global seismic parameters,
(the frequency where the smoothed amplitude of the
oscillations peak) and (the large frequency separation), are
frequently used in grid-based modeling searches. However, the methods by which
is calculated from observed data and how is
calculated from stellar models are not the same. Typically for observed stars,
especially for those with low signal-to-noise data, is calculated
by taking the power spectrum of a power spectrum, or with autocorrelation
techniques. However, for stellar models, the actual individual mode frequencies
are calculated and the average spacing between them directly determined. In
this work we try to determine the best way to combine model frequencies in
order to obtain that can be compared with observations. For this
we use stars with high signal-to-noise observations from Kepler as well as
simulated TESS data of Ball et al. (2018). We find that when determining
from individual mode frequencies the best method is to use the
modes with either no weighting or with a Gaussian weighting around
.Comment: 14 pages, 8 figures, accepted for publication in Ap
Solar cycle variation of ν<sub>max</sub> in helioseismic data and its implications for asteroseismology
The frequency, , at which the envelope of pulsation power
peaks for solar-like oscillators is an important quantity in asteroseismology.
We measure for the Sun using 25 years of Sun-as-a-Star Doppler
velocity observations with the Birmingham Solar-Oscillations Network (BiSON),
by fitting a simple model to binned power spectra of the data. We also apply
the fit to Sun-as-a-Star Doppler velocity data from GONG and GOLF, and
photometry data from VIRGO/SPM on the ESA/NASA SOHO spacecraft. We discover a
weak but nevertheless significant positive correlation of the solar with solar activity. The uncovered shift between low and high activity,
of , translates to an uncertainty of 0.8 per cent in
radius and 2.4 per cent in mass, based on direct use of asteroseismic scaling
relations calibrated to the Sun. The mean in the different
datasets is also clearly offset in frequency. Our results flag the need for
caution when using in asteroseismology.Comment: 6 pages, 4 figures, published in MNRAS Letters, 2020, vol 493, pages
L49 - 53 Corrected error in metadata list of author