28 research outputs found
Buoyancy-driven oscillations in helio- and asteroseismology
This thesis focuses on the application of asteroseismology to red giants observed with Kepler alongside searching for solar g-modes using the Birmingham Solar Oscillations Network (BiSON). In the case of the Sun, solar gravity modes are highly sought after because they can shed light on the inner rotation profile of the Sun. This thesis contains work showing how the low frequency regime of BiSON data has been cleaned enabling the search to be made in BiSON data without instrumental artefacts. Moving onwards along the stars evolution, thanks to space mission such as Kepler and CoRoT tens of thousands of red giant stars have been observed allowing huge ensemble investigations. The ability to use high-quality, long datasets as constraints to shorter and noiser datasets has been investigated through fitting the background power of 6000 Kepler red giants. Red giants also offer the opportunity to study the inclination angle distribution of stars to confirm that the distribution conforms to the expected isotropy used in many simulations. This can be extended to inferring the obliquity through asteroseismology, as applied to a red-giant, M-dwarf eclipsing binary. This offering a means to probe obliquity distributions in in a different regime to that using traditional spectroscopic techniques
Mixed-mode Ensemble Asteroseismology of Low-Luminosity Kepler Red Giants
We present measurements of the dipole mode asymptotic period spacing
(), the coupling factor between p- and g- modes (), the g-mode
phase offset (), and the mixed-mode frequency rotational splitting
() for 1,074 low-luminosity red giants from the
Kepler mission. Using oscillation mode frequencies extracted from each star, we
apply Bayesian optimization to estimate from the power spectrum
of the stretched period spectrum and to perform the subsequent forward
modelling of the mixed-mode frequencies. With our measurements, we show that
the mode coupling factor shows significant anti-correlation with both
stellar mass and metallicity, and can reveal highly metal-poor stars. We
present the evolution of up the lower giant branch up to before
the luminosity bump, and find no significant trends in or
with stellar mass and metallicity in our sample.
Additionally, we identify six new red giants showing anomalous distortions in
their g-mode pattern. Our data products, code, and results are provided in a
public repository.Comment: Accepted in the Astrophysical Journal. Code repository is at
https://github.com/jsk389/BOChaMM and associated peakbagging data is publicly
available at https://zenodo.org/record/788863
HD-TESS: An Asteroseismic Catalog of Bright Red Giants within TESS Continuous Viewing Zones
We present HD-TESS, a catalog of 1,709 bright () red giants from
the Henry Draper (HD) Catalog with asteroseismic measurements based on
photometry from NASA's Transiting Exoplanet Survey Satellite (TESS). Using
light curves spanning at least six months across a single TESS observing cycle,
we provide measurements of global asteroseismic parameters
( and ) and evolutionary state for each star in
the catalog. We adopt literature values of atmospheric stellar parameters to
estimate the masses and radii of the giants in our catalog using asteroseismic
scaling relations, and observe that HD-TESS giants on average have larger
masses compared to Kepler red giants. Additionally, we present the discovery of
oscillations in 99 red giants in astrometric binary systems, including those
with subdwarf or white dwarf companions. Finally, we benchmark radii from
asteroseismic scaling relations against those measured using long-baseline
interferometry for 18 red giants and find that correction factors to the
scaling relations improve the agreement between asteroseismic and
interferometric radii to approximately 3%.Comment: 20 pages, 12 figures. Accepted for publication in the Astronomical
Journal. Table of asteroseismic masses and radii is available as an ancillary
fil
KOI-3890: A high mass-ratio asteroseismic red-giantM-dwarf eclipsing binary undergoing heartbeat tidal interactions
KOI-3890 is a highly eccentric, 153-day period eclipsing, single-lined
spectroscopic binary system containing a red-giant star showing solar-like
oscillations alongside tidal interactions. The combination of transit
photometry, radial velocity observations, and asteroseismology have enabled the
detailed characterisation of both the red-giant primary and the M-dwarf
companion, along with the tidal interaction and the geometry of the system. The
stellar parameters of the red-giant primary are determined through the use of
asteroseismology and grid-based modelling to give a mass and radius of
and
respectively. When combined with
transit photometry the M-dwarf companion is found to have a mass and radius of
and
. Moreover, through
asteroseismology we constrain the age of the system through the red-giant
primary to be . This provides a constraint on
the age of the M-dwarf secondary, which is difficult to do for other M-dwarf
binary systems. In addition, the asteroseismic analysis yields an estimate of
the inclination angle of the rotation axis of the red-giant star of
degrees. The obliquity of the system\textemdash the
angle between the stellar rotation axis and the angle normal to the orbital
plane\textemdash is also derived to give degrees
showing that the system is consistent with alignment. We observe no radius
inflation in the M-dwarf companion when compared to current low-mass stellar
models.Comment: 11 pages, 5 figures, accepted for publication in MNRA
Bayesian hierarchical inference of asteroseismic inclination angles
The stellar inclination angle-the angle between the rotation axis of a star
and our line of sight-provides valuable information in many different areas,
from the characterisation of the geometry of exoplanetary and eclipsing binary
systems, to the formation and evolution of those systems. We propose a method
based on asteroseismology and a Bayesian hierarchical scheme for extracting the
inclination angle of a single star. This hierarchical method therefore provides
a means to both accurately and robustly extract inclination angles from red
giant stars. We successfully apply this technique to an artificial dataset with
an underlying isotropic inclination angle distribution to verify the method. We
also apply this technique to 123 red giant stars observed with
. We also show the need for a selection function to account
for possible population-level biases, that are not present in individual
star-by-star cases, in order to extend the hierarchical method towards
inferring underlying population inclination angle distributions.Comment: 20 pages, 12 figures, accepted for publication in MNRA
Gaussian Process modelling of granulation and oscillations in red-giant stars
The analysis of photometric time series in the context of transiting planet
surveys suffers from the presence of stellar signals, often dubbed "stellar
noise". These signals, caused by stellar oscillations and granulation, can
usually be disregarded for main-sequence stars, as the stellar contributions
average out when phase-folding the light curve. For evolved stars, however, the
amplitudes of such signals are larger and the timescales similar to the transit
duration of short-period planets, requiring that they be modeled alongside the
transit. With the promise of TESS delivering on the order of light
curves for stars along the red-giant branch, there is a need for a method
capable of describing the "stellar noise" while simultaneously modelling an
exoplanet's transit. In this work, a Gaussian Process regression framework is
used to model stellar light curves and the method validated by applying it to
TESS-like artificial data. Furthermore, the method is used to characterize the
stellar oscillations and granulation of a sample of well-studied
\textit{Kepler} low-luminosity red-giant branch stars. The parameters
determined are compared to equivalent ones obtained by modelling the power
spectrum of the light curve. Results show that the method presented is capable
of describing the stellar signals in the time domain and can also return an
accurate and precise measurement of , i.e., the frequency of
maximum oscillation amplitude. Preliminary results show that using the method
in transit modelling improves the precision and accuracy of the ratio between
the planetary and stellar radius, . The method's implementation is
publicly available.Comment: Accepted for publication in MNRAS; 12 pages, 10 figures, 2 table
Kepler-432: a red giant interacting with one of its two long period giant planets
We report the discovery of Kepler-432b, a giant planet ()
transiting an evolved star with an orbital period of days. Radial velocities (RVs) reveal that
Kepler-432b orbits its parent star with an eccentricity of , which we also measure independently with
asterodensity profiling (AP; ), thereby confirming
the validity of AP on this particular evolved star. The well-determined
planetary properties and unusually large mass also make this planet an
important benchmark for theoretical models of super-Jupiter formation.
Long-term RV monitoring detected the presence of a non-transiting outer planet
(Kepler-432c; days), and adaptive optics imaging revealed a nearby
(0\farcs87), faint companion (Kepler-432B) that is a physically bound M dwarf.
The host star exhibits high signal-to-noise asteroseismic oscillations, which
enable precise measurements of the stellar mass, radius and age. Analysis of
the rotational splitting of the oscillation modes additionally reveals the
stellar spin axis to be nearly edge-on, which suggests that the stellar spin is
likely well-aligned with the orbit of the transiting planet. Despite its long
period, the obliquity of the 52.5-day orbit may have been shaped by star-planet
interaction in a manner similar to hot Jupiter systems, and we present
observational and theoretical evidence to support this scenario. Finally, as a
short-period outlier among giant planets orbiting giant stars, study of
Kepler-432b may help explain the distribution of massive planets orbiting giant
stars interior to 1 AU.Comment: 22 pages, 19 figures, 5 tables. Accepted to ApJ on Jan 24, 2015
(submitted Nov 11, 2014). Updated with minor changes to match published
versio
Synergy between asteroseismology and exoplanet science:an outlook
Space-based asteroseismology has been playing an important role in the
characterization of exoplanet-host stars and their planetary systems. The
future looks even brighter, with space missions such as NASA's TESS and ESA's
PLATO ready to take on this legacy. In this contribution, we provide an outlook
on the synergy between asteroseismology and exoplanet science, namely, on the
prospect of conducting a populational study of giant planets around oscillating
evolved stars with the TESS mission.Comment: 8 pages, 11 figures, 1 table; To appear in the Proceedings of PHOST
"Physics of Oscillating Stars" - a conference in honour of Prof. H.
Shibahashi, 2-7 Sep 2018, Banyuls-sur-mer, France; Edited by J. Ballot, S.
Vauclair and G. Vauclai
TESS asteroseismology of the known red-giant host stars HD 212771 and HD 203949
International audienc