92 research outputs found
Finger on the pulse of asteroseismology
Warrick Ball highlights some recent discoveries in the context of the past,
present and future of asteroseismology.Comment: Author-produced version of article accepted in Astronomy & Geophysic
A novel analytic atmospheric relation for stellar models
Stellar models often use relations between the temperature and optical
depth to evaluate the structure of their optically-thin outer layers. We
fit a novel analytic function to the Hopf function of a
radiation-coupled hydrodynamics simulation of near-surface convection with
solar parameters by Trampedach et al. (2014). The fit is accurate to within
0.82 per cent for the solar simulation and to within 13 per cent for all the
simulations that are not on either the low-temperature or low-gravity edges of
the grid of simulations.Comment: 3 pages, 1 figur
Model-independent measurement of internal stellar structure in 16 Cygni A and B
We present a method for measuring internal stellar structure based on
asteroseismology that we call "inversions for agreement." The method accounts
for imprecise estimates of stellar mass and radius as well as the relatively
limited oscillation mode sets that are available for distant stars. By
construction, the results of the method are independent of stellar models. We
apply this method to measure the isothermal sound speeds in the cores of the
solar-type stars 16 Cyg A and B using asteroseismic data obtained from Kepler
observations. We compare the asteroseismic structure that we deduce against
best-fitting evolutionary models and find that the sound speeds in the cores of
these stars exceed those of the models.Comment: Accepted for publication in The Astrophysical Journa
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Quasi-stars and the Schönberg–Chandrasekhar limit
The mechanism by which the supermassive black holes that power bright quasars
at high redshift form remains unknown. One possibility is that, if fragmentation is
prevented, the monolithic collapse of a massive protogalactic disc proceeds via a
cascade of triaxial instabilities and leads to the formation of a quasi-star: a growing
black hole, initially of typical stellar-mass, embedded in a hydrostatic giant-like
envelope. Quasi-stars are the main object of study in this dissertation. Their envelopes
satisfy the equations of stellar structure so the Cambridge STARS code is modified to
model them. Analysis of the models leads to an extension of the classical Schönberg–
Chandrasekhar limit and an exploration of the implications of this extension for the
evolution of main-sequence stars into giants.
In Chapter 1, I introduce the problem posed by the supermassive black holes that
power high-redshift quasars. I discuss potential solutions and describe the conditions
under which a quasi-star might form. In Chapter 2, I outline the Cambridge STARS
code and the modifications that are made to model quasi-star envelopes.
In Chapter 3, I present models of quasi-stars where the base of the envelope is
located at the Bondi radius of the black hole. The black holes in these models are
subject to a robust upper fractional mass limit of about one tenth. In addition, the
final black hole mass is sensitive to the choice of the inner boundary radius of the
envelope. In Chapter 4, I construct alternative models of quasi-stars by drawing
from work on convection- and advection-dominated accretion flows around black
holes. To improve the accuracy of my models, I incorporate corrections owing to
special and general relativity into a variant of the STARS code that includes rotation.
The evolution of these quasi-stars is qualitatively different from those described in
Chapter 3. Most notably, the core black holes are no longer subject to a fractional
mass limit and ultimately accrete all of the material in their envelopes.
In Chapter 5, I demonstrate that the fractional mass limit found in Chapter 3, for
the black holes in quasi-stars, is in essence the same as the Schönberg–Chandrasekhar
limit. The analysis demonstrates how other similar limits are related and that limits
exist under a wider range of circumstances than previously thought. A test is provided
that determines whether a composite polytrope is at a fractional mass limit. In Chapter
6, I apply this test to realistic stellar models and find evidence that the existence of
fractional mass limits is connected to the evolution of stars into the red giants
Seismic signatures of stellar magnetic activity — what can we expect from TESS?
Asteroseismic methods offer a means to investigate stellar activity and activity cycles as well as to identify those properties of stars which are crucial for the operation of stellar dynamos. With data from CoRoT and Kepler, signatures of magnetic activity have been found in the seismic properties of a few dozen stars. Now, NASA's Transiting Exoplanet Survey Satellite (TESS) mission offers the possibility to expand this, so far, rather exclusive group of stars. This promises to deliver new insight into the parameters that govern stellar magnetic activity as a function of stellar mass, age, and rotation rate. We derive a new scaling relation for the amplitude of the activity-related acoustic (p-mode) frequency shifts that can be expected over a full stellar cycle. Building on a catalog of synthetic TESS time series, we use the shifts obtained from this relation and simulate the yield of detectable frequency shifts in an extended TESS mission. We find that, according to our scaling relation, we can expect to find significant p-mode frequency shifts for a couple hundred main-sequence and early subgiant stars and for a few thousand late subgiant and low-luminosity red giant stars
A synthetic sample of short-cadence solar-like oscillators for TESS
NASA's Transiting Exoplanet Survey Satellite (TESS) has begun a two-year
survey of most of the sky, which will include lightcurves for thousands of
solar-like oscillators sampled at a cadence of two minutes. To prepare for this
steady stream of data, we present a mock catalogue of lightcurves, designed to
realistically mimic the properties of the TESS sample. In the process, we also
present the first public release of the asteroFLAG Artificial Dataset
Generator, which simulates lightcurves of solar-like oscillators based on input
mode properties. The targets are drawn from a simulation of the Milky Way's
populations and are selected in the same way as TESS's true Asteroseismic
Target List. The lightcurves are produced by combining stellar models,
pulsation calculations and semi-empirical models of solar-like oscillators. We
describe the details of the catalogue and provide several examples. We provide
pristine lightcurves to which noise can be added easily. This mock catalogue
will be valuable in testing asteroseismology pipelines for TESS and our methods
can be applied in preparation and planning for other observatories and
observing campaigns.Comment: 14 pages, 6 figures, accepted for publication in ApJS. Archives
containing the mock catalogue are available at
https://doi.org/10.5281/zenodo.1470155 and the pipeline to produce it at
https://github.com/warrickball/s4tess . The first public release of the
asteroFLAG Artificial Dataset Generator v3 (AADG3) is described at
https://warrickball.github.io/AADG3
Overview and Validation of the Asteroseismic Modeling Portal v2.0
The launch of NASA's Kepler space telescope in 2009 revolutionized the
quality and quantity of observational data available for asteroseismic
analysis. While Kepler was able to detect solar-like oscillations in hundreds
of main-sequence and subgiant stars, the Transiting Exoplanet Survey Satellite
(TESS) is now making similar observations for thousands of the brightest stars
in the sky. The Asteroseismic Modeling Portal (AMP) is an automated and
objective stellar model-fitting pipeline for asteroseismic data, which was
originally developed to use models from the Aarhus Stellar Evolution Code
(ASTEC). We briefly summarize an updated version of the AMP pipeline that uses
Modules for Experiments in Stellar Astrophysics (MESA), and we present initial
modeling results for the Sun and several solar analogs to validate the
precision and accuracy of the inferred stellar properties.Comment: 3 pages, 1 table, AAS Journals accepted. Software available at
https://github.com/travismetcalfe/amp
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