Stellar ages and stellar rotation

Angus, R

Abstract:: Stellar ages will play a big role in the next generation of astronomy. Useful to exoplaneteers and galactic archaeologists alike, this relatively under-exploited stellar property is currently limited by the precision of dating techniques. The work presented in this thesis contributes incrementally to a greater understanding of rotation period decay in Main Sequence (MS) stars as a proxy for stellar age. Inferring stellar ages from rotation periods, 'gyrochronology', is the only d...
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APA Style

Angus, R. (2016). Stellar ages and stellar rotation [PhD thesis]. University of Oxford.

MLA Style

Angus, R. Stellar Ages and Stellar Rotation. 2016. University of Oxford, PhD thesis.

Chicago Style

Angus, R. 2016. “Stellar Ages and Stellar Rotation.” PhD thesis, University of Oxford.
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+ Angus, R More by this author

Division:: MPLS
Department:: Physics
Sub department:: Astrophysics
Department:: University of Oxford
Role:: Author

Contributors

+ Aigrain, S

Department:: University of Oxford
Role:: Supervisor

+ The Leverhulme Trust More from this funder

DOI:: 10.5287/ora-qo5n6vq1o
Type of award:: DPhil
Level of award:: Doctoral
Awarding institution:: University of Oxford

Language:: English
Keywords:: Bayesian

Star spots

Stellar ages

Kepler

Stellar rotation

Statistics

Flicker

Gaussian processes

Signal processing

Probabilistic

Stars

Exoplanets
Subjects:: Bayesian inference

Gaussian processes

Statistics

Survey astronomy

Data analysis

Astronomy

Hierarchical Bayesian Modelling

Stellar Physics

Time series analysis
UUID:: uuid:07f78624-adfe-4775-9d3e-dbc63636156a
Deposit date:: 2016-08-29
ARK identifier:: ark:/29072/ora_07f78624adfe47759d3edbc63636156a

Related item:: Calibrating gyrochronology using Kepler asteroseismic targets
Description:: Among the available methods for dating stars, gyrochronology is a powerful one because it requires knowledge of only the star's mass and rotation period. Gyrochronology relations have previously been calibrated using young clusters, with the Sun providing the only age dependence, and are therefore poorly calibrated at late ages. We used rotation period measurements of 310 Kepler stars with asteroseismic ages, 50 stars from the Hyades and Coma Berenices clusters and 6 field stars (including the Sun) with precise age measurements to calibrate the gyrochronology relation, whilst fully accounting for measurement uncertainties in all observable quantities. We calibrated a relation of the form P = An × (B - V - c)b, where P is rotation period in days, A is age in Myr, B and V are magnitudes and a, b and n are the free parameters of our model. We found a = 0.40^{+0.3}_{-0.05}, b = 0.31^{+0.05}_{-0.02} and n = 0.55^{+0.02}_{-0.09}. Markov Chain Monte Carlo methods were used to explore the posterior probability distribution functions of the gyrochronology parameters and we carefully checked the effects of leaving out parts of our sample, leading us to find that no single relation between rotation period, colour and age can adequately describe all the subsets of our data. The Kepler asteroseismic stars, cluster stars and local field stars cannot all be described by the same gyrochronology relation. The Kepler asteroseismic stars may be subject to observational biases; however, the clusters show unexpected deviations from the predicted behaviour, providing concerns for the overall reliability of gyrochronology as a dating method.
Related item:: Systematics-insensitive Periodic Signal Search with K2
Description:: From pulsating stars to transiting exoplanets, the search for periodic signals in K2 data, Kepler’s two-wheeled extension, is relevant to a long list of scientific goals. Systematics affecting K2 light curves due to the decreased spacecraft pointing precision inhibit the easy extraction of periodic signals from the data. We here develop a method for producing periodograms of K2 light curves that are insensitive to pointing-induced systematics; the Systematics-insensitive Periodogram (SIP). Traditional sine-fitting periodograms use a generative model to find the frequency of a sinusoid that best describes the data. We extend this principle by including systematic trends, based on a set of “eigen light curves,” following Foreman-Mackey et al., in our generative model as well as a sum of sine and cosine functions over a grid of frequencies. Using this method we are able to produce periodograms with vastly reduced systematic features. The quality of the resulting periodograms are such that we can recover acoustic oscillations in giant stars and measure stellar rotation periods without the need for any detrending. The algorithm is also applicable to the detection of other periodic phenomena such as variable stars, eclipsing binaries and short-period exoplanet candidates. The SIP code is available at https://github.com/RuthAngus/SIPK2.
Related item:: Probabilistic Inference of Basic Stellar Parameters: Application to Flickering Stars
Description:: The relations between observable stellar parameters are usually assumed to be deterministic. That is, given an infinitely precise measurement of independent variable, “x”, and some model, the value of dependent variable, “y” can be known exactly. In practice this assumption is rarely valid and intrinsic stochasticity means that two stars with exactly the same “x” will have slightly different “y”s. The relation between short-timescale brightness fluctuations (flicker) of stars and both surface gravity and stellar density are two such stochastic relations that have until now been treated as deterministic ones. We recalibrate these relations in a probabilistic framework, using hierarchical Bayesian modeling to constrain the instrinsic scatter in the relations. We find evidence for additional scatter in the relationship, that cannot be accounted for by the observational uncertainties alone. The scatter in surface gravity and stellar density does not depend on flicker, suggesting that using flicker as a proxy for {log}g and ρ ⋆ is equally valid for dwarf and giant stars, despite the fact that the observational uncertainties tend to be larger for dwarfs.

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Copyright holder:: Angus, R

Licence:: Terms and Conditions of Use for Oxford University Research Archive

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Stellar ages and stellar rotation

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