Time-series analysis of red giant stars

Details Time-series analysis of red giant stars Time-series analysis of red giant stars

Aug 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006phdt........38s&link_type=abstract

University of Sydney

Astronomy and Astrophysics

Astrophysics

Asteroseismology, Red Giant Stars, Stellar Clusters: M67 (Ngc2682), Stars: Xi Hya, Photometry

Scientific paper

Stars are the building blocks of the Universe and, as such, are essential to understanding many aspects of astrophysics. Our understanding of cosmology, galaxies and planetary formation all depend on stellar evolution. To understand how stars evolve we need to investigate stars in different evolutionary stages.
The goals of the work presented in this thesis are to investigate the prospects of applying asteroseismic techniques to studying the interiors of red giant stars. The main achievements and conclusions of this thesis are:
(1) We developed a new technique to measure the mode lifetime of solar-like oscillations. The results point towards a short mode lifetime (roughly 2 days) for the red giant xi Hya, which contradicts the theoretical value (roughly 20 days) by Houdek & Gough (2002). This discrepancy could be due to the lack of a proper theory for convection in a pulsating environment, which might be important to better understand the driving and damping of solar-like oscillations in evolved stars where the convection is expected to be quite vigorous.
(2) Our simulations of solar-like oscillations in red giant stars showed that large variations are expected in the Fourier spectrum of the oscillations in a given star observed at different epochs. This is an important observation, which should be taken into account when evaluating observations of solar-like oscillations such as mode amplitude, frequency and lifetime.
(3) Simulations of the red giant xi Hya strongly indicate that no frequencies can be detected unambiguously from the velocity measurements published by Frandsen et al. (2002). This is due to both the non-continuous coverage of the oscillations and the apparently short mode lifetime.
(4) Based on measurements of the mode lifetime and oscillation periods in main-sequence stars and the red giant xi Hya, there seems to be a steep decline in the quality factor (the period mode-lifetime ratio) towards evolved stars (because the period increases while the mode-lifetime stays roughly the same). This implies that the number of coherent oscillation periods that we can observe will be low for evolved stars, which ultimately limits the precision by which we can determine their frequencies. However, this result needs confirmation from additional observations of either the same star with more continuous data coverage or, preferably, of many red giant stars.
(5) We obtained photometric time series of the open cluster M67 during a multi-site campaign lasting 43 days. The nine telescopes (0.6-2.1 metre) collected in total 560 hours of time series of the cluster. On the best nights the noise was limited by irreducible terms (scintillation and photon noise), and reached down to 0.5 mmag per minute of integration.
(6) Our photometric observations of 20 red giant stars in the open cluster M67 showed excess power in the Fourier spectra consistent with solar-like oscillations. The location of the excess power of three different groups of stars (grouped according to their luminosity) matched with expectations. In three stars, the excess power was more or less in agreement with the expected amplitudes based on L/M-scaling. Simulations further supported that a significant part of the excess seen at low frequencies could be of stellar origin for most target stars. It was not possible to obtain unambiguous detection of a characteristic frequency separation. The signal-to-noise and lack of a clear frequency pattern did not support further extraction of individual peaks. The limitations of this data set was mainly due to the apparent presence of significant non-white noise, which however, could not be quantified or separated clearly from the stellar signal.

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