Asteroseismological observations of eta Cassiopeiae A with the Palomar east arm echelle spectrograph

Citation

Peri, Michal Leah (1995) Asteroseismological observations of eta Cassiopeiae A with the Palomar east arm echelle spectrograph. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/VKRJ-A345. https://resolver.caltech.edu/CaltechETD:etd-05212004-105738

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Asteroseismology is the study of acoustic oscillations in stars. Such oscillations probe the stellar interior, and thus carry information on the physical conditions in regions of the star that are not accessible through ordinary observational techniques. Oscillations have not yet been measured for any star other than the Sun, however, because they are quite small. The solar p-modes generate Doppler shifts of [...], as well as variations [...] in surface intensity. Theory predicts that the corresponding variations will be at most a few times larger in other solar-type stars. The goal of this thesis is to extend the methods of asteroseismology to the levels where one can reasonably expect to detect oscillations on solar-type stars. Doppler spectroscopy is the preferred method for asteroseismological observations. The fundamental limitations on sensitivity imposed by photon noise can be alleviated by looking at bright stars with large telescopes and by observing a large number of spectral lines simultaneously. The observations require high resolution over a broad wavelength regime with excellent stability on the short timescales [...] associated with p-mode periodicities. To obtain such observations, this project began with the design and construction of a new high-resolution spectrometer for Palomar 200" Hale Telescope. The East Arm Echelle Spectrograph, which is coupled by optical fiber to prime focus, produces a resolution of [...] over most of the visible spectrum. The design was optimized for making high-precision radial velocity measurements, incorporating fiber-optic double scrambling for enhanced stability and a molecular iodine absorption cell for Doppler calibration. To enhance utility to other astronomical applications, the instrument also offers a low-resolution mode with R=20000. Specialized data reduction software was developed to compensate for the echelle order curvature and spectral line tilt. The analysis code extracts spectral Doppler shift, carefully isolating the stellar signal from atmospheric perturbation and instrumental instabilities. The algorithms were designed to be robust and automated for processing large numbers of frames required for asteroseismological timeseries. Instrumental performance was assessed from timeseries of solar radial velocity measurements. The spectrograph readily measures motion associated with the Earth's rotation. Configured for asteroseismology observations, the instrument demonstrated an rms sensitivity of 1.14 m/s per frame over [...] 1/2 hour timescales. This precision is within a factor of [...] 2 of the photon noise limit attainable for single measurements, indicating the instrument provides excellent stability over timescales associated with stellar acoustic oscillations. In August 1993 we obtained a six-night timeseries of observations on the solar twin [eta] Cas. The measurements carried an rms radial velocity scatter of [...] 3 m/s per 60 second exposure. The power spectrum derived from this data showed a mean power spectral density corresponding to a Doppler amplitude of 10.8 cm/s over the frequency range 450-4450 [...]. Unfortunately, we found no clear evidence of p-mode oscillations. Interpretation of the data is complicated by the lack of specific predictions for the star's seismicity. Nevertheless, we set a very conservative threshold for the absence of oscillations at a Doppler amplitude of 30-35 cm/s. This limit represents a very substantial gain over previous asteroseismological observations. The [eta] Cas results push the state of the art by factor of two, on a much fainter star than had been previously observed. This accomplishment increases the number of stars that can be reasonably targeted for study by an order of magnitude. Our conservative threshold lies well within the range of acoustic behavior predicted for main-sequence dwarf stars and is the first to approach the level of precision required to observe pulsations of solar amplitude. Although we did not see p-modes in [eta] Cas, we believe that the methodology developed in this thesis is capable of detecting asteroseismological phenomena on other solar-type candidate stars.

Thesis Files