A Search for Planets around Red Stars

Open Access
Gettel, Sara Jeanne
Graduate Program:
Astronomy and Astrophysics
Doctor of Philosophy
Document Type:
Date of Defense:
September 21, 2012
Committee Members:
  • Alexander Wolszczan, Dissertation Advisor
  • Lawrence William Ramsey, Committee Member
  • John David Mathews, Committee Member
  • Mercedes Richards, Committee Member
  • Kevin Luhman, Committee Member
  • Jason Wright, Committee Member
  • elanet detection
  • evolved stars
  • radial velocity
  • telluric line
  • calibration
Our knowledge of planets around other stars has expanded drastically in recent years, from a handful Jupiter-mass planets orbiting Sun-like stars, to encompass a wide range of planet masses and stellar host types. In this thesis, I review the development of radial velocity planet searches and present results from projects focusing on the detection of planets around two classes of red stars. The first project is part of the Penn State - Torun Planet Search (PTPS) for substellar companions to K giant stars using the Hobby-Eberly Telescope (HET). The results of this work include the discovery of planetary systems around five evolved stars. These systems illustrate several of the differences between planet detection around giants and Solar-type stars, including increased masses and a lack of short period planets. One planet has a nearly six year orbit, the longest announced to date around a giant star, with an amplitude approaching the limits of detectability due to stellar ‘jitter’. Two more of these systems also show long-term radial velocity trends which are likely caused by the presence of an additional, more distant binary companion. The remaining two systems show increased radial velocity noise, typical of giant systems. Finally I show that, if the stellar jitter is caused by p-mode oscillations, the amplitude of this noise is anti-correlated with metallicity. The second project focuses on the expansion of the current radial velocity calibration methods to a new wavelength regime. The absorption cell technique is modified to use the telluric O2 and water vapor bands found between ∼6000-9000 A. These features have been found to be stable to ∼10 m s−1 and allow access to the increased red flux of low-mass and evolved stars. I carry out a mock planet search of six early M dwarfs that are known to be radial velocity stable, providing a recoverable null result. Measurements are also made of several telluric standards, to improve the characterization of the atmospheric conditions at the time of observation. Radial velocities are measured by forward modeling the observations as a combination of a best-fit model telluric spectrum and a deconvolved stellar template, convolved with a best-fit point-spread function (PSF). These measurements are tested using a small number of blocks and compared to analogous measurements made using the standard iodine calibration. This small sample of blocks is then extrapolated to the full wavelength range, yielding a precision of ∼20 m s−1 for the iodine calibration and ∼30 m s−1 for telluric calibration. These relatively modest precisions may be improved in the future both by improving portions of the PSF modeling and deconvolution algorithms, and by increasing the signal-to-noise ratio (S/N) of the observations. Nevertheless, it is reassuring to obtain relatively similar results with the two calibration methods and even with the present level of precision, telluric calibration would be able to detect a Neptune-mass planet in the habitable zone of an M dwarf.