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All Subjects: habitability
Creators: Shkolnik, Evgenya
Creators: Osby, Ella

Photometric Color Correction of the Star-Planet Activity Research CubeSat (SPARCS)

Description

The Star Planet Activity Research CubeSat (SPARCS) will be a 6U CubeSat devoted to photometric monitoring of M dwarfs in the far-ultraviolet (FUV) and near-ultraviolet (NUV) (160 and 280 nm respectively), measuring the time-dependent spectral slope, intensity and evolution of M dwarf stellar UV radiation. The delta-doped detectors baselined for SPARCS have demonstrated more than five times the in-band quantum efficiency of the detectors of GALEX. Given that red:UV photon emission from cool, low-mass stars can be million:one, UV observation of thes stars are susceptible to red light contamination. In addition to the high efficiency delta-doped detectors, SPARCS will include red-rejection filters to help minimize red leak. Even so, careful red-rejection and photometric calibration is needed. As was done for GALEX, white dwarfs are used for photometric calibration in the UV. We find that the use of white dwarfs to calibrate the observations of red stars leads to significant errors in the reported flux, due to the differences in white dwarf and red dwarf spectra. Here we discuss the planned SPARCS calibration model and the color correction, and demonstrate the importance of this correction when recording UV measurements of M stars taken by SPARCS.

ContributorsOsby, Ella (Author) / Shkolnik, Evgenya (Thesis director) / Ardila, David (Committee member) / School of Earth and Space Exploration (Contributor) / Barrett, The Honors College (Contributor)

Created2021-05

A Photometric and Spectroscopic Analysis of the High-Energy Evolution of K Stars

Description

The balance between relative numbers, lifetime, and habitable zone (HZ) size of K stars (0.6 – 0.9 M⊙) in comparison with M (0.08 – 0.6 M⊙) and G (0.9 – 1.1 M⊙) stars makes them candidates to host “super-habitable” planets. Understanding the high- energy radiation environment of planets around these stars is crucial, since ultraviolet (UV) and X-ray radiation may cause severe photodissociation and ionization of the atmosphere, with the potential for complete erosion. In this thesis, I present the first broad study of the UV and X-ray evolution of K stars. I first focused on Galaxy Evolution Explorer (GALEX) and Ro ̈ntgen Satellit (ROSAT) photometric UV and X-ray evolutions of K stars and compared this with the age evolution of both early- (0.35 – 0.6 M⊙) and late-M (0.08 – 0.35 M⊙) stars. I found that the fractional UV and X-ray flux from M and K stars is similar; however, the wider and farther HZs of K stars mean that there is less incident UV radiation on HZ planets. Next, I led a spectroscopic study of 41 K stars using Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) data to show that the UV line and continua emission show no decrease in flux beyond 650 Myr whereas early-M star flux declines by 150 Myr; therefore, the K star intrinsic UV flux is greater than early-M stars after this time. I suggest that this phenomenon is related to K star rotational spin-down stalling. Lastly, I revisited the GALEX and ROSAT data with newly-available distances from the Gaia mission for both K and M stars. I find that the UV flux for K stars is an order of magnitude higher for M stars at all ages and the flux in their respective HZs is similar. However, K star X-ray flux is an order of magnitude less in the HZ than for M stars. The age of decline shows a dependency on wavelength, a phenomenon which is not seen in either the early- or late-M star data. These results suggest thatK stars may not exhibit quite the advantage as HZ planet host stars as the scientific community originally thought.

ContributorsRichey-Yowell, Tyler (Author) / Shkolnik, Evgenya (Thesis advisor) / Patience, Jennifer (Committee member) / Jacobs, Daniel (Committee member) / Bowman, Judd (Committee member) / Young, Patrick (Committee member) / Arizona State University (Publisher)

Created2022

The Diversity of Chemical Composition and the Effects on Stellar Evolution and Planetary Habitability

Description

I present a catalog of 1,794 stellar evolution models for solar-type and low-mass stars, which is intended to help characterize real host-stars of interest during the ongoing search for potentially habitable exoplanets. The main grid is composed of 904 tracks, for 0.5-1.2 M_sol at scaled metallicity values of 0.1-1.5 Z_sol and specific elemental abundance ratio values of 0.44-2.28 O/Fe_sol, 0.58-1.72 C/Fe_sol, 0.54-1.84 Mg/Fe_sol, and 0.5-2.0 Ne/Fe_sol. The catalog includes a small grid of late stage evolutionary tracks (25 models), as well as a grid of M-dwarf stars for 0.1-0.45 M_sol (856 models). The time-dependent habitable zone evolution is calculated for each track, and is strongly dependent on stellar mass, effective temperature, and luminosity parameterizations. I have also developed a subroutine for the stellar evolution code TYCHO that implements a minimalist coupled model for estimating changes in the stellar X-ray luminosity, mass loss, rotational velocity, and magnetic activity over time; to test the utility of the updated code, I created a small grid (9 models) for solar-mass stars, with variations in rotational velocity and scaled metallicity. Including this kind of information in the catalog will ultimately allow for a more robust consideration of the long-term conditions that orbiting planets may experience.

In order to gauge the true habitability potential of a given planetary system, it is extremely important to characterize the host-star's mass, specific chemical composition, and thus the timescale over which the star will evolve. It is also necessary to assess the likelihood that a planet found in the "instantaneous" habitable zone has actually had sufficient time to become "detectably" habitable. This catalog provides accurate stellar evolution predictions for a large collection of theoretical host-stars; the models are of particular utility in that they represent the real variation in stellar parameters that have been observed in nearby stars.

ContributorsTruitt, Amanda Rosendall (Author) / Young, Patrick (Thesis advisor) / Anbar, Ariel (Committee member) / Desch, Steven (Committee member) / Patience, Jennifer (Committee member) / Shkolnik, Evgenya (Committee member) / Arizona State University (Publisher)

Created2017