X-ray Activity Variations and Coronal Abundances of the Star-Planet Interaction candidate HD 179949

Abstract

We carry out detailed spectral and timing analyses of the $Chandra$ X-ray data of HD 179949, a prototypical example of a star with a close-in giant planet with possible star-planet interaction (SPI) effects. We find a low coronal abundance $A(\mathrm{Fe})/A(\mathrm{H})\approx 0.2$ relative to the solar photospheric baseline of Anders & Grevesse (1989), and significantly lower than the stellar photosphere as well. We further find low abundances of high First Ionization Potential (FIP) elements $A(\mathrm{O})/A(\mathrm{Fe})\lesssim 1$, $A(\mathrm{Ne})/A(\mathrm{Fe})\lesssim 0.1$, but with indications of higher abundances of $A(\mathrm{N})/A(\mathrm{Fe})\gg 1,A(\mathrm{Al})/A(\mathrm{Fe})\lesssim 10$. We estimate a FIP bias for this star in the range $\approx -0.3$ to $-0.1$, larger than the $\lesssim -$0.5 expected for stars of this type, but similar to stars hosting close-in hot Jupiters. We detect significant intensity variability over time scales ranging from 100 s - 10 ks, and also evidence for spectral variability over time scales of 1-10 ks. We combine the $Chandra$ flux measurements with $Swift$ and XMM-$Newton$ measurements to detect periodicities and determine that the dominant signal is tied to the stellar polar rotational period, consistent with expectations that the corona is rotational-pole dominated. We also find evidence for periodicity at both the planetary orbital frequency and at its beat frequency with the stellar polar rotational period, suggesting the presence of a magnetic connection between the planet and the stellar pole. If these periodicities represent an SPI signal, it is likely driven by a quasi-continuous form of heating (e.g., magnetic field stretching) rather than sporadic, hot, impulsive flare-like reconnections.

26 pages, 12 figures, 11 tables. Accepted for publication in the Astrophysical Journal

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