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(Fe)/AH) ≈ 0.2 relative to the solar photospheric baseline of Anders & Grevesse, and significantly lower than the stellar photosphere as well. We further find low abundances of high first ionization potential (FIP) elements A(O)/A(Fe) ≲ 1, A(Ne)/A(Fe) ≲ 0.1, but with indications of higher abundances of A(N)/A(Fe) ≫ 1, A(Al)/A(Fe) ≲ 10. We estimate a FIP bias for this star in the range ≈ − 0.3 to −0.1, larger than the ≲ −0.5 expected for stars of this type, but similar to stars hosting close-in hot Jupiters. We detect significant intensity variability over timescales ranging from 100 s to 10 ks, and also evidence for spectral variability over timescales 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.