On the correlation between stellar chromospheric flux and the surface gravity of close-in planets

Abstract

Aims. The chromospheric emission of stars with close-in transiting planets has been found to correlate with the surface gravity of their planets. Stars with low-gravity planets have on average a lower chromospheric flux. Methods. We propose that this correlation is due to the absorption by circumstellar matter that comes from the evaporation of the planets. Planets with a lower gravity have a greater mass-loss rate, which leads to a higher column density of circumstellar absorption and in turn explains the lower level of chromospheric emission observed in their host stars. We estimated the required column density and found that planetary evaporation can account for it. We derived a theoretical relationship between the chromospheric emission as measured in the core of the Ca II H&K lines and the planet gravity.Results. We applied this relationship to a sample of transiting systems for which both the stellar Ca II H&K emission and the planetary surface gravity are known and found a good agreement, given the various sources of uncertainties and the intrinsic variability of the stellar emissions and planetary evaporation rates. We consider implications for the radial velocity jitter applied to fit the spectroscopic orbits and for the age estimates of planetary systems based on the chromospheric activity level of their host stars.

The GAPS programme with HARPS-N at TNG

“…The empirical relation by Wright (2005) predicts a typical stellar jitter of ∼ 4 m s −1 for such a star as TrES-4. We note, however, that the very low value of the chromospheric emission is in line with the correlation found by Hartman (2010) and could be explained as the effect of absorption in the Ca ii H and K line cores by material evaporated from the low-gravity planet (Lanza 2014;Figueira et al 2014). Based on the absence of bump progression in the mean line profiles of HARPS-N and archival Keck data (determined with the Donati et al 1997 technique), we also rule out the possibility that the amplitude of the RV curve of TrES-4 is modified by nonradial stellar pulsations typical of γ Dor variables (Kaye et al 1999), which have been detected in a few cases in stars with stellar parameters similar to those of TrES-4 (Uytterhoeven et al 2014).…”

Suppressed Far-UV Stellar Activity and Low Planetary Mass Loss in the WASP-18 System*

“…Haswell et al (2012) and Fossati et al (2013) concluded that extrinsic absorption by material local to the WASP-12 system, and presumably escaping from the planet, is the most likely cause of the unexpected broad depression present at the cores of the Mg II h&k and Ca II H&K resonance lines (note that this depression is always present, regardless of the planet's orbital phase; Haswell et al 2012;Nichols et al 2015). General support to the idea that circumstellar material may be present around stars hosting close-in planets has been given by Cohen et al (2011), France et al (2013), Lanza (2014), Matsakos et al (2015), Fossati et al (2015c), Carroll-Nellenback et al (2016, and Staab et al (2017). Pillitteri et al (2014) reported the results of Chandra X-ray observations of WASP-18, which led to a non-detection with an upper limit of log L X ([erg s…”

A Decade of Hα Transits for HD 189733 b: Stellar Activity versus Absorption in the Extended Atmosphere

“…Lanza (2014) posited that Ca II absorption in stellar prominences fed by planetary mass loss could be responsible for the correlation between planetary surface gravity and logR HK found by Hartman (2010) (see also Fossati et al 2015). Indeed, detections of multiple transitions of neutral calcium have been claimed in the atmosphere of HD 209458 b (Astudillo-Defru & Rojo 2013).…”

“…However, transiting gas (e.g., previously evaporated material) not associated with the atmosphere could cause such changes. In this case, the atmosphere would show no absorption and the external feature (e.g., a condensation or accretion stream; Lai et al 2010;Lanza 2014) that they occur preferentially when the planet is in-or near-transit. We find it more likely that these changes are due variable absorption in the circumplanetary environment since stochastic changes in the stellar activity level, as a result of star-planet interactions, should be observable at orbital phases not associated with the transit.…”