Absorbing gas around the WASP-12 planetary system

TL;DR

This study investigates the unusual absence of Mg2 h&k line emission in WASP-12, attributing it to circumstellar gas absorption from planetary material rather than stellar inactivity, with implications for detecting evaporating exoplanets.

Contribution

It provides evidence that circumstellar gas from evaporating hot Jupiters can cause spectral line anomalies, offering a new method to identify such systems.

Findings

Extrinsic absorption explains line core anomalies in WASP-12.

ISM absorption is insufficient to account for observed features.

Circumstellar gas may cause low activity indices in stars with evaporating planets.

Abstract

Near-UV observations of the planet host star WASP-12 uncovered the apparent absence of the normally conspicuous core emission of the Mg2 h&k resonance lines. This anomaly could be due either to (1) a lack of stellar activity, which would be unprecedented for a solar-like star of the imputed age of WASP-12; or (2) extrinsic absorption, from the intervening interstellar medium (ISM) or from material within the WASP-12 system itself, presumably ablated from the extreme hot Jupiter WASP-12b. HIRES archival spectra of the Ca2 H&K lines of WASP-12 show broad depressions in the line cores, deeper than those of other inactive and similarly distant stars and similar to WASP-12's Mg2 h&k line profiles. We took high resolution ESPADONS and FIES spectra of three early-type stars within 20' of WASP-12 and at similar distances, which show the ISM column is insufficient to produce the broad Ca2 depression observed in WASP-12. The EBHIS H1 column density map supports and strengthens this conclusion. Extrinsic absorption by material local to the WASP-12 system is therefore the most likely cause of the line core anomalies. Gas escaping from the heavily irradiated planet could form a stable and thick circumstellar disk/cloud. The anomalously low stellar activity index (logR'HK) of WASP-12 is evidently a direct consequence of the extra core absorption, so similar HK index deficiencies might signal the presence of translucent circumstellar gas around other stars hosting evaporating planets.