WASP-12, shrouded in mystery or just cold gas?

TL;DR

This study models the distribution and ionization state of magnesium around WASP-12, revealing that most MgII absorption likely originates from cold clumps formed by cooling instabilities in the gas torus, challenging previous assumptions.

Contribution

It combines hydrodynamical simulations and observations to quantify MgII absorption, proposing a cooling instability mechanism to explain observed absorption levels.

Findings

Most MgII is in the ISM, not the torus.

The torus temperature favors higher ionization states of Mg.

Cooling instabilities may produce MgII-rich cold clumps.

Abstract

Observations of the planet-hosting star WASP-12 show a distinctive depression in the \ion{Mg}{ii} and \ion{Ca}{ii} resonance lines. This has been interpreted as a marker of atmospheric loss from the close-in hot Jupiter WASP-12b and the resulting formation of a gas torus around the star. In this paper we quantify the \ion{Mg}{ii} absorption from this torus, compared to that provided by the stellar wind, the stellar astrosphere and the ISM. To do this we piece together the full density profile of \ion{Mg}{ii} from WASP-12 to an observer on Earth using a combination of hydrodynamical simulations and observations. We find that the bulk of the gas along the line of sight is contained within a dense torus close to WASP-12. However, the temperatures in this torus are sufficient to promote Mg into a doubly (\ion{Mg}{iii}) or higher ionized state. As a result, the singly ionized fraction (\ion{Mg}{ii}) is low. We find that most of the \ion{Mg}{ii} is not in the torus but in the ISM. Despite this, the total column density of \ion{Mg}{ii} is two orders of magnitude lower than required to explain observations of the system. To resolve this discrepancy, we note that the torus gas is at a temperature where it will cool efficiently. We speculate that the onset of the cooling instability will cause the torus to fragment, forming cold clumps with a higher fraction of \ion{Mg}{ii}, capable of explaining the observed absorption.