Search for gas from the disintegrating rocky exoplanet K2-22b

Abstract

Context. The red dwarf star K2-22 is transited every 9.14 hours by an object which is best explained by being a disintegrating rocky exoplanet featuring a variable comet-like dust tail. While the dust is thought to dominate the transit light curve, gas is also expected to be present, either from being directly evaporated off the planet or by being produced by the sublimation of dust particles in the tail. Aims. Both ionized calcium and sodium have large cross-sections, and although present at low abundance, exhibit the strongest atomic absorption features in comets. We therefore also identify these species as the most promising tracers of circumplanetary gas in evaporating rocky exoplanets and search for them in the tail of K2-22 b to constrain the gas-loss and sublimation processes in this enigmatic object. Methods. We observed four transits of K2-22 b with X-shooter on the Very Large Telescope operated by ESO to obtain time series of intermediate-resolution (R ∼ 11400) spectra. Our analysis focussed on the two sodium D lines (588.995 nm and 589.592 nm) and the Ca + triplet (849.802 nm, 854.209 nm, and 866.214 nm). The stellar calcium and sodium absorption was removed using the out-oftransit spectra. We searched for planet-related absorption in the velocity rest frame of the planet, which changes from approximately −66 to +66 km s −1 during the transit. Results. Since K2-22 b exhibits highly variable transit depths, we analysed the individual nights and their average. By injecting signals we reached 5σ upper limits on the individual nights that range from 11% -13% and 1.7% -2.0% for the sodium and ionized calcium absorption of the tail, respectively. Night 1 was contaminated by its companion star so we considered weighted averages with and without Night 1 and quote conservative 5σ limits without Night 1 of 9% and 1.4%, respectively. Assuming their mass fractions to be similar to those in the Earth's crust, these limits correspond to scenarios in which 0.04% and 35% of the transiting dust is sublimated and observed as absorbing gas. However, this assumes the gas to be co-moving with the planet. We show that for the high irradiation environment of K2-22 b, sodium and ionized calcium could be quickly accelerated to 100s of km s −1 owing to radiation pressure and entrainment by the stellar wind, making these species much more difficult to detect. No evidence for such possibly broad and blue-shifted signals are seen in our data. Conclusions. Future observations aimed at observing circumplanetary gas should take into account the possible broad and blue-shifted velocity field of atomic and ionized species.

Dust formation in the outflows of catastrophically evaporating planets

“…Ito & Ikoma 2021), and note that gas absorption lines have not yet been detected in systems with dusty tails (e.g. Ridden-Harper et al 2019).…”

Constraining the Bulk Composition of Disintegrating Exoplanets Using Combined Transmission Spectra from JWST and SPICA

“…Outside the midinfrared range, Lyman-α lines in the ultraviolet (e.g., Vidal-Madjar et al 2003;Lecavelier Des Etangs et al 2010;Ehrenreich et al 2015) and the absorption feature of excited metastable helium at 1083 nm (Seager & Sasselov 2000;Oklopčić & Hirata 2018;Spake et al 2018) are potentially useful to study whether the disintegrating planets possess an escaping atmosphere of hydrogen and helium. Detection of the line absorption features of sodium and calcium atoms would give us constraints on the gas loss and sublimation processes at work on disintegrating planets (Ridden-Harper et al 2019).…”

A possible transit of a disintegrating exoplanet in the nearby multiplanet system DMPP-1

“…Metal-rich gas must co-exist with the dusty effluents from CDEs (Rappaport et al 2012;Ridden-Harper et al 2019). A search for traces of this circumstellar gas in the Na D and Ca II infrared triplet lines in spectra of K2-22 yielded null results and a range of upper limits (Ridden-Harper et al 2019).…”

“…Metal-rich gas must co-exist with the dusty effluents from CDEs (Rappaport et al 2012;Ridden-Harper et al 2019). A search for traces of this circumstellar gas in the Na D and Ca II infrared triplet lines in spectra of K2-22 yielded null results and a range of upper limits (Ridden-Harper et al 2019). DMPP-1 is already known to exhibit circumstellar absorption (Staab et al 2019;Haswell et al 2019), so if the marginal transit detection reported herein does prove to be a CDE, this system will be the best yet known to trace the gas loss and sublimation processes discussed in Ridden-Harper et al (2019).…”