Strong H$\alpha$ emission and signs of accretion in a circumbinary planetary mass companion from MUSE

TL;DR

This study reports strong Hα and Hβ emission indicating active accretion in a young circumbinary planetary companion, providing insights into its formation and age, with implications for understanding accretion in low-mass systems.

Contribution

First spectroscopic characterization of the accreting circumbinary planetary companion 2MASS J01033563-5515561 (AB)b using MUSE, revealing active accretion signs and estimating its mass.

Findings

Detected strong Hα and Hβ emission lines.

Estimated accretion rate from multiple methods.

Indications of a potentially younger age than expected.

Abstract

Context. Intrinsic H$\alpha$ emission can be advantageously used to detect substellar companions because it improves contrasts in direct imaging. Characterising this emission from accreting exoplanets allows for the testing of planet formation theories. Aims. We characterise the young circumbinary planetary mass companion 2MASS J01033563-5515561 (AB)b (Delorme 1 (AB)b) through medium-resolution spectroscopy. Methods. We used the new narrow-field mode (NFM) for the MUSE integral-field spectrograph, located on the ESO Very Large Telescope, during science verification time to obtain optical spectra of Delorme 1 (AB)b. Results. We report the discovery of very strong H$\alpha$ and H$\beta$ emission, accompanied by He I emission. This is consistent with an active accretion scenario. We provide accretion rate estimates obtained from several independent methods and find a likely mass of $12-15$ M$_{\rm Jup}$ for Delorme 1 (AB)b. This is also consistent with previous estimates. Conclusions. Signs of active accretion in the Delorme 1 system might indicate a younger age than the $\sim 30-40$ Myr expected from a likely membership in Tucana-Horologium (THA). Previous works have also shown the central binary to be overluminous, which gives further indication of a younger age. However, recent discoveries of active discs in relatively old ($\sim 40$ Myr), very low-mass systems suggests that ongoing accretion in Delorme 1 (AB)b might not require in and of itself that the system is younger than the age implied by its THA membership.