A transmission spectrum of the planet candidate WD 1856+534 b and a lower limit to its mass^★

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
e-mail: ras@iac.es
² Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
³ Gran Telescopio Canarias (GTC), 38712, Breña Baja, La Palma, Spain
⁴ Department of Physics, The University of Warwick, Coventry CV4 7AL, UK

Received: 15 January 2021
Accepted: 29 March 2021

Abstract

The cool white dwarf WD 1856+534 was found to be transited by a Jupiter-sized object with a mass at or below 14 M_Jup. We used the GTC telescope to obtain and analyse the photometry and low-resolution spectroscopy of six transits of WD 1856+534 b, with the intention of deriving the slope of the transmission spectrum. Such a slope, assuming a cloud-free atmosphere dominated by Rayleigh scattering of the particles in its atmosphere, could be translated into an estimation of the mass of WD 1856+534 b. However, the resultant transmission spectrum is essentially flat and therefore permits only the determination of lower mass limits of 2.4 M_Jup at the 2σ level, or 1.6 M_Jup at 3σ. These limits have implications for some of the formation scenarios proposed for the object. We elaborate on the potential effects of clouds and hazes in our estimations, based on previous studies of Jupiter and Titan. In addition, we detected an Hα absorption feature in the combined spectrum of the host white dwarf, which leads to the assignation of a DA classification and allows the derivation of an independent set of atmospheric parameters. Furthermore, the epochs of five transits were measured with sub-second precision, which demonstrates that additional objects more massive than ≈5 M_Jup and with periods longer than O(100) days could be detected through the light-time effect.