X-ray flares of the young planet host Ds Tucanae A

Abstract

The discovery of planets around young stars has spurred novel studies of the early phases of planetary formation and evolution. Stars are strong emitters at X-ray and UV wavelengths in their first billion of years and this strongly affects the evaporation, thermodynamics, and chemistry in the atmospheres of the young planets orbiting around them. In order to investigate these effects in young exoplanets, we observed the 40 Myr old star DS Tuc A with XMM-Newton. We recorded two X-ray bright flares, with the second event occurring about 12 ks after the first one. Their duration, from the rise to the end of the decay, was about 8 - 10 ks in soft X-rays (0.3-10 keV). The flares were also recorded in the 200-300 nm band with the UVM2 filter of the Optical Monitor. The duration of the flares in UV was about 3 ks. The observed delay between the peak in the UV band and in X-rays is a probe of the heating phase, followed by evaporation and an increase in the density and emission of the flaring loop. The coronal plasma temperature at the two flare peaks reached 54-55 MK. Diagnostics based on the temperatures and timescales of the flares applied to these two events have allowed us to infer a loop length of 5 - 7 x 10(10) cm, which is about the extent of the stellar radius. We also inferred the values of electron density at the flare peaks of 2.3 - 6.5 x 10(11) cm(-3), along with a minimum magnetic field strength on the order of 300-500 G that is needed to confine the plasma. The energy released during the flares was on the order of 5 - 8 x 10(34) erg in the bands 0.3 - 10 keV and 0.9 - 2.7 x 10(33) erg in the UV band (200-300 nm). We speculate that the flares were associated with coronal mass ejections (CMEs) that hit the planet about 3.3 h after the flares, which dramatically increased the rate of evaporation for the planet. From the RGS spectra, we retrieved the emission measure distribution and the abundances of coronal metals during the quiescent and flaring states, respectively. Finally, we inferred a high electron density measurement, which is in agreement with the inferences drawn from time-resolved spectroscopy and EPIC spectra, as well as the analysis of RGS spectra during the flares.