Brightness and mass accretion rate evolution during the 2022 burst of EX Lupi

Abstract

Context. EX Lupi is the prototype by which EXor-type outbursts have been defined. It has experienced multiple accretion-related bursts and outbursts throughout the past decades, and the study of these events has greatly extended our knowledge about their effects. Notably, this star experienced a new burst in 2022. Aims. We aim to investigate whether the recent brightening was caused by temporarily increased accretion or by a brief decrease in the extinction and study the evolution of the EX Lupi system throughout this event. Methods. We used multi-band photometry to create color-color and color-magnitude diagrams to exclude the possibility that the brightening could be explained by a decrease in extinction. We obtained spectra using the X-shooter instrument of the Very Large Telescope (VLT) to determine the Lacc and Ṁacc during the peak of the burst and after its return to quiescence using two different methods: empirical relationships between line luminosity and Lacc, and a slab model of the whole spectrum. We examined the 130-yr light curve of EX Lupi to provide statistics on the number of outbursts experienced during this period of time. Results. Our analysis of the data taken during the 2022 burst confirmed that a change in extinction is not responsible for the brightening. Our two approaches in calculating the Ṁacc were in agreement and resulted in values that are two orders of magnitude above what had previously been estimated for EX Lupi using only a couple of individual emission lines, thus suggesting that EX Lupi is a strong accretor even when in quiescence. We determined that in 2022 March, the Ṁacc increased by a factor of seven with respect to the quiescent level. We also found hints that even though the Ṁacc had returned to near pre-outburst levels, certain physical properties of the gas (i.e., temperature and density) had not returned to the quiescent values. Conclusions. We found that the mass accreted during this three-month event was 0.8 lunar masses, which is approximately half of what is accreted during a year of quiescence. We calculated that if EX Lupi remains as active as it has been for the past 130 yr, during which it has experienced at least three outbursts and ten bursts, then it will deplete the mass of its circumstellar material in less than 160 000 yr.