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

Cruz-Sáenz de Miera, F. and Kóspál, Á. and Ábrahám, P. and Claes, R. A. B. and Manara, C. F. and Wendeborn, J. and Fiorellino, E. and Giannini, T. and Nisini, B. and Sicilia-Aguilar, A. and Campbell-White, J. and Alcalá, J. M. and Banzatti, A. and Szabó, Zs. M. and Lykou, F. and Antoniucci, S. and Varga, J. and Siwak, M. and Park, S. and Nagy, Zs. and Kun, M. (2023) Brightness and mass accretion rate evolution during the 2022 burst of EX Lupi. ASTRONOMY & ASTROPHYSICS. ISSN 0004-6361 (In Press)

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Abstract

Context. EX Lupi is the prototype by which EXor-type outbursts were defined. It has experienced multiple accretion-related bursts and outbursts throughout the last decades, whose study have greatly extended our knowledge about the effects of these types of events. This star experienced a new burst in 2022. Aims. We aim to investigate whether this recent brightening was caused by temporarily increased accretion or by a brief decrease in the extinction, and to 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 M˙ acc 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 year 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 M˙ acc 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 M˙ acc increased by a factor of 7 with respect to the quiescent level. We also found hints that even though the M˙ acc had returned to almost its 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 years, during which it has experienced at least 3 outbursts and 10 bursts, then it will deplete the mass of its circumstellar material in less than 160 000 yr.

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