EXORCISM: A Spectroscopic Survey of Young Eruptive Variables (EXor and Candidates)
T. Giannini, A. Giunta, Manuele Gangi, R. Carini, D. Lorenzetti, S. Antoniucci, A. Caratti o Garatti, L. P. Cassará, B. Nisini, A. Rossi, V. Testa, F. Vitali
Abstract
Abstract We present an optical/near-IR survey of 11 variable young stars (EXors and EXor candidates) aimed at deriving and monitoring their accretion properties. About 30 optical and near-infrared spectra ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="fraktur">R</mml:mi> <mml:mo>∼</mml:mo> <mml:mn>1500</mml:mn> <mml:mo>–</mml:mo> <mml:mn>2000</mml:mn> </mml:math> ) were collected between 2014 and 2019 with the Large Binocular Telescope (LBT). From the spectral analysis we have derived the accretion luminosity ( L acc ) and mass accretion rate ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mi>acc</mml:mi> </mml:mrow> </mml:msub> </mml:math> ), the visual extinction ( A V ), the temperature and density of the permitted line formation region ( T , n H ), and the signature of the outflowing matter. Two sources (ASASSN-13db and iPTF15afq) have been observed in outburst and quiescence, three during a high level of brightness (XZ Tau, PV Cep, and NY Ori), and the others in quiescence. These latter have L acc and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mi>acc</mml:mi> </mml:mrow> </mml:msub> </mml:math> in line with the values measured in classical T Tauri stars of similar mass. All sources observed more than once present L acc and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mi>acc</mml:mi> </mml:mrow> </mml:msub> </mml:math> variability. The most extreme case is ASASSN-13db, for which <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mi>acc</mml:mi> </mml:mrow> </mml:msub> </mml:math> decreases by two orders of magnitude from the outburst peak in 2015 to quiescence in 2017. Also, in NY Ori L acc decreases by a factor 25 in one year. In 80% of the sample we detect the [O i ] 6300 Å line, a tracer of mass loss. From the variability of the H α /[O i ] 6300 Å ratio, we conclude that mass accretion variations are larger than mass loss variations. From the analysis of the H i recombination lines, a correlation is suggested between the density of the line formation region, and the level of accretion activity of the source.