EXORCISM: a spectroscopic survey of young eruptive variables (EXor and candidates)

TL;DR

This spectroscopic survey of young eruptive stars investigates their accretion and outflow properties, revealing significant variability in accretion rates and signatures of mass loss, with implications for understanding stellar evolution.

Contribution

The study provides detailed spectroscopic measurements of 11 young eruptive stars, highlighting accretion variability and establishing correlations between accretion activity and line formation region density.

Findings

Accretion rates vary significantly, with some stars showing decreases by two orders of magnitude.

80% of sources exhibit the [OI]630 nm line, indicating active mass loss.

Mass accretion variations are generally larger than mass loss variations.

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 ($\Re$ $\sim$ 1500-2000) were collected between 2014-2019 with the Large Binocular Telescope (LBT). From the spectral analysis we have derived the accretion luminosity and mass accretion rate, the visual extinction, the temperature and density of the permitted line formation region, 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 accretion luminosity and mass accretion rates in line with the values measured in classical T Tauri stars of similar mass. All sources observed more than once present accretion variability. The most extreme case is ASASSN-13db, for which the mass accretion rate decreases by two orders of magnitude from the outburst peak in 2015 to quiescence in 2017. Also, in NY Ori the accretion luminosity decreases by a factor 25 in one year. In 80% of the sample we detect the [OI]630 nm line, a tracer of mass loss. From the variability of the H$\alpha$/[OI]630, ratio, we conclude that mass accretion variations are larger than mass loss variations. From the analysis of the HI recombination lines a correlation is suggested between the density of the line formation region and the level of accretion activity of the source.