On the 2015 outburst of the EXor variable star V1118 Ori

TL;DR

The 2015 outburst of V1118 Ori, a typical EXor variable star, showed increased brightness, emission lines, and higher accretion and mass loss rates, with the event being less energetic than previous outbursts.

Contribution

This study provides detailed spectroscopic and photometric analysis of the 2015 outburst of V1118 Ori, highlighting changes in emission lines and accretion rates compared to quiescence and past outbursts.

Findings

Brightness increased by about 3 mag in V band.

Mass accretion rate increased by an order of magnitude.

Spectral lines indicative of enhanced accretion and outflow activity.

Abstract

After a long-lasting period of quiescence of about a decade, the source V1118 Ori, one of the most representative members of the EXor variables, is now outbursting. Since the initial increase of the near-infrared flux of about 1 mag (JHK bands) registered on 2015 September 22, the source brightness has remained fairly stable. We estimate DeltaV about 3 mag with respect to the quiescence phase. An optical/near-IR low-resolution spectrum has been obtained with the Large Binocular Telescope instruments MODS and LUCI2, and compared with a spectrum of similar spectral resolution and sensitivity level taken during quiescence. Together with the enhancement of the continuum, the outburst spectrum presents a definitely higher number of emission lines, in particular HI recombination lines of the Balmer, Paschen, and Brackett series, along with bright permitted lines of several species, forbidden atomic lines, and CO ro-vibrational lines. Both mass accretion and mass loss rates have significantly increased (by to about an order of magnitude, mass accretion rate = 1.2-4.8 10^-8 M_sun/yr, mass loss rate = 0.8-2 10^-9 M_sun/yr) with respect to the quiescence phase. If compared with previous outbursts, the present one appears less energetic. Alternatively, it could already be in the fading phase (with the maximum brightness level reached when the source was not visible), or, viceversa, still in the rising phase.