ON THE 2015 OUTBURST OF THE EXor VARIABLE V1118 ORI

Abstract

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 ΔV ∼ 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 a 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 H i 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 about an order of magnitude: M ˙ acc ?> = 1.2–4.8 10−8 M ⊙ yr−1, M ˙ loss ?> = 0.8–2 10−9 M ⊙ yr−1), 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.