Infrared spectroscopy of eruptive variable protostars from VVV

TL;DR

This study presents infrared spectra of 37 eruptive variable protostars from the VVV survey, revealing diverse accretion signatures and proposing a new class called MNors for these intermediate objects.

Contribution

The paper introduces a new classification, MNors, for eruptive YSOs with intermediate outburst durations and mixed features, expanding understanding of episodic accretion.

Findings

15/18 eruptive YSOs show high accretion signatures

Most eruptive variables differ from classical FUor and EXor types

Eruptive YSOs with bright state observations have higher accretion luminosities

Abstract

In a companion work (Paper I) we detected a large population of highly variable Young Stellar Objects (YSOs) in the Vista Variables in the Via Lactea (VVV) survey, typically with class I or flat spectrum spectral energy distributions and diverse light curve types. Here we present infrared spectra (0.9--2.5 $\mu$m) of 37 of these variables, many of them observed in a bright state. The spectra confirm that 15/18 sources with eruptive light curves have signatures of a high accretion rate, either showing EXor-like emission features ($\Delta$v=2 CO, Br$\gamma$) and/or FUor-like features ($\Delta$v=2 CO and H$_{2}$O strongly in absorption). Similar features were seen in some long term periodic YSOs and faders but not in dippers or short-term variables. The sample includes some dusty Mira variables (typically distinguished by smooth Mira-like light curves), 2 cataclysmic variables and a carbon star. In total we have added 19 new objects to the broad class of eruptive variable YSOs with episodic accretion. Eruptive variable YSOs in our sample that were observed at bright states show higher accretion luminosities than the rest of the sample. Most of the eruptive variables differ from the established FUor and EXor subclasses, showing intermediate outburst durations and a mixture of their spectroscopic characteristics. This is in line with a small number of other recent discoveries. Since these previously atypical objects are now the majority amongst embedded members of the class, we propose a new classification for them as MNors. This term (pronounced emnor) follows V1647 Ori, the illuminating star of McNeil's Nebula.