Investigating episodic accretion in a very low-mass young stellar object^⋆

¹ Dublin Institute for Advanced Studies (DIAS), School of Cosmic Physics, Astronomy and Astrophysics Section, 31 Fitzwilliam Place, Dublin 2, Ireland
e-mail: cstock@cp.dias.ie
² Trinity College Dublin, School of Physics, College Green, Dublin 2, Ireland
³ School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
⁴ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
⁵ Department of Space, Earth & Environment, Chalmers University of Technology, 412 93 Gothenburg, Sweden

Received: 11 July 2020
Accepted: 10 September 2020

Abstract

Context. Very low-mass Class I protostars have been investigated very little thus far. Variability of these young stellar objects (YSOs) and whether or not they are capable of strong episodic accretion is also left relatively unstudied.

Aims. We investigate accretion variability in IRS 54 (YLW52), a Class I very low-mass protostar with a mass of M_⋆ ∼ 0.1 − 0.2 M_⊙.

Methods. We obtained spectroscopic and photometric data with VLT/ISAAC and VLT/SINFONI in the near-infrared (J, H, and K bands) across four epochs (2005, 2010, 2013, and 2014). We used accretion-tracing lines (Paβ and Brγ) and outflow-tracing lines (H₂ and [Fe II]) to examine physical properties and kinematics of the object.

Results. A large increase in luminosity was found between the 2005 and 2013 epochs of more than 1 magnitude in the K band, followed in 2014 by a steep decrease. Consistently, the mass accretion rate (Ṁ_acc) rose by an order of magnitude from ∼10⁻⁸ M_⊙ yr⁻¹ to ∼10⁻⁷ M_⊙ yr⁻¹ between the two early epochs. The visual extinction (A_V) has also increased from ∼15 mag in 2005 to ∼24 mag in 2013. This rise in A_V in tandem with the increase in Ṁ_acc is explained by the lifting up of a large amount of dust from the disc of IRS 54, following the augmented accretion and ejection activity in the YSO, which intersects our line of sight due to the almost edge-on geometry of the disc. Because of the strength and timescales involved in this dramatic increase, this event is believed to have been an accretion burst possibly similar to bursts of EXor-type objects. IRS 54 is the lowest mass Class I source observed to have an accretion burst of this type, and therefore potentially one of the lowest mass EXor-type objects known so far.