Investigating episodic accretion in a very low-mass young stellar object
  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. We
investigate accretion variability in IRS54, a Class I very low-mass protostar
with a mass of M$_{\star}$ ~ 0.1 - 0.2 M$_{\odot}$. 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 (Pa$\beta$ and Br$\gamma$) and outflow-tracing lines
(H$_2$ and [FeII] to examine physical properties and kinematics of the object.
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 ($\dot{M}_{acc}$) rose by an order of
magnitude from ~ 10$^{-8}$ M$_{\odot}$ yr$^{-1}$ to ~ $10^{-7}$ M$_{\odot}$
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 $\dot{M}_{acc}$ is explained by the lifting up of a
large amount of dust from the disc of IRS54, 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. IRS54 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.
