Probing episodic accretion with chemistry: CALYPSO observations of IRAM
  04191+1522. Results from the CALYPSO IRAM-PdBI survey
  Context. The process of mass accretion in the earliest phases of star
formation is still not fully understood: Does the accretion rate smoothly
decline with the age of the protostar or are there short, intermittent
accretion bursts? Aims. We aim to explore whether or not the observed C$^{18}$O
and N$_2$H$^+$ emission pattern towards the VeLLO IRAM 04191+1522 can be
understood in the framework of a scenario where the emission is chemically
tracing a past accretion burst. Methods.We used high-angular-resolution Plateau
de Bure Interferometer (PdBI) observations of C$^{18}$O and N$_2$H$^+$ towards
IRAM 04191+1522 that were obtained as part of the CALYPSO IRAM Large Program.
We model these observations using a chemical code with a time-dependent
physical structure coupled with a radiative transfer module, where we allow for
variations in the source luminosity. Results. We find that the N$_2$H$^+$ line
emission shows a central hole, while the C$^{18}$O emission is compact. The
morphology of these two lines cannot be reproduced with a constant luminosity
model based on the present-day internal luminosity (0.08 L$_{sun}$). However,
the N$_2$H$^+$ peaks are consistent with a constant-luminosity model of 12
L$_{sun}$. Using a model with time-dependent temperature and density profiles,
we show that the observed N$_2$H$^+$ peak emission could indeed be caused by a
past accretion burst. Such a burst should have occurred a couple of hundred
years ago. Conclusions. We suggest that an accretion burst occurred in IRAM
04191+1522 in the recent past. If such bursts are common and sufficiently long
in VeLLOs, they could lead to higher accretion onto the central object than
their luminosity suggests. For IRAM 04191 in particular, our results yield an
estimated final mass of 0.2 - 0.25 M$_{sun}$ by the end of the Class 0 phase,
which would make this object a low-mass star rather than a brown dwarf.
