ALMA and NACO observations towards the young exoring transit system
  J1407 (V1400 Cen)
  Our aim was to directly detect the thermal emission of the putative exoring
system responsible for the complex deep transits observed in the light curve
for the young Sco-Cen star 1SWASP J140747.93-394542.6 (V1400 Cen, hereafter
J1407), confirming it as the occulter seen in May 2007, and to determine its
orbital parameters with respect to the star. We used the Atacama Large
Millimeter/submillimeter Array (ALMA) to observe the field centred on J1407 in
the 340 GHz (Band 7) continuum in order to determine the flux and astrometric
location of the ring system relative to the star. We used the VLT/NACO camera
to observe the J1407 system in March 2019 and to search for the central
planetary mass object at thermal infrared wavelengths. We detect no point
source at the expected location of J1407, and derive an upper limit $3\sigma$
level of $57.6~\mu\rm{Jy}$. There is a point source detected at an angular
separation consistent with the expected location for a free-floating ring
system that occulted J1407 in May 2007, with a flux of $89~\mu\rm{Jy}$
consistent with optically thin dust surrounding a massive substellar companion.
At 3.8 microns with the NACO camera, we detect the star J1407 but no other
additional point sources within 1.3 arcseconds of the star, with a lower bound
on the sensitivity of $6M_{Jup}$ at the location of the ALMA source, and down
to $4M_{Jup}$ in the sky background limit. The ALMA upper limit at the location
of J1407 implies that a hypothesised bound ring system is composed of dust
smaller than $1\rm{~mm}$ in size, implying a young ring structure. The detected
ALMA source has multiple interpretations, including: (i) it is an unbound
substellar object surrounded by warm dust in Sco-Cen with an upper mass limit
of $6M_{Jup}$, or (ii) it is a background galaxy.
