IRAM 04191+1522: a compact proto-brown dwarf binary candidate

Very low-luminosity objects in nearby star-forming regions have been identified as promising proto-brown dwarf candidates. The study of their multiplicity can shed light on the dominant formation mechanism of these substellar objects. We aim at studying the multiplicity of the very low luminosity object IRAM 04191+1522. To do so, we have obtained 0.89mm ALMA observations with a very extended configuration, achieving an angular resolution of ~0.04 arcsec (6 au at 140 pc). We have complemented our data with new VLA observations, and ALMA archival data at 1.3mm. As a result, we resolve IRAM04191+1522 into a close binary candidate for the first time. The binary is detected in the ALMA continuum data with a projected separation of ~80 mas, or 11 au at a distance of 140 pc. The two sources are oriented in the East-West direction, with the eastern component being brighter and more extended than the western one, which is marginally resolved. The analysis of C18O(2-1) archival data reveals gaseous material in rotation around the binary, presumably from a circumbinary disk with ~27 au of radius centered on the faintest ALMA component. A fit of the position-velocity diagram allows us to estimate a total dynamical mass for the system of 50+-40 MJup. Therefore, we classify IRAM04191 as a tight proto-brown dwarf binary candidate. The VLA data reveals the detection of a single object closer to the western ALMA source, and with a spectral index consistent with a radio jet.