Characterization of Molecular Outflows in the Substellar Domain

Abstract

We report here our latest search for molecular outflows from young brown dwarfs and very low-mass stars in nearby star-forming regions. We have observed three sources in Taurus with the Submillimeter Array and the Combined Array for Research in Millimeter-wave Astronomy at 230 GHz frequency to search for CO J = 2 → 1 outflows. We obtain a tentative detection of a redshifted and extended gas lobe at about 10 arcsec from the source GM Tau, a young brown dwarf in Taurus with an estimated mass of 73 M J , which is right below the hydrogen-burning limit. No blueshifted emission around the brown dwarf position is detected. The redshifted gas lobe that is elongated in the northeast direction suggests a possible bipolar outflow from the source with a position angle of about 36 • . Assuming that the redshifted emission is outflow emission from GM Tau, we then estimate a molecular outflow mass in the range from 1.9 × 10 −6 M to 2.9 × 10 −5 M and an outflow mass-loss rate from 2.7 × 10 −9 M yr −1 to 4.1 × 10 −8 M yr −1 . These values are comparable to those we have observed in the young brown dwarf ISO-Oph 102 of 60 M J in ρ Ophiuchi and the very low-mass star MHO 5 of 90 M J in Taurus. Our results suggest that the outflow process in very low-mass objects is episodic with a duration of a few thousand years and the outflow rate of active episodes does not significantly change for different stages of the formation process of very low-mass objects. This may provide us with important implications that clarify the formation process of brown dwarfs.

Bipolar molecular outflow of the very low-mass star Par-Lup3-4

“…The outflow velocity measured for Par-Lup3-4 is slightly higher (6 km/s) but on the same order as the velocities observed in other VLM stars and BDs, which are between 1 to 4.7 km/s (Phan-Bao et al 2008;Kauffmann et al 2011;Phan-Bao et al 2014). Low-mass stars have outflows with velocities in the range between 10-100 km/s (Arce et al 2007, and references therein).…”

“…Par-Lup3-4 has a molecular outflow mass of ∼10 −6 M that is in the range of the observed values for other VLM stars and BDs that span 10 −4 to 10 −6 M (Phan- Bao et al 2014). As pointed out by Phan-Bao et al (2014), the VLM outflow values are at least one order of magnitude lower than the values obtained for low-mass protostars in a similar evolutionary status.…”

“…The outflow mass-loss rate for a typical lowmass protostar ranges from 8.9 × 10 −9 to 10 −4 M yr −1 M yr −1 , although the median value is 10 −7 M yr −1 (Levreault 1988b). The mass-loss rate for VLM stars and BDs is lower, with values lying between 2.5 × 10 −9 and 2 × 10 −7 M yr −1 (Phan- Bao et al 2014). The mass-loss rate for Par-Lup3-4 is even lower than the expected value for VLM stars and BDs, but this might be an effect of the potentially missed flux emission from the whole outflow extent that we do not observe.…”

“…Common and notable characteristics of outflows from these sources include a very small physical size (600-1000 au), and a low outflow velocity (<5 km/s). Other characteristics, such as the ratio between wind mass-loss and accretion rate (Phan-Bao et al 2014), have been studied to search for trends among VLM stars and BDs with evolution. The observational uncertainties are still large, however.…”

Using binary statistics in Taurus-Auriga to distinguish between brown dwarf formation processes

“…The faint object SMM2E will most likely remain a substellar object (30 − 35 M Jupiter , Palau et al 2014), just as a 73 M Jupiter object in Taurus (Phan-Bao et al 2014). These authors interpret their respective objects to be a scaleddown version of a forming star.…”