Sub-stellar companions of the young weak-line TTauri Star DoAr21

The compact, non-thermal emission in DoAr21 has been studied with the VLBA to investigate the possibility that the residuals of the astrometry fitting are due to the reflex motion induced by a possible companion. We find that the fitting of VLBA astrometric observations of DoAr21 improves significantly by adding the orbital motions of three companions. We obtain an improved distance to the source of $134.6\pm1.0$ pc, and estimate that the central star, DoAr21, has a mass of about $2.04\pm0.70~M_{\odot}$. We suggest that DoAr21 represents a unique case where two sub-stellar companions, DoAr21b and DoAr21c ($m_{b}\sim 35.6\pm27.2~M_{jup}$ and $m_{c}\sim44.0\pm13.6~M_{jup}$, respectively), have been found associated to a relatively low mass, pre-main sequence star. In addition, we find that this WTTau star is an astrometric double system, having a low-mass star companion, DoAr21$B$ ($m_{B}\sim0.35\pm0.12~M_{\odot}$), in a relatively eccentric orbit. The orbit of this low-mass stellar companion is compact, while the Brown Dwarfs are located in external orbits. DoAr21$c$ has the strongest astrometric signature in the periodogram, while DoAr21$B$ has a weak but significant signature. On the other hand, the astrometric signature of DoAr21$b$ does not appear in the periodogram, however, this Brown Dwarf was directly detected in some of the VLBA observations. The estimated orbital periods of DoAr21$B$, DoAr21$b$ and DoAr21$c$ are $P_{B}\sim92.92\pm0.02$, $P_{b}\sim450.9\pm3.8$ and $P_{c}\sim1013.5\pm25.3$ days, respectively. Since the estimated age of this young star is about 0.4$-$0.8 Myrs, the detected Brown Dwarf companion is among the youngest companions observed to date.