Discovering substellar companions with VLBI astrometry
The detection and physical characterisation of young brown dwarfs is the first step towards understanding the mechanism that is responsible for forming substellar objects. A number of brown dwarf companions has been discovered by high-precision radial velocity surveys, because of their large signatures that easily exceed the radial velocity measurement precision that is often designed to search for exoplanets. However, the high levels of magnetic activity that are often observed in young stars produce a variability of the order of several hundreds of m/s and hamper the detection of substellar companions around these targets with radial velocities. High-precision astrometry offers one promising alternative to search for young substellar companions because it is less sensitive to stellar activity as compared to the radial velocity method. The astrometric sensitivity increases with the period (and separation) of the companion and is therefore more efficient in regimes where the radial velocity method has reduced precision (see Figure 1).
Figure 1: Space of orbital parameters and masses that can be investigated with radial velocities and astrometry. The red line indicates the radial velocity signature for a precision of 660 m/s which corresponds to the radial velocity scatter observed in the 2Myr old star V830Tau (see e.g. Damasso et al. 2020). The green solid line indicates the astrometric signature of the companion for an astrometric precision of 0.1 mas that can be achieved with VLBA observations based on our previous experience. Our calculations assume a host star with 1Msun located at a distance of 130 pc which is representative of our targets.
We will search for brown dwarf companions around young stars using high-precision Very Long Baseline Interferometry (VLBI) astrometric observations. In particular, we aim to detect long-period (> 10yr) substellar companions covering a region of the space of parameters (see Figure 1) that (i) has not yet been sufficiently explored in the literature, (ii) cannot be done with Gaia data alone, and (iii) is not covered by the radial velocity method.
We will use multi-epoch observations collected with the Very Long Baseline Array (VLBA) to measure very precise positions and infer the existence of substellar companions around our targets (see Figure 2). VLBA observations have been shown to be sensitive to the quiescent and flaring radio emission produced by brown dwarfs, so that they can be directly detected in our observations. We will solve for the full orbital solution of each system and compute dynamical masses for the substellar companions that can be directly detected in our observations.
Figure 2: Astrometry fit for two binary systems including the orbital motion of the system in the solution. Left panel: the extended time baseline of VLBA observations for the HPTau G2 system allows one to infer the existence of the secondary component (not detected in the VLBA images). Right panel: astrometric solution with the orbital motion of the V1023 Tau system based on the stellar positions of the secondary component directly detected in the VLBA images (see Galli et al. 2018).