Notes on Mesolensing of a Background Galaxy by Luhman's Dwarf

Einstein radius of nearby stars lensing very distant sources

Nearby stars have two properties that make them potentially useful as gravitational lenses. The first is that their proximity makes their Einstein radius for very distant sources large enough to be resolved by modern telescopes. When the distance between the source and lens \(D_{\rm LS} \approx D_{\rm S}\), the distance to the source, we are in the mesolensing regime: \[\theta_E = \sqrt{\frac{4GM}{c^2} \frac{D_{\rm LS}}{D_{\rm L}D_{\rm S}}} \approx 90.3 \mbox{mas} \sqrt{\frac{(M/M_\odot)}{(D_{\rm L}/\mbox{pc})}}\]

The second is that their high proper motions mean they will lens very faint, extragalactic background sources more frequently than more distant stars.

The possibility of mesolensing by vB10 was discussed by (Lépine et al., 2012), and the overall statistics of detecting such events has been analyzed by (Stefano 2008, Stefano 2008a).

Finite lens sizes

One potential complication is the physical size of the star itself, which projects to an angular size of \[\theta_*=R/D_{\rm L}=4.65 \mbox{mas} \frac{R/R_\odot}{D_{\rm L}/pc}\] Using the rough rule of thumb that \(R/R_\odot \approx M/M_\odot\), the angular size of the star thus equal to its Einstein radius at a distance of \[D_{\rm L} = 5.7\times10^{-4} \mbox{pc} (M/M_\odot)\] So we see that all dwarf stars except the Sun have Einstein radii larger than their angular radii. Brown dwarfs have size \(\sim 0.1 R_\odot\), which even at 2.5 pc yields \(\sim\)0.2 mas, much smaller than its Einstein ring.