We present the analysis of MOA-2007-BLG-197Lb , the first brown dwarf companion to a Sun-like star detected through gravitational microlensing .
The event was alerted and followed-up photometrically by a network of telescopes from the PLANET , MOA , and \mu \mathrm { FUN } collaborations , and observed at high angular resolution using the NaCo instrument at the VLT .
From the modelling of the microlensing light curve , we derived basic parameters such as , the binary lens separation in Einstein radius units ( s \simeq 1.13 ) , the mass ratio q = ( 4.732 \pm 0.020 ) \times 10 ^ { -2 } and the Einstein radius crossing time ( t _ { \mathrm { E } } \simeq 82 \mathrm { d } ) .
Because of this long time scale , we took annual parallax and orbital motion of the lens in the models into account , as well as finite source effects that were clearly detected during the source caustic exit .
To recover the lens system ’ s physical parameters , we combined the resulting light curve best-fit parameters with ( J,H,K _ { s } ) magnitudes obtained with VLT NaCo and calibrated using IRSF and 2MASS data .
From this analysis , we derived a lens total mass of 0.86 \pm 0.04 \mathrm { M } _ { \sun } and a lens distance of D _ { \mathrm { L } } = 4.2 \pm 0.3 \mathrm { kpc } .
We find that the companion of MOA-2007-BLG-197L is a brown dwarf of 41 \pm 2 \mathrm { M _ { J } } observed at a projected separation of a _ { \perp } = 4.3 \pm 0.1 \mathrm { AU } , and orbits a 0.82 \pm 0.04 \mathrm { M } _ { \sun } G-K dwarf star .
We then placed the companion of MOA-2007-BLG-197L in a mass-period diagram consisting of all brown dwarf companions detected so far through different techniques , including microlensing , transit , radial velocity , and direct imaging ( most of these objects orbit solar-type stars ) .
To study the statistical properties of this population , we performed a two-dimensional , non-parametric probability density distribution fit to the data , which draws a structured brown dwarf landscape .
We confirm the existence of a region that is strongly depleted in objects at short periods and intermediate masses ( P \lesssim 30 \mathrm { d } , M \sim 30 - 60 \mathrm { M _ { J } } ) , but also find an accumulation of objects around P \sim 500 \mathrm { d } and M \sim 20 \mathrm { M _ { J } } , as well as another depletion region at long orbital periods ( P \gtrsim 500 \mathrm { d } ) and high masses ( M \gtrsim 50 \mathrm { M _ { J } } ) .
While these data provide important clues on the different physical mechanisms of formation ( or destruction ) that shape the brown dwarf desert , more data are needed to establish their relative importance , in particular as a function of host star mass .
Future microlensing surveys should soon provide more detections , in particular for red dwarf hosts , thus uniquely complementing the solar-type host sample .