MOA-2007-BLG-197: exploring the brown dwarf desert
Ranc, C and Cassan, A and Albrow, MD and Kubas, D and Bond, IA and Batista, V and Beaulieu, JP and Bennett, DP and Dominik, M and Dong, S and Fouque, P and Gould, A and Greenhill, J and Jorgensen, UG and Kains, N and Menzies, J and Sumi, T and Bachelet, E and Coutures, C and Dieters, S and Dominis Prester, D and Donatowicz, J and Gaudi, BS and Han, C and Hundertmark, M and Horne, K and Kane, SR and Lee, C-U and Marquette, J-B and Park, B-G and Pollard, KR and Sahu, KC and Street, R and Tsapras, Y and Wambsganss, J and Williams, A and Zub, M and Abe, F and Fukui, A and Itow, Y and Masuda, K and Matsubara, Y and Muraki, Y and Ohnishi, K and Rattenbury, N and Saito, To and Sullivan, DJ and Sweatman, WL and Tristram, PJ and Yock, PCM and Yonehara, A, MOA-2007-BLG-197: exploring the brown dwarf desert, Astronomy and Astrophysics, 580 Article A125. ISSN 0004-6361 (2015) [Refereed Article]
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 μFUN\ 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 ≃ 1.13\), the mass ratio q = (4.732 ± 0.020) × 10-2 and the Einstein radius crossing time (tE ≃ 82 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,Ks) magnitudes obtained with VLT NaCo and calibrated using IRSF and 2MASS data. From this analysis, we derived a lens total mass of 0.86 ± 0.04 M⊙ and a lens distance of DL = 4.2 ± 0.3 kpc. We find that the companion of MOA-2007-BLG-197L is a brown dwarf of 41 ± 2 MJ observed at a projected separation of a⊥ = 4.3 ± 0.1 AU, and orbits a 0.82 ± 0.04 M⊙ 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 ≲ 30 d, M ~ 30−60 MJ), but also find an accumulation of objects around P ~ 500 d and i>M ~ 20 MJ, as well as another depletion region at long orbital periods (P ≳ 500 d) and high masses (M ≳ 50 MJ). 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.
planets and satellites: detection, gravitational lensing: macro, brown dwarfs