+Statistical Properties of Brown Dwarf Companions around Solar Type Stars

Ma, Bo Department of Astronomy University of Florida

“From Stars to Life”, April 2013 Gainesville, FL

Reference: Ma & Ge, 2013, arxiv:1303.6442

+Background

 Brown dwarfs (BD) are in the mass range of approximately 13-80 Jupiter masses, having sufficient masses to burn deuterium but not enough to burn hydrogen in their inner cores (Burrows et al. 1997).

 The first discovery of a bona-fide BD (Rebolo et al. 1995; Nakajima et al. 1995; Oppenheimer et al. 1995; Basri et al. 1996; Rebolo et al. 1996) came in the same year as the discovery of the first extra-solar planet around a solar type star, 51 Peg b

+Background

 Brown dwarf desert, a paucity of brown dwarf companions relative to planets within 3 AU around main-sequence FGKM stars (Marcy & Butler 2000; Grether & Lineweaver 2006).

 The California & Carnegie Planet Search measured an occurrence rate of 0.7%±0.2% from their sample of ~1000 target stars (Vogt et al. 2002, Patel et al. 2007).

 Sahlmann et al. (2011a) obtained an upper limit of 0.6% for the frequency of close BD companions based on the uniform stellar sample of the CORALIE planet search, which contains 1600 solar type stars within 50 pc.

+How do they form

 Current understanding of BD formation: the BDs may form either like a star (relatively high mass BD) or like a planet (low mass BD).

 We want to look for evidence which could support such an idea about BD formation.

 We searched literature and found a total of 65 BD companions around solar type star with known orbits.

Ma & Ge (2013)

+Period-Mass relation

A low density region in the period-mass diagram

+Cumulative Distribution of Mass

A clear depletion of close BDs in the range of 30-55 Jupiter Mass

Serves as a natural dividing line between two BD sub-sample. (Sahlmann et al. 2011)

+Period-Eccentricity Relation

The probability that these two BD samples are drawn from the same distribution is 1.7%.

When comparing our two BD samples with binary sample from Halbwachs et al (2003), 18% and 0.1% respectively.

+Mass–Eccentricity Relation

If eccentricity is excited via scattering-scattering model in the disk, then more massive one will tend to have lower eccentricity

Evidence of formation in a disk?

A mass limit for planets?

38 Jupiter mass (Mordasini et al. 2009)

+Host Star Metallicity Distribution

Relatively metal poor when comparing with giant planet host star

Against core accretion model

core Giant planet BD

+Main Results:

 BD companions are almost depleted at P<100 days and 30 MJup<M<55MJup in the period-mass diagram

 High mass BDs have eccentricity distribution similar to that of binaries

 Low mass BDs have eccentricity distribution consistent with forming in a circumstellar disk.

  Host stars of BD companions are metal poor when comparing with host stars of giant planets, suggesting a formation scenario at least partly different from the core-accretion model.

+Scenario of BD formation:

 BD companion with mass above ~40 MJup may form similar to stellar binary systems;

 BD companions with masses below ~40 MJup may form in a protoplanetary disk through the disk-instability scenario, or other model that does not favor metal rich star.

 A future larger BD sample will be very helpful to test these ideas.

 Comments are welcome!

Thank you!

Ma & Ge, 2013, arxiv:1303.6442