Wide planetarymass companions of lowmass dwarfs

First, an aside: recent months have seen the imaging of low-mass companions to several young stars in the Solar Neighborhood. Neuhauser et al. (2005) have identified a faint, common proper motion (cpm) companion of GQ Lupi, a -0.45 M0 member of the Lupus I group (t — 1-2 Myrs); the companion lies at a separation of -100 AU and has a likely mass between 1 and 42 Jupiter masses (MJ). Similarly, Chauvin et al. (2005b) have discovered a faint cpm companion to AB Pic, a K2 dwarf, member of the -30 Myr-old Tucana-Horologium association; the companion is -245 AU distant, with a likely mass of 13-14 Mj. Finally, and most intriguingly, Chauvin et al. (2005a) have shown that 2MASS J1207334-393254, a -35 MJ brown dwarf member of the -10-Myr-old TW Hydrae association, has a wide cpm companion, separation -60 AU, with a likely mass of only 2-5 MJ.

Should we classify these low-mass companions as planets or brown dwarfs? In my opinion, all of these objects are brown dwarfs. The mass estimates themselves, based on evolutionary models, offer no discrimination, since, while they overlap with the planetary regime, it remains unclear whether they lie beyond a lower limit to the brown dwarf mass

Separation (AU)

Figure 1. Wide planetary-mass companions of low-mass dwarfs: total mass as a function of separation in binary systems (adapted from Burgasser et al. 2003). The crosses plot data for stellar binaries, while solid points identify systems with brown dwarf components; the solid line outlines a linear relation between maximum separation and total mass, while the dotted line plots ttmax <x Mt2ot. The stars mark the location of 2M1207AB, GQ Lupi and AB Pic AB.

Separation (AU)

Figure 1. Wide planetary-mass companions of low-mass dwarfs: total mass as a function of separation in binary systems (adapted from Burgasser et al. 2003). The crosses plot data for stellar binaries, while solid points identify systems with brown dwarf components; the solid line outlines a linear relation between maximum separation and total mass, while the dotted line plots ttmax <x Mt2ot. The stars mark the location of 2M1207AB, GQ Lupi and AB Pic AB.

spectrum (if such exists). The large distance between each companion and its primary is a more telling parameter. There is little question that the Solar System planets formed from the Sun's protoplanetary disk; it seems reasonable to apply the same criterion in categorizing extrasolar planets. This test offers no discriminatory power at small separations, as with the 11 MJ companion of HD 114762, which has an orbital semi-major axis of 0.3 AU. However, one might argue that disks are likely to have problems forming massive companions at large radii. Thus, it seems unlikely that a 35 MJ brown dwarf would possess a protoplanetary disk sufficiently massive that it could form a 2-5 MJ companion at a distance of 60 AU from the primary.

Nonetheless, this debate over terminology should not be allowed to obscure the significance of these discoveries, particularly 2M1207AB. Recent analyses show that the maximum separation of binaries in the field appears to correlate with the total mass (Burgasser et al. 2003). The newly discovered systems lie at the extremes of this distribution (Figure 1). 2M1207AB is particularly notable, with not only a projected separation, A, four times higher than comparable brown dwarf binaries, but also a very low mass ratio, q = < 0.14. All of these systems would be extremely difficult to detect at ages exceeding ~108 years; thus, their absence among field binaries could reflect either dynamical evolution (and system disruption), or observational selection effects.

10

7 stars

• •

* •

• _ • _ • • •• •• • • • • • *

i.C/yV*. * • • •

.. . * • TrES-l

• i i

i i 1 i i i i 1 i i

Was this article helpful?

0 0

Post a comment