Rapid Formation of Super-Earths around M Dwarf Stars

While the recent microlensing discoveries of super-Earths orbiting two Mdwarf stars have been taken as support for the core accretion mechanism ofgiant planet formation, we show here that these planets could also have beenformed by the competing mechanism of disk instability, coupled withphotoevaporative loss of their gaseous envelopes by a strong external source ofUV radiation, i.e., an O star. M dwarfs that form in regions of futurehigh-mass star formation would then be expected to have super-Earths orbitingat distances of several AU and beyond, while those that form in regions oflow-mass star formation would be expected to have gas giants at thosedistances. Given that most stars are born in the former rather than in thelatter regions, M dwarfs should have significantly more super-Earths than gasgiants, as seems to be indicated by the microlensing surveys.Comment: 13 pages, 2 figures, 1 table. Astrophysical Journal (Letters), in pres