Protostars and Planets VI, Heidelberg, July 15-20, 2013

Poster 2H028

Formation scenarios and observational detection of giant planet and brown dwarf embryos on wide orbits

Vorobiev, Eduard (Institute of Astrophysics, University of Vienna, Tuerkenschanzstrasse 17, 1180, Vienna, Austria and Research Institute of Physics, Southern Federal University, Rostov-on-Don, 344090, Russia )
Dunham, Michael (Department of Astronomy, Yale University, New Haven, CT 06520, USA)
Zakhozhay, Olga (Main Astronomical Observatory, National Academy of Sciences of Ukraine, Kyiv, Ukraine)

We numerically studied the formation and likelihood of observational detection of giant planet (GP) and brown dwarf (BD) embryos formed via gravitational fragmentation of protostellar disks. While most models showed disk fragmentation with fragments spaning a mass range from about a Jupiter mass to very-low-mass protostars, only 6 models out of 60 revealed the formation of quasi-stable, wide-orbit GP/BD embryos. The low probability for the fragment survival is in agreement with the low frequency of observational detection of wide-orbit companions and is caused by efficient inward migration/ejection/dispersal mechanisms that operate in the embedded phase of star formation. Disk fragmentation produced GP/BD embryos with masses in the 3.5-43 M_J range, covering the whole mass spectrum of directly imaged, wide-orbit companions to (sub-)solar mass stars. On the other hand, our modeling failed to produce embryos on orbital distances < 170 AU, whereas several directly imaged companions were found at smaller orbits down to a few AU. Disk fragmentation also failed to produce wide-orbit companions around stars with mass < 0.7 M_sun and multicomponent systems, both in disagreement with observations. Disk fragmentation is unlikely to explain the whole observed spectrum of wide-orbit companions to (sub-)solar-mass stars and other formation mechanisms, for instance, dynamical scattering of closely packed companions onto wide orbits, should be invoked to account for the whole spectrum of observed wide-orbit companions. We explore the possibility of observational detection of the GP/BD embryos in disks viewed through the outflow cavity and demonstrate that one hour of integration time with the Atacama Larger Millimeter Array is sufficient to detect the fragments with masses as low as 2.0 M_J at radial distances up to 800 AU. For the adopted resolution of our simulated ALMA images of 0.1 arcsec, the fragments can be detected at distances down to 50 AU. At smaller distances, the fragments usually merge with the central density peak. Some of the most massive fragments, regardless of their orbital distance, can produce characteristic peaks at \\approx 5 mkm and hence their presence can be indirectly inferred from the observed spectral energy distributions of protostars.

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