80 research outputs found
The Formation of Brown Dwarfs: Observations
We review the current state of observational work on the formation of brown
dwarfs, focusing on their initial mass function, velocity and spatial
distributions at birth, multiplicity, accretion, and circumstellar disks. The
available measurements of these various properties are consistent with a common
formation mechanism for brown dwarfs and stars. In particular, the existence of
widely separated binary brown dwarfs and a probable isolated proto-brown dwarf
indicate that some substellar objects are able to form in the same manner as
stars through unperturbed cloud fragmentation. Additional mechanisms such as
ejection and photoevaporation may play a role in the birth of some brown
dwarfs, but there is no observational evidence to date to suggest that they are
the key elements that make it possible for substellar bodies to form.Comment: Protostars and Planets V, in pres
The Prototypical Young L/T-Transition Dwarf HD 203030B Likely Has Planetary Mass
Upon its discovery in 2006, the young L7.5 companion to the solar analog HD
203030 was found to be unusual in being 200 K cooler than older late-L
dwarfs. HD 203030B offered the first clear indication that the effective
temperature at the L-to-T spectral type transition depends on surface gravity:
now a well-known characteristic of low-gravity ultra-cool dwarfs. An initial
age analysis of the G8V primary star indicated that the system was 130--400 Myr
old, and so the companion between 12--31 . Using moderate
resolution near-infrared spectra of HD 203030B, we now find features of very
low gravity comparable to those of 10--150 Myr-old L7--L8 dwarfs. We also
obtained more accurate near infrared and {\sl Spitzer}/IRAC photometry, and
find a color of mag---comparable to those
observed in other young planetary-mass objects---and a luminosity of
log() dex. We further reassess the
evidence for the young age of the host star, HD 203030, with a more
comprehensive analysis of the photometry and updated stellar activity
measurements and age calibrations. Summarizing the age diagnostics for both
components of the binary, we adopt an age of 100 Myr for HD 203030B and an age
range of 30--150 Myr. Using cloudy evolutionary models, the new companion age
range and luminosity result in a mass of 11 with a range of 8--15
, and an effective temperature of K.Comment: 12 pages, 7 figures, accepted for publication in A
The Formation and Early Evolution of Low-mass Stars and Brown Dwarfs
The discovery of large numbers of young low-mass stars and brown dwarfs over
the last decade has made it possible to investigate star formation and early
evolution in a previously unexplored mass regime. In this review, we begin by
describing surveys for low-mass members of nearby associations, open clusters,
star-forming regions and the methods used to characterize their stellar
properties. We then use observations of these populations to test theories of
star formation and evolution at low masses. For comparison to the formation
models, we consider the initial mass function, stellar multiplicity,
circumstellar disks, protostellar characteristics, and kinematic and spatial
distributions at birth for low-mass stars and brown dwarfs. To test the
evolutionary models, we focus on measurements of dynamical masses and empirical
Hertzsprung-Russell diagrams for young brown dwarfs and planetary companions.Comment: Final published version at http://www.annualreviews.org/journal/astr
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