131 research outputs found
Demographics of Transition Objects
The unusual properties of transition objects (young stars with an optically
thin inner disc surrounded by an optically thick outer disc) suggest that
significant disc evolution has occured in these systems. We explore the nature
of these systems by examining their demographics, specifically their stellar
accretion rates (Mdot) and disc masses (Mdisc) compared to those of accreting T
Tauri stars of comparable age. We find that transition objects in Taurus occupy
a restricted region of the Mdot vs. Mdisc plane. Compared to non-transition
single stars in Taurus, they have stellar accretion rates that are typically
~10 times lower at the same disc mass and median disc masses ~4 times larger.
These properties are anticipated by several proposed planet formation theories
and suggest that the formation of Jovian mass planets may play a significant
role in explaining the origin of at least some transition objects. Considering
transition objects as a distinct demographic group among accreting T Tauri
stars leads to a tighter relationship between disc masses and stellar accretion
rates, with a slope between the two quantities that is close to the value of
unity expected in simple theories of disc accretion.Comment: 11 pages, 4 figures, to appear in MNRA
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
HD 101088, An Accreting 14 AU Binary in Lower Centaurus Crux With Very Little Circumstellar Dust
We present high resolution (R=55,000) optical spectra obtained with MIKE on
the 6.5 m Magellan Clay Telescope as well as Spitzer MIPS photometry and IRS
low resolution (R~60) spectroscopy of the close (14 AU separation) binary, HD
101088, a member of the ~12 Myr old southern region of the Lower Centaurus Crux
(LCC) subgroup of the Scorpius-Centaurus OB association. We find that the
primary and/or secondary is accreting from a tenuous circumprimary and/or
circumsecondary disk despite the apparent lack of a massive circumbinary disk.
We estimate a lower limit to the accretion rate of > 1x10^-9 solar masses per
year, which our multiple observation epochs show varies over a timescale of
months. The upper limit on the 70 micron flux allows us to place an upper limit
on the mass of dust grains smaller than several microns present in a
circumbinary disk of 0.16 moon masses. We conclude that the classification of
disks into either protoplanetary or debris disks based on fractional infrared
luminosity alone may be misleading.Comment: 8 pages, 2 figures, ApJ accepte
The Highly Dynamic Behavior of the Innermost Dust and Gas in the Transition Disk Variable LRLL 31
We describe extensive synoptic multi-wavelength observations of the
transition disk LRLL 31 in the young cluster IC 348. We combined four epochs of
IRS spectra, nine epochs of MIPS photometry, seven epochs of cold-mission IRAC
photometry and 36 epochs of warm mission IRAC photometry along with multi-epoch
near-infrared spectra, optical spectra and polarimetry to explore the nature of
the rapid variability of this object. We find that the inner disk, as traced by
the 2-5micron excess stays at the dust sublimation radius while the strength of
the excess changes by a factor of 8 on weekly timescales, and the 3.6 and
4.5micron photometry shows a drop of 0.35 magnitudes in one week followed by a
slow 0.5 magnitude increase over the next three weeks. The accretion rate, as
measured by PaBeta and BrGamma emission lines, varies by a factor of five with
evidence for a correlation between the accretion rate and the infrared excess.
While the gas and dust in the inner disk are fluctuating the central star stays
relatively static. Our observations allow us to put constraints on the physical
mechanism responsible for the variability. The variabile accretion, and wind,
are unlikely to be causes of the variability, but both are effects of the same
physical process that disturbs the disk. The lack of periodicity in our
infrared monitoring indicates that it is unlikely that there is a companion
within ~0.4 AU that is perturbing the disk. The most likely explanation is
either a companion beyond ~0.4 AU or a dynamic interface between the stellar
magnetic field and the disk leading to a variable scale height and/or warping
of the inner disk.Comment: Accepted to ApJ. 10 pages of text, plus 11 tables and 13 figures at
the en
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