6 research outputs found
Global dynamics of radiatively inefficient accretion flows: advection versus convection
We obtain global solutions of radiatively inefficiently accretion flows
around black holes. Whether and where convection develops in a flow are
self-consistently determined with the mixing-length theory. The solutions can
be divided into three types according to the strength of normal viscosity. Type
I solution corresponds to large viscosity parameter \alpha \ga 0.1, which is
purely advection-dominated and with no convection, and has been extensively
studied in the literature. Type II solution is for moderate ,
which has a three-zone structure. The inner zone is advection-dominated, the
middle zone is convection-dominated and ranges from a few tens to a few
thousands of gravitational radii, and the outer zone is convectively stable and
matches outward a Keplerian disc. The net energy flux throughout the flow is
inward as in type I solution. Type III solution which is for small \alpha \la
0.001 consists of two zones as Abramowicz et al. suggested previously: an
inner advection-dominated zone and an outer convection-dominated zone,
separated at a radius of a few tens of gravitational radii. This type of
solution has an outward net energy flux. In both type II and III solutions the
radial density profile is between the 1/2 law of self-similar
convection-dominated accretion flow model and the 3/2 law of self-similar
advection-dominated accretion flow model, and the efficiency of energy release
is found to be extremely low. Our results are in good agreement with those of
recent numerical simulations.Comment: 13 pages, 2 figures, accepted for publication in MNRA
Stellar dynamical evidence against a cold disc origin for stars in the Galactic Centre
Observations of massive stars within the central parsec of the Galaxy show
that, while most stars orbit within a well-defined disc, a significant fraction
have large eccentricities and / or inclinations with respect to the disc plane.
Here, we investigate whether this dynamically hot component could have arisen
via scattering from an initially cold disc -- the expected initial condition if
the stars formed from the fragmentation of an accretion disc. Using N-body
methods, we evolve a variety of flat, cold, stellar systems, and study the
effects of initial disc eccentricity, primordial binaries, very massive stars
and intermediate mass black holes. We find, consistent with previous results,
that a circular disc does not become eccentric enough unless there is a
significant population of undetected 100--1000 Msun objects. However, since
fragmentation of an eccentric disc can readily yield eccentric stellar orbits,
the strongest constraints come from inclinations. We show that_none_ of our
initial conditions yield the observed large inclinations, regardless of the
initial disc eccentricity or the presence of massive objects. These results
imply that the orbits of the young massive stars in the Galactic Centre are
largely primordial, and that the stars are unlikely to have formed as a
dynamically cold disc.Comment: 5 pages, 6 colour figures. MNRAS Letters in press. (v2: very minor
changes
The shortest period field contact binary
Photometric and spectroscopic results for the contact binary GSC 01387-00475
(ASAS 083128+1953.1) are presented. The existence of this binary with the
orbital period of P = 0.2178 d strengthens the argument that the cut-off of the
period distribution for contact binaries - until now defined by CC Comae - is
very sharp. The only case of a still shorter period is known in a globular
cluster where more compact contact configurations are in fact expected. While
the spectroscopic orbit of GSC 01387-00475 is well defined, the low orbital
inclination of the binary and the presence of a spectroscopic companion
contributing about 1/3 of the total light conspire to reduce the photometric
variability to ~0.09 mag. The photometric data are currently inadequate to
identify the source of the small amplitude (0.02 - 0.03 mag) intrinsic
variability of the system.Comment: to appear in MNRA
OGLE 2003-BLG-235/MOA 2003-BLG-53: A planetary microlensing event
We present observations of the unusual microlensing event OGLE
2003-BLG-235/MOA 2003-BLG-53. In this event a short duration (~7 days) low
amplitude deviation in the light curve due a single lens profile was observed
in both the MOA and OGLE survey observations. We find that the observed
features of the light curve can only be reproduced using a binary microlensing
model with an extreme (planetary) mass ratio of 0.0039 +/- (11, 07) for the
lensing system. If the lens system comprises a main sequence primary, we infer
that the secondary is a planet of about 1.5 Jupiter masses with an orbital
radius of ~3 AU.Comment: 13 pages, 3 colour figures. To appear in Astrophysical Journal
Letters (May 2004
Discovery of a Cool Planet of 5.5 Earth Masses Through Gravitational Microlensing
In the favoured core-accretion model of formation of planetary systems, solid
planetesimals accumulate to build up planetary cores, which then accrete
nebular gas if they are sufficiently massive. Around M-dwarf stars (the most
common stars in our Galaxy), this model favours the formation of Earth-mass to
Neptune-mass planets with orbital radii of 1 to 10 astronomical units (AU),
which is consistent with the small number of gas giant planets known to orbit
M-dwarf host stars. More than 170 extrasolar planets have been discovered with
a wide range of masses and orbital periods, but planets of Neptune's mass or
less have not hitherto been detected at separations of more than 0.15 AU from
normal stars. Here we report the discovery of a 5.5 (+5.5/-2.7) M_earth
planetary companion at a separation of 2.6 (+1.5/-0.6) AU from a 0.22
(+0.21/-0.11) M_solar M-dwarf star. (We propose to name it OGLE-2005-BLG-390Lb,
indicating a planetary mass companion to the lens star of the microlensing
event.) The mass is lower than that of GJ876d, although the error bars overlap.
Our detection suggests that such cool, sub-Neptune-mass planets may be more
common than gas giant planets, as predicted by the core accretion theory