16 research outputs found
Boundary layer on the surface of a neutron star
In an attempt to model the accretion onto a neutron star in low-mass X-ray
binaries, we present two-dimensional hydrodynamical models of the gas flow in
close vicinity of the stellar surface. First we consider a gas pressure
dominated case, assuming that the star is non-rotating. For the stellar mass we
take M_{\rm star}=1.4 \times 10^{-2} \msun and for the gas temperature K. Our results are qualitatively different in the case of a
realistic neutron star mass and a realistic gas temperature of
K, when the radiation pressure dominates. We show that to get the stationary
solution in a latter case, the star most probably has to rotate with the
considerable velocity.Comment: 7 pages, 7 figure
A self-consistent model of a 22 GHz water maser in a dusty environment near late-type stars
We study the conditions for operation of the 22 GHz ortho-water maser in a
dusty medium near late-type stars. The main physical processes, such as
exchange of energy between dust and gas in the radiation field of a star,
radiative cooling by water molecules and pumping of water masers are described
self-consistently. We show that the presence of dust grains of various types
(or of one type with size distribution) strongly affects the maser action. The
pumping mechanism based on the presence of the dust of different optical
properties is able to explain water masers in the silicate carbon star V778
Cyg. However, the masers in the winds from asymptotic giant branch stars
require an additional source of heating, for instance due to the dust drift
through the gas.Comment: 15 pages, 15 figures, A&A, in pres
Water masers in dusty environments
We study in details a pumping mechanism for the lambda=1.35 cm maser
transition 6_16 -> 5_23 in ortho-water based on the difference between gas and
dust temperatures. The upper maser level is populated radiatively through 4_14
-> 5_05 and 5_05 -> 6_16 transitions. The heat sink is realized by absorbing
the 45 mum photons, corresponding to the 5_23 -> 4_14 transition, by cold dust.
We compute the inversion of maser level populations in the optically thick
medium as a function of the hydrogen concentration, the gas-to-dust mass ratio,
and the difference between the gas and the dust temperatures. The main results
of numerical simulations are interpreted in terms of a simplified four-level
model. We show that the maser strength depends mostly on the product of
hydrogen concentration and the dust-to-water mass ratio but not on the size
distribution of the dust particles or their type. We also suggest approximate
formulae that describe accurately the inversion and can be used for fast
calculations of the maser luminosity. Depending on the gas temperature, the
maximum maser luminosity is reached when the water concentration N_water ~
10^6-10^7 cm^-3 times the dust-to-hydrogen mass ratio, and the inversion
completely disappears at density just an order of magnitude larger. For the
dust temperature of 130 K, the 6_16 -> 5_23 transition becomes inverted already
at the temperature difference of Delta T ~1 K, while other possible masing
transitions require a larger Delta T > 30 K. We identify the region of the
parameter space where other ortho- and para-water masing transitions can
appear.Comment: 14 pages, 10 figures; submitted to A&
Oxygen-rich disk in the V778 Cyg system resolved
Various scenarios have been proposed to explain the presence of silicate
features associated with carbon stars, such as V778 Cyg. We have attempted to
constrain these theories by means of mapping water maser mission from V778 Cyg.
The 22 GHz water maser emission from this star has been mapped using MERLIN
with an astrometric accuracy of 25 mas. The spatially- and
kinematically-resolved maser complex is displaced by about 190 mas from the
position of the C-star as measured 10 years earlier using Tycho. Our
simulations and analysis of available data show that this position difference
is unlikely to be due to proper motion if V778 Cyg is at the assumed distance
of 1.4 kpc. The maser components seem to form a distorted S-shaped structure
extended over 18 mas with a clear velocity gradient. We propose a model which
explains the observed water maser structure as an O-rich warped disk around a
companion of the C-star in V 778 binary system, which is seen almost edge-on.
Analysis of observational data, especially those obtained with MERLIN, suggests
that V778 Cyg (and, by implication, other silicate carbon stars) are binary
systems composed of a C-rich star and a companion which stores circumstellar
O-rich material.Comment: 4 pages, 3 figures, A&A in pres