13 research outputs found
Apparent Sizes and Spectral Line Profiles for Spherical and Disk Masers: Solutions to the Full Equations
Calculations are performed for the spectral line profiles and images of
astrophysical maser radiation that emerges from isolated spheres and thin disks
viewed edge-on. In contrast to previous investigations where various
approximations are made, the full equations are solved here for the
frequency-dependent radiative transport that includes the thermal motion of the
molecules. The spectral line profiles for spheres and disks are found to
rebroaden to the full thermal Doppler breadth with increasing saturation in
essentially the same way as is well known to occur for a linear maser. The
variation with frequency in the apparent angular sizes of masing spheres and
thin disks is found to be negligible at frequencies within the spectral line
where the flux is significant. Calculations also are performed for spherical
and disk masers that are not isolated, but for which the seed radiation for the
masers is incident from one side as would occur when a strong continuum source
is on the far side of the masers. Again, the spectral line profiles are found
to rebroaden to the full thermal breadths with increasing saturation and there
are no significant variations in the apparent angular sizes with frequency.
However, the full rebroadening does occur at somewhat higher saturation and the
variation of the apparent angular sizes as a function of the degree of
saturation is quite different from that of the isolated masers. Spheres and
disks have served as idealized geometries with which to examine possible
deviations from the linear approximation for astrophysical masers.Comment: accepted for Astrophysical Journa
Polarization of Astronomical Maser Radiation. IV. Circular Polarization Profiles
Profile comparison of the Stokes parameters and is a powerful tool
for maser data analysis, providing the first direct methods for unambiguous
determination of (1) the maser saturation stage, (2) the amplification optical
depth and intrinsic Doppler width of unsaturated masers, and (3) the
comparative magnitudes of Zeeman splitting and Doppler linewidth. Circular
polarization recently detected in OH 1720 MHz emission from the Galactic center
appears to provide the first direct evidence for maser saturation.Comment: 14 pages, 1 Postscript figures (included), uses aaspp4.sty. To appear
in Astrophysical Journa
Selfgravitating Gas Spheres in a Box and Relativistic Clusters: Relation between Dynamical and Thermodynamical Stability
We derive a variational principle for the dynamical stability of a cluster as
a gas sphere in a box. Newtonian clusters are always dynamically stable and,
for relativistic clusters, the relation between dynamical and thermodynamical
instabilities is analyzed. The boundaries between dynamically and
thermodynamically stable and unstable models are found numerically for
relativistic stellar systems with different cut off parameters. A criterion
based on binding energy curve is used for determination of the boundary of
dynamical stability.Comment: 10 figure
Exact analytical solution of the collapse of self-gravitating Brownian particles and bacterial populations at zero temperature
We provide an exact analytical solution of the collapse dynamics of
self-gravitating Brownian particles and bacterial populations at zero
temperature. These systems are described by the Smoluchowski-Poisson system or
Keller-Segel model in which the diffusion term is neglected. As a result, the
dynamics is purely deterministic. A cold system undergoes a gravitational
collapse leading to a finite time singularity: the central density increases
and becomes infinite in a finite time t_coll. The evolution continues in the
post collapse regime. A Dirac peak emerges, grows and finally captures all the
mass in a finite time t_end, while the central density excluding the Dirac peak
progressively decreases. Close to the collapse time, the pre and post collapse
evolution is self-similar. Interestingly, if one starts from a parabolic
density profile, one obtains an exact analytical solution that describes the
whole collapse dynamics, from the initial time to the end, and accounts for non
self-similar corrections that were neglected in previous works. Our results
have possible application in different areas including astrophysics,
chemotaxis, colloids and nanoscience
Cosmological expansion and local physics
The interplay between cosmological expansion and local attraction in a
gravitationally bound system is revisited in various regimes. First, weakly
gravitating Newtonian systems are considered, followed by various exact
solutions describing a relativistic central object embedded in a Friedmann
universe. It is shown that the ``all or nothing'' behaviour recently discovered
(i.e., weakly coupled systems are comoving while strongly coupled ones resist
the cosmic expansion) is limited to the de Sitter background. New exact
solutions are presented which describe black holes perfectly comoving with a
generic Friedmann universe. The possibility of violating cosmic censorship for
a black hole approaching the Big Rip is also discussed.Comment: 17 pages, LaTeX, to appear in Phys. Rev.
Various Modes of Helium Mixing in Globular Cluster Giants and Their Possible Effects on the Horizontal Branch Morphology
It has been known for a long time that some red giants in globular clusters
exhibit large star-to-star variations in the abundances of light elements that
are not exhibited by field giants. This fact can be taken as evidence that the
extra mixing mechanism(s) that operate in globular cluster giants may be
consequences of star-star interactions in the dense stellar environment. In
order to constrain the extra mixing mechanism(s), we study the influence of
helium enrichment along the red giant branch (RGB) on the evolution of stars
through the horizontal branch. Three possible modes of helium enrichment are
considered, associated with close encounters of stars in the globular clusters.
We show that as a consequence of the variations in the core mass as well as in
the total mass due to mass loss, the color of horizontal branch models are
distributed over almost all range of horizontal branch. The results are
discussed in relation to the scenario for the origin of the abundance anomalies
and for the effects on the morphology of horizontal branch. We argue that the
star-star interactions can explain not only the source of angular momentum of
rapid rotation but also provide a mechanism for the bimodal distribution of
rotation rates in some globular clusters. We also propose the time elapsed from
the latest core collapse phase during the gravo-thermal oscillations as the
second parameter to explain the variations in HB morphology among the globular
clusters.Comment: 57 pages, 16 figures, to be published on June 2006 in Ap
Cosmic Black-Hole Hair Growth and Quasar OJ287
An old result ({\tt astro-ph/9905303}) by Jacobson implies that a black hole
with Schwarzschild radius acquires scalar hair, ,
when the (canonically normalized) scalar field in question is slowly
time-dependent far from the black hole, with
time-independent. Such a time dependence could arise in
scalar-tensor theories either from cosmological evolution, or due to the slow
motion of the black hole within an asymptotic spatial gradient in the scalar
field. Most remarkably, the amount of scalar hair so induced is independent of
the strength with which the scalar couples to matter. We argue that Jacobson's
Miracle Hair-Growth Formula implies, in particular, that an
orbiting pair of black holes can radiate {\em dipole} radiation, provided only
that the two black holes have different masses. Quasar OJ 287, situated at
redshift , has been argued to be a double black-hole binary
system of this type, whose orbital decay recently has been indirectly measured
and found to agree with the predictions of General Relativity to within 6%. We
argue that the absence of observable scalar dipole radiation in this system
yields the remarkable bound on the
instantaneous time derivative at this redshift (as opposed to constraining an
average field difference, , over cosmological times), provided
only that the scalar is light enough to be radiated --- i.e. m \lsim 10^{-23}
eV --- independent of how the scalar couples to matter. This can also be
interpreted as constraining (in a more model-dependent way) the binary's motion
relative to any spatial variation of the scalar field within its immediate
vicinity within its host galaxy.Comment: 20 page
Dynamical Processes in Globular Clusters
Globular clusters are among the most congested stellar systems in the
Universe. Internal dynamical evolution drives them toward states of high
central density, while simultaneously concentrating the most massive stars and
binary systems in their cores. As a result, these clusters are expected to be
sites of frequent close encounters and physical collisions between stars and
binaries, making them efficient factories for the production of interesting and
observable astrophysical exotica. I describe some elements of the competition
among stellar dynamics, stellar evolution, and other processes that control
globular cluster dynamics, with particular emphasis on pathways that may lead
to the formation of blue stragglers.Comment: Chapter 10, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Relativistic Dynamics and Extreme Mass Ratio Inspirals
It is now well-established that a dark, compact object (DCO), very likely a
massive black hole (MBH) of around four million solar masses is lurking at the
centre of the Milky Way. While a consensus is emerging about the origin and
growth of supermassive black holes (with masses larger than a billion solar
masses), MBHs with smaller masses, such as the one in our galactic centre,
remain understudied and enigmatic. The key to understanding these holes - how
some of them grow by orders of magnitude in mass - lies in understanding the
dynamics of the stars in the galactic neighbourhood. Stars interact with the
central MBH primarily through their gradual inspiral due to the emission of
gravitational radiation. Also stars produce gases which will subsequently be
accreted by the MBH through collisions and disruptions brought about by the
strong central tidal field. Such processes can contribute significantly to the
mass of the MBH and progress in understanding them requires theoretical work in
preparation for future gravitational radiation millihertz missions and X-ray
observatories. In particular, a unique probe of these regions is the
gravitational radiation that is emitted by some compact stars very close to the
black holes and which could be surveyed by a millihertz gravitational wave
interferometer scrutinizing the range of masses fundamental to understanding
the origin and growth of supermassive black holes. By extracting the
information carried by the gravitational radiation, we can determine the mass
and spin of the central MBH with unprecedented precision and we can determine
how the holes "eat" stars that happen to be near them.Comment: Update from the first version, 151 pages, accepted for publication @
Living Reviews in Relativit
Relativistic Binaries in Globular Clusters
Galactic globular clusters are old, dense star systems typically containing
10\super{4}--10\super{7} stars. As an old population of stars, globular
clusters contain many collapsed and degenerate objects. As a dense population
of stars, globular clusters are the scene of many interesting close dynamical
interactions between stars. These dynamical interactions can alter the
evolution of individual stars and can produce tight binary systems containing
one or two compact objects. In this review, we discuss theoretical models of
globular cluster evolution and binary evolution, techniques for simulating this
evolution that leads to relativistic binaries, and current and possible future
observational evidence for this population. Our discussion of globular cluster
evolution will focus on the processes that boost the production of hard binary
systems and the subsequent interaction of these binaries that can alter the
properties of both bodies and can lead to exotic objects. Direct {\it N}-body
integrations and Fokker--Planck simulations of the evolution of globular
clusters that incorporate tidal interactions and lead to predictions of
relativistic binary populations are also discussed. We discuss the current
observational evidence for cataclysmic variables, millisecond pulsars, and
low-mass X-ray binaries as well as possible future detection of relativistic
binaries with gravitational radiation.Comment: 88 pages, 13 figures. Submitted update of Living Reviews articl