802 research outputs found
Negative feedback effects on star formation history and cosmic reionization
After considering the effects of negative feedback on the process of star
formation, we explore the relationship between star formation process and the
associated feedback, by investigating how the mechanical feedback from
supernovae(SNe) and radiative feedback from luminous objects regulate the star
formation rate and therefore affect the cosmic reionization.Based on our
present knowledge of the negative feedback theory and some numerical
simulations, we construct an analytic model in the framework of the Lambda cold
dark matter model. In certain parameter regions, our model can explain some
observational results properly. In large halos(T_vir>10000 K), both mechanical
and radiative feedback have a similar behavior: the relative strength of
negative feedback reduces as the redshift decreases. In contrast, in small
halos (T_vir<10000 K$) that are thought to breed the first stars at early time,
the radiative feedback gets stronger when the redshift decreases. And the star
formation rate in these small halos depends very weakly on the star-formation
efficiency. Our results show that the radiative feedback is important for the
early generation stars. It can suppress the star formation rate considerably.
But the mechanical feedback from the SNe explosions is not able to affect the
early star formation significantly. The early star formation in small-halo
objects is likely to be self-regulated. The radiative and mechanical feedback
dominates the star formation rate of the PopII/I stars all along. The feedback
from first generation stars is very strong and should not be neglected.
However, their effects on the cosmic reionization are not significant, which
results in a small contribution to the optical depth of Thomson scattering.Comment: 12 pages,6 figure
Observe matter falling into a black hole
It has been well known that in the point of view of a distant observer, all
in-falling matter to a black hole (BH) will be eventually stalled and "frozen"
just outside the event horizon of the BH, although an in-falling observer will
see the matter falling straight through the event horizon. Thus in this "frozen
star" scenario, as distant observers, we could never observe matter falling
into a BH, neither could we see any "real" BH other than primordial ones, since
all other BHs are believed to be formed by matter falling towards singularity.
Here we first obtain the exact solution for a pressureless mass shell around a
pre-existing BH. The metrics inside and interior to the shell are all different
from the Schwarzschild metric of the enclosed mass. The metric interior to the
shell can be transformed to the Schwarzschild metric for a slower clock which
is dependent of the location and mass of the shell. Another result is that
there does not exist a singularity nor event horizon in the shell. Therefore
the "frozen star" scenario is incorrect. We also show that for all practical
astrophysical settings the in-falling time recorded by an external observer is
sufficiently short that future astrophysical instruments may be able to follow
the whole process of matter falling into BHs. The distant observer could not
distinguish between a "real" BH and a "frozen star", until two such objects
merge together. It has been proposed that electromagnetic waves will be
produced when two "frozen stars" merge together, but not true when two "real"
bare BHs merge together. However gravitational waves will be produced in both
cases. Thus our solution is testable by future high sensitivity astronomical
observations.Comment: 7 pages, 2 figures. Proceeding of the conference "Astrophysics of
Compact Objects", 1-7 July, Huangshan, China. Abridged abstrac
Possible evidence that pulsars are quark stars
It is a pity that the real state of matter in pulsar-like stars is still not
determined confidently because of the uncertainty about cold matter at
supranuclear density, even 40 years after the discovery of pulsar. Nuclear
matter (related to neutron stars) is one of the speculations for the inner
constitution of pulsars even from the Landau's time more than 70 years ago, but
quark matter (related to quark stars) is an alternative due to the fact of
asymptotic freedom of interaction between quarks as the standard model of
particle physics develops since 1960s. Therefore, one has to focus on
astrophysical observations in order to answer what the nature of pulsars is. In
this presentation, I would like to summarize possible observational
evidence/hints that pulsar-like stars could be quark stars, and to address
achievable clear evidence for quark stars in the future experiments.Comment: 6 pages, 2 figures; a talk at the international conference
"Astrophysics of Compact Objects" (July 1-7, 2007; Huangshan, China);
http://vega.bac.pku.edu.cn/rxxu/publications/index_C.htm. A mistake in Fig.1
is corrected; Correction of typo
Phenomenology of Gamma-Ray Jets
We discuss some phenomenological aspects of -ray emitting jets. In
particular, we present calculations of the -sphere and -sphere for
various target photon fields, and employ them to demonstrate how -ray
observations at very high energies can be used to constraint the Doppler factor
of the emitting plasma and the production of VHE neutrinos. We also consider
the implications of the rapid TeV variability observed in M87 and the TeV
blazars, and propose a model for the very rapid TeV flares observed with HESS
and MAGIC in some blazars,that accommodates the relatively small Doppler
factors inferred from radio observations. Finally, we briefly discuss the
prospects for detecting VHE neutrinos from relativistic jets.Comment: Proceedings, Huangshan meeting on "Astrophysics of Compact Objects
Dynamics of Magnetized Spherical Accretion Flows
Transonic accretion flow with self-consistent treatment of random magnetic
field is presented.Comment: in proceedings to "Astrophysics of Compact Objects", Huangshan,
China, 200
On the Efficiency of Thermal Conduction in Galaxy Clusters
Galaxy clusters host a large reservoir of diffuse plasma with
radially-varying temperature profiles. The efficiency of thermal conduction in
the intracluster medium (ICM) is complicated by the existence of turbulence and
magnetic fields, and has received a lot of attention in the literature.
Previous studies suggest that the magnetothermal instability developed in outer
regions of galaxy clusters would drive magnetic field lines preferentially
radial, resulting in efficient conduction along the radial direction. Using a
series of spherically-symmetric simulations, here we investigate the impact of
thermal conduction on the observed temperature distributions in outer regions
of three massive clusters, and find that thermal conduction substantially
modifies the ICM temperature profile. Within 3 Gyr, the gas temperature at a
representative radius of typically decreases by ~10 - 20% and the
average temperature slope between and drops by ~ 30 -
40%, indicating that the observed ICM would not stay in a long-term equilibrium
state in the presence of thermal conduction. However, X-ray observations show
that the outer regions of massive clusters have remarkably similar
radially-declining temperature profiles, suggesting that they should be quite
stable. Our study thus suggests that the effective conductivity along the
radial direction must be suppressed below the Spitzer value by a factor of 10
or more, unless additional heating sources offset conductive cooling and
maintain the observed temperature distributions. Our study provides a
smoking-gun evidence for the suppression of parallel conduction along magnetic
field lines in low-collisionality plasmas by kinetic mirror or whistler
instabilities.Comment: Slightly revised version, accepted for publication in ApJ. 11 pages,
7 figure
"Black Star" or Astrophysical Black Hole?
Recently wide publicity has been given to a claim by T. Vachaspati that
"black holes do not exist", that the objects known as black holes in
astrophysics should rather be called "black stars" and they not only do not
have event horizons but actually can be the source of spectacular gamma ray
bursts. In this short essay (no flimsier than the original preprint where these
extravagant claims appeared) I demonstrate that these ill-considered claims are
clearly wrong. Yet they present a good occasion to reflect on some well known
but little discussed conceptual difficulties which arise when applying
relativistic terminology in an astrophysical context.Comment: Poster presented at "Compact Objects" meeting in Hunagshan, China,
2-7 July 2007. To be published in the AIP Conference Proceeding serie
A simple proof of exponential decay in the two dimensional percolation model
Kesten showed the exponential decay of percolation probability in the
subcritical phase for the two-dimensional percolation model. This result
implies his celebrated computation that for bond percolation in the
square lattice, and site percolation in the triangular lattice, respectively.
In this paper, we present a simpler proof for Kesten's theorem.Comment: 9 pages and one figur
Neutron star magnetospheres: the binary pulsar, Crab and magnetars
A number of disparate observational and theoretical pieces of evidence
indicate that, contrary to the conventional wisdom, neutron stars' closed field
lines are populated by dense, hot plasma and may be responsible for producing
some radio and high energy emission. This conclusion is based on eclipse
modeling of the binary pulsar system PSR J0737-3039A/B (Lyutikov & Thompson
2005), a quantitative theory of Crab giant pulses (Lyutikov 2007) and a number
of theoretical works related to production of non-thermal spectra in magnetars
through resonant scattering. In magnetars, dense pair plasma is produced by
twisting magnetic field lines and associated electric fields required to lift
the particles from the surface. In long period pulsars, hot particles on closed
field lines can be efficiently trapped by magnetic mirroring, so that
relatively low supply rate, e.g. due to a drift from open field lines, may
result in high density. In short period pulsars, magnetic mirroring does not
work; large densities may still be expected at the magnetic equator near the
Y-point.Comment: Proceedings, Huangshan meeting "Astrophysics of Compact Objects
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