1,386 research outputs found
Planet Consumption and Stellar Metallicity Enhancements
The evolution of a giant planet within the stellar envelope of a
main-sequence star is investigated as a possible mechanism for enhancing the
stellar metallicities of the parent stars of extrasolar planetary systems.
Three-dimensional hydrodynamical simulations of a planet subject to impacting
stellar matter indicate that the envelope of a Jupiter-like giant planet can be
completely stripped in the outer stellar convection zone of a solar-mass star.
In contrast, Jupiter-like and less massive Saturn-like giant planets are able
to survive through the base of the convection zone of a 1.22 solar-mass star.
Although strongly dependent on details of planetary interior models, partial or
total dissolution of giant planets can result in significant enhancements in
the metallicity of host stars with masses between about 1.0 and 1.3 solar
masses. The implications of these results with regard to planetary orbital
migration are briefly discussed.Comment: 11 pages, 2 figures, accepted for ApJ Letter
Self-Consistent Data Analysis of the Proton Structure Function g1 and Extraction of its Moments
The reanalysis of all available world data on the longitudinal asymmetry A||
is presented. The proton structure function g1 was extracted within a unique
framework of data inputs and assumptions. These data allowed for a reliable
evaluation of moments of the structure function g1 in the Q2 range from 0.2 up
to 30 GeV2. The Q2 evolution of the moments was studied in QCD by means of
Operator Product Expansion (OPE).Comment: Proceeding of 3rd International Symposium on the
Gerasimov-Drell-Hearn Sum Rule and its extensions, Old Dominion University,
Norfolk, Virginia June 2-5, 200
Higher twist analysis of the proton g_1 structure function
We perform a global analysis of all available spin-dependent proton structure
function data, covering a large range of Q^2, 1 < Q^2 < 30 GeV^2, and calculate
the lowest moment of the g_1 structure function as a function of Q^2. From the
Q^2 dependence of the lowest moment we extract matrix elements of twist-4
operators, and determine the color electric and magnetic polarizabilities of
the proton to be \chi_E = 0.026 +- 0.015 (stat) + 0.021/-0.024 (sys) and \chi_B
= -0.013 -+ 0.007 (stat) - 0.010/+0.012 (sys), respectively.Comment: 6 pages, 2 figures, to appear in Phys. Lett.
Where are the Baryons?
New, high resolution, large-scale, cosmological hydrodynamic galaxy formation
simulations of a standard cold dark matter model (with a cosmological constant)
are utilized to predict the distribution of baryons at the present and at
moderate redshift. It is found that the average temperature of baryons is an
increasing function of time, with most of the baryons at the present time
having a temperature in the range 10^{5-7} K. Thus, not only is the universe
dominated by dark matter, but more than one half of the normal matter is yet to
be detected. Detection of this warm/hot gas poses an observational challenge,
requiring sensitive EUV and X-ray satellites. Signatures include a soft, cosmic
X-ray background, apparent warm components in hot clusters due to both
intrinsic warm intra-cluster gas and warm inter-cluster gas projected onto
clusters along the line of sight, absorption lines in X-ray and UV quasar
spectra [e.g., O VI (1032,1038)A lines, OVII 574 eV line], strong emission
lines (e.g., O VIII 653 eV line) and low redshift, broad, low column density
\lya absorption lines. We estimate that approximately 1/4 of the
extragalactic soft X-ray background (SXRB) (at 0.7 keV) arises from the
warm/hot gas, half of it coming from and three-quarters from ,
so the source regions should be identifiable on deep optical images.Comment: ApJ in press, revised (fig 3 is in jpg). Whole paper including
fig3.ps can be obtained at
"http://astro.princeton.edu/~cen/PAPERS_TO_APPEAR/64
Dark Matter Capture in the First Stars: a Power Source and Limit on Stellar Mass
The annihilation of weakly interacting massive particles can provide an
important heat source for the first (Pop. III) stars, potentially leading to a
new phase of stellar evolution known as a "Dark Star". When dark matter (DM)
capture via scattering off of baryons is included, the luminosity from DM
annihilation may dominate over the luminosity due to fusion, depending on the
DM density and scattering cross-section. The influx of DM due to capture may
thus prolong the lifetime of the Dark Stars. Comparison of DM luminosity with
the Eddington luminosity for the star may constrain the stellar mass of zero
metallicity stars; in this case DM will uniquely determine the mass of the
first stars. Alternatively, if sufficiently massive Pop. III stars are found,
they might be used to bound dark matter properties.Comment: 19 pages, 4 figures, 3 Tables updated captions and graphs, corrected
grammer, and added citations revised for submission to JCA
Photo-production of Nucleon Resonances and Nucleon Spin Structure Function in the Resonance Region
The photo-production of nucleon resonances is calculated based on a chiral
constituent quark model including both relativistic corrections H{rel} and
two-body exchange currents, and it is shown that these effects play an
important role. We also calculate the first moment of the nucleon spin
structure function g1 (x,Q^2) in the resonance region, and obtain a
sign-changing point around Q^2 ~ 0.27 {GeV}^2 for the proton.Comment: 23 pages, 5 figure
Role of clusters of galaxies in the evolution of the metal budget in the Universe
Using the guidelines on SN element production provided by XMM-Newton, we
summarize the results of ASCA observations on the element abundance in groups
and clusters of galaxies. We show that while the metal production in groups
could be described by a stellar population with a standard local IMF, clusters
of galaxies require a more top-heavy IMF. We attribute an excess heavy element
production to an IMF evolution with redshift. Dating the galaxy formation in
clusters by observations of the star-formation rate, we conclude that the IMF
variations have occurred preferentially at z>~4. We further combine our
metallicity measurements with the mass function of clusters to estimate the
role of clusters in the evolution of the metal content of the Universe. We
argue that at no epoch stars are a major container of metals, unless groups of
galaxies are not representative for the star-formation. This lends further
support for the reduced (0.6 solar) mass-averaged oxygen abundance in the
stellar population.Comment: 8 pages, 2003, ApJ, 594, September 1 issu
Turbulent Molecular Cloud Cores: Rotational Properties
The rotational properties of centrally condensed, turbulent molecular cloud
cores with velocity fields that are characterized by Gaussian random fields are
investigated. It is shown that the observed line width-size relationship can be
reproduced if the velocity power spectrum is a power-law with P(k)=k**n and
n=-3 to -4. The line-of-sight velocity maps of these cores show velocity
gradients that can be interpreted as rotation. For n=-4, both, the deduced
values of the angular velocity Omega=1.6 km/s/pc * (R/0.1 pc)**0.5 and the
scaling relations between Omega and the core radius R are in very good
agreement with the observations. As a result of the dominance of long
wavelength modes, the cores also have a net specific angular momentum with an
average value of j=7*(10**20)*(R/0.1 pc)**(1.5) cm**2/s with a large spread.
Their internal dimensionless rotational parameter is beta=0.03, independent of
the scale radius R. In general, the line-of-sight velocity gradient of an
individual turbulent core does not provide a good estimate of its internal
specific angular momentum. We find however that the distribution of the
specific angular momenta of a large sample of cores which are described by the
same power spectrum can be determined very accurately from the distribution of
their line-of-sight velocity gradients Omega using the simple formula
j=p*Omega*R*R where p depends on the density distribution of the core and has
to be determined from a Monte-Carlo study. Our results show that for centrally
condensed cores the intrinsic angular momentum is overestimated by a factor of
2-3 if p=0.4 is used.Comment: 23 pages, 7 figures, Astrophysical Journal, in pres
The counterrotating core and the black hole mass of IC1459
The E3 giant elliptical galaxy IC1459 is the prototypical galaxy with a fast
counterrotating stellar core. We obtained one HST/STIS long-slit spectrum along
the major axis of this galaxy and CTIO spectra along five position angles. We
present self-consistent three-integral axisymmetric models of the stellar
kinematics, obtained with Schwarzschild's numerical orbit superposition method.
We study the dynamics of the kinematically decoupled core (KDC) in IC1459 and
we find it consists of stars that are well-separated from the rest of the
galaxy in phase space. The stars in the KDC counterrotate in a disk on orbits
that are close to circular. We estimate that the KDC mass is ~0.5% of the total
galaxy mass or ~3*10^9 Msun. We estimate the central black hole mass M_BH of
IC1459 independently from both its stellar and its gaseous kinematics. Some
complications probably explain why we find rather discrepant BH masses with the
different methods. The stellar kinematics suggest that M_BH = (2.6 +/-
1.1)*10^9 Msun (3 sigma error). The gas kinematics suggests that M_BH ~
3.5*10^8 Msun if the gas is assumed to rotate at the circular velocity in a
thin disk. If the observed velocity dispersion of the gas is assumed to be
gravitational, then M_BH could be as high as ~1.0*10^9 Msun. These different
estimates bracket the value M_BH = (1.1 +/- 0.3)*10^9 Msun predicted by the
M_BH-sigma relation. It will be an important goal for future studies to assess
the reliability of black hole mass determinations with either technique. This
is essential if one wants to interpret the correlation between the BH mass and
other global galaxy parameters (e.g. velocity dispersion) and in particular the
scatter in these correlations (believed to be only ~0.3 dex). [Abridged]Comment: 51 pages, LaTeX with 19 PostScript figures. Revised version, with
three new figures and data tables. To appear in The Astrophysical Journal,
578, 2002 October 2
On the Importance of Electroweak Corrections for Majorana Dark Matter Indirect Detection
Recent analyses have shown that the inclusion of electroweak corrections can
alter significantly the energy spectra of Standard Model particles originated
from dark matter annihilations. We investigate the important situation where
the radiation of electroweak gauge bosons has a substantial influence: a
Majorana dark matter particle annihilating into two light fermions. This
process is in p-wave and hence suppressed by the small value of the relative
velocity of the annihilating particles. The inclusion of electroweak radiation
eludes this suppression and opens up a potentially sizeable s-wave contribution
to the annihilation cross section. We study this effect in detail and explore
its impact on the fluxes of stable particles resulting from the dark matter
annihilations, which are relevant for dark matter indirect searches. We also
discuss the effective field theory approach, pointing out that the opening of
the s-wave is missed at the level of dimension-six operators and only encoded
by higher orders.Comment: 25 pages, 6 figures. Minor corrections to match version published in
JCA
- …