80 research outputs found
Detection of Tidal Disruption Events around Direct Collapse Black Holes at High Redshifts with the James Webb Space Telescope
This is the third sequel in a series discussing the discovery of various
types of extragalactic transients with the {\it James Webb Space Telescope} in
a narrow-field ( deg), moderately deep ( mag)
survey. In this part we focus on the detectability and observational
characteristics of Direct Collapse Black Holes (DCBH) and Tidal Disruption
Events (TDE) around them. We use existing models for DCBH accretion
luminosities and spectra as well as for TDE light curves, and find that
accreting DCBH seeds may be bright enough for detection up to with
JWST NIRCam imaging, TDEs of massive ( Msol) stars around them
can enhance the chance for discovering them as transient objects, although the
rates of such events is low, a few per survey time. TDEs around non-accreting
black holes of Msol may also be detected at redshifts in
the redder NIRCam bands between 3 and 5 microns. It is also shown that
accreting DCBHs appear separate from supernovae (SNe) on the NIRCam color-color
plot, but TDEs from quiescent black holes fall in nearly the same color range
as Superluminous Supernovae (SLSNe), which makes them more difficult to
identify.Comment: 17 pages, 11 figures, ApJ in pres
The Effects of the Peak-Peak Correlation on the Peak Model of Hierarchical Clustering
In two previous papers a semi-analytical model was presented for the
hierarchical clustering of halos via gravitational instability from peaks in a
random Gaussian field of density fluctuations. This model is better founded
than the extended Press-Schechter model, which is known to agree with numerical
simulations and to make similar predictions. The specific merger rate, however,
shows a significant departure at intermediate captured masses. The origin of
this was suspected as being the rather crude approximation used for the density
of nested peaks. Here, we seek to verify this suspicion by implementing a more
accurate expression for the latter quantity which accounts for the correlation
among peaks. We confirm that the inclusion of the peak-peak correlation
improves the specific merger rate, while the good behavior of the remaining
quantities is preserved.Comment: ApJ accepted. 15 pages, including 4 figures. Also available at
ftp://pcess1.am.ub.es/pub/ApJ/effectpp.ps.g
Binary Star Origin of High Field Magnetic White Dwarfs
White dwarfs with surface magnetic fields in excess of MG are found as
isolated single stars and relatively more often in magnetic cataclysmic
variables. Some 1,253 white dwarfs with a detached low-mass main-sequence
companion are identified in the Sloan Digital Sky Survey but none of these is
observed to show evidence for Zeeman splitting of hydrogen lines associated
with a magnetic field in excess of 1MG. If such high magnetic fields on white
dwarfs result from the isolated evolution of a single star then there should be
the same fraction of high field white dwarfs among this SDSS binary sample as
among single stars. Thus we deduce that the origin of such high magnetic fields
must be intimately tied to the formation of cataclysmic variables. CVs emerge
from common envelope evolution as very close but detached binary stars that are
then brought together by magnetic braking or gravitational radiation. We
propose that the smaller the orbital separation at the end of the common
envelope phase, the stronger the magnetic field. The magnetic cataclysmic
variables originate from those common envelope systems that almost merge. We
propose further that those common envelope systems that do merge are the
progenitors of the single high field white dwarfs. Thus all highly magnetic
white dwarfs, be they single stars or the components of MCVs, have a binary
origin. This hypothesis also accounts for the relative dearth of single white
dwarfs with fields of 10,000 - 1,000,000G. Such intermediate-field white dwarfs
are found preferentially in cataclysmic variables. In addition the bias towards
higher masses for highly magnetic white dwarfs is expected if a fraction of
these form when two degenerate cores merge in a common envelope. Similar
scenarios may account for very high field neutron stars.Comment: 6 pages, 1 figure, accepted by MNRA
Observational biases in Lagrangian reconstructions of cosmic velocity fields
Lagrangian reconstruction of large-scale peculiar velocity fields can be
strongly affected by observational biases. We develop a thorough analysis of
these systematic effects by relying on specially selected mock catalogues. For
the purpose of this paper, we use the MAK reconstruction method, although any
other Lagrangian reconstruction method should be sensitive to the same
problems. We extensively study the uncertainty in the mass-to-light assignment
due to luminosity incompleteness, and the poorly-determined relation between
mass and luminosity. The impact of redshift distortion corrections is analyzed
in the context of MAK and we check the importance of edge and finite-volume
effects on the reconstructed velocities. Using three mock catalogues with
different average densities, we also study the effect of cosmic variance. In
particular, one of them presents the same global features as found in
observational catalogues that extend to 80 Mpc/h scales. We give recipes,
checked using the aforementioned mock catalogues, to handle these particular
observational effects, after having introduced them into the mock catalogues so
as to quantitatively mimic the most densely sampled currently available galaxy
catalogue of the nearby universe. Once biases have been taken care of, the
typical resulting error in reconstructed velocities is typically about a
quarter of the overall velocity dispersion, and without significant bias. We
finally model our reconstruction errors to propose an improved Bayesian
approach to measure Omega_m in an unbiased way by comparing the reconstructed
velocities to the measured ones in distance space, even though they may be
plagued by large errors. We show that, in the context of observational data, a
nearly unbiased estimator of Omega_m may be built using MAK reconstruction.Comment: 29 pages, 21 figures, 6 tables, Accepted by MNRAS on 2007 October 2.
Received 2007 September 30; in original form 2007 July 2
Casimir effect: running Newton constant or cosmological term
We argue that the instability of Euclidean Einstein gravity is an indication
that the vacuum is non perturbative and contains a condensate of the metric
tensor in a manner reminiscent of Yang-Mills theories. As a simple step toward
the characterization of such a vacuum the value of the one-loop effective
action is computed for Euclidean de Sitter spaces as a function of the
curvature when the unstable conformal modes are held fixed. Two phases are
found, one where the curvature is large and gravitons should be confined and
another one which appears to be weakly coupled and tends to be flat. The
induced cosmological constant is positive or negative in the strongly or weakly
curved phase, respectively. The relevance of the Casimir effect in
understanding the UV sensitivity of gravity is pointed out.Comment: Final, slightly extended version, to appear in Classical and Quantum
Gravit
Low-Mass Binary Induced Outflows from Asymptotic Giant Branch Stars
A significant fraction of planetary nebulae (PNe) and proto-planetary nebulae
(PPNe) exhibit aspherical, axisymmetric structures, many of which are highly
collimated. The origin of these structures is not entirely understood, however
recent evidence suggests that many observed PNe harbor binary systems, which
may play a role in their shaping. In an effort to understand how binaries may
produce such asymmetries, we study the effect of low-mass (< 0.3 M_sun)
companions (planets, brown dwarfs and low-mass main sequence stars) embedded
into the envelope of a 3.0 M_sun star during three epochs of its evolution (Red
Giant Branch, Asymptotic Giant Branch (AGB), interpulse AGB). We find that
common envelope evolution can lead to three qualitatively different
consequences: (i) direct ejection of envelope material resulting in a
predominately equatorial outflow, (ii) spin-up of the envelope resulting in the
possibility of powering an explosive dynamo driven jet and (iii) tidal
shredding of the companion into a disc which facilitates a disc driven jet. We
study how these features depend on the secondary's mass and discuss
observational consequences.Comment: 24 pages, 6 figures, submitted to MNRA
Infall Regions of Galaxy Clusters
In hierarchical clustering, galaxy clusters accrete mass through the
aggregation of smaller systems. Thus, the velocity field of the infall regions
of clusters contains significant random motion superimposed on radial infall.
Because the purely spherical infall model does not predict the amplitude of the
velocity field correctly, methods estimating the cosmological density parameter
Omega_0 based on this model yield unreliable biased results. In fact, the
amplitude of the velocity field depends on local dynamics and only very weakly
on the global properties of the universe. We use N-body simulations of flat and
open universes to show that the amplitude of the velocity field of the infall
regions of dark matter halos is a direct measure of the escape velocity within
these regions. We can use this amplitude to estimate the mass of dark matter
halos within a few megaparsecs from the halo center. In this region dynamical
equilibrium assumptions do not hold. The method yields a mass estimate with
better than 30% accuracy. If galaxies trace the velocity field of the infall
regions of clusters reliably, this method provides a straightforward way to
estimate the amount of mass surrounding rich galaxy clusters from redshift data
alone.Comment: 26 pages, AAS Latex macros v4.0, to appear in The Astrophysical
Journal, June 1, 1997 issue, Vol. 48
CLASH-VLT: The mass, velocity-anisotropy, and pseudo-phase-space density profiles of the z=0.44 galaxy cluster MACS 1206.2-0847
We use an unprecedented data-set of about 600 redshifts for cluster members,
obtained as part of a VLT/VIMOS large programme, to constrain the mass profile
of the z=0.44 cluster MACS J1206.2-0847 over the radial range 0-5 Mpc (0-2.5
virial radii) using the MAMPOSSt and Caustic methods. We then add external
constraints from our previous gravitational lensing analysis. We invert the
Jeans equation to obtain the velocity-anisotropy profiles of cluster members.
With the mass-density and velocity-anisotropy profiles we then obtain the first
determination of a cluster pseudo-phase-space density profile. The kinematics
and lensing determinations of the cluster mass profile are in excellent
agreement. This is very well fitted by a NFW model with mass M200=(1.4 +- 0.2)
10^15 Msun and concentration c200=6 +- 1, only slightly higher than theoretical
expectations. Other mass profile models also provide acceptable fits to our
data, of (slightly) lower (Burkert, Hernquist, and Softened Isothermal Sphere)
or comparable (Einasto) quality than NFW. The velocity anisotropy profiles of
the passive and star-forming cluster members are similar, close to isotropic
near the center and increasingly radial outside. Passive cluster members follow
extremely well the theoretical expectations for the pseudo-phase-space density
profile and the relation between the slope of the mass-density profile and the
velocity anisotropy. Star-forming cluster members show marginal deviations from
theoretical expectations. This is the most accurate determination of a cluster
mass profile out to a radius of 5 Mpc, and the only determination of the
velocity-anisotropy and pseudo-phase-space density profiles of both passive and
star-forming galaxies for an individual cluster [abridged]Comment: A&A in press; 22 pages, 19 figure
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