138 research outputs found
Unseen Progenitors of Luminous High-z Quasars in the R_h=ct Universe
Quasars at high redshift provide direct information on the mass growth of
supermassive black holes and, in turn, yield important clues about how the
Universe evolved since the first (Pop III) stars started forming. Yet even
basic questions regarding the seeds of these objects and their growth mechanism
remain unanswered. The anticipated launch of eROSITA and ATHENA is expected to
facilitate observations of high-redshift quasars needed to resolve these
issues. In this paper, we compare accretion-based supermassive black hole
growth in the concordance LCDM model with that in the alternative
Friedmann-Robertson Walker cosmology known as the R_h=ct universe. Previous
work has shown that the timeline predicted by the latter can account for the
origin and growth of the > 10^9 M_sol highest redshift quasars better than that
of the standard model. Here, we significantly advance this comparison by
determining the soft X-ray flux that would be observed for Eddington-limited
accretion growth as a function of redshift in both cosmologies. Our results
indicate that a clear difference emerges between the two in terms of the number
of detectable quasars at redshift z > 6, raising the expectation that the next
decade will provide the observational data needed to discriminate between these
two models based on the number of detected high-redshift quasar progenitors.
For example, while the upcoming ATHENA mission is expected to detect ~0.16
(i.e., essentially zero) quasars at z ~ 7 in R_h=ct, it should detect ~160 in
LCDM---a quantitatively compelling difference.Comment: 14 pages, 11 figures, 1 table. Accepted for publication in Ap
A Numerical Assessment of Cosmic-ray Energy Diffusion through Turbulent Media
How and where cosmic rays are produced, and how they diffuse through various
turbulent media, represent fundamental problems in astrophysics with far
reaching implications, both in terms of our theoretical understanding of
high-energy processes in the Milky Way and beyond, and the successful
interpretation of space-based and ground based GeV and TeV observations. For
example, recent and ongoing detections, e.g., by Fermi (in space) and HESS (in
Namibia), of -rays produced in regions of dense molecular gas hold
important clues for both processes. In this paper, we carry out a comprehensive
numerical investigation of relativistic particle acceleration and transport
through turbulent magnetized environments in order to derive broadly useful
scaling laws for the energy diffusion coefficients.Comment: Accepted for publication in Ap
Star Formation at the Galactic Center
Molecular clouds at the Galactic center (GC) have environments considerably
different from their disk counterparts. The GC may therefore provide important
clues about how the environment affects star formation. Interestingly, while
the inner 50 parsecs of our Galaxy include a remarkable population of high-mass
stars, the initial mass function (IMF) appears to be consistent with a Salpeter
slope down to ~ 1 solar mass. We show here that the loss of turbulent pressure
due to ambipolar diffusion and the damping of Alfven and fast MHD waves can
lead to the formation of dense condensations exceeding their Jeans limit. The
fragmentation and subsequent collapse of these condensations is similar to the
diffusion-driven protostellar collapse mechanism expected to occur within
nearby "regular" molecular clouds. As such, a Salpeter IMF at the GC is not
surprising, though the short dynamical timescales associated with the GC
molecular clouds may help explain the lower star formation efficiency observed
from this region.Comment: Accepted for publication in PAS
Effects of Turbulence on Cosmic Ray Propagation in Protostars and Young Star/Disk Systems
The magnetic fields associated with young stellar objects are expected to
have an hour-glass geometry, i.e., the magnetic field lines are pinched as they
thread the equatorial plane surrounding the forming star but merge smoothly
onto a background field at large distances. With this field configuration,
incoming cosmic rays experience both a funneling effect that acts to enhance
the flux impinging on the circumstellar disk and a magnetic mirroring effect
that acts to reduce that flux. To leading order, these effects nearly cancel
out for simple underlying magnetic field structures. However, the environments
surrounding young stellar objects are expected to be highly turbulent. This
paper shows how the presence of magnetic field fluctuations affects the process
of magnetic mirroring, and thereby changes the flux of cosmic rays striking
circumstellar disks. Turbulence has two principle effects: 1) The (single)
location of the magnetic mirror point found in the absence of turbulence is
replaced with a wide distribution of values. 2) The median of the mirror point
distribution moves outward for sufficiently large fluctuation amplitudes
(roughly when at the location of the turbulence-free mirror
point); the distribution becomes significantly non-gaussian in this regime as
well. These results may have significant consequences for the ionization
fraction of the disk, which in turn dictates the efficiency with which disk
material can accrete onto the central object. A similar reduction in cosmic ray
flux can occur during the earlier protostellar stages; the decrease in
ionization can help alleviate the magnetic braking problem that inhibits disk
formation.Comment: Accepted for publication in The Astrophysical Journa
A Theory of the IMF for Star Formation in Molecular Clouds
We present models for the initial mass function (IMF) for stars forming
within molecular clouds. These models use the idea that stars determine their
own masses through the action of powerful stellar outflows. This concept allows
us to calculate a semi-empirical mass formula (SEMF), which provides the
transformation between initial conditions in molecular clouds and the final
masses of forming stars. For a particular SEMF, a given distribution of initial
conditions predicts a corresponding IMF. We consider several different
descriptions for the distribution of initial conditions in star forming
molecular clouds. We first consider the limiting case in which only one
physical variable -- the effective sound speed -- determines the initial
conditions. In this limit, we use observed scaling laws to determine the
distribution of sound speed and the SEMF to convert this distribution into an
IMF. We next consider the opposite limit in which many different independent
physical variables play a role in determining stellar masses. In this limit,
the central limit theorem shows that the IMF approaches a log-normal form.
Realistic star forming regions contain an intermediate number of relevant
variables; we thus consider intermediate cases between the two limits. Our
results show that this picture of star formation and the IMF naturally produces
stellar mass distributions that are roughly consistent with observations. This
paper thus provides a calculational framework to construct theoretical models
of the IMF.Comment: 34 pages, 7 figures available on reques
Self-Similar Collapse Solutions for Cylindrical Cloud Geometries and Dynamic Equations of State
A self-similar formalism for the study of the gravitational collapse of
molecular gas provides an important theoretical framework from which to explore
the dynamics of star formation. Motivated by the presence of elongated and
filamentary structures observed in giant molecular clouds, we build upon the
existing body of work on cylindrical self-similar collapse flows by including
dynamic equations of state that are different from the effective equation of
state that produces the initial density distribution. We focus primarily on the
collapse of initial states for which the gas is at rest and everywhere
overdense from its corresponding hydrostatic equilibrium profile by a factor
, and apply our results toward the analysis of star formation within
dense, elongated molecular cores. An important aspect of this work is the
determination of the mass infall rates over a range of the parameters which
define the overall state of the gas -- the overdensity parameter , the
index of the static equation of state, and the index of the
dynamic equation of state. While most of the parameter space explored in this
work leads to solutions for which the underlying equations do not become
singular, we do include a discussion on how to treat cases for which solutions
pass smoothly through the singular surface. In addition, we also present a
different class of collapse solutions for the special case .Comment: Accepted for publication to PAS
Focusing of Alfvénic power in neutron star magnetospheres
Highly dynamic magnetospheric perturbations in neutron star environments can naturally account for the features observed in gamma‐ray burst spectra. However, if GRB’s have an extragalactic origin, as is implied by the uniform yet spatially truncated distribution observed by the BATSE experiment, then noncatastrophic isotropic emission mechanisms may be ruled out on energetic and timing arguments. As such, we consider MHD processes which can produce strongly anisotropic γ‐rays with an observable flux out to distances of ∼1–2 Gpc. In particular, we show that sheared Alfvén waves propagating along open magnetospheric field lines at the poles of magnetized neutron stars transfer their energy dissipationally to the charges generating the current which sustains the field misalignment, and thereby focus their power into a spatial region (i.e., the shear) that can be many times smaller than that of the crustal disturbance. This produces a strong (observable) flux enhancement along certain directions. © 1994 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87628/2/515_1.pd
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