206 research outputs found
Distributions of Long-Lived Radioactive Nuclei Provided by Star Forming Environments
Radioactive nuclei play an important role in planetary evolution by providing
an internal heat source, which affects planetary structure and helps facilitate
plate tectonics. A minimum level of nuclear activity is thought to be necessary
--- but not sufficient --- for planets to be habitable. Extending previous work
that focused on short-lived nuclei, this paper considers the delivery of
long-lived radioactive nuclei to circumstellar disks in star forming regions.
Although the long-lived nuclear species are always present, their abundances
can be enhanced through multiple mechanisms. Most stars form in embedded
cluster environments, so that disks can be enriched directly by intercepting
ejecta from supernovae within the birth clusters. In addition, molecular clouds
often provide multiple episodes of star formation, so that nuclear abundances
can accumulate within the cloud; subsequent generations of stars can thus
receive elevated levels of radioactive nuclei through this distributed
enrichment scenario. This paper calculates the distribution of additional
enrichment for K, the most abundant of the long-lived radioactive
nuclei. We find that distributed enrichment is more effective than direct
enrichment. For the latter mechanism, ideal conditions lead to about 1 in 200
solar systems being directly enriched in K at the level inferred for the
early solar nebula (thereby doubling the abundance). For distributed enrichment
from adjacent clusters, about 1 in 80 solar systems are enriched at the same
level. Distributed enrichment over the entire molecular cloud is more
uncertain, but can be even more effective.Comment: 24 pages, 8 figures, accepted for publication in Ap
High Energy Cosmic-ray Diffusion in Molecular Clouds: A Numerical Approach
The propagation of high-energy cosmic rays through giant molecular clouds
constitutes a fundamental process in astronomy and astrophysics. The diffusion
of cosmic-rays through these magnetically turbulent environments is often
studied through the use of energy-dependent diffusion coefficients, although
these are not always well motivated theoretically. Now, however, it is feasible
to perform detailed numerical simulations of the diffusion process
computationally. While the general problem depends upon both the field
structure and particle energy, the analysis may be greatly simplified by
dimensionless analysis. That is, for a specified purely turbulent field, the
analysis depends almost exclusively on a single parameter -- the ratio of the
maximum wavelength of the turbulent field cells to the particle gyration
radius. For turbulent magnetic fluctuations superimposed over an underlying
uniform magnetic field, particle diffusion depends on a second dimensionless
parameter that characterizes the ratio of the turbulent to uniform magnetic
field energy densities. We consider both of these possibilities and parametrize
our results to provide simple quantitative expressions that suitably
characterize the diffusion process within molecular cloud environments. Doing
so, we find that the simple scaling laws often invoked by the high-energy
astrophysics community to model cosmic-ray diffusion through such regions
appear to be fairly robust for the case of a uniform magnetic field with a
strong turbulent component, but are only valid up to TeV particle
energies for a purely turbulent field. These results have important
consequences for the analysis of cosmic-ray processes based on TeV emission
spectra associated with dense molecular clouds.Comment: Accepted for publication in The Astrophysical Journa
Assessing the Feasibility of Cosmic-Ray Acceleration by Magnetic Turbulence at the Galactic Center
The presence of relativistic particles at the center of our galaxy is
evidenced by the diffuse TeV emission detected from the inner
of the Galaxy. Although it is not yet entirely clear whether the origin of the
TeV photons is due to hadronic or leptonic interactions, the tight correlation
of the intensity distribution with the distribution of molecular gas along the
Galactic ridge strongly points to a pionic-decay process involving relativistic
protons. In earlier work, we concluded that point-source candidates, such as
the supermassive black hole Sagittarius A* (identified with the HESS source
J1745-290), or the pulsar wind nebulae dispersed along the Galactic plane,
could not account for the observed diffuse TeV emission from this region.
Motivated by this result, we consider here the feasibility that the cosmic rays
populating the Galactic Center (GC) region are accelerated in situ by magnetic
turbulence. Our results indicate that even in a highly conductive environment,
this mechanism is efficient enough to energize protons within the intercloud
medium to the \ga TeV energies required to produce the HESS emission.Comment: Accepted for publication in Ap
High-Energy Activity in the Unusually Soft TeV Source HESS J1804-216 toward the Galactic Center
In recent years, apparent anisotropies in the ~EeV cosmic ray (CR) flux
arriving at Earth from the general direction of the galactic center have been
reported from the analysis of AGASA and SUGAR data. The more recently
commissioned Auger Observatory has not confirmed these results. HESS has now
detected an unusually soft TeV source roughly coincident with the location of
the previously claimed CR anisotropy. In this paper, we develop a model for the
TeV emission from this object, consistent with observations at other
wavelengths, and examine the circumstances under which it might have
contributed to the EeV cosmic ray spectrum. We find that the supernova
remnant G8.7-0.1 can plausibly account for all the known radiative
characteristics of HESS J1804-216, but that it can accelerate cosmic rays only
up to an energy GeV. On the other hand, the pulsar (PSR J1803-2137)
embedded within this remnant can in principle inject EeV protons into the
surrounding medium, but it cannot account for the broadband spectrum of HESS
J1804-216. We therefore conclude that although G8.7-0.1 is probably the source
of TeV photons originating from this direction, there is no compelling
theoretical motivation for expecting a cosmic ray anisotropy at this location.
However, if G8.7-0.1 is indeed correctly identified with HESS J1804-216, it
should also produce a GeV flux detectable in a one-year all sky survey
by GLAST
Diffusive Cosmic Ray Acceleration at the Galactic Centre
The diffuse TeV emission detected from the inner of the Galaxy
appears to be strongly correlated with the distribution of molecular gas along
the Galactic ridge. Although it is not yet entirely clear whether the origin of
the TeV photons is due to hadronic or leptonic interactions, the tight
correlation of the intensity distribution with the molecular gas strongly
points to a pionic-decay process involving relativistic protons. But the
spectrum of the TeV radiation---a power law with index ---cannot be accommodated easily with the much steeper distribution of
cosmic rays seen at Earth. In earlier work, we examined the possible sources of
these relativistic protons and concluded that neither the supermassive black
hole Sagittarius A* (identified with the HESS source J1745-290), nor several
pulsar wind nebulae dispersed along the Galactic plane, could produce a TeV
emission profile morphologically similar to that seen by HESS. We concluded
from this earlier study that only relativistic protons accelerated throughout
the inter-cloud medium could account for the observed diffuse TeV emission from
this region. In this paper, we develop a model for diffusive proton
acceleration driven by a turbulent Alfv\'enic magnetic field present throughout
the gaseous medium. Though circumstantial, this appears to be the first
evidence that at least some cosmic rays are accelerated diffusively within the
inner pc of the Galaxy.Comment: Accepted for publication in MNRAS letter
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