1,735 research outputs found
The Life and Times of Extremal Black Holes
Charged extremal black holes cannot fully evaporate through the Hawking
effect and are thus long lived. Over their lifetimes, these black holes take
part in a variety of astrophysical processes, including many that lead to their
eventual destruction. This paper explores the various events that shape the
life of extremal black holes and calculates the corresponding time scales.Comment: 9 pages, LaTeX, accepted to General Relativity and Gravitatio
Magnetic and Gravitational Disk-Star Interactions: An Interdependence of PMS Stellar Rotation Rates and Spin-Orbit Misalignments
The presence of giant gaseous planets that reside in close proximity to their
host stars may be a consequence of large-scale radial migration through the
proto-planetary nebulae. Within the context of this picture, significant
orbital obliquities characteristic of a substantial fraction of such planets
can be attributed to external torques that perturb the disks out of alignment
with the spin axes of their host stars. Therefore, the acquisition of orbital
obliquity exhibits sensitive dependence on the physics of disk-star
interactions. Here, we analyze the primordial excitation of spin-orbit
misalignment of Sun-like stars, in light of disk-star angular momentum
transfer. We begin by calculating the stellar pre-main sequence rotational
evolution, accounting for spin-up due to gravitational contraction and
accretion as well as spin-down due to magnetic star-disk coupling. We devote
particular attention to angular momentum transfer by accretion, and show that
while generally subdominant to gravitational contraction, this process is
largely controlled by the morphology of the stellar magnetic field (i.e.
specific angular momentum accreted by stars with octupole-dominated surface
fields is smaller than that accreted by dipole-dominated stars by an order of
magnitude). Subsequently, we examine the secular spin-axis dynamics of
disk-bearing stars, accounting for the time-evolution of stellar and disk
properties and demonstrate that misalignments are preferentially excited in
systems where stellar rotation is not overwhelmingly rapid. Moreover, we show
that the excitation of spin-orbit misalignment occurs impulsively, through an
encounter with a resonance between the stellar precession frequency and the
disk-torquing frequency. Cumulatively, the model developed herein opens up a
previously unexplored avenue towards understanding star-disk evolution and its
consequences in a unified manner.Comment: 18 pages, 7 figures, accepted to Ap
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
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
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