765 research outputs found
Palomar 13: An Unusual Stellar System in the Galactic Halo
We have measured Keck/HIRES radial velocities for 30 candidate red giants in
the direction of Palomar 13: an object traditionally cataloged as a compact,
low-luminosity globular cluster. From a sample of 21 confirmed members, we find
a systemic velocity of 24.1 km/s and a projected, intrinsic velocity dispersion
of 2.2 km/s. Although small, this dispersion is several times larger than that
expected for a globular cluster of this luminosity and central concentration.
Taken at face value, this dispersion implies a mass-to-light ratio of ~ 40 (in
solar units) based on the best-fit King-Michie model. The surface density
profile of Palomar 13 also appears to be anomalous among Galactic globular
clusters -- depending upon the details of background subtraction and
model-fitting, Palomar 13 either contains a substantial population of
"extra-tidal" stars, or it is far more spatially extended than previously
suspected. The full surface density profile is equally well-fit by a
King-Michie model having a high concentration and large tidal radius, or by a
NFW model. We examine -- and tentatively reject -- a number of possible
explanations for the observed characteristics of Palomar 13 (e.g., velocity
"jitter" among the red giants, spectroscopic binary stars, non-standard mass
functions, modified Newtonian dynamics), and conclude that the two most
plausible scenarios are either catastrophic heating during a recent
perigalacticon passage, or the presence of a massive dark halo. Thus, the
available evidence suggests that Palomar 13 is either a globular cluster which
is now in the process of dissolving into the Galactic halo, or a faint,
dark-matter-dominated stellar system (ABRIDGED).Comment: 31 pages, 13 postscript figures and 1 color gif image. Also available
at http://www.physics.rutgers.edu/ast/ast-rap.html. Accepted for publication
in the Astrophysical Journa
Bar-halo Friction in Galaxies I: Scaling Laws
It has been known for some time that rotating bars in galaxies slow due to
dynamical friction against the halo. However, recent attempts to use this
process to place constraints on the dark matter density in galaxies and
possibly also to drive dark matter out of the center have been challenged. This
paper uses simplified numerical experiments to clarify several aspects of the
friction mechanism. I explicitly demonstrate the Chandrasekhar scaling of the
friction force with bar mass, halo density, and halo velocity dispersion. I
present direct evidence that exchanges between the bar and halo orbits at major
resonances are responsible for friction and study both individual orbits and
the net changes at these resonances. I also show that friction alters the phase
space density of particles in the vicinity of a major resonance, which is the
reason the magnitude of the friction force depends on the prior evolution. I
demonstrate that bar slow down can be captured correctly in simulations having
modest spatial resolution and practicable numbers of particles. Subsequent
papers in this series delineate the dark matter density that can be tolerated
in halos of different density profiles.Comment: 15 pages, 13 figures, to appear in ApJ - major revisions from version
Monster black holes
A combination of ground-based and spacecraft observations has uncovered two
black holes of 10 billion solar masses in the nearby Universe. The finding
sheds light on how these cosmic monsters co-evolve with galaxies.Comment: 2 pages, 1 figure, LaTeX. Published in Nature "News & Views
Core Collapse via Coarse Dynamic Renormalization
In the context of the recently developed "equation-free" approach to
computer-assisted analysis of complex systems, we extract the self-similar
solution describing core collapse of a stellar system from numerical
experiments. The technique allows us to side-step the core "bounce" that occurs
in direct N-body simulations due to the small-N correlations that develop in
the late stages of collapse, and hence to follow the evolution well into the
self-similar regime.Comment: 5 pages, 3 figure
Growth and migration of solids in evolving protostellar disks I: Methods and Analytical tests
This series of papers investigates the early stages of planet formation by
modeling the evolution of the gas and solid content of protostellar disks from
the early T Tauri phase until complete dispersal of the gas. In this first
paper, I present a new set of simplified equations modeling the growth and
migration of various species of grains in a gaseous protostellar disk evolving
as a result of the combined effects of viscous accretion and photo-evaporation
from the central star. Using the assumption that the grain size distribution
function always maintains a power-law structure approximating the average
outcome of the exact coagulation/shattering equation, the model focuses on the
calculation of the growth rate of the largest grains only. The coupled
evolution equations for the maximum grain size, the surface density of the gas
and the surface density of solids are then presented and solved
self-consistently using a standard 1+1 dimensional formalism. I show that the
global evolution of solids is controlled by a leaky reservoir of small grains
at large radii, and propose an empirically derived evolution equation for the
total mass of solids, which can be used to estimate the total heavy element
retention efficiency in the planet formation paradigm. Consistency with
observation of the total mass of solids in the Minimum Solar Nebula augmented
with the mass of the Oort cloud sets strong upper limit on the initial grain
size distribution, as well as on the turbulent parameter \alphat. Detailed
comparisons with SED observations are presented in a following paper.Comment: Submitted to ApJ. 23 pages and 13 figure
A Prescription for Building the Milky Way's Halo from Disrupted Satellites
We develop a semi-analytic method for determining the phase-space population
of tidal debris along the orbit of a disrupting satellite galaxy and illustrate
its use with a number of applications. We use this method to analyze Zhao's
proposal that the microlensing events towards the Large Magellanic Cloud (LMC)
might be explained by an appropriately placed tidal streamer, and find that his
scenarios lead either to unacceptably high overdensities (10 -- 100%) in faint
star counts (apparent magnitudes 17.5 -- 20.5) away from the Galactic plane or
short timescales for the debris to disperse (10^8 years). We predict that the
tidal streamers from the LMC and the Sagittarius dwarf galaxy currently extend
over more than in azimuth along their orbits. Assuming that each
satellite has lost half of its primordial mass, we find that the streamers will
have overdensities in faint star counts of 10 -- 100% and < 1% respectively,
and conclude that this mass loss rate is unlikely for the LMC, but possible for
Sagittarius. If the Galaxy has accreted one hundred
objects (comparable to its current population of globular clusters) at
distances of 20 -- 100 kpc during its lifetime then 10% of the sky will now be
covered by tidal streamers.Comment: 35 pages, LaTeX, 12 postscript figures included. Submitted to Ap
Collapsing Layers on Schwarzschild-Lemaitre Geodesics
We discuss Israel layers collapsing inward from rest at infinity along
Schwarzschild-Lemaitre geodesics. The dynamics of the collapsing layer and its
equation of state are developed. There is a general equation of state which is
approximately polytropic in the limit of very low pressure. The equation of
state establishes a new limit on the stress-density ratio.Comment: To appear in Phys. Rev. D 1
Formation of Semi-relativisitc Jets from Magnetospheres of Accreting Neutron Stars: Injection of Hot Bubbles into a Magnetic Tower
We present the results of 2.5-dimensional resistive magnetohydrodynamic (MHD)
simulations of the magnetic interaction between a weakly magnetized neutron
star and its accretion disk. General relativistic effects are simulated by
using the pseudo-Newtonian potential. We find that well-collimated jets
traveling along the rotation axis of the disk are formed by the following
mechanism: (1) The magnetic loops connecting the neutron star and the disk are
twisted due to the differential rotation between the neutron star and the disk.
(2) Twist injection from the disk initiates expansion of the loop. (3) The
expanding magnetic loops create a magnetic tower in which accelerated disk
material travel as collimated bipolar jets. The propagation speed of the
working surface of the jet is the order of 10% of the speed of light (). (4) Magnetic reconnection taking place inside the expanding magnetic
loops injects hot bubbles intermittently into the magnetic tower. The ejection
speed of the bubble is the order of the local Alfv\'{e}n speed of the launching
point and in our simulations. (5) The hot bubbles moving inside the
tower catch up with the working surface of the jet. High energy electrons
created by the magnetic reconnection are a plausible source of radio emission.
Our model can explain the formation process of a narrow jet from a weakly
magnetized (|{\boldmathB_{*}}|\le 10^{9} gauss) neutron star and the
correlation between radio flares of the core and of the lobe observed in Sco
X-1.Comment: 6 pages, 3 figures, to appear in ApJ, uses emulateapj.cls and
apjfonts.sty. A paper with high-resolution figures and movies available at
http://www2.yukawa.kyoto-u.ac.jp/~ykato/research
Long-Lived Double-Barred Galaxies From Pseudo-Bulges
A large fraction of barred galaxies host secondary bars that are embedded in
their large-scale primary counterparts. These are common also in gas poor
early-type barred galaxies. The evolution of such double-barred galaxies is
still not well understood, partly because of a lack of realistic -body
models with which to study them. Here we report a new mechanism for generating
such systems, namely the presence of rotating pseudo-bulges. We demonstate with
high mass and force resolution collisionless -body simulations that
long-lived secondary bars can form spontaneously without requiring gas,
contrary to previous claims. We find that secondary bars rotate faster than
primary ones. The rotation is not, however, rigid: the secondary bars pulsate,
with their amplitude and pattern speed oscillating as they rotate through the
primary bars. This self-consistent study supports previous work based on
orbital analysis in the potential of two rigidly rotating bars. The pulsating
nature of secondary bars may have important implications for understanding the
central region of double-barred galaxies.Comment: Paper submitted to ApJ
Dwarf Cepheids in the Carina Dwarf Spheroidal Galaxy
We have discovered 20 dwarf Cepheids (DC) in the Carina dSph galaxy from the
analysis of individual CCD images obtained for a deep photometric study of the
system. These short-period pulsating variable stars are by far the most distant
(~100 kpc) and faintest (V ~ 23.0) DCs known. The Carina DCs obey a
well-defined period-luminosity relation, allowing us to readily distinguish
between overtone and fundamental pulsators in nearly every case. Unlike RR Lyr
stars, the pulsation mode turns out to be uncorrelated with light-curve shape,
nor do the overtone pulsators tend towards shorter periods compared to the
fundamental pulsators. Using the period-luminosity (PL) relations from Nemec et
al. (1994 AJ, 108, 222) and McNamara (1995, AJ, 109, 1751), we derive (m-M)_0 =
20.06 +/- 0.12, for E(B-V) = 0.025 and [Fe/H] = -2.0, in good agreement with
recent, independent estimates of the distance/reddening of Carina. The error
reflects the uncertainties in the DC distance scale, and in the metallicity and
reddening of Carina. The frequency of DCs among upper main sequence stars in
Carina is approximately 3%. The ratio of dwarf Cepheids to RR Lyr stars in
Carina is 0.13 +/- 0.10, though this result is highly sensitive to the
star-formation history of Carina and the evolution of the Horizontal Branch. We
discuss how DCs may be useful to search effectively for substructure in the
Galactic halo out to Galactocentric distances of ~100 kpc.Comment: 20 pages of text, 7 figure
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