3,851 research outputs found
Aspherical Core-Collapse Supernovae in Red Supergiants Powered by Nonrelativistic Jets
We explore the observational characteristics of jet-driven supernovae by
simulating bipolar-jet-driven explosions in a red supergiant progenitor. We
present results of four models in which we hold the injected kinetic energy at
a constant ergs across all jet models but vary the specific
characteristics of the jets to explore the influence of the nature of jets on
the structure of the supernova ejecta. We evolve the explosions past
shock-breakout and into quasi-homologous expansion of the supernova envelope
into a red supergiant wind. The oppositely-directed, nickel-rich jets give a
large-scale asymmetry that may account for the non-spherical excitation and
substructure of spectral lines such as H and He I 10830\AA. Jets with a
large fraction of kinetic to thermal energy punch through the progenitor
envelope and give rise to explosions that would be observed to be asymmetric
from the earliest epochs, inconsistent with spectropolarimetric measurements of
Type II supernovae. Jets with higher thermal energy fractions result in
explosions that are roughly spherical at large radii but are significantly
elongated at smaller radii, deep inside the ejecta, in agreement with the
polarimetric observations. We present shock breakout light curves that indicate
that strongly aspherical shock breakouts are incompatible with recent {\it
GALEX} observations of shock breakout from red supergiant stars. Comparison
with observations indicates that jets must deposit their kinetic energy
efficiently throughout the ejecta while in the hydrogen envelope. Thermal
energy-dominated jets satisfy this criterion and yield many of the
observational characteristics of Type II supernovae.Comment: 21 pages, 19 figures, submitted to ApJ on 4 Nov 200
Multidimensional Simulations of Rotating Pair Instability Supernovae
We study the effects of rotation on the dynamics, energetics and Ni-56
production of Pair Instability Supernova explosions by performing rotating
two-dimensional ("2.5-D") hydrodynamics simulations. We calculate the evolution
of eight low metallicity (Z = 10^-3, 10^-4 Zsun) massive (135-245 Msun) PISN
progenitors with initial surface rotational velocities 50% that of the critical
Keplerian value using the stellar evolution code MESA. We allow for both the
inclusion and the omission of the effects of magnetic fields in the angular
momentum transport and in chemical mixing, resulting in slowly-rotating and
rapidly-rotating final carbon-oxygen cores, respectively. Increased rotation
for carbon-oxygen cores of the same mass and chemical stratification leads to
less energetic PISN explosions that produce smaller amounts of Ni-56 due to the
effect of the angular momentum barrier that develops and slows the dynamical
collapse. We find a non-monotonic dependence of Ni-56 production on rotational
velocity in situations when smoother composition gradients form at the outer
edge of the rotating cores. In these cases, the PISN energetics are determined
by the competition of two factors: the extent of chemical mixing in the outer
layers of the core due to the effects of rotation in the progenitor evolution
and the development of angular momentum support against collapse. Our 2.5-D
PISN simulations with rotation are the first presented in the literature. They
reveal hydrodynamic instabilities in several regions of the exploding star and
increased explosion asymmetries with higher core rotational velocity.Comment: 31 pages, 23 figures, accepted for publication in the Ap
Exploratory wind tunnel tests of a shock-swallowing air data sensor at a Mach number of approximately 1.83
The test probe was designed to measure free-stream Mach number and could be incorporated into a conventional airspeed nose boom installation. Tests were conducted in the Langley 4-by 4-foot supersonic pressure tunnel with an approximate angle of attack test range of -5 deg to 15 deg and an approximate angle of sideslip test range of + or - 4 deg. The probe incorporated a variable exit area which permitted internal flow. The internal flow caused the bow shock to be swallowed. Mach number was determined with a small axially movable internal total pressure tube and a series of fixed internal static pressure orifices. Mach number error was at a minimum when the total pressure tube was close to the probe tip. For four of the five tips tested, the Mach number error derived by averaging two static pressures measured at horizontally opposed positions near the probe entrance were least sensitive to angle of attack changes. The same orifices were also used to derive parameters that gave indications of flow direction
Galaxy threshing and the origin of intracluster stellar objects
We numerically investigate dynamical evolution of non-nucleated dwarf
elliptical/spiral galaxies (dE) and nucleated ones (dE,Ns) in clusters of
galaxies in order to understand the origin of intracluster stellar objects,
such as intracluster stars (ICSs), GCs (ICGCs), and ``ultra-compact dwarf''
(UCDs) recently discovered by all-object spectroscopic survey centred on the
Fornax cluster of galaxies. We find that the outer stellar components of a
nucleated dwarf are removed by the strong tidal field of the cluster, whereas
the nucleus manages to survive as a result of its initially compact nature. The
developed naked nucleus is found to have physical properties (e.g., size and
mass) similar to those observed for UCDs. We also find that the UCD formation
processes does depend on the radial density profile of the dark halo in the
sense that UCDs are less likely to be formed from dwarfs embedded in dark
matter halos with central `cuspy' density profiles. Our simulations also
suggest that very massive and compact stellar systems can be rapidly and
efficiently formed in the central regions of dwarfs through the merging of
smaller GCs. GCs initially in the outer part of dE and dE,Ns are found to be
stripped to form ICGCs.Comment: 6 pages and 3 figures (JPG file for Fig. 1), in the proceedings of
IAU 217 ``Recycling intergalactic and interstellar matter'
Ultra-compact dwarf galaxies: a new class of compact stellar system discovered in the Fornax Cluster
We have used the 2dF spectrograph on the Anglo-Australian Telescope to obtain
a complete spectroscopic sample of all objects in the magnitude range, Bj= 16.5
to 19.8, regardless of morphology, in an area centred on the Fornax Cluster of
galaxies. Among the unresolved targets are five objects which are members of
the Fornax Cluster. They are extremely compact stellar systems with scale
lengths less than 40 parsecs. These ultra-compact dwarfs are unlike any known
type of stellar system, being more compact and significantly less luminous than
other compact dwarf galaxies, yet much brighter than any globular cluster.Comment: To appear in IAU Symposium 207: Extragalactic Star Cluster
HST Observations of Gravitationally Lensed Features in the Rich Cluster Ac114
Deep Hubble Space Telescope images of superlative resolution obtained for the
distant rich cluster AC114 (z=0.31) reveal a variety of gravitational lensing
phenomena for which ground-based spectroscopy is available. We present a
luminous arc which is clearly resolved by HST and appears to be a lensed z=0.64
sub-L star spiral galaxy with a detected rotation curve. Of greatest interest
is a remarkably symmetrical pair of compact blue images separated by 10 arcsec
and lying close to the cluster cD. We propose that these images arise from a
single very faint background source gravitationally lensed by the cluster core.
Deep ground-based spectroscopy confirms the lensing hypothesis and suggests the
source is a compact star forming system at a redshift z=1.86. Taking advantage
of the resolved structure around each image and their very blue colours, we
have identified a candidate third image of the same source roughly 50 arcsec
away. The angular separation of the three images is much larger than previous
multiply-imaged systems and indicates a deep gravitational potential in the
cluster centre. Resolved multiply-imaged systems, readily recognised with HST,
promise to provide unique constraints on the mass distribution in the cores of
intermediate redshift clusters.Comment: submitted to ApJ, 6 pages (no figures), uuencoded Postscript,
compressed TAR of Postscript figures available via anonymous ftp in
users/irs/figs/ac114_figs.tar.gz on astro.caltech.edu. PAL-IRS-
Galaxy threshing and the formation of ultra-compact dwarf galaxies
Recent spectroscopic and morphological observational studies of galaxies
around NGC 1399 in the Fornax Cluster (Drinkwater et al. 2000b) have discovered
several `ultra-compact dwarf' galaxies with intrinsic sizes of 100 pc
and absolute band magnitudes ranging from -13 to -11 mag. In order to
elucidate the origin of these enigmatic objects, we perform numerical
simulations on the dynamical evolution of nucleated dwarf galaxies orbiting NGC
1399 and suffering from its strong tidal gravitational field. Adopting a
plausible scaling relation for dwarf galaxies, we find that the outer stellar
components of a nucleated dwarf are totally removed. This is due to them being
tidally stripped over the course of several passages past the central region of
NGC 1399. The nucleus, however, manages to survive. We also find that the size
and luminosity of the remnant are similar to those observed for ultra-compact
dwarf galaxies, if the simulated precursor nucleated dwarf has a mass of
. These results suggest that ultra-compact dwarf galaxies
could have previously been more luminous dwarf spheroidal or elliptical
galaxies with rather compact nuclei.Comment: 9 pages 4 figures,2001, ApJL, 552, 10
Passive spiral formation from halo gas starvation: Gradual transformation into S0s
Recent spectroscopic and high resolution -imaging observations have
revealed significant numbers of ``passive'' spiral galaxies in distant
clusters, with all the morphological hallmarks of a spiral galaxy (in
particular, spiral arm structure), but with weak or absent star formation.
Exactly how such spiral galaxies formed and whether they are the progenitors of
present-day S0 galaxies is unclear. Based on analytic arguments and numerical
simulations of the hydrodynamical evolution of a spiral galaxy's halo gas
(which is a likely candidate for the source of gas replenishment for star
formation in spirals), we show that the origin of passive spirals may well be
associated with halo gas stripping. Such stripping results mainly from the
hydrodynamical interaction between the halo gas and the hot intracluster gas.
Our numerical simulations demonstrate that even if a spiral orbits a cluster
with a pericenter distance 3 times larger than the cluster core radius,
80 % of the halo gas is stripped within a few Gyr and, accordingly,
cannot be accreted by the spiral. Furthermore, our study demonstrates that this
dramatic decline in the gaseous infall rate leads to a steady increase in the
parameter for the disk, with the spiral arm structure, although persisting,
becoming less pronounced as the star formation rate gradually decreases. These
results suggest that passive spirals formed in this way, gradually evolve into
red cluster S0s.Comment: 13 pages 4 figures (fig.1 = jpg format), accepted by Ap
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