800 research outputs found
Substructure in the Andromeda Galaxy Globular Cluster System
In the most prominent current scenario of galaxy formation, galaxies form
hierarchically through the merger of smaller systems. Such mergers could leave
behind dynamical signatures which may linger long after the event. In
particular, the globular cluster system (GCS) of a merging satellite galaxy may
remain as a distinct sub-population within the GCS of a massive galaxy. Using
the latest available globular cluster velocities and metallicities, we present
the results of a search for grouping in the GCS of our nearest large spiral
galaxy neighbor, M31. A modified friends-of-friends algorithm is used to
identify a number of possible merger remnants in projected position, radial
velocity and [Fe/H] parameter space. Numerical simulations are used to check
that such merger remnants are indeed plausible over the timescales of interest.
The identification of stellar streams associated with these groups is required
in order to confirm that they represent merger remnants.Comment: 19 pages, 3 figures, accepted for publication in the Ap
A Wide Symbiotic Channel to Type Ia Supernovae
As a promising channel to Type Ia supernovae (SNe Ia), we have proposed a
symbiotic binary system consisting of a white dwarf (WD) and a low mass
red-giant (RG), where strong winds from the accreting WD play a key role to
increase the WD mass to the Chandrasekhar mass limit. Here we propose two new
evolutionary processes which make the symbiotic channel to SNe Ia much wider.
(1) We first show that the WD + RG close binary can form from a wide binary
even with such a large initial separation as . Such
a binary consists of an AGB star and a low mass main-sequence (MS) star, where
the AGB star is undergoing superwind before becoming a WD. If the superwind at
the end of AGB evolution is as fast as or slower than the orbital velocity, the
wind outflowing from the system takes away the orbital angular momentum
effectively. As a result the wide binary shrinks greatly to become a close
binary. Therefore, the WD + RG binary can form from much wider binaries than
our earlier estimate. (2) When the RG fills its inner critical Roche lobe, the
WD undergoes rapid mass accretion and blows a strong optically thick wind. Our
earlier analysis has shown that the mass transfer is stabilized by this wind
only when the mass ratio of RG/WD is smaller than 1.15. Our new finding is that
the WD wind can strip mass from the RG envelope, which could be efficient
enough to stabilize the mass transfer even if the RG/WD mass ratio exceeds
1.15. With the above two new effects (1) and (2), the symbiotic channel can
account for the inferred rate of SNe Ia in our Galaxy.Comment: 29 pages including 14 firgures, to be published in ApJ, 521, No.
A New Evolutionary Path to Type Ia Supernovae: Helium-Rich Super-Soft X-Ray Source Channel
We have found a new evolutionary path to Type Ia supernovae (SNe Ia) which
has been overlooked in previous work. In this scenario, a carbon-oxygen white
dwarf (C+O WD) is originated, not from an asymptotic giant branch star with a
C+O core, but from a red-giant star with a helium core of . The helium star, which is formed after the first common envelope
evolution, evolves to form a C+O WD of with transferring
a part of the helium envelope onto the secondary main-sequence star. This new
evolutionary path, together with the optically thick wind from mass-accreting
white dwarf, provides a much wider channel to SNe Ia than previous scenarios. A
part of the progenitor systems are identified as the luminous supersoft X-ray
sources or the recurrent novae like U Sco, which are characterized by the
accretion of helium-rich matter. The white dwarf accretes hydrogen-rich,
helium-enhanced matter from a lobe-filling, slightly evolved companion at a
critical rate and blows excess matter in the wind. The white dwarf grows in
mass to the Chandrasekhar mass limit and explodes as an SN Ia. A theoretical
estimate indicates that this channel contributes a considerable part of the
inferred rate of SNe Ia in our Galaxy, i.e., the rate is about ten times larger
than the previous theoretical estimates for white dwarfs with slightly evolved
companions.Comment: 19 pages including 12 figures, to be published in ApJ, 519, No.
M87, Globular Clusters, and Galactic Winds: Issues in Giant Galaxy Formation
New VRI photometry is presented for the globular clusters in the innermost
140'' of the M87 halo. The results are used to discuss several issues
concerning the formation and evolution of globular cluster systems in
supergiant ellipticals like M87. (1) we find no significant change in the
globular cluster luminosity function (GCLF) with galactocentric radius, for
cluster masses M < 10^5 solar masses, indicating that the main effects of
dynamical evolution may be only on lower-mass clusters. (2) Within the core
radius (1') of the globular cluster system, the metallicity distribution is
uniform, but at larger radii the mean metallicity declines steadily as Z ~
r^-0.9. (3) The various options for explaining the existence of high specific
frequency galaxies like M87 are evaluated, and scaling laws for the GCSs in
these galaxies are given. Interpretations involving secondary evolution
(formation of many globular clusters during mergers, intergalactic globular
clusters, etc.) are unlikely to be the primary explanation for high-S_N
galaxies. (4) We suggest that central-supergiant E galaxies may have formed in
an exceptionally turbulent or high-density environment in which an early,
powerful galactic wind drove out a high fraction of the protogalactic gas, thus
artificially boosting the specificComment: 67 pages, 17 figures. To appear in Astronomical Journal, in press for
May 1998. Preprints also available from W.Harris; send e-mail request to
[email protected]
The X-ray Faint Early-Type Galaxy NGC4697
We analyze archival ROSAT HRI, ROSAT PSPC, and ASCA data of the X-ray faint
early-type galaxy NGC4697. The joint ROSAT PSPC + ASCA spectrum is fit by a
two-component thermal model, a MEKAL model with kT_{MEKAL}=0.26^{+0.04}_{-0.03}
keV with low metallicity and a bremsstrahlung model with
kT_{BREM}=5.2^{+3.0}_{-1.6} keV. A similar model was found to fit the spectra
of another faint early-type galaxy (NGC4382) and the bulge of M31. We interpret
this soft emission as a combination of emission from a soft component of low
mass X-ray binaries (LMXBs) and from a low temperature interstellar medium,
although the relative contributions of the two components could not be
determined. Twelve point sources were identified within 4' of NGC4697, of which
11 are most likely LMXBs associated with the galaxy. The soft X-ray colors of
four of the LMXBs in NGC4697 support the claim that LMXBs possess a soft
spectral component. Finally, we present a simulation of what we believe the
Chandra data of NGC4697 will look like.Comment: 10 pages, uses emulateapj.sty, accepted by Astrophysical Journa
Synchronized Formation of Sub-Galactic Systems at Cosmological Reionization: Origin of Halo Globular Clusters
Gas rich sub-galactic halos with mass Mt <= 10^7.5 Msun, while incapable of
forming stars due to lack of adequate coolants, contain a large fraction of
baryonic mass at cosmological reionization. We show that the reionization of
the universe at z=10-20 has an interesting physical effect on these halos. The
external radiation field causes a synchronous inward propagation of an
ionization front towards each halo, resulting in an inward, convergent shock.
The resident gas of mass Mb~10^4-10^7 Msun in low spin (initial dimensionless
spin parameter lambda <= 0.01) halos with a velocity dispersion sigmav <=
11km/s would be compressed by a factor of ~100 in radius and form self
-gravitating baryonic systems. Under the assumption that such compressed
gaseous systems fragment to form stars, the final stellar systems will have a
size 2-40pc, velocity dispersion 1-10km/s and a total stellar mass of M*
10^3-10^6 Msun. The characteristics of these proposed systems seem to match the
observed properties of halo globular clusters. The expected number density is
consistent with the observed number density of halo globular clusters. The
observed mass function of slope ~-2 at the high mass end is predicted by the
model. Strong correlation between velocity dispersion and luminosity (or
surface brightness) and lack of correlation between velocity dispersion and
size, in agreement with observations, are expected. Metallicity is, on average,
expected to be low and should not correlate with any other quantities of
globular clusters, except that a larger dispersion of metallicity among
globular clusters is expected for larger galaxies. The observed trend of
specific frequency with galaxy type may be produced in the model. We suggest
that these stellar systems are seen as halo globular clusters today.Comment: accepted to ApJ, 7 ApJ page
Rheophysics of dense granular materials : Discrete simulation of plane shear flows
We study the steady plane shear flow of a dense assembly of frictional,
inelastic disks using discrete simulation and prescribing the pressure and the
shear rate. We show that, in the limit of rigid grains, the shear state is
determined by a single dimensionless number, called inertial number I, which
describes the ratio of inertial to pressure forces. Small values of I
correspond to the quasi-static regime of soil mechanics, while large values of
I correspond to the collisional regime of the kinetic theory. Those shear
states are homogeneous, and become intermittent in the quasi-static regime.
When I increases in the intermediate regime, we measure an approximately linear
decrease of the solid fraction from the maximum packing value, and an
approximately linear increase of the effective friction coefficient from the
static internal friction value. From those dilatancy and friction laws, we
deduce the constitutive law for dense granular flows, with a plastic Coulomb
term and a viscous Bagnold term. We also show that the relative velocity
fluctuations follow a scaling law as a function of I. The mechanical
characteristics of the grains (restitution, friction and elasticity) have a
very small influence in this intermediate regime. Then, we explain how the
friction law is related to the angular distribution of contact forces, and why
the local frictional forces have a small contribution to the macroscopic
friction. At the end, as an example of heterogeneous stress distribution, we
describe the shear localization when gravity is added.Comment: 24 pages, 19 figure
Infrared Properties of Cataclysmic Variables from 2MASS: Results from the 2nd Incremental Data Release
Because accretion-generated luminosity dominates the radiated energy of most
cataclysmic variables, they have been ``traditionally'' observed primarily at
short wavelengths. Infrared observations of cataclysmic variables contribute to
the understanding of key system components that are expected to radiate at
these wavelengths, such as the cool outer disk, accretion stream, and secondary
star. We have compiled the J, H, and Ks photometry of all cataclysmic variables
located in the sky coverage of the 2 Micron All Sky Survey (2MASS) 2nd
Incremental Data Release. This data comprises 251 systems with reliably
identified near-IR counterparts and S/N > 10 photometry in one or more of the
three near-IR bands.Comment: 2 pages, including 1 figure. To appear in the proceedings of The
Physics of Cataclysmic Variables and Related Objects, Goettingen, Germany.
For our followup ApJ paper (in press), also see
http://www.ctio.noao.edu/~hoard/research/2mass/index.htm
Density waves and density fluctuations in granular flow
We simulate the granular flow in a narrow pipe with a lattice-gas automaton
model. We find that the density in the system is characterized by two features.
One is that spontaneous density waves propagate through the system with
well-defined shapes and velocities. The other is that density waves are so
distributed to make the power spectra of density fluctuations as
noise. Three important parameters make these features observable and they are
energy dissipation, average density and the rougness of the pipe walls.Comment: Latex (with ps files appended
- …