890 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 very luminous, highly extinguished, very fast nova - V1721 Aquilae
Fast novae are primarily located within the plane of the Galaxy, slow novae
are found within its bulge. Because of high interstellar extinction along the
line of sight many novae lying close to the plane are missed and only the
brightest seen. One nova lying very close to the Galactic plane is V1721
Aquilae, discovered in outburst on 2008 September 22. Spectra obtained 2.69
days after outburst revealed very high expansion velocities (FWHM ~6450 km/s).
In this paper we have used available pre- and post-outburst photometry and
post-outburst spectroscopy to conclude that the object is a very fast,
luminous, and highly extinguished A_V=11.6+/-0.2) nova system with an average
ejection velocity of ~3400 km/s. Pre-outburst near-IR colours from 2MASS
indicate that at quiescence the object is similar to many quiescent CNe and
appears to have a main sequence/sub-giant secondary rather than a giant. Based
on the speed of decline of the nova and its emission line profiles we
hypothesise that the axis ratio of the nova ejecta is ~1.4 and that its
inclination is such that the central binary accretion disc is face-on to the
observer. The accretion disc's blue contribution to the system's near-IR
quiescent colours may be significant. Simple models of the nova ejecta have
been constructed using the morphological modelling code XS5, and the results
support the above hypothesis. Spectral classification of this object has been
difficult owing to low S/N levels and high extinction, which has eliminated all
evidence of any He/N or FeII emission within the spectra. We suggest two
possibilities for the nature of V1721 Aql: that it is a U Sco type RN with a
sub-giant secondary or, less likely, that it is a highly energetic bright and
fast classical nova with a main sequence secondary. Future monitoring of the
object for possible RN episodes may be worthwhile, as would archival searches
for previous outbursts.Comment: 9 pages 10 figures, accepted for publication in A&A. Abstract has
been slightly shortened from published versio
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
Dynamics of the Globular Cluster System Associated with M87 (NGC 4486). II. Analysis
We present a dynamical analysis of the globular cluster system associated
with M87 (= NGC 4486), the cD galaxy near the dynamical center of the Virgo
cluster. The analysis utilizes a new spectroscopic and photometric database
which is described in a companion paper (Hanes et al. 2001). Using a sample of
278 globular clusters with measured radial velocities and metallicities, and
new surface density profiles based on wide-field Washington photometry, we
study the dynamics of the M87 globular cluster system both globally --- for the
entire cluster sample --- and separately --- for the metal-rich and metal-poor
globular cluster samples. This constitutes the largest sample of radial
velocities for pure Population II tracers yet assembled for any galaxy. We
discuss the implications of our findings for models for the formation of giant
elliptical galaxies, globular cluster systems, and the Virgo cluster.
(ABRIDGED)Comment: 28 pages, 19 postscript figures, 1 jpeg image. See
http://www.physics.rutgers.edu/ast/ast-rap.html to download the manuscript
with higher quality figures. Accepted for publication in the Astrophysical
Journa
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
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
Low Mass X-ray Binaries and Globular Clusters in Early-Type Galaxies
(Abridged) A high fraction of the Low Mass X-ray Binaries (LMXBs) in
early-type galaxies are associated with globular clusters (GCs). Here, we
discuss the correlations between LMXBs and GCs in a sample of four early-type
galaxies. There is some evidence that the fraction of LMXBs associated with GCs
(f_X-GC) increases along the Hubble sequence from spiral bulges to S0s to Es to
cDs. On the other hand, the fraction of globular clusters which contain X-ray
sources appears to be roughly constant at f_GC-X ~ 4%. There is a strong
tendency for the X-ray sources to be associated with the optically more
luminous GCs. However, this correlation is consistent with a constant
probability of finding a LMXB per unit optical luminosity; it seems to result
primarily from the larger number of stars in optically luminous GCs. The
probability of finding a bright LMXB per unit optical luminosity in the GCs is
about 1.5e-7 LMXBs per L_solar,I for L_X >~ 1e38 erg/s, and rises to about
2.0e-7 LMXBs per L_solar,I at lower X-ray luminosities, L_X >~ 3e37 erg/s. This
frequency appears to be roughly constant for different galaxies, including the
bulges of the Milky Way and M31. There is a tendency for the X-ray sources to
be found preferentially in redder GCs. This seems to indicate that the
evolution of X-ray binaries in a GC is affected either by the metallicity or
age of the GC, with younger and/or more metal rich GCs having more LMXBs. There
is a weak tendency for the brightest LMXBs, whose luminosities exceed the
Eddington luminosity for a 1.4 M_solar neutron star, to avoid GCs. That may
indicate that black hole X-ray are somewhat less likely to be found in GCs, as
seems to be true in our Galaxy.Comment: Astrophysical Journal, 595, in press. 44 pages with 16 embedded
Postscript 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
Discovery of a Probable CH Star in the Globular Cluster M14 and Implications for the Evolution of Binaries in Clusters
We report the discovery of a probable CH star in the core of the Galactic
globular cluster M14, identified from an integrated-light spectrum of the
cluster obtained with the MOS spectrograph on the CFHT. From a high- resolution
echelle spectrum of the same star obtained with the Hydra fiber positioner and
bench spectrograph on the WIYN telescope, we measure a radial velocity of
km s. Although this velocity is inconsistent with
published estimates of the systemic radial velocity of M14 (eg, km s), we use high-precision Hydra velocities for 20 stars
in the central 2.6 arcminutes of M14 to calculate improved values for the
cluster mean velocity and one-dimensional velocity dispersion: km
s and km s, respectively. Both the star's location
near the tip of the red giant branch in the cluster color magnitude diagram and
its radial velocity therefore argue for membership in M14. Since the
intermediate-resolution MOS spectrum shows not only enhanced CH absorption but
also strong Swan bands of C, M14 joins Omega Cen as the only globular
clusters known to contain classical CH stars. Although evidence for its
duplicity must await additional radial velocity measurements, the CH star in
M14 is probably, like all field CH stars, a spectroscopic binary with a
degenerate (white dwarf) secondary. The candidate and confirmed CH stars in M14
and Omega Cen, and in a number of Galactic dSph galaxies, may then owe their
existence to the long timescales for the shrinking and coalescence of hard
binaries in low-concentration environments.Comment: Accepted to the Astrophysical Journal Letters. 13 pages, AAS LaTeX
and three postscript figures (numbers 2,3,4). Entire paper (including Figure
1) available at http://www.hia.nrc.ca/DAO/SCIENCE/science.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
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