6,826 research outputs found
The morphology of the Magellanic Clouds revealed by stars of different age: results from the DENIS survey
The spatial distribution of sources populating different regions of the
colour-magnitude diagram (I-J, I) extracted from the DENIS catalogue towards
the Magellanic Clouds (DCMC -- Cioni et al. 2000) reveal significantly
different morphologies. Each region is associated to a different age group. The
Large Magellanic Cloud (LMC) shows an extended circular shape with a prominent,
off center bar, a nucleus and irregular spiral arms. The Small Magellanic Cloud
shows a perturbated structure with a prominent central concentration of stars.
Old and young populations are offset from one another.Comment: 4 pages and 7 figures, accepted for publication in A&A Journal
Letter
Comments on photonic shells
We investigate in detail the special case of an infinitely thin static
cylindrical shell composed of counter-rotating photons on circular geodetical
paths separating two distinct parts of Minkowski spacetimes--one inside and the
other outside the shell--and compare it to a static disk shell formed by null
particles counter-rotating on circular geodesics within the shell located
between two sections of flat spacetime. One might ask whether the two cases are
not, in fact, merely one
SPH Simulations of Direct Impact Accretion in the Ultracompact AM CVn Binaries
The ultracompact binary systems V407 Vul (RX J1914.4+2456) and HM Cnc (RX
J0806.3+1527) - a two-member subclass of the AM CVn stars - continue to pique
interest because they defy unambiguous classification. Three proposed models
remain viable at this time, but none of the three is significantly more
compelling than the remaining two, and all three can satisfy the observational
constraints if parameters in the models are tuned. One of the three proposed
models is the direct impact model of Marsh & Steeghs (2002), in which the
accretion stream impacts the surface of a rapidly-rotating primary white dwarf
directly but at a near-glancing angle. One requirement of this model is that
the accretion stream have a high enough density to advect its specific kinetic
energy below the photosphere for progressively more-thermalized emission
downstream, a constraint that requires an accretion spot size of roughly
1.2x10^5 km^2 or smaller. Having at hand a smoothed particle hydrodynamics code
optimized for cataclysmic variable accretion disk simulations, it was
relatively straightforward for us to adapt it to calculate the footprint of the
accretion stream at the nominal radius of the primary white dwarf, and thus to
test this constraint of the direct impact model. We find that the mass flux at
the impact spot can be approximated by a bivariate Gaussian with standard
deviation \sigma_{\phi} = 164 km in the orbital plane and \sigma_{\theta} = 23
km in the perpendicular direction. The area of the the 2\sigma ellipse into
which 86% of the mass flux occurs is roughly 47,400 km^2, or roughly half the
size estimated by Marsh & Steeghs (2002). We discuss the necessary parameters
of a simple model of the luminosity distribution in the post-impact emission
region.Comment: 24 pages, 5 figures, Accepted for publication in Ap
Thermal Conduction in Systems out of Hydrostatic Equilibrium
We analyse the effects of thermal conduction in a relativistic fluid, just
after its departure from hydrostatic equilibrium, on a time scale of the order
of thermal relaxation time. It is obtained that the resulting evolution will
critically depend on a parameter defined in terms of thermodynamic variables,
which is constrained by causality requirements.Comment: 16 pages, emTex (LaTex 2.09). To appear in Classical and Quantum
Gravit
CO Emission in Low Luminosity, HI Rich Galaxies
We present 12CO 1-0 observations of eleven low luminosity M_B > -18),
HI--rich dwarf galaxies. Only the three most metal-rich galaxies, with
12+log(O/H) ~ 8.2, are detected. Very deep CO spectra of six extremely
metal-poor systems (12+log(O/H) < 7.5) yield only low upper limits on the CO
surface brightness, I_CO < 0.1 K km/s. Three of these six have never before
been observed in a CO line, while the others now have much more stringent upper
limits. For the very low metallicity galaxy Leo A, we do not confirm a
previously reported detection in CO, and the limits are consistent with another
recent nondetection. We combine these new observations with data from the
literature to form a sample of dwarf galaxies which all have CO observations
and measured oxygen abundances. No known galaxies with 12+log(O/H) < 7.9 (Z <
0.1 solar) have been detected in CO. Most of the star-forming galaxies with
higher (12+log(O/H) > 8.1) metallicities are detected at similar or higher I_CO
surface brightnesses. The data are consistent with a strong dependence of the
I_CO/M_H_2 = X_CO conversion factor on ambient metallicity. The strikingly low
upper limits on some metal-poor galaxies lead us to predict that the conversion
factor is non-linear, increasing sharply below approximately 1/10 of the solar
metallicity (12+log(O/H) < 7.9).Comment: 25 pages, 4 figures, 3 tables. Accepted for publication in AJ Tables
replaced -- now formated for landscape orientatio
Breakdown of the linear approximation in the perturbative analysis of heat conduction in relativistic systems
We analyze the effects of thermal conduction in a relativistic fluid just
after its departure from spherical symmetry, on a time scale of the order of
relaxation time. Using first order perturbation theory, it is shown that, as in
spherical systems, at a critical point the effective inertial mass density of a
fluid element vanishes and becomes negative beyond that point. The impact of
this effect on the reliability of causality conditions is discussed.Comment: 11 pages (Latex2.09) To appear in Physics Letters
On the dual interpretation of zero-curvature Friedmann-Robertson-Walker models
Two possible interpretations of FRW cosmologies (perfect fluid or dissipative
fluid)are considered as consecutive phases of the system. Necessary conditions
are found, for the transition from perfect fluid to dissipative regime to
occur, bringing out the conspicuous role played by a particular state of the
system (the ''critical point '').Comment: 13 pages Latex, to appear in Class.Quantum Gra
High Excitation Molecular Gas in the Magellanic Clouds
We present the first survey of submillimeter CO 4-3 emission in the
Magellanic Clouds. The survey is comprised of 15 6'x6' maps obtained using the
AST/RO telescope toward the molecular peaks of the Large and Small Magellanic
Clouds. We have used these data to constrain the physical conditions in these
objects, in particular their molecular gas density and temperature. We find
that there are significant amounts of molecular gas associated with most of
these molecular peaks, and that high molecular gas temperatures are pervasive
throughout our sample. We discuss whether this may be due to the low
metallicities and the associated dearth of gas coolants in the Clouds, and
conclude that the present sample is insufficient to assert this effect.Comment: 18 pages, 3 figures, 5 tables. To appear in Ap
The Spatial Distribution of Atomic Carbon Emission in the Giant Molecular Cloud NGC 604-2
We have mapped a giant molecular cloud in the giant HII region NGC 604 in M33
in the 492 GHz ^3P_1 -- ^3P_0 transition of neutral atomic carbon using the
James Clerk Maxwell Telescope. We find the distribution of the [CI] emission to
be asymmetric with respect to the CO J=1--0 emission, with the peak of the [CI]
emission offset towards the direction of the center of the HII region. In
addition, the line ratio I_{[CI]}/I_{CO} is highest (~ 0.2) facing the HII
region and lowest (< 0.1) away from it. These asymmetries indicate an edge-on
morphology where the [CI] emission is strongest on the side of the cloud facing
the center of the HII region, and not detected at all on the opposite side This
suggests that the sources of the incident flux creating C from the dissociation
of CO are the massive stars of the HII region. The lowest line ratios are
similar to what is observed in Galactic molecular clouds, while the highest are
similar to starburst galaxies and other regions of intense star formation. The
column density ratio, N(C)/N(H_2) is a few times 10^{-6}, in general agreement
with models of photodissociation regions.Comment: Accepted for publication in ApJ. 8 pages, 5 figures, 3 table
Dynamical Masses in Luminous Infrared Galaxies
We have studied the dynamics and masses of a sample of ten nearby luminous
and ultraluminous infrared galaxies (LIRGS and ULIRGs), using 2.3 micron CO
absorption line spectroscopy and near-infrared H- and Ks-band imaging. By
combining velocity dispersions derived from the spectroscopy, disk
scale-lengths obtained from the imaging, and a set of likely model density
profiles, we calculate dynamical masses for each LIRG. For the majority of the
sample, it is difficult to reconcile our mass estimates with the large amounts
of gas derived from millimeter observations and from a standard conversion
between CO emission and H_2 mass. Our results imply that LIRGs do not have huge
amounts of molecular gas (10^10-10^11 Msolar) at their centers, and support
previous indications that the standard conversion of CO to H_2 probably
overestimates the gas masses and cannot be used in these environments. This in
turn suggests much more modest levels of extinction in the near-infrared for
LIRGs than previously predicted (A_V~10-20 versus A_V~100-1000). The lower gas
mass estimates indicated by our observations imply that the star formation
efficiency in these systems is very high and is triggered by cloud-cloud
collisions, shocks, and winds rather than by gravitational instabilities in
circumnuclear gas disks.Comment: 14 pages, 2 figures, accepted to Ap
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