757 research outputs found
Interstellar extinction towards the inner Galactic Bulge
DENIS observations in the J (1.2 micron) and K_S (2.15 micron) bands together
with isochrones calculated for the RGB and AGB phase are used to draw an
extinction map of the inner Galactic Bulge. The uncertainty in this method is
mainly limited by the optical depth of the Bulge itself. A comparison with
fields of known extinction shows a very good agreement. We present an
extinction map for the inner Galactic Bulge (approx. 20 sq. deg.)Comment: 4 pages, 4 figures, accepted for publication in A&A as a letter, see
also http://www-denis.iap.fr/articles/extinction
Gas Rich Dwarf Spheroidals
We present evidence that nearly half of the dwarf spheroidal galaxies (dSph
and dSph/dIrr) in the Local Group are associated with large reservoirs of
atomic gas, in some cases larger than the stellar mass. The gas is sometimes
found at large distance (~10 kpc) from the center of a galaxy and is not
necessarily centered on it. Similarly large quantities of ionized gas could be
hidden in these systems as well. The properties of some of the gas reservoirs
are similar to the median properties of the High-Velocity Clouds (HVCs); two of
the HI reservoirs are catalogued HVCs. The association of the HI with the dwarf
spheroidals might thus provide a link between the HVCs and stars. We show that
the HI content of the Local Group dSphs and dIrrs exhibits a sharp decline if
the galaxy is within 250 kpc of either the Milky Way or M31. This can be
explained if both galaxies have a sufficiently massive x-ray emitting halo that
produces ram-pressure stripping if a dwarf ventures too close to either giant
spiral. We also investigate tidal stripping of the dwarf galaxies and find that
although it may play a role, it cannot explain the apparent total absence of
neutral gas in most dSph galaxies at distances less than 250 kpc. For the
derived mean density of the hot gas, n_0 = 2.5e-5 cm^-2, ram-pressure stripping
is found to be more than an order of magnitude more effective in removing the
gas from the dSph galaxies. The hot halo, with an inferred mass of 1e10 solar
masses, may represent a reservoir of ~1000 destroyed dwarf systems, either HVCs
or true dwarf galaxies similar to those we observe now.Comment: AASTex preprint style, 27 pages including 12 figures. Submitted to
ApJ. See also http://astro.berkeley.edu/~robisha
Recommended from our members
A New Calibration Of Galactic Cepheid Period-Luminosity Relations From B To K Bands, And A Comparison To LMC Relations
Context. The universality of the Cepheid period-luminosity (PL) relations has been under discussion since metallicity effects were assumed to play a role in the value of the intercept and, more recently, of the slope of these relations. Aims. The goal of the present study is to calibrate the Galactic PL relations in various photometric bands (from B to K) and to compare the results to the well-established PL relations in the LMC. Methods. We use a set of 59 calibrating stars, the distances of which are measured using five different distance indicators: Hubble Space Telescope and revised Hipparcos parallaxes, infrared surface brightness and interferometric Baade-Wesselink parallaxes, and classical Zero-Age-Main-Sequence-fitting parallaxes for Cepheids belonging to open clusters or OB stars associations. A detailed discussion of absorption corrections and projection factor to be used is given. Results. We find no significant difference in the slopes of the PL relations between LMC and our Galaxy. Conclusions. We conclude that the Cepheid PL relations have universal slopes in all photometric bands, not depending on the galaxy under study (at least for LMC and Milky Way). The possible zero-point variation with metal content is not discussed in the present work, but an upper limit of 18.50 for the LMC distance modulus can be deduced from our data.McDonald Observator
Cepheid Period-Radius and Period-Luminosity Relations and the Distance to the LMC
We have used the infrared Barnes-Evans surface brightness technique to derive
the radii and distances of 34 Galactic Cepheid variables. Radius and distance
results obtained from both versions of the technique are in excellent
agreement. The radii of 28 variables are used to determine the period-radius
relation. This relation is found to have a smaller dispersion than in previous
studies, and is identical to the period-radius relation found by Laney & Stobie
from a completely independent method, a fact which provides persuasive evidence
that the Cepheid period-radius relation is now determined at a very high
confidence level. We use the accurate infrared distances to determine
period-luminosity relations in the V, I, J, H and K passbands from the Galactic
sample of Cepheids. We derive improved slopes of these relations from updated
LMC Cepheid samples and adopt these slopes to obtain accurate absolute
calibrations of the PL relation. By comparing these relations to the ones
defined by the LMC Cepheids, we derive strikingly consistent and precise values
for the LMC distance modulus in each of the passbands which yield a mean value
of DM (LMC) = 18.46 +- 0.02.
Our results show that the infrared Barnes-Evans technique is very insensitive
to both Cepheid metallicity and adopted reddening, and therefore a very
powerful tool to derive accurate distances to nearby galaxies by a direct
application of the technique to their Cepheid variables, rather than by
comparing PL relations of different galaxies, which introduces much more
sensitivity to metallicity and absorption corrections which are usually
difficult to determine.Comment: LaTeX, AASTeX style, 9 Figures, 10 Tables, The Astrophysical Journal
in press (accepted Oct. 14, 1997). Fig. 3 replace
Self-Averaging Scaling Limits of Two-Frequency Wigner Distribution for Random Paraxial Waves
Two-frequency Wigner distribution is introduced to capture the asymptotic
behavior of the space-frequency correlation of paraxial waves in the radiative
transfer limits. The scaling limits give rises to deterministic transport-like
equations. Depending on the ratio of the wavelength to the correlation length
the limiting equation is either a Boltzmann-like integral equation or a
Fokker-Planck-like differential equation in the phase space. The solutions to
these equations have a probabilistic representation which can be simulated by
Monte Carlo method. When the medium fluctuates more rapidly in the longitudinal
direction, the corresponding Fokker-Planck-like equation can be solved exactly.Comment: typos correcte
Mean Escape Time in a System with Stochastic Volatility
We study the mean escape time in a market model with stochastic volatility.
The process followed by the volatility is the Cox Ingersoll and Ross process
which is widely used to model stock price fluctuations. The market model can be
considered as a generalization of the Heston model, where the geometric
Brownian motion is replaced by a random walk in the presence of a cubic
nonlinearity. We investigate the statistical properties of the escape time of
the returns, from a given interval, as a function of the three parameters of
the model. We find that the noise can have a stabilizing effect on the system,
as long as the global noise is not too high with respect to the effective
potential barrier experienced by a fictitious Brownian particle. We compare the
probability density function of the return escape times of the model with those
obtained from real market data. We find that they fit very well.Comment: 9 pages, 9 figures, to be published in Phys. Rev.
ExELS: an exoplanet legacy science proposal for the ESA Euclid mission. II. Hot exoplanets and sub-stellar systems
The Exoplanet Euclid Legacy Survey (ExELS) proposes to determine the
frequency of cold exoplanets down to Earth mass from host separations of ~1 AU
out to the free-floating regime by detecting microlensing events in Galactic
Bulge. We show that ExELS can also detect large numbers of hot, transiting
exoplanets in the same population. The combined microlensing+transit survey
would allow the first self-consistent estimate of the relative frequencies of
hot and cold sub-stellar companions, reducing biases in comparing "near-field"
radial velocity and transiting exoplanets with "far-field" microlensing
exoplanets. The age of the Bulge and its spread in metallicity further allows
ExELS to better constrain both the variation of companion frequency with
metallicity and statistically explore the strength of star-planet tides.
We conservatively estimate that ExELS will detect ~4100 sub-stellar objects,
with sensitivity typically reaching down to Neptune-mass planets. Of these,
~600 will be detectable in both Euclid's VIS (optical) channel and NISP H-band
imager, with ~90% of detections being hot Jupiters. Likely scenarios predict a
range of 2900-7000 for VIS and 400-1600 for H-band. Twice as many can be
expected in VIS if the cadence can be increased to match the 20-minute H-band
cadence. The separation of planets from brown dwarfs via Doppler boosting or
ellipsoidal variability will be possible in a handful of cases. Radial velocity
confirmation should be possible in some cases, using 30-metre-class telescopes.
We expect secondary eclipses, and reflection and emission from planets to be
detectable in up to ~100 systems in both VIS and NISP-H. Transits of ~500
planetary-radius companions will be characterised with two-colour photometry
and ~40 with four-colour photometry (VIS,YJH), and the albedo of (and emission
from) a large sample of hot Jupiters in the H-band can be explored
statistically.Comment: 18 pages, 16 figures, accepted MNRA
ExELS: an exoplanet legacy science proposal for the ESA Euclid mission. II. Hot exoplanets and sub-stellar systems
The Exoplanet Euclid Legacy Survey (ExELS) proposes to determine the
frequency of cold exoplanets down to Earth mass from host separations of ~1 AU
out to the free-floating regime by detecting microlensing events in Galactic
Bulge. We show that ExELS can also detect large numbers of hot, transiting
exoplanets in the same population. The combined microlensing+transit survey
would allow the first self-consistent estimate of the relative frequencies of
hot and cold sub-stellar companions, reducing biases in comparing "near-field"
radial velocity and transiting exoplanets with "far-field" microlensing
exoplanets. The age of the Bulge and its spread in metallicity further allows
ExELS to better constrain both the variation of companion frequency with
metallicity and statistically explore the strength of star-planet tides.
We conservatively estimate that ExELS will detect ~4100 sub-stellar objects,
with sensitivity typically reaching down to Neptune-mass planets. Of these,
~600 will be detectable in both Euclid's VIS (optical) channel and NISP H-band
imager, with ~90% of detections being hot Jupiters. Likely scenarios predict a
range of 2900-7000 for VIS and 400-1600 for H-band. Twice as many can be
expected in VIS if the cadence can be increased to match the 20-minute H-band
cadence. The separation of planets from brown dwarfs via Doppler boosting or
ellipsoidal variability will be possible in a handful of cases. Radial velocity
confirmation should be possible in some cases, using 30-metre-class telescopes.
We expect secondary eclipses, and reflection and emission from planets to be
detectable in up to ~100 systems in both VIS and NISP-H. Transits of ~500
planetary-radius companions will be characterised with two-colour photometry
and ~40 with four-colour photometry (VIS,YJH), and the albedo of (and emission
from) a large sample of hot Jupiters in the H-band can be explored
statistically.Comment: 18 pages, 16 figures, accepted MNRA
Scalar Quantum Field Theory in Disordered Media
A free massive scalar field in inhomogeneous random media is investigated.
The coefficients of the Klein-Gordon equation are taken to be random functions
of the spatial coordinates. The case of an annealed-like disordered medium,
modeled by centered stationary and Gaussian processes, is analyzed. After
performing the averages over the random functions, we obtain the two-point
causal Green's function of the model up to one-loop. The disordered scalar
quantum field theory becomes qualitatively similar to a
self-interacting theory with a frequency-dependent coupling
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