160 research outputs found
On the usefulness of finding charts Or the runaway carbon stars of the Blanco & McCarthy field 37
We have been recently faced with the problem of cross--identifying stars
recorded in historical catalogues with those extracted from recent fully
digitized surveys (such as DENIS and 2MASS). Positions mentioned in the old
catalogues are frequently of poor precision, but are generally accompanied by
finding charts where the interesting objects are flagged. Those finding charts
are sometimes our only link with the accumulated knowledge of past literature.
While checking the identification of some of these objects in several
catalogues, we had the surprise to discover a number of discrepancies in recent
works.The main reason for these discrepancies was generally the blind
application of the smallest difference in position as the criterion to identify
sources from one historical catalogue to those in more recent surveys. In this
paper we give examples of such misidentifications, and show how we were able to
find and correct them.We present modern procedures to discover and solve
cross--identification problems, such as loading digitized images of the sky
through the Aladin service at CDS, and overlaying entries from historical
catalogues and modern surveys. We conclude that the use of good finding charts
still remains the ultimate (though time--consuming) tool to ascertain
cross--identifications in difficult cases.Comment: 4 pages, 1 figure, accepted by A&
Cosmic rays from Leptonic Dark Matter
If dark matter possesses a lepton number, it is natural to expect the
dark-matter annihilation and/or decay mainly produces the standard model
leptons, while negligible amount of the antiproton is produced. To illustrate
such a simple idea, we consider a scenario that a right-handed sneutrino dark
matter decays into the standard model particles through tiny R-parity violating
interactions. Interestingly enough, charged leptons as well as neutrinos are
directly produced, and they can lead to a sharp peak in the predicted positron
fraction. Moreover, the decay of the right-handed sneutrino also generates
diffuse continuum gamma rays which may account for the excess observed by
EGRET, while the primary antiproton flux can be suppressed. Those predictions
on the cosmic-ray fluxes of the positrons, gamma rays and antiprotons will be
tested by the PAMELA and FGST observatories.Comment: 21 pages, 4 figures, 2 tables, updated plots including PAMELA dat
The Contribution of Fermi Gamma-Ray Pulsars to the local Flux of Cosmic-Ray Electrons and Positrons
We analyze the contribution of gamma-ray pulsars from the first Fermi-Large
Area Telescope (LAT) catalogue to the local flux of cosmic-ray electrons and
positrons (e+e-). We present new distance estimates for all Fermi gamma-ray
pulsars, based on the measured gamma-ray flux and pulse shape. We then estimate
the contribution of gamma-ray pulsars to the local e+e- flux, in the context of
a simple model for the pulsar e+e- emission. We find that 10 of the Fermi
pulsars potentially contribute significantly to the measured e+e- flux in the
energy range between 100 GeV and 1 TeV. Of the 10 pulsars, 2 are old EGRET
gamma-ray pulsars, 2 pulsars were discovered with radio ephemerides, and 6 were
discovered with the Fermi pulsar blind-search campaign. We argue that known
radio pulsars fall in regions of parameter space where the e+e- contribution is
predicted to be typically much smaller than from those regions where Fermi-LAT
pulsars exist. However, comparing the Fermi gamma-ray flux sensitivity to the
regions of pulsar parameter space where a significant e+e- contribution is
predicted, we find that a few known radio pulsars that have not yet been
detected by Fermi can also significantly contribute to the local e+e- flux if
(i) they are closer than 2 kpc, and if (ii) they have a characteristic age on
the order of one mega-year.Comment: 21 pages, 6 figures, accepted for publication in JCA
Detection of an inner gaseous component in a Herbig Be star accretion disk: Near- and mid-infrared spectro-interferometry and radiative transfer modeling of MWC 147
We study the geometry and the physical conditions in the inner (AU-scale)
circumstellar region around the young Herbig Be star MWC 147 using
long-baseline spectro-interferometry in the near-infrared (NIR K-band,
VLTI/AMBER observations and PTI archive data) as well as the mid-infrared (MIR
N-band, VLTI/MIDIobservations). The emission from MWC 147 is clearly resolved
and has a characteristic physical size of approx. 1.3 AU and 9 AU at 2.2 micron
and 11 micron respectively (Gaussian diameter). The spectrally dispersed AMBER
and MIDI interferograms both show a strong increase in the characteristic size
towards longer wavelengths, much steeper than predicted by analytic disk models
assuming power-law radial temperature distributions. We model the
interferometric data and the spectral energy distribution of MWC 147 with 2-D,
frequency-dependent radiation transfer simulations. This analysis shows that
models of spherical envelopes or passive irradiated Keplerian disks (with
vertical or curved puffed-up inner rim) can easily fit the SED, but predict
much lower visibilities than observed; the angular size predicted by such
models is 2 to 4 times larger than the size derived from the interferometric
data, so these models can clearly be ruled out. Models of a Keplerian disk with
optically thick gas emission from an active gaseous disk (inside the dust
sublimation zone), however, yield a good fit of the SED and simultaneously
reproduce the absolute level and the spectral dependence of the NIR and MIR
visibilities. We conclude that the NIR continuum emission from MWC 147 is
dominated by accretion luminosity emerging from an optically thick inner
gaseous disk, while the MIR emission also contains contributions from the
outer, irradiated dust disk.Comment: 44 pages, 15 figures, accepted for publication in The Astrophysical
Journal. The quality of the figures was slightly reduced in order to comply
with the astro-ph file-size restrictions. You can find a high-quality version
of the paper at http://www.mpifr-bonn.mpg.de/staff/skraus/papers/mwc147.pd
The CDS information hub
The Centre de Donnees astronomiques de Strasbourg (CDS) provides homogeneousaccess to heterogeneous information of various origins: information aboutastronomical objects in Simbad; catalogs and observation logs in VizieR and inthe catalogue service; reference images and overlays in Aladin; nomenclature inthe Dictionary of Nomenclature; Yellow Page services; the AstroGLU resourcediscovery tool; mirror copies of other reference services; and documentation.With the implementation of links between the CDS services, and with otheron--line reference information, CDS has become a major hub in the rapidlyevolving world of information retrieval in astronomy, developing efficienttools to help astronomers to navigate in the world-wide `Virtual Observatory'under construction, from data in the observatory archives to results publishedin journals. The WWW interface to the CDS services is available at:http://cdsweb.u-strasbg.fr
An inverse method to interpret colour-magnitude diagrams
An inverse method is developed to determine the star formation history, the
age-metallicity relation, and the IMF slope from a colour-magnitude diagram.
The method is applied to the Hipparcos HR diagram. We found that the thin
disk of our Galaxy shows a peak of stellar formation 1.6 Gyr ago. The stars
close to the Sun have a solar metallicity and a mean IMF index equal to 3.2.
However, the model and the evolutionary tracks do not correctly reproduce the
horizontal giant branch.Comment: 14 pages, 11 figures. To be published in Astronomy & Astrophysic
Multi-wavelength analysis of the dust emission in the Small Magellanic Cloud
We present an analysis of dust grain emission in the diffuse interstellar
medium of the Small Magellanic Cloud (SMC). This study is motivated by the
availability of 170 microns ISOPHOT data covering a large part of the SMC, with
a resolution enabling to disentangle the diffuse medium from the star forming
regions. After data reduction and subtraction of Galactic foreground emission,
we used the ISOPHOT data together with HiRes IRAS data and ATCA/Parkes combined
HI column density maps to determine dust properties for the diffuse medium. We
found a far infrared emissivity per hydrogen atom 30 times lower than the Solar
Neighborhood value. The modeling of the spectral energy distribution of the
dust, taking into account the enhanced interstellar radiation field, gives a
similar conclusion for the smallest grains (PAHs and very small grains)
emitting at shorter wavelength. Assuming Galactic dust composition in the SMC,
this result implies a difference in the gas-to-dust ratio (GDR) 3 times larger
than the difference in metallicity. This low depletion of heavy elements in
dust could be specific of the diffuse ISM and not apply for the whole SMC dust
if it results from efficient destruction of dust by supernovae explosions.Comment: 11 pages, 10 figures. Accepted for publication in Astronomy &
Astrophysic
Cosmic Super-Strings and Kaluza-Klein Modes
Cosmic super-strings interact generically with a tower of relatively light
and/or strongly coupled Kaluza-Klein (KK) modes associated with the geometry of
the internal space. In this paper, we study the production of spin-2 KK
particles by cusps on loops of cosmic F- and D-strings. We consider cosmic
super-strings localized either at the bottom of a warped throat or in a flat
internal space with large volume. The total energy emitted by cusps in KK modes
is comparable in both cases, although the number of produced KK modes may
differ significantly. We then show that KK emission is constrained by the
photo-dissociation of light elements and by observations of the diffuse gamma
ray background. We show that this rules out regions of the parameter space of
cosmic super-strings that are complementary to the regions that can be probed
by current and upcoming gravitational wave experiments. KK modes are also
expected to play an important role in the friction-dominated epoch of cosmic
super-string evolution.Comment: 35pp, 5 figs, v2: minor modifications and Refs. added, matches
published versio
A New Approach to Searching for Dark Matter Signals in Fermi-LAT Gamma Rays
Several cosmic ray experiments have measured excesses in electrons and
positrons, relative to standard backgrounds, for energies from ~ 10 GeV - 1
TeV. These excesses could be due to new astrophysical sources, but an
explanation in which the electrons and positrons are dark matter annihilation
or decay products is also consistent. Fortunately, the Fermi-LAT diffuse gamma
ray measurements can further test these models, since the electrons and
positrons produce gamma rays in their interactions in the interstellar medium.
Although the dark matter gamma ray signal consistent with the local electron
and positron measurements should be quite large, as we review, there are
substantial uncertainties in the modeling of diffuse backgrounds and,
additionally, experimental uncertainties that make it difficult to claim a dark
matter discovery. In this paper, we introduce an alternative method for
understanding the diffuse gamma ray spectrum in which we take the intensity
ratio in each energy bin of two different regions of the sky, thereby canceling
common systematic uncertainties. For many spectra, this ratio fits well to a
power law with a single break in energy. The two measured exponent indices are
a robust discriminant between candidate models, and we demonstrate that dark
matter annihilation scenarios can predict index values that require "extreme"
parameters for background-only explanations.Comment: v1: 11 pages, 7 figures, 1 table, revtex4; v2: 13 pages, 8 figures, 1
table, revtex4, Figure 4 added, minor additions made to text, references
added, conclusions unchanged, published versio
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