114 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&
A Physically-Motivated Photometric Calibration of M Dwarf Metallicity
The location of M dwarfs in the V-K_s--M_Ks color-magnitude diagram (CMD) has
been shown to correlate with metallicity. We demonstrate that previous
empirical photometric calibrations of M dwarf metallicity exploiting this
correlation systematically underestimate or overestimate metallicity at the
extremes of their range. We improve upon previous calibrations in three ways.
We use both a volume-limited and kinematically-matched sample of F and G dwarfs
from the Geneva-Copehnagen Survey (GCS) to infer the mean metallicity of M
dwarfs in the Solar Neighborhood, we use theoretical models of M dwarf
interiors and atmospheres to determine the effect of metallicity on M dwarfs in
the V-K_s--M_Ks CMD, and we base our final calibration purely on
high-resolution spectroscopy of FGK primaries with M dwarf companions. As a
result, we explain an order of magnitude more of the variance in the
calibration sample than previous photometric calibrations. We
non-parametrically quantify the significance of the observation that M dwarfs
that host exoplanets are preferentially in a region of the V-K_s--M_Ks plane
populated by metal-rich M dwarfs. We find that the probability p that
planet-hosting M dwarfs are distributed across the V-K_s--M_Ks CMD in the same
way as field M dwarfs is p = 0.06 +/- 0.008. Interestingly, the subsample of M
dwarfs that host Neptune and sub-Neptune mass planets may also be
preferentially located in the region of the V-K_s--M_Ks plane populated by
high-metallicity M dwarfs. The probability of this occurrence by chance is p =
0.40 +/- 0.02, and this observation hints that low-mass planets may be more
likely to be found around metal-rich M dwarfs. An increased rate of low-mass
planet occurrence around metal-rich M dwarfs would be a natural consequence of
the core-accretion model of planet formation. (abridged)Comment: 10 pages, 4 figures, and 1 table in A&A format; accepted for
publication in A&
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
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
Crossing the Gould Belt in the Orion vicinity
We present a study of the large-scale spatial distribution of 6482 RASS X-ray
sources in approximately 5000 deg^2 in the direction of Orion. We examine the
astrophysical properties of a sub-sample of ~100 optical counterparts, using
optical spectroscopy. This sub-sample is used to investigate the space density
of the RASS young star candidates by comparing X-ray number counts with
Galactic model predictions. We characterize the observed sub-sample in terms of
spectral type, lithium content, radial and rotational velocities, as well as
iron abundance. A population synthesis model is then applied to analyze the
stellar content of the RASS in the studied area. We find that stars associated
with the Orion star-forming region do show a high lithium content. A population
of late-type stars with lithium equivalent widths larger than Pleiades stars of
the same spectral type (hence younger than ~70-100 Myr) is found widely spread
over the studied area. Two new young stellar aggregates, namely "X-ray Clump
0534+22" (age~2-10 Myr) and "X-ray Clump 0430-08" (age~2-20 Myr), are also
identified. The spectroscopic follow-up and comparison with Galactic model
predictions reveal that the X-ray selected stellar population in the direction
of Orion is characterized by three distinct components, namely the clustered,
the young dispersed, and the widespread field populations. The clustered
population is mainly associated with regions of recent or ongoing star
formation and correlates spatially with molecular clouds. The dispersed young
population follows a broad lane apparently coinciding spatially with the Gould
Belt, while the widespread population consists primarily of active field stars
older than 100 Myr. We expect the "bi-dimensional" picture emerging from this
study to grow in depth as soon as the distance and the kinematics of the
studied sources will become available from the future Gaia mission.Comment: 17 pages, 13 figures, 4 tables. Accepted for publication in Astronomy
and Astrophysics. Abstract shortene
Implications of the Fermi-LAT diffuse gamma-ray measurements on annihilating or decaying Dark Matter
We analyze the recently published Fermi-LAT diffuse gamma-ray measurements in
the context of leptonically annihilating or decaying dark matter (DM) with the
aim to explain simultaneously the isotropic diffuse gamma-ray and the PAMELA,
Fermi and HESS (PFH) anomalous data. Five different DM
annihilation/decay channels , , , , or (the latter
two via an intermediate light particle ) are generated with PYTHIA. We
calculate both the Galactic and extragalactic prompt and inverse Compton (IC)
contributions to the resulting gamma-ray spectra. To find the Galactic IC
spectra we use the interstellar radiation field model from the latest release
of GALPROP. For the extragalactic signal we show that the amplitude of the
prompt gamma-emission is very sensitive to the assumed model for the
extragalactic background light. For our Galaxy we use the Einasto, NFW and
Isothermal DM density profiles and include the effects of DM substructure
assuming a simple subhalo model. Our calculations show that for the
annihilating DM the extragalactic gamma-ray signal can dominate only if rather
extreme power-law concentration-mass relation is used, while more
realistic relations make the extragalactic component comparable or
subdominant to the Galactic signal. For the decaying DM the Galactic signal
always exceeds the extragalactic one. In the case of annihilating DM the PFH
favored parameters can be ruled out only if power-law relation is
assumed. For DM decaying into or the PFH favored DM parameters
are not in conflict with the Fermi gamma-ray data. We find that, due to the
(almost) featureless Galactic IC spectrum and the DM halo substructure,
annihilating DM may give a good simultaneous fit to the isotropic diffuse
gamma-ray and to the PFH data without being in clear conflict with the
other Fermi-LAT gamma-ray measurements.Comment: Accepted for publication in JCAP, added missing references, new Figs.
9 \& 10, 35 page
Mixed Higgsino Dark Matter from a Reduced SU(3) Gaugino Mass: Consequences for Dark Matter and Collider Searches
In gravity-mediated SUSY breaking models with non-universal gaugino masses,
lowering the SU(3) gaugino mass |M_3| leads to a reduction in the squark and
gluino masses. Lower third generation squark masses, in turn, diminish the
effect of a large top quark Yukawa coupling in the running of the higgs mass
parameter m_{H_u}^2, leading to a reduction in the magnitude of the
superpotential mu parameter (relative to M_1 and M_2). A low | mu | parameter
gives rise to mixed higgsino dark matter (MHDM), which can efficiently
annihilate in the early universe to give a dark matter relic density in accord
with WMAP measurements. We explore the phenomenology of the low |M_3| scenario,
and find for the case of MHDM increased rates for direct and indirect detection
of neutralino dark matter relative to the mSUGRA model. The sparticle mass
spectrum is characterized by relatively light gluinos, frequently with
m(gl)<<m(sq). If scalar masses are large, then gluinos can be very light, with
gl->Z_i+g loop decays dominating the gluino branching fraction. Top squarks can
be much lighter than sbottom and first/second generation squarks. The presence
of low mass higgsino-like charginos and neutralinos is expected at the CERN
LHC. The small m(Z2)-m(Z1) mass gap should give rise to a visible
opposite-sign/same flavor dilepton mass edge. At a TeV scale linear e^+e^-
collider, the region of MHDM will mean that the entire spectrum of charginos
and neutralinos are amongst the lightest sparticles, and are most likely to be
produced at observable rates, allowing for a complete reconstruction of the
gaugino-higgsino sector.Comment: 35 pages, including 26 EPS figure
Constraining Very Heavy Dark Matter Using Diffuse Backgrounds of Neutrinos and Cascaded Gamma Rays
We consider multi-messenger constraints on very heavy dark matter (VHDM) from
recent Fermi gamma-ray and IceCube neutrino observations of isotropic
background radiation. Fermi data on the diffuse gamma-ray background (DGB)
shows a possible unexplained feature at very high energies (VHE), which we have
called the "VHE Excess" relative to expectations for an attenuated power law
extrapolated from lower energies. We show that VHDM could explain this excess,
and that neutrino observations will be an important tool for testing this
scenario. More conservatively, we derive new constraints on the properties of
VHDM for masses of 10^3-10^10 GeV. These generic bounds follow from cosmic
energy budget constraints for gamma rays and neutrinos that we developed
elsewhere, based on detailed calculations of cosmic electromagnetic cascades
and also neutrino detection rates. We show that combining both gamma-ray and
neutrino data is essential for making the constraints on VHDM properties both
strong and robust. In the lower mass range, our constraints on VHDM
annihilation and decay are comparable to other results; however, our
constraints continue to much higher masses, where they become relatively
stronger.Comment: 33 pages, 21 figures, accepted for publication in JCA
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