27,523 research outputs found
A low-mass stellar companion of the planet host star HD75289
We report on the detection of a new low-mass stellar companion of HD75289, a
G0V star that harbors one known radial-velocity planet (Udry et al. 2000).
Comparing an image of 2MASS with an image we obtained with SofI at the ESO
3.58m NTT three years later, we detected a co-moving companion located
21.465+-0.023arcsecs (621+-10AU at 29pc) east of HD75289. A second SofI image
taken 10 months later confirmed the common proper motion of HD75289B with its
host star. The infrared spectrum and colors of the companion are consistent
with an M2 to M5 main-sequence star at the distance of HD75289. No further
(sub)stellar companion down to H = 19mag could be detected. With the SofI
detection limit we can rule out additional stellar companions beyond 140AU and
substellar companions with masses m > 0.050Msun from 400AU up to 2000AU.Comment: accepted in A&
Orion revisited. II. The foreground population to Orion A
Following the recent discovery of a large population of young stars in front
of the Orion Nebula, we carried out an observational campaign with the DECam
wide-field camera covering ~10~deg^2 centered on NGC 1980 to confirm, probe the
extent of, and characterize this foreground population of pre-main-sequence
stars. We confirm the presence of a large foreground population towards the
Orion A cloud. This population contains several distinct subgroups, including
NGC1980 and NGC1981, and stretches across several degrees in front of the Orion
A cloud. By comparing the location of their sequence in various color-magnitude
diagrams with other clusters, we found a distance and an age of 380pc and
5~10Myr, in good agreement with previous estimates. Our final sample includes
2123 candidate members and is complete from below the hydrogen-burning limit to
about 0.3Msun, where the data start to be limited by saturation. Extrapolating
the mass function to the high masses, we estimate a total number of ~2600
members in the surveyed region. We confirm the presence of a rich, contiguous,
and essentially coeval population of about 2600 foreground stars in front of
the Orion A cloud, loosely clustered around NGC1980, NGC1981, and a new group
in the foreground of the OMC-2/3. For the area of the cloud surveyed, this
result implies that there are more young stars in the foreground population
than young stars inside the cloud. Assuming a normal initial mass function, we
estimate that between one to a few supernovae must have exploded in the
foreground population in the past few million years, close to the surface of
Orion A, which might be responsible, together with stellar winds, for the
structure and star formation activity in these clouds. This long-overlooked
foreground stellar population is of great significance, calling for a revision
of the star formation history in this region of the Galaxy.Comment: Accepted for publication in A&
Dynamics and Constraints of the Massive Gravitons Dark Matter Flat Cosmologies
We discuss the dynamics of the universe within the framework of Massive
Graviton Dark Matter scenario (MGCDM) in which gravitons are geometrically
treated as massive particles. In this modified gravity theory, the main effect
of the gravitons is to alter the density evolution of the cold dark matter
component in such a way that the Universe evolves to an accelerating expanding
regime, as presently observed. Tight constraints on the main cosmological
parameters of the MGCDM model are derived by performing a joint likelihood
analysis involving the recent supernovae type Ia data, the Cosmic Microwave
Background (CMB) shift parameter and the Baryonic Acoustic Oscillations (BAOs)
as traced by the Sloan Digital Sky Survey (SDSS) red luminous galaxies. The
linear evolution of small density fluctuations is also analysed in detail. It
is found that the growth factor of the MGCDM model is slightly different
() from the one provided by the conventional flat CDM
cosmology. The growth rate of clustering predicted by MGCDM and CDM
models are confronted to the observations and the corresponding best fit values
of the growth index () are also determined. By using the expectations
of realistic future X-ray and Sunyaev-Zeldovich cluster surveys we derive the
dark-matter halo mass function and the corresponding redshift distribution of
cluster-size halos for the MGCDM model. Finally, we also show that the Hubble
flow differences between the MGCDM and the CDM models provide a halo
redshift distribution departing significantly from the ones predicted by other
DE models. These results suggest that the MGCDM model can observationally be
distinguished from CDM and also from a large number of dark energy
models recently proposed in the literature.Comment: Accepted for publication in Physical Review D (12 pages, 4 figures
The Dynamical State fo the Starless Dense Core FeSt 1-457: A Pulsating Globule?
High resolution molecular line observations of CS, HCO+, C18O and N2H+ were
obtained toward the starless globule FeSt 1-457 in order to investigate its
kinematics and chemistry. The HCO+ and CS spectra show clear self-reversed and
asymmetric profiles across the face of the globule. The sense of the observed
asymmetry is indicative of the global presence of expansion motions in the
outer layers of the globule. These motions appear to be subsonic and
significantly below the escape velocity of the globule. Comparison of our
observations with near-infrared extinction data indicate that the globule is
gravitationally bound. Taken together these considerations lead us to suggest
that the observed expansion has its origin in an oscillatory motion of the
outer layers of the globule which itself is likely in a quasi-stable state near
hydrostatic equilibrium. Analysis of the observed linewidths of CO and N2H+
confirm that thermal pressure is the dominant component of the cloud's internal
support. A simple calculation suggests that the dominant mode of pulsation
would be an l = 2 mode with a period of 0.3 Myr. Deformation of the globule due
to the large amplitude l = 2 oscillation may be responsible for the
double-peaked structure of the core detected in high resolution extinction
maps. Detailed comparison of the molecular-line observations and extinction
data provides evidence for significant depletion of C18O and perhaps HCO+ while
N2H+ may be undepleted to a cloud depth of about 40 magnitudes of visual
extinction.Comment: to appear in ApJ vol 665 20 August 2007
Alguns insetos úteis ao algodoeiro.
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