8,182 research outputs found
Long-term radial-velocity variations of the Sun as a star: The HARPS view
Stellar radial velocities play a fundamental role in the discovery of
extrasolar planets and the measurement of their physical parameters as well as
in the study of stellar physical properties. We investigate the impact of the
solar activity on the radial velocity of the Sun using the HARPS spectrograph
to obtain measurements that can be directly compared with those acquired in the
extrasolar planet search programs. We use the Moon, the Galilean satellites,
and several asteroids as reflectors to measure the radial velocity of the Sun
as a star and correlate it with disc-integrated chromospheric and magnetic
indexes of solar activity that are similar to stellar activity indexes. We
discuss in detail the systematic effects that affect our measurements and the
methods to account for them. We find that the radial velocity of the Sun as a
star is positively correlated with the level of its chromospheric activity at
about 95 percent significance level. The amplitude of the long-term variation
measured in the 2006-2014 period is 4.98 \pm 1.44 m/s, in good agreement with
model predictions. The standard deviation of the residuals obtained by
subtracting a linear best fit is 2.82 m/s and is due to the rotation of the
reflecting bodies and the intrinsic variability of the Sun on timescales
shorter than the activity cycle. A correlation with a lower significance is
detected between the radial velocity and the mean absolute value of the
line-of-sight photospheric magnetic field flux density. Our results confirm
similar correlations found in other late-type main-sequence stars and provide
support to the predictions of radial velocity variations induced by stellar
activity based on current models.Comment: 11 pages, 7 figures, 2 tables, 1 Appendix; accepted by Astronomy and
Astrophysic
Detection of a population gradient in the Sagittarius Stream
We present a quantitative comparison between the Horizontal Branch morphology
in the core of the Sagittarius dwarf spheroidal galaxy (Sgr) and in a wide
field sampling a portion of its tidal stream (Sgr Stream), located tens of kpc
away from the center of the parent galaxy. We find that the Blue Horizontal
Branch (BHB) stars in that part of the Stream are five times more abundant than
in the Sgr core, relative to Red Clump stars. The difference in the ratio of
BHB to RC stars between the two fields is significant at the 4.8 sigma level.
This indicates that the old and metal-poor population of Sgr was preferentially
stripped from the galaxy in past peri-Galactic passages with respect to the
intermediate-age metal rich population that presently dominates the bound core
of Sgr, probably due to a strong radial gradient that was settled within the
galaxy before its disruption. The technique adopted in the present study allows
to trace population gradients along the whole extension of the Stream.Comment: 4 pages, 3 .ps figures (fig. 1 at low resolution); Accepted for
publication by A&A Letter
FLAMES and XSHOOTER spectroscopy along the two BSS sequences of M30
We present spectroscopic observations acquired with FLAMES and XSHOOTER at
the Very Large Telescope for a sample of 15 Blue Straggler Stars (BSSs) in the
globular cluster (GC) M30. The targets have been selected to sample the two BSS
sequences discovered, with 7 BSSs along the blue sequence and 8 along the red
one. No difference in the kinematical properties of the two groups of BSSs has
been found. In particular, almost all the observed BSSs have projected
rotational velocity lower than ~30 km/s, with only one (blue) fast rotating BSS
(>90 km/s), identified as a W UMa binary. This rotational velocity distribution
is similar to those obtained in 47 Tucanae and NGC 6397, while M4 remains the
only GC studied so far harboring a large fraction of fast rotating BSSs. All
stars hotter than ~7800 K (regardless of the parent BSS sequence) show iron
abundances larger than those measured from normal cluster stars, with a
clearcut trend with the effective temperature. This behaviour suggests that
particle trasport mechanisms driven by radiative levitation occur in the
photosphere of these stars, as already observed for the BSSs in NGC 6397.
Finally, 4 BSSs belonging to the red sequence (not affected by radiative
levitation) show a strong depletion of [O/Fe], with respect to the abundance
measured in Red Giant Branch and Horizontal Branch stars. This O-depletion is
compatible with the chemical signature expected in BSSs formed by mass transfer
processes in binary systems, in agreement with the mechanism proposed for the
formation of BSSs in the red sequence.Comment: Accepted for publication in Ap
Mathematical models in landscape ecology: stability analysis and numerical tests
Si presentano vari modelli matematici per la valutazione della qualita' ecologica di un sistema ambientale. Si presenta poi un nuovo modello denominato PANDORA 2.0 e si mostra un'applicazione nel territorio della provincia di Viterb
Non-dynamic origin of the acoustic attenuation at high frequency in glasses
The sound attenuation in the THz region is studied down to T=16 K in glassy
glycerol by inelastic x-ray scattering. At striking variance with the decrease
found below 100 K in the GHz data, the attenuation in the THz range does not
show any T dependence. This result i) indicates the presence of two different
attenuation mechanisms, active respectively in the high and low frequency
limits; ii) demonstrates the non-dynamic origin of the attenuation of THz sound
waves, and confirms a similar conclusion obtained in SiO2 glass by molecular
dynamics; and iii) supports the low frequency attenuation mechanism proposed by
Fabian and Allen (Phys.Rev.Lett. 82, 1478 (1999)).Comment: 3 pages, 5 Figures, To be published in PR
Constraining Cosmological Models by the Cluster Mass Function
We present a comparison between two observational and three theoretical mass
functions for eight cosmological models suggested by the data from the recently
completed BOOMERANG-98 and MAXIMA-1 cosmic microwave background (CMB)
anisotropy experiments as well as peculiar velocities (PVs) and type Ia
supernovae (SN) observations. The cosmological models have been proposed as the
best fit models by several groups. We show that no model is in agreement with
the abundances of X-ray clusters at .On the
other hand, we find that the BOOM+MAX+{\sl COBE}:I, Refined Concordance and
MDM are in a good agreement with the abundances of optical clusters.
The P11 and especially Concordance models predict a slightly lower abundances
than observed at . The BOOM+MAX+{\sl COBE}:II
and PV+CMB+SN models predict a slightly higher abundances than observed at
. The nonflat MAXIMA-1 is in a fatal conflict
with the observational cluster abundances and can be safely ruled out.Comment: 17 pages, 2 figures, reference added, figures changes, substantial
revision mad
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