582 research outputs found
Measurement of the radial velocity of the Sun as a star by means of a reflecting solar system body. The effect of the body rotation
Minor bodies of the solar system can be used to measure the spectrum of the
Sun as a star by observing sunlight reflected by their surfaces. To perform an
accurate measurement of the radial velocity of the Sun as a star by this
method, it is necessary to take into account the Doppler shifts introduced by
the motion of the reflecting body. Here we discuss the effect of its rotation.
It gives a vanishing contribution only when the inclinations of the body
rotation axis to the directions of the Sun and of the Earth observer are the
same. When this is not the case, the perturbation of the radial velocity does
not vanish and can reach up to about 2.4 m/s for an asteroid such as 2 Pallas
that has an inclination of the spin axis to the plane of the ecliptic of about
30 degrees. We introduce a geometric model to compute the perturbation in the
case of a uniformly reflecting body of spherical or triaxial ellipsoidal shape
and provide general results to easily estimate the magnitude of the effect.Comment: 14 pages, 5 figures, 3 tables, accepted by Experimental Astronom
Abundances in Damped Ly-alpha Galaxies
Damped Ly_alpha galaxies provide a sample of young galaxies where chemical
abundances can be derived throughout the whole universe with an accuracy
comparable to that for the local universe. Despite a large spread in redshift,
HI column density and metallicity, DLA galaxies show a remarkable uniformity in
the elemental ratios rather suggestive of similar chemical evolution if not of
an unique population. These galaxies are characterized by a moderate, if any,
enhancement of alpha-elements over Fe-peak elemental abundance with [S/Zn]
about 0 and [O/Zn] about 0.2, rather similarly to the dwarfs galaxies in the
Local Group. Nitrogen shows a peculiar behaviour with a bimodal distribution
and possibly two plateaux. In particular, the plateau at low N abundances
([N/H] < -3), is not observed in other atrophysical sites and might be evidence
for primary N production by massive stars.Comment: To appear in the Proceedings of the ESO/Arcetri Workshop on "Chemical
Abundances and Mixing in Stars in the Milky Way and its Satellites", eds., L.
Pasquini and S. Randich (Springer-Verlag Series, "ESO Astrophysics Symposia"
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
Pre-MS depletion, accretion and primordial 7Li
We reconsider the role of pre-main sequence (pre-MS) Li depletion on the
basis of new observational and theoretical evidence: i) new observations of
Halpha emissions in young clusters show that mass accretion could be continuing
till the first stages of the MS, ii) theoretical implications from
helioseismology suggest large overshooting values below the bottom of the
convective envelopes. We argue here that a significant pre-MS 7Li destruction,
caused by efficient overshoot mixing, could be followed by a matter accretion
after 7Li depletion has ceased on MS thus restoring Li almost to the pristine
value. As a test case we show that a halo dwarf of 0.85 Msun with an extended
overshooting envelope starting with an initial abundance of A(Li) = 2.74 would
burn Li completely, but an accretion rate of the type 1e-8xe^{-t/3e6} Msun
yr would restore Li to end with an A(Li) = 2.31. A self-regulating
process is required to produce similar final values in a range of different
stellar masses to explain the PopII Spite plateau. However, this framework
could explain why open cluster stars have lower Li abundances than the
pre-solar nebula, the absence of Li in the most metal poor dwarfs and a number
of other features which lack of a satisfactory explanation.Comment: To be published in Memorie della Societ\`a Astronomica Italiana
Supplementi Vol. 22, Proceedings of Lithium in the cosmos, Iocco F.,
Bonifacio P., Vangioni E., ed
Chemical Evolution of Damped Ly alpha galaxies: The [S/Zn] abundance ratio at redshift z > 2
Relative elemental abundances, and in particular the alpha/Fe ratio, are an
important diagnostic tool of the chemical evolution of damped Ly alpha systems
(DLAs). The S/Zn ratio is not affected by differential dust depletion and is an
excellent estimator of the alpha/Fe ratio. We report 6 new determinations of
sulphur abundance in DLAs at zabs greater than or equal to 2 with already known
zinc abundances. The combination with extant data from the literature provides
a measure of the S/Zn abundance ratio for a total of 11 high redshift DLA
systems. The observed [S/Zn] ratios do not show the characteristic [alpha/Fe]
enhancement observed in metal-poor stars of the Milky Way at comparable level
of metallicity ([Zn/H] ~ -1). The behaviour of DLAs data is consistent with a
general trend of decreasing [S/Zn] ratio with increasing metallicity [Zn/H].
This would be the first evidence of the expected decrease of the alpha/Fe ratio
in the course of chemical evolution of DLA systems. However, in contrast to
what observed in our Galaxy, the alpha/iron-peak ratio seems to attain solar
values when the metallicity is still low ([Zn/H] < -1) and to decrease below
solar values at higher metallicities. The behaviour of the alpha/Fe ratio
challenges the frequently adopted hypothesis that high redshift DLAs are
progenitors of spiral galaxies and favours instead an origin in galaxies
characterized by low star formation rates, in agreement with the results from
imaging studies of low redshift DLAs, where the candidate DLA galaxies show a
variety of morphological types including dwarfs and LSBs and only a minority of
spirals.Comment: ApJ (accepted
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