3,045 research outputs found
Chemical signatures of planets: beyond solar-twins
Elemental abundance studies of solar twin stars suggest that the solar
chemical composition contains signatures of the formation of terrestrial
planets in the solar system, namely small but significant depletions of the
refractory elements. To test this hypothesis, we study stars which, compared to
solar twins, have less massive convective envelopes (therefore increasing the
amplitude of the predicted effect) or are, arguably, more likely to host
planets (thus increasing the frequency of signature detections). We measure
relative atmospheric parameters and elemental abundances of a late-F type dwarf
sample (52 stars) and a sample of metal-rich solar analogs (59 stars). We
detect refractory-element depletions with amplitudes up to about 0.15 dex. The
distribution of depletion amplitudes for stars known to host gas giant planets
is not different from that of the rest of stars. The maximum amplitude of
depletion increases with effective temperature from 5650 K to 5950 K, while it
appears to be constant for warmer stars (up to 6300 K). The depletions observed
in solar twin stars have a maximum amplitude that is very similar to that seen
here for both of our samples. Gas giant planet formation alone cannot explain
the observed distributions of refractory-element depletions, leaving the
formation of rocky material as a more likely explanation of our observations.
More rocky material is necessary to explain the data of solar twins than
metal-rich stars, and less for warm stars. However, the sizes of the stars'
convective envelopes at the time of planet formation could be regulating these
amplitudes. Our results could be explained if disk lifetimes were shorter in
more massive stars, as independent observations indeed seem to suggest.Comment: Astronomy and Astrophysics, in press. Full tables available in the
source downloa
A possible signature of terrestrial planet formation in the chemical composition of solar analogs
Recent studies have shown that the elemental abundances in the Sun are
anomalous when compared to most (about 85%) nearby solar twin stars. Compared
to its twins, the Sun exhibits a deficiency of refractory elements (those with
condensation temperatures Tc>900K) relative to volatiles (Tc<900K). This
finding is speculated to be a signature of the planet formation that occurred
more efficiently around the Sun compared with the majority of solar twins.
Furthermore, within this scenario, it seems more likely that the abundance
patterns found are specifically related to the formation of terrestrial
planets. In this work we analyze abundance results from six large independent
stellar abundance surveys to determine whether they confirm or reject this
observational finding. We show that the elemental abundances derived for solar
analogs in these six studies are consistent with the Tc trend suggested as a
planet formation signature. The same conclusion is reached when those results
are averaged heterogeneously. We also investigate the dependency of the
abundances with first ionization potential (FIP), which correlates well with
Tc. A trend with FIP would suggest a different origin for the abundance
patterns found, but we show that the correlation with Tc is statistically more
significant. We encourage similar investigations of metal-rich solar analogs
and late F-type dwarf stars, for which the hypothesis of a planet formation
signature in the elemental abundances makes very specific predictions. Finally,
we examine a recent paper that claims that the abundance patterns of two stars
hosting super-Earth like planets contradict the planet formation signature
hypothesis. Instead, we find that the chemical compositions of these two stars
are fully compatible with our hypothesis.Comment: To appear in Astronomy and Astrophysic
Stellar Chemical Abundances: In Pursuit of the Highest Achievable Precision
The achievable level of precision on photospheric abundances of stars is a
major limiting factor on investigations of exoplanet host star characteristics,
the chemical histories of star clusters, and the evolution of the Milky Way and
other galaxies. While model-induced errors can be minimized through the
differential analysis of spectrally similar stars, the maximum achievable
precision of this technique has been debated. As a test, we derive differential
abundances of 19 elements from high-quality asteroid-reflected solar spectra
taken using a variety of instruments and conditions. We treat the solar spectra
as being from unknown stars and use the resulting differential abundances,
which are expected to be zero, as a diagnostic of the error in our
measurements. Our results indicate that the relative resolution of the target
and reference spectra is a major consideration, with use of different
instruments to obtain the two spectra leading to errors up to 0.04 dex. Use of
the same instrument at different epochs for the two spectra has a much smaller
effect (~0.007 dex). The asteroid used to obtain the solar standard also has a
negligible effect (~0.006 dex). Assuming that systematic errors from the
stellar model atmospheres have been minimized, as in the case of solar twins,
we confirm that differential chemical abundances can be obtained at sub-0.01
dex precision with due care in the observations, data reduction and abundance
analysis.Comment: Accepted for publication in ApJ; 13 pages, 6 figures, 7 table
Observational signatures for depletion in the Spite plateau: solving the cosmological Li discrepancy?
We present Li abundances for 73 stars in the metallicity range -3.5 < [Fe/H]
< -1.0 using improved IRFM temperatures (Casagrande et al. 2010) with precise
E(B-V) values obtained mostly from interstellar NaI D lines, and high-quality
equivalent widths (errors ~ 3%). At all metallicities we uncover a
fine-structure in the Li abundances of Spite plateau stars, which we trace to
Li depletion that depends on both metallicity and mass. Models including atomic
diffusion and turbulent mixing seem to reproduce the observed Li depletion
assuming a primordial Li abundance ALi = 2.64 dex (MARCS models) or 2.72
(Kurucz overshooting models), in good agreement with current predictions (ALi =
2.72) from standard BBN. We are currently expanding our sample to have a better
coverage of different evolutionary stages at the high and low metallicity ends,
in order to verify our findings.Comment: In press, Light elements in the Universe, Proceedings IAU Symposium
No. 268, 2010. C. Charbonnel, M. Tosi, F. Primas & C. Chiappini, ed
Line formation in solar granulation VI. [C I], C I, CH and C2 lines and the photospheric C abundance
The solar photospheric carbon abundance has been determined from [C I], C I,
CH vibration-rotation, CH A-X electronic and C2 Swan electronic lines by means
of a time-dependent, 3D, hydrodynamical model of the solar atmosphere.
Departures from LTE have been considered for the C I lines. These turned out to
be of increasing importance for stronger lines and are crucial to remove a
trend in LTE abundances with the strengths of the lines. Very gratifying
agreement is found among all the atomic and molecular abundance diagnostics in
spite of their widely different line formation sensitivities. The mean of the
solar carbon abundance based on the four primary abundance indicators ([C I], C
I, CH vibration-rotation, C_2 Swan) is log C = 8.39 +/- 0.05, including our
best estimate of possible systematic errors. Consistent results also come from
the CH electronic lines, which we have relegated to a supporting role due to
their sensitivity to the line broadening. The new 3D based solar C abundance is
significantly lower than previously estimated in studies using 1D model
atmospheres.Comment: Accepted for A&A, 13 page
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