17,070 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
Collaborative Hierarchical Sparse Modeling
Sparse modeling is a powerful framework for data analysis and processing.
Traditionally, encoding in this framework is done by solving an l_1-regularized
linear regression problem, usually called Lasso. In this work we first combine
the sparsity-inducing property of the Lasso model, at the individual feature
level, with the block-sparsity property of the group Lasso model, where sparse
groups of features are jointly encoded, obtaining a sparsity pattern
hierarchically structured. This results in the hierarchical Lasso, which shows
important practical modeling advantages. We then extend this approach to the
collaborative case, where a set of simultaneously coded signals share the same
sparsity pattern at the higher (group) level but not necessarily at the lower
one. Signals then share the same active groups, or classes, but not necessarily
the same active set. This is very well suited for applications such as source
separation. An efficient optimization procedure, which guarantees convergence
to the global optimum, is developed for these new models. The underlying
presentation of the new framework and optimization approach is complemented
with experimental examples and preliminary theoretical results.Comment: To appear in CISS 201
Oxygen Abundances in Nearby FGK Stars and the Galactic Chemical Evolution of the Local Disk and Halo
Atmospheric parameters and oxygen abundances of 825 nearby FGK stars are
derived using high-quality spectra and a non-LTE analysis of the 777 nm O I
triplet lines. We assign a kinematic probability for the stars to be thin-disk
(P1), thick-disk (P2), and halo (P3) members. We confirm previous findings of
enhanced [O/Fe] in thick-disk (P2>0.5) relative to thin-disk (P1>0.5) stars
with [Fe/H]<-0.2, as well as a "knee" that connects the mean [O/Fe]-[Fe/H]
trend of thick-disk stars with that of thin-disk members at [Fe/H]>-0.2.
Nevertheless, we find that the kinematic membership criterion fails at
separating perfectly the stars in the [O/Fe]-[Fe/H] plane, even when a very
restrictive kinematic separation is employed. Stars with "intermediate"
kinematics (P1<0.7, P2<0.7) do not all populate the region of the [O/Fe]-[Fe/H]
plane intermediate between the mean thin-disk and thick-disk trends, but their
distribution is not necessarily bimodal. Halo stars (P3>0.5) show a large
star-to-star scatter in [O/Fe]-[Fe/H], but most of it is due to stars with
Galactocentric rotational velocity V-200 km/s
follow an [O/Fe]-[Fe/H] relation with almost no star-to-star scatter. Early
mergers with satellite galaxies explain most of our observations, but the
significant fraction of disk stars with "ambiguous" kinematics and abundances
suggests that scattering by molecular clouds and radial migration have both
played an important role in determining the kinematic and chemical properties
of solar neighborhood stars.Comment: ApJ, in press. Complete tables 2-6 are available in the source
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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
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