99 research outputs found
High precision abundances in the 16 Cyg binary system: a signature of the rocky core in the giant planet
We study the stars of the binary system 16 Cygni to determine with high
precision their chemical composition. Knowing that the component B has a
detected planet of at least 1.5 Jupiter masses, we investigate if there are
chemical peculiarities that could be attributed to planet formation around this
star. We perform a differential abundance analysis using high resolution (R =
81,000) and high S/N (~700) CFHT/ESPaDOnS spectra of the 16 Cygni stars and the
Sun; the latter was obtained from light reflected of asteroids. We determine
differential abundances of the binary components relative to the Sun and
between components A and B as well. We achieve a precision of about 0.005 dex
and a total error ~0.01 dex for most elements. The effective temperatures and
surface gravities found for 16 Cyg A and B are Teff = 5830+/-7 K, log g =
4.30+/-0.02 dex, and Teff = 5751+/-6 K, log g = 4.35+/-0.02 dex, respectively.
The component 16 Cyg A has a metallicity ([Fe/H]) higher by 0.047+/-0.005 dex
than 16 Cyg B, as well as a microturbulence velocity higher by 0.08 km/s. All
elements show abundance differences between the binary components, but while
the volatile difference is about 0.03 dex, the refractories differ by more and
show a trend with condensation temperature, which could be interpreted as the
signature of the rocky accretion core of the giant planet 16 Cyg Bb. We
estimate a mass of about 1.5-6 M_Earth for this rocky core, in good agreement
with estimates of Jupiter's core.Comment: ApJ Letters. Press release: http://cfht.hawaii.edu/en/news/16CygAB
2MASS J18082002-5104378: The brightest (V=11.9) ultra metal-poor star
Context. The most primitive metal-poor stars are important for studying the
conditions of the early galaxy and are also relevant to big bang
nucleosynthesis. Aims. Our objective is to find the brightest (V<14) most
metal-poor stars. Methods. Candidates were selected using a new method, which
is based on the mismatch between spectral types derived from colors and
observed spectral types. They were observed first at low resolution with EFOSC2
at the NTT/ESO to obtain an initial set of stellar parameters. The most
promising candidate, 2MASS J18082002-5104378 (V=11.9), was observed at high
resolution (R=50 000) with UVES at the VLT/ESO, and a standard abundance
analysis was performed. Results. We found that 2MASS J18082002-5104378 is an
ultra metal-poor star with stellar parameters Teff = 5440 K, log g = 3.0 dex,
vt = 1.5 km/s, [Fe/H] = -4.1 dex. The star has [C/Fe]<+0.9 in a 1D analysis, or
[C/Fe]<=+0.5 if 3D effects are considered; its abundance pattern is typical of
normal (non-CEMP) ultra metal-poor stars. Interestingly, the star has a binary
companion. Conclusions. 2MASS J1808-5104 is the brightest (V=11.9) metal-poor
star of its category, and it could be studied further with even higher S/N
spectroscopy to determine additional chemical abundances, thus providing
important constraints to the early chemical evolution of our Galaxy.Comment: A&A Letter
Shallow extra mixing in solar twins inferred from Be abundances
Lithium and beryllium are destroyed at different temperatures in stellar
interiors. As such, their relative abundances offer excellent probes of the
nature and extent of mixing processes within and below the convection zone. We
determine Be abundances for a sample of eight solar twins for which Li
abundances have previously been determined. The analyzed solar twins span a
very wide range of age, 0.5-8.2 Gyr, which enables us to study secular
evolution of Li and Be depletion. We gathered high-quality UVES/VLT spectra and
obtained Be abundances by spectral synthesis of the Be II 313 nm doublet. The
derived beryllium abundances exhibit no significant variation with age. The
more fragile Li, however, exhibits a monotonically decreasing abundance with
increasing age. Therefore, relatively shallow extra mixing below the convection
zone is necessary to simultaneously account for the observed Li and Be behavior
in the Sun and solar twins
Revisiting the 16 Cygni planet host at unprecedented precision and exploring automated tools for precise abundances
The binary system 16 Cygni is key in studies of the planet-star chemical
composition connection, as only one of the stars is known to host a planet.
This allows us to better assess the possible influence of planet interactions
on the chemical composition of stars that are born from the same cloud and
thus, should have a similar abundance pattern. In our previous work, we found
clear abundance differences for elements with Z between both components
of this system, and a trend of these abundances as a function of the
condensation temperature (T), which suggests a spectral chemical
signature related to planet formation. In this work we show that our previous
findings are still consistent even if we include more species, like the
volatile N and neutron capture elements (Z 30). We report a slope with
T of dex K, that is good agreement
with both our previous work and recent results by Nissen and collaborators. We
also performed some tests using ARES and iSpec to automatic measure the
equivalent width and found T slopes in reasonable agreement with our
results as well. In addition, we determine abundances for Li and Be by spectral
synthesis, finding that 16 Cyg A is richer not only in Li but also in Be, when
compared to its companion. This may be evidence of planet engulfment,
indicating that the T trend found in this binary system may be a chemical
signature of planet accretion in the A component, rather than a imprint of the
giant planet rocky core formation on 16 Cyg B.Comment: 11 pages, 5 figures, accepted for publication in A&
Chemical inhomogeneities in the Pleiades: signatures of rocky-forming material in stellar atmospheres
The aim of Galactic archaeology is to recover the history of our Galaxy
through the information encoded in stars. An unprobed assumption of this field
is that the chemical composition of a star is an immutable marker of the gas
from which it formed. It is vital to test this assumption on open clusters,
group of stars formed from the same gas. Previous investigations have shown
that unevolved stars in clusters are chemically homogeneous within the typical
uncertainties of these analysis, i.e. 15 of the elemental abundances. Our
strictly differential analysis on five members of the Pleiades allows us to
reach precisions of 5 for most elements and to unveil chemical anomalies
within the cluster that could be explained by planet engulfment events. These
results reveal that the evolution of planetary systems may alter the chemical
composition of stars, challenging our capability of tagging them to their
native environments, and also paving the way for the study of planetary
architectures and their evolution, through the chemical pattern of their host
stars.Comment: 10 pages, 2 figures. Accepted for publication in Ap
The Solar Twin Planet Search II. A Jupiter twin around a solar twin
Through our HARPS radial velocity survey for planets around solar twin stars,
we have identified a promising Jupiter twin candidate around the star HIP11915.
We characterize this Keplerian signal and investigate its potential origins in
stellar activity. Our analysis indicates that HIP11915 hosts a Jupiter-mass
planet with a 3800-day orbital period and low eccentricity. Although we cannot
definitively rule out an activity cycle interpretation, we find that a planet
interpretation is more likely based on a joint analysis of RV and activity
index data. The challenges of long-period radial velocity signals addressed in
this paper are critical for the ongoing discovery of Jupiter-like exoplanets.
If planetary in nature, the signal investigated here represents a very close
analog to the solar system in terms of both Sun-like host star and Jupiter-like
planet.Comment: 8 pages, 5 figures; A&A accepted; typos corrected in this versio
The Solar Twin Planet Search. V. Close-in, low-mass planet candidates and evidence of planet accretion in the solar twin HIP 68468
[Methods]. We obtained high-precision radial velocities with HARPS on the ESO
3.6 m telescope and determined precise stellar elemental abundances (~0.01 dex)
using MIKE spectra on the Magellan 6.5m telescope. [Results]. Our data indicate
the presence of a planet with a minimum mass of 26 Earth masses around the
solar twin HIP 68468. The planet is a super-Neptune, but unlike the distant
Neptune in our solar system (30 AU), HIP 68468c is close-in, with a semi-major
axis of 0.66 AU, similar to that of Venus. The data also suggest the presence
of a super-Earth with a minimum mass of 2.9 Earth masses at 0.03 AU; if the
planet is confirmed, it will be the fifth least massive radial velocity planet
discovery to date and the first super-Earth around a solar twin. Both
isochrones (5.9 Gyr) and the abundance ratio [Y/Mg] (6.4 Gyr) indicate an age
of about 6 billion years. The star is enhanced in refractory elements when
compared to the Sun, and the refractory enrichment is even stronger after
corrections for Galactic chemical evolution. We determined a NLTE Li abundance
of 1.52 dex, which is four times higher than what would be expected for the age
of HIP 68468. The older age is also supported by the low log(R'HK) (-5.05) and
low jitter. Engulfment of a rocky planet of 6 Earth masses can explain the
enhancement in both lithium and the refractory elements. [Conclusions]. The
super-Neptune planet candidate is too massive for in situ formation, and
therefore its current location is most likely the result of planet migration
that could also have driven other planets towards its host star, enhancing thus
the abundance of lithium and refractory elements in HIP 68468. The intriguing
evidence of planet accretion warrants further observations to verify the
existence of the planets that are indicated by our data and to better constrain
the nature of the planetary system around this unique star.Comment: A&A, in pres
18 Sco: a solar twin rich in refractory and neutron-capture elements. Implications for chemical tagging
We study with unprecedented detail the chemical composition and stellar
parameters of the solar twin 18 Sco in a strictly differential sense relative
to the Sun. Our study is mainly based on high resolution (R ~ 110 000) high S/N
(800-1000) VLT UVES spectra, which allow us to achieve a precision of about
0.005 dex in differential abundances. The effective temperature and surface
gravity of 18 Sco are Teff = 5823+/-6 K and log g = 4.45+/-0.02 dex, i.e., 18
Sco is 46+/-6 K hotter than the Sun and log g is 0.01+/-0.02 dex higher. Its
metallicity is [Fe/H] = 0.054+/-0.005 dex and its microturbulence velocity is
+0.02+/-0.01 km/s higher than solar. Our precise stellar parameters and
differential isochrone analysis show that 18 Sco has a mass of 1.04+/-0.02M_Sun
and that it is ~1.6 Gyr younger than the Sun. We use precise HARPS radial
velocities to search for planets, but none were detected. The chemical
abundance pattern of 18 Sco displays a clear trend with condensation
temperature, showing thus higher abundances of refractories in 18 Sco than in
the Sun. Intriguingly, there are enhancements in the neutron-capture elements
relative to the Sun. Despite the small element-to-element abundance differences
among nearby n-capture elements (~0.02 dex), we successfully reproduce the
r-process pattern in the solar system. This is independent evidence for the
universality of the r-process. Our results have important implications for
chemical tagging in our Galaxy and nucleosynthesis in general.Comment: ApJ, in pres
The Solar Twin Planet Search I. Fundamental parameters of the stellar sample
Context. We are carrying out a search for planets around a sample of solar twin stars using the HARPS spectrograph. The goal of this project is to exploit the advantage offered by solar twins to obtain chemical abundances of unmatched precision. This survey will enable new studies of the stellar composition - planet connection.Aims. We determine the fundamental parameters of the 88 solar twin stars that have been chosen as targets for our experiment.Methods. We used the MIKE spectrograph on the Magellan Clay Telescope to acquire high resolution, high signal-to-noise ratio spectra of our sample stars. We measured the equivalent widths of iron lines and used strict differential excitation/ionization balance analysis to determine atmospheric parameters of unprecedented internal precision: Ï(Teff) = 7? K, Ï(log? g) = 0.019, Ï([Fe/H]) = 0.006? dex, Ï(vt) = 0.016? km? s-1. Reliable relative ages and highly precise masses were then estimated using theoretical isochrones.Results. The spectroscopic parameters we derived are in good agreement with those measured using other independent techniques. There is even better agreement if the sample is restricted to those stars with the most internally precise determinations of stellar parameters in every technique involved. The root-mean-square scatter of the differences seen is fully compatible with the observational errors, demonstrating, as assumed thus far, that systematic uncertainties in the stellar parameters are negligible in the study of solar twins. We find a tight activity-age relation for our sample stars, which validates the internal precision of our dating method. Furthermore, we find that the solar cycle is perfectly consistent both with this trend and its star-to-star scatter.Conclusions. We present the largest sample of solar twins analyzed homogeneously using high quality spectra. The fundamental parameters derived from this work will be employed in subsequent work that aims to explore the connections between planet formation and stellar chemical composition
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