274 research outputs found
Age consistency between exoplanet hosts and field stars
Transiting planets around stars are discovered mostly through photometric
surveys. Unlike radial velocity surveys, photometric surveys do not tend to
target slow rotators, inactive or metal-rich stars. Nevertheless, we suspect
that observational biases could also impact transiting-planet hosts. This paper
aims to evaluate how selection effects reflect on the evolutionary stage of
both a limited sample of transiting-planet host stars (TPH) and a wider sample
of planet-hosting stars detected through radial velocity analysis. Then, thanks
to uniform derivation of stellar ages, a homogeneous comparison between
exoplanet hosts and field star age distributions is developed. Stellar
parameters have been computed through our custom-developed isochrone placement
algorithm, according to PARSEC models. The notable aspects of our algorithm
include the treatment of element diffusion, activity checks in terms of
and , and the evaluation of the stellar evolutionary
speed in the Hertzsprung-Russel diagram in order to better constrain age.
Working with TPH, the observational stellar mean density allows
us to compute stellar luminosity even if the distance is not available, by
combining with the spectroscopic . The median value of
the TPH ages is Gyr. Even if this sample is not very large, however,
the result is very similar to what we found for the sample of spectroscopic
hosts, whose modal and median values are [3, 3.5) Gyr and Gyr,
respectively. Thus, these stellar samples suffer almost the same selection
effects. We also conclude that the age of our Sun is consistent with the age
distribution of solar neighbourhood MS stars with spectral types from late F to
early K, regardless of whether they harbour planets or not. We considered the
possibility that our selected samples are older than the average disc
population.Comment: 13 pages, 1 longtable, 9 figures. Accepted by A&
Bright Stars and Metallicity Spread in the Globular Cluster Omega Centauri
The globular cluster Omega Centauri (NGC 5139) is the most massive and
brightest cluster in our Galaxy. It has also a moderately high mass to light
ratio (3.6) and an anomalous flattening (0.83) for a globular cluster. This
cluster is also very interesting because it is one of a few examples of
globular clusters with a measurable spread in the metal abundance (see Da Costa
& Willumsen 1981, Norris et al. 1996, and Suntzeff and Kraft 1996 and
references therein) and then it offers a unique, big sample of nearby stars
having all the same distance and reddening but showing different metallicity
(and age ?) effects. A recent paper by Norris et al. (1997) shows also an
interesting correlation between kinematics and metal abundance.Comment: 5 pages, 3 eps figure, pasconf.sty included, Proceedings of the Third
Mount Stromlo Symposium: the Galactic Hal
FSR1767 - a new globular cluster in the Galaxy
The globular cluster (GC) nature of the recently catalogued candidate FSR
1767 is established in the present work. It results as the closest GC so far
detected in the Galaxy. The nature of this object is investigated by means of
2MASS colour-magnitude diagrams (CMDs), the stellar radial density profile
(RDP) and proper-motions (PM). The properties are consistent with an
intermediate metallicity (\feh\approx-1.2) GC with a well-defined turnoff
(TO), red-giant branch (RGB) and blue horizontal-branch (HB). The distance of
FSR 1767 from the Sun is \ds\approx1.5 kpc, and it is located at the
Galactocentric distance \rgc\approx5.7 kpc. With the space velocity
components , FSR 1767 appears to be a
Palomar-like GC with \mv\approx-4.7, that currently lies pc below
the Galactic plane. The RDP is well represented by a King profile with the core
and tidal radii \rc=0.24\pm0.08 pc and \rt=3.1\pm1.0 pc, respectively, with
a small half-light radius \rh=0.60\pm0.15 pc. The optical absorption is
moderate for an infrared GC, , which together with its central
direction and enhanced contamination explains why it has so far been
overlooked.Comment: 6 pages, 7 figures. MNRAS Letters, accepte
Revising the ages of planet-hosting stars
This article aims to measure the age of planet-hosting stars (SWP) through
stellar tracks and isochrones computed with the \textsl{PA}dova \&
T\textsl{R}ieste \textsl{S}tellar \textsl{E}volutionary \textsl{C}ode (PARSEC).
We developed algorithms based on two different techniques for determining the
ages of field stars: \emph{isochrone placement} and \emph{Bayesian estimation}.
Their application to a synthetic sample of coeval stars shows the intrinsic
limits of each method. For instance, the Bayesian computation of the modal age
tends to select the extreme age values in the isochrones grid. Therefore, we
used the isochrone placement technique to measure the ages of 317 SWP. We found
that of SWP have ages lower than 0.5 Gyr. The age distribution peaks
in the interval [1.5, 2) Gyr, then it decreases. However, of the
stars are older than 11 Gyr. The Sun turns out to be a common star that hosts
planets, when considering its evolutionary stage. Our SWP age distribution is
less peaked and slightly shifted towards lower ages if compared with ages in
the literature and based on the isochrone fit. In particular, there are no ages
below 0.5 Gyr in the literature.Comment: 16 pages, 18 figures. Accepted by A&
On the astronomical origin of the Hallstatt oscillation found in radiocarbon and climate records throughout the Holocene
An oscillation with a period of about 2100-2500 years, the Hallstatt cycle,
is found in cosmogenic radioisotopes (C-14 and Be-10) and in paleoclimate
records throughout the Holocene. Herein we demonstrate the astronomical origin
of this cycle. Namely, this oscillation is coherent to the major stable
resonance involving the four Jovian planets - Jupiter, Saturn, Uranus and
Neptune - whose period is p=2318 yr. The Hallstatt cycle could derive from the
rhythmic variation of the circularity of the solar system disk assuming that
this dynamics could eventually modulate the solar wind and, consequently, the
incoming cosmic ray flux and/or the interplanetary/cosmic dust concentration
around the Earth-Moon system. The orbit of the planetary mass center (PMC)
relative to the Sun is used as a proxy. We analyzed how the instantaneous
eccentricity vector of this virtual orbit varies from 13,000 B. C. to 17,000 A.
D.. We found that it undergoes kind of pulsations as it clearly presents
rhythmic contraction and expansion patterns with a 2318 yr period together with
a number of already known faster oscillations associated to the planetary
orbital stable resonances. We found that a fast expansion of the Sun-PMC orbit
followed by a slow contraction appears to prevent cosmic rays to enter within
the system inner region while a slow expansion followed by a fast contraction
favors it. Similarly, the same dynamics could modulate the amount of
interplanetary/cosmic dust falling on Earth. These would then cause both the
radionucleotide production and climate change by means of a cloud/albedo
modulation. Other stable orbital resonance frequencies (e.g. at periods of 20
yr, 45 yr, 60 yr, 85 yr, 159-171-185 yr, etc.) are found in radionucleotide,
solar, aurora and climate records, as determined in the scientific literature.
Thus, the result supports a planetary theory of solar and/or climate variation.Comment: 36 pages, 14 figures, 1 tabl
Capture of field stars by globular clusters in dense bulge regions
The recent detection of a double Red Giant Branch in the optical
color-magnitude diagram (CMD) of the bulge globular cluster HP1 (Ortolani et
al. 1997), a more populated metal-poor steep one corresponding to the cluster
itself, and another metal-rich curved, led us to explore in the present Letter
the possibility of capture of field stars by a globular cluster orbiting in
dense bulge regions over several gigayears. Analytical arguments, as well as
N-body calculations for a cluster model of 10^5 solar masses in a bulge-like
environment, suggest that a significant fraction of cluster stars may consist
of captures. Metal-poor globular clusters in the inner bulge, like HP1,
contrasting at least in Delta [Fe/H] = 1.0 dex with respect to the surrounding
metal-rich stars, are ideal probes to further test the capture scenario. In
turn, if this scenario is confirmed, the double RGB of HP1 could provide direct
estimates of blanketing amounts, which is fundamental for the photometric
calibration of metal-rich stellar populations.Comment: 6 pages, 2 included figures, aas2pp4,sty Latex style. To be published
in Astrophysical Journal Letter
SMC west halo: a slice of the galaxy that is being tidally stripped? Star clusters trace age and metallicity gradients
(ABRIDGED) The evolution and structure of the Magellanic Clouds is presently
under debate. The classical scenario where both the Large and Small Magellanic
Clouds (LMC, SMC) are orbiting the Milky Way has been challenged by an
alternative where the LMC and SMC are in their first close passage to our
Galaxy. Detailed studies of stellar populations in the galaxies should
constrain the proposed scenarios. In particular, the west halo of the SMC was
recently characterized with radial trends in age and metallicity which
indicates tidal disruption. We increase the sample of star clusters in the west
halo of the SMC with homogeneous age, metallicity, and distance derivations, to
determine better age and metallicity gradients. Comparisons of observed and
synthetic V,(B-V) colour-magnitude diagrams are used to derive parameters for
west halo star clusters. We derived age and metallicity for the reference
cluster NGC 152 compatible with literature parameters. Age and metallicity
gradients are confirmed in the west halo: 2.6 +/- 0.6 Gyr/deg and -0.19 +/-
0.09 dex/deg, respectively. Age-metallicity relation for the west halo has low
dispersion in metallicity and it is compatible with a burst model of chemical
enrichment. All WH clusters seem to follow the same predicted stellar
distribution, with exception of AM-3 that should belong to the counter-bridge.
Bruck 6 is only 130 +/- 40 Myr old and it could have been formed during a
recent tidal interaction of the SMC-LMC. We suggest that it is crucial to split
the SMC cluster population in groups: main body, wing/bridge, counter-bridge
and west halo. This is the way to analyse the complex star formation and
dynamical history of our neighbour. In particular we show that west halo has
clear age and metallicity gradients and age-metallicity relation, also
compatible with the dynamical model of tidal influence of the LMC over the SMC.Comment: 17 pages, 16 figures, 6 tables, in press in A&
FORS2/VLT survey of Milky Way globular clusters I. Description of the method for derivation of metal abundances in the optical and application to NGC 6528, NGC 6553, M 71, NGC 6558, NGC 6426 and Terzan 8
(abridged) We have observed almost 1/3 of the globular clusters in the Milky
Way, targeting distant and/or highly reddened objects, besides a few reference
clusters. A large sample of red giant stars was observed with FORS2@VLT/ESO at
R ~ 2,000. The method for derivation of stellar parameters is presented with
application to six reference clusters. We aim at deriving the stellar
parameters effective temperature, gravity, metallicity and alpha-element
enhancement, as well as radial velocity, for membership confirmation of
individual stars in each cluster. We analyse the spectra collected for the
reference globular clusters NGC 6528, NGC 6553, M 71, NGC 6558, NGC 6426 and
Terzan 8. They cover the full range of globular cluster metallicities, and are
located in the bulge, disc and halo. Full spectrum fitting techniques are
applied, by comparing each target spectrum with a stellar library in the
optical region at 4560-5860 A. We employed the library of observed spectra
MILES, and the synthetic library by Coelho et al. (2005). Validation of the
method is achieved through recovery of the known atmospheric parameters for 49
well-studied stars that cover a wide range in the parameter space. We adopted
as final stellar parameters (effective temperatures, gravities, metallicities)
the average of results using MILES and Coelho et al. libraries. We identified 4
member stars in NGC 6528, 13 in NGC 6553, 10 in M 71, 5 in NGC 6558, 5 in NGC
6426 and 12 in Terzan 8. Radial velocities, Teff, log(g), [Fe/H] and
alpha-element enhancements were derived. We derived abundances for NGC 6426
from spectroscopy for the first time. The method proved to be reliable for red
giant stars observed with resolution R ~ 2,000, yielding results compatible
with high-resolution spectroscopy. The derived alpha-element abundances show
[A/Fe] vs. [Fe/H] consistent with that of field stars at the same
metallicities.Comment: 22 pages, 21 figures, accepted for publication in A&
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