193 research outputs found
The chemical composition of nearby young associations: s-process element abundances in AB Doradus, Carina-Near, and Ursa Major
Recently, several studies have shown that young, open clusters are
characterised by a considerable over-abundance in their barium content. In
particular, D'Orazi et al. (2009) reported that in some younger clusters
[Ba/Fe] can reach values as high as ~0.6 dex. The work also identified the
presence of an anti-correlation between [Ba/Fe] and cluster age. For clusters
in the age range ~4.5 Gyr-500 Myr, this is best explained by assuming a higher
contribution from low-mass asymptotic giant branch stars to the Galactic
chemical enrichment. The purpose of this work is to investigate the ubiquity of
the barium over-abundance in young stellar clusters. We analysed
high-resolution spectroscopic data, focusing on the s-process elemental
abundance for three nearby young associations, i.e. AB Doradus, Carina-Near,
and Ursa Major. The clusters have been chosen such that their age spread would
complement the D'Orazi et al. (2009) study. We find that while the s-process
elements Y, Zr, La, and Ce exhibit solar ratios in all three associations, Ba
is over-abundant by ~0.2 dex. Current theoretical models can not reproduce this
abundance pattern, thus we investigate whether this unusually large Ba content
might be related to chromospheric effects. Although no correlation between
[Ba/Fe] and several activity indicators seems to be present, we conclude that
different effects could be at work which may (directly or indirectly) be
related to the presence of hot stellar chromospheres.Comment: Accepted for publication in MNRA
Chemical composition of the Taurus-Auriga association
The Taurus-Auriga association is perhaps the most famous prototype of a
low-mass star forming region, surveyed at almost all wavelengths.
Unfortunately, like several other young clusters/associations, this T
association lacks an extensive abundance analysis determination. We present a
high-resolution spectroscopic study of seven low-mass members of Taurus-Auriga,
including both weak-lined and classical T Tauri stars designed to help robustly
determine their metallicity. After correcting for spectral veiling, we
performed equivalent width and spectral synthesis analyses using the GAIA set
of model atmospheres and the 2002 version of the code MOOG. We find a solar
metallicity, obtaining a mean value of [Fe/H]=0.05. The
-element Si and the Fe-peak one Ni confirm a solar composition. Our
work shows that the dispersion among members is well within the observational
errors at variance with previous claims. As in other star forming regions, no
metal-rich members are found, reinforcing the idea that old planet-host stars
form in the inner part of the Galactic disc and subsequently migrate.Comment: In press on A\&
The debris disk host star HD 61005: a member of the Argus Association?
HD 61005 is a nearby young solar type star that shows a large infrared excess
due to a debris disk. The disk has been recently imaged from ground and space,
with indications of several components. Some characteristics of the disk
suggest the presence of planetary companions around the star, that remain
undetected in deep adaptive optics imaging. For a better understanding of the
system we aim to refine the determination of the stellar parameters, with
emphasis on the stellar age and system orientation. We used ASAS and Hipparcos
photometry and FEROS spectra to determine the rotation period, radial and
rotational velocity, chromospheric emission, effective temperature, and
chemical composition. We find no indication of any misalignment between the
star rotation axis and the disk. The standard age calibrations applied to
several indicators yield an age close to that of the Pleiades (120 Myr);
however the kinematic properties strongly support its membership in the younger
(40 Myr) Argus association, which also includes the IC 2391 open cluster.
Detailed comparison of the properties of HD 61005 and IC 2391 members shows
that the characteristics of HD 61005 are compatible with membership to the
Argus association, once its rather slow rotation is taken into account, because
lithium and other age indicators are somewhat correlated with stellar rotation
at a fixed age. We also identify systematic differences between the field and
cluster population of the Argus association, which are probably selection
effects, so we suggest that additional members with slower rotation and lower
activity level are waiting to be identified.Comment: A&A, in press, 13 pages, 11 figure
Aluminum abundances of multiple stellar generations in the globular cluster NGC 1851
We study the distribution of aluminum abundances among red giants in the
peculiar globular cluster NGC 1851. Aluminum abundances were derived from the
strong doublet Al I 8772-8773 A measured on intermediate resolution FLAMES
spectra of 50 cluster stars acquired under the Gaia-ESO public survey. We
coupled these abundances with previously derived abundance of O, Na, Mg to
fully characterize the interplay of the NeNa and MgAl cycles of H-burning at
high temperature in the early stellar generation in NGC 1851. The stars in our
sample show well defined correlations between Al,Na and Si; Al is
anticorrelated with O and Mg. The average value of the [Al/Fe] ratio steadily
increases going from the first generation stars to the second generation
populations with intermediate and extremely modified composition. We confirm on
a larger database the results recently obtained by us (Carretta et al. 2011a):
the pattern of abundances of proton-capture elements implies a moderate
production of Al in NGC 1851. We find evidence of a statistically significant
positive correlation between Al and Ba abundances in the more metal-rich
component of red giants in NGC 1851.Comment: Astronomy and Astrophysics, in pres
The connection between missing AGB stars and extended horizontal branches
Recent surveys confirm early results about a deficiency or even absence of
CN-strong stars on the asymptotic giant branch (AGB) of globular clusters
(GCs), although with quite large cluster-to-cluster variations. In general,
this is at odds with the distribution of CN band strengths among first ascent
red giant branch (RGB) stars. Norris et al. proposed that the lack of CN-strong
stars in some clusters is a consequence of a smaller mass of these stars that
cannot evolve through the full AGB phase. In this short paper we found that the
relative frequency of AGB stars can change by a factor of two between different
clusters. We also find a very good correlation between the minimum mass of
stars along the horizontal branch (Gratton et al. 2010) and the relative
frequency of AGB stars, with a further dependence on metallicity. We conclude
that indeed the stars with the smallest mass on the HB cannot evolve through
the full AGB phase, being AGB-manque'. These stars likely had large He and N
content, and large O-depletion. We then argue that there should not be AGB
stars with extreme O depletion, and few of them with a moderate one.Comment: 5 Pages, 2 figures, A&A Accepte
Characterizing HR3549B using SPHERE
Aims. In this work, we characterize the low mass companion of the A0 field
star HR3549. Methods. We observed HR3549AB in imaging mode with the the NIR
branch (IFS and IRDIS) of SPHERE@VLT, with IFS in YJ mode and IRDIS in the H
band. We also acquired a medium resolution spectrum with the IRDIS long slit
spectroscopy mode. The data were reduced using the dedicated SPHERE GTO
pipeline, purposely designed for this instrument. We employed algorithms such
as PCA and TLOCI to reduce the speckle noise. Results. The companion was
clearly visible both with IRDIS and IFS.We obtained photometry in four
different bands as well as the astrometric position for the companion. Based on
our astrometry, we confirm that it is a bound object and put constraints on its
orbit. Although several uncertainties are still present, we estimate an age of
~100-150 Myr for this system, yielding a most probable mass for the companion
of 40-50MJup and T_eff ~300-2400 K. Comparing with template spectra points to a
spectral type between M9 and L0 for the companion, commensurate with its
position on the color-magnitude diagram.Comment: Accepted by A&A, 13 pages, 10 Figures (Figures 9 and 10 degraded to
reduce the dimension
Implications of the discovery of AF Lep b. The mass-luminosity relation for planets in the ÎČ Pic Moving Group and the LâT transition for young companions and free-floating planets
This is the final version. Available on open access from EDP Sciences via the DOI in this recordContext. Dynamical masses of young planets aged between 10 and 200 Myr detected in imaging play a crucial role in shaping models of giant planet formation. Regrettably, only a few such objects possess these characteristics. Furthermore, the evolutionary pattern of young sub-stellar companions in near-infrared colour-magnitude diagrams might diverge from free-floating objects, possibly due to differing formation processes.
Aims. The recent identification of a giant planet around AF Lep, part of the ÎČ Pic moving group (BPMG), encouraged us to re-examine these points.
Methods. We considered updated dynamical masses and luminosities for the sub-stellar objects in the BPMG. In addition, we compared the properties of sub-stellar companions and free-floating objects in the BPMG and other young associations remapping the positions of the objects in the colour-magnitude diagram into a dustiness-temperature plane.
Results. We found that cold-start evolutionary models do not reproduce the mass-luminosity relation for sub-stellar companions in the BPMG. This aligns rather closely with predictions from âhot startâ scenarios and is consistent with recent planet formation models. We obtain rather good agreement with masses from photometry and the remapping approach compared to actual dynamical masses. We also found a strong suggestion that the near-infrared colour-magnitude diagram for young companions is different from that of free-floating objects belonging to the same young associations.
Conclusions. If confirmed by further data, this last result would imply that cloud settling â which likely causes the transition between L and T spectral type â occurs at a lower effective temperature in young companions than in free-floating objects. This might tentatively be explained with a different chemical composition.Royal Societ
A Stromgren view of the multiple populations in globular clusters
We discuss a variety of photometric indices assembled from the uvby Stromgren
system. Our aim is to examine the pros and cons of the various indices to find
the most suitable one(s) to study the properties of multiple populations in
globular clusters (GCs) discovered by spectroscopy. We explore in particular
the capabilities of indices like m_1 and c_y at different metallicities. We
define a new index delta_4=(u-v)-(b-y) to separate first and second stellar
generations in GCs of any metal abundance, since it keeps the sensitivity to
multiple stellar populations over all the metallicity range and at the same
time minimizes the sensitivity to photometric errors. We detecte clear
differences in the red giant branches of the GCs examined, like skewness or
bi/multi-modality in color distribution. We connect the photometric information
with the spectroscopic results on O, Na abundances we obtained in our survey of
GCs. Finally, we compute the effects of different chemical composition on the
Stromgren filters and indices using synthetic spectra.Comment: Accepted for publication on Astronomy and Astrophysics. Figures 1,3,5
degraded in resolutio
Multiple populations in Omega Centauri: a cluster analysis of spectroscopic data
Omega Cen is composed of several stellar populations. Their history might
allow us to reconstruct the evolution of this complex object. We performed a
statistical cluster analysis on the large data set provided by Johnson and
Pilachowski (2010). Stars in Omega Cen divide into three main groups. The
metal-poor group includes about a third of the total. It shows a moderate O-Na
anticorrelation, and similarly to other clusters, the O-poor second generation
stars are more centrally concentrated than the O-rich first generation ones.
This whole population is La-poor, with a pattern of abundances for n-capture
elements which is very close to a scaled r-process one. The metal-intermediate
group includes the majority of the cluster stars. This is a much more complex
population, with an internal spread in the abundances of most elements. It
shows an extreme O-Na anticorrelation, with a very numerous population of
extremely O-poor and He-rich second generation stars. This second generation is
very centrally concentrated. This whole population is La-rich, with a pattern
of the abundances of n-capture elements that shows a strong contribution by the
s-process. The spread in metallicity within this metal-intermediate population
is not very large, and we might attribute it either to non uniformities of an
originally very extended star forming region, or to some ability to retain a
fraction of the ejecta of the core collapse SNe that exploded first, or both.
As previously noticed, the metal-rich group has an Na-O correlation, rather
than anticorrelation. There is evidence for the contribution of both massive
stars ending their life as core-collapse SNe, and intermediate/low mass stars,
producing the s-capture elements. Kinematics of this population suggests that
it formed within the cluster rather than being accreted.Comment: Accepted for publication in Astronomy and Astrophysic
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