156 research outputs found
Analysis of Red Supergiants in VDBH 222
Abstract
Recent surveys uncovered new young massive clusters (YMCs) that host dozens of red supergiants (RSGs) in the inner Milky Way. These clusters are ideal for studying the most recent and violent star formation events in the inner Galaxy. However, due to the high extinction that affects the Galactic plane, they need to be studied through infrared (IR) spectroscopy. IR spectra of RSGs have proven to be powerful tools for obtaining chemical abundances. We present the first [Fe/H] measurement (−0.07 ± 0.02) for the YMC VdBH 222 through analysis of its RSGs using Very Large Telescope/X-shooter spectra. We find no evidence for multiple stellar populations in this YMC, contrary to what is routinely observed in older massive clusters.</jats:p
Carbon stars in the X-shooter Spectral Library
We provide a new collection of spectra of 35 carbon stars obtained with the
ESO/VLT X-shooter instrument as part of the X-shooter Spectral Library project.
The spectra extend from 0.3m to 2.4m with a resolving power above
8000. The sample contains stars with a broad range of (J-K) color and
pulsation properties located in the Milky Way and the Magellanic Clouds. We
show that the distribution of spectral properties of carbon stars at a given
(J-K) color becomes bimodal (in our sample) when (J-K) is larger than about
1.5. We describe the two families of spectra that emerge, characterized by the
presence or absence of the absorption feature at 1.53m, generally
associated with HCN and CH. This feature appears essentially only in
large-amplitude variables, though not in all observations. Associated spectral
signatures that we interpret as the result of veiling by circumstellar matter,
indicate that the 1.53m feature might point to episodes of dust production
in carbon-rich Miras.Comment: 29 pages, 21 figures, 9 tables, Accepted for publication in A&
THESEUS1 and RALF34 monitor cell wall integrity
The cell wall is a rigid network being the first barrier between a plant cell and its environment, but at the same time has to be a dynamic network whose cell growth and shape is given by deposition and remodeling of the cell wall. Maintaining cell wall integrity (CWI) is essential for correct plant development and stress response. Members of the family of Catharanthus roseus receptor-like kinase 1-like (CrRLK1L) proteins have been shown to play a role in cell wall homeostasis, mechanoperception, CWI maintenance and growth control. One of the 17 members in Arabidopsis, THESEUS1 (THE1), was identified in a suppressor screen of a cellulose deficient mutant, revealing that the reduction in growth is part of a THE1-mediated compensatory response to cell wall perturbation (Hématy et al., 2007).
Interestingly, several CrRLK1L members have been shown to be receptors for Rapid Alkalinisation Factor (RALF) peptides. RALFs are on average 50 amino acids highly basic, cysteine-rich peptides, most of which are predicted to be cleaved from a highly acidic prodomain by a subtilisin protease. Recently, THE1 has been identified as a receptor for RALF34 (Gonneau et al., 2018). However, this peptide might not be the only THE1 ligand, since ralf34 loss-of-function mutants do not phenocopy all aspects of the1 mutants. RALF24 and RALF31 clustered together with RALF34, based on expression values across different tissues. We generated CRISPR/Cas9 mutants on RALF24, RALF31 and RALF34 with the aim to study which THE1 responses depend on these peptides. Our data suggest that RALF34 could not be the ligand for CWI response of THE1. The presence of RALF34 is acting negatively through THE1, inhibiting its response to cell wall damage.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech
Carbon stars in the X-shooter Spectral Library. II. Comparison with models
In a previous paper, we assembled a collection of medium-resolution spectra
of 35 carbon stars, covering optical and near-infrared wavelengths from 400 to
2400 nm. The sample includes stars from the Milky Way and the Magellanic
Clouds, with a variety of colors and pulsation properties. In the
present paper, we compare these observations to a new set of high-resolution
synthetic spectra, based on hydrostatic model atmospheres. We find that the
broad-band colors and the molecular-band strengths measured by
spectrophotometric indices match those of the models when is bluer
than about 1.6, while the redder stars require either additional reddening or
dust emission or both. Using a grid of models to fit the full observed spectra,
we estimate the most likely atmospheric parameters , ,
and C/O. These parameters derived independently in the
optical and near-infrared are generally consistent when . The
temperatures found based on either wavelength range are typically within
100K of each other, and and are consistent
with the values expected for this sample. The reddest stars ( 1.6)
are divided into two families, characterized by the presence or absence of an
absorption feature at 1.53\,m, generally associated with HCN and
CH. Stars from the first family begin to be more affected by
circumstellar extinction. The parameters found using optical or near-infrared
wavelengths are still compatible with each other, but the error bars become
larger. In stars showing the 1.53\,m feature, which are all
large-amplitude variables, the effects of pulsation are strong and the spectra
are poorly matched with hydrostatic models. For these, atmospheric parameters
could not be derived reliably, and dynamical models are needed for proper
interpretation.Comment: 25 pages (11 pages main part, figures and tables in appendix
Modelling simple stellar populations in the near-ultraviolet to near-infrared with the X-shooter Spectral Library (XSL)
We present simple stellar population models based on the empirical X-shooter
Spectral Library (XSL) from NUV to NIR wavelengths. The unmatched
characteristics of relatively high resolution and extended wavelength coverage
( nm, ) of the XSL population models bring us closer to
bridging optical and NIR studies of intermediate and old stellar populations.
It is now common to find good agreement between observed and predicted NUV and
optical properties of stellar clusters due to our good understanding of the
main-sequence and early giant phases of stars. However, NIR spectra of
intermediate-age and old stellar populations are sensitive to cool K and M
giants. The asymptotic giant branch, especially the thermally pulsing
asymptotic giant branch, shapes the NIR spectra of Gyr old stellar
populations; the tip of the red giant branch defines the NIR spectra of
populations with ages larger than that. We construct sequences of the average
spectra of static giants, variable-rich giants, and C-rich giants to include in
the models separately. The models span the metallicity range
and ages above 50 Myr, a broader range in the NIR than in other models based on
empirical spectral libraries. Our models can reproduce the integrated optical
colours of the Coma cluster galaxies at the same level as other semi-empirical
models found in the literature. In the NIR, there are notable differences
between the colours of the models and Coma cluster galaxies. The XSL models
expand the range of predicted values of NIR indices compared to other models
based on empirical libraries. Our models make it possible to perform in-depth
studies of colours and spectral features consistently throughout the optical
and the NIR range to clarify the role of evolved cool stars in stellar
populations.Comment: 30 pages, 26 figures, accepted to Astronomy & Astrophysics, models
will be available on http://xsl.astro.unistra.fr/ upon publishin
The Gaia-ESO survey: mapping the shape and evolution of the radial abundance gradients with open clusters
The spatial distribution of elemental abundances and their time evolution are
among the major constraints to disentangle the scenarios of formation and
evolution of the Galaxy. We used the sample of open clusters available in the
final release of the Gaia-ESO survey to trace the Galactic radial abundance and
abundance to iron ratio gradients, and their time evolution. We selected member
stars in 62 open clusters, with ages from 0.1 to about 7~Gyr, located in the
Galactic thin disc at Galactocentric radii from about 6 to 21~kpc. We analysed
the shape of the resulting [Fe/H] gradient, the average gradients [El/H] and
[El/Fe] combining elements belonging to four different nucleosynthesis
channels, and their individual abundance and abundance ratio gradients. We also
investigated the time evolution of the gradients dividing open clusters in
three age bins. The[Fe/H] gradient has a slope of -0.054 dex~kpc-1. We saw
different behaviours for elements belonging to different channels. We found
that the youngest clusters in the inner disc have lower metallicity than their
older counterpart and they outline a flatter gradient. We considered some
possible explanations, including the effects of gas inflow and migration. We
suggested that it might be a bias introduced by the standard spectroscopic
analysis producing lower metallicities in low gravity stars. To delineate the
shape of the `true' gradient, we should limit our analysis to stars with low
surface gravity logg>2.5 and xi<1.8 km~s-1. Based on this reduced sample, we
can conclude that the gradient has minimally evolved over the time-frame
outlined by the open clusters, indicating a slow and stationary formation of
the thin disc in the latest Gyr. We found a secondary role of clusters'
migration in shaping the gradient, with a more prominent role of migration for
the oldest clusters.Comment: 25 pages, 14 figures and 4 tables in the main text, 3 figures and 7
tables in the Appendix. Accepted for publication in A&
The Gaia -ESO Survey: Galactic evolution of lithium at high metallicity
Context. Reconstructing the Galactic evolution of lithium (Li) is the main tool used to constrain the source(s) of Li enrichment in the Galaxy. Recent results have suggested a decline in Li at supersolar metallicities, which may indicate reduced production. Aims. We exploit the unique characteristics of the Gaia-ESO Survey open star cluster sample to further investigate this issue and to better constrain the evolution of Li at high metallicity. Methods. We trace the upper envelope of Li abundance versus metallicity evolution using 18 clusters and considering members that should not have suffered any Li depletion. Results. At variance with previous claims, we do not find any evidence of a Li decrease at high metallicity. The most metal-rich clusters in the sample ([Fe/H] = ∼0.3) actually show the highest Li abundances, with A(Li) > 3.4. Our results clearly show that previous findings, which were based on field stars, were affected by selection effects. The metal-rich population in the solar neighbourhood is composed of relatively old and cool stars that have already undergone some Li depletion; hence, their measured Li does not represent the initial interstellar medium abundance, but a lower limit to it
Gaia-ESO survey: Lithium abundances in open cluster Red Clump stars
Context. It has recently been suggested that all giant stars with masses below 2 M⊙ suffer an episode of surface lithium enrichment between the tip of the red giant branch (RGB) and the red clump (RC).
Aims. We test if the above result can be confirmed in a sample of RC and RGB stars that are members of open clusters.
Methods. We discuss Li abundances in six open clusters with ages between 1.5 and 4.9 Gyr (turn-off masses between 1.1 and 1.7 M⊙). We compare these observations with the predictions of different models that include rotation-induced mixing, thermohaline instability, mixing induced by the first He flash, and energy losses by neutrino magnetic moment.
Results. In six clusters, we find close to 35% of RC stars have Li abundances that are similar or higher than those of upper RGB stars. This can be a sign of fresh Li production. Because of the extra-mixing episode connected to the luminosity bump, the expectation has been for RC stars to have systematically lower surface Li abundances. However, we cannot confirm that this possible Li production is ubiquitous. For about 65% of RC giants, we can only determine upper limits in abundances that could be hiding very low Li content.
Conclusions. Our results indicate the possibility that Li is being produced in the RC, at levels that would not typically permit the classification of these the stars as Li rich. The determination of their carbon isotopic ratio would help to confirm that the RC giants have suffered extra mixing followed by subsequent Li enrichment. The Li abundances of the RC stars can be qualitatively explained by the models including an additional mixing episode close to the He flash.</jats:p
The Gaia-ESO Survey: Galactic evolution of lithium from iDR6
We exploit the unique characteristics of a sample of open clusters (OCs) and
field stars for which high-precision 7Li abundances and stellar parameters are
homogeneously derived by the Gaia-ESO Survey (GES). We derive possibly
undepleted 7Li abundances for 26 OCs and star forming regions with ages from
young to old spanning a large range of Galactocentric distances, which allows
us to reconstruct the local late Galactic evolution of lithium as well as its
current abundance gradient along the disc. Field stars are added to look
further back in time and to constrain 7Li evolution in other Galactic
components. The data are then compared to theoretical tracks from chemical
evolution models that implement different 7Li forges. We find that the upper
envelope of the 7Li abundances measured in field stars of nearly solar
metallicities traces very well the level of lithium enrichment attained by the
ISM as inferred from observations of cluster stars. We confirm previous
findings that the abundance of 7Li in the solar neighbourhood does not decrease
at supersolar metallicity. The comparison of the data with the chemical
evolution model predictions favours a scenario in which the majority of the 7Li
abundance in meteorites comes from novae. Current data also seem to suggest
that the nova rate flattens out at later times. This requirement might have
implications for the masses of the white dwarf nova progenitors and deserves
further investigation. Neutrino-induced reactions taking place in core-collapse
supernovae also produce some fresh lithium. This likely makes a negligible
contribution to the meteoritic abundance, but could be responsible for a mild
increase of the 7Li abundance in the ISM of low-metallicity systems that would
counterbalance the astration processes.Comment: 17 pages, 10 figures, 3 tables (full tables 1 and 3 only available at
the CDS), accepted for publication in Astronomy and Astrophysic
The Gaia-ESO Survey: Asymmetric expansion of the Lagoon Nebula cluster NGC 6530 from GES and Gaia DR2
The combination of precise radial velocities from multi-object spectroscopy and highly accurate proper motions from Gaia DR2 opens up the possibility for detailed 3D kinematic studies of young star-forming regions and clusters. Here, we perform such an analysis by combining Gaia-ESO Survey spectroscopy with Gaia astrometry for ∼900 members of the Lagoon Nebula cluster, NGC 6530. We measure the 3D velocity dispersion of the region to be 5.35+0.39 -0.34 km s-1, which is large enough to suggest the region is gravitationally unbound. The velocity ellipsoid is anisotropic, implying that the region is not sufficiently dynamically evolved to achieve isotropy, though the central part of NGC 6530 does exhibit velocity isotropy that suggests sufficient mixing has occurred in this denser part. We find strong evidence that the stellar population is expanding, though this is preferentially occurring in the declination direction and there is very little evidence for expansion in the right ascension direction. This argues against a simple radial expansion pattern, as predicted by models of residual gas expulsion. We discuss these findings in the context of cluster formation, evolution, and disruption theories.NJW acknowledges an STFC Ernest Rutherford Fellowship (grant number ST/M005569/1). RJP acknowledges support from the Royal Society in the form of a Dorothy Hodgkin Fellowship. AB acknowledges support from ICM (Iniciativa Científica Milenio) via the Núcleo Milenio de Formación Planetaria. EJA acknowledges support from the Spanish Government Ministerio de Ciencia, Innovacion y Universidades though grant AYA2016-75 931-C2-1 and from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709)
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