Aims. We searched the first set of data released by the CoRoT mission for red stars showing long-period variability (LPV). These stars are likely highly evolved stars on the upper giant branch. The ...high time resolution of the CoRoT data allows us to study the light curves of these variables in unprecedented detail. Methods. Light curves of the reddest stars were extracted from the CoRoT public archive. The search for LPV was done by visual inspection. Instrumental artefacts were removed, and the final light curves were inspected for signs of variability on time scales of hours to a few days. Such abrupt and irregular light curve changes have been reported in the literature to occur in these stars, but their origin is not yet understood. Results. We detected 52 LPV candidates in four CoRoT fields, most of them previously unknown. No signs of any short-time light changes on time scales between 20 min and a few days and an amplitude exceeding a few mmag could be detected. We conclude that if these short-time variations are physical reality, they must be quite rare events, and we derive a 95% confidence upper limit of 0.15 events per star per year. Any deviations from a perfectly smooth long-time variation are occurring on time scales longer than approximately 10 days.
Context. Recent studies have detected multiple populations in globular clusters. The massive globular cluster ω Cen hosts at least three populations of different metallicity. The most metal-rich one ...is thought to show also an overabundance of He. These differences should become visible in the structure, evolution, and pulsation of its stars. Aims. We aim to study the effects of the different starting compositions of the three populations in ω Cen on the most luminous red giants in this cluster. Methods. The long-periodic variability of evolved stars in ω Cen opens a comparably easy access window to the structure and composition of these objects. We made a detailed search for long-period variables (LPVs) in ω Cen leading to the detection of many new variables and period determinations for a significant number of them. Periods and luminosities were then compared with the most recent pulsation models for these kinds of stars. Results. Some of the LPVs belong to each of the main metallicity sub-groups of ω Cen. Almost all detected variables with periods are fundamental mode pulsators. For the metal-rich group, our comparison with pulsation models favours a high He abundance of Y = 0.4. Conclusions. Our study can be considered an independent piece of evidence supporting a high helium abundance among the metal-rich stars in ω Cen.
We present an evaluation of the performance of an automated classification of the Hipparcos periodic variable stars into 26 types. The sub-sample with the most reliable variability types available in ...the literature is used to train supervised algorithms to characterize the type dependencies on a number of attributes. The most useful attributes evaluated with the random forest methodology include, in decreasing order of importance, the period, the amplitude, the V−I colour index, the absolute magnitude, the residual around the folded light-curve model, the magnitude distribution skewness and the amplitude of the second harmonic of the Fourier series model relative to that of the fundamental frequency. Random forests and a multi-stage scheme involving Bayesian network and Gaussian mixture methods lead to statistically equivalent results. In standard 10-fold cross-validation (CV) experiments, the rate of correct classification is between 90 and 100 per cent, depending on the variability type. The main mis-classification cases, up to a rate of about 10 per cent, arise due to confusion between SPB and ACV blue variables and between eclipsing binaries, ellipsoidal variables and other variability types. Our training set and the predicted types for the other Hipparcos periodic stars are available online.
Context. The presence of two stellar populations in the Milky Way bulge has been reported recently, based on observations of giant and dwarf stars in the inner and intermediate bulge.Aims. We aim at ...studying the abundances and kinematics of stars in the outer Galactic bulge, thereby providing additional constraints on formation models of the bulge.Methods. Spectra of 401 red giant stars in a field at (l,b) = (0°, −10°) were obtained with the FLAMES-GIRAFFE spectrograph at the VLT. Stars of luminosities down to below the two bulge red clumps are included in the data set. From these spectra we measured general metallicities, abundances of iron and the α-elements, and radial velocities of the stars. The abundances were derived from an interpolation and fitting procedure within a grid of COMARCS model atmospheres and spectra. These measurements as well as photometric data were compared to simulations with the Besançon and TRILEGAL models of the Galaxy.Results. We confirm the presence of two populations among our sample stars: i) a metal-rich one at M/H ~ +0.3, comprising about 30% of the sample, with low velocity dispersion and low α-abundance, and ii) a metal-poor population at M/H ~ −0.6 with high velocity dispersion and high α-abundance. The metallicity difference between the two populations, a systematically and statistically robust figure, is Δ M/H = 0.87 ± 0.03. The metal-rich population could be connected to the Galactic bar. We identify this population as the carrier of the double red clump feature. We do not find a significant difference in metallicity or radial velocity between the two red clumps, a small difference in metallicity being probably due to a selection effect and contamination by the metal-poor population. The velocity dispersion agrees well with predictions of the Besançon Galaxy model, but the metallicity of the “thick bulge” model component should be shifted to lower metallicity by 0.2 to 0.3 dex to well reproduce the observations. We present evidence that the metallicity distribution function depends on the evolutionary state of the sample stars, suggesting that enhanced mass loss preferentially removes metal-rich stars. We also confirm the decrease of α-element over-abundance with increasing metallicity.Conclusions. Our sample is consistent with the existence of two populations, one being a metal-rich bar, the second one being more like a metal-poor classical bulge with larger velocity dispersion.
Context. A small number of K-type giants on the red giant branch (RGB) is known to be very rich in lithium (Li). This fact is not accounted for by standard stellar evolution theory. The exact phase ...and mechanism of Li enrichment is still a matter of debate. Aims. Our goal is to probe the abundance of Li along the RGB, from its base to the tip, to confine Li-rich phases that are supposed to occur on the RGB. Methods. For this end, we obtained medium-resolution spectra with the FLAMES spectrograph at the VLT in GIRAFFE mode for a large sample of 401 low-mass RGB stars located in the Galactic bulge. The Li abundance was measured in the stars with a detectable Li 670.8 nm line by means of spectral synthesis with COMARCS model atmospheres. A new 2MASS (J − KS) − Teff calibration from COMARCS models is presented in the Appendix. Results. Thirty-one stars with a detectable Li line were identified, three of which are Li-rich according to the usual criterion (log ϵ(Li) > 1.5). The stars are distributed all along the RGB, not concentrated in any particular phase of the red giant evolution (e.g. the luminosity bump or the red clump). The three Li-rich stars are clearly brighter than the luminosity bump and red clump, and do not show any signs of enhanced mass loss. Conclusions. We conclude that the Li enrichment mechanism cannot be restricted to a clearly defined phase of the RGB evolution of low-mass stars (M ~ 1 M⊙), contrary to earlier suggestions from disk field stars.
The intermediate-age Magellanic Cloud clusters NGC 1978 and 419 are each found to contain substantial numbers of pulsating asymptotic giant branch (AGB) stars, both oxygen rich and carbon rich. Each ...cluster also contains two pulsating AGB stars which are infrared sources with a large mass-loss rate. Pulsation masses have been derived for the AGB variables, from the lowest luminosity O-rich variables to the most evolved infrared sources. It is found that the stars in NGC 1978 have a mass of 1.55 M⊙ early on the AGB while the NGC 419 stars have a mass of 1.87 M⊙ early on the AGB. These masses are in good agreement with those expected from the cluster ages determined by main-sequence turn-off fitting. Non-linear pulsation models fitted to the highly evolved AGB stars show that a substantial amount of mass-loss has occurred during the AGB evolution of these stars. An examination of the observed mass-loss on the AGB, and the AGB tip luminosities, shows that in both clusters the mass-loss rates computed from the formula of Vassiliadis & Wood reproduce the observations reasonably well. The mass-loss rates computed from the formula of Blöcker terminate the AGB in both clusters at a luminosity which is much too low.
An analysis of high-resolution near-infrared spectra of a sample of 45 asymptotic giant branch (AGB) stars towards the Galactic bulge is presented. The sample consists of two subsamples, a larger one ...in the inner and intermediate bulge, and a smaller one in the outer bulge. The data are analysed with the help of hydrostatic model atmospheres and spectral synthesis. We derive the radial velocity of all stars, and the atmospheric chemical mix (Fe/H, C/O, 12C/13C, Al, Si, Ti, and Y) where possible. Our ability to model the spectra is mainly limited by the (in)completeness of atomic and molecular line lists, at least for temperatures down to T
eff ≈ 3100 K. We find that the subsample in the inner and intermediate bulge is quite homogeneous, with a slightly subsolar mean metallicity and only few stars with supersolar metallicity, in agreement with previous studies of non-variable M-type giants in the bulge. All sample stars are oxygen-rich, C/O < 1.0. The C/O and carbon isotopic ratios suggest that third dredge-up (3DUP) is absent among the sample stars, except for two stars in the outer bulge that are known to contain technetium. These stars are also more metal-poor than the stars in the intermediate or inner bulge. Current stellar masses are determined from linear pulsation models. The masses, metallicities and 3DUP behaviour are compared to AGB evolutionary models. We conclude that these models are partly in conflict with our observations. Furthermore, we conclude that the stars in the inner and intermediate bulge belong to a more metal-rich population that follows bar-like kinematics, whereas the stars in the outer bulge belong to the metal-poor, spheroidal bulge population.
Context. The pulsation periods of long period variables (LPVs) depend on their mass and helium abundance as well as on their luminosity and metal abundance. Comparison of the observed periods of LPVs ...in globular clusters with models is capable of revealing the amount of mass lost on the giant branch and the helium abundance. Aims. We aim to determine the amount of mass loss that has occurred on the giant branches of the low metallicity globular clusters NGC 362 and NGC 2808. We also aim to see if the LPVs in NGC 2808 can tell us about helium abundance variations in this cluster. Methods. We have used optical monitoring of NGC 362 and NGC 2808 to determine periods for the LPVs in these clusters. We have made linear pulsation models for the pulsating stars in these clusters taking into account variations in mass and helium abundance. Results. Reliable periods have been determined for 11 LPVs in NGC 362 and 15 LPVs in NGC 2808. Comparison of the observed variables with models in the log P–K diagram shows that mass loss of ~0.15−0.2 M⊙ is required on the first giant branch in these clusters, in agreement with estimates from other methods. In NGC 2808, there is evidence that a high helium abundance of Y ~ 0.4 is required to explain the periods of several of the LPVs. Conclusions. It would be interesting to determine periods for LPVs in other Galactic globular clusters where a helium abundance variation is suspected to see if the completely independent test for a high helium abundance provided by the LPVs can confirm the high helium abundance estimates.
Context. The bolometric luminosity is one of the most fundamental stellar parameters that is generally not directly observable. When integrating whole spectral energy distributions, bolometric ...corrections are widely used. These corrections are typically specified for a well defined object type, a measured photometric colour, and the colour index range for which it is calibrated. Aims. We provide this bolometric correction for the near-infrared colours of red late giants. Methods. We compare bolometric luminosities derived from fits to spectral energy distributions covering the visual and near, mid, and far infrared to different near-infrared colour indices to define subgroups within our stellar sample and finally obtain specific bolometric corrections. Results. For well-defined subgroups of different near-infrared colours and atmospheric chemistry, we present four distinct bolometric colour relations and compare these to earlier results from the literature.
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