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High precision Kepler photometry is used to explore the details of AGB light curves. Since AGB variability has a typical time scale on order of a year we present methods for the removal of long term trends and quarterly changes in Kepler data. While undergoing long period variations of many magnitudes, the light curves are shown to be smooth at the millimagnitude level over much shorter time intervals. No flares or other rapid events were detected down to the sub-day time scale.

We obtained high-resolution near-IR spectra of 45 AGB stars located in the Galactic bulge. The aim of the project is to determine key elemental abundances in these stars to help constrain the formation history of the bulge. A further aim is to link the photospheric abundances to the dust species found in the winds of the stars. Here we present a progress report of the analysis of the spectra.

Context. The period-luminosity diagram (PLD) has proven to be a powerful tool for studying populations of pulsating red giants. Gaia Data Release 2 (DR2) provides a large data set including many long-period variables (LPVs) on which this tool can be applied. Aims. We investigate the location of LPVs from the Large and Small Magellanic Clouds in the PLD using various optical and infrared luminosity indicators from Gaia and 2MASS, respectively. We thereby distinguish between stars of different masses and surface chemistry. Methods. The data set taken from the Gaia DR2 catalogue of LPVs allows for a homogeneous study from low- to high-mass LPVs. These sources are divided into sub-populations of asymptotic giant branch (AGB) stars according to their mass and their O- or C-rich nature using the Gaia-2MASS diagram developed by our group. This diagram uses a Wesenheit index WBP, RP based on Wesenheit functions in the Gaia and 2MASS photometric bands. Four different luminosity indicators are used to study the period-luminosity (P–L) relations. Results. We provide the first observational evidence of a P–L relation offset for both fundamental and 1O pulsators between low- and intermediate-mass O-rich stars, in agreement with published pulsation predictions. Among the luminosity indicators explored, sequence C′ is the narrowest in the P–WBP, RP diagram, and is thus to be preferred over the other PLDs for the determination of distances using LPVs. The majority of massive AGB stars and red supergiants form a smooth extension of sequence C of low- and intermediate-mass AGB stars in the P–WBP, RP diagram, suggesting that they pulsate in the fundamental mode. All results are similar in the two Magellanic Clouds. [ABSTRACT FROM AUTHOR]

Analyzing 41 targets data of the Kepler K2 Campaign 2 mission suspected to be Long Period Variables (LPVs), we developed a method for the prediction of periods longer than the observation period of 77.48d using the 3500 data points provided by K2. The 'Self-Flat-Field' method (K2SFF or SFF) of the ' Kepler K2 High Level Science Product' (K2HLSP) corrected the instrumental effects best. [ABSTRACT FROM AUTHOR]

Context. The abundances of the three main isotopes of oxygen are altered in the course of the CNO-cycle. When the first dredge-up mixes the burning products to the surface, the nucleosynthesis processes can be probed by measuring oxygen isotopic ratios. Aims. By measuring 16O/17O and 16O/18O in red giants of known mass we compare the isotope ratios with predictions from stellar and galactic evolution modelling. Methods. Oxygen isotopic ratios were derived from the K-band spectra of six red giants. The sample red giants are open cluster members with known masses of between 1.8 and 4.5 M⊙. The abundance determination employs synthetic spectra calculated with the COMARCS code. The effect of uncertainties in the nuclear reaction rates, the mixing length, and of a change in the initial abundance of the oxygen isotopes was determined by a set of nucleosynthesis and mixing models using the FUNS code. Results. The observed 16O/17O ratios are in good agreement with the model results, even if the measured values do not present clear evidence of a variation with the stellar mass. The observed 16O/18O ratios are clearly lower than the predictions from our reference model. Variations in nuclear reaction rates and mixing length parameter both have only a very weak effect on the predicted values. The 12C/13C ratios of the K giants studied implies the absence of extra-mixing in these objects. Conclusions. A comparison with galactic chemical evolution models indicates that the 16O/18O abundance ratio underwent a faster decrease than predicted. To explain the observed ratios, the most likely scenario is a higher initial 18O abundance combined with a lower initial 16O abundance. Comparing the measured 18O/17O ratio with the corresponding value for the interstellar medium points towards an initial enhancement of 17O as well. Limitations imposed by the observations prevent this from being a conclusive result. [ABSTRACT FROM AUTHOR]

Context. Asymptotic giant branch (AGB) stars play a key role in the enrichment of galaxies with heavy elements. Due to their large amplitude variability, the measurement of elemental abundances is a highly challenging task that has not been solved in a satisfactory way yet. Aims. Following our previous work we use hydrostatic and dynamical model atmospheres to simulate observed high-resolution nearinfrared spectra of 12 variable and non-variable red giants in the globular cluster 47 Tuc. The 47 Tuc red giants are independently well-characterized in important parameters (mass, metallicity, luminosity). The principal aim was to compare synthetic spectra based on the dynamical models with observational spectra of 47 Tuc variables. Assuming that the abundances are unchanged on the upper giant branch in these low-mass stars, our goal is to estimate the impact of atmospheric dynamics on the abundance determination. Methods. To estimate abundances, we measured the equivalent widths of selected features in observed spectra and compared the results with predictions from a set of hydrostatic and dynamical model atmospheres resembling 47 Tuc AGB stars in their fundamental parameters. Our study includes lines of 12CO, 13CO, OH, and Na. Furthermore, we investigated the variations in line intensities over a pulsation cycle. Results. We present new measurements of the C/O and 12C/13C ratio for 5 non-variable red giants in 47 Tuc. The equivalent widths measured for our 7 variable stars strongly differ from the non-variable stars and cannot be reproduced by either hydrostatic or dynamical model atmospheres. Nevertheless, the dynamical models fit the observed spectra of long-period variables much better than any hydrostatic model. For some spectral features, the variations in the line intensities predicted by dynamical models over a pulsation cycle give similar values as a sequence of hydrostatic models with varying temperature and constant surface gravity. Conclusions. Our study of the dynamical effects on abundance determination visible in these well-characterized cluster stars prepares the ground for the long-term goal of deriving abundances for variable AGB stars in general. [ABSTRACT FROM AUTHOR]

ABSTRACT We present an automated classification of stars exhibiting periodic, non-periodic and irregular light variations. The Hipparcos catalogue of unsolved variables is employed to complement the training set of periodic variables of Dubath et al. with irregular and non-periodic representatives, leading to 3881 sources in total which describe 24 variability types. The attributes employed to characterize light-curve features are selected according to their relevance for classification. Classifier models are produced with random forests and a multistage methodology based on Bayesian networks, achieving overall misclassification rates under 12 per cent. Both classifiers are applied to predict variability types for 6051 Hipparcos variables associated with uncertain or missing types in the literature. [ABSTRACT FROM AUTHOR]

Context. New instrumental capabilities and the wealth of astrophysical information extractable from the near-infrared wavelength region have led to a growing interest in the field of high resolution spectroscopy at 1-5 μm. Aims. We aim to provide a library of observed high-resolution and high signal-to-noise-ratio near-infrared spectra of stars of various types throughout the Hertzsprung-Russell diagram. This is needed for the exploration of spectral features in this wavelength range and for comparison of reference targets with observations and models. Methods. High quality spectra were obtained using the CRIRES near-infrared spectrograph at ESO's VLT covering the range from 0.97 μm to 5.3μm at high spectral resolution. Accurate wavelength calibration and correction for telluric lines were performed by fitting synthetic transmission spectra for the Earth's atmosphere to each spectrum individually. Results. We describe the observational strategy and the current status and content of the library which includes 13 objects. The first examples of finally reduced spectra are presented. This publication will serve as a reference paper to introduce the library to the community and explore the extensive amount of material. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

ABSTRACT 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. [ABSTRACT FROM AUTHOR]

The CRIRES-POP project (Lebzelter et al. 2012) will provide a public database of high-res, high S:N, NIR spectra of stars spanning the HR diagram. A relevant part of this library for the evolved star community will be the high quality reference spectra of several cool evolved giants. These atlases will be valuable references for future research, and will include comprehensive line identifications, abundance measurements, and isotope ratios, alongside stellar parameters derived through the inclusion of other data sources. All CRIRES observations have been completed, and reduction and analysis of the spectra, including improved telluric subtraction and wavelength calibration, is ongoing. We present here preliminary results of the first atlas to be produced, that of K1 III giant 10 Leo. [ABSTRACT FROM AUTHOR]

Time series of near-IR spectra are used to explore the cool molecular region between the photosphere and expanding circumstellar shell in large amplitude (Mira) AGB variables. The ~1000 K Mira "MOLsphere" is shown to undergo large amplitude velocity changes that are aperiodic with time scales many times longer than the photospheric pulsation. [ABSTRACT FROM AUTHOR]

We examined the light variations of AGB variables of the two Local Group members NGC147 and NGC185. A photometric monitoring in the i-band was carried out with the Nordic Optical Telescope at La Palma over ≈2.5 yrs resulting in roughly 35 data points for the period analysis. We found 513 long period variables (LPVs) in NGC185 and 213 LPVs in NGC147. Additional single-epoch Ks-band photometry was obtained for 387 (NGC 185, 323 with periods) and 182 (NGC 147, 147 with periods) LPVs, respectively. For a large fraction of these stars we also have an indication of the atmospheric chemistry (carbon- or oxygen-rich) from narrow band photometry (see Nowotny et al. 2003). The resulting Ks -logP diagrams for both systems show a well populated sequence of fundamental mode pulsators (sequence C, cf. Ita et al. 2004). In NGC185 we also see a number of LPVs pulsating in the first overtone mode (sequence C'). Interestingly, such stars are missing in NGC147. We speculate that this lack of shorter period variables in NGC147 could originate from a difference in the star formation history (SFH) of the two galaxies, with NGC147 containing a smaller population of intermediate-age stars. [ABSTRACT FROM AUTHOR]