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Register a new userInsights from the APOKASC Determination of the Evolutionary State of Red-Giant Stars by consolidation of different methods
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Yvonne Elsworth Professor
Publication date
2019
fields
Physics
and research's language is
English
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The internal working of low-mass stars is of great significance to both the study of stellar structure and the history of the Milky Way. Asteroseismology has the power to directly sense the internal structure of stars and allows for the determination of the evolutionary state -- i.e. has helium burning commenced or is the energy generated only by the fusion in the hydrogen-burning shell? We use observational data from red-giant stars in a combination (known as APOKASC) of asteroseismology (from the textit{Kepler} mission) and spectroscopy (from SDSS/APOGEE). The new feature of the analysis is that the APOKASC evolutionary state determination is based on the comparison of diverse approaches to the investigation of the frequency-power spectrum. The high level of agreement between the methods is a strong validation of the approaches. Stars for which there is not a consensus view are readily identified. The comparison also facilitates the identification of unusual stars including those that show evidence for very strong coupling between p and g cavities. The comparison between the classification based on the spectroscopic data and asteroseismic data have led to a new value for the statistical uncertainty in APOGEE temperatures. These consensus evolutionary states will be used as an input for methods that derive masses and ages for these stars based on comparison of observables with stellar evolutionary models (`grid-based modeling) and as a training set for machine-learning and other data-driven methods of evolutionary state determination
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Determining the ages of red-giant stars is a key problem in stellar astrophysics. One of the difficulties in this determination is to know the evolutionary state of the individual stars -- i.e. have they started to burn Helium in their cores? That is the topic of this paper. Asteroseismic data provide a route to achieving this information. What we present here is an highly autonomous way of determining the evolutionary state from an analysis of the power spectrum of the light curve. The method is fast and efficient and can provide results for a large number of stars. It uses the structure of the dipole-mode oscillations, which have a mixed character in red-giant stars, to determine some measures that are used in the categorisation. It does not require that all the individual components of any given mode be separately characterised. Some 6604 red giant stars have been classified. Of these 3566 are determined to be on the red-giant branch, 2077 are red-clump and 439 are secondary-clump stars. We do not specifically identify the low metallicity, horizontal-branch stars. The difference between red-clump and secondary-clump stars is dependent on the manner in which Helium burning is first initiated. We discuss that the way the boundary between these classifications is set may lead to mis-categorisation in a small number of stars. The remaining 522 stars were not classified either because they lacked sufficient power in the dipole modes (so-called depressed dipole modes) or because of conflicting values in the parameters.
Context: The Kepler space mission is reaching continuous observing times long enough to start studying the fine structure of the observed p-mode spectra. Aims: In this paper, we aim to study the signature of stellar evolution on the radial and p-dominated l=2 modes in an ensemble of red giants that show solar-type oscillations. Results: We find that the phase shift of the central radial mode (eps_c) is significantly different for red giants at a given large frequency separation (Dnu_c) but which burn only H in a shell (RGB) than those that have already ignited core He burning. Even though not directly probing the stellar core the pair of local seismic observables (Dnu_c, eps_c) can be used as an evolutionary stage discriminator that turned out to be as reliable as the period spacing of the mixed dipole modes. We find a tight correlation between eps_c and Dnu_c for RGB stars and no indication that eps_c depends on other properties of these stars. It appears that the difference in eps_c between the two populations becomes if we use an average of several radial orders, instead of a local, i.e. only around the central radial mode, Dnu to determine the phase shift. This indicates that the information on the evolutionary stage is encoded locally, in the shape of the radial mode sequence. This shape turns out to be approximately symmetric around the central radial mode for RGB stars but asymmetric for core He burning stars. We computed radial modes for a sequence of RG models and find them to qualitatively confirm our findings. We also find that, at least in our models, the local Dnu is an at least as good and mostly better proxy for both the asymptotic spacing and the large separation scaled from the model density than the average Dnu. Finally, we investigate the signature of the evolutionary stage on the small frequency separation and quantify the mass dependency of this seismic parameter.
The successful launches of the CoRoT and Kepler space missions have led to the detections of solar-like oscillations in large samples of red-giant stars. The large numbers of red giants with observed oscillations make it possible to investigate the properties of the sample as a whole: ensemble asteroseismology. In this article we summarise ensemble asteroseismology results obtained from data released by the Kepler Science Team (~150,000 field stars) as presented by Hekker et al. (2011b) and for the clusters NGC 6791, NGC 6811 and NGC 6819 (Hekker et al. 2011a) and we discuss the importance of such studies.
Frequencies of acoustic and mixed modes in red giant stars are now determined with high precision thanks to the long continuous observations provided by the NASA Kepler mission. Here we consider the eigenfrequencies of nineteen low-luminosity red giant stars selected by Corsaro et al. (2015) for a detailed peak-bagging analysis. Our objective is to obtain stellar parameters by using individual mode frequencies and spectroscopic information. We use a forward modelling technique based on a minimization procedure combining the frequencies of the p modes, the period spacing of the dipolar modes, and the spectroscopic data. Consistent results between the forward modelling technique and values derived from the seismic scaling relations are found but the errors derived using the former technique are lower. The average error for log g is 0.002 dex, compared to 0.011 dex from the frequency of maximum power and 0.10 dex from the spectroscopic analysis. Relative errors in the masses and radii are on average 2 and 0.5 per cent respectively, compared to 3 and 2 per cent derived from the scaling relations. No reliable determination of the initial helium abundances and the mixing length parameters could be made. Finally, for our grid of models with a given input physics, we found that low-mass stars require higher values of the overshooting parameter.
Spectra for 2D stars in the 1.5D approximation are created from synthetic spectra of 1D non-local thermodynamic equilibrium (NLTE) spherical model atmospheres produced by the PHOENIX code. The 1.5D stars have the spatially averaged Rayleigh-Jeans flux of a K3-4 III star, while varying the temperature difference between the two 1D component models ($Delta T_{mathrm{1.5D}}$), and the relative surface area covered. Synthetic observable quantities from the 1.5D stars are fitted with quantities from NLTE and local thermodynamic equilibrium (LTE) 1D models to assess the errors in inferred $T_{mathrm{eff}}$ values from assuming horizontal homogeneity and LTE. Five different quantities are fit to determine the $T_{mathrm{eff}}$ of the 1.5D stars: UBVRI photometric colors, absolute surface flux SEDs, relative SEDs, continuum normalized spectra, and TiO band profiles. In all cases except the TiO band profiles, the inferred $T_{mathrm{eff}}$ value increases with increasing $Delta T_{mathrm{1.5D}}$. In all cases, the inferred $T_{mathrm{eff}}$ value from fitting 1D LTE quantities is higher than from fitting 1D NLTE quantities and is approximately constant as a function of $Delta T_{mathrm{1.5D}}$ within each case. The difference between LTE and NLTE for the TiO bands is caused indirectly by the NLTE temperature structure of the upper atmosphere, as the bands are computed in LTE. We conclude that the difference between $T_{mathrm{eff}}$ values derived from NLTE and LTE modelling is relatively insensitive to the degree of the horizontal inhomogeneity of the star being modeled, and largely depends on the observable quantity being fit.
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