A Unified tool to estimate Distances, Ages and Masses (UniDAM) from spectrophotometric data

Galactic archaeology - the study of the formation and evolution of the Milky Way by reconstructing its past from its current constituents - requires precise and accurate knowledge of stellar parameters for as many stars as possible. To achieve this a number of large spectroscopic surveys have been undertaken and are still ongoing. So far consortia carrying out the different spectroscopic surveys have used different tools to determine stellar parameters of stars from their derived effective temperatures (Teff), surface gravities (log g) and metallicities ([Fe/H]) possibly combined with photometric, astrometric, interferometric or asteroseismic information. Here we aim to homogenise the stellar characterisation by applying a unified tool to a large set of publicly available spectrophotometric data. We use spectroscopic data from a variety of large surveys combined with infra-red photometry from 2MASS and AllWISE and compare these in a Bayesian manner with PARSEC isochrones to derive probability density functions (PDFs) for stellar masses, ages and distances. We treat PDFs of pre-helium-core burning, helium-core burning and post helium-core burning solutions as well as different peaks in multi-modal PDFs (i.e. each unimodal sub-PDF) of the different evolutionary phases separately. For over 2.5 million stars we report mass, age and distance estimate for each evolutionary phase and unimodal sub-PDF. We report Gaussian, skewed Gaussian, truncated Gaussian, modified truncated exponential distribution or truncated Student's t-distribution functions to represent each sub-PDF, allowing to reconstruct detailed PDFs. Comparisons with stellar parameter estimates from the literature show good agreement within uncertainties. We present UniDAM - the unified tool applicable to spectrophotometric data of different surveys to obtain a homogenised set of stellar parameters

Isochrone fitting in the Gaia era

Context. Currently galactic exploration is being revolutionized by a flow of new data: Gaia provides measurements of stellar distances and kinematics; growing numbers of spectroscopic surveys provide values of stellar atmospheric parameters and abundances of elements; and Kepler and K2 missions provide asteroseismic information for an increasing number of stars. Aims. In this work we aim to determine stellar distances and ages using Gaia and spectrophotometric data in a consistent way. We estimate precisions of age and distance determinations with Gaia end-of-mission and TGAS parallax precisions. Methods. To this end we incorporated parallax and extinction data into the isochrone fitting method used in the Unified tool to estimate Distances, Ages, and Masses (UniDAM). We prepared datasets that allowed us to study the improvement of distance and age estimates with the inclusion of TGAS and Gaia end-of-mission parallax precisions in isochrone fitting. Results. Using TGAS parallaxes in isochrone fitting we are able to reduce distance and age estimate uncertainties for TGAS stars for distances up to 1 kpc by more than one third, compared to results based only on spectrophotometric data. With Gaia end-of-mission parallaxes in isochrone fitting we will be able to further decrease our distance uncertainties by about a factor of 20 and age uncertainties by a factor of two for stars up to 10 kpc away from the Sun. Conclusions. We demonstrate that we will be able to improve our distance estimates for about one third of stars in spectroscopic surveys and to decrease log(age) uncertainties by about a factor of two for over 80% of stars as compared to the uncertainties obtained without parallax priors using Gaia end-of-mission parallaxes consistently with spectrophotometry in isochrone fitting .Comment: 15 pages, 3 figures, 4 table

Selection functions of large spectroscopic surveys

Context. Large spectroscopic surveys open the way to explore our Galaxy. In order to use the data from these surveys to understand the Galactic stellar population, we need to be sure that stars contained in a survey are a representative subset of the underlying population. Without the selection function taken into account, the results might reflect the properties of the selection function rather than those of the underlying stellar population. Aims. In this work, we introduce a method to estimate the selection function for a given spectroscopic survey. We apply this method to a large sample of public spectroscopic surveys. Methods. We apply a median division binning algorithm to bin observed stars in the colour-magnitude space. This approach produces lower uncertainties and lower biases of the selection function estimate as compared to traditionally used 2D-histograms. We run a set of simulations to verify the method and calibrate the one free parameter it contains. These simulations allow us to test the precision and accuracy of the method. Results. We produce and publish estimated values and uncertainties of selection functions for a large sample of public spectroscopic surveys. We publicly release the code used to produce the selection function estimates. Conclusions. The effect of the selection function on distance modulus and metallicity distributions of stars in surveys is important for surveys with small and largely inhomogeneous spatial coverage. For surveys with contiguous spatial coverage the effect of the selection function is almost negligible.Comment: 12 pages, 11 figures, 1 tabl

Asteroseismic Study on Cluster Distance Moduli for RGB Stars in NGC 6791 and NGC 6819

Stellar distance is an important basic parameter in stellar astrophysics. Stars in a cluster are thought to be formed coevally from the same interstellar cloud of gas and dust. They are therefore expected to have common properties. These common properties strengthen our ability to constrain theoretical models and/or to determine fundamental parameters, such as stellar mass, metal fraction, and distance when tested against an ensemble of cluster stars. Here we derive a new relation based on solar-like oscillations, photometric observations, and the theory of stellar structure and evolution of red giant branch stars to determine cluster distance moduli through the global oscillation parameters $\Delta\nu$ and $\nu_{\rm max}$, and photometric data \textit{V}. The values of $\Delta\nu$ and $\nu_{\rm max}$ are derived from \textit{kepler} observations. At the same time, it is used to interpret the trends between \textit{V} and $\Delta\nu$. From the analyses of this newly derived relation and observational data of NGC 6791 and NGC 6819 we devise a method in which all stars in a cluster are regarded as one entity to determine the cluster distance modulus. This approach fully reflects the characteristic of member stars in a cluster as a natural sample. From this method we derive true distance moduli of $13.09\pm0.10$ mag for NGC 6791 and $11.88\pm0.14$ mag for NGC 6819. Additionally, we find that the distance modulus only slightly depends on the metallicity [Fe/H] in the new relation. A change of 0.1 dex in [Fe/H] will lead to a change of 0.06 mag in the distance modulus.Comment: 9 pages, 6 figures, 4 tables, accepted Ap

New asteroseismic scaling relations based on Hayashi track relation applied to red-giant branch stars in NGC 6791 and NGC 6819

Stellar mass $M$, radius $R$, and gravity $g$ are important basic parameters in stellar physics. Accurate values for these parameters can be obtained from the gravitational interaction between stars in multiple systems or from asteroseismology. Stars in a cluster are thought to be formed coevally from the same interstellar cloud of gas and dust. The cluster members are therefore expected to have some properties in common. These common properties strengthen our ability to constrain stellar models and asteroseismically derived $M$, $R$ and $g$ when tested against an ensemble of cluster stars. Here we derive new scaling relations based on a relation for stars on the Hayashi track ($\sqrt{T_{\rm eff}} \sim g^pR^q$) to determine the masses and metallicities of red giant branch stars in open clusters NGC 6791 and NGC 6819 from the global oscillation parameters $\Delta\nu$ (the large frequency separation) and $\nu_{\rm max}$ (frequency of maximum oscillation power). The $\Delta\nu$ and $\nu_{\rm max}$ values are derived from \kepler\ observations. From the analysis of these new relations we derive: (1) direct observational evidence that the masses of red giant branch stars in a cluster are the same within their uncertainties, (2) new methods to derive $M$ and $z$ of the cluster in a self consistent way from $\Delta\nu$ and $\nu_{\rm max}$, with lower intrinsic uncertainties, (3) the mass dependence in the $\Delta\nu$ - $\nu_{\rm max}$ relation for red giant branch stars.Comment: open clusters and associations: individual (NGC 6791, NGC 6819) -- stars: late-type -- stars: fundamental parameters -- stars: interiors -- stars: oscillations -- asteroseismolog

A New Method for the Asteroseismic Determination of the Evolutionary State of Red-Giant Stars

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.Comment: Accepted for publication in MNRAS. There will be an online table associated with the publicatio

Automated determination of g-mode period spacing of red-giant stars

The Kepler satellite has provided photometric timeseries data of unprecedented length, duty cycle and precision. To fully analyse these data for the tens of thousands of stars observed by Kepler, automated methods are a prerequisite. Here we present an automated procedure to determine the period spacing of gravity modes in red-giant stars ascending the red-giant branch. The gravity modes reside in a cavity in the deep interior of the stars and provide information on the conditions in the stellar core. However, for red giants the gravity modes are not directly observable on the surface, hence this method is based on the pressure-gravity mixed modes that present observable features in the Fourier power spectrum. The method presented here is based on the vertical alignment and symmetry of these mixed modes in a period echelle diagram. We find that we can obtain reliable results for both model frequencies and observed frequencies. Additionally, we carried out Monte Carlo tests to obtain realistic uncertainties on the period spacings with different set of oscillation modes (for the models) and uncertainties on the frequencies. Furthermore, this method has been used to improve mode detection and identification of the observed frequencies in an iterative manner.Comment: Accepted for publication in MNRA