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

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

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

First, Second and Third Massive Stars in Open Clusters

The goal of this paper is to study possibilities of using first, second and third massive stars in open clusters to estimate total cluster mass and membership. We built estimator functions with the use of numerical simulations and analytical approximations and studied the precision and error distribution of the obtained estimator functions. We found that the distribution of the mass of first, second and third massive stars shows strong power-law tails at the high-mass end, thus it is better to use median or mode values instead of average ones. We show that the third massive star is a much better estimator then the first as it is more precise and less dependent on parameters such as maximum allowed stellar mass.Comment: 24 pages, 5 figures, 5 tables, to appear in Ap

Periodic alternating 0,π0,\pi-junction structures as realization of ϕ\phi-Josephson junctions

We consider the properties of a periodic structure consisting of small alternating 0- and pi- Josephson junctions. We show that depending on the relation between the lengths of the individual junctions, this system can be either in the homogeneous or in the phase-modulated state. The modulated phase appears via a second order phase transition when the mismatch between the lengths of the individual junctions exceeds the critical value. The screening length diverges at the transition point. In the modulated state, the equilibrium phase difference in the structure can take any value from -pi to pi (phi-junction). The current-phase relation in this structure has very unusual shape with two maxima. As a consequence, the field dependence of the critical current in a small structure is very different from the standard Fraunhofer dependence. The Josephson vortex in a long structure carries partial magnetic flux, which is determined by the equilibrium phase.Comment: 4 pages, 3 figues, submitted to Phys. Rev.

Josephson junction between anisotropic superconductors

The sin-Gordon equation for Josephson junctions with arbitrary misaligned anisotropic banks is derived. As an application, the problem of Josephson vortices at twin planes of a YBCO-like material is considered. It is shown that for an arbitrary orientation of these vortices relative to the crystal axes of the banks, the junctions should experience a mechanical torque which is evaluated. This torque and its angular dependence may, in principle, be measured in small fields, since the flux penetration into twinned crystals begins with nucleation of Josephson vortices at twin planes.Comment: 6 page