S. Ak

Local stellar kinematics from RAVE data – I. Local standard of rest

We analyze a sample of 82850 stars from the RAVE survey, with well-determined velocities and stellar parameters, to isolate a sample of 18026 high-probability thin-disc dwarfs within 600 pc of the Sun. We derive space motions for these stars, and deduce the solar space velocity with respect to the Local Standard of Rest. The peculiar solar

Transformations between 2MASS, SDSS and BVRI photometric systems: bridging the near-infrared and optical

We present colour transformations for the conversion of the Two Micron All Sky Survey (2MASS) photometric system to the Johnson–Cousins UBVRI system and further into the Sloan Digital Sky Survey (SDSS) ugriz system. We have taken SDSS gri magnitudes of stars measured with the 2.5-m telescope from SDSS Data Release 5 (DR5), and BVRI and JHKs magnitudes from Stetson’s catalogue and Cutri et al., respectively. We matched thousands of stars in the three photometric systems by their coordinates and obtained a homogeneous sample of 825 stars by the following constraints, which are not used in previous transformations: (1) the data are dereddened, (2) giants are omitted and (3) the sample stars selected are of the highest quality. We give metallicity, population type and transformations dependent on two colours. The transformations provide absolute magnitude and distance determinations which can be used in space density evaluations at short distances where some or all of the SDSS ugriz magnitudes are saturated. The combination of these densities with those evaluated at larger distances using SDSS ugriz photometry will supply accurate Galactic model parameters, particularly the local space densities for each population.

Estimation of Galactic model parameters in high latitudes with 2MASS

Context. In general, studies focused on the Milky Ways structure show a range of values when deriving different Galactic parameters, such as radial scalelengths, vertical scaleheights, or local space densities. Those values are also dependent on the Galactic coordinates under consideration for the corresponding analysis, as a direct consequence of observing a structure (our Galaxy) that is far from being as smooth and well-behaved as models usually treat. Aims. In this paper, we try to find any dependence of the Galactic structural parameters on the Galactic longitude for either the thin disc or the thick disc of the Milky Way that would indicate possible inhomogeneities or asymmetries in those Galactic components. Methods. Galactic model parameters have been estimated for a set of 36 high-latitude fields with Two Micron All Sky Survey (2MASS) photometry. Possible variations with the Galactic longitude in either the scaleheight and the local space density of these components are explored. Results. Galactic model parameters for the different fields show that they are Galactic longitude-dependent. The thick disc scaleheight changes from ∼800 pc at 150° < l < 230° to ∼1050 pc at |l| < 30°. A plausible explanation for this finding might be the effect of the flare on this Galactic component, which changes the scaleheight (h z ) with Galactocentric distance (R) following the approximate law: h z (R) = (940 ± 20) x [1 - (0.12 ± 0.02)(R - R ⊙ )]. The effect of the flare is more prominent in some lines of sight than in others, producing the observed changes in the parameters with the Galactic coordinates used to derive them.

Local stellar kinematics from RAVE data – II. Radial metallicity gradient

We investigate radial metallicity gradients for a sample of dwarf stars from the RAdial Velocity Experiment (RAVE) Data Release 3 (DR3). We select a total of approximately 17 000 F-type and G-type dwarfs, using a selection of colour, log g and uncertainty in the derived space motion, and calculate for each star a probabilistic (kinematic) population assignment to a thick or thin disc using space motion and additionally another (dynamical) assignment using stellar vertical orbital eccentricity. We additionally subsample by colour, to provide samples biased toward young thin-disc and older thin-disc stars. We derive a metallicity gradient as a function of Galactocentric radial distance, i.e. d[M/H]/dRm =− 0.051 ± 0.005 dex kpc −1 , for the youngest sample, F-type stars with vertical orbital eccentricities ev ≤ 0.04. Samples biased toward older thin-disc stars show systematically shallower abundance gradients.

Estimation of Galactic Model Parameters in High Latitudes with SDSS

We estimated the Galactic model parameters for a set of 36 high-latitude fields included in the currently available Data Release 5 (DR 5) of the Sloan Digital Sky Survey (SDSS), to explore their possible variation with the Galactic longitude. The thick disc scaleheight moves from ~550 pc at 120 < l < 150° to ~720 pc at 250 < l < 290°, while the thin disc scaleheight is as large as ~195 pc in the anticenter direction and ~15% lower at |l| < 30°. Finally, the axis ratio (c/a) of the halo changes from a mean value of ~0.55 in the two first quadrants of the Galaxy to ~0.70 at 190 < l < 300°. For the halo, the reason for the dependence of the model parameters on the Galactic longitude arises from the well known asymmetric structure of this component. However, the variation of the model parameters of the thin and thick discs with Galactic longitude originates from the gravitational effect of the Galactic long bar. Moreover, the excess of stars in quadrant I (quadrant III) over quadrant IV (quadrant II) is in agreement with this scenario.

A new absolute magnitude calibration with 2MASS for cataclysmic variables

Using reliable trigonometric measurements, we find that the absolute magnitude of cataclysmic variables depends on the orbital period and de-reddened (J −H)0 and (H −Ks)0 colours of 2MASS (Two Micron All Sky Survey) photometric system. The calibration equation covers the ranges 0.032 d < Porb ≤ 0.454 d , −0.08 < (J −H)0 ≤ 1.54, −0.03 < (H −Ks)0 ≤ 0.56 and 2.0 < MJ < 11.7; It is based on trigonometric parallaxes with relative errors of (��/�) ≤ 0.4. By using the period-luminositycolours (PLCs) relation, we estimated the distances of cataclysmic variables with orbital periods and 2MASS observations and compared them with distances found from other methods. We suggest that the PLCs relation can be a useful statistical tool to estimate the distances of cataclysmic variables.

Luminosity–colour relations for thin‐disc main‐sequence stars

In this study we present the absolute magnitude calibrations of thin-disc main-sequence stars in the optical (M V ), and in the near-infrared (Mj). Thin-disc stars are identified by means of Padova isochrones, and absolute magnitudes for the sample are evaluated via the newly reduced Hipparcos data. The obtained calibrations cover a large range of spectral types: from A0 to M4 in the optical and from A0 to M0 in the near-infrared. Also, we discuss the effects of binary stars and evolved stars on the absolute magnitude calibrations. The usage of these calibrations can be extended to the estimation of galactic model parameters for the thin disc individually, in order to compare these parameters with the corresponding ones estimated by Χ 2 min statistics (which provides galactic model parameters for thin and thick discs and halo simultaneously) to test any degeneracy between them. The calibrations can also be used in other astrophysical researches where distance plays an important role in that study.

New Metallicity Calibration Down to [Fe/H] = −2.75 dex

We have taken 88 dwarfs, covering the colour-index interval 0.37 ≤ (B–V)0 ≤ 1.07 mag, with metallicities –2.70 ≤ [Fe/H] ≤ +0.26 dex, from three different sources for new metallicity calibration. The catalogue of Cayrel de Stroble et al. (2001), which includes 65% of the stars in our sample, supplies detailed information on abundances for stars with determination based on high-resolution spectroscopy. In constructing the new calibration we have used as ‘corner stones’ 77 stars which supply at least one of the following conditions: (i) the parallax is larger than 10 mas (distance relative to the Sun less than 100 pc) and the galactic latitude is absolutely higher than 30°; (ii) the parallax is rather large, if the galactic latitude is absolutely low and vice versa. Contrary to previous investigations, a third-degree polynomial is fitted for the new calibration: [Fe/H] = 0.10 – 2.76δ – 24.04δ2 + 30.00δ3. The coefficients were evaluated by the least-squares method, without regard to the metallicity of Hyades. However, the constant term is in the range of metallicity determined for this cluster, i.e. 0.08 ≤ [Fe/H] ≤ 0.11 dex. The mean deviation and the mean error in our work are equal to those of Carney (1979), for [Fe/H] ≥ –1.75 dex where Carneys calibration is valid.

A charge‐coupled device study of high‐latitude Galactic structure: testing the model parameters

We interpret published charge-coupled device (CCD) UBVI data to deduce the stellar density distribution and metallicity distribution function in the region of 2-8 kpc from the Galactic plane, and compare our results to several star-count models. A feature of extant star-count models is degeneracy between the adopted scaleheights of the thin and thick discs, and their local normalization. We illustrate the utility of this small data set, and future larger sets (e.g. Sloan Digital Sky Survey, SDSS), by explicitly considering consistency between the derived density laws, and the implied solar neighbourhood luminosity function. Our data set, from Hall et al.s 1996 paper (I = 52°, b = -39°), contains 566 stars, selected to be consistent with stellar loci in colour-colour diagrams. The effective apparent V-magnitude interval is 15.5 ≤ V o ≤ 20.5. Our analysis supports the parametrization of the recent (SDSS) galaxy model of Chen et al., except in preferring the stellar halo axial ratio to be η = 0.84. Photometric metal abundances have been derived for 329 stars with (B - V) 0 ≤ 1.0 using a new calibration. This shows a multimodal distribution with peaks at [Fe/H] = -0.10, -0.70 and -1.50 and a tail down to -2.75 dex. The vertical distance-dependent metallicity distribution function, if parametrized by a single mean value, can be described by a metallicity gradient d[Fe/H]/dz ∼ -0.2 dex kpc - 1 for the thin disc and thick disc, and d[Fe/H]/dz ∼ -0.1 dex kpc - 1 for the inner halo, to z = 8 kpc. However, the data are better described as the sum of three discrete distribution functions, each of which has a small or zero internal gradient. The changing mix of thin disc, thick disc and halo populations with distance from the plane generates an illusion of a smooth gradient.

Galactic model parameters for field giants separated from field dwarfs by their 2MASS and V apparent magnitudes

We present a method which separates field dwarfs and field giants by their 2MASS and V apparent magnitudes. This method is based on spectroscopically selected standards and is hence reliable.We applied it to stars in two fields, SA 54 and SA 82, and we estimated a full set of Galactic model parameters for giants including their total local space density. Our results are in agreement with the ones given in the recent literature. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)