Yuksel Karatas

Dynamical evolution of active detached binaries on the log Jo–log M diagram and contact binary formation

Orbital angular momentum (OAM, J o ), systemic mass (M) and orbital period (P) distributions of chromospherically active binaries (CAB) and W Ursae Majoris (W UMa) systems were investigated. The diagrams of log J o -logP, logM-logP and log J o -log M were formed from 119 CAB and 102 W UMa stars. The log J o -log M diagram is found to be most meaningful in demonstrating dynamical evolution of binary star orbits. A slightly curved borderline (contact border) separating the detached and the contact systems was discovered on the log J o -log M diagram. Since the orbital size (a) and period (P) of binaries are determined by their current J o , M and mass ratio, q, the rates of OAM loss (d log J o /dt) and mass loss (d log M/dt) are primary parameters to determine the direction and the speed of the dynamical evolution. A detached system becomes a eontact system if its own dynamical evolution enables it to pass the contact border on the log J o -log M diagram. The evolution of q for a mass-losing detached system is unknown unless the mass-loss rate for each component is known. Assuming q is constant in the first approximation and using the mean decreasing rates of Jo and M from the kinematical ages of CAB stars, it has been predicted that 11, 23 and 39 per cent of current CAB stars would transform to W UMa systems if their nuclear evolution permits them to live 2, 4 and 6 Gyr, respectively.

Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution

The kinematics of 237 chromospherically active binaries (CABs) were studied. The sample is heterogeneous with different orbits and physically different components from F to M spectral-type main-sequence stars to G and K giants and supergiants. The computed U, V, W space velocities indicate that the sample is also heterogeneous in velocity space. That is, both kinematically younger and older systems exist among the non-evolved main sequence and the evolved binaries containing giants and subgiants. The kinematically young (0.95 Gyr) subsample (N = 95), which is formed according to the kinematical criteria of moving groups, was compared with the rest (N = 142) of the sample (3.86 Gyr) to investigate any observational clues of binary evolution. Comparing the orbital period histograms between the younger and older subsamples, evidence was found supporting the finding of Demircan that the CABs lose mass (and angular momentum) and evolve towards shorter orbital periods. The evidence of mass loss is noticeable on the histograms of the total mass (M h + M c ), which is compared between the younger (only N = 53 systems available) and older subsamples (only N = 66 systems available). The orbital period decrease during binary evolution is found to be clearly indicated by the kinematical ages of 6.69, 5.19 and 3.02 Gyr which were found in the subsamples according to the period ranges of log P ≤ 0.8, 0.8 < log P ≤ 1.7 and 1.7 < log P ≤ 3, respectively, among the binaries in the older subsample.

Mass loss and orbital period decrease in detached chromospherically active binaries

The secular evolution of the orbital angular momentum (OAM), the systemic mass (M = M1+M2) and the orbital period of 114 chromospherically active binaries (CABs) were investigated after determining the kinematical ages of the sub-samples which were set according to OAM bins. OAMs, systemic masses and orbital periods were shown to be decreasing by the kinematical ages. The first order decreasing rates of OAM, systemic mass and orbital period have been determined as u J = 3.48× 10 10 yr 1 per systemic OAM, u M = 1.30× 10 10 yr 1 per systemic mass and u P = 3.96 × 10 10 yr 1 per orbital period respectively from the kinematical ages. The ratio of dlog J/dlog M = 2.68, which were derived from the kinematics of the present sample, implies that there must be a mechanism which amplifies the angular momentum loss ¯ A = 2.68 times in comparison to isotropic angular momentum loss of hypothetical isotropic wind from the components. It has been shown that simple isotropic mass loss from the surface of a component or both components would increase the orbital period.

Kinematics, ages and metallicities for F- and G-type stars in the solar neighbourhood

A new metallicity distribution and an age–metallicity relation are presented for 437 nearby F and G turn-off and sub-giant stars selected from radial velocity data of Nidever et al. Photometric metallicities are derived from uvby− Hβ photometry, and the stellar ages from the isochrones of Bergbusch & VandenBerg as transformed to uvby photometry using the methods of Clem et al. The X (stellar population) criterion of Schuster et al., which combines both kinematic and metallicity information, provides 22 thick-disc stars. σW= 32 ± 5 km s−1, 〈Vrot〉= 154 ± 6 km s−1 and 〈[M/H]〉=−0.55 ± 0.03 dex for these thick-disc stars, which is in agreement with values from previous studies of the thick disc. α-element abundances which are available for some of these thick-disc stars show the typical α-element signatures of the thick disc, supporting the classification procedure based on the X criterion. Both the scatter in metallicity at a given age and the presence of old, metal-rich stars in the age–metallicity relation make it difficult to decide whether or not an age–metallicity relation exists for the older thin-disc stars. For ages greater than 3 Gyr, our results agree with the other recent studies that there is almost no correlation between age and metallicity, Δ([M/Fe])/Δ(age) =−0.01 ± 0.005 dex Gyr−1. For the 22 thick-disc stars there is a range in ages of 7–8 Gyr, but again almost no correlation between age and metallicity. For the subset of main-sequence stars with extra-solar planets, the age–metallicity relation is very similar to that of the total sample, very flat, the main difference being that these stars are mostly metal-rich, [M/H]≳−0.2 dex. However, two of these stars have [M/H]∼−0.6 dex and have been classified as thick-disc stars. As for the total sample, the range in ages for these stars with extra-solar planetary systems is considerable with a nearly uniform distribution over 3 ≲ age ≲ 13 Gyr.

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)

Metallicities and ages of stellar populations at a high Galactic latitude field

We present an analysis of UBVRI data from the selected area SA 141. By applying recalibrated methods of measuring ultraviolet excess (UVX), we approximate abundances and absolute magnitudes for 368 stars over 1.3 deg 2 out to distances over 10 kpc. With the density distribution constrained from our previous photometric parallax investigations and with sufficient accounting for the metallicity bias in the UVX method, we are able to compare the vertical abundance distribution to those measured in previous studies. We find that the abundance distribution has an underlying uniform component consistent with previous spectroscopic results that posit a monometallic thick disc and halo with abundances of [Fe/H] = -0.8 and -1.4, respectively. However, there are a number of outlying data points that may indicate contamination by more metal-rich halo streams. The absence of vertical abundance gradients in the Galactic stellar populations and the possible presence of interloping halo streams would be consistent with expectations from merger models of Galaxy formation. We find that our UVX method has limited sensitivity in exploring the metallicity distribution of the distant Galactic halo, owing to the poor constraint on the UBV properties of very metal-poor stars. The derivation of metallicities from broad-band UBV photometry remains fundamentally sound for the exploration of the halo but is in need of both improved calibration and superior data.

New intrinsic-colour calibration for uvby-β photometry

Abstract A new intrinsic-colour calibration ( ( b - y ) o - β ) is presented for the uvby-β photometric system, making use of re-calibrated Hipparcos parallaxes and published reddening maps. This new calibration for ( b - y ) o - β , our Eq. (1) , has been based upon stars with d Hip 70 pc in the photometric catalogues of Schuster and Nissen (1988) , Schuster et al., 2004 , Schuster et al., 2006 , provides a small dispersion, ± 0.009 , and has a positive “standard” + 2.239 Δ β coefficient, which is not too different from the coefficients of Crawford (1975a, +1.11) and of Olsen (1988, +1.34) . For 61 stars with spectra from CASPEC, UVES/VLT, and FIES/NOT databases, without detectable Na I lines, the average reddening value 〈 E ( b - y ) 〉 = - 0.001 ± 0.002 shows that any zero-point correction to our intrinsic-colour equation must be minuscule.

A New Procedure for the Photometric Parallax Estimation

We present a new procedure for photometric parallax estimation. The data for 1236 stars provide calibrations between the absolute magnitude offset from the Hyades main-sequence and the ultraviolet-excess for eight different (B–V)0 colour-index intervals, (0.3 0.4), (0.4 0.5), (0.5 0.6), (0.6 0.7), (0.7 0.8), (0.8 0.9), (0.9 1.0) and (1.0 1.1). The mean difference between the original and estimated absolute magnitudes and the corresponding standard deviation are rather small, +0.0002 and ±0.0613 mag. The procedure has been adapted to the Sloan photometry by means of colour equations and applied to a set of artificial stars with different metallicities. The comparison of the absolute magnitudes estimated by the new procedure and the canonical one indicates that a single colour–magnitude diagram does not supply reliable absolute magnitudes for stars with large range of metallicity.