Yazar "Ak, S." seçeneğine göre listele

Listeleniyor 1 - 2 / 2
  • [ X ]
    Öğe
    Spatial distribution and galactic model parameters of cataclysmic variables
    (Elsevier Science Bv, 2008) Ak, T.; Bilir, S.; Ak, S.; Eker, Z.
    The spatial distribution, galactic model parameters and luminosity function of cataclysmic variables (CVs) in the solar neighbourhood have been determined from a carefully established sample of 459 CVs. The sample contains all of the CVs with distances computed from the period-luminosity-colours (PLCs) relation of CVs which has been recently derived and calibrated with 2MASS photometric data. It has been found that an exponential function fits best to the observational z-distributions of all of the CVs in the sample, non-magnetic CVs and dwarf novae, while the sech(2) function is more appropriate for nova-like stars and polars. The vertical scaleheight of CVs is 158 +/- 14 pc for the 2MASS J-band limiting apparent magnitude of 15.8. On the other hand, the vertical scaleheights are 128 +/- 20 and 160 +/- 5 pc for dwarf novae and nova-like stars, respectively. The local space density of CVs is found to be similar to 3 x 10(-5) pc(-3) Which is in agreement with the lower limit of the theoretical predictions. The luminosity function of CVs shows an increasing trend toward higher space densities at low luminosities, implying that the number of short-period systems should be high. The discrepancies between the theoretical and observational population studies of CVs will almost disappear if for the z-dependence of the space density the sech(2) density function is used. (C) 2007 Elsevier B.V. All rights reserved.
  • [ X ]
    Öğe
    The Galactic kinematics of cataclysmic variables
    (Springer, 2015) Ak, T.; Bilir, S.; Ozdonmez, A.; Soydugan, Faruk; Soydugan, Esin; Puskullu, C.; Ak, S.
    Kinematical properties of CVs were investigated according to population types and orbital periods, using the space velocities computed from recently updated systemic velocities, proper motions and parallaxes. Reliability of collected space velocity data was refined by removing 34 systems with largest space velocity errors. The 216 CVs in the refined sample were shown to have a dispersion of 53.70 +/- 7.41 kms(-1) corresponding to a mean kinematical age of 5.29 +/- 1.35 Gyr. Population types of CVs were identified using their Galactic orbital parameters. According to the population analysis, seven old thin disc, nine thick disc and one halo CV were found in the sample, indicating that 94 % of CVs in the Solar Neighbourhood belong to the thin-disc component of the Galaxy. Mean kinematical ages 3.40 +/- 1.03 and 3.90 +/- 1.28 Gyr are for the nonmagnetic thin-disc CVs below and above the period gap, respectively. There is not a meaningful difference between the velocity dispersions below and above the gap. Velocity dispersions of the non-magnetic thin-disc systems below and above the gap are 24.95 +/- 3.46 and 26.60 +/- 4.18 kms(-1), respectively. This result is not in agreement with the standard formation and evolution theory of CVs. The mean kinematical ages of the CV groups in various orbital period intervals increase towards shorter orbital periods. This is in agreement with the standard theory for the evolution of CVs. Rate of orbital period change was found to be dP/dt =-1.62(+/- 0.15) x10(-5) sec yr(-1).