MBCAA Observatory

KZ Hya: a pulsating binary (improved)

Observed: 9 Feb, 11, 14 March, 19 Apr 2007

Michel Bonnardeau
10 Feb 2007
Revised binary model and new observations 18 March 2007
Updated and revised 21 Apr 2007
Correction 29 Sep 2010

Abstract

This pulsating star is in a binary system with an 8-yr orbital period. The times of maximum are compared with an improved ephemeris and the parameters of the binary are derived.

Correction

The period of 8.4yr used in this analysis is not correct, although it gives a fairly good fit with the available data. With more observations, Kim et al (2007, Ap Sp Sc 312 41) obtained a period of 24.77yr.

Introduction

KZ Hya is a pulsating star with a period of 85mn. It is in a binary system with a period of 8.4yr. In 2006 I improved the existing ephemeris with my 2004-2006 observations. These 2007 observations are a follow-up to check the ephemeris.

2004 observations.
2005-2006 observations and derivation of an ephemeris.

Observations

The observations are carried out with the same setup as in 2004-2006:

  • 203mm SC f/6.3 telescope, Rc filter and SBIG ST7E camera (KAF401E CCD);
  • each exposure has a duration of 60s;
  • the comparison star is Tycho 6638-00764 with an assumed Rc magnitude of 9.800;
  • the check stars are K: GSC 6638-00842 and K1: GSC 6638-00639.
  • An example of a light curve:


    Red: KZ Hya, Blue: the K1 check star, shifted by -2.6 mag. The error bars are +/- the 1-sigma statistical uncertainties.

    All the light curves are HERE.

    6 pulses are measured:

    ToM (HJD - 2454000) uncertainties (day)
    141.5583 0.001
    171.4927 0.001
    174.409 0.001
    174.468 0.001
    174.5275 0.001
    210.41270.001

    2004-2006 observations

    In 2004 and 2005-2006 I observed 11 times of maximum. I re-determine these ToM with a better precision (2004 and 2005-2006):

    ToM (HJD - 2453000) uncertainties (day)
    121.41150.001
    123.3750.0015
    144.38150.0015
    728.72750.001
    729.67950.001
    761.57750.0015
    761.63750.002
    808.41350.001
    808.4730.001
    808.5320.001
    833.4080.001

    Comparison with the ephemeris for the pulsations

    The 90 times of maximum of Liu Yanying et al (1991) [LY91] and my 11 measurements of 2004-2006 were used to derive the pulsation parameters: P=0.059,511,264d, t0=2,442,516.15584HJD, beta=-3.38*10^-13. With this ephemeris, one has the O-C diagram:


    Red circles: my 2004-2006 measurements;
    Black circles: my 2007 measurements;
    Blue squares: from ASAS data; Magenta diamond: from ROTSE data; Green diamond: from AAVSO data.
    The ASAS, ROTSE, AAVSO data are not individual times of maximum but average values over 1 yr; they were derived HERE.

    The binary motion (revised)

    In 2006 I derived an ephemeris that used the equations of LY91 for the binary motion. Now, I use improved equations to fit (by hand) the parameters for the light-travel time effect. The results are:

    Orbital period 7.934 yr
    a*sin(i) (semi-major axis * sin(inclination)) 0.889 au
    Eccentricity 0.455
    Periastron longitude (from node line) 333
    Time of passage at periastron 2,442,895.9 HJD (19760427.4)

    The resulting O-C diagram in phase:



    Red dots: the times of maximum of LY91;
    Red circles: my 2004-2006 measurements;
    Black circles: my 2007 measurements;
    Cyan curve: the binary light-travel time effect.


    and as a function of time:



    Red dots: the 90 times of maximum of LY91;
    Red circles: my 2004-2006 measurements;
    Black circles: my 2007 measurements;
    Blue: from ASAS data;
    Magenta: from ROTSE data;
    Green: from AAVSO data;
    Cyan curve: the binary light-travel time effect.
    The ASAS, ROTSE, AAVSO data are not individual times of maximum but average values over 1 yr; they were derived HERE.

    The differences with the 2006 calculations come from:

  • I use the exact equation (5) instead of the approximation valid only for e->0;
  • in my calculations the distance of the star from the center of gravity of the binary system varies continuously;
  • the longitude of the periastron is defined differently (a 180 difference);
  • a factor of 2 was missing for the eccentricity.
  • Reference

    Liu Yanying, Jiang Shiyang, Cao Ming (1991) IBVS 3606 [LY91].

    Astronomical notes

    Light-travel time effects in binary systems.

    Pulsating stars and its links.

    Technical notes

    Telescope and camera configuration.

    Computer and software configuration.



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