KZ Hya: a pulsating binary

Observed: 23, 24 Dec 2005, 25 Jan, 13 Mar, 7 Apr 2006

Michel Bonnardeau
15 May 2006

Abstract

Time-series of this pulsating SXPHE star in a binary system are presented and compared with other observations.

Introduction

KZ Hya is a SXPHE pulsating star with a period of 85mn. The times of maximum variations show a wavy pattern interpreted as the effect of an orbital motion with a period of 9yr.

I already observed this star in 2004. Here are new observations.

Observations

These 2005-2006 observations were carried out with a 203 mm SC telescope, a Cousins red filter and a SBIG ST7E camera (KAF401E CCD) (nearly the same setup as in 2004). 466 images were obtained. The exposures have all a duration of 60s.

The comparison star is Tycho 6638-00764 with an assumed Rc magnitude of 9.800. Two check stars are used:

An example of a light curve:


Red: KZ Hya, Black: the check star K1 magnitudes shifted by -2.5 mag. The error bars are +/- the 1 sigma statistical uncertainties.

All the light curves are HERE.

Height pulses are observed at the following times of maximum:

HJD - 2453000	uncertainty
728.728	0.001
729.678	0.001
761.577	0.002
761.636	0.002
808.413	0.001
808.474	0.001
808.532	0.001
833.407	0.001

Phase analysis

For the 8 observed pulses and 3 other pulses observed in 2004, an ephemeris may be derived:

HJD(E) = T0 + P0*E
with T0 = 2,453,728.72692
and P0 = 0.059,511,55 +/- 0.00000046 day

This gives the O-C diagrams:

This gives the phase plot (2005-2006 data only):


Note the bump at phase -0.3/+0.7.

Comparison with ASAS, AAVSO and ROTSE observations

KZ Hya was observed from 2000 to 2006 by ASAS (Pojmanski (2002)). The measurements are not dense enough to identify individual pulses but, by folding the data with the period P0 over a selected date range, a phase shift from my measurements may be obtained. This is done HERE and the results are:

Average epoch (-2450000)	Pulse max delay (times P0)	Uncertainty (times P0)
1963	0.13	0.04
2402	0.09	0.04
2727	0.05	0.05
3076	0.01	0.05
3462	0.04	0.04
3717	0.00	0.07

The same way, I obtain a phase shift from my ephemeris out of AAVSO data. This is done HERE and the result is:

Average epoch (-2450000)	Pulse max delay (times P0)	Uncertainty (times P0)
1627	0.14	0.06

I also obtain a phase shift out of 1999-2000 SkyDot/ROTSE-I/NSVS measurements. This is done HERE and the result is:

Average epoch (-2450000)	Pulse max delay (times P0)	Uncertainty (times P0)
1574	0.2	0.08

The O-C diagram shows that KZ Hya has been speeding up these last years:


Red circles: my observations, Blue diamonds: ASAS, Green square: AAVSO, Purple square: ROTSE.

2002 observations

KZ Hya was observed in 2002 by Doncel et al (2004). 12 times of maximum are reported. These maxima are way off my ephemeris and of the ASAS observations at the same epoch. I suspect the publication to be in error: the reported times of maximum are daylight times for the observatory location!

Comparison with early observations

Between 1975 and 1991, 90 times of maximum were measured and reported in 4 publications:

Date ranges	Nb of measurements	References
1975-1979	25	Przybylski & Bessel (1979)
1984	38	Hobart et al (1985)
1978, 1984	4	McNamara & Budge (1985)
1981-1991	23	Liu Yanying et al (1991) [LY91]

According to LY91, KZ Hya is not only a pulsating star but also a binary with a 9-yr orbital period, from the O-C residuals interpreted as Doppler shifts. They derived the ephemeris:

HJD(E) = T1 + P1*E + 0.5*beta1*E^2 + binary(HJD)
with T1=2,442,516.15576 +/- 0.0003
P1=0.059,511,036 +/- 0.00000017 day
beta1=(2.92 +/- 0.18)*10^-12
and with binary(HJD) the orbital modulation.

The resulting O-C diagram (Note that there are errors in the computation of E and of O-C in LY91; the correct values are in Doncel et (2004)):


Black diamonds: the 90 times of maximum from which the LY91 ephemeris is derived; Red dots: my 11 times of maximum; Blue dots: from ASAS observations; Green dot: from AAVSO observations; Magenta dot: from ROTSE observations. The cyan line is the binary function of LY91 with a 9 yr period.

Ephemeris from all the data

From the 90 times of maximum compiled in LY91 and my 11 times of maximum, I derive the ephemeris:

HJD(E) = T2 + P2*E + 0.5*beta2*E^2 + binary(HJD)
with T2 = 2,442,516.15584
P2 = 0.059,511,264 day
beta2 = -3.38*10^-13

I also modified (by hand) the parameters of the binary function so as to have a good fit. The main difference with LY91 is that the period is 8.382yr instead of 9.262.
All the paramerers are HERE.
The resulting O-C diagram with an average dispersion of 23 seconds:


Red dots: the 90+11 times of maximum from which the ephemeris is derived; Blue dots: from ASAS observations; Green dot: from AAVSO observations; Magenta dot: from ROTSE observations. Cyan line: the orbital binary function.

Discussion

The pulsations of KZ Hya appear to be very constant (the beta2 term is very small).

My observations support the duplicity model of KZ Hya. Assuming the total mass of the 2 stars is 2 solar masses, the diameter of the orbit is 5.02au and the inclination is 17°.

The angular separation is certainly much smaller than 0.01", much too small to be observed. The companion may show up spectroscopicaly, however this was not noticed by McNamara & Budge (1985).

References

Doncel F., Troche A., Noguchi T. (2004) UNPSA 15 29 (available from NASA ADS); Ap & Sp Sc 290 399 (I have no access to it).

Hobart M.A, Peniche R., Peña J.H. (1985) Rev. Mexicana Astron. Astrof. 11 19.

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

McNamara D.H., Budge K.G. (1985) PASP 97 322.

Pojmanski G. (2002) Acta Astronomica 52 397.

Przybylski A., Bessel M.S. (1979) MNRAS 189 377.

Acknowledgement

The variable star observations from the AAVSO International Database contributed by observers worldwide are acknowledged.

Technical notes

Telescope and camera configuration.

Computer and software configuration.