MBCAA Observatory

VX Hya long term variability

Observed: 24, 25, 26, 27 Dec 2006

Michel Bonnardeau
29 Jan 2008
Updated 19 Feb 2008

Abstract

Time-series for this pulsating star with multiple oscillations are obtained. They are compared with the ephemeris of Fitch (1966): there is a delay of 1.2 hour. I also compile various observations and derive an O-C diagram from 1954 to 2007.

Introduction

VX Hydrae is a pulsating star of the delta Scuti type, with multiple periodicities. I already observed it in 2005 and found it late by 1.4 hour when compared with the 1954 ephemeris. Here are new observations:

Observations

The observations were carried out with a setup similar to the one used in 2005:

203mm SC telescope, Johnson B and V filters in a filter wheel, SBIG ST7E camera (KAF401E CCD);

162 usable images were obtained with the V filter, 87 with the B one;

each exposure is 200s long, whatever the filter is;

the comparison star is Tycho 5482-01347. However, Tycho2 does not give reliable B magnitudes for this field. The adopted magnitudes for the comparison are then B=12.172 V=11.580;

the check star is GSC 5482-01054 with the observed magnitudes V=12.850 (average 1-sigma statistical uncertainty=0.013, standard deviation=0.020) and B=13.442 (0.028, 0.038).

An example of a light curve:

Green: with the V filter, Blue: with the B filter.
The upper 2 light curves are for VX Hya; the lower ones are for the check star, shifted by -1.6mag in B, -1.2mag in V.
The error bars are +/- the 1-sigma statistical uncertainties.
The peak in the check star V light curve is because of a satellite.

All the light curves are HERE.

Comparison with the ephemeris

VX Hya was monitored from 1954 to 1965 and a model of the light curve was derived (Fitch (1966)). This model involves 2 oscillations:

n1=4.476495 1/day (period=5.36 h)
n2=5.789730 1/day (period=4.15 h)
T=2,435,000.0 (14 Sept 1954)

and a long-period modulation:

frequency nL=0.000227 1/day (period=12 yr)
TL=2,434,860.0 (27 Apr 1954)

the following way:

with <V(t)>=10.696 mag the average magnitude. The coefficients a,b,c,d for the 1955-1959 period are given below. They do not fit correctly the 1964-1965 period.

Although this model is not supposed to work after 1959, I obtained a good fit of my 2005 observations, provided the T parameter is 1.428 hour late.

The model also fit the 2006 observations, with T 1.241 hour late and
<V(t)>=10.683 mag:

Green: the V measurements, Red: the model.

Long term variability

I compile a number of maxima from the light curves, compare them with the Fitch (1966) model, and I derive a O-C diagram:

In 2005, I observed 3 maxima:

Date ToM (HJD-2,453,000) O-C (mn)

20050313 443.348 94

20050316 446.457 85

20050317 447.335 84

In 2006 I observed 2 maxima:

Date ToM (HJD-2,454,000) O-C (mn)

20061226 96.684 66

20061227 87.563 71

Breger (1977) published the light curves for 2 maxima:

Date ToM O-C (mn)

19750113 2,442,426.4236 55

19750117 2,442,430.4168 49

Klingenberg et al (2006) reported 5 times of maxima for 2003-2005, with no light curves:

Date ToM (HJD - 2452000) O-C (mn)

20030406 735.6067 70

20040112 1016.9122 91

20040418 1113.6364 93

20050407 1467.6958 89

20050416 1476.5976 ?

For the last time of maximum, I cannot find a match with the Fitch's prediction.

From the light curves in the AAVSO International Database, 27 maxima can be identified, from 2003 to 2007:

Date ToM (JD - 2452000) heliocentric
correction (s) ToM (HJD - 2452000) O-C (mn) Observer

20030406 735.602 335.7 735.6059 69 DKS

20030629 819.86 -254.4 819.8571 86 BIW

20040112 1016.91 333 1016.9138 93 DKS

20040216 1051.975 447.6 1051.9802 94 BIW

20040418 1113.635 261.8 1113.6380 95 DKS

20050323 1452.9 396.4 1452.9046 97 BIW

20050407 1467.692 327.8 1467.6958 89 DKS

20060202 1768.767 421.8 1768.7719 73 DKS

20060204 1771.005 427.1 1771.0099 73 BIW

20060207 1773.74 434.7 1773.7450 91 DKS

20060208 1774.803 436.9 1774.8081 72 DKS

20060209 1775.69 438.6 1775.6951 78 DKS

20060210 1776.853 440.6 1776.8581 85 DKS

20060210 1777.044 440.9 1777.0491 75 BIW

20060302 1796.718 447.3 1796.7232 77 DKS

20060303 1797.5935 446.3 1797.5987 72 DKS

20060304 1798.782 444.8 1798.7871 ? DKS

20060305 1799.647 443.7 1799.6521 87 DKS+GHS

20060306 1800.715 442 1800.7201 74 DKS

20060317 1811.915 416.7 1811.9198 78 BIW

20060318 1812.605 413.7 1812.6098 90 DKS

20060319 1813.673 410.5 1813.6777 77 DKS

20060326 1820.592 384.8 1820.5964 76 DKS

20060401 1826.627 358.2 1826.6311 74 DKS

20060425 1850.98 217.5 1850.9825 73 BIW

20070115 2115.918 351 2115.9221 65 DKS

20070228 2159.759 449.2 2159.7642 ? DKS

For the light curves at 20060304 and 20070228, I cannot find a match with the Fitch's prediction.

From the Table 1 of Fitch (1966), 15 maxima can be identified:

Date ToM (HJD - 2,435,000) O-C (mn)

19541228 104.998 -18

19541230 107.0417 -5

19550123 130.928 12

19560303 535.722 -3

19560319 551.7977 0

19560408 571.6719 0

19561228 835.938 0

19570126 864.7578 -10

19571222 1194.975 -3

19571227 1199.8408 -13

19571229 1201.885 -9

19581212 1549.924 -19

19590307 1634.783 -3

19640305 3459.6837 -21

19650507 3887.685 -9

The maxima are detected "by eye" from the Table 1, so the differences with the ephemeris are only approximative.

The resulting O-C diagrams:

Blue circles: Fitch (1966), Magenta squares: Breger (1977), Green squares: Klingengerg et al (2006), Brown circles: AAVSO, Red circles: my observations.