VX Hya (2008) free fall?
Observed: 7-13 Feb 2008
20 Feb 2008
Updated 2, 21 March 2008, 13 Feb 2009 (slope of B-V vs V)
Time-series for this pulsating star with multiple oscillations
are obtained. They are compared with the ephemeris of Fitch (1966): they
are too early by 3.7 hours. This is a strong change from previous, recent
VX Hydrae is a pulsating star probably of the delta Scuti type, with multiple
periodicities. I already observed it in 2005
and in 2006 and found it late by
1.4-1.2 hour when compared with the 1954 ephemeris. Here are new observations:
The observations were carried out with a setup similar to the one used
in 2005 and 2006:
203mm SC telescope, Johnson B and V filters in a filter wheel, SBIG
ST7E camera (KAF401E CCD);
346 usable images were obtained with the V filter, 337 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=12.171 V=11.580;
the check star is GSC 5482-01054 with the observed magnitudes V=12.858
(average 1-sigma statistical uncertainty=0.014, standard deviation=0.019)
and B=13.438 (0.031, 0.039).
An example of a light curve:
Green: with the V filter, Blue: with the B filter. The upper curves
are VX Hya, the bottom curves are the check star shifted of -1.2mag in
V, -1.6 in B. The error bars are +/- the 1-sigma statistical uncertainties.
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
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, and of my 2006
observations, with T 1.241 hour late.
I can also fit these 2008 observations, but the T parameter has to be
3.700 hour early and <V(t)>=10.675 mag:
Red: Fitch's function, Green: the V observations.
The B and V magnitudes being measured alternatively with a filter wheel,
and not simultanously, the B magnitudes at the times of the V magnitudes
are computed by interpolation. The origin of the B magnitudes is somewhat
uncertain because the B magnitude of the comparison star is uncertain.
The magnitudes are not transformed, however with my set up the transformation
corrections are very small for B, V.
An example of color light curve:
Blue: the V-B magnitude differences; Green: the V magnitudes shifted by -10.5 magnitudes;
Red: the Fitch ephemeris (3.7h too early), shifted by -10.5 magnitudes.
All the color light curves are HERE.
The diagram for B-V versus V is linear with the star becoming bluer
when it is brighter. (This is unlike the usual loop diagram for pulsating
The slope of B-V vs V is 0.339.
Long term variability
I compare each maxima (the first is actually a minimum) I observed with
the Fitch ephemeris and I obtain a O-C value:
||ToM (HJD -
I compiled a number of times of maxima
from Fitch (1966), Breger (1977), Klingenberg et al (2007), the
AAVSO and my 2005 and 2006
measurements. I then derive the O-C diagrams:
Blue circles: Fitch (1966), Magenta squares: Breger (1977), Green squares:
Klingengerg et al (2006), Brown circles: AAVSO, Red circles: my observations.
So, in 2007-2008 it looks like there is a strong change in the pulsation
periods and/or the phases.
From the extrapolations of the 2003-2007 measurements, one would expect
that O-C being late by ~1h. I tried to fit the data with such a value of T but
that did not give a good match. Also note that the average of the 5.36h and 4.15h is 4.75h,
and that being early by 3.7h is then somewhat equivalent as being 1h late.
Breger M. (1977) PASP 89 55.
Fitch W.S. (1966) ApJ 143 852.
Klingenberg G., Dvorak S.W., Robertson C.W. (2006) IBVS 5701.
The use of the AAVSO International
Database for this amateur research is acknowledged.
My 2005 observations and comparison with
the Fitch's model.
My 2006 observations and other observations
and comparison with the Fitch's model.
Telescope and camera configuration.
Computer and software configuration.