COMMISSION 27 OF THE I.A.U.
INFORMATION BULLETIN ON VARIABLE STARS
Number 1294
Konkoly Observatory
Budapest
1977 June 20
SEVEN COLOUR PHOTOMETRY OF THE DELTA SCUTI STAR 27 Vir
27 Vir (HR 4824, A5, V=6.30) was observed during 5 hours
in 1975 by Bartolini et al. (1975). They suggested that the star
is a Delta Scuti variable with a period of 0.05d.
We planned to monitor this star during a two week observing
run in the second part of February 1977, but due to bad
weather good measurements could only be obtained during the
nights of February 26/27 and 27/28. The star was observed differentially
using the Geneva U B V B_1 B_2 V_1 G photometer attached to
the 1-meter telescope of Observatoire de Lyon at Gornergrat
Observatory. Comparison stars were HR 4861 (A0V, V=6.4) and HR 4855
(A3, V=6.05).
During the night of February 26 ten measurements in the sense
first comparison star, program star, second comparison star were
made in each colour. From the magnitude differences between
HR 4861 and HR 4855 we infer that the mean error on one measurement
amounts to 0.005m magnitudes in all colours.
The measurements were made during a time span of almost 6 hours,
which, according to the period of Bartolini et al. covers five
periods. Using the method of least squares we approximated the
magnitude differences 27 Vir - HR 4861 by a trigonometric polynomial
with unknown frequencies and coefficients. The mean value
of the location of the well-defined maxima in the frequency
spectrum in every colour yielded a period P= 0.042d +- 0.0015d with
regression coefficients lying between 0.7 and 0.9. Figure 1
shows the resulting phase-diagrams for P=0.042d.
During the following night (Feb. 27/28) 11 measurements in the
B2 filter were obtained during a time span of 1h30m. The mean
error on one measurement as derived from the differences between
the comparison stars is 0.004m. The resulting light curve is
shown in Figure 2.
It is evident that a period of 0.042d cannot fit the observations.
A least squares fit yields an optimal representation
by a sine curve for P=0.056d. The fitted curve with amplitude
(half-range) of 0.013m is shown in Figure 2.
These two discrete P-values, and also the shape of the lightcurves
obtained by Bartolini et al. indicate that this Delta
Scuti star too exhibits multiple periodic light variations.
Continuous monitoring is needed in order to extract two or more
periods from the measurements.
Acknowledgement
The author is indebted to the Geneva and Lyon Observatories for
the permission to use the 1-meter telescope and photometric
equipment.
C. STERKEN
Astrophysical Institute
Vrije Universiteit Brussel
Pleinlaan 2
B-1050 Brussels, Belgium
Reference:
Bartolini, C., Piccioni, A., Silveri, P.: I.B.V.S. No. 981, 1975
[FIGURE 1]
Fig. 1 : Phase diagrams constructed with P=0.042d for the
differential measurements 27 Vir - HR 4861 on JD 2443201.
[FIGURE 2]
Fig. 2 : B_2 measurements on JD 2443202 and the fitted sine-curve.