COMMISSION 27 OF THE I. A. U.
INFORMATION BULLETIN ON VARIABLE STARS
Number 1638
Konkoly Observatory
Budapest
1979 July 10
ON THE PERIOD AND VELOCITY CURVE OF THE BETA CEPHEI STAR HR6684
In a recent paper McNamara (1978) derived a velocity curve
for the beta Cep star HR6684 (=HD 163472) that differs considerably
from those obtained by Pike (1374,1978) and Bolton, Percy and
Shemilt (1975, hereafter BPS). McNamara does not consider the
differences in velocity amplitude to be real, though he does
suggest that his observations indicate some asymmetry in the velocity
curve. The purpose of this note is to show that the differences
found by McNamara are due to the method of analysis and the
use of a slightly incorrect period.
Inspection of Figure 1 of McNamara (1978) shows that the
sinusoidal velocity curve has been fit to the data assuming the
descending branch crosses the gamma-velocity at the time of maximum
light predicted by the ephemeris of Morton and Hansen (1974),
Max.light= JD_hel 2441442.048 + 0.1398903d*E.
BPS showed that this is the correct relationship between the light
and velocity curves. However, McNamara's velocities were obtained
almost 58.000 cycles before the epoch of the light ephemeris, so
even a small error in the period could produce a substantial phase
shift between the true and computed ephemerides.
I have fitted sine curves to all three sets of radial velocity
data with the gamma-velocity, amplitude, and time of maximum radial
velocity as free parameters. The results are given in Table I.
Table I
Element McNamara Pike BPS
V_0 (km/s) -16.6(4) 0.2(6) -15.5(3)
K (km/s) 7.2(8) 8.2(11) 8.8(5)
T(JD_hel2400000+) 33332.92116(3) 42225.6705(2) 42229.74375(8)
epsilon (km/s) 2.5 3.8 1.4
O-C (d) -0.08876 -0.008 +0.00844
n (cycles) -57968 +5602 +5631
V_0 is the gamma-velocity, K the velocity semi-amplitude, T the JD_hel
of maximum velocity, epsilon the RMS scatter about the fitted curves,
O-C difference between the observed and predicted time of maximum
velocity according to the ephemeris of Morton and Hansen, and n
is the number of cycles elapsed between the epoch of the ephemeris
and the epoch of the observations. The standard errors of the fitted
quantities in the sense of the uncertainty in the last digit
quoted are given in parentheses following the parameters. I have
used the measures from Pike (1978) rather than the earlier measures
Pike (1974). This accounts for the widely discrepant gamma-velocity
for this data set.
None of the velocity curves show any evidence of asymmetry,
and there is excellent agreement among the velocity semi-amplitudes.
If we ignore the velocity curve of Pike, which was obtained only
4 days before that of BPS and is of much lower accuracy, then the
O-C's indicate that the period given by Morton and Hansen should
be increased by 1.53 x 10^-6 day to 0.13989183d.
C.T. BOLTON
David Dunlap Observatory
P.O. Box 360
Richmond Hill, Ontario
Canada L4C 4Y6
References:
Bolton, C.T., Percy, J.R., and Shemilt, R.E. 1975, Pub.A.S.P.,
87, 595 (BPS) [BIBCODE 1975PASP...87..595B ]
McNamara, D.H. 1978, Pub.A.S.P., 90, 96 [BIBCODE 1978PASP...90...96M ]
Morton, A.E. and Hansen, H.K. 1974, Pub.A.S.P., 86, 943 [BIBCODE 1974PASP...86..943M ]
Pike, C.D. 1974, Pub.A.S.P., 86, 681 [BIBCODE 1974PASP...86..681P ]
Pike, C.D. 1978, Mon.Not. R.A.S., 184, 265 [BIBCODE 1978MNRAS.184..265P ]