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Volume II Number 19

A STUDY OF THE VARIABLE STARS IN THE GLOBULAR CLUSTER MESSIER 14

II. PERIODS AND LIGHT CURVES OF THE SECOND GROUP OF TWENTY VARIABLES

HELEN SAWYER HOGG AND AMELIA WEHLAU

1968 TORONTO, CANADA PRINTED AT THE UNIVERSITY OF TORONTO PRESS p

A STUDY OF THE VARIABLE STARS IN THE GLOBULAR CLUSTER MESSIER 14

II. Periods and Light Curves of the Second Group of Twenty Variables

By Helen Sawyer Hogg and Amelia Wehlau

The data forming the basis of this program, and its purpose, have already been described in the Publications of the David Dun/a Observatory, vol. II, no. 17, 1966. In the present paper we give the observations for the second group of twenty variables in Messier 14, NGC 6402, with their mean light curves. A summary of this material was presented to the American Astronomical Society (Sawyer Hogg and Wehlau, 1965). The cluster is now shown to contain four long- period Cepheids, which means that it is exceeded in the number of Cepheids, among globular clusters of our own Galaxy, only by Omega

Centauri. Therefore we describe the period-luminosity relation it defines.

This second group consists of the following variables: numbers 3, 8, 12, 13, 14, 17, 18, 20, 27, 31, 34, 37, 51, 59, 61, 62, 68, 71, 75 and 76.

Table I gives, for each of these variables, the maximum, minimum and mean magnitudes, the epoch of maximum (usually the nearest maximum just before J.D. 2438200 in the 1963 series of observations), and the period, followed by the value of Beta, and remarks, when pertinent. For several stars for which our observations do not completely rule out an alternate period, the date of maximum is that actually observed. Of this group of 20 variables, only three are not RR Lyrae stars.

One of these is a Cepheid, Variable 7(5, whose discovery was reported in our first paper. Its period now is calculated as 1.89003 days, and it raises to four the number of Cepheids in the cluster with periods determined. The other two non-RR Lyrae stars, Variable 17 and Variable 27 are probably not members of the cluster. Variable 17 has a period of 12.097 days with an amplitude of only 0.65 magnitude, a sine-type light curve, and the largest period change so tar found among our

7 variables. Its value of Beta is +35 X 10 day day. This variable is about two magnitudes brighter than the RR Lyrae stars. Its spectrum was investigated by Joy (1949) and found to vary between F8 and G2, and his measured radial velocities were +9 and —IS km sec.

49:: 494 Helen Sawyer Hogg and Amelia Wehlau

TABLE I Elements of Twenty Variables Variable Stars in Globular Cluster Messier 14 495 are few and poor, he questioned the cluster membership of the variable both because of the poor velocity agreement, and the distance of the star from the cluster centre. Further data are needed to assign a definite type of variability for this star. Variable 27 has the second greatest distance from the cluster centre of all the 76 variables. It is a long-period variable, at maximum about one magnitude brighter than the RR Lyrae stars at maximum. A period of 167.0 days seems to represent our observations, but since the distribution of the plates is not favourable for the determination of such long periods, we cannot yet rule out a period of 308.3 days. Seventeen of the variables are RR Lyrae type, with periods between

0.263721 and 0.686071 days. Four of these stars, Variables 3, 13, 18 and 34, show period changes. Of these changes, two are positive and two negative, with the values of Beta falling between -f-41 and —23. None of the RR Lyrae stars reported in our first paper showed period changes. For several of the RR Lyrae stars it proved difficult to decide among several related periods, as indicated in the table. In the case of Variable 62, we are greatly indebted to Dr. R. Margoni who generously supplied us with his unpublished measures from his

Asiago plates to help in the period selection. Although it is tempting to draw a frequency distribution for the 34 RR Lyrae periods so far determined, this would not be meaningful until a higher proportion of periods have been found for the variable stars in this cluster.

TABLE II Cepheid Variable Stars ;

496 Helen Sawyer Hogg and Amelia Wehlau

Fig. 1 —The period-luminosity relationship as defined by variable stars in Messier 14. Ordinate, mean apparent photographic magnitude; abscissa, logarithm of the period. The large filled circles represent the four recognized long-period Cepheids the small dots are the 31 RR Lyrae ab stars whose periods have thus far been determined.

Cepheid, it is not included. With a mean of the 31 RR Lyrae stars at

17.74 m pg and log P = —0.25, the points indicate a slope of about — 1.6, which compares with —1.74 obtained by Dickens and Carey (1967) in their paper discussing globular-cluster Cepheids. Some of the scatter in the magnitudes of the RR Lyrae stars is certainly real;

part of it may be due to the effect of image crowding or to inclusion of field stars. Until the period determination of the variables in this

cluster is more nearly complete, it cannot be known if more Cepheids

are contained in it. this cluster, If we assume Mvg = 0.6 for the RR Lyrae stars in from the average mean magnitude of the 31 RR Lyrae ab stars, the apparent

photographic distance modulus is 17.1. Kron and Alayall (1960) give a visual absorption of 1.5 (Case I) or 1.8 (Case II). Correcting the Variable Stars in Globular Cluster Messier 1/+ 497

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Fig. 2-—The mean light curves for 10 variables, arranged in decreasing length of period. photographic modulus by —0.6 to a visual modulus oi 16.5 and accept- ing Case II, we obtain a corrected distance modulus oi 14.7 correspond- ing to a distance of 8.5 kpc. The light curves of these twenty variables, in order oi decreasing length of period, are shown in figures 2 and 3. The points are the com- puted weighted means of all observations at intervals in phase of 0.04 of the period of the star. Observations with colons in Tabic III have 49S Helen Sawyer Hogg and Amelia Wehlau

5 5 5

Fig. 3—The mean light curves of 10 variables, arranged in decreasing length of period.

been assigned half weight. A filled circle represents at least three good observations, but averages about ten such. Open circles represent mean points derived from observations which are few in number or of low weight. The observations for these twenty variables are contained in Table III, which gives the number of the plate, the heliocentric Julian Day, X

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in the second group are substantially fainter than any in Paper I. This means that measures for them on many of the early plates, both of the Dominion Astrophysical Observatory and of the David Dunlap Observatory until the mirror was aluminized before the 1942 season, give uncertain results because the variable is close to the limiting magnitude of the plate. We found that in such cases the eye estimates made by one of us (H.S.H.) were as reliable as the photometer measures made by the other (A.W.) and resulted in less scatter in the light curve. Accordingly, for the two variables for which the effect was most noticeable, Variables 20 and 62, the published measures are usually means of photometer and eye estimates.

It is a pleasure to affirm again our appreciation to the National Research Council of Canada for its continuing generous support of this program, and to express once more our gratitude to Dr. R. Margoni who most kindly sent us his unpublished measures from Asiago plates for one variable.

References

Dickens, R. J. and Carey, J. V. 1967, Roy. Obs. Bull., no. 129, p. E 335. Joy, A. H. 1949, A p. J., vol. 110, p. 105; -1//. !!'. and Pal. Rcpr., no. 5. Kron, G. E. and Mayall, N. U. 1960, A.J., vol. 65, no. 10, p. 581. Mayall, N. U. 1946, Ap. J., vol. 104, p. 290; Lick Cont., Ser. 2, no. 15.

Sawyer Hogg, H. and Wehlau, A. 1965, .4.7., vol. 70, no. 9, p. 678.