Non-Periodic Phenomena in Variable Stars
IAU Colloquium, Budapest, 1968
THE THIRD CATALOGUE OF VARIABLE STARS IN GLOBULAR
CLUSTERS
HELEN SAWYER HOGG
David Dunlap Observatory, Ontario, Canada
A subtitle of this paper might be "Some Notes on the Observational
Program on these stars at the David Dunlap Observatory." These two
topics are so interwoven that it is hard to separate one from the other.
The observational program has been running since the 74-inch telescope
went into operation in 1935, and the first catalogue was published in 1939.
At first glance the variables in globular clusters seem to have little
to do with the topic of this colloquium, Non-Periodic Variables, for
most of the variables in globular clusters are definitely periodic.
Nevertheless there are a few which are not, two or three U Geminorum
stars which might be cluster members, and of particular interest three
novae, two of which almost certainly are members. The first was Nova T
Sco, discovered visually by Pogson in M 80 in 1860. It changed the whole
appearance of the cluster, as it was 7th magnitude at maximum. Then in
1949 Mrs. Margaret Mayall discovered the spectrum of a nova near NGC
6553, but there is doubt as to its cluster membership. The third was
discovered five years ago by Dr. Amelia Wehlau in M 14 on plates I had
taken with the 74-inch David Dunlap reflector in 1938. This nova hovered
near mag. 16.0 on eight plates taken during one week in June, 1938, but
it may very well have been several magnitudes brighter earlier. As this
nova has already been reported at a former IAU meeting, it will not be
discussed further now.
However, the discovery of this nova prompted us to start a systematic
search by blink microscope of hundreds of globular cluster plates in our
collection. Before the discovery of the nova our philosophy had been that once a
cluster has been thoroughly blinked for variables, with plates over a considerable
interval of time, there was nothing further to be gained by searching.
Now it seems important to try to ascertain the frequency with which novae
do occur in globular clusters. To do this, a careful search (probably blink
microscope is the easiest) is necessary on thousands of plates. With the help
of assistants we have now spent some hundreds of hours blinking hundreds
of plates. So far the search has yielded no more novae, but a few new variables
have resulted from it. We hope to continue with the checking of all the
globular cluster plates at the David Dunlap Observatory, bearing in mind that
the nova we did discover was not of spectacular appearance, - a 16th magnitude
star in the central region of a globular cluster, no brighter than some of
the variables. To get a statistical frequency for novae in globular clusters, it
will be necessary to search thousands of plates over long periods of time.
In the 33 years since the program with the 74-inch reflector began, we
have missed only 4 cluster observing seasons. We have plates on about 50
globular clusters, but by no means have all of these been observed every
season. In addition a 19-inch telescope has been used, and four
additional seasons had been obtained earlier, from 1931 through 1934,
with the 72-inch telescope of the Dominion Astrophysical Observatory,
Victoria, B.C. We have about 4800 globular cluster plates at the David
Dunlap Observatory. An instrumental development this year may be of
interest. On our new building on the University of Toronto campus in the
heart of the city, on the 16th floor a new 16-inch Boller and Chivens
reflector is in operation. We are taking plates on globular clusters at
the Cassegrain focus with a scale of 0.5' to the millimeter, which
compares quite favorably with the 22" per millimeter scale of the 74-inch.
The 16-inch has an advantage in that it is located only a mile
from the northern shore of Lake Ontario and hence can photograph in the
southern sky looking across 25 miles of dark lake. To see the region
toward the galactic center our 74-inch now has to look across many miles
of brilliant city lights. It is hoped to continue these programs with
both instruments for many years to come.
Table I
Variable Stars in Globular Clusters
(September 1, 1968)
NGC No. Vars. No. Periods No. RR Lyr No. Others All Types
104 11 5 2 3+6a M,I, RR
288 1 1 0 1 SR
362 14 10 7 3 RR, C, M
1261 6 0 -
1851 9+1s 0 -
1904 5+2s 3 3 1 RR, SR
2298 0+6u 0 -
2419 36 2 2+28a 0+5a RR, I
2808 0+4u 0 -
Pal 3 1 0 1 0 RR
3201 77+5u 59 58 1+1a RR, E, M
Pal 4 2 2 0 2 M
4147 16 15 15 0 RR
4372 0 O
4590 38 36 35 1 RR, M (field)
4833 10 9 6 3 RR, M (field)
5024 44 33 33 0+1a RR, C
5053 10 10 10 0 RR
5139 171+2s+4f 154 138 16+5a RR, C, RV, M, I, SR
5272 189 176 173 3 RR, SR, C, EW
5286 7 0 -
5466 22+22s 18 18 0 RR
5634 7 1 1+6a RR
5694 0 0 O
Pal 5 5 5 5 RR
IC4499 6 0 -
5824 27 9 9+16u RR
5897 4 0 0+4a RR
5904 97 93+1a 90+2a 3+1a RR, C, SR, UG
5927 2+1s+12f 0 -
5986 5 0 3 1 RR
6093 8 3+1a 0 4 C, N, M (field)
6121 43 41 41 1 RR, SR
6144 1 0 -
6171 24+16f 18 18 0+1a RR, M
6205 10+2s 6 2 4+2a RR, C, I
6218 1 1 0 1 C
6229 22 16 15 1 RR, C
6235 2 0 -
6254 3 2 0 2 C, SR
6266 83+6u 74+4u 74 1 RR
6273 4 0 -
6284 6 0 -
6287 3 0 -
6293 5 0 -
6304 7+4f 0 3a RR
6333 13 11 11 RR
6341 15 13 12 1 RR, EW (field)
6352 3+9f 0 -
6356 5 0 -
HP1 4 0 -
6362 32 0 -
6366 2 0+1u 1u RR
6397 3 3 1 2 RR, SR, M (field)
6402 77 40 34 6 RR, C, M, N
6426 12 10 10 RR
6522 9 8 8 1 RR, I (field)
6528 0 0 -
6535 1 0 -
6539 0+1u 0 -
6541 1 0 0 0+1a M
6553 6 3+1a 3 0+3a RR, M, N
6558 9+14f 0 4a RR
11276 5 1 1 4 RR, SR, M
6569 5+3f 0 -
6584 0 0 O
6624 3+10f 0 -
6626 17+1u 2+3u 3u 3 RR, C, RV, UG
6637 5+5f 0 1a 1 RR
6656 27+2u+1f 22+4u 18 9 RR, M, SR
6681 12+4f 0 2a RR
6712 19+1s 15 9 6+3a RR, SR, M, E
(field) UG
6715 80 37 34 3+2a RR, C, SR, E
(field)
6723 19+5u 19 24 0 RR
6752 1 0 -
6760 4 0 -
6779 12 4 2 4 C, SR, RV, RR
(field)
6809 6 5 5 RR
6838 4 0+1u 0 2 E, SR
6864 11 0 -
6934 51 0+44u 0+44u RR
6981 39 28 28 RR
7006 72+3s 27 26+20a 2 RR, SR
7078 102+1f 77 74 3 RR, C
7089 21 21 17 4 RR, C, RV
7099 4 3 3 1 RR, UG
Pal 12 3 0 2a 1 RR
Pal 13 4 4 4 0 RR
7492 2 2 2 0 RR
Notes: a = assumed; f = field; u = unpublished.
Of the variables in the third catalogue, there is no doubt that most of them
are the RR Lyrae types recognized in these clusters by Professor S. I. Bailey
of Harvard almost 70 years ago. However, there are about 30 Type II Cepheids,
and a few long period stars, but no actual members so far with period greater
than 220^d, as pointed out by Dr. M. W. Feast who has made a major contribution
in indentifying with his radial velocity work some of these stars as actual
cluster members, such as CH Scuti in NGC 6712.
Since Bailey's time, however, and particularly in recent years, we have
gained a rather different impression of the RR Lyrae stars in respect to
their period changes. As the interval of time spanned by plates of
different observatories increases, more and more period changes are
being detected, and their importance in the scheme of stellar evolution
is recognized. Period changes are becoming one of the most fascinating
aspects of variables in clusters. They also make the publication of the
catalogue more complicated as to choice of period and epoch for listing
because not all the values can be published in such a catalogue.
The third catalogue of variables in globular clusters has been in
preparation for some time and it is expected to deliver the manuscript
to the printer this winter. It will be similar in format to the second
catalogue of 1955 and will be a David Dunlap Observatory publication. It
is no coincidence that a substantial proportion of the new information
has been contributed by astronomers who are in this gathering here
today. Strongholds of research in this field have been at the Konkoly
Observatory of our host institution here under Professor Detre, with the
work of Dr. J. Balazs-Detre, Dr. Szeidl, Mr. Lovas, and Mrs. Barlai; and
at Asiago under Professor Rosino, whose contribution is prodigious, with
Dr. Margoni and Dr. Mammano, and Dr. Christine Coutts who is a link
between Asiago and David Dunlap. I wish to take this opportunity of
thanking publicly those workers in this field who are present here today
for the wonderful cooperation they have given, me in supplying me with
their important material, both published and unpublished.
In all, 89 clusters have now been searched for variables, with a total of
1754 variables published, and 61 unpublished or suspected. Of these clusters,
4 have no variables, and in 3 others the variables are unpublished. In 49
clusters 1155 periods have been published, and 60 additional periods remain
unpublished in 3 more clusters. Of these periods, 1086 are of RR Lyrae stars in
47 clusters, and there are 132 more such periods unpublished or assumed for a
total of 53 clusters. In 36 clusters there are 105 other types of variables
with periods determined, and 31 more periods assumed for a total of 41 clusters.
A problem of increasing importance is to distinguish between cluster members
and field stars, and it is not yet possible to treat this problem in a uniform
way from cluster to cluster. Table I gives a brief summary of present data.
References which form the basis of this table will be published in the catalogue.
DISCUSSION
Bakos: Are you considering a patrol of globular clusters for novae?
Sawyer Hogg: Not really. Fairly large scale plates would be necessary
for an efficient survey. We are trying to get series of plates on
different clusters, which we will examine.
Herbig: Is there any hope of detecting the remnant of T Sco in M 80?
Sawyer Hogg: I think that years ago some of the Mt. Wilson astronomers,
perhaps Dr. Baade, looked into it, and thought not. It would be very
difficult now to identify the nova in the center of this compact cluster.
Feast: It seems to me extremely important to try to determine periods or
quasi-periods for semiregular red variables in globular clusters. In 47
Tuc beside the 200 day Mira stars there are sveral semiregular stars for
which Arp derived quasi-periods of the order of 50 to 150 days. It would
be valuable to have more data on these stars as well as similar stars in
the clusters. This should help in understanding the evolution of red
variables.
Sawyer Hogg: I certainly agree with this. There are some variables of this
type which we are trying to follow.