PROBLEMS OF VARIABLE STARS IN GLOBULAR CLUSTERS
by L. ROSINO, Asiago
Variable stars in globular clusters represent a class of objects
which are very important from many points of view. They are indicators
of type, because the presence of even one single variable in a cluster
is sufficient condition to conclude that the cluster, independently of
its appearance, belongs to the globular type. They give a good
indication of the distance of the system to which they belong. Moreover,
the study of the color of the variable stars in a cluster brings the
possibility, if there is any interstellar absorption, of deriving the
color excess of the system; while the abundance of RR Lyrae-type
variables, which are concentrated in the horizontal branch of the H-R
diagram, offers an idea of the aspect of such a diagram without the
necessity of measuring the colors of individual stars. This is not all.
Evidently the frequency of variables is an isolated system and the
distribution of types and periods, must be dependent in some way from
the initial constitution of the cluster and from the actual degree of
evolution. Hence, by collecting data on the variables of globular
clusters, we may find finally some signs of the evolutional path of such
systems. Particularly sensitive to small changes of physical conditions,
due to evolution, are probably the RR Lyrae variables, the periods of
which can be derived with the greatest precision. Secular variations of
these periods should therefore be determined with the highest accuracy.
Finally, the variable stars of the globular clusters can give an indication
of the extension of the clusters or oŁ the degree of dissociation. Sometimes
variable stars are found rather far from the centre of the cluster suggesting
that either the limits of these systems are greater than commonly
accepted, or some stars are actually escaping from the cluster itself.
With the preceding points in mind, we may look now to the work that
has been done on the variables of globular clusters and to the future workers.
In my opinion a plan for the complete survey of the variable stars on the
globular clusters of our Galaxy should attain the following objectives, in order
of increasing difficulties: 1. Search of variable stars in unexamined clusters.
2. Determination of mean magnitudes and types. 3. Accurate study of the
periods. 4. Determination of light-curves in two colors and position of the
variables of each cluster on the H-R diagram. I would like to illustrate
now this plan of work, with particular consideration for the researches that
we are doing at Asiago with the 120 cm telescope.
1. Search of variable stars
The Second Catalogue of variable stars in globular clusters of Helen
Sawyer (1955) gives an extensive data on the variable stars of 72
clusters for which there is a record of search. Although nearly fifteen
of these clusters seem not to have been accurately searched, further
work will not probably bring to substantial differences. In the same
Catalogue we find that 34 globular clusters have never been observed for
variables. 25 of these are south of declination -25 deg and therefore
very difficult to observe with large instruments at our latitudes. Nine
are north of -25 deg and will be gradually included in the program of
Asiago. I would like to add that recently I have been in South Africa,
at the Radcliffe Observatory of Pretoria, with the specific purpose of
observing, with the 74-inch telescope, some of the southern globular
clusters. The following objects were included in my program: NGC 5824,
6304, 6569 and 6637, never searched before for variables; NGC 5986,
6558, 6715 (M 54) and 6864 (M 75), in which some variables were already
known. The research has been fruitful. By a first blinking of 150
plates obtained in June and July at Pretoria, eighteen new variable
stars have been found in NGC 5824 and 42 in NGC 6715; from 1 to 5
variables have been identified in each of the other clusters. This is
sufficient to demonstrate that there is still much to do in the globular
clusters of the southern hemisphere. It would be highly desirable that
the researches on globular clusters south of -25 deg would be soon
completed.
In addition to the clusters of our Galaxy recognized as globular,
there are clusters of uncertain type which may be galactic (population I)
as well as globular (population II). The presence of variables in these,
clusters is sufficient to decide that they are globular independently from
their appearance.
Variable stars have never been found, in fact, in galactic clusters
of population I. I would like to present here the case of two clusters
of this kind which I have recently studied: the anonimous cluster N 5 of
the list of Abell (PASP 67, 259, 1955) and NGC 6558. The first, although
very open and loose, was suspected by Baade, since 1950, to be a cluster
of faint stars of population II, because of its high galactic latitude
(+44 deg); the discovery of eight RR Lyrae variable stars on Loiano
plates was decisive and removed any residual doubt on the true nature of
the system. The second, namely NGC 6558, is a poor object with moderate
concentration in an exceedingly rich field of the Milky Way. It was
classified by Shapley and Collinder as galactic. In 1950, by blinking
some plates taken with the 82-inch reflector of the McDonald
Observatory, I found, near the centre of the cluster, two variables,
probably RR Lyrae. However, the question of the classification of the
cluster was still doubtful, because the variables could belong to the
rich galactic field. But on the plates which I took at the Radcliffe
Observatory, four more variables were found. The cluster is therefore of
the globular type.
It is possible that many other clusters, now considered as galactic
(population I) are really globular (population II). For instance, in the
Collinder Catalogue of Galactic Clusters there are at least ten objects
which look suspect. Also the 13 new globular clusters found by Abell
during the Palomar Survey and listed in the Publications of the Pacific
(op. cit.) should be accurately studied. They are extremely faint
objects which look rather peculiar. Two of them (No. 4 and No. 13) are
now under observation at Asiago, and appear to be at the limit of the
possibilities of the telescope of 120 cm. In each of them variable stars
have been discovered on our plates. (2 variables with period of nearly
100 days in the Abell cluster No. 4; and 4 RR Lyrae variables in the
Abell cluster No. 13)
2. Determination of mean magnitudes and types
The next step, after the search of variable stars in globular clusters,
is the determination of mean magnitudes and types. RR Lyrae variables
are easily identified by the rapidity of the light-variation. Cepheids with
periods of 1 to 3 days, W Virginis variables with periods of 15-18 days,
RV Tauri, semiregulars of 100 days, long period variables and bright irregulars
are also easily found, when they are present, provided the observer has
sufficient material. All these variables are representative of population II.
Novae are not frequent; in fact only two cases have been recorded: in M 80
and in NGC 6553, but the last is doubtful; on the other hand a control is
extremely difficult and a sudden apparition of a nova in a cluster can easily
escape the discovery. Two U Geminorum stars have been found in globular
clusters: one by Oosterhoff in M5 and one by the writer in M30. Both are
rather distant from the centre and may eventually be field stars. It would be
interesting to control for the presence of faint stars of the U Gem-type some
of the nearest globular clusters using the largest instruments.
The determination of median magnitudes for the RR Lyrae and Cepheids
in globular clusters is always an important point. By the photoelectric
work of the Mount Wilson and Palomar observers we dispose now of good
photographic and photovisual sequences down to the faintest stars, in SA
and in globular clusters. However, the problem of the transfer of comparison
sequences to faint and condensed clusters and the determination of median
magnitudes for variables is not simple, from the observational point of view.
Of the 72 clusters of the Sawyer's Second Catalogue only 52 contain data on
the median magnitudes of the variable stars. Some of these data should be
revised. So, there is still much work to do for a good photometric calibration
of individual globular clusters. When the median magnitude of the variables
is known we may try to derive (in absence of local absorption) the distance
of the cluster. However, the recent works of Arp, Baum and Sandage rise some
doubts on the existence of an unique period-luminosity relation in different
systems. The situation from this point of view is not so optimistic as
it was some years ago.
3. Periods
I intend to speak here in particular of the RR Lyrae variables. The
derivation of the periods of these stars should be done with the following
two problems in mind:
a) Frequency distribution oŁ periods
b) Secular variation of periods
Problem a), in principle, is not a difficult problem. Fifty or sixty plates,
well distributed over an interval of two-three months should be sufficient to
give periods approximate to the minute. However, what is really important
is to evitate spurious periods. And this can be obtained only if the observer
disposes of long night-runs of observations. When this is not possible, because
of the season or the position of the cluster, a careful discussion of the
observations should be made. There are several methods for finding out spurious
periods; nevertheless when observations have been done exclusively near
the meridian the possibility of errors is always to keep present. I would like
to emphasize to this regard the utility of international collaboration. Two or
three observers, working together at different geographic longitudes, may
be able to decide rapidly all the dubious cases.
The problem b), secular variation of the period, implies the determinations
of periods at least to the sixth decimal of day. This is only possible
when the observer has a series of many plates distributed on several years.
Many of the already known periods need badly a revision; moreover, there
are more than five hundred RR Lyrae variables, the periods of which have
never been determined.
In conclusion, there is very much to do on the periods of variable stars
in globular clusters. What is important, for future work, is to give not simply
periods, but good periods, with the greatest possible precision. Collaboration
in this field would be highly desirable.
4. Two colors light-curves of variable stars in globular clusters
The next step, in the study of variable stars in globular clusters, is the
derivation of good light-curves in one or two colors. The study of the
lightcurves is important from two points of view: a) To verify whether the
lightcurves are correlated with other physical characters and what characters.
b) To derive the position of the variable stars in the H-R diagram and to
find the "region of instability" of the RR Lyrae variables in the horizontal
branch.
About point a) it is well known that such correlation exists, for instance,
between the light-curve and the periods. Variable stars of different periods
have different light curves. However, the careful analysis of the light-curve
has been made only for some of the richest clusters (M 5, omega Centauri, M 4).
It should be extended to the other clusters and a comparison between them
should be made. Light-curves sometimes change from cycle to cycle, particularly
when the star is an RR Lyrae-c. Although it is obviously rather difficult to
observe photoelectrically the variable stars of globular cluster, it would be
very interesting to follow these changes by photoelectric means to see whether
they are periodical or not.
Light-curves of variable stars with period greater than one day have
been recently studied by Arp, with conclusion of great importance. A sudden
flare was found, for instance, in the light curve of the variable No. 2 of M 13
(which has the unusual period of five days). Some of the sixteen-days Cepheids
were found to exhibit alternative periods. A correlation between the form
of the light-curves and the period-luminosity relation was also suspected.
This field is very promising for future researches.
The position of the variable stars of a certain cluster on the H-R
diagram can be obtained by means of two colors light-curves. Observations
of this kind are rather difficult, because of the field errors and also because
it is necessary to obtain, in the determination of photographic and
photovisual magnitudes, a high degree of precision. Very important results have
been reached recently by Roberts and Sandage on the RR Lyrae variables
of M 3. The region of instability in the horizontal branch is that of the stars
which have a color-index lying between -0.005 and +0.235 and an apparent
visual magnitude lying between 15.54 and 15.70. Amplitude and periods
were found to be dependent from the colors. These researches should be now
extended to other globular clusters. At Asiago we have started a program
of blue and yellow observations of the variables of the globular cluster M 15.
I hope to give the first conclusion in the next winter. The experience in M 15
will show whether to extend the researches with the same instruments to
M 5, or not.