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OCR for page 270
Close Binary Stars in Globular Clusters
BRUCE MAROON
University of Washington
ABSTRACT
Although close binary stars are thought theoretically to play a major
role in globular cluster dynamics, virtually no non-degenerate close binaries
are known in clusters. We review He status of observations in this area, and
report on two new programs which are finally yielding candidate systems
suitable for further study. One of the objects, a close eclipsing system in
Cen, is also a blue straggler, thus finally providing firm evidence that
globular cluster blue stragglers really are binary stars.
INTRODUCTION
A growing body of theoretical work indicates that a small number of
close binary stars dominate the dynamic evolution of globular star clusters
fag, Elson et at 1987~. For convenience, we here define a "close" binary
as one with an orbital penod of a few days or less. Such systems may
have binary orbital velocities of several hundred lon s~i, compared with a
typical single star caster orbital velocity of just a few In sol. Therefore,
literally just a handful of close binaries in a globular cluster can store as
much kinetic energy of motion as possessed by the total of all 105 single
stars in the cluster! This energy may be liberated by three-body encounters
of cluster single stars with the binaries, and this source term is likely to be
the dominant counterbalance of ldnetic energy sinks of stars escaping from
the cluster. Cluster binaries may be pnmordial, or formed later through a
variety of encounter processes, especially during the collapse of the cluster
core due to the so called "gravothermal catastrophe".
270
OCR for page 271
HIGH-ENERGY ASTROPHYSICS
271
Elson et al. (1987) have pouted out the analogy of this energy source
to thermonuclear fusion in stars: in this case, the cluster collapses, forms
binaries, and then "burns" the orbital kinetic energy via encounters. A
problem that observers may have with this elegant theoretical scenario is
that virtually no close binary stars are known in globular clusters! Only
one spectroscopic or eclipsing close binary, an object reported later in this
paper, is known in any globular cluster. A small fraction of globular clusters
are known to contain neutron star binary systems, evidencing themselves as
either the X-ray bursters near the cluster centers, or as millisecond radio
pulsars (although note that several of the latter systems are known to have
binary periods of months rather than days, and thus store negligible kinetic
energy). As only the very smallest fraction of globular cluster stars end
their lives as neutron stars, however, one might expect binaries containing
white dwarfs to be far more abundant in clusters (Verbunt and Meylan
1988), and systems with giant, subgiant, and main sequence components
yet more common.
SEARCHES FOR EVOLVED GLOBULAR CLUSTER BINARIES
White dwarf close binaries in globular clusters are probably most easily
found if they are undergoing mass exchange. The analog of such a system
in the field, outside of clusters, is the cataclysmic variable (CV). Of course,
both novae and dwarf novae easily draw attention to themselves during
their intense optical outbursts, but even in quiescence such systems can
be located through a variety of techniques. The optical spectra of CVs
in quiescence are highly distinctive (strong, broad emission lines), as are
the colors (distinct ultraviolet excess). A decade of study has also revealed
quiescent CVs to be prominent soft X-ray sources, with luminosities in the
range 103° through 1032 erg s~: (C6rdova and Mason 1983, 1984~.
Novae are relatively rare in the field, and so one might expect only a
very small number in globular clusters. Indeed, only two such objects have
ever been reported: one in NGC 6093 in the year 1860 (Pogson 1860; Luther
1860), and one in 1938 in M 14 (Hog" and Wehlau 1964; Shara et al. 1986),
and cluster membership for the latter, although probable, is still uncertain.
However, dwarf novae are more common in the field (Patterson 1984), and
one might hope that the analog of these mass exchange red dwarf~white
dwarf systems might be found in globular clusters. In fact, only two dwarf
novae are known in globular clusters. These stars, both discovered long
ago during outbursts, have been recently recovered in quiescence: V101
in M5 (Margon et al. 1981), and V4 in M30 (Margon and Downes 1983~.
These two objects are poorly studied to date; although at least V101 is
known to stat be undergoing outbursts (Share et al. 1987), neither has ever
been caught in outburst by a spectroscopist, and neither has been observed
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AMERICAN AND SOVIET PERSPECTIVES
for X-ray emission. Thus definite examples of white dwarf close binaries in
globular clusters remain rare and poorly understood.
The suggestion by Hertz and Grindlay (1983a, b) that the low-
luminosibr X-ray sources observed outside of the cores of several globular
clusters by the Einstein Observatory are mass-exchange close binary systems
containing a white dwarf, analogous to the field cataclysmic variables (CVs),
is therefore of great interest. These authors derive a luminosity function
which implies that the galactic globular cluster system contains a total of
~103 such objects, a conclusion also reached by Droll (1984) and Hertz
and Wood (1985~. EXOSAT observations have confirmed a few of these
sources (Verbunt et al. 1986; Koch-Miramond and Auriere 1987; Auriere
et al. 1989~.
It is sometimes not amply enough stressed that all of the evidence
which identifies these systems as white dwarf close binaries is quite indirect,
based primarily on the fact that these sources have considerably lower X-
ray luminosities than the highly luminous sources located within a few
arcseconds of the cluster cores; the latter are now widely agreed to be
neutron star binary systems (Lewin 1980; Lewin and Joss 1983; Grindlay
et al. 1984~. In particular, in spite of several sensitive attempts, there has
never been an optical identification of a single one of these low-luminosity
cluster X-ray sources.
Is there reason to suspect this evidence, and/or do reasonably plausible
competing models exist? Verb unt et al. (1984) have pointed out the curious
fact that all of these "low luminosity" sources, If in fact cluster members,
have X-ray luminosities an order of magnitude larger than the classical
U Gem systems in the held, despite their supposed physical similarity to
those systems. Furthermore, if these systems share the most basic properly
of the known dwarf novae (be., optical outbursts) it seems surprising that
there could be a population of 103 of them in the Galaxy, and not one
has ever been reported in optical outburst, despite a century of intensive
photographic study of globular clusters for variable stars. Crowding is not
likely to totally suppress discovery of the outbursts, if the sources reported
by Hertz and Grindlay (1983b) are indicative of the class; the majority of
them are many core radii from the cluster centers, and inspection of catalogs
of cluster variable stars All, Hogg 1973) show that there are numerous
single variables cataloged equally close to the core, at magnitudes equivalent
to that that would be reached by a globular cluster CV in outburst. The
two known cluster CVs cited above were in fact initially found exactly this
way, despite their location in relatively crowded regions of the cluster.
Verbunt et al. (1984) have suggested that the sources may instead
be quiescent X-ray transients. Some of these sources may have an even
simpler explanation: Hertz and Gnndlay (~1983a) calculate that ~3.3 of
the 8 sources they discuss may be background QSOlAGN superpositions
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HIGH-ENERGY ~TROP~ICS
273
or foreground stellar coronal sources. Furthermore, three clusters are
reported to have spatially extended X-ray emission as well (Hartwick et
al. 1982), raising the possibility that some of the reported point sources
are confused or nonexistent. Margon and Bolte (1987) showed that if the
X-ray sources reported by Hertz and Grindlay (1983a, by in the cluster
Centaun were indeed like the field dwarf novae, they might be optically
identified due to their peculiar colors; yet these workers were unable to
locate any such counterparts (see also Shara et al. l988~. We do know that
the cluster CV M5 V101, with (U-B) =-0.9 in quiescence,: does show
the distinctive color of the field CVs, so this negative result is significant.
Thus, the nature of the low-luminosity cluster X-ray sources remains terribly
uncertain, and it is far from obvious that all, or even many, are truly close
binary stars.
Cluster binaries with red giant components at least have the virtue of
being bright enough for easy spectroscopic observation. Gunn and Griffin
(l979), working at a radial velocity precision of 1 An s~i, failed to find
even a single binary candidate amongst extensive observations of 111 giants
in M 3 spanning five years. However, a few successes are finally emerging.
Pryor et al. (1988a) have shown that vZ 164 in M 3 is probably a binary with
a period of a few years, and Pryor et al. (1989) report a handful of further
candidate binaries, all with periods of years. Although these systems are at
least true cluster binaries in the classical sense, their very long periods of
course imply that they store negligible orbital kinetic energy, and cannot
contribute to the energy budget of cluster, the problem which introduces
this paper.
The only other candidate for a short period globular cluster binary of
which we are aware is V78 ~ the direction of ~ Cen (Bailey 1902~. This
eclipsing Algol with a 1.2& period appears frequently in the literature with
comments of varying degrees of certitude that it is a cluster member fag,
Geyer 1967, 1971; Sistero 1968), but based on radial velocity it is now
known definitely not to be a cluster member (Geyer and Vogt 19783.
The very limited success in finding cluster binaries with evolved com-
panions, either degenerate stars or red giants, has led my colleagues and
me to initiate a number of programs aimed at finding binaries with main
sequence componenm. We believe we have met with success through two
different approaches.
AN ECLIPSING BLUE STRAGGLER IN A GLOBULAR CLUSTER
Blue stragglers, especially in globular clusters, have long rankled both
1 Multicolor photometry for MS V101 appears in Richer and Fahlman (1987), although the ob-
ject is anonymous in that paper; it is the most extreme ultraviolet point in their Figure 11.
OCR for page 274
274
AMERICAN AND SOVIET PERSPECTIVES
observers and theorists; their position on the color-magnitude diagram, with
normal main sequence color and luminosity, but lying far hotter than the
cluster tumors, is simply inconsistent with stellar evolution. Explanations
abound, but there are few hard facts to back up these theories. Specifically,
for more than 20 years they have been said to be close binaries ~IcCrea
1964; Hoyle 1964), with their odd colorlluminosity resulting from an ex-
tensive past episode of mass transfer, but there is no direct evidence for
this: not a single globular cluster straggler is an eclipsing or spectroscopic
binary. Alternative theones, involving long-lived single stars undergoing
odd evolution, are also viable (Wheeler 1979~. Some evidence that blue
stragglers in NGC 5466 and NGC 5053 have a peculiarly concentrated
spatial distnbution, and thus a higher primordial mass than other evolved
stars in the cluster, has been provided recently (Nemec and Harris 1987;
Nemec and Cohen 1989~. As has been previously stressed, however, this
does not directly discriminate between binary mass exchange and mixing
theories (Mathieu and Latham 1988~.
Some hope of addressing the problem arose when the eclipsing system
NJ~ 5 was identified In the field of co Cen, and it was pointed out that the
star lies near the blue straggler area of the cluster color-magn~tude diagram
(Niss et al. 1978~. This object may be identified with entry no. 5876 in the
Woolley catalogue (Woolley 19663, and star 5642 of Dickens et al. (1988~.
The period is 1.4&, and there is a deep (1.2 may) primary eclipse. However,
a strange twist of fate appears to have quenched interest in the system:
based on early photometry and the shape of the light curve, it was concluded
that NJL 5 is probably not a cluster member (tiller 1978~. No spectra have
ever been published for this object, although one unpublished spectrum has
been cited as compatible with membership (Jensen and Jorgensen 1985~.
As it was obtained in the green and shows only two spectral lines (Jorgensen
1987), it must surely be regarded as inconclusive.
We obtained spectra of NJL 5 on five nights in 1988 at moderate `2Ay
spectral resolution, using the RGO spectrograph and IPCS detectors at
the Anglo-Australian Telescope. The bright cluster horizontal branch star
ROA 4153 (Cannon and Stobie 1973), of color similar to NJL 5, was used
as a radial velocity standard. The average of the five velocities of Nob 5
at various phases throughout the 1.4& photometric period is 8 ~ 7 hen s~i
relative to the comparison star velocity, and thus, given the high velocity of
Cen, 232 lan s-i (Meylan and Mayor 1986), the object is most certainly
a cluster member. Our individual absolute velocity determinations for both
NJL 5 and ROA 4153, although less precise than the cross correlation
results, also clearly indicate that both stars are cluster members. NJL 5
is also almost certainly a blue straggler: the spectral type is near AS, and
it is far below the cluster horizontal branch (DaCosta et al. 1986~. The
spectrum is quite ordinary: the dominant features are Balmer adsorptions,
OCR for page 275
HIGH-ENERGY ASTROPHYSICS
275
with both stellar and interstellar Ca K lines also visible. There is no sign of a
secondary spectrum. We have also obtained one low resolution red (~5500
- lo,000A) spectrum of NJL 5 with the Faint Object Red Spectrograph
at the AAT, and again see no obvious sign of a secondary, although
considerably more extensive phase coverage is desirable to strengthen this
conclusion.
As a historical footnote, we point out that the (B - V) color for the
star listed in the Woolley (1966) catalogue differs from the normal color
of the system by 1 may, for reasons which are unclear. A correct color
appears in more recent photometry (Jensen and Jorgensen 1985; Dickens
et al. 1988).
NJL 5 now becomes very important: not only is it the first and
only globular cluster blue straggler Mown to be a binary, but the very
large eclipse depth shows the inclination is near 90°, and so the orbit is
soluble and all system parameters may ultimately be obtained. Our existing
moderate resolution spectroscopy is already tantalizing. Our five spectra
have good phase coverage, and no radial velocity variations are convincingly
detected; a conservative upper limit on the orbit K velocity is 30 km sol.
This already implies a very high mass ratio and an exotic, undermassive
secondary (recall that the unseen companion has radius comparable to
the AS star to cause the deep primary eclipse). Thus it appears that
the solution of the NJL 5 orbit may confirm the widely held but unproven
scenario that blue stragglers result from a previous episode of very extensive
mass transfer. There are several bright Algols in the field that have periods
and light curves similar to NJL 5 and presage the probable parameters:
RY Aqr (Heft 1987) and RT Per (Mancuso et al. 1977) both have q ~ 0.2,
with K~ 30 and 60 km s-l, respectively. The low amplitude we infer for
the radial velocity variations is presumably responsible for the failure of
many previous observers to detect the binarity of blue stragglers in old open
clusters (Stryker and Hrivnak 1984~. We have already obtained and are
currently analyzing the necessary spectroscopy at much higher resolution
to define the orbital parameters of NJL 5.
This work on NJL 5 was done collaboratively with Russell Cannon,
and will be described more fully elsewhere (Margon and Cannon 1989~.
FAINT MAIN SEQUENCE BINARIES IN GLOBULAR CLUSTERS
What are the prospects for detection of main sequence binaries of later
spectral type than blue stragglers, and thus at a less odd stage of evolution?
A conceptually simple way of identifying such binaries in globular clusters
has been known for many years (Steinlin 1956; Fernie and Rosenberg 1961~:
if a cluster color-magnitude (C-M) diagram can be obtained with very high
photometric precision, and if nature cooperates with a metallicity spread
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276
AMERICAN AND SOVIET PERSPECTIVES
OCR for page 277
HIGH-ENERGY ASTROPHYSICS
277
on the cluster ages, metall~cities, chemical homogeneity, etc., have been
published for NGC 362 (Bolte 1987a) and NGC 7099 (Bolte 1987b, 1989~.
The photometry, obtained with the CIIO 4m telescope, reaches far down
onto the main sequence, and we do indeed find that its intrinsic width
is unresolved at our level of precision, thereby enabling a search for the
"parallel" main sequence due to putative binaries. For each of our four
clusters, we have fields at three different core radii, potentially allowing
a search for mass segregation as well. The quality of these data very
substantially exceeds anything in the previous literature on these clusters,
and a second main sequence separated by ~0.75 mag in luminosity would
clearly stand out, greatly exceeding both the uncertainties in our photometry
and the intrinsic (as yet unresolved) main sequence breadth. Sadly, no such
sequence is apparent in any of the fields in any of our four clusters, with
one exception: the most central field in NGC 288.
Perhaps with hindsight, NGC 288 is an ideal globular cluster for a
search for main sequence binanes. It has a modest distance modulus,
~14.5 may, permitting precise photometry far down on the main sequence.
There is virtually no foreground contamination at this galactic latitude
(b = - 89°!~. Most important of all, the object, although unquestionably
a globular cluster by any definition, is sufficiently sparse to be virtually
transparent almost all the way into the core. Thus, crowding is minimal in
the innermost field, exacter where one expects the binaries to sink In Figure
1 we show our C-M diagram of the central field in NGC 288. One sees
immediately approximated one dozen stars displaced upward in luminosity
from the otherwise cleanly defined main sequence. The displaced stars
are spatially well separated by our reduction programs, so we have no
particular reason to suspect that their distinguishing characteristic is merely
poor quality photometry. As an interpretive aid, we show with the solid
line where the fiducial main sequence would fall if displaced upward in
luminosity by 0.75 may, as expected for a sequence of equal mass binaries.
The dozen outlying objects fall quite precisely on this sequence. This
coincidence may be a conspiracy of nature, but if so it is particulars cruel
that it occurs in the one field where we should have the best chance of
seeing the binary sequence, nameh,r the most central field we possess with
precision photometry. We thus do not suggest that NGC 288 is in any way
unique, but merely that if all clusters in fact possess a few of these objects,
it is quite logical that we would see them in this cluster first.
One would be naive to believe that the objects offset from the main
sequence in Figure 1 are binaries strictly on the grounds of our photometry.
Surely either eclipses or radial velocity variations must be required to con-
vincingly argue that the first population of globular cluster main-sequence
binaries has been discovered. Eclipses would be obsenationally straight-
forward to search for, but statistically unlikelier to be observed. A negative
OCR for page 278
278
V
AMERICAN AND SOVIET PERSPECTIVES
16
18
20
22
24
I I I I I I r
.
I I I 1
o
o
_
o
o
o
o
o
O O
o
O O
O ~
O ~`
o ooo
oo o
o
o
o
o
O ~' 00 0 0
O ~ O
0 0
O
_ | l l I L
o
o
o o
1 1 1
NGC 288
o
o
~ o
- _ ~r ~ ~
°~8
oo~co o
° o °o~O. o~OO
0 0 o8°~°~°O
-
o o° _
o o
o
-
.5
(B V)
1
FIGURE 1 A color-magnitude diagram for NGC 288, denved from CCD photomet~y
obtained at the prime focus of the 4m telescope of the CeIro Tololo Interamerican
Obsenratory. I-he solid line is a locus off~;et by 0.75 mag ~om a fiducial main sequence.
The dozen or so candidate main-sequence binaries, Iying quite close to this line, are readily
apparent.
OCR for page 279
HIGH-ENERGY ASTROPHYSICS
279
result from an extensive series of eclipse observations would probably not
be informative. Radial velocity variations, on the other hand, are certainly
present if the objects are truly binanes, and, as always subject to the un-
known inclination factors, will be substantial in the close binaries that we
seek Of course, with our dozen or so candidates, it is quite unlikely that
all have unfavorable inclinations.
In collaboration with M. Bolte of the Dominion Astrophysical Obser-
vatory, we have begun a program to obtain repeated observations of radial
velocities for the ten or so brightest, best separated objects on this parallel
man-sequence in NGC 288. This is a difficult observational problem: we
seek a large number of velocities of accuracy ~10 km s-i on objects of
~ > 19.5. Until very recently, one would be hard pressed to find in the lit-
erature a spectrum of any cluster main sequence star, much less a program
of repeated observations of many objects at reasonably high dispersion. The
recent development in instrumentation that has made this program feasible
is the introduction of multiple-object fiber-fed spectrographs at several of
the world's 4m-class telescopes. This project is of course ideal for such an
instrument, as all of the candidate stars are in immediate proximity, lying
on the same 3' x 5' CCD chip, and we desire all of the radial velocities
simultaneously, not knowing which are true binaries, and which have favor-
able inclinations. Further, we can employ the many otherwise unused fibers
to obtain spectra of brighter cluster members in the immediate proximity,
which are not only of astrophysical interest, but provide a large number of
local velocity standards,2 as well as a varieW of control objects so important
to this precision work, e.g. anonymous stars of comparable magnitude and
crowding to our candidate binaries. The colors of the candidates indicate
spectral Apes near solar, and so we may anticipate ample absorption lines
for radial velocity determination.
We are conducting our observations using the fiber-optic coupled multi-
object spectrograph (FOCAP) (Gray 1986; Party and Gray 1986) at the 3.9m
Anglo-Australian Telescope. This system uses approximately 60 fibers to
feed as many star or sly spectra to the no' offal RGO spectrograph, using the
Image Photon Counting System (IPCS) as a two-dimensional detector. We
use a grating which yields ~1000A of spectral coverage, with 2~ resolution,
centered near A4200 to take advantage of the region of peak efficiency of
the IPCS. We have thus far obtained observations at two epochs in 1987 88
which convince us that the project is Just!) feasible in excellent observing
conditions; typical integration times to obtain ~100 counts pixels above
sly at this dispersion on these faint stars are 12,000 sec. In Figure 2, we
show an example of one of the spectra so obtained of one of our main
2 Several dozen giants and horizontal branch stem in NGC 288 already have velocities determined
to an accurapyof ~.6 k n s 1 via radialvelocity spectrometer observations (P~yoretaL 19~.
OCR for page 280
280
AMERICAN AND SOVIET PERSPECTIVES
6000
4000
an
o
2000
o
100
50
o
T I ~I T I l I
AL80
I I ~1 11
4000 4500
Wavelength (I)
1 11 1 1 1 1 ,.l
- uB1
4000
4500
Wavelength (I)
FIGURE 2 To of the sixty spectra obtained simultaneously in the NGC 288 field
at the Anglo-A~lstralian Telescope in 1988 July, using the fiber-optic coupled multi-
object spectrograph (FOCAP) and the Image Photon Counting System (IPCS) detector.
Lower panel: One of the candidate I-sequence binaries, MB1, an object with V =
19.2, (B-V) = 0.50; despite the modest signal-to-noise ratio of the spectrum, one can
clearly see Ht. H7, and Ca H & K absorption lines. Upper panel: one of the bright radial
velocity standard stars in the cluster, AL80, a giant with V = 13.89, (B-V) = 1.14
(AIcaino and Liller l980~; a precise (~0.7 km s-1) velocity for this star has been obtained
by Pryor et aL (1988~) with a radial velocity spectrometer. All of the features and
structure in this spectrum are actual absorption lines in the object. At this dispersion,
cross~orrelation of the two stars can yield velocities of accuracy ~6 km s-1 if there is
reasonable signal in the program objects
OCR for page 281
HIGH-ENERGY ASTROPHYSICS
281
sequence binary candidates, together with that of one of the bright giant
radial velocity standards in the cluster. Cross-correlation analysis allows us
to derive velocities for the binary candidates to an accuracy of ~6 lain sol.
As the necessary multiple-epoch observations are still under way, these
results must be viewed simply as a progress report at this time. However,
for the candidates where we have sufficient signal, we can already say,
based on their derived velocities, that these faint stars are indeed cluster
members; their sing position on the C-M diagram cannot be explained
merely by superposition of a non-member. Thus we are encouraged by the
prospect that current instrumentation is indeed adequate to reveal radial
velocity variations in main sequence binaries in this and similar clusters in
the near future.
CONCLUSION
In an lateral of only a few years, we have progressed from knowing of
literally no non-degenerate binary stars in globular clusters, to a short list
of candidates of a variety of luminosities. Whether the very crowded inner
cores of the clusters currently hide a population of close binaries that would
otherwise be easily distinguished by their odd colors and luminosities will
presumably become clear with even the first handful of multicolor images
from the Hubble Space Telescope. Thus prospects are bright that in the next
few years, observations will finally catch up to, and hopefully overtake, the
theory of close binary stars in globular clusters.
ACKNOWLEDGEMENTS
Most of what little I know about globular clusters and binaries therem
has been taught to me by Michael Bolte, and I am indebted to him for his
continued collaborative efforts in much of the work described here. I thank
Russell Cannon and all of the staff of the Anglo-Australian Observatory
for hosting me during the sabbatical year where much of this work was
done; Warrick Couch was particularly essential in accomplishing many of
these observations. I appreciate useful discussions with P. Hut, D. Popper,
C. Pryor, and H. Richer. E. ballots has aided in the data reduction.
I was a Visiting Astronomer at Cerro lblolo Interamencan Observatory,
National Optical Astronomy Obse~vatones, which is operated by AURA,
Inc., under contract to the NSF. This work has been supported in part by
NASA Contract NAS5-29293 and NASA Grant NGT-70050.
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-~-
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lump Mention. ~mp~i=1 kumal go: 10~19.
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~mno~1 Stem ~ the PadOc By: 7~8.
Hear, 1. E., R. O. Value, ~ ~ H~eo, R. D Orson, ~ ~n Ludlow B. aged
and D. Ego. 1~. ^ Day ~or_~ilude diadem far me Door star dialer
~=~1~7~ PI of He ~1~DO~ Idea of the Parsec ad: ~1&
Hills, J. G. 1~. En~unle~ Open away and single ~ and 1beir egg on He
Swamis column ~ Cellar plea. ~tmnomi=1 loop ~ I.
Hogs, H. 3. 1~3. ^ laid Bulge of adage sag iD globular Cusps ~mpd~Dg ~19
=~ PI ~~ ~ 0~ 0~_ 3, ha. ~1~.
flogs, H. S., and ~ Pablo. 1~. ^ pbologmpbic n~ in the glower clever flier
1! frugal of the ~1 ~-nomi=1 S-eV ~ 0~ S~ 16~1~.
ape, E 1~. Owes and solar ~ludon. 1 Oslo Bulledn ha: 91.
]en~n, ~ S., and H. E. Lena. 19~. Cog band ~ and ~ ligb~u~= far He
echoing gnaw ~ ~ in Omega redbud. Logo and ~l~p~io 3upp~ment
ha.
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E=~ ~l~no~ and ~lmp~o 183: 1~.
# ~ 1~ ~ ~ -~ ~ b ~ ~ _ ~ _
~ Doper ~= _~1 ]-m~ Ha: Id.
Pages 31~0 In: Aped D., and B. Shadow Ada. Globular Ousle=. Om~dge
~9 P_ ~
In, ~ H. G., and ~ C Ha. 1~. K~ buses and the X~ Cup of He
_~ _~ ~>~R1
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Odes M. H. 1~ ~ flag binary iD He Beld of w Am. I^~ali~ Bulletin on
adage Sag 15~.
= E. 1~. gong On =~ndedi~en slemeD. amiss Den as:
a.
In, ~ ~ amp, ad 0. I. 1~. We ~ Hem ~ phi a~ is
ligb1~. ~tm~io and Saw Science 47: I.
Carol, B., and at. B~1= 1~. We l~lumin~iV X- Cup in Omega Into.
~1 amp ~ Ha:
~~on, B., and R. Onn=. 1~) ~ aiming Nue simpler in ~ Maraud. 0~1
1~ Ha.
Bags, B., and ~ ~ Dana. lS3. ^ Hand =~lc gamble in a globular gales
~mp~~1 puma -~- Ha: ~1~.
If=, B., R. A. Dane, and 1. a. Ou=. 1981 ~ V101: a dam ~ Cam
globular deep ~1~ ]~mal ~1~ ad: ~
Adieu, ~ D., and D. ~ Tam. 1~. We s~1ia1 disldbution of Epic Ins
ad blue Plea in Ha. Ages 67~76 In: Odadl~ ~ H. O., and ~ 0. D^
Philip beak. P ~ of IT -~ slum 1~, globular Ouster Xylems
flies. ~= D0~L
Yucca, ~ H. 1~. Lends ~n-=quen~ of Some stellar tousles Only NoU=s of
I__ 1~.
~an, O., ad at. _ 1~. Sages of ~am1~1 pappies of globular dales. II.
~ mud=, =1~9 Aspen and mad ~ ~ redbud ad ~ ~. ^1mno~
ad ~l-~= lag 1~=
_T ~ ~ C ~ 1~ ~ _~ ~ ~ _r~
~1 puma ~& 1~.
OCR for page 284
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Representative terms from entire chapter:
globular clusters
