Far-Ultraviolet Surveys of Globular Clusters: Searching for the Products of Stellar Collisions and Near Misses

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by  C. Knigge

Toappearin“IAUColl. 194: InteractingBinariesintheGalaxyandBeyond(2003)”RevMexAA(SC)
FAR-ULTRAVIOLET SURVEYS OF GLOBULAR CLUSTERS: SEARCHING
FOR THE PRODUCTS OF STELLAR COLLISIONS AND NEAR MISSES
C. Knigge, 1 D.R. Zurek, 2 M.M. Shara, 2 K.S. Long, 3 R.L. Gilliland, and 3 P.A. Charles 1
RESUMEN
El resumen ser´a traducido al espan˜ol por los editores.
Far-ultraviolet (FUV) observations with the Hubble Space Telescope are an excellent way to find and
studythehot,bluestellarpopulationsinthecoresofglobularclusters. Thesepopulationsincludedynamically-
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formed blue stragglers and interacting binaries (such as cataclysmic variables), i.e. the products of stellar
0
0 collisions and near misses. Using the cluster 47 Tuc as an example, we show how the combination of FUV
2 imaging and slitless spectroscopy can be used to uncover and study these populations.

n
ABSTRACT
a
J Far-ultraviolet (FUV) observations with the Hubble Space Telescope are an excellent way to find and

8 studythehot,bluestellarpopulationsinthecoresofglobularclusters. Thesepopulationsincludedynamically-
2
formed blue stragglers and interacting binaries (such as cataclysmic variables), i.e. the products of stellar

collisions and near misses. Using the cluster 47 Tuc as an example, we show how the combination of FUV
1
v imaging and slitless spectroscopy can be used to uncover and study these populations.
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Key Words: STARS: BLUE STRAGGLERS — GLOBULAR CLUSTERS: INDIVIDUAL (47 TU-
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CANAE) — STARS: NOVAE, CATACLYSMIC VARIABLES — ULTRAVIOLET: GEN-
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1 ERAL — STARS: WHITE DWARFS
0
4 1. INTRODUCTION of the cluster core. The tremendous difference in
0 crowding between these images nicely illustrates
Globularclusters(GCs)arefantasticstellarcrash
/ the ease with which hot stellar populations can be
h test laboratories. Violent encounters between bina-
p detected in the FUV. The color-magnitude diagram
ries and single stars in dense cluster cores give rise
- produced from this data is shown in Figure 2.
o to exotic stellar populations, such as blue stragglers
r (BSs), cataclysmic variables(CVs) andlow-massX- As discussed in Knigge et al. (2002), the FUV
t
s ray binaries. All of these objects have consider- imagingdatahasalreadyallowedusto: (i)findFUV
a
ably bluer spectral energy distributions than “nor- counterpartstoalloftheChandraCVcandidatesin
:
v mal”clustermembers. CVsandyoungwhite dwarfs the core of this cluster; (ii) confirm the CV nature
Xi (WDs), inparticular,radiatemuchoftheirluminos- ofthesecandidatesbasedonvariabilityandlocation
ityinthefar-UV.Observationsinthiswavebandare in the far-UV colour-magnitude diagram; (iii) sug-
r
a therefore well suited to the detection and character- gest several additional CV candidates; (iv) detect a
ization of these important stellar species. We have clean, well-populated far-UV BSS sequence; (v) de-
therefore embarkedon a programto study the cores tect numerous young WDs on the upper end of the
ofGCsatFUVwavelengths,usingbothimagingand cooling curve.
slitless spectroscopy with STIS and ACS onboard
Since crowding is not a serious problem in the
HST. Here, we present results from observations of
FUV, it is possible to carry out multi-object slitless
our first target, 47 Tucanae (for details, see Knigge
spectroscopy even in the dense core of a globular
et al. 2003; 2003)
clusterlike47Tuc. Thisisanextremelyefficientway
of spectroscopically confirming interesting sources,
2. RESULTS
such as CV candidates. The power of this approach
Figure 1 shows a comparison of FUV isdemonstratedinFigure3,whichshowstheslitless
(STIS/F25QTZ; a bandpass centered around 2-D spectral image of 47 Tuc and the 1-D extracted
1600 ˚A) and U-band (WFPC2/F336W) images spectrumofthebrightestsource,theeclipsingbinary
AKO 9. The strong C iv and He ii emission lines in
1UniversityofSouthampton, UK.
2AmericanMuseumofNaturalHistory,USA. immediately suggest that AKO 9 is a cataclysmic
3SpaceTelescopeScienceInstitute, USA. variable (see Knigge et al. 2003).
1
2 KNIGGE ET AL.
Fig.1. LeftPanel: A25′′×25′′far-UVimageofthecoreof
47Tuc. Theclustercenterismarkedasacross. Thepo-
sitionsofpreviouslyknownblueobjects(squares),Chan-
drax-raysources(bigcircles) andCV candidates(small
circles) are marked. The four confirmed CVs within the
FUV field are labelled with their names. Right Panel:
The WFPC2/F336W image of the same field. The dif-
ference in crowding is obvious.
Fig. 3. Top Panel: The 2-D spectral image obtained
from our slitless FUV spectroscopy of the field shown
in Fig. 1. The brightest source is the cataclysmic vari-
able AKO 9, which shows clear evidence for line emis-
sion.Bottom Panel: Theextracted1-DFUVspectrumof
AKO 9 (dataaffected by eclipses was excluded).
3. CONCLUSIONS & THE FUTURE
Far-ultraviolet surveys are an excellent tool for
studying the most interesting hot stellar popula-
tionsinglobularclusters,includingthedynamically-
formedinteractingbinaries. Here,wehavepresented
a snapshot of our work in this area, but there is
much more to come. For example, we already have
preliminary spectroscopic confirmation of two addi-
tional CV candidates in 47 Tuc, FUV observations
Fig. 2. The FUV/optical color-magnitude diagram de- of several other clusters are being analysed, and ad-
rived from the data in Fig. 1. Variable FUV sources ditional observations are already scheduled. Watch
are marked with black circles, and the four previously this space!
known or suspected CVs are labeled with their names.
The diagonal line marks the FUV completeness limit. REFERENCES
The other lines in the diagram indicate theexpected lo-
Knigge, C., Zurek, D. R., Shara, M. M., & Long, K. S.
cationsofvariousstellarpopulations. Unmarkedsources
2002, ApJ, 579, 752
between the WD and main sequences are potential new
Knigge, C., Zurek, D. R., Shara, M. M., Long, K. S., &
CV candidates.
Gilliland, R.L. 2003, ApJ, 599, 1320
P.A. Charles and C. Knigge: School of Physics & Astronomy, University of Southampton, Southampton SO17
1BJ, UK ([email protected],[email protected]).
R.L.GillilandandK.S.Long: SpaceTelescopeScienceInstitute,3700SanMartinDrive,Baltimore,MD21218
([email protected],[email protected]).
M.M. Shara and D.R. Zurek: American Museum of Natural History, 79th Street and Central Park West, New
York, NY 10024 ([email protected],[email protected]).