Globular Clusters in the Far UV*

1. Globular Clusters
In
The Far-Ultraviolet
Collaborators
Christian Knigge (Southampton U)
Thomas Maccarone (Texas Tech)
Michael Shara (AMNH)
Andrea Dieball (U of Bon)
Knox Long (STScI)
David Pooley (Trinity U.)
Grace Thomson (private sector)
NGC 6681 (M70)

2. Stellar Exotica
Blue Stragglers
Sub-subgiants
From Bailyn (1995)
From Geller et al (2017)

3. Solheim (2010)
Rob Haynes 2001
Pearson Prentice Hall, Inc

4. Where it all started (47 Tuc)
Knigge et al (2002)
Spectroscopically confirmed 3
cataclysmic variables
Identified the previously puzzling
variable AKO9 as a cataclysmic
variable analogous to GK Per.
Resulted in the identification of
hot white dwarfs, blue stragglers
and candidate cataclysmic
variables

5. Other work
NGC 2808 - Dieball et al (2005)
• Identified ~40 white dwarfs, ~60 blue stragglers, and ~60 cataclysmic variable candidates
NGC 6093 - Dieball et al (2010)
• Identified ~117 blue and extreme horizontal branch stars, ~75 blue stragglers, ~32 white dwarfs,
and ~59 main sequence - white dwarf candidates (or cataclysmic variable candidates)
NGC 6397 - Shara et al (2005)
• Found two cataclysmic variable candidates from Grindlay et al (2001) appear to have variations
between epochs that are consistent with these sources being dwarf novae
NGC 6397 - Dieball et al (2017)
• Identified 3 likely blue horizontal branch stars, eleven blue stragglers, and 16 white dwarf and/or
white dwarf binaries
NGC 6752 - Thomson et al (2012)
• Identified numerous horizontal branch stars, blue stragglers and white dwarfs. They found 87
sources populating the region between the white dwarfs and the main sequence
NGC 7078 - Dieball et al (2005)
• Identified a Far-ultraviolet counterpart to the low-mass X-ray binary M15 X-2. Clearly detected a
period of 22.58 minutes. Dieball et al (2007) found 41 sources that show signs of variability.

6. Other telescopes?
Subramaniam et al (2017)

7. NGC 1851
The experiment (i.e. data)

8. Bootstrapping from the
FUV to the Optical
SBC
F140LP
WFPC2
F336W
WFPC2
F555W
ACS/WFC
F814W

9. The CMDs
-1 -0.5 0 0.5 1 1.5
V-I (Vega)
12
13
14
15
16
17
18
19
20
21
22
23
V(Vega)
MS
SGB
RGB
AGB
HB
WD

10. The Color-Color plot

14. RR Lyrae

15. Blue Stragglers

16. Everything else

17. The ultracompact X-ray source
10’’
N1851-FUV1
4U 0513-40
N
E

18. A Suspected AM Can
An AM CVn is a double degenerate binary with a period less than about 70 minutes

19. Period is 18.05 minutes

21. 0.1 1 10
Radius (SBC pixels)
0
500
1000
1500
2000
2500
3000
NumberofMatches(Normalizedbyarea)
No emission lines which are
expected if the object is a
Symbiotic binary
Object is in the tail of the
matched objects distribution

22. NGC 1851 summary
There are 36 candidate variables
• 12 candidate RR Lyrae
• 13 candidates are on the Blue Horizontal Branch
• 5 are identified as Blue Stragglers and are likely SX Phoenicis pulsators
• 6 are apparently on the main sequence in the optical
• 1 is an AM CVn
• 1 (though not discovered here) is an Ultra Compact X-ray binary
The Photometry indicates that there are a number of sources which
have an excess of flux in the ultraviolet. This may indicate a
companion of a hot white dwarf or Helium core.

23. NGC 6681
Why this cluster and the data
85 Epochs of Far-ultraviolet and
Near-ultraviolet imaging

24. The search
It has been assumed that
dwarf novae are typically
in eruption 15% of the
time (i.e. U Gem).
Perhaps a better or more
typical dwarf novae is
the WZ Sge type which
erupts for ~1 month
every ~30 years. It
therefore spends 0.3%
of its time in eruption.
Dwarf Novae
The probability to detect (or catch an eruption) is
Pdetect = 1-(1-d) N

25. What does finding no eruptions mean?
We are essentially 100% complete for dwarf novae with duty cycles
greater then 5%. We are still 60% complete for duty cycles of 1%.
If we consider only epochs spaced by more than a month (N=47) then we
are >99% complete for duty cycles greater than 10% and ~38% complete
for duty cycles of 1%. Even if the duty cycle is lower (0.3%) we would still
have a detection efficiency of 13%.
A comparison to 47 Tucanae is possible as is it has also been observed
over many epochs/years (Shara et al. in prep). 47 Tuc is 8 times more
massive than NGC 6681 so we might expect to see ~0.5 dwarf novae with
a simple scaling by cluster mass. On the other hand scaling by the stellar
encounter rate would imply that we should have found 4 dwarf novae.
It is possible to reconcile the absence of detected dwarf novae with
theoretical predictions (Ivanova et al 2006) if all the dwarf novae duty
cycles comparable to or lower than WZ Sge-like systems.