Far-Ultraviolet Surveys of Globular Clusters

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Far-Ultraviolet Surveys of Globular Clusters

• Christian Knigge
• Astronomy Group
• School of Physics Astronomy
• University of Southampton

Collaborators Andrea Dieball (Southampton) David
Zurek (AMNH) Mike Shara (AMNH) Knox Long
(STScI) Ron Gilliland (STScI)
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Whats so special about the FUV?
it gets rid of the chaff!
U (3500A)
Far-UV (1600A)
Knigge et al. (2002)
FUV imaging is a nice way to find and study hot
and/or dynamically-formed stellar populations in
GCs!
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Without the chaff, whats left?Dynamically-forme
d Systems and Other FUV Emitters
(M3 Buonanno et al 93)

• Many Dynamically-Formed Systems Are Extremely
  Blue!
• Blue Stragglers higher mass MS stars
• CVs/LMXBs accretion heating
• EHBs extremely thin H envelope

EHB
Only a few other blue objects
Blue stragglers

• which are interesting as well!
• Young WDs not yet cooled down
• Blue HB Stars thin H envelope
• UV Bright Stars PAGB, eg central stars of PNe

CVs LMXBs
Main sequence
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47 Tuc The First Deep FUV Survey
Knigge et al. (2002)

• The Core of 47 Tuc

• The Core of 47 Tuc
• FUV counterparts to all 4 original Chandra CVs in
  the FUV field

• The Core of 47 Tuc
• FUV counterparts to all 4 original Chandra CVs in
  the FUV field
• 12 additional, new CV candidates

• The Core of 47 Tuc
• FUV counterparts to all 4 original Chandra CV in
  the FUV field
• 12 additional, new CV candidates
• 19 Blue Stragglers

• The Core of 47 Tuc
• FUV counterparts to all 4 original Chandra CVs in
  the FUV field
• 12 additional, new CV candidates
• 19 Blue Stragglers
• 17 Young WDs

• The Core of 47 Tuc
• FUV counterparts to all 4 original Chandra CVs in
  the FUV field
• 12 additional, new CV candidates
• 19 Blue Stragglers
• 17 Young WDs
• still more WDs CVs to be found

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UPDATE FUV Counterparts to Chandra Sources in 47
Tuc

• Easy CVs The 4 original Chandra CVs

Knigge, Dieball et al. in prep.
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UPDATE FUV Counterparts to Chandra Sources in 47
Tuc

• Hard CVs New FUV Counterpart Identifications

Knigge, Dieball et al. in prep.
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UPDATE Revised CMD with New CVs
Knigge, Dieball et al. in prep.

• Have now at least 8 CVs in 1/3 of cluster core
• Faintest CVs probably still not seen in X-ray and
  optical data (maybe there in FUV?)
• Approaching consistency with theoretical
  predictions! (Well, maybe)
• What about the new, FUV-only candidates?
• If CVs, dynamical formation scenarios would be
  looking even better

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The FUV-only CV candidates

• No central concentration!
• At first sight inconsistent with CVs or
  non-interacting WD/MS binaries
• SMC Interlopers?
• lt 1 expected statistically
• False matches?
• Only about 3 expected statistically
• He WDs?
• Have been seen in a few clusters
• But expected to be formed via binary evolution!
• Dark companion probably WD or NS?
• Central concentration expected and observed!
• Mix of several populations?
• The nature of these sources remains unclear

Dieball, Knigge et al. in prep.
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NGC 2808 FUV Insights into Horizontal Branch
Stars and Dynamically-formed Stellar Populations

• Distance
• d 10.2 kpc
• Metallicity
• Fe/H -1.15
• Reddening
• E(B-V) 0.18
• Dynamical interaction rate comparable to 47 Tuc
• 16 core radius
• Core is fully covered by FUV mosaic!
• FUV data originally obtained for calibration
  purposes!
• Initial analysis of data by Brown et al. (2001)
  with focus on FUV HB morphology

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Flash-mixing on the WD Cooling Curve in NGC 2808

• UV Horizontal Branch morphology is qualitatively
  different from optical one
• Discovery of sub-luminous hot horizontal branch
  stars
• Delayed He Flash with Dredge-up on the WD Cooling
  Curve?
• Supported by spectroscopy (Moehler et al. 2004)
• Explains gap in optical HB distribution
• Requires strongly enhanced mass loss on RGB

Brown et al. 2001
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NGC 2808 Revisited Blue Stragglers, White
Dwarfs and Cataclysmic Variables

• Re-analysis of the FUV data
• Pushing deeper to find faint FUV sources
• Focus on dynamically-formed and related
  populations
• Results
• 40 WDs
• 60 Blue Straggles (incl. 3 variables)
• 60 CV candidates (incl. 2 variables)
• Consistent with dynamical formation
• But what about central concentration?

Dieball et al. 2005
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Radial Distributions of FUV Populations in NGC
2808
Dieball et al. 2005

• The FUV-selected CV candidates are strongly
  centrally-concentrated
• Comparable to Blue Straggler population
• Good news for CV / binary interpretation of these
  sources
• So whats going on in 47 Tuc?!

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NGC 1851FUV Variability Across the CMD
Optical

• Distance
• d 12.1 kpc
• Metallicity
• Fe/H -1.22
• Reddening
• E(B-V) 0.02
• Dynamical interaction rate again comparable to 47
  Tuc
• 3.6 core radius
• Innermost regions difficult to resolve even with
  HST
• Reduced crowding in FUV is particularly useful
• FUV data originally obtained to search for
  variability in a known LMXB (Homer et al. 2001)
• Visual inspection reveals massive variability
  amongst some FUV sources

Far-UV
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Massive FUV Variability Amongst RR Lyraes in NGC
1851

• Observed amplitudes of up to 4 mags!
• With hindsight, this could have been expected
• Variability is mainly due to temperature
  variations
• so amplitude proportional to dB/dT
• FUV band is on exponential Wien tail of SED
• so large dB/dT
• and huge FUV amplitude

Downes et al. 2004
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GALEX observations of a field RR Lyrae

• GALEX has observed the same phenomenon amongst
  field RR Lyrae
• Modelling shows that FUV amplitude is quite
  sensitive to metallicity
• A new metallicity indicator for globular
  clusters?
• May also be able to use period/FUV-amplitude
  relation for distance determinations

Wheatley et al. 2005
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NGC 1851 RevisitedVariability across the FUV HR
Diagram
Zurek et al. in prep.
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NGC 1851 RevisitedVariability across the FUV HR
Diagram

• What are all these variable stars?
• RR Lyrae
• BSS
• SX Phe
• RHB / RGB / AGB
• Pop II Cepheids? (W Vir / BL Her)
• MSTO
• Binaries?
• BHB
• Binaries?
• A New Class?
• WD region and sources without optical
  counterparts

Pulsators in the Instability Strip
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Latest Results I Discovery of an Ultracompact
X-ray Binary in M15
Dieball et al. (2005)

• M15 XB-2 has a bright FUV counterpart

• FUV data shows pulsations with period 22.6 minutes

• Pulsations are consistent with being coherent
  over gt 3000 cycles

• ?Period is orbital
• M15 XB-2 is an ultracompact XRB (NSHeWD)
• Only the 3rd confirmed UCXRB in a globular
  cluster!
• For additional info, see poster and talk by
  Andrea Dieball

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Latest Results II A Blue Straggler with an
X-ray-active Companion
Knigge et al. (2005)

• FUV/Chandra astrometry yields a 0.06
  astrometric match between a Blue Straggler and an
  X-ray source in 47 Tuc

• Archival HST data reveals 1.56-day periodic
  signal in I-band

• At least 3 stars are required to form
  this system!

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BSS-7 Spectral Energy Distribution
FUV flux is an excellent BS thermometer
Knigge et al. (2005)
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Summary Conclusions

• FUV surveys are an excellent way to find and
  study hot and/or dynamically-formed stellar
  populations in globular clusters
• They allow convenient and efficient X-ray
  counterpart searches and X-ray source
  identifications in cluster cores
• Results from the first three clusters are helping
  to uncover the long missing cataclysmic
  variable population
• FUV data has shed new light on controversies
  surrounding anomalous horizontal branch
  morphologies
• Pulsating stars often display massive FUV
  variability that might be exploited to measure
  cluster parameters
• Time-resolved FUV imaging allows detection,
  classification and period determination for
  interacting binaries, including CVs, LMXBs and
  UCXBs
• FUV-Chandra source matching has led to the
  discovery of the first Blue Straggler with at
  least 3 progenitors in a globular cluster
• FUV surveys should become part of the standard
  arsenal of observational techniques that we use
  to study globular clusters

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Outline

• Introduction
• Whats so special about the FUV waveband?
• Dynamically-formed stellar population in the FUV
• A Tale of Three Clusters
• 47 Tuc
• The first deep look at a globular cluster in the
  far-UV
• NGC 2808
• A FUV census of the hot stellar populations
• NGC 1851
• A deep FUV variability study
• Latest Results on Exotic FUV Emitters
• The ultra-compact binary M15 XB-2
• A Blue Straggler with an X-ray active companion
  in 47 Tuc

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Hey, that worked pretty well... so lets really
go crazy!
Multi-object far-UV spectroscopy in the core of a
globular cluster
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Spectroscopic Confirmation of the First 3 CVs
Much more to come!