The small Blue Straggler star population in the The small Blue Straggler star population in the dense Galactic Globular Cluster NGC 6752dense Galactic Globular Cluster NGC 6752

E.SabbiE.Sabbi

Dipartimento di Astronomia,Università di Bologna,via Ranzani 1,I-40126 Bologna,ItalyDipartimento di Astronomia,Università di Bologna,via Ranzani 1,I-40126 Bologna,Italy elena.sabbi@unibo.itelena.sabbi@unibo.it

F.R.FerraroF.R.Ferraro Francesco.ferraro3@unibo.itFrancesco.ferraro3@unibo.it

A.Sills A.Sills

Department of Physics and Astronomy,McMaster University,1280 Main Street West,Department of Physics and Astronomy,McMaster University,1280 Main Street West, Hamilton,ON L8S 4M1,CanadaHamilton,ON L8S 4M1,Canada asills@mcmaster.caasills@mcmaster.ca andand R.T.RoodR.T.Rood Astronomy Dept.,University of Virginia Charlottesville VA 22903-0818,USAAstronomy Dept.,University of Virginia Charlottesville VA 22903-0818,USA

rtr@virginia.edurtr@virginia.edu

• IntroductionIntroduction

• Observations and data analysisObservations and data analysis

• The CMD and the BSS candidatesThe CMD and the BSS candidatesThe HST dataThe HST dataThe WFI dataThe WFI data

• BSS radial distributionBSS radial distribution

• Comparison with other GCsComparison with other GCs

• Collisional ModelsCollisional Models

• Summary and DiscussionSummary and Discussion

IntroductionIntroduction• Two mechanisms of making BSSTwo mechanisms of making BSS

Mass transfer between or the merger of two stars Mass transfer between or the merger of two stars in a primordial binaryin a primordial binary

Collisions in regions of very high stellar densityCollisions in regions of very high stellar densityDirect collisionsDirect collisionsCreated as collisions harden primordial binaries until Created as collisions harden primordial binaries until

they mergethey mergeResulting when binaries are produced in a collision Resulting when binaries are produced in a collision

and merge laterand merge later

IntroductionIntroduction

• Provide new insights not only dynamical Provide new insights not only dynamical interaction and evolution of individual interaction and evolution of individual stars ,but also of the cluster as a wholestars ,but also of the cluster as a whole Segregation toward the center of the more Segregation toward the center of the more

massive stars (or binaries)massive stars (or binaries) Binaries are thought to play a fundamental role in Binaries are thought to play a fundamental role in

the core collapsethe core collapse——delay the collapse of the coredelay the collapse of the core

IntroductionIntroduction

• GGC NGC 6752GGC NGC 6752Host many faint X-ray sources in the coreHost many faint X-ray sources in the coreAt least five millisecond pulsars (MSPs)At least five millisecond pulsars (MSPs)

Two of which are in the outer part—most radially diTwo of which are in the outer part—most radially distant MSPs gravitationally bound to a GCstant MSPs gravitationally bound to a GC

Two of the three central MSPs display an anomalouTwo of the three central MSPs display an anomalous accelerations acceleration

The existence of a binary black hole of intermeThe existence of a binary black hole of intermediate mass could be a viable possibilitydiate mass could be a viable possibility

Observations and data analysisObservations and data analysis

• Two data setsTwo data sets High resolution set-a series of high-resolution High resolution set-a series of high-resolution

WFPC2-HSTWFPC2-HST image ,using the image ,using the F555W(V),F336W(U)and F255W(mid-UV) filtersF555W(V),F336W(U)and F255W(mid-UV) filtersPlanetary camera(PC)(0’’.046/pixel) centered on the Planetary camera(PC)(0’’.046/pixel) centered on the

cluster centercluster centerWide Field (WF) cameras(0’’1/pixel) sampled the Wide Field (WF) cameras(0’’1/pixel) sampled the

surrounding outer regionssurrounding outer regions Wide Field set-Wide Field Imager (WFI) (B,V,I)Wide Field set-Wide Field Imager (WFI) (B,V,I)

8 CCD chips (each with a field of view of 8’ 8 CCD chips (each with a field of view of 8’ ×16’)giving a global field of view of 33’ ×34’×16’)giving a global field of view of 33’ ×34’

The CMD and the BSS candidatesThe CMD and the BSS candidates

• The HST dataThe HST data The main contributors to the UV emission of GThe main contributors to the UV emission of G

Cs are the hot stars which populate the horizoCs are the hot stars which populate the horizontal branch (HB) and the BSSntal branch (HB) and the BSSThe ( ) plane is an ideal tool for selThe ( ) plane is an ideal tool for sel

ecting BSSecting BSSSelect 28 ‘safe’ BSS in the HST-WFPC2 FoVSelect 28 ‘safe’ BSS in the HST-WFPC2 FoV

Brighter than ~18(0.3 mag brighter than the cluster TBrighter than ~18(0.3 mag brighter than the cluster TO)O)

Bluer than ~0.76Bluer than ~0.76

336255,255 mmm

255m

336255 mm

The CMD and the BSS candidatesThe CMD and the BSS candidates

• The WFI dataThe WFI data Consider the stars contained within the tidal r ~Consider the stars contained within the tidal r ~

16’.67( ~1000”)16’.67( ~1000”) Select 15 BSS candidates from the (V,B-V) CMSelect 15 BSS candidates from the (V,B-V) CM

DDBrighter than V ~16.9Brighter than V ~16.9Bluer than (B-V) ~0.44Bluer than (B-V) ~0.44

BSS radial distributionBSS radial distribution

• Combine the two samples (HST+WFI)Combine the two samples (HST+WFI) Require the same limiting magnitudeRequire the same limiting magnitude

Use F555W (HST) and V (WFI)Use F555W (HST) and V (WFI)Select 19 BSS sample from the BBS-HST brighter Select 19 BSS sample from the BBS-HST brighter

than V ~16.9than V ~16.9

• Use HB and RGB as reference to study the Use HB and RGB as reference to study the BSS radial distributionBSS radial distribution The two samples are well matchedThe two samples are well matched 87 HB and 255 RGB in HST and 264 HB and 87 HB and 255 RGB in HST and 264 HB and

1984 RGB in WFI1984 RGB in WFI

BSS radial distributionBSS radial distribution

• The inner sample (HST)The inner sample (HST) The exact value of the limiting magnitude chosen The exact value of the limiting magnitude chosen

does not affect our resultsdoes not affect our results More concentrated than the RGB and the HB More concentrated than the RGB and the HB

stars selected in the same areastars selected in the same area

• The outer sample (WFI)The outer sample (WFI) Not statistically different (less than 1σ)Not statistically different (less than 1σ) Could be due to the fact that the number of BSS Could be due to the fact that the number of BSS

is too small to draw a definite conclusionis too small to draw a definite conclusion

BSS radial distributionBSS radial distribution

• Comparison with M3 and 47Tuc Comparison with M3 and 47Tuc In M3 and 47TucIn M3 and 47Tuc

The BSS are more concentrated than the reference The BSS are more concentrated than the reference population in the central regions.population in the central regions.

In the outer regions with the BSS being less In the outer regions with the BSS being less concentrated than the reference populationconcentrated than the reference population

Less well defined in NGC 6752Less well defined in NGC 6752Could not show up in a sample of this sizeCould not show up in a sample of this size

BSS radial distributionBSS radial distribution

• Computed the radial behavior of the Computed the radial behavior of the BSS relative frequencyBSS relative frequency Reach the maximum in the innermost annulus F Reach the maximum in the innermost annulus F

~0.42~0.42 Decrease to less then 0.04 as r increaseDecrease to less then 0.04 as r increase In the most external annuli the distribution In the most external annuli the distribution

shows a small upturn reaching ~0.2shows a small upturn reaching ~0.2

HB

BSSBSSHB N

NF

BSS radial distributionBSS radial distribution

• Normalized the BSS number to the RGB popNormalized the BSS number to the RGB populationulation

• Follows the bimodality observed in M3,M55 aFollows the bimodality observed in M3,M55 and recently in 47Tucnd recently in 47Tuc

• Extensive surveys in the outer region of other Extensive surveys in the outer region of other cluster are needed before we can conclude thcluster are needed before we can conclude that this is the “natural” radial distribution of BSat this is the “natural” radial distribution of BSS in globularsS in globulars

Comparison with other GCsComparison with other GCs

• Apply the same criteria adopted by Ferraro Apply the same criteria adopted by Ferraro et al.et al. Shift in magnitude and color to match the Shift in magnitude and color to match the

MS of M3MS of M3 Bright BSS with magnitude brighter than Bright BSS with magnitude brighter than

19.0 are considered19.0 are considered

• One single mass King model and two single One single mass King model and two single mass King modelsmass King models

255m

255m

Collisional ModelsCollisional Models• The models are described in detail in Sills & The models are described in detail in Sills &

Bailyn (1999)Bailyn (1999) Assume that all the BSS were formed via direct stAssume that all the BSS were formed via direct st

ellar collisions between two stars during an encoellar collisions between two stars during an encounter between a single star and a binary system.unter between a single star and a binary system.

The out put of these simulations is the probability The out put of these simulations is the probability that a collision between stars of specific masses that a collision between stars of specific masses will occurwill occur

Collisional ModelsCollisional Models• We must question our assumptions about the We must question our assumptions about the

formation mechanism for the most luminous formation mechanism for the most luminous BSSBSS Maybe the product of three stars coming together Maybe the product of three stars coming together

(mass greater than twice the TO mass)(mass greater than twice the TO mass) Some mixing mechanism, such as rotation, the Some mixing mechanism, such as rotation, the

stars can be significantly bluer, brighter, and stars can be significantly bluer, brighter, and have longer MS lifetimeshave longer MS lifetimes

Not formed through collisions at all ,but rather Not formed through collisions at all ,but rather through the merger of the two components of a through the merger of the two components of a primordial binary systemprimordial binary system

Summary and DiscussionSummary and Discussion

• A bimodal radial distribution(M3&47Tuc), A bimodal radial distribution(M3&47Tuc), more BSS surveys, covering the full spatial more BSS surveys, covering the full spatial extent of the host cluster, are necessary to extent of the host cluster, are necessary to determine just how common bimodality isdetermine just how common bimodality is

• A large binary population and high stellar A large binary population and high stellar density should lead to efficient production of density should lead to efficient production of BSS, yet we observe the reverse. BSS, yet we observe the reverse.

Summary and DiscussionSummary and Discussion

• Cluster’s dynamic stateCluster’s dynamic state M80 has very large BSS population and maybe at M80 has very large BSS population and maybe at

the onset of core contraction but models suggest the onset of core contraction but models suggest that BSS formation ended a few Gyr ago.that BSS formation ended a few Gyr ago.

NGC 6752 is undergoing a post-collapse bounce, NGC 6752 is undergoing a post-collapse bounce, a more advanced dynamical state than M80a more advanced dynamical state than M80

• Large number of luminous and hot BSS in the Large number of luminous and hot BSS in the cluster core casts some doubt on our model fcluster core casts some doubt on our model formation mechanismormation mechanism