9 Proper Motion with HST: Searching for High-Velocity Stars

in the Core of the Globular Cluster 47 Tucanae

Georges Meylan, Dante Minniti, Carlton Pryor, Christopher G. Tinney, E. Sterl Phinney, Bruce Sams

This document was prepared for submittal to Science with the Hubble Space Telescope - XI

Space Telescope Science Institute, 1996 Conference,

Paris, France, December 44,1995

February 13,1996

Thisisa preprintofapaperintended forpublicationina journalorpmceedings. Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author.

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Portions of this document may be fllegible in electronic image products. Smages are produced from the best available original document

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1.1.

One of Llie most intriguing Tea.tures of globular cluster dynainical evolution is i t s instability. Wlietlier viewed as arising from two-body relaxation (ItiPnoii 19611, the mass-segregal.ion instaI>iIity (Spitzer 1969); or the gra.votherma1 iiistability (.4ntonov 1962, IJ!;~ide~i-B~ll k \,2'ood 19tjS), the evolution of t.he core of a globu1a.r cluster is expected to reach a. state o f \.cry IiigIi cf(:nsity i n a finite time: a plieiiomeiion called "cor(: collapse?." The liigli iieiisitics rcsult.irig froiii core collapse iiicrcase hot 11 e~icount(?r ancl collision r:ites arid C;LII

actually modify the st.el1a.r i>opulation i n tlie cluster ceiit.er, producing sucli exotic objects

for ii wvi(:\vj a.rid possibly even affecting such traditional cluster properties a.7 the slia.lxt of t , l i ( ! Iioi-izontal I>railcli (11jorppvslii & Piotto 1993). "flit st,c:ilar popula.tioii can i n t ur i i

i 11 f l u eiice t lie d y 11 aiiiical evoi u tio 11.

i i is no\$' [vel1 esta1)lisiied that I>ina.ry st.ars c,oiitrol t hc late stages of corc coliapse (see Hut. et a.1. 1992 for a review). Gravitational encounters betweeii hard binaries and passing sta.rs leave the binary more tiglitly bound, with the released energy "heating" the b inary ' s e i ivirqni i ient . f3inaries are concentrated i n the core due to inass segrega.tioii and thrg ci\ll

ciitla!;. ! r a l t . and even reverse the collapse. Primordial binaries provide a finite 'fossil fuel" use the Iiina.ries are eveiitually destroyed by being driven t.o coalescence, by being pted I)? an encounter xvitli other bina.ries. or li!. lieiiig ejected from t h e cluster by tlie

recoil from a strong intera.ct.ion. At very high core densities, new binaries can I>e formed bj. encounters between three single stars (Heggie 1975) or hy "tidal-capture") i n which t.wo stars approach so closely that they dissipate enough of their relative kinet.ic energy through t,idai oscillations to remain bound (Fabian: Pringle, k Rees 1975).

i- !ifoi.tuiiately! tlie hiliar!. popiilat,ion i n deiise cluster cores is difliciil: to s tudy ob- ioiially. Tkcausc of cron:dir,g. radia.1 velocity xa rc l i e s arc confined 1.0 i fie few brigtit

giants (e.g., Gebhardt et al. 1993). The broadening of t,he iiiairi sequence by binaries is only sensi ti \:e to systems with two 1 u mi no us stell a r corn ponenty . N u mcrical si m ulat-ions predic.t. tliat I:iii.iiy binaries in dense coi'cs \vi11 contain at. lcasi, oiie lieavy, da.rk, tvliite dwarf 01'

iieutroii s tar. The numbers arid kinds of esotic stellar ol~jects i n the core arc a useful probe of 111. i,iii;irJ- population because they a.re produced 11~. st.el1a.r collisions that occur during gravitational encounters involving binaries. But these exotic objects are ra.re, making small iiuii i t x r statist.ics a. problem. Anotlier possible probe of the binary popula.tioIi is suggested by t l i t discovery of two high-velocity stars in the core of t.he globular cluster 4; 'Tuc (Meylaii c i :ti. 1991). These stars very like1.v fia.ined their' iiigli \.(>locities from e r i c a i i ~ ~ ~ o r ~ \q;i1.li h au l binaric:.5.

Binary Stars and Globular C l u s t e r Dyiiainical Evolution

; i s l ; ,1,114.1y . . . S-r; ty sources: I)iue stra,gglers (13%) ~ and iiiiIli-stc.oiid pulsars (SW h~leylait 1 W I

I .

I .2. Iz 7 Tuca nae

O i l ( : (ji' I I i c closest ( U 2 4.6 kpc; (771 - M)l., = I ;j.:jj ai!ci iiios? ii-ia.,sivc ( jW 2 10'' , , U T ! ) g I ~ i ) - tilai. ciiisters i i i our Galaxy is 47 'i'i~c. It. has l ~ e e ~ i iiiteii.~i~~.eI,y studied, Imt.11 photornetrica.ll~ aiid ki:ienia.ticallg, for decades. 'f his cluster is relatii.elL. iiietal-rich ( [ F c / f f ] 2 -0.7), has ;III +ge of 2 13 Gyr (Hesser et al. 1987), has low reddening ( E ( U - V ) ZY 0.04), a.nd h a s oiily small galactic contamination. The high concentrat.ioii ( c 2 2. 1) of 47 Tuc suggests that. i i is d J. i 1 a i n icall 2; evolved and ) a1 t ho ugh we1 1 fi t ted 1)). 1x1 u I ti- i n a s aiiiso t ro pic Ki jig- Mi ch ic riiodek: that, it. is evolving through a sequence of quasi-equilibri\im states towards i t s ul- tinlate fa.te of core collapse (Meylan 1989). There is an increasing amouiit of direct and indirect oliservational evidence for stellar encounters and collisioIis iiL t h e core, of 47 'L'uc. 'l'lic pre\:iously-kno\~rii single X-ray source has been resolved by ROSAT into a.t least five components (Hasinger et al. 1994). Besides the high-ve1ocit.y stars: other evidence for biiia,- rics i n -17 Tuc comes from the eleven millisecond pulsax (Manchester et a.1. 19911 Robinson PI. ai . lil!tS) and tlie very high density of ccntrall:--clirstcreci 13Ss obse rv~d by I-IST (I'nresce

3

et al. 1991). With one of the richest populations of collision products, 47 TUC is clearly an ideal laboratory for confronting theoretical predictions with observations.

Radial velocities measured for 550 stars within two core radii of the center of 47 TIIC (Gcbliardt e t al. 1995) have increased the sample of high-velocity stars from 2 to 8. With velocities more than 32 kin s-I from the cluster mean (about three times the core velocity dispersion of 10 km s-l), these stars are moving a t close to the cluster escape velocity. 'rhc original high frequency of rapidly moving sta.rs (2 out of 50) stimulated theoretical studies of t l i r ~ i r expected numbers (Phinney IC Sigurdsson 1991, Sigurdsson IC Phinney 1995). While t!irt la.rger saniple yields a loiver frequency, the studies have demonstra.ted t h a t , t l i c : n u n i b c ~ o f irigli-velocity stars is a powerful tool for probing conditions i n the core and testing the d ~ ~ n a n i i ~ a l niodels. Howwer, larger sa.inples a re needed to exploit this t.ool.

1.3. Proper Motions with HST

Consequently, we are using the Planetary G'a.mera. to obtain a complete census of Iiigli- 1.elocit.y stars inside half a core ra.dius of 47 'I'uc. down t.o a limit , of Ci N 22. This ceii- sus includes stars on the red giant branch (RGB), subgiant branch, red horizontal branch (RHI3): upper and lower main sequence (down to - K3V), and white d\irarfs (see Figure 2 lwlon,). W e do not, need to take ima.ges wi th different filters: sinctt good color iiiforiiizt.i(iii for the stars i n the core of 47 ' h c is already a.vaila.l)le from previous 16'1' p r o g r a m (e.$.: Guhathakur ta et al. 1992).

Ikperieiicc obtained wit l i the tccliniqiics of CCD astroinetrg: 011 t.lie ground Iias sIio~?.ii that CCDs ntake almost ideal devices for failit, relative astrometry. Astrometric positions Ivitli a precision of 5 mas/epoch are now being routinely measured from the ground (Monet, et al. 1992; Tinney 1993). In the absence of atmospheric seeing (the fundamental liniitation for ground-based CCD astrornetry), there is no reason why a suitably-designed astrometric prc'gra~ii witli \VIFPC2 cannot ac1iiei.c p~e~i~i : io l i s liinitcti only by photoit statistics. 'I'liis

illpails that. measuring proper motions with 1 - 0 u ncertairit its of 0.2 mas/?;ear sl~ould lw e\.able. \\'e liave a conserva.tive approacll, a.iiiiing firs[, for the high-velocity sta.rs, but .

est.imates siioiv that we should be ahlc to nica-511re proper motions for most of the stars i i i

0 1 1 r fr3,mes.

1.4. Scientific Goals

Given the strong limits imposed by seeing on tile ground-based acquisition of radial velocitks i n the core of 47 Tuc, HST is the only telescope able to provide: i n the very core of the cluster: a kinema.tic sample consist.ing of thousands of stars. The preserit. census of proper inot.ions \vi11 provide uiiique constraints on: E relasatioii processes (possible a.nisot ropy of t h c \-docity dispersiori). * col!isioii and ejection ra.tes (rnea.iiingfu1 frhciioii of !iigii-veIocity sli j1.s f r o i n i i large s a i i l l j l ( b ) .

0 iiic i;,rin of the velocity distribution i n t , l 1 ( 3 c01.c as ; I I ' i i i i c t , i o i i of' io sttllar. riiass. aiid * iiiii.5~ segregation.

Yatural by-products of this project. aIi.c!aciy oi>ti:inwi f ro i i i pliotoiiict.rg of the: PCI . iYk.2. \VF3: a.nd iYF4 frames are interesting pci- .ye and are discussed i n Section .3 I~elo\\~.

2. The Data

2.1. The Observatioiis

1 he iifteen WFI'C2 f r a m e constituting the first. epoch observations for tliis project were obtained by KST 011 October 25, 1995 (Cycle 5): with the P C field c.ent,ered on the core of 47 Tuc. Each exposure lasted 350 seconds and was ta.ken th rough t,he F300W filter, whicli h a 5 a passband similar to U . The fina.1 combined irnagc has total exposure time of 1.46 l i o i i r arid reaches F3OOSY = 25 mag for :j-g dettctioiis.

7 ,

4

We selected the F300W filter in order to reduce the range of brightness [)resented by the stars i n t,lie field, t h u s preventing the saturation of pixels by red giants and allowing litore or less a11 stages of stel1a.r evolutioii i n the cliist.er i o lie seen. This filter allows tl i t .

detectioii of iuain-sequence (MS) stars with a. wide ni‘ass range siniultaneously wi th TCG 13, RIiI3, AGB, B S ; and white dwarf (WD) sta,rs, without a.tiy of these stages being SO briglit coinpared to the others that multiple exposures with different observing times are required. For a similar number of stars detected, the dynamic range needed with the F300W filter is oiilg about G inagnitudcs; coinpared with 10 ma.gnit.udes in the optica.1 (c.g.. Ij(:t\YcClI t,hc IX:I3 and 1,lie \VI) sequence). The I tHB a.nd I3S stars are hrigIit(;sI. (see Figurc 2). Swiipliiig pimblcrns a r e also reduced i n the IJV cornpa.red ~ v i t l i optical passbands. We prefer t he w i d t I ; filt.er F30n\17 1.0 tlic ,Johnson ( J filter 13313\17 Ixcause of the liigliei. t l irouglipii t (a gaiii o f ;timiit. 0.5 iiia.g) a.iid a. reduced red leak, both crit.ical for t l ~ e faintest oljjccts. ‘1’11~ r c ~ l li.i~.l< i i i til(: I~~’30O‘t‘i~’ filter is not it problem for. o i i r proposed scier1c.c:: since C,OIOI. i i if()riii:i .t ,ioii

j‘: avitilal)le. ’l’lie iiiajor disadvaiitagc of the F300\~1~ filter is that i t . is less \veil ~ . ; t l i I~ra t~c~l t l i a i i ot.tier \YFPC2 filters. Again, this does not affect our scient,ific results.

2.2. T h e P h o t o m e t r y

?;;)(\ 01 1-1: V:W ol)t.;l.iired wit.h tlic T);\OPI!OT T i p c k ~ g r ~ \\.it h i l i i l ? A l ? . . \ t w i t S:OOO s i axs ’wrc ideiitiiied i n the PC1 field and about 23:000 stars i n the other t,hrcc (W1-2, \TI%: N7F’4) fields. for 2 iota1 of more than 30;OOO sta.rs. The results reported here were obtained iising aqcrt.iire j i i.iotonletry witli a 2-pixel radius and the skj. defined by a i aiiritilus ivit.11 10- and 13-pixel radii. \lie followed the calibration procedure described hy Holtzntan et al. (1995, their Table 10). Tjrpical photometric errors a.re cTF3001tf = 0.01, 0.04, and 0.25 at F300M’ = 16: 20. arid 23; respectively. Based on art.ificial star experiments, ive estiina.t,e tltat the dat.a are 100% complete down to F30OI.I’ = 19 a.nd 50% complete at. F300\5’ = 22.

2 .3 .

CCL> astro1iietr.v from tlie ground has shown tha t three fuItda11lc11tal issues inust bc a& clressed i.? designing the observing prograni for Iiigh-precisic+ii rci1a.t ivc ;i.st.roiiicLr>’. Once ail t.!irt:c issues are so1i.ed: obtaining positions accurate to better tfiari 1%; of a. pisel cati cltSi1.v be a.chkved: pro\.ided t1ia.t the required photon counting 1iinit.s (= 10: 000 photons) cam be rcaci1ed. e The first requirement is that the astrometry must be carried out i n a. dZfle7~2tial mode. ‘Tra.ditiona1 phot.ographic astrometry lias always been limited by the need for a detailed kriowledge of t.hc field distortions (if the field is not fla.t). If it, cannot he guarant.eed (I

pi~iori wheri. t l i ~ a~Iruinet . r ic sta.rs will be p l a c d i:i t liat field. it is essciitial to lx ahle t i ) i1;itt.e~ tlic iicM i o liigli precision. On tlit grouiiti. CCDs obvia1.c t.Iiis p roh le i i i hy a.IIo\viijs, oiic to al\vi:?-s place tlic target objects ba.ck on the sa.mc place O I I the CC1) ti) u . i t l i i i i a feu. pixels; at ivliich poilit the field ciistortioiis cancei out a.nd high p r -ioii i:aii he Ititcf\c<i. ‘l‘liis st.rateg\. is a(.iopted f o r our observatioiis: all of our fra.mes are niatie \vi1 I1 an ideiitic;tl roll a.iigle aiid I)?. placing the stars at the same position oil tlie PCl, e The secod reyuireore~it is that the PSF iiiust be <\:ell-sampled by t.lie CCD. It. has beeri realized i n recent >‘ears that CCDs do not have uniform sensit.ivity across a pixel. I n tlir worst case: the sei-isitivit); can va.r\; by as niucli a5 5% across a. single pixel. ‘This ohviously lias serious iiiiplicatioiis for a program whicli a.iins to measure positiotis 1.0 a.11 acci1ra.c): of better than 1% oi ̂a pixel. Because IiST images are undersampled even by the PCI., ‘IVC

lia,ve designed a prograin of 15 exposures per epoch that incorporates a dithering pa.ttern of s u b-pisel shifts ii;it li dirt‘erent values in order to improve the spatia.1 resolution. Tliis enables US to regain the oversampling of the PC1 images required for high-precision astromctry.

The last requirement is to have a distant reference fra.me. Fortunately, this is a simple problem to solve i n the case of 47 Tuc. A background of stars in the SMC provides a rich reference frariie of 5:tars wliich caii be easily identified 011 color-niag~iitridc dia.gra.ms (see

The Astroinet,ry: Measuring Proper _Motions \~:it.h HST

5

3 . -2 s t rc p 11 y s icn 1 R > - prod uc t P :

3 .2 . Color-Magnitude Diagrams

Figure 2 prese1it.s two color-magnitude dia.grains for the core of 47 'i'uc jl'C1 c1at.a oir!y). :'ill stages of stellar evoIut,ioii i n the cluster arc prcsent i n our da.ta.: '1'0 s i - 0.8 .t.l I.. lower M S sta.rs with M 2 0.5 M,: RGI3 stars; HB stars : AG13 stars . 135 sta.rs. ; I l l t i IVD stars. (Though WDs mus'i. be present i n oilr data, we a.re riot able to show theiii i : i 13%. 2 because the complement.arg optical photometry is not deep eiiough yet..)

with

G

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.... . I . . : I ' '

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. . . . . . ,. i . ._ ;/ ,._ .......... ..__ 1 7 1 ....................... . . 1 : j i 7 6 . . . ' . . .

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. . . . . . . . . . . . . . . . . . . . .

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b'igure 3 . Examples of light curves for 6 variable blue stragglers. bppcr paiieis: pulsating variables of SX ['he type. LonFer panels: sem-detached eclipsing h i n a r k , for n-liicli Giliiiaiid CI a!. (199.7) and Ednlonds et ai. (1996) s!io\\; tllc P I : : ~ ~ { . T : m h I .

7

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4. Coriclrisioi~

W-7405-Eng-48.

9

References