INAF, Osservatorio Astronomico di Roma

1. Globular Clusters: a laboratory for binary stellar evolution F. D'Antona INAF, Osservatorio Astronomico di Roma XI Advanced School of Astrophysics, Brazil, 1-6 September 2002

2. Summary * Low mass X ray binaries and millisecond pulsars GCs * The case of PSR J1740-5340 and the helium remnant WDs

3. King, Cool & Piotto HST data The global HR diagram of NGC6397

4. The other stellar remnants in GCs • White dwarfs are today’s stellar remnants in GC (M evolving ~ 0.8Mo • What about the remnants of the more massive stars? We have already seen that inermediate masses influence the chemistry of the GCs through selfpollution from their low v winds: these stars too (M<6Mo) evolve into (more massive) WDs • 3) More massive stars have evolved into supernovae in the first phases of life of the clusters: but we see their remnant neutron stars population through the huge millisecond pulsar population and the (few) low mass Xray binaries

5. Binaries containing neutron stars Millisecond radio Pulsars Low mass companion B ~ 108 – 109 G Low mass X-ray binary Low mass companion B ~ 108 – 109 G

6. Three are ultracompact! X-ray binaries with known Porb in GCs a Deutsch, Margon, & Anderson 2000. c Ilovaisky et al. 1993. d in’t Zand et al. 2000. e Sansom et al. 1993. f Homer et al. 1996. g van der Klis et al. 1993. h Stella et al. 1987.

7. X-ray imaging of the core of 47 Tuc (Chandra) Grindlay et al. 2001 Science 70Ks exposure with resolution <1”: 108 faint (Lx=1030-33 erg s-1 X-ray sources located in the central 2’x2.5’, >half with Lx<1030.5 All the 15 MSPs are identified. The authors suggest: 50% are MSPs 30% accreting WDs 15% MS binaries in flare outbursts 2-3 quiescent LMXBs with NS

8. The Pulsar population from D.R. Lorimer 2001

9. The millisecond Pulsar population • ~1400 radio pulsars known • ~100 have at least one of the properties: • * very short pulse period (~77 have P<12ms) • * relatively weak magnetic field (~46 have B<1010G) • * are found in binaries (~66) • * are located in a globular cluster (~50) • 3) One major class of binary radio-pulsar in the disk have low mass companions (0.1 –0.4 Mo) and nearly circular orbit. • Porb goes from a fraction of day to 103 days • 4) Most of the wide systems thus seem to have low mass helium white dwarfs companion, remnant of mass transfer starting when the secondary star is a subgiant or a giant with a helium core.

10. List of MSPs in GCs

11. MSPs in Globular Clusters

12. Why so many MSPs in Globular Clusters? In the field of the Galaxy, it is necessary that : 1)the primordial binary in which the NS is formed survives the SN explosion; 2) The binary parameters allow, later on, a phase of mass transfer which spins the pulsar up to ms periods GCs are the best place to produce binary systems containing a NS even a long time after the NS independent formation

13. (Why so many neutron stars in Globular Clusters?) (In the following we will not discuss a key problem: the embarassing presence of so many NSs in GCs. In fact, in the Galactic disk, <v>~200Km/s (implying some “kick” velocity at the SN event). Being vesc~25Km/s typically from Gcs, a low fraction of NS should be retained in the clusters. For this reasons some invoke an AIC scenario for the NS formation)

14. Bound system of two stars formed from an unbound configuration: a sink of orbital energy is required: Mechanisms for binary formation • Collision involving three stars; one takes up the excess energy and escapes (very small probability); • Exchange interaction of a NS with an existing binary: the NS replaces one of the components; • Tidal capture: deformation of a normal star by close passage of a compact star takes away kinetic energy of the orbit, then dissipated through oscillations and heating of the envelope. If DEk exceeds the total positive energy of the initial orbit, a bound system is formed.

15. Which mechanism dominates? • Exchange encounters: • Direct exchange • Resonance encounter in which a temporary triple is formed, followed by the ejection of the third component (more frequent than i). For equal masses M • Gbin ~ nNS nbin M a/v inf(v inf =vel. at large separation) • Tidal captures: (require d<3R) • Gbin ~ nNS n(m+M) 3R/v inf • (cross section is favoured for exchange encounters -a versus 3R-, but nbin<<n)

16. 1) MS of RG companion (relatively distant encounters) 2) Binaries with WD companions, from direct collision with RGs Tidal capture 1) Mass transfer will begin soon, leading either to larger or to shorter Porb, depending on the nuclear evolution of the acquired companion. The system becomes a LMXB 2) GR - AML can bring the system into contact at very short (minutes) Porb. Mass transfer increases P leading to a system like the 11m binary in NGC 6624 (But also another channel..)

17. 1)LMXB phase preceding the MSP stage; 2) mass transfer stops 3) the radio MSP emerges Orbital evolution: * Pinitial>Pbifurcation: nuclear evolution drives mass exchange: MS  RG  RLO NS spun up  Porb increases  He WD + MSP ** Pinitial<P bifurcation: systemic AML drives mass exchange during MS  Porb decreases  when MSP appears, the strong MSP radiation evaporates the companion (!) single MSP Old Neutron stars spun up by accretion from a companion Recycling model for MSPs

18. Bifurcation period The evolutionary meaning of the bifurcation is: has the secondary enough time to grow a helium core? In that case, it will evolve towards large orbits

19. The bifurcation period from 100 binary sequences Podsiadlowski, Rappaport & Pfhal 2002

20. The known binary periods of LMXBs in GCs, ranging from 17hr to 11min …. Again about the bifurcation period in GCs seem all in favour of tidal capture which leads naturally to periods 13 –17hr. These values are below or very close to Pbif , so that the systems will evolve towards shorter Porb In addition, partial hydrogen burning in the core of the secondary, may lead to very short Porb (with the secondary transformed into a degenerate white dwarf having a hydrogen-helium intermediate composition, see Ergma and Fedorova 1998, Podsiadlowski et al. 2002, and the following lesson)

21. Most binary MSPs have long orbital periods and mass function identifying the companions as low mass helium white dwarfs The “normal” MSP companions *Porb’s are compatible with the evolution with mass transfer from a subgiant or giant, which finally leaves a helium WD remnant of mass 0.45 >~ M/Msun>~0.2 *if we notice that the radius is determined mainly by the core mass Mc, and put R=R 2,R a relation (Porb,Mc,M2) is obtained * Pfin is then obtained by assuming Mc=M2 (at which stage Mdot stops)

22. Is the recycling model reasonable? As we have seen, the majority of binary MSP have long Porb and He-WD companions, as predicted by evolution at Pinitial>Pbifurcation . (Caveat: in Gcs the LMXBs with known Porb are all at short Porb, and GC MSPs have Porb<2.6days -apart from two at hundreds of days- It is possible that in GCs the long period binaries have been broken by further encounters) The answer is YES! The recycling model makes sense

23. Open Problems But: 1)Birthrates of LMXBs << MSPs; 2)why we do not find MSP modulation in many LMXBs? 3)Where are the submilllisecond pulsars? Proposed solutions: 1) Mass transfer very large in LMXBs; 2)Accretion and spin up inhibited by propeller

24. Birthrate of LMXBs ~1/10 – 1/100 than birthrate of MSPs The problem of the birthrates *this is found both in the disk (Kulkarni and Narayan 1988) and in the GCs (Fruchter and Goss 1990); *observationally exhacerbated by the fact that the radio MSP population is not complete, the LMXBs are all known The result depends mainly on taking t ~ 109yr as lifetime of the LMXB phase IRRADIATION by the X ray emission enhances the mass transfer and may be at least part of the solution

25. There are now only 3 LMXBs (transients) which show X-ray millisecond modulation The lack of X-ray MSPs SAX J1808.4-3658: Ps=2.5ms Porb=2hr (Wijnands & van derl Klis 1998) XTE J1751-306: Ps=2.3ms Porb=42m (Markwardt et al.2002) XTE J0929-314 Ps=5.4ms, Porb=43.6m (Galloway et al. 2002) The “standard” LMXB phase is, after all, NOT the main acceleration phase of the MSP? Again this may be linked to the evolutionary timescale of LMXBs

26. In the standard LMXBs evolution, enough mass may be accreted to lead to Ps<1ms The lack of sub-ms pulsars • Either conservative mass transfer leads often to accretion induced collapse of the NS to Black Hole (for mass MNS<~2Msun for a “soft” EOS, to MNS>~3Msun for the stiffest EOS) (Cook Shapiro Teukolski ); • Or –in short Porb systems- the detection of the radio pulsar is hampered by the strong orbital Doppler modulation of the radio signal • Or not enough matter is accreted (see later)

27. When Rm>Rcorotation, the matter can not be accreted (Illarionov and Sunayev 1975) The propeller does not work! Consequence: We need to expel matter far away from the NS surface! 1)But in these systems Rm~RNS, so there is a huge energy requirements to eject matter 2) Even with ad hoc tuning, the maximum efficiency of a propeller is <50%

28. When the radiation pressure of the rotating magnetic dipole becomes large enough, it prevents accretion directly at the inner Lagrangian point! The radio-ejection (Burderi et al. 2001) • Requiremnt: • Mass transfer must stop (or be very much reduced) to allow the radio pulsar switch on • 2) Ps short enough that P psr > P matter

29. The first MSP in an interacting binary: J1740-5340 and in a globular cluster! is observed during the radio-ejection phase? (Burderi D’Antona & Burgay 2002)