Binary Origin of Blue Stragglers Xuefei CHEN Yunnan Observatory, CHINA

outline Pros and cons for binary origin of blue straggler stars (BSS) Binary evolutionary channels Binary population synthesis Long-orbital period BSS

Pros and Cons for binary origin of BSS Surface abundance of BSS (Mucciarelli’ talk) Carbon/Oxygen depleted in 47 Tuc (Ferraro et al. 2006) but not in M4 (Lovisi te al, 2010) Radial distribution (Ferraro’s talk) Bimodal for many GCs and dSphs, but in Omega Cen, NGC 2419 and Palomar 14 : the same as that of other cluster stars Even in the core of GCs: there is a clear correlation between the number of BSS found in the core and the total stellar mass contained within it. (Knigge, Leigh and Sills, 2009, Nature) Correlation between the BSS specific frequency and the binary fraction (Sollima et al. 2008, A&A,481,701) Theoretical studies and comparison with observations (the redder sequence of M30, Dalessandro’s talk)

Conclusively, binaries seem to dominate, or at least play an important role, for BSS formation in open clusters and in the field, as well as in dwarf spheroidal galaxies and globular clusters.

Binary evolution mass transfer (MT) merger of two components coalescence of contact binaries (W UMa) dynamical unstable MT (rapid) angular momentum loss of low-mass binaries (AML) Binary collisions binary-binary, binary-single Binary Origin of BSS

Mass transfer (MT) a MS, or HG or FGB/AGB stars progenitor of BSS Lu et al. 2010, MNRAS, 409, , case B Chen & Han, 2004, MNRAS, 355, 1182 Tian et al, 2006, A&A, 455, 247 Chen & Han, 2008, 387, 1416 Lu et al. 2010, MNRAS, 409, 1013

MT (2)-surface abundance Initially, the accreted matter is from the surface of the mass donor. However, with age increasing, some material in the core of the mass donor may also be carried onto the surface, then transferred to the accretor. In this case, what will we see? — Thermohaline-mixng. Chen & Han,2004,MNRAS,355, 1182 Signals of central burning of the donor should appear in surface abundance of BSS !

MT (3)-Location on the CMD Lu et al. 2010, MNRAS

Binary merger: coalescence of contact binaries Webbink 1976 Material of the companion (the lower one) stores (likely homogeneours) in the envelope of the merger. -strong He enhancement of the final object! FK com Contact semidetacted contact semidetacted … Evolution of W UMa: Note that the common envelope is convective ! Li, Han & Zhang, 2004, MNRAS,351,137 Li, Han & Zhang, 2004, MNRAS, 355, 1381 Li, Han & Zhang, 2005, MNRAS, 360, 272

The remnants may lie to the left region of ZAMS defined by normal stars - It is common in observations ! Binary coalescence (2): Chen & Han, 2008, MNRAS, 384, 1263

Binary merger: dynamically unstable MT (rapid coalescence) Common envelope The core of the donor (H burning ) The secondary (MS) A massive MS star ! The mass ratio > 4 or even larger ! (Ge et al. in preparation) The secondary is nearly unevolved ! The material of the secondary stays in the core of the remnant! Very similar to that of stellar collision.

Binary Merger: AML of low-mass binaries Some low-mass binaries with late-type components show clear signs of magnetic activity, which indicates that the systems evolve by way of a scenario implying AML by magnetic braking (Mestel 1984). The AML is at the expense of the orbital angular momentum, resulting in orbital decay and the components approaching each other. A detached binary, then, may become a contact binary and finally coalesce at or before the cluster age (Stepien 1995). If AML is not considered, such binaries are likely detached and nearly unevolved, then have no contributions to BSS. But in old clusters, these binaries may be much important !

NGC 2682 AML Binary coalescence Chen & Han, 2008, MNRAS, 384, 1263

Binary Population Synthesis : ISED Beta=0.5 for mass transfer in HG Beta=1 in HG Chen & Han, 2009, MNRAS, 395, 1822

AML of low-mass binaries

CMDs

colours

We only obtained one fifth of observed blue stragglers !!! Birthrate

Long-orbital-period blue stragglers FGB/AGB WDBSS MS Rappaport et al For giant stars with degenerate cores, there exists a Mc - R relation.

The companion mass-period relation During stable RLOF, the donor is Roche-lobe filling. Given a Mc, R, RL and P can be determined ， i.e. there is a Mc-P relation The RLOF stops when donor’s envelope collapses, and we then have a Mf-Pf relation The relation does NOT depend on dJ

Chen, Han, Deca, Podsiadlowski, 2012, MNRAS, submitted Mwd P(days) Chen & Han, 2008, MNRAS, 387, 1416

Is there a companion mass- orbital period relation for long-orbital-period BSS observationally ? HR3220 (a field BS): P=899d, Mcomp=0.42(+0.09,-0.05)Msun (Fuhrmann et al. 2011, MNRAS, 416, 391) NGC 188: a binary star fraction of 76 per cent

Many thanks ！