The blue stragglers formed via mass transfer in old open clusters

B. Tian, L. Deng, Z. Han, and X.B. Zhang

astro-ph:0604290

What’s ‘Blue straggler’?

• lie above and blueward the main sequence turn off (MSTO) in the Color-Magnitude Diagram (CMD)

• observed in almost all star clusters, dwarf galaxies and presumably in all other stellar systems (Stryker 1993)

• Remnants of stellar merger events, or still binary systems with luminous blue components

• Can not be predicted by the single-star evolution theory

Mechanisms for BS formation

• Hills & Day (1976) proposed that the remnant of a collision between two main-sequence stars could produce a blue straggler.

• All types of colliding encounters including single-single, binary-single, and binary-binary have been investigated by subsequent studies (Ouelette & Pritchet 1998;Lombardi et al. 2002; Fregeau et al. 2004).

• Mass transfer in close binary systems was presented for the first time by McCrea in 1964.

Primordial BS (Davies et al. 2004)

the primary could transfer material to the secondary through the inner Lagrangian point after becoming a red giant and filling up its Roche lobe.

The material is stripped off of the envelope of the primary and is accommodated on the surface of the secondary.

Then, as the secondary is gaining material gradually, it can became a more massive main-sequence star with a hydrogen-rich envelope.

This “modified” (or, newly formed) star can stay on the upper extension of the main sequence until finishing hydrogen burning in the core.

As a result of mass transfer, the star has its main-sequence lifetime doubled compared to a normal star with the same mass.

Why study?

• visible tracers of binary populations, and additionally they provide an opportunity for learning how interactions in binary systems affect stellar evolution (Pols & Marinus 1994).

• appreciable frequency of single-single star collisions generally occur only in the cores of the densest clusters (Hills & Day 1976)

• observational constraints on stellar collision cross sections for tidal captures, direct collisions, and the corresponding productions in dense stellar environment (Bailyn 1995; Lombardi et al. 2002)

Mass transfer types

• case A: hydrogen burning in the core—a BS in a very short-period Algol system —ends in coalescence of the two MS stars—single BSs

• case B: with the rapid core contraction preceding helium ignition —mass-transfer occurs in slightly wider binaries and results in a short-period circular binary containing a BS with a white dwarf companion.

• case C: with helium having been ignited in the core—BSs in longer period binaries—mass transfer or by accretion of material from an AGB star wind.

The model of primordial blue stragglers

neglected the influences due to tidal evolution, magnetic braking, and stellar spins on the evolution of binary systems (for simplicity) ?

donor

accretor

ZAMS

Mass exchange

Mass ratio=1observed

Mass transfer terminatedCore-H burning of the secondary ends

donor

accretor

most of the lifetime of the binary after mass transfer starts is spent on the blue side of the CMD

(defined as (B-V) < 0.545 – the color of MSTO of the 4.0Gyr theoretical isochrone).

BS (1.4M_sun+0.9M_sun) can last a long enough time (1.228Gyrs) in the target region to be observed

Monte-Carlo simulations of the primordial BSs in M67

• Grid: donor: 0.1-2M_sun,0.1Msun

accretor: 0.1-donor,0.1Msun

orbit separation: 1.0-50.0R_sun,1.0R_sun

age: 1.0-6.0Gyr,1.0Gyr• 12000 primordial binaries at its birth (Hurley er al. 2005)• solar abundance• the orbital eccentricities of close binaries are all assumed to

be e = 0• IMF of the binary:• uniform distribution of mass ratio (Hurley et al. 2001)• flat distribution of orbital separations (Pols &Marinus 1994)

19PBs experiencing mass exchange

4 in the region of BS

15 progenitors or decendants of PBSs

decendants

Most of them shoule be photometric binaries, some of them could be low mass primordial stars that also experienced mass exchange

9 above the upper limit of the simulatived PBSs region

other formation mechanisms but not exclusive

1) Most massive binaries

2) Short orbital separation

High eccentricity

1) dynamical encounters within the cluster environment and perturbations from nearby stars or binaries could alter the orbit parameters of the primordial binaries (Hurley 2005)

2) Or created via wind accretion in wide binaries.

1)11% primordial BSs among 1984 PBs experiencing mass transfer2) Most of these PBs stay in between the turn-off and about 2 magnitudes below the turn off of the 4.0Gyr theoretical isochrone.

I

II

I) The bluer the color of the system, the longer the period.

II) Too little samples

Conclusions

• direct collisions are considered as the principal mechanism to form BSs in a dense stellar environment (Fregeau et al. 2004), while mass transfer in a close binary system is regarded as the main way to form BSs in a sparse environment (Mathys 1991)

• BS (1.4M_sun+0.9M_sun) can last a long enough time (1.228Gyrs) in the target region to be observed.

• BSs with short periods entering the primordial BS region should have a mass-transfer origin.

• Period distribution of the PBs should be studied in the future.