Stars Collide……Stars Collide……

……..and then what?..and then what?

An overview of the advancements in modelling of stellar collisions, filtered through the biases

of Alison Sills

MODEST-6, Northwestern, August 2005

What can you get?What can you get?

MSMS RGRG WDWD NSNS

MSMS BS RG

sdB

RG

CV

LMXB

MSP

RGRG RG

He WD

CV

sdB

He WD

TZ

LMXB

MSP

WDWD CV

He WD

SN Ia

GRB

NSNS GRB

Table stolen from talks by Charles Bailyn, 1995-2000

Added a new kind of starAdded a new kind of star

PMSPMS MSMS RGRG WDWD NSNS

PMSPMS PMS MS RG RG TZ

MSMS BSBS RGRG

sdBsdB

RGRG

CVCV

LMXBLMXB

MSPMSP

RGRG RGRG

He WDHe WD

CVCV

sdBsdB

He WDHe WD

TZTZ

LMXBLMXB

MSPMSP

WDWD CVCV

He WDHe WD

GRBGRB

NSNS GRBGRB

Simulations done by Laycock & Sills 2005

Everything you wanted to know Everything you wanted to know about MS-MS collisionsabout MS-MS collisions

Freitag & Benz 2005

•Most masses•Most velocities•Most impact parameters•SPH simulations with ~few thousand particles•Results: mass loss, energy loss, deflection angle, angular momentum loss, merger or disruption

0.1

74.2

M2 in M

M1 in M0.1 74.2

Freitag & Benz 2005

dmin

Vrel

What happens after the collision?What happens after the collision?

We already knew that MS-MS collision products We already knew that MS-MS collision products retain chemical info about parents, are puffed up retain chemical info about parents, are puffed up and take a ~ thermal time to settle to main and take a ~ thermal time to settle to main sequence (e.g. Sills et al. 1997, 2001)sequence (e.g. Sills et al. 1997, 2001)

Evolution after the collision is like truncated main Evolution after the collision is like truncated main sequence evolution, and otherwise normalsequence evolution, and otherwise normal

Angular momentum problem solvable with disks Angular momentum problem solvable with disks and/or winds (Sills, Adams & Davies 2005)and/or winds (Sills, Adams & Davies 2005)

What happens with populations of What happens with populations of collisions?collisions?

M67 from Hurley et al: very nice fit to data; M67 from Hurley et al: very nice fit to data; no blue stragglers from single-single no blue stragglers from single-single collisioncollision

(Ultra-)simple cluster models + (slightly (Ultra-)simple cluster models + (slightly more) detailed collision models more) detailed collision models distributions of blue stragglers in CMD distributions of blue stragglers in CMD

(yes, I’ve been presenting these for a long time, but they’re still really (yes, I’ve been presenting these for a long time, but they’re still really interesting and they still tell us something)interesting and they still tell us something)

Core: 0 – 23”Mass function x = -8Formation time: 7 Gyr ago to nowKS test: 93%

Outside: 130 – 1200”Mass function x = 1.35Formation time: 7 – 1.2 Gyr agoKS test: 35%

Mid: 23 – 130”Mass function x = -3Formation time: 7 – 0.6 Gyr agoKS test: 44%

Monkman & Sills 2006

young, massive !

older, less massive ?

old, light ??

47 Tuc data from Ferraro et al & Piotto et alHST + ground-based

Beyond Blue StragglersBeyond Blue Stragglers Stripping of giants (Adams, Davies & Sills 2004) Stripping of giants (Adams, Davies & Sills 2004)

– not the dominant way of getting rid of bright – not the dominant way of getting rid of bright giantsgiants

Increased interest in mergers/collisions in Increased interest in mergers/collisions in context of massive star formation (Bally & context of massive star formation (Bally & Zinnecker 0502485, Bonnell & Bate 0506689)Zinnecker 0502485, Bonnell & Bate 0506689)

Runaway collisions Runaway collisions IMBH (SM & SPZ, MF & IMBH (SM & SPZ, MF & FR) – depends on knowing radii of products after FR) – depends on knowing radii of products after collision (Fregeau et al 2004, see Marc’s talk)collision (Fregeau et al 2004, see Marc’s talk)

Ultra-compact X-Ray Binaries (see Jamie’s talk)Ultra-compact X-Ray Binaries (see Jamie’s talk)

The Story So FarThe Story So FarWe basically understand blue stragglers if We basically understand blue stragglers if

they are collision products (and perhaps they are collision products (and perhaps some of them are….)some of them are….)

We can model and evolve collisions of We can model and evolve collisions of non-main sequence stars, and we will if non-main sequence stars, and we will if you make usyou make us

But we’d rather be modeling binary But we’d rather be modeling binary mergers (if we could only figure out how)mergers (if we could only figure out how)