Stellar collisions and their products Melvyn B. Davies Department of Astronomy and Theoretical Physics Lund University www.astro.lu.se
KEY IDEA #1 Collision rate depends on V.
Stellar encounter timescales Cross section is given by τ enc 10 11 yr σ = πr 2 min 10 5 /pc 3 1 + 2G(M 1 + M 2 ) R min V 2 Timescale for a given star to undergo an encounter is n M M R Rmin V 10km/s
KEY IDEA # 2 What gets hit depends on V.
What can collide with what? MS RG WD NS MS RG BS Interacting Binaries WD NS GRB
Stellar radius vs cumulative number of collisions low-mass star in a globular cluster (Sills, Adams, and Davies 2005)
KEY IDEA # 3 Outcome of collision depends on V.
A stellar collision between two MS stars 0 < 10 < 600 (Sills, Adams, and Davies 2005)
Outcomes of collisions in globular clusters Mass loss: 1-10 % (more for MS-CO) Typical delta V ~100km/s for collisions, so collisions lead to mergers Capture for Rmin ~ 3 Rstar (Fabian, Pringle, and Rees 1975)
RG-BH collisions Stopping power ~escape speed of RG (Bailey & Davies 1999). RG-BH collision with v =800kms -1,Rmin=10R, 1M giant, 10M BH (Dale et al. 2009) However: RG-MS/NS/WD leave a RG (because 1=2)
KEY IDEA # 4 Encounters change binary properties.
Encounter timescales involving binaries Timescale for a given star to undergo an encounter with a binary is τ enc 10 11 yr 10 5 /pc 3 n M M R V Rmin 10km/s Where now Rmin is roughly the size of the binary, which can be much larger than the radius of a star. Can have situation where 1+1 ' 2+1
Concepts concerning binary-single encounters Hard-soft boundary V1 d hs ' 6AU 10km/s 2 Soft binaries get broken up Hard binaries get harder Thermal distribution of eccentricities Clean exchanges: lowest-mass star ejected Stellar collisions occur during encounters
Possible outcomes of encounters between a binary and a single star. a) b) c) d) e) f)
KEY IDEA # 5 Blue stragglers are made in two ways.
Ways to make blue stragglers Produced in collisions/mergers between two single stars (during 1+1, 2+1, or 2+2 encounters) Produced via mass transfer within binaries.
(Sills, Adams, and Davies 2005)
How does blue straggler formation rate scale with cluster mass? Collision rate between main-sequence stars given by coll / 2 r 3 c / 2 r 3 c p / M c 2 rc 3 p / f c 2 r 1/2 h Mtot /r h Mtot /r h r 3 c M 3/2 tot Number of blue stragglers made in collisions N bs,coll / M 3/2 tot Number of blue stragglers made in binaries N bs,bin / f bs,bin M tot
There are roughly the same number of blue stragglers in all clusters! (e.g. Piotto et al. 2004)
Making blue stragglers through binary evolution 1. MS MS MS 2. MS MS 3. Evolved donor MS 4. Blue Straggler (Davies, Piotto, & De Angeli 2004)
Predicted BS numbers from both collisions and binaries (Davies, Piotto, and De Angeli 2004)
The effect of encounters on binaries Encounters reduce the number of binaries which can make BSs today (Davies, Piotto, & De Angeli 2004) See also Milone et al. 2012 (Hut, McMillan, & Romani 1992)
KEY IDEA # 6 Clusters are factories producing interacting binaries.
Interacting binaries are made dynamically (Pooley et al. 2003)
How to make an interacting binary Tidal capture (single-single encounter) Collisions involving a red giant making a common envelope system Encounters between binaries and single stars which lead to clean exchanges
How to make an IMXB within stellar clusters 1. MS MS MS 2. MS MS NS 3. NS Evolved donor 4. NS Intermediate-mass X-ray binary cf. Cygnus X-2 (Kolb et al. 2000) (Davies & Hansen 1998)
IMXBs make MSPs in the past IMXB Production Rate extinct systems systems visible today 5 10 15 Time (Gyr)
KEY IDEA # 7 Are clusters factories producing compact binaries?
Producing compact binaries outside of clusters Initial main-sequence - main-sequence binary Most massive star evolves and transfers envelope... leaving a helium-star - main-sequence star binary Helium star explodes in the first supernova explosion... to leave a neutron-star - main-sequence binary Second star evolves into a giant, and transfers mass unstably... forming a tight helium-star - neutron-star binary Helium star transfers mass unstably, forming a very tight binary Finally the core explodes as a supernova,... to leave an ultra-compact double neutron-star binary (Church et al. 2011)
Producing BH-BH binaries in the field (Belczynski et al. 2016) (De Mink et al. 2016; Marchant et al. 2016)
Producing compact binaries within clusters 1. NS MS MS 2. NS NS MS 3. NS NS 4. NS NS (eg. Davies 1995)
An example of how binary-single scattering can produce a tight BH-BH binary which then spirals in due to the emission of gravitational radiation (Rodriguez et al. 2016)
Offsets sometimes larger than expected 060502B - burst occurred outside host galaxy P (R >r) 0.1 0.01 10 3 10 4 Field NS NS binaries Field BH NS binaries 1 10 100 Projected radius r/kpc It could be that the progenitor formed dynamically in a globular cluster. (Church et al. 2011) Also looking at offsets for WD-NS systems (Church et al. in prep.)
So which dominates, field or cluster? Common Rare Rare Common
Summary: 1) Collision rate depends on V. 2) What gets hit depends on V. 3) Outcome of collision depends on V. 4) Encounters change binary properties. 5) Blue stragglers are made in two ways. 6) Clusters are factories producing interacting binaries. 7) Are clusters factories producing compact binaries?