The Impact of Stellar Collisions in the Galactic Centre

Transcription

1 The Impact of Stellar Collisions in the Galactic Centre Melvyn B. Davies Lund Observatory Ross Church, Serge Nzoke, James Dale, Daniel Malmberg, Marc Freitag

2 The Galactic Centre (Genzel et al 2003)

3 The missing red giants Recall that on the HR diagram, lines of constant K magnitude are diagonal Tracks for 1-8M stars shown The missing objects are the big, bright giants towards the tip of the giant branch Missing objects 4M 3M 2M 1M K=10.5 K=12 K=15

4 New observations: early vs late-type stars (Buchholz et al 2009) (Do et al 2009) (Bartko et al 2009)

5 Missing giants in the Galactic Centre The GC cluster is a very dense system Stellar collisions certainly occur there Collisions may be able to explain the missing giants Can use this to study stellar population including objects we cannot see (eg black holes)

6 GC is a dense environment Plots from Schoedel et al 2007 of dynamical enclosed mass estimates Clearly have a large number of stars packed into a small volume, n 10 7 pc -3 (like a globular cluster...)

7 Stellar collisions If, in a given volume of space, the number density of stars is n, the mean velocity is v, and the cross section for collisions is σ, the collision rate per star is nσv In the region dominated by the SMBH, v is approximately Keplerian v = 400 ( ) 1 r 2 km/s 0.1pc

8 Stellar collisions σ given by σ = πr 2 min ( 1 + 2G(M ) 1 + M 2 ) R min V 2 Collision timescale given by t coll,un f ocussed yr ( n ) 1 ( v ) 1 ( R 10 7 pc kms 1 R ) 2

9 What is n(r) in the Galactic Centre? What do we assume about the star formation in the Galactic Centre? 1) Can say star formation follows regular (Miller- Scalo) IMF and consider regular stellar cluster - OR - 2) Can say that stellar population built up from a stack of discs, each following the flatter IMF observed for the young disc in the GC

10 1) Result for a regular IMF Using a regular IMF, and building a cuspy cluster with a Monte Carlo code. Computing collision rates using Eggleton evolution of stellar radii. Post-MS encounters dominate, then MS-MS, more than MS-BH RG-MS collisions not so effective (Bailey & Davies 1999) (Freitag et al 2008)

11 How much mass is lost in collisions with BH? RG-BH collision with v =800kms -1,Rmin=10R, 1M giant, 10M BH (Dale et al, 2009)

12 Effects of RG-BH collision The core is ejected, taking some of the envelope with it 13% of the envelope mass is retained by the core - to evolve to become visible in the middle 10.5<K<12 band, this giant needs to retain >40% of the envelope Thus at least some collisions involving BHs can remove some of the visible RGs. BHs are more effective than MSs or WDs as much more of the RG envelope is removed. Although RG-BH collisions cannot explain depletion of brightest RGs. (Dale et al, 2009)

13 MS-BH collisions MS-BH collision with v =874kms -1,Rmin=0.5R, 1M MS star, 10M BH (Dale et al, in prep)

14 Summary for regular IMF RG-BH collisions can remove some RGs in the very centre of the galaxy (within 0.04 pc) MS-BH collisions can destroy stars but too infrequent MS-MS collisions do not help: ones become twos but halves become ones (unless there are no halves)

15 Will now consider the effects of a flatter IMF There are fewer low-mass stars Most stars have evolved to become BHs, NSs, or WDs The dominant collision rate for MS stars involve compact remnants (ie BHs, NSs, or WDs)

16 1) Result for a flatter IMF Using a flatter IMF, and building a cluster from a stack of discs. Computing collision rates using Eggleton evolution of stellar radii. Encounters between MS and remnants dominate over MS-MS encounters.

17 Theoretical calculations - gamma = 0.45 r/arcseconds Surface density / arcsec Early type stars Late type stars (with collisions) Late type stars (no collisions) r/pc

18 Theoretical calculations - gamma = 1.05, BHs eaten r/arcseconds Surface density / arcsec Early type stars Late type stars (with collisions) Late type stars (no collisions) r/pc

19 Summary for flatter IMF WDs, NSs and BHs dominate population MS-CO collisions dominate and can destroy stars Could frequency be high enough to flatten surface density of late-type stars? Total mass could be a problem: but BHs may feed MBH Need to look at dynamical effects (eg mass segregation) Would have a GREY CLUSTER at the galactic centre