Star formation near Sgr A*

Transcription

1 Star formation near Sgr A* Sergei Nayakshin University of Leicester

2 Plan Stay away from the S-stars (Schoedel et al. 02, Ghez et al 03) of the inner arcsecond (see T. Alexander talk) Sgr A* young stars most likely formed in a circular massive gaseous disk an eccentric gaseous ring Top-heavy IMF for stellar disks Theory/early simulations of AGN star-forming disks.

3 Infall of a massive cluster Gerhard, Portegies Zwart, McMillan, Hansen, Milosavljević, Kim, Figer, Morris, Levin, Gürkan, Rasio, Need ~ 10^5 to 10^6 M_sun star cluster, to infall rapidly enough Need to be within < 30 pc of the GC Need IMBH of ~ 10^4 M_sun to delay cluster core destruction. Two such events are needed Kim, Figer & Morris 2004

4 No trace of cluster disruption o Cluster disruption leaves a trail of stars But the disks have outer edges (Paumard et al 06) o Stars should be on rather eccentric orbits due to scatterings with the IMBH (Levin, Wu & Thommes) The inner clock-wise disk stars have small eccentricities

5 Looking for low mass YSO near Sgr A* with Chandra Inner ~ 40 parsec of the Galaxy (Baganoff et al. 2003) YSO's should produce a bright X-ray background emission Method: using Chandra's results, put upper limits on the emission/mass of all young stars. YSO s quiescent X-ray emission could be hidden in a 30 parsec disk But the X-ray flares could be seen Muno et al no more than ~ 5000 YSO Cluster model is ruled out (Nayakshin & Sunyaev 2005)

6 Star formation in an accretion disc Paczynski 1978, Kolykhalov & Sunyaev 1980, Shlossman & Begelman 1989, Collin & Zahn 1999, Goodman et al. 2003, Levin 2006 Q = M_bh H / M_disk R < 1 (Toomre 1964) M_disk > M_bh (H/R) and a rapid enough cooling (Gammie01, Rice, Lodato 2005, Rafikov 2005) Gadget-2 run (code written by V. Springel, MPA)

7 Two (?) stellar rings (Genzel et al. 2003) view of the rings on the sky

8 Constraints on stellar rings from orbits Nayakshin 2005 warping of a disk by an inclined stellar ring

9 Disk(s) Too much warping destroys disks

10 Maximum mass of stellar disks 1) Orbital modelling (Nayakshin, Dehnen, Cuadra & Genzel 06) 2) Paumard et al. (2006) estimate total mass of stellar disks to be ~ 3 thousand M_sun. 3) Consistent with Q=1 disk model. Nayakshin 2006

11 Disk(s) Disks are co-eval within ~ 2 Million years, which is very long! These are present-day orbits Was the CCW system an initially flat and thin eccentric ring?

12 Top-heavy IMF 1) Observed stellar mass M_hm > ,000 M_sun 2) K-band luminosity function: the data require a mass function flatter than a Salpeter function by 1 to 1.5 dex. Paumard et al. (2006) 3) X-ray YSO constrains

13 Top-Heavy IMF For standard IMF (i.e. That of ONC), predicted X-ray luminosity of low mass YSO is Chandra upper limit: Stars with M < 3 Msun are under-abundant by a factor of ~ 20 Note: the YSO could not have left the inner parsec, so this is robust.

14 Top-Heavy IMF: why? Is the Jeans mass larger in an accretion disc? - No Masses of first stars in a disc. Levin 2006 The disk did not just collapsed in a single orbital time gives hope to survival of QSO accretion disks

15 Top - heavy IMF: Slow fragmentation or SF Feedback? Levin 2006: gas clumps do not collapse until they grow much larger by agglomeration. M_dot > M_Edd, Disc is heated up from inside by the stars Q increases to > 1, Further disc fragmentation stops, But accretion onto proto-stars continues Nayakshin 2006

16 Preliminary results of SPH simulations Use P-Gadget2 (cf. V. Springel and T. Di Matteo talks). Modifications are by Jorge Cuadra. (2-10) x 10^6 particles sink particles to model individual star formation and accretion finite collapse time for gas can merge clumps, clumps t cool = t_c = beta/omega Confirmed Rice et al 05: no fragmentation for beta > 6 Eddington-limited accretion rate

17 Initially flat but non-circular disk

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23 IMF of stars formed in simulations Solid : β = 3, circular disk.. β = 1, circular disk β = 3, eccentric disk

24 Conclusions AGN disks can form stars, and with a top-heavy IMF Data are consistent with one circular and one eccentric disks Sgr A* gaseous disk(s) were consumed by star formation Top-heavy IMF (it it general for AGN disks?) Theory/Simulations of star-forming disks are in childhood Problem: How do SMBH get their food if star formation steals it???