Young Stars In The Galactic Center

1. Young Stars In The Galactic Center Audra K. Hernandez High Energy Astrophysics Discussion Group Friday 10th Feb.

2. Topics of Discussion • Galactic Center Parameters • What Kind of stars are there in the GC • 2 rings of young stars • Why is star formation hard at GC? • Formation scenarios • Infall-bad • Old low mass stars that accrete-bad • In Situ-good • S Stars

3. Galactic Center: Sgr A* • R < 100” ~ 10’’ : Near compact radio source the overall surface density and surface brightness increases. • R < 10” ~ 1”: Surface brightness continues to increase, stellar surface number density drops off. • Core radius: ~0.34+/- 0.2 pc Genzel et al. (2003)

4. Several different populations in central pc. • Red giants in old component (1-10 Gyr) (K>13) • ~Dozen luminous blue giants - recent star formation within 2-7 Myr. (K~9-12) • “A number” of bright AGB stars sample an intermediate mass and age. (K~10-12) • Dust-embedded stars with nearly featureless near-IR spectra • Stellar mean velocities indicate a central compact mass. • Objects enclosed with in S2 pericenter approach of 17 lt-yr, v>5000 km s-1, is 3.5 x 106 Msun • Densities of hypothetical non-black hole objects too high to be stable. Genzel et al. (2003)

5. What stars are found in GC? • He I stars • O and B stars with abnormally strong He lines • Hydrogen deficient • Loss (or depletion) of H envelope leaves He core exposed. • Probably due to stellar winds -> Wolf-Rayet stars. Lu et al. (2005), Genzel et al. (2003)

6. He I stars • ~40 stars within central pc. • Identified by K spectra • Blue supergiants (Of) • Luminous blue variables (LBVs) • Wolf-Rayet (WN/C) stars • Based on Ott et al. (2003), 5 of 7 stars at r < 3” with K < 11.5, 6 of 11 with K < 12. • Masses ranging 30-120 Msun. • Ages of 2-7 Myr • Distances limited to 1” -10” from SBH. • R < 0.5”, with AO-assisted spectroscopy, several young stars exhibit HI Br absorption. Stars clearly hot at have MS identification of O8/B0. • Ks ~ 14 Lu et al. (2005), Genzen et al (2003)

7. Two Kinematic Components Lu et al. (2005), Genzen et al (2003

8. Lu et al. (2005), Genzen et al (2003

9. Disks • Clockwise • i= -120o and phi = -60o • 14 stars: 4 Of, 5 WNL, 1 WNE, 4 WCL. • 3 velocities • Thin rotating disk 2’’-4’’ from center. • Period of circulation ~ 2000yr • Counterclockwise • I=-40o phi=160o • 12 stars: 2 Of/LBV, 3 WNL, 6 WCL, 1 WCE. • Thin disk 4’’-7’’. Lu et al. (2005), Genzen et al (2003

10. Lu et al. (2005), Genzen et al (2003

11. Is Star Formation Possible? • “Standard” Star formation models are forbidden around SMBH due to huge tidal forces. • BH would shear gas clouds with densities higher than the highest density cores of observed GMCs. • Need nH > 1011 cm-3 R0.1-3. • Scenarios: • Infall-no good • “old” low mass stars with accretion-no good • In Situ

12. Infall Scenario • ‘normal’ (Nayakshin et al (2005) star formation at several parsecs away from GC in a massive cluster that then spirals in. • This would avoid the need for excessive gas density in order to form stars. • Cluster would orbit through background stars, decay through friction, and settle in center containing ~ only He I stars.

13. Problems • Star cluster would need to be very massive: M ~ 106 Msun and very compact. • 3 orders of magnitude brighter than stars in ONC. Thus, the expected low-mass stars spiraling in cluster is ~1000 that of the ONC. • This does not match the observed diffuse X-ray emission. • The standard galactic IMF would predict hundreds to thousands of He I stars rather than the dozens observed. • Most of the stars would be peeled of in the central pc!

14. The Capture and Growth of ‘old ‘ low-mass stars • Artymowicz et al (1993) showed that star clusters close to quasars can be captured by the disk……Stars can then grow by accretion. • Good: disk does not need to be self gravitating to work provided: • There is enough stars • Stars are trapped quickly • Bad: we only have a few tens of stars and stars would not be born in just a few million years. Nayakshin et al. (2005)

15. In Situ: • If the disk mass exceeds a “fraction of a percent or so” of the SMBH the tidal density limit can be overcome. • Stars can then be formed directly if radiative cooling is efficient enough. • Disk can become gravitationally unstable when gas mass is greater than 104 Msun. Nayakshin et al (2005)

16. N-body Simulations • Nayakshin et al. (2005): guessed initial geometrical arrangement based on present day observed configuration and followed evolution for 3 Myr. • They find minimum mass 5 x 103 Msun . Thus, rings are close to being unstable. • The total mass of stars formed should be close to the original gas mass. The observed mass, through assuming a standard Salpeter (1955) IMF, is around 104 Msun……disk gravitationally unstable! Nayakshin et al (2005)

17. Comparisons with ONC • ONC: • ~1400 low mass stars emit LON=1.2x 1033 erg s-1 in X-ray. • Use to compare to YSO X-ray emission in Sgr A*. • The larger clockwise disk is believed to be 20 time larger than all the massive stars in the ONC -> Lexp = 2.5 x 1034 ergs. But, Lobs=1.2 x 10 33 erg s-1. • Thus if stars are formed in situ, the galactic IMF needs to be abandoned. Nayakshin et al. (2005)

18. Problems with the IMF • Observed X-ray emission is low for both infall and in situ senarios. • Infall: problem solved if IMF top heavy by allowing 99% cluster mass to be in massive stars…..Would over produce massive stars that are not seen. • In situ: only need to suppress low-mass by 10% or so. • Could indicate IMF is not universal….especially in extreme cases involving BHs.

19. S-Stars: inner 0.5” • Stars not co-aligned with two disks of massive young stars at 1”-10”. • First observations w/ SINFONI

20. 90 % of all K< 16 stars are eerily stars with spectral properties identical to normal, main-sequence B0-B9 stars. • The orientations of the stellar orbits appear to be random. • Given their normal properties, they must have formed or been built in their present location. • Most recent distance to GC is from S2 orbit: R = 7.62 +/- 0.32 kpc. • Genzel et al (2003) and Ghez et al. (2004) first reported detections of variable IR emission. • Probably due hot or relativistic gas near the event horizon.

22. References • Einsenhauer et al. (2005), ApJ, 628:246 • Genzel et al. (2003), ApJ, 394:812 • Lu et al. (2005), ApJ, 625:L51 • Nayakshin et al. (2005), Mon. Not. R. Astron. Soc., 364: L23 • Nayakshin et al. (2005), A&A, 437:437 • Nayakshin et al. (2006), Mon. Not. R. Astron. Soc., 10.1111/j.1365-2966.2005.09906.x .