Young Stars In The Galactic Center

Audra K. HernandezHigh Energy Astrophysics

Discussion GroupFriday 10th Feb.

Audra K. HernandezHigh Energy Astrophysics

Discussion GroupFriday 10th Feb.

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

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)

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)

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)

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)

Two Kinematic Components

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

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

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

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

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

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.

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!

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)

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)

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)

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)

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.

S-Stars: inner 0.5” Stars not co-aligned with two disks of

massive young stars at 1”-10”. First observations w/ SINFONI

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.

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 .