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

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young stars in the galactic center

Young Stars In The Galactic Center

Audra K. Hernandez

High Energy Astrophysics Discussion Group

Friday 10th Feb.

topics of discussion
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
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
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
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
Two Kinematic Components

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

  • 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

is star formation possible
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
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.
  • 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
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
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
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
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
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
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.
  • 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 .