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Young Stars In The Galactic Center
Audra K. HernandezHigh Energy Astrophysics
Discussion GroupFriday 10th Feb.
Audra K. HernandezHigh Energy Astrophysics
Discussion GroupFriday 10th Feb.
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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
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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)
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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)
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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)
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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)
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Two Kinematic Components
Lu et al. (2005), Genzen et al (2003
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Lu et al. (2005), Genzen et al (2003
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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
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Lu et al. (2005), Genzen et al (2003
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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
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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.
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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!
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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)
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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)
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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)
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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)
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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.
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S-Stars: inner 0.5” Stars not co-aligned with two disks of
massive young stars at 1”-10”. First observations w/ SINFONI
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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.
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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 .