Download Star Formation in the Galactic Center

Young Stars In The Galactic
Center
Audra K. Hernandez
High Energy Astrophysics Discussion
Group
Friday 10th Feb.
Topics of Discussion
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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
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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
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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, mainsequence 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 .
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