Tuesday Afternoon Talks

4:15-4:35 pm Tobias Fritz (Germany): Nuclear Cluster of the Milky Way (15 + 5 minutes)

Model fitting of nuclear cluster: Sersic, etc.
  • unusually bright nuclear cluster for its size

Globular cluster origin of nuclear cluster

Questions:

  • Q: Round component consistent with inspiraling globular cluster. What about spectral properties?
  • A: No. Metallicity difficult.

4:35-4:55 pm Tuan Do (Canada): Measuring the physical properties of the Milky Way Nuclear Star Cluster with 3D stellar kinematics (15 + 5 minutes)

Dynamics of CMZ stars
  • Massive black hole affects dynamics of a cluster
  • testing presence of cusps + old stars have no cusp + young stars have a cusp, but not dynamically relaxed
  • what is the true stellar density profile? + can't determine by counting + kinematics can help + old stars randomly distributed + young stars concentrated
  • did the stats right + no binning + individual likelihoods + unfortunately, this is new
  • flat slope, no anisotropy
  • correlation between parameters, particularly the slope and BH mass
  • edge of core poorly constrained
  • infalling IMBH or SMBH destroys stellar cusp + observational constraints on IMBHs poor
  • QUESTION: Do the fits improve at all if you impose a GC distance? + Answered: Can merge with PDF from stellar orbits

Question

  • Q: Inconsistent with 7.7 kpc distance to CMZ?
  • A: no, consistent within 1-sigma
  • Q: How do you contrain R_0 with this approach?
  • A: radial velocities give absolute scale. Proper motions must also have some scale. Match the scales to get R_0
  • Q: Don't use any information outside 0.5 parsecs. Shouldn't you include it? I get the right mass if I do
  • A: Will help, but velocity dispersion dominated by nearest stars
  • Q: Anisotropy in cluster. Get rid of red supergiants somehow? Tangential or radial?
  • A: Hard to know. Very little anisotropy.

4:55-5:15 pm Mark Wardle (Australia): Star Formation within 0.5 pc of SgrA* (15 + 5 minutes)

"Highest I've ever been for a talk"

High densities needed to overcome tidal shear
  • big cloud -> Hoyle-Littleton accretion + self-intersecting orbit on the backside of the cloud
  • central hole because accreted?
  • high magnetic pressure in disk: 10x greater than gas
  • B-fields suppress fragmentation
  • Black hole takes bite out of cloud
  • optically thick disk: sigma T_eff^4 cooling + heating due to starlight, accretion
  • twisting knobs to figure out where accretion disk turns into fragmenting disk + inside, magnetically active / supported + outside (0.04 pc) fragmenting + further out: grav stable
  • maybe high accretion rate drove fermi bubble
  • accretion rate consistent with 10^6 Msun/Gyr -> build Sgr A*

Questions

  • Q Mitch: Grav unstable region globally unstable?
  • A: Has to do with the disk height. h/r very small, not an issue
  • Q Fred Lo: We don't see any of this observationally. All history?
  • A: Yes, this all happened a few Myr ago.
  • Q: Eddington accretion: what happens to rest of star formation and the disk?
  • A: I think it's OK - star formation 10^4-10^5 years, but dump time 10^5-10^6 yrs
  • Q: Bonnel & Rice did simulations. Why need more?
  • A: B-fields? Eqn of state? Radiative cooling? Crazy enough initial conditions?
  • A Farhad: didn't include radial distribution of stars
  • Q: How do you start with 0-angular-momentum cloud?
  • A: Lots of junk around, maybe collisions do it. I don't have a good answer. Depends on how big a fluffy cloud you use.
  • Q Fred Lo: What observational signature remains?
  • A: Maybe kinematics

5:15-5:35 pm Andrea Ghez (USA): Probing General Relativity with Short Period Stars at the Galactic Center (15 + 5 minutes)

Why continue studies of GC stars?

  • Test GR

  • Role of BH in galaxies

  • Depth of potential 100x greater, on mass scale 10^6 larger than other tests

  • simplest tests + Relativistic redshift (easy) + Precession of periapse

  • SO-2 "star of the show" + next close approach in 2018.5 + relativistic signal ~200 km/s + need very accurate keplerian orbit first + radial velocity from BrG

  • absolute reference frame is tough + masers give the reference frame, but require larger field

  • full orbital coverage with astrometry + 13-parameter model

  • Systematic drift in reference frame + There is a major source of systematic error + Spatial variation in PSF (AO issue) is the worst systematic + drift term prevents bias + 5 km/s uncertainty projected by 2018

    • 5-sigma GR detection

  • GEMS to make reference frame

  • TMT could do 2-yr orbits

Questions

  • Q Ostriker: Could the SMBH be sloshing? Can you allow for that? What about acceleration?
  • A: Mark Reid did calculation. Limit ~3 km/s for present BH velocity. Problem: assume linear velocities, but that is not always a safe assumption. If BH accelerating, would create "nonphysical" accelerations.
  • Q: Many things spiral into center, will lead to oscillations in BH motion
  • Q Stocke: How big an effect is blending with unseen stars?
  • A: We simulate it. But, trying to simulate something we don't understand. <0.1 mas
  • Q: Contributions from resonant relaxation, encounters with other stars.
  • A: Skipped the slide that shows this. For GR, insignificant, but for precession of periapse, have to worry.
  • Q: What about the velocity component?
  • A: small effect on velocity.

Discussion Section

  • Q: IMFs shallower than salpeter.
  • A Lu: Hard to decouple from dynamical history. push on spectroscopy to lower masses. Decouple young and old populations
  • A Fritz: instrument sensitivity limit... VLT can detect CO bandheads...?
  • How well do we know it's 2 populations, or maybe it's just 1 population? Assumptions disagree, not measurements
  • Stocke: Background is changing in center. Can you use fluctuation analysis to determine brightness of lower mass stars?
  • Lu: Very challenging for young stars because they're a small fraction of the total luminosity. Foal(?) et al did good work on older star populations
  • Arches cluster: Hui Dong found 4 runaways of same mass as the 12 stars in the Arches cluster. Maybe as many runaways as cluster stars? Half of cluster stars kicked out. Do dynamical models predict this?
  • [silence]
  • Find massive stars in "other 3/4" (I'm lost...). Need spectroscopy
  • Ghez: Role of binary stars in ejections.
  • Missing half of cluster mass?
  • Ghez: Role of interaction with black hole? Clusters not evolving in isolation
  • Arches 2.5 Myr old. Largest stars could just start... evolving?
  • Most difficult objects to explain are LBVs. LBVs outside of cores of clusters. How do you get very massive binaries out of cores? All LBVs found are outside cores.
  • Role of binaries. New evolutionary models. LBVs that must be <2 Myr, WCs must be >4 Myr. Contradiction?
  • New evolutionary models say many stars come from rejuvenation. Massive blue stragglers.
  • In situ formation? What fraction should be expected to be formed in situ?
  • How confident are we about mass-luminosity relation for stars?
  • Lu: On main sequence, confident, but post-main-sequence is uncertain. We drop them in our analysis. Pre-main sequence is a problem in Arches. Arches fitted with only main-sequence stars
  • very difficult with rotation included
  • Distance to the Galactic center "decreasing at an alarming rate". Does determination conflict with any other indicators? What does that mean?
  • Ghez: Mark Reid likes lower R_0. All consistent within uncertainties
  • Do simulations for S stars include interactions between stars?
  • Ghez: Not in most recent version, but in an earlier version
  • How does this test of GR compare with other tests of GR around Sgr A*?
  • Ghez: Event Horizon Telescope, GRAVITY.
  • Different systematics.
  • Are HST data consistent with BH at center?
  • Tuan Do: Same reference frame. BH defined to be at center. Velocity can drift, but not position

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