Advanced Photonics in Application
to Astrophysical Problems
Electronic Proceedings of the
Third Joint NSF-DFG Research Conference
Washington, DC, USA • 10 - 12 June 2007
IR-IF Observations of the
Central Parsec of our Galaxy
J.-U. Pott
W.M.Keck Observatory
University of California, Los Angeles, CA, USA &
Universität zu Köln, Germany
Editors:
Julian Christou, Wolfgang J. Duschl,
Andreas Eckart, Stefan Krückeberg,
and Thomas Rimmele
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proved nor endorsed. IR-IF observations of the
central parsec of our Galaxy
−
DFG/NSF Astrophysics Research conference
June 2007
Jörg-Uwe Pott
W.M. Keck Observatory, UCLA &
University of Cologne, Germany
Outline of the presentation:
Outline fo the presentation
• Introduction to the Galactic Center (GC; central pc)
as accessible to actual IR-Interferometers (VLTI/KI)
• Observational results achieved with the VLTI at 2 and
10 μm (AMBER/MIDI)
• Outlook for GC science with the Keck Interferometer
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
The center of the Milky Way:
– Harbors closest massive black hole (MBH, 3.6*106M~) at
only ~8 kpc distance (IF: 1 mas ~40 μpc ~8 AU)
– Next similar galactic nucleus (M31 Andromeda) is 100x
farther
– Unique to study MBH-host interaction: stellar winds, tidal
forces, binary and multiple star systems, dust formation,
bow shock study, IMF, central stellar cusp, BH outflow ...
-> e.g. Posters by K. Muzic and L. Meyer, previous talks
– We have to understand star and dust formation in the
immediate vicinity of a MBH and investigate its radiative
properties
to
understand
spatially
unresolved
observations of extragalactic nuclei
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
25mag of visual extinction require IR observations:
the central cluster in NIR:
– Lots of hot and massive stars ionizing the local ISM
– Indications for favoured massive star formation,
‘top-heavy IMF‘
– Solar metallicity, ongoing star formation, Avis≈25
K-Naco
< GCIRS 7, K=6.5
< Sgr A*
Schodel et al. 2007
J&K two color
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Radio-Jet with VLBI: Baum+97
Currently limited uv-coverage of current LA-OLBI
avoid model-independent imaging:
but there is
more to do…
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Limited uv-coverage data is exploited best if combined
with single-telescope and IF multiwavelenth data:
Example: our VLTI/MIDI campaign
to investigate the enigma of GCIRS3:
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Limited uv-coverage data is exploited best if combined
with single-telescope and IF multiwavelenth data:
Example: our VLTI/MIDI campaign
to investigate the enigma of GCIRS3:
• Very red, bright, and dust embedded source only 5” away from
SgrA*
• Similar prop as nearby Quintuplet cocoon stars (e.g. Moneti+01)
• WolfRayet dust forming candidate, but lacking spectral
classification; Lbol,dust ≈ 5e4 L~
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Limited uv-coverage data is exploited best if combined
with single-telescope and IF multiwavelenth data:
•
•
•
•
Example: our VLTI/MIDI campaign
to investigate the enigma of GCIRS3:
Very red, bright, and dust embedded source only 5” away from
SgrA*
Similar prop as nearby Quintuplet cocoon stars (e.g. Moneti+01)
WolfRayet dust forming candidate, but lacking spectral
classification; Lbol,dust ≈ 5e4 L~
Diffraction limited 8m telescopes only can avoid serious source
confusion in the MIR regime; speckle IF and aperture masking
can be used to emphasize the high spatial frequencies
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Limited uv-coverage data is exploited best if combined
with single-telescope and IF multiwavelenth data:
•
•
•
•
•
•
Example: our VLTI/MIDI campaign
to investigate the enigma of GCIRS3:
Very red, bright, and dust embedded source only 5” away from
SgrA*
Similar prop as nearby Quintuplet cocoon stars (e.g. Moneti+01)
WolfRayet dust forming candidate, but lacking spectral
classification; Lbol,dust ≈ 5e4 L~
Diffraction limited 8m telescopes only can avoid serious source
confusion in the MIR regime; speckle IF and aperture masking
can be used to emphasize the high spatial frequencies
Full IRλ≤20μm-SED now available at sufficient spatial resolution
Can we establish a MIR reference star for the central
arcseconds to calibrate ISM silicate extinction towards SgrA*?
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Enigma of GCIRS3, multi-λ:
1. IR-SED constrains temp. and dust chemistry
(Pott et al. 2006;07sub)
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Enigma of GCIRS3, multi-λ:
add.silicate abs. corr.
1. IR-SED constrains temp. and dust chemistry
2. Spectrum shows strong silicate abs.
(Pott et al. 2006;07sub)
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Enigma of GCIRS3, multi-λ:
add.silicate abs. corr.
1. IR-SED constrains temp. and dust chemistry
2. Spectrum shows strong silicate abs.
3. Mean visibility constrains overall geometry:
two shells (20 & 50mas)
(Pott et al. 2006;07sub)
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Enigma of GCIRS3, multi-λ:
add.silicate abs. corr.
1. IR-SED constrains temp. and dust chemistry
2. Spectrum shows strong silicate abs.
3. Mean visibility constrains overall geometry: 4. Visibility spectrum localizes the silicate
two shells (20 & 50mas)
(Pott et al. 2006;07sub)
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
New insights into the enigma:
– RT-modelling of IRS3 suggests the embedded star to be
a cool carbon-rich dust forming giant
– Outer 50m“-shell is externally heated-> stellar interaction
– No intrinsic silicate absorption: ISM extinction towards
the central parsec has to be reviewed at high angular
resolution for λ ≥ 8μm.
Outlook:
Pott+’06,‘07
•A
lot of warm dust in the
MIR-central parsec: shockheated material, ionized gas
• MIR-OLBI can be used to
study the dust formation in
situ
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
Let‘s change to the NIR
MIR
NIR
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Why to use a non-imaging NIR-OLBI for GC research:
– PSF size shrinks by an order of magnitude to ~5mas
– Narrow-angle astrometry of ~50 μas is feasible
– Single telescope confusion limit: K~16mag (seeing
limited) and K~17-18mag (AO-limited at 8m class
telescopes)
Schödel+‘07
Stellar cusp due to the
gravitational potential of
the
SMBH
appears
increases
source
confusion around SgrA*
in stellar number counts
-> talk by R. Schödel:
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Potential sources for (phase-referenced) large
aperture NIR-long baseline interferometry:
[data: Ghez+’98,‘05]
– Current limit: Klim~9-10mag, VLTI/UT is currently vibration
limited (Petrov+‘07), KI imposes a J~7mag limit
– Already potential Klim~11 would help
– Off-axis phase-referencing with Keck/ASTRA and
VLTI/PRIMA and will dramatically increase No. of targets
– Even SgrA* NIR-flares (Genzel03,Eckart04ff,Ghez05) are
within reach
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Current phase of the project:
studying GCIRS7 as phase reference (PR)
NIR
dust emission →
MIR-10μm MIDI obs
– GC is at the current limit: off-axis, and faint AO-GS,
even GCIRS7 is faint for working LA-OLBI systems
– PR should be stable and small: red M1 supergiant,
Teff=3600K, exp. diameter: 0.5mas, if NO hot dust!
– Facing the real life: start to observe and gain
experience (e.g.: understand to do „lucky fringing“...)
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
First observational results:
Special thanks to S. Kraus, MPIfR
VLTI/AMBER K-band:
Visibility could be
measured and
calibrated at
short baseline (U3-U4),
but still low SNR
0.8--
Raw visibilities: where is my banana?
best 10%
HD 153368
best 30%
best 10%
best 30%
GCIRS 7
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
First observational results:
Special thanks to S. Kraus, MPIfR
VLTI/AMBER K-band:
Visibility could be
measured and
calibrated at
short baseline (U3-U4),
but still low SNR
0.8--
Think positive and interpret: Data might indicate:
– Calibrated visibility slightly increase with wavelength
– 0.9≤Vis≤0.95 -> target slightly resolved -> 2mas
– Extended structure or binary (-> variable visibility) is
possible -> further observations are needed
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Feasibility of LA-OLBI observations of GCIRS7 with
current instrumentation was demonstrated
– Phase referencing on GCIRS7 appears to be possible
– Aggravating circumstances (high airmass, baseline
tracking problems, strong piston error due to LR-mode,
bright calibrator etc. in Mar06, exceptionally short
coherence times below 1ms in May07) avoided the
measurement of ClosurePhase, longer baseline
visibilities and higher accuracy so far
– Extended structure might exist -> variability on IF scales
– New telescope time was awarded to redo this pioneering
study and measure the long baseline visibilities with KI in
two weeks (June07).
– New data will help to simulate source confusion biases
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Science cases for the upcoming phase-referencing
facilities (KI/ASTRA and VLTI/PRIMA):
•
Search for binaries in the gravitational potential of the
MBH
•
Deviations from Keplerian orbits including the search
for new orbiting stars, even closer to SgrA*: add subarcsec accuracy positions to the AO-data and use the
better point-source sensitivity of the interferometer to
resolve short-period orbits.
•
Visibility measurement and centroid motion of
accretion flares to probe the existing disk+spot
models of the central accretion zone(Genzel+03,
Ghez+04,05, Eckart+04,06)
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Probing stellar binarity / multiplicity
•
At the moment we know of only one binary system in the central
parsec, confirmed via periodic lightcurves and spectra
(IRS16SW, e.g. Martins+06)
IRS16SW
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Probing stellar binarity / multiplicity
•
•
•
•
•
At the moment we know of only one binary system in the central
parsec, confirmed via periodic lightcurves and spectra
(IRS16SW, e.g. Martins+06)
Standard stellar environment show binary fractions of 60-70% for
intermediate-high mass stars (Kobulnicky+06,Duquennoy+91)
-> binary fraction probes StarFormation
Most bright stars in the central parsec are massive OB ones
(„Paradox of youth“, Ghez 2003, Eisenhauer+05) -> we expect
many binaries
MIR and Xray studies suggest many colliding wind binaries
(Muno+05 find an excess in the central parsec)
Finding less than usual binaries in the very center would back
the unproven theory about the origin of the hypervelocity stars,
as being kicked-out from a binary system due to close BHencounter (e.g. Ginsburg+07, Yelda in prep).
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Deviations from Keplerian orbits
•
(Rubilar&Eckart01)
•
Periapse revolution results from extended enclosed mass
distribution and General Relativity (GR) -> probe GR in an
unprecedented
regime
of
strength
(rel.
parameter
γ(r):=RSchwarz/r≥1.5·10-3 at periapse for known orbiting stars,
might increase to 1·10-2 for closer stars (Zucker+06; the most
relativistic white dwarf Sirius B* probes γ~5·10-4 (Barstow+05)
and the Hulse-Taylor pulsar probes γ~5·10-6 (Taylor+89));
SourceConfusion is critical!
(Olling+01)
S2 expected apoapse shift:
~0.4mas/rev.
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Deviations from Keplerian orbits
•
Periapse revolution results from extended enclosed mass
distribution and General Relativity (GR) -> probe GR in an
unprecedented
regime
of
strength
(rel.
parameter
γ(r):=RSchwarz/r≥1.5·10-3 at periapse for known orbiting stars,
might increase to 1·10-2 for closer stars (Zucker+06; the most
relativistic white dwarf Sirius B* probes γ~5·10-4 (Barstow+05)
and the Hulse-Taylor pulsar probes γ~5·10-6 (Taylor+89));
SourceConfusion is critical!
•
More than two non-Keplerian orbits decouple the degeneracy
between extended mass and GR induced periapse shifts
(Rubilar+01).
Possible orbit deflections by stellar BH encounters probe dark
mass properties of the central stellar cusp (Weinberg+05)
Precise orbits + stellar radial velocity data decouple BH mass
and GC distance Ro, increased Ro accuracy helps to precise
measurements of the galactic dark matter halo shape.
(Olling+01)
•
•
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
•) Intro •) Observations •) Outlook
Observing the IR-flares of the BH accretion disc
•
current spatially unresolved multiwavelengths observations suggest
radiation
coming
a
few
RSchwarz of
a
spinning
BH
(Genzel+03,Ghez+04,Eckart+‘04), evoked by synchrotron emission
processes.
•
a phase-referenced visibility will challenge these models and is the
currently the ultimate probe for measuring the compactness of the
radiation
40
(Hornstein+07)
apparently the NIR-color is independent30
from the strength of the IR emission,
which challenges the existing
20
synchrotron models (Hornstein+07).
The smaller interferometric PSF
10
can help to better constrain
the color during the quiescent
0
14.5
state of emission
CDF (%)
•
fraction of Time
NIR Flare emission
15.0
15.5
16.0
Elevated Emission Level (K’ mag)
16.5
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu
Stay tuned!
Thank You and Mahalo
for your attention!
Any questions?
Jörg-Uwe Pott, ‘IR-Interferometry of the GC’; contact: jpott@keck.hawaii.edu