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					 Astrometry from VERA to SKA



               Hiroshi Imai
Graduate School of Science and Engineering,
           Kagoshima University
  SKA-JP Astrometry Sub-Working Group
                   Contents
• Current VLBI astrometry
  – VERA (CH3OH, H2O, SiO maser sources)
  – VLBA/EVN/LBA (BeSSeL, Gould's Belt, Pulsar, …)
• Possible astrometry with SKA
  – Wide-field astrometry
  – Multi-frequency astrometry
  – Deep space astrometry
  – Astrometry for transient objects
• Challenging issues towards SKA
  – High resolution network construction and operation
  – Data calibration and processing
  – Target sources and their numbers after GAIA era
VERA (VLBI Exploration of Radio Astrometry)


• First VLBI array dedicated for radio astrometry
• Dual-beam system used for phase compensation against
  atmospheric fluctuation within 0.3-2.2 deg separation
• CH3OH(6.7GHz), H2O(22GHz), SiO(43GHz) maser sources
• Operation for ~5,000 hours/year in the next decade
• Total bandwidth: 512 MHz (1 Gbps) ⇒ 1GHz (4 Gbps)
                             VERA astrometry
•   1%-level accuracy within 500 pc
•   10%-level within 5 kpc, up to 15 kpc
•   Papers in PASJ special issues (2008, 2011)
•   Astrometry towards ~500 maser sources within 10 years




SiO masers in Orion Source I (Kim et al. 2008)   H2O masers in T Lep (Nakagawa et al. 2011)
      Current scientific targets in VERA
• High-mass star forming regions (H2O, CH3OH)
• low-mass young stellar objects (YSOs), (H2O)
• Evolved stars (H2O, SiO)

• Maser sources at sites of
   star formation and stellar mass loss
• Galactic plane for the Galactic dynamics
• Nearby (~1 kpc) the Solar system for distance-scale
  calibration (e.g. Mira’s P-L relation)
• AGN, Blazers for core-shifts with frequency
• X-ray binary for SN kick
               VLBA/HSA/EVN/LBA
• Multi-purpose VLBI arrays
• VLBA Large Projects for astrometry
  – BeSSeL: H2O and CH3OH masers towards
     ~400 high-mass star forming regions
  – Gould's Belt astrometry: continuum sources towards
     ~200 YSOs
  – PSRPI: astrometry towards ~140 pulsars
        ~1000-1500 hours/year for astrometry?
• LBA towards the southern hemisphere
• Radio astrometry in any field (e.g. planet search)


                                 Spitzer Gould’s Belt Survey
         Possible astrometry with SKA
• Astrometry for non-thermal and thermal sources
• Wide-field astrometry
  – in-beam, multi-beam, multi-field delay tracking
• Wide-band/multi-frequency astrometry
  – wide-band receiving, flexible spectroscopy
• Deep space astrometry
  – high sensitivity
• Astrometry for transient objects
  – snapshot, blind survey, flexible operation (ToO)
• Earth rotation and geodesy
  – Maintenance of reference frames
         Empirical astrometric accuracies (VERA/VLBA)
                   and expectations to SKA
                    VERA                  VLBA                 SKA (expectation)
Dishes              4 × 20 m              10 × 25 m            5 000 x 15 m
                                                               (1° FoV, 100 m2
                                                               @1GHz)
Effective Ae        630 m2 @22 GHz        2950 m2 @22 GHz      500 000 m2 @10 GHz
Baseline SEFD       1760 Jy               500 Jy               940 Jy, 17 Jy (station)
(antenna pair)
Baseline SEFD       250 Jy with GBT       130 Jy with GBT      2.4 Jy with core
(with large tel.)                                                  (0.34 Jy)
Baseline length     2 300 km              8 600 km             ~3 000 km (+α)
N baselines         6                     45 (within VLBA)     50 (core – stations)
                                                               600 (within stations)
Maser astrometry    ~20 μas for ~10 Jy    ~20 μas for ~1 Jy    ~10 μas for ~20 mJy

Continuum           ~20 μas for ~70 mJy   ~20 μas for ~5 mJy   ~10 μas for ~25 μJy
astrometry          (Δν=256 MHz)          (Δν=512 MHz)                   (Δν=8 GHz)
    Astrometric specification @ 10μas
           updated with SKA
• Reference source candidates: 30 mJy⇒1 mJy @8 GHz
• Super-synthesis: 2 hours ⇒ 10 min
• Target maser flux for 10 min: 1 Jy ⇒ 20 mJy
• Target continuum flux for 10 min: 5 mJy ⇒ 25 μJy
• Geodetic VLBI: residual monitoring in semi-real-time
~20 tels., ~500 scans/day ⇒ ~50 sta., 50×10 scans/30
  min
• Angular resolution: θVLBA ~ 1/3 θSKA ~ θSKA +α
            Wide-field astrometry with SKA
Permitted phase coherence angle
within atmospheric fluctuation
for 10-μas level astrometry
Δθ(target – reference) < 2 deg @22 GHz
Δθ(target – reference) < 6 deg @6 GHz          ASKAP focal plane phased
                                               array FoV=30 deg2 @1.5GHz

~30,000 reference sources
                          with S8GHz > 1 mJy
Δθ(reference – reference) ~0.7 deg

Multi-reference, in-beam astrometry
                          is possible.
Wide-field astrometry is still necessary
   and possible for estimation/
   correction of zenith delay residuals.
    Data correlation for SKA wide-field astrometry
• Time-average smearing in data correlation
    dD dt        D e                             0.015
              t          t  1, t[s] 
        c             c                     [ 10GHz ]D 1000km  1deg 

• Targeted astrometry with multi-field correlation
      Toward known/selected sources (Nfield < 100)
– NIR/MIR sources (MSX, AKARI)
– molecular cloud cores (NANTEN, ASTE/AzTEC, GASKAP)
• Blind astrometry with wild-field correlation
           Toward unknown sources (Nfield> 1 000)
– Stellar OH masers in the Galactic halo (HIPPARCOS, GAIA)
– γ-ray bursts from the objects that are not QSOs
– Astrometric micro-lensing events towards the Galactic
  bulge and LMC/SMC (P~1/103 [stars] for 10 μas)
                                                            (Onishi 1995)
     Multi-frequency astrometry with SKA

• Spectral lines per observation: 1 or 2 lines ⇒ >2 lines
• Multi-frequency astrometry at mid-band
Masers [MHz]
   – OH: 1612, 1665, 1667, 1720, 4751, 4766, 6031, 6035, 13441
   – CH3OH: 6669, 12179
   – H2O: 22235, NH3: 23694, 23723, 23870 (high-band)
thermal lines ……
   – CH3OH: 834, ... , NH2CHO: 1539, CH3OCHO: 1065 [MHz]
   – recombination lines
         Targets of SKA line astrometry
• Astrometry towards maser sources
  – low/intermediate-mas young stellar objects (YSOs)
     – nearby regions (in TMC, Ophiucus, Serpens, up to ~5 kpc)
  – high-mass YSOs
     – Galactic plane, LMC, M33, M31, up to ~10 Mpc
  – Evolved stars (Mira variables, OH/IR stars)
     – Galactic halo, bulge, LMC, SMC, up to ~100 kpc
• Thermal lines: mas-level astrometry
  – Mainly proper motion measurements
  – CH3OH, NH2CHO, CH3OCHO
  – Recombination lines in HII regions and
     planetary nebulae
          Wide band astrometry with SKA
      Tb  1.4 10 B [Jy/mas ]  10GHz               
                      4                2                 2


• Non-thermal continuum sources
  –   25 μJy/(10μas)2 ⇒ Tb~3×109 K c.f. R●~1 AU ⇒10 μas @100kpc
  –   Nearby super massive black holes ⇒ Deep space astrometry
  –   Micro quasars
  –   Pulsars ⇒ Kemeya-san’s talk
  –   Gyro-synchrotron radiation from
       YSOs, brown dwarfs and planets
• Thermal continuum sources (@10GHz)
  – 25 μJy/mas2 ⇒ Tb~3×105 K c.f. R*~0.1 AU ⇒1 mas @ 100 pc
             O-type stars (10 μas level astrometry)
  – 25 μJy/(10 mas)2 ⇒ Tb~3×103 K c.f. R*~1 AU ⇒10 mas @ 100
    pc
             red giants (100 μas level astrometry)
    Deep space astrometry with SKA
– The Magellanic System
  • Proper motions, trigonometric parallaxes (SKA + α)
– M31 (Andromeda Galaxy)
  • Proper motions (~100 km/s)
  ~50 μas/yr, dependent on cosmological model
  • SKA site dependent (low elevation)
– Galaxies in the Local Group
  • Proper motions (~10 μas/yr) (SKA + α)
– Virgo cluster
  • Cluster proper motions (~1 μas/yr) (SKA + α)
– Quasars, sources at the cosmological scale
  • Stability of the celestial reference frame
  • Stability of metric (e.g. physical constants)
 Astrometry for transient objects with SKA

• Contribution to quick source identification
  – Good advantage thanks to the wide field of view
  – Astrometry, spectroscopy, SED in 1-10 GHz
  – Depending on SKA operation modes
• Target sources for radio transient objects
  – Radio super novae, γ-ray burst after grows
  – Stellar outbursts and flares (YSOs, evolved stars)
  – Photometric micro-lensing events
  – Extra-Terrestrial Intelligence (ETI) signals
     What can we learn/obtain from
           SKA astrometry?

• 3D visualization of the movements of stars,
  interstellar gas clumps, and galaxies, including
  exotic objects.
• History of the Universe probed by these
  movements in the whole sky.
• Evolution of the time-space surrounding
   the Earth, the Solar System, and the Milky Way.
      Challenging issues towards SKA
Data calibration and processing
 – Wide angle astrometry
    • Near field astrometry dependent on reference sources
    • Monitoring instrumental and atmospheric excess path delays
    • Observation scheduling
 geodetic-type operation?, multiple beam direction?
    • Multiple-field delay tracking
 – Maintenance of reference frames
    • Meet the “VLBI2010” specification for geodesy
    • Quick measurements of Earth orientation parameters
        and their real-time feedback to data correlation
    • Contribution to ICRF maintenance
    • Identification of reference sources nearby the Galactic plane
        Challenging issues towards SKA
High resolution network construction and
  operation
   – SKA (<3000 km) alone
     • Higher sensitivity (by a factor of ~100)
     • ~1 mas @22GHz, ~3 mas @6.7 GHz, ~18 mas @1.6 GHz
  – Global array: ~10,000 km with VLBA, EVN, APT
     • Compatibility in system operation and signal recording
     • Dependent on Earth’s rotation, limited obs. efficiency
  – SKA + spacecraft: >20,000 km
     • More efficient (u,v)-plane recovery (<30,000 km)
     • Poor sensitivity with small spacecrafts (by a factor of ~10)
     • high costs (excluding operation cost)
        – Spacecraft: 200M USD for 10-m dish for 5 years
        – Ground radio telescope: 20M USD for 30-m dish for 20 years
       Challenging issues towards SKA
• Target source number after GAIA era
  – GAIA: σ~24 μas for V<15 mag, ~108 stars
  – SIM: σ~4 μas all over the sky, >>104 stars
  – JASMIN (2020~): σ~10 μas for KW<11 mag, ~105 stars
              towards MW bulge
  – SKA: How many optical/IR invisible radio objects?




         GAIA          SIM                Nano-JASMIN FM
              From VERA to SKA

• Domestic meeting on astrometry and the Galaxy
  – 1995 October 23—24
  – 2000 December 4—5

• There still exist many remained explorations
  proposed in 1990’s, which should come true
  with SKA!

				
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posted:10/3/2012
language:English
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