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VLBI observations of H2O masers towards the
high-mass Young Stellar Objects in AFGL 5142
Ciriaco Goddi Università di Cagliari,
INAF-Osservatorio Astronomico di Cagliari (Italy)
Collaborators Luca Moscadelli INAF,
Osservatorio Astronomico di Cagliari Walter Alef
Max-Planck-Institut für Radioastronomie
(Bonn) Jan Brand IRA-CNR, Istituto di
Radioastronomia di Bologna
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Star Forming Regions
Theory

• High resolution observations at radio, millimetre
  and FIR wavelengths
• Thermal line observations by mm and radio
  connected interferometers (e.g., OVRO, VLA)
• linear resolutions of ?1000 AU, insufficient
  to resolve the disk structure and to study the
  root'' of the jet
• VLBI observations of maser lines (e.g., 22 GHz
  H2O 6.7 and 12 GHz CH3OH)
• permit to study the gas structure and
  kinematics nearby the YSO with a linear
  resolution of few AU

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The case of AFGL 5142 Previous observations
stronlgy suggest the presence of an high-mass YSO
Hunter et al. (1995) VLA 8.4 GHz thermal
continuum source (interpreted as
free-free emission from an
ionized wind)
CO bipolar outflow
H2 NIR emission jet.

• Hunter et al. (1999) OVRO SiO jet and HCO
  outflow
• OVRO 88 GHz
  source (coincident with the 8.4 GHz source)
• The radio flux and the bolometric luminosity of
  the source both indicate
• the presence of a massive object (M ??10 M?).

Zhang et al. (2002) VLA NH3 compact structure
(diameter 1800 AU), interpreted
as a rotating disk surrounding
a high-mass young star.
Hunter et al. (1995 1999) a cluster of VLA 22
GHz water masers
associated with the continuum
sources. The VLA angular resolution (0.1
arcsec) is inadequate to determine the detailed
spatial distribution and the proper motions of
the maser spots.
VLBI water maser observations are needed!!
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Observations
Data reduction
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Identification of maser features

• Every channel map has been searched for emission
  above a conservative
• detection threshold (in the range 5-10 ?)
• The detected maser spots have been fitted with
  two-dimensional elliptical
• Gaussians (intensities in the range 0.3-17 Jy
  beam-1)
• A maser feature is considered real if it is
  detected in at least three contiguous
• channels (spectral FWHM gt 0.3 km/s), with a
  position shift of the intensity
• peak from channel to channel smaller than the
  FWHM size.

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Measured proper motions
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Comparison of VLBI results with previous
interferometric observations
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Kinematics of the masing gas Simple
interpretation The detected maser features are
tracing the flow motion in the innermost portion
of the molecular outflow BUT
Diameter 50'', vel. dispersion 100 km s-1,
(assuming a Hubble flow) rate dispersion2 km
s-1 arcsec-1 vel. dispersion8 km s-1, distance
0.35'' vel. dispersion1.7 km s-1, distance 1''
Large scale outflow
VLBI Group I
VLBI Group II
The whole VLBI maser distribution can not be
directly associated with the large-scale
molecular outflow. The two groups are tracing a
more complex structure!
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Group I

• It is found closer ( 500-1000 AU) to the
  expected location of the massive YSO,
• where an accretion disk and/or the base of the
  jet should be found
• It has an elongated spatial distribution (close
  to that of proper motion orientation)
• edge-on rotating disk or outflow motion along
  the elongation axis?

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Group I
We tested the kinematics fitting two models
Keplerian disk and conical outflow.
Only the keplerian disk model produces an
acceptable solution!

• The best fit disk almost edge-on and (on the
  sky) parallel with the elongation axis
• Disk radius ? 800 AU (in agreement with expected
  values for massive stars)
• MYSO (38 ??20) M? the central object is a
  massive YSO, compatible with
• previous core (Hunter et al 1999) and disk
  (Zhang et al 2001) mass estimates

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• Group II
• There are too few observables to test
  meaningfully a kinematical model.
• Group II might be associated with a distinct (as
  yet undetected) YSO.
• The positions and the LOF velocities of these
  features are in agreement
• with the blue-shifted lobe of the (SiO and
  HCO) molecular outflow.
• Their emission is excited by the interaction of
• the gas outflowing from the YSO with the
• ambient gas of the progenitor molecular core.

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• Conclusions
• Using the EVN we have observed water masers
  towards the massive SFR
• AFGL 5142 for four epochs (Oct 1996 Nov 1997)
• We have identified the water maser emission
  centers and calculated the proper
• motions for persistent features.
• Group I features could arise on the surface of a
  nearly edge-on keplerian disk
• Maser features of Group II might be excited by
  the interaction of the gas
• outflowing from the YSO with the ambient gas.

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• Final remarks
• Only 5-7 antennae, out of the 11 presently
  available to observe at 22.2 GHz,
• took part in each run in 1996-1997
• Our EVN observations were able to measure the
  proper motions of strong
• (? 0.3 Jy/beam) and long-living (1 yr) water
  maser features

• Future work
• We have proposed and obtained four epochs of 22
  GHz VLBA observations
• Advantages
• shorter time separation (1 month vs 3-4 months
  of EVN) between two
• consecutive epochs
• higher sensitivity (10 antennas vs 5-7 of our
  EVN epochs)