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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
Star Forming Regions
GMC
Theory:
collapsing core
jet
Angular momentum conservation
Accretion disk
jet
Observations
Low-mass YSOs: high angular resolution observations, from the millimeter to the optical
(HST), have revealed the existence of disk/jet systems, confirming the theory
High-mass YSOs: On average more distant from the Sun (1 kpc) and during the ZAMS
phase still enshrouded in dust and gas envelopes (optical and NIR observations impracticable)
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
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!!
Observations
Data reduction
Array: EVN (Medicina, Cambridge, Onsala,
Effelsberg, Metsahovi, Noto, Jodrell
and Shanghai)
Transition rest frequency = 22235.080 Mhz
Reduction package: NRAO AIPS
Channel map sky area: 4''4''
Velocity range: [-10.5, 0.7] km s-1.
Observational epochs: Oct 1996, and June,
Clean beam FWHM: 2.1 1.1 mas.
Sept, Nov 1997
Integration time: 13 scans of 6.5 minutes
Bandwidth = 1 MHz
Spectral channels = 112
Velocity resolution = 0.12 km s-1
Polarizations = LCP & RCP
Correlator = MKIII (Bonn, Germany)
RMS noise level: 0.02-0.27 Jy beam-1.
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 > 0.3 km/s), with a position shift of the intensity
peak from channel to channel smaller than the FWHM size.

26 maser “features” over the four epochs
A final set of 12 distinct “features”, 7 out of these observed for more than one epoch
Measured proper motions
Comparison of VLBI results with previous
interferometric observations
OVRO outflows
(Hunter et al. 1999)
Group I of VLBI masers
8.4 GHz continuum
88 Ghz continuum
□
1992 VLA H2O
.
1998 VLA H2O
Proper motions
Group II of VLBI masers
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:
Large scale outflow
Diameter ~ 50'', vel. dispersion ~100 km s-1, (assuming a Hubble flow)
rate dispersion~2 km s-1 arcsec-1
VLBI Group I
vel. dispersion~8 km s-1, distance ~ 0.35''
VLBI Group II
vel. dispersion~1.7 km s-1, distance ~1''
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!
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?

Group I
We tested the kinematics fitting two models:
Keplerian disk and conical outflow.
Only the keplerian disk model produces
an acceptable solution!
..
...
Maser H2O
Proper motions
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
Group II
There
Group
The
are too few observables to test meaningfully a kinematical model.
II might be associated with a distinct (as yet undetected) YSO.
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
Ambient gas
the gas outflowing from the YSO with the
ambient gas of the progenitor molecular core.
Maser H2O
Red shifted
lobe
Blue-shifted
lobe
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.
AFGL 5142 is a good example of a massive (proto)star, possibly
associated with a keplerian disk and jet/outflow system
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)