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 CagliariWalter Alef: Max-Planck-Institut für Radioastronomie (Bonn)Jan Brand: IRA-CNR, Istituto di Radioastronomia di Bologna

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 CH

3OH):

permit to study the gas structure and kinematics nearby the YSO with a linear resolution of few AU

Star Forming RegionsStar Forming Regionsjet

Accretion disk

jet

Angular momentum conservationcollapsing core

GMC

Theory:Theory:

ObservationsObservationsLow-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)

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).

Hunter et al. (1995): • VLA 8.4 GHz thermal continuum source (interpreted as free-free emission from an ionized wind); • CO bipolar outflow; • H

2 NIR emission jet.

VLBI water maser observations are needed!!VLBI water maser observations are needed!!

The case of AFGL 5142The case of AFGL 5142Previous observations stronlgy suggest the presence of an high-mass YSO:

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.

ObservationsObservations

Array: EVN (Medicina, Cambridge, Onsala, Effelsberg, Metsahovi, Noto, Jodrell and Shanghai)

Transition rest frequency = 22235.080 Mhz

Observational epochs: Oct 1996, and June,

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)

Data reduction

Reduction package: NRAO AIPSChannel map sky area: 4''4''

Velocity range: [-10.5, 0.7] km s-1.

Clean beam FWHM: 2.1 1.1 mas.

RMS noise level: 0.02-0.27 Jy beam-1.

Identification of maser featuresIdentification 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 epochsA final set of 12 distinct “features”, 7 out of these observed for more than one epoch

Measured proper motionsMeasured proper motions

Comparison of VLBI results with previous interferometric observations

8.4 GHz continuum

88 Ghz continuum

□ 1992 VLA H2O

. 1998 VLA H2O

Proper motions

OVRO outflows (Hunter et al. 1999)

Group I of VLBI masers

Group II of VLBI masers

Kinematics of the masing gasSimple 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 dispersion~2 km s-1 arcsec-1

vel. dispersion~8 km s-1, distance ~ 0.35''

vel. dispersion~1.7 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!

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 foundIt 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

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) Mthe central object is a massive YSO, compatible with

previous core (Hunter et al 1999) and disk (Zhang et al 2001) mass estimates

Group IWe tested the kinematics fitting two models: Keplerian disk and conical outflow.

Only the keplerian disk model produces an acceptable solution!

. .. . . Proper motions

Maser H H22OO

Group IIThere 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.Maser HMaser H

22OO

Ambient gasRed shifted lobe

Blue-shifted lobe

ConclusionsUsing 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

Future workWe 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)

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