VLBI observations of H 2 O 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

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 H 2 O; 6.7 and 12 GHz CH 3 OH): permit to study the gas structure and kinematics nearby the YSO with a linear resolution of few AU Star Forming Regions jet Accretion disk jet Angular momentum conservation collapsing core GMCTheory: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)

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!! The case of AFGL 5142 Previous observations stronlgy suggest the presence of an high-mass YSO: Zhang et al. (2002): VLA NH 3 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.

Observations Array: EVN (Medicina, Cambridge, Onsala, Effelsberg, Metsahovi, Noto, Jodrell and Shanghai) Transition rest frequency = 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 AIPS Channel 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: Jy beam -1.

Identification of maser features 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: 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 Measured proper motions

Comparison of VLBI results with previous interferometric observations 8.4 GHz continuum 88 Ghz continuum □ 1992 VLA H 2 O VLA H 2 O  Proper motions OVRO outflows (Hunter et al. 1999) Group I of VLBI masers 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: 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 (~ 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

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) M YSO = (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 I We tested the kinematics fitting two models: Keplerian disk and conical outflow. Only the keplerian disk model produces an acceptable solution!.... Proper motions H 2 O Maser H 2 O

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. Maser H 2 O Ambient gas 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

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