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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

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slide1

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

slide2

Star Forming Regions

GMC

jet

collapsingcore

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)

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

slide3

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!!

slide4

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)

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: 0.02-0.27 Jy beam-1.

Observations

Data reduction

slide5

26 maser “features” over the four epochs

  • A final set of 12 distinct “features”, 7 out of these observed for more than one epoch

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.

slide7

Comparison of VLBI results with previous interferometric observations

OVROoutflows

(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

slide8

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

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

vel. dispersion~1.7 km s-1, distance ~1''

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!

slide9

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?

slide10

Group I

. .

. . .

Maser H2O

Proper motions

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-onand (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

slide11

Red shifted lobe

Ambient gas

Maser H2O

Blue-shifted lobe

  • 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.
slide12

AFGL 5142 is a good example of a massive (proto)star, possibly

associated with a keplerian disk and jet/outflow system

  • 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.

slide13

Final remarks

  • Only 5-7 antennae, out of the 11 presently available to observe at 22.2 GHz,

took part in each runin 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)