EVN observations of OH maser burst in OH17.7-2.0 M. Szymczak Torun Centre for Astronomy collaborators: A. Bartkiewicz (Torun Centre for Astronomy) E. Gerard (Paris Observatory)

Target characteristics OH17.7-2.0 (IRAS18276-1431) central star earlier than K5, Te=4000 –104 K (Le Bertre 1987) bimodal IR energy distribution O-rich proto-planetary nebula (Kwok 1993) small OH flux variations (Herman & Habing 1985) non-variability or only slight variations at NIR (Le Bertre 1989) no SiO masers (Nyman et al.1998) disappearance of water maser in 1990 (Engels 1997) ellipsoidal shape at 2.2m: 2.5x2arcsec, PA~20o OH circumstellar shell of radius 250-650mas (Bains et al.2003)

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Unusual outburst of ~73kms–1 feature at 1612MHz

Variations in the 1612MHz flux density of selected features

No variations in the 1665/1667MHz flux densities

Motivation of VLBI observations where is an active region to determine its properties causes and mechanisms of the outburst – a local enhancement of mass loss, pump rate, OH abundance, – amplification of background continuum source – magnetic beaming, overlap of Zeeman components – interaction of a fast wind with remnant AGB shell

Observations 8 telescopes (Cm,Jb,Ef,Mc,On,Tr,Hh,Wb) of the EVN 2005 March 3 (1st epoch) 1612MHz OH line phase-referencing observations JIVE correlator, dual circular polzn spectral resolution (0.5MHz/1024ch) ~0.09kms–1 beam 64 x 20mas, PA=8o 1s ~ 10mJy beam–1 for single circular polzn. OH flux monitoring with the Nancay telescope

Comparison of the 1612 MHz spectra

Averaged channel maps 48.0/2 48.3/5 48.6/2 48.8/3 49.1/3 49.4/4 49.8/4 50.1/4 50.5/4 50.9/4 51.2/4 51.7/7 54.5/40 68.2/11 69.0/6 69.6/6 70.2/9 70.8/4 71.2/5 71.6/5 71.9/2 72.3/5 72.6/2 72.8/3 73.2/6 73.7/4 74.0/4 74.4/3 74.6/3 74.9/3

V = 47.96kms–1

V = 48.28kms–1

V = 48.60kms–1

V = 48.83kms–1

V = 49.10kms–1

V = 49.41kms–1

V = 49.77kms–1

V = 50.15kms–1

V = 50.50kms–1

V = 50.86kms–1

V = 51.23kms–1

V = 51.73kms–1

V = 54.46kms–1

V = 68.19kms–1

V = 68.96kms–1

V = 69.58kms–1

V = 70.18kms–1

V = 70.85kms–1

V = 71.18kms–1

V = 71.63kms–1

V = 71.95kms–1

V = 72.27kms–1

V = 72.59kms–1

V = 72.81kms–1

V = 73.22kms–1

V = 73.68kms–1

V = 74.04kms–1

V = 74.36kms–1

V = 74.63kms–1

V = 74.90kms–1

OH17.7-2.0 1612MHz blue-shifted emission 0 moment

OH17.7-2.0 1612MHz red-shifted emission 0 moment

Spot diagram outburst region 4 components of the maser emission: – spherical shell – biconical interaction region – equatorial region – stellar image bipolar outflow with an axis at PA ~10o outburst region is ~400mas to the S of the central star; 2 1016cm for D=3.4kpc outburst region

Expanding shell model OUTBURST stellar image V* = 61.6 kms-1 Vexp=13.8 kms-1 stellar image

Summary of results two OH shells of radii ~250 and ~650mas (1.3 1016 and 3.3 1016cm) were detected => episodic mass loss (time scale of 460years) the bursting emission comes from two unresolved spots of Tb1011K located at 2 1016cm from the central star bipolar outflow; polar cavities oriented along PA ~10o and equatorial region at PA ~100o stellar image due to amplification of background stellar photons

Conclusions strong support for the model of interacting stellar winds interactions of bipolar outflow with the remnant shell quench the maser inside the biconical region and improve /induce the maser conditions at the biconical surface the outburst appears to be a result of these interactions, – the excitation of blobs in the remnant shell VLBA polarimetric studies are required to explain a possible role of B field in this phenomenon further EVN observations to monitor the outburst