J. Chapman (ATNF); M. Cohen (Berkeley, US); Q. Parker (Macquarie University / Anglo-Australian Observatory); R. Deacon (University of Sydney); R. Sault (ATNF)

V1018 Sco is a well known OH/IR star with strong infrared and radio maser emission. The OH 1612-MHz maser spectrum of V1018 Sco, shown in Figure 1, has a double-peaked profile that is characteristic of asymptotic giant branch (AGB) stars. The AGB stars are evolved pulsating stars which have high mass-loss rates and strong stellar winds. The OH maser emission from AGB stars occurs from the outer regions of an expanding circumstellar envelope. For V1018 Sco the stellar wind has an expansion velocity of 15 kilometres per second, while the OH emission is detected from an exceptionally large shell with a diameter approximately 11,000 times larger than the distance between the sun and earth. Monitoring observations of the OH maser emission with the Parkes radio telescope have shown that the central star has large amplitude pulsations with a period of about 1500 days.

During a systematic search for Galactic Planetary Nebulae (Parker et al. 2005), a survey exposure showed a faint almost circularly-symmetric ionized nebula that was almost centred on the position of the OH maser emission from V1018 Sco. This position also coincides with a strong infrared source. The optical nebula has an outer diameter of 39 arcsec, approximately 10 times larger than the OH shell. Follow up long-slit spectroscopy confirmed the presence of the nebula with the detection of Hα, [NII] and [SII] emission lines, typical of planetary nebulae.

The detection of an ionised planetary nebula around a still-pulsating AGB star was remarkable. V1018 Sco is the only known source where a planetary nebula has been detected around an AGB star that is still pulsating and thus still in the AGB stage of evolution. AGB stars are the precursors of planetary nebulae and represent an earlier stage of stellar evolution. Models of stellar evolution predict that towards the end of the AGB phase, stars lose so much mass from their outer envelopes that they can no longer support strong pulsations. As a star evolves away from the AGB, the pulsations cease and the star changes from losing mass in slow dense winds to losing mass in a hotter, faster wind. During the post-AGB phase, the hot winds sweep up the remnant material and the swept-up shells become visible as planetary nebulae as the central stars become hot enough to ionise the shells.

To investigate the nebula around V1018 Sco, radio continuum emission observations were taken with the Compact Array using four observing bands at 3, 6, 13 and 20 cm. As shown in Figure 2, two regions of radio continuum emission were detected with stronger emission from the western part of the nebula (Source A), and fainter emission to the north (Source B). Both sources were offset by about 10 arcsec from the stellar position.

Figure 3 shows the spectral energy distribution for Source A and Source B. In both cases, a power law fitted to the data gives a slope (or "spectral index") of about -0.8, and this shows that the radio continuum emission is strongly non-thermal. Source A while was detected in all four bands while source B was only detected at 3, 6 and 13 cm. This can be interpreted as showing the source B is likely to be located towards the back of the nebula, so that the 20-cm radio emission is strongly absorbed within the nebula. Source A is likely to be located towards the front of the nebula.

The spectral index of the nonthermal radio emission from V1018 Sco is similar to that detected from Wolf Rayet stars. In the much more massive Wolf-Rayet stars, nonthermal radio continuum emission is detected from shocks that are generated when a wind from a massive Wolf Rayet star collides violently with the wind from a second massive star. For V1018 Sco, the presence of nonthermal emission also indicates a wind-wind collision.

For V1018 Sco, the Compact Array data suggest a previously unobserved phase where a planetary nebula has just started to form around an AGB star that is still pulsating. A fast wind from the central star has recently "turned on" and this ploughs into the slow AGB wind creating shocks, non-thermal emission and a compressed shell which is visible as an ionised optical nebula. Such wind-wind collisions are likely to play an important role in shaping the nebula during the transition of a star as it evolves from the AGB to become a planetary nebula.

References

Cohen, M., Parker, Q. A., Chapman, J. M., 2005, MNRAS, 357, 1189

Cohen, M., Chapman, J.M., Deacon, R.M., Sault, R. J., Parker, Q. A., Green, A., 2006, MNRAS, in press

Parker, Q. A., et al. 2005, MNRAS 362, 689

Figure 1: An averaged OH 1612-MHz spectral profile of V1018 Sco, from 36 separate epochs of observations taken with the Parkes radio telescope. The two emission peaks correspond to the strongest maser emission seen from the front and back of the expanding circumstellar envelope.

Figure 2: A Compact Array image obtained in 2005 showing the 6-cm radio continuum emission from V1018 Sco. The large circle indicates the approximate size of the optical planetary nebula. The ring has a diameter of 39 arcsec and is centred on the position of the central star which is marked by a small cross. The peak flux density of the 6-cm emission is 3 milliJansky.

Figure 3: The spectral energy distribution of the two radio continuum sources A and B. The solid lines are power law fits to the data. The negative slope of the lines (with a spectral index of -0.8) shows that the emission is largely nonthermal.