Discovery of a planetary nebula around the OH/IR star V1018 Sco

The OH/IR star

V1018 Sco (IRAS 17317-3331, OH354.88-0.54) 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 where the maser emission occurs from the outer regions of an expanding circumstellar envelope. Welty et al. (1987) have imaged the OH 1612-MHz maser emission with the VLA and shown that it occurs in a number of clumps distributed within a shell of angular diameter ~ 3.3 arcsec. The OH shell has strongest emission in an east-west plane and is weaker to north and south. We interpret this as a well-filled molecular torus around the star with less dense emission along a polar axis.

Figure 2: The OH 1612-MHz light curve from the Parkes monitoring data, plotted using a weighted average of the emission from the blue-and red-shifted emission peaks. The best-fit curve for a period of 1486 days is also shown. This is obtained assuming a combination of a fundamental period and a single harmonic.

Figure 2 shows an OH light curve of V1018 Sco, obtained from 36 epochs of Parkes monitoring data, over a five-year period. The light curve shows that the star is a large-amplitude, long-period variable with an exceptionally long pulsation period of 1486 days. The light travel time across the OH envelope has been obtained using a phase lag technique (Chapman et al. 1995). The measured phase lag of 60 days corresponds to a linear size of the OH shell of 11,000 AU. The phase-lag linear diameter and the angular diameter yield a stellar distance of 3.2 kpc, with an estimated precision of about 20%.

The OH maser properties of V1018 Sco are characteristic of maser emission from an OH/IR star with a high expansion velocity (14.9 km s^-1), an exceptionally long pulsation period (1486 days) and an extremely large OH shell (11,000 AU). The large-amplitude pulsations demonstrate that the star is still pulsating strongly, and has not yet left the AGB stage of evolution, while the large shell size, long stellar period and Galactic location indicate a fairly massive star with an initial mass above 4 M_sun.

Discovery of the planetary nebula

During a systematic search for Galactic Planetary Nebulae, as part of the MASH (Macquarie-AAO-Strasbourg-Ha) project (Parker et al. 2003), a survey exposure of field HA630 showed a faint almost circularly symmetric Hα nebula. Subsequent identifications showed that the position of the centre of the nebula aligns precisely with the central position of the OH 1612-MHz masers. This position also coincides with a strong infrared source, detected as a point source in the MSX and IRAS mid-far infrared surveys.



Figure 3: The weak circular feature at the centre of the image shows an Hα ring, centred on the stellar position, detected as part of the MASH survey for planetary nebulae. The image size is 128 arcsec in declination and 140 arcsec in right ascension. To enhance the faint nebula, the image shown is a quotient image formed from the ratio of the Hα image to a short-red exposure of the same region. This effectively removes most of emission from point sources in the region.

The nebula, shown in Figure 3, 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.

Radio continuum and H₂O maser emission: first results

Following a target-of-opportunity request, in January 2004 we observed V1018 Sco at 3 and 6 cm using the ATCA for ~ 8 hours in the 6A configuration. Radio continuum was detected at both 3 and 6 cm with total flux densities of 0.5 and 2.2 mJy respectively, corresponding to spectral index of 0.8. The radio continuum thus appears to be strongly non-thermal. The brightest continuum emission is seen at both 3 and 6 cm as an arc-like extended feature located 14 arcsec southwest of the stellar position, on the inner edge of the optically visible nebula. To better define the radio continuum structure and to look for weaker, more extended emission, further radio continuum observations at 3, 6, 13 and 20 cm are in progress.

Figure 4: A spectrum of the 22-GHz water maser emission from V1018 Sco, obtained from a short observation taken with the Compact Array on 14 November 2004.

More recently, in November 2004 we obtained a detection of H₂O maser emission from V1018 Sco, from a short Compact Array observation. The spectrum shows two maser emission features at velocities of 2.4 and +25 km s^-1 (Figure 4). In AGB stars, the water masers are collisionally excited in the inner, denser regions of the circumstellar shells, at densities up to 10¹¹ cm^-3. In general the water masers are located closer to the stars than the OH masers. However, for a small number of post-AGB stars, high velocity water maser emission has been detected that is associated with high-velocity, collimated molecular outflows. In such cases the water masers may be located beyond the OH maser region. An excellent example of this is seen in the AGB star W43A (Imai et al. 2002). Such jet-like flows may mark the beginning of the transition of an AGB star to becoming a planetary nebula, and play an important role in shaping the planetary nebulae.

Discussion

The detection of an ionised planetary nebula around a still-pulsating AGB star is 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. 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.

In V1018 Sco we may be witnessing a hitherto unobserved phase where a planetary nebula has just started to form around an AGB star. We speculate that a fast wind has recently turned on and is able to penetrate lower density regions in the cooler AGB wind. For this scenario, collisional excitation may occur in accumulated material, previously shed during the AGB phase, far out in the circumstellar envelope. The continuum emission may occur from the wind-wind collision between the jet-like outflow and the cooler AGB wind, while the water masers may be associated with the jet, with the maser emission either from a post-shock region at the end of the jet, or from within the jet.

The optical, infrared and OH maser properties of V1018 Sco are discussed in more detail in a recent paper (Cohen et al. 2005). A second paper that includes the radio continuum and water maser emission results is in progress.

References

Chapman, J., Habing, H., Killeen, N., 1995, ASP Conf Ser., 83, p113
Cohen, M., Parker Q., Chapman, J., MNRAS, 2005, in press (paper I)
Imai, H., Obara, K., Diamond, P., Omodaka, T., Sasao, T., Nature, 417, 829
Parker, Q.A., Hartley, M., Russeil, D., Acker, A., Morgan, D.H., Beaulieu, S., Morris, R., Phillipps, S., Cohen, M., 2003, "A rich new vein of Planetary Nebulae from the AAO/UKST Halpha survey". In: Planetary Nebulae: Their evolution and role in the Universe, ASP Conf. Ser. P25 (published on behalf of the IAU by the Astronomical Socieity of the Pacific), (eds. Dopita, Kwok & Sutherland)
Welty, A., Fix, J., Mutel, R., 1987, ApJ, 318, 852

Jessica Chapman (ATNF), Martin Cohen (Berkeley, USA), Quentin Parker (Macquarie University / Anglo-Australian Observatory), Rachel Deacon (University of Sydney) and Bob Sault (ATNF)
(Jessica.Chapman@csiro.au)