IRAS-17423-1755 - A MASSIVE POST-AGE STAR EVOLVING INTO THE PLANETARY-NEBULA STAGE

IRAS 17423-1755 has been recognized as a new bipolar nebula during a m ulti-wavelength observational program of unidentified IRAS sources wit h far infrared colours similar to those of known planetary nebulae. B, V, R and H alpha CCD images show a clearly marked bipolar structure w ith a total extension of similar to 11 arcsec. The spectrum of the cor e shows strong emission lines of HI (Balmer and Paschen series), HeI, FeII, [FeII], OI, CaII and [CaII], Strong P-Cygni profiles are clearly seen in the Balmer lines and in some other emission lines, indicating the presence of a strong mass outflow. This is confirmed by the prese nce of a very steep density gradient in the nebula, strong near infrar ed excess and the detection of highly symmetric bipolar emission at ve ry large velocities in the lobes (greater than or similar to 425 km s( -1)). The highest velocity, however, is observed in the innermost regi on of the bipolar outflow, where a jet-like structure is detected with v = 870 km s(-1), while its velocity decreases to 750 km s(-1) a few arcsecs away from the central star. This has been interpreted as the r esult of sporadic mass loss events with a time-dependent ejection velo city. The position-velocity diagram is well reproduced assuming an inc lination angle of 150 degrees. The emission observed in the lobes show s an extraordinary line width and double-peaked profiles, indicating t hat the emission arises from the cooling region behind a bow-shock. Fr om the high values of the [NII]/H alpha ratios, we deduce that the out flowing material is nitrogen enriched gas of stellar origin, which can only be explained if IRAS 17423-1755 is an evolved star and not a you ng stellar object. The morphology and kinematics closely resemble thos e observed in well known bipolar proto-planetary nebulae, while the lu minosity is far below the values found in known LBV's (Luminous Blue V ariables). The characteristics of the OH maser emission found in IRAS 17423-1755 are consistent with the presence of an equatorial disk of n eutral material (perpendicular to the bipolar axis) expanding at v(e) = 50 km s(-1), which could be the responsible for the collimation of t he outflow.