IRAS 17423-1755: a massive post-AGB star evolving into the planetary nebula stage?

Riera, A.; Garcia-Lario, P.; Manchado, A.; Pottasch, S. R.; Raga, A. C.
Referencia bibliográfica

Astronomy and Astrophysics, v.302, p.137

Fecha de publicación:
IRAS 17423-1755 has been recognized as a new bipolar nebula during a multi-wavelength observational program of unidentified IRAS sources with far infrared colours similar to those of known planetary nebulae. B, V, R and Hα CCD images show a clearly marked bipolar structure with a total extension of ~11 arcsec. The spectrum of the core 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 presence of a very steep density gradient in the nebula, strong near infrared excess and the detection of highly symmetric bipolar emission at very large velocities in the lobes (>~425km/s). The highest velocity, however, is observed in the innermost region of the bipolar outflow, where a jet-like structure is detected with v=870km/s, while its velocity decreases to 750km/s a few arcsecs away from the central star. This has been interpreted as the result of sporadic mass loss events with a time-dependent ejection velocity. The position-velocity diagram is well reproduced assuming an inclination angle of 150deg. The emission observed in the lobes shows an extraordinary line width and double-peaked profiles, indicating that the emission arises from the cooling region behind a bow-shock. From the high values of the [NII]/Hα ratios, we deduce that the outflowing material is nitrogen enriched gas of stellar origin, which can only be explained if IRAS 17423-1755 is an evolved star and not a young stellar object. The morphology and kinematics closely resemble those observed in well known bipolar proto-planetary nebulae, while the luminosity is far below the values found in known LBV's (Luminous Blue Variables). The characteristics of the OH maser emission found in IRAS 17423-1755 are consistent with the presence of an equatorial disk of neutral material (perpendicular to the bipolar axis) expanding at v_e_=50km/s, which could be the responsible for the collimation of the outflow.