Bipolar, Collimated Outbursts in the Planetary Nebula Hb 4

Abstract

Spatially resolved profiles of the Hα and [N II] 6548, 6584 A emission lines obtained with the Manchester Echelle Spectrometer combined with the 2.1 m San Pedro Martir telescope have revealed the remarkable velocity structure in the peculiar planetary nebula Hb 4. The compact, elliptical core shows a moderate splitting in the [N II] 6584 A line profiles of only 2Vexp = 43 km s-1. However, the detection of high velocities in the nebular line profiles from the elongated, ionized knots protruding symmetrically on both sides of the core give direct evidence of the presence of highly collimated outflows in Hb 4. The line profiles in these symmetric, elongated knots are complex, with radial velocities of ±150 km s-1 with respect to the systemic radial velocity and detached emission regions at further extreme radial velocities. The long-slit spectra are analyzed with a parameterized description of the emissivity and velocity field of the fast-moving, collimated, ionized flow. The observed profiles are simulated in detail by a model of the emission from a bow shock at the head of the jet followed by a faster high-density component. In Hb 4 we are probably observing the first stages of intermittent outflows that could develop into episodic two-sided jets, as observed in the case of the planetary nebula Fleming 1.