Astron. Astrophys. 355, 1103-1114 (2000)

Optical long-slit spectroscopy and imaging of OH 231.8+4.2

C.  Sánchez Contreras ^1,3, V.  Bujarrabal ¹, L.F.  Miranda ² and M.J.  Fernández-Figueroa <sup>3

1</sup> Observatorio Astronómico Nacional (IGN), Ap. 1143, 28800 Alcalá de Henares, Spain ([sanchez, bujarrabal]@oan.es)
² Instituto de Astrofísica de Andalucía, CSIC, Ap. 3004, C/ Sancho Panza s/n, 18080 Granada, Spain (lfm@iaa.es)
³ Departamento de Astrofísica, Facultad CC. Físicas, Universidad Complutense, 28040 Madrid, Spain ([sanchez, mjf ]@astrax.fis.ucm.es)

Received 4 June 1999 / Accepted 28 December 1999

Abstract

We present optical long-slit spectra and complementary broad and narrow band images of the bipolar proto-planetary nebula OH 231.8+4.2. Absolute J2000 coordinates have been calculated for our maps from the position of nearby stars. Our maps of the optical continuum show the spatial distribution of the starlight scattered by dust grains. This component is found to be highly elongated along the nebular axis, with a structure very similar to that of the molecular emission. Flux variations with time of the red continuum emission are detected. Our long-slit spectroscopy of H and other atomic lines reveals wide spectral profiles and, in general, a complex spatial and spectral emission distribution. The emission arises from two broad lobes and is shifted toward shorter (north lobe) and longer wavelengths (south lobe), indicating that the gas is flowing outwards at high velocity. The clumpiness of the emission nebula is remarkable. A simple model has been used to describe the complex structure and kinematics of this source. Our model suggests that, in addition to the two extended, hollow lobes identified in the H images, a smaller, bubble-like expanding structure should lie inside the south lobe. A comparison of the emission line spectrum with predictions of theoretical shock models confirms that the optical lines have been shock excited. We have estimated the electron density of the lobes and deduced the total ionized mass of the nebula, obtaining a low value of 510^-4 . We interpret the shaping and evolution of OH 231.8+4.2 in the wind interaction scenario for planetary nebulae formation. The peculiar structure and kinematics of the molecular outflow and the ionized envelope are explained in terms of a shock regime transition: the well collimated molecular outflow and the different components of the optical nebula would consist of circumstellar material swept-up by a unique shock in a radiative, and non-radiative regime, respectively. Finally, we briefly discuss the controversial evolutionary status of OH 231.8+4.2.

Key words: stars: AGB and post-AGB stars: individual: OH 231.8+4.2 stars: mass-loss stars: circumstellar matter stars: late-type

Send offprint requests to: C.  Sánchez Contreras

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Contents

• 1. Introduction
• 2. Observations and data reduction
  • 2.1. Optical imaging
  • 2.2. Optical long-slit spectroscopy
• 3. Observational results
  • 3.1. Optical imaging
  • 3.2. The spectrum of the reflection nebulosity of OH 231.8+4.2
  • 3.3. Long-slit spectroscopy
• 4. Structure and kinematics of OH 231.8+4.2
• 5. Physics of the emission nebula
  • 5.1. Excitation conditions: Importance of shocks
  • 5.2. Electron density and ionized mass estimations
• 6. Shaping and evolution of OH 231.8+4.2
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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