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Burnham's nebula (HH 255), a peculiar Herbig-Haro object
* 1 and
J. Solf 2
Received 26 March 1996 / Accepted 27 June 1996
Burnham's nebula (HH 255) has been consistently identified as a Herbig-Haro object, i.e. a radiating shock wave. Spectrophotometry shows that at least a considerable part of this shock wave must have a velocity of 90 km s-1 or more. Recently, position-velocity diagrams have shown that the centroid radial velocity and the radial velocity dispersion are unusually small in HH 255 causing problems in the detailed shock wave interpretation. A bow shock like structure is excluded.
In order to get some insight into the physics of this enigmatic HH object we have studied the degree of ionization, the kinematic state and the electron density as a function of position near the main axis of HH 255. We find that the ionization (visible in a steep increase in O , N ions and in a decrease in and Ca ) occurs in a very narrow (1 0 - 1 5 wide) region which is centered at S of T Tau. In the same narrow region the centroid radial velocity drops rapidly to zero and the radial velocity dispersion has a maximum and then drops rapidly to 25 km s-1 (FWHM). Beyond 4 5 S of T Tau HH 255 looks like a pure recombination zone. The [O III ] lines and then the [N II ] lines become gradually weak in comparison to neutral and low-ionization lines. However, this interesting region, which forms the bulk of HH 255, is about 100 times wider than the theoretical prediction for the recombination region of a plane shock with the same post-shock density and the same shock velocity as HH 255. Consequently an interpretation of the zero radial velocity region as a plane shock seen edge-on is also excluded.
Key words: stars: individual: T Tau stars: pre-main sequence ISM: individual: Burnham's nebula ISM: jets and outflows ISM: kinematics and dynamics
Send offprint requests to: J. Solf
Online publication: July 8, 1998