The central star of the planetary nebula Abell 78.

Abstract

The authors have studied the ultraviolet spectrum of the nucleus of Abell 78, one of the two planetaries known to contain zones of nearly pure helium. Abell 78 is strikingly similar in many respects to Abell 30, the other planetary with embedded helium knots. The ultraviolet-optical color temperature, which the authors tentatively identify with the effective temperature, almost certainly lies between 67,000K and 130,000K. The limitations are set primarily by the error in the interstellar extinction. The best two choices of extinction now available yield 77,000 and 84,000 K. From the current evolutionary tracks and distance, these temperatures imply a mass of 0.56-0.58 M_sun;. The O V and N V P Cygni lines have terminal velocities similar to the value of 4200 km s^-1 measured earlier for C IV. A new possible P Cygni line is also seen at λ1310, which is tentatively identified as Si II.