A 2 micron H2Spectral survey of proto-planetary nebulae.
KELLY D.M. and HRIVNAK B.J.
Abstract (from CDS):
We measured 2.1-2.3 µm spectra for a mostly complete sample of known proto-planetary nebulae (PPNe) at declinations north of -30°. This spectral range includes the H2emission lines 1-0 S(1), 1-0 S(0), 2-1 S(1), 2-1 S(2), and 3-2 S(3). We detected H2emission from 16 of the 51 objects in our survey. Twelve of these are new detections, doubling the number of PPNe with detected H2emission. We find that H2 emission commences at a spectral class of mid-G in PPNe with bipolar morphologies and optically thick equatorial regions; in fact, all bipolar F-G PPNe with an optically thick equatorial region have collisionally excited H2emission. Radiative excitation becomes important in PPNe when the central star reaches an early B spectral type, just before photoionization of the nebula and the commencement of the planetary nebula (PN) phase. Almost all of the PPNe with B central stars show H2emission, with either a purely radiative or a mixed collisional plus radiative spectrum. Since H2emission is rare in nonbipolar PNe, the destruction of the H2in the nonbipolars must be roughly coincident with the photoionization of the nebula. As with H2-emitting PNe, the bipolar H2-emitting PPNe are found at low Galactic latitudes. Optically thin H2-emitting nebulae are not similarly restricted to low latitudes. Brγ emission is detected in 15 of the sources, including all those of B spectral type.
Stars: Circumstellar Matter - Infrared: Stars - ISM: Planetary Nebulae: General - Stars: AGB and Post-AGB - Stars: Mass Loss