2005ApJ...629.1040K

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.