Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005apj...629.1040k&link_type=abstract
The Astrophysical Journal, Volume 629, Issue 2, pp. 1040-1054.
Astronomy and Astrophysics
Astronomy
19
Stars: Circumstellar Matter, Infrared: Stars, Ism: Planetary Nebulae: General, Stars: Agb And Post-Agb, Stars: Mass Loss
Scientific paper
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 H2 emission lines 1-0 S(1), 1-0 S(0), 2-1 S(1), 2-1 S(2), and 3-2 S(3). We detected H2 emission from 16 of the 51 objects in our survey. Twelve of these are new detections, doubling the number of PPNe with detected H2 emission. 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 H2 emission. 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 H2 emission, with either a purely radiative or a mixed collisional plus radiative spectrum. Since H2 emission is rare in nonbipolar PNe, the destruction of the H2 in 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.
Hrivnak Bruce J.
Kelly Douglas M.
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