Proto-planetary nebulae

Table 6.2 gives an overview of some of the stages of stellar evolution where carbon and/or s-process anomalies occur (see Fig. 3.37). The C/O ratio increases down the series. In addition to the types listed there, there are infrared carbon stars such as IRC +10216,1 proto-planetary nebulae and a whole zoo of peculiar carbon stars, including J stars (strong 13C as in the case of HD 52432 shown in Fig. 1.7) and hydrogen-deficient carbon stars which can be cool, e.g. R Cor Bor, RY Sag and HD 137613 shown in Fig. 1.7, or hot (when they look like extreme helium stars) such stars may have lost their envelopes by binary mass transfer, or they may be born-again AGB stars. 

Calculated spectra for hydrogenated fullerenes have been published in comparison with the unidentified infrared emission bands (Webster 1991 Stoldt et al. 2001). The infrared spectrum of C60H36 has been compared also with the infrared features of other astrophysical objects like the proto-planetary nebulae (Cataldo 2003a, b). An inventory about fullerenes and hydrogenated derivatives in the interstellar... 

Note-. A This work. Khanna et al. [98] have measured infrared band extinctions of crystalline C4H2. Materny and Kiefer [99] have determined the Raman spectrum of 4Ol(s) crystals. Cernicharo et al. [79] have observed the 628 cm band in the proto-planetary nebula CRL 618. 

Molecules are frequently observed in the outer envelopes of red supergiants with surface temperatures less than about 5000 K. These stars have strong stellar winds, of order 10 to 10 M yr with velocities typically less than 50 km s . A few hotter stars, such as 89 Herculis and HD 161796, have also been found to show CO emission these and related stars are proto-planetary nebula objects in the process of becoming white dwarf stars. For some very highly evolved dusty stars, strong far-IR emission, indicative of dust, is accompanied by maser emission. These are the so-called OH/IR stars, which are most frequently Mira variables. Some evolved stars also show SiO masers. A few stars, such as the extreme supergiant IRC -t-10216, are veritable chemical factories, displaying almost all of the molecular species observed in comets and in dense interstellar molecular clouds. 

Abstract. We resolved dust shells at 10.5 and 12.5 pm around the post asynq>totic pant branch stars HD 161796 and AFGL 2343, two oxygen-ridi proto-planetary nebulae with very cold IRAS colors. The shell of AFGL 2343 appears as a drculax ring with 4" — 5" diameter, while HD 161796 appears bipolar with 2" diameter, bmer shell radii indicate that both stars left the asymptotic giant brand about 200 years ago. 

We can estimate the time elapsed since each star left the AGB from the observed inner radii of the dust shells. The shell expansion velocity is obtsdned from published CO observations. For HD 161796, with D=1.2 kpc d Vexp = 12 km sec, the observed inner radius of 0.5" implies that the star stopped losing mass and left the AGB 240 years ago. For AFGL 2343, the relevant parameters are D=1.5 kpc, Vexp = 33 km sec, and 0.75" inner radius, meaning that the star left the AGB 170 years ago. Therefore, if AFGL 2343 is a proto-planetary nebula, it is lazier than HD 161796, whidi left the AGB about the same time as AFGL 2343, but already has a hotter central star (F2-5) than AFGL 2343 (G5). 

MID-IR (8-13 m) IMAGES OF THE 21 /tm, CARBON RICH PROTO-PLANETARY NEBULAE... 

Key words 21/on Proto-Planetary Nebulae - Carbon Dust PAHs - Stellar Evolution... 

Fig. 4.7. 3He/H in simple Galactic H n regions, i.e. those thought to be reasonably well represented by homogeneous spherical models (Balser et al. 1999), and one planetary nebula, as a function of their oxygen abundance. 3He/H is plotted on a logarithmic scale relative to the proto-solar value of 1.5 x 10-5. After Bania, Rood and Balser (2002). Reprinted by permission from Macmillan Publishers Ltd. Courtesy Tom Bania.

Astronomical observations of molecular clouds and young stellar objects provide the basis for our understanding of the early solar system (Cameron, 1995 Alexander et al., 2001). The first stage in this process is when a fragment of an interstellar molecular cloud collapses to form a disk-like nebula, or proto-planetary disk. This process normally takes... 

Key words 10 pm observations - nebulae proto-planetary - stars drcumsteUar matter - stars supergiants... 

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