Physics
Scientific paper
Mar 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996lpi....27..573h&link_type=abstract
Lunar and Planetary Science, volume 27, page 573
Physics
2
Aluminum-26, Isotopes: Carbon, Isotopes: Nitrogen, Silicon Carbide: Presolar, Stars: Agb
Scientific paper
The isotopic characteristics of most presolar SiC grains (mainstream grains) from primitive chondrites are consistent with an origin in low-mass AGB stars. Small groups of grains require a different origin, such as the X grains which appear to have come from supernovae. Grains with 12C/l3C ratios below ~20, which make up 5-10% of the presolar SiC in chondrites, are not currently understood. Standard models of partial H burning at the base of the stratified stellar envelope during the Main Sequence followed by First, Second, and Third Dredge-up cannot produce 12C/l3C ratios below ~20 because: 1) homogenization of the envelope dilutes material processed by H burning (l2c/l3c z 3.5) with unprocessed envelope material (12c/l3c ~ 89), and 2) 12c is mixed into the envelope from the He shell during Third Dredge-up, the process which produces a carbon star. These grains also tend to have higher (26AI/27Al)o and 15N/I4N ratios than mainstream SiC grains. Low 12C/l3C ratios can be produced by hot bottom burning at the base of the convective envelope during the AGB phase, which occurs in stars more massive than ~4 M solar mass, but if too much nuclear processing occurs, most of the C is converted to 14N and the star cannot become a carbon star. Models of slower deep circulation below the standard convective envelope, which transports material into the H-burning zone in 1-2 M solar mass stars, appear capable of producing low 12C/l3C and 18O/~fiO ratios while still permitting the stars to become carbon stars. But, since 26AI is efficiently produced only at temperatures above ~30x106 degrees, higher 26AI is not predicted by a deep circulation model in 1-2 M solar mass stars. The base of the convective envelope may achieve such temperatures in more massive stars during hot bottom burning, allowing extra production of 26Al, but no AGB model appears capable of explaining high 15N/I4N ratios, particularly when the ratios exceed the solar value.
Huss Gary R.
Wasserburg Gerald J.
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