Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..507f&link_type=abstract
Meteoritics, vol. 30, no. 5, page 507
Other
1
Gravitational Collapse, Isotopic Anomalies, Nucleosynthesis, Shock Wave, Solar Nebula
Scientific paper
The evidence for live 26Al in Allende refractory inclusions [1] implies that no more than about 1 Myr elapsed between the nucleosynthesis of the 26Al and its incorporation into cm-sized inclusions in the solar nebula [2]. A supernova was immediately suggested as the source of the 26Al, and the supernova shock front was implicated both as a means for transporting the 26Al from the supernova to the presolar cloud, and for triggering the collapse of the cloud by the impact of the shock [3]. Evidence of other exotic stellar environments comes from isotopic anomalies in presolar grains, which suggest grain formation in novae, Wolf-Rayet stars, or asymptotic giant branch (AGB) stars [4]. Nucleosynthesis in the hydrogen- and helium-burning layers of an AGB star can account for the approximate abundances of several short-lived radionuclei (26Al, 60Fe, and 107Pd) found in chondritic meteorites, assuming a mass mixing ratio of about 100:1 between the presolar cloud and the AGB star ejecta [2]. The same 100:1 mixing ratio is necessary to explain the solar system abundance of 3He if the 3He was derived from the planetary nebula phase of an AGB star [5]. 3D hydrodynamical models of the interaction of stellar shock waves with dense cloud cores suggest a mixing ratio of about 100:1 between the mass of the presolar cloud and the mass of the material from the shock wave that is injected into the collapsing protostellar cloud [6]. The agreement between these three independent estimates of mixing ratios is very remarkable, especially considering the diverse techniques employed in their derivations. AGB stars are also capable of synthesizing the 41Ca [7] that has recently been inferred to have been present in the solar nebula [8]. The presence of live 41Ca in the solar nebula would shorten the time interval between synthesis and crystallization of refractory inclusions to about 0.5 to 0.7 Myr [7]. Considering that the 'standard theory' of star formation involves a 10 Myr period of quasistatic contraction prior to the onset of the rapid collapse phase [9], the 41Ca time constraint further strengthens the need for collapse to be triggered by the arrival of the stellar shock front. A supernova has been re-proposed as the source of 26Al and other short-lived radionuclei [10]; in order to achieve the proper dilution of supernova ejecta with the presolar cloud, a distance of a few to about 10 parsecs is inferred [10]. In order to learn whether AGB stars or supernovae are better suited to triggering the collapse of the presolar cloud, we have developed a 2D gravitational hydrodynamics code and used it to study the interaction of shock waves with dense cloud cores [11]. We reproduced previous simulations of the impact of an adiabatic shock wave (appropriate for a nearby supernova), showing that in this case the cloud is destroyed by small-scale instabilities [12,13]. We find that the key factor permitting cloud collapse rather than destruction is the shock thermodynamics -- isothermal shocks (appropriate for AGB star winds or distant supernovae) can lead to sustained protostellar collapse [11]. A distant (10 or more parsecs) supernova shock has just about enough momentum to induce collapse. However, planetary nebulae appear to be somewhat deficient in momentum, and require the compressive effects of warm post-shock gas to trigger collapse. References: [1] Lee T. et al. (1976) GRL, 3, 109. [2] Wasserburg G. J. et al. (1994) Astrophys. J., 424, 412. [3] Cameron A. G. W. and Truran J. W. (1977) Icarus, 30, 447. [4] Anders E. and Zinner E. (1993) Meteoritics, 28, 490. [5] Palla F. (1995) Workshop on Galactic Star Formation and Early Stellar Evolution, Ringberg Castle, Germany. [6] Boss A. P. (1995) Astrophys. J., 439, 224. [7] Wasserburg G. J. et al. (1995) Astrophys. J. Lett., 440, L101. [8] Srinivasan G. et al. (1994) Astrophys. J. Lett., 431, L67. [9] Shu F. H. et al. (1987) Annu. Rev. Astron. Astrophys., 25, 23. [10] Cameron A. G. W. et al. (1995) Astrophys. J., in press. [11] Foster P. N. and Boss A. P. (1995) Astrophys. J., in preparation. [12] Klein R. I. et al. (1994) Astrophys. J., 420, 213. [13] Mac Low, M.-M. et al. (1994) Astrophys. J., 433, 757.
Boss Alan P.
Foster Prudence N.
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