[PPS06-P02] Li-Be-B and Al-Mg isotopic compositions of Ca, Al-rich inclusions: Implications for the evolution of the proto-solar disk.
Keywords:Ca, Al-rich inclusion, cosmic ray spallation processes, lithium-beryllium-boron isotopic ratios
We studied eight CAIs from the Sayh al Uhaymir 290 (CH) and the Isheyevo (CH/CBb) chondrites. CAIs in CH and CH/CBb chondrites show highly variable initial 10Be/9Be ratios ranging from 1.7 to 40.5 x 10-4. These CAIs have nearly chondritic Li isotopic compositions independent of their 10Be/9Be ratios. In addition, all CAIs studied here show no resolvable excesses in 26Mg decayed from a short-lived radionuclide 26Al.
In contrast to the case for CAIs in CH and CH/CBb chondrites, CAIs in CV chondrites show a relatively narrow range of 10Be/9Be ratios (4.6 - 12.2 x 10-4 [3-9]) and have nearly canonical 26Al/27Al ratios (26Al/27Al = 4 - 5 x 10-5 [e.g., 8]). To explain the differences between CH and CH/CBb CAIs and CV CAIs, we propose that the variations in 10Be/9Be ratios may reflect fluctuations of the cosmic ray flux from early active Sun. Considering a relationship between accretion rates and X-ray luminosities in T Tauri stars [10, 11], episodic accretion events (i.e., FUori-type or EXori-type outbursts [e.g., 12, 13]) would be a candidate of the cause for the variation in the number flux of protons. If this is correct, the variations in 10Be/9Be ratios of CAIs would give important constraints on the evolution of the proto-solar disk. Observations suggest that FUori-type outbursts are confined to the first few tens of thousands of years , which correspond to class I at the stage of the protoplanetary disk evolution . We propose that high and variable 10Be/9Be ratios recorded in CH and CH/CBb CAIs reflect episodic cosmic ray fluxes caused by FUori-type outbursts. On the other hand, relatively low 10Be/9Be ratios recorded in CV CAIs may reflect less episodic accretion events, possibly the EXori-type outbursts, which are confined to the evolution stage a few million years after the formation of the protoplanetary disk (= class II). If the above argument is correct, the injection of 26Al in the solar system would have occurred between the evolutionary stages class I and class II of the proto-Solar disk.
 Feigelson et al. (2002) ApJ, 572, 335-349.  Wolk et al. (2005) ApJ. suppl. ser. 160, 423-449.  McKeegan et al. (2000) Science, 289, 1334-1337.  Sugiura et al. (2001) MAPS, 36, 1397-1408.  MacPherson et al. (2003) GCA, 67, 3165-3179.  Chaussidon et al. (2006) GCA, 70, 224-245.  Wielandt et al. (2012) ApJL, 748, L25 (7pp).  Srinivasan & Chaussidon (2013) EPSL, 374, 11-23.  Sossi et al. (2017) Nat. Astron., 1, 0055.  Preibisch et al. (2005) ApJ, 160, 401-422.  Telleschi et al. (2007) A&A, 468, 425-442.  Hartmann & Kenyon (1996) ApJ, 34, 207-240.  Herbig, 2007 ApJ, 217, 629-715.  Schulz (2012) From dust to Stars and planets 2nd edition, Springer.  Evans et al. (2009) ApJ, 181, 321-350.