Trans-iron composition and volatility

Recent results from the TIGER balloon instrument, combining results of two flights for a total of 50 days[1] and results from almost 14 years of data of the CRIS instrument on the ACE spacecraft [2] give individual-element abundances extending up to Sr (Z=38). When the source abundances of the trans-iron elements are compared with solar-system abundances [3], the factor of ∼ 4 differences between volatile and refractory elements previously noted with elements of lower Z disappears. However when the cosmic-ray source is compared with a mixture of ∼80% solar-system composition [3] and ∼20% outflow and ejecta of massive stars [4], a neat ordering that depends on the volatile/refractory distinction clearly emerges, extending from the lighter elements through these elements with Z >= 38. This 80%/20% mixture is consistent with cosmic-ray acceleration in OB associations, as was indicated by the isotopic composition of the cosmic rays [5]. Refractory elements, those likely to be found in interstellar grains, are enhanced by a factor of ∼4 relative to volatile elements, and the efficiency of acceleration appears to increase with increasing atomic number; both of these facts provide important constraints on any detailed model of cosmic-ray acceleration. The next step in extending and improving the trans-iron composition will be balloon flights of Super-TIGER, an instrument similar to TIGER but with four times the area [6]. The first flight of Super-TIGER, over Antarctica, is scheduled for launch in December 2012.