Oxygen isotope anomalies of the Sun and the original environment of the Solar system
J.-E. Lee, 1 E. A. Bergin, 2 and J. R. Lyons 3
1Department of Astronomy and Space Science/ Sejong University, Korea
2Department of Astronomy/ The University of Michigan, USA
3
Department of Earth and Space Sciences/ University of California, USA

Most stars form in clusters [1] even though our star, the Sun, seems alone at present with theclosest stellar neighbor at 1.3 pc. It is not certain, however, whether the Sun formed in acluster or in isolation. Some memory of the environment where the Sun formed might exist
within the remnants of the formation of the Solar System. In this regard, primitive meteorites,
interplanetary dust particles, and comets have been considered as the best sources to search
for memory of conditions prior to and during formation, since they escaped from significant
physical and chemical processing and therefore may preserve some relic of interstellar
material. For instance, the inferred presence of short-lived radionuclides, especially 60Fe, inmeteorites [2] suggests that the Sun formed near a massive star, which went through the corecollapse of a supernova and provided the radionuclide to the solar nebula [3] or to the protosolar molecular cloud [4]. This possibility of the Sun formation in proximity to massive stars
is also supported by the prominent dearth of objects beyond the semi-major axes of theKuiper Belt, which might be caused by the dynamical truncation though stellar encounters[5][6] or by the external photoevaporation [7].
If the Sun formed in a cluster, as described above, the ultraviolet radiation field around theproto-Sun would have been enhanced by 4 to 5 orders of magnitude compared to the standard
local interstellar radiation field. Therefore, this enhanced radiation environment must havealso left some evidences in chemistry affected by photolysis. It has been discovered thatoxygen isotopes in calcium aluminum rich inclusions (CAIs) embedded in primitive
meteorites had different ratios from those seen in terrestrial rocks [8], where the oxygenisotopic fractionation depends on mass. Recent theories suggest this mass-independent
fractionation recorded in meteorites can be understood as a result from the isotopic-selective
photodissociation of CO, either in the Solar Nebula [9][10] or parent cloud [11]. The
photodissociation of CO is strongly coupled with the strength of the far-ultraviolet (FUV)
radiation field as well as the CO column density. The studying of oxygen isotope ratios in theSolar system, therefore, will place strong constraints on its formation environment.
The oxygen isotope composition of the Sun is central to understanding the oxygen isotope
evolution of the Solar system as recorded in meteorites. Recent preliminary Genesis results[12] of the direct Solar wind measurements show that the Solar oxygen isotope ratios aresimilar to the initial bulk values (δ18,17SMOW = -50 permil), i.e., isotopically light compared tothe Standard Mean Ocean Water (SMOW). However, the Solar wind isotope measurements inlunar metal grains have yielded dramatically different results; δ18,17SMOW = -50 permil in one
case [13] and δ18,17SMOW = +50 permil in the other [14].
Based on the CO self-shielding model in a collapsing low mass star forming cloud, we haveshowed that the Solar oxygen isotopic anomalies could vary depending on the strength of thesurrounding radiation field (G0) when the Sun formed [15]. When constrained by oxygenisotope anomalies measured in meteorites, comets, and the Solar wind (Genesis), our model
independently suggests that the Sun formed in the vicinity of massive stars, which isconsistent with other evidences such as short-lived radionuclides and the dynamical propertiesof planets and Kuiper Belt objects.

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