Protostars and Planets VI, Heidelberg, July 15-20, 2013
Observations of the Nitrogen Isotopic Composition around Low-Mass Protostars
Wampfler, Susanne F. (Centre for Star and Planet Formation and Niels Bohr Institute, University of Copenhagen, Denmark)
Jørgensen, Jes K. (Niels Bohr Institute and Centre for Star and Planet Formation, University of Copenhagen, Denmark)
Bisschop, Suzanne (Centre for Star and Planet Formation and Niels Bohr Institute, University of Copenhagen, Denmark)
Bizzarro, Martin (Centre for Star and Planet Formation, University of Copenhagen, Denmark)
A key goal for observational studies of low-mass star formation is to understand the origin and evolution of the solar system. For this purpose, the observed properties of the gas and dust in star-forming regions are compared to the physical and chemical characteristics of the solar system. Direct information about the composition of the protosolar disk can be obtained from measurements of the isotopic composition of different solar system materials. While the isotopic composition of most elements is fairly homogeneous, volatiles like hydrogen, oxygen, and nitrogen show significant variations. Among the most extreme variations are those found in the 15N/14N abundance ratio. Recent solar wind measurements (Marty et al. 2011) have demonstrated that the rocky bodies show an enhancement of 15N compared to the Sun and Jupiter.
Various scenarios have been proposed to explain the observed variations within the solar system and also between different star-forming regions. They include a nucleosynthetic origin through chemical evolution of the galaxy, photo-chemical self-shielding, and chemical fractionation through isotope exchange reactions at low temperatures. It is still unclear whether the large 15N/14N variations observed in the solar system are a relict from intrinsic processes in the protosolar disk, or whether the isotopic signature was inherited from a pre-solar history, or even a combination of both. Observations of the 15N/14N ratio in nearby star-forming regions to test the hypothesis that chemical fractionation could be responsible for the solar system variations are however sparse.
We present measurements of the 15N/14N ratio around three protostars carried out with the APEX telescope in isotopologues of HCN and HNC. Most of our derived 15N/14N ratios differ from the solar (and local interstellar medium) value, pointing at the occurrence of local processing. Moreover, the source with the lowest temperatures in the outer envelope shows the highest 15N-enrichment, which is an indication for chemical fractionation. However, spatially resolved observations are needed to distinguish between the photo-dissociation and chemical fractionation scenarios.
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