title: Observations of nitrogen isotope fractionation in deeply embedded protostars.
abstract: BACKGROUND
  The terrestrial planets, comets, and meteorites are significantly enriched in15N compared to the Sun and Jupiter. While the solar and jovian nitrogen isotope ratio is believed to represent the composition of the protosolar nebula, a still unidentified process has caused15N-enrichment in the solids. Several mechanisms have been proposed to explain the variations, including chemical fractionation. However, observational results that constrain the fractionation models are scarce. While there is evidence of15N-enrichment in prestellar cores, it is unclear how the signature evolves into the protostellar phases.
  
  
  OBJECTIVE
  The aim of this study is to measure the14N/15N ratio around three nearby, embedded low- to intermediate-mass protostars.
  
  
  METHODS
  Isotopologues of HCN and HNC were used to probe the14N/15N ratio. A selection ofJ= 3-2 and 4-3 transitions of H13CN, HC15N, HN13C, and H15NC was observed with the Atacama Pathfinder EXperiment telescope (APEX). The14N/15N ratios were derived from the integrated intensities assuming a standard12C/13C ratio. The assumption of optically thin emission was verified using radiative transfer modeling and hyperfine structure fitting.
  
  
  RESULTS
  Two sources, IRAS 16293A and R CrA IRS7B, show15N-enrichment by a factor of ~1.5-2.5 in both HCN and HNC with respect to the solar composition. IRAS 16293A falls in the range of typical prestellar core values. Solar composition cannot be excluded for the third source, OMC-3 MMS6. Furthermore, there are indications of a trend toward increasing14N/15N ratios with increasing outer envelope temperature.
  
  
  CONCLUSIONS
  The enhanced15N abundances in HCN and HNC found in two Class 0 sources (14N/15N ~ 160-290) and the tentative trend toward a temperature-dependent14N/15N ratio are consistent with the chemical fractionation scenario, but14N/15N ratios from additional tracers are indispensable for testing the models. Spatially resolved observations are needed to distinguish between chemical fractionation and isotope-selective photochemistry.
authors: Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 København K, Denmark ; Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 København Ø, Denmark.
authors: S F
authors: SF
authors: Wampfler
authors: Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 København Ø, Denmark ; Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 København K, Denmark.
authors: J K
authors: JK
authors: Jørgensen
authors: Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 København K, Denmark.
authors: M
authors: M
authors: Bizzarro
authors: Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 København K, Denmark ; Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 København Ø, Denmark.
authors: S E
authors: SE
authors: Bisschop
keywords: abundances - ISM; astrochemistry - ISM; formation - stars; general; molecules - stars