Isotopic ratios are keys to understanding the origin and early evolution of the solar system in the context of Galactic nucleosynthesis. The large range of measured 14 N/ 15 N isotopic ratios in the solar system reflects distinct reservoirs of nitrogen whose origins remain to be determined. We have directly measured a C 14 N/C 15 N abundance ratio of 323 ± 30 in the disk orbiting the nearby young star TW Hya. This value, which is in good agreement with nitrogen isotopic ratios measured for prestellar cores, likely reflects the primary present-day reservoir of nitrogen in the solar neighbourhood. These results support models invoking novae as primary 15 N sources as well as outward migration of the Sun over its lifetime, and suggest that comets sampled a secondary, 15 N-rich reservoir during solar system formation.

The isotopic ratio of nitrogen in nearby protoplanetary disks, recently measured in CN and HCN, indicates that a fractionated reservoir of volatile nitrogen is available at the earliest stage of comet formation. This reservoir also presents a 3:1 enrichment in 15 N relative to the elemental ratio of 330, identical to that between the solar system comets and the protosun, suggesting that similar processes are responsible for the fractionation in the protosolar nebula (PSN) and in these PSN analogs. However, where, when, and how the fractionation of nitrogen takes place is an open question. Previously obtained HCN/HC 15 N abundance ratios suggest that HCN may already be enriched in 15 N in prestellar cores, although doubts remain on these measurements, which rely on the double-isotopologue method. Here we present direct measurements of the HCN/H 13 CN and HCN/HC 15 N abundance ratios in the L1498 prestellar core based on spatially resolved spectra of HCN(1-0), (3-2), H 13 CN(1-0), and HC 15 N(1-0) rotational lines. We use state-of-the-art radiative transfer calculations using ALICO, a 1D radiative transfer code capable of treating hyperfine overlaps. From a multiwavelength analysis of dust emission maps of L1498, we derive a new physical structure of the L1498 cloud. We also use new, high-accuracy HCN-H 2 hyperfine collisional rates, which enable us to quantitatively reproduce all the features seen in the line profiles of HCN(1-0) and HCN(3-2), especially the anomalous hyperfine line ratios. Special attention is devoted to derive meaningful uncertainties on the abundance ratios. The obtained values, HCN/H 13 CN=45±3 and HCN/HC 15 N=338±28, indicate that carbon is heavily fractionated in HCN, but nitrogen is not. For the H 13 CN/HC 15 N abundance ratio, our detailed study validates to some extent analyses based on the single excitation temperature assumption. Comparisons with other measurements from the literature suggest significant core-to-core variability. Furthermore, the heavy 13 C enrichment we found in HCN could explain the superfractionation of nitrogen measured in solar system chondrites.

We present results derived from the first multi-chord stellar occultations by the transneptunian object (50000) Quaoar, observed on 2011 May 4 and 2012 February 17, and from a single-chord occultation observed on 2012 October 15. If the timing of the five chords obtained in 2011 were correct, then Quaoar would possess topographic features (crater or mountain) that would be too large for a body of this mass. An alternative model consists in applying time shifts to some chords to account for possible timing errors. Satisfactory elliptical fits to the chords are then possible, yielding an equivalent radius R equiv = 555±2.5 km and geometric visual albedo p V = 0.109±0.007. Assuming that Quaoar is a Maclaurin spheroid with an indeterminate polar aspect angle, we derive a true oblateness of = 0.087 +0.0268 −0.0175 , an equatorial radius of 569 +24 −17 km, and a density of 1.99 ± 0.46 g cm −3 . The orientation of our preferred solution in the plane of the sky implies that Quaoar's satellite Weywot cannot have an equatorial orbit. Finally, we detect no global atmosphere around Quaoar, considering a pressure upper limit of about 20 nbar for a pure methane atmosphere.