The ¹²C₂/¹²C¹³C isotopic ratio in comets C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR)^⋆

¹ University of Franche-Comté, Observatoire des Sciences de l’Univers THETA, Institut UTINAM - UMR CNRS 6213, BP 1615, 25010 Besançon Cedex, France
e-mail: rousselot@obs-besancon.fr
² F.R.S.-FNRS, Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 août 17, 4000 Liège, Belgium

Received: 22 March 2012
Accepted: 5 July 2012

Abstract

Context. Measuring the carbon isotope abundance ratio in comets allows one to constrain the conditions in the outer protosolar nebula. Different measurements of the ¹²C/¹³C ratio, using various molecules, have already been published for different solar system objects, such as the Sun, the Earth, the Moon, asteroids, planets, or comets. So far, all these measurements are consistent with ¹²C/¹³C ~ 90, but significant differences have been observed. This ratio is remarkably constant in comets (91.0 ± 3.6) for studies based on the CN radical, but it presents stronger variations in studies based on other radicals.

Aims. This paper aims at measuring the ¹²C/¹³C ratio in two bright Oort cloud comets using the ${}^{\mathrm{12}}\mathrm{C}_{\mathrm{2}}$ and ${}^{\mathrm{12}}\mathrm{C}^{\mathrm{13}}\mathrm{C}$ emission lines and an improved method. The ratios will be compared to those obtained for the same comets with another radical, CN.

Methods. We used the (2,1) and (1,0) bandheads of the ${}^{\mathrm{12}}\mathrm{C}^{\mathrm{13}}\mathrm{C}$, near 4723 and 4745 Å to measure the ¹²C/¹³C ratio and compared their intensity to the ${}^{\mathrm{12}}\mathrm{C}_{\mathrm{2}}$ lines of the same bands. We developed a model for interpreting observational data obtained at high resolution (~70 000) using the 8.2-m Kueyen telescope (UT2) of the Very Large Telescope (VLT) with the Ultraviolet and Visual Echelle Spectrograph (UVES) in two comets: C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR).

Results. Our modeling has provided ¹²C/¹³C = 85 ± 20 for C/2002 T7 (LINEAR) and 80 ± 20 for C/2001 Q4 (NEAT). These values are compatible with previous measurements performed with the CN radical.