The12C2/12C13C isotopic ratio in comets C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR)

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 12 C/ 13 C 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 12 C/ 13 C ∼ 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 12 C/ 13 C ratio in two bright Oort cloud comets using the 12 C 2 and 12 C 13 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 12 C 13 C, near 4723 and 4745 A to measure the 12 C/ 13 C ratio and compared their intensity to the 12 C 2 lines of the same bands. We developed a model for interpreting observational data obtained at high resolution (∼70000) 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 12 C/ 13 C = 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.