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

Context. Measuring the carbon isotope abundance ratio in comets allows one to constrain the conditions in the outer protosolar nebula. Different measurements of the C-12/C-13 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 C-12/C-13 similar to 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 C-12/C-13 ratio in two bright Oort cloud comets using the C-12(2) and (CC)-C-12-C-13 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 (CC)-C-12-C-13, near 4723 and 4745 angstrom to measure the C-12/C-13 ratio and compared their intensity to the C-12(2) lines of the same bands. We developed a model for interpreting observational data obtained at high resolution (similar to 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 C-12/C-13 = 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.