Letter to the Editor

Doubly substituted isotopologues of HCCCN in TMC-1: Detection of D¹³CCCN, DC¹³CCN, DCC¹³CN, DCCC¹⁵N, H¹³C¹³CCN, H¹³CC¹³CN, HC¹³C¹³CN, HCC¹³C¹⁵N, and HC¹³CC¹⁵N^⋆

¹ Observatorio Astronómico Nacional (IGN), C/ Alfonso XII 3, 28014 Madrid, Spain
e-mail: b.tercero@oan.es
² Observatorio de Yebes (IGN), Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain
³ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, 92190 Meudon, France
⁴ Grupo de Astrofísica Molecular, Instituto de Física Fundamental, CSIC, C/ Serrano 123, 28006 Madrid, Spain
e-mail: jose.cernicharo@csic.es

Received: 12 December 2023
Accepted: 22 January 2024

Abstract

We report the first detection in space of a complete sample of nine doubly substituted isotopologues of HCCCN towards the cyanopolyyne peak of TMC-1 using observations of the QUIJOTE¹ line survey taken with the Yebes 40 m telescope. We detected D¹³CCCN, DC¹³CCN, DCC¹³CN, DCCC¹⁵N, H¹³C¹³CCN, H¹³CC¹³CN, HC¹³C¹³CN, HCC¹³C¹⁵N, and HC¹³CC¹⁵N through their J = 4 − 3 and J = 5 − 4 lines in the 7 mm window. In addition, we present an extensive analysis of the emission of HCCCN and its singly substituted isotopologues through a large velocity gradient model of the lines detected at 7 mm and 3 mm using the Yebes 40 m and the IRAM 30 m telescopes, respectively. The derived column densities for all the isotopologues are consistent in the two spectral bands for an H₂ volume density of 1 × 10⁴ cm⁻³ and a kinetic temperature of 10 K. Whereas we observed a ¹³C fractionation for HCC¹³CN and other double isotopologues with a ¹³C atom adjacent to the nitrogen atom, we derived similar C/¹³C abundance ratios for the three ¹³C substituted species of DCCCN. This suggests additional chemical discrimination for deuterated isotopologues of HCCCN. Finally, we present the spatial distribution of the J = 4 − 3 and J = 5 − 4 lines from the singly substituted species observed with the Yebes 40 m telescope. The emission peak of the spatial distribution of DCCCN appears to be displaced by ∼40″ with respect to that of HCCCN and the ¹³C and ¹⁵N isotopologues. In addition to a different formation route for the deuterated species, we could also expect that this differentiation owing to the deuterium fractionation is more efficient at low temperatures, and therefore, that deuterated species trace a colder region of the cloud.