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

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 QUIJOTE 1 line survey taken with the Yebes 40 m telescope. We detected D 13 CCCN, DC 13 CCN, DCC 13 CN, DCCC 15 N, H 13 C 13 CCN, H 13 CC 13 CN, HC 13 C 13 CN, HCC 13 C 15 N, and HC 13 CC 15 N 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 H 2 volume density of 1 × 10 4 cm −3 and a kinetic temperature of 10 K. Whereas we observed a 13 C fractionation for HCC 13 CN and other double isotopologues with a 13 C atom adjacent to the nitrogen atom, we derived similar C/ 13 C abundance ratios for the three 13 C 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 13 C and 15 N 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.

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