2023A&A...680L...4C

In this work, we present the detection of twelve doublets with quantum numbers of N = 12-11 to N = 17-16 of the ν₁₁(μ²Σ) vibrationally excited state of C₆H towards TMC-1. This marks the first time that an excited vibrational state of a molecule has been detected in a cold starless core. The data are part of the QUIJOTE line survey gathered with the Yebes 40m radio telescope. The line intensities have been aptly reproduced with a rotational temperature of 6.2 ± 0.4 K and a column density of (1.2 ± 0.2)×10¹¹ cm^–2. We also analysed the ground state transitions of C₆H, detecting fourteen lines with quantum numbers of J = 23/2-21/2 to J = 35/2 for each of the two ²Π_3/2 and ²Π_1/2 ladders. It is not possible to model the intensities of all the transitions of the ground state simultaneously using a single column density. We considered the two ladders as two different species and found that the rotational temperature is the same for both ladders, T_rot(²Π_3/2)=T_rot(²Π_1/2)=6.2 ± 0.2, achieving a result that is comparable to that of the ν₁₁(²μΣ) state. The derived column densities are N(²Π_3/2) = (6.2 ± 0.3)×10¹² cm^–2 and N(²Π_1/2) = (8.0 ± 0.4)×10¹⁰ cm^–2. The fraction of C₆H molecules in its ²Π_3/2, ²Π_1/2, and ν₁₁(μ²Σ) states is 96.8%, 1.3%, and 1.9%, respectively. Finally, we report that this vibrational mode has also been detected towards the cold cores Lupus-1A and L1495B, as well as the low-mass star forming cores L1527 and L483, with fractions of C₆H molecules in this mode of 3.8%, 4.1%, 14.8%, and 6%, respectively.