Title: Detection of vibrationally excited C6H in the cold prestellar core TMC-1 with the QUIJOTE line survey

Abstract: In this work, we present the detection of twelve doublets with quantum numbers of N=12-11 to N=17-16 of the v11 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 +/- 0.4K and a column density of (1.2+/-0.2)e11 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 2Pi_3/2 and 2Pi_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, Trot(2Pi_3/2)=Trot(2Pi_1/2)=6.2+/-0.2, achieving a result that is comparable to that of the v11 state. The derived column densities are N(2Pi_3/2)(6.2+/-0.3)e12cm-2 and N(2Pi_1/2)=(8.0+/-0.4)e10cm-2. The fraction of C6H molecules in its 2Pi_3/2, 2Pi_1/2, and v11 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.