Abundance and excitation of molecular anions in interstellar clouds

Abstract

We present new observations of molecular anions with the Yebes 40 m and IRAM 30 m telescopes toward the cold, dense clouds TMC-1 CP, Lupus-1A, L1527, L483, L1495B, and L1544. We report the first detections of C3N− and C5N− in Lupus-1A as well as C4H− and C6H− in L483. In addition, we detected new lines of C6H− toward the six targeted sources, of C4H− toward TMC-1 CP, Lupus-1A, and L1527, and of C8H− and C3N− in TMC-1 CP. Excitation calculations using recently computed collision rate coefficients indicate that the lines of anions accessible to radiotelescopes run from subthermally excited to thermalized as the size of the anion increases, with the degree of departure from thermalization depending on the H2 volume density and the line frequency. We noticed that the collision rate coefficients available for the radical C6H are not sufficient to explain various observational facts, thereby calling for the collision data for this species to be revisited. The observations presented here, together with observational data from the literature, have been used to model the excitation of interstellar anions and to constrain their abundances. In general, the anion-to-neutral ratios derived here agree with the literature values, when available, within 50% (by a factor of two at most), except for the C4H−/C4H ratio, which shows higher differences due to a revision of the dipole moment of C4H. From the set of anion-to-neutral abundance ratios derived two conclusions can be drawn. First, the C6H−/C6H ratio shows a tentative trend whereby it increases with increasing H2 density, as we would expect on the basis of theoretical grounds. Second, the assertion that the higher the molecular size, the higher the anion-to-neutral ratio is incontestable; furthermore, this supports a formation mechanism based on radiative electron attachment. Nonetheless, the calculated rate coefficients for electron attachment to the medium size species C4H and C3N are probably too high and too low, respectively, by more than one order of magnitude.

A 3 mm Spectral Line Study of the Central Molecular Zone Infrared Dark Cloud G1.75-0.08

“…The values of η MB range from 0.20 to 0.31 over the observed frequency range. 1 The absolute calibration uncertainty was adopted to be 10% following the practice of studies that employ the Yebes Q-band observations (e.g., [28][29][30]). However, this should be taken as a lower limit to the calibration uncertainty in the higher frequency W band.…”

Computed Rotational Collision Rate Coefficients for Recently Detected Anionic Cyanopolyynes

“…It is interesting to note here that, in a recent work by Agúndez et al (2023), excitation calculations were carried out using computed collision rate coefficients for a series of cyanopolyynes and polyynes. Their results indicate that the Figure 16.…”

“…The computed rate coefficients originate from the collisional interactions of these anions with He atoms and H 2 molecules, both species well known to be present in significant abundances in the same ISM environments. More specifically, the relative abundance of He with respect to H 2 is taken to be about 0.17 (Agúndez et al 2023) and is usually considered to have about a 20% contribution to the state-changing probabilities in comparison with that of the neutral molecular partner. The abundance of the latter is considered to have a local density of the order of 10 4 cm −3 (Agúndez et al 2023) and is therefore relevant for its contribution to the rotational thermalization or subthermalization of the emission lines observed for the two anionic cyanopolyynes.…”