2007MNRAS.374..852S

In this paper, we report the detection of a series of radio recombination lines (RRLs) in absorption near 26 MHz arising from the largest bound carbon atoms detected in space. These atoms, which are more than a million times larger than the ground-state atoms, are undergoing delta transitions (n ∼ 1009, Δn = 4) in the cool tenuous medium located in the Perseus arm in front of the supernova remnant, Cassiopeia A (Cas A). Theoretical estimates had shown that atoms which recombined in tenuous media are stable up to quantum levels n ∼ 1500. Our data indicate that we have detected radiation from atoms in states very close to this theoretical limit. We also report high signal-to-noise ratio detections of α, β and γ transitions in carbon atoms arising in the same clouds. In these data, we find that the increase in linewidths with quantum number (approximately proportional to n⁵) due to pressure and radiation broadening of lines is much gentler than that expected from existing models which assume a power-law background radiation field. This discrepancy had also been noted earlier. The model linewidths had been overestimated since the turnover in radiation field of Cas A at low frequencies had been ignored. In this paper, we show that, once the spectral turnover is included in the modelling, the slower increase in linewidth with quantum number is naturally explained.