Radio recombination lines from the largest bound atoms in space

Abstract

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, Delta n=4) in the cool tenuous medium located in the Perseus arm in front of the supernova remnant, Cassiopeia A. Theoretical estimates had shown that atoms which recombined in tenuous media are stable up to quantum levels n~1500. Our data indicates that we have detected radiation from atoms in states very close to this theoretical limit. We also report high signal-to-noise detections of alpha, beta and gamma transitions in carbon atoms arising in the same clouds. In these data, we find that the increase in line widths with quantum number (proportional to n^5) due to pressure and radiation broadening of lines is much gentler than expected from existing models which assume a power law background radiation field. This discrepancy had also been noted earlier. The model line widths had been overestimated since the turnover in radiation field of Cassiopeia A at low frequencies had been ignored. In this paper, we show that, once the spectral turnover is included in the modeling, the slower increase in line width with quantum number is naturally explained.Comment: 5 pages, 4 figures, accepted for publication in MNRA

Low-frequency Carbon Radio Recombination Lines. I. Calculations of Departure Coefficients

“…Carbon radio recombination lines have been observed in the interstellar medium of our Galaxy towards two types of clouds: diffuse clouds (e.g. : Konovalenko & Sodin 1981;Erickson et al 1995;Roshi et al 2002;Stepkin et al 2007;Oonk et al 2014) and photodissociation regions (PDRs), the boundaries of HII regions and their parent molecular clouds (e.g. : Natta et al 1994;Wyrowski et al 1997;Quireza et al 2006).…”

Radio recombination lines at decametre wavelengths

“…Second, most of our comments have concerned α-transition lines. In deep integrations, β-, γ-, and potentially δ-transition lines can be detected (Stepkin et al 2007). The resulting spectral occupancy would be much higher and the ability to fold or remove the lines could be reduced substantially.…”

“…Since then, carbon RRLs have been measured towards Cas A at frequencies from 14 to 1420 MHz (Payne et al 1994, and references therein). A recent study by Stepkin et al (2007) measured α, β, γ, and even δ carbon-line transitions at frequencies near 20 MHz. The lines were measured up to n ∼ 1005, which is the highest n-bound state ever detected.…”

H 3 + at the interface between astrochemistry and astroparticle physics

“…Astronomical observations at decametre wavelengths have revealed absorption lines of interstellar carbon atoms in states as high as n = 1009 in Dn = 4 transitions that flank the stronger n ≈ 630 Dn = 1 lines near 26 MHz frequency [44]. At n = 1009, the binding energy of the state is only −0.089 cm −1 relative to C + (2p 2 P o 1/2 ), and the radius of the classical electron orbit is a startling 54 mm.…”