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Please use this identifier to cite or link to this item: http://hdl.handle.net/2289/2205

Title: Radio recombination lines from the largest bound atoms in space
Authors: Stepkin, S.V.
Konovalenko, A.A.
Kantharia, N.G.
Udaya Shankar, N.
Keywords: ISM: atoms
ISM: clouds
radio lines: ISM
Issue Date: Jan-2007
Publisher: Blackwell Publishing for the Royal Astronomical Society
Citation: Monthly Notices of the Royal Astronomical Society, 2007, Vol.374, p 852-856.
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, 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.
Description: Restricted Access. An open-access version is available at arXiv.org (one of the alternative locations)
URI: http://hdl.handle.net/2289/2205
ISSN: 0035-8711
Alternative Location: http://adsabs.harvard.edu/cgi-bin/bib_query?2007MNRAS.374..852S
http://arxiv.org/abs/astro-ph/0610407
http://dx.doi.org/10.1111/j.1365-2966.2006.11190.x
Copyright: 2007 Royal Astronomical Society
Appears in Collections:Research Papers (A&A)

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