Two consecutive rotational transitions of the long cyanopolyyne HC 11 N, J = 39 → 38 and 38 → 37, have been detected in the cold dust cloud TMC-1 at the frequencies expected from recent laboratory measurements by Travers et al. (1996), and at about the expected intensities. The astronomical lines have a mean radial velocity of 5.8(1) km s −1 , in good agreement with the shorter cyanopolyynes HC 7 N and HC 9 N observed in this very sharp-line source [5.82(5) and 5.83(5) km s −1 , respectively]. The column density of HC 11 N is calculated to be 2.8 × 10 11 cm −2 . The abundance of the cyanopolyynes decreases smoothly with length to HC 11 N, the decrement from one to the next being about 6 for the longer carbon chains.

ABSTRACT. New, low-noise receivers have allowed detection, in several giant H II regions, of RydbergRydberg transitions of hydrogen that cover a large range of *n-values in a single observing window. This, in turn, allows lines covering a large range in principal quantum number n to be detected simultaneously with the same antenna beam. We have employed a new frequency-switching technique which allows a very precise determination of the line widths. We have used this technique with the NRAO 140 foot telescope to observe lines in W51 and Orion A near 6 GHz, with *n-values that vary by a factor of D21 (*n \ 1ÈD21) and corresponding n-values that vary by a factor of 2.7 (n \ 102È274). By generating Voigt line proÐles using GriemÏs theory of impact broadening by electrons, inserting them into a telescope data Ðle, and processing them in a manner identical to that of the telescope data, we have been able to examine how the observing and reduction techniques a †ect both the line widths and line areas as n increases. For n ¹ 180, *n ¹ 6, our restored line widths and areas give densities of and 4000 cm~3 in W51 and Orion A, respectively. N e \ 2500 These densities are higher than reported previously with a 5@ beamwidth. For higher n-values we are unable to Ðt our data using GriemÏs theory. For n [ 180, *n [ 6, our telescope-measured line widths fall rapidly below predicted values, while the line areas simultaneously increase above predicted values. This behavior of the line area as the line widths decrease is inconsistent with GriemÏs theory or an instrumental e †ect. Observations of Orion A at 17.6 GHz, with a beamwidth, require a density in excess of cm~3 1@ .7 N e \ 20,000 to Ðt. Although the detected lines cover a range in n and *n from 71 to 177 and 1 to 17, respectively, there is no evidence for a line width decrease at the high *n-values. We conclude from this that the line narrowing seen at 6 GHz is related to the principal quantum number.

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