* Is it possible to get the upper frequency limit pushed to 27.5GHz? At 27.478 GHz, there's the ammonia (9,9) line located. It is third lowest ortho-ammonia transition [after the (3,3) and the (6,6) lines]. To determine the temperature of ortho-ammonia, one needs at least two lines but the (3,3) line can, under many conditions, turn into a maser and may therefore become not useful. The only lines left are then (6,6) @ 25.056 GHz and the (9,9) line. Also note that the ortho ammonia lines are typically much stronger than the adjacent para-ammonia lines, i.e., (9,9) is much stronger than (8,8) or (10,10). Thus it is also a preferred, relatively bright line to measure high temperatures. (Juergen Ott)

I'd like to add that, at least for the Galactic Center, the HC3N (3-2) line is one of the very brightest in K-band. It's about as bright as the overall brightest line in K-band (after the occasional water maser spots) ammonia (3,3). The HC3N (3-2) frequency is about 27.3 GHz. Note that such a high rotational quantum number is unusual at such a low frequency and thus HC3N is one of the few molecules for which the GBT can observe the entire rotational ladder. For almost any other molecular species, the rotational ladder starts at a few millimeters. A high frequency end of 27.5 GHz will cover HC3N (3-2) but the original design with an upper limit of 26GHz will not, which would be a real pitty.

The HC3N (2-1) line, on the other hand is at 18.2 GHz, just at the lower end of K-band. It would be fantastic to be able to observe both, the (3-2) and the (2-1) line with the same K-band FPA. In particular, it's currently tested to observe the HC3N line at larger redshifts. So any additional frequency coverage below 18.2GHz would help observing the (2-1) line (e.g. a frequency coverage down to 17GHz would provide a redshift coverage of up to 0.07, 16GHz would provide 0.13 redshift range). But then a single pixel will probably be enough to do this redshift studies at the low frequency end. (Juergen Ott 09 Oct 2007)

-- JuergenOtt - 09 Oct 2007
Topic revision: r1 - 2007-10-09, JuergenOtt
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