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The distance and structure of the Coalsack. I - Photometric data
The results of UBV and H-beta photometry for 284 stars of spectral typesB to F located in the area of the dark cloud Coalsack are presented.Most of the present stars are brighter than 10.0 mag and are classifiedin the MK system of Houk and Cowley (1975). For each star, the bestvalues for the extinction and distance modulus are provided, withtypical estimated errors of + or - 0.08 mag for the extinction and + or- 0.32 mag for the distance modulus. Possible misclassifications andspectral peculiarities are noted for several of the stars.
| Preferred frequency for interstellar communications
It is suggested that the 4829.659-MHz line frequency of the1(11)-to-1(10) rotational transition of formaldehyde is a plausiblefrequency to monitor for the purpose of detecting interstellarcommunications. The formaldehyde line is of interest because, as aresult of the antimaser effect, it exhibits a very low excitationtemperature. The background radiation temperature in the direction ofdark clouds is below the 3 K background temperature; the backgroundnoise temperature, lowered approximately 1 K, is lower than at any otherfrequency band.
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