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Description
In the late 1960s Gerhard Ackermann published two lists of extremely red stars (1968ZA.....69..130A and 1970A&A.....8..315A), including what appeared to be reliable spectral classifications for a few hundred stars. The two published papers provide details of the observing and analysis, made between June and October 1968 at the Heidelberg observatory with the 25cm f/3.6 Schmidt camera together with a 10-degree UBK7 prism yielding a dispersion of 3500 {AA}/mm at the atmospheric A band (7700{AA}). Exposures on ammonia-hypersensitized Kodak I-N emulsion + RG665 filter were obtained along with direct plates on I-N emulsion as well as 103a-E + OG550, totalling 101 exposures in all. The spectra were classified by following the precepts of Cameron & Nassau (1955ApJ...122..177C) and Nassau & Velghe (1964ApJ...139..190N). Ackermann initiated photoelectric observations at I and K, but obviously the Heidelberg weather was not conducive to pursuing this. Because the calibration of the photometry on the direct plates was by necessity rather crude, Ackermann's photometric results are not reported here -- the procedures used were sufficient to identify the reddest stars, but not for much else. Approximate V magnitudes were added for all the stars -- for the most part these are merely averages of the photo-blue and -red magnitudes in both GSC-2.2 and USNO-B1.0, but their zero-point and scale should be reliable at the few-tenths magnitude level in this range. Coordinates were drawn from either UCAC2 or 2MASS depending mainly on Declination, since UCAC2 extends only to about +40 Dec in this area. The UCAC2 positions should be good to better than 0.1", and those from 2MASS to 0.2". In a few cases, such as gross overexposure in 2MASS, positions are from various Schmidt survey catalogues, as indicated with each entry. The first 13 stars of the catalog come from the concluding volume of the ZA paper; in the 1970 paper, Ackermann refers to these stars using the acronym HDK, which is adopted here. The published table gave rather rough arcminute-precision positions (for equinox 1965). Star 6 was recovered 1 degree east of the nominal place, and identified with the variable star WX CMi. Along with the positions and IDs, the table shows V magnitudes and the spectral types given in the original paper. Some of the stars are within the region covered by the ASAS-3 survey, so reliable V magnitudes are available along with beautiful lightcurves. Ackermann's second much longer list contains rough photometry from the POSS-I prints for some 400 very red stars in the Cygnus starcloud, and for 254 of these spectral types were also determined. The stars without spectral classifications comprise a diverse group including both red/reddened late-type stars and greatly obscured hotter stars, but no real astrophysical information is given for any of them beyond sheer redness -- these stars are omitted from the catalog. In consultation with Prof Ackermann, the following changes have been made to the spectral types: for stars shown in the original table with a hyphen between two types (e.g. M5-M6), this was to show that the type was uncertain to that degree (i.e. M5 or M6), for which the 'slash' notation of Nancy Houk was preferred, and thus M5-M6 becomes M5/6. One star, 78-0-144, exhibited a range in types, and it is shown as M7-M9. Ackermann also indicated a number of stars as being of very-late type with VO present (separable from the A-band), but where the star was too faint to classify accurately. He agrees these are likely to be in the range M7 to M9, and again these are shown in the slash-style notation as M7/9:, with the colon added to indicate uncertainty since the types were not explicit in the original, and sometimes a question mark for those already marked as uncertain in the original. These changes were made so that machine parsing schemes will be better able to handle the classifications.
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