DJM (MacConnell 1992) collected I-N spectrum plates of dispersion 3400 Å with the Curtis Schmidt Camera at CTIO; the deepest reach I 13.5. The plate scale is 966 , and each covers 5. They cover the galactic latitude belt along the full southern galactic plane. In the years 1986-92, DJM classified about 14200 IRAS sources (identified as well as unidentified) in the R.A range to with falling in the area of the available plates. These constitute nearly 40 of all IRAS PSC sources of this type. The majority of these IRAS sources have cool stellar photospheres in the visible region, and the above colour criterion ensures that their CSE are warmer than 300 K. Stars earlier than type M3 have TiO bands too weak to be seen on these plates and so cannot be classified.
|Figure 1: Reduced-scale version of a I-N objective prism spectrum plate obtained at a dispersion of 3400 Å with the Curtis Schmidt Camera at CTIO, Chile. The deepest plates reach I 13.5. Each plate covers 5. The plate scale is . The different symbols on the photographic print refer to IRAS sources with .The size of the symbol is proportional to . These symbols refer to sources as listed below: 1 = 1000, star = 1100, x = 1110, * = 1111, triangle = 1101, x with bar on top = 1010, x with bar top/bottom = 1011, infinity = 1001. The four numbers on the right of the sign "='' refer in the order to the flux density of these sources at 12, 25, 60 and 100 m respectively, where "0'' means flux density is below the detection threshold and "1'' means flux density is above it|
The plates were illuminated at a light table and examined at a magnification of 12X with a binocular microscope. Overlay plots to the plate scale and at the plate centers were generated for all plates at IPAC. Figure 1 shows a reduced-scale version of a plate on the appropriate IRAS plot. Stars from the SAO Catalog were also plotted to ensure the registration of plate with plot; small crosses denote positions of SAO stars. There was seldom any doubt as to which object on the plate corresponded to a given IRAS source even in the crowded galactic plane areas. The classifications and other data were stored on a microcomputer running a commercial program. Positions of the IRAS sources were measured with an x-y digitizer, and the measures were converted to equatorial co-ordinates using a specialized program on the PC which wrote the IRAS Name of the source into a database record and paused for entry of the classification and other data. These classifications are on file with the Astronomical Data Center (ADC) at the Goddard Space Flight Center, and most are contained in Version 2.1 of the IRAS PSC where they are designated as Catalog # 43. The sources classified in this way may be broken down into 10 groups, and the statistics based on a sample of 10500 classifications are listed in Table 1. The uncertainty in a type is subclass.
We examine here the data of only the unidentified IRAS sources which could be classified as O-rich stars (spectral type M) i) to search for correlations between spectral types and IRAS colours, and ii) to determine distributions of the various spectral classes in the two-colour diagrams.
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