IRAS Low-Resolution Spectrometer Catalog Format

[Adapted from the Explanatory Supplement]

The spectra in the Low-Resolution Spectrometer (LRS) Catalog are divided into two bands (8-13 microns and 11-22 microns). The file LRS_HEADER.DAT lists only the wavelengths corresponding to each channel of the spectra (see Table 1 below). The file LRS_SPECTRA.DAT contains the spectra and associated header information (see Table 2). The spectra are in order of increasing right ascension of the corresponding point source. As described below, the spectra are identified by the name of the corresponding source in the Point Source Catalog. Both files have 80-character ASCII records.

        Table 1.  Catalog Header File

Start	Name		Description			Units		Format
Byte
0	ANGLE		Angle from center of slit	arcmin		100F8.4
			(one number per sample)
800	LAMBDA1		Wavelength in 8-13 micron	micron		100F8.4
			corresponding to each sample
1600	LAMBDA2		Wavelength in 11-22 micron	micron		100F8.4
			corresponding to each sample

Distance from Slit: ANGLE(100)

As described in detail in Chapter IX of the Explanatory Supplement, each sample in the spectrum corresponds to a certain in-scan distance of the source from the centerline of the spectrometer entrance aperture. ANGLE lists these angular distances.

Wavelength Calibration: LAMBDA1, LAMBDA2

There is a non-linear relation between the displacement of the source from the centerline of the spectrometer and the sampled wavelength. The wavelengths corresponding to each sample (or ANGLE) are given in LAMBDA1 and LAMBDA2 for the two wavelength bands. The beginning and end of each spectrum contain measurements that lie outside of the wavelength coverage of the instrument but which can be used for baseline determination. The wavelength values corresponding to these values of sample number (or ANGLE) are set to 0.

        Table 2.  Spectrum Records

Start	Name		Description				Units		Format
Byte
0	NAME		Source name						11A1
11	HOURS		RA 1950  				hrs		I2
13	MINUTE		RA 1950					min		I2
15	SECOND		RA 1950					deci-sec					I3
18	DSIGN		Declination sign			±  		A1
19	DECDEG		DEC 1950				arc deg		I2
21	DECMIN		DEC 1950				arc min		I2
23	DECSEC		DEC 1950				arc sec		I2
25	FLUX		Averaged non-color corrected		Jy		4E9.3
			flux densities (1 value
			per band)
61	NSPECTRA	No. of observed spectra					I2
63	NACCEPT		No. of observed spectrum halves				2I2
67	LRSCHAR		Characterization of spectrum				2I1
69	SPQUAL		Quality of 8-13 micron and 11-22			2I1
71	VAR		Percent of variability likelihood			I2
73	NID		No. of associations					I2
	(<25)
75	IDTYPE		Type of association					I1
76	SPARE		4 spare bytes						4A1
---- New Record ----
80	BASELINE	Average of outer 20 samples of		scaled by SCALE	4I4
			spectra.  Short and long wave-
			length end for each spectrum half
			(from catalog)
96	NOISE		RMS noise per sample (one		scaled by SCALE	2I4
			value per spectrum half)
104	SNR		Signal-to-noise ratio (average 				2E10.3
			signal in spectrum part divided by
			noise, one value per spectrum half
124	ASYMM		Relative baseline asymmetry				2E10.3
			(difference of left and right
			baselines divided by average siganl;
			one value per band)
144	SRATIO		Ratio of integrated LRS flux to 12			F5.2
			micron survey flux
149	SCALE		Scale factor for all flux		W m-2 micron-1	E11.5
			densities	
----New Record----
160	SPECTRUM	100 samples for each of the		scaled by SCALE	200I4
			two bands (8-13 micron, 11-22
			micron) scaled by SCALE
----New Record----
960	ID#1		Association field from main catalog			40A1
1000	ID#2		Additional records as required

The entries starting at bytes 0 through 25 are identical to the items with the same names in the Point Source Catalog. This is also the case for the entries starting at bytes 71, 73, 75, and 960. Their explanations will not be duplicated here, but may be found in the description of the format of the PSC.

Number of Spectra: NSPECTRA, NACCEPT

NSPECTRA is the number of spectra observed for the source; NACCEPT is the number of 8-13 micron and the 11-22 micron spectrum halves ultimately averaged to make the entry in the catalog.

Characterization of the Spectrum: LRSCHAR

A description of the method of characterization is given in Section IX.D of the Explanatory Supplement. Table 3 lists the spectral classes used to characterize the spectrum.

Quality of the Spectrum Halves: SPQUAL

Depending on the signal-to-noise ratio of the 8-13 micron and 11-22 micron halves of the spectra, the number of accepted spectrum halves and the difference in level of the baselines on either side of the spectrum halves, a quality digit is assigned to each half of the spectrum: 1 indicates good quality, 2 moderate quality, and 3 barely acceptable.

Scale Factor for All Spectrum Flux Densities: SCALE

Multiplying the integers BASELINE, NOISE, and SPECTRUM by the factor SCALE converts the values into units of W m-2 micron-1.

Baseline of Spectrum Halves: BASELINE(4)

These four values are the averages of samples of 1 through 20 (short wavelength end) and 81 through 100 (long wavelength end) of both the 8-13 micron and 11-22 micron halves of the spectrum.

RMS Noise: NOISE

Using the twenty samples on the long wavelength end used for BASELINE, the RMS noise per spectrum half is determined.

Signal-to-Noise Ratio: SNR

The average value of the samples in the wavelength ranges 8-13 microns and 11-22 microns, respectively, are divided by the NOISE values determined.

Baseline Asymmetry: ASYMM

This value indicates by what fraction of the average signal the baselines on the short and long wavelength sides of the spectrum halves differ. A large baseline asymmetry indicates that a confusing source may have contaminated the spectrum. The baseline asymmetry is usually large near the Galactic plane.

LRS/Survey Flux Ratio: SRATIO

The ratio of the integrated flux (after convolution of the spectral flux densities with the 12 micron bandpass of the survey instrument) in the spectrum and the 12 micron survey flux is given in this item. Normally this value should be close to unity (section IX.C of the Explanatory Supplement). As the 11-22 micron part of the LRS spectrum hardly overlaps with the 25 micron survey band, a ratio of LRS/survey for this band is not significant.

The Spectrum: SPECTRUM(200)

The integer values of the spectrum; they must be multiplied by SCALE for conversion to W m-2 micron-1. The wavelengths corresponding to the 100 samples given for the 8-13 micron and the 11-22 micron halves of the spectrum are given in the catalog header file LRS_HEADER.DAT. Values for non-significant wavelengths are set to zero. For baseline interpolation, either the sample numbers or the ANGLE (distance in arcmin from the spectrometer center line) can be used.

        Table 3.  Spectral Classification Scheme

Class    Characteristic                       Number    Typical objects
On        other class                           363      unknown
          -subclasses n=0,2,3,4:  see text
          -subclass n=1:  blue, low S/N
          -subclass n=5:  red, low S/N
1n          blue, featureless                    2249      stars with spectral
          -subclass:  n=2 times                            type earlier than M5
          spectral index
2n        blue, 10 micron emission             1738      stars with not-too-thick
          -subclass: n=band strength                       oxygen-rich envelopes
3n        blue, 10 micron absorption            230      stars with thick
          -subclass:  n=band strength                      oxygen-rich envelopes
4n         blue, 11 micron emission             542      stars with carbon-rich
          -subclass:  n=band strength                      envelopes
5n         red, no line or 10 micron band        64      unknown
          -subclass:  n=2 times spectral index
6n         red, 10 micron emission               78      stars with very thick
          -subclass:  n=band strength                      oxygen-rich envelopes
7n         red, 10 micron absorption             67      stars with extremely thick
          -subclass:  n=band strength                      oxygen-rich envelopes and
                                                           hot spots in molecular clouds
8n        11.3 micron emission line              71      compact H II regions and
          -subclass n=0:  no atomic line                   planetary nebulae
          -other subclasses:  strongest line
          [Ne II], 12.8 micron, n=1
          [S III], 18.8 micron, n=2
          [Ar III], 9.0 micron, n=3
          [S IV], 10.5 micron, n=4
          [Ne III], 15.5 micron, n=5
          [Ne V], 14.5 micron, n=6
9n        same as class 8, but without           50      unknown
          11.3 micron line

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