IRAS Explanatory Supplement
VI. Flux Reconstruction and Calibration
C. Absolute Calibration
C.2 Point Source Calibration

C.2.a Stellar Calibration


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The absolute calibration at 12 µm was set so that the color corrected flux density of -Tau at that wavelength was 448 Jy, in agreement with ground based measurements by Rieke et al. (1984) at 12 µm. -Tau was chosen on the basis that it was the primary stellar source in the absolute calibration of Rieke et al. and that it was well measured, with high signal to noise ratio, by IRAS. Although the absolute scale was set using observations of this one star, the IRAS measurements of a significant subset of the stars used by Rieke et al, are in excellent agreement with the ground based observations, In Table VI.C.2 the flux densities at 12 µm of stars measured using the pointed mode of IRAS are compared with the results of Rieke et al. at 10.6 µm. In this table, and in the following discussion, the flux densities obtained from IRAS have been color corrected assuring the energy distribution follows that of a hot blackbody. The flux densities obtained from the ground based observations have been extrapolated from 10.6 µm to 12 µm again assuming the energy distribution of the star follows that of a hot blackbody. The average ratio of the ground based flux densities to those obtained by IRAS is 1.01 ± 0.01.

Comparison with Ground-Based Observations
Table VI.C.2
Star
fv(Rieke et al.)*
fv(IRAS, 12 µm)
alpha Tau 1.01
alpha Aur 1.01
alpha CMi 0.98
beta Gem 1.01
alpha Boo 1.00
gamma Dra 1.05
alpha Lyr 1.00
* extrapolated to 12 µm using hot black body energy distribution

[12 µm] - [25 µm] = -0.03 mag
[25 µm] - [60 µm] = -0.03 mag

(VI.C.2)

In order to extrapolate the absolute calibration which was established for the 12 µm band to the longer IRAS wavelengths, it is convenient to define a magnitude system for inter-comparisons of the photometry in different wavelength bands which normalizes out the energy continuum of hot stars. Because the energy distributions of the stars are very nearly black bodies, the magnitude system has been defined such that zero magnitude corresponds to a flux density in Janskys:

fv(0.0 mag) =1.57 × 10-16 × Bv(l0,000 K)

where Bv is the Planck function in Jy sr-1. for the IRAS effective wavelengths fv[0.00 mag] is 28.3, 6.73, 1.19 and 0.43 Jy at 12, 25, 60 and 100 µm. On this systemalpha-Lyr has a 12 µm magnitude of [12 µm] (alpha-Lyr) = 0.02 mag if it behaves as a 10,000 K blackbody beyond 2.2 µm and [12 µm] (alpha-Tau) = -3.00 mag.

The extrapolation of the stellar model to 25 µm was based on the compilation of solar photometry presented by Vernazza, Avrett, and Loeser (1976). A comparison of several stellar model calculations (Gustafsson, et al. 1975; Kurucz, 1979; Bell, 1984) with the data in Vernazza et al. predicted a smaller color difference, by about 2%, in the models than observed for the Sun. The models did show, however, that stars with a wide range of effective temperatures and surface gravities have the same [12 µm] - [25 µm] and [25 um] - [60 µm] colors. The following colors, obtained from the observations of the solar fluxes, were thus adopted for the average of a set of calibration stars.

In Table VI.C.3 the stars used to develop the calibration are listed along with the resultant magnitudes from IRAS pointed observations. The stars were selected because reliable models were available for these stars and because there was no obvious excess at 60 µm relative to alpha-Tau.

IRAS Magnitudes of Calibration Stars in Pointed Mode1
Table VI.C.3
Star [12 µm]
(mag)
[25 µm]
(mag)
[60 µm]
(mag)
alpha Tau -3.00 -2.97 -2.95
alpha Aur -1.91 -1.91 -1.86
alpha Car -1.40 -1.38 -1.33
alpha CMa -1.36 -1.32 -1.27
alpha CMi -0.74 -0.72 -0.70
beta Gem -1.20 -1.18 -1.17
alpha Leo 1.66 1.72 --
alpha Boo -3.15 -3.10 -3.11
1 See Section VI.C.2.a for definition of magnitude scale.

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