ANALYTIC APPROXIMATIONS TO THE 2-60 MU-M INFRARED CONTINUA FOR STANDARD CALIBRATION STARS - WITH APPLICATION TO THE CALIBRATION OF SPECTROSCOPY AND PHOTOMETRY, AND THE DETERMINATION OF EFFECTIVE TEMPERATURE AND ANGULAR SIZE FROM IR MEASUREMENTS

Convenient two-parameter analytic expressions approximating the long-wavelength (2-60-mu-m) infrared continua expected from over a dozen stellar calibration standards are derived. These analytic spectral functions are based on the implicit scaling of a semiempirical solar atmospheric temperature profile to differing effective temperatures, a procedure believed to be theoretically valid for stars with effective temperature lower than 6000 K. The generalized result is written in the form of a Planck function with a brightness temperature that is a function of both observing wavelength and effective temperature. This function is then fit to the best empirical flux data available on the individual standard stars. The resulting spectral curves are nearly as simple to use as straight blackbody approximations, but are consistent with actual infrared measurements and stellar theory. Though intended primarily for calibration applications where continuous spectral distributions must be estimated, the process of fitting to empirical data also produces estimates of effective temperature and angular diameter for cool stars which correlate very well with results obtained by other means (to within a few percent). This suggests that such fitting of the theoretical curve to infrared data could also be useful as a technique for studying the properties of these stars themselves (similar to the presently used infrared flux method of Blackwell et al. 1990). The estimated probable error in absolute flux values generated using these fits is about +/- 3% below 10-mu-m, growing to +/- 5% in the vicinity of 25-mu-m and 6% at 60-mu-m. On average, monochromatic stellar fluxes obtained using the fits are the same as those obtained using the standard IRAS photometry and calibration except for a + 1% bias (at 12, 25 and 60-mu-m) (Beichmann et al. 1988). Average short-wavelength monochromatic fluxes differ from the calibrations of Campins et al. ( 1 98 5) by about - 1% at 2.22-mu-m, and + 1% at 3.54-mu-m. Relative flux ratios computed using the continua approximations for a variety of stars agree with relative photometry on those stars to within the given measurement accuracies ( +/- 1% to +/- 2% ) indicating that the relative errors in the predictions themselves are probably 1% or better. Only for the star alpha Boo does the predicted flux relative to other stars show evidence of departing significantly from the actual measured value; an apparent 3%-4% deep depression near 20-mu-m is indicated relative to the computed continuum. Recalibration of 2-5.5-mu-m spectroscopy from Strecker et al. 1979, is demonstrated, showing that one can extract information on molecular absorption bands observed in giant stars by dividing out the underlying continua. Using the spectral fitting functions, a smoothed system of expected photometric magnitudes is generated for hypothetical narrow passbands centered at 2.2, 3.5, 4.8, 11, 22, and 60-mu-m. This facilitates rapid calibration of results reported in magnitudes in standard K, L, M, N, and Q bands. In such applications substitution of this fit-derived flux estimate at the effective wavelength of each band for the individual empirical estimates presently available should provide a way of getting the best calibration of photometry in any one band based on a combination of the available measurements in all bands. This is clearly advantageous even when continua are not needed explicitly.