IR Standards
Frank Valdes wrote on Oct 20, 1998
Q: I am going to do some observing with NICMOS III spectrometer (at Gurushikhar Telescope near Ahmadabad)in the J H K bands during 11-14 Oct and obtain spectra in these bands for some 60 F G and K type stars. Do you have any reference or digital fluxes for standard stars in these bands which we can install at the IRAF library at IUCAA so that after I come back I can use the fluxes to do the flux calibration of the program stars. A: You have reopened a discussion which involved a number of people, which started with a question by Margaret Hanson about the existence of a near-IR spectrophotometric library. My answer, which was confirmed by several others at CTIO and U Arizona, is that no one has done the same sort of observations that were done in the optical by Phil Massey and others to generate the standards one finds in the IRAF ondestds directory. There appear to be several reasons for this. 1. It is a lot of hard work. Phil elaborated for me the amount of observing with the IRS and other instruments which went into the list(s) he has published. He cheerfully admitted that the issue of telluric absorption lines, which is of great significance in the IR, but not a particular problem in the optical, except at a few wavelengths, was handled by simply choosing his bandpasses to miss any such features. 2. It can involve a lot of observing time. Allocation committees are generally not inclined, at least at KPNO, to give time to standards or calibration programs on oversubscribed telescopes unless one can make a compelling scientific case. In the optical, one can often provide strong arguments for the importance of combining spectra from groundbased and spaceborne UV instruments. One approach to spectrophotometric calibration in the IR has been that noted by Lazaro et al (1994, MNRAS, 269, 365). They used the absolute calibration data of Mountain et al (1985, A&A, 151, 399) fitted to a Kurucz model atmosphere for Vega to obtain a hypothetical Vega flux calibration spectrum. They also note the absolute calibration data of Cohen et al (1991, A. J., 104, 1650), but did not employ their data. Megessier (1995 , A&A, 296, 771) has raised some questions about the accuracy of this technique (i.e., relying on model atmospheres to establish flux calibration). He refers to a number of additional references and established an empirical blackbody relationship, which he noted was not meant to imply anything deep about the effective temperature of Vega. You might want to look through these references. My one recent publication of IR spectra in a flux scale used the calibration of Megessier, primarily because it was the most recent and was at least a reference point for the fluxes. I was forced to use the published photometry for these objects, since my spectra were not in any way calibrated due to slit losses. My approach was to convert the spectra, after dividing by a standard spectrum, to a magnitude scale and scale the integrated magnitude over the wavelength bandpass of the photometric filter to the published value. This is quickly described in A. J. 115, 2059, 1998. It might be an interesting idea to put the Megessier calibration of Vega into the IRAF library, but I am not sure how to do this. My perusal of the standards files show that the values must be in some sort of strange units, since the "magnitude" of Vega varies from 1.191 at 3200 A, to slightly negative, and back to 0.507 at 8370 A. Dick Joyce
Last post on Oct 20, 1998