Absolute Physical Calibration in the Infrared

Abstract

We determine an absolute calibration for the Multiband Imaging Photometer for Spitzer 24 µm band and recommend adjustments to the published calibrations for Two Micron All Sky Survey (2MASS), Infrared Array Camera (IRAC), and IRAS photometry to put them on the same scale. We show that consistent results are obtained by basing the calibration on either an average A0V star spectral energy distribution (SED), or by using the absolutely calibrated SED of the Sun in comparison with solar-type stellar photometry (the solar analog method). After the rejection of a small number of stars with anomalous SEDs (or bad measurements), upper limits of ∼1.5% root mean square (rms) are placed on the intrinsic infrared (IR) SED variations in both A-dwarf and solar-type stars. These types of stars are therefore suitable as general-purpose standard stars in the IR. We provide absolutely calibrated SEDs for a standard zero magnitude A star and for the Sun to allow extending this work to any other IR photometric system. They allow the recommended calibration to be applied from 1 to 25 µm with an accuracy of ∼2%, and with even higher accuracy at specific wavelengths such as 2.2, 10.6, and 24 µm, near which there are direct measurements. However, we confirm earlier indications that Vega does not behave as a typical A0V star between the visible and the IR, making it problematic as the defining star for photometric systems. The integration of measurements of the Sun with those of solar-type stars also provides an accurate estimate of the solar SED from 1 through 30 µm, which we show agrees with theoretical models.

An absolutely calibratedTeffscale from the infrared flux method

“…From a pragmatic point of view, this improves the consistency in determining T eff . Also, such changes are within the 2MASS quoted errors and for the K S band we remark the agreement with the 0.2% increase found by Rieke et al (2008) and discussed in Sect. 2.2.…”

“…Johnson 1965;Campins et al 1985;Rieke et al 2008). This rationale can be extended to other physical properties, namely using the solar effective temperature T eff, = 5777 K as the average value for solar-type stars (e.g.…”

“…(a) Average and standard deviation of the colours in Rieke et al (2008) and obtained from solar-type stars or computed convolving various solar spectra with the appropriate filter curves and using their revised absolute physical calibration. The empirical colours in Table 7 are also in agreement with the synthetic ones, computed using the same zero points and absolute calibration for Vega used in the IRFM to derive our T eff scale.…”

EXPLORATIONS BEYOND THE SNOW LINE:SPITZER/IRS SPECTRA OF DEBRIS DISKS AROUND SOLAR-TYPE STARS

“…All of these stars have fractional excesses that extend well above the 2σ threshold at longer wavelengths, and are significant at the 3σ level in the 30-34 μm band. similar conclusion applies if we fit a solar photosphere (Rieke et al 2008) to the IRAC 3.6 and 4.8 μm data. Extending the Kurucz photosphere to longer wavelengths yields a marginal excess of about 5% at IRAC 7.8 μm that carries through to IRS SL1 and MIPS 24 μm.…”

Interferometric observations of rapidly rotating stars

“…For example, the particulars of the analysis of the spectral energy distribution of Vega already noted above ) significantly revises our view of this fundamental standard (Rieke et al 2008).…”