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Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| Flatfielding and photometric accuracy of the first Hubble Space Telescope Wide Field Camera
Long exposures with the original Hubble Space Telescope (HST) Wide FieldCamera (WFC) through the F555W and F785LP filters show gradients in thebackground following standard pipeline calibration. We show that thesegradients also appear in stellar photometry, and thus must bepredominantly the result of inaccurate flatfielding at a level of 10 to20%. Color errors may be even larger. Applying corrections to theflatfield frames based on the background structure leads to an improvedaccuracy of approximately 4% for single-measurement photometry within asingle CCD chip, compared to the approximately 10% accuracy suggested byprevious studies. We have reanalyzed the F555W and F785LP calibrationphotometry to derive zero points appropriate for corrected data; thesenew zero points have internal consistency at a level of approximately1.2%, based on comparison between the chip-to-chip offsets and the skylevels observed in corrected images. This indicates that relativephotometry approaching 1 to 2% is achievable with the WFC. The new zeropoint values for corrected data are 22.90, 23.04, 23.04, and 22.96(F555W), and 21.56, 21.64, 21.44, and 21.47 (F785LP) for chips WF1-WF4,respectively. Comparison is made with other zero points, and theapplicability of 'delta flats' is briefly discussed.
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