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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.
| Variable central stars of young planetary nebulae. I. Photometric multisite observations of IC 418.
We report the results of a photometric multisite campaign devoted to HD35914, the variable central star of the Planetary Nebula IC 418. Fromthe analysis of 120 hours of data acquired with a variety of techniques,we find that HD 35914 exhibits two distinct kinds of variability:irregular light modulation with a time scale of days, as well as cyclicvariations with a time scale of 6.5 hours. The short-term variations arenot strictly periodic, and cannot be reasonably explained bymultiperiodicity; they appear to be semiregular. The star is generallyredder when it is brighter; this behavior appears to be connected withthe long-term variability. A re-analysis of most of the older dataobtained for HD 35914 by various researchers suggests that the basicbehavior of the star did not change during the last 15 years. Wecarefully discuss all the possible causes for the light variations ofthe star. Rotational modulation of surface features cannot explain theobservations, and binarity is unlikely. Pulsations may be excited, butwind variability (or a combination of both) can also not be ruled out.
| Photometric observations of the nucleus of IC 418.
We discuss the observational characteristics of a recently proposedclass of variable central stars of planetary nebulae. We summarizeprevious observations of the nucleus of IC 418, a prototype star of thisclass, and we analyze new photometry of this object, obtained in 1989and 1991. We confirm that the characteristic timescale of thephotometric variations is a few hours. No single period can fit all ofour observational runs, implying that the variability is eitherirregular or multimodal. We discuss two possible explanations of theobserved phenomena: (i) an unstable, radiatively driven wind and (ii)pulsational instability driven by the ?-mechanism. We alsoconsider the possibility that the observed variability is produced by acombination of both mechanisms and that they may influence each other.
| uvby beta Photometry of Stars of "Astrophysical Interest"
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