Accurate fundamental parameters for lower main-sequence stars

Abstract

We derive an empirical effective temperature and bolometric luminosity calibration for G and K dwarfs, by applying our own implementation of the Infrared Flux Method to multiband photometry. Our study is based on 104 stars for which we have excellent BV(RI ) C JHK S photometry, excellent parallaxes and good metallicities.Colours computed from the most recent synthetic libraries (ATLAS9 and MARCS) are found to be in good agreement with the empirical colours in the optical bands, but some discrepancies still remain in the infrared. Synthetic and empirical bolometric corrections also show fair agreement.A careful comparison to temperatures, luminosities and angular diameters obtained with other methods in the literature shows that systematic effects still exist in the calibrations at the level of a few per cent. Our Infrared Flux Method temperature scale is 100-K hotter than recent analogous determinations in the literature, but is in agreement with spectroscopically calibrated temperature scales and fits well the colours of the Sun. Our angular diameters are typically 3 per cent smaller when compared to other (indirect) determinations of angular diameter for such stars, but are consistent with the limb-darkening corrected predictions of the latest 3D model atmospheres and also with the results of asteroseismology.Very tight empirical relations are derived for bolometric luminosity, effective temperature and angular diameter from photometric indices.We find that much of the discrepancy with other temperature scales and the uncertainties in the infrared synthetic colours arise from the uncertainties in the use of Vega as the flux calibrator. Angular diameter measurements for a well-chosen set of G and K dwarfs would go a long way to addressing this problem.

Synthetic stellar photometry – I. General considerations and new transformations for broad-band systems

“…Should its theoretical spectrum F λ (see next Section) be modelled with sufficient precision, and its angular diameter θ accurately measured, its absolute flux would be straightforward to obtain. In practice, its pole-on and rapidly rotating nature, as well as its suspected variability (although longward of 12µm, Aumann et al 1984, and thus of no impact on the colours investigated here) complicate things (also see the discussion in Casagrande et al 2006). In practice, the best approach resorts to the use of a composite absolutely calibrated spectrum for different wavelength regions.…”

“…Brighter than this threshold the difference between the asinh and Pogson magnitudes is negligible and rapidly goes to zero. This makes the Pogson formulation outlined 7 The only difference is the BV(RI) C absolute calibration in Casagrande et al (2010), which is based on earlier HST spectrophotometry of α Lyr (as reported in Table A2 of Casagrande et al 2006), producing T eff values that differ by only a few Kelvin. However, this difference essentially cancels out when the solar twins are used in the calibrating process; i.e., the absolute calibration derived from Bohlin (2007) agrees with that based on the solar twins.…”

“…where ZP ζ is derived for each bandpass ζ using a star of known absolute flux f λ = f and observed magnitude m VEGA = m ,ζ (e.g., Bessell et al 1998;Girardi et al 2002;Casagrande et al 2006, where our adopted magnitudes for Vega are reported in the third column of Table 1). Thus…”

Revisiting the helium abundance in globular clusters with multiple main sequences

“…Recent observations also seem to support a MS broadening in 47 Tuc (Anderson et al 2009 Casagrande et al (2006) to translate the dereddened maximum observed colour split between the bMS and rMS returns Δ log T eff = 0.0185 implying ΔY = 0.144 and Y bMS = 0.39 (Fig. 2).…”

“…More recently, Casagrande et al (2006Casagrande et al ( , 2007 further improved the analysis by compiling a much larger sample of K dwarfs with bolometric magnitudes and effective temperatures derived via InfraRed Flux Method (IRFM). The implementation adopted for the IRFM resorted to homogeneous multi-band photometry to determine in a fully self-consistent, (almost) model independent way T eff and M Bol .…”