Bolometric correction and spectral energy distribution of cool stars in Galactic clusters

Synthetic BVR_CI_C JHK photometry from the derived spectral energy distributions allowed us to obtain robust temperature (T_eff) estimates for each star, within +/-100K or less. According to the appropriate temperature estimate, blackbody extrapolation of the observed spectral energy distribution allowed us to assess the unsampled flux beyond the wavelength limits of our survey. For the bulk of our red giants, this fraction amounted to 15 per cent of the total bolometric luminosity, a figure that raises up to 30 per cent for the coolest targets (T_eff <~ 3500K). Overall, we obtain stellar M_bol values with an internal accuracy of a few percentages. Even neglecting any correction for lost luminosity etc., we would be overestimating M_bol by <~0.3mag, in the worst cases. Making use of our new data base, we provide a set of fitting functions for the V and K BC versus T_eff and versus (B - V) and (V - K) broad-band colours, valid over the interval 3300 <= T_eff <= 5000K, especially suited for red giants.

The analysis of the BC_V and BC_K estimates along the wide range of metallicity spanned by our stellar sample shows no evident drift with [Fe/H]. Things may be different for the B-band correction, where the blanketing effects are more and more severe. A drift of Δ(B - V) versus [Fe/H] is in fact clearly evident from our data, with metal-poor stars displaying a `bluer' (B - V) with respect to the metal-rich sample, for fixed T_eff.

Our empirical bolometric corrections are in good overall agreement with most of the existing theoretical and observational determinations, supporting the conclusion that (a) BC_K from the most recent studies are reliable within <~+/-0.1 over the whole colour/temperature range considered in this paper, and (b) the same conclusion apply to BC_V only for stars warmer than ~=3800K. At cooler temperatures the agreement is less general, and MARCS models are the only ones providing a satisfactory match to observations, in particular in the BC_V versus (B - V) plane.

Based on observations made at La Palma, at the Spanish Observatorio del Roque de los Muchachos of the IAC, with the Italian Telescopio Nazionale Galileo (TNG) operated by the Fundación Galileo Galilei of INAF.

E-mail: alberto.buzzoni@oabo.inaf.it