Temperature, gravity, and bolometric correction scales for non-supergiant OB stars

Abstract

Context. Precise and accurate determinations of the atmospheric parameters effective temperature and surface gravity are mandatory to derive reliable chemical abundances in OB stars. Furthermore, fundamental parameters like distances, masses, radii, luminosities can also be derived from the temperature and gravity of the stars. Aims. Atmospheric parameters recently determined at high precision with several independent spectroscopic indicators in non-local thermodynamic equilibrium, with typical uncertainties of ∼300 K for temperature and of ∼0.05 dex for gravity, are employed to calibrate photometric relationships. This is in order to investigate whether a faster tool to estimate atmospheric parameters can be provided. Methods. Temperatures and gravities of 30 calibrators, i.e. well-studied OB main sequence to giant stars in the solar neighbourhood, are compared to reddening-independent quantities of the Johnson and Strömgren photometric systems, assuming normal reddening. In addition, we examine the spectral and luminosity classification of the star sample and compute bolometric corrections. Results. Calibrations of temperatures and gravities are proposed for various photometric indices and spectral types. Once the luminosity of the stars is well known, effective temperatures can be determined at a precision of ∼400 K for luminosity classes III/IV and ∼800 K for luminosity class V. Furthermore, surface gravities can reach internal uncertainties as low as ∼0.08 dex when using our calibration to the Johnson Q-parameter. Similar precision is achieved for gravities derived from the β-index and the precision is lower for both atmospheric parameters when using the Strömgren indices [c1] and [u − b]. In contrast, external uncertainties are larger for the Johnson than for the Strömgren calibrations. Our uncertainties are smaller than typical differences among other methods in the literature, reaching values up to ±2000 K for temperature and ±0.25 dex for gravity, and in extreme cases, +6000 K and ±0.4 dex, respectively. A parameter calibration for sub-spectral types is also proposed. Moreover, we present a new bolometric correction relation to temperature based on our empirical data, rather than on synthetic grids. Conclusions. The photometric calibrations presented here are useful tools to estimate effective temperatures and surface gravities of non-supergiant OB stars in a fast manner. This is also applicable to some single-line spectroscopic binaries, but caution has to be taken for undetected double-lined spectroscopic binaries and single objects with anomalous reddening-law, dubious photometric quantities and/or luminosity classes, for which the systematic uncertainties may increase significantly. We recommend to use these calibrations only as a first step of the parameter estimation, with subsequent refinements based on spectroscopy. A larger sample covering more uniformly the parameter space under consideration will allow refinements to the present calibrations.

B fields in OB stars (BOB): The discovery of a magnetic field in a multiple system in the Trifid nebula, one of the youngest star forming regions

“…The presence of the weak He ii 4686 Å line in the central strongest component indicates a spectral type B1 (for normal He content) or B1.5 (for enhanced He), implying an effective temperature of about 24 000-26 000 K (Nieva 2013). Moreover, the He lines appear in normal strength for the T eff estimated from the metal lines.…”

Critical study of the distribution of rotational velocities of Be stars

“…In these cases we could not do better than conform with the apparent bolometric luminosity estimated from the bolometric absolute magnitude M bol = M V + BC(T eff ) with M bol = +4.742 mag, where M V is the visual absolute magnitude and BC(T eff ) the bolometric correction (Flower 1996;Torres 2010;Nieva 2013). The magnitude M V was determined using the apparent magnitude V of an emissionless phase.…”

“…log L app /L with those given by de Jager & Nieuwenhuijzen (1987) and with the bolometric luminosities derived indirectly using calibrations of visual absolute magnitudes M V . In this last case, M V is transformed into absolute bolometric magnitude using the T eff (λ 1 , D) and bolometric corrections (Flower 1996;Torres 2010;Nieva 2013) with M bol = +4.742 mag. The calibrations of M V against the spectral classes used are from Wegner (2006), Deutschman et al (1976), and Aller et al (1982).…”

Fundamental properties of nearby single early B-type stars

“…Again, this is in agreement with the DEB data. Note that a discussion of the T eff -spectral-type relation is omitted here as this has been investigated in detail by Nieva (2013).…”