Context. Effective temperatures of early-type supergiants are important to test stellar atmosphere-and internal structure-models of massive and intermediate mass objects at different evolutionary phases. However, these T eff values are more or less discrepant depending on the method used to determine them. Aims. We aim to obtain a new calibration of the T eff parameter for early-type supergiants as a function of observational quantities that are: a) highly sensitive to the ionization balance in the photosphere and its gas pressure; b) independent of the interstellar extinction; c) as much as possible model-independent. Methods. The observational quantities that best address our aims are the (λ 1 , D) parameters of the BCD spectrophotometric system. They describe the energy distribution around the Balmer discontinuity, which is highly sensitive to T eff and log g. We perform a calibration of the (λ 1 , D) parameters into T eff using effective temperatures derived with the bolometric-flux method for 217 program stars, whose individual uncertainties are on average |ΔT eff |/T f eff = 0.05. Results. We obtain a new and homogeneous calibration of the BCD (λ 1 , D) parameters for OB supergiants and revisit the current calibration of the (λ 1 , D) zone occupied by dwarfs and giants. The final comparison of calculated with obtained T eff values in the (λ 1 , D) calibration show that the latter have total uncertainties, which on average are T eff /T f eff ±0.05 for all spectral types and luminosity classes. Conclusions. The effective temperatures of OB supergiants derived in this work agree on average within some 2000 K with other determinations found in the literature, except those issued from wind-free non-LTE plane-parallel models of stellar atmospheres, which produce effective temperatures that can be overestimated by up to more than 5000 K near T eff = 25 000 K. Since the stellar spectra needed to obtain the (λ 1 , D) parameters are of low resolution, a calibration based on the BCD system is useful to study stars and stellar systems like open clusters, associations or stars in galaxies observed with multi-object spectrographs and/or spectro-imaging devices.
Context. Recent studies of O-type stars have demonstrated that discrepant mass-loss rates are obtained when different diagnostic methods are employed. Fitting the unsaturated UV resonance lines (e.g., P v) gives drastically lower values than obtained from the Hα emission. Wind inhomogeneity (so-called "clumping") may be the main cause of this discrepancy. Aims. In a previous paper, we presented 3D Monte-Carlo calculations for the formation of scattering lines in a clumped stellar wind. In the present paper we select five O-type supergiants (from O4 to O7) and test whether the reported discrepancies can be resolved this way. Methods. In the first step, the analyses started with simulating the observed spectra with Potsdam Wolf-Rayet (PoWR) non-LTE model atmospheres. The mass-loss rates are adjusted to fit to the observed Hα emission lines best. For the unsaturated UV resonance lines (i.e., P v) we then applied our 3D Monte-Carlo code, which can account for wind clumps of any optical depths ("macroclumping"), a non-void interclump medium, and a velocity dispersion inside the clumps. The ionization stratifications and underlying photospheric spectra were adopted from the PoWR models. The properties of the wind clumps were constrained by fitting the observed resonance line profiles.Results. Our results show that with the mass-loss rates that fit Hα (and other Balmer and He ii lines), the UV resonance lines (especially the unsaturated doublet of P v) can also be reproduced with no problem when macroclumping is taken into account. There is no need to artificially reduce the mass-loss rates or to assume a subsolar phosphorus abundance or an extremely high clumping factor, unlike what was claimed by other authors. These consistent mass-loss rates are lower by a factor of 1.3 to 2.6, compared to the mass-loss rate recipe from Vink et al. Conclusions.Macroclumping resolves the previously reported discrepancy between Hα and P v mass-loss diagnostics.
Context. The periodicity of 5.5 years for some observational events occurring in η Carinae manifests itself across a large wavelength range and has been associated with its binary nature. These events are supposed to occur when the binary components are close to periastron. To detect the previous periastron passage of η Car in 2003, we started an intensive, ground-based, optical, photometric observing campaign. Aims. We continued observing the object to monitor its photometric behavior and variability across the entire orbital cycle. Methods. Our observation program consisted of daily differential photometry from CCD images, which were acquired using a 0.8 m telescope and a standard BVRI filter set at La Plata Observatory. The photometry includes the central object and the surrounding Homunculus nebula. Results. We present up-to-date results of our observing program, including homogeneous photometric data collected between 2003 and 2008. Our observations demonstrated that η Car has continued increasing in brightness at a constant rate since 1998. In 2006, it reached its brightest magnitude (V ∼ 4.7) since about 1860. The object then suddenly reverted its brightening trend, fading to V = 5.0 at the beginning of 2007, and has maintained a quite steady state since then. We continue the photometric monitoring of η Car in anticipation of the next "periastron passage", predicted to occur at the beginning of 2009. Conclusions.
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