Quantitative H and K band spectroscopy of Galactic OB-stars at medium resolution

In this paper we have analyzed 25 Galactic O and early B-stars by means of H and K band spectroscopy, with the primary goal to investigate to what extent a lone near-IR spectroscopy is able to recover stellar and wind parameters derived in the optical. Most of the spectra have been taken with SUBARU IRCS, at a resolution of 12,000, and with a very high S/N (200 or better). In order to synthesize the strategic H/He lines, we have used our recent, line-blanketed version of FASTWIND. First we investigated the predicted behaviour of the strategic lines. In contradiction to what one expects from the optical in the O-star regime, almost all photospheric H/HeI/HeII H/K band lines become stronger if the gravity decreases. Concerning H and HeII, this finding is related to the behaviour of Stark broadening as a function of electron density, which in the line cores is different for members of lower (optical) and higher (IR) series. Regarding HeI, the predicted behaviour is due to some subtle NLTE effects resulting in a stronger overpopulation of the lower level when the gravity decreases. For most of our objects, we obtained good fits, except for the line cores of Br_gamma in early O-stars with significant mass-loss, where this discrepancy might be an indirect effect of clumping. After having derived the stellar and wind parameters from the IR, we have compared them to results from previous optical analyses. Overall, the IR results coincide in most cases with the optical ones within the typical errors usually quoted for the corresponding parameters, i.e, an uncertainty in Teff of 5%, in log g of 0.1 dex and in Mdot of 0.2 dex, with lower errors at higher wind densities. Outliers above the 1-sigma level where found in four cases with respect to log g and in two cases for Mdot.