Analysis of Galactic late-type O dwarfs: more constraints on the weak wind problem ^*,**

¹ LAM-UMR 6110, CNRS & Univ. de Provence, 38 rue Fréderic Joliot-Curie, 13388 Marseille, France e-mail: wagner.marcolino@oamp.fr
² GRAAL-UMR 5024, CNRS & Univ. de Montpellier II, Place Bataillon, 34095 Montpellier, France
³ Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA
⁴ Department of Astronomy, University of Maryland, College Park, MD 20742, USA

Received: 4 November 2008
Accepted: 17 February 2009

Abstract

Aims. We investigate the stellar and wind properties of a sample of late-type O dwarfs. Previous analyses of such stars have found very low mass-loss rates; rates much lower than predicted by theory (the weak wind problem).

Methods. Far-UV to optical spectra of five Galactic O stars were analyzed: HD 216898 (O9IV/O8.5V), HD 326329 (O9V), HD 66788 (O8V/O9V), ζ Oph (O9.5Vnn), and HD 216532 (O8.5V((n))). We used a grid of TLUSTY models to obtain effective temperatures, gravities, rotational velocities, and to identify wind lines. Wind parameters for each object were obtained using expanding atmosphere models calculated with the CMFGEN code.

Results. The spectra of our sample have primarily a photospheric origin. A weak wind signature is seen in C iv 1548, 1551, from which mass-loss rates consistent with previous CMFGEN results for O8-O9V stars were derived ( yr^-1). A discrepancy of roughly two orders of magnitude is found between these mass-loss rates and the values predicted by theory (), confirming a breakdown or a steepening of the modified wind momentum-luminosity relation at log . We have estimated the carbon abundance for the stars of our sample and concluded that its value cannot be reduced to sufficiently small values to solve the weak wind problem. Upper limits on were established for all objects using lines of different ions: P v 1118, 1128, C iii , N v 1239, 1243, Si iv 1394, 1403, and N iv λ1718. All the values obtained are in disagreement with theoretical predictions, bringing support to the reality of weak winds. Together with C iv 1548, 1551, the use of N v 1239, 1243 results in the lowest mass-loss rates: the upper limits indicate that must be less than about -1.0 dex . Upper mass-loss rate limits obtained for other transitions are also low: they indicate that must be less than about (-0.5 ± 0.2) dex . We studied the behavior of the Hα line with different mass-loss rates. For two stars, only models with very low 's provide the best fit to the UV and optical spectra. We also explored ways to fit the observed spectra with the theoretical mass-loss rates. By using large amounts of X-rays, we could reduce the predicted wind emission to the observed levels. However, unrealistic X-ray luminosities had to be used (log ). The validity of the models used in our analyses is discussed.