Probing the weak wind phenomenon in Galactic O-type giants

Abstract

Aims. Analyses of Galactic late O dwarfs (O8-O9.5V stars) raised the "weak wind problem": spectroscopic mass-loss rates (Ṁ) are up to two orders of magnitude lower than the theoretical values. We investigated the stellar and wind properties of Galactic late O giants (O8-O9.5III stars). These stars have luminosities log(L /L ) ∼ 5.2, which is the critical value (onset of weak winds) proposed in the literature. Methods. We performed a spectroscopic analysis of nine O8-O9.5III stars in the ultraviolet (UV) and optical regions using the model atmosphere code CMFGEN. Results. Stellar luminosities were adopted using calibrations from the literature. Overall, our model spectral energy distributions agree well with the observed ones considering parallaxes from the latest GAIA data release (DR2). The effective temperature derived from the UV region agrees well with the ones from the optical. As expected, the analysis of the Hertzsprung-Russell (HR) diagram shows that our sample is more evolved than late O dwarfs. From the UV region, we foundṀ ∼ 10 −8 − 10 −9 M yr -1 overall. This is lower by ∼ 0.9 − 2.3 dex than predicted values based on the (global) conservation of energy in the wind. The mass-loss rates predicted from first principles, based on the moving reversing layer theory, agree better with our findings, but it fails to match the spectroscopiċ M for the most luminous OB stars. The region of log(L /L ) ∼ 5.2 is critical for both sets of predictions in comparison with the spectroscopic mass-loss rates. CMFGEN models with the predictedṀ (the former one) fail to reproduce the UV wind lines for all the stars of our sample. We reproduce the observed Hα profiles of four objects with ourṀ derived from the UV. Hence, lowṀ values (weak winds) are favored to fit the observations (UV + optical), but discrepancies between the UV and Hα diagnostics remain for some objects. Conclusions. Our results indicate weak winds beyond the O8-9.5V class, since the region of log(L /L ) ∼ 5.2 is indeed critical to the weak wind phenomenon. Since O8-O9.5III stars are more evolved than O8-9.5V, evolutionary effects do not seem to play a role in the onset of the weak wind phenomenon. These findings support that theṀ (for low luminosity O stars) in use in the majority of modern stellar evolution codes must be severely overestimated up to the end of the H-burning phase. Further investigations must evaluate the consequences of weak winds in terms of physical parameters for massive stars (e.g., angular momentum and CNO surface abundances).

UV Spectroscopy of Massive Stars

“…Similar stars, with very low mass-loss rates, were found in the study of MSC-N81 by [149] who suggested that the wind-luminosity relation for dwarfs breaks down when L < 3 × 10 5 L . Similar "weak-wind" stars are found in the Galaxy [150,151] although the upper L < 2 × 10 5 limit to the phenomenon may be a little lower than in the SMC. [151] suggest that evolution may not be a crucial parameter-they found that O9-O9.5III stars also exhibited weak winds.…”

“…Similar "weak-wind" stars are found in the Galaxy [150,151] although the upper L < 2 × 10 5 limit to the phenomenon may be a little lower than in the SMC. [151] suggest that evolution may not be a crucial parameter-they found that O9-O9.5III stars also exhibited weak winds. The "weak wind" problem is also seen for non-supergiant B stars [152].…”

New predictions for radiation-driven, steady-state mass-loss and wind-momentum from hot, massive stars

“…Regarding the two excluded low-luminosity stars, these seem to indicate a weak wind effect also at Galactic metallicity. Also observational studies of Galactic dwarfs (Martins et al 2005;Marcolino et al 2009) and giants (de Almeida et al 2019) find that the onset of the weak wind problem seems to lie around log(L * /L ) ≈ 5.2, where such an onset was already indicated in the data provided by Puls et al (1996). Although our mass-loss rates in this regime are significantly lower than those of Vink 2000), they are still not as low as those derived from these observational studies.…”

Intensity interferometry of P Cygni in the H α emission line: towards distance calibration of LBV supergiant stars

“…The Balmer lines, in particular Hα, seems to be better reproduced using a slightly higher value for the mass-loss rate (Ṁ = 4.0 × 10 −5 M yr -1 with f ∞ = 0.5). However, this difference is encompassed by the typically uncertainties onṀ found from spectroscopic analysis of massive stars in literature (see, e.g., [7]). It is beyond the scope of this paper to derive the stellar and wind parameters of P Cyg, as performed by [36] and [35].…”