New self-consistent wind parameters to fit optical spectra of O-type stars observed with the HERMES spectrograph

¹ Departamento de Ciencias, Facultad de Artes Liberales, Universidad Adolfo Ibáñez, Av. Padre Hurtado 750, Viña del Mar, Chile
e-mail: alex.gormaz@uv.cl
² Instituto de Física y Astronomía, Universidad de Valparaíso, Av. Gran Bretaña 1111, Casilla, 5030 Valparaíso, Chile
³ Centro de Astrofísica, Universidad de Valparaíso, Av. Gran Bretaña 1111, Casilla, 5030 Valparaíso, Chile
⁴ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
⁵ Departamento de Espectroscopía, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata (UNLP), Paseo del Bosque S/N (B1900FWA), La Plata, Argentina
⁶ Instituto de Astrofísica de La Plata, CCT La Plata, CONICET-UNLP, Paseo del Bosque S/N (B1900FWA), La Plata, Argentina
⁷ Centro de Investigación DAiTA Lab, Facultad de Estudios Interdisciplinarios, Universidad Mayor, Alonso de Córdova 5495, Santiago, Chile

Received: 7 October 2021
Accepted: 17 February 2022

Abstract

Aims. We performed a spectral fitting for a set of O-type stars based on self-consistent wind solutions, which provide mass-loss rate and velocity profiles directly derived from the initial stellar parameters. The great advantage of this self-consistent spectral fitting is therefore the reduction of the number of free parameters to be tuned.

Methods. Self-consistent values for the line-force parameters (k, α, δ)_sc and subsequently for the mass-loss rate, M_sc, and terminal velocity, υ_∞‚sc, are provided by the m-CAK prescription introduced in Paper I, which is updated in this work with improvements such as a temperature structure T(r) for the wind that are self-consistently evaluated from the line-acceleration. Synthetic spectra were calculated using the radiative transfer code FASTWIND, replacing the classical β-law for our new calculated velocity profiles v(r) and therefore making clumping the only free parameter for the stellar wind.

Results. We found that self-consistent m-CAK solutions provide values for theoretical mass-loss rates of the order of the most recent predictions of other studies. From here, we generate synthetic spectra with self-consistent hydrodynamics to fit and obtain a new set of stellar and wind parameters for our sample of O-type stars (HD 192639, 9 Sge, HD 57682, HD 218915, HD 195592, and HD 210809), whose spectra were taken with the high-resolution echelle spectrograph HERMES (R = 85 000). We find a satisfactory global fit for our observations, with a good accuracy for photospheric He I and He II lines and a quite acceptable fit for H lines. Although this self-consistent spectral analysis is currently constrained in the optical wavelength range alone, this is an important step towards the determination of stellar and wind parameters without using a β-law. Based on the variance of the line-force parameters, we establish that our method is valid for O-type stars with T_eff ≥ 30 kK and log g ≥ 3.2. Given these results, we expect that the values introduced here are helpful for future studies of the stars constituting this sample, together with the prospect that the m-CAK self-consistent prescription may be extended to numerous studies of massive stars in the future.