STScI Preprint #1315

The Ionization in the Winds of O Stars and the Determination of Mass Loss Rates from Ultraviolet Lines

Empirical ionization fractions of CIV, NV, SiIV, and empirical ionization plus excitation fractions of CIII*  and NIV* in the winds of 34 O stars and one B star have been derived. We combined the mass loss rates derived from radio measurements and from H with the line fitting of ultraviolet resonance lines and subordinate lines by means of the SEI method. The dependence of the empirical ionization fractions <q> on the stellar effective temperature and on the mean wind density is discussed. This sets constraints for the models of the ionization in the winds of hot stars. The ionization and excitation fractions can be expressed in terms of an empirical radiation temperature. The radiation temperatures scale with T_eff and we derive empirical relations for T_rad as a function of T_eff. The radiation temperatures are on the order of 0.5 to 0.9 T_eff with significant differences between the ions. The derived relations between the ionization fractions and the stellar parameters have an uncertainty of 0.2 dex for SiIV, NV and CIII*, and about 0.26 dex for NIV*. For CIV we can only derive an expression for the mean ionization fraction in the wind if the mass loss rate is small, dM/dt < 10^-6 M/yr, because the CIV lines are usually saturated for higher mass loss rates. The resulting expressions for T_rad can be used to derive the mass loss rates from studies of ultraviolet P Cygni profiles in the range of stellar parameters studied here: 30,000 T_eff 50,500 K, 5.2 log L/L_* 6.4 and -7.5 log dM/dt -4.6 M/yr. An accuracy is about a factor two or better can be reached, depending on the lines that are used, and on the accuracy of the line fits and the stellar parameters. The SIV lines give the most reliable mass loss rates because the abundance is about the same for all O stars, the lines saturate only for high mass loss rates, the doublet lines only partly overlap and the mass loss rate is proportional to the square root of the column density. The radiation temperature of NV shows a surprisingly strict relation with T_eff with a scatter of only T_rad/T_eff=0.01. The mass loss rate cannot be derived from the NV lines because the column density of the NV ions in the wind is independent of dM/dt. A consistency check and a test of the method for the stars HD 14749 and HD 190429 show that the mass loss rate derived from the UV lines with the ionization fractions of this paper agree very well with the mass loss rate derived from new radio flux measurements.

Status:

Appeared in: The Astrophysical Journal

Affiliations:

¹Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²Astronomical Institute, Princetonplein 5, NL-3584 CC Utrecht, The Netherlands
³SRON Lab. for Space Research, Sorbonnelaan 2, NL-3584 CA Utrecht, The Netherlands
⁴Sternwarte, Universitat Muenchen Scheinerstrasse 1 D 8000, Muenchen 80, FRG
⁵Astronomical Institute Anton Pannekoek University of Amsterdam Kruislaan 403 NL-1098 SJ, Amsterdam, The Netherlands