Spectroscopy and mass loss diagnostic: observations
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Recent studies of massive O-type stars present clear evidences of inhomogeneous and clumped winds. O-type (H-rich) central stars of planetary nebulae (CSPNs) are in some ways the low mass–low luminosity analogous of those massive stars. In this contribution, we present preliminary results of our on-going multi-wavelength (FUV, UV and optical) study of the winds of Galactic CSPNs. Particular emphasis will be given to the clumping factors derived by means of optical lines (Hα and Heii 4686) and “classic” FUV (and UV) lines.
We exploit time-series $FUSE$ spectroscopy to {\it uniquely} probe spatial structure and clumping in the fast wind of the central star of the H-rich planetary nebula NGC~6543 (HD~164963). Episodic and recurrent optical depth enhancements are discovered in the P{\sc v} absorption troughs, with some evidence for a $\sim$ 0.17-day modulation time-scale. The characteristics of these features are essentially identical to the discrete absorption components' (DACs) commonly seen in the UV lines of massive OB stars, suggesting the temporal structures seen in NGC~6543 likely have a physical origin that is similar to that operating in massive, luminous stars. The mechanism for forming coherent perturbations in the outflows is therefore apparently operating equally in the radiation-pressure-driven winds of widely differing momenta ($\mdot$$v_\infty$$R_\star^{0.5}$) and flow times, as represented by OB stars and CSPN.
This paper outlines a newly-developed method to include the effects of time variability in the radiative transfer code CMFGEN. It is shown that the flow timescale is often large compared to the variability timescale of LBVs. Thus, time-dependent effects significantly change the velocity law and density structure of the wind, affecting the derivation of the mass-loss rate, volume filling factor, wind terminal velocity, and luminosity. The results of this work are directly applicable to all active LBVs in the Galaxy and in the LMC, such as AG Car, HR Car, S Dor and R 127, and could result in a revision of stellar and wind parameters. The massloss rate evolution of AG Car during the last 20 years is presented, highlighting the need for time-dependent models to correctly interpret the evolution of LBVs.
We discuss the results of time-resolved spectroscopy of three presumably single Population I Wolf-Rayet stars in the Small Magellanic Cloud, where the ambient metallicity is $\sim 1/5 Z_\odot$. We were able to detect and follow numerous small-scale wind-embedded inhomogeneities in all observed stars. The general properties of the moving features, such as their velocity dispersions, emissivities and average accelerations, closely match the corresponding characteristics of small-scale inhomogeneities in the winds of Galactic Wolf-Rayet stars.
The influence of the wind to the total continuum of OB supergiants is discussed. For wind velocity distributions with β > 1.0, the wind can have strong influence to the total continuum emission, even at optical wavelengths. Comparing the continuum emission of clumped and unclumped winds, especially for stars with high β values, delivers flux differences of up to 30% with maximum in the near-IR. Continuum observations at these wavelengths are therefore an ideal tool to discriminate between clumped and unclumped winds of OB supergiants.
We present the latest results on the observational dependence of the mass-loss rate in stellar winds of O and early-B stars on the metal content of their atmospheres, and compare these with predictions. Absolute empirical rates for the mass loss of stars brighter than 10$^{5.2} L_{\odot}$, based on H$\alpha$ and ultraviolet (UV) wind lines, are found to be about a factor of two higher than predictions. If this difference is attributed to inhomogeneities in the wind this would imply that luminous O and early-B stars have clumping factors in their H$\alpha$ and UV line forming regime of about a factor of 3--5. The investigated stars cover a metallicity range $Z$ from 0.2 to 1 $Z_{\odot}$. We find a hint towards smaller clumping factors for lower $Z$. The derived clumping factors, however, presuppose that clumping does not impact the predictions of the mass-loss rate. We discuss this assumption and explain how we intend to investigate its validity in more detail.
We report on new mass-loss rate estimates for O stars in six massive binaries using the amplitude of orbital-phase dependent, linear-polarimetric variability caused by electron scattering off free electrons in the winds. Our estimated mass-loss rates for luminous O stars are independent of clumping. They suggest similar clumping corrections as for WR stars and do not support the recently proposed reduction in mass-loss rates of O stars by one or two orders of magnitude.
Clumping in O-star winds
(2007)