Search

Context. High spectral resolution and long exposure times are providing unprecedented levels of data quality of massive stars at X-ray wavelengths. Aims. A key diagnostic of the X-ray emitting plasma are the fir lines for He-like triplets. In particular, owing to radiative pumping effects, the forbidden-to-intercombination line luminosity ratio, R = f∕i, can be used to determine the proximity of the hot plasma to the UV-bright photospheres of massive stars. Moreover, the era of large observing programs additionally allows for investigation of line variability. Methods. This contribution is the second to explore how variability in the line ratio can provide new diagnostic information about distributed X-rays in a massive star wind. We focus on wind integration for total line luminosities, taking account of radiative pumping and stellar occultation. While the case of a variable stellar radiation field was explored in the first paper, the effects of wind variability are emphasized in this work. Results. We formulate an expression for the ratio of line luminosities f∕i that closely resembles the classic expression for the on-the-spot result. While there are many ways to drive variability in the line ratio, we use variable mass loss as an illustrative example for wind integration, particularly since this produces no variability for the on-the-spot case. The f∕i ratio can be significantly modulated owing to evolving wind properties. The extent of the variation depends on how the timescale for the wind flow compares to the timescale over which the line emissivities change. Conclusions. While a variety of factors can ellicit variable line ratios, a time-varying mass-loss rate serves to demonstrate the range of amplitude and phased-dependent behavior in f∕i line ratios. Importantly, we evaluate how variable mass loss might bias measures of f∕i. For observational exposures that are less than the timescale of variable mass loss, biased measures (relative to the time-averaged wind) can result; if exposures are long, the f∕i ratio is reflective of the time-averaged spherical wind. [ABSTRACT FROM AUTHOR]

Context. Line ratios in "fir" triplets of helium-like ions have proven to be a powerful diagnostic of conditions in X-ray emitting gas surrounding massive stars. Recent observations indicate that these ratios can be variable with time. Aims. The possibilities for causes of variation in line ratios are limited: changes in the radiation field or changes in density, which would result in radiational or collisional level-pumping, respectively; and changes in mass-loss or geometry, which could cause variations in X-ray absorption effects. In this paper, we explore the first of these potential causes of variability: changes in the radiation field. We therefore explore the conditions necessary to induce variability in the ratio R = f/ i via this mechanism. Methods. To isolate the radiative effect, we use a heuristic model of temperature and radius changes in variable stars in the B and O range with low-density, steady-state winds. We then model the changes in emissivity of X-ray emitting gas close to the star due to differences in level-pumping owing to the availability of UV photons at the location of the gas. Results. We find that under these conditions, variability in R is dominated by the stellar temperature. Although the relative amplitude of variability is roughly comparable for most lines at most temperatures, detectable variations are limited to a few lines for each spectral type. We predict that variable values in R due to stellar variability must follow predictable trends found in our simulations. Conclusions. Our heuristic model uses radial pulsations as a mode of stellar variability that maximizes the amplitude of variation in R. This model is robust enough to provide a guide to which ions will provide the best opportunity for observing variability in the f/i ratio at different stellar temperatures, and the correlation of that variability with other observable parameters. In real systems, however, the effects would be more complex than in our model, with differences in phase and suppressed amplitude in the presence of non-radial pulsations. This, combined with the range of amplitudes produced in our simulations, suggests that changes in R across many lines concurrently are not likely to be produced by a variable radiation field. [ABSTRACT FROM AUTHOR]

Highlights from the article: Keywords: stars: early-type; stars: massive; stars: mass-loss; stars: winds, outflows; X-rays: stars; errata, addenda .