VLA Observations of ζ Aurigae: Confirmation of the Slow Acceleration Wind Density Structure

Abstract

Studies of the winds from single K and early M evolved stars indicate that these flows typically reach a significant fraction of their terminal velocity within the first couple of stellar radii. The most detailed spatially resolved information of the extended atmospheres of these spectral types comes from the Aur eclipsing binaries. However, the wind acceleration inferred for the evolved primaries in these systems appears significantly slower than for stars of similar spectral type. Since there are no successful theories for mass loss from K and early M evolved stars, it is important to place strong empirical constraints on potential models and determine whether this difference in acceleration is real or an artifact of the analyses. We have undertaken a radio continuum monitoring study of Aurigae ( K4 Ib + B5 V) using the Very Large Array to test the wind density model of Baade et al. that is based on Hubble Space Telescope (HST ) Goddard High Resolution Spectrograph ultraviolet spectra. Aur was monitored at centimeter wavelengths over a complete orbital cycle, and flux variations during the orbit are found to be of similar magnitude to variations at similar orbital phases in the adjacent orbit. During eclipse, the flux does not decrease, showing that the radio emission originates from a volume substantially larger than R 3 K $ (150 R ) 3 surrounding the B star. Using the one-dimensional density model of the K4 Ib primary's wind derived from HST spectral line profile modeling and electron temperature estimates from previous optical and new HST studies, we find that the predicted radio fluxes are consistent with those observed. Three-dimensional hydrodynamic simulations indicate that the accretion flow perturbations near the B star do not contribute significantly to the total radio flux from the system, consistent with the radio eclipse observations. Our radio observations confirm the slow wind acceleration for the evolved K4 Ib component. Aur's velocity structure does not appear to be typical of single stars with similar spectral types. This highlights the need for more comprehensive multiwavelength studies for both single stars, which have been sadly neglected, and other Aur systems to determine if its wind properties are typical.

Orbitally modulated photoexcited Si i emission in the eclipsing composite-spectrum binary ζ Aurigae

“…HST K-star eclipse spectra show that ζ Aur A's chromospheric heating and velocity fields are typical of single stars with similar spectral-types, e.g. λ Vel (K4 Ib; Carpenter et al 1999;Harper et al 2005), thus supporting a broader applicability of such a study. The illuminating B-star radiation field is well characterized through a combination of direct observations and synthetic photospheric spectra, and given recent improvements in atomic data, including photoionization crosssections of neutral species, other photoexcitation processes may also be examined.…”

“…To estimate conditions deeper in, we can take advantage of the property that the chromospheric heating in ζ Aur, as judged from the intrinsic K-star emission observed in low-opacity lines during the eclipse of the B star, is similar to that of other similar single, evolved stars (Schröder et al 1988;Eaton 1993;Harper et al 2005). The semi-empirical chromospheric model of the eclipsing system 22 Vul (G5: Ib:+ B8 V; Griffin et al 1993) has a temperature minimum, T min = 3550 K, with a hydrogen column density N H 4 × 10 24 cm −1 (Marshall 1996), while in the case of the single K4 II bright giant α TrA, N H 10 24 cm −1 (Harper 1992).…”

A NEW VLA-HIPPARCOSDISTANCE TO BETELGEUSE AND ITS IMPLICATIONS

“…To gain physical insight, quantitative studies require accurate stellar parameters: for example, the distance leads to the luminosity and when combined with an angular diameter gives the physical radius and effective temperature; luminosity and isotopic abundances provide estimates of the stellar age and mass. Except for ζ Aurigae eclipsing binary systems, e.g., Bennett et al (1996) and Harper et al (2005), accurate stellar parameters for cool supergiants are not available.…”

A Model for the Chromosphere/Wind of 31 Cygni and Its Implications for Single Stars

“…Again, the telling absorptions are in the UV, so most of our knowledge of these stars has come from IUE (e.g., Eaton 1996) with incremental improvements from HST (e.g., Baade et al 1996). More recently Harper et al (2005) have used radio observations of ζ Aur to verify the atmospheric structure they derive from UV lines.…”