P. D. Bennett

GHRS Observations of Cool, Low-Gravity Stars. V. The Outer Atmosphere and Wind of the Nearby K Supergiant λ Velorum*

UV spectra of λ Velorum taken with the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope are used to probe the structure of the outer atmospheric layers and wind and to estimate the mass-loss rate from this K5 Ib-II supergiant. VLA radio observations at λ = 3.6 cm are used to obtain an independent check on the wind velocity and mass-loss rate inferred from the UV observations. Parameters of the chromospheric structure are estimated from measurements of UV line widths, positions, and fluxes and from the UV continuum flux distribution. The ratios of optically thin C II] emission lines indicate a mean chromospheric electron density of log Ne ≈ 8.9 ± 0.2 cm-3. The profiles of these lines indicate a chromospheric turbulence (v0 ≈ 25-36 km s-1), which greatly exceeds that seen in either the photosphere or wind. The centroids of optically thin emission lines of Fe II and of the emission wings of self-reversed Fe II lines indicate that they are formed in plasma approximately at rest with respect to the photosphere of the star. This suggests that the acceleration of the wind occurs above the chromospheric regions in which these emission line photons are created. The UV continuum detected by the GHRS clearly traces the mean flux-formation temperature as it increases with height in the chromosphere from a well-defined temperature minimum of 3200 K up to about 4600 K. Emission seen in lines of C III] and Si III] provides evidence of material at higher than chromospheric temperatures in the outer atmosphere of this noncoronal star. The photon-scattering wind produces self-reversals in the strong chromospheric emission lines, which allow us to probe the velocity field of the wind. The velocities to which these self-absorptions extend increase with intrinsic line strength, and thus height in the wind, and therefore directly map the wind acceleration. The width and shape of these self-absorptions reflect a wind turbulence of ≈9-21 km s-1. We further characterize the wind by comparing the observations with synthetic profiles generated with the Lamers et al. Sobolev with Exact Integration (SEI) radiative transfer code, assuming simple models of the outer atmospheric structure. These comparisons indicate that the wind in 1994 can be described by a model with a wind acceleration parameter β ~ 0.9, a terminal velocity of 29-33 km s-1, and a mass-loss rate ~ 3 × 10-9 M☉ yr-1. Modeling of the 3.6 cm radio flux observed in 1997 suggests a more slowly accelerating wind (higher β) and/or a higher mass-loss rate than inferred from the UV line profiles. These differences may be due to temporal variations in the wind or from limitations in one or both of the models. The discrepancy is currently under investigation.

The Masses and Radii of the Eclipsing Binary F Aurigae

We present a full determination of the fundamental stellar and orbital parameters of the eclipsing binary ζ Aurigae (K4 Ib + B5 V) using recent observations with the Hubble Space Telescope Goddard High Resolution Spectrograph (GHRS) and the Mark III long-baseline optical interferometer. The information obtained from spectroscopic and interferometric measurements is complementary, and the combination permits a complete determination of the stellar masses, the absolute semimajor axis of the orbit, and the distance. A complete solution requires that both components be visible spectroscopically, and this has always been difficult for the ζ Aur systems. The ζ Aur K star primary presents no difficulty, and accurate radial velocities are readily obtainable in the optical. However, the B star secondary is more problematic. Ground-based radial velocity measurements are hampered by the difficulty of working with the composite spectrum in the blue-violet region, the small number of suitable lines in the generally featureless optical spectrum of the B star, and the great width of the few available lines (the Balmer lines of hydrogen and a few weak He I lines) due to rapid rotation. We avoid the worst of these problems by using GHRS observations in the ultraviolet, where the K star Flux is negligible and the intrinsic B star spectrum is more distinctive, and obtain the most accurate determination of the B star radial velocity amplitude to date. We also analyze published photometry of previous eclipses and near-eclipse phases of ζ Aur in order to obtain eclipse durations, which fix the length of the eclipse chord and therefore determine the orbit inclination. The long-baseline interferometry (LBI) yields, in conjunction with the spectroscopic solution, the distance to the system and thus the absolute stellar radius of the resolved K supergiant primary star, ζ Aur A. The secondary is not resolved by LBI, but its angular (and absolute) radius is found by Fitting the model stellar flux plus an interstellar extinction model to the flux-calibrated GHRS data. We Find MK = 5.8 ± 0.2 M☉, MB = 4.8 ± 0.2 M☉, RK = 148 ± 3 R☉ and RB = 4.5 ± 0.3 RB for the masses and radii of the ζ Aur stars. We determine the distance to ζ Aur to be 261 ± 3 pc. Additionally, we refine the stellar parameters of the B star secondary presented in the 1995 spectroscopic study of Bennett, Brown, & Linsky. We also determine the effective temperature of the K star primary using values of the bolometric flux, angular diameter, and interstellar extinction derived in this study. The positions of the ζ Aur stars on the theoretical H-R diagram are compared to current evolutionary model tracks, and the resulting good agreement provides a strong check of the internal self-consistency of this analysis and the accuracy of the theoretical models. The ζ Aurigae stars are confirmed to be coeval with an age of 80 ± 15 Myr.

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

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 ζ 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 ζ 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. ζ 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 ~ (150 R⊙)3 surrounding the B star. Using the one-dimensional density model of the K4 Ib primarys 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. ζ Aurs 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 ζ Aur systems to determine if its wind properties are typical.

MG Emission from Hybrid-Chromosphere Stars: 1.5 Decades of Chromospheric Variability Monitoring

We present an analysis of the available long-wavelength, high-dispersion spectra of seven hybridchromosphere stars obtained with the International Ultraviolet Explorer between 1978 and 1993. Our investigation of the variability of the Mg n h and k resonance doublet demonstrates that the emissionline fluxes are not rotationally modulated with the periods previously suggested by Brosius, Mullan, & Stencel. Furthermore, we find no evidence in the Mg n data to corroborate the multiple periodicities reported in the Ca n emissions of hybrid stars by Rao et al. Examination of 40 pairs of closely spaced Mg n observations failed to reveal the presence of any strong chromospheric flaring on the sample stars. Significant (20%-40%) nonperiodic Mg n flux variability, on timescales of days to years, is observed in six of the seven stars. The flux variations occasionally are accompanied by dramatic changes in the morphology of the Mg n profiles, indicating variable stellar-wind absorption. We argue that the variability observed is consistent with stochastic changes associated with the slow growth and decay of chromospheric active regions and the gradual evolution of the physical conditions in the winds. It is our opinion that no compelling argument has yet been made for periodic variability in the chromospheric diagnostics

An Atlas of Far-ultraviolet Spectra of the Zeta Aurigae Binary 31 Cygni with Line Identifications

The ζ Aurigae system 31 Cygni (K4 Ib + B4 V) was observed by the FUSE satellite during total eclipse and at three phases during chromospheric eclipse. We present the coadded, calibrated spectra and atlases with line identifications. During total eclipse, emission from high ionization states (e.g., Fe III and Cr III) shows asymmetric profiles redshifted from the systemic velocity, while emission from lower ionization states (e.g., Fe II and O I) appears more symmetric and is centered closer to the systemic velocity. Absorption from neutral and singly ionized elements is detected during chromospheric eclipse. Late in chromospheric eclipse, absorption from the K star wind is detected at a terminal velocity of ∼80 km s{sup –1}. These atlases will be useful for interpreting the far-UV spectra of other ζ Aur systems, as the observed FUSE spectra of 32 Cyg, KQ Pup, and VV Cep during chromospheric eclipse resemble that of 31 Cyg.

Spatial Extension in the Ultraviolet Spectrum of VV Cephei

Two-dimensional processing of Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) echelle spectra of the eclipsing M2 Iab + B binary system, VV Cep, has revealed extended emission in all 21 epochs observed, ranging from midtotality through the first quadrature. In the ultraviolet accessible to STIS (1150- 3150 ?), essentially all observed photons originate from the hot component. In total eclipse, a pure emission spectrum is seen, with strong lines from lower levels up to ~2 eV cut by blue-shifted absorption in the M supergiant wind. Out of eclipse, a continuum is observed upon which a rich absorption spectrum of mostly neutral and singly-ionized iron group elements is superimposed. Out of eclipse, the strongest absorption features sit atop the emission profiles seen during eclipse. Far from eclipse, the neutral lines disappear, and the strongest of the remaining absorption lines develop complex inverse P-Cygni profiles. The continuum and spectrally extended blue wings of the inverse P-Cygni spectral lines show a spatial extension characteristic of the HST/STIS response to an unresolved source. However, the emission components of spectral lines lying longward of the blue-shifted wind absorption typically show flux extending beyond the unresolved hot component, implying formation over an extended region much larger than the size of the binary orbit. This extended line emission is believed to be formed by scattering of photons from the hot components continuum by ions in ground and low-lying metastable levels in the extended wind of the M supergiant. Extension is observed out of eclipse for lines with lower levels up to approximately 4 eV, but the greatest degree of extension is observed for lines with lower levels less than 2 eV. Radial velocity differences across the spatially extended emission correlate with the aperture position angle, consistent with model predictions for wind flow in a binary system in which the wind outflow is comparable to the orbital velocity, and indicate an orbital plane oriented from +11? E of N (receding) through ?169? (approaching).

An Ultraviolet Spectral Atlas of VV Cephei during Total Eclipse

The first observations of the ultraviolet spectrum of the binary system VV Cep (M2 Iab + B?) during total eclipse have been made with the HST. A rich emission-line spectrum is seen, with over two thousand emission features present. Shortward of 1600 A, a weak continuum presumably due to Rayleigh scattering of the hot component is present, strengthening markedly shortward of 1500 A. The continuum of the M supergiant becomes apparent longward of 2650 A. Numerous circumstellar and interstellar absorption features are also seen. We present an atlas of the spectrum from 1300-3160 A as observed near midtotality and at two epochs closer to egress, when gas surrounding the hot component has begun to emerge from eclipse and the flux on the short-wavelength edges of many emission lines has increased. This atlas has been annotated with line identifications. The majority of the emission lines arise from singly ionized elements in the iron group with upper levels up to 14 eV, and generally correspond to absorption features seen out of eclipse. Circumstellar and interstellar absorption features seen out of eclipse remain in absorption during totality and are also annotated on the atlas. Well over 90% of the emission features have plausible identifications, but there are many blends. Only a few hundred features appear to be free enough from blending for useful measurement of the peak flux in the line. We present a list of these unblended features.

Ultraviolet Eclipse Observations and Fundamental Parameters of the Binary HR 2554 (G6 II+A1 V)*

HR 2554 is an eclipsing binary system that contains a G6 II primary star and an A dwarf secondary. We have obtained ultraviolet spectra during two eclipses of HR 2554 in 1994 March–April and 1995 May using the HST Goddard High Resolution Spectrograph (GHRS) to study the atmospheric structure of the primary and to better establish the properties of the binary system. We have determined accurate orbital elements for both components, using new and existing optical spectra for the primary and the GHRS spectra for the secondary, in conjunction with eclipse photometry presented in 1992 by Schroder & Hunsch. The derived orbital period is 195.258 ± 0.003 days, the derived radii are 31.3 ± 0.9 R⊙ and 1.9 ± 0.1 R⊙, and the derived masses are 3.14 ± 0.17 M⊙ and 1.98 ± 0.11 M⊙, respectively. The resulting positions of HR 2554 in the Hertzsprung-Russell (H-R) diagram are somewhat inconsistent with the independent evolution of the component stars; in particular, the G star primary is both overluminous and too blue for its mass. The difficulties might be overcome if the metallicity of the system were unusually low ([Fe/H] ~ -1), but this possibility appears to be ruled out by the [Fe/H] = -0.04 determination of Eggen. We suspect HR 2554 has undergone a more complicated evolutionary history involving mass transfer than has been considered previously.

The Special Case of VV Cephei

VV Cephei (M2 Iab + B0–2 V) is the only known ζ Aur binary that has an M-type supergiant. Studies of its atmospheric eclipses provide a valuable extension to what can be learned by eclipse mapping, but are fraught with many challenges. The space-UV absorption spectrum is extremely rich, and line blending is severe. Wind interaction and accretion effects are prominent, and the spectrum of the B star companion is heavily veiled by overlying Balmer continuum emission, which usually exceeds that of the B-star’s continuum flux in the UV. An accretion region immediately around the B star gives rise to a rich spectrum of lines, some of which sometimes show prominent inverse P Cygni profiles. The chromospheric absorption spectrum shows structure in the line profiles that persists throughout chromospheric eclipse, and implies the existence of large-scale supersonic, complex velocity flows across the whole extended chromosphere of the M supergiant. Spectrum formation in the atmosphere of VV Cep is more complex than in the other ζ Aur binaries owing to the much higher rate of mass loss from the primary. This chapter summarizes the fundamental stellar and orbit parameters of VV Cep, and presents key results found from three decades of UV astronomy. It also presents a simple model of the chromosphere and wind.

NLTE radiative transfer in the extended atmospheres and winds of cool stars

The mechanism responsible for driving the ubiquitous winds of cool giant and supergiant stars remains to be established. To this end, we are constructing semi-empirical models of the extended outer atmospheres (‘chromospheres’) and winds of selected red supergiants. These models are constrained by analyses of the UV line spectra of single stars, and of red supergiants in binaries that eclipse their main-sequence companions: the ζ Aur and VV Cep stars. These detached binaries are well-separated, with no evidence of mass transfer. The C II] 2325 A line profiles of the binaries are similar to those of comparable single stars, suggesting that the chromospheres remain relatively unperturbed by binarity. However, it is unclear how much binarity disturbs the wind: binary observations suggest a gradual acceleration (β ∼ 3), but line profile analyses of single red supergiants imply a rapid acceleration (β