Lee Anne M. Willson

From wind to superwind : the evolution of mass-loss rates for Mira models

Dynamical atmosphere models were calculated for a large grid of variables with Mira-like properties satisfying the Iben radius-luminosity-mass relationship for evolving AGB stars. Their masses ranged from 0.7 to 2.4 M ○. , and their periods from 150 to 800 days. All were fundamental-mode pulsators, had solar metallicity, and included effects of dust. As a natural consequence of evolutionary changes in stellar parameters, the mass-loss rate increases as an approximately exponential function of time

First Surface-resolved Results with the Infrared Optical Telescope Array Imaging Interferometer: Detection of Asymmetries in Asymptotic Giant Branch Stars

We have measured non-zero closure phases for about 29% of our sample of 56 nearby Asymptotic Giant Branch (AGB) stars, using the 3-telescope Infrared Optical Telescope Array (IOTA) interferometer at near-infrared wavelengths (H band) and with angular resolutions in the range 5-10 milliarcseconds. These nonzero closure phases can only be generated by asymmetric brightness distributions of the target stars or their surroundings. We discuss how these results were obtained, and how they might be interpreted in terms of structures on or near the target stars. We also report measured angular sizes and hypothesize that most Mira stars would show detectable asymmetry if observed with adequate angular resolution.We have measured nonzero closure phases for about 29% of our sample of 56 nearby asymptotic giant branch (AGB) stars, using the three-telescope Infrared Optical Telescope Array (IOTA) interferometer at near-infrared wavelengths (H band) and with angular resolutions in the range 5-10 mas. These nonzero closure phases can only be generated by asymmetric brightness distributions of the target stars or their surroundings. We discuss how these results were obtained and how they might be interpreted in terms of structures on or near the target stars. We also report measured angular sizes and hypothesize that most Mira stars would show detectable asymmetry if observed with adequate angular resolution.

Comparison between mass-losing and standard solar models

The effects of main-sequence mass loss on the evolution, interior structure, and surface composition of the sun are studied. Modeling results are presented for three mass-losing solar models of initial mass two solar, plus a standard one solar mass comparison model. It is shown that the solar models can be evolved to produce present-day solar models that do not differ radically in structure and composition from standard solar models. The mass-losing models require low initial He-4 abundances and predict high surface He-3 abundances that are near the limits, yet still within the bounds, of observational estimates. The mass-losing models have somewhat higher B-8 neutrino fluxes, but standard solar models also fail to reproduce observations by a wide margin. For mass-losing models, protosolar Li and Be are completely destroyed, requiring a mechanism for partial replenishment to observed surface abundances. 34 references.

Angular momentum and mass loss for hot stars

Why a Meeting on Angular Momentum and Mass Loss for Hot Stars?.- Angular Momentum and Mass Loss and Stellar Evolution.- The Effects of Rotation on Stellar Structure and Evolution.- New Evolutionary Aspects of Mass Loss and Angular Momentum.- Mass Loss During the Evolution of Massive Stars.- Pre-Main Sequence Stages.- Angular Momentum Loss in Pre-Main Sequence Objects and the Initial Angular Momentum of Stars.- The Herbig Ae and Be Stars: Mass and Angular Momentum Losses.- Eccentric Spiral Modes in Disks Associated with Young Stellar Objects.- Evidence that Wolf-Rayet Stars are Pre-Main Sequence Objects.- A and F Stars near the Main Sequence.- Rotation, Pulsation and Atmospheric Phenomena in A-Type Stars.- Pulsation Studies of a 1.8 M? Delta Scuti Model.- Post-Main Sequence Evolution of Binary Am Stars.- A Statistical Study of Main Sequence A and F Stars: Testing the Main Sequence Mass Loss Hypothesis.- O, B and Be Stars.- Basic Magnetic Rotator Theory with Application to the Angular Momentum Driven Winds of B[e] and Wolf Rayet Stars.- The Connection between Rotation and the Winds of Be Stars.- UV Glimpse of OB Stars.- Nonspherical Radiation Driven Wind Models Applied to Be Stars.- A Simple Criterion to Identify Rapidly Rotating Stars Viewed at Small to Intermediate Inclination Angles.- Intensive Photometric Campaign on Be Stars: Behaviour of Short-Term Periodic Variations and its Relationship to Pulsation and Mass Loss.- Constraints on the Thickness of Be Star Disks Derived from Combined IR Excess and Optical Polarimetry Data.- On the Correlation between Pulsation Amplitude and Shell Activity in the Be star ? Eridani.- B[e] Supergiants: Continuum Polarization by Electron Scattering in Rotationally Distorted, Radiation Driven Stellar Winds.- Rotational Evolution of Hot Stars due to Mass Loss and Magnetic Fields.- New Facts About the Variability of 45 ? Persei.- Long-Term Study of Stellar-Wind Variability of O Stars.- HeII ?1640 as a Diagnostic for Assessing the Extent of Rapid Rotation in Be Stars.- Some Examples of the Role of Stellar Rotation in Hot Star Winds.- HD 193077 -- A Fast Rotating Wolf-Rayet Star.- How Effective is Rotation in Enhancing the Rate of Mass Loss in Early Type Stars?.- Rotation and Pulsation-Mode-Selection in B-Type Stars.- The Angular Momentum-Loss and the Differential Rotation in B and Be Stars.- Very Luminous and Very Massive Stars.- The Role of Axial Symmetry in the Upper Part of the HRD: B[e] Supergiants and LBVs.- Effects of Mass Loss on Late Stages of Massive Star Evolution.- V444 Cygni and CQ Cephei, Key Wolf-Rayet Binary Stars.- Rotation of Hot Stars After They Cool Off.- Winds, Mass Loss and Rotation in Central Stars of Planetary Nebulae.- White Dwarf Mass Loss, Rotation, Individual Masses and the Identification of the White Dwarf Remnants of Upper Main Sequence Stars.- Evolved Stars as Probes of Main Sequence Angular Momentum and Mass Loss.- The Bizarre Kinematics of Planetary Nebula NGC 7009, and Some Thoughts on the Transfer of Stellar Angular Momentum to Planetary Nebulae.- Chromospheric H? Activity in ? ORI.- Stellar Winds in A-Type Supergiants.- Special Section: A Debate Concerning the Nature of Wolf-Rayet Stars.- The Evidence that Wolf-Rayet Stars are in a Late Stage of Evolution.- Why Wolf-Rayet Stars Should Not Be Considered To Be Evolved Cores of Massive Stars.- Author index.- Keyword index.- Astronomical index.

Secular Evolution in Mira Variable Pulsations

Stellar evolution theory predicts that asymptotic giant branch (AGB) stars undergo a series of short thermal pulses that significantly change their luminosity and mass on timescales of hundreds to thousands of years. These pulses are confirmed observationally by the existence of the short-lived radioisotope technetium in the spectra of some of these stars, but other observational consequences of thermal pulses are subtle and may only be detected over many years of observations. Secular changes in these stars resulting from thermal pulses can be detected as measurable changes in period if the star is undergoing Mira pulsations. It is known that a small fraction of Mira variables exhibit large secular period changes, and the detection of these changes among a larger sample of stars could therefore be useful in evolutionary studies of these stars. The American Association of Variable Star Observers (AAVSO) International Database currently contains visual data for over 1500 Mira variables. Light curves for these stars span nearly a century in some cases, making it possible to study the secular evolution of the pulsation behavior on these timescales. In this paper we present the results of our study of period change in 547 Mira variables using data from the AAVSO. We use wavelet analysis to measure the period changes in individual Mira stars over the span of available data. By making linear fits to the period versus time measurements, we determine the average rates of period change, d ln P/dt, for each of these stars. We find nonzero d ln P/dt at the 2 σ significance level in 57 of the 547 stars, at the 3 σ level in 21 stars, and at the level of 6 σ or greater in eight stars. The latter eight stars have been previously noted in the literature, and our derived rates of period change largely agree with published values. The largest and most statistically significant d ln P/dt are consistent with the rates of period change expected during thermal pulses on the AGB. A number of other stars exhibit nonmonotonic period change on decades-long timescales, the cause of which is not yet known. In the majority of stars, the period variations are smaller than our detection threshold, meaning the available data are not sufficient to unambiguously measure slow evolutionary changes in the pulsation period. It is unlikely that more stars with large period changes will be found among heretofore well-observed Mira stars in the short term, but continued monitoring of these and other Mira stars may reveal new and serendipitous candidates in the future.

First images of R aquarii and its asymmetric H2O shell

We report imaging observations of the symbiotic long-period Mira variable R Aquarii (R Aqr) at near-infrared and radio wavelengths. The near-infrared observations were made with the IOTA imaging interferometer in three narrowband filters centered at 1.51, 1.64, and 1.78 μm, which sample mainly water, continuum, and water features, respectively. Our near-infrared fringe visibility and closure phase data are analyzed using three models. (1) A uniform disk model with wavelength-dependent sizes fails to fit the visibility data, and is inconsistent with the closure phase data. (2) A three-component model, consisting of a Mira star, water shell, and an off-axis point source, provide a good fit to all data. (3) A model generated by a constrained image reconstruction analysis provides more insight, suggesting that the water shell is highly nonuniform, i.e., clumpy. The VLBA observations of SiO masers in the outer molecular envelope show evidence of turbulence, with jetlike features containing velocity gradients.

Continuous and burst-like accretion on to substellar companions in Mira winds

ABSTRACT We present numerical hydrodynamical modeling of the effects of a giant planetor brown dwarf companion orbiting within the extended atmosphere and wind for-mation zone of an approximately solar-mass Mira variable star. The large-scale, time-dependent accretion flows within the radially oscillating and outflowing circumstellargas around Miras are related to Bondi-Hoyle-Lyttleton flows, but have not, to ourknowledge, been previously modelled. The new models presented in this paper illus-trate the changes in accretion and wake dynamics as the companion mass is variedover a range from 10 to 50 Jupiter masses (M J ), and generalize the results of thesingle model we presented in an earlier paper.The character of the accretion onto the companion changes greatly as the com-panion mass is increased. At the lowest companion masses considered here, a lowcontinuous rate of mass accretion is punctuated by large, nearly periodic bursts of ac-cretion. When the companion mass is large, the mass accretion has both a continuouspart, and a rapidly varying, nearly stochastic part. Surprisingly, the angular momen-tum of the accreted gas shows an opposite trend with mass, varying nearly periodicallyat large companion masses, and stochastically at low masses. These trends can be un-derstood as the result of the interplay between the shocks and radial oscillations inthe circumstellar gas, and the wake flow behind the companion. Boundary conditionsalso affect the character of the accretion. The equation of state, however, is found tohave little effect, at least for gamma-law gases, with gamma in the range from 1 to5/3.Models with accretion bursts may produce observable optical brightenings, andmay affect SiO maser emission, as we suggested in previous papers. Interruptions ofcontinuous accretion, or shadowing effects, could give rise to bursts of dimming inthe optical. Such dimming effects are likely to be correlated with bursts and opticalflashes, helping to explain some rather mysterious Hipparcos observations.Key words:stars: variables: stars – AGB and Post-AGB – accretion, accretion discs – masers –circumstellar matter – planetary systems: stars – winds.

Models of Planets and Brown Dwarfs in Mira Winds

We present numerical hydrodynamical models of the effects of planets or brown dwarfs orbiting within the extended atmosphere and wind formation zone of Mira variables. We find time-dependent wake dynamics and episodic accretion phenomena that may give rise to observable optical events and affect SiO maser emission.

THE STRUCTURE AND KINEMATICS OF THE ENVELOPE AROUND U ORI FROM IOTA OBSERVATIONS

Many of the Mira stars observed with adequate spatial resolution show detectable asymmetry. This asymmetry can be caused by an asymmetric stellar photosphere and/or asymmetric envelope around the star and can be the origin of asymmetries in the subsequent planetary nebula. In this paper, we present the results of long baseline interferometric observations of the Mira-type star U Ori at 1.51 (H2O band), 1.64 (pseudocontinuum), and 1.78 (H2O band) μm in 2005. We performed model-independent image reconstruction of the envelope around the star using measured visibilities and closure phases. The images show asymmetric structure of the U Ori envelope that is similar to the structure of 22 GHz H2O masers obtained by Vlemmings et al. in 2003. Further comparison of near infrared images with available radio maps gives some evidence for differential rotation of the envelope with rotational velocities varying between 3 and 5 km s–1. Finally, we discuss the geometric and kinematic structure of the U Ori envelope based on a model of an almost face-on expanding and rotating disk around the star.

An IRAS survey of main-sequence B, A, and F stars

Results of an IRAS survey of main-sequence B, A, and F stars are presented. During the search of the IRAS Point Source Catalog, 686 high quality positional correlations with IRAS point sources were found from the survey database of 7871 stars. The IRAS flux densities associated with these stars were color corrected and reduced to magnitudes from which infrared color excesses could be determined. Four groups of stars are studied ; (1) main-sequence, single stars, (2) main-sequence, close binary stars, (3) subgiants, and (4) spectrally peculiar stars. 20% of the first group stars show an infrared color excess in at least one of the 12, 25, or 60 μm IRAS bands.