Lee Anne Willson

• Iowa State University

Using the data set of Riebel et al. for 6889 pulsating AGB stars in the LMC, we have derived formulae for mass-loss rate as a function of luminosity and pulsation period, or luminosity and mass, in three ways, for each of five subsets of data: fundamental-mode oxygen-rich stars, first-overtone-mode oxygen-rich stars, fundamental-mode carbon stars,...

Using the Riebel et al. (2012) data set for 6,889 pulsating AGB stars in the LMC, we have derived formulae for mass-loss rate as a function of luminosity and pulsation period or luminosity and mass in three ways, for each of five subsets of data: fundamental mode oxygen rich stars, first overtone mode oxygen rich stars stars, fundamental mode carbo...

On November 15, 1894, Arvid Högbom, geologist, presented a paper at a meeting of the Swedish Chemical Society (“Kemistsamfundet) in Uppsala. His title: “On the probability of secular changes in the atmosphere’s carbonic acid concentration”. The possibility that changes in the carbon dioxide concentration would produce changes in the surface tempera...

We present one-dimensional hydrodynamical models that suggest a new mechanism for generating spiral structures around evolved stars. In these models, the pulsation of the star initiates the mass outflow and generates shock waves from a static atmosphere. An orbiting companion perturbs the outflow, creating a long-period series of shocks with differ...

Mass loss on the AGB removes most of the envelope and leaves a compact remnant to become a white dwarf and perhaps first the central star of a planetary nebula. The envelope mass provides an upper limit on the material available to form the PN, and the terminal mass loss rate plus the small remnant mass left on the core determines how much of that...

Mira variables share essential characteristics: High visual amplitude, periods of hundreds of days, red colors (spectral types M, S, and C), and the presence of emission lines at some phases. They are fundamental mode pulsators, with progenitor masses ranging from >1 to several solar masses. In this review, we summarize what is known from modeling...

(Abstract only) Planetary nebulae are the result of mass loss from an AGB star (specifically, a Mira variable or post-Mira infrared source) that is swept up by a later fast wind and/or ionized when the central star becomes hot. The central stars of planetary nebulae are the naked cores of the former AGB star. Not all AGB stars form PNe, however, an...

We present the initial-final mass relation derived from 10 white dwarfs in wide binaries that consist of a main sequence star and a white dwarf. The temperature and gravity of each white dwarf was measured by fitting theoretical model atmospheres to the observed spectrum using a χ 2 fitting algorithm. The cooling time and mass was obtained using th...

Stars that have reached the tip of the asymptotic giant branch (AGB) are as large and luminous as they can ever be. The mass loss time scale is shorter than the luminosity evolution time scale for mass loss rates exceeding 5x10-6(M/MSun) solar masses/year. At the same time the mass outflow is visible to observers since the column density is high. W...

To investigate the importance of physical processes in the stellar atmosphere on the mass loss rates of AGB stars, we have run a substantial grid of dynamical atmosphere models using a code that approximates non-LTE, dust formation, and radiative transfer via one or two parameters each, and using R(L, M, Z, l/H) to investigate the importance of low...

We present some 1-D hydrodynamical models that are capable of generating ring structures around evolved stars. In these models, the pulsation of the star is considered. It initiates the flow and generates shock waves from a static atmosphere. A secondary period is introduced by an orbiting low mass companion. It creates a series of shocks with diff...

This is the fourth paper in a series of multi-epoch observations at 7 mm wavelength of the SiO masers in several asymptotic giant branch stars from a sample of Mira variable stars showing evidence of asymmetric structure in the infrared. These stars have been observed interferometrically in the infrared by IOTA and with VLBA measurements of the SiO...

This is the third paper in a series of multi-epoch observations at 7 mm wavelength of the SiO masers in several asymptotic giant branch stars from a sample of Mira variable stars showing evidence of asymmetric structure in the infrared. These stars have been observed interferometrically in the infrared by Infrared Optical Telescope Array and with V...

Theoretical studies of mass loss from evolved stars have either emphasized detailed dust nucleation and growth or non-equilibrium physics. Bowen (1988ApJ...329..299B) showed that with non-LTE coupling to the radiation field, slower cooling and reheating, and thus departure from radiative equilibrium during compression and expansion, a refrigerated...

This is the second paper in a series of multi-epoch observations of the SiO masers at 7 mm wavelength in several asymptotic giant branch stars from a sample of Mira variable stars showing evidence of asymmetric structure in the infrared. These stars have been observed interferometrically in the infrared by the Infrared Optical Telescope Array and w...

This is the first paper in a series of multi-epoch observations of the SiO masers at 7 mm wavelength in several asymptotic giant branch (AGB) stars. This is a sample of Mira variable stars showing evidence of asymmetric structure in the infrared which were observed interferometrically in the infrared by Infrared Optical Telescope Array and with Ver...

We look to observations to tell us the dependence of mass loss rates on stellar parameters - L, R, M, T_{eff}, and Z - but what observations of mass loss rates tell us instead is the parameters of stars that have reached an interesting rate of mass loss. In order to find the dependence of the mass loss rates on stellar parameters - the slopes d log...

In the concluding session of this conference, the participants met to discuss the directions that the ``cool star'' research community should take over the next decade. The authors were the moderators of this session.

The Bowen code, developed in the 1980s and 1990s, was optimized for studying the mass loss mechanism as a system. It allowed inclusion of pulsation with shock development, non-LTE coupling of the gas to the radiation field and departures from radiative equilibrium, grain formation and acceleration by radiation, and radiative acceleration on molecul...

Planetary nebulae are formed by illumination of material lost during the Mira evolutionary phase together with a smaller amount of material coming from a later, fast wind. While textbooks tell us this is the fate of all intermediate-mass stars, there are problems in accounting for (a) the rapid evolution in effective temperature required for this t...

It has long been clear that most, if not all, of the mass loss experienced by stars from 0.8 to 8 solar masses occurs near the tip of the AGB and/or the RGB. Evolutionary studies have incorporated empirical mass loss laws but theoretical models suggest quite different dependence of mass loss rate on stellar parameters. Here we explore what happens...

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 obser...

Recent observations of mass loss from a fraction of the stars along the RGB in 47 Tuc are at odds with our understanding of single-star mechanisms for mass loss as these kick in only at or above the tip of the RGB. The observations are well fitted by mass loss rates matched to the evolutionary time scale for the stars (dlnM/dt ≈ dlnR/dt), sugges...

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 asymmet...

Many formulae have been proposed to describe how stars lose mass. Most of them were determined by fitting observational data, but a few developed from theoretical models. We may define the deathline as where , where X is a variable that follows the evolutionary track. For the stars near the tip of the asymptotic giant branch (AGB) this may be expre...

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...

The well-observed semiregular variable RU Vulpeculae has undergone a substantial change in period over the past fifty-five years. The discovery period of ~155 days has undergone a continuous change to its current value of 108 days. The amplitude and stability of the light curve have changed as well; the pulsations are much less regular and have a l...

Observations tend to select mass loss rates near the critical rate, Ṁcrit = M /L. There are two reasons for this. In some situations, such as near the tip of the AGB, the mass loss rate is very sensitive to stellar parameters. In this case, stars with Ṁ ≪ Ṁcrit have dust-free, hard-to-measure mass loss rates while stars with Ṁ ≫ Ṁcrit do not surviv...

It has long been clear that most, if not all, of the mass loss experienced by stars from 0.8 to 8 solar masses occurs near the tip of the AGB and/or the RGB. Evolutionary studies have incorporated empirical mass loss laws but theoretical models suggest quite different dependence of mass loss rate on stellar parameters. We are combining evolutionary...

We report imaging observations of the symbotic long--period variable R

Longtime Harvard Curator of Astronomical Photographs and AAVSO officer Martha Hazen passed away on 23 December 2006 at Hingham, Massachusetts, after a short illness due to acute myelogenous leukemia. One of four children of Harold Locke and Katherine (neé Salisbury) Hazen, Martha was born in Cambridge, Massachusetts, on 15 July 1931, and raised in...

Mass loss rate formulae are derived from observations or from suites of models. For theoretical models, the following have all been identified as factors greatly influencing the atmospheric structure and mass loss rates: Pulsation with piston amplitude scaling appropriately with stellar L; dust nucleation and growth, with radiation pressure and gra...

From modeling and observations of mass loss from cool stars we have learned that the mass loss rate can be a very steep function of stellar parameters; that when this is the case, selection effects dominate empirical mass loss formulae; and that a variety of formulae tested against observed mass loss rates give the same answer for ``which stars are...

Optical interferometric observations of Mira stars with adequate

The potentially observable consequences of the pulsation/rotation-induced mass loss from main-sequence A and F stars proposed by Willson et al. (1987) are discussed, reviewing the results of recent investigations. Particular attention is given to (1) evidence for a deficiency in A stars and an excess of F and G stars, as predicted by the theory, (2...

It is not yet possible to construct models that include details of all processes that play a role in Asymptotic Giant Branch tip mass loss. However, the very sensitivity of mass loss rates to stellar parameters, that causes problems for modellers, also provides the means for synthesizing populations, because we can interpret observed mass loss rate...

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 b...

Pulsation in one or many radial and/or non-radial modes may be a universal characteristic of stars. The possible consequences of pulsation include an increase in the atmospheric scale height (levitation), the heating of a region of the atmosphere to a temperature well in excess of the radiative equilibrium temperatur, and the driving of substantial...

We have run models of intermediate mass stars (5, 6, 7, and 8 M with Y=0.28, Z=0.02) with pulsationally driven mass loss occurring in the Cepheid instability strip. We used the new 12C(,)16O rates of Caughlan et. al. (1985). The enhanced rate extends the tip of the blue loop, allowing the 5 and 6 Mp models to re-enter the Cepheid strip, unlike the...

This review stresses four ways that pulsation affects the atmosphere of a star with consequences for the appearance of the star. These are: Motion in the form of standing waves, traveling waves, and shocks; Extension or levitation of the atmosphere; Heating and cooling of the gas; Mass loss. The motion is directly observable (at least in some parts...

Janet Mattei appears among the authors on over 70 papers concerning Mira variables or related classes of red variables. She contributed to the planning or execution of some of the research in this area, but more often appeared as representative for the thousands of AAVSO observers world wide. Janet also helped to start several observational program...

Stellar evolution theory predicts that asymptotic giant branch stars undergo a series of short thermal pulses that significantly change their luminosity and mass on timescales of hundreds to thousands of years. Secular changes in these stars resulting from thermal pulses can be detected as measurable changes in period if the star is undergoing Mira...

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We investigate the small class of long-period Mira variables exhibiting two distinct maxima during each pulsation cycle, and compare them to the more general case of Miras with humps or bumps in their light curve. We present a list of known or suspected Miras with double maxima and their light curves, describe their general characteristics, and con...

As director of the American Association of Variable Star Observers for thirty years, Janet Mattei led the organization through a series of major improvements and in the process, helped and influenced amateur and professional astronomers around the world. Having successfully tackled many challenges, however, Janet lost her battle with acute myelogen...

We have carried out a program of hydrodynamic modeling to study the interactions between brown dwarfs or massive planets and the extended, oscillating atmospheres of Mira parent stars. We also consider observational consequences, including optical microvariability and effects on SiO masers.

The AGB terminates with an episode of mass loss lasting a few times 105 years. The mechanism for this mass loss is pulsation combined with radiation pressure on grains for most stars with metallicity greater than about 30% of solar. The mass loss rate increases very steeply with increasing luminosity and radius and decreasing mass. The mass loss ra...

Numerical models of the dynamically extended atmospheres of long-period variable or Mira stars have shown that their winds have a very simple, power-law structure when averaged over the pulsation cycle. This structure is stable and robust despite the pulsational wave disturbances, and appears to be strongly self-regulated. Observational studies sup...

We present numerical hydrodynamical modelling of the effects of a giant planet or brown-dwarf companion orbiting within the extended atmosphere and wind formation zone of an approximately solar-mass Mira variable star. The large-scale, time-dependent accretion flows within the radially oscillating and outflowing circumstellar gas around Miras are r...

Speakers for this session were selected according to our perceptions about the frontier areas in dynamical atmospheres, the atmospheres of very cool stars, and stellar winds. This summary of posters and topics raised in the round table discussion opens with a brief summary of current issues in modeling pulsating stars and winds. Observational const...

Short time scale variations have been reported for a few Miras, including sudden 0.2 magnitude or more brightening in the visual lasting from a few hours to a few days. Are these flashes real? Are they hot? What are they? Almost all the natural time scales for variation in the atmosphere and wind of a Mira variable are months to years long. What co...

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 which may give rise to observable optical events and affect SiO maser emission. Comment: 4 pages, 3 figures, fol...

We've been working on a series of web-based courses (www.polaris.iastate.edu) on gasic astronomical concepts, such as coordinate systems and the diurnal motion of the sky. I will discuss the philosopy (sic) behind this project and show one or two examples of what we have developed so far.

This summer three undergraduate students worked with me on three projects using AAVSO light curves together with infrared light curves from the South Africa Astronomical Observatory. Sean Patterson, Bradley University junior, worked on O-C diagrams at different wavelengths, and in the process of doing this he rediscovered the Harrington effect. Car...

A new three-dimensional hydrodynamics code is under development to model the propagation of acoustically generated waves in stellar atmospheres, in particular the solar atmosphere. These waves are generated using an artificial pulsing boundary, representing the top of the convection zone. The resulting shock waves and subsequent oscillations and at...

This review emphasizes the mass loss processes that affect the fates of single stars with initial masses between one and nine solar masses. Just one epoch of mass loss has been clearly demonstrated to be important for these stars; that is the episode that ends their evolution up the asymptotic giant branch. Quite a clear picture of this evolutionar...

Over the past couple of decades, many astronomers have looked for formulae relating mass loss rates to the properties of stars. Such "mass loss relations" have been derived from observed mass loss rates and from theoretical computations. To compare these relations, the predicted mass loss rate vs. luminosity for each is found using a single set of...

What do AAVSO's favorite stars have to do with the ultimate fate of the earth? We have been learning about the most extreme conditions that the planets will encounter as the Sun ages. In about 7 Gyr, when the Sun is a Mira, the Earth will most likely be in serious trouble. Some results of recent calculations are presented showing what is most likel...

If production in stellar flares can be ruled out as a significant source of the Li, Be, and B isotopes observed in stellar atmospheres, then observed abundances provide very stringent constraints on stellar mass loss before and during the main sequence phase. Also, stellar surface abundances of these isotopes are often invoked as constraints on Big...

Stellar pulsation is associated with some of the most dramatic epochs of mass loss from stars. Detailed numerical modeling calculations that reproduce the observed characteristics of such mass losing stars necessarily include a number of key physical processes. The reader is referred to detailed calculations of dynamical atmospheres for Mira variab...

Red variables are traditionally classified into Mira, semiregular (SR), and slow irregular (L) variables. The Mira variables are the best defined subgroup, whereas SR and L stars are more numerous. The SR subgroup is additionally subdivided into: SRa variables, which feature regular variability with smaller pulsation amplitudes than Miras; SRb vari...

An investigation of the atmospheric structure of non-Mira, asymptotic giant branch stars through NLTE radiative transfer modeling applied to hydrodynamic models is discussed. Synthetic spectra resulting from these calculations were compared with IUE observations of these stars to test the validity of the models. The development of the hydrodynamic...

Consistently with a trend observed in recent past triennial reports, progress in the theory of stellar atmospheres continues to be made in two different directions: (i) the traditional areas of continuum and line radiation transfer, line blanketing, atomic physics and atmospheric structures controlled by the joint conditions of radiative and hydros...

Detailed calculations of the mass loss process for pulsating red giants illustrate clearly the pitfalls in the use of empirical mass loss laws, including the popular Reimers' relation and the more recently developed relation for long period variables by Vassiliadis and Wood (Ap. J., 413, 641). When studied in detail, mass loss rates prove very sens...

Measurement of observable quantities of stars, variable or otherwise, is only part of the process of understanding their nature. First of all, quite a lot of theory-based analysis goes into a measurement of a desired basic property for the star (such as mass, radius, or luminosity). Second, theoretical models are used to check relationships among t...

ROSAT PSPC pointed observations of PU Vulpeculae on 1992 November 10-12 UT give a count rate of 4.00 +- 0.75 X 10^-3 cts/s in the energy range from 0.1 to 1.8 keV. The count rate outside this energy range is approximately zero. An analysis of the energy spectrum, with the requirement that the result be consistent with the observed reddening of abou...

In populations with ages between about 1 and 10 Gyr, the brightest stars include vigorously pulsating and mass-losing Miras. Their location at the tip of the AGB coincides closely with the superwind phase that ends their ascent. The (L, T, M) where the superwind sets in is governed by a complex interplay of physical effects, as has been demonstrate...

We have carried out numerical hydrodynamic / thermodynamic modeling calculations for the atmospheres of pulsating stars with Cepheid-like parameters. These prove to have a complex structure that includes multiple shocks and an extended outer region with coronal temperatures and densities. In this paper we describe non-LTE radiative transfer calcula...

We present a movie showing the time evolution of NLTE synthetic spectra representative of Mira-type variable stars, e.g. S Car (K7--M4 IIIe). The spectra of hydrodynamic models were calculated using PANDORA. The screen is formatted into an upper and a lower panel for comparisons between different thermodynamic and spectral features. The movie begin...

Within about a month after its 1988 outburst, AS 296 showed a strong continuum, emission lines of singly ionized metals, and deep absorption lines also of the ionized metals. The emission lines indicate an expansion velocity of only about 50 km/s. The absorption lines are those of a high-density shell as indicated by the presence of Mg II, 448 I A,...

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 solar masses, and their periods from 150 to 800 days. All were fundamental-mode pulsators, had solar metallicity, and included effects...

Results are presented of an IRAS survey of main-sequence B, A, and F stars, based on three primary sources as a data base: the Bernacca-Perinotto Catalog of Stellar Rotational Velocities, the Michigan Spectral Catalog, and the Bright Star Catalog. The stars in the data base are divided into four categories: (1) main-sequence single stars, (2) main-...

Both the amplitude and period of the light variations of the semiregular long-period variables RV And, S Aql, and U Boo dramaticaly decreased, and then suddenly increased, in recent years. These observations, in conjunction with spectroscopic data, suggest that these stars may have switched from fundamental to overtone pulsation and back again. The...

A database of IRAS observations, vsini measurements, and visual spectral types/optical colors to use to test the hypothesis that pulsation and rotation contribute to mass loss along the main sequence was developed. If the hypothesis of main sequence mass loss linked to rotation and pulsation is correct, then these infrared excesses should correlate...

CO radio map data are presented for the bright carbon stars R Scl, U Ant, S Sct, and TT Cyg. The stars are found to have large circumstellar envelopes which, at least in the last three cases, are geometrically thin and clearly detached from the stars, indicating that the mass loss has occurred episodically. The possibility that these episodes are t...

This is the second in a series of meetings organized jointly by Iowa State University and the University of Trieste. The theme for these workshops is “dynamical phenomena in stars and circumstellar envelopes”. Here I review the justification for the series and for the topic selected for this meeting, and note some relevant recent developments in ou...

The focus of this meeting was on the mass and angular momentum evolution of stars that on the main sequence have spectral types earlier than F, i.e. those stars that are mostly rapidly rotating. Because the mass loss and angular momentum history of a star determine the present mass and angular momentum, the topics included pre-main sequence, main s...

The possibility that main sequence stars of spectral types A and F could lose evolution-altering amounts of mass due to a combination of pulsation and rotation has been suggested by Willson, Bowen, and Struck-Marcell (=WBS, 1987). These stars would evolve down the main sequence on a timescale that is less than or on the order of the nuclear evoluti...

In an effort to understand the formation of non-spherical morphologies, we are studying the possibilities for efficient transfer of the bulk of a star’s angular momentum to its planetary nebula. The kinematics of planetary nebula (PN) NGC 7009 (“the Saturn Nebula”) have been mapped using an echelle spectrograph (single-slit mode), with a 3” spatial...

Fundamental unsolved problems of stellar astrophysics include the effects of angular momentum on stellar structure and evolution, the nature and efficiency of the processes by which angular momentum is redistributed within and lost from stars, and the role that stellar rotation plays in enhancing or driving stellar mass loss. There appears to be a...