Chromospheres and Winds of Cool Supergiants

TALK (Keynote)

Philip D. Bennett 1, 2

1Eureka Scientific, Inc.

2Saint Mary's University

Massive stars lose material via stellar winds on evolutionary timescales in the supergiant phases of their lives. In particular, stars between about 3 and 40 M(sun) spend a significant fraction of their post-main sequence lifetime as red supergiants of spectral classes G-M. The upper limit to red supergiant masses appears to be constrained by mass loss that increases rapidly with stellar mass. Although mass loss from red supergiants has been observed since the 1930s, we still don't understand the mechanism that drives it. Unfortunately, most observations of mass loss from supergiant stars include the entire, unresolved, circumstellar envelope, and provide no spatial information, especially about the inner part of the wind where mass loss starts. One way to achieve spatially resolved observations is to observe the select group of red supergiants with main-sequence companions (typically B stars) in eclipsing orbits. As the hot, and much smaller (in physical size, not mass) companion moves behind the supergiant during eclipse ingress, the line of sight sweeps through successively deeper layers of the supergiant's extended outer envelope. This circumstellar envelope superimposes an absorption upon the continuum of the hot companion, and this "chromospheric" eclipse spectrum can be used to infer the density, velocity and ionization state along the line of sight. By repeated observation of the spectrum of the binary through eclipse, the structure of the supergiant's outer atmosphere can be derived. All of this was realized decades ago. But, despite this potential, the early promise of the binary method to reveal atmospheric structure in red supergiants has never been fully achieved. There are many reasons for this: the additional data reduction complication caused by the need to separate composite spectra, the uncertain perturbing effect of the companion on the structure of the circumstellar envelope being probed, the difficulty of removing light scattered by the circumstellar envelope into the line of sight (which appears as emission), and sometimes, because of the sheer complexity of the chromospheric absorption line spectrum observed (e.g. VV Cephei). The situation has improved considerably with the advent of ultraviolet observations from space, which mostly obviate the need to disentangle composite spectra. The International Ultraviolet Explorer has provided a huge amount of spectroscopic data of limited resolution and signal-to-noise, while the Hubble Space Telescope GHRS and STIS spectrographs have obtained a limited number of observations of superb spectral resolution and signal-to-noise. The Far Ultraviolet Spectroscopic Explorer has also contributed results shortward of Lyman-alpha. In this talk, I will summarize the present state of the binary method, and present observations, results and prospects for several red supergiant binaries.

The Very Strange Star U Antlia = HR 4153

TALK (Contributed)

William P. Bidelman 1, Charles Cowley 2

1Case Western Reserve University

2University of Michigan

U Antlia is one of the brightest carbon stars in the sky. It is classified as an N0 carbon star and an Lb irregular variable. This star has a very unique spectra which will be discussed in this presentation. CO radio line maps of this star (Oloffson et.al, 1996, Astron. & Astrophys, 311, 587) show that this star has a detached circumstellar shell which is expanding at 18.1 km/s and is relatively young (4000 years old).

The Stellar Imager (SI) - A Mission to Resolve Stellar Surfaces, Interiors, and Magnetic Activity

TALK (Keynote)

Kenneth G. Carpenter 1, Karel Schrijver 2, Margarita Karovska 3, and the SI Vision Mission Team

1NASA-Goddard Space Flight Center

2Lockheed Martin Advanced Technology Center

3Smithsonian Astrophysical Observatory

The Stellar Imager (SI) is a UV/Optical, Space-Based Interferometer designed to enable 0.1 milli-arcsecond (mas) spectral imaging of stellar surfaces and, via asteroseismology, stellar interiors and of the Universe in general. The ultra-sharp images of the Stellar Imager will revolutionize our view of many dynamic astrophysical processes by transforming point sources into extended sources, and snapshots into evolving views. SI's science focuses on the role of magnetism in the Universe, particularly on magnetic activity on the surfaces of stars like the Sun. SI's prime goal is to enable long-term forecasting of solar activity and the space weather that it drives. SI will also revolutionize our understanding of the formation of planetary systems, of the habitability and climatology of distant planets, and of many magneto-hydrodynamically controlled processes in the Universe. SI is included as a "Flagship and Landmark Discovery Mission" in the 2005 NASA Sun Solar System Connection (SSSC) Roadmap and as a candidate for a "Pathways to Life Observatory" in the NASA Exploration of the Universe Division (EUD) Roadmap (May, 2005). In this paper we discuss the science goals and technology needs of, and the baseline design for, the SI Mission (http://hires.gsfc.nasa.gov/si/) and its ability to image the Biggest, Baddest, Coolest Stars.

Statistical Study of Over 100 Mira Variables with PTI

TALK (Keynote)

Michelle J. Creech-Eakman 1, Robert R. Thompson 2

1New Mexico Tech., Socorro, NM

2CHARA Array, Georgia State Univ., Mt. Wilson Observatory, CA

Optical interferometry is revolutionizing our understanding of manyareas in stellar evolution. Nowhere is this more evident than amongthe large, cool stars, which are easily accessible to most ground-basedinterferometers shortly after achieving first light. However, thethe real trick to observing these objects is to get dedicated observing time,especially for monitoring changes in variable sources like miras.Starting in 1999, we undertook a 5-year monitoring campaign of approximately 100 mira variables with the Palomar Testbed Interferometer.The data represent about 25 nights/year of observations over the rangeof chemical subtypes with pulsation periods varying from 200-500 days.With these data we have identified clear evidence for pulsation, foundmolecular layers whose apparent position depends upon opacity/chemistry,and can begin to make statistical statements about pulsations and the phaserelationship between optical and near-infrared light curves. Thesenew findings will be shown in light of other information being gatheredby interferometers world-wide to set the stage for a consistent pictureof their pulsational behavior.

Photometric Variability in Post-AGB Stars


Gary D. Henson, W. Ryan Deskins

Department of Physics, Astronomy, and Geology, East Tennessee State University

At the end of the asymptotic giant branch (AGB) stage of stellar evolution the mass loss rate has risen to such high values that virtually the entire hydrogen-rich envelope is expelled from the central star. The mass loss ends and by definition the star now enters its post-AGB (or proto-planetary nebula) phase. The youngest post-AGB stars are not well studied since they are generally highly obscured by their circumstellar shells. In some cases, the heavily obscured post-AGB stars are most likely the more massive stars which could be evolving rapidly to the PN stage. For the later spectral type and more heavily obscured stars, little is known about the presence and nature of any variability. We have been performing BVRI photometry on a select group of post-AGB stars and a few other unique infrared sources of interest. Variability, with a large range of amplitudes and time scales, is clearly present in most sources, but not all. We will summarize the results of approximately three years of photometric monitoring for this sample of stars.

From Biggest and Coolest to Smallest and Hottest: Proto-Planetary Nebulae Transitional Objects

TALK (Keynote)

Bruce J. Hrivnak

Valparaiso University

Proto-planetary nebulae (PPNe) are objects in transition between the AGB and PN phases of stellar evolution. During this short-lived (few thousand year) phase, the extended atmosphere of the star detaches and the star is surrounded by an expanding envelope of gas and dust. This dust absorbs and obscures the visible light from the star, but it re-radiates it in the thermal infrared. PPNe were first found in significant numbers in the mid-1980s by analyzing the IRAS database. Over the last decade, high-resolution spectroscopy has revealed the atmospheric abundances of the central stars and high resolution imaging with the Hubble Space Telescope has reveled the morphology of the envelope. Similarly, mid-IR spectroscopy has reveled the chemistry of the dust and mid-IR imaging has shown directly its location. This talk will review the properties of PPNe, which consist of cool (G), large stars surrounded by their expending detached envelopes. Emphasis will be on new results for near and mid-IR imaging and spectroscopy, including Spitzer Space Telescope data. This research is supported by the NSF and by NASA funding through the JPL to support Spitzer observations.

Studying the Pulsational Properties of Proto-Planetary Nebulae


Bruce J. Hrivnak, Wenxian Lu

Valparaiso University

We have carried out a 13 year study of light variability in 24 PPNs. We find that all of them vary in light. Periods are found for eight of these, ranging from 44 to 140 days. Others show longer-term variability and others shorter-term variability without clear periodicity. This variability is shown to be due to pulsation in the stars. We will display the light curves and discuss what we learn about PPNs from these results. This work is supported by grants from NSF and the Indiana Space Grant Consortium.

Polarization from the Structured Envelopes of Cool Evolved Stars


Richard Ignace, Gary D. Henson

Department of Physics, Astronomy, and Geology, East Tennessee State University

Cool evolved star atmospheres and wind flows are dynamic, and there are signifcant sources of scattering opacity, from free electrons in chromospheres to Rayleigh scattering by molecular opacity to dusty winds. Motivated by various evidence for aspherical radiative and density distributions, we discuss models to describe fixed and variable polarimetric signals from unresolved sources of this stellar class. For optically thin scattering, the polarization is dominated by essentially two factors, one that describes the envelope structure in terms of geometry and another that sets the overall amount of scattering material. We consider the extent to which current and future observing facilities will be capable of measuring predicted polarization levels, and how such measurements can aid our understanding of processes governing the dynamics occurring in the atmospheres and circumstellar environs of these stars.

On the Distribution of Dust in the "Born-again" Planetary Nebula A30


Florian Kerber 1, M. Roth 2, T. Rauch 3, N. Ageorges 4, G.C. Clayton 5, O. De Marco 6, J. Koller 7

1European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany

2Las Campanas Observatory, Carnegie Institution of Washington, Colina El Pino, Casilla 601, La Serena, Chile

3Institut fuer Astronomie und Astrophysik, Eberhard-Karls-Universitaet, Sand 1, D-72076 Tuebingen, Germany

4European Southern Observatory, Av. Alonso de Cordova 3107, Santiago, Chile

5Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA

6American Museum of Natural History, New York, NY 10024, USA

7Los Alamos National Laboratory, Space Science and Applications, Los Alamos, NM 87545, USA

The planetary nebula (PN) A30 consists of two nebular shells, one old, spherical, hydrogen-rich PN and a second, younger, hydrogen-poor, and dust-rich nebula which is the result of a very late thermal pulse (VLTP), a helium-shell flash that occured long after the central (CS) star had left the asymptotic giant branch (AGB). During the VLTP the CS returned to the AGB and became a "born-again" giant for a few years. During this extremely fast episode of stellar evolution a final mass-loss phase created the second, dusty PN a few thousand years ago. Such a VLTP should occur in 20% of all post-AGB stars according to theory but only a handful of "born-again" PNe are known, a discrepancy that remains unexplained so far. In the case of A30 the "born-again" PN is highly filamentary and the individual knots clearly show signs of erosion from the fast wind of the - yet again - hot CS, such as "cometary" tails. While optical imaging ([OIII]-gas emission) obtained with the HST has provided excellent spatial resolution, near infrared imaging (emission from dust) had been very limited in resolution so far. With our new PANIC/Magellan data we quite literally see the other side of the coin and as a consequence, for the first time we are able to shed light on this complex interplay between gas and dust in this PN. We will use the data to better understand the effect of single-photon heating of small dust grains by the hard UV-radiation field of the CS. A30 forms an evolutionary sequence with V4334 Sgr (10 yrs after the flash) and V605 Aql (100 yrs) and, hence, provides valuable insight into the physics of the still poorly understood "born-again" PNe.

IRS Spectra of Unusual Evolved Objects in the Magellanic Clouds


Kathleen E. Kraemer 1, G. C. Sloan 2, J. Bernard-Salas 2, P. R. Wood 3, S. D. Price 1, A. A. Zijlstra 4, M. P. Egan 5, and J. R. Houck 2

1Air Force Research Laboratory

2Cornell University

3Australian National University

4University of Manchester

5National Geo-Spatial Intelligence Agency

We present mid-infrared spectra from evolved sources in the Large and Small Magellanic Clouds (LMC and SMC) observed with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. Many targets in the project were selected based on their mid-infrared colors, with little additional information available, and in this sample, we observed more carbon stars than originally anticipated. We also observed several unusual objects. Two sources in the SMC have nearly featureless mid-IR spectra and are thought to be R Coronae Borealis candidates, only a handful of which are known in the SMC. We have identified a post-asymptotic giant branch (post-AGB) object in the SMC which shows a rare combination of spectral features, and in the LMC we find that an object typically described as a planetary nebula is actually still in the short-lived, post-AGB phase of stellar evolution. We also describe a newly detected feature at ~14 microns often associated with strong silicate emission or absorption.

The Coolest Stars in the Clouds: Unusual Red Supergiants in the Magellanic Clouds

TALK (Keynote)

Emily M. Levesque 1, P. Massey 2, K. A. G. Olsen 3, B. Plez 4

1Institute for Astronomy, University of Hawaii

2Lowell Observatory

3National Optical Astronomy Observatories

4GRAAL, Universite Montpellier II

Red supergiants (RSGs) are a He-burning phase in the evolution of moderately high mass stars (10-25 solar masses). The evolution of these stars, particularly at low metallicities, is still poorly understood. The latest-type RSGs in the Magellanic Clouds are cooler than the current evolutionary tracks allow, occupying the region to the right of the Hayashi limit where stars are no longer in hydrodynamic equilibrium. We have discovered four Cloud RSGs in this region that display remarkably similar unusual behavior. All of them show considerable variations in their V magnitudes and effective temperatures (and spectral types). Two of these stars, HV 11423 and [M2002] SMC 055188, have been observed in an M4.5 I state, considerably later and cooler than any other supergiant in the SMC. These stars suffer dramatic physical changes on timescales of months - when they are at their warmest, they are also brighter, larger, more luminous, and show an increased amount of extinction. This variable extinction is characteristic of the effects of circumstellar dust, and can be connected with sporadic dust production from these stars in their cooler states. We suggest that these unusual properties are indicative of an unstable (and short-lived) evolutionary phase not previously associated with RSGs, and consider the implications such behavior could have for our understanding of the latest stages of massive star evolution in low-metallicity environments.

Of Photons, Gas, and Dust: The Mira Mixmaster

TALK (Keynote)

Donald G. Luttermoser

Department of Physics, Astronomy, and Geology, East Tennessee State University

The physical processes in the atmospheres of asymptotic giant branch (AGB) stars have many important attributes. Their large mass-loss rates impact their evolution and enriches the metal content in our galaxy. Virtually all AGB stars vary in brightness, which includes the Mira-type variables. The "mira" variability is thought to be caused by pulsations which produce a 'shocked' atmosphere. Excitation and ionization collisional rates are small in comparison to radiative rates, due to the low densities, which makes the LTE approximation often invalid. The cool atmospheric temperatures of these stars allow molecules and dust to form which further complicates the picture -- the formation of these species may not be in equilibrium either. Atmospheric modeling of these stars has followed two different approaches: (1) the hydrostatic (HS) method and (2) the hydrodynamic (HD) method. Each of these techniques has its limitations. Dust is often seen in these stars and dust formation is an important component to the chemistry of these atmospheres and to the mass loss. This talk will highlight what has been done to date in modeling the atmospheres of these stars and make suggestions as to what should be done in future modeling attempts.

A High-Resolution UV Spectral Atlas for Mira Variable Stars


Donald G. Luttermoser

Department of Physics, Astronomy, and Geology, East Tennessee State University

The ultraviolet (UV) spectra of Mira variable stars have been studied for over two decades. The International Ultraviolet Explorer (IUE) space telescope observed these stars at both low and high dispersion. Later the Hubble Space Telescope (HST) obtained high-dispersion spectra both with the High Resolution Spectrograph (HRS) and the Space Telescope Imaging Spectrograph (STIS). This poster displays a STIS spectrum of the cool Mira-type variable star R Leo taken on 31 December 1998. On this date R Leo was at phase 0.37 in its light-curve cycle. This spectrum shows a large number of emission lines and identifications are made for nearly 200 of these features. Many of these emission lines were previously unrecorded in IUE and HRS spectra of Miras, such as the Fe II (UV33, 35, 158, 160, 161, 180, 181), Mn II (UV38), V II (UV43, 73), Zr II (UV58), and the Ni II (UV36) multiplets. The electron density diagnostic multiplet of C II] (UV0.01) gives an electron density of 109 cm-3 for R Leo at this phase. This is similar to the electron density found for the Mira star R Hya at phase 0.26 obtained with a HST/HRS spectrum. Finally, the photospheric spectrum was detected from 2980 Angstroms (the long wavelength cut-off) down to 2450 Angstroms. Prominent absorption features in this photospheric spectrum also are identified.

The Physical Properties of Red Supergiants

TALK (Keynote)

Philip Massey 1, Bertrand Plez 2, Emily M. Levesque 3, K. A. G. Olsen 4, D. R. Silva 5, G. C. Clayton 6

1Lowell Observatory

2GRAAL, Universite de Montpellier II, CNRS

3Institute for Astronomy, University of Hawaii

4Gemini Science Center, National Optical Astronomy Observatory

5Thirty Meter Telescope

6Department of Physics and Astronomy, Louisiana State

University Red supergiants (RSGs) are an important but poorly understood phase in the evolution of massive stars. Such stars are neither the most massive nor the most luminous, but they are the physically largest, and provide a critical test of stellar evolutionary theory. For many years, the "observed" placement of RSGs in the H-R diagram was at odds with evolutionary models. Using the new generation of MARCS stellar atmosphere and newly acquired spectrophotometry, we have determined the effective temperatures, bolometric luminosities, and effective stellar radii for a large sample of RSGs in the Milky Way, Magellanic Clouds, and M31. Our results now bring the physical properties of these stars into much better agreement with the Geneva evolutionary tracks, although there is clearly some unexplained problems at the lowest metallicities. We find that RSGs have higher reddening than neighboring OB stars, and suggest in that in extreme cases circumstellar dust provides up to a couple of magnitudes of extra V- band extinction. This is in accord with our expectations given the observed mass-loss rates and condensation radii. In the solar neighborhood AGBs contribute most of the dust to the ISM, but in metal-poor galaxies at large look back times we expect that RSGs are a major contributor. We will summarize the results of the past few years, as well as providing a look at our most recent work on M31 RSGs.

Post-Red Supergiants

TALK (Keynote)

Rene Oudmaijer 1, Ben Davies 2, Willem-Jan de Wit 1, Mitesh Patel 3

1School of Physics and Astronomy, University of Leeds, UK

2Center for Imaging Science, Rochester, USA

3Astrophysics Group, Imperial College, London, UK

The yellow hypergiants are found in a stage between the massive Red Supergiants and the Wolf-Rayet stars, objects thought to be Gamma Ray Burst progenitors. This presentation will address current issues concerning the evolution of massive stars, concentrating on the transitional post-Red Supergiant phase. Few yellow hypergiants are known and even fewer show evidence for having evolved off the Red Supergiant branch. Indeed, only two such rare objects with a strong infrared excess due to circumstellar dust, the remnant of the previous mass losing phase, are known, IRC +10420 and HD 179821. The talk will review their properties, discuss their real-time evolution and the geometry of their circumstellar material. Finally, recent results employing near-infrared interferometry, integral field spectroscopy and polarimetry will be presented.

Globular Cluster Age Determination Using the Red Giant Branch Luminosity Function

TALK (Keynote)

Nathaniel Paust

NASA/Space Telescope Science Center, U.S.A.

While isochrone fitting is the routine method used to determine cluster ages, it is not the only possible method. Indeed recent observations of M92 (NGC 6341), M3 (NGC 5272), M13 (NGC 6205), and M14 (NGC 6402) have shown that the shape of the red giant branch (RGB) luminosity function (LF) can be used to simultaneously determine the distance modulus and age of clusters with gigayear precision. In addition to revealing these ages, LF fitting combined with a Monte Carlo approach can reveal the resulting age uncertainty which results from uncertainties in reaction rates, opacities, and other inputs into stellar evolution models.

Photometric Variability of Pulsating Red Giants

TALK (Contributed)

John R. Percy

Department of Astronomy and Astrophysics University of Toronto, Canada

I will review the many forms of photometric variability of pulsating red giants (PRGs), including the recent and current work of my group. The topics include: (i) identification of periods from long-term photometric monitoring, using Fourier and self-correlation techniques; (ii) multiperiodicity of PRGs, including its application to mass determination; (iii) pulsation modes in PRGs, determination, systematics, and comparison with results from large-scale surveys; (iv) the nature and cause of the irregularity of PRGs; (v) amplitude variations in PRGs; (vi) the nature and cause of the long secondary periods in PRGs; (vi) very long-term variability in PRGs. Much of the data that we use is collected with small photometric telescopes, either robotic, or operated by skilled amateur astronomers through the AAVSO; by merging data from these two sources, we can work with datasets of 20 years or more. And most of the analysis has been done by undergraduate research students, who derive important educational benefits. Acknowledgement: supported by the Natural Sciences and Engineering Research Council of Canada.

Mass Loss from Evolved Stars in Local Group Galaxies: The View from the Spitzer Space Telescope

TALK (Keynote)

Gregory C. Sloan

Cornell University

The Spitzer Space Telescope has carried out spectrographic surveys of evolved stars in the Magellanic Clouds and several other Local Group galaxies, covering a range of ages, initial masses, and metallicities. I will describe these surveys, which are dominated by carbon stars in the final superwind phases of their evolution. The Spitzer spectroscopy has confirmed that the mass loss during this evolutionary phase depends strongly on metallicity, which has significant implications for the quantity and composition of mass returned to the interstellar medium and also for supernovae rates in metal-poor environments.

Warm Era COBE DIRBE Infrared Light Curves of Dusty Evolved Stars


Beverly J. Smith1, Stephan D. Price2

1Department of Physics, Astronomy, and Geology, East Tennessee State University

2Air Force Research Laboratory

We have extracted post-cryogen infrared light curves of evolved stars from the DIRBE instrument on the COBE satellite, extending the cryogenic light curves an additional 2.77 years. These warm-era data have not previously been exploited. We present some example post-cryogenic DIRBE light curves, and compare with optical light curves from the AAVSO and other sources.

The Circumstellar/Interstellar Boundary Around Cool Evolved Stars

TALK (Keynote)

Robert E. Stencel

University of Denver, Dept. Physics & Astronomy

The mapping of a circumstellar-interstellar bow shock around the AGB star, R Hya, using Spitzer MIPS 70 micron imaging (Ueta et al. 2006) has transformed our perception of how shells merge with the galactic environment. This helps place claims from the IRAS era in context with exciting new far infrared images being collected with the AKARI sky survey and pointed observations.

The Wilson-Bappu Effect - 50 Years Later


Robert E. Stencel

University of Denver, Dept. Physics & Astronomy

Wilson and Bappu (1957) published an empirical correlation between the FHWM of the emission core of the CaII K-Line at 393nm and the intrinsic luminosity among late-type dwarf, giant, and supergiant stars. Later on, Stencel (1977) extended this luminosity calibrator by using so-called wing emission lines found in the wings of the H and K lines. Efforts to extend these techniques to the brightest supergiants in local group galaxies were frustrated by the limits of photographic coude spectra at the time, even on 4-meter telescopes. With the advent of CCD spectroscopy and S/N possible with 8-meter telescopes, we here explore the potential for extragalactic hypergiant star distance calibration.

Giant Stars, Astrophysics, and Amateur Astronomy

TALK (Contributed)

Matthew R. Templeton

American Association of Variable Star Observers, 49 Bay State Road, Cambridge, MA 02138

Organized variable star research organizations have existed for over a century now, and thousands of observers world-wide contribute hundreds of thousands of multiwavelength observations every year. The data archives of organizations like the AAVSO are a rich resource for data-mining, and thanks to technology now include both visual data and multiwavelength, high-precision, calibrated photometry, accessible in (near) real-time. Here, I will highlight some of the new and growing observing programs of the AAVSO including a revitalized photoelectric program and a nascent near-IR photometry group. I will also highlight some historically underutilized data archives for classes of interest to the cool giant star community.

Interferometric Observations of Supergiants: Direct Measures of the Very Largest Stars

TALK (Contributed)

Gerard T. van Belle

Michelson Science Center, California Institute of Technology

I will present angular diameters for 42 luminosity class I stars and 32 luminosity class II stars that have been interferometrically determined with the Palomar Testbed Interferometer. Derived values of radius and effective temperature are established for these objects, and an empirical calibration of these parameters for supergiants will be presented as a functions of spectral type and colors. For the effective temperature versus V-K color, I find an empirical calibration with a median deviation of delta T = 70K in the range of 0.7 < V-K < 5.1 for LC I stars; for LC II, the median deviation is delta T = 120K from 0.4 < V-K < 4.3. Directly determined diameters range up to 400 R(sun), though are limited by poor distance determinations, which dominate the error estimates.

Big, Cool, and Losing Mass: Dependence of Mass Loss Rates on L, R, M and Z

TALK (Keynote)

Lee Anne Willson

Iowa State University

We look to observations to tell us the dependence of mass loss rates on stellar parameters - L, R, M, Teff, 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 dlogMdot/dlogL, dlogMdot/dlogM, and dlogMdot/dlogR - we need to look to other kinds of data, such as the distribution stars vs. L or Mdot or R, or the duration of the mass loss process. Mass loss from the biggest, coolest stars is highly sensitive to the stellar parameters, and this results in a narrow "death-zone" of mass loss rates sufficient to strip the star of its envelope. Recognizing this pattern, we can draw some strong conclusions about mass loss on the AGB, and also on the RGB, for various populations.

The Biggest Stars of All

TALK (Keynote)

Robert F. Wing

Ohio State University

The classical method of estimating the sizes of stars, which dates back to the early use of the Hertzsprung-Russell diagram, employs spectroscopic indicators of surface temperature and luminosity. The largest stars are those closest to the upper-right "corner" of the HR diagram. Although recent technological advances have provided direct measurements of the sizes of several very large stars, and computational advances have placed the atmospheres of normal cool supergiants on a secure footing, the classical approach is still useful because it can easily be applied to stars too faint for direct measurements and too extreme for representation by models. I will give examples of stars that appear to be larger than any that have been measured directly or modelled successfully. The main problem with the classical approach is that the calibrations of spectroscopic temperature and luminosity criteria are of unknown validity when extrapolated to extreme cases.

Evolved Stars and Superwinds in Metal-Poor Galaxies in the Local Group

Albert A. Zijlstra 1, M. Matsuura 2, E. Lagadec 1, G.C. Sloan 3

1University of Manchester, Manchester, UK

2National Astronomical Observatory of Japan, Tokyo, Japan

3Cornell University, Ithaca, NY, USA

TALK (Keynote)

Most stars end their lives with a phase of spectacular mass loss, the so-called superwind. The physics of this process is still poorly understood. It determines the final mass of stars and, as a consequence, the lower mass limit of supernovae. Theoretical models suggest that this process is strongly dependent on metallicity, but before now, this has not been confirmed observationally. We have conducted observations of these superwind stars in the Magellanic Clouds, the Sagittarius dwarf elliptical galaxy, the Fornax dwarf spheroidal, and other Local Group galaxies, using the IRS on the Spitzer Space Telescope and the VLT in Chile. These systems enable a comparison of late stellar evolution, mass-loss rates, and dust production over a range of metallicities, ages, and progenitor mass.