IAU Colloquium No. 175:
"The Be Phenomenon in Early-Type Stars"

Give me fruitful error any time, full of seeds, bursting with its own correction. --- Italian economist Pareto to Kepler I. Introduction Be stars are nonsupergiant B stars which at some time have displayed emission in their Balmer lines. They are comprised of at least three subgroups: "classical," Herbig Be, and interacting binaries. Among the classical Be subgroup (putatively single, post-ZAMS), the early and late Be stars seem to show somewhat differing properties and to have an unclear mutual relationship. With some oversimplification, the early classicals exhibit strong and variable winds and a tendency for rapid (Prot) spectral and light variability. The ``Be phenomenon" is the episodic ejection of mass, as evidenced by emission in the optical and IR emission lines of hydrogen and less excited ions. The ejected mass stabilizes generally into flattened decretion disks. Among the early-classicals the formation of variable Discrete Absorption Components in the UV resonance lines is also in important attribute. Although Be stars stand out by their comparatively high rotational velocities, their mean rotation rates are actually only 1/2-2/3 of the "critical" velocities at which centrifugal force balances gravitation at the equator. Thus, the 1931 Struve picture of surface instabilities caused by a combination of centrifugal and evolutionary expansion forces is inadequate to explain mass loss from these stars, except in close binaries where contemporaneous mass exchange occurs. The key problems in understanding this phenomenon are how to eject quantities of mass in a nonsteady fashion and how to enhance the specific angular momentum of ejected material so it can attain a stable orbit. These issues continue to challenge and motivate researchers on Be and related groups of variable hot stars. A great deal of study has focused on the early-classicals because of their rapid variability. IAUC No. 175, entitled "The Be Phenomenon among Early-Type Stars," was held in Alicante, Spain on 28 June-2 July, 1999. It was dedicated to the careers of Drs. Arne Slettebak and Carlos & Mercedes Jaschek. Sadly, the first two scientists died suddenly several weeks before the meeting. Our sadness about their passings was helped by the our memories of the many happy personal and professional times we had with these remarkable people and also by the recollection that one of the two Slettebak children married the Jaschek's daughter. Thus, in a real way this conference was an additional child to both families (as Mirek Plavec once put it, a case of astronomical incest -- but we are quite sure the good kind!). The conference was sponsored by the Departments of Astronomy of the Universities of Valencia and Alicante, Spain and the Círculo Astronómico del Mediterráneo. The LOC chair was Dr. Juan Fabregat. The Scientific Organizing Committee, chaired by the author, consisted of 11 members, including astronomers from four continents, one woman, and a number of young astronomers. The meeting was attended by some 130 participants from 30 countries and six continents. The proceedings will be published in the ASP Conference Series and will be edited by M.A. Smith, H. Henrichs, and J. Fabregat. On the afternoon immediately following the close of the meeting, Dr. R. Linfield chaired a "rump" session to exchange information between the Be community and the NASA/JPL "Space Technology-3" (ST-3) project on spaceborne interferometry. Over the last 25 years there have been three IAU meetings on Be stars: IAUS No. 70 (Bass River, Mass., 1975), IAUS No. 98 (Munich, 1981) and IAUC No. 92 (Boulder, 1986). Recent IAU-sponsored meetings on related topics were held last year in Heidelberg and in 1993 in Juan les Pins, France. Several other international meetings have been held during this interval in a variety of venues. II. Recent Progress on Be Stars Since the 1993 meeting substantial progress has been made in understanding Be stars, Be disks, and Be outbursts. Recent developments include the following: results from recent infrared (ISO) and X-ray (Rosat, ASCA, RXTE, SAX) satellites, the establishment of the IUE Final Archives with NEWSIPS data. Moreover, data from old (IUE, Copernicus and Voyager) and new (Astro & ORFEUS) UV missions are becoming available at the STScI MAST center, construction of optical echelle spectrometers and their application to multi-line time-series observations, the imaging of Be star disks and the development of the first spatio-kinematic models, the maturing of the 1-armed spiral disk models to a degree sufficient to be tested and to be compared with other dynamical models of disks. For example, the just cited imaging of disks is consistent with Keplerian orbits and contradicts the large outflow velocities predicted by the Wind Compressed Disk model, extension of wind theory to 2.5 dimensions and the addition of density clumping to HD models, the widespread (but not universal) recognition that nonradial pulsations (NRPs) are active in some or many early-classical Be stars, the development and distribution of computer programs to compute time sequences of line profile variations due to NRPs. The code includes rotational and thermodynamic effects in the modal eigenfunctions, the observed correspondence between coadded velocity amplitudes of nonradial pulsation (NRP) modes and outbursts in the B2e star Cen. The possible semiregularity of outbursts in 1 or 2 other early Be stars has also been raised, the entrance into the field of a number of young astronomers with an unusually well developed array of talents. III. Scientific Highlights of the Conference The summary below is undoubtedly colored by the author's own perceptions. Other participants may have come away from the meeting with different impressions. a) General Highlights An underlying theme of the meeting was that the Be phenomenon becomes less bewildering if one considers the star and disk together as a mutally interacting system. Thus, the "New Missions" session was important in demonstrating how multiwavelength, polarization, and imaging studies can lead to converging pictures of well observed objects as dynamic entities. In addition, to comprehend the observations may well require theoretical progress in a wide variety of areas such as radiative transfer (over many wavelength regimes), stellar interiors and evolution, hydrodynamics, the dissipation of magnetic free energy, and nonlinear, nonadiabatic nonradial pulsations. A noteworthy attribute of the Alicante meeting was the devoting of a session to discussing evidence for and attributes of magnetic fields. Speakers discussed physical conditions leading to flares on other stars, the development of theory of regenerating magnetic fields applicable to Be stars, and indirect or direct evidence for magnetic fields in a few Be stars. A significant announcement was of the direct detection of a variable magnetic field in one Be star (see below). Ironically, another talk underscored that it will be difficult in the near future to detect even moderately-sized fields (kG) on rapidly rotating stars. Another fascinating report by two groups was that the percentages of Be stars among B stars vary in Galactic and Magellanic Cloud clusters, increasing with age and decreasing metallicity. It is not yet clear from these new findings whether a given B star's probability of becoming a Be star is affected by age or if more rapidly rotating stars are formed in low metallicity protoclusters. b) Putting It All Together (Well, not quite yet...) For the last session the SOC developed a list of several questions to guide general audience discussion on two key topics: (1) which mechanism(s), NRP, magnetism, and/or "other," were responsible for the atmospheric instabilities leading to the Be phenomenon, and (2) are the disk kinematics consistent with Keplerian orbits? (The answer seems to be yes to question 2.) The first question was preceded by a canvassing of audience opinion about the mechanism(s) that produces the Be phenomenon in isolated Be stars. At the 1986 Boulder meeting, the overwhelming majority of the participants expressed their opinion through a vote that at least three mechanisms must be present to explain the Be phenomenon. At Alicante in 1999 almost half the participants believed that two processes might be sufficient to explain the episodic mass expulsions of single Be stars. In 1986 almost no one believed that a single mechanism was sufficient whereas in Alicante 8 out of 100 believed that magnetism (all magnetic advocates, I believe) is somehow responsible. These simple figures do not reflect the wide diversity of opinion, from staunch advocates of either of the two camps asking for a more compelling need before a second mechanism is posited on one hand, to the opinion by a slim majority that it was too early to venture an opinion on the other. Of course scientific paradigms are not settled by such votes, but the pursuit of discovery, the construction and use of instrumentation, and the drive of this work into neighboring subfields all are influenced by the sentiment they reflect. As such, this snapshot of the Be community's opinion is of more than casual interest. Certainly, it represents some shift from a more conservative opinion of the past. In this reviewer's opinion, the increased acceptance in 1999 of a role for magnetic fields in classical Be stars is a confluence of several factors: the announcement in the meeting of a modulating magnetic field in the slowly rotating Be star  Cephei by Zeeman observations. The period of this field, 12 days, matches the rotation period suspected from earlier UV studies, aperiodic line profile transients and, in two stars, X-ray flares indicate that high energy processes can be present. Such processes cannot easily be explained by the tapping of a thermal energy pool in the photosphere, the observation of dynamic corotating clouds over  Cas and possibly other Be stars. Such structures are observed over many He-rich Bp stars which are known to be magnetic. Yet one wonders: could a slight hint of optimism be illusory? No one believes that a rash of magnetic field detections are forthcoming. c) What's Next?: Narrowing the Options Many participants believed that there need not be a competition between NRP and magnetic fields. Perhaps pulsations triggers the ejection, but then magnetism supplies the necessary energy and facilitate the angular momentum transfer to a disk, a popular speculation. It is also conceivable that energy and angular momentum are provided in indirect ways. For example, several participants suggested that toroidal or tesseral NRP modes can produce vorticity. Vorticity might regenerate magnetic fields from an initial fossil field. This too is speculation, but if it is on the right track there could be a large payoff in terms of a more generalized dynamo theory. In order to separate fact from conjecture, a number of observational issues must be settled. One of them is the correct interpretation of the frequency difference of Be stars in clusters. Second, it is critical to discriminate between the efficacy of magnetism and pulsation to generate time-dependent mass loss. Third, new multi-line echelle observations of spectral line profile ``transients" may be applied to compare the signatures in low and highly excited ions of ejected mass events: does such mass return?, is it organized into collimated flows? Fourth, most of the ejected mass into disks is still not seen, so one needs to learn how to compare the continuous and discontinous components of mass loss during an outburst. Fifth, new work needs to be carried out on NRPs and/or magnetic-proxy signatures in Bn stars. Sixth, clues provided by the rapid spectroscopic variability appear not to be present for the late-Be stars, so will we still find diagnostics by observing these stars more intensively? Seventh and finally, the interacting Be binaries provide not only unambiguous glimpses into stellar evolution but also evidence of processes which may still find a place in the understanding of the single Be stars. Near/mid-infrared and radio observations can continue to probe the extent and kinematics of Be disks not visible by H emission. These will provide us a better picture of both their kinematic and thermodyamic properties with distance and latitude from the star. They will also tell us whether our "snapshots" of Be stars from 1867 to 1999 are indicative of their mass loss over far longer timescales. Perhaps they will soon provide an answer to another key question: what fraction of initially ejected matter is returned to the star's surface and how steadily it is ejected - questions about which, remarkably, we have few clues. The exquisite agreement of the position angles of the major axes of Be disks determined from both optical interferometry and polarimetry has demonstrated the accuracy and utility of both techniques. One can expect a rich harvest of information on the interaction of the star and disk will be obtained from next generation interferometric projects, both on the ground (NPOI, VLTI, Keck, CHARA) and in space (ST-3). As an example, the detection or nondetection of a void between the star and the disk would address the issues of whether significant matter falls back to the star from the disk and provide clues to the transfer of angular momentum to the disk. IV. Conclusions Two ways of judging a meeting's success are by the participation by the people who actually do research (generally the younger astronomers) and by the percentage of the total participants still attending the final session. By both yardsticks the meeting was a success. Participants were urged to go back to their local communities and discuss Be stars with their colleagues in the context not of one more type of peculiar star but as objects which show evidence of just about every physical processes thought to be operating in other nondegenerate stars across the HR Diagram. For that matter, the timescales and wavelength ranges over which variability is seen are similar to those in Active Galactic Nuclei. This conference was a culmination of work between the SOC and LOC for over two years. I would like to thank each of the members of both committees, the speakers, and a group of six poster paper summarizers who were instrumental in bringing new results learned from the posters to the general session.

Last modified: August 2, 1999