Active B Star Commission Proposal

 

 

The IAU Working Group on Active B Stars (WGABS), formerly the WG on Be Stars, was established under Commission 29 at the 17th General Assembly in Montreal, Canada in1979, and has since been in continuous operation. The initial focus on classical Be stars was changed to active (effectively meaning all non-degenerate) B stars at the 22nd General Assembly in The Hague, Netherlands in 1994 to reflect the broader research interests of the membership. Since then, the WGABS has increasingly engaged in an activity level that is more indicative for a Commission than for a WG, including a Newsletter publication beyond a simple abstract service, hosting meetings, and supporting reviews.

 

According to reform guidelines "Commissions are...to be more focused", and "work usefully and effectively together to achieve specific goals, perform significant roles, or for some other particular purpose of interest to the community or to society." The WGABS current activities and future plans express this spirit, as detailed below. We note that our administrative and election procedures, as well as the WGABS IAU membership of about 130 distributed world-wide, already meet the requirements for a Commission. In turn, WGs in the new structure are supposed to have "specific deliverables", i.e. are meant to be more task oriented and of narrower scope than the current WGABS. Given the above and the increasing relevance of Active B Stars for the investigation of physical processes well beyond these objects, as crossroads of many physical processes, the WGABS, currently a WG of IAU Division G, seeks elevation to Commission status.

 

 

 

The study of active B stars includes many focused sub-disciplines including stellar evolution, rotation, pulsations, disk formation, winds, magnetic fields, and binarity. The circumstellar environments surrounding many of these stars offer a valuable probe that can not only help understand the physics for these particular types of stars but can help provide greater understanding about disks and winds in many other astrophysical phenomena including external galaxies. We envision establishing Working Groups focused on the above-mentioned sub-disciplines.

 

Be stars, the original topic of the WGABS, are rapidly rotating stars, intermittently surrounded by a mass-loss disk. Due to their proximity such stars offer excellent test-beds for the study of disks, ubiquitous in astronomical objects. E.g., the evolution (but not formation) of the disks is governed by viscosity. With current capabilities (see supporting material) the viscosity as a function of gas temperature and irradiation from the central star can be studied, which will provide important input physics for the wider astronomical community. Descriptive toy models, not only for the stellar environment but also for the stars themselves, have been superseded by models into which we can incorporate more and more physics.

 

Rapidly rotating stars, of which Be stars are only one class, provide a unique opportunity to study the effects of rotation on stellar evolution. The interplay between these processes, as a star's interior is spun up, is not completely understood, and these stars offer a valuable probe. Also the evolutionary status of many active B stars that populate the upper main sequence is not well understood, and evolutionary connections between these groups, for instance between Be, B[e] and other rapidly rotating B supergiants, are ambiguous.

 

The magnetic B stars hold a unique position, being the only magnetic stars possessing co-rotating magnetospheres. While in earlier stars the vigorous wind disrupts the closed field lines, magnetospheres in later type stars do not form due to the lack of ionization of the circumstellar material. Observed densities are about two orders of magnitudes smaller than predicted by straightforward MHD, pointing to a significant lack of understanding of this fundamental physics. It is among the B stars stars where the most rapidly rotating (non-degenerate) magnetic stars are found, and again in the last decade has seen the advent of increasingly sophisticated physical models.

 

Due to space driven advances in asteroseismology, interest in pulsating B stars has surged. The classes of beta Cephei and slowly pulsating B stars (SPB) are well known, whether Be stars and newly detected early A-type pulsators form extensions of the SPBs is under investigation. While pulsational excitation, non-radial mode selection, and amplitude growth is reasonably understood for slowly rotating B stars, these questions are open for the fast rotators. Advances are expected from both the evolution of the computational technology as well as from future and ongoing space missions. The possibility of investigating the stellar interior of rapid rotators by means of asteroseismology will boost the understanding of stellar evolution for these stars.

 

Also B-type supergiants, being potential distance calibrators, re-gained scientific focus due to space based photometry missions. The pulsations, not being long-term and coherent but nevertheless omnipresent, are expected to affect their stellar wind. A firm confirmation would be, next to magnetic fields discussed above, the second observed mechanism of photosphere-wind connection in early type stars.

 

Considering that more than half of the B stars have a close companion, the importance of binarity in Be phenomenon continues to be debated. O-type subdwarf secondaries that appear to be the stripped-down remnants of earlier mass transfer have already been confirmed around three bright Be stars. Other Be stars are Algol-type binaries. About 60% of high mass X-ray binaries are Be/X-ray binaries, i.e. they contain an active B-type star with a compact companion, usually a neutron star. Measurements of the disk inclination in Be/X-ray and gamma-ray binaries, made possible by insights gained from more nearby Be stars, will improve constraints on the supernova kick velocity distribution. Ongoing collaborations are investigating the stellar/compact companion interactions, identifying the mechanism for particle acceleration, and determining the nature of the compact companions. The recent discovery of the first Be+black hole binary has opened a new dimension for research. These projects promote collaboration between the active B star and high energy astrophysics communities.

 

The common theme, apart from being defined through spectral type and activity, is that their stellar evolution is set apart from both hotter and more massive main sequence objects (which will end as supernovae) and cooler and less massive objects (which possess convective envelopes instead of convective cores) by rather sharp transitions. Equally important, the physical processes governing the circumstellar environment are, of course, as well acting in other stars, but their relative importance in Active B Stars make these stars uniquely suited to study these processes in detail.

 

 

 

The field of active B stars has numerous connections with fundamental physics and other stellar objects, and it is likely that both Commission- and Inter-Commission WGs will be formed. With several of the suggested and existing WGs and Commissions there would be a certain overlap, yet the field of active B stars, as defined above, is sufficiently distinct in terms both observational efforts, in terms of physical phenomena, and theoretical and modeling approaches, to warrant the formation of an independent Commission.

 

 

 

The current WG activities are a baseline on which a future Commission would build and expand. Beyond the sponsoring of IAU supported meetings, the WGABS has coordinated other meetings, supported the publication of two invited reviews, and regularly submits scientific reports to the IAU. The WG's own publication, the Be Star Newsletter (since 1980) is undergoing a transformation from a traditional page and volume based publication into a collaborative resource making use of modern information technology. This includes, for instance, the hosting of conference video recordings to make the talks available to members of the community who were not able to attend, and a forum to discuss current events and ongoing observational campaigns.

 

In the next few years, the Active B Star community plans to hold several meetings, including a proposal submitted for an IAU Symposium 2016 in Brazil. As well in 2016 a workshop with the theme of "disk and winds" is currently in the planning stages, and a focused B[e] workshop is being considered for 2017. We expect that beyond these three also other conference organizers will make use of our offer to host video recordings of talks.

 

Several of the WGABS OC members are currently involved in proposals for new projects (like for the space-based UV-VIS spectropolarimeter Arago) and policy documents (like a whitebook on the "The future of Interferometry in Europe"), for which the input drafts shall be circulated and discussed in the OC and possibly input requested from the community. This is a concrete example of community involvement for the future.

 

The expansion of the Active B Star website will continue with the WGABS site becoming a hub to link research activities in a meta-database for archive and similar data. This includes observational data archives, and as well amateur astronomers contributions, and may include the development of a virtual observatory (VO) interface for observational and model data, for which a funding proposal will soon be submitted.

 

The WGABS has long encouraged the collaboration with amateur communities and their member regularly present public talks to general audiences. As a Commission we would maintain these efforts.