December 2021
Speaker: Dr. Luqian Wang
Title: Search for hot subdwarf companions of rapidly rotating Be stars
Abstract: Be stars are B-type main-sequence stars, and their spectra display Balmer emission features that are resulted from the circumstellar decretion disk. Be stars are also featured by their near-critical rotation. Growing evidence suggests that the rapid rotation is likely a consequence of close binary interaction such that the massive formal component of a binary system stripped of its envelope as it evolved, and the gainer star spun up from a past mass transfer scenario through a Roche-lobe overflow. The evolutionary avenue for such a binary system may end up with a now-observed rapidly rotating Be star with an evolved, stripped, He-core burning companion star (sdO). By utilizing the far-ultraviolet (FUV) spectroscopy from the International Ultraviolet Explorer (IUE), we detected the hot sdO companions in Be binaries of HR 2142 and 60 Cygni. Further searches for the sdO companion of 264 Be stars from the IUE spectroscopy led to the detection of an additional twelve Be+sdO binary candidate systems, which corresponds to a ~6% detection rate among the sample. We confirmed the signature of the subdwarf companions in ten Be+sdO binaries and measured their physical parameters from the Hubble Space Telescope/STIS FUV spectroscopy. Follow-up spectroscopy is needed to complete the orbital and physical solutions to portrait the evolutionary history of these Be+sdO binary systems, and such information is important to constrain the binary formation scenario for rapidly rotating Be populations.
November 2021
Speaker: Dr. Barnali Das
Title: Coherent radio emission as a unique probe for hot magnetic stars
Abstract: Magnetic early-type stars are characterized by the presence of large-scale surface magnetic fields that lead to the formation of a magnetosphere around the stars. This magnetosphere is the site of production of several phenomena that generates radiation over a wide range of the electromagnetic spectrum. Among them, the most recently discovered (and hence least understood) phenomenon is the generation of coherent radio emission that is observed as periodic pulses over sub-GHz to GHz frequencies. We recently coined the name of 'Main sequence Radio Pulse emitter’ (MRP) for the hot magnetic stars that can emit radio pulses. The mechanism by which they produce radio pulses is the electron cyclotron maser emission (ECME), which is also the mechanism behind the Earth’s auroral kilometric emission. In my talk, I will describe the unique characteristics of MRPs that set them apart from other ECME emitting objects. The highlight of my talk will be the different information which one can extract about the host star from the observation of ECME. This includes (but not limited to) inference of rotation period evolution, and estimation of plasma density and distribution in the stellar magnetosphere. In addition, I will also talk about radio flares and 'giant pulse’ that we have recently observed from a MRP at sub-GHz frequencies. I will discuss the possible source of these flares and their implication about the magnetic field-stellar wind interaction.
October 2021
Speaker: Dr. Anahí Granada
Title: Revisiting the angular momentum loss rates for Be stars in the single star scenario.
Abstract: Rapid surface rotation is key for the Be phenomenon to develop, though how close to critical is still under debate. The interplay between rapid stellar rotation and other mechanisms allow mass and angular momentum to be transferred outwards and eventually develop a viscous decretion disk surrounding the central star. In order to investigate the mass and angular momentum loss rates in critically rotating B stars, in 2013 we combined the results from stellar evolution calculations and simple disk prescriptions to provide tables with the loss rates we obtained. Since then, different studies in the literature showed that these values may overestimate the angular momentum loss rates derived from observations by more than one order of magnitude. In this presentation I will show that neither improving the precision of angular momentum conservation within the star nor exploring different transport coefficient efficiencies in the evolutionary calculations modifies our 2013 results significantly. In the light of accumulating evidence indicating that many Be stars rotate somehow below the critical limit, we propose to use a new simple parametric prescription for mechanical mass loss that leads to smaller angular momentum loss rates, in better agreement with data available in the literature. This kind of prescription could also help to understand the mechanisms involved in the appearance of the Be phenomenon.
August 2021
Speaker: Dr. Robert Klement
Title: Interferometric View on Classical Be Stars - Revealing Close Binarity
Abstract: Both optical/IR and radio interferometry have been instrumental in revealing the nature of classical Be stars. For example, the angular resolution of the edge-on disk of psi Per achieved by the Very Large Array at centimeter wavelengths was the first direct confirmation of a flattened envelope. With optical interferometry, Be stars and their disks are routinely resolved and those with larger angular extent can be studied in great detail or even imaged in a model-independent way. Today, the still enigmatic classical Be stars remain among favorite targets of state-of-the-art interferometric facilities like VLTI and CHARA. Near-critical rotation, which is an essential property of Be stars, could have been acquired by means of mass and angular momentum transfer in a close binary. The originally more massive component in this case loses a large fraction of its mass and becomes an evolved remnant stripped of its outer envelope. The present-day Be star, on the other hand, is rejuvenated and spun up, which leads to a possible subsequent formation of the characteristic mass-loss disk. The observational test for this scenario is that Be stars should not have main-sequence dwarf companions, and if the binary system remained bound after a possible supernova explosion of the evolved component, we should be able to observe a population of Be binaries with evolved companions, or possibly Be stars that are merger products. Detecting faint evolved companions to Be stars poses many difficulties, but luckily the question can also be addressed with interferometry, which is currently capable of directly detecting companions down to an angular separation of ~0.5 milliarcsec and up to a contrast ratio of ~6 magnitudes in the near-IR. To address the question of whether we can detect faint evolved companions around nearby Be stars, I have recently started a CHARA/MIRC-X interferometric survey of Be stars, and I will present some promising initial results. Recently, we also looked into the possible importance of hierarchical triple systems with a Be star, of which only about a dozen examples are currently known. I will present our full orbital solution of the triple system nu Gem - with only the second dynamical mass measurement of a classical Be star - and its possible implications.
July 2021
Speaker: Dr. Julia Bodensteiner
Title: On the possible binary origin of classical Be stars
Abstract: Binary interactions play an important role in the evolution of massive stars. Yet, the complex interaction physics as well as the outcome of the interactions remain poorly understood. A century-old question addresses the origin of the Be phenomenon, which occurs in ~20% of the early-type stars. Observationally, classical Be stars are classified as B-type stars with Balmer line emission, indicative of a circumstellar disk, which strongly correlates with rapid rotation of the star. The processes that lead to such high rotation rates are, however, still widely debated. In my talk I will discuss the different formation channels proposed for the formation of Be stars, especially the binary channel which invokes previous mass-transfer in binary systems as the origin of their spin-up. I will present observational evidence that suggests that the binary channel is predominant in the formation of massive Be stars. I will furthermore show that the few known Be binaries are indeed exotic systems with stripped or compact companions, and that there is a lack of reported main-sequence companions.
June 2021
Speaker: Dr. Matthew E. Shultz
Title: Magnetic B-type Stars as Magnetospheric Exemplars
Abstract: Strong magnetic fields can trap the radiatively driven, ionized winds of hot stars, giving rise to large, corotating circumstellar magnetospheres. These intricate structures can be detected at multiple wavelengths, with available diagnostics at X-ray, ultraviolet, visible, near-infrared, and radio wavelengths, each probing a distinct magnetospheric component. Just as the Keplerian decretion disks of classical Be stars serve as laboratories for disk physics, hot star magnetospheres show promise as magnetospheric laboratories. The early B-type stars in particular stand out due to the simultaneous availability of all known diagnostics. In this talk, I review the key role played by rapid rotation in shaping the distribution and flow of magnetospheric plasma, explaining how centrifugally driven magnetic reconnection may be the unifying mechanism driving optical, X-ray, and gyrosynchrotron emission from rapidly rotating, strongly magnetic B stars.
May 2021
Speaker: Dr. Jonathan Labadie-Bartz
Title: Space photometry of classical Be stars: rapid rotation, pulsation, and links to mass ejection
Abstract: Classical Be stars are near-critical rotators which occasionally eject mass and angular momentum to form circumstellar 'decretion' disks. Space photometry has revealed that Be stars are pulsators as a rule, and evidence is mounting that pulsation plays a role in the mass ejection mechanism. In this talk, light curves from the TESS satellite are used to study the variability of ~500 Be stars. Virtually all Be stars are variable, with the vast majority showing `frequency groups' (which represent multi-mode pulsation). About 20% of the sample shows brightening events that reflect mass ejection episodes, often with associated enhancements in the strength of the frequency groups. In some stars, there is evidence of non-linear coupling of individual pulsation modes which can control the timing of mass outbursts. Time-series spectroscopy simultaneous with the TESS observations for selected targets provides crucial information for interpreting the photometric variability. Correlations between different variability patterns are studied to better understand the overall behavior of the Be star population.