November 2022
Speaker: Dr. Chen Wang
Title: What can we learn from the multiple MS components detected in young star clusters?
Abstract: Recent high-precision Hubble Space Telescope (HST) photometry reveals that the main-sequences (MSs) of young star clusters in the color-magnitude diagram are split into several components. This challenges the traditional view of star clusters containing coeval stars born with identical initial conditions. We use detailed single and binary evolution models to investigate the origin of these multiple MS components. We find that binary mergers may produce slowly-rotating rejuvenated stars, which appear younger and bluer than other cluster members. In contrast, stable mass transfer creates near-critically-rotating stars, which coincides with the observed red Be sequence in young star clusters. Our results lead us to conclude that stars may either form by accretion, or through a binary merger, and that both paths lead to distinctly different spins, magnetic fields, and stellar mass distributions.
October 2022
Speaker: Amanda Rubio
Title: Decretion and accretion in Be star binary systems
Abstract: Accretion disks are common in astrophysical systems, from AGN to protostellar disks. The disks of Be stars (rapidly rotating Main Sequence B-type stars) are special: they are discretion disks, built from matter ejected by the central object. When in a binary system, the companion can affect the Be disk in many ways, exciting density waves and even causing truncation. It can also accrete material from it, as is the case for Be X-ray binaries, whose X-ray emission is powered by accretion onto a compact companion. In our work we simulate Be binaries with a smoothed particle hydrodynamics (SPH) code (Okazaki et al. 2002) in order to investigate the effects of the companion on the dynamics of the disk, the process through which it accretes matter, and how the system loses mass as whole. We employ a modified version of the code, specially updated by us to increase resolution in low density areas of the system, such as the outer disk and around the companion. We find that disk structures around the secondary are only formed for disks with low viscosity and short binary periods, while no consistent structure is formed for more viscous disks. In these cases, mass is accreted by the companion as it flows past it. Furthermore, a circumbinary disk is formed around the system for all simulations, which was never before seen in simulations for Be binaries but agrees with recent observational findings of radio emission from these types of system, where an ad-hoc circumbinary disk model was employed. Our study paves the way for a better understanding of X-ray emission in Be X-ray binaries, and offers an insight in how hidden companions of Be stars can be detected observationally.
June 2022
Speaker: Dr. Ylva Götberg
Title: Stars stripped in binaries: from theory to observation
Abstract: Stars stripped of their hydrogen-rich envelopes via interaction with a binary companion are predicted to be responsible for most hydrogen-poor supernovae and also constitute two necessary steps in the formation of neutron stars that merge in gravitational wave events. In addition, because these stripped stars are the exposed cores of their progenitors, they are so hot that most of their radiation is emitted in ionizing wavelengths. This can both affect the appearance of bursty star-forming galaxies and contribute to cosmic reionization. Despite their importance, only one intermediate-mass stripped star has been published to date. Based on theoretical predictions, we designed an observational search for stars stripped in binaries in the Large and Small Magellanic Clouds. I will share the first results from this ongoing survey, including the identification and characterization of a first observed sample. Being the exposed helium cores of evolved massive stars, these observed stripped stars offer the opportunity to explore physical processes typically embedded in the deep interiors of massive stars.
May 2022
Speaker: Dr. Shigeyuki Karino
Title: Be-type high-mass X-ray binaries as progenitors of ULXs
Abstract: Since the detection of X-ray pulses from ultraluminous X-ray sources (ULXs), neutron stars have been considered as their central objects. Recent observations suggest that at least some ULXs are special cases of Be-HMXBs. To investigate the conditions under which neutron stars achieve mass-accretion rates beyond the Eddington limit, we use the framework of mass-accretion models for BeHMXBs. With simple accretion models, we show that a Be-HMXB may become a ULX if the magnetic field of the neutron star and the density of the Be disc meet certain conditions. Moreover, we adopt binary evolution models and investigate the duration for which BeHMXBs can be observed as transient ULXs. The results indicate that BeHMXBs could be ULXs for a typical duration of 1 Myr. Comparisons with nearby observed BeHMXBs indicate that many binary systems have the potential to become ULXs during their evolution. Particularly, a BeHMXB system tends to become a ULX when the Be donor has a dense disc. Because BeHMXBs are common objects and a significant number of them can become ULXs, we expect that a reasonable fraction of the ULXs could consist of evolved BeHMXBs.
April 2022
Speaker: Dr. Catalina Arcos
Title: Long-term variability in Be stars: features of the outer disks
Abstract: Be stars are massive stars surrounded by a thin equatorial gas disk in quasi-Keplerian rotation. By studying the emission lines from their spectra, information about the features of the inner and outer part of the disk can be obtained. Moreover, these stars are the only objects studied so far with the characteristic of forming, maintaining and dissipating a disk, making them one of the most attractive stellar objects to be studied. The mechanism that transfers the mass and angular momentum from the star to the disk is still under debate, but once the material is placed in the circumstellar orbit it is governed by viscosity. In this taIk, I will present follow up variability of a sample of southern Be stars in order to constraint the size of the emitting region, disk density and inclination angle of outer disks, as well as stellar properties, with the aim of seek for correlations between the central star and the disk phase.
March 2022
Speaker: Dr. Yaël Nazé
Title: The mysterious gamma Cas stars
Abstract: Long thought to be a prototype of Be stars, the first detected case of such objects, gamma Cas, was found to be actually atypical amongst them! Indeed, it emits very bright and extremely hard X-rays with a luminosity intermediate between that of normal massive stars and that of high-mass X-ray binaries. Recent efforts in X-ray monitoring have revealed that such objects are not uncommon hence gamma Cas stars are important ionizing sources, particularly at low metallicities where Be stars abound. The properties of these stars will be reviewed using the latest observational results.
February 2022
Speaker: Dr. Kareem El-Badry (Harvard)
Title: Binary mass transfer, Be stars, and the search for stellar-mass black holes
Abstract: Many massive stars have emission lines that originate in circumstellar “decretion” disks around stars rotating at close to their break-up velocity. There is no consensus on how these stars came to rotate so rapidly, but the recently several lines of evidence have suggested that one important channel is mass transfer and spin-up in close binaries. I will discuss a recently identified class of mass transfer binary stars caught in a short-lived phase following envelope stripping. I will also summarize a search for the progenitors of such binaries I carried out using the APOGEE survey, with the goal of constraining the fraction of Be stars that formed by mass transfer. Finally, I will describe how mass transfer binaries can masquerade as stellar-mass black holes, and will also summarize ongoing efforts to find dormant stellar mass black holes via light curve variability.