December 2023
Speaker: Dr. Anatoly Miroshnichenko (University of North Carolina at Greensboro)
Title: Stars with Be and B[e] phenomena: search for binarity
Abstract: Stars with masses of 3-20 Msun at birth spend their main sequence lifetime as those of spectral type B. Single B-type stars run stellar winds that are thought to be too weak to explain large amounts of circumstellar matter. The latter manifests itself through the presence of emission lines and infrared excesses, which are defining features of the Be and B[e] phenomena. The B-type stars, which represent the Be and B[e] phenomenon, often exhibit double-peaked emission-line profiles formed in disk-shaped circumstellar envelopes. Be stars rotate fast and contain only gas in their disks, while B[e] stars exhibit a strong excess of infrared radiation that manifests the presence of dusty particles in addition to the gas. The origin of both phenomena is still under debate, but recent results of their investigation strongly suggest that the presence of a second star in the system may play an important role in triggering the circumstellar material accumulation. Current views of the phenomena will be introduced, and examples of objects showing them will be presented.
November 2023
Speaker: Dr. Pablo Reig (Institute of Astrophysics, Foundation for Research and Technology-Hellas)
Title: Dissecting Be/X-ray binaries
Abstract: BeXB consist of a neutron star orbiting a Be star, that is, a spectral type B star with an equatorial disk formed from material ejected from the star's photosphere. X-rays result from the accretion of material from this circumstellar disk by the neutron star. Virtually all BeXB are X-ray pulsars. Accreting X-ray pulsars undergo giant X-ray outbursts during which the X-ray luminosity increases by several orders of magnitude. Multiwavelength studies are essential to understand the properties of these systems. The optical and IR bands provide information about the Be star, while the high energy radiation is produced in the vicinity of the neutron star. In this talk I will present selected results of my work on Be/X-ray binaries over the past ~20 years. I will present the results of a recent study that show that Be stars in BeXB are also non-radial pulsators. I will also explain how to use the variability in the X-ray continuum emission to obtain information about the accretion states. Finally, I will discuss the observational evidence that supports the presence of truncated and warped disks during X-ray outbursts.
October 2023
Speaker: Dr. Yanina Cochetti (Instituto de Astrofisica de La Plata)
Title: IR spectroscopy of Be stars
Abstract: The IR spectra of Be stars display plenty of hydrogen recombination lines, which constitute a great tool for obtaining information on the physical and dynamic structure of different regions within the circumstellar envelope. With the aim of characterising those circumstellar envelopes, we obtained quasi-simultaneous spectra in the J, H, K, and L bands of a sample of Be stars with medium resolution with the spectroscopic facilities at the Gemini and Las Campanas observatories. By measuring different parameters of the observed hydrogen recombination lines, we diagnosed the physical conditions in the circumstellar environment and defined new complementary criteria to classify Be stars according to their disc opacity. Moreover, the obtained spectra provide valuable observational material for analysis and modelling. With this aim, we started to compute a grid of synthetic spectra covering the near-IR spectral range with the HDUST code. In this seminar, I will present the results based on the IR spectroscopical observations and what we have performed thus far regarding the line modelling.
September 2023
Title: Unveiling the evolutionary state of three B supergiant stars: PU Gem, ε CMa, and η CMa
Speaker: Dr. Julieta Sanchez Arias (Czech Academy of Sciences)
Abstract: We aim to combine asteroseismology, spectroscopy, and evolutionary models to establish a comprehensive picture of the evolution of Galactic blue supergiant stars (BSG). To start such an investigation, we selected in this work three BSG for our analysis: HD 42087 (PU Gem), HD 52089 (ϵ CMa) and HD 58350 (η CMa). These stars show pulsations and were suspected to be either preceding or succeding the red supergiant stage. For our analysis, we utilized the 2-min cadence TESS data to study the photometric variability and obtained new spectroscopic observations at the CASLEO observatory. We calculated CMFGEN non-LTE radiative transfer models and derived stellar and wind parameters using the iterative spectral analysis pipeline XTGRID. The spectral modelling was limited to changing only the effective temperature, surface gravity, CNO abundances, and mass-loss rates. Finally, we compared the derived metal abundances with predictions from Geneva stellar evolution models. The frequency spectra of all three stars suggest either stochastic oscillations, nonradial strange modes, or a rotational splitting. We conclude that the rather short sectoral observing windows of TESS prevent establishing a reliable mode identification of low frequencies connected to mass loss variability. The spectral analysis confirmed gradual changes in the mass-loss rates, and the derived CNO abundances comply with the values reported in the literature. We were able to achieve a quantitative match with stellar evolution models for the stellar masses and luminosities. However, the spectroscopic surface abundances turned out to be inconsistent with theoretical predictions. The stars show N enrichment, typical for CNO cycle processed material, but the abundance ratios do not reflect the associated levels of C and O depletion.
May 2023
Speaker: Dr. Yong Shao
Title: Population synthesis of Galactic Be stars
Abstract: Be stars are rapidly rotating B stars and their optical spectra show Hα emission lines that originate in a circumstellar decretion disk. The formation of Be stars involves single-star and binary-interaction channels. Based on recent observations, there is growing evidence that binary interaction dominates the formation of Be stars. Until now, more than 100 Be stars have been observed to have compact companions including subdwarf O/B stars, white dwarfs, neutron stars and black holes. These Be-star binaries present a golden sample to constrain the evolution of massive binaries that relates to some important phenomena such as supernova explosions and gravitational waves. In this talk, I will introduce the observational clues and theoretical models on Be-star formation. In particular, I will talk about the population properties of various types of Be-star binaries in the Milky Way.
April 2023
Speaker: Dr. Jim Fuller (California Institute of Technology)
Title: The magnetic hearts of stars
Abstract: Magnetic fields are frequently observed at the surfaces of stars, but we have a poor understanding of their configurations deep within stars. I will show how to compute the structures of magnetically distorted stars that are in both hydrostatic and thermal equilibrium. These models suggest internal field strengths several times larger than surface field strengths. The magnetic field also produces a quadrupolar distortion in the star’s surface temperature, but this is too small to explain observed photometric modulations of magnetic main sequence stars and white dwarfs. In thousands of low-mass red giant stars, Kepler asteroseismic data has revealed gravity mode oscillations that are sensitive probes of the stars’ degenerate cores. I will show how gravity modes (or lack thereof) can also reveal strong magnetic fields in the cores of red giants, which are common within "retired" A stars but are absent in their lower-mass counterparts. Additionally, the slow core rotation measured for red giants cannot be explained by existing hydrodynamic angular momentum transport mechanisms. Instead, magnetic torques likely dominate angular momentum transport, and I will show a modified version of the Tayler-Spruit dynamo can largely explain internal rotation rates of main sequence stars, red giants, and white dwarfs.
March 2023
Speaker: Dr. Sally Oey (University of Michigan)
Title: OBe Star Kinematics: Evidence Supporting Binary Interactions
Abstract: The binary-origin model for classical OBe stars has gained much traction in recent years. I will present evidence in support of this scenario from our studies of field OB and OBe stars in the Small Magellanic Cloud. Based on statistical, spatial, and kinematic observations, we find that OBe stars appear to be dominated by a population that, compared to normal OB stars, preferentially contains runaways and walkaways ejected from clusters. The spatial and kinematic properties of OBe stars also mirror those of high-mass X-ray binaries, which are similar, post-SN binary systems. Finally, I will present an extreme Oe star, AzV 493, which appears to be a very early-type, eccentric system with strong photometric and spectroscopic variability.
February 2023
Speaker: Dr. Nazma Islam (University of Maryland, BC, and the X-ray Astrophysics Lab at NASA's Goddard Space Flight Center)
Title: Superorbital modulations in supergiant High Mass X-ray binaries
Abstract: In addition to pulsations and orbital period, a third type of periodicity is observed in many neutron star X-ray binaries. While the first two periodicities are attributed to the NS and binary rotation respectively, the causes of super-orbital modulation are still not fully understood. Long term observations of Supergiant High Mass X-ray binaries (sgHMXBs) with Swift/BAT have detected super-orbital modulations in 6 sources. The accretion in these systems are driven by strong stellar winds, which provide a less regularly structured environment compared to an accretion disk. In this talk, I will present results from long term Swift/BAT and pointed NuSTAR observations, which are used to provide constraints of the various models explaining the mechanisms driving these flux modulations.
January 2023
Speaker: Dr. Sebastian Kamann (Liverpool John Moores University)
Title: The fingerprint of stellar rotation on the colour-magnitude diagrams of young star clusters
Abstract: Young star clusters show features in their colour-magnitude diagrams that contradict the notion that they formed as simple stellar populations, such as split main sequences and extended main sequence turn-offs. Previously interpreted as evidence for prolonged star formation episodes, it has recently become clear that instead, these features are caused by stellar rotation. Using the example of NGC 1850, a 100 Myr old massive star cluster in the Large Magellanic Cloud that has been extensively observed with the MUSE integral field spectrograph, we have recently demonstrated the link between the rotational velocities of stars and their photometric colours. We revealed a clear bimodality of slow and fast rotating stars populating either side of the split main sequence. Among its turn-off population, NGC 1850 further hosts a large population of fast rotating Be stars, and the MUSE spectroscopy enables us to study the properties of their decretion disks as well as their impact on the observed photometry. My talk will also include an overview of the theories that have been put forward in order to explain the observed stellar rotational velocities that we observe.