Upcoming seminars
February 4, 2025 at 10:30 AM
Probing the magnetospheres of chemically peculiar stars through the dips in their light curves
Zdeněk Mikulášek
(ÚTFA PřF MU Brno)
The advent of extensive photometric surveys such as Kepler or TESS missions has enabled the recent unexpected unveiling of short-term dips in the phase light curves of the majority of magnetic chemically peculiar stars of the upper main sequence. We explain this characteristic type of stellar variability caused by repeating transits of semi-transparent structures of stellar plasma trapped in the corotating magnetospheres of stars with a global magnetic field. In the light curves of such stars, we detect dips with a typical depth from fractions of mmag to several mmag, the configuration of which is persistent in the scale of decades. The occurrence of photometric dips, frequency, and prominence in the light curves of rotationally modulated objects allows us to judge magnetospheres’ dimension and strength, making this analysis a universally available instrument for diagnosing stellar magnetic fields. We will demonstrate dip analysis on notorious mCP stars such as V901 Ori, CU Vir, V545 Lyr, 56 Ari, or EE Dra.
Held seminars in 2024
December 2, 2024 at 10:30 AM
Two-temperature accretion flows around compact objects
Shilpa Sarkar
(Harish-Chandra Research Institute, HRI, India)
Accretion is one of the most efficient processes by which the gravitational potential energy of matter can be converted into radiation. This phenomenon provides us with an explanation of the huge amount of energy liberated and high luminosities observed in Active Galactic Nuclei, X-ray binaries, etc. Therefore, modelling of these accretion flows is necessary to understand the underlying physical processes present in these systems. The soup of protons and electrons in these ionised flows are bound together by weak Coulomb force. Additionally, in most of the astrophysical cases, the infall timescales are much shorter. This makes the species settle down into two different temperature distributions, hence, the name two-temperature flows. However, this theory suffers from a serious problem of degeneracy. Compared to one-temperature flows, there is one more variable in the two-temperature system — the extra temperature. However, there is no increase in the number of equations of motion. Thus, no unique solution exists, for a given set of constants of motion; or in other words, the system is degenerate! Different values of Tp/Te ratio supplied at any boundary, would generate different kinds of solutions with drastically different topologies as well as spectra. In addition, there is no known principle dictated by plasma physics which may constrain the relation between these two-temperatures. This degeneracy is irrespective of the type of the central object and is generic to two-temperature flows. We propose for the first time, an entropy maximisation formulation using the first principles. Using this methodology, we were able to constrain degeneracy and a unique solution with maximum entropy was selected following the second law of thermodynamics. Thereafter, we analysed the spectrum of these unique solutions for different accreting systems like black holes and neutron stars.
October 15, 2024 at 1:00 PM
Magnetic Massive Star Winds and Spin-Down
Asif ud-Doula
(Pennsylvania State University, Dunmore, USA)
Massive stars (at least eight times as massive as the Sun) possess strong stellar winds driven by radiation. An increasing number of these massive stars have been confirmed to have global magnetic fields. Such magnetic fields can have significant influence on the dynamics of these stellar winds which are strongly ionized. Such interaction of the wind and magnetic field can generate copious amount of X-rays, they can spin the star down, they can also help form large scale disk-like structures. In this presentation I will discuss the nature of such radiatively-driven winds and how they interact with magnetic fields with a particular focus on spin-down effects.
October 8, 2024 at 1:00 PM
The WINE collaboration: a Warm Giants planets survey
Marcelo Tala Pinto
(Universidad Adolfo Ibanez, Santiago, Chile)
Warm giants, planets broadly defined as planets orbiting their host star with periods longer than 10 days, remained elusive to ground-based transit searches, mainly due to the constrains imposed by the daily cycle. While some detections were possible with the Kepler+K2 missions, only with TESS it has been possible to systematically characterize them, either through the detection of periodic transiting signals or single transiters. By complementing transit observations with radial velocity (RV) measurements, it is possible to provide a detailed characterization of the dynamical properties of the planetary system. The Warm gIaNts with TESS (WINE) collaboration is a dedicated survey to identify, confirm and characterize warm giants using TESS data and ground-based photometric and spectroscopic follow-up facilities, with the main goal of building a giant planets database to constrain theories of planetary formation and evolution. We have detected more than 20 Warm giants and we have been following-up dozens of exoplanet candidates. In this talk I will provide an overview of the survey, highlighting the most relevant results of this project.
September 16, 2024 at 9:00 AM
Cosmic butterflies: the product of tempestuous stellar marriages
David Jones
(Instituto de Astrofisica de Canarias)
Planetary nebulae are some of the most strikingly beautiful astrophysical phenomena known, gracing many a glossy-paged, coffee-table book and earning them the nickname “cosmic butterflies”. Classical stellar evolutionary theory states that all intermediate mass stars should produce a planetary nebula, forming as the star leaves the Asymptotic Giant Branch and evolves towards the white dwarf phase. While it remains the standard for astronomy textbooks, this paradigm has gradually become less and less accepted by the planetary nebula community. As such, it is now clear that a significant fraction of planetary nebulae originate from a binary evolutionary pathway, with some theories even going as far as to say that binarity may be a prerequisite to form a planetary nebula. In this seminar, I will begin by outlining the fundamental pathways for binary evolution and how they each may relate to the formation of planetary nebulae. I will then go on to discuss some of the more interesting developments in the field, with particular emphasis on post-common-envelope central stars and what they can tell us about the common envelope phase itself. Finally, I will discuss the growing connections between planetary nebulae and other binary phenomena including novae and type Ia supernovae.
June 19, 2024 at 1:00 PM
The mass distribution of hot subdwarf stars in wide binaries
Francisco Molina
(University of Postdam)
Abstract: Obtaining the masses of hot subdwarf (sdB) stars stands as a primary objective in the study of properties of wide sdB binary systems. The minimum masses of the components are derived from the radial-velocity curves based on spectroscopic data. However, they are contingent upon the unknown orbital inclinations. We utilize the fitting of stellar atmosphere models to the composite SED and stellar evolution models for determining the absolute masses of the cool companions within hot subdwarf binary systems with fully solved orbits. This approach enables the estimation of the mass of the subdwarf star by using the mass ratio parameter. Consequently, we present the first mass distribution of sdB stars in wide binary systems. The distribution reveals significant differences when compared to published distributions for single-lined sdB stars or sdB stars in close binaries. It also shows a large group of sdBs with masses significantly higher than those predicted by theoretical studies.
May 28, 2024 at 1:00 PM
Five new eclipsing binaries with low mass companions
Jozef Lipták
(Astronomical Institute of CAS, Charles University)
Low-mass stars of spectral types K and M are the most common in the Galaxy. However, their physical characteristics are not well described by theoretical models. The number of dwarf stars with well-established masses, radii, and temperatures has started to grow with the advent of space-based photometric missions and is now around 100. I will present the journey of five originally exoplanet candidates from the TESS mission, identified later as eclipsing binaries with help of OES and TCES. For two of the systems, we were able to derive stellar parameters with errors down to a few percent. The M dwarf secondaries can be used further in theoretical model calibration. The other systems have grazing eclipses, somewhat limiting the description but showing other interesting phenomena such as pulsations or rotational variability.
March 14, 2024 at 1:30 PM
Massive stars in transition phases in the near-IR
María Laura Arias
(Institute of Astrophysics of La Plata (CONICET-UNLP); Facultad de Ciencias Astronomicas y Geofisicas (FCAG); La Plata National University (UNLP), Argentina)
After leaving the main sequence, massive stars enter short-lived phases during which they shed substantial amounts of mass into their surroundings through episodic events or strong radiatively-driven winds. Consequently, these objects become embedded in dense environments, which are discernible through numerous characteristic features in their line and continuum spectra. Near-infrared observations have proven to be a powerful tool for investigating these stars. For example, CO molecular emission around 2.3 microns allows for the tracing of the structure and dynamics of the outer parts of the disks, while H recombination lines contribute to diagnosing the physical conditions of the inner disk. Similarly, the modeling of the Br alpha line can be used to study the winds and derive mass-loss rates. To contribute to the study of circumstellar material and gain insights into the nature of central objects, we are constructing a near-infrared spectroscopic database of massive stars in such transitional phases using GEMINI facilities. In this presentation, I will discuss the various possibilities offered by the near-IR region, as well as present some results from our ongoing research on the topic.
March 7, 2024 at 1:30 PM
Stellar Winds from B Supergiant Stars: Exploring the Delta-Slow Hydrodynamic Solution
Roberto O. J. Venero
(Institute of Astrophysics of La Plata (CONICET-UNLP); Facultad de Ciencias Astronomicas y Geofisicas (FCAG); La Plata National University (UNLP), Argentina)
The B-type supergiant stars form a heterogeneous group of objects, within which various types of peculiar stars coexist in brief and distinctly different evolutionary stages. Ordering the evolutionary sequences for these diverse stellar groups and assigning each star its respective phase poses a challenge. Nonetheless, the strong stellar winds of these stars produce observable spectral features that could serve as valuable diagnostic tools in addressing this question. In order to utilize these characteristics, it is necessary to know the structure of the wind in detail. The fundamental hydrodynamic equations governing radiation-driven winds (m-CAK theory) predict three distinct solutions for rotating stellar winds. These solutions include the ‘fast’ solution (or classical), the ‘Omega-slow’ solution (applicable to rapidly rotating stars), and the ‘delta-slow’ solution (pertaining to winds with ionization changes). These solutions are primarily distinguished by the terminal speed achieved by the wind, with the ‘fast’ wind regime exhibiting the highest values for this parameter.
Excluding the ‘Omega-slow’ solutions due to the absence of fast rotation in B supergiants, the ‘fast’ and ‘delta-slow’ solutions are distributed in distinct domains within the classical parameter space of the radiation force (k, alpha, and delta). In this presentation, I will illustrate the distribution of these domains and, more specifically, asses the ability of the delta-slow solution in predicting the fundamental spectral characteristics observed in B supergiants.
February 15, 2024 at 1:30 PM
Unraveling the complex nature of FS CMa stars
Nela Dvořáková
(Astronomical Institute of Charles University)
FS CMa type stars form a group of rare B type stars with extremely strong emission lines, presence of forbidden lines and strong IR excess, pointing to a complex circumstellar environment of gas and dust. They are variable on timescales of days, months or even years. Binarity was the offered explanation for some of their observed properties. However, only some FS CMa stars are proven binaries. Recently, a promising lead was uncovered in the form of a strong magnetic field found in IRAS 17449+2320. Together with unusually high space velocity, we are now pursuing a different scenario: stellar mergers. They provide a natural explanation for such strong magnetic field, the slow rotation of the FS CMa type stars as well as the presence of the dust. Supporting evidence can be found in interferometric observations of HD 50138, which cannot be explained well with a binary model. Our recent analysis of a series of N-body simulations shows that more than 50 % of mergers are B type stars and that lower to intermediate mass stars provide an especially important channel of a merger formation.
January 25, 2024 at 1:00 PM
Multiwavelength monitoring of the High-mass X-ray binary Cygnus X-1
Maïmouna Brigitte
(Astronomical Institute of the Czech Academy of Sciences; Charles University)
First discovered in 1964, the high-mass X-ray binary Cygnus X-1 contains a 21.2 solar-mass black hole and a 40.6 solar-mass (B0 or O9.7 Iab-type) star – also known as HDE 226868. For over 60 years, the system has been widely studied in X-rays and optical wavelengths. I will present new results obtained from the optical monitoring of the binary, performed between March-July 2022 and April-September 2023 simultaneously with X-ray observations. The optical observations were made using the 2m-Perek telescope located in Ondrejov in the Czech Republic. We employed the new Echelle spectrograph with a typical resolution of 0.25 A per pixel. We combined in total 27 observations covering wavelengths from near UV to near IR with high spectral resolution to disentangle different emission and absorption lines. The variable profiles of Hydrogen and Helium lines are used to study the structure and kinematics of the photosphere and circumstellar matter in the system. The strong correlation between X-ray and optical emission is crucial to understand the relationship between the accretion disk and the stellar wind.