November 23, 2023 at 1:30 PM
From transits and dynamical stability of exoplanets to eclipsing binaries
Pavol Gajdoš
(Astronomical Institute of the Czech Academy of Sciences)
This presentation will consist of two parts. At the beginning, I will present an overview of my previous results. I will focus mainly on transiting exoplanets and their orbital dynamics. Kepler-410Ab shows significant TTV changes but RV measurements excluded our hypothesis about additional stellar companion. Systems WASP-92, WASP-93, and WASP-118 allow the existence of other planets from a dynamical point of view. Three planets in the Kepler-18 systems are close to the mean-motion resonance which can cause significant changes in the orbital parameters of the planets. Kepler-23 shows very chaotic behaviour, we tried to find out the source of this instability. In the second part, I will introduce my current research on eclipsing binary KIC 7023917. It is one of nearly three thousand other binaries in the well-known Kepler Eclipsing Binary Catalog. However, it has some uncommon features. We focused on this system because we detected fast anti-correlated changes on the O-C diagram which could suggest the presence of apsidal motion. But, these changes are very fast for apsidal motion and the orbit of this binary is circular. Detailed analysis of the Kepler and TESS light curves reveals deformation of the light curve by short-periodic pulsations and the O’Connell effect caused by stellar spot(s). Moreover, we collected ground-based photometric and spectroscopic data to better characterise this system. Here, I present our initial study of this system and a possible explanation of the observed O-C diagram.
October 26, 2023 at 1:30 PM
Spectral modeling of OB-type stars and wind momentum luminosity relationship
Natalia Machuca
(Instituto de Fisica y Astronomia, Universidad de Valparaiso, Chile)
Radiation-driven winds theory predicts a relationship between wind momentum and stellar luminosity, called Wind momentum Luminosity Relationship (WLR). In the last years, different hydrodynamical solutions have been proposed for wind theory that seems to affect this relationship. In this seminar, I’ll present a preliminary result of the WLR using the wind hydrodynamic consistently with the NLTE radiative transport calculations. For this purpose, I developed a code that uses a χ2 test to determine the model that most closely fits some observed spectral lines of 90 OB-type stars using the ISOSCELES grid. This grid is based on the hydrodynamic code HYDWIND and the radiative transfer code FASTWIND, then our hydrodynamic solutions provide a velocity profile with a better justification than the β-law. Also, our first results show that fast solutions describe more accurate O-type stars, while δ-slow solutions recreate in a better way the spectral lines of B supergiant stars. Finally, I’ll present my contribution during my secondment here at Ondrejov.
presentation hereAugust 29, 2023 at 2:15 PM
Precession of the orbital planes and rotational axes in transiting exoplanets
Dmytro Orikhovskyi
(Comenius University in Bratislava / Astronomický ústav SAV)
Orbital planes of numerous close-in exoplanets are not perpendicular to the host-star spin axis. This means that the current orbital plane of the exoplanet was probably altered after the system was formed. Projected spin-orbit misalignment can be measured in transiting exoplanets using so called Rossiter-McLaughlin effect affecting radial velocity of the host star during the transit. For fast-rotating host stars, where the radial velocities cannot be often measured with a sufficient precision, the planet signature can be found in the mean line profiles. Analysis of the profiles during the transit enables us to determine the projected spin axis-orbital plane misalignment. If the inclination angle of stellar spin axis is known from a high-precision photometry, the true misalignment (stellar obliquity) can be determined. Some misaligned objects (e.g. Kepler-13Ab) were found to show precession of the exoplanet orbit caused by the tides due to the rotationally-deformed parent star. The precession causes changes of the transit duration (TDV) due to the shift of the the transit chord across the stellar surface. Exoplanet’s orbit precession is always connected with precession of the parent’s star rotational axis due to the conservation of the total angular momentum but its amplitude is much smaller. Its analysis brings us information on the internal structure of the star. The primary goal of the diploma thesis was to find exoplanets with spin axis-orbital plane misalignment and to detect those which show transit duration variations (TDV) due to the orbital plane precession. This work used high-precision satellite photometry from TESS for objects selected from the ExoFOP website. The analysis of the light-curve asymmetries lead to detection of several objects where spin-orbit misalignment is plausible. One object significantly changing its transit depth was found. Light-curve modelling lead to improvement of transit ephemerides for 64 objects. Several objects were found to show interesting peculiarities and deserve future investigation.
August 29, 2023 at 1:30 PM
MWC 645: A Puzzling FS CMa-type Star
Andrea Torres
(Universidad Nacional de La Plata, Argentina)
The B[e] phenomenon encompasses a diverse group of stars encircled by gaseous and dusty envelopes, sharing similar physical characteristics. Among these stars, FS CMa-type objects are suspected binary systems that might be undergoing mass-transfer processes, potentially explaining their substantial surrounding material. Our study aims to contribute insights into a recently confirmed binary star system, MWC 645, which could be experiencing active mass transfer. Through spectroscopic analysis, we identify atomic and molecular features, derive quantitative properties of line profiles, and detect CO molecular bands in absorption at 1.62 μm and 2.3 μm for the first time. We set an upper limit of the effective temperature for the cooler binary component and find a correlation between enhanced Hα emission and decreased optical brightness, potentially linked to mass-ejection events or increased mass loss. Additionally, we outline the envelope’s global properties, which could cause brightness fluctuations due to variable extinction, and briefly discuss various hypothetical scenarios.
presentation hereJune 29, 2023 at 1:30 PM
Modeling diatomic molecules in stellar disk
Rodolfo Vallverdú
(Universidad Nacional de La Plata, Argentina)
Stellar disks have temperatures low enough to allow for the formation of molecules. The presence of these molecules is detected through the observation of emission bands in the spectrum of the star and its disk. To identify which molecule corresponds to these bands, as well as determine some physical parameters of the disk, it is necessary to have synthetic spectra for comparison with observations. In this talk, I will discuss the code I have been developing to generate these synthetic spectra, its results, and its scope.
presentation hereMay 22, 2023 at 1:30 PM
Evolution of massive stars with new hydrodynamic wind models
Alex C. Gormaz-Matamala
(Centrum Astronomiczne im. Mikołaja Kopernika, Warsaw, Poland; Universidad Adolfo Ibáñez, Viña del Mar, Chile)
We present evolutionary models for a set of massive stars, introducing a new prescription for the mass loss rate obtained from hydrodynamic calculations in which the wind velocity profile and the line-acceleration are obtained in a self consistently way. Evolutionary models with the new recipe for mass loss retain more stellar mass through their evolution, which is expressed in larger radii and consequently more luminous tracks over the Hertzsprung-Russell diagram. Also, models with self-consistent winds predict a weaker braking in the rotational velocity and a more marked drift redwards of the evolutionary tracks across the HRD, as a direct consequence of the differences in the stellar angular momentum loss and in the rotational mixing. Together with the prediction of higher masses at the end of the main sequence, self-consistent tracks also predict a distribution of rotational velocities for Galactic O-type stars more in agreement with the diagnostics of recent surveys. Other hypothetical implications, such as the masses of Ofpe stars at the Galactic Centre or the contribution of the isotope Al-26 to the ISM, are open to discussion.
May 16, 2023 at 1:30 PM
Exploring the origin and evolution of hydrogen-deficient supergiants
Anirban Bhowmick
(VIT Bhopal University)
The peculiar characteristics of the enigmatic hydrogen(H) deficient supergiants suggest an evolutionary path very different from that of an H-normal star. Being extremely rare, these enigmatic objects are challenging to study, but a thorough knowledge of their chemical composition across their different samples can provide clues to their evolution. Broadly classified into four main categories- 1) the Extreme Helium stars (EHes), 2) the R Coronae Borealis Stars RCBs, 3) the Hydrogen-Deficient carbon stars HdCs, and the 4) DY Persei variables they span a wide range of temperature — from the coolest DY Persei variables (Teff~3500 K) to the hot Extreme Helium Stars (EHes) (Teff>14000 K). Based on their observed surface properties, two formation scenarios were proposed — 1) The Double Degenerate (DD) merger scenario involving the merger of two low-mass double-degenerate white dwarfs and 2) the FF scenario involving a late or final He shell flash in a post-AGB star. However, apart from H-deficiency, some of these stars share unique abundance anomalies, which tilt the balance towards the DD merger scenario and suggest a common evolutionary connection. While the cooler members — the cool RCBs and HdCs exhibit enhancement of 18O relative to 16O and depletion of 13C with respect to 12C, the hotter RCBs and the cool EHes showed enrichment of fluorine (F) with respect to iron (Fe) by 800-8000 times than solar, in their atmospheres. While the DD scenario can account for the observed abundance anomalies, these anomalies were not observed entirely across the sample, mainly in the cool DY Pers, and the hot EHes. In this talk, I explain my work investigating the evolutionary connection between the different members of H-deficient supergiants with a focus on the two extreme members of the group, the DY Persei variables and the hot EHes. Using multiwavelength (optical and NIR) spectroscopic observations obtained from 2m HCT, IAO India, as well as archival data (ESO-FEROS and UVES), I explore the status of the chemical peculiarities across the sample and investigate whether the proposed DD merger scenario can be accepted as the final formation scenario.
February 16, 2023 at 1:00 PM
Resolving HD327083
Paula Marchiano
(Universidad Nacional de La Plata, Argentina)
B[e] stars are a massive peculiar star showing an important excess in the near infrared due to the presence of circumstellar dust around. Besides, due to their mass loss these objects deposit huge amounts of mass and energy into their environment and enrich it with chemically processed material. HD 327083 exhibits these and other characteristics of the B[e] phenomenon, but when we study this object with more detail…we ask : Is HD327083 a truly B[e] supergiant?
January 26, 2023 at 1:30 PM
Massive stars and their hydrodynamic wind regimes
Ignacio Araya
(Universidad Mayor, Chile)
The study of massive stars is significant in several fields of astrophysics. These objects play a dynamical and kinematic role in the interstellar medium, depositing a large amount of energy and momentum via their strong stellar winds. This talk will focus on these powerful winds, called line-driven winds. Based on the original “CAK” model, I will summarise the main hydrodynamic (stationary and time-dependent) wind solutions under varied conditions (e.g., due to rapid rotation) and their applications to different types of massive stars. Finally, I will present our undergoing work to derive the stellar and wind parameters from massive stars.