## Department seminars

15 April 2021

### Large grids of model atmospheres for a rapid analysis of stellar spectra

#### Janos Zsargó

(Instituto Politécnico Nacional, Mexico)

We present a database of 43 340 atmospheric models for stars with stellar masses between 9 and 120 M☉, covering the region of the OB main-sequence and Wolf-Rayet stars in the Hertzsprung-Russell diagram. The models were calculated using the stellar atmosphere code CMFGEN. The parameter space has six dimensions: the effective temperature, the luminosity, the metallicity, and three stellar wind parameters: velocity law, the terminal velocity, and the volume filling factor. For each model, we also calculate synthetic spectra in the UV, optical, and near-IR regions. We also present the results of the reanalysis of ∊ Ori using our grid to demonstrate the benefits of databases of precalculated models. Our analysis succeeded in reproducing the best-fit parameter ranges of the original study, although our results favor the higher end of the mass-loss range and a lower level of clumping. Our results indirectly suggest that the resonance lines in the UV range are strongly affected by the velocity-space porosity, as has been suggested by recent theoretical calculations and numerical simulations.

26 March 2021

### Moving-mesh radiation hydrodynamics and an application to wind-reprocessed transients

#### Diego Calderón

(Charles University, Prague)

The development of surveys with high cadence on large fields has improved significantly in the last decades. This has made possible conducting detailed studies of many transient phenomena such as tidal disruption events, supernovae, luminous red novae among others. However, such improvements have posed a major challenge for explaining their nature, specifically on understanding how these sources are being powered. Numerical simulations of such processes are very challenging mainly due to the need of coupling radiation with hydrodynamics while, at the same time, covering a wide dynamic range spatially and temporarily. In order to overcome this problem, we developed a new module for coupling radiation into the moving-mesh hydrodynamics code JET. The moving-mesh nature of the code allows us to perform simulations with a wide dynamic range of more than five orders of magnitude spatially as well as temporarily. We present the code and its first applications for modelling wind-reprocessed transients.

11 February 2021

### PLATOSpec, new spectrograph for La Silla

#### Petr Kabáth

(ASU)

I will present the new spectrograph PLATOSpec planned for ESO La Silla observatory, Chile. PLATOSpec will be a modern echelle spectrograph sensitive in blue wavelengths with resolving power of R=70000 which will be mounted at E152 telescope (former ESO telescope). Main goal of the project is the support of the upcoming PLATO space mission. Scientific topics covered with PLATOSpec will range from exoplanets till stellar physics and beyond. Currently, the project is in the telescope upgrade stage and the spectrograph is undergoing the Design Review phase. Full operations are planned for the end of 2023. The project is led by the AsU in partnership with Pontifica Universidad Catolica de Chile, Chile and Thueringer Landessternwarte Tautenburg, Germany. I will describe the current status of the project, its time line and planned operations scheme. PLATOSpec and OES spectrographs will be compared and the accuracies will be described and put into context with other instruments.

10 December 2020

### Symbiotic binaries: the mystery of cool giants and hot dwarfs

#### Jaroslav Merc

(Charles University, Prague; P. J. Šafárik University, Košice, Slovakia)

Symbiotic stars are interacting binaries consisting of an evolved, cool giant transferring mass to a hot companion - a white dwarf or rarely a neutron star. The presence of both ionized and neutral regions in their surroundings, interacting winds, jets, accretion disks, or dust forming regions make them extraordinary astrophysical laboratories for studying various aspects of the interaction and evolution in binary systems. Although some of the symbiotic systems are studied for more than a hundred years, there are still several open questions concerning the mechanisms of activity, individual components, and their evolution. We will briefly review the important information on symbiotic stars and present the results connected with the New Online Database of Symbiotic Variables and long-term monitoring of selected objects.

12 November 2020

### Constraining the Rapid Neutron-Capture Process with Meteoritic I-129 and Cm-247

#### Andrés Yagüe López

(Konkoly Observatory, Budapest, Hungary)

Among all radioactive isotopes produced in the Galaxy, a small number of them have relatively short mean lives between 0.1 and 100 Myr. Early Solar System abundances of these radioisotopes can be determined through meteoritic analysis and, due to their short half lives, give us insight into the sites and processes that produced them. In this talk, I discuss the ratio of two of these short-lived radioisotopes, I-129 and Cm-247. I also show how, due to their remarkably similar half lives, they give us a unique opportunity to constrain the physical conditions of the last rapid neutron-capture process event that contributed to the enrichment of the pre-solar nebula.

15 October 2020

### ALMA and the Cool Universe

#### Abhijeet Borkar

(ASU)

The Atacama Large Millimeter/submillimeter Array (ALMA) is currently the largest radio telescope in the world, a complete imaging and spectroscopic instrument operating in the high frequency radio regime. It is located in northern Chile as a partnership between Europe (ESO), North America (NRAO, USA and NRC, Canada) and East Asia (NAOJ, Japan, ASIAA, Taiwan, and KASI, South Korea). ALMA provides an extensive breadth and depth of science, from cosmology & high redshift Universe, planet & star formation, to solar system objects, and offers capabilities which are unique and complimentary to major terrestrial and space telescope of the modern era. In this talk, I will provide an overview of ALMA telescope, operations, science capabilities & highlights and how you can use ALMA for your favourite science. I will also review the role of the Czech ALMA Regional Center node and preparations for the proposal Cycle.

24 September 2020

### Nebulosities of the symbiotic binary R Aquarii

#### Tiina Liimets

(ASU)

I will give an overview of the fascinating nebulae around one of the closest known symbiotic star R Aquarii. Together with historic and more recent observational data I will present results of our own long-term monitoring of these intriguing nebulosities: an ancient arcs, hour-glass nebula and puzzling, still active jet.

14 July 2020

### Wray 15-906: a post-red supergiant luminous blue variable discovered with WISE, Herschel and SALT

#### Olga Maryeva

(ASU)

Presently we know two evolutionary paths leading massive stars to Wolf-Rayet (WR) stage. Stars witn M>40Msun after the end of hydrogen burning in the core come to WR through the phase of Luminous blue variables (LBV). On the other hand, stars with lower mass (~30Msun) before LBV phase cross the stage of red supergiant (RSG). Theory predicts what the latters may explode as supernova (SN) directly after LBV phase showing before explosion the spectrum of WN11 type. However in practice in our Galaxy there are only three stars of WN11 type and all of them have higher initial masses, and therefore they did not pass through RSG phase. In my talk I will present results of study of recently discovered Galactic candidate luminous blue variable Wray 15-906. I will show how spectral classification was performed, and how stellar parameters were calculated. Estimated location on Hertzsprung-Russell diagram shows that Wray 15-906 is a post-red supergiant star. Collected spectral data together with results of modelling show that properties of Wray 15-906 are very similar to predicted ones for a star with initial mass of ≈25Msun, which will pass through WN11h stage right before exploding as a supernova.

11 June 2020

### Asteroseismology

#### Julieta Sanchez Arias

(ASU)

Variable stars offer a great opportunity to study their interiors through asteroseismology. This powerful astrophysics tool allows us to derive stellar parameters such as the mass, the radius, the metallicity and the age by the comparison between theoretical models and the frequency spectrum derived from observations. In this opportunity, I will present the challenges in the modelling of hybrids delta Stc-gamma Dor stars and the analysis of the light curve of one particular hybrid star with surface activity. In addition, I will introduce asteroseismic tools to distinguish two different kinds of variable stars, which usually lie in the same region of the seismic HR diagram: delta Sct stars and the precursors of the so-called extremely low mass white dwarf stars. Finally, I will present a brief overview of the potential of asteroseismology in massive stars.

30 January 2020

### Self-consistent solutions for line-driven winds using Lambert W-function

#### Alex C. Gormaz-Matamala

Hot massive stars present strong stellar winds which are driven by absorption, scattering and reemission of photons by the ions of the atmosphere (line-driven winds). A better comprehension of this phenomenon and a more accurate calculation of hydrodynamics and radiative acceleration is required to determine accurate mass-loss rates, and hence constrain evolutionary tracks of hot massive stars. The equation of motion for the stellar winds of a hot massive star is solved analytically by using the Lambert W-function. To solve radiative transfer equation in the stellar atmosphere and to calculate the radiative acceleration g_line(r) we use the non-LTE code CMFGEN. Since the acceleration depends now only on radial coordinate, it can be used to solve analytically the equation of motion by means of the Lambert W-function. An iterative procedure between the solution of the radiative transfer and the equation of motion is executed in order to obtain a consistent result.

29 October 2019

### Common envelope transients: buried in infrared

Most stars in our Universe live in binaries.  Unstable mass transfer from one star to another can lead to the formation of a shared gaseous non co-rotating shell where both stars orbit: the common envelope. The end of this phase is marked by the quick spiral-in of the secondary star towards its companion, leading to violent interactions between the components. The whole, or part of the binary's common envelope gets ejected, and the pair may even completely merge. This last phase has been serendipitously witnessed as astrophysical transients called luminous red novae (LRNe), allowing us to study the progenitor stars, the energetics of the outburst and the properties of the ejected material. In my talk, I will provide an overview of LRNe, their progenitor systems and their main formation scenarios, explored by recent theoretical models. Observations of these common-envelope transients show that the previous mass loss from the system is crucial to extract the angular momentum from the system. At later times, as the emission quickly fades in the optical bands, the infrared signature remains bright, revealing the formation of cold dust shells reprocessing the light of the newly coalesced star.

17 October 2019

### A Planet Search among Kepler Giant Stars

#### Marie Karjalainen

(IAC/The MAGIC Telescopes, La Palma, Spain)

Planet searches around intermediate mass K giant stars may provide us with important clues on a dependence of planet formation on stellar mass. To date over 120 exoplanets (3 % of the total) have been discovered orbiting giant stars. Unlike for a main-sequence star it is more problematic to determine the stellar mass of a giant star. Evolutionary tracks for stars covering a wide range of masses all converge to the similar region of the H-R diagram. Fortunately, the stellar mass can be derived from solar-like oscillations. The Kepler space mission was monitoring a sample of over 13,000 red-giant stars which can be used for asteroseismic studies. This represents a unique sample for planet searches as a planetary detection would mean that we can determine reliable stellar properties via asteroseismic analysis, characteristics not well known for many other planet-hosting giant stars. For this reason, in 2010 we started a planet-search program among 95 Kepler asteroseismic-giant stars, and we would like to give an overview of the roject with actual results here.

27 September 2019

### Simultaneous J, H, K and L band spectroscopic observations of galactic Be stars

#### Yanina Cochetti

(Universidad Nacional de La Plata, Argentina)

Be stars are rapidly rotating non-supergiant B-type stars whose spectra show or have shown Hα line in emission. Their infrared region is characterized by moderate flux excess and the presence of hydrogen recombination lines. Because of the small contribution of the photospheric absorption to hydrogen infrared lines, the near-infrared spectral region provides a powerful tool to infer physical properties, morphology, and dynamics of the circumstellar envelopes, which cannot easily be acquired from other spectral regions. In this talk, I will present near-infrared medium-resolution spectroscopic observations of a sample of 22 Galactic Be stars, with different spectral subtypes and luminosity classes. I will describe the main characteristics observed in the J, H, K and L bands (especially the hydrogen recombination lines from Paschen, Brackett, Pfund and Humphreys series) and the properties of the circumstellar environment derived from the shape and intensity of the line profiles.

24 September 2019

### Astrochemistry

#### Despina Panoglou

(Observatorio Nacional, Rio de Janeiro, Brazil)

Both the atmosphere and interior of Earth are largely molecular. Atoms do not exist free in the terrestrial atmosphere and whatever knowledge regarding atoms observed in space has to be acquired under laboratory conditions. On the contrary, out of Earth atoms are the norm. Whenever molecules are observed, an investigation of their existence plays a key role on the study of the ambiance where they are found. As chemistry studies the formation and destruction of bonds between atoms, astrochemistry is the study of synthesis of molecules in extraterrestrial environments. In this talk I give a short review on the reactions that involve the most common species, including ionizing agents, carbon chemistry, gas-dust interactions and photochemistry.

5 September 2019

### Radiation Line-driven wind theory vs observations

#### Lydia Cidale

(Universidad Nacional de La Plata, Argentina)

The classical theory of radiatively driven stellar winds often fairly represents the observed stellar wind conditions of massive stars. However, some discrepancies are still found between the parameters predicted by the theory and those observed in mid-B, late-B, and A-type supergiants. Inclusion of rotation and ionization in the models brought a remarkable progress in the development of the theory of stellar winds. Three types of stationary wind regimes are currently known: the classical fast solution, the $\Omega$--slow solution that arises for fast rotators, and the $\delta$--slow solution that takes place in highly ionized winds. I discuss these hydrodynamical solutions in the context of the observed wind properties of B and Be stars.

23 July 2019

### Spectral and photometric properties of UX Ori type star WW Vul

(Shamakhy Astrophysical Observatory, Azerbaijan)

1 July 2019

### First results of tunable-filter observations with MaNGaL

#### Alexei Moiseev

(Special Astrophysical Observatory, Nizhnij Arkhyz, Russia)

The Mapper of Narrow Galaxy Lines (MaNGaL) was developed in the Special Astrophysical Observatory of the Russian Academy of Sciences at the 1-m SAO RAS telescope and 2.5-m telescope of the Caucasus Observatory of the Sternberg Astronomical Institute of the Lomonosov Moscow State University. The instrument is based on the  low-order scanning Fapry-Perot interferometer (FPI) working as a narrow (bandwidth ~1.5 nm) filter precisely positioned at the selected emission lines. We present  the results of the first observations with MaNGaL of various Galactic nebulae and emission-line galaxies: star-formation regions and planetary nebulae, ionized cones around active galactic nuclei and galactic wings. The benefits and disadvantages of the tunable-filter observations as compared with other 3D-spectroscopy methods are considered.

21 June 2019

### Modeling molecules in stellar enviroments

#### Rodolfo Vallverdú

(Universidad Nacional de La Plata, Argentina)

30 May 2019

### All you ever wanted to know about hot subdwarfs

#### Maja Vuckovic

(Instituto de Física y Astronomía, Facultad de Ciencias,  Universidad de Valparaíso, Chile)

Hot subdwarfs stars of spectral type B (sdB) are associated with the so-called Extended Horizontal Branch forming a blue extension to the Horizontal Branch (HB). These stars correspond to the low-mass (about 0.47 Msun) objects burning He in their cores. However, they differ from HB stars mainly at the level of their residual H-rich envelope, which has been strongly reduced during the prior evolution phase, leaving only a tiny layer less massive than 0.02 Msun. As a consequence, sdB stars remain hot and compact (Teff ~ 22 000-40 000 K, log g ~ 5.2-6.2) throughout their He-burning lifetime, and never ascend the Asymptotic Giant Branch before reaching the white dwarf cooling tracks. They play an important role in our understanding of binary evolution, stellar atmospheres and interiors, and of the Galaxy itself. They are also known to pulsate and their asteroseismic properties are allowing us to probe their interiors and measure the sizes of their convective cores. In this talk I will present the current NASA Transiting Exoplanet Survey Satellite (TESS) mission and its' contribution to the field of hot subdwarfs. I will show our fresh asteroseismic results from the TESS — we have analyzed a total of 615 hot subdwarfs and candidates that have been observed in the first 8 sectors of the TESS space mission with a cadence of 2-min and found periodicities in about 50 out of which 34 are new pulsators.

16 May 2019

### A 5D map of the nearest open clusters from high-mass stars down to the substellar regime

#### Nicolas Lodieu

(Instituto de Astrofisica de Canarias, Spain)

We present a 5D map of four of the nearest clusters to the Sun: Alpha Persei (d~178 pc, 85 Myr), the Pleiades (d~135 pc; 125 Myr), the Hyades (d~46 pc; 650 Myr; Lodieu et al. 2019), and Praesepe (d~187 pc; 590 Myr). We identified bona-fide kinematic members from high-mass stars down to the hydrogen-burning limit and below (depending on the distance and age of the cluster) in the second data release of Gaia. We revised the physical sizes of the clusters, and inferred updated mean distances and velocities. We derive the luminosity and mass functions and compare them to the log-normal form of the Chabrier field mass function. We also looked at the 3D spatial distribution of members and produced movies of the new members in 3D space. We find that high-mass stars tend to be located in the central regions of the clusters while low-mass stars are more frequent beyond the half-mass radii. We clearly confirm the presence of a stream in the Hyades and the Pleiades. We also compare the age of these clusters, from the literature, with the ages that we obtain from a few white dwarfs belonging to the clusters (Lodieu et al. 2019a,b).

17 December 2018

### On the rotation period of the O giant ξ Persei: a magnetic star?

#### Natallia Sudnik

(The Maxim Tank Belarusian State Pedagogical University)

Many spectral lines in OB stars show unexplained variability on a rotational timescale. This occurs for example in the so-called discrete absorption components (DACs) in UV wind-line profiles and in many wind-sensitive optical lines. This variability is generally considered to be cyclical (like sunspots), rather than periodic. The absence of strict periodicity is in accordance with the lack of evidence for a permanent magnetic field, with typical upper limits of ∼ 300 G. We aim to identify regions in spectra of the O7.5III(n)((f)) star ξ Persei that are formed very close to the star and which suffer a minimum of contamination with disturbing features like doublet overlap or irregular surface phenomena which may prevent the detection of a periodic signal. We present strong evidence for a rotation period of 2.0406 d of the ξ Per, derived from the N IV λ1718 wind line in 12 yr of IUE data. Since this period can be ruled out as due to nonradial pulsations, we predict that ξ Per has a (corotating) magnetic dipole field. We calculate the most favorable phase to attempt new magnetic measurements. In contrast to earlier work, we can exclude ∼ 4 d as the rotation period, since this is constrained by the stellar parameters. The sinusoidal behavior implies that only one magnetic pole is visible, which gives i ∼ 56, and hence β should be near 90− i = 34. We propose that the azimuthal distribution of strong DACs corresponds to the location of a magnetic pole on the surface. The crucial observational test consists of detecting a magnetic field.

5 December 2018

### Using hot subdwarf binaries to constrain RLOF models

#### Joris Vos

(The Institute of Physic and Astronomy, University of Potsdam, Germany)

Hot subdwarf B (sdB) stars are evolved core helium burning stars that have lost most of their hydrogen envelope due to binary interaction on the red giant branch. As sdB stars in wide binary systems can only be created by stable Roche lobe overflow, they are a great test sample to constrain the theoretical models for stable mass loss on the red giant branch. We have setup a long term monitoring program using high resolution spectrographs on different telescopes to create a sample of solved long period sdB binaries. An important advantage of using wide sdB binaries in these studies is that all of them are double lined binaries, and the GAIA data shows that it is a uniform population of canonical sdB stars. This way the sdB+MS binaries provide much stronger constraints on theoretical models than many other systems. The first results of our observing program are now available. We found two main features in the orbital parameters. The majority of the systems has eccentric orbits with systems on longer orbital period having a higher eccentricity. As these systems have undergone mass loss near the tip of the RGB, circularisation theory predicts them to be circularized. Our observations suggest that efficient eccentricity pumping mechanisms are active during the mass loss phase. A second finding is a strong correlation between the mass ratio and the orbital period. Using binary evolution models, this relation is used to derive an upper limit on the critical mass ratio for stable RLOF which depends on the orbital period of the system. Furthermore a split in the P-q relation seems to indicate two different groups with somewhat different formation histories.

28 November 2018

### HDUST and SHELLSPEC codes - two tools for modelling the stellar observables

(Tartu Observatory, Estonia)

31 October 2018

### Binary stars as the key to understand planetary nebulae

#### Henri Boffin

(ESO Garching, Germany)

Binarity and mass transfer appear to play a key role in the shaping and, most likely, in the formation of planetary nebulae (PNe), thereby explaining the large fraction of axisymmetric morphologies. I present the binary hypothesis for PNe and its current status. Recent discoveries have led to a dramatic increase in the number of post-common envelope binary central stars of PNe, thereby allowing us to envisage statistical studies. Moreover, these binary systems let us study in detail the mass transfer episodes before and after the common envelope, and I present the evidences for mass transfer - and accretion - prior to the common envelope phase.

14 August 2018

### Spectral research of young stars at Shamakhy Astrophycial Observatory

(Shamakhy Astrophycial Observatory, Azerbaijan)

7 February 2018

### OPERA tool for reduction of echelle spectra from OES

#### Mgr. Roaman Grossova

(Masaryk University, Brno, Czech Republic)

OPERA is an open-source software package developed by a CFHT scientific group in Hawaii for reducing echelle data from ESPaDOnS. Later on, it was adapted to several other instruments, including OES. In comparison with the reduction software IRAF, OPERA was able to solve a long-standing problem of tilted lines in the OES spectra. A user-friendly GUI framework was developed for the fast routine reduction of your scientific data.

27 September 2017

### Binary stars with RR Lyrae component - why so rare?

#### Marek Skarka

(Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Science)

Large portion of stars of all spectral types is bound in binary systems. However, there is one spectacular exception. Among 100 000 catalogized stars of RR Lyrae type only several tens of binary candidates are known and none of them has been unambiguously confirmed yet. Could this lack of binaries be real? Why it is so difficult to discover RR Lyrae star in binary system? Are there any observational limitations? What methods are suitable for the search for such binaries? I will discuss all these questions, give an overview about the progress in last three years, and will show why many of the candidates could actually be false positives.

23 August 2017

### Instrumentation at Astronomical Institute in the High Tatras-Slovakia

#### Martin Vanko

(Astronomical Institute of Slovak Academy of Sciences)

Astronomical Institute of the Slovak Academy of Sciences (AI SAS) runes several observatories located at different places in the High Tatras. Our telescopes are equipped with new high-end post-focus instruments producing valuable data in the field of the stellar and exoplanetary astronomy. First results will be presented in my talk. Finally, I will present existing and future projects between our Institutes.

18 May 2017

### Blue atmosphere or stellar activity – is the blue atmosphere of the exoplanet GJ 3470 b real?

#### Silvia Kunz

(Thueringer Landessternwarte Tautenburg, Germany)

One of the big challenges in the field of exoplanet atmospheres is to distinguish the influence of stellar activity on transit measurements from real effects that can be seen in photometric measurements in different bands. Oshagh et al. (2014) assumed that an increase of planetary radii in the blue part of the spectrum can be explained with the presence of a hot plage region on the stellar surface. In particular they stated that the measured blue atmosphere of the sub-Uranus-mass low-density planet GJ 3470b could be mimicked by a plage region that covers only 2.56% of the star’s surface. We have developed a method to exclude the influence of plage regions on transit measurements. The Ca II H,K lines are tracers of stellar activity – especially of plage regions. If plage regions were occulted those lines should vary during transit. We therefore have observed one transit of GJ 3470 b with the high-resolution UVES spectrograph at the 8.2 m Very Large Telescope. We have found that the difference of the Ca II H,K lines in- and out-of-transit is only 0.67 ± 0.22% and have determined a magnetic filling factor of about 10–15%. In order to confirm the Rayleigh scattering slope we have analyzed the planet‘s lightcurve by observing three transits with the low-resolution OSIRIS spectrograph at the 10.4 m Gran Telescopio Canarias. With those almost simultaneous measurements of GJ3470b‘s transit we  were able to confirm its Rayleigh scattering slope towards the blue."