IAU Commission G5 Talks

(This seminar is organized by the IAU G5 commission on stellar and planetary atmospheres) 

Task-based computing is a method where computational problems are split
   into a large number of semi-independent tasks (cf.
   2018MNRAS.477..624N). The method is a general one, with application not
   limited to traditional grid-based simulations; it can be applied with
   advantages also to particle-based and hybrid simulations, which involve
   both particles and fields. The main advantages emerge when doing
   simulations of very complex and / or multi-scale systems, where the
   cost of updating is very unevenly distributed in space, with perhaps
   large volumes with very low update cost and small but important regions
   with large update costs.

   Possible applications in the context of stellar atmospheres include
   modelling that covers large scales, such as whole active regions on the
   Sun or even the entire Sun, while at the same time allows resolving
   small-scale details in the photosphere, chromosphere, and corona. In
   the context of planetary atmospheres, models of pebble-accreting hot
   primordial atmospheres that cover all scales, from the surfaces of
   Mars- and Earth-size embryos to the scale heights of the surrounding
   protoplanetary disks, have already been computed (2018MNRAS.479.5136P,
   2019MNRAS.482L.107P), and one can envision a number of applications
   where the task-based computing advantage is leveraged, for example to
   selectively do the detailed chemistry necessary to treat atmospheres
   saturated with evaporated solids, or to do complex cloud chemistry
   combined with 3-D radiative transfer.

   In the talk I will give a quick overview of the principles behind
   task-based computing, and then use both already published and still
   on-going work to illustrate how this may be used in practice. I will
   finish by discussing how these methods could be applied with great
   advantage to problems such as non-equilibrium ionization, non-LTE
   radiative transfer, and partial redistribution diagnostics of spectral
   lines.

The ExoMol project (www.exomol.com) provides comprehensive spectroscopic data (line lists) for the study of atmospheres of exoplanets and other hot bodies.  These line lists serve as input for models of radiative transport through hot atmospheres and are useful for a variety of terrestrial applications. The basic form of the database is extensive line lists; these are supplemented with partition functions, state lifetimes, cooling functions, Landé g-factors, temperature-dependent cross sections, opacities, k-coefficients and pressure broadening parameters. Currently containing 80 molecules and 190 isotopologues totaling over 700 billion transitions, the database covers infrared, visible and UV wavelengths. The field of the HR spectroscopy of exoplanets is growing extremely fast and urgently demands molecular data of high precision. Failure to detect molecules in atmospheres of exoplanets is often attributed to the lack of the underlying quality of
the line positions.  These developments have led us to begin a systematic attempt to improve the accuracy of the line positions for the line lists contained in the database. Our new ExoMolHD project aims to provide comprehensive line lists to facilitate their use in characterization of exoplanets using high resolution Doppler shift spectroscopy. Progress on this objective will be presented.

The discoveries of thousands of extrasolar planets have revealed an astonishing diversity in their physical characteristics - orbital properties, masses, radii, temperatures, and host stars. Exoplanets known today range from super Jupiters to Earth-size rocky planets over a wide range of temperatures, including several in the habitable zones of their host stars. Recent advances in atmospheric spectroscopy of exoplanets are leading to unprecedented insights into their atmospheric properties. I will discuss some of these developments in atmospheric characterisation of exoplanets and their implications for understanding atmospheric processes, formation mechanisms, and habitability across a wide range of planetary bulk properties. A survey of theoretical and observational directions in the field will be presented along with some open questions on the horizon.

Zoom:

https://rediris.zoom.us/j/86558203608?pwd=OXQyMmVMQTI2M2VSRUZldThkR0JqUT09

Meeting ID: 865 5820 3608

Passcode: 055970

Youtube. https://youtu.be/GvXv2RMbFz8

The new generation of spectrometers designed for extreme precision radial velocities enable correspondingly precise stellar spectroscopy. It is now fruitful to theoretically explore what the information content would be if stellar spectra could be studied with spectral resolutions of a million or more, and to deduce what signatures remain at lower resolutions. Hydrodynamic models of stellar photospheres predict how line profiles shapes, asymmetries, and convective wavelength shifts vary from disk center to limb. Corresponding high-resolution spectroscopy across spatially resolved stellar disks is now practical using differential observations during exoplanet transits, thus enabling the testing of such models. A most demanding task is to understand and to model spectral microvariability toward the radial-velocity detection of also low-mass planets in Earth-like orbits around solar-type stars. Observations of the Sun-as-a-star with extreme precision spectrometers now permit searches for spectral-line modulations on the level of a part in a thousand or less, feasible to test against hydrodynamic models of various solar features.

The field of Galactic archaeology has been very active in recent years, with a major influx of data from the Gaia satellite and large spectroscopic surveys. The major science questions in the field include Galactic structure and dynamics, the accretion history of the Milky Way, chemical tagging, and age-abundance relations. I will give an overview of GALAH as a large spectroscopic survey, and describe how it is complementary to other ongoing and future survey projects. I will also discuss recent science highlights from the GALAH team and compelling questions for future work.

Accurate non-LTE modelling of stellar spectra, and thus the derivation of accurate stellar properties such as chemical composition, temperature, and surface gravity, depends on reliable atomic data.  While inelastic electron collisions are very important even in FGK stars, the relative importance of the more abundant yet less efficient hydrogen collisions has for a long time been a major source of uncertainty.  Over the last decades, considerable progress has been made on the theoretical side, as well as with experiments for the important charge transfer processes involving hydrogen, both covering a significant number of astrophysically important atoms.  I will review this progress, and discuss where further progress might be needed.

Accelerating Computational Modeling via Neural Networks: Application to Exoplanet Atmospheric Retrieval

IIn physics and astronomy, computationally expensive forward models are often an integral part of preparing experiments/observations, analyzing data, and/or planning future instrumentation/telescopes.  In many of these cases, machine learning (ML) models, such as neural networks (NNs), can offer a significant reduction in compute time with minimal loss in accuracy.  We demonstrate this approach on the problem of exoplanet atmospheric retrieval, which involves on the order of 10^5 -- 10^6 radiative transfer (RT) model evaluations.  We find that the ML RT approach yields the same scientific conclusions as the traditional method, while requiring ~1000x less compute cost for typical setups.  We present our open-source software packages that implement this technique, and we discuss broader applications of this NN surrogate modeling approach.

The first magnetic Helium-sdOs: which mergers are magnetic?

Magnetic fields play an important role throughout stellar evolution, and among white dwarfs, the end stage of 95% of all stars, the fraction of strongly magnetic systems is larger than 20%. The origins of magnetic white dwarfs are still under discussion, but it is likely that a significant fraction of them are formed by stellar mergers.

Several types of merger remnants are thought to ignite helium fusion, such as the merger of a helium-WD (He-WD) with a second He-WD, a He/C/O hybrid WD, or a low-mass main sequence star, thus forming a hot subdwarf star. The majority of hot subdwarf stars are helium burning stars with very thin or no hydrogen envelopes. In particular, most of the hot and helium-poor He-sdO stars are thought to be formed by mergers. However, out of hundreds of hot subdwarfs studied over several decades, none showed detectable magnetic fields.
This changed recently, when four almost identical magnetic He-sdO stars were discovered, with mean field strengths between 300 and 500kG.

Why are these stars magnetic while vast majority of other He-sdOs are non-magnetic? This question is still open. In this talk I will give a short introduction to He-sdO stars and their formation and then try to highlight the differences between the four magnetic stars and their non-magnetic cousins.

We present a sample of 734 ultracool dwarfs using LAMOST DR7 spectra, i.e.,  those having a spectral type of or later than M6, including an L0.  All of these red or brown dwarfs are within 360 pc, with a Gaia G magnitude brighter than 19.2 mag, a BP-RP color redder than 2.5 mag  and an absolute G magnitude fainter than 9 mag. Their stellar parameters  (Teff, log g, and [M/H]) are consistent with being the Galactic thin-disk  population, which is further supported by their kinematics using LAMOST radial velocity plus Gaia proper motion and parallax.  A total of 77 are detected  with the lithium absorption line at 6708 A, signifying youth and substellar nature. We report on their kinematic ages estimated by the velocity dispersion. Thirty five close pairs are identified, of which the binarity of six is discovered for the first time.

Massive stars (at least eight times as massive as the Sun) possess strong stellar winds driven by radiation. With the advent of the so called MiMeS collaboration, 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.

https://youtu.be/jKmifm17bno

Unlike the giant planets in the solar system, "hot Jupiter" giant exoplanets are subject to intense stellar irradiation, which drives their atmospheres from ~100 K (in the solar system) to ~1000 to 3000 K.  In this talk I will review several aspects of their atmospheric physics and chemistry, with a particular emphasis on the cooler "warm Jupiters" below 1000 K where we expect to see some interesting transitions in atmospheric chemistry and clouds with JWST.  I will discuss new JWST spectra and modeling work on 1100 K exoplanet WASP-39b, which will be published in Nature in a few days time, as part of the JWST Early Release Science (ERS) program.  The spectra show some expected things but also some very interesting surprises!

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Zoom: https://rediris.zoom.us/j/82072236238?pwd=MWFRSXFnQ0lUN3ZMTzgzTGQxS0ZGUT09

Meeting ID: 820 7223 6238 
Passcode: 648570

YouTube: https://youtu.be/jbFP6L-7mxo

Spectroscopic analyses of stellar chemical compositions are model-dependent, and shortcomings in the models often limit the accuracy of the final results.  For late-type stars like our Sun, two of the main problems in present-day methods are that they assume the stellar atmosphere is a) one-dimensional (1D) and hydrostatic, and b) satisfies local thermodynamic equilibrium (LTE).  We can relax these assumptions simultaneously by performing detailed 3D non-LTE radiative transfer post-processing of 3D radiative-hydrodynamic model stellar atmospheres.  I shall give a brief overview of this approach, and illustrate its impact on carbon, oxygen, and iron abundances in late-type stars.

 Gaia all-sky scanning and multi-epoch photometric and spectroscopic observations offer unprecedented opportunities to detect and characterise magnetic activity and rotation of low-mass stars. In the second Gaia data release, rotation period and rotational modulation amplitude for ≈150,000 low-mass stars were provided from the analysis of the first 22-month of data collection. The third Gaia data release, based on the analysis of the first 34 months of data collection, contains rotational modulation data for ≈500,00 low-mass stars and an activity index for ≈2 million low-mass stars, this latter derived from the analysis of the CaII infrared triplet (IRT) observed by the RVS instrument. These constitute the largest catalogues of magnetic activity and rotational period to date. Such a large number of all-sky measurements represents a gold mine for studies related to stellar rotation, magnetic activity, and mass accretion. The analysis of the period-amplitude diagram of the rotational modulation has already provided evidence of distinct magneto-rotational regimes in the early phase of stellar evolution and of rapid transitions between them. Such evidence challenges a dependence on rotation only and suggests an important role of the rotational history in the early phase of the magneto-rotational evolution. The activity index derived from the CaII IRT clearly show three regimes, confirming suggestions made by previous authors on much smaller datasets. For the first time, a dramatic change in the activity distribution is found for Teff < 3500 K, with a dominance of low activity stars close to the transition between partially- and fully-convective stars and a rise in activity down into the fully-convective regime.

Planets orbiting close to hot stars experience intense extreme-ultraviolet radiation, potentially leading to atmosphere evaporation and to thermal dissociation of molecules. However, this extreme regime remains mainly unexplored due to observational challenges. Only a single known ultra-hot giant planet, KELT-9b, receives enough ultraviolet radiation for molecular dissociation, with a day-side temperature of ~4,600K. An alternative approach uses irradiated brown dwarfs as hot-Jupiter analogues. With atmospheres and radii similar to those of giant planets, brown dwarfs orbiting close to hot Earth-sized white dwarf stars can be directly detected above the glare of the star.

In this talk I will present the discovery of an extremely irradiated low-mass companion to the hot white dwarf WD0032–317, focusing on the observational aspects of the discovery. Our analysis indicates a day-side temperature of ~8,000K, and a day-to-night temperature difference of ~6,000K. The amount of extreme-ultraviolet radiation received by WD0032–317B is equivalent to that received by planets orbiting close to stars as hot as late B-type stars, and about 5,600 times higher than that of KELT-9b. With a mass of ~75–88 Jupiter masses, this near-hydrogen-burning-limit object is potentially one of the most massive brown dwarfs known.

The Institute for Astrophysics and Geophysics (IAG) in Göttingen operates a Fourier-Transform Spectrograph (FTS) that can be fed by either a 50cm siderostat or an integrating sphere. In 2016, we published a solar flux atlas (disk-integrated) at wavelengths 405-2300nm. Being largely comparable to the Kitt Peak spectral atlases in terms of resolution and signal-to-noise, our focus lies on an accurate calibration of frequencies (wavelengths) because we are interested in absolute convective blueshift of solar surface features.
Recently, we published an atlas of the quiet solar surface at different limb positions (spatially resolved Sun) and investigated convective velocities across the solar surface and in different spectral lines (Ellwarth et al., 2023 and in press). Furthermore, we studied the impact of centre-to-limb variation on exoplanet transit observations (Reiners et al., 2023). We are currently collecting data of active solar regions with the goal to measure convective blueshift as a function of limb position and magnetic field strength. In the talk, I will provide an overview of our program with a focus on the challenges we met in our attempts to determine Doppler velocities at the solar surface at m/s accuracy.

Zoom link: https://rediris.zoom.us/j/97531318437?pwd=TEpGZGVkNkpBcXBLWVlNTGt6bkdNUT09
Meeting ID: 975 3131 8437

Passcode: 147922

Activity of cool dwarf stars can reveal itself in the form of high-energy radiation (eg, enhanced X-ray coronal emission, flares) and particles (eg, winds, coronal mass ejections). Together, these phenomena shape the space weather around (exo)planets. Because most of the known exoplanets have significantly closer orbital distances than solar system planets, they are often embedded in much harsher particle and radiation environments, leading to stronger interactions between the exoplanet and its surrounding environment. In this talk I will present an overview of how stellar activity and outflows can induce and shape atmospheric escape in exoplanets. I will focus mostly on close-in gas giant planets, whose escaping atmospheres are somewhat easier to observe. I will then discuss how the observability of atmospheric escape, through spectroscopic transits, evolve on billions of years timescales.

The James Webb Space Telescope has shown us that we cannot consider exoplanet atmospheres as globally uniform objects in chemical equilibrium. On the contrary: day and night sides can have a completely different composition and disequilibrium effects such as mixing and photochemistry will affect chemical abundances, and hence, our understanding of how these planets were formed.

In this talk, I will present new chemical models that incorporate disequilibrium chemistry and three-dimensionality in hot Jupiter atmospheres. I will show that photochemistry can have a global impact and may explain some enigmatic observations on ultra-hot planets. Finally, I will discuss the important effect that photodissociation and ionization have on the atmospheric structure of exoplanets.

Elemental abundances of Sun-like stars are crucial for understanding the detailed properties of their planets. However, measuring elemental abundances in M stars is challenging due to their faintness and pervasive molecular features in optical spectra. To address this, elemental abundances of Sun-like stars have been proposed to constrain those of M stars by scaling [X/H] with measured [Fe/H] – a practice is yet to be well tested. Here we compile elemental abundances for 43 M dwarfs for 10 major rock-forming elements (Fe, C, O, Mg, Si, Al, Ca, Na, Ni, and Ti) from high-resolution near-infrared stellar surveys (APOGEE, CARMENES and Subaru/IRD). We perform bootstrap-based linear regressions on the M dwarfs to determine the trends of [X/H] vs. [Fe/H] and compare them with GK dwarfs (from GALAH + APOGEE). A 2-sample, multivariate Mahalanobis Distance test is applied to assess the significance of differences in [X/H]—[Fe/H] trends for individual elemental pairs between M and GK dwarfs. The null hypothesis of no significant difference in chemical trends between M and GK dwarfs is strongly rejected for all elements except Si, for which rejection is marginal, and Na and Ni, for which results are inconclusive. This suggests that assuming no difference may lead to biased results and inaccurate constraints on rocky planets around M dwarfs. Therefore, it is crucial for both the stellar and exoplanet communities to recognise these differences. To better understand these differences, we advocate for dedicated modelling techniques for M dwarf atmospheres and more homogeneous abundance analyses. Our statistically constrained trends of [X/H]—[Fe/H] for M dwarfs offer a new constraint on estimating M-dwarf elemental abundances given measured [Fe/H], aiding in detailed characterisation of M dwarf-hosted rocky worlds in the era of JWST, PLATO and ELT.

Albeit rare in absolute numbers, massive stars are shaping our cosmic history as they are connected to many astrophysical key processes. Commonly defined as stars with an initial mass of more than 8 times the mass of our Sun, massive stars are the progenitors of black holes and neutron stars, reaching all nuclear burning stages before eventually undergoing their inevitable core collapse. In their comparably short life, these luminous objects have an enormous impact on their galactic environment, enriching the surrounding medium with momentum, matter, and ionizing radiation. This so-called "feedback" of massive stars is a building block for the evolution of galaxies, initiating and inhibiting further star formation. In the "afterlives" of massive stars, black holes and neutron stars can merge with each other, giving rise a to Gravitational Wave events. Yet, overall textbook picture typically drawn of massive stars is rather sketchy and often also at odds with observational constraints. New frontiers such as the strong metal-enrichment in high-redshift galaxies discovered by JWST or the black hole statistics obtained from Gravitational Waves only add further pieces to the enigmatic massive star puzzle.

For a better understanding of massive stars, it is essential to properly determine their parameters and feedback. For young and hot massive stars, many properties are only accessible via spectroscopy. Their quantitative measurements and predictions rely on suitable models for stellar atmospheres, which requires sophisticated simulations to account for their non-equilibrium conditions and strong stellar winds. In this talk, I will introduce the techniques and challenges of atmosphere modelling for hot, massive stars and their winds. Afterwards, I will present a selection of the research efforts within my group demonstrating the range of empirical and theoretical applications of modern non-LTE stellar atmosphere models, such as the analysis of important landmarks of massive star evolution, the search for "hidden" post-interaction binaries, or theoretical insights on radiation-driven winds. Finally, I will give an outlook on current observational challenges and theoretical insights from 2D and 3D simulations raising a new need to reconsider some of the current paradigms in massive star atmosphere modelling.