arXiv subjects

Thorsten Balduin

Publications and source records attributed to Thorsten Balduin.

3 recordsLinked to original sources

arXiv2024

A grid of self-consistent MSG (MARCS-StaticWeather-GGchem) cool stellar, sub-stellar, and exoplanetary model atmospheres

Computation of a grid of self consistent 1D model atmospheres of cool stars, sub-stellar objects and exoplanets in the effective temperature range 300K to 3000K, including cloud formation, chemical non-equilibrium effects, and stellar irradiation. The models are called MSG, because they are based on an iterative coupling between three well tested codes, the MARCS stellar atmosphere code, the StaticWeather cloud formation code and the GGchem chemical equilibrium code. It includes up-to-date molecular and atomic opacities, cloud formation and advanced chemical equilibrium calculations, and involves new numerical methods at low temperatures to allow robust convergence. The coupling between the MARCS radiative transfer and GGchem chemical equilibrium computations has made it possibly effectively to reach convergence based on electron pressure for the warmer models and gas pressure for the cooler models, enabling self-consistent modelling of stellar, sub-stellar and exoplanetary objects in a very wide range of effective temperatures. Here we describe the basic details of the models, with illustrative examples of cloudy and irradiated models as well as models based on non-equilibrium chemistry. The qualitative changes in the relative abundances of TiO, H2O, CH4, NH3, and other molecules in our models follow the observationally defined M, L, T (and Y) sequences, but reveal more complex and depth dependent abundance changes, and therefore a spectral classification depending on more parameters. The self consistent coupling to Static-Weather cloud computations, allows detailed comparison between nucleation and observed relative dimming of different spectral bands, with advanced applications for new identification methods of potential exoplanetary biology.

astro-ph.EP↗

arXiv2023

On the origin of V-shaped polarisation spectra in molecular clouds

We extend previous theoretical works to gain a better understanding of the origin of observed polarisation degree spectra of molecular clouds, which show a so-called V-shape, i.e. a pronounced minimum around 350~$\mu$m. For this purpose, we present results of two-phase dust models investigated with POLARIS. We also provide a guideline to calculate individual dust temperatures for different grain types in POLARIS. We show that V-shaped polarisation spectra can only be obtained if two dust phases, one dense and cold as well as one warm and dilute phase, are present along the line of sight. We find that the V-shape is the stronger pronounced the larger the density and temperature contrast between both phases is. In contrast to previous results, no correlation between the alignment efficiency of silicate grains and the dust temperature is required; carbonaceous grains are in general assumed to be not aligned with the magnetic field. By matching our model results with actual observations of V-shaped polarisation spectra, we show that in UV-illuminated regions (here the warm and dilute phase) carbon grain destruction might take place. This leads to a more pronounced V-shape with a minimum around 300~$\mu$m. In addition, we show that the dust spectral index and temperature of silicate grains affects the steepness of the polarisation spectrum at long wavelengths. Finally, we present a first polarisation spectrum obtained from a 3D, magneto-hydrodynamical molecular cloud simulation. It shows a flattening or even weakly pronounced minimum around 350~$\mu$m demonstrating the potential of such complex 3D simulations to study polarisation spectra.

astro-ph.GA↗

arXiv2023

Size-dependent charging of dust particles in protoplanetary disks Can turbulence cause charge separation and lightning?

Protoplanetary disk are the foundation of planet formation. Lightning can have a profound impact on the chemistry of planetary atmospheres. The emergence of lightning in a similar manner in protoplanetary disks, would substantially alter the chemistry of protoplanetary disks. We aim to study under which conditions lightning could emerge within protoplanetary disks. We employ the ProDiMo code to make 2D thermo-chemical models of protoplanetary disks. We included a new way of how the code handles dust grains, which allows the consideration of dust grains of different sizes. We investigate the chemical composition, dust charging behaviour and charge balance of these models, to determine which regions could be most sufficient for lightning. We identify 6 regions within the disks where the charge balance is dominated by different radiation processes and find that the emergence of lightning is most probable in the lower and warmer regions of the midplane. This is due to the low electron abundance ($n_{\rm e}/n_{\rm\langle H \rangle}<10^{-15}$) in these regions and dust grains being the most abundant negative charge carriers ($ n_{\rm Z}/n_{\rm\langle H \rangle}> 10^{-13}$). We find that $\rm NH4^+$ is the most abundant positive charge carrier in those regions at the same abundances as the dust grains. We then develop a method of inducing electric fields via turbulence within this mix of dust grains and $\rm NH_4^+$. The electric fields generated with this mechanism are however several orders of magnitude weaker than required to overcome the critical electric field.

astro-ph.EP↗