Models of stellar atmospheres

1D models

Model atmospheres of cool stars

A one-dimensional (1D) model atmosphere is essentially a comprehensive table that details the temperature, pressure, and other properties of the gas as they vary with depth below the stellar surface. Uppsala astronomers maintain and develop a large data base of 1D model atmospheres called MARCS, which is freely available online. These models are widely used for interpreting the spectra of cool stars and also serve as boundary conditions for models of stellar interiors.

Models of stellar atmospheres are necessary for studies of many of the properties of stars. Prominently the composition of all the chemical elements is encoded in their spectra. The light emanating from the stellar surface is created by the interaction of light with the atoms and molecules in the stellar surface layers. This light originates in the heat that continuously escapes from the nuclear reactions in the stellar centre. The model atmospheres have also been used to study the atmospheres of extreme stars such as red supergiants, where accurately determining effective temperatures and bolometric luminosities is critical. By comparing our models with observations, we are able to refine the temperature scale for these stars, achieving better consistency with stellar evolutionary tracks and providing an independent check on the accuracy of our models.

Our research has significantly expanded the MARCS grid, covering a broad range of stellar parameters including effective temperatures from 2500K to 8000K, surface gravities, and varying chemical compositions. These models incorporate recent updates in atomic and molecular data, ensuring accurate representation of the physical processes in the atmospheres of late-type and cool stars. We have also explored the effects of sphericity, molecular blanketing, and carbon enhancement, finding both regularities and departures in the behavior of cool star atmospheres. This detailed grid of models is crucial for analyzing stellar spectra and contributes to large-scale surveys, such as those conducted by SDSS-III and APOGEE, both of which aim to map the chemical evolution of the Galaxy.

In addition to the 1D models, our work extends to non-local thermodynamic equilibrium (non-LTE) calculations, which account for the complexities of stellar atmospheres that cannot be captured under the LTE assumption. These non-LTE models, particularly for elements like hydrogen, carbon, and oxygen, have been integrated into large spectroscopic surveys, providing more precise stellar abundance measurements and revealing important insights into the chemical evolution of the Milky Way.

Advanced 1D model atmospheres of early-type stars

One-dimensional theoretical codes are commonly employed to model atmospheres of early-type stars, assuming solar or scaled-solar chemical composition. Such tools are generally not applicable to B, A, and F stars with anomalous surface abundances. In these stars line opacity differs considerably from normal stars and also varies substantially from one star to another, depending on individual chemical composition and occasional presence of strong magnetic fields. The properties of stellar atmospheres may also vary significantly across the stellar surface due to horizontal chemical and magnetic inhomogeneities.

We use an advanced line-by-line opacity sampling 1-D stellar model atmosphere code LLMODELS which allows to treat the effects of non-solar abundances, modification of the line opacity due to Zeeman splitting, magneto-hydrostatic equilibrium, and an inhomogeneous vertical and horizontal distribution of chemical elements. Inclusion of these effects enables a considerably more accurate modelling of stellar spectra than possible with standard solar or scaled-solar composition models.

In our recent studies, we have applied these sophisticated models to a range of early-type stars, these stars may have strong magnetic fields and unusual chemical properties. For example, spectropolarimetric investigations of stars like the young, double-lined spectroscopic binary V1878 Ori show that even stars with similar fundamental characteristics can display vastly different global magnetic field structures. By employing techniques such as Zeeman Doppler Imaging (ZDI), we can create detailed maps of magnetic field topologies. When combined with our LLMODELS code, these maps offer a deeper insight into the atmospheric and magnetic complexities of these stars.

Our research also includes the measurement of surface magnetic fields in intermediate-mass T Tauri stars. Using high-resolution near-infrared spectra and analyzing Zeeman broadening in magnetically sensitive spectral lines, we can estimate magnetic field strengths and explore their effects on the atmospheres of these stars. Our results suggest that intermediate-mass T Tauri stars generally have weaker magnetic fields compared to their lower-mass counterparts, a finding that has important implications for models of pre-main-sequence stellar evolution.

We continue to systematically apply our advanced model atmospheres to derive fundamental parameters and chemical composition amongst other stellar properties, allowing for more accurate and detailed studies of chemically peculiar and magnetically complex early-type stars.

3D models

The "weather" in the atmosphere of a cool star like the Sun is shaped by overturning convective flows, pulsating waves, rotating tornadoes, and small-scale turbulent motions. The movie above, created from a 3D computer simulation using the CO5BOLD code by Uppsala astronomers, illustrates these processes. It captures the light intensity of a small patch of the solar surface: bright granules bring hot material from the star's interior to the surface, where it cools and sinks back into the darker intergranular lanes. Similar to stationary 1D model atmospheres, dynamic 3D models help analyze observed stellar spectra, offering insights into temperature fluctuations and stellar motions. Comparing observed and synthetic spectra deepens our understanding not just of stellar atmospheres, but also of the interior and outer layers.

Uppsala astronomers study the atmospheres of Sun-like stars as well as those with more extreme characteristics. For example, asymptotic giant branch (AGB) stars experience significant mass loss driven by radiation pressure on dust. Advanced 3D radiation hydrodynamics simulations as done with the CO5BOLD code, which we develop and utilize, show that large convection cells and pulsations in these stars produce nearly spherical shock waves, which push material into cooler regions where dust can form. These simulations reveal that, while spherical symmetry is sometimes valid, the resulting dust shells often have non-radial structures that can be detected using interferometric techniques.

The modeling of AGB stars has shown that convection and pulsations naturally arise, influencing the formation and behavior of dust-driven winds. The shocks generated by these dynamic processes create a patchy, nonspherical atmosphere, leading to a clumpy distribution of dust clouds. These intricate 3D morphologies, with simultaneous infall and outflow regions near the star, differ notably from the smooth outflows predicted by 1D models. Understanding the interplay of these processes is crucial for grasping mass-loss mechanisms, especially in cases where 1D models may fail to predict winds.