Exploring Millimeter Indicators of Solar-Stellar Activity

The activity in the outer layers of the Sun and other stars remains poorly understood due to the difficulties with interpreting available observations. The Atacama Large Millimeter/sub-millimeter Array (ALMA), which is a telescope array located high in the Chilean Andes, observes at wavelengths around one millimetre that are thus much longer than for visible light. Observing at these long wavelengths usually comes at the price of a much lower spatial resolution and hence the level of details that can be seen. ALMA is a technological leap forward and now enables observations of our Sun at a resolution that comes close to that of today's solar telescopes for visible light. One of the main advantages is that the temperature of the gas in the outer layers of the Sun can be much more directly inferred from ALMA observations than from observations with other telescopes. In addition, ALMA observations of the Sun can reveal the three-dimensional structure of the gas and even the magnetic field in the outer layers. The same tools can be applied to (spatially unresolved) observations of other stars, too. ALMA thus provides new means to observe and eventually understand the active nature of the Sun and other stars. The EMISSA project aims to utilize ALMA's new capabilities for a reassessment of activity in stars using a comparative study with the Sun as a basic reference. In the first phase of the EMISSA project, the ALMA Science Archive was searched for suitable observations of the Sun and other stars. A large number of spatially resolved ALMA observations of the entire Sun have been processed and compared with similar observations for spectral indicators of solar activity (here H alpha and Ca II K). The aim was to find statistical correlations between these indicators and the temperatures derived with ALMA for the Sun. The ALMA observations of the other stars were then supplemented with observations at radio wavelengths and in the infrared to ultraviolet range. The resulting sample consists of 12 main-sequence stars and the Sun-as-a-star as a reference. Despite the relatively small number, the sample covers already the relevant range from hotter (spectral type A) to cool main-sequence stars (spectral type M). For these stars, the spectral energy distribution over the whole wavelength range was constructed and compared to model atmospheres calculated with the PHOENIX code. As these models contain only a photosphere and a declining temperature stratification above, the differences between the observations and these models indicate the temperatures in the upper atmosphere of the considered stars. The gathered observations indeed proof the existence of a hot chromosphere for the cooler stars and a lack thereof for the hotter stars in the sample. The sample demonstrates the potential of mm observations for the study of stellar atmospheres and their activity. Based on the aforementioned mm continuum observations for stars of different spectral types across the main-sequence, a new potential stellar activity indicator was designed, which can provide a robust quantitative mapping between the activity and the physical properties of a star’s atmosphere, in particular the thermal stratification of its chromosphere. The resulting alpha_mm spectral index utilizes observations in the 30 - 1000 GHz range. A comparison of the corresponding values to other activity indicators for the stellar sample demonstrates that, despite unavoidable uncertainties, the alpha_mm values can differentiate well between different cool stars where other indicators fail. A systematic survey that exploits the advantages of this new indicator requires new multi-frequency observations of a larger set of stars. Applying for more observing time at various mm and radio observing facilities has therefore been an integral part of the project activities. The EMISSA team has succeeded in obtaining more data and coordinated additional multi-observatory campaigns in 2023. New results from these data will be published in 2024. Next to the observations, new numerical 3D simulations of stellar atmospheres for different spectral types and corresponding synthetic mm observations were produced and analysed, providing a first systematic overview of the activity of main sequence stars. In addition, detailed 3D simulations of the Sun were used for the computation of intensity data for important spectral lines such as H alpha and Ca II H and K, which are commonly used as activity indicators. A quantitative comparison to corresponding mm intensities revealed the connection of these indicators and how mm observations of the Sun and thus other stars can be used for a more precise and robust characterization of the thermal properties and atmospheric activity of stars in general. As part of the project, 1 Ph.D. and 2 M.Sc. were successfully defended.

Prosjektet bidro til å utvide forskningen på solaktivitet ved UiO utover Sola til andre stjerner. De produserte databasene og metodene legger grunnlaget for fremtidige studier, systematiske observasjonskampanjer og internasjonalt samarbeid. Langsiktig sett vil forskningen bidra til en bedre forståelse av virkningen av sol-/stjerneaktivitet på nærliggende planeter og romvær i vårt solsystem. Det siste har mulige implikasjoner for samfunnet på sikt.

The activity of the Sun and stars remains poorly understood due to the difficulties with interpreting available observable activity indicators. The Atacama Large Millimeter/sub-millimeter Array (ALMA), which is a true leap forward for observations at (sub-)mm wavelengths, now offers a solution in the form of millimeter (mm) radiation based indicators for stellar activity, which are much easier to interpret than commonly used indicators at shorter wavelengths. With a spatial resolution of solar observations close to that of optical solar telescopes, major advantages of the mm wavelength range can be exploited: The mm continuum radiation is an essentially linear measure of the local gas temperature in thin layers in the solar/stellar atmosphere, where the formation height increases with wavelength, thus facilitating measurements of the height-dependent (3D) thermal structure. In addition, ALMA's polarisation measurements will allow for reconstructing the (3D) magnetic field, while recombination and molecular lines (e.g., CO) provide complementary diagnostic tools for the thermal, kinetic and magnetic plasma state of the probed atmospheric gas. The same tools can be applied to (spatially unresolved) observations of stars, while the Sun serves as an important reference. The aim of the EMISSA project is to exploit ALMA's novel capabilities for a re-evaluation of stellar activity by means of a comparative solar-stellar study with the Sun-as-a-star case as a fundamental reference. A sequence of state-of-the-art 3D numerical simulations of stellar atmospheres and corresponding synthetic observables will be used to prepare and interpret simultaneous multi-wavelength observations of the Sun and a selected sample of other stars with ALMA and other observatories. The proposed systematic comparison of mm-based activity indicators and 'classical' indicators promises a new view on stellar activity and the physical processes causing it.