2:30 PM - 2:45 PM
[PCG19-14] Forward modeling of the stellar XUV emission: from the Sun to young solar-type stars and pre-main-sequence stars
Keywords:Sun, solar-type stars, pre-main-sequenece stars, XUV radiation
X-ray and extreme ultraviolet (EUV) emissions from stars are known to drive heating and chemical reactions in planetary atmospheres and protoplanetary disks. Understanding a stellar XUV spectrum is therefore essential for modeling the thermochemical structure and dissipative process of the surrounding planets and protoplanetary disks. However, direct measurement of the XUV spectrum is challenging due to the strong interstellar absorption of EUV photons, which constitutes a significant part of XUV. To address this, several methods have been proposed to estimate EUV spectrum using empirical correlations with observable quantities. These methods, however, are not based on physical principles and can lead to errors exceeding an order of magnitude. Consequently, there is a demand for a new XUV estimation method based on physical reasoning rather than empirical approaches.
In this study, we propose a new estimation method for XUV radiation based on the Differential Emission Measure (DEM), a physical quantity representing radiation intensity at various temperatures. While DEM, a function of temperature, can be observationally constrained within limited temperature ranges, it is impossible to observe across all temperature ranges causing EUV radiation. To address this, we calculate DEM using numerical simulations. This requires quantitatively solving the stellar atmospheric formation, particularly the coronal heating problem. Although this is known to require extensive computational resources, we have resolved this issue by successfully incorporating the heating process phenomenologically into a computationally efficient one-dimensional magnetohydrodynamic (MHD) model.
We demonstrate that the solar XUV spectrum can be accurately reproduced using a DEM calculated from the MHD model. A key advantage of our model is its reliance solely on magnetic flux as an input parameter, making it applicable to any low-mass star with measured magnetic flux. We applied the model to young solar-type stars, confirming its validity even for stars with magnetic flux over 100 times that of the Sun. Additionally, we extended our calculations to pre-main-sequence stars, showing that the model reproduces the typical X-ray luminosity. These results indicate that our method for estimating XUV emissions through numerical simulations is feasible from the pre-main-sequence to the main-sequence stage.
In this study, we propose a new estimation method for XUV radiation based on the Differential Emission Measure (DEM), a physical quantity representing radiation intensity at various temperatures. While DEM, a function of temperature, can be observationally constrained within limited temperature ranges, it is impossible to observe across all temperature ranges causing EUV radiation. To address this, we calculate DEM using numerical simulations. This requires quantitatively solving the stellar atmospheric formation, particularly the coronal heating problem. Although this is known to require extensive computational resources, we have resolved this issue by successfully incorporating the heating process phenomenologically into a computationally efficient one-dimensional magnetohydrodynamic (MHD) model.
We demonstrate that the solar XUV spectrum can be accurately reproduced using a DEM calculated from the MHD model. A key advantage of our model is its reliance solely on magnetic flux as an input parameter, making it applicable to any low-mass star with measured magnetic flux. We applied the model to young solar-type stars, confirming its validity even for stars with magnetic flux over 100 times that of the Sun. Additionally, we extended our calculations to pre-main-sequence stars, showing that the model reproduces the typical X-ray luminosity. These results indicate that our method for estimating XUV emissions through numerical simulations is feasible from the pre-main-sequence to the main-sequence stage.