Coronae of Zero/Low-metal, Low-mass Stars

Abstract

Recent theoretical studies suggest the existence of low-mass zero-metal stars in the current universe. In order to study the basic properties of the atmosphere of low-mass first stars, we perform one dimensional magnetohydrodynamical simulations for the heating of coronal loops on low-mass stars with various metallicities. While the simulated loops are heated up to ≥ 10 6 K by the dissipation of Alfvénic waves originating from the convective motion irrespectively of the metallicity, the coronal properties sensitively depend on the metallicity. Lower-metal stars create hotter and denser coronae because the radiative cooling is suppressed. The zero-metal star gives more than 40 times higher coronal density than the solar-metallicity counterpart, and as a result, the UV and X-ray fluxes from the loop are several times higher than those of the solar metallicity star. We also discuss the dependence of the coronal properties on the length of the simulated coronal loops.

Coronal Properties of Low-mass Population III Stars and the Radiative Feedback in the Early Universe

“…We note that the dependence of 𝜏 cool on 𝑇 is different for different temperature ranges; for 𝑇 > 10 6 K, Λ ∝ 𝑇 1/2 so that 𝜏 cool ∝ 𝑇 1/2 𝜌 −1 . On the other hand, for 𝑇 < 10 6 K, Λ ∝ 𝑇 −1 in the case of zero metallicity (Sutherland & Dopita 1993;Washinoue & Suzuki 2019) and thus 𝜏 cool ∝ 𝑇 2 𝜌 −1 . For both temperature ranges, 𝜏 cool increases with temperature so that the gas should be thermally unstable (Balbus 1995).…”

“…The loop model and simulations are basically similar to those in Washinoue & Suzuki (2019); we consider a single magnetic flux tube that is anchored at the photosphere and take the coordinate 𝑠 along the loop. We inject velocity perturbations 𝛿𝑣 in three directions from the footpoints to excite MHD waves.…”

“…The coronae of low-mass Pop III stars possibly give a significant contribution to the early cosmic reionization because they emit ionizing photons continuously for a sufficiently long duration due to the long lifetime. Recently, the dependence of coronal property on stellar metallicity has been investigated by MHD simulations for the coronal heating in open magnetic flux tubes (Suzuki 2018) and closed magnetic loops (Washinoue & Suzuki 2019); both studies show that lower-metallicity stars give hotter and denser coronae because the radiative cooling is suppressed. It is also reported that zero-metal coronae emit strong UV and X-rays compared with the solar-metal coronae, which implies the importance of their radiative feedback at high-redshift.…”

Corona and XUV emission modelling of the Sun and Sun-like stars

“…As noted in Table 1, the coordinate along the axis of the flux tube is denoted by s. Spatial variation only along the loop is assumed to be nonzero, that is, ∂/∂s 0. Similar one-dimensional flux tube models are used in the models of solar spicules (Hollweg et al 1982;Kudoh & Shibata 1999;Matsumoto & Shibata 2010), solar and stellar coronal heating (Moriyasu et al 2004;Washinoue & Suzuki 2019), and solar wind acceleration (Suzuki & Inutsuka 2005;Shoda et al 2018).…”

“…Whether our theoretical predictions apply to lowermass stars (M dwarfs in particular) needs to be investigated in future. In light of lower metallicity found in old M-type stars, the metallicity dependence of the XUV emission needs to be studied as well (Washinoue & Suzuki 2019). According to the Figure 14 of Johnstone et al (2021), a single power law between L X and L EUV is applicable to low-mass stars with various spectral types, and thus, it is possible that Eq.…”