Day 1684

Tuesday.

2004.01795
On the contribution of quiet Sun magnetism to solar irradiance variations: constraints on quiet Sun variability and grand minimum scenarios
Rempel

While the quiet Sun magnetic field shows only little variation with the solar cycle, long-term variations cannot be completely ruled out from first principles. We investigate the potential effect of quiet Sun magnetism on spectral solar irradiance through a series of small-scale dynamo simulations with zero vertical flux imbalance ($\langle B_z\rangle=0$) and varying levels of small-scale magnetic field strength, and one weak network case with an additional flux imbalance corresponding to a flux density of $\langle B_z\rangle=100$ G. From these setups we compute the dependence of the outgoing radiative energy flux on the mean vertical magnetic field strength in the photosphere at continuum optical depth $\tau=1$ ($\langle \vert B_z\vert\rangle_{\tau=1}$). We find that a quiet Sun setup with a mean vertical field strength of $\langle \vert B_z\vert\rangle_{\tau=1}=69$ G is about $0.6~\%$ brighter than a non-magnetic reference case. We find a linear dependence of the outgoing radiative energy flux on the mean field strength $\langle \vert B_z\vert\rangle_{\tau=1}$ with a relative slope of $1.4\cdot 10^{-4}$ G$^{-1}$. With this sensitivity, only a moderate change of the quiet Sun field strength by $10\%$ would lead to a total solar irradiance variation comparable to the observed solar cycle variation. While this does provide strong indirect constraints on possible quiet Sun variations during a regular solar cycle, it also emphasizes that potential variability over longer time scales could make a significant contribution to longer-term solar irradiance variations.

2004.02364

Formation of massive stars under protostellar radiation feedback: very metal-poor stars

Fukushima, et al

We study the formation of very metal-poor stars under protostellar radiative feedback effect. We use cosmological simulations to identify low-mass dark matter halos and star-forming gas clouds within them. We then follow protostar formation and the subsequent long-term mass accretion phase of over one million years using two-dimensional radiation-hydrodynamics simulations. We show that the critical physical process that sets the final mass is formation and expansion of a bipolar HII region. The process is similar to the formation of massive primordial stars, but radiation pressure exerted on dust grains also contributes to halting the accretion flow in the low-metallicity case. We find that the net feedback effect in the case with metallicity $Z = 10^{-2}~Z_{\odot}$ is stronger than in the case with $Z \sim 1~Z_{\odot}$. With decreasing metallicity, the radiation pressure effect becomes weaker, but photoionization heating of the circumstellar gas is more efficient owing to the reduced dust attenuation. In the case with $Z = 10^{-2}~Z_{\odot}$, the central star grows as massive as 200 solar-masses, similarly to the case of primordial star formation. We conclude that metal-poor stars with a few hundred solar masses can be formed by gas accretion despite the strong radiative feedback.