Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userCoronae of Zero/Low-Metal Low-Mass Stars
60
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic 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 performed 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 Alfvenic 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 ten times higher coronal density than the solar-metallicity counterpart, and as a result, the UV and X-ray fluxes from the loop are (1-5) 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
rate research
Read More
We investigated stellar winds from zero/low-metallicity low-mass stars by magnetohydrodynamical simulations for stellar winds driven by Alfven waves from stars with mass $M_{star}=(0.6-0.8)M_{odot}$ and metallicity $Z=(0-1)Z_{odot}$, where $M_{odot}$ and $Z_{odot}$ are the solar mass and metallicity, respectively. Alfvenic waves, which are excited by the surface convection, travel upward from the photosphere and heat up the corona by their dissipation. For lower $Z$, denser gas can be heated up to the coronal temperature because of the inefficient radiation cooling. The coronal density of Pop.II/III stars with $Zle 0.01Z_{odot}$ is 1-2 orders of magnitude larger than that of the solar-metallicity star with the same mass, and as a result, the mass loss rate, $dot{M}$, is $(4.5-20)$ times larger. This indicates that metal accretion on low-mass Pop.III stars is negligible. The soft X-ray flux of the Pop.II/III stars is also expected to be $approx (1-30)$ times larger than that of the solar-metallicity counterpart owing to the larger coronal density, even though the radiation cooling efficiency is smaller. A larger fraction of the input Alfvenic wave energy is transmitted to the corona in low $Z$ stars because they avoid severe reflection owing to the smaller density difference between the photosphere and the corona. Therefore, a larger fraction is converted to the thermal energy of the corona and the kinetic energy of the stellar wind. From this energetics argument, we finally derived a scaling of $dot{M}$ as $dot{M}propto L R_{star}^{11/9}M_{star}^{-10/9}T_{rm eff}^{11/2}left[max (Z/Z_{odot},0.01)right]^{-1/5}$, where $L$, $R_{star}$, and $T_{rm eff}$ are stellar luminosity, radius, and effective temperature, respectively.
We use a suite of SPH simulations to investigate the susceptibility of protoplanetary discs to the effects of self-gravity as a function of star-disc properties. We also include passive irradiation from the host star using different models for the stellar luminosities. The critical disc-to-star mass ratio for axisymmetry (for which we produce criteria) increases significantly for low-mass stars. This could have important consequences for increasing the potential mass reservoir in a proto Trappist-1 system, since even the efficient Ormel et al. (2017) formation model will be influenced by processes like external photoevaporation, which can rapidly and dramatically deplete the dust reservoir. The aforementioned scaling of the critical $M_d/M_*$ for axisymmetry occurs in part because the Toomre $Q$ parameter has a linear dependence on surface density (which promotes instability) and only an $M_*^{1/2}$ dependence on shear (which reduces instability), but also occurs because, for a given $M_d/M_*$, the thermal evolution depends on the host star mass. The early phase stellar irradiation of the disc (for which the luminosity is much higher than at the zero age main sequence, particularly at low stellar masses) can also play a key role in significantly reducing the role of self-gravity, meaning that even Solar mass stars could support axisymmetric discs a factor two higher in mass than usually considered possible. We apply our criteria to the DSHARP discs with spirals, finding that self-gravity can explain the observed spirals so long as the discs are optically thick to the host star irradiation.
The cosmological lithium problem, that is, the discrepancy between the lithium abundance predicted by the Big Bang nucleosynthesis and the one observed for the stars of the Spite plateau, is one of the long standing problems of modern astrophysics. Recent hints for a possible solution involve lithium burning induced by protostellar mass accretion on Spite plateau stars. The purpose of this paper is to analyze the effect of protostellar accretion on low metallicity low-mass stars with a focus on PMS lithium evolution. We computed the evolution from the protostar to the MS phase of accreting models with final masses of 0.7 and 0.8 M$_odot$, and three metallicities Z=0.0001, Z=0.0010, and Z=0.0050. The effects of changing the main parameters affecting accreting models, that is the accretion energy (cold versus hot accretion), the initial seed mass $M_{seed}$ and radius $R_{seed}$, and the mass accretion rate $dot{m}$, have been investigated in detail. As for the main stellar properties and the surface $^7 Li$ abundance, hot accretion models converge to standard non-accreting ones within 1 Myr, regardless of the actual value of $M_{seed}$, $R_{seed}$, and $dot{m}$. Also, cold accretion models with a relatively large $M_{seed}$ ($gtrsim 10~M_{jup}$) or $R_{seed}$ ($gtrsim 1~R_odot$) converge to standard non-accreting ones in less than about 10-20~Myr. A drastically different evolution occurs whenever a cold protostellar accretion process starts from small values of $M_{seed}$ and $R_{seed}$ ($M_{seed}sim 1~M_{jup}$, $R_{seed} lesssim 1~R_odot$). These models almost entirely skip the standard Hayashi track evolution and deplete Li before the end of the accretion phase. The exact amount of depletion depends on the actual combination of the accretion parameters ($dot{m}$, $M_{seed}$, and $R_{seed}$), achieving in some cases the complete exhaustion of Li in the whole star.
Low-mass stars play a key role in many different areas of astrophysics. In this article, I provide a brief overview of the evolution of low-mass stars, and discuss some of the uncertainties and problems currently affecting low-mass stellar models. Emphasis is placed on the following topics: the solar abundance problem, mass loss on the red giant branch, and the level of helium enrichment associated to the multiple populations that are present in globular clusters.
We present an analysis of K2 light curves (LCs) from Campaigns 4 and 13 for members of the young ($sim$3 Myr) Taurus association, in addition to an older ($sim$30 Myr) population of stars that is largely in the foreground of the Taurus molecular clouds. Out of 156 of the highest-confidence Taurus members, we find that 81% are periodic. Our sample of young foreground stars is biased and incomplete, but nearly all (37/38) are periodic. The overall distribution of rotation rates as a function of color (a proxy for mass) is similar to that found in other clusters: the slowest rotators are among the early M spectral types, with faster rotation towards both earlier FGK and later M types. The relationship between period and color/mass exhibited by older clusters such as the Pleiades is already in place by Taurus age. The foreground population has very few stars, but is consistent with the USco and Pleiades period distributions. As found in other young clusters, stars with disks rotate on average slower, and few with disks are found rotating faster than $sim$2 d. The overall amplitude of the light curves decreases with age and higher mass stars have generally lower amplitudes than lower mass stars. Stars with disks have on average larger amplitudes than stars without disks, though the physical mechanisms driving the variability and the resulting light curve morphologies are also different between these two classes.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
