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2

A gravitating object in a disk creates a wake on the inner edge of its orbit as well on the outer edge of its orbit. The inner wake is the leading wake, while the outer wake is trailing. The torques created by these two wakes would cancel, if the disk had no shear. Yet disks in astrophysical contexts are subject to Keplerian shear - and thus the torques ...


4

Not really my area, but this question is probably related to the planetary migration in circumstellar disks. In this case, the migration is caused by gravitational interactions between the planet and the gas in the disk. There are two explanations for this effect Impulse approximation: consider a parcel of gas in the corrotating frame. If the gas is close ...


17

This is the Newtonian model of gravity. It is a very good model, it is used for accurate calculating the motion of objects in the solar system to a very high degree of accuracy. However, for very strong gravitational fields you need to use Einstein's model, which accounts for things like the constant speed of light for all observers. I'm not going to go into ...


2

The answer below is a combination of the first answer with a cross-check from Wikipedia (for the obliquity of the ecliptic plane specifically) and here (the formula for ecliptic longitude of the Sun in the first answer uses 0.918994643 to multiply sin(2 * g * pi) in the final term instead of 0.020, so I used the factor below, but I am not sure which is ...


31

There is basically an upper limit to the mass of a star because their luminosity is so great that the radiation pressure prevents the accretion of further mass. However, the upper limit depends on the composition of the accreting material. This is because the effect of the radiation depends on the opacity of the material - stuff that is more metal-rich is ...


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In this case itseems to mean that the depth of the line is 7 times its error bar below the continuum level. Impossible to answer. You say it can't be done, but the authors say that they fitted a Gaussian. You either use a rough estimate (attributable to Cayrel de Strobel 1988) of $$\Delta {\rm EW} \sim 1.5\frac{\sqrt{RP}}{{\rm SNR}},$$ where $R$ is the ...


3

According to Lodders (2003, https://arxiv.org/pdf/1010.2746 ) the relative abundance of helium to hydrogen is $A({\rm He})=10.925$, on a logarithmic scale where the hydrogen number abundance is 12. So this would mean a helium to hydrogen ratio, by number, of $10^{10.925-12}=0.08414$. i.e. 8.4% (your source uses 10.93, not 10.99, hence a very slightly ...


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