Commit c1a22e4e by Parameswaran Ajith

adding the tutorials.

parents
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\title{ICTS Graduate Course: Tools of Astrophysics (PHY410.5)}
\author{Parameswaran~Ajith}\email{ajith@icts.res.in}
\author{Rajaram Nityananda} \email{rajaram.nityananda@icts.res.in}
\affiliation{International Centre for Theoretical Sciences, Tata Institute of Fundamental Research, Bangalore 560089, India.}
\author{Muhammed Irshad (tutor)}\email{muhammed.irshad@icts.res.in}
\affiliation{International Centre for Theoretical Sciences, Tata Institute of Fundamental Research, Bangalore 560089, India.}
\bigskip
\date{\today}
\maketitle
\section{Astrophysics across scales}
\input{astro_scale.tex}
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\bibliography{Lab}
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cp Lab.pdf /Users/pajith/Sites/ICTS/P._Ajiths_Homepage/Teaching_files
publish:
/Users/pajith/Sites/ICTS/P._Ajiths_Homepage/updateweb.sh
\subsection{Virial Equilibrium}
As we discussed in the class, the {virial theorem} provides a unifying description of equilibrium configurations across a wide range of astrophysical systems. You are encouraged to make {order-of-magnitude estimates}, clearly state assumptions, and focus on physical scalings rather than numerical prefactors. Unless stated otherwise, assume spherical symmetry and equilibrium. The virial theorem may be written schematically as
\begin{equation}
K = -\frac{1}{2} U ,
\end{equation}
where $K$ is the total kinetic (or pressure-support) energy and $ U$ is the potential (binding) energy. We disscussed in the class that, in different systems, the pressure support is provided by different effects: 1) random motions of dark matter particles in dark matter halos, 2) thermal pressure of the gas in molecular clouds, 3) Coulomb energy of the protons in a cold planet, 4) degeneracy pressure of electrons in a white dwarf.
\subsubsection{Problems}
\begin{enumerate}
\item Consider a virialized dark matter halo of total mass $M$ and radius $R$. The halo is supported by random motions characterized by a circular velocity $v_c$.
\begin{enumerate}
\item[(a)] Using the virial theorem, estimate the relation between $M$, $R$, and $v_c$.
\item[(b)] Assume the halo virializes at redshift $z$, with mean density
$ \rho_{\rm vir} \simeq 200 \rho_c(z)$, where $\rho_c(z) = \frac{3H^2(z)}{8\pi G}. $
Express the characteristic halo mass in terms of $v_c$ and the Hubble parameter $H(z)$.
\item[(c)] Estimate the halo mass for the following representative values: $v_c \sim 200 ~ \mathrm{km ~s^{-1}}$ ~ $H_0 \sim 70 ~ \mathrm{km,s^{-1} ~Mpc^{-1}}$.
\item Why does the characteristic mass of a virialized dark matter halo depend on the Hubble parameter at the epoch of collapse?
\end{enumerate}
\item A molecular cloud of mass $M$, radius $R$, and temperature $T$ is supported by thermal pressure.
\begin{enumerate}
\item[(a)] Use the virial theorem to estimate $M$ in terms of $T$ and $R$.
\item[(b)] Using the sound speed $c_s$ and a characteristic size $R \sim c_s/\sqrt{G\rho}$, derive the scaling of the Jeans mass with $c_s$ and the density $\rho$.
\item[(c)] Estimate the Jeans mass for a typical molecular cloud with: $T = 10,\mathrm{K}$, number density $n = 100,\mathrm{cm^{-3}}$ and mean molecular weight $\mu = 2.3$.
\item Why does increasing the temperature of a molecular cloud tend to suppress gravitational collapse?
\end{enumerate}
\item Consider a cold, solid planet composed of atoms with atomic number $Z$ and mass number $A$. The planet has mass $M$ and radius $R$.
\begin{enumerate}
\item[(a)] Estimate the gravitational binding energy. Assume the typical Coulomb (electrostatic) energy per atom is $E_C \sim \frac{Z^2 e^2}{a}$, where $a$ is the characteristic interatomic spacing. Estimate the total Coulomb energy of the planet in terms of $M$ and $R$.
\item[(b)] Using virial balance between gravity and Coulomb energy, estimate the maximum mass of a stable planet.
\item[(c)] Using representative rocky material parameters $Z \sim 10$ $A \sim 20$ estimate the maximum planetary mass and compare it to the mass of Jupiter.
\item Why does electron degeneracy pressure not play an important role in setting the mass scale of ordinary planets?
\end{enumerate}
\item A white dwarf is supported against gravitational collapse by electron degeneracy pressure.
\begin{enumerate}
\item[(a)] Estimate the gravitational energy. For relativistic degenerate electrons, assume the characteristic kinetic energy is
$K \sim N_e p_F c,$ where $p_F \sim \hbar n_e^{1/3}$ is the Fermi momentum and $n_e$ is the electron number density.
\item[(b)] Express the total degeneracy energy in terms of $M$ and $R$, assuming $N_e \sim M/m_p$.
\item[(c)] Using the virial theorem, estimate the maximum mass of a white dwarf supported by electron degeneracy pressure.
\item Why does the maximum mass of a white dwarf depend only on fundamental constants and not on its radius?
\end{enumerate}
\end{enumerate}
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