= Solution
Direct evidence for <stellar feedback> includes expanding ionized shells and superbubbles around young associations, hot X-ray-emitting gas in <supernova remnant>[supernova remnants], broad or split emission lines, and blueshifted absorption showing cool and warm <galactic outflow>[outflows]. P-Cygni profiles reveal massive-star winds, while extraplanar filaments and metal-enriched gas demonstrate transport away from star-forming disks. Indirect evidence includes the galaxy mass--metallicity relation, low baryon fractions and suppressed star formation in dwarf galaxies, chemically enriched circumgalactic gas, and correlations of outflow speed and mass loading with the <star formation rate>.
Rapid gas removal changes the gravitational potential before stellar and dark-matter orbits can respond adiabatically. Positions and velocities are initially unchanged, but orbital binding energies rise; orbits expand and become more eccentric, and some particles escape. Repeated burst--outflow--reaccretion cycles can irreversibly transfer energy to collisionless matter and turn a central dark-matter cusp into a core. This matters because dwarf-galaxy rotation curves are used to test dark-matter microphysics: a feedback-made core can mimic a non-cold or self-interacting dark-matter signature.
Write the initial potential energy as $W=-aGM^2/R$. The <virial theorem> gives $T=-W/2$. If a well-mixed fraction $\epsilon$ remains after instantaneous mass loss, the immediate kinetic and potential energies are $T_a=\epsilon T$ and $W_a=\epsilon^2W$, so
$$
E_a=T_a+W_a
=a\frac{GM^2}{R}\left(\frac\epsilon2-\epsilon^2\right).
$$
After revirialization at $R'$, $E_f=W_f/2=-aG\epsilon^2M^2/(2R')$. Equating energies gives the <impulsive mass-loss expansion law>
$$
\boxed{\frac{R'}R=\frac{\epsilon}{2\epsilon-1}.}
$$
The remnant is bound only for $\epsilon>1/2$; loss of half or more of the gravitating mass disrupts this idealized system.
For many infinitesimal, individually revirialized losses, put $\epsilon=1+dM/M$ in the impulsive result. To first order, $dR/R=-dM/M$. Integration yields the <adiabatic mass-loss expansion law>
$$
\boxed{MR=\text{constant},\qquad \frac{R'}R=\frac1\epsilon.}
$$
Slow loss causes finite expansion for every positive remaining mass fraction and has no sharp disruption threshold.
Finally consider an initially circular orbit of radius $R$ around a point mass $M$. Its speed and specific angular momentum obey $v^2=GM/R$ and $h^2=GMR$. Immediately after $M\to\epsilon M$, these remain unchanged, while the new specific energy is
$$
E'=\frac{GM}{R}\left(\frac12-\epsilon\right).
$$
Using the <orbital eccentricity> relation $e^2=1+2E'h^2/(G^2\epsilon^2M^2)$ gives
$$
\boxed{e=\frac{1-\epsilon}{\epsilon}.}
$$
It is an ellipse for $\epsilon>1/2$, parabolic at $\epsilon=1/2$, and unbound for smaller $\epsilon$, in agreement with the virial argument.
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