  Nonrelativistic electrostatic unmagnetized shocks are frequently observed in
laboratory plasmas and they are likely to exist in astrophysical plasmas. Their
maximum speed, expressed in units of the ion acoustic speed far upstream of the
shock, depends only on the electron-to-ion temperature ratio if binary
collisions are absent. The formation and evolution of such shocks is examined
here for a wide range of shock speeds with particle-in-cell (PIC) simulations.
The initial temperatures of the electrons and the 400 times heavier ions are
equal. Shocks form on electron time scales at Mach numbers between 1.7 and 2.2.
Shocks with Mach numbers up to 2.5 form after tens of inverse ion plasma
frequencies. The density of the shock-reflected ion beam increases and the
number of ions crossing the shock thus decreases with an increasing Mach
number, causing a slower expansion of the downstream region in its rest frame.
The interval occupied by this ion beam is on a positive potential relative to
the far upstream. This potential pre-heats the electrons ahead of the shock
even in the absence of beam instabilities and decouples the electron
temperature in the foreshock ahead of the shock from the one in the far
upstream plasma. The effective Mach number of the shock is reduced by this
electron heating. This effect can potentially stabilize nonrelativistic
electrostatic shocks moving as fast as supernova remnant (SNR) shocks.
