Habitable zone around white dwarfs?

...the original question of where what we call
the habitable zone would be for a white dwarf. Surely it's
largely a matter of luminosity and distance.

Phineas T Puddleduck wrote:
Given a 5000-1000K surface T for a WD, its going to be AU and above.

We don't need to put in numbers to get a rough idea. If the white
dwarf temperature is the same as the Sun's, the habitable zone will be
where the star has an angular size of half a degree. (This is much
smaller than an AU.) For an object of fixed angular size, the tidal
force it exerts is proportional to the object's density. Thus, if I'm
remembering white dwarf density right, the tidal force in the
habitable zone will be about 100 times the Sun's tidal force on
Earth. That should be big enough to lock the planet's rotation but
not big enough to tear it apart.

If the white dwarf is hotter, the radius of the habitable zone will be
larger, and tidal forces will be smaller.