The z = 0.0777 C III Absorber Towards PHL 1811 as a Case Study of a Low
  Redshift Weak Metal Line Absorber
  We consider the physical conditions and origin of the z = 0.0777 absorption
system observed in C III, C II, Si III, C IV, O VI, and H I absorption along
the line of sight towards the quasar PHL 1811. We analysed the HST/STIS and
FUSE spectra of this quasar and compared the results to Cloudy photoionization
and collisional ionization models in order to derive densities, temperatures,
and metallicities of the absorbing gas. The absorption can be explained by two
C III clouds, offset by 35 km/s in velocity, with metallicities of ~one-tenth
the solar value. One cloud has a density of order n_H = 1.2 +0.9 -0.5 * 10^-3
cm^-3 (thickness 0.4 +0.3 -0.2 kpc) and produces the observed C II and Si III
absorption, while the other has a density of order n_H = 1.2 +0.9 -0.5 * 10^-5
cm^-3 (thickness 80 +70 -40 kpc) and gives rise to the observed weak C IV
absorption. Cloud temperatures are ~14,000 +3000 -2000 K and ~34,000 -4000
+2000 K for photoionized models. Although collisionally ionized clouds with T ~
70,000 K are possible, they are less likely because of the short cooling
time-scales involved. Previous studies revealed no luminous galaxy at the
absorber's redshift, so it is probably related to tidal debris, ejected
material, a dwarf galaxy, or other halo material in a galaxy group. Our models
also indicate that one of the two clouds would produce detectable weak Mg II
absorption if spectral coverage of that transition existed. We predict what the
system would look like at z ~ 1 when the ionizing background radiation was more
intense. We find that at z ~ 1 the denser component resembles a C IV absorber.
The second C III cloud in this z = 0.0777 absorber may be analogous to a subset
of the more diffuse O VI absorbers at higher redshift.
