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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  Hubble Space Telescope /Space Telescope Imaging Spectrograph and  Far Ultraviolet Spectroscopic Explorer  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 −1  in velocity, with metallicities of approximately one‐tenth the solar value. One cloud has a density of the order of   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 the order of   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 +2000  −4000  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.

The z = 0.0777 C iii absorber towards PHL 1811 as a case study of a low‐redshift weak metal line absorber