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This is the second of two papers reporting observations and analysis of theunusually bright (m_b=14.4), luminous (M_B=-25.5), nearby (z=0.192) narrow-linequasar PHL 1811. The first paper reported that PHL 1811 is intrinsically X-rayweak, and presented a spectral energy distribution (SED). Here we present HSTSTIS optical and UV spectra, and ground-based optical spectra. The optical andUV line emission is very unusual. There is no evidence for forbidden orsemiforbidden lines. The near-UV spectrum is dominated by very strong FeII andFeIII, and unusual low-ionization lines such as NaID and CaII H&K are observed.High-ionization lines are very weak; CIV has an equivalent width of 6.6A, afactor of ~5 smaller than measured from quasar composite spectra. An unusualfeature near 1200A can be deblended in terms of Ly\alpha, NV, SiII, and CIII*using the blueshifted CIV profile as a template. Photoionization modeling showsthat the unusual line emission can be explained qualitatively by the unusuallysoft SED. Principally, a low gas temperature results in inefficient emission ofcollisionally excited lines, including the semiforbidden lines generally usedas density diagnostics. The emission resembles that of high-density gas; inboth cases this is a consequence of inefficient cooling. PHL 1811 is veryunusual, but we note that quasar surveys are generally biased against findingsimilar objects.

The Intrinsically X-ray Weak Quasar PHL 1811. II. Optical and UV Spectra and Analysis