Non-Axisymmetric Pillar Structures due to Embedded Stars in AGN Accretion Discs

Abstract

Gravitationally unstable outer regions of AGN discs may host in situ formation of massive stars whose heating produces localized vertical "pillars" on the disc surface, in the same region that emits the broad lines. Here we investigate the reverberation-mapping signatures that such pillars would produce. We compute velocity-delay maps $Ψ(λ, τ)$, time-evolving emission-line spectra, and continuum reverberation lag spectra for these structures. We model each pillar as a 2D Gaussian perturbation to the disc height under lamp-post irradiation and weight the line response with Cloudy photoionization grids covering the major BLR lines. We show H$α$, Mg II, and C IV as representative examples. A single pillar produces a localized hotspot in $Ψ$ and a clear "S-wave" pattern in the time-evolving spectrum that persists, though diluted, for $N \sim 100$ pillars. Emission-line fluxes are suppressed in shadows behind the pillars, but when normalized by H$β$ (or by the local continuum) the line-to-H$β$ ratios move in opposite directions: Mg II/H$β$ rises while C IV/H$β$ drops. This contrast distinguishes pillars from purely kinematic features that would modulate all lines by similar factors. Across scans of the pillar shape and lamp-post height, the pillar models produce an optical continuum reverberation lag spectrum of the observed magnitude. The once-per-orbit modulation grows with $\sin i$ from foreshortening, and at sufficiently high inclination the pillar's body geometrically self-occults its lamp-facing inner slope, adding a sharper near/far contrast. These signatures provide observational diagnostics for detecting in situ star formation in AGN discs through velocity-resolved reverberation mapping campaigns.