A purely passive central mass. A fixed mass at the centre enters only the gravity source (Δφ + Gρ = 0, with ρ = gas + a fixed central bump) — it is never part of the fluid field, so it pulls gas in but never deletes it: no sink, mass conserved, no evacuated ring. The gas simply falls in and piles up into a dense central blob. The spin slider sets the gas's orbital velocity (spin 0 = radial infall).   Δφ + Gρ = 0  |  ρ_t + div m = 0  |  m_t + div(m v) + grad p − νΔv = ρ gradφ  |  e_t + div(e v) + p div v − φ div m = 0;   p = γe, v = m/|ρ|.   M_c sets the central mass (live), G its well depth. Red = mass; white dots = fluid tracers (Lagrangian, seeded ∝ density, periodic); periodic box.