Rapid cooling of neutron stars by hyperons and Δ isobars

We show that direct Urca processes with hyperons and/or nucleon isobars can occur in dense matter as long as the concentration of Λ hyperons exceeds a critical value that is less than 3% and is typically about 0.1%. The neutrino luminosities from the hyperon Urca processes are about 5-100 times less than the typical luminosity from the nucleon direct Urca process, if the latter process is not forbidden, but they are larger than those expected from other sources. These new direct Urca processes provide avenues for rapid cooling of neutron stars which invoke neither exotic states nor the large proton fraction (of order 0.11-0.15) required for the nucleon direct Urca process.