A Primordial Origin for the Gas-Rich Debris Disks Around Intermediate-Mass Star

While most debris disks consist of dust with little or no gas, a fraction have significant amounts of gas detected via emission lines of CO, ionized carbon, and/or atomic oxygen. Almost all such gaseous debris disks known are around A-type stars with ages up to 50 Myr. We show using semi-analytic disk evolution modeling that this can be understood if the gaseous debris disks are remnant protoplanetary disks that have become depleted of fine dust. Photoelectric heating by the A-stars’ FUV radiation is then inefficient, while the stars’ EUV and X-ray emission is weak owing to a lack of surface convective zones capable of driving magnetic activity. In this picture, stars outside the range of spectral types from A through early B cannot have such long-lived gas disks. Less-massive stars have stronger magnetic activity in the chromosphere, transition region, and corona with resulting EUV and X-ray emission, while more-massive stars have photospheres hot enough to produce EUV radiation. In both cases, primordial disk gas is likely to photoevaporate well before 50 Myr. These results come from disk evolution models where we treat internal accretion stresses, MHD winds, and photoevaporation by EUV and X-ray photons at luminosities that are functions of the stellar mass and age. A key issue this work leaves open is how some disks become depleted in fine dust so that FUV photoevaporation slows. Candidates include grains’ growth, settling, radial drift, radiation force, and incorporation into planetary systems.