Accretion-related properties of Herbig Ae/Be stars

Abstract

Context. The mass accretion rate Msun;acc) is a key parameter that has not accurately been determined for a wide sample of Herbig Ae/Be (HAeBe) stars until recently. Aims: We look for trends relating Msunacc and the stellar ages (t), spectral energy distributions (SEDs), and disk masses for a sample of 38 HAeBe stars, comparing them to analogous correlations found for classical T Tauri stars. Our goal is ultimately to shed light on the timescale and physical processes that drive the evolution of intermediate-mass pre-main sequence objects. Methods: Mass accretion rates obtained by us in a previous work were related to several stellar and disk parameters: the age of the stars was compiled from the literature, SEDs were classified according to their shape and the wavelength where the IR excess starts, near- and mid-IR colour excesses were computed, and disk masses were estimated from mm fluxes. Results: Msun;acc decreases with the stellar age, showing a dissipation timescale τ = 1.3+1.0-0.5 Myr from an exponential law fit, while a power law yields Msunacc(t) ∝ t - η, with η = 1.8+1.4-0.7. This result is based on our whole HAeBe sample (1-6 Msun), but the accretion rate decline most probably depends on smaller stellar mass bins. The near-IR excess is higher and starts at shorter wavelengths (J and H bands) for the strongest accretors. Active and passive disks are roughly divided by 2 × 10-7 Msun yr-1. The mid-IR excess and the SED shape from the Meeus et al. classification are not correlated with Msunacc. Concerning disk masses, we find dot{M_acc ∝ M_disk1.1 ± 0.3}. Most stars in our sample with signs of inner dust dissipation typically show accretion rates ten times lower and disk masses three times smaller than the remaining objects. Conclusions: The trends relating Msunacc with the near-IR excess and Mdisk extend those found for T Tauri stars, and are consistent with viscous disk models. The differences in the inner gas dissipation timescale, and the relative position of the stars with signs of inner dust clearing in the Msunacc - Mdisk plane, could be suggesting a slightly faster evolution, and that a different process - such as photoevaporation - plays a more relevant role in dissipating disks in the HAeBe regime compared to T Tauri stars. Our conclusions must consider the mismatch between the disk mass estimates from mm fluxes and the disk mass estimates from accretion, which we also find in HAeBe stars. Appendix A is only available in electronic form at http://www.aanda.org