This work focuses on the massive counterparts of classical T-Tauri stars, the so-called Herbig Ae/Be (HAeBe) stars. These are the most massive objects to experience an optically visible pre-main sequence (PMS) phase, bridging the transition between low and high-mass young stars. Our knowledge about the HAeBe properties is much more limited than that for lower-mass objects. The main reason is the comparatively small sample, caused by the fast evolution of massive objects to the main-sequence, and by the fact that star formation process shows favour for the less-massive objects, as the shape of the initial-mass function suggests.
The general aim of this thesis is to contribute to our knowledge of HAeBe stars, in particu- lar, the behaviour of the circumstellar atomic gas, the accretion properties, and the evolution and physical mechanisms driving the star-disk interaction. This research is mainly based on multi-epoch optical spectra and simultaneous optical-near infrared (nIR) photometry of a representative sample of 38 HAeBe stars, as well as on their stellar parameters and spectral energy distributions. The main achievements of this work are summarized as follows:
- We have carried out the most complete characterization to date of the circumstellar behaviour of HAeBe stars in the optical, analysing multi-epoch (intervals of hours-days- months) and averaged spectra in Hα, [OI]6300, NaID and HeI5876. These spectra, and the simultaneous photometry, have allowed us to estimate line fluxes and to assess whether the observed equivalent-width variations are caused by changes in the stellar continuum or by variations of the circumstellar gas itself. These data constitute one of the largest existing sets to study some of the variability properties of intermediate-mass PMS stars, which led us to find significant differences between the variability behaviour of HAe and HBe stars, as well as between HAeBes and classical T-Tauri stars.
- We have applied magnetospheric-accretion shock modelling to reproduce the observed Balmer excess from multi-epoch Johnson’s UB photometry, deriving the most reliable estimates of the accretion rates for a wide sample of HAeBe stars to date. We provided empirical expressions relating the accretion and the Hα, [OI]6300 and Brγ luminosities, being the first time that these calibrations are accurately obtained for the HAeBe regime. In contrast, we found that the Hα line width at 10% of peak intensity is not a valid accretion tracer for the HAeBes, unlike for lower mass stars. In addition, the accretion-rate changes from multi-epoch Balmer excess measurements seem to be uncorrelated to the simultaneous variability of the Hα and [OI]6300 lines. This led us to suggest that the origin of the empirical calibrations between the accretion and line luminosities could not be driven by the influence of accretion on the emission lines, but by a common dependence on the stellar luminosity.
- Finally, we have looked for trends relating the accretion rates with several stellar and disk properties. We have estimated inner gas dissipation timescales for the HAeBe regime, and report similar correlations than those for lower mass T Tauri stars. In particular, we find trends relating the mass accretion rate with the nIR excess and disk mass that point to simple viscous disk models explaining them. However, we find slightly faster inner gas dissipation timescales for our sample, and indications suggesting that a different physical process -such as photoevaporation- plays a major role dissipating disks around HAeBes.