What's happening around Herbig Ae stars?: investigating circumstellar activity in young intermediate mass stars with optical and near-infrared spectroscopy

Abstract

We have investigated the optical and near-infrared spectral behavior of the intermediate mass, pre-main sequence Herbig Ae stars, with particular emphasis on variability and evolution. The evolutionary state of Herbig Ae (HAe) stars is intermediate between very young, pre-main sequence objects and the young main sequence Vega, or beta Pic-like, stars. The majority of HAe stars are estimated to span a small range in ages, between 2 and 6 million years. Near-infrared spectra reveal an evolutionary sequence from strong Brackett emission in the youngest sources to photospheric absorption in main sequence Vega-like stars. HAe stars exhibit nearly featureless spectra in the near-infrared, a combination of photospheric absorption veiled by strong excess continuum, and weak Brackett line emission. In the evolutionary sequence, the hot gas creating the Brackett emission disappears before the hot dust responsible for the thermal emission. The data suggest that the strength of the Brackett emission decreases with age between 1 and 10 Myrs, while the thermal emission drops for stars >10 Myrs. Our high resolution spectra of Brgamma in 2 HAe stars indicate that the Brackett emission is associated with accreting gas. Therefore, near-infrared Brackett emission offers an important diagnostic of inner disk evolution in late pre-main sequence intermediate mass stars.A large fraction (∼25--50%) of HAe stars, called UXORs after the prototype UX Ori, exhibit large optical variability (DeltaV > 1.5 mag), commonly attributed to obscuration of the central star by circumstellar dust. Optical spectra of the UXOR star RR Tau over factor of 10 in brightness (V = 11 to V ∼ 13.5 mag), reveal a system in which the photosphere of the star (Balmer wings and weak metal lines) and the wind flux ([OI], [FeII], and Halpha lines) are unaffected by the brightness minima while permitted metal lines (FeII, CaII, OI, NaI) change from absorption to emission when the star fades. We suggest a model that attributes the metal absorption to a higher density region obscured with the star, while the metal emission lines come from a more extended unobscured region. More importantly, this result exemplifies the value of spectral monitoring in constraining the location of obscuring material and its effects on the circumstellar environment.