Abstract Details

Name: Pratheeksha Nayak
Affiliation: Indian Institute of Space Science and Technology
Conference ID: ASI2017_1294
Title : OI Lines in Herbig Ae/Be Stars
Authors and Co-Authors : Pratheeksha Nayak (Indian Institute of Space Science and Technology), Blesson Mathew (Tata Institute of Fundamental Research), Manoj Puravankara (Tata Institute of Fundamental Research)
Abstract Type : Poster
Abstract Category : Stars,ISM and the Galaxy
Abstract : Herbig Ae/Be (HAeBe) stars are intermediate mass (2M☉< M < 8M☉) pre-main sequence stars surrounded by accretion disks. The inner most region of the accretion disks is a reservoir for the production of several emission lines seen in the optical and near-IR spectra. We investigated the dominant excitation mechanism for the most prominent OI line at λ8446 Å observed in the spectra of HAeBe stars. Several different mechanisms have been proposed for the OI line formation, which include Lyman beta fluorescence, collisional excitation, recombination and continuum fluorescence. In Classical Be stars, the main sequence analogs of HAeBe stars and which possess ionised decretion disks, Lyβ fluorescence has been identified as the primary excitation mechanism for OI λ8446 Å line (Mathew et al. 2012b). In HAeBe stars with accretion disks, the situation is less clear. We analysed optical (5500-9000 Å) and near-IR (Y & J band) spectra of a large sample of HAeBe stars to study the excitation conditions of the OI λ8446 Å line. Excess OI emission above photospheric absorption is observed in 23 optical and 3 near-IR spectra. We measured emission strengths of Hα, OI λ7774 and OI λ8446 in these. The observed line flux values were compared with theoretical estimates to identify the dominant excitation mechanism. Our analysis indicate that Lyβ fluorescence is likely the dominant mechanism for OI λ8446 Å excitation in HAeBe stars, if the line originates in the hot (> 5000 K) and dense (ne > 10^12 cm^-3) magnetospheric accretion columns. Interestingly, we find a strong correlation between OI λ8446 line flux and the accretion rates estimated from Hα. This opens up the possibility of using OI λ8446 as an accretion indicator, which, unlike Hα, is less affected by contributions from jets/outflows.