By Parshati Patel
Thesis defended on April 25, 2016
Department of Physics and Astronomy, Western University
Thesis advisors: Dr. Aaron Sigut and Dr. John Landstreet
Thesis link: http://ir.lib.uwo.ca/etd/3692/
Herbig Ae/Be stars are intermediate mass (~2 to 20 Msun) pre-main sequence stars that inherit a circumstellar disk of dust and gas from their formation phase. The region of this disk closest to the star is entirely gaseous because dust evaporates at the high temperatures there, and this gaseous region is currently poorly understood. Using non-LTE circumstellar disk codes to model the optical and near-infrared spectra of five, early-type (B0Ve to B2Ve) Herbig Be stars obtained with the Canada-France-Hawaii telescope ESPaDOnS instrument, the density structure of each inner gaseous disks was determined and estimates of the disk masses and sizes were obtained. In the modeling, the photoionizing radiation of the central B star was assumed to be disk’s sole source of energy input.
For the four Herbig B2e stars, BD+65 1637, HD 76534, HD 114981 and HD 21669, reasonable matches were found for all emission line profiles considered individually; however, for each star, no disk density model based on a single power law for the equatorial density was able to simultaneously fit all of the observed emission lines in the spectrum. For BD+65 1637, the equatorial disk density law was estimated to fall as 10-10(R*/R)3 g cm-3 in a 50 R* disk, and this model provided a reasonable match to the overall line shapes and strengths. The stars HD 76534, HD 114981 and HD 216629 required a similar density model to that of BD+65 1637, but in a smaller, 25 R* disk. The overall implied masses of these inner gaseous disks are in the range of ~5.7 x 10-8 to 1.2 x 10-9 M*.
For BD+65 1637, the metal lines, Fe II and especially the Ca II IR triplet lines, required higher disk densities than implied by the hydrogen Balmer lines, with the disk density falling more slowly as 10-10(R*/R)2 g cm-3. In general, for all stars, the metal lines of Ca II and Fe II required higher disk densities than the Balmer lines to match the observed line profiles. A more complex disk density distribution is likely required to reconcile this difference and refine the match to the spectra of these stars.
The spectrum of the Herbig B0e star MWC 137 is dominated by very strong emission lines in comparison to the Herbig B2e stars. Preliminary results show that the models require extended disks to reproduce the observed Ca II and Fe II metal emission line profiles. For hydrogen Balmer lines, no disk models extending up to 200 R* were capable of reproducing the observed line strengths, indicating that even larger disks are required and/or the observed hydrogen lines are contaminated by hydrogen recombination emission from the surrounding H II region.
Taken as a whole, the analysis of these five, early-type HBe stars suggest that the optical and near-IR emission lines in their spectra can be adequately accounted for by an inner, entirely gaseous, disk in Keplerian rotation, heated solely by the photoionizing energy input of the central star, and requiring only a tiny fraction ~10-7 of the central star’s mass.