Literature Review #6: Parameters Herbig Ae/Be and Vega-type stars

B. Montesinos, C. Eiroa, A. Mora, B. Merin

Previous studies involving the modellin of circumstellar disks, including the previously discussed Manoj et al. (2006), have used basic parameters for the central star such as assuming solar metallicity. This will likely lead to incorrect results since the energy received by the disk from the star, determined by the star’s mass, radius, and effective temperature, affects disk characteristics such as geometry and contribution to the SED. The goal of the paper by Montesino et al. is to determine the stellar characteristics of 27 Herbig Ae/Be stars. The characteristics they found are effective temperature, surface gravity, and metallicity. This was interesting for me to read about because, as the paper points out in its introduction, that the circumstellar disks for T Tauri stars and Herbig Ae/Be stars dissipate at different rates. The disk of a classical T Tauri star will be gone when the star is 5-7 Myr while the disk of a HAeBe star will be gone when the star is 3 Myr. I don’t know, and I think nobody knows, why the lifetime of the disks is so different from the low mass to intermediate mass stars. An accurate model of CSDs based on accurate source parameters is very likely key to understanding this.

The basic method that the authors used to characterize the properties of the stars is to compare observed spectra with synthetic spectra. They observed the sample set using the Calar Alto Faint Object Spectrograph (CAFOS) to get intermediate resolution spectra to use Balmer line profiles in order to estimate stellar gravities. To determine the metallicities of the stars high-resolution echelle spectra was taken with the Utrech Echelle Spectrograph and the William Herschel Telescope.

The first parameter that needed to be found was the effective temperature of each star. To do this the authors compared observed spectral energy distributions with a grid of low-resolution synthetic spectra with different effective temperatures and then choose the best fit.

Stellar gravities, which is the surface gravity of the star not the potential well created by the star, were found by comparing the wings of the Balmer lines with synthetic profiles from Kurucz (1993). Since the wings of the lines are related to how the gas is moving a high surface gravity corresponds to a broader line.

Metallicities were found by comparing synthetic spectra found using SYNTHE (Sbordone et al 2004) with observed high-resolution spectra. Basically the made different spectra with the same effective temperature and surface gravity of the stars computed for different metallicities with the observed spectrum.

In the paper the authors specifically point out RR Tau for the difficulties involved in using the above methods to determine its parameters. Because RR Tau varies in a complex way the authors tried to single out spectral features that had as little variation as possible, features that would originate from the photosphere, to determine the parameters.

The paper presents a single SED for RR Tau and I don’t think it can ever be completely accurate to present a single thing of anything for RR Tau. I understand that they tried to minimize the influence of the variation but I think it would be better to make SEDs of RR Tau at many different places in its brightness. I talked to Hugh about the idea of doing this with HYPERION and my Gemini observations but he said that because my wavelength region is so narrow it’s unlikely that the SED shape will change that much. So he suggested I do full spectral fits instead of SEDs using line indices. So I’m going to start looking into that.

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