The Be Star 66 Ophiuchi: 60 Years of Disk Evolution

TL;DR

This study models the 60-year disk evolution of the Be star 66 Ophiuchi using observational data and advanced simulations, revealing detailed properties of disk dissipation and structure.

Contribution

It provides the first comprehensive 60-year analysis of 66 Ophiuchi's disk evolution combining observations with hydrodynamic and radiative transfer models.

Findings

Disk dissipation lasted approximately 21 years.

The initial disk density was about 2.5×10⁻¹¹ g/cm³.

Outer disk remained bright in radio after dissipation.

Abstract

We use a time-dependent hydrodynamic code and a non-LTE Monte Carlo code to model disk dissipation for the Be star 66 Ophiuchi. We compiled 63 years of observations from 1957 to 2020 to encompass the complete history of the growth and subsequent dissipation of the star's disk. Our models are constrained by new and archival photometry, spectroscopy and polarization observations, allowing us to model the disk dissipation event. Using Markov chain Monte Carlo methods, we find 66 Oph is consistent with standard B2Ve stellar properties. We computed a grid of 61568 Be star disk models to constrain the density profile of the disk before dissipation using observations of the H$\alpha$ line profile and SED. We find at the onset of dissipation the disk has a base density of $2.5\times10^{-11}\ \rm{g\ cm^{-3}}$ with a radial power-law index of $n=2.6$. Our models indicate that after 21 years of disk dissipation 66 Oph's outer disk remained present and bright in the radio. We find an isothermal disk with constant viscosity with an $\alpha = 0.4$ and an outer disk radius of $\sim$115 stellar radii best reproduces the rate of 66 Oph's disk dissipation. We determined the interstellar polarization in the direction of the star in the V-band is $p=0.63 \pm 0.02\%$ with a polarization position angle of $\theta_{IS}\approx85.7 \pm 0.7^\circ$. Using the Stokes QU diagram, we find the intrinsic polarization position angle of 66 Oph's disk is $\theta_{int}\approx98 \pm 3^\circ$.