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Register a new userThe Be Star 66 Ophiuchi: 60 Years of Disk Evolution
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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 stars 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 $2.5times10^{-11} rm{g cm^{-3}}$ with a radial power-law index of $n=2.6$. Our models indicate that after 21 years of disk dissipation 66 Ophs 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 Ophs 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 Ophs disk is $theta_{int}approx98 pm 3^circ$.
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In this paper we present results of the spectroscopic analysis of H$alpha$ line profile of the Be star 60 Cygni. We present time evolution of the equivalent width of the H$alpha$ line profiles during years 1992 - 2016 and $V/R$ variation during years 1995 - 2016. We analyzed data from Ondv{r}ejov Observatory and from BeSS Database. The circumstellar disk of the star was present twice during years 1992 - 2016 and the second cycle shows stronger emission activity. We found out that the formation of the disk takes longer time than the disk extinction (extinction is much steeper than the formation) and that there is no evident period of changes in $V/R$ variation.
The short-lived $^{26}$Al and $^{60}$Fe radionuclides are synthesized and expelled in the interstellar medium by core-collapse supernova events. The solar systems first solids, calcium-aluminium refractory inclusions (CAIs), contain evidence for the former presence of the $^{26}$ Al nuclide defining the canonical $^{26}$Al/$^{27}$ Al ratio of $sim5 times10^{-5}$. A different class of objects temporally related to canonical CAIs are CAIs with fractionation and unidentified nuclear effects (FUN CAIs), which record a low initial $^{26}$Al/$^{27}$Al of $10^{-6}$. The contrasting level of $^{26}$Al between these objects is often interpreted as reflecting the admixing of the $^{26}$Al nuclide during the early formative phase of the Sun. We use giant molecular cloud (GMC) scale adaptive mesh-refinement numerical simulations to trace the abundance of $^{26}$Al and $^{60}$Fe in star-forming gas during the early stages of accretion of individual low mass protostars. We find that the $^{26}$Al/$^{27}$Al and $^{60}$Fe/$^{56}$Fe ratios of accreting gas within a vicinity of 1000 AU of the stars follow the predicted decay curves of the initial abundances at time of star formation without evidence of spatial or temporal heterogeneities for the first 100 kyr of star formation. Therefore, the observed differences in $^{26}$Al/$^{27}$Al ratios between FUN and canonical CAIs are likely not caused by admixing of supernova material during the early evolution of the proto-Sun. Selective thermal processing of dust grains is a more viable scenario to account for the heterogeneity in $^{26}$Al/$^{27}$Al ratios at the time of solar system formation.
The observed emission lines of Be stars originate from a circumstellar Keplerian disk that are generally well explained by the Viscous Decretion Disk model. In an earlier work we performed the modeling of the full light curve of the bright Be star $omega$ CMa (Ghoreyshi et al. 2018) with the 1-D time-dependent hydrodynamics code SINGLEBE and the Monte Carlo radiative-transfer code HDUST. We used the V -band light curve that probes the inner disk through four disk formation and dissipation cycles. This new study compares predictions of the same set of model parameters with time-resolved photometry from the near UV through the mid-infrared, comprehensive series of optical spectra, and optical broad-band polarimetry, that overall represent a larger volume of the disk. Qualitatively, the models reproduce the trends in the observed data due to the growth and decay of the disk. However, quantitative differences exist, e.g., an overprediction of the flux increasing with wavelength, too slow decreases in Balmer emission-line strength that are too slow during disk dissipation, and the discrepancy between the range of polarimetric data and the model. We find that a larger value of the viscosity parameter alone, or a truncated disk by a companion star, reduces these discrepancies by increasing the dissipation rate in the outer regions of the disk.
A global disk oscillation implemented in the viscous decretion disk (VDD) model has been used to reproduce most of the observed properties of the well known Be star $zeta$ Tau. 48 Librae shares several similarities with $zeta$ Tau -- they are both early-type Be stars, they display shell characteristics in their spectra, and they exhibit cyclic $V/R$ variations -- but has some marked differences as well, such as a much denser and more extended disk, a much longer $V/R$ cycle, and the absence of the so-called triple-peak features. We aim to reproduce the photometric, polarimetric, and spectroscopic observables of 48 Librae with a self-consistent model, and to test the global oscillation scenario for this target. Our calculations are carried out with the three-dimensional NLTE radiative transfer code HDUST. We employ a rotationally deformed, gravity-darkened central star, surrounded by a disk whose unperturbed state is given by the VDD model. A two-dimensional global oscillation code is then used to calculate the disk perturbation, and superimpose it on the unperturbed disk. A very good, self-consistent fit to the time-averaged properties of the disk is obtained with the VDD. The calculated perturbation has a period $P = 12$ yr, which agrees with the observed period, and the behaviour of the $V/R$ cycle is well reproduced by the perturbed model. The perturbed model improves the fit to the photometric data and reproduces some features of the observed spectroscopic data. Some suggestions to improve the synthesized spectroscopy in a future work are given.
We study the effect of density distribution evolution on the global one-armed oscillation modes in low viscosity disks around isolated and binary Be stars. Observations show that some Be stars exhibit evidence of formation and dissipation of the equatorial disk. In this paper, we first calculate the density evolution in disks around isolated Be stars. To model the formation stage of the disk, we inject mass at a radius just outside the star at a constant rate for 30-50 years. As the disk develops, the density distribution approaches the form of the steady disk solution. Then, we turn off the mass injection to model the disk dissipation stage. The innermost part of the disk starts accretion, and a gap forms between the star and the disk. Next, we calculate the one-armed modes at several epochs. We neglect the effect of viscosity because the time-scale of oscillations is much shorter than the disk evolution time-scale for low viscosity. In the disk formation stage, the eigenfrequency increases with time toward the value for the steady state disk. On the other hand, one-armed eigenmodes in dissipating Be disks have significantly higher eigenfrequencies and narrower propagation regions. Observationally, such a change of mode characteristics can be taken as an evidence for gap opening around the star. In binary Be stars, the characteristics of the disk evolution and the eigenmodes are qualitatively the same as in isolated Be stars, but quantitatively they have shorter evolution time-scales and higher eigenfrequencies, which is in agreement with the observed trend.
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