DANIEL MOSER FAES
An interferometric view of hot star disks
Abstract
Optical long baseline interferometry was recently established as a technique capable of resolving stars and their circumstellar environments at the milliarcsecond (mas) resolution level. This high-resolution opens an entire new window to the study of astrophysical systems, providing information inaccessible by other techniques.
Astrophysical disks are observed in a wide variety of systems, from galaxies up to planetary rings, commonly sharing similar physical processes. Two particular disk like systems are studied in the thesis: (i) B He-rich stars that exhibits magnetic fields in order of kG and that trap their winds in structures called magnetospheres; and (ii) Be stars, fast rotating stars that create circumstellar viscous disks.
This study uses the interferometric technique to investigate both the photosphere proper and the circumstellar environment of these stars. The objective is to combine interferometry with other observational techniques (such as spectroscopy and polarimetry) to perform a complete and well-constrained physical description of these systems. This description is accompanied by radiative transfer models performed by the hdust code.
The first firm detection of a hot star magnetosphere in continuum linear polarization is reported, as a result of the monitoring campaign of Sigma Ori E at the Pico dos Dias Observatory (OPD/LNA). The polarimetric data was modeled by a single-scattering "dumbbell+disk" model, that describes the magnetosphere as constituted of two blob-like structures (at the intersection to the rotation and magnetic equators) and a circumstellar disk at the magnetic equator. The polarimetric modeling predicts a lower blob to disk mass ratio than the expect from the RRM (Rigid Rotating Magnetosphere) model, that provides a good spectroscopic description of these magnetospheres.
In addition to Sigma Ori E, we report the first polarimetric detections of the magnetospheres of HR 7355 and HR 5907. Our analysis indicate that these structures share similar properties, such as polarimetric modulation amplitude and mass distribution. In the case of the HR 5907, we also report the first interferometric detection of a magnetosphere with differential phase amplitude of about 3 degrees with AMBER-VLTI/ESO.
A new interferometric phenomenon, called CQE-PS (Central Quasi-Emission Phase Signature), was described and identified as a useful tool for the study of Be circumstellar disks, in particular for shell stars. Departures from a S-shaped differential phases occur when the disk obscures part of the stellar photosphere. This disk absorption alters the phase signal mainly near the rest wavelengths and can introduce a central reversal in the phases profile. This phenomenon can be used to probe disk size, density and radial slope. It may even provide an estimate for the (maximum) stellar angular size.
The photosphere of the Be star Achernar was studied in detail using high-precision visibilities and closure phase information from PIONIER-VLTI/ESO. This is the first precise photospheric characterization of a Be star and is, up to date, the characterization of the star with the highest mass and rotation rate. The variable line profile of Achernar and high-precision photometric frequencies are analyzed in the light of these new parameters. These results have great significance for stellar models and for the Be stars, such as the determination of the gravitational darkening coefficient.
We present the BeAtlas project as part of BeACoN group from IAG-USP. It consists of a systematic grid of Be stars models generated by the hdust code aiming at a comprehensive investigation of Be stars and their state-of-art modeling, the VDD (Viscous Decretion Disk). As first project application, we argue that Achernar is not a typical main sequence star, displaying characteristics of a star leaving the main sequence: it exhibits an big size and a luminosity higher than the expected for its mass.
The recent outburst of Achernar was investigated and modeled. For the analysis of the activity, a large set of observational data was obtained, such as AMBER spectro-interferometry, FEROS spectroscopy (ESO) and optical polarimetry (OPD/LNA). These observations contain the first spectro-interferometric study of a Be star disk evolution, angularly resolving the growing disk.
The secular evolution of the newly formed disk is characterized and the VDD modeling prescription was employed. The AMBER interferometric analysis shows no evidence of a polar wind in this active phase. The Halpha spectroscopy exhibits a slow and gradual evolution, and reaches a near stationary regime just after approx. 1.6 years. This feature will allow to estimate the disk viscous diffusion coefficient with dynamic VDD modeling.
Keywords: stars: individual (Achernar), stars: fundamental parameters, techniques: interferometric, circumstellar matter, stars: emission-line, Be, stars: magnetic field