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ASAMBA:
AsteroSeismic Approach Towards Understanding Massive Blue SupergiAnts

Final Report Summary - ASAMBA (ASAMBA:AsteroSeismic Approach Towards Understanding Massive Blue SupergiAnts)

1. Background: ASAMBA (AsteroSeismic Approach Towards Understanding Massive Blue SupergiAnts) aimed at providing deeper insight into the internal workings of hot massive B-type stars. This idea came to existence due to the availability of high-precision NASA Kepler space photometry and state-of-the-art one-dimensional stellar models for intermediate and massive B-type stars. The basic methodology is to confront the observed stellar pulsation frequencies with the predicted pulsation modes from theory. In this process, we tune few global parameters of stars to minimize the difference between the observed and model frequencies.

Asteroseismology is the observation, analysis and inference of the (flux and spectral) variability of stars due to radial/non-radial oscillations. The cause of surface stellar variability are the standing waves that form near the cores of stars, travel all the way towards the surface, and bounce back inward. Thus, these waves reveal the internal physics of stars, thanks to their very high sensitivity to the physical conditions at different depths in stars. We are currently living in the golden era of asteroseismology given the availability of high-quality observations and stellar models.

2. Scope: The ASAMBA project focused on the following three critical aspects in stellar physics which are the major sources of uncertainties on the physics of stars:
- The size, mass and the mixing induced by the "overshooting" bubbles from convective cores into radiative regions.
- The mixing in the radiative region (between convective core and surface, where the modes propagate).
- The impact of iron and nickel opacity on stellar evolution and pulsations.

For our study, we used four years of Kepler light curves of two late B-type stars KIC 10526294 (Papics, Moravveji et al. 2014; Moravveji et al. 2015) and KIC 7760680 (Papics et al. 2015; Moravveji et al. 2016). The former is a very slowly-rotating slowly-pulsating B star exhibiting 19 dipole gravity modes. The slow rotation allows excluding the complicated impacts of rotation on stellar structure, evolution and pulsation and simplified the study. The latter star rotates moderately (period of half-a-day), while exhibiting 36 dipole gravity-mode pulsations, the longest set of modes found so far in a B star. This was the first time that two Kepler B stars were so thoroughly exploited.

3. Achievements: In the case of both B stars, I succeeded to put very tight constraints on the mass, metallicity, core overshooting size/mass, age, extra diffusive mixing and rotation rate. This claim stems from a very successful fitting of the observed pulsation periods (or equivalently pulsation frequencies) with the theoretically predicted values from 1D stellar evolution and pulsation models. In the attached Figure-1 and Figure-2 files, I demonstrate the quality of the matching the observed period spacing (period difference between two consecutive modes) versus mode periods.

Based on the obtained match between the two profiles for both stars, the following conclusions are drawn:
- The relative difference between the observed pulsation frequencies, and stellar models are better than half-a-percent (which is remarkable).
- Rotation increases the extent of the overshooting zone.
- The strength of the overshooting mixing declines radially from the convective core.
- It is possible to nail down the unknown overshooting parameter in Kepler stars (regardless of their spin) to better than 5 percent.
- The diffusive mixing beyond the convective core is two to three orders of magnitude lower than predicted.
It is noteworthy that these conclusions drawn above are in agreement with modern theories of stellar convection (Arnett & Moravveji, 2017) and numerical simulations (Browning et et al. 2004).

Since 2015, the Iron (and Nickel) opacity has been majorly revisited. Numerical simulations and direct laboratory measurements of Iron (and Nickel) opacity in high temperature and density conditions (resembling solar interior) clearly show that these two elements are much more opaque than what we previously thought. I offered a third angle to consolidate this fact, by employing stellar pulsations in massive stars which depend sensitively on Iron opacity.

I compiled new opacity tables by enhancing the Iron and Nickel opacity by 75% (according to the simulations/measurements). Then, updated stellar models were computed using the MESA code, and were fed into GYRE code for non-adiabatic mode stability analysis of radial/non-radial modes. The locus of models which exhibited excited modes were drawn (the so called instability strips) in a diagram with surface temperature on the abscissa and the surface gravity on the ordinate. The position of confirmed pulsating O- and B-type stars were were overplotted on this diagram, showing a striking match between the two, and providing an additional evidence that Iron (and Nickel) opacities must be significantly increased when computing stellar models. Without such an opacity enhancement, it is not possible to explain why the target stars exhibit surface oscillations in photometry and high-resolution spectroscopy.
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