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Mixing and Angular Momentum tranSport of massIvE stars

Periodic Reporting for period 3 - MAMSIE (Mixing and Angular Momentum tranSport of massIvE stars)

Reporting period: 2019-01-01 to 2020-06-30

Computations of the chemical enrichment of galaxies and of the Universe as a whole, rely heavily on the outcome of stellar evolutionary theory of massive stars (e.g. Maeder 2009, Langer 2012). Essentially all heavy elements are delivered by stars that get born with several to tens of times the mass of the Sun (hereafter termed massive stars). Theoretical predictions for the evolutionary properties of such massive stars depend strongly on poorly known phenomena like convective overshooting, internal differential rotation, internal magnetic elds, the stellar wind, etc. The joint consequences of these processes for the mixing of chemical species and for the transport and loss of angular momentum throughout the stellar lifetime remain largely unknown. Furthermore, the inclusion of angular momentum transport and turbulent entrainment from the convective core regions are currently still lacking in stellar evolution theory while they hold the potential to explain various recently observed phenomena that have hitherto no explanation in terms of current models (e.g. Tkachenko et al. 2014).

With MAMSIE, we take a novel approach by constructing models for stellar interiors of carefully selected massive stars through the development and application of asteroseismic modelling methods based on detected and identied gravity-mode oscillations. In contrast to pressure-mode oscillations propagating through the envelopes of stars, gravity modes probe the entire stellar interior, all the way from the surface to the core. A large number of such modes has recently been detected by our team in a few carefully selected young massive stars from the CoRoT and Kepler missions. Moreover, hundreds of massive stars in all evolutionary stages were submitted by the MAMSIE team as NASA Guest Observer programme and have recently been observed by the successor of Kepler, termed the K2 mission (Howell et al. 2014).
"Some of the most important publications in our work packages are summarized below.

Van Reeth T., Tkachenko A., Aerts C., 2016, “Interior rotation of a sample of γ-Doradus stars from ensemble modelling of their gravity-mode period spacings”, Astronomy and Astrophysics, 593, A120:

Near-core rotation in massive stars remains a critical aspect in the understanding of their interior layers and general evolutions. Its measurement is difficult due to lack of observational data. Hence, we aimed to constrain the near-core rotation rates for a sample of γ Doradus stars using a newly developed method. Herein the calculated asymptotic period spacing was combined with high-precision spectroscopy to fit the observed period spacing patterns.
The applied method resulted in a successful chief outcome where for a majority of the γ Doradus stars the rotation rates were determined, as well as the modes corresponding to their detected pulsation frequencies. Despite its current limitations, the proposed method forms a crucial step towards a more detailed asteroseismic modelling.

Moravveji E., Townsend R. H. D., Aerts C., Mathis S., 2016, ""Sub-inertial Gravity Modes in the B8V Star KIC 7760680 Reveal Moderate Core Overshooting and Low Vertical Diffusive Mixing"", The Astrophysical Journal, 823, 130:

In this paper, we carried out a thorough forward asteroseismic modelling of KIC 7760680, the richest slowly pulsating B star discovered to that date and furthermore also one of the best-understood stars of the SPB star class observed with the Kepler satellite.
Our research has pointed out that KIC 7760680 has a moderate rotation rate (25% of the critical Roche rate). Moderate convective core overshooting at its core boundary is required to match the detected pulsation frequencies. Not only is thus recognized that convective core overshooting can coexist together with moderate rotation, we have also demonstrated that stellar rotation increases convective overshooting from the core and leads to gravity-inertial modes.

Aerts C., Símon-Díaz S., Bloemen S., Debosscher J., Pápics P. I., Bryson S., Still M., Moravveji E., Williamson M. H., Grundahl F., Fredslund Andersen M., Antoci V., Pallé P. L., Christensen Dalsgaard J., Rogers T. M., 2017, “Kepler sheds new and unprecedented light on the variability of a blue supergiant: Gravity waves in the O9.5Iab star HD 188209”, Astronomy & Astrophysics, 602, A32

In this work, we used the Kepler spacecraft far beyond its nominal performance by studying scattered-light photometry of the bright blue supergiant HD 188209. The latter is the only massive supergiant that was monitored with the nominal Kepler mission during a total time base of four years and about equally long in ground-based spectroscopy.
A major conclusion of the work performed is that the range of detected frequencies in the space photometry and in ground-based spectroscopy are the same. All the detected frequency spectra point towards variability occurring in the photosphere of the blue supergiant and consistently propagating into the bottom of the stellar wind. While we could not find a simple point-to-point relationship between the photometric and spectroscopic data that were taken simultaneously, we did find full consistency in the frequency range caused by these independent data sets.

Aerts C., Van Reeth T., Tkachenko A., 2017, “The Interior Angular Momentum of Core Hydrogen Burning Stars from Gravity-mode Oscillations”, The Astrophysical Journal, 847, L7

Rotation has a significant impact on the life of a star; it induces a multitude of hydrodynamical processes that affect stellar structure and its evolution. The angular momentum distribution inside stars and its change during stellar life remains nevertheless a major uncertainty in the theory of stellar evolution.
In this research, we composed a sample of 67 stars in the core hydrogen burning phase, with an asteroseismic estimate of the near-core rotation rate derived from gravity-mode oscillations detected in space photometry. This survey resulted in the ability to place the observed near-core rotation rates in an evolutionary context and to come to the important conclusion that the core rotation must drop drastically before or during the short phase between the end of the core hydrogen burning and the onset of core helium burning.

Buysschaert B., Neiner C., Briquet M., Aerts C., 2017, “Magnetic characterization of the SPB/β Cep hybrid pulsator HD 43317”, Astronomy & Astrophysics, 605, A104
Buysschaert B., Aerts C., Bowman D. M., Johnston C., Van Reeth T., Pedersen M. G., Mathis S., Neiner C., 2018, “Forward seismic modelling of the pulsating magnetic B-type star HD 43317”, Astronomy & Astrophysics

The target of the executed study is SPB/ β Cepheid HD 43317, the only magnetic early-type star with a rich frequency spectrum of gravity modes. Its chemical surface abundances agree with the solar abundances and some co-rotating He abundance spots at the stellar surface.
Chief outcome of this research contains several magnetic characteristics of HD 44317. Our results point to a dipolar magnetic field without any substantial quadrupolar contribution. We were further able to improve derivation of its rotation period, by combined CoRoT photometry and spectropolarimetry and deduced that the dipolar magnetic field of HD 43317 is between approximately 1 and 1.5 kG, with an uncertain inclination angle. Comparing the derived strength for the magnetic field with theoretical criteria indicates that the surface magnetic field of HD 43317 is sufficiently strong to impose a uniform rotation in the radiative layer of the star. Our subsequent asteroseismic modelling of the star is the only one so far including both the Coriolis and Lorentz forces, pointing out that the latter is negligible with respect to the former. This key novel conclusion implies that asteroseismic modelling can be focused on rotating non-magnetic stars.

Aerts C., Bowman D. M., Simón-Díaz S., Buysschaert B., Johnston C., Moravveji E., Beck P. G., De Cat P., Triana S., Aigrain S., Castro N., Huber D., White T., 2018, ""K2 photometry and HERMES spectroscopy of the blue supergiant ρ Leo: rotational wind modulation and low-frequency waves"", Monthly Notices of the Royal Astronomical Society, 476, 1234-1241

Blue supergiants are in the least understood stage of the evolution of massive stars. Lack of understanding of this stage is unfortunate, since the successors of these stars play a key role in the chemical evolution of their host galaxy. We unravelled the dominant causes of the low-frequency variability of the blue supergiant ρ Leo from combined 80 days K2 halo photometry and 1800 days high-resolution HERMES spectroscopy. The low-amplitude variations in velocity and brightness observed this way were interpreted as due to internal gravity waves, caused by convective driving or by an opacity mechanism in the envelope.
Our study illustrated the power of combined high-cadence uninterrupted space photometry with a time base of months and ground-based high-resolution spectroscopy covering several years and is a guide to our future research on blue supergiants during the second part of the ERC project.

In essence, the way our project was expected to have major impact on stellar evolution theory is on excellent track and has not changed from the original set up and definition in the DoA. We did already encounter some (pleasant!) surprises revealed to us by the detected gravity modes in selected stars, due to which the paths forward are more optimistic than anticipated as the project moves along. During the first two years of MAMSIE, we have been able to perform more dissemination in terms of publications and talks at international conferences than foreseen originally. Recognition of the results at international level is recognised by the invitation to write a review paper in Annual Reviews of Astronomy & Astrophysics, the journal with the highest impact factor in Universe Science, summarizing our ERC MAMSIE results. We also did a lot of training and outreach activities in which MAMSIE results were integrated, not only for astronomers, but also for schools and the broad public.

Most of our focus since the beginning of the project has been on Work Package 1 (WP1), since the other packages need its results. In this context, we have been focusing on the search for a wide variety of pulsating stars, hunting to derive their fundamental properties from space photometry and ground-based spectroscopy. The newly started PhD projects in WP1 are well on track and overall far better progress is made in WP1 than anticipated, including a re-orientation of Work Package 2 (WP2) towards a novel integrated mathematical modelling tool for stellar parameter estimation for gravity-mode pulsators. Our new methodology paper was under revision at the time of this intermediate report and will be included in subsequent reporting. It relies better state-of-the-art mathematical tools than adopted so far in the literature and constitutes a major milestone in our project.

In Work Package 3 (WP3), our goal is to replace the current too basic treatment of mixing with a new one, based on new physical ingredients. The latter concerns the characteristics and effect of internal gravity waves on chemical mixing in the radiative envelope of massive stars. Our work of observing a star that was not on active silicon of the Kepler CCD cameras led to an entirely new opportunity of using satellite data to do so, meanwhile followed by other teams. We will engage in detailed studies of internal gravity waves in a large sample of stars from our successful NASA Guest Observer proposals.

Lastly, feeding off data gathered by WP1 and WP2, and our modelling in WP2 and WP3, allowed us to integrate the aim Work Package 4 (WP4) to combine a 3D hydrodynamical description of turbulent entrainment with 1D stellar structure computations into an overall modelling scheme, which is currently being implemented on the supercomputing facilities at our disposal in Flanders, to which the MAMSIE team managed to get competitive access.