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Dr Giovanni Mirouh


Research Fellow in Stellar Astrophysics

Academic and research departments

Department of Physics.

My publications

Publications

Handler Gerald, Pigulski Andrzej, DaszyDska-Daszkiewicz Jadwiga, Irrgang Andreas, Kilkenny David, Guo Zhao, Przybilla Norbert, Aliçavu_ Filiz Kahraman, Kallinger Thomas, Pascual-Granado Javier, Niemczura Ewa, Ró|aDski Tomasz, Chowdhury Sowgata, Buzasi Derek L., Mirouh Giovanni M., Bowman Dominic M., Johnston Cole, Pedersen May G., Simón-Díaz Sergio, Moravveji Ehsan, Gazeas Kosmas, De Cat Peter, Vanderspek Roland K., Ricker George R. (2019) Asteroseismology of Massive Stars with the TESS Mission: The Runaway ² Cep Pulsator PHL 346 = HN Aqr, The Astrophysical Journal Letters 873 (1) IOP Publishing
We report an analysis of the first known ² Cep pulsator observed by the Transiting Exoplanet Survey Satellite (TESS) mission, the runaway star PHL 346 = HN Aqr. The star, previously known as a singly periodic pulsator, has at least 34 oscillation modes excited, 12 of those in the g-mode domain and 22 p modes. Analysis of archival data implies that the amplitude and frequency of the dominant mode and the stellar radial velocity were variable over time. A binary nature would be inconsistent with the inferred ejection velocity from the Galactic disk of 420 km s?1, which is too large to be survivable by a runaway binary system. A kinematic analysis of the star results in an age constraint (23 ± 1 Myr) that can be imposed on asteroseismic modeling and that can be used to remove degeneracies in the modeling process. Our attempts to match the excitation of the observed frequency spectrum resulted in pulsation models that were too young. Hence, asteroseismic studies of runaway pulsators can become vital not only in tracing the evolutionary history of such objects, but to understand the interior structure of massive stars in general. TESS is now opening up these stars for detailed asteroseismic investigation.
Pedersen May G., Chowdhury Sowgata, Johnston Cole, Bowman Dominic M., Aerts Conny, Handler Gerald, Cat Peter De, Neiner Coralie, David-Uraz Alexandre, Buzasi Derek, Tkachenko Andrew, Simón-Díaz Sergio, Moravveji Ehsan, Sikora James, Mirouh Giovanni M., Lovekin Catherine C., Cantiello Matteo, DaszyDska-Daszkiewicz Jadwiga, Pigulski Andrzej, Vanderspek Roland K., Ricker George R. (2019) Diverse Variability of O and B Stars Revealed from 2-minute Cadence Light Curves in Sectors 1 and 2 of the TESS Mission: Selection of an Asteroseismic Sample, The Astrophysical Journal 872 (1) IOP Publishing
Uncertainties in stellar structure and evolution theory are largest for stars undergoing core convection on the main sequence. A powerful way to calibrate the free parameters used in the theory of stellar interiors is asteroseismology, which provides direct measurements of angular momentum and element transport. We report the detection and classification of new variable O and B stars using high-precision short-cadence (2 minutes) photometric observations assembled by the Transiting Exoplanet Survey Satellite (TESS). In our sample of 154 O and B stars, we detect a high percentage (90%) of variability. Among these we find 23 multiperiodic pulsators, 6 eclipsing binaries, 21 rotational variables, and 25 stars with stochastic low-frequency variability. Several additional variables overlap between these categories. Our study of O and B stars not only demonstrates the high data quality achieved by TESS for optimal studies of the variability of the most massive stars in the universe, but also represents the first step toward the selection and composition of a large sample of O and B pulsators with high potential for joint asteroseismic and spectroscopic modeling of their interior structure with unprecedented precision.
Mirouh Giovanni M, Angelou George C, Reese Daniel R, Costa Guglielmo (2018) Mode classification in fast-rotating stars using a convolutional neural network: model-based regular patterns in ´ Scuti stars, Monthly Notices of the Royal Astronomical Society: Letters 483 (1) pp. L28-L32 Oxford University Press (OUP)
Oscillation modes in fast-rotating stars can be split into several subclasses, each with their own properties. To date, seismology of these stars cannot rely on regular pattern analysis and scaling relations. However, recently there has been the promising discovery of large separations observed in spectra of fast-rotating ´ Scuti stars; they were attributed to the island-mode subclass, and linked to the stellar mean density through a scaling law. In this work, we investigate the relevance of this scaling relation by computing models of fast-rotating stars and their oscillation spectra. In order to sort the thousands of oscillation modes thus obtained, we train a convolutional neural network isolating the island modes with 96 per cent accuracy. Arguing that the observed large separation is systematically smaller than the asymptotic one, we retrieve the observational ”½--Á¯¯¯ scaling law. This relation will be used to drive forward modelling efforts, and is a first step towards mode identification and inversions for fast-rotating stars.
Balona L A, Handler G, Chowdhury S, Ozuyar D, Engelbrecht C A, Mirouh G M, Wade G A, David-Uraz A, Cantiello M (2019) Rotational modulation in TESS B stars, Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP)
Light curves and periodograms of 160 B stars observed by the TESS space mission and 29 main-sequence B stars from Kepler and K2 were used to classify the variability type. There are 114 main-sequence B stars in the TESS sample, of which 45 are classified as possible rotational variables. This confirms previous findings that a large fraction (about 40 percent) of A and B stars may exhibit rotational modulation. Gaia DR2 parallaxes were used to estimate luminosities, from which the radii and equatorial rotational velocities can be deduced. It is shown that observed values of the projected rotational velocities are lower than the estimated equatorial velocities for nearly all the stars, as they should be if rotation is the cause of the light variation. We conclude that a large fraction of main-sequence B stars appear to contain surface features which cannot likely be attributed to abundance patches.