Stochastic Gravito-Inertial Modes Discovered by CoRoT in the Hot Be Star HD?51452

C. Neiner1, M. Floquet2, R. Samadi1, F. Espinosa Lara3, Y. Frémat4, S. Mathis5,1, B. Leroy1, B. de Batz1, M. Rainer6, E. Poretti6, P. Mathias3, J. Guarro Fló7, C. Buil8, J. Ribeiro9, E. Alecian1, L. Andrade10, M. Briquet1,11, P. D. Diago12,14, M. Emilio13, J. Fabregat14, J. Gutiérrez-Soto12,15,1, A.-M. Hubert2, E. Janot-Pacheco10, C. Martayan16, T. Semaan2, J. Suso14, J. Zorec17,

1 LESIA, UMR 8109 du CNRS, Observatoire de Paris, UPMC, Univ. Paris Diderot, 5 place Jules Janssen, 92195 Meudon Cedex, France
2 GEPI, UMR 8111 du CNRS, Observatoire de Paris, Univ. Paris Diderot, 5 place Jules Janssen, 92195 Meudon Cedex, France
3 Université de Toulouse, UPS-OMP, IRAP, CNRS, 14 avenue Edouard Belin, 31400 Toulouse, France
4 Royal Observatory of Belgium, 3 avenue circulaire, 1180 Brussels, Belgium
5 Laboratoire AIM Paris-Saclay, CEA/DSM-CNRS-Université Paris Diderot; IRFU/SAp, Centre de Saclay, 91191 Gif-sur-Yvette Cedex, France
6 INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807, Merate (LC), Italy
7 ARAS, Astronomical Ring for Access to Spectroscopy
8 Castanet Tolosan Observatory, 6 place Clemence Isaure, 31320 Castanet Tolosan, France
9 R. Venezuela 29, 3 Esq., 1500-618 Lisboa, Portugal
10 University of Sao Paulo, Brazil
11 Institut d'Astrophysique et de Géophysique, Université de Liège, 17 allée du 6 août, Bât B5c, 4000 Liège, Belgium
12 Valencian International University (VIU), Prolongación C/ José Pradas Gallen s/n, edificio B piso 2, 12006 Castellón de la Plana, Spain
13 Observatório Astronômico/DEGEO, Universidade Estadual de Ponta Grossa, Av. Carlos Cavalcanti, 4748 Ponta Grossa, Paraná, Brazil
14 Observatori Astronòmic de la Universitat de València, Edifici Instituts d'Investigatió, Poligon La Coma, 46980 Paterna, València, Spain
15 Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n 18008, Granada, Spain
16 ESO, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
17 Institut d'Astrophysique de Paris, UMR 7095, CNRS, Université Pierre & Marie Curie, 98bis Boulevard Arago 75014 Paris, France

Be stars are rapidly rotating stars with a circumstellar decretion disk. They usually undergo pressure and/or gravity pulsation modes excited by the ?-mechanism, i.e. an effect of the opacity of iron-peak elements in the envelope of the star. In the Milky Way, p-modes are observed in stars that are hotter than or equal to the B3 spectral type, while g-modes are observed at the B2 spectral type and cooler. We observed a B0e star, HD?51452, with the high-precision, high-cadence photometric CoRoT satellite and high-resolution, ground-based HARPS and SOPHIE spectrographs to study its pulsations in great detail. We also used the lower resolution spectra available in the BeSS database. We analyzed the CoRoT and spectroscopic data with several methods: Clean-NG, FreqFind, and a sliding window method. We also analyzed spectral quantities, such as the violet over red (V/R) emission variations, to obtain information about the variation in the circumstellar environment. We calculated a stellar structure model with the ESTER code to test the various interpretation of the results. We detect 189 frequencies of variations in the CoRoT light curve in the range between 0 and 4.5 c~d$^{-1}$. The main frequencies are also recovered in the spectroscopic data. In particular we find that HD?51452 undergoes gravito-inertial modes that are not in the domain of those excited by the ?-mechanism. We propose that these are stochastic modes excited in the convective zones and that at least some of them are a multiplet of r-modes (i.e. subinertial modes mainly driven by the Coriolis acceleration). Stochastically excited gravito-inertial modes had never been observed in any star, and theory predicted that their very low amplitudes would be undetectable even with CoRoT. We suggest that the amplitudes are enhanced in HD?51452 because of the very rapid stellar rotation. In addition, we find that the amplitude variations of these modes are related to the occurrence of minor outbursts. Thanks to CoRoT data, we have detected a new kind of pulsations in HD?51452, which are stochastically excited gravito-inertial modes, probably due to its very rapid rotation. These modes are probably also present in other rapidly rotating hot Be stars.

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