MOST found evidence for solar-type oscillations in the K2 giant star HD 20884

Abstract We found evidence for radial p-modes and nonradial mixed modes in the oscilla-tion spectrum of the K giant HD20884 based on 20.6 days of nearly continuoushigh-precisionphotometryobtainedbytheCanadianmicrosatelliteMOST 1 .Os-cillation frequencies range from 5-31µHz (periods of about 2.3d - 9hr) withluminosity amplitudes between about 300 and 950 ppm and mode lifetimes ex-ceeding 10 days are indicated. The mode identifications are based on searchesof a large grid of models for a best fit to the frequencies and temperature ofHD20884. The latter is better constrained now by spectroscopy obtained atthe David Dunlap Observatory as part of this work. Individual Objects: HD20884, HD20790, κ 1 Ceti, ǫOph, ξHya, HD146490 Introduction Our understanding of the Sun’s structure has been revolutionised over the last threedecades by helioseismology. This technique allows investigations of the solar interiorby observing p-modes at the stellar surface. Observing sun-like oscillations in otherstars was hampered for a long time by the extremely small pulsation amplitudes. Forstars cooler and more luminous than the Sun, the expected amplitudes are greater andshould be more easily observable. However, the larger radii of red giants extend theirpulsation periods from ∼5 minutes in the Sun to a range of several hours to severaldays. This again complicates groundbased detections and frequency identifications,especially due to daily aliasing.As ultra-precise rapid photometry from space and high-precision radial velocitymeasurements from the ground became available, detections of stochastically drivenoscillations were reported for several giants (see Bouchy & Carrier 2003 for a review).Examples include the G9.5 giant ǫOph, based on radial velocities from the CORALIEand ELODIE spectrographs (De Ridder et al. 2006) and Fabry Imaging photometryfromtheMOSTsatellite(Barbanetal. 2007), andtheG7giantξHya, basedonradialvelocity measurements with the CORALIE spectrograph (Frandsen et al. 2002), orthe K2.5 giant star GSC09137-03505 where Kallinger et al. (2005) found oscillations,based on Fine Guidance Sensor photometry by the Hubble Space Telescope.If pulsating red giants do indeed have radial and observable nonradial modes withrelatively long lifetimes, then these modes will enable to constrain the deep interiorof red giant stars, as well as set limits to the excitation mechanisms. But recentinvestigations (e.g., Barban et al. 2007 or Stello et al. 2006 ) suggest that only radialmodes with very short lifetimes (< 3 days) should be observable in red giants. Onthe other hand, Kallinger et al. (2008b) report strong evidence for the existence ofradial and nonradial modes in ǫOph (with lifetimes longer than 10 days) based on