Abstract
We analyse time-series observations from the BRITE-Constellation of the well-known β Cephei type star θ Ophiuchi. Seven previously known frequencies were confirmed and 19 new frequency peaks ...were detected. In particular, high-order g modes, typical for the SPB (Slowly Pulsating B-type star) pulsators, are uncovered. These low-frequency modes are also obtained from the 7-yr SMEI light curve. If g modes are associated with the primary component of θ Oph, then our discovery allows, as in the case of other hybrid pulsators, to infer more comprehensive information on the internal structure. To this aim we perform in-depth seismic studies involving simultaneous fitting of mode frequencies, reproducing mode instability, and adjusting the relative amplitude of the bolometric flux variations. To explain the mode instability in the observed frequency range a significant increase of the mean opacity in the vicinity of the Z-bump is needed. Moreover, constraints on mass, overshooting from the convective core and rotation are derived. If the low-frequency modes come from the speckle B5 companion then taking into account the effects of rotation is enough to explain the pulsational mode instability.
Abstract
This is the final photometric study of TW Hya based on new MOST satellite observations. During 2014 and 2017, the light curves showed stable 3.75- and 3.69-d quasi-periodic oscillations, ...respectively. Both values appear to be closely related to the stellar rotation period, as they might be created by changing visibility of a hotspot formed near the magnetic pole directed towards the observer. These major light variations were superimposed on a chaotic, flaring-type activity caused by hotspots resulting from unstable accretion – a situation reminiscent of that in 2011, when TW Hya showed signs of a moderately stable accretion state. In 2015, only drifting quasi-periods were observed, similar to those present in 2008–2009 data and typical for magnetized stars accreting in a strongly unstable regime. A rich set of multicolour data was obtained during 2013–2017 with the primary aim of characterizing the basic spectral properties of the mysterious occultations in TW Hya. Although several possible occultation-like events were identified, they are not as well defined as in the 2011 MOST data. The new ground-based and MOST data show a dozen previously unnoticed flares, as well as small-amplitude 11 min–3 h brightness variations, associated with ‘accretion bursts’. We cannot exclude the possibility that the shortest 11–15 min variations could also be caused by thermal instability oscillations in an accretion shock.
Towards the end of their lives, stars like the Sun greatly expand to become red giant stars. Such evolved stars could provide stringent tests of stellar theory, as many uncertainties of the internal ...stellar structure accumulate with age. Important examples are convective overshooting and rotational mixing during the central hydrogen-burning phase, which determine the mass of the helium core, but which are not well understood. In principle, analysis of radial and non-radial stellar oscillations can be used to constrain the mass of the helium core. Although all giants are expected to oscillate, it has hitherto been unclear whether non-radial modes are observable at all in red giants, or whether the oscillation modes have a short or a long mode lifetime, which determines the observational precision of the frequencies. Here we report the presence of radial and non-radial oscillations in more than 300 giant stars. For at least some of the giants, the mode lifetimes are of the order of a month. We observe giant stars with equally spaced frequency peaks in the Fourier spectrum of the time series, as well as giants for which the spectrum seems to be more complex. No satisfactory theoretical explanation currently exists for our observations.
Through photometric monitoring of the extended transit window of HD 97658b with the MOST space telescope, we have found that this exoplanet transits with an ephemeris consistent with that predicted ...from radial velocity measurements. The mid-transit times are 5.6sigma earlier than those of the unverified transit-like signals reported in 2011, and we find no connection between the two sets of events. The transit depth together with our determined stellar radius (Rlow * = (ProQuest: Formulae and/or non-USASCII text omitted) Rmiddot in circle) indicates a (ProQuest: Formulae and/or non-USASCII text omitted) R+ in circle super-Earth. When combined with the radial velocity determined mass of 7.86 + or - 0.73 M sub(+ in circle), our radius measure allows us to derive a planet density of (ProQuest: Formulae and/or non-USASCII text omitted) g cm super(-3). Models suggest that a planet with our measured density has a rocky core that is enveloped in an atmosphere composed of lighter elements. The star of the HD 97658 system is the second brightest known to host a transiting super-Earth, facilitating follow-up studies of this not easily daunted, warm and likely volatile-rich exoplanet.
Observations of β Lyr in four months of 2018 by three BRIght Target Explorer (BRITE) Constellation satellites, the red-filter BRITE-Toronto and BRITE-Heweliusz, and the blue-filter BRITE-Lem, ...permitted a first, limited look into the light-curve variability in two spectral bands. The variations were found to be well correlated outside the innermost phases of the primary eclipses with the blue variations appearing to have smaller amplitudes than the red; this reduction may reflect their presumed origin in the cooler, outer parts of the accretion disk. This result must be confirmed with more extensive material as the current conclusions are based on observations spanning slightly less than three orbital cycles of the binary. The assumption of an instrumental problem and the applied corrections made to explain the unexpectedly large amplitude of the red-filter light curve observed with the BRITE-Toronto satellite in 2016 are fully confirmed by the 2018 results.
BRITE-Constellation is devoted to high-precision optical photometric monitoring of bright stars, distributed all over the Milky Way, in red and/or blue passbands. Photometry from space avoids the ...turbulent and absorbing terrestrial atmosphere and allows for very long and continuous observing runs with high time resolution and thus provides the data necessary for understanding various processes inside stars (e.g., asteroseismology) and in their immediate environment. While the first astronomical observations from space focused on the spectral regions not accessible from the ground it soon became obvious around 1970 that avoiding the turbulent terrestrial atmosphere significantly improved the accuracy of photometry and satellites explicitly dedicated to high-quality photometry were launched. A perfect example is BRITE-Constellation, which is the result of a very successful cooperation between Austria, Canada and Poland. Research highlights for targets distributed nearly over the entire HRD are presented, but focus primarily on massive and hot stars.
Abstract
η Car is a massive, eccentric binary with a rich observational history. We obtained the first high-cadence, high-precision light curves with the BRITE-Constellation nanosatellites over 6 ...months in 2016 and 6 months in 2017. The light curve is contaminated by several sources including the Homunculus nebula and neighbouring stars, including the eclipsing binary CPD −59°2628. However, we found two coherent oscillations in the light curve. These may represent pulsations that are not yet understood but we postulate that they are related to tidally excited oscillations of η Car's primary star, and would be similar to those detected in lower mass eccentric binaries. In particular, one frequency was previously detected by van Genderen et al. and Sterken et al. through the time period of 1974–1995 through timing measurements of photometric maxima. Thus, this frequency seems to have been detected for nearly four decades, indicating that it has been stable in frequency over this time span. These pulsations could help provide the first direct constraints on the fundamental parameters of the primary star if confirmed and refined with future observations.
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