BRITE (BRIght Target Explorer) Constellation, the first nanosatellite mission applied to astrophysical research, is a collaboration among Austria, Canada and Poland. The fleet of satellites (6 launched; 5 functioning) performs precise optical photometry of the brightest stars in the night sky. A pioneering mission like BRITE -with optics and instruments restricted to small volume, mass and power in several nanosatellites, whose measurements must be coordinated in orbit -poses many unique challenges. We discuss the technical issues, including problems encountered during on-orbit commissioning (especially higher-thanexpected sensitivity of the CCDs to particle radiation). We describe in detail how the BRITE team has mitigated these problems, and provide a complete overview of mission operations. This paper serves as a template for how to effectively plan, build and operate future low-cost niche-driven space astronomy missions. and multi-filter capability, for a sample of the brightest stars, which tend to be the most intrinsically luminous (i.e., massive and/or highly evolved). BRITE Constellation extends the parameter space of space photometry missions, with nearly all-sky coverage in two wavelength ranges of hundreds of the most luminous stars in the Galaxy -all at relatively low cost (Weiss et al. 2014) . Three partner nations (Austria, Canada and Poland) each contributed a pair of nanosatellites (mass 7 kg; 3-axis-stablized). The BRITE network is designed to collect optical photometry of millimagnitude precision (Popowicz et al. 2016, in prep; hereafter Paper III) in light curves of high cadence (20 -25 s between consecutive exposures) and long duration (up to 6 months) through red and blue filters. The features of the six BRITE nanosatellites are listed in Table 1; only five are currently operating in orbit. The Austrian satellites are UniBRITE (UBr) and BRITE-Austria (BAb), the Polish are BRITE-Lem (BLb) and BRITE-Heweliusz (BHr), and the Canadian are BRITE-Toronto (BTr) and BRITE-Montréal (BMb, which did not deploy correctly into orbit); where r and b refer to the satellites equipped with red and blue filters, respectively. This is Paper II in a series of publications that address the technical aspects of the BRITE mission. The first paper in the series, Weiss et al. (2014), shall hereafter be referred to as Paper I. This paper provides a comprehensive history of the development of BRITE, the overall design of each satellite, and an explanation of the objectives that have been the driving forces behind the mission. Paper III in the series will be a description of the BRITE data reduction pipeline. BRITE's prime directive is to observe bright stars (V ≤ 4 mag), and shed light on their internal and surface dynamics. Among the benefits that BRITE offers are:• A test bed for future astronomical surveys with small satellites. The combination of cutting-edge science with small low-cost instruments in space has come ≈ $600 million price tag (Borucki 2016), albeit with many more limitations, providing the opportunit...
Context. The BRITE mission is a pioneering space project aimed at the long-term photometric monitoring of the brightest stars in the sky by means of a constellation of nano-satellites. Its main advantage is high photometric accuracy and time coverage inaccessible from the ground. Its main drawback is the lack of cooling of the CCD and the absence of good shielding that would protect sensors from energetic particles. Aims. The main aim of this paper is the presentation of procedures used to obtain high-precision photometry from a series of images acquired by the BRITE satellites in two modes of observing, stare and chopping. The other aim is comparison of the photometry obtained with two different pipelines and comparison of the real scatter with expectations. Methods. We developed two pipelines corresponding to the two modes of observing. They are based on aperture photometry with a constant aperture, circular for stare mode of observing and thresholded for chopping mode. The impulsive noise is a serious problem for observations made in the stare mode of observing and therefore in the pipeline developed for observations made in this mode, hot pixels are replaced using the information from shifted images in a series obtained during a single orbit of a satellite. In the other pipeline, the hot pixel replacement is not required because the photometry is made in difference images. Results. The assessment of the performance of both pipelines is presented. It is based on two comparisons, which use data from six runs of the UniBRITE satellite: (i) comparison of photometry obtained by both pipelines on the same data, which were partly affected by charge transfer inefficiency (CTI), (ii) comparison of real scatter with theoretical expectations. It is shown that for CTI-affected observations, the chopping pipeline provides much better photometry than the other pipeline. For other observations, the results are comparable only for data obtained shortly after switching to chopping mode. Starting from about 2.5 years in orbit, the chopping mode of observing provides significantly better photometry for UniBRITE data than the stare mode.Conclusions. This paper shows that high-precision space photometry with low-cost nano-satellites is achievable. The proposed methods, used to obtain photometry from images affected by high impulsive noise, can be applied to data from other space missions or even to data acquired from ground-based observations.
Context. Asteroseismology of massive pulsating stars of β Cep and SPB types can help us to uncover the internal structure of massive stars and understand certain physical phenomena that are taking place in their interiors. We study β Centauri (Agena), a triple system with two massive fast-rotating early B-type components which show p-and g-mode pulsations; the system's secondary is also known to have a measurable magnetic field. Aims. This paper aims to precisely determine the masses and detect pulsation modes in the two massive components of β Cen with BRITE-Constellation photometry. In addition, seismic models for the components are considered and the effects of fast rotation are discussed. This is done to test the limitations of seismic modeling for this very difficult case. Methods. A simultaneous fit of visual and spectroscopic orbits is used to self-consistently derive the orbital parameters, and subsequently the masses, of the components. Time-series analysis of BRITE-Constellation data is used to detect pulsation modes and derive their frequencies, amplitudes, phases, and rates of frequency change. Theoretically-predicted frequencies are calculated for the appropriate evolutionary models and their stability is checked. The effects of rotational splitting and coupling are also presented. Results. The derived masses of the two massive components are equal to 12.02 ± 0.13 and 10.58 ± 0.18 M . The parameters of the wider, A-B system, presently approaching periastron passage, are constrained. Analysis of the combined blue-and red-filter BRITEConstellation photometric data of the system revealed the presence of 19 periodic terms, of which eight are likely g modes, nine are p modes, and the remaining two are combination terms. It cannot be excluded that one or two low-frequency terms are rotational frequencies. It is possible that both components of β Cen are β Cep/SPB hybrids. An attempt to use the apparent changes of frequency to distinguish which modes originate in which component did not succeed, but there is potential for using this method when more BRITE data become available. Conclusions. Agena seems to be one of very few rapidly rotating massive objects with rich p-and g-mode spectra, and precisely known masses. It can therefore be used to gain a better understanding of the excitation of pulsations in relatively rapidly rotating stars and their seismic modeling. Lacking proper mode identification, the pulsation frequencies found in β Cen cannot yet be used to constrain the internal structure of the components, but it may be possible to achieve this in the future with the use of spectroscopy and spectropolarimetry. In particular, these kinds of data can be used for mode identification since they provide new radial velocities. In consequence, they may help to improve the orbital solution, derive more precise masses, magnetic field strength and geometry, inclination angles, and reveal rotation periods. They may also help to assign pulsation frequencies to components. Finally, the case studied here ...
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