By / par Gregg Wade (Canadian PI for BRITE)
(Cassiopeia – Spring / printemps 2019)
BRITE-Constellation is an international space astronomy mission consisting of a fleet of 20x20x20 cm nanosatellites dedicated to precision optical photometry of bright stars in two photometric colours. The mission continues in full science operations, with 25 data releases to BRITE target PIs having already taken place, and many datasets available in the public domain from the BRITE public archive.
The BRITE mission is a collaboration between Canadian, Austrian and Polish astronomers and space scientists. The Canadian partners represent University of Toronto, Université de Montréal, Bishop’s University, and Royal Military College of Canada. The mission was built, and the Canadian satellites operated by, the University of Toronto Institute for Aerospace Studies Space Flight Lab (UTIAS-SFL). The Canadian Space Agency funded the construction of the Canadian satellites, and continues to support their day-to-day operations.
There are five operating BRITE satellites in the Constellation, collecting data on various sky fields in a coordinated programme to obtain well-sampled, longterm continuous (~6 months) light curves in both red and blue bandpasses.
As this issue of Cassiopeia went to press, here was the status of the sky assignments for the BRITE cubesats:
- BRITE Toronto (Canada): Toronto observes with a red filter. It is currently observing the Ori/Tau field, switching between the two fields each orbit.
- BRITE Lem (Poland): Lem observes with a blue filter. It is also observing the Ori/Tau field.
- BRITE Heweliusz (Poland): Heweliusz observes with a red filter. This satellite is observing CMa/Pup II field. As implied by the numeral ‘II’, the current campaign on this field represents a revisit of a previously-observed field.
- BRITE Austria (Austria): BRITE Austria observes with a blue filter. It is observing the Ori/Tau field and the Vel/Pup V field.
- UniBRITE (Austria): UniBRITE observes with a red filter. This satellite is also observing the Ori/Tau field and the Vel/Pup V field.
The BRITE Constellation observing programme from early 2017 through to early 2020 has been planned by the BRITE Executive Science Team (BEST), and details are available on the BRITE photometry Wiki page.
Recent Science Results
“Seismic modelling of early B-type pulsators observed by BRITE: I. θ Ophiuchi” (P. Walczak et al. MNRAS, in press). In this paper the authors analyse BRITE time-series observations of the well known β Cephei type star θ Ophiuchi. Seven previously known frequencies are confirmed and nineteen new frequency peaks are detected. In particular, high-order g modes, typical of the Slowly Pulsating B-type (SPB) pulsators, are uncovered. These low-frequency modes are also detected in the 7-year-long 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 star’s internal structure. With this aim the authors perform in-depth seismic studies. 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.
“Stellar masses from granulation and oscillations of 23 bright red giants observed by BRITE – Constellation” (Kallinger et al. A&A, in press). The study of stellar structure and evolution depends crucially on accurate stellar parameters. The photometry from space telescopes has provided superb data that allowed asteroseismic characterisation of thousands of stars. However, typical targets of space telescopes are rather faint and complementary measurements are difficult to obtain. On the other hand, the brightest, otherwise well-studied stars, are lacking seismic characterization. Aims: Our goal is to use the granulation and/or oscillation time scales measured from photometric time series of bright red giants (1.6≤Vmag≤5.3) observed with BRITE to determine stellar surface gravities and masses. Methods: We use probabilistic methods to characterize the granulation and/or oscillation signal in the power density spectra and the autocorrelation function of the BRITE time series. Results: We detect a clear granulation and/or oscillation signal in 23 red giant stars and extract the corresponding time scales from the power density spectra as well as the autocorrelation function of the BRITE time series. To account for the recently discovered non-linearity of the classical seismic scaling relations, we use parameters from a large sample of Kepler stars to re-calibrate the scalings of the high- and low-frequency components of the granulation signal. We develop a method to identify which component is measured if only one granulation component is statistically significant in the data. We then use the new scalings to determine the surface gravity of our sample stars, finding them to be consistent with those determined from the autocorrelation signal of the time series. We further use radius estimates from the literature to determine the stellar masses of our sample stars from the measured surface gravities. We also define a statistical measure for the evolutionary stage of the stars.
“BRITE photometry of the massive post-RLOF system HD 149404G” (Rauw et al. 2019, A&A 621,15). HD 149404 is an evolved non-eclipsing O-star binary that has previously undergone a Roche lobe overflow interaction. Understanding some key properties of the system requires a determination of the orbital inclination and of the dimensions of the components. The authors used the BRITE-Heweliusz satellite to collect photometric data of HD 149404. Additional photometry was retrieved from the SMEI archive. These data were analysed using a suite of period search tools. The orbital part of the lightcurve was modelled with the nightfall binary star code. The Gaia-DR2 parallax of HD 149404 was used to provide additional constraints. The periodograms revealed a clear orbital modulation of the lightcurve with a peak-to-peak amplitude near 0.04 mag. The remaining non-orbital part of the variability is consistent with red noise. The lightcurve folded with the orbital period reveals ellipsoidal variations, but no eclipses. The minimum when the secondary star is in inferior conjunction is deeper than the other minimum, likely due to mutual reflection effects between the stars. Combined with the Gaia-DR2 parallaxes, the photometric data indicate an orbital inclination in the range of 23°-31° and a Roche lobe filling factor of the secondary larger than or equal to 0.96. The authors conclude that the luminosity of the primary star is consistent with its present-day mass, whereas the more evolved secondary appears over-luminous for its mass. It is confirmed that that the primary’s rotation period is about half the orbital period. Both features most probably stem from the past Roche lobe overflow episode.
Conferences, Resources and Social Media
The BRITE team is spearheading the organization of a conference entitled “Stars and their Variability, Observed from Space”, to occur in Vienna, Austria from August 19 – 23, 2019. Preregistration is available on the conference website.
The BRITE Public Data Archive, based in Warsaw, Poland, at the Nikolaus Copernicus Astronomical Centre, can be accessed at brite.camk.edu.pl/pub/index.html.
The mission Wiki (including information on past, current and future fields) can be accessed at brite.craq-astro.ca/.
BRITE Constellation is on Facebook, at @briteconstellation.
The BRITE International Advisory Science Team
The BRITE International Advisory Science Team (BIAST), which consists of BRITE scientific PIs, technical authorities, amateur astronomers, and mission fans, advises the mission executive on scientific and outreach aspects of the mission. If you’re interested to join BIAST, contact Canadian BRITE PI Gregg Wade: firstname.lastname@example.org.
BRITE-Austria and UniBRITE recently celebrated their 6th anniversary in orbit.
The BRITE Executive Science Team recently adopted the principles of the EAS Ethics Statement and Guidelines for Good Practice.