###### Abstract

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.

###### keywords:

space photometry; stellar structure; stellar evolution; stellar environment;
nanosatellites

7 6 199 Academic Editors: Laszlo Szabados and Nikolay N. Samus 17 May 2021 8
June 2021 16 June 2021 https://doi.org/10.3390/ universe7060199 Space
Photometry with BRITE-Constellation § Space Photometry with BRITE-
Constellation Werner W. Weiss 1,*, Konstanze Zwintz 2, Rainer Kuschnig 3,
Gerald Handler 4, Anthony F. J. Moffat 5, Dietrich Baade 6, Dominic M. Bowman
7, Thomas Granzer 8, Thomas Kallinger 1, Otto F. Koudelka 3, Catherine C.
Lovekin 9, Coralie Neiner 10, Herbert Pablo 11, Andrzej Pigulski 12, Adam
Popowicz 13, Tahina Ramiaramanantsoa 14, Slavek M. Rucinski 15, Klaus G.
Strassmeier 8 and Gregg A. Wade 16 Werner W. Weiss, Konstanze Zwintz, Rainer
Kuschnig, Gerald Handler, Anthony F. J. Moffat, Dietrich Baade, Dominic M.
Bowman, Thomas Granzer, Thomas Kallinger, Otto F. Koudelka, Catherine C.
Lovekin, Coralie Neiner, Herbert Pablo, Andrzej Pigulski, Adam Popowicz,
Tahina Ramiaramanantsoa, Slavek M. Rucinski, Klaus G. Strassmeier and Gregg A.
Wade Weiss, W.W.; Zwintz, K.; Kuschnig, R.; Handler, G.; Moffat, A.F.J.;
Baade, D.; Bowman, D.M.; Granzer, T.; Kallinger, T.; Lovekin, C.C.; et al.
Correspondence<EMAIL_ADDRESS>BRITE-Constellation was designed,
built, launched, and is operated and supported by the Austrian Research
Promotion Agency (FFG), the University of Vienna, the Technical University of
Graz, the University of Innsbruck, the Canadian Space Agency (CSA), the
University of Toronto Institute for Aerospace Studies (UTIAS), the Foundation
for Polish Science & Technology (FNiTP MNiSW) and the National Science Centre
(NCN).

##  1 A Brief Flashback

The first successful launch of a satellite in 1957 (Sputnik NASA History
Office (1997)) triggered a new era of astronomical observing techniques which
enormously expanded the research potential of astrophysics, mainly because the
terrestrial atmosphere could be overcome. Consequently, the first space
observations focused on spectral regions, which were not accessible from the
ground. Eight years after Sputnik, Proton satellites observed cosmic
$\gamma$-rays (1965). The Orbiting Astronomical Observatory (OAO Code et al.
(1970)) from NASA were the first operational telescopes in space. After a
power failure of OAO-1 right after the launch in 1966, OAO-2 was launched in
1968, and a follow-up, OAO-3 (Copernicus), was launched in 1972. These OAO-
satellites provided a wealth of insight into the variability of stars and
intricate details of the interstellar matter. The Astronomical Netherlands
Satellite (ANS, 1974 van Duinen et al. (1975)) conducted photometric
observations of variable stars in the UV, followed by the NASA/ESA
International Ultraviolet Explorer (IUE Boggess et al. (1978)) in 1978.

Already in the early days of space astronomy, the obvious scientific success
accelerated the development of space instrumentation for all spectral ranges,
taking advantage of avoiding a turbulent and absorbing atmosphere, not to
mention clouds and a day/night rhythm. The desire to observe even fainter
targets required the launch of space telescopes with increasing size—very
similar to most ground based observatories. A still scientifically productive
example is the amazing Hubble Space Telescope (HST Hall (1982) ), launched in
1990.

It needed nearly 30 years after the dawn of space telescopes for projects
explicitly dedicated to “simple” stars to become a reality. A most prominent
example is Hipparcos van Leeuwen et al. (1997) , an ESA mission with an
aperture of 29 cm, launched in 1989, with the goal of determining high
precision parallaxes of a large number of stars in our neighbourhood. The
entire sky was scanned during three years, which resulted in up to 110 data
points per star in the final Hipparcos and Tycho catalogues. The data have
proven to be a treasure chest for detecting stellar variability (Kallinger and
Weiss (2002) and many more). The follow-up ESA mission Gaia ((Brown A.,
2018)), using a 145 $\times$ 50 cm telescope, was launched in 2013 and
enormously increased our knowledge about the 4D picture of our Galaxy.

Projects in the early stages of space telescopes focused on highly ranked
targets devoted to the evolutionary aspects of galaxies, cosmology,
interstellar nebulae and their role in stellar evolution, solar system planets
and other hot topics. Monitoring classical variable or allegedly constant
stars continuously over many hours, days or even months, was hindered due to
the high pressure on telescope time. Fortunately, space telescopes need
pointing and guiding equipment, usually provided by small auxiliary
telescopes. One of the first star trackers, “abused” for stellar photometry,
was the Fine Guidance Sensor (FGS) of the HST, as described by Kuschnig et al.
(1997, 1998); Zwintz et al. (1999); Weiss et al. (2000); Zwintz et al. (2000)
and in workshop proceedings of the Space Telescope Science Institute (Kuschnig
et al. (1996)).

Already in 1982, a first proposal for a space photometer dedicated to stellar
variability and activity, Evris, was submitted to CNES (Mangeney et al.
(1982)) and was developed further as a passenger instrument for the USSR-
Mars94 mission. It was intended to be active during the cruise time to Mars
(Vuillemin et al. (1998)). However, launch of the Mars94 mission was delayed
to 1996 and ended in disaster, because of a rocket failure, which crashed
Mars96 in the Chilean Andes together with Evris. Fortunately, the experience
gained during development of Evris was not lost: already in 1993, the French
team had submitted a larger follow-up seismology mission, CoRoT (Schneider et
al. (1998); Weiss and Baglin (2000)), which was launched by ESA in 2006 and
was active until 2014.

Asteroseismology experienced a boom towards the end of the last century, as it
became obvious how much one can learn about stellar structure and evolution
with this tool, as well as how one can test complex astrophysical concepts,
with important implications for astrophysics in general. However, excellent
data were necessary for such investigations, that is, data which cover as
continuously as possible a long time span and with mmag accuracy or better.

A textbook-like example is $\zeta$ Pup (Figure 1), which was observed
simultaneously from space by two satellites and which illustrates the bonus of
higher photometric accuracy (TESS) counter-weighted by longer data sets
(BRITE). Another example is $\alpha$ Cir (Section 4.12 and Figure 20). The
shorter TESS run of $\zeta$ Pup has broader Fourier peaks ($\approx$1.5 months
vs. $\approx$4.5 months long data sets), but also shows many Fourier peaks,
which appear to be less prominent in a longer run as is illustrated in the
time resolved frequency analysis (lower part of Figure 2). Evidently,
stochastic stellar variability dominates in the shorter run, while the 1.78 d
cyclic variability of the star is much more well-defined and well-covered in
the longer observing run (see also Figure 1). The changing amplitude of the
1.78 d signal indicates that the signal is likely due to (bright) spots of
this O4If(n) type star that come and go. Sometimes the signal is not even
there (see the BHr time-frequency diagram of Figure 2). Whenever there are
overlapping TESS and BHr observations, they follow each other relatively well
and have roughly the same amplitudes, except at the beginning of each subset
of the TESS observations, which is due to systematics in TESS data around
gaps. More about $\zeta$ Pup is presented later in Section 4.1, including
comments on the physics that is probably involved.

Figure 1: $\zeta$ Pup observed by BRITE-Heweliusz, a component of BRITE-
Constellation, (black dots, red filter) and with TESS in sector 7 (first line
in blue) and in sector 8 (second line in red).

In addition to long continuous data sets, photometers operated in space have
an advantage by avoiding day-time gaps in low-Earth orbits, inaccessibility of
stars during certain seasons, and noise introduced by a turbulent atmosphere
(Weiss (2007)). The need for such data was subsequently boosted further by the
discovery of exoplanets.

The CNES mission CoRoT provides a perfect example of a “small” satellite,
which produced top science with a rather small budget, although nowhere nearly
as small as that for BRITE-Constellation. Not surprisingly, the scientific
community was interested in generating more such satellites, but competition
with trendy space projects was intense, as is illustrated by the tortuous path
after CoRoT. PRISMA (Lemaire et al. (1992)) was developed to extend CoRoT and
was accepted in 1993 as an ESA Horizon-2000 M2 project study, but finally lost
the race in 2002 against the $\gamma$-ray satellite INTEGRAL. The study team
did not give up and produced a Horizon-2000 M3 proposal, STARS (Jones et al.
(1996)), but lost again in 2009 against the cosmic background explorer Planck.
The next attempt was Eddington (Favata et al. (2000); Roxburgh (2002)), an ESA
Flexi Mission, but the gravitational wave detector Lisa settled its problems
for a planned launch in 2015, and consequently Eddington had to step back.
Later, the launch of LISA was delayed and is now scheduled for 2034. But
finally, PLATO was proposed in 2007 and selected in 2014 as an ESA Cosmic
Vision mission (PLATO-Consortium Plato-Consortium (2017)), driven by an
exploding interest in exoplanets. Launch is scheduled for 2026. It took 20
years after CoRoT till a follow-up, Plato, finally was decided and about 30
years till-hopefully-first data will be available!

Figure 2: Time dependent frequency spectra of $\zeta$ Pup obtained from data
presented in Figure 1 (Ramiaramanantsoa et al., in preparation) and with a
sliding time window of 12 days. The colour scale represents signal power
normalized to the maximum power in the windowed discrete Fourier transforms of
the BHr data.

Outside Europe, similar efforts were also successful. Soon after the crash of
Evris, an Announcement of Opportunity (AO) for Small Payloads was distributed
in 1996 by the Canadian Space Agency (CSA), which was responded to in 1997
with a proposal for MOST (Rucinski et al. (1997)). This satellite was launched
in 2003 as Canada’s first space telescope, and with an aperture of 15 cm it
was the smallest space telescope in orbit at that time. While designed only
for a nominal lifetime of one year, it collected under the directorship of
Jaymie Matthews (UBC) scientifically useful data till January 2018, that is,
for more than 15 years. Even after the CSA operations, funding ended in 2014,
MOST was frequently activated for pay-per-view observers.

Paying tribute to the exploding interest in exoplanets after the detection of
51b Peg, NASA decided in 2001 to fund a space telescope, Kepler, dedicated to
the discovery of exoplanets (Borucki et al. (2010)). At that time only 80
exoplanets were known, a number which increased dramatically after Kepler’s
launch in 2009. Reaction-wheel failures in 2012 and 2013 resulted in a
modified mission, Kepler-K2, which finally ended the mission in 2018, after
the discovery of more than 2600 exoplanets and delivering an enormous amount
of data for asteroseismology.

The Wide Field Infrared Explorer (WIRE) reminds one of the HST’s Fine Guidance
Sensors as auxiliary equipment with a potential for space photometry. WIRE
(Hacking et al. (1997)) was launched in 1999, but due to a premature ejection
of the telescope cover, all cryogen quickly evaporated and made IR observation
impossible. Fortunately, the star tracker was still working and contributed
successfully to asteroseismology until the decommissioning of WIRE in 2011.
This substantially exceeded the four months of the originally planned lifetime
of the IR mission. Another mission producing photometric data for
asteroseismology as a side-product to its main research goal is the Solar Mass
Ejection Imager (SMEI) on board Coriolis (Eyles et al. (2003)), which was
operational from 2003 to 2011 in a sun synchronous polar orbit with 102 min
period.

The follow-up mission to Kepler is TESS, which was first discussed in 2005 and
launched in 2018 by NASA, just after ending the Kepler mission. TESS (Ricker
et al. (2014) focuses on the stars brighter than those observed by Kepler and
the K2 follow-up, and it covers a sky area 400 times larger than that
monitored by Kepler. As an example of the relevance of TESS data for
asteroseismology we refer, for example, to Cunha et al. (2019), Antoci et al.
(2019), Bowman (2020) and Burssens et al. (2020).

More information about HST, Kepler, Gaia and TESS will be presented in
dedicated chapters of this journal volume.

##  2 The Birth of BRITE-Constellation

The development of BRITE-Constellation can be traced to the origins of the
Canadian microsatellite MOST Rucinski et al. (1997), which was designed by
Slavek Rucinski and Kieran Carroll (University of Toronto, UT), starting with
construction in 1998, and successfully utilized by the team led by Jaymie
Matthews (UBC) after launch in 2003 till 2014. Robert Zee (Manager of UT Space
Flight Laboratory, SFL) wanted to continue the momentum created by the success
of MOST and asked Rucinski in 2002 the non-trivial question, if nanosatellites
could be of relevance for astronomy. One has to keep in mind that at that time
nanosatellites were young and rarely utilized for research, with primary
interest as an engineering experimentation exercise and looking down, not up,
for Earth-atmosphere and -surface research. Nevertheless, a design concept for
a single CANX-3 satellite was developed in 2004 by SFL and a small team of
Canadian astronomers as a first fully three-axis stabilized satellite of 20
$\times$ 20 $\times$ 20 cm size, containing a telescope with 3 cm aperture
(Figure 3).

Figure 3: Basic structure of the BRITE satellites. Source: SFL.

Another root of origin is with Werner Weiss (University of Vienna, UoV) who
was co-I of Evris, later of CoRoT and also a member of the MOST team. The
latter membership closed the loop to CanX-3. The failure of the Evris-launch
contrasted dramatically with the anticipated research potential for
asteroseismology, an expectation, which was later confirmed by CoRoT and MOST.
Hence, the pressure to produce a space telescope optimised for bright stars
grew. Luckily, the Austrian Ministry of Science and Technology established in
March 2005 a program for improving the infrastructure of Austrian
Universities, to which UoV submitted a proposal for UniBRITE. This was
accepted in October 2005 and, one month later, UniBRITE was ordered at SFL,
based on their concept of CanX-3.

A third root is with Otto Koudelka (Technical University Graz, TUG). The
Austrian Space Agency (ASA) issued in 2005 a call for the 3rd Austrian Space
Programme. Two nanosatellite proposals were in the queue: one of the Institute
for Astrophysics (UoV), dedicated to asteroseismology (Weiss (2007)), and
another from the Institute of Communication Networks and Satellite
Communications (TUG), for developing and building a cubesat. ASA suggested to
merge these initiatives, which resulted in a proposal with Koudelka at TUG as
the PI, and which was approved by ASA in 2006. This was the birth of the first
satellite built in Graz (and Austria): BRITE-Austria, also called TUGSat-1.
The link to MOST is highlighted in a sentence of the proposal: BRITE-Austria
will extend and supplement the spectacularly successful Canadian
microsatellite MOST into the domain of nanosatellites.

As the Austrian BRITE’s were accepted for funding, Slavek Rucinski felt that
Poland (his country of birth) with its rapidly improving economy should join.
When the Canadian part of the project appeared to be in limbo due to CSA
dragging its heels regarding funding (from 2006 until 2011), he started
pushing his colleagues and former students in Poland to follow the Austrian
example. Aleksander Schwarzenberg-Czerny (Copernicus Astronomical Center,
Warsaw, CAMK and former PhD student of Rucinski) was able to obtain funding
for two BRITE satellites at the end of 2009 and, hence, he provides the fourth
root of BRITE-Constellation.

The pressure on the Canadian Space Agency (CSA) increased considerably, after
Austria funded UniBRITE and BRITE-Austria, and after Poland funded BRITE-
Heweliusz and BRITE-Lem. Finally, CSA accepted in 2011 the two Canadian
BRITE’s (former CanX-3): BRITE-Toronto and BRITE-Montreal.

In this way, BRITE-Constellation was born with six satellites.

##  3 BRITE-Constellation

The goal of BRITE-Constellation website (2021) was to provide high-precision
photometric monitoring of very bright ($\lesssim$4 mag) stars in two optical
wavelength bands (colours), that is, blue and red, and for up to six months,
the maximum feasible time in an affordable low-Earth orbit. Various concepts
have been discussed and finally a single telescope, optimized for a given
passband was chosen with no moving elements on board, thus reducing risk, but
which required one spacecraft per filter.

The proceedings of the First BRITE Workshop Zwintz and Kaiser (2008) provide
an overview to the technical and scientific issues, which were discussed and
decided before launch in 2013. The situation of the six (five active)
components of BRITE-Constellation after launch (Table 3) are described in
Weiss et al. (2014); Deschamps et al. (2009); Koudelka et al. (2014) and
various aspects of BRITE-data reduction in Pablo (2016); Popowicz et al.
(2017); Popowicz (2018).

[H] Launch and orbital information for the BRITE nanosats. BRITE-Montréal did
not separate from the launch vehicle and is not operational. The red filter
covers 550–700 nm, and the blue filter 400–450 nm.  Owner Name Filter ID
Launch Date Orbit (km) Period (min) Austria UniBRITE red UBr 25 Feb. 2013
$781\times 766$ 100.37 BRITE-Austria blue BAb 25 Feb. 2013 $781\times 766$
100.36 Poland BRITE-Heweliusz red BHr 19 Aug. 2014 $612\times 640$ 97.10
BRITE-Lem blue BLb 21 Nov. 2013 $600\times 900$ 99.57 Canada BRITE-Toronto red
BTr 19 June 2014 $629\times 577$ 98.24 BRITE-Montréal blue 19 June 2014 n/a 2

Conferences were organised almost every year to discuss updates and new
aspects of the mission. Most importantly, they allowed for vivid scientific
discussions which helped to shape the focus of BRITE-Constellation. The first
science conference took place in 2015 in Gdańsk, Poland, one year later in
Innsbruck Zwintz and Poretti (2016), and in 2017 at Lac Taureau, Canada. The
conference in Vienna “Stars and their Variability, Observed from Space-
Celebrating the 5th Anniversary of BRITE-Constellation” in August 2019
provided the most recent status report proc (2019).

###  3.1 Instrumentation

The BRITE instruments consist of a multi-lens telescope with an aperture of 3
cm, optimised for the red (550–700 nm) or the blue (400–450 nm) wavelength
range (Figure 4, red design). The unvignetted field of view (FOV) is about 24∘
in diameter and the optics were chosen to provide slightly out-of-focus
stellar images for improved S/N, an experience acquired from MOST. For the two
Austrian, the two Canadian and the blue Polish instruments a 5-lens system was
developed. The red Polish instrument (BHr) has a four-lens design, which
results in a shorter telescope, but with a smaller FOV of 20∘. A baffle in
front of each telescope reduces off-axis stray-light from bright sources,
including the Sun, Moon and Earth.

Figure 4: Camera scheme for the UniBRITE  and BRITE-Toronto red instruments
with a nearly vignetting free field-of-view of 24∘. The red or blue filters
are placed at the entrance pupil of the 5-lens camera to assure a constant
filter function over the entire FOV. The blue cameras have slightly modified
lens radii and separations to optimise the image quality for the needed
wavelength range. Source: SFL.

The same interline frame-transfer CCDs, a Kodak KAI 11002-M (4048 $\times$
2672 pixels and 9 $\upmu$m pixel size) chip, are used for each BRITE. This is
an off-the-shelf product which includes all read-out electronics and
preamplifiers on a header board behind the chip. Attractive features, besides
the modest price, is the low dark current at high temperatures (0∘–30∘ C),
which allows one to avoid a cooling system, and a low read-out noise and power
consumption. This CCD has been successfully used on the ground in SBIG
Cameras, but never in the radiation environment of space. In order to avoid
pixel saturation, the CCD is positioned out-of-focus, which, together with the
optical design, results in about 8-pixel-wide on-axis stellar images. Off-
center images have a more complex shape, as is shown in Figure 5. The scale is
about 27′′–30′′ per pixel, increasing slightly towards the edge due to image
distortion.

Figure 5: Full frame image of the Orion field taken with UniBRITE (UBr) in
December 2013. The stars which have been selected for photometric time series
are indicated. Subframes (24 $\times$ 24 pixels) which contain a full PSF,
were stored in memory for a later download to the ground. Typical subframes in
the center and close to the edge of the field are presented in the right
panel.

###  3.2 Photometry and Data Processing

The BRITE mission requirements were set in 2005 such that the instruments
shall observe a selected star-field for at least 15 min per orbit. Outside
that time interval, scattered light from the Earth and Sun would be
encountered. Data from up to 15 stars per field shall be collected for up to
100 days. In reality the BRITE satellites typically collect data from 24 to 28
stars during 20 to 40 min per orbit over a time base of about 160 days. The
exposure times typically vary between 1 and 5 s and every 21 s subframes were
read out. All functioning BRITE satellites were launched into low-earth polar
orbits with periods close to 100 min.

Target fields can be occulted by the Earth during part of the orbit. After the
field becomes visible again and its distance from the earth-limb exceeds a
critical angle, the Attitude Control System (ACS) of the satellite re-points
the star field in the camera FOV. To obtain top-quality photometry, the ACS
must assure stable pointing of the PSF during the entire observing run close
to the same pixels (flat-field exposures are not possible), which typically is
achieved within 1.5′ rms ($\approx$3 pixels). An example of such a photometric
cadence is show in Figure 6. Whenever possible, a satellite setup was chosen,
allowing one to observe a second field during an orbit, when the first field
was invisible for the satellite.

Figure 6: A typical photometric sequence of 44 Cyg (Zwintz (2017)), observed
by BRITE-Toronto in the field during a single orbit (left) and the data
sequence during six consecutive orbits (right), indicating intrinsic light
variations.

After the first BRITE satellites (UBr and BAb) were launched and the first
images were recorded, features appeared which were not present in the
laboratory: pixels and even entire columns with increased dark (thermal)
signal, that is, “hot pixels” and “warm columns” (Figure 7). These flaws were
distributed over a significant fraction of the CCD, in the FOV as well as
outside with no light access. The defects became stronger during successive
weeks, even at the same CCD operating temperature. The signal of “warm
columns” ranged from 100 to 500 ADUs above nominal background. One ADU
corresponds at 20∘ C to 3.2 detected electrons. For a “hot” pixel the signal
(even without illumination) is more than 100 ADUs above median background and
it can even get close to saturation ($\approx$1200 ADUs). All BRITE satellites
suffer from these radiation defects, believed to arise mainly from proton
collisions, which accumulate over time and adversely affect the data reduction
and quality.

Figure 7: Illustration of the chopping procedure. Top: empty rasters (24
$\times$ 36 pixel subsets of a frame). The left and middle raster was off-set
horizontally by about 0.15∘ . Bottom: same as for top row, but with the
telescope moved to a nearby star in the raster. Right column: absolute values
of the raster differences. All images here were taken at about +20 ∘C
operating temperature. The values on top of the upper row are the % of pixels
which reveal a dark current higher than 100 ADU, compared to the median
background of all pixels in the respective raster.

As is described in Popowicz et al. (2017); Popowicz (2018), a very efficient
technique to overcome the mentioned detector flaws and to significantly
improve the accuracy of the CCD photometry is the “chopping” technique, which
was introduced to the observing procedure in November 2014 and installed in
February 2015 as the default observing mode for all satellites. This mode
replaced the previously used “stare” mode. In the chopping mode a satellite is
shifted between exposures back and forth, so that for every second raster-
image the star is positioned in the other part of the raster (Figure 7).
Finally, the difference of two subsequent rasters contains essentially only
information relating to the stellar brightness and all local background
features are close to being eliminated.

Data reduction of all BRITE photometry is the responsibility of the Data
Reduction and Quality Control (DRQC) team (see Section 3.3). The data
corrected for, for example, the flux values with the CCD temperature and x and
y pointing positions on the CCD, are archived and forwarded to the Principal
Investigators (PI) for further de-correlation. De-correlation methods have
been developed by Pigulski and documented as the “BRITE Cookbook”, which can
be accessed together with the software code at
https://www.pta.edu.pl/pliki/proc/vol8/v8p175.pdf. Examples of BRITE
photometry are presented in Figures 6, 13–16, 18, 21 and 23.

###  3.3 Organisation and Operation

Organisation and operation of BRITE-Constellation relies on six interacting
teams (Figure 8), which are:

Figure 8: Organisation structure of BRITE-Constellation. BEST: BRITE Executive
Science Team, MC: Mission Control, SatOp: Satellite Operation, DRQC: Data
Reduction & Quality Control, BIAST: BRITE International Advisory Science Team

  * •

BEST (BRITE Executive Science Team) is the ruling body of BRITE-Constellation.
It consists of two voting members per satellite, nominated by the three member
countries (Austria, Canada and Poland) which funded the BRITE satellites. BEST
elects additional non-voting experts, presently 15. BEST releases 6 to 12
months before a new observing campaigns starts a BRITE Observing Plan (BOP),
which typically covers 12 to 14 months of operation. The BOP defines which
satellite is assigned to which field and for how long (Figures 9 and 10). The
rather long lead-time allows the PI’s to organise supplementary observations
from the ground or from space.

  * •

MC (Mission Control) team is headed by Rainer Kuschnig (IKS, TU-Graz, formerly
IfA Uni-Vienna) and is responsible for the execution of BOP by providing
satellite orientation and instrument setup data. To ensure a maximum
efficiency of BRITE-Constellation, a frequent quality control of all data
generated with all active satellites is another core activity of MC. Such
tests are applied at least twice a week and reported to BEST every second
week. In case of problems, MC interacts directly with the corresponding
satellite operator in charge.
A very short turn-around time between data check and satellite operation is
possible, because BRITE-Constellation observes “only” up to 60 stars during a
campaign and basically a single person inspects the data nearly in real time.
The obvious benefit is a fast response to unexpected stellar variability. The
best and most outstanding example is the serendipitous data collection from
Nova Carinae 2018. Almost instantly it was apparent that BRITE-Constellation
had caught the nova days before it was discovered visually. Hence, this early
volatile phase could be covered by BRITE-Constellation in an unprecedented
manner, as is explained in Section 4.16.

  * •

SatOp (Satellite Operation) teams are other key elements of the mission.
Satellite operators are in charge of controlling the national spacecraft via
the ground stations, of which one is in Austria at TU-Graz, one in Canada at
SFL-Toronto and a third one in Poland at CAMK-Warsaw. However, in case of
emergency, communication is possible from each of the ground stations to any
satellite to ensure uninterrupted satellite control and data management. This
was, and still is, usually required during harsh weather conditions at
particular ground stations or during maintenance periods.

  * •

DRQC (Data Reduction and Quality Control) is another core element of the
mission. The data received from each BRITE satellite on a daily basis is
delivered by SatOp to MC for a preliminary quality check. Once a campaign on a
given field is finished, all raw data are ASCII formatted with a FITS-like
header and made available to DRQC, which generates pipeline-reduced data files
(supervised by Adam Popowicz, Silesian University of Technology, Gliwice)
Popowicz et al. (2017), and performs quality control (supervised by Bert
Pablo, AAVSO). The original data, the raw science data (ASCII) files and the
time series datasets are then submitted to the BRITE Data Archive (maintained
by Andrzej Pigulski, University of Wroclaw). Most of the archive can be
accessed publicly, but some data are still protected for a limited time for
the corresponding PIs. The BRITE Public Data Archive can be found at
https://brite.camk.edu.pl/pub/index.html.

  * •

BIAST (BRITE International Advisory Science Team) is an informal group of
presently 60 scientists, who have already successfully proposed relevant
observations and/or are planning this in the future. Hence, BIAST members have
expertise in BRITE data, have published the results and can advise BEST in
optimising the observing program.

  * •

GBOT (Ground-Based Observing Team), which is headed by Konstanze Zwintz (U.
Innsbruck), provides a platform for BRITE scientists and observers worldwide
to support collaboration and to maximize the scientific output of BRITE-
Constellation.

Figure 9: Sky map highlighting the fields observed thus far by at least one
BRITE satellite.  Figure 10: Temporal distribution of the observations of all
five active BRITE satellites until the end of 2020. The data obtained in the
stare and chopping observing modes are shown with unfilled and filled bars,
respectively.

###  3.4 Present Status

The various star fields observed with various BRITE satellites since launch
and until 2020 are presented in Figure 9. Which satellite observed a field in
which period, either in chopping or stare mode, is indicated in Figure 10.

As of March 2021, 705 individual stars have been observed so far, often
contemporaneously in two colours, and almost 6 million image-rasters of target
stars have been produced. Most observations occurred in fields close to the
Galactic plane, where the density of very bright stars in the FOV is high,
allowing a proper choice of guide stars by the much less sensitive guiding
telescope (Figure 9). Also many of the primary targets listed in the early
BRITE proposals were located in this area, for example, the bright OB, B and
Be stars in Orion, Carina, Centaurus or Sagittarius.

The observing strategy of BEST during the past eight years focused not only on
stars of primary interest to the BRITE-community, like 6-month campaigns on
hot, massive and intrinsically bright stars, but also to re-observe high
profile targets essentially every possible season. The best examples are the
brightest stars in the Orion field, which have been selected for the first
campaign starting in December 2013 and which are currently being observed for
the eighth time (Figure 10). These datasets are certainly jewels of the BRITE-
Constellation legacy program.

Even though the early BRITE science program focused on O to B (including Be)
type stars, it also includes now objects beyond this range in the HR diagram
(Figure 12), which is indebted to wide field photometry, reaching by default
many stars and of different type. For example, cool red-giants have been
observed, although not originally considered a priority, but the first data
analysis led already to a relevant publication. An excellent example is
$\beta$ Pictoris (Section 4.11). Finally it should be mentioned that TESS
obtained data for stars which BRITE satellites observed simultaneously. An
example was already given in Section 1 with $\zeta$ Puppis.

For all BRITE satellites the nominal lifetime was two years. Hence, the still
active satellites exceed this limit more than three times, which illustrates
the high engineering quality. Nevertheless, BRITE-Constellation encountered
technical problems described in the following.

The photometric accuracy is limited primarily due to stabilisation problems of
BRITE satellites, but also by problems related to increasing CCD defects
(Popowicz (2018); Popowicz and Farah (2020)). The development, e.g., of the
normalised detector dark current with time is presented in Figure 11.
Obviously, satellites with either a tungsten or a light weight borotron shield
suffer significantly lower thermal noise increase compared to unshielded CCDs.
Moreover, the sensors probably received different radiation doses during
launch, as is indicated in Figure 11 by the onset of the linear
approximations.

Figure 11: Temporal development of the CCD dark currents.

The status of individual BRITE satellites can be summarized as:

  * •

BRITE-Toronto (BTr), is in good condition and produces among the best data,
despite a significant amount of radiation damage. Primary target stars can be
placed on the CCD where the background is least noisy.

  * •

BRITE-Heweliusz (BHr), is working very well in general; some observing fields
seem to cause problems for the pointing system, but usually alternative
orientations of the field (different guide stars) can be chosen. It also has
the least amount of radiation damage due to a better shielding of the CCD.

  * •

BRITE-AUSTRIA (BAb) produces scientifically relevant data, even after more
than eight years in orbit and an enhanced radiation environment. To obtain the
best photometric consistency over the lifetime of BRITE-Constellation, this
satellite has been assigned to observe every year essentially the same set of
fields in Orion and Sagittarius.

  * •

UniBRITE (UBr), was working well until June 2019, despite its high grade of
radiation damage. However, it failed after that date and a failure analysis
led by SFL and conducted by IKS TU-Graz concluded that one of the three
reaction wheels seems to be damaged and cannot be used for stabilising the
spacecraft. A repair concept is being developed.

  * •

BRITE-Lem (BLb), worked well until April 2020 when it consistently failed to
get into fine pointing. This is very likely due to a damaged reaction wheel.
However more tests are still to be conducted to come to a firm decision.

In conclusion, presently three of the five functioning BRITE-Constellation
satellites are still operational: BHr and BTr are producing very good data and
BAb still useful photometry. BEST expects to continue the mission until at
least in 2022, depending on unpredictable technical failures, for example, of
the reaction wheels. Attempts to recover the other two BRITEs will continue.

##  4 Key Results of the Mission and Scientific Highlights

Since its launch BRITE-Constellation has obtained measurements for 705
individual targets in 60 currently completed fields (Figure 9) of which many
overlap. A large fraction of the targets was observed in more than one field
which yields total time bases of up to eight years for several stars (Figure
10). As of March 2021, 11.5 % of all targets observed by BRITE-Constellation
are included in one or more peer-reviewed publications. BRITE data of many
other targets are still being actively analyzed and will be the topics of
additional future papers. In the following, selected research highlights based
on BRITE-Constellation data are presented, mostly sorted from most massive to
least massive stars. The individual objects are also indicated in Figure 12.

Figure 12: HR diagram of the stars brighter than 6th mag in $V$ (grey dots).
Stars for which BRITE photometry was collected are shown by black dots. The
objects discussed in Section 4 are marked as open red symbols where the larger
symbol stands as a representation of the 23 red giants discussed in Section
4.15. Indicative instability domains for several types of pulsators are shown
as colored ellipses. Be stars cover much of the $\beta$ Cep and SPB domains

###  4.1 The Link between Stellar and Wind Variability in Very Massive Stars

High-precision photometry of the runaway early-O-type supergiant $\zeta$
Puppis (Figures 1, 2 and 13) revealed that a previously-proposed rotation
period of 5.1 d is incorrect and the period actually is 1.78 d, which agrees
much better with a model for the rotational evolution. Figure 13 also
indicates that the large, real scatter beyond the 1.78 d modulation, is
probably due to stochastically varying short-lived bright regions in the
photosphere arising in a subsurface convection zone, which led to clumps in
the wind. An alternative supposition is that the stochastic variability arises
from gravity waves at the internal radiative/convection border. This is
supported by hydrodynamic simulations showing gravity waves causing stochastic
variability in the photospheres of main sequence OB stars (Bowman et al.
(2019)).

Figure 13: Left: Fourier transform of the red and blue 2014/15 BRITE
photometry of $\zeta$ Puppis (see Figure 10 for VelPup I-VI runs). Middle:
Corresponding rotation light curve (P = 1.78 d). Coloured points are 0.04
phase bins with 1 $\sigma$ error. Note the large, real scatter, which could be
due to stochastically varying short-lived bright regions in the photosphere
which lead to clumps in the wind. Right: Comparison diagram for the phased
blue and red light curves (top), and distribution of orthogonal distances with
a Gaussian fit (bottom). (Figure 5 of Ramiaramanantsoa et al. (2018)).

The top diagram in the right column of Figure 13 shows that both kinds of
bright spots show the same variability amplitude in the BRITE blue and red
filters, implying insensitivity to the expected hotter nature of the spots
compared to their adjacent areas in the stellar photosphere. The reason for
this is that the Rayleigh-Jeans tails of the stellar emission spectrum are
sampled at significantly longer wavelengths, compared to the UV maximum peak.
The bottom diagram in the right column is consistent with the photometric
precision of the data.

The findings yielded by the 2014/2015 observing campaign on $\zeta$ Puppis may
be an important resolution of a long-standing puzzle indicating subsurface
convection as the main source of the two types of wind variability (quasi-
periodic co-rotating interaction regions-CIRs-and stochastic clumps), which
previously was not considered possible in such hot stars.

After this study of $\zeta$ Puppis, parallel observations were obtained in
2018/19 using BRITE in the optical and Chandra in X-rays (Nichols et al.
(2021)). Both satellites confirm a 1.78 d period (Figure 14), which is thought
to be the result of bright photospheric spots driving CIRs in the stellar
wind, with the X-rays arising somewhat further out in the wind, where the CIR
shock is strongest. Alternatively, as noted above, the stochastic component of
variability could arise from gravity waves arriving from a much deeper zone.

Figure 14: $\zeta$ Puppis observed in the visible with BRITE and in X-rays
with Chandra, folded with the period of 1.78 d. The multi-wavelength light
curve behaviour presumably illustrates the effects of Corotating Interaction
Regions. The cyan arrows indicate the primary and the blue arrows the
secondary maximum. There is a significant shift in the times of maximum due to
a large delay or a smaller shift but mismatch in which is primary and
secondary maximum (Figure 3 of Nichols et al. (2021)).

A very recent investigation was made on about 60 bright galactic Wolf-Rayet
stars using combined data sets from MOST, BRITE and TESS by Lenoir-Craig et
al. (submitted to ApJ and in Lenoir-Craig et al. (2020)). Fourier analysis of
the light curves reveals an important trend of enhanced stochastic variability
at low frequencies ($\lesssim$1 cd-1) with the spectrally-modelled
hydrostatic-core temperatures (T∗), much like a preceding ground-based
spectral variability study by Chené et al. (2019, 2020). Both studies support
the idea that the stochastic variability seen in all WR stars arises in clump
formation and propagation in their strong winds, such that, surprisingly,
hotter WR stars with faster winds show less variability and hence less
clumping. This can be explained by the triggering of the clumps in subsurface
convective zones that are deeper and stronger in cooler WR stars. This may or
may not conflict with the heretofore theory of clump formation by wind
instabilities, which are expected to be stronger in hotter, faster WR winds.

Other targets with similar science relevance are WR 40 (WN8h; Ramiaramanantsoa
et al. (2019)), V973 Sco (O8Iaf; Ramiaramanantsoa et al. (2018)) and
$\gamma^{2}$ Vel (WC8+O7.5III-V; Richardson et al. (2017)). They were among
those prominently observed by BRITE-Constellation during several runs and
helped to investigate the dynamics of winds and their relation to variations
occurring at the stellar (hydrostatic) surface.

###  4.2 The Heartbeat of Stars: $\iota$ Orionis and $\epsilon$ Lupi

Heartbeat stars are a class of eccentric binaries which are characterized by
tidally excited oscillations (TEO) with distinct amplitude changes at
periastron. They are uniquely interesting for the study of massive stars,
because they allow for full binary solutions without eclipses and provide
access to asteroseismology of objects where pulsation is rare. Using BRITE-
Constellation, the well-studied binary system $\iota$ Ori (O9III+B1 III/IV)
was the first massive star ever in which TEOs were discovered, and which
opened a whole new avenue to studying massive star interiors (Pablo et al.
(2017)). The data in Figure 15 are phased to periastron (phase = 1.0, with P =
29.13376 d) and binned to 0.0025 in phase.

Another unique heartbeat star discovered with BRITE-Constellation was
$\epsilon$ Lupi. This system is the only known doubly magnetic massive binary
(Shultz et al. (2015)). Pablo et al. (2019) were able to determine masses and
radii despite an orbital inclination of $\approx$20∘. This allows one to
explore the interesting interplay between magnetism and tidal effects in the
evolution of such a system.

Figure 15: Binned and phased data of $\iota$ Ori, obtained with BRITE-
Constellation, covering two years. The data clearly show tidally excited
oscillations, most prominently from 0.5–0.8 in phase, as well as a strong
heartbeat signal (0.95–1.05 in phase). The blue points are shifted by a
constant flux for clarity.

The value of BRITE heartbeat stars also extends to the upper reaches of the HR
diagram with the enigmatic and highly eccentric binary system $\eta$ Car,
although the length of the period combined with mass loss have made it
difficult to characterize any heartbeat signal at periastron. Using two
separate BRITE observations, Richardson et al. (2018) were able to confirm
oscillation frequencies, which appear to be stable over the past four decades
(van Genderen et al. (1995), Sterken et al. (1996)). These frequencies share
many similarities with TEOs, though this identification will need more data to
confirm.

###  4.3 The Riddle of Betelgeuse

The red supergiant Betelgeuse is not only one of the biggest stars in the sky,
but also one of the most puzzling. Long-term photometry and radial velocity
studies reveal semi-regular stellar pulsation periods of 420 d, and possibly
superposed by a cycle of 8.7 years (Goldberg (1984); Dupree et al. (1987);
Smith et al. (1989)). In comparison, Kiss et al. (2006) report $388\pm 30$
days as a pulsation period and a $5.6\pm 1.1$ years cycle, using AAVSO-V data
obtained almost during an entire century (1918–2006).

Curiously, its high apparent brightness makes Betelgeuse a difficult target
for ground-based photometry, as big telescopes suffer from over-exposure. This
gap is now filled with high-quality BRITE photometry (Figure 16), augmented
with spectroscopic data obtained during more than 10 years at the STELLA
robotic observatory, which is one of the biggest fully robotic telescopes
worldwide (Strassmeier et al. (2004)). Only automated observing procedures
allow scheduling of almost daily visits of the same star, each lasting no
longer than 5 min and stretching over more than a decade. More than 2000
individual, high-resolution spectra have been collected and automatically
reduced.

As Figure 16 illustrates, the radial velocity variations follow in general
closely the photometry, suggesting a physical link between photometric and
radial velocity variations. A splitting of the main pulsation period into P
$\approx 420$ d and P $\approx 335$ d, as seen in the RV-data, is proven
beyond doubt by BRITE photometry, which provides the required cadence and
precision. This is work in progress-adding another season will hopefully put
these findings on solid grounds.

Figure 16: Comparison of light and RV variations of Betelgeuse. From top to
bottom: STELLA RV data, BRITE-blue, AAVSO-V absolute photometry, and BRITE-red
photometry. Error bars on BRITE magnitudes and on STELLA RV are too small to
be visible.

During the grand dimming event in the 2020/21 season, an extraordinarily
excursion of the photometry to the RV-data can be seen. The photometric
amplitude by far outstretches the already high RV amplitude. An analysis of
HST UV-data of this period (Dupree et al. (2020)) hints to a big plume of dust
being emitted from the surface of the star and subsequently drifting into the
line of sight, thereby enhancing the photometric minimum.

Betelgeuse is approaching its end of life as a star, commonly believed to be a
supernova progenitor. Last year’s dimming event sparked estimates that an
explosion may be imminent within the next 100,000 years. But observations and
models are currently not refined enough to prove whether Betelgeuse will end
in a type IIb, II-L, or II-P supernova (Meynet et al. (2013)). Hence, new
observations are needed to better estimate mass and rotation rate in order to
pin down Betelgeuse’s future path. BRITE-Constellation will participate in
these campaigns.

###  4.4 Evolving Pulsation of the Slowly Rotating Magnetic $\beta$ Cephei
Pulsator $\xi^{1}$ CMa

$\xi^{1}$ CMa is a remarkable magnetic early B-type star that is distinguished
in several ways: it rotates extremely slowly ($P_{\rm rot}\sim 30$ y; Shultz
et al. (2017)), it is the only magnetic B-star known to exhibit detectable
H$\alpha$ emission from a dynamical magnetosphere Shultz et al. (2017), and
its optical and X-ray magnetospheric emission are modulated according to its
$\sim$0.2 d radial pulsation period (Shultz et al. (2017); Oskinova et al.
(2014)).

Building on work by Pigulski (1992); Jerzykiewicz (1999) and Shultz et al.
(2017), BRITE-Constellation photometry (BLb, BHr, BTr) of $\xi^{1}$ CMa was
employed by Wade et al. (2020), as one of the most recent anchor points to
monitor the evolution of its pulsation period. Combining over one century of
photometric and radial velocity monitoring, they concluded that the period
evolution of $\xi^{1}$ CMa consists of a secular period lengthening of roughly
0.3 s/century that can be satisfactorily understood as a consequence of
expansion due to stellar evolution. An additional period evolution—more rapid
and of lower amplitude—remains unexplained, and the authors speculate that it
may be a consequence of rotational modulation or evolution that is restricted
to relatively rapid, short-term episodes, rather than uniform long-term
changes. Binarity can be ruled out, because the corresponding RV variations
would have been easily detected.

###  4.5 The Triple System $\beta$ Centauri

Massive stars, with initial masses greater than 8 $M_{\odot}$, are among the
least understood, but they are extremely important, because they produce the
majority of heavy elements. A fascinating BRITE-Constellation target is the
triple system $\beta$ Centauri (Figure 17)—also named Agena–consisting of a
massive binary ($\beta$ Cen AaAb: B1 II and B1 III) with an eccentric orbit
and a more distant and 3 mag fainter companion, also of B-type (Pigulski et
al. (2016)).

Figure 17: The triple system $\beta$ Centauri, with two example orbits
(excentricity of 0.6 and 0.8). Adapted from Figures 1 and 2 of Pigulski et al.
(2016).

$\beta$ Cen B was discovered in 2011 as a magnetic star (Alecian et al.
(2011)). With 17 detected p and g modes, the close massive binary system
becomes one of about a dozen known hybrid $\beta$ Cep/SPB stars with such a
rich frequency spectrum. Furthermore, its binarity provides a very precise
determination of the masses of the components, but complicates seismic
modeling, because the modes need to be safely assigned to one of the
components, which—in addition—are relatively fast rotating.

The case of $\beta$ Cen illustrates the potential of BRITE-Constellation data
for the detection of rich-frequency spectra of small-amplitude modes in
pulsating stars.

###  4.6 Long-Period Oscillations in the $\beta$ Cephei Pulsators $\nu$
Eridani and $\theta$ Ophiuchi

Thanks to the long-term stability of BRITE, Handler et al. (2017)) detected
several previously unknown long-period signals corresponding to gravity-mode
oscillations of the $\beta$ Cephei pulsator $\nu$ Eridani. Daszyńska-
Daszkiewicz et al. (2017, 2018) demonstrated that present standard pulsation
models cannot reproduce the observed frequency range of g-mode pulsations,
which is likely due to shortcomings in the underlying stellar physics data, in
particular of opacities.

Upon the detection of a large number of g-mode pulsations in the BRITE data of
another $\beta$ Cephei star, $\theta$ Ophiuchi, Walczak et al. (2019) arrived
at an identical conclusion (with the caveat that a B5 companion star could be
responsible for the g modes), namely that opacities need to be increased
between 30% and 145% (!!) in the range log $T=5.06$–$5.47$ to reproduce the
observations. Obviously, the use of correct opacity data is important for
modelling of all kinds of stars. Hence, the implied revision of these data
impacts stellar physics in general.

###  4.7 The Ellipsoidal SPB Variable $\pi^{5}$ Orionis

BRITE observations of the ellipsoidal variable $\pi^{5}$ Orionis (Jerzykiewicz
et al. (2020)) revealed that the primary star belongs to the class of Slowly
Pulsating B (SPB) stars. Within the modes of pulsation, there is a re-
occurring splitting of twice the orbital frequency. This is interpreted as
perturbation of nonradial pulsation modes by the equilibrium tide exerted by
the companion. The behaviour of the two tidally disturbed pulsation modes is
largely consistent with axisymmetric dipole modes ($l=1,m=0$). These findings
have two important and interesting consequences:

  * –

$\pi^{5}$ Ori is the first SPB star in which tidal perturbations have been
identified and

  * –

these perturbations facilitate the identification of nonradial pulsation
modes.
BRITE allowed a valuable proof-of-concept of mode identification to be carried
out, which opened up tidal asteroseismology of SPB stars in multiple systems.

###  4.8 Be Stars

The BRITE database is rich in Be-star observations because there are many
bright Be stars and, for B-type stars, the blue- and red-sensitive BRITE
satellites achieve roughly equal S/N, unless an extreme reddening is present.
The combination of the frequency resolution and quality of BRITE observations
over several seasons with the long-term behaviour documented by SMEI has
achieved qualitatively new insights into the so-called Be phenomenon.

Two central questions about Be stars (see Rivinius et al. (2013) for a review)
are:

  * (i)

How do Be stars maintain their Keplerian decretion disks where the eponymous
emission lines form and that, without regular replenishment, dissipate within
a year?

  * (ii)

How have Be stars acquired their $\gtrsim$75% critical rotation? One way to
explain the latter is mass transfer in a close binary. The former primaries
often appear as hot, subluminous sdO stars that are challenging to detect even
in UV spectra (Wang et al. (2018)) and contribute little flux in the BRITE
passbands. However, first photometric Doppler shifts derived from BRITE and
SMEI data spanning 25 years have set an upper limit of $\sim$1 $M_{\odot}$ on
the mass of a putative companion of $\nu$ Pup (Baade et al. (2018b)).

BRITE-Constellation has been instrumental in confirming earlier suggestions
that Be disks are fed by discrete mass-loss outbursts driven by the
superposition of several low-order non-radial pulsation modes or by recently
detected stochastically-excited pulsations, transporting angular momentum from
the stellar core to the surface (Neiner et al. (2020)). Although originally
detected in H$\alpha$ line profiles of $\mu$ Cen (Rivinius et al. (1998)),
optical photometry is a better tracer of outbursts because the $V$-band flux
responds sensitively to varying amounts of ejecta causing electron scattering
and free-bound recombination (Haubois et al. (2012)). In fact, in $\mu$ Cen,
outbursts have up to 100 times higher amplitude than the underlying non-radial
pulsation and can render the pulsations undetectable (Baade et al. (2016)). In
$\eta$ Cen (Baade et al. (2016)), 28 Cyg (Baade et al. (2018a)), and 25 Ori
(Baade et al. (2018)), BRITE found closely spaced NRP frequencies the
difference between which corresponds to the repeat frequency of the outbursts.
During an outburst, the combined amplitude of the involved non-radial
pulsation modes grows nonlinearly, demonstrating that the outbursts are
pulsation powered far beyond mere mode beating. Hierarchically nested
frequency groups can drive repetitive outbursts on timescales from weeks to
years (Figure 18), and the frequency groups typical of Be stars can be
understood as difference frequencies (g0), non-radial pulsation frequencies
proper (g1), and sum/harmonic frequencies (g2) (Baade et al. (2018)). So-
called Štefl frequencies first found in emission lines (Štefl et al. (1998))
probably are orbital frequencies in the innermost inhomogeneous disk (Baade et
al. (2016)).

For shorter timescales/lower amplitudes, TESS (Labadie-Bartz et al. (2020,
2021)) has confirmed the correlation between increased non-radial pulsation
amplitude and mean brightness. Similarly tight networks of selected non-radial
pulsation frequencies do not seem to be known from other stars, and the
outbursts may enable Be stars to escape an angular-momentum crisis possibly
caused by the contracting core (Baade and Rivinius (2020)). The detection of
non-radial pulsation modes by BRITE (Borre et al. (2020)) and TESS (Labadie-
Bartz et al. (2021)) has also terminated decades-long speculations that the
best-known Be star, $\gamma$ Cas, shows rotational variability due to a
magnetic field (Smith and Henry (2021)).

Figure 18: Blue (blue symbols) and red (red symbols) BRITE light curves of the
Be star 25 Ori (cf., Baade et al. (2018)) from 2014/15 (top) and 2016/17
(bottom). In either season, two outbursts separated by $\sim$78 d occurred
which corresponds to the difference frequency of 0.0129 c/d between many pairs
of non-radial pulsation modes. The black curves are a sine fit to the 2014/15
light curve outside the outbursts with frequency 0.1777 c/d which is another
difference frequency in multiple non-radial pulsation frequency pairs. During
the outbursts in 2014/15, the light is modulated with 0.1777 c/d, less clearly
so in 2016/17. The change in mean magnitude after the outbursts is probably
due to increased scattering and free-bound transitions in the subsequently
dissipating ejecta.

###  4.9 $\beta$ Lyrae: A Binary with a Hidden Component?

A highlight binary is $\beta$ Lyrae, which consists of a B6-8II bright giant
(3 $M_{\odot}$) and an invisible, more massive companion (13 $M_{\odot}$)
producing the primary eclipses. The bright giant loses mass to the more
massive object at a rate that induces a fast period change of 19 s per year.
There were no previous studies of the intrinsic variability of the $\beta$
Lyrae system available which were credible, sufficiently continuous, and
uniform, because of the day-gaps in ground-based observations, which coincided
with the prevalent time-scales of the intrinsic variability in this 12.9-day
orbital-period binary.

The BRITE data extending over slightly more than 10 full orbital revolutions
of the binary provided the first usable time series, reaching substantially
beyond the intrinsic time scales and permitting utilisation of tools well
developed for studies of variability of active galactic nuclei and quasars.
Analysis of the BRITE time series shows typically three to five instability
events per binary orbit, showing a slightly stronger serial correlation than
the red noise (Rucinski et al. (2018, 2019)). The two-parameter Damped-Random-
Walk (DRW) model of the fluctuations (Kelly et al. (2009); Zu et al. (2013)),
characterised by the red-noise spectrum at time scales shorter than the de-
correlation time scale $\tau$ and white noise at longer time scales, agrees
very well with the data.

The fluctuations are characterised by the amplitude of the stochastic signal
of $1.3\,\%$, expressed relative to the maximum flux from the binary, while
the de-correlation length of the random disturbances is characterised by a
typical value of $\tau$ = 0.88 days. The invisible companion is the most
likely source of the instabilities. Unexpectedly, the time scale of the
intrinsic variability—most likely associated with the thermal time scale of
mass-transfer instabilities—appears to follow the same dependence on the mass
of the accreting object as is observed for active galactic nuclei and quasi-
stellar objects, which are five to nine orders of magnitude more massive than
the $\beta$ Lyrae torus-hidden component.

###  4.10 HD 201433—A Rosetta-Stone SPB Star in a Multiple System

Rotation is a still incompletely understood key process of stellar evolution
(Aerts et al. (2019)). If stars locally conserved angular momentum, their
cores would spin up and the surviving compact objects would spin much faster
than is actually observed. This implies that present standard models are
incomplete and miss essential processes and correct timescales. A first step
towards solving this problem is to detect how angular momentum is distributed
inside stars, as a function of various parameters including age. Prime
candidates for such studies, and more easily understood than B-type stars, are
subgiant and red giant stars, as they convey the rotational history of the
earlier stages of evolution and pulsate with mixed p/g modes that carry
information about the deep stellar interior, as is argued in Kallinger et al.
(2017) and illustrated in Figure 19.

Figure 19: Mean core (dashed lines) and envelope (dotted lines) rotation rate
during the evolution of YREC models (from the TAMS to the RGB) with various
masses (colour coded) assuming local conservation of angular momentum and
rigid rotation on the main sequence. The rotation rate and stellar radius are
given relative to their respective values on the TAMS. The filled symbols
correspond to the relative envelope rotation rates of various stars with a
given mass and radius. The core rotation rate (open symbols) is determined
from this value and the observed core-to envelope rotation gradient (Figure 19
of Kallinger et al. (2017)).

BRITE-Toronto observed in 2015 the SPB star HD 201433 continuously for 156
days Kallinger et al. (2017). The peaks in the Fourier spectrum of the BRITE
observations turned out to be broader than expected, which triggered the
development of a new Bayesian-based frequency determination technique with a
resolution beyond the formal Rayleigh-criterion. As a proof, three
rotationally split triplets are identified in the nearly half-year long BRITE-
data, with central frequencies and splittings agreeing well with those
extracted from the nearly 8 years of SMEI observations.

A science highlight of the HD 201433 BRITE-photometry is a trend of splitting
becoming more common towards longer periods, which implies a non-rigid
internal rotation profile, as is elaborated in Kallinger et al. (2017). For a
detailed investigation, a dense grid of MESA models (Paxton et al., 2011,
2013) and their non-adiabatic pulsation modes were computed by Kallinger et
al. (2017). Using classical $\chi^{2}$ techniques and other statistical
methods, a representative model (3.05 $M_{\odot}$ and 2.6 $R_{\odot}$) was
identified that reproduces best the observed frequencies.

The pulsation modes that are accessible to the seismic analysis probe the
radiative envelope of HD 201433 from the boundary of the convective core at
about 0.11 R∗ up to about 0.98 R∗. The Bayesian analysis of various rotation
profiles provides strong evidence for a slowly (292 $\pm$ 76 d) and rigidly
rotating envelope, topped by a thin and significantly more rapidly rotating
surface layer, which covers about the outer 4% of the radius (Figure 19). In
conclusion, BRITE-Constellation data provide strong evidence for non-rigid
internal rotation in a main-sequence star, which still is rarely presented in
the literature.

###  4.11 The Young Star $\beta$ Pictoris and Its Exoplanetary System

Exoplanet properties crucially depend on their host star’s parameters. The
$\beta$ Pic system includes a wide, dense circumstellar disk that is seen
edge-on and two giant gas planets ($\beta$ Pic b and c) that are only
grazingly eclipsing the host star. BRITE-Constellation data have been used to
search for a transiting planet. This puts limits on the $\beta$ Pic system, as
possible planets must be larger than 0.6 (0.75, 1.0) RJupiter for periods of
less than 5 (10, 20) days (Lous et al. (2018)).

Furthermore, the predicted transit of the Hill sphere of $\beta$ Pic b
triggered an international observing campaign in 2017–2018 including the
BRITE-Constellation nanosats. No dimming caused by the Hill sphere transit was
observed in any of the involved photometric instruments, where the precision
of the BRITE photometry would allow detection of a drop in intensity by only
0.5% in the time of interest (Kenworthy et al. (2021)). In the spectroscopic
observations, some signs of the Hill sphere transit have been detected (e.g.,
in the Ca II H & K lines) illustrating that the material in the planet’s Hill
sphere is not sufficiently dense to dim the stellar light enough to be
photometrically detected from the ground. In addition, in 1981 anomalous
fluctuations of the flux coming from the $\beta$ Pic system were originally
interpreted as being caused by foreground material that transited the stellar
disk. Recently, based on the observations conducted within the $\beta$ Pic
Hill sphere transit campaign, Kenworthy et al. (2021) showed that this 1981
event did not originate from the transit of a circumplanetary disk.

The high-quality BRITE-Constellation photometry for $\beta$ Pictoris obtained
since 2015 provided crucial constraints on the properties of the exoplanet
host star itself (Zwintz et al. (2019)). The first asteroseismic analysis
using multi-color space photometry yielded a precision of 2% in mass and
radius for $\beta$ Pictoris, determined the inclination angle to be 89.1∘
(which agrees with the inclination angle of the disk of 88.1∘), and identified
the 15 pulsation frequencies as three $\ell$ = 1, six $\ell$ = 2 and six
$\ell$ = 3 p-modes.

###  4.12 The roAp Star $\alpha$ Cir

$\alpha$ Cir is the brightest rapidly oscillating (roAp) star with a magnetic
field. It was discovered in 1981 by Kurtz and Cropper (1981) and since then,
many publications dealt with photometric and spectroscopic properties,
including the magnetic field (see, e.g., Holdsworth and Brunsden (2020) and
Weiss et al. (2016, 2020)).

$\alpha$ Cir is a text-book illustration for an advantage of nanosatellites
dedicated to photometry (e.g., Weiss (2007)), as they allow one to observe
stars over a long time span. Even if the accuracy of individual data points is
inferior to that of larger instruments, long observations of targets result in
more accurate frequency spectra. Figure 2 of Weiss et al. (2020) presents
light curves observed by five different satellites with apertures ranging from
3 cm (BRITE-blue) to effective 10 cm (TESS) and filter bandwidths of 55 nm and
400 nm, centred on 425 nm and about 800 nm, respectively, (see Table 1 of
Weiss et al. (2020)). The larger the aperture and filter bandwidth, the more
accurate the photometry, but if frequency-resolution is important, the picture
changes drastically in favour of data obtained over 3 years even with a
smaller aperture telescope (Figure 20).

Figure 20: Fourier amplitude spectra of the TESS, WIRE and BRITE photometry,
centred on the main pulsation frequency ($f_{1}$) of $\alpha$ Cir. BTr14,
BTr16, Bb⋆14, and Bb⋆16 are the red and blue BRITE data, obtained during the
years 2014 and 2016, respectively. Bb⋆ represents the combined blue data
obtained with BRITE-Austria and BRITE-Lem. The BRITE-blue amplitudes are
divided by two (!!) for better comparison with the other data (adapted Figure
6 of Weiss et al. (2020)).

Combining the times of maximum from BRITE-red and WIRE data, results in
$f_{1}$ = 210.993264(5) d-1, which is, with an error in the corresponding
period of 0.01 msec, the most accurately determined dominant pulsation period
of any roAp star to date. The main pulsation frequency ($f_{1})$ can be
identified with an $\ell=1$ mode, and two additional frequencies likely come
from two consecutive radial $\ell=0$ modes Weiss et al. (2020).

At least three surface spots can be identified for $\alpha$ Cir; the TESS data
even suggest a fourth spot. The best-fit (minimum $\chi^{2}$) set of
parameters differs significantly from that inferred from the marginal
distributions of the parameters, which hints at a noticeable skewness of the
probability distribution of the Bayesian photometric imaging in the considered
ten-dimensional configuration space. Obviously, spot latitudes are less well
determined than longitudes, as expected. To our knowledge, this is the first
time that Bayesian-based evidence of models differing in the number of spots
has been quantitatively determined Weiss et al. (2020).

###  4.13 $\beta$ Cas: The First $\delta$ Scuti Pulsator with a Dynamo
Magnetic Field

One of the cooler BRITE-Constellation targets showing pulsations and a
magnetic field is the F2 type star $\beta$ Cas, which is also one of the
objects in the BRITE legacy fields (Zwintz et al. (2020)). $\beta$ Cas is
quite an unusual star in several aspects:

  * (i)

It shows only two independent $\delta$ Scuti type p-mode frequencies. As
$\delta$ Scuti stars are usually known to show up to hundreds of individual
frequencies, this challenges the asteroseismic interpretation. Why only two
frequencies can be detected with a total time base of over 2.5 years is still
unclear.

  * (ii)

$\beta$ Cas is one of the few $\delta$ Scuti stars known to date to show a
measurable magnetic field at all Zwintz et al. (2020). The three other
magnetic $\delta$ Scuti stars are HD 188774 (Lampens et al. (2013)), $\rho$
Pup (Neiner et al. (2017)) and HD 41641 (Thomson-Paressant et al. (2021)).

  * (iii)

Additionally, the magnetic field structure of $\beta$ Cas is quite complex and
almost certainly of dynamo origin. One may speculate that the presence of this
dynamo field is related to the unusual lack of numerous $\delta$ Scuti
frequencies.

All this makes $\beta$ Cas a powerful test bench for modelling of dynamo
processes in thin convective envelopes of F-type stars.

###  4.14 Rotation, Pulsation, Orbits and Eclipses in the Constellation of
Auriga

The Auriga field is an excellent example of an arrangement typically chosen
for observations with BRITE-Constellation. One or two key targets determine
the orientation of a BRITE satellite and in the same field additional targets
with a large mass-range maximise the science output.

Rotation and pulsation periods across the Hertzsprung-Russell diagram are of
top priority for understanding stellar activity as a function of time.
Continuous photometry with up to three BRITE satellites was obtained for 12
targets, primarily in the Aur/Per I field, and subjected to a period search
(Strassmeier et al. (2020)). The bright active star, Capella, was found to be
constant in the red bandpass with an rms of just 1 mmag over 176 d, but showed
a 10.1 $\pm$ 0.6 d periodicity in the blue, which is interpreted to be the
rotation period of its active and hotter secondary star (Figure 21). Its
position in the Hertzsprung gap suggests ongoing changes in its internal
structure. It is expected that this has a profound impact on the visible
surface and can explain its fast rotation.

Figure 21: Phase plots for the red (top) and blue data (bottom) with the best-
fit 10.1 d period for Capella. The blue data are dominated by the hotter G0
component while the red data are dominated by the cooler G8 component. The
rotation period of the cool component is near the orbital period of 104 days
(adapted Figure 3 of Strassmeier et al. (2020)).

Results for the other targets in Auriga include:

  * (i)

The main pulsation period of the F0 supergiant $\varepsilon$ Aur is detected
by a multi-harmonic fit of the 152-day long light curve. This is noteworthy,
because the RVs observed contemporaneously with the Stella spectrograph
revealed a clear 68 d period. Although the light curve showed two minima
separated by 74 d, a single period of that duration would not fit the data
adequately. These RVs indicated that the (stellar) disk-integrated pulsations
seem to revert when maximum or minimum light is reached, that is, the star is
apparently most contracted when brightest and most expanded when faintest.

  * (ii)

An ingress of an eclipse of the $\zeta$ Aur binary system was covered and a
precise timing for its eclipse onset derived. We obtained a possible 70 d
period from the outside-eclipse light-curve fits of the proposed tidally-
induced, nonradial pulsations of this ellipsoidal K4 supergiant.

  * (iii)

$\eta$ Aur was identified as an SPB star with a main period of 1.289 $\pm$
0.001 d. Five more periods are seen in the BRITE photometry and three of these
are also seen in the RV data. The amplitude ratios as well as the phase lags
between brightness and RV periods reflect those expected from low-degree
gravity modes of SPB stars. $\eta$ Aur is, thus, among the brightest SPB stars
known.

  * (iv)

Rotation of the magnetic Ap star $\theta$ Aur is easily detected by photometry
and spectroscopy with a period of 3.6189 $\pm$ 0.0001 d and 3.6177 $\pm$
0.0006 d, respectively. The RVs of this star show a striking non-sinusoidal
shape with a large amplitude of 7 kms-1, which is likely due to the line-
profile deformations from the inhomogeneous surface distribution of its
chemical elements. Such a non-sinusoidal shape likely explains the small
period difference and suggests that the two periods are actually in agreement.

  * (v)

Photometric rotation periods are also confirmed for the magnetic Ap star IQ
Aur of 2.463 d and for the solar-type star $\kappa^{1}$ Cet of 9.065 d, and
also for the B7 HgMn giant $\beta$ Tau of 2.74 d. The latter remains uncertain
because it was reconstructed only with the very small amplitude of 0.54 mmag.

  * (vi)

Revised orbital solutions are derived for the eclipsing SB2 binary $\beta$
Aur, which replaces the initial orbit from 1948, and for the RS CVn binary
V711 Tau for which a spot-corrected orbital solution was achieved. The two K
giants $\nu$ Aur and $\iota$ Aur are found with long-term trends in both the
light curve and the RVs. $\nu$ Aur could be a long-period eccentric SB1 system
with a low-mass companion for which a provisional orbital solution is
predicted with a period of 20 yr and an eccentricity of 0.7. The RV variations
of the hybrid giant $\iota$ Aur are of even lower amplitude (0.7 kms-1) but
shorter period ($\approx$4 yrs) and are more likely due to surface
oscillations. Long-term brightness trends were seen for both stars and appear
related with the RVs.

###  4.15 Stellar Masses of Red Giants from Their Granulation Signal

A sample of 23 RG stars in the range 1.6 < V < 5.0 and distributed all over
the sky was investigated by Kallinger et al. (2019), and a clear granulation
and/or oscillation signal was found. Each star was observed almost
continuously by at least one of the five BRITE satellites for up to 173 d.

Even though plenty of information is available in the literature for these
bright stars, neither surface gravity (log g) nor mass is sufficiently well
known. Granulation and/or oscillation timescales, deduced from BRITE-
Constellation observations, help to determine model-independent estimates of
log g with two different methods (Kallinger et al. (2016)). Using precise
radii from the literature, mostly from interferometric angular diameters and
Gaia parallaxes, the mass of the stars can be estimated from log g, derived
from BRITE-data, which are dominated by the granulation signal.

The stellar masses presented in Figure 22 range from about 0.7 to more than
$8\,M_{\odot}$ and have formal uncertainties of about 10% to 20%, which covers
the observational errors as well as the known uncertainties of the used
scaling relations. One might question whether simple scaling relations hold
for low-mass giants with about $10\,M_{\odot}$ to high-mass giants with more
than $200\,R_{\odot}$, but this is difficult to estimate due to missing
independent and reliable mass estimates. Even though there might still be some
unknown systematic effects in the scaling relations, they appear to be at
least good enough to disentangle low-mass stars from high-mass stars.

Figure 22: Hertzsprung–Russell diagram (left) and Kiel diagram (right) with
red giants observed by BRITE-Constellation (grey-filled circles). The small
dots show MIST stellar evolution models for solar composition with the mass
colour coded. Blue-filled circles mark stars for which solar-type oscillations
have been found in the BRITE-Constellation data (adapted Figure 10 from
Kallinger et al. (2019)).

Comparison of the masses derived through the scaling relations with parameters
from a large grid of stellar models also allows one to evaluate statistically
the relative evolutionary state of the individual stars , that is, to
distinguish low-mass red-clump stars from high-mass red giants.

In recent years, the seismology of red giants has grown to become an important
field in stellar astrophysics, providing the unique opportunity to probe the
interior structure of evolved stars (Chaplin and Miglio (2013)). In general,
seismic scaling relations have become indispensable for determining mass and
radius of stars with a convective envelope.

###  4.16 Complete Coverage of Nova Carinae 2018 (ASASSN-18fv)

This first-time ever observation of a complete nova eruption came about by
chance. The BRITE-Constellation had just monitored 18 stars continuously over
several weeks in the constellation Carina, when BRITE-Mission-Control (MC, see
Section 3.2) recognised a sudden brightening of a field star (inserts in
Figure 23). A quick search among the top sky-news announcements indicated a
new star, discovered by the All-Sky Automated Survey for Supernovae (ASASSN)
as ASASSN-18fv (Figure 23).

Figure 23: BRITE-Toronto photometry of Nova Carinae 2018 (grey: individual
observations with red filter, red: orbit averages). Blue: Fermi-LAT (Atwood et
al. (2009)) $\gamma$-ray observations. Inserts: raster centred on HD 92063,
taken on 1 March 2018 before nova eruption (left), and on 14 April 2018 during
nova eruption (right). Scale is 27′′ per pixel, and exposure times are 4 s.

The cooperation of BRITE-Constellation with the international community is
reported, e.g., in Aydi et al. (2020) and resulted in unprecedented
simultaneous space observations in a broad wavelength range and with BRITE
starting even before the actual outburst.

A shock model of Metzger et al. (2014) predicts that in addition to
$\gamma$-rays, the shocked gas should emit mostly in X-rays, which will be
absorbed by the dense nova ejecta ahead of the shocked gas, reprocessed to
lower energies, and escape in the optical. This process indicates a source for
the bolometric luminosity of the nova, in addition to the remnant nuclear
burning on the white dwarf surface. Shocks occur in many transient phenomena,
such as Type IIn supernovae, tidal disruption events, stellar mergers,
superluminous supernovae, etc. Hence, shock interactions may contribute
substantially to the bolometric luminosities of these events, but direct
observational evidence has been lacking. The BRITE-Constellation observations
were unique in this context and helped to provide insights in many previously
poorly observed and understood phases of novae evolution, see for example,
Hounsell et al. (2016); Aydi et al. (2020).

The well-sampled BRITE light curve (Figure 23) clearly resolves a series of
distinct short-lasting flares of the order of one to two days, but which were
poorly resolved from the ground. $\gamma$-rays indicate a series of flares,
similar to those in the optical regime, which suggest:

  * (i)

The fact that the flares occur simultaneously in time in both BRITE bands
implies that they very likely share the same origin, that is, shocks, because
they power the $\gamma$-rays. Consequently, shocks are also powering some of
the optical emission.

  * (ii)

Doubling of the luminosity of the nova during the flares, implies that the
shocks power a substantial fraction of the nova luminosity.

  * (iii)

$\gamma$-ray and optical light curves (Figure 23) were very well sampled and
indicate a time lag of approximately 5 h. This is an additional confirmation
that the optical emission originates in the shocks. $\gamma$-rays escape from
the shocks with little absorption, but it takes a few hours to reprocess the
X-rays and to emit the energy in the optical regime, exactly as observed.

Fortunately, BRITE-Constellation observed this nova even before it was
discovered, providing “smoking-gun evidence” for the shock model.

##  5 Summary

BRITE-Constellation has outlasted its minimum design-lifetime by several
factors. While it is tempting to terminate the mission, it would be a real
pity for humanity to do this, instead of allowing further observations to form
a legacy for astronomy. The cost is truly modest compared to most other space
missions, especially in relation to the valuable science that BRITE has
accomplished and still can accomplish.

BRITE-Constellation’s uniqueness lies first in the small sizes of the
individual satellites that are capable of three-axis stabilization and
providing a pointing stability accurate enough for astrophysical observations.
Second, BRITE-Constellation is an outstanding and unique space mission because
of its possibility to observe stars simultaneously in two designated pass-
bands and up to 6 months contiguously.

The big success of BRITE-Constellation is reflected—as of March 2021—in 42
peer-reviewed publications and many more conference papers that address a
variety of scientific topics from the most massive stars to cool red giants
and novae. Here, we have highlighted some of the key results as part of a
brief overview.

All authors have contributed substantially to the work reported. All authors
have read and agreed to the published version of the manuscript.

We are grateful to BEST, consisting of two voting members per satellite,
supplemented by presently 15 non-voting members, who managed BRITE-
Constellation successfully during nearly a decade. We acknowledge with thanks
the variable star observations from the AAVSO International Database
contributed by observers worldwide and used in this research. The authors from
Poland acknowledge assistance by BRITE PMN grant 2011/01/M/ST9/05914 and GH
thanks the Polish NCN for support (grant 2015/18/A/ST9/00578). AFJM and GAW
acknowledge Discovery Grant support from the Natural Sciences and Engineering
Research Council (NSERC) of Canada. APo was supported by SUT grant no.
02/140/RGJ21/0012 and Statutory Activities grant no. BK-225/RAu-11/2021.

Not applicable.

Not applicable.

Data access is provided by https://brite.camk.edu.pl/pub/index.html.

The authors declare no conflict of interest.

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