29 research outputs found
A detailed X-ray investigation of zeta Puppis IV. Further characterization of the variability
Previously, the X-ray emission of zeta Puppis was found to be variable with
light curves harbouring "trends" with a typical timescale longer than the
exposure length. The origin of these changes was proposed to be linked to
large-scale structures in the wind, but further characterization of the
variability at high energies was needed. Since then, a number of new X-ray
observations have become available. Furthermore, a cyclic behaviour with a
1.78d period was identified in long optical photometric runs, which is thought
to be associated with the launching mechanism of large-scale wind structures.
We analysed these new X-ray data, revisited the old data, and compared X-ray
with optical data, including when simultaneous. We found that the behaviour in
X-rays cannot be explained in terms of a perfect clock because the amplitude
and shape of its variations change with time. For example, zeta Puppis was much
more strongly variable between 2007 and 2011 than before and after this
interval. Comparing the X-ray spectra of the star at maximum and minimum
brightness yields no compelling difference beyond the overall flux change: the
temperatures, absorptions, and line shapes seem to remain constant, well within
errors. The only common feature between X-ray datasets is that the variation
amplitudes appear maximum in the medium (0.6-1.2keV) energy band. Finally, no
clear and coherent correlation can be found between simultaneous X-ray and
optical data. Only a subgroup of observations may be combined coherently with
the optical period of 1.78d, although the simultaneous optical behaviour is
unknown. The currently available data do not reveal any obvious, permanent, and
direct correlation between X-ray and optical variations. The origin of the
X-ray variability therefore still needs to be ascertained, highlighting the
need for long-term monitoring in multiwavelengths, i.e. X-ray, UV, and optical.Comment: accepted for publication by A&
BRITE-Constellation: Data processing and photometry
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. 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. We developed two pipelines
corresponding to the two modes of observing. 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. This paper shows that high-precision space
photometry with low-cost nano-satellites is achievable. The proposed meth- ods,
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
The chaotic wind of WR 40 as probed by BRITE
Among Wolf-Rayet stars, those of subtype WN8 are the intrinsically most
variable. We have explored the long-term photometric variability of the
brightest known WN8 star, WR 40, through four contiguous months of
time-resolved, single-passband optical photometry with the BRIght Target
Explorer (BRITE) nanosatellite mission. The Fourier transform of the observed
light-curve reveals that the strong light variability exhibited by WR 40 is
dominated by many randomly-triggered, transient, low-frequency signals. We
establish a model in which the whole wind consists of stochastic clumps
following an outflow visibility promptly rising to peak brightness upon clump
emergence from the optically thick pseudo-photosphere in the wind, followed by
a gradual decay according to the right-half of a Gaussian. Free electrons in
each clump scatter continuum light from the star. We explore a scenario where
the clump size follows a power-law distribution, and another one with an
ensemble of clumps of constant size. Both scenarios yield simulated light
curves morphologically resembling the observed light curve remarkably well,
indicating that one cannot uniquely constrain the details of clump size
distribution with only a photometric light curve. Nevertheless, independent
evidence favours a negative-index power law, as seen in many other
astrophysical turbulent media.Comment: 11 pages, 8 figures, 3 tables; Monthly Notices of the Royal
Astronomical Society (MNRAS), in pres
Constraining the Physical Properties of Stellar Coronal Mass Ejections with Coronal Dimming: Application to Far Ultraviolet Data of Eridani
Coronal mass ejections (CMEs) are a prominent contributor to solar system
space weather and might have impacted the Sun's early angular momentum
evolution. A signal diagnostic of CMEs on the Sun is coronal dimming: a drop in
coronal emission, tied to the mass of the CME, that is the direct result of
removing emitting plasma from the corona. We present the results of a coronal
dimming analysis of Fe XII 1349 A and Fe XXI 1354 A emission from
Eridani ( Eri), a young K2 dwarf, with archival far-ultraviolet
observations by the Hubble Space Telescope's Cosmic Origins Spectrograph.
Following a flare in February 2015, Eri's Fe XXI emission declined
by %. Although enticing, a scant 3.8 min of preflare observations
allows for the possibility that the Fe XXI decline was the decay of an earlier,
unseen flare. Dimming nondetections following each of three prominent flares
constrain the possible mass of ejected Fe XII-emitting (1 MK) plasma to less
than a few g. This implies that CMEs ejecting this much or more
1 MK plasma occur less than a few times per day on Eri. On the Sun,
g CMEs occur once every few days. For Eri, the mass loss
rate due to CME-ejected 1 MK plasma could be , well below
the star's estimated 30 mass loss rate (wind + CMEs). The
order-of-magnitude formalism we developed for these mass estimates can be
broadly applied to coronal dimming observations of any star.Comment: 27 pages, 22 figures, accepted to Ap
Dynamically Controlling Image Integration Onboard the Star-Planet Activity Research CubeSat (SPARCS)
The Star-Planet Activity Research CubeSat (SPARCS) is a 6U CubeSat astronomical observatory underdevelopment and will be entirely dedicated to the photometric monitoring of the flaring activity of M dwarfs at near-UV (258 nm – 308 nm) and far-UV (153 nm–171 nm) wavelengths. The SPARCS science pay load is composed of a 9-cm telescope that projects a 40’ field-of-view onto two UV-optimized delta-doped charge-coupled devices (CCDs), which are controlled by a dedicated payload processor board. Given that M dwarf flares in the UV are expected to be capable of reaching amplitudes ∼14,000 times above their quiescent flux, with durations that can be as short as a couple of minutes, the SPARCS payload processor is designed to be able to dynamically adjust the imaging system’s integration times and gains on the fly to reduce CCD pixel saturation issues when flaring events are detected. The SPARCS payload processor is a BeagleBone Black (BBB) with a protective Pumpkin Motherboard Module 2, and runs a custom fully Python-based software to perform active detector thermal control, manage science observations, and apply near-real time image processing to autonomously adjust the exposure times and gains of the detectors upon flare detection. Here we present the approach adopted for that automated dynamic exposure control, as well as its pre-flight tests and performance using simulated M dwarf light curves and full-frame images in the two SPARCS passbands
Spectroscopy, MOST Photometry, and Interferometry of MWC 314: Is it an LBV or an interacting binary?
MWC 314 is a bright candidate luminous blue variable that resides in a fairly
close binary system, with an orbital period of 60.7530.003 d. We observed
MWC 314 with a combination of optical spectroscopy, broad-band ground- and
space-based photometry, as well as with long baseline, near-infrared
interferometry. We have revised the single-lined spectroscopic orbit and
explored the photometric variability. The orbital light curve displays two
minima each orbit that can be partially explained in terms of the tidal
distortion of the primary that occurs around the time of periastron. The
emission lines in the system are often double-peaked and stationary in their
kinematics, indicative of a circumbinary disc. We find that the stellar wind or
circumbinary disc is partially resolved in the K\prime-band with the longest
baselines of the CHARA Array. From this analysis, we provide a simple,
qualitative model in an attempt to explain the observations. From the
assumption of Roche Lobe overflow and tidal synchronisation at periastron, we
estimate the component masses to be M1 M and M2
M, which indicates a mass of the LBV that is extremely low. In addition
to the orbital modulation, we discovered two pulsational modes with the MOST
satellite. These modes are easily supported by a low-mass hydrogen-poor star,
but cannot be easily supported by a star with the parameters of an LBV. The
combination of these results provides evidence that the primary star was likely
never a normal LBV, but rather is the product of binary interactions. As such,
this system presents opportunities for studying mass-transfer and binary
evolution with many observational techniques.Comment: 26 pages, 7 figures, 5 tables, 2 appendices with 7 additional tables
and 2 additional figures. Accepted for publication in MNRA
MOST detects corotating bright spots on the mid-O type giant {\xi} Persei
We have used the MOST (Microvariability and Oscillations of STars)
microsatellite to obtain four weeks of contiguous high-precision broadband
visual photometry of the O7.5III(n)((f)) star {\xi} Persei in November 2011.
This star is well known from previous work to show prominent DACs (Discrete
Absorption Components) on time-scales of about 2 d from UV spectroscopy and NRP
(Non Radial Pulsation) with one (l = 3) p-mode oscillation with a period of 3.5
h from optical spectroscopy. Our MOST-orbit (101.4 min) binned photometry fails
to reveal any periodic light variations above the 0.1 mmag 3-sigma noise level
for periods of hours, while several prominent Fourier peaks emerge at the 1
mmag level in the two-day period range. These longer-period variations are
unlikely due to pulsations, including gravity modes. From our simulations based
upon a simple spot model, we deduce that we are seeing the photometric
modulation of several co-rotating bright spots on the stellar surface. In our
model, the starting times (random) and lifetimes (up to several rotations) vary
from one spot to another yet all spots rotate at the same period of 4.18 d, the
best-estimated rotation period of the star. This is the first convincing
reported case of co-rotating bright spots on an O star, with important
implications for drivers of the DACs (resulting from CIRs - Corotating
Interaction Regions) with possible bright-spot generation via a breakout at the
surface of a global magnetic field generated by a subsurface convection zone.Comment: 9 pages, 4 figures, 2 tables, MNRAS in pres