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Several long-period binaries with a carbon-rich Wolf-Rayet star and an O star produce dust in their wind collisions. In eccentric binaries, this is seen most strongly near periastron passage. The exact conditions leading to dust creation require orbital properties to be determined, which is difficult owing to their long periods. Recently, the binary system WR 125 (WC7+O9III) began a dust creation episode seen through an infrared outburst first detected by NEOWISE-R, which was the first outburst detected since 1991. We present new near- and mid-infrared photometry, which we use to show consistency between the two outbursts and derive an orbital period of 28.12$^{+0.10}_{-0.05}$ yr. We use a long time-series of optical spectra to place the first constraints on its orbital elements, on the assumption that this system will produce dust near periastron. The orbit has a mild eccentricity of 0.29$\pm$0.12 and is only derived for the Wolf-Rayet component, as the O star's radial velocities have noise that is likely larger than the expected semi-amplitude of the orbit. We also present SOFIA/FORCAST grism spectroscopy to examine the infrared spectral energy distribution (SED) of the dust during this outburst, comparing its properties to other WCd binaries, deriving a dust temperature of 580 K in 2021. This collection of observations will allow us to plan future observations of this system and place the system in the context of dust-creating Wolf-Rayet binaries., Comment: accepted to ApJ

Eta Carinae underwent the Great Eruption in the 1840s and a Lesser Eruption in the 1890s. Its apparent spectrum, modified by intervening ejecta, the Homunculus and Little Homunculus, continues to evolve but contains information pertaining to events in the 19th century. The LOS spectrum contains narrow absorption velocities, from -122 to -1665 km/s: rungs of a broken ladder caused by shells formed by the interacting winds. Estimated shell origin dates correlate with origin dates of expanding emission structures preceding the Great Eruption. The LOS absorption velocities extend the record post Great Eruption to the Lesser Eruption. We suggest that these shells originated from a binary merger within a triple system. Shells formed not only from periastron passages of the current secondary, but also from ear-like extensions preceding and following the periastron event. Additional models need to be considered., Comment: 18 pages, 5 figures, 1 table

We analyse $\textit{XMM-Newton}$ RGS spectra of Wolf-Rayet (WR) 140, an archetype long-period eccentric WR+O colliding wind binary. We evaluate the spectra of O and Fe emission lines and find that the plasmas emitting these lines have the largest approaching velocities with the largest velocity dispersions between phases 0.935 and 0.968 where the inferior conjunction of the O star occurs. This behaviour is the same as that of the Ne line-emission plasma presented in our previous paper. We perform diagnosis of electron number density $n_{\rm e}$ using He-like triplet lines of O and Ne-like Fe-L lines. The former results in a conservative upper limit of $n_{\rm e} \lesssim 10^{10}$-10$^{12}$ cm$^{-3}$ on the O line-emission site, while the latter can not impose any constraint on the Fe line-emission site because of statistical limitations. We calculate the line-of-sight velocity and its dispersion separately along the shock cone. By comparing the observed and calculated line-of-sight velocities, we update the distance of the Ne line-emission site from the stagnation point. By assuming radiative cooling of the Ne line-emission plasma using the observed temperature and the local stellar wind density, we estimate the line-emission site extends along the shock cone by at most $\pm$58 per cent (phase 0.816) of the distance from the stagnation point. In this framework, excess of the observed velocity dispersion over the calculated one is ascribed to turbulence in the hot-shocked plasma at earlier orbital phases of 0.816, 0.912, and 0.935, with the largest velocity dispersion of 340-630 km s$^{-1}$ at phase 0.912., Comment: 16 pages, 9 figures, Accepted for publication in MNRAS

Changes in the flux and spectrum of Eta Carinae since 1900 have been attributed to the evolution of the central binary by some. Others suggest evolution in the occulting ejecta. The brightness jump in the 1940s, which coincided with the appearance of narrow forbidden emission lines, may have been caused by the clearing and ionization of intervening circumstellar ejecta. The brightening changed at a slower pace up through forty years later. Here we continue earlier studies focused on the long-term showing that the forbidden line emission increased in the early 1990s with no noticeable increase in the brightness of the Homunculus. We interpret that the increase in narrow line emission is due to decreased extinction in the LOS from the central binary to the Weigelt clumps. In 2000, the central stellar core increased in brightness at a faster rate without associated changes in the Homunculus. By 2018, hundreds of narrow-line absorptions from singly-ionized metals in our LOS from Eta Carinae disappeared, thought to be caused by increased ionization of metals. These three events (1990, 2000, and 2018) are explained by the dissipation of circumstellar material within the Homunculus close to the binary. Combining these changes with the steadiness of the Homunculus and the primary winds over the past four decades indicates that circumstellar ejecta in our direction have been cleared., Comment: 26 pages, 8 figures, to appear in ApJ

We present infrared aperture masking interferometry (AMI) observations of newly formed dust from the colliding winds of the massive binary system Wolf-Rayet (WR) 137 with JWST using the Near Infrared Imager and Slitless Spectrograph (NIRISS). NIRISS AMI observations of WR 137 and a point-spread-function calibrator star, HD~228337, were taken using the F380M and F480M filters in 2022 July and August as part of the Director's Discretionary Early Release Science (DD-ERS) program 1349. Interferometric observables (squared visibilities and closure phases) from the WR 137 "interferogram" were extracted and calibrated using three independent software tools: ImPlaneIA, AMICAL, and SAMpip. The analysis of the calibrated observables yielded consistent values except for slightly discrepant closure phases measured by ImPlaneIA. Based on all three sets of calibrated observables, images were reconstructed using three independent software tools: BSMEM, IRBis, and SQUEEZE. All reconstructed image combinations generated consistent images in both F380M and F480M filters. The reconstructed images of WR 137 reveal a bright central core with a $\sim300$ mas linear filament extending to the northwest. A geometric colliding-wind model with dust production constrained to the orbital plane of the binary system and enhanced as the system approaches periapsis provided a general agreement with the interferometric observables and reconstructed images. Based on a colliding-wind dust condensation analysis, we suggest that dust formation within the orbital plane of WR 137 is induced by enhanced equatorial mass-loss from the rapidly rotating O9 companion star, whose axis of rotation is aligned with that of the orbit., Comment: 18 pages, 8 figures, Accepted for publication in ApJ. Updated plotting error in Fig. 2

The bright WN4 star EZ CMa exhibits a 3.77 day periodicity in photometry, spectroscopy, and polarimetry but the variations in the measurements are not strictly phase-locked, exhibiting changes in reference times, amplitudes, and the shape of the variability happening over times as short as a few weeks. Recently, 137 days of contiguous, variable photometry from BRITE-Constellation was interpreted as caused either by large-scale dense wind structures modulated by rotation, or by a fast-precessing binary having a slightly shorter 3.626 day orbital period and a fast apsidal motion rate of $1315^\circ\,\text{yr}^{-1}$. We aim at testing the latter hypothesis through analysis of spectroscopy and focus on the N\,{\sc v} $\lambda\,4945$ line. We derive an orbital solution for the system and reject the 3.626 day period to represent the variations in the radial velocities of EZ CMa. An orbital solution with an orbital period of 3.77 days was obtained but at the cost of an extremely high and thus improbable apsidal motion rate. Our best orbital solution yields a period of $3.751\pm0.001$\,days with no apsidal motion. We place our results in the context of other variability studies and system properties. While we cannot fully reject the precessing binary model, we find that the corotating interaction region (CIR) hypothesis is better supported by these and other data through qualitative models of CIRs., Comment: accepted to MNRAS

WR 137 (HD 192641) is a binary system consisting of a carbon-rich Wolf-Rayet star and an Oe companion star in a 13-year orbit. Near periastron, the winds of the two stars collide and form carbonaceous dust. We obtained three mid-infrared grism spectra of the system with SOFIA and FORCAST during the last year of SOFIA's operations in July 2021, February 2021, and May 2022 (Cycle 9). Within these spectra, we have identified several wind lines from He I, He II, C III, and C IV that are emitted from the Wolf-Rayet wind as well as a weak emission feature around 6.3-6.4 $\mu$m that may have shifted its peak flux from 6.29 to 6.41$\mu$m through this time period. The weak feature grew as the continuum dust emission grew while the WR emission appeared to decline due to lower contrast with the continuum. Furthermore, we observe that the peak of the feature shifts to redder wavelengths during the observations. We compare this feature to the UIR feature and other emission lines identified in dusty WC binaries. For WR 137, we speculate that mixing of the winds in the system with the Oe star's disk is important for starting the dust formation and that it is less important as dust formation continues. Previous infrared photometry shows "mini-eruptions" of dust production which could then be explained with variations of the Oe star disk., Comment: 13 pages, accepted to ApJ

Accurately quantifying the rates dM/dt at which massive stars lose mass is essential to any understanding of their evolution. All dM/dt estimates to date assume wind clumping factors; not allowing for clumping leads to overestimates of dM/dt and underestimates of lifetimes and masses when these stars explode as supernovae. Mid-IR spectroscopy suggested that the wind of the nearest Wolf-Rayet star, Gamma2 Vel, is resolved with a Full Width at 10 per cent intensity of 0.5 arcsec, or 171 AU at the 342 pc distance of the star. As the Zorro speckle imager on Gemini-South is capable of 0.02 arcsec resolution, we have used it to image Gamma2 Vel at two orbital phases (0.30 and 0.44) with two narrowband and two intermediate-band filters in an attempt to resolve its wind. Our observations demonstrate that the wind of Gamma2 Vel may be resolved as a 0.07 arcsec westward elongation through an 832 nm filter at orbital phase 0.3 . If confirmed, this is the smallest scale (24 AU) at which a WR star wind asymmetry has been directly imaged. Similar imaging at multiple phases is needed to determine if the asymmetry is due to stochastic wind clumping, co-rotating interaction regions or colliding-wind, cone-shaped shocks., Comment: 7 pages, 4 Figures, in press in MNRAS

Wolf-Rayet (WR) 140 is the archetypal periodic dust-forming colliding-wind binary that hosts a carbon-rich WR (WC) star and an O-star companion with an orbital period of 7.93 years and an orbital eccentricity of 0.9. Throughout the past several decades, multiple dust-formation episodes from WR 140 have been observed that are linked to the binary orbit and occur near the time of periastron passage. Given its predictable dust-formation episodes, WR 140 presents an ideal astrophysical laboratory for investigating the formation and evolution of dust in the hostile environment around a massive binary system. In this paper, we present near- and mid-infrared (IR) spectroscopic and imaging observations of WR 140 with Subaru/SCExAO+CHARIS, Keck/NIRC2+PyWFS, and Subaru/COMICS taken between 2020 June and Sept that resolve the circumstellar dust emission linked to its most recent dust-formation episode in 2016 Dec. Our spectral energy distribution (SED) analysis of WR 140's resolved circumstellar dust emission reveals the presence of a hot ($T_\mathrm{d}\sim1000$ K) near-IR dust component that is co-spatial with the previously known and cooler ($T_\mathrm{d}\sim500$ K) mid-IR dust component composed of $300-500$ {\AA}-sized dust grains. We attribute the hot near-IR dust emission to the presence of nano-sized ("nanodust") grains and suggest they were formed from grain-grain collisions or the rotational disruption of the larger grain size population by radiative torques in the strong radiation field from the central binary. Lastly, we speculate on the astrophysical implications of nanodust formation around colliding-wind WC binaries, which may present an early source of carbonaceous nanodust in the interstellar medium., Comment: 21 pages, 8 figures, Accepted for publication in ApJ

Previous STIS long-slit observations of Eta Carinae identified numerous absorption features in both the stellar spectrum, and in the adjacent nebular spectra, along our line-of-sight. The absorption features became temporarily stronger when the ionizing FUV radiation field was reduced by the periastron passage of the secondary star. Subsequently, dissipation of a dusty structure in our LOS has led to a long-term increase in the apparent magnitude of \ec, an increase in the ionizing UV radiation, and the disappearance of absorptions from multiple velocity-separated shells extending across the foreground Homunculus lobe. We use HST/STIS spectro-images, coupled with published infrared and radio observations, to locate this intervening dusty structure. Velocity and spatial information indicate the occulter is ~1000 au in front of Eta Carinae. The Homunculus is a transient structure composed of dusty, partially-ionized ejecta that eventually will disappear due to the relentless rain of ionizing radiation and wind from the current binary system along with dissipation and mixing with the ISM. This evolving complex continues to provide an astrophysical laboratory that changes on human timescales., Comment: 20 pages, 10 figures, submitted to ApJ

The binary eta Carinae is the closest example of a very massive star, which may have formed through a merger during its Great Eruption in the mid-nineteenth century. We aimed to confirm and improve the kinematics using a spectroscopic data set taken with the CTIO 1.5 m telescope over the time period of 2008-2020, covering three periastron passages of the highly eccentric orbit. We measure line variability of H-alpha and H-beta, where the radial velocity and orbital kinematics of the primary star were measured from the H-beta emission line using a bisector method. At phases away from periastron, we observed the He II 4686 emission moving opposite the primary star, consistent with a possible Wolf-Rayet companion, although with a seemingly narrow emission line. This could represent the first detection of emission from the companion., Comment: 11 pages, accepted to MNRAS

Massive colliding-wind binaries that host a Wolf-Rayet (WR) star present a potentially important source of dust and chemical enrichment in the interstellar medium (ISM). However, the chemical composition and survival of dust formed from such systems is not well understood. The carbon-rich WR (WC) binary WR~140 presents an ideal astrophysical laboratory for investigating these questions given its well-defined orbital period and predictable dust-formation episodes every 7.93 years around periastron passage. We present observations from our Early Release Science program (ERS1349) with the James Webb Space Telescope (JWST) Mid-Infrared Instrument (MIRI) Medium-Resolution Spectrometer (MRS) and Imager that reveal the spectral and spatial signatures of nested circumstellar dust shells around WR~140. MIRI MRS spectroscopy of the second dust shell and Imager detections of over 17 shells formed throughout the past $\gtrsim130$ years confirm the survival of carbonaceous dust grains from WR~140 that are likely carriers of "unidentified infrared" (UIR)-band features at 6.4 and 7.7 $\mu$m. The observations indicate that dust-forming WC binaries can enrich the ISM with organic compounds and carbonaceous dust., Comment: Published in Nature Astronomy on Oct 12, 2022; 21 pages, 5 figures, 2 tables

The Na D absorption doublet in the spectrum of $\eta$ Carinae is complex, with multiple absorption features associated with the Great Eruption (1840s), the Lesser Eruption (1890s), and interstellar clouds. The velocity profile is further complicated by the P Cygni profile originating in the system's stellar winds and blending with the He I $\lambda$5876 profile. The Na D profile contains a multitude of absorption components, including those at velocities of $-$145 km s$^{-1}$, $-$168 km s$^{-1}$, and $+$87 km s$^{-1}$ that we concentrate on in this analysis. Ground-based spectra recorded from 2008 to 2021 show significant variability of the $-$145 km s$^{-1}$ absorption throughout long-term observations. In the high ionization phases of $\eta$ Carinae prior to the 2020 periastron passage, this feature disappeared completely but briefly reappeared across the 2020 periastron, along with a second absorption at $-$168 km s$^{-1}$. Over the past few decades, $\eta$ Car has been gradually brightening demonstrated to be caused by a dissipating occulter. The decreasing absorption of the $-$145 km s$^{-1}$ component, coupled with similar trends seen in absorptions of ultraviolet resonant lines, indicate that this central occulter was possibly a large clump associated with the Little Homunculus or another clump between the Little Homunculus and the star. We also report on a foreground absorption component at $+$87 km s$^{-1}$. Comparison of Na D absorption in the spectra of nearby systems demonstrates that this red-shifted component likely originates in an extended foreground structure consistent with a previous ultraviolet spectral survey in the Carina Nebula., Comment: 8 figures, 13 pages plus online data, accepted to ApJ

Stellar-mass black holes are the final remnants of stars born with more than 15 solar masses. Billions are expected to reside in the Local Group, yet only few are known, mostly detected through X-rays emitted as they accrete material from a companion star. Here, we report on VFTS 243: a massive X-ray faint binary in the Large Magellanic Cloud. With an orbital period of 10.4-d, it comprises an O-type star of 25 solar masses and an unseen companion of at least nine solar masses. Our spectral analysis excludes a non-degenerate companion at a 5-sigma confidence level. The minimum companion mass implies that it is a black hole. No other X-ray quiet black hole is unambiguously known outside our Galaxy. The (near-)circular orbit and kinematics of VFTS 243 imply that the collapse of the progenitor into a black hole was associated with little or no ejected material or black-hole kick. Identifying such unique binaries substantially impacts the predicted rates of gravitational-wave detections and properties of core-collapse supernovae across the Cosmos., Comment: Accepted to Nature Astronomy, 64 pages, 15 figures, 2 tables; ESO press release: https://www.eso.org/public/news/eso2210/; Nat Asr paper URL: https://www.nature.com/articles/s41550-022-01730-y

FUV spectra of Eta Car, recorded across two decades with HST/STIS, document multiple changes in resonant lines caused by dissipating extinction in our line of sight. The FUV flux has increased nearly ten-fold which has led to increased ionization of the multiple shells within the Homunculus and photo-destruction of molecular hydrogen. Comparison of observed resonant line profiles with CMFGEN model profiles allows separation of wind-wind collision and shell absorptions from the primary wind, P Cygni profiles.The dissipating occulter preferentially obscured the central binary and interacting winds relative to the very extended primary wind. We are now able to monitor changes in the colliding winds with orbital phase. High velocity transient absorptions occurred across the most recent periastron passage, indicating acceleration of the primary wind by the secondary wind which leads to a downstream, high velocity bowshock that is newly generated every orbital period. There is no evidence of changes in the properties of the binary winds., Comment: 36 pages, 22 figures, accepted Astrophysical Journal

WR22 = HD 92740 is a bright (V = 6.4 $mag$), intrinsically luminous, double-line WN7h + O9III-V binary exhibiting one sharp 8\% deep eclipse near periastron in its elliptical (e = 0.6) 80-day orbit, when the WR-star passes in front of the O star, with no secondary eclipse. We apply two models (L96, A13) to probe the optical space-based light curves from {\em BRITE-Constellation}, including three separate, complete eclipses, that show increased (o-c) scatter compared to the rest of the observations outside the eclipses, likely due to O-star light encountering WR wind-clumps. L96 is a simple atmospheric-eclipse model, often applied to close WR+O binaries, where the O-star is considered a point-source. A13 considers a finite-disk O-star and allows for atmospheric, photospheric and reflection components to the eclipse, permitting a better characterization of its shape through a more physically realistic description of the structures for both stars in WR22. Nevertheless, A13 is still susceptible to uncertainties in the luminosity of the O-star before unique values for the orbital inclination and WR mass-loss rate can be estimated. We present solutions for the two extremes of the O-star, O9V and O9III. As photometry alone cannot allow us to discriminate between these, we compared our results to the spectral models found in the literature and determined the correct solution to be O9V. Our best-fit A13 Model 1 gives $i = 83.5 \pm 0.4^{\circ}$, with $\dot M_{\rm WR} = (1.86 \pm 0.2) \times 10^{-5} \dot M_{\odot}/yr$. The flux ratio in the red {\em BRITE} band in this model is $F_{\rm O}/F_{\rm WR} = 0.064\pm 0.002$., Comment: 15 pages, 11 figures

Luminous Blue Variables (LBVs) are massive stars that are likely to be a transitionary phase between O stars and hydrogen-free classical Wolf-Rayet stars. The variability of these stars has been an area of study for both professional and amateur astronomers for more than a century. In this paper, we present five years of precision photometry of the classical LBV P Cygni taken with the BRITE-Constellation nanosatellites. We have analyzed these data with Fourier analysis to search for periodicities that could elucidate the drivers of variability for these stars. These data show some long-timescale variability over the course of all six calendar years of observations, but the frequencies needed to reproduce the individual light curves are not consistent from one year to the next. These results likely show that there is no periodic phenomenon present for P Cygni, meaning that the variability is largely stochastic. We interpret the data as being caused by internal gravity waves similar to those seen in other massive stars, with P Cygni exhibiting a larger amplitude and lower characteristic frequency than the main-sequence or blue supergiant stars previously studied. These results show evidence that LBVs may be an extrapolation of the blue supergiants, which have previously been shown to be an extension of main-sequence stars in the context of the stochastic low-frequency photometric variability., Comment: 10 pages, accepted for publication in MNRAS

Gravitationally bound companions to stars enable determinations of their masses, and offer clues to their formation, evolution and dynamical histories. So motivated, we have carried out a speckle imaging survey of eight of the nearest and brightest Wolf-Rayet (WR) stars to directly measure the frequency of their resolvable companions, and to search for much fainter companions than hitherto possible. We found one new, close companion to each of WR 113, WR 115 and WR 120 in the separation range 0.2" - 1.2". Our results provide more evidence that similar-brightness, close companions to WR stars are common. More remarkably, they also demonstrate that the predicted, but much fainter and thus elusive companions to WR stars are now within reach of modern speckle cameras on 8m class telescopes by finding the first example. The new companion to WR 113 is just 1.16" distant from it, and is 8 magnitudes fainter than the WR star. The empirical probability of a chance line-of-sight of the faint companion at the position of WR 113 is < 0.5%, though we cannot yet prove or disprove if the two stars are gravitationally bound. If these three new detections are physical companions we suggest, based on their narrowband magnitudes, colors, reddenings and GAIA distances that the companions to WR113, WR 115 and WR 120 are an F-type dwarf, an early B-type dwarf, and a WNE-type WR star, respectively., Comment: 14 pages, 4 figures, accepted for publication in MNRAS

The orbital periods of most eclipsing cataclysmic binaries are not undergoing linear secular decreases of order a few parts per billion as expected from simple theory. Instead, they show several parts per million increases and decreases on timescales of years to decades, ascribed to magnetic effects in their donors, triple companions, or both. To directly test the triple companion hypothesis, we carried out a speckle imaging survey of six of the nearest and brightest cataclysmic variables. We found no main sequence companions earlier than spectral types M4V in the separation range 0.02" - 1.2", corresponding to projected linear separations of 2 - 100 AU, and periods of 3 - 1000 years. We conclude that main sequence triple companions to CVs are not very common, but cannot rule out the presence of the faintest M dwarfs or close brown dwarf companions., Comment: 8 pages, 1 figure, accepted in MNRAS

We present optical and infrared (IR) light curves of NaSt1, also known as Wolf-Rayet (WR) 122, with observations from Palomar Gattini-IR (PGIR), the Zwicky Transient Facility (ZTF), the Katzman Automatic Imaging Telescope (KAIT), the Asteroid Terrestrial-impact Last Alert System (ATLAS), and the All-Sky Automated Survey for Supernovae (ASAS-SN). We identify a $P=309.7\pm0.7$ d photometric period from the optical and IR light curves that reveal periodic, sinusoidal variability between 2014 July and 2021 July. We also present historical IR light curves taken between 1983 July and 1989 May, which show variability consistent with the period of the present-day light curves. In the past, NaSt1 was brighter in the $J$ band with larger variability amplitudes than the present-day PGIR values, suggesting that NaSt1 exhibits variability on longer ($\gtrsim$ decade) timescales. Sinusoidal fits to the recent optical and IR light curves show that the amplitude of NaSt1's variability differs at various wavelengths and also reveal significant phase offsets of $17.0\pm2.5$ d between the ZTF $r$ and PGIR $J$ light curves. We interpret the $310$ d photometric period from NaSt1 as the orbital period of an enshrouded massive binary. We suggest that the photometric variability of NaSt1 may arise from variations in the line-of-sight optical depth toward circumstellar optical/IR emitting regions throughout its orbit due to colliding-wind dust formation. We speculate that past mass transfer in NaSt1 may have been triggered by Roche-lobe overflow (RLOF) during an eruptive phase of an Ofpe/WN9 star. Lastly, we argue that NaSt1 is no longer undergoing RLOF mass transfer., Comment: 20 Pages, 7 Figures, 5 Tables, 1 Table Attachment, Accepted to ApJ

We present spectroscopy of the P~Cygni profile of the 1.083-micron He I line in the WC7 + O5 colliding-wind binary (CWB) WR 140 (HD 193793), observed in 2008, before its periastron passage in 2009, and in 2016-17, spanning the subsequent periastron passage. Both absorption and emission components showed strong variations. The variation of the absorption component as the O5 star was occulted by the wind-collision region (WCR) sets a tight constraint on its geometry. While the sightline to the O5 star traversed the WCR, the strength and breadth of the absorption component varied significantly on time-scales of days. An emission sub-peak was observed on all our profiles. The variation of its radial velocity with orbital phase was shown to be consistent with formation in the WCR as it swung round the stars in their orbit. Modelling the profile gave a measure of the extent of the sub-peak forming region. In the phase range 0.93-0.99, the flux in the sub-peak increased steadily, approximately inversely proportionally to the stellar separation, indicating that the shocked gas in the WCR where the line was formed was adiabatic. After periastron, the sub-peak flux was anomalously strong and varied rapidly, suggesting formation in clumps down-stream in the WCR. For most of the time, its flux exceeded the 2-10-keV X-ray emission, showing it to be a significant coolant of the shocked wind., Comment: 19 pages, 14 figures, accepted for publication in the MNRAS

We present updated orbital elements for the Wolf-Rayet (WR) binary WR\,140 (HD\,193793; WC7pd + O5.5fc). The new orbital elements were derived using previously published measurements along with {\color{black}160} new radial velocity measurements across the 2016 periastron passage of WR 140. Additionally, four new measurements of the orbital astrometry were collected with the CHARA Array. With these measurements, we derive stellar masses of $M_{\rm WR} = 10.31\pm0.45 M_\odot$ and $M_{\rm O} = 29.27\pm1.14 M_{\odot}$. We also include a discussion of the evolutionary history of this system from the Binary Population and Spectral Synthesis (BPASS) model grid to show that this WR star likely formed primarily through mass loss in the stellar winds, with only a moderate amount of mass lost or transferred through binary interactions., Comment: 10 pages, 5 figures

We present Spitzer/IRAC observations of dust formation from six extragalactic carbon-rich Wolf-Rayet (WC) binary candidates in low-metallicity (Z $\lesssim0.65$ Z$_\odot$) environments using multi-epoch mid-infrared (IR) imaging data from the SPitzer InfraRed Intensive Transients Survey (SPIRITS). Optical follow-up spectroscopy of SPIRITS~16ln, 19q, 16df, 18hb, and 14apu reveals emission features from C IV $\lambda5801\text{-}12$~and/or the C III-IV $\lambda4650$ He II $\lambda4686$~blend that are consistent with early-type WC stars. We identify SPIRITS~16ln as the variable mid-IR counterpart of the recently discovered colliding-wind WC4+O binary candidate, N604-WRXc, located in the sub-solar metallicity NGC 604 H II~region in M33. We interpret the mid-IR variability from SPIRITS~16ln as a dust-formation episode in an eccentric colliding-wind WC binary. SPIRITS~19q, 16df, 14apu, and 18hb exhibit absolute [3.6] magnitudes exceeding one of most IR-luminous dust-forming WC systems known, WR~104 (M$_\mathrm{[3.6]}\lesssim-12.3$). An analysis of dust formation in the mid-IR outburst from SPIRITS~19q reveals a high dust production rate of $\dot{M}_d\gtrsim2\times10^{-6}$ M$_\odot$ yr$^{-1}$, which may therefore exceed that of the most efficient dust-forming WC systems known. We demonstrate that efficient dust-formation is feasible from early-type WC binaries in the theoretical framework of colliding-wind binary dust formation if the systems host an O-type companion with a high mass-loss rate ($\dot{M}\gtrsim1.6\times10^{-6}$ M$_\odot$ yr$^{-1}$). This efficient dust-formation from early-type WC binaries highlights their potential role as significant sources of dust in low-metallicity environments., Comment: 28 pages, 10 Figures, and 5 Tables. Accepted to ApJ on 2021 Jan 4 (Replacement of version submitted to arXiv on 2020 Nov 19)

Analysis of the recent long exposure Chandra X-ray observation of the early-type O star zeta Pup shows clear variability with a period previously reported in optical photometric studies. These 813 ks of HETG observations taken over a roughly one year time span have two signals of periodic variability: a high significance period of 1.7820 +/- 0.0008 day, and a marginal detection of periodic behavior close to either 5 day or 6 day period. A BRITE-Constellation nanosatellite optical photometric monitoring, using near-contemporaneous observations to the Chandra data, confirms a 1.78060 +/- 0.00088 day period for this star. The optical period coincides with the new Chandra period within their error ranges, demonstrating a link between these two wavebands and providing a powerful lever for probing the photosphere/wind connection in this star. The phase lag of the X-ray maximum relative to the optical maximum is approximately phi=0.45, but consideration of secondary maxima in both datasets indicates possibly two hot spots on the star with an X-ray phase lag of phi=0.1 each. The details of this periodic variation of the X-rays are probed by displaying a phased and trailed X-ray spectrum and by constructing phased light curves for wavelength bands within the HETG spectral coverage, ranging down to bands encompassing groups of emission lines. We propose that the 1.78 day period is the stellar rotation period and explore how stellar bright spots and associated co-rotating interacting regions or CIRs could explain the modulation of the optical and X-ray output for this star and their phase difference., Comment: 17 pages; 10 figures

We attempt to determine the driver for clumping in hot-star winds by extending the measure of the spectral variability level of Galactic Wolf-Rayet stars to by far the hottest known among them, the WN2 star WR 2 and the WO2 stars WR 102 and WR 142. These three stars have T* = 140 kK and 200 kK, the last two being well above the bulk of WR stars with T* ~ 40-120 kK. This full temperature range for WR stars is much broader than that of their O-star progenitors (~30-50 kK), so is better suited to look for any temperature dependence of wind clumping. We have obtained multiple observations with high signal-to-noise, moderate-resolution spectroscopy in search of smallscale variability in the strong emission lines from the dense winds of these three extreme stars, and find a very low-level of variability in both stars. Temperature and terminal velocity are correlated, so faster winds show a lower variability, though this trend goes against any predictions made involving Line Deshadowing Instability (LDI) only, implying that instabilities intrinsic to LDI are not the main source of wind clumping. Instead, it could be taken as support for the suggestion that clumps are caused by a sub-surface convection zone (SSCZ) at T ~ 170 kK, since such an SSCZ would have little opportunity to operate under the hydrostatic surface of these hottest WR stars. It is still possible, however, that an SSCZ-related driver could interact with nonlinear line instability effects to enhance or possibly even produce clumps., Comment: 17 pages, 6 figures, 5 tables, ApJ, in press

WR 112 is a dust-forming carbon-rich Wolf-Rayet (WC) binary with a dusty circumstellar nebula that exhibits a complex asymmetric morphology, which traces the orbital motion and dust formation in the colliding winds of the central binary. Unraveling the complicated circumstellar dust emission around WR 112 therefore provides an opportunity to understand the dust formation process in colliding-wind WC binaries. In this work, we present a multi-epoch analysis of the circumstellar dust around WR 112 using seven high spatial resolution (FWHM $\sim0.3-0.4''$) N-band ($\lambda \sim12$ $\mu$m) imaging observations spanning almost 20 years and includes newly obtained images from Subaru/COMICS in Oct 2019. In contrast to previous interpretations of a face-on spiral morphology, we observe clear evidence of proper motion of the circumstellar dust around WR 112 consistent with a nearly edge-on spiral with a $\theta_s=55^\circ$ half-opening angle and a $\sim20$-yr period. The revised near edge-on geometry of WR 112 reconciles previous observations of highly variable non-thermal radio emission that was inconsistent with a face-on geometry. We estimate a revised distance to WR 112 of $d = 3.39^{+0.89}_{-0.84}$ kpc based on the observed dust expansion rate and a spectroscopically derived WC terminal wind velocity of $v_\infty= 1230\pm260$ km s$^{-1}$. With the newly derived WR 112 parameters we fit optically-thin dust spectral energy distribution models and determine a dust production rate of $\dot{M}_d=2.7^{+1.0}_{-1.3}\times10^{-6}$ M$_\odot$ yr$^{-1}$, which demonstrates that WR 112 is one of the most prolific dust-making WC systems known., Comment: 17 pages, 9 figures, 1 animated gif, accepted for publication in ApJ

Classical novae are thermonuclear explosions that occur on the surfaces of white dwarf stars in interacting binary systems (Bode & Evans 2008). It has long been thought that the luminosity of classical novae is powered by continued nuclear burning on the surface of the white dwarf after the initial runaway (Gallaher & Starrfield 1978). However, recent observations of GeV $\gamma$-rays from classical novae have hinted that shocks internal to the nova ejecta may dominate the nova emission. Shocks have also been suggested to power the luminosity of events as diverse as stellar mergers (Metzger & Pejcha 2017), supernovae (Moriya et al. 2018), and tidal disruption events (Roth et al. 2016), but observational confirmation has been lacking. Here we report simultaneous space-based optical and $\gamma$-ray observations of the 2018 nova V906 Carinae (ASASSN-18fv), revealing a remarkable series of distinct correlated flares in both bands. The optical and $\gamma$-ray flares occur simultaneously, implying a common origin in shocks. During the flares, the nova luminosity doubles, implying that the bulk of the luminosity is shock-powered. Furthermore, we detect concurrent but weak X-ray emission from deeply embedded shocks, confirming that the shock power does not appear in the X-ray band and supporting its emergence at longer wavelengths. Our data, spanning the spectrum from radio to $\gamma$-ray, provide direct evidence that shocks can power substantial luminosity in classical novae and other optical transients., Comment: 49 pages including supplementary information. Accepted and published in Nature Astronomy

We have obtained a very deep exposure (813 ks) of $\zeta\,$Puppis (O4 supergiant) with the Chandra/HETG Spectrometer. Here we report on analysis of the 1-9 \r{A} region, especially well suited for Chandra, which has a significant contribution from continuum emission between well separated emission lines from high-ionization species. These data allow us to study the hottest plasma present through the continuum shape and emission line strengths. Assuming a powerlaw emission measure distribution which has a high-temperature cut-off, we find that the emission is consistent with a thermal spectrum having a maximum temperature of 12 MK. This implies an effective wind shock velocity of $900\,\mathrm{km\,s^{-1}}$, well below the wind terminal speed of $2250\,\mathrm{km\,s^{-1}}$. For X-ray emission which forms close to the star, the speed and X-ray flux are larger than can be easily reconciled with strictly self-excited line-deshadowing-instability models, suggesting a need for a fraction of the wind to be accelerated extremely rapidly right from the base. This is not so much a dynamical instability as a nonlinear response to changing boundary conditions., Comment: 18 pages, 4 figures; accepted for publication in the Astrophysical Journal

Context. Be stars are physically complex systems that continue to challenge theory to understand their rapid rotation, complex variability and decretion disks. $\gamma$ Cassiopeiae ($\gamma$ Cas) is one such star but is even more curious because of its unexplained hard thermal X-ray emission. Aims. We aim to examine the optical variability of $\gamma$ Cas and thereby to shed more light on its puzzling behaviour. Methods. Three hundred twenty-one archival H$\alpha$ spectra from 2006 to 2017 are analysed to search for frequencies corresponding to the 203.5 day orbit of the companion. Space photometry from the SMEI satellite from 2003 to 2011 and the BRITE-Constellation of nano-satellites between 2015 and 2019 is investigated in the period range from a couple of hours to a few days. Results. The orbital period of the companion of 203.5 days is confirmed with independent measurements from the structure of the H$\alpha$ line emission. A strong blue/red asymmetry in the amplitude distribution across the H$\alpha$ emission line could hint at a spiral structure in the decretion disk. With the space photometry, the known frequency of 0.82 d$^{-1}$ is confirmed in data from the early 2000s. A higher frequency of 2.48 d$^{-1}$ is present in the data from 2015 to 2019 and possibly also in the early 2000s. A third frequency at 1.25 d$^{-1}$ is proposed to exist in both SMEI and BRITE data. The only explanation covering all three rapid variations seems to be nonradial pulsation. The two higher frequencies are incompatible with rotation., Comment: 13 pages, 12 figures. Accepted for publication in A&A

Some massive, merging black holes (BH) may be descendants of binary O stars. The evolution and mass transfer between these O stars determines the spins of their progeny BH. These will be measurable with future gravitational wave detectors, incentivizing the measurement of the spins of O stars in binaries. We previously measured the spins of O stars in Galactic Wolf-Rayet (WR) + O binaries. Here we measure the vsini of four LMC and two SMC O stars in WR + O binaries to determine whether lower metallicity might affect the spin rates. We find that the O stars in Galactic and Magellanic WR + O binaries display average vsini = 258 +/- 18 km/s and 270 +/- 15 km/s, respectively. Two LMC O stars measured on successive nights show significant line width variability, possibly due to differing orbital phases exhibiting different parts of the O stars illuminated differently by their WR companions. Despite this variability, the vsini are highly super-synchronous but distinctly subcritical for the O stars in all these binaries; thus we conclude that an efficient mechanism for shedding angular momentum from O stars in WR + O binaries must exist. This mechanism, probably related to Roche lobe overflow-created dynamo magnetic fields, prevents nearly 100% breakup spin rates, as expected when RLOF operates, as it must, in these stars. A Spruit-Tayler dynamo and O star wind might be that mechanism., Comment: 8 pages, 13 figures, accepted in MNRAS

Wolf-Rayet stars have strong, hot winds, with mass-loss rates at least a factor of ten greater than their O-star progenitors, although their terminal wind speeds are similar. In this paper we use the technique of multiband linear polarimetry to extract information on the global asymmetry of the wind in a sample of 47 bright Galactic WR stars. Our observations also include time-dependent observations of 17 stars in the sample. The path to our goal includes removing the dominating component of wavelength-dependent interstellar polarization (ISP), which normally follows the well-known Serkowski law. We include a wavelength-dependent ISP position angle parameter in our ISP law and find that 15 stars show significant results for this parameter. We detect a significant component of wavelength-independent polarization due to electron scattering in the wind for 10 cases, with most WR stars showing none at the $\sim$0.05% level precision of our data. The intrinsically polarized stars can be explained with binary interaction, large-scale wind structure, and clumping. We also found that 5 stars out of 19 observed with the Str\"omgren $b$ filter (probing the complex $\lambda$4600--4700 emission line region) have significant residuals from the ISP law and propose that this is due to binary illumination or wind clumping. We provide a useful catalogue of ISP for 47 bright Galactic WR stars and upper limits on the possible level of intrinsic polarization., Comment: 28 pages, 8 figures, 4 machine-readable tables, 1 figure set (47 images). Accepted in AJ

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 press

Observations of Beta Lyr in four months of 2018 by three BRITE Constellation satellites (the red-filter BTr and BHr, and the blue-filter BLb) permitted a first, limited look into the light-curve variability in two spectral bands. The variations were found to be well correlated outside the innermost primary minima with the blue variations appearing to have smaller amplitudes than the red; this reduction may reflect their presumed origin in the cooler, outer parts of the accretion disk. This result must be confirmed with more extensive material as the current conclusions are based on observations spanning slightly less than three orbital cycles of the binary. The assumption of an instrumental problem and the applied corrections made to explain the unexpectedly large amplitude of the red-filter light-curve observed with the BTr satellite in 2016 are fully confirmed by the 2018 results., Comment: 11 pages, 3 figures

Cosmic-ray acceleration has been a long-standing mystery and despite more than a century of study, we still do not have a complete census of acceleration mechanisms. The collision of strong stellar winds in massive binary systems creates powerful shocks, which have been expected to produce high-energy cosmic-rays through Fermi acceleration at the shock interface. The accelerated particles should collide with stellar photons or ambient material, producing non-thermal emission observable in X-rays and gamma-rays. The supermassive binary star eta Carinae drives the strongest colliding wind shock in the solar neighborhood. Observations with non-focusing high-energy observatories indicate a high energy source near eta Carinae, but have been unable to conclusively identify eta Carinae as the source because of their relatively poor angular resolution. Here we present the first direct focussing observations of the non-thermal source in the extremely hard X-ray band, which is found to be spatially coincident with the star within several arc-seconds. These observations show that the source of non-thermal X-rays varies with the orbital phase of the binary, and that the photon index of the emission is similar to that derived through analysis of the gamma-ray spectrum. This is conclusive evidence that the high-energy emission indeed originates from non-thermal particles accelerated at colliding wind shocks., Comment: 36 pages, 7 figures, 2 tables

We analyse time-series observations from the BRITE-Constellation of the well known $\beta$ Cephei type star $\theta$ Ophiuchi. Seven previously known frequencies were confirmed and nineteen new frequency peaks were detected. In particular, high-order g modes, typical for the SPB (Slowly Pulsating B-type star) pulsators, are uncovered. These low-frequency modes are also obtained from the 7-year SMEI light curve. If g modes are associated with the primary component of $\theta$ Oph, then our discovery allows, as in the case of other hybrid pulsators, to infer more comprehensive information on the internal structure. To this aim we perform in-depth seismic studies involving simultaneous fitting of mode frequencies, reproducing mode instability and adjusting the relative amplitude of the bolometric flux variations. To explain the mode instability in the observed frequency range a significant increase of the mean opacity in the vicinity of the $Z$-bump is needed. Moreover, constraints on mass, overshooting from the convective core and rotation are derived. If the low-frequency modes come from the speckle B5 companion then taking into account the effects of rotation is enough to explain the pulsational mode instability., Comment: 15 pages, 10 figures

The interacting binary Eta Carinae remains one of the most enigmatic massive stars in our Galaxy despite over four centuries of observations. In this work, its light curve from the ultraviolet to the near-infrared is analysed using spatially resolved HST observations and intense monitoring at the La Plata Observatory, combined with previously published photometry. We have developed a method to separate the central stellar object in the ground-based images using HST photometry and applying it to the more numerous ground-based data, which supports the hypothesis that the central source is brightening faster than the almost-constant Homunculus. After detrending from long-term brightening, the light curve shows periodic orbital modulation ($\Delta V$ $\sim$ 0.6 mag) attributed to the wind-wind collision cavity as it sweeps around the primary star and it shows variable projected area to our line-of-sight. Two quasi-periodic components with time scales of 2-3 and 8-10 yr and low amplitude, $\Delta V$ $<$ 0.2 mag, are superimposed on the brightening light-curve, being the only stellar component of variability found, which indicates minimal stellar instability. Moreover, the light curve analysis shows no evidence of `shell ejections' at periastron. We propose that the long-term brightening of the stellar core is due to the dissipation of a dusty clump in front of the central star, which works like a natural coronagraph. Thus, the central stars appear to be more stable than previously thought since the dominant variability originates from a changing circumstellar medium. We predict that the brightening phase, due mainly to dust dissipation, will be completed around 2032 $\pm$ 4 yr, when the star will be brighter than in the 1600's by up to $\Delta V$ $\sim$ 1 mag., Comment: 26 pages, 19 figures. Accepted to appear on MNRAS

The most violent strange mode pulsations (SMPs) are expected in classical WR stars, the bare, compact helium-burning cores of evolved massive stars, where SMPs are predicted to manifest themselves in cyclic photometric variability with periods ranging from minutes to hours and amplitudes in the several to tens of mmag range. Since WR\,2 is one of the hottest WR stars known, it should make it an excellent candidate for SMPs. However, our high-precision near infrared photometric campaign shows no periodic variations with an amplitude higher than 2.5 mmag on a timescale of ~6 hours, which is more than an order of magnitude smaller than the theoretical predictions., Comment: 3 pages, 1 figure, accepted in AAS Research Note

We briefly summarize the BRITE observations of classical Cepheids so far., Comment: presented at the 3rd BRITE Science Conference, 7-10 August 2017, Auberge du Lac Taureau, QC, Canada

Context. We investigate small-amplitude light variations in FU Ori occurring in timescales of days and weeks. Aims. We seek to determine the mechanisms that lead to these light changes. Methods. The visual light curve of FU Ori gathered by the MOST satellite continuously for 55 days in the 2013-2014 winter season and simultaneously obtained ground-based multi-colour data were compared with the results from a disc and star light synthesis model. Results. Hotspots on the star are not responsible for the majority of observed light variations. Instead, we found that the long periodic family of 10.5-11.4 d (presumably) quasi-periods showing light variations up to 0.07 mag may arise owing to the rotational revolution of disc inhomogeneities located between 16-20 solar radii. The same distance is obtained by assuming that these light variations arise because of a purely Keplerian revolution of these inhomogeneities for a stellar mass of 0.7 solar mass. The short-periodic (3-1.38 d) small amplitude (0.01 mag) light variations show a clear sign of period shortening, similar to what was discovered in the first MOST observations of FU Ori. Our data indicate that these short-periodic oscillations may arise because of changing visibility of plasma tongues (not included in our model), revolving in the magnetospheric gap and/or likely related hotspots as well. Conclusions. Results obtained for the long-periodic 10-11 d family of light variations appear to be roughly in line with the colour-period relation, which assumes that longer periods are produced by more external and cooler parts of the disc. Coordinated observations in a broad spectral range are still necessary to fully understand the nature of the short-periodic 1-3 d family of light variations and their period changes., Comment: Accepted to A&A

Stochastically-triggered photospheric light variations reaching $\sim$$40$ mmag peak-to-valley amplitudes have been detected in the O8Iaf supergiant V973 Scorpii as the outcome of two months of high-precision time-resolved photometric observations with the BRIght Target Explorer (BRITE) nanosatellites. The amplitude spectrum of the time series photometry exhibits a pronounced broad bump in the low-frequency regime ($\lesssim$$0.9$ d$^{-1}$) where several prominent frequencies are detected. A time-frequency analysis of the observations reveals typical mode lifetimes of the order of $5-10$ days. The overall features of the observed brightness amplitude spectrum of V973 Sco match well with those extrapolated from two-dimensional hydrodynamical simulations of convectively-driven internal gravity waves randomly excited from deep in the convective cores of massive stars. An alternative or additional possible source of excitation from a subsurface convection zone needs to be explored in future theoretical investigations., Comment: 17 pages, 12 figures, 3 tables; Monthly Notices of the Royal Astronomical Society (MNRAS), in press

Photometric instabilities of $\beta$ Lyr were observed in 2016 by two red-filter BRITE satellites over more than 10 revolutions of the binary, with $\sim$100-minute sampling. Analysis of the time series shows that flares or fading events take place typically 3 to 5 times per binary orbit. The amplitudes of the disturbances (relative to the mean light curve, in units of the maximum out-of-eclipse light-flux, f.u.) are characterized by a Gaussian distribution with $\sigma=0.0130\pm0.0004$ f.u. Most of the disturbances appear to be random, with a tendency to remain for one or a few orbital revolutions, sometimes changing from brightening to fading or the reverse. Phases just preceding the center of the deeper eclipse showed the most scatter while phases around secondary eclipse were the quietest. This implies that the invisible companion is the most likely source of the instabilities. Wavelet transform analysis showed domination of the variability scales at phase intervals $0.05-0.3$ (0.65--4 d), with the shorter (longer) scales dominating in numbers (variability power) in this range. The series can be well described as a stochastic Gaussian process with the signal at short timescales showing a slightly stronger correlation than red noise. The signal de-correlation timescale $\tau=(0.068\pm0.018)$ in phase or $(0.88\pm0.23)$~d appears to follow the same dependence on the accretor mass as that observed for AGN and QSO masses 5--9 orders of magnitude larger than the $\beta$~Lyr torus-hidden component., Comment: 28 pages, 10 figures, accepted by AJ: 3 May 2018

$\eta$ Car is a massive, eccentric binary with a rich observational history. We obtained the first high-cadence, high-precision light curves with the BRITE-Constellation nanosatellites over 6 months in 2016 and 6 months in 2017. The light curve is contaminated by several sources including the Homunculus nebula and neighboring stars, including the eclipsing binary CPD$-$59$^\circ$2628. However, we found two coherent oscillations in the light curve. These may represent pulsations that are not yet understood but we postulate that they are related to tidally excited oscillations of $\eta$ Car's primary star, and would be similar to those detected in lower-mass eccentric binaries. In particular, one frequency was previously detected by van Genderen et al. and Sterken et al. through the time period of 1974 to 1995 through timing measurements of photometric maxima. Thus, this frequency seems to have been detected for nearly four decades, indicating that it has been stable in frequency over this time span. These pulsations could help provide the first direct constraints on the fundamental parameters of the primary star if confirmed and refined with future observations., Comment: 8 pages, 4 figures, accepted to MNRAS

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&A

From $5.5$ months of dual-band optical photometric monitoring at the $1$ mmag level, BRITE-Constellation has revealed two simultaneous types of variability in the O4I(n)fp star $\zeta$ Puppis: one single periodic non-sinusoidal component superimposed on a stochastic component. The monoperiodic component is the $1.78$ d signal previously detected by Coriolis/SMEI, but this time along with a prominent first harmonic. The shape of this signal changes over time, a behaviour that is incompatible with stellar oscillations but consistent with rotational modulation arising from evolving bright surface inhomogeneities. By means of a constrained non-linear light curve inversion algorithm we mapped the locations of the bright surface spots and traced their evolution. Our simultaneous ground-based multi-site spectroscopic monitoring of the star unveiled cyclical modulation of its He II $\lambda4686$ wind emission line with the $1.78$-day rotation period, showing signatures of Corotating Interaction Regions (CIRs) that turn out to be driven by the bright photospheric spots observed by BRITE. Traces of wind clumps are also observed in the He II $\lambda4686$ line and are correlated with the amplitudes of the stochastic component of the light variations probed by BRITE at the photosphere, suggesting that the BRITE observations additionally unveiled the photospheric drivers of wind clumps in $\zeta$ Pup and that the clumping phenomenon starts at the very base of the wind. The origins of both the bright surface inhomogeneities and the stochastic light variations remain unknown, but a subsurface convective zone might play an important role in the generation of these two types of photospheric variability., Comment: 44 pages, 28 figures, 6 Tables; To appear in the Monthly Notices of the Royal Astronomical Society (MNRAS)

Photometric time series of the $\gamma$ Dor star 43 Cyg obtained with the BRITE-Constellation nano-satellites allow us to study its pulsational properties in detail and to constrain its interior structure. We aim to find a g-mode period spacing pattern that allows us to determine the near-core rotation rate of 43 Cyg and redetermine the star's fundamental atmospheric parameters and chemical composition. We conducted a frequency analysis using the 156-days long data set obtained with the BRITE-Toronto satellite and employed a suite of MESA/GYRE models to derive the mode identification, asymptotic period spacing and near-core rotation rate. We also used high-resolution, high signal-to-noise ratio spectroscopic data obtained at the 1.2m Mercator telescope with the HERMES spectrograph to redetermine the fundamental atmospheric parameters and chemical composition of 43 Cyg using the software Spectroscopy Made Easy (SME). We detected 43 intrinsic pulsation frequencies and identified 18 of them to be part of a period spacing pattern consisting of prograde dipole modes with an asymptotic period spacing $\Delta \Pi_{l=1}$ of $2970^{+700}_{-570} \rm s$. The near-core rotation rate was determined to be $f_{\rm rot} = 0.56^{+0.12}_{-0.14}\,\rm d^{-1}$. The atmosphere of 43 Cyg shows solar chemical composition at an effective temperature of 7150 $\pm$ 150 K, a log g of 4.2 $\pm$ 0.6 dex and a projected rotational velocity, $v {\rm sin}i$, of 44 $\pm$ 4 kms$^{-1}$. The morphology of the observed period spacing patterns shows indications of the presence of a significant chemical gradient in the stellar interior., Comment: 9 pages, 8 figures, accepted by A&A

We report on the first multi-color precision light curve of the bright Wolf-Rayet binary $\gamma^2$ Velorum, obtained over six months with the nanosatellites in the BRITE- Constellation fleet. In parallel, we obtained 488 high-resolution optical spectra of the system. In this first report on the datasets, we revise the spectroscopic orbit and report on the bulk properties of the colliding winds. We find a dependence of both the light curve and excess emission properties that scales with the inverse of the binary separation. When analyzing the spectroscopic properties in combination with the photometry, we find that the phase dependence is caused only by excess emission in the lines, and not from a changing continuum. We also detect a narrow, high-velocity absorption component from the He I $\lambda$5876 transition, which appears twice in the orbit. We calculate smoothed-particle hydrodynamical simulations of the colliding winds and can accurately associate the absorption from He I to the leading and trailing arms of the wind shock cone passing tangentially through our line of sight. The simulations also explain the general strength and kinematics of the emission excess observed in wind lines such as C III $\lambda$5696 of the system. These results represent the first in a series of investigations into the winds and properties of $\gamma^2$ Velorum through multi-technique and multi-wavelength observational campaigns., Comment: 16 pages, 14 figures, additional measurements to be included in online dataset. Accepted for publication in MNRAS

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.

Here we report the recovery of the binary underlying the classical nova of 11 March 1437 A.D. whose age is independently confirmed by proper motion-dating, and show that in the 20th century it exhibits dwarf nova eruptions. The four oldest recovered classical novae are now all dwarf novae. Taken together they strongly suggest that mass transfer rates decrease by an order of magnitude or more in the centuries after a classical nova event, greatly slowing the evolution, and lengthening the lifetimes of these explosive binary stars., Comment: 30 pages, 7 Figures