Orbital Cycles Research Articles

Environmental changes in Arctic Chukotka during Marine Isotope Stages 38‐31: implications for the Early to Middle Pleistocene transition

Lake El'gygytgyn in far northeastern Russia provides one of the few records that can improve understanding of terrestrial responses to climatic changes during the Early to Middle Pleistocene transition (EMPT; 1400–400 ka). Palynological data from MIS 38‐31 (~1260 to 1060 ka) help clarify the complex nature of that transition. In contrast to the evergreen conifer forest of the MIS 31 ‘super’ interglaciation, Larix‐Betula‐Alnus forest with shrub tundra characterized MIS 37 and MIS 35. The Larix forest‐tundra of MIS 33 represents an even cooler interglaciation. The landscapes of MIS 38, MIS 36 and early MIS 34 were a tundra mosaic with areas of Larix‐forest tundra. Although marine records suggest that MIS 38 and MIS 36 were exceptionally harsh glaciations, the decrease in woody taxa (particularly Alnus and Pinus) indicate the coolest conditions at El'gygytgyn occurred during MIS 32 and late MIS 34. When combined with other palynological data from El'gygytgyn, the EMPT portion of the record indicates that Betula was important in the vegetation during both cool and warm periods prior to MIS 24. Following the build‐up of global ice volumes ~900 ka, the taxon was characteristic of warm intervals only, marking the establishment of the herb‐dominated tundra, which typifies the remainder of the Pleistocene glaciations. The more complex successional patterns in post‐900 ka vegetation suggest greater climate variability as ice volume increased. Pinus pumila became a significant component of interglacial vegetation following the shift from a 41 ka to a quasi‐100 ka orbital cycle, indicating an increase in snow cover and a lessening of aridification, at least in this part of the Arctic.

Astronomically forced climate evolution in a saline lake record of the middle Eocene to Oligocene, Jianghan Basin, China

Following the early Eocene climatic optimum some 50 million years ago, the marine paleoclimate record indicates that a long interval of global cooling took place, punctuated by a series of short-term warming reversals and culminating with the glaciation of Antarctica at the Eocene-Oligocene transition. We investigate a saline lake record from the Qianjiang and Jinghezhen formations, Jianghan Basin, China for the corresponding continental climate response. A 20.25 Myr long astronomical time scale is constructed based on 405-kyr orbital eccentricity cycle tuning of gamma ray (GR) series measured from the Qianjiang and Jinghezhen formations. The halite-rich interval between 41-40.4 Ma correlates well with the Middle Eocene Climate Optimum recorded in the deep-sea δ18O record, confirming predominantly halite deposition during the warming. Silt-mudstone-rich intervals were deposited during ∼2.4 Myr orbital eccentricity minima. The evidence indicates a history of alternating fresh water (humid/cool) and saline water (dry/hot) lake cycles paced by ∼100 kyr orbital eccentricity cycles. Analysis of GR series from the deepest part of the lake indicates strong astronomical forcing of halite deposition throughout the Middle-Late Eocene, and a shift to siliciclastic deposition and cooler climates at ∼33.9 Ma. Early Oligocene wet/cool climate in the Jianghan Basin signals East Asian summer monsoon intensification, and a reduction from northwest winter monsoon in response to Tibetan Plateau uplift. Northern Hemisphere summer insolation minima in the Middle-Late Eocene with temperatures that are 4.6°C higher than present may provide an analogy for near-future climate change in Jianghan Basin.

Enhanced Mediterranean water cycle explains increased humidity during MIS 3 in North Africa

Abstract. We report a new fluid inclusion dataset from northeastern Libyan speleothem SC-06-01, which is the largest speleothem fluid inclusion dataset for North Africa to date. The stalagmite was sampled in Susah Cave, a low-altitude coastal site, in Cyrenaica, on the northern slope of the Jebel Al-Akhdar. Speleothem fluid inclusions from the latest Marine Isotope Stage (MIS) 4 and throughout MIS 3 (∼67 to ∼30 kyr BP) confirm the hypothesis that past humid periods in this region reflect westerly rainfall advected through the Atlantic storm track. However, most of this moisture was sourced from the western Mediterranean, with little direct admixture of water evaporated from the Atlantic. Moreover, we identify a second moisture source likely associated with enhanced convective rainfall within the eastern Mediterranean. The relative importance of the western and eastern moisture sources seems to differ between the humid phases recorded in SC-06-01. During humid phases forced by precession, fluid inclusions record compositions consistent with both sources, but the 52.5–50.5 kyr interval forced by obliquity reveals only a western source. This is a key result, showing that although the amount of atmospheric moisture advections changes, the structure of the atmospheric circulation over the Mediterranean does not fundamentally change during orbital cycles. Consequently, an arid belt must have been retained between the Intertropical Convergence Zone and the midlatitude winter storm corridor during MIS 3 pluvials.

Photometry of β Lyrae in 2018 by the BRITE Satellites

Abstract Observations of β Lyr in four months of 2018 by three BRIght Target Explorer (BRITE) Constellation satellites, the red-filter BRITE-Toronto and BRITE-Heweliusz, and the blue-filter BRITE-Lem, 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 phases of the primary eclipses 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 BRITE-Toronto satellite in 2016 are fully confirmed by the 2018 results.

Control chaos to different stable states for a piecewise linear circuit system by a simple linear control

Abstract In this paper, we mainly investigate chaos control of a piecewise linear circuit system. According to the characteristic of this system, we modify Hwang’s linear continuous controller and obtain a more simple controller consisting of two parts, by which we find from theory the extent of control parameter when chaotic motion is controlled to equilibrium manifold, equilibrium point, periodic orbit or limit cycle. Numerical simulation also verifies the method is effective.

The Likely Fermi Detection of the Be X-Ray Binary GRO J1008-57

Abstract In our search for γ-ray emission from Be X-ray binaries from the analysis of the data obtained with the Large Area Telescope onboard the Fermi Gamma-Ray Space Telescope, we find likely detection of GRO J1008−57. The binary has an orbital period of 249.48 days, and it is only significantly detected in its orbital phase 0.8–0.9 (>4σ). Further light-curve analysis indicates that the detection is probably largely due to an emitting event in one orbital cycle around year 2012–2013, following a giant X-ray outburst of the source. This property of having occasional γ-ray emitting events is similar to that seen in another high-mass X-ray binary (HMXB) 4U 1036−56. However, models considering possible γ-ray emission from an accreting neutron star have difficulty in explaining the observed ∼1034 erg s−1 luminosity of the source, unless the distance was largely overestimated. Further observational studies are required, in order to more clearly establish the high-energy emission properties of GRO J1008−57 or similar HMXBs and find clues for understanding how γ-ray emission is produced from them.

Wavelet-based verification of a relative paleointensity record from the North Pacific

We present a relative paleointensity (RPI) record for the last ~ 1.1 Myr estimated from a sediment core in the central North Pacific, with quality verification using wavelet analysis. Rock magnetic analysis reveals that a stable remanence is carried mainly by single-domain (SD) biogenic magnetite and pseudo-SD detrital magnetite and that concentration- and grain-size-related bulk magnetic parameters vary by a factor of 3, initially meeting a conventional standard for RPI estimation. However, a further test using wavelet spectra of RPI proxies normalized by anhysteretic remanent magnetization (ARM) or isothermal remanent magnetization (IRM) shows intermittent orbital contamination at period of 100 kyr. A part of the 100-kyr orbital frequency in ARM-normalized RPI has a coherence and physical relationship with the normalizer. For the same time interval, a prominent 100-kyr cycle in the ARM/IRM wavelet spectra is also coherent with RPI, indicating that relative changes in biogenic and detrital magnetite were not well compensated by ARM normalization. For the selected RPI proxy using IRM normalization, a significant physical relationship with the lithology of magnetic minerals was not detected in the wavelet analysis, and thus, its intermittent orbital cycles could be of climatic origin, probably induced by non-magnetic factors. Nevertheless, our RPI record is consistent with global RPI stacks and provides a successful reconstruction of paleointensity with geomagnetic field origin in the North Pacific.

Integrated bio- and cyclostratigraphy of Middle Triassic (Anisian) ramp deposits, NW Bulgaria

Abstract A cyclostratigraphic interpretation of peritidal to shallow-marine ramp deposits of the early Middle Triassic (Anisian) Opletnya Member exposed in outcrops along the Iskar River gorge, NW Bulgaria, is presented. Based on facies trends and bounding surfaces, depositional sequences of several orders can be identified. New biostratigraphic data provide a time frame of the studied succession with placement of the boundaries of the Anisian substages and show that the Aegean (early Anisian) substage lasted about 1.6 Myr. In the corresponding interval in the two studied sections, 80 elementary sequences are counted. Five elementary sequences compose a small-scale sequence. The prominent cyclic pattern of the Opletnya Member can thus be interpreted in terms of Milankovitch cyclicity: elementary sequences represent the precession (20-kyr) cycle and small-scale sequences the short eccentricity (100-kyr) cycle in the Milankovitch frequency band. Medium-scale sequences are defined based on lithology but only in two cases can be attributed to the long eccentricity cycle of 405 kyr. The transgressive-regressive facies trends within the sequences of all scales imply that they were controlled by sea-level changes, and that these were in tune with the climate changes induced by the orbital cycles. However, the complexity of facies and sedimentary structures seen in the Opletnya Member also implies that additional factors such as lateral migration of sediment bodies across the ramp were active. In addition, three major sequence boundaries have been identified in the studied sections, which can be correlated with the boundaries Ol4, An1, and An2 of the Tethyan realm.

Detection of Strong Precession Cycles from the Late Pliocene Sedimentary Records of Northeastern Tibetan Plateau

AbstractRecent studies demonstrate that aridification is sensitive to eccentricity forcing based on the northeastern Tibetan Plateau Cenozoic fluvio‐lacustrine stratigraphic records. However, it is unknown whether this is a bias associated with the fact that higher frequency periods normally have larger age model uncertainties, which tend to decrease spectral power of higher frequency orbital cycles (precession and obliquity) and enlarge lower frequency ones (eccentricity). Here we detect strong and well‐resolved (~23 and ~19 kyr) precession cycles in the magnetic susceptibility record of late Pliocene fluvio‐lacustrine sediments from the Qaidam Basin, northwestern China. We interpret the magnetic susceptibility as a proxy for the degree of chemical weathering in this region, based on the positive correlation between low‐frequency magnetic susceptibility and frequency‐dependent magnetic susceptibility. The different responses of magnetic susceptibility and a halite content proxy to orbital forcing suggest that aridification is more sensitive to eccentricity forcing than precipitation, but precipitation is more sensitive to precession forcing. This study indicates that different climatic aspects have different sensitivity to orbital forcing. Furthermore, a comparison of the magnetic susceptibility variations in the western Qaidam Basin and global ice volume proxy records suggests ice volume control of Qaidam Basin precipitation and chemical weathering at million‐year timescales.

Orbital controls on Namib Desert hydroclimate over the past 50,000 years

Abstract Despite being one of the world’s oldest deserts, and the subject of decades of research, evidence of past climate change in the Namib Desert is extremely limited. As such, there is significant debate regarding the nature and drivers of climate change in the low-latitude drylands of southwestern Africa. Here we present data from stratified accumulations of rock hyrax urine that provide the first continuous high-resolution terrestrial climate record for the Namib Desert spanning the past 50,000 yr. These data, spanning multiple sites, show remarkably coherent variability that is clearly linked to orbital cycles and the evolution and perturbation of global boundary conditions. Contrary to some previous predictions of southwestern African climate change, we show that orbital-scale cycles of hydroclimatic variability in the Namib Desert region are in phase with those of the northern tropics, with increased local summer insolation coinciding with periods of increased aridity. Supported by climate model simulations, our analyses link this to variations in position and intensity of atmospheric pressure cells modulated by hemispheric and land-sea temperature gradients. We conclude that hydroclimatic variability at orbital time scales is driven by the combined influence of direct low-latitude insolation forcing and the influence of remote controls on the South Atlantic anticyclone, with attendant impacts on upwelling and sea-surface temperature variations.

CRTS J035010.7 + 323230, a new eclipsing polar in the cataclysmic variable period gap

ABSTRACT We report the discovery of a new eclipsing polar, CRTS J035010.7+323230 (hereafter CRTS J0350+3232). We identified this cataclysmic variable (CV) candidate as a possible polar from its multiyear Catalina Real-Time Transient Survey (CRTS) optical light curve. Photometric monitoring of 22 eclipses in 2015 and 2017 was performed with the 2.1-m Otto Struve Telescope at McDonald Observatory. We derive an unambiguous high-precision ephemeris. Strong evidence that CRTS J0350 + 3232 is a polar comes from optical spectroscopy obtained over a complete orbital cycle using the Apache Point Observatory 3.5-m telescope. High velocity Balmer and He ii λ4686Å emission-line equivalent width ratios, structures, and variations are typical of polars and are modulated at the same period, 2.37 h (142.3 min), as the eclipse to within uncertainties. The spectral energy distribution and luminosity is found to be comparable to that of AM Herculis. Pre-eclipse dips in the light curve show evidence for stream accretion. We derive the following tentative binary and stellar parameters assuming a helium composition white dwarf and a companion mass of 0.2 M⊙: inclination i = 74.68° ± 0.03°, semimajor axis a = 0.942 ± 0.024 R⊙, and masses and radii of the white dwarf and companion, respectively: M1 = 0.948 $^{+0.006}_{-0.012}$ M⊙, R1 = 0.00830 $^{+0.00012}_{-0.00006}$ R⊙, and R2 = 0.249 ± 0.002 R⊙. As a relatively bright (V ∼ 17–19 mag), eclipsing, period-gap polar, CRTS J0350 + 3232 will remain an important laboratory for the study of accretion and angular momentum evolution in polars.

Oscillations of global sea-level elevation during the Paleogene correspond to 1.2-Myr amplitude modulation of orbital obliquity cycles

The mechanisms involved in the formation of 1- to 2-Myr-long third-order stratigraphic sequences through the Phanerozoic have been extensively debated, but the main underlying forcings remain uncertain. In this study, we investigated third-order sequences of Paleocene-Eocene age in the East China Sea Shelf Basin, identifying a prominent ∼1.2-Myr periodicity in the 405-kyr-tuned gamma ray profile of the WP-1 borehole. We infer that this ∼1.2-Myr cycle, which corresponds to long-period amplitude modulation of obliquity, was the main driver of third-order sea-level changes and sequence development. This modulation entails variation in the range of obliquity amplitude between a maximum of 2.4° (i.e. 24.5°–22.1°) and a minimum of 0.3° (i.e. 23.3°–23.0°) as a result of interactions among the three primary parameters of Earth's orbit (i.e., eccentricity, obliquity, and precession). We anchored our floating astronomical time scale (ATS) to the T42 seismic horizon, a major sequence boundary between the Lin Feng and Mingyuefeng formations that corresponds to the top of Nannofossil Zone NP8 (reported age of 57.7 Ma). This yields a fixed ATS for the late Paleocene-to-middle Eocene interval spanning 60.84 to 48.71 Ma. Finally, we evaluated our findings relative to the third-order sequence stratigraphy of the East China Sea Shelf Basin and to records of early Cenozoic eustasy and global events.

Isotopic records of climate seasonality in equid teeth

We investigate how oxygen isotopes in equid teeth can be used as a record of seasonality. First, we use in situ laser ablation and conventional microsampling techniques to understand time-averaging of environmental signals in intra-tooth isotope profiles in modern feral horse teeth (n = 5) from Mongolia, where there is a large seasonal gradient in the oxygen isotopic composition of precipitation. We demonstrate that laser ablation can be used to sample inner, middle, and outer enamel layers in large mammalian herbivore teeth. The inner enamel layer records less attenuated isotopic signals than other layers, as predicted by the mineralization patterns, but intra-tooth signal amplitude is similar for laser and conventional sampling methods. Second, we use modern zebra teeth (n = 21) collected in eastern Africa to evaluate how intra-tooth oxygen isotope variation relates to rainfall patterns in the tropics. We show that the intra-tooth isotopic range increases with intra-annual precipitation isotopic range, which in turn relates to aridity in the equatorial bimodal rainfall region but is influenced by other hydroclimate processes in the region as a whole. Finally, we address isotopic seasonality during the Early Pleistocene in eastern Africa using oxygen isotopes in fossil equid teeth from southwestern Kenya (n = 11) and northern Tanzania (n = 5). We find variable isotopic seasonality in the past, similar to present-day eastern Africa, consistent with the notion that hominins and other mammals were able to accommodate environmental variability on intra-annual scales in addition to well-documented orbital cycles.

Orbital control on cyclical organic matter accumulation in Early Silurian Longmaxi Formation shales

High resolution (939 samples) total organic carbon content (TOC) analyses were conducted on the Shuanghe Section of ∼152.6 ​m in the Changning area, Sichuan Basin. The sampling section was divided into two units considering the distinct-different deposit environment and sediments accumulation rate. The lower part (Unit 1) and the peer part (Unit 2) with high resolution sample spacing (0.08–0.4 ​m) enables the identification of the precession cycle in two sedimentary sequences with distinct different sedimentary accumulation rate. MTM Power spectral analyses on untuned TOC series reveals significant peaks exceeding above the 95% confidence level and shows that both Unit 1 and Unit 2 have recorded Milankovitch cycles of 405 kyr long eccentricity, short eccentricity, obliquity and precession. The floating astronomical time scale (ATS) was constructed on the Shuanghe Section in the Early Silurian (∼439.673–444.681 ​Ma), and which was calibrated by 405 kyr long eccentricity cycles. The total duration of the Wufeng and Longmaxi shales is 5.01 Myr. The floating ATS used for estimating the duration of the graptolite zones and each stage in the study interval. Finally, we postulated two models that could verify the linkage between orbital cycle and organic accumulation. To make sure whether productivity or preservation is the main factor that under long eccentricity control, the phase correlation between the obtained filtered signal and the theoretical orbital solution should be made clear in the further research.

Second Outburst of the Yellow Symbiotic Star LT Delphini

We present the results of our photoelectric $UBV$ observations of the yellow symbiotic star LT Del over 2010-2018. The binary system LT Del, which consists of a bright K giant and a compact hot star with a temperature of $\sim$100 000 K, has an orbital period of 476 days. In 2017 the variable experienced a second low-amplitude ($\Delta V\sim 0.^{m}7$) outburst in the history of its studies whose maximum occurred at an orbital phase of 0.15$\pm$0.05. The outburst duration was $\sim$60 days. The $B-V$ and $U-B$ colors in the outburst became noticeably bluer. A difference in the photometric behavior of the star in the 1994 and 2017 outburst has been detected. In the orbital cycle preceding the 2017 outburst a secondary minimum with a depth of $0.^{m}15$ and $0.^{m}20$ appeared in the $B$ and $V$ light curves, respectively, whose cause is discussed. The phase light and color curves are presented and explained; the position of the star on the color-color diagram is interpreted. We have estimated the parameters of the cool and hot components of the system based on the distance determination from Gaia DR2.

Geomusic as a New Pedagogical and Outreach Resource: Interpreting Geoheritage with All the Senses

The scientific, rational approach to the knowledge of Earth can be complemented and enhanced with an emotional approach by means of arts. Sonification of sedimentary series, by converting distinct lithology, facies or geochemical parameters into notes, and bed thickness into duration of sounds, provides a new viewpoint on both their sequential features and the cultural meaning of geoheritage. A total of 14 musical compositions have been achieved according to that procedure, based on successions of diverse ages and sedimentary environments within the Iberian Peninsula. Some of these successions exhibit cyclic features that have been analyzed by a number of authors. Cyclostratigraphy shows how certain sedimentary patterns can reveal climatic oscillations related to periodic variations of Earth orbital cycles. Geomusic elaborated from sonification of such sedimentary cycles could be therefore linked with Music of the Spheres postulated by Pythagoras in ancient Greece. Its hidden message deals with asking for a New Culture of Earth, for a renewed, friendly relationship with our planet. Its applied development could extend to soundtracks of scientific documentaries, background music at museums or geoparks, or performances at outreach events, or as a motivating factor in Earth Sciences learning.

Updated orbital ephemeris of the ADC source X 1822-371: a stable orbital expansion over 40 years

Aims. Source X 1822-371 is an eclipsing compact binary system with a period close to 5.57 h and an orbital period derivative Ṗorb of 1.51(7)×10−10 s s−1. The very high value of Ṗorb is compatible with a super-Eddington mass transfer rate from the companion star, as suggested by X-ray and optical data. The XMM-Newton observation taken in 2017 allows us to update the orbital ephemeris and verify whether the orbital period derivative has been stable over the past 40 yr. Methods. We added two new values obtained from the Rossi-XTE (RXTE) and XMM-Newton observations performed in 2011 and 2017, respectively, to the X-ray eclipse arrival times from 1977 to 2008. We estimated the number of orbital cycles and the delays of our eclipse arrival times spanning 40 yr, using as reference time the eclipse arrival time obtained from the RXTE observation taken in 1996. Results. Fitting the delays with a quadratic model, we found an orbital period Porb = 5.57062957(20) h and a Ṗorb value of 1.475(54)×10−10 s s−1. The addition of a cubic term to the model does not significantly improve the fit quality. We also determined a spin-period value of Pspin = 0.5915669(4) s and its first derivative Ṗspin = − 2.595(11) × 10−12 s s−1. Conclusions. Our results confirm the scenario of a super-Eddington mass transfer rate; we also exclude a gravitational coupling between the orbit and the change in the oblateness of the companion star triggered by the nuclear luminosity of the companion star.

Modelling the periodical variations in multiband polarization and photometry for discs of binary Be stars

The tidal interaction of a Be star with a binary companion forms two spiral arms that cause orbital modulation of the Be disc structure. The aim of this work is to identify observables in which this modulation is apparent. The structure of a Be disc in a coplanar circular binary system is computed with a smoothed-particle hydrodynamics code, and a radiation transfer code calculates the spectral energy distribution. Line depolarisation was confirmed, with polarisation profiles nearly reverse to emission-line profiles. The continuum flux maximizes for pole-on discs, but photometric variability maximizes for edge-on discs. The linear polarisation exhibits one or two maxima per orbital cycle. While polarisation variability in visible passbands is important only at low inclinations, infrared bands may demonstrate high orbital variability even at large inclinations. More evident is the modulation in the polarisation angle (PA) for low inclinations. The latter can be used to track azimuthal asymmetries for pole-on discs, where the spectroscopic variability in the violet-to-red (V/R) emission-component ratio disappears. PA reversals coincide with phases where V/R=1, tracking lines of sight directed towards regions where the approaching and receding arms overlap. Continuum flux and polarisation are mostly in phase for neighbouring wavelength regions. It is suggested that studies of non-symmetric discs distorted by tidal forces from a secondary star may be used to study disc variabilities of other origins.

Facies and depositional environments of the Upper Muschelkalk (Schinznach Formation, Middle Triassic) in northern Switzerland

Subsurface sedimentary strata in northern Switzerland, such as the Middle Triassic Upper Muschelkalk, are attracting interest as potential reservoirs for CO2 sequestration and for geothermal energy production. Characterizing facies in such strata aids prediction of reservoir properties in unexplored areas. Although well studied elsewhere, the Swiss Upper Muschelkalk has received little attention despite containing the southern-most deposits of the Central European Basin. The Upper Muschelkalk represents the deposits of a storm-dominated, homoclinal carbonate ramp, developed during a basin-wide 3rd-order transgressive–regressive cycle. Our facies analyses of nine boreholes across northern Switzerland reveal 12 lithofacies, eight lithofacies associations and four types of metre-scale 5th-order cycles corresponding to at least 23 short orbital eccentricity cycles. During the 3rd-order transgression, crinoidal bioherms developed across Switzerland followed by deep-ramp environments. Subsequently, tempestites were deposited up to and after the basin-wide maximum flooding surface. Lateral tempestite correlations indicate that Switzerland lay within an open-marine, mid-ramp environment during almost half of the depositional history. Mid-ramp deposits pass upwards to prograding shelly shoals, which sheltered a back-shoal lagoon containing patchy oolitic shoals. At the top of the Upper Muschelkalk, back-shoal sediments give way to coastal sabkha facies, which were overlain by oolitic shoals during a marine transgression. Shortly thereafter the top of the Upper Muschelkalk was dolomitized by brines from an overlying hypersaline environment that was later removed by a basin-wide erosive event. Overall, the paucity of porous shoal facies, unlike in southern Germany, has resulted in poor primary reservoir properties in the Upper Muschelkalk of Switzerland.

Long uninterrupted photometric observations of the Wolf-Rayet star EZ CMa by the Toronto BRITE satellite reveal a very fast apsidal motion

Context. The variability of the Wolf-Rayet star EZ CMa has been documented for close to half a century, and a clear periodicity of ∼3.7 days is established. However, all attempts to prove that it is a binary have failed because the photometric, spectroscopic, and polarimetric variations are not coherent over more than a few orbital cycles. Aims. In this Letter we show that the lack of coherence in the variability can be explained with a very rapid apsidal motion in a binary orbit. Methods. We measured the times of minima in a recently published exceptionally long photometric light curve obtained by the Toronto BRITE satellite. The apsidal motion and the system eccentricity are determined from the length of the time intervals between these minima, which alternate in their duration, following a pattern that is clearly associated with apsidal motion. These minima are superposed on brightness enhancements of the emission from a shock zone, which occur at about the times of periastron phases. Results. We determine the orbital periodicity, Pa = 3.63 d, and the period of the apsidal motion, U ≃ 100 d, which together yield an average sidereal period of Ps = 3.77 d. The eccentricity is found to be close to 0.1. The rate of periapsis retreat changes significantly over the period of observation and is determined to be −16° P−1a at the beginning of the observing period and −10° P−1a at the end. Conclusions. We demonstrate that by introducing a fast apsidal motion, the basic photometric variability is very well explained. The binary nature of EZ CMa is now established. This might imply that other apparently single Wolf-Rayet stars that emit hard X-rays, similar to EZ CMa, are also binaries.