Dimming Events Research Articles

Young Exoplanet Transit Initiative follow-up observations of the T Tauri star CVSO 30 with transit-like dips

ABSTRACT The T Tauri star CVSO 30, also known as PTFO 8-8695, was studied intensively with ground-based telescopes as well as with satellites over the last decade. It showed a variable light curve with additional repeating planetary transit-like dips every ∼10.8 h. However, these dimming events changed in depth and duration since their discovery and from autumn 2018 on, they were not even present or near the predicted observing times. As reason for the detected dips and their changes within the complex light curve, e.g. a disintegrating planet, a circumstellar dust clump, stellar spots, possible multiplicity, and orbiting clouds at a Keplerian co-rotating radius were discussed and are still under debate. In this paper, we present additional optical monitoring of CVSO 30 with the meter class telescopes of the Young Exoplanet Transit Initiative in Asia and Europe over the last 7 yr and characterize CVSO 30 with the new Early Data Release 3 of the European Space Agency-Gaia mission. As a result, we describe the evolution of the dimming events in the optical wavelength range since 2014 and present explanatory approaches for the observed variabilities. We conclude that orbiting clouds of gas at a Keplerian co-rotating radius are the most promising scenario to explain most changes in CVSO 30’s light curve.

Scattering and sublimation: a multiscale view of µm-sized dust in the inclined disc of HD 145718

ABSTRACT We present multi-instrument observations of the disc around the Herbig Ae star, HD 145718, employing geometric and Monte Carlo radiative transfer models to explore the disc orientation, the vertical and radial extent of the near-infrared (NIR) scattering surface, and the properties of the dust in the disc surface and sublimation rim. The disc appears inclined at 67–71°, with position angle, PA = −1.0 to 0.6°, consistent with previous estimates. The NIR scattering surface extends out to ${\sim}75\,$ au and we infer an aspect ratio, hscat(r)/r ∼ 0.24 in J band; ∼0.22 in H band. Our Gemini Planet Imager images and VLTI + CHARA NIR interferometry suggest that the disc surface layers are populated by grains ≳λ/2π in size, indicating these grains are aerodynamically supported against settling and/or the density of smaller grains is relatively low. We demonstrate that our geometric analysis provides a reasonable assessment of the height of the NIR scattering surface at the outer edge of the disc and, if the inclination can be independently constrained, has the potential to probe the flaring exponent of the scattering surface in similarly inclined (i ≳ 70°) discs. In re-evaluating HD 145718’s stellar properties, we found that the object’s dimming events – previously characterized as UX Or and dipper variability – are consistent with dust occultation by grains larger, on average, than found in the ISM. This occulting dust likely originates close to the inferred dust sublimation radius at $0.17\,$ au.

Stars on the Verge: Analyses of the Complex Light Variations of the Hyper-luminous Red Supergiant VY Canis Majoris: On the Nature of the Star’s “Great Dimming” Episodes

Abstract VY Canis Majoris (VY CMa) is one of the largest, most luminous red supergiant (RSG) stars in the Galaxy. This huge oxygen-rich RSG may be on the verge of becoming a bright core-collapse Type-II supernova. Over the 20th century, VY CMa has been undergoing episodic large quasi-periodic light variations. It has experienced two “great dimming” episodes during ∼1920–1950 and ∼1985–1995. During these, the star underwent large (1–2.5 mag) quasi-periodic light variations with dominant periods ranging between ∼1415 and 1565 days. In addition to experiencing large quasi-periodic light variations, VY CMa became fainter by 1.0–1.5 mag during the 1920–1950 episode. There is evidence that the star’s brightness decreased from ∼6.5–7.0 to ∼8.0–8.5 mag from 1850 to after 1900. These long-term dimming events may arise from dust formation from previously expelled gas. The light variations and the “dimmings” of VY CMa far surpass the recent “great dimming” of Betelgeuse.

Highly Structured Inner Planetary System Debris around the Intermediate Age Sun-like Star TYC 8830 410 1

Abstract We present a detailed characterization of the extremely dusty main-sequence star TYC 8830 410 1. This system hosts inner planetary system dust (T dust ≈ 300 K) with a fractional infrared luminosity of ∼1%. Mid-infrared spectroscopy reveals a strong, mildly crystalline solid-state emission feature. TYC 8830 410 1 (spectral type G9 V) has a 49.5″ separation M4-type companion comoving and co-distant with it, and we estimate a system age of ∼600 Myr. TYC 8830 410 1 also experiences “dipper”-like dimming events as detected by the All-Sky Automated Survey for Supernovae, Transiting Exoplanet Survey Satellite, and characterized in more detail with the Las Cumbres Observatory Global Telescope. These recurring eclipses suggest at least one roughly star-sized cloud of dust orbits the star in addition to assorted smaller dust structures. The extreme properties of the material orbiting TYC 8830 410 1 point to dramatic dust-production mechanisms that likely included something similar to the giant impact event thought to have formed the Earth–Moon system, although hundreds of millions of years after such processes are thought to have concluded in the solar system. TYC 8830 410 1 holds promise to deliver significant advances in our understanding of the origin, structure, and evolution of extremely dusty inner planetary systems.

Atmosphere of Betelgeuse before and during the Great Dimming event revealed by tomography

Context. Despite being the best studied red supergiant star in our Galaxy, the physics behind the photometric variability and mass loss of Betelgeuse is poorly understood. Moreover, recently the star has experienced an unusual fading with its visual magnitude reaching a historical minimum. The nature of this event was investigated by several studies where mechanisms, such as episodic mass loss and the presence of dark spots in the photosphere, were invoked. Aims. We aim to relate the atmospheric dynamics of Betelgeuse to its photometric variability, with the main focus on the dimming event. Methods. We used the tomographic method which allowed us to probe different depths in the stellar atmosphere and to recover the corresponding disk-averaged velocity field. The method was applied to a series of high-resolution HERMES observations of Betelgeuse. Variations in the velocity field were then compared with photometric and spectroscopic variations. Results. The tomographic method reveals that the succession of two shocks along our line-of-sight (in February 2018 and January 2019), the second one amplifying the effect of the first one, combined with underlying convection and/or outward motion present at this phase of the 400 d pulsation cycle, produced a rapid expansion of a portion of the atmosphere of Betelgeuse and an outflow between October 2019 and February 2020. This resulted in a sudden increase in molecular opacity in the cooler upper atmosphere of Betelgeuse and, thus, in the observed unusual decrease of the star’s brightness.

An Asymmetric Eclipse Seen toward the Pre-main-sequence Binary System V928 Tau

Abstract K2 observations of the weak-lined T Tauri binary V928 Tau A and B show the detection of a single, asymmetric eclipse, which may be due to a previously unknown substellar companion eclipsing one component of the binary with an orbital period >66 days. Over an interval of about 9 hr, one component of the binary dims by around 60%, returning to its normal brightness about 5 hr later. From modeling of the eclipse shape, we find evidence that the eclipsing companion may be surrounded by a disk or a vast ring system. The modeled disk has a radius of 0.9923 ± 0.0005 R *, with an inclination of 56.°78 ± 0.°03, a tilt of 41.°22 ± 0.°05, an impact parameter of −0.2506 ± 0.0002 R *, and an opacity of 1.00. The occulting disk must also move at a transverse velocity of 6.637 ± 0.002 R * day−1, which, depending on whether it orbits V928 Tau A or B, corresponds to approximately 73.53 or 69.26 km s−1. A search in ground-based archival data reveals additional dimming events, some of which suggest periodicity, but no unambiguous period associated with the eclipse observed by K2. We present a new epoch of astrometry that is used to further refine the orbit of the binary, presenting a new lower bound of 67 yr, and constraints on the possible orbital periods of the eclipsing companion. The binary is also separated by 18″ (∼2250 au) from the lower-mass CFHT-BD-Tau 7, which is likely associated with V928 Tau A and B. We also present new high-dispersion optical spectroscopy that we use to characterize the unresolved stellar binary.

Revealing the Spectroscopic Variations of FU Orionis Object V960 Mon with High-resolution Spectroscopy

Abstract We present the results of the high-resolution spectroscopy of the FU Orionis-type star V960 Mon. The brightness of V960 Mon decreased continuously after the outburst was detected in 2014 November. During this dimming event, we carried out medium-resolution spectroscopic monitoring observations and found that the equivalent width of the absorption features showed variations. To further investigate the spectroscopic variations, we conducted a high-resolution spectroscopic observation of V960 Mon with the Subaru Telescope and the High Dispersion Spectrograph on 2018 January 8 and 2020 February 1. By comparing this spectrum with the archival data of the Keck Observatory and the High Resolution Echelle Spectrometer taken between 2014 and 2017, we found that the absorption profiles changed as V960 Mon faded. The line profile of absorption lines such as Fe i and Ca i can be explained by a sum of the spectra of the disk atmosphere and the central star. The model spectrum created to explain the variations of the line profiles suggests that the effective temperature of the central star is ∼5500 K, which is comparable to that of the pre-outburst phase with a distance of 1.6 kpc with Gaia. The spectrum also shows that the effective temperature of the disk atmosphere decreased as V960 Mon faded. The variations of the Hα and Ca ii lines (8498.0, 8542.1 Å) also show that the V960 Mon spectrum became central-star dominant.

Standing on the Shoulders of Giants: New Mass and Distance Estimates for Betelgeuse through Combined Evolutionary, Asteroseismic, and Hydrodynamic Simulations with MESA

Abstract We conduct a rigorous examination of the nearby red supergiant Betelgeuse by drawing on the synthesis of new observational data and three different modeling techniques. Our observational results include the release of new, processed photometric measurements collected with the space-based Solar Mass Ejection Imager instrument prior to Betelgeuse’s recent, unprecedented dimming event. We detect the first radial overtone in the photometric data and report a period of 185 ± 13.5 days. Our theoretical predictions include self-consistent results from multi-timescale evolutionary, oscillatory, and hydrodynamic simulations conducted with the Modules for Experiments in Stellar Astrophysics software suite. Significant outcomes of our modeling efforts include a precise prediction for the star’s radius: . In concert with additional constraints, this allows us to derive a new, independent distance estimate of pc and a parallax of mas, in good agreement with Hipparcos but less so with recent radio measurements. Seismic results from both perturbed hydrostatic and evolving hydrodynamic simulations constrain the period and driving mechanisms of Betelgeuse’s dominant periodicities in new ways. Our analyses converge to the conclusion that Betelgeuse’s ≈400 day period is the result of pulsation in the fundamental mode, driven by the κ-mechanism. Grid-based hydrodynamic modeling reveals that the behavior of the oscillating envelope is mass-dependent, and likewise suggests that the nonlinear pulsation excitation time could serve as a mass constraint. Our results place α Orionis definitively in the early core helium-burning phase of the red supergiant branch. We report a present-day mass of 16.5–19 M ⊙—slightly lower than typical literature values.

Possible Time Correlation between Jet Ejection and Mass Accretion for RW Aur A*

Abstract For the active T-Taur star RW Aur A we have performed long-term (∼10 yr) monitoring observations of (1) jet imaging in the [Fe II] 1.644 μm emission line using Gemini-NIFS and VLT-SINFONI; (2) optical high-resolution spectroscopy using CFHT-ESPaDOnS; and (3) V-band photometry using the CrAO 1.25-m telescope and AAVSO. The latter two observations confirm the correlation of time variabilities between (A) the Ca II 8542 Å and O I 7772 Å line profiles associated with magnetospheric accretion, and (B) optical continuum fluxes. The jet images and their proper motions show that four knot ejections occurred at the star over the past ∼15 yr with an irregular interval of 2–6 yr. The timescale and irregularity of these intervals are similar to those of the dimming events seen in the optical photometry data. Our observations show a possible link between remarkable (ΔV < −1) photometric rises and jet knot ejections. Observations over another few years may confirm or reject this trend. If confirmed, this would imply that the location of the jet launching region is very close to the star (r ≲ 0.1 au) as predicted by some jet launching models. Such a conclusion would be crucial for understanding disk evolution within a few astronomical units of the star, and therefore possible ongoing planet formation at these radii.

RW Aur B: a modest UX Ori-type companion of the famous primary

ABSTRACT The secondary of the famous young binary RW Aur is much less studied than the primary. To rectify this shortcoming, we present here the results of UBVRIJHK photometric, VRI polarimetric and optical spectral observations of RW Aur B. The star demonstrates chaotic brightness variations in the optical band, with irregular, short (∼1-d) dimmings with an amplitude ΔV up to 1.3 mag. The dimmings are accompanied by an increase in the linear polarization (up to 3 per cent in the I band), presumably as a result of the scattering of stellar radiation by dust in the circumstellar disc, which means that RW Aur B can be classified as a UX Ori-type star. We conclude that the observed excess emission at λ ≲ 0.45 μm and longwards of ~2 μm as well as the variability of fluxes and profiles of the H i, He i and Na i D emission lines are due to the accretion process. At the same time, emission components of Ca ii lines indicate that RW Aur B has a powerful chromosphere. Assuming solar elemental abundances, we find the following parameters for the star: Teff = 4100–4200 K, AV = 0.6 ± 0.1 (out of the dimming events), L* ≈ 0.6 ${\rm L}_\odot ,\, R_* \approx 1.5$ ${\rm R}_\odot ,\, M\approx 0.85$ M$_\odot ,\, \dot{M}_{\rm acc}\lt 5\times 10^{-9}$ M⊙ yr−1. Finally, we discuss possible reasons for the different levels of accretion activity of the RW Aur binary components and present arguments in favour of the fact that the components are gravitationally bound.

Spatially Resolved Ultraviolet Spectroscopy of the Great Dimming of Betelgeuse

Abstract The bright supergiant Betelgeuse (Alpha Orionis, HD 39801) experienced a visual dimming during 2019 December and the first quarter of 2020, reaching an historic minimum 2020 February 7–13. During 2019 September–November, prior to the optical dimming event, the photosphere was expanding. At the same time, spatially resolved ultraviolet spectra using the Hubble Space Telescope/Space Telescope Imaging Spectrograph revealed a substantial increase in the ultraviolet spectrum and Mg ii line emission from the chromosphere over the southern hemisphere of the star. Moreover, the temperature and electron density inferred from the spectrum and C ii diagnostics also increased in this hemisphere. These changes happened prior to the Great Dimming Event. Variations in the Mg ii k-line profiles suggest material moved outwards in response to the passage of a pulse or acoustic shock from 2019 September through November. It appears that this extraordinary outflow of material from the star, likely initiated by convective photospheric elements, was enhanced by the coincidence with the outward motions in this phase of the ∼400 day pulsation cycle. These ultraviolet observations appear to provide the connecting link between the known large convective cells in the photosphere and the mass ejection event that cooled to form the dust cloud in the southern hemisphere imaged in 2019 December, and led to the exceptional optical dimming of Betelgeuse in 2020 February.

Early warning signals indicate a critical transition in Betelgeuse

Context.Critical transitions occur in complex dynamical systems when the system dynamics undergoes a regime shift. These can often occur with little change in the mean amplitude of the system response prior to the actual time of transition. The recent dimming and brightening event in Betelgeuse occurred as a sudden shift in the brightness and has been the subject of much debate. Internal changes or an external dust cloud have been suggested as reasons for this change in variability.Aims.We examine whether the dimming and brightening event of 2019–20 could be due to a critical transition in the pulsation dynamics of Betelgeuse by studying the characteristics of the light curve prior to transition.Methods.We calculated the quantifiers hypothesized to rise prior to a critical transition for the light curve of Betelgeuse up to the dimming event of 2019–20. These included the autocorrelation at lag-1, variance, and the spectral coefficient calculated from detrended fluctuation analysis, in addition to two measures that quantify the recurrence properties of the light curve. Significant rises are confirmed using the Mann-Kendall trend test.Results.We see a significant increase in all quantifiers (p < 0.05) prior to the dimming event of 2019–20. This suggests that the event was a critical transition related to the underlying nonlinear dynamics of the star.Conclusions.Together with results that suggest a minimal change inTeffand IR flux, a critical transition in the pulsation dynamics might be a reason for the unprecedented dimming of Betelgeuse. The rise in the quantifiers we studied prior to the dimming event supports this possibility.

Coronal Dimmings Associated with Coronal Mass Ejections on the Solar Limb

Abstract We present a statistical analysis of 43 coronal dimming events associated with Earth-directed coronal mass ejections (CMEs) that occurred during the period of quasi-quadrature of the Solar Dynamics Observatory (SDO) and Solar Terrestrial Relations Observatory (STEREO) satellites. We studied coronal dimmings that were observed above the limb by STEREO Extreme Ultraviolet Imager and compared their properties with the mass and speed of the associated CMEs. The unique position of the satellites allowed us to compare our findings with the results from Dissauer et al., who studied the same events observed against the solar disk by the SDO Atmospheric Imaging Assembly. Such statistics is done for the first time and confirms the relation of coronal dimmings and CME parameters for the off-limb viewpoint. The observations of dimming regions from different lines of sight reveal a similar decrease in the total extreme ultraviolet intensity (c = 0.60 ± 0.14). We find that the (projected) dimming areas are typically larger for off-limb observations (mean value of 1.24 ± 1.23 × 1011 km2 against 3.51 ± 0.71 × 1010 km2 for on-disk), with a correlation of c = 0.63 ± 0.10. This systematic difference can be explained by the (weaker) contributions to the dimming regions higher up in the corona that cannot be detected in the on-disk observations. The off-limb dimming areas and brightnesses show very strong correlations with the CME mass (c = 0.82 ± 0.06 and 0.75 ± 0.08), whereas the dimming area and brightness change rate correlate with the CME speed (c ∼ 0.6). Our findings suggest that coronal dimmings have the potential to provide early estimates of the mass and speed of Earth-directed CMEs, relevant for space weather forecasts, for satellite locations at both L1 and L5.

A low-mass stellar companion to the young variable star RZ Psc

ABSTRACT RZ Psc is a young Sun-like star with a bright and warm infrared excess that is occasionally dimmed significantly by circumstellar dust structures. Optical depth arguments suggest that the dimming events do not probe a typical sightline through the circumstellar dust, and are instead caused by structures that appear above an optically thick mid-plane. This system may therefore be similar to systems where an outer disc is shadowed by material closer to the star. Here, we report the discovery that RZ Psc hosts a $0.12\, \mathrm{ M}_\odot$ companion at a projected separation of 23 au. We conclude that the disc must orbit the primary star. While we do not detect orbital motion, comparison of the angle of linear polarization of the primary with the companion’s on-sky position angle provides circumstantial evidence that the companion and disc may not share the same orbital plane. Whether the companion severely disrupts the disc, truncates it, or has little effect at all will require further observations of both the companion and disc.

Coronal Response to Magnetically Suppressed CME Events in M-dwarf Stars

Abstract We report the results of the first state-of-the-art numerical simulations of coronal mass ejections (CMEs) taking place in realistic magnetic field configurations of moderately active M-dwarf stars. Our analysis indicates that a clear, novel, and observable, coronal response is generated due to the collapse of the eruption and its eventual release into the stellar wind. Escaping CME events, weakly suppressed by the large-scale field, induce a flare-like signature in the emission from coronal material at different temperatures due to compression and associated heating. Such flare-like profiles display a distinctive temporal evolution in their Doppler shift signal (from red to blue), as the eruption first collapses toward the star and then perturbs the ambient magnetized plasma on its way outwards. For stellar fields providing partial confinement, CME fragmentation takes place, leading to rise and fall flow patterns which resemble the solar coronal rain cycle. In strongly suppressed events, the response is better described as a gradual brightening, in which the failed CME is deposited in the form of a coronal rain cloud leading to a much slower rise in the ambient high-energy flux by relatively small factors (∼2–3). In all the considered cases (escaping/confined) a fractional decrease in the emission from midrange coronal temperature plasma occurs, similar to the coronal dimming events observed on the Sun. Detection of the observational signatures of these CME-induced features requires a sensitive next generation X-ray space telescope.

Exploring the dimming event of RW Aurigae A through multi-epoch VLT/X-shooter spectroscopy

Context. RW Aur A is a classical T Tauri star that has suddenly undergone three major dimming events since 2010. The reason for these dimming events is still not clear. Aims. We aim to understand the dimming properties, examine accretion variability, and derive the physical properties of the inner disc traced by the CO ro-vibrational emission at near-infrared wavelengths (2.3 μm). Methods. We compared two epochs of X-shooter observations, during and after the dimming. We modelled the rarely detected CO bandhead emission in both epochs to examine whether the inner disc properties had changed. The spectral energy distribution was used to derive the extinction properties of the dimmed spectrum and compare the infrared excess between the two epochs. Lines tracing accretion were used to derive the mass accretion rate in both states. Results. The CO originates from a region with physical properties of T = 3000 K, NCO = 1 × 1021 cm−2 and vk sin i = 113 km s−1. The extinction properties of the dimming layer were derived with the effective optical depth ranging from τeff ~2.5−1.5 from the UV to the near-IR. The inferred mass accretion rate Ṁacc is ~1.5 × 10−8 M⊙ yr−1 and ~2 × 10−8 M⊙ yr−1 after and during the dimming respectively. By fitting the spectral energy distribution, additional emission is observed in the infrared during the dimming event from dust grains with temperatures of 500–700 K. Conclusions. The physical conditions traced by the CO are similar for both epochs, indicating that the inner gaseous disc properties do not change during the dimming events. The extinction curve is flatter than that of the interstellar medium, and large grains of a few hundred microns are thus required. When we correct for the observed extinction, the mass accretion rate is constant in the two epochs, suggesting that the accretion is stable and therefore does not cause the dimming. The additional hot emission in the near-IR is located at about 0.5 au from the star and is not consistent with an occulting body located in the outer regions of the disc. The dimming events could be due to a dust-laden wind, a severe puffing-up of the inner rim, or a perturbation caused by the recent star-disc encounter.

Relativistic Beaming as a Probe of Stellar and Planetary Masses

The primary method of extra-solar planet (exoplanet) detection and characterization is through planetary transits. These events occur when a planet is observed to pass in front of it’s host star with respect to the observers line of sight, which causes a small dimming event. Transits alone yield information on the orbital properties such as period, inclination, semi-major axis as well as physical properties such as the planetary radius. With high-precision photometry, a new photometric effect has emerged as a probe of short-period exoplanet masses. This effect is known as relativistic Doppler beaming (or boosting), and has been used to estimate the masses and densities of numerous exoplanets and stars in binary systems. Here, this effect is discussed in detail along with the prospect of utilizing it with next generation space-based telescopes that will be devoted to the detection and characterization of exoplanets. Prospects for the characterization of binary systems will also be examined.

Statistics of Coronal Dimmings Associated with Coronal Mass Ejections. II. Relationship between Coronal Dimmings and Their Associated CMEs

Abstract We present a statistical study of 62 coronal dimming events associated with Earth-directed coronal mass ejections (CMEs) during the quasi-quadrature period of STEREO and the Solar Dynamics Observatory (SDO). This unique setting allows us to study both phenomena in great detail and compare characteristic quantities statistically. Coronal dimmings are observed on-disk by the SDO/Atmospheric Imaging Assembly and the Helioseismic and Magnetic Imager, while the CME kinematics during the impulsive acceleration phase is studied close to the limb with STEREO/EUVI and COR, minimizing projection effects. The dimming area, its total unsigned magnetic flux, and its total brightness, reflecting properties of the total dimming region at its final extent, show the highest correlations with the CME mass (c ∼ 0.6–0.7). Their corresponding time derivatives, describing the dynamics of the dimming evolution, show the strongest correlations with the CME peak velocity (c ∼ 0.6). The highest correlation of c = 0.68 ± 0.08 is found with the mean intensity of dimmings, indicating that the lower the CME starts in the corona, the faster it propagates. No significant correlation between dimming parameters and the CME acceleration was found. However, for events where high-cadence STEREO observations were available, the mean unsigned magnetic field density in the dimming regions tends to be positively correlated with the CME peak acceleration (c = 0.42 ± 0.20). This suggests that stronger magnetic fields result in higher Lorentz forces providing stronger driving force for the CME acceleration. Specific coronal dimming parameters correlate with both CME and flare quantities providing further evidence for the flare-CME feedback relationship. For events in which the CME occurs together with a flare, coronal dimmings statistically reflect the properties of both phenomena.

High-resolution spectroscopy of Boyajian’s star during optical dimming events

Boyajian's star is an apparently normal main sequence F-type star with a very unusual light curve. The dipping activity of the star, discovered during the Kepler mission, presents deep, asymmetric, and aperiodic events. Here we present high resolution spectroscopic follow-up during some dimming events recorded post-Kepler observations, from ground-based telescopes. We analise data from the HERMES, HARPS-N and FIES spectrographs to characterise the stellar atmosphere and to put some constraints on the hypotheses that have appeared in the literature concerning the occulting elements. The star's magnetism, if existing, is not extreme. The spots on the surface, if present, would occupy 0.02% of the area, at most. The chromosphere, irrespective of the epoch of observation, is hotter than the values expected from radiative equilibrium, meaning that the star has some degree of activity. We find no clear evidence of the interstellar medium nor exocoments being responsible for the dimmings of the light curve. However, we detect at 1-2 sigma level, a decrease of the radial velocity of the star during the first dip recorded after the \emph{\emph{Kepler}} observations. We claim the presence of an optically thick object with likely inclined and high impact parameter orbits that produces the observed Rossiter-McLaughlin effect.

The ‘Big Dipper’: the nature of the extreme variability of the AGN SDSS J2232−0806

Abstract SDSS J2232−0806 (the ‘Big Dipper’) has been identified as a ‘slow-blue nuclear hypervariable’: a galaxy with no previously known active nucleus, blue colours, and large-amplitude brightness evolution occurring on a time-scale of years. Subsequent observations have shown that this source does indeed contain an active galactic nucleus (AGN). Our optical photometric and spectroscopic monitoring campaign has recorded one major dimming event (and subsequent rise) over a period of around 4 yr; there is also evidence of previous events consistent with this in archival data recorded over the last 20 yr. Here we report an analysis of the 11 optical spectra obtained to date and we assemble a multiwavelength data set including infrared, ultraviolet, and X-ray observations. We find that an intrinsic change in the luminosity is the most favoured explanation of the observations, based on a comparison of continuum and line variability and the apparent lagged response of the hot dust. This source, along with several other recently discovered ‘changing-look’ objects, demonstrate that AGNs can exhibit large-amplitude luminosity changes on time-scales much shorter than those predicted by standard thin accretion disc models.