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Extreme debris discs are characterized by unusually strong mid-infrared excess emission, which often proves to be variable. The warm dust in these discs is of transient nature and is likely related to a recent giant collision occurring close to the star in the terrestrial region. Here we present the results of a 877 days long, gap-free photometric monitoring performed by the Spitzer Space Telescope of the recently discovered extreme debris disc around TYC 4209-1322-1. By combining these observations with other time-domain optical and mid-infrared data, we explore the disc variability of the last four decades with particular emphasis on the last 12 yr. During the latter interval the disc showed substantial changes, the most significant was the brightening and subsequent fading between 2014 and 2018 as outlined in WISE data. The Spitzer light curves outline the fading phase and a subsequent new brightening of the disc after 2018, revealing an additional flux modulation with a period of ~39 days on top of the long-term trend. We found that all these variations can be interpreted as the outcome of a giant collision that happened at an orbital radius of ~0.3 au sometime in 2014. Our analysis implies that a collision on a similar scale could have taken place around 2010, too. The fact that the disc was already peculiarly dust rich 40 yr ago, as implied by IRAS data, suggests that these dust production events belong to a chain of large impacts triggered by an earlier even more catastrophic collision., Comment: 18 pages, 12 figures, published in MNRAS

Historically, FU Orionis-type stars are low-mass, pre-main sequence stars. The members of this class experience powerful accretion outbursts and remain in an enhanced accretion state for decades or centuries. V1515 Cyg, a classical FUor, started brightening in the 1940s and reached its peak brightness in the late 1970s. Following a sudden decrease in brightness it stayed in a minimum state for a few months, then started a brightening for several years. We present results of our ground-based photometric monitoring complemented with optical/NIR spectroscopic monitoring. Our light curves show a long-term fading with strong variability on weekly and monthly time scales. The optical spectra show P Cygni profiles and broad blue-shifted absorption lines, common properties of FUors. However, V1515 Cyg lacks the P Cygni profile in the Ca II 8498 \r{A} line, a part of the Ca infrared triplet (IRT), formed by an outflowing wind, suggesting that the absorbing gas in the wind is optically thin. The newly obtained near-infrared spectrum shows the strengthening of the CO bandhead and the FeH molecular band, indicating that the disk has become cooler since the last spectroscopic observation in 2015. The current luminosity of the accretion disk dropped from the peak value of 138 $L_{\odot}$ to about 45 $L_{\odot}$, suggesting that the long-term fading is also partly caused by the dropping of the accretion rate., Comment: 26 pages, 19 figure, accepted for publication in The Astrophysical Journal

Among the low-mass pre-main sequence stars, a small group called FU Orionis-type objects (FUors) are notable for undergoing powerful accretion outbursts. V1057 Cyg, a classical example of an FUor, went into outburst around 1969-1970, after which it faded rapidly, making it the fastest fading FUor known. Around 1995, a more rapid increase in fading occurred. Since that time, strong photometric modulations have been present. We present nearly 10 years of source monitoring at Piszk\'estet\H{o} Observatory, complemented with optical/near-infrared photometry and spectroscopy from the Nordic Optical Telescope, Bohyunsan Optical Astronomy Observatory, Transiting Exoplanet Survey Satellite, and the Stratospheric Observatory for Infrared Astronomy. Our light curves show continuation of significant quasi-periodic variability in brightness over the past decade. Our spectroscopic observations show strong wind features, shell features, and forbidden emission lines. All of these spectral lines vary with time. We also report the first detection of [S II], [N II], and [O III] lines in the star., Comment: 37 pages, 20 figures

We present extensive observations of SN 2018zd covering the first $\sim450$\,d after the explosion. This SN shows a possible shock-breakout signal $\sim3.6$\,hr after the explosion in the unfiltered light curve, and prominent flash-ionisation spectral features within the first week. The unusual photospheric temperature rise (rapidly from $\sim 12,000$\,K to above 18,000\,K) within the earliest few days suggests that the ejecta were continuously heated. Both the significant temperature rise and the flash spectral features can be explained with the interaction of the SN ejecta with the massive stellar wind ($0.18^{+0.05}_{-0.10}\, \rm M_{\odot}$), which accounts for the luminous peak ($L_{\rm max} = [1.36\pm 0.63] \times 10^{43}\, \rm erg\,s^{-1}$) of SN 2018zd. The luminous peak and low expansion velocity ($v \approx 3300$ km s$^{-1}$) make SN 2018zd to be like a member of the LLEV (luminous SNe II with low expansion velocities) events originated due to circumstellar interaction. The relatively fast post-peak decline allows a classification of SN 2018zd as a transition event morphologically linking SNe~IIP and SNe~IIL. In the radioactive-decay phase, SN 2018zd experienced a significant flux drop and behaved more like a low-luminosity SN~IIP both spectroscopically and photometrically. This contrast indicates that circumstellar interaction plays a vital role in modifying the observed light curves of SNe~II. Comparing nebular-phase spectra with model predictions suggests that SN 2018zd arose from a star of $\sim 12\,\rm M_{\odot}$. Given the relatively small amount of $^{56}$Ni ($0.013 - 0.035 \rm M_{\odot}$), the massive stellar wind, and the faint X-ray radiation, the progenitor of SN 2018zd could be a massive asymptotic giant branch star which collapsed owing to electron capture., Comment: Accepted for publication in MNRAS, 20 pages, 11 figures

Studying chromospheric activity of contact binaries is an important way of revealing the magnetic activity processes of these systems. An efficient but somewhat neglected method for that is to follow the changes of the H$\alpha$ line profiles via optical spectroscopy. Our goal was to perform a comprehensive analysis based on the optical spectral signs of chromospheric activity on the largest sample of contact binaries to date. We collected echelle spectra on 12 bright contact binaries and derived new radial velocity curves from our observations. For quantifying the apparent chromospheric activity levels of the systems, we subtracted self-constructed synthetic spectra from the observed ones and measured the equivalent widths of the residual H$\alpha$-profiles at each observed epoch. Our well-sampled data set allowed us to study the short-term variations of chromospheric activity levels as well as to search for correlations between them and some basic physical parameters of the systems. Fitting the radial velocity curves, we re-determined the mass ratios and systemic velocities of all observed objects. We found that chromospheric activity levels of the studied systems show various changes during the orbital revolution: we see either flat, or one-peaked, or two-peaked distributions of equivalent width vs. the orbital phase. The first case means that the activity level is probably constant, while the latter two cases suggest the presence of one or two active longitudes at the stellar surfaces. Our correlation diagrams show that mean chromospheric activity levels may be in connection with orbital periods, B$-$V color indices, inverse Rossby numbers, and temperature differences of the components. At the same time, no clear trend is visible with respect to mass ratios, inclinations and fill-out factors of the systems. A- and W-type contact binaries in our sample show similar distributions., Comment: 13 pages, 10 figures, 3 tables, accepted for publication in Astronomy and Astrophysics

We present a photometric study of 17 Type Ia supernovae (SNe) based on multi-color (Bessell BVRI) data taken at Piszkesteto mountain station of Konkoly Observatory, Hungary between 2016 and 2018. We analyze the light curves (LCs) using the publicly available LC-fitter SNooPy2 to derive distance and reddening information. The bolometric LCs are fit with a radiation-diffusion Arnett-model to get constraints on the physical parameters of the ejecta: the optical opacity, the ejected mass and the expansion velocity in particular. We also study the pre-maximum (B-V) color evolution by comparing our data with standard delayed detonation and pulsational delayed detonation models, and show that the Ni56 masses of the models that fit the (B-V) colors are consistent with those derived from the bolometric LC fitting. We find similar correlations between the ejecta parameters (e.g. ejecta mass, or Ni56 mass vs decline rate) as published recently by Scalzo et al. (2019)., Comment: 28 pages, 14 figures, accepted for publication in ApJ

V582 Aur is a pre-main sequence FU Orionis type eruptive star, which entered a brightness minimum in 2016 March due to changes in the line-of-sight extinction. Here, we present and analyze new optical $B$, $V$, $R_C$ and $I_C$ band multiepoch observations and new near-infrared $J$, $H$ and $K_S$ band photometric measurements from 2018 January$-$2019 February, as well as publicly available mid-infrared WISE data. We found that the source shows a significant optical$-$near-infrared variability, and the current brightness minimum has not completely finished yet. If the present dimming originates from the same orbiting dust clump that caused a similar brightness variation in 2012, than our results suggest a viscous spreading of the dust particles along the orbit. Another scenario is that the current minimum is caused by a dust structure, that is entering and leaving the inner part of the system. The WISE measurements could be consistent with this scenario. Our long-term data, as well as an accretion disk modeling hint at a general fading of V582 Aur, suggesting that the source will reach the quiescent level in $\sim$80 years., Comment: 8 pages, 4 figures, accepted for publication in ApJ

We present an exquisite, 30-min cadence Kepler (K2) light curve of the Type Ia supernova (SN Ia) 2018oh (ASASSN-18bt), starting weeks before explosion, covering the moment of explosion and the subsequent rise, and continuing past peak brightness. These data are supplemented by multi-color Pan-STARRS1 and CTIO 4-m DECam observations obtained within hours of explosion. The K2 light curve has an unusual two-component shape, where the flux rises with a steep linear gradient for the first few days, followed by a quadratic rise as seen for typical SNe Ia. This "flux excess" relative to canonical SN Ia behavior is confirmed in our $i$-band light curve, and furthermore, SN 2018oh is especially blue during the early epochs. The flux excess peaks 2.14$\pm0.04$ days after explosion, has a FWHM of 3.12$\pm0.04$ days, a blackbody temperature of $T=17,500^{+11,500}_{-9,000}$ K, a peak luminosity of $4.3\pm0.2\times10^{37}\,{\rm erg\,s^{-1}}$, and a total integrated energy of $1.27\pm0.01\times10^{43}\,{\rm erg}$. We compare SN 2018oh to several models that may provide additional heating at early times, including collision with a companion and a shallow concentration of radioactive nickel. While all of these models generally reproduce the early K2 light curve shape, we slightly favor a companion interaction, at a distance of $\sim$$2\times10^{12}\,{\rm cm}$ based on our early color measurements, although the exact distance depends on the uncertain viewing angle. Additional confirmation of a companion interaction in future modeling and observations of SN 2018oh would provide strong support for a single-degenerate progenitor system., Comment: 20 pages, 9 figures, 2 tables. Submitted to APJ Letters on 31 Jul 2018, Accepted for publication on 31 Aug 2018

Supernova (SN) 2018oh (ASASSN-18bt) is the first spectroscopically-confirmed type Ia supernova (SN Ia) observed in the $Kepler$ field. The $Kepler$ data revealed an excess emission in its early light curve, allowing to place interesting constraints on its progenitor system (Dimitriadis et al. 2018, Shappee et al. 2018b). Here, we present extensive optical, ultraviolet, and near-infrared photometry, as well as dense sampling of optical spectra, for this object. SN 2018oh is relatively normal in its photometric evolution, with a rise time of 18.3$\pm$0.3 days and $\Delta$m$_{15}(B)=0.96\pm$0.03 mag, but it seems to have bluer $B - V$ colors. We construct the "uvoir" bolometric light curve having peak luminosity as 1.49$\times$10$^{43}$erg s$^{-1}$, from which we derive a nickel mass as 0.55$\pm$0.04M$_{\odot}$ by fitting radiation diffusion models powered by centrally located $^{56}$Ni. Note that the moment when nickel-powered luminosity starts to emerge is +3.85 days after the first light in the Kepler data, suggesting other origins of the early-time emission, e.g., mixing of $^{56}$Ni to outer layers of the ejecta or interaction between the ejecta and nearby circumstellar material or a non-degenerate companion star. The spectral evolution of SN 2018oh is similar to that of a normal SN Ia, but is characterized by prominent and persistent carbon absorption features. The C II features can be detected from the early phases to about 3 weeks after the maximum light, representing the latest detection of carbon ever recorded in a SN Ia. This indicates that a considerable amount of unburned carbon exists in the ejecta of SN 2018oh and may mix into deeper layers., Comment: 48 pages, 23 figures. This paper is part of a coordinated effort between groups. Accepted for publication in ApJ

V582 Aur is an FU Ori-type young eruptive star in outburst since $\sim$1985. The eruption is currently in a relatively constant plateau phase, with photometric and spectroscopic variability superimposed. Here we will characterize the progenitor of the outbursting object, explore its environment, and analyse the temporal evolution of the eruption. We are particularly interested in the physical origin of the two deep photometric dips, one occurred in 2012, and one is ongoing since 2016. We collected archival photographic plates, and carried out new optical, infrared, and millimeter wave photometric and spectroscopic observations between 2010 and 2017, with high sampling rate during the current minimum. Beside analysing the color changes during fading, we compiled multiepoch spectral energy distributions, and fitted them with a simple accretion disk model. Based on pre-outburst data and a millimeter continuum measurement, we suggest that the progenitor of the V582 Aur outburst is a low-mass T Tauri star with average properties. The mass of an unresolved circumstellar structure, probably a disk, is 0.04 M$_{\odot}$. The optical and near-infrared spectra demonstrate the presence of hydrogen and metallic lines, show the CO bandhead in absorption, and exhibit a variable H$\alpha$ profile. The color variations strongly indicate that both the $\sim$1 year long brightness dip in 2012, and the current minimum since 2016 are caused by increased extinction along the line of sight. According to our accretion disk models, the reddening changed from $A_V$=4.5 mag to 12.5 mag, while the accretion rate remained practically constant. Similarly to the models of the UXor phenomenon of intermediate and low-mass young stars, orbiting disk structures could be responsible for the eclipses., Comment: 20 pages, 11 figures, accepted for publication in ApJ

The optical observations of Ic-4 supernova (SN) 2016coi/ASASSN-16fp, from $\sim 2$ to $\sim450$ days after explosion, are presented along with analysis of its physical properties. The SN shows the broad lines associated with SNe Ic-3/4 but with a key difference. The early spectra display a strong absorption feature at $\sim 5400$ \AA\ which is not seen in other SNe~Ic-3/4 at this epoch. This feature has been attributed to He I in the literature. Spectral modelling of the SN in the early photospheric phase suggests the presence of residual He in a C/O dominated shell. However, the behaviour of the He I lines are unusual when compared with He-rich SNe, showing relatively low velocities and weakening rather than strengthening over time. The SN is found to rise to peak $\sim 16$ d after core-collapse reaching a bolometric luminosity of Lp $\sim 3\times10^{42}$ \ergs. Spectral models, including the nebular epoch, show that the SN ejected $2.5-4$ \msun\ of material, with $\sim 1.5$ \msun\ below 5000 \kms, and with a kinetic energy of $(4.5-7)\times10^{51}$ erg. The explosion synthesised $\sim 0.14$ \msun\ of 56Ni. There are significant uncertainties in E(B-V)host and the distance however, which will affect Lp and MNi. SN 2016coi exploded in a host similar to the Large Magellanic Cloud (LMC) and away from star-forming regions. The properties of the SN and the host-galaxy suggest that the progenitor had $M_\mathrm{ZAMS}$ of $23-28$ \msun\ and was stripped almost entirely down to its C/O core at explosion., Comment: Accepted for publication in MNRAS. Updated to reflect the published version, minor typographical changes only

In this paper we present visible range light curves of the irregular Uranian satellites Sycorax, Caliban, Prospero, Ferdinand and Setebos taken with Kepler Space Telescope in the course of the K2 mission. Thermal emission measurements obtained with the Herschel/PACS and Spitzer/MIPS instruments of Sycorax and Caliban were also analysed and used to determine size, albedo and surface characteristics of these bodies. We compare these properties with the rotational and surface characteristics of irregular satellites in other giant planet systems and also with those of main belt and Trojan asteroids and trans-Neptunian objects. Our results indicate that the Uranian irregular satellite system likely went through a more intense collisional evolution than the irregular satellites of Jupiter and Saturn. Surface characteristics of Uranian irregular satellites seems to resemble the Centaurs and trans-Neptunian objects more than irregular satellites around other giant planets, suggesting the existence of a compositional discontinuity in the young Solar system inside the orbit of Uranus., Comment: Accepted for publication in the Astronomical Journal

We present the K2 light curves of a large sample of untargeted Main Belt asteroids (MBAs) detected with the Kepler space telescope. The asteroids were observed within the Uranus superstamp, a relatively large, continuous field with low stellar background designed to cover the planet Uranus and its moons during Campaign 8 of the K2 mission. The superstamp offered the possibility to obtain precise, uninterrupted light curves of a large number of MBAs and thus to determine unambiguous rotation rates for them. We obtained photometry for 608 MBAs, and were able to determine or estimate rotation rates for 90 targets, of which 86 had no known values before. In an additional 16 targets we detected incomplete cycles and/or eclipse-like events. We found the median rotation rate to be significantly longer than that of the ground-based observations indicating that the latter are biased towards shorter rotation rates. Our study highlights the need and benefits of further continuous photometry of asteroids., Comment: 14 pages, 10 figures, plus an 11-page appendix with light curve plots, accepted for publication in ApJS. Light curve data accessible from: http://konkoly.hu/KIK/data_en.html#K2_MB_ast. Changes to v1 (submitted version): more detailed statistics, to v2: added this link to our video of the asteroids passing through during the campaign: https://www.youtube.com/watch?v=gBQbyl1SM-w

Due to the failure of the second reaction wheel, a new mission was conceived for the otherwise healthy Kepler space telescope. In the course of the K2 Mission, the telescope is staring at the plane of the Ecliptic, hence thousands of Solar System bodies cross the K2 fields, usually causing extra noise in the highly accurate photometric data. In this paper we follow the someone's noise is another one's signal principle and investigate the possibility of deriving continuous asteroid light curves, that has been unprecedented to date. In general, we are interested in the photometric precision that the K2 Mission can deliver on moving Solar System bodies. In particular, we investigate space photometric optical light curves of main-belt asteroids. We study the K2 superstamps covering the M35 and Neptune/Nereid fields observed in the long cadence (29.4-min sampling) mode. Asteroid light curves are generated by applying elongated apertures. We use the Lomb-Scargle method to find periodicities due to rotation. We derived K2 light curves of 924 main-belt asteroids in the M35 field, and 96 in the path of Neptune and Nereid. The light curves are quasi-continuous and several days long. K2 observations are sensitive to longer rotational periods than usual ground-based surveys. Rotational periods are derived for 26 main-belt asteroids for the first time. The asteroid sample is dominated by faint (>20 mag) objects. Due to the faintness of the asteroids and the high density of stars in the M35 field, only 4.0% of the asteroids with at least 12 data points show clear periodicities or trend signalling a long rotational period, as opposed to 15.9% in the less crowded Neptune field. We found that the duty cycle of the observations had to reach ~60% in order to successfully recover rotational periods., Comment: 9 pages, 9 figures, 6 tables, to be published in Astronomy and Astrophysics, for the full tables of observed asteroids and their rotational parameters see http://www.konkoly.hu/KIK/data_en.html

We present fully covered phased light curves for 56 Jovian Trojan asteroids as acquired by the K2 mission of the Kepler space telescope. This set of objects has been monitored during Campaign 6 and represents a nearly unbiased subsample of the population of small Solar System bodies. We derived precise periods and amplitudes for all Trojans, and found their distributions to be compatible with the previous statistics. We point out, however, that ground-based rotation periods are often unreliable above 20h, and we find an overabundance of rotation periods above 60h compared with other minor planet populations. From amplitude analysis we derive a rate of binarity of 20$\pm$ 5%. Our spin rate distribution confirms the previously obtained spin barrier of ~5h and the corresponding ~0.5 g cm$^{-3}$ cometary-like density limit, also suggesting a high internal porosity for Jovian Trojans. One of our targets, asteroid 65227 exhibits a double rotation period, which can either be due to binarity or the outcome of a recent collision., Comment: 14 pages, 13 figures, 2 tables, to be published in Astronomy and Astrophysics