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The theoretical relations between the orbital period and mean densities of component stars of contact binaries are deduced. It is found that the orbital period of a contact binary is inversely proportional to the square root of the mean densities of component stars with the slopes as functions of mass ratio and fill-out factor of system. A test of 148 well-studied contact binaries with physical parameters determined from comprehensive photometric and spectroscopic solutions shows that the upper limit of secondaries' mean densities depends strongly on the total masses, M 1 + M 2, of the contact systems. For a low-mass contact system with |$(M_{}+M_{2})\le 2\, \mathrm{M}_{\odot }$|⁠ , the maximum mean density of the secondary is strictly lower than that of a 5 Gyr star with a mass of (M + M 2)/2. Taking this as the basic constraint, the derived period–density relation was applied to simulate the orbital period distribution of low-mass contact binaries over the total-mass range of 1–2 |$\, \mathrm{M}_{\odot }$|⁠. It reproduces a period cut-off at 0.12–0.16 d, depending on the fill-out factor. [ABSTRACT FROM AUTHOR]

CSS1603+19 is a cataclysmic variable (CV) with an orbital period of 81.96 min, near the minimal period of CVs. It is unusual in having a strong mid-infrared excess inconsistent with thermal emission from a brown dwarf companion. Here, we present time-resolved multiwavelength observations of this system. WISE photometry indicates that the mid-infrared excess displays a one-magnitude eclipsing-like variability during the orbit. We obtained near-infrared and optical spectroscopy using Gemini, MDM, and APO telescopes. Near-infrared spectra show possible cyclotron features indicating that the white dwarf has a magnetic field of about 5 MG. Optical and near-infrared spectra display double-peaked emission lines, with both components showing strong radial velocity variations during the orbital period and with the broad component leading the narrow component stably by about 0.2 of the orbital phase. We construct a physical model informed by existing observations of the system and determine that one component likely originates from the accretion column on to the magnetized white dwarf in synchronous rotation with the orbital motion and the other from the Roche overflow point. This allows us to constrain the masses of the binary components to be M 1 > 0.24 M⊙ for the white dwarf accretor and M 2 = 0.0644 ± 0.0074 M⊙ for the donor. We classify the system as an AM Herculis star, or a polar. It has likely completed its stint on the period gap, but has not yet gone through the period bounce. [ABSTRACT FROM AUTHOR]

Precise measurements of stellar parameters are required in order to develop our theoretical understanding of stellar structure. These measurements enable errors and uncertainties to be quantified in theoretical models and constrain the physical interpretation of observed phenomena, such as the inflated radii of low-mass stars. We use newly-available TESS (Transiting Exoplanet Satellite Survey) light curves combined with published radial velocity measurements to improve the characterization of 12 low mass eclipsing binaries composed of an M dwarf accompanied by a brighter F/G star. We present and analyse ground-based simultaneous four-colour photometry for two targets. Our results include the first measurements of the fundamental properties of two of the systems. Light curve and radial velocity information were converted into the physical parameters of each component of the systems using an isochrone fitting method. We also derive the effective temperatures of the M dwarfs, almost tripling the number of such measurements. The results are discussed in the context of radius inflation. We find that exquisite precision in the age estimation of young objects is required to determine their inflation status. However, all but three of the objects are securely located among the main sequence, demonstrating radius inflation and the necessity to develop our understanding of the complex physical processes governing the evolution of low-mass stars. We investigated the hypothesis that luminosity is unaffected by the inflation problem but the findings were not conclusive. [ABSTRACT FROM AUTHOR]

I report updates to the substellar mass–radius diagram for 11 transiting brown dwarfs (BDs) and low-mass stars published before the third data release from the Gaia mission (Gaia DR3). I re-analyse these transiting BD systems whose physical parameters were published between 2008 and 2019 and find that when using the parallax measurements from Gaia DR3, seven BDs show significant differences in their radius estimate or an improvement in the radius uncertainty. This has important implications for how these BDs are used to test substellar evolutionary models in the mass–radius diagram. The remaining four BDs show mass–radius estimates that are consistent with their previous pre-Gaia DR3 measurements. The seven BDs that show significant deviation from the original mass–radius measurements are AD 3116b, CoRoT-3b, CoRoT-15b, EPIC 201702477b, Kepler-39b, KOI-205b, and KOI-415b. Of these, AD 3116b is a known member of the Praesepe cluster at an age of 600 Myr. Additionally, some of the previously smallest known transiting BDs, KOI-205b and KOI-415b, are not as small as once thought, leaving the mass–radius region for the very oldest BDs relatively sparse as a result of this work. [ABSTRACT FROM AUTHOR]

One of the puzzles to have emerged from the Kepler and TESS missions is the existence of unexplained dips in the light curves of a small fraction of rapidly rotating M dwarfs in young open clusters and star-forming regions. We present a theoretical investigation of one possible explanation – that these are caused by dust clouds trapped in the stellar magnetic fields. The depth and duration of the observed dips allow us to estimate directly the linear extent of the dust clouds and their distances from the rotation axis. The dips are found to be between 0.4 and 4.8 per cent. We find that their distance is close to the co-rotation radius: the typical location for stable points where charged particles can be trapped in a stellar magnetosphere. We estimate the charge acquired by a dust particle due to collisions with the coronal gas and hence determine the maximum grain size that can be magnetically supported, the stopping distance due to gas drag, and the time-scale on which dust particles can diffuse out of a stable point. Using the observationally derived magnetic field of the active M dwarf V374 Peg, we model the distribution of these dust clouds and produce synthetic light curves. We find that for 1μm dust grains, the light curves have dips of 1–3 per cent and can support masses of order of 1012 kg. We conclude that magnetically trapped dust clouds (potentially from residual disc accretion or tidally disrupted planetesimal or cometary bodies) are capable of explaining the periodic dips in the Kepler and TESS data. [ABSTRACT FROM AUTHOR]

Studies of T Tauri discs inform planet formation theory; observations of variability due to occultation by circumstellar dust are a useful probe of unresolved, planet-forming inner discs, especially around faint M dwarf stars. We report observations of 2M0632, an M dwarf member of the Carina young moving group that was observed by Transiting Exoplanet Survey Satellite over two 1-yr intervals. The combined light curve contains >300 dimming events, each lasting a few hours, and as deep as 40 per cent (0.55 magnitudes). These stochastic events are correlated with a distinct, stable 1.86-d periodic signal that could be stellar rotation. Concurrent ground-based, multiband photometry show reddening consistent with interstellar medium-like dust. The star's excess emission in the infrared and emission lines in optical and infrared spectra reveal a T Tauri-like accretion disc around the star. We confirm membership of 2M0632 in the Carina group by a Bayesian analysis of its Galactic space motion and position. We combine stellar evolution models with Gaia photometry and constraints on T eff, luminosity, and the absence of detectable lithium in the photosphere to constrain the age of the group and 2M0632 to 40–60 Myr, consistent with earlier estimates. 2M0632 joins a handful of long-lived discs which challenge the canon that disc lifetimes are ≲10 Myr. All known examples surround M dwarfs, suggesting that lower X-ray/ultraviolet irradiation and slower photoevaporation by these stars can dramatically affect disc evolution. The multiplanet systems spawned by long-lived discs probably experienced significant orbital damping and migration into close-in, resonant orbits, and perhaps represented by the TRAPPIST-1 system. [ABSTRACT FROM AUTHOR]

Data for 405 contact binary systems extracted from a recent compilation are supplemented by information from the literature for an additional ten ultrashort period (P < 0.2 d) binaries. This is used to calibrate period–luminosity–temperature relations for contact binaries. The derived relations are applied to a standard mean temperature–luminosity tabulation to obtain predicted periods for contact systems over the spectral range M9.5–A2. Minimum periods in the range 0.08–0.15 d follow, depending on the precise period–luminosity–temperature model fitted. [ABSTRACT FROM AUTHOR]

The rotational evolution of a young stellar population can give informations about the rotation pattern of more evolved clusters. Combined with rotational period values of thousands of young stars and theoretical propositions about the redistribution and loss of stellar angular momentum, it allows us to trace the rotational history of stars according to their mass. We want to investigate how internal and environmental changes on single stars can change the rotational evolution of a young stellar population. We run Monte Carlo simulations of a young cluster composed by solar mass stars of 0.5, 0.8 and 1.0 M⊙ from 1 to 550 Myr taking into account observational and theoretical parameters. In order to compare our results with the observations we run Kolmogorov-Smirnov tests. Our standard model is able to reproduce some clusters younger than h Per and marginally M37, which is 550 Myr old. Varying the disk fraction or the initial period distribution did not improve the results. However, when we run a model with a finer mass grid the Pleiades can be also reproduced. Changing the initial mass distribution to be similar to the empirical ONC mass function also gives good results. Modeling the evolution of a young synthetic cluster from pre-main sequence to early main sequence considering physical mechanisms of extraction and exchange of angular momentum can not be achieved successfully for all clusters for which we have enough rotational data. Clusters of about the same age present different rotational behaviors due perhaps to differences in their initial conditions. [ABSTRACT FROM AUTHOR]

We present the discovery and characterization of an eclipsing binary identified by the Next Generation Transit Survey in the ∼115-Myr-old Blanco 1 open cluster. NGTS J0002−29 comprises three M dwarfs: a short-period binary and a companion in a wider orbit. This system is the first well-characterized, low-mass eclipsing binary in Blanco 1. With a low mass ratio, a tertiary companion, and binary components that straddle the fully convective boundary, it is an important benchmark system, and one of only two well-characterized, low-mass eclipsing binaries at this age. We simultaneously model light curves from NGTS, TESS , SPECULOOS, and SAAO, radial velocities from VLT/UVES and Keck/HIRES, and the system's spectral energy distribution. We find that the binary components travel on circular orbits around their common centre of mass in P orb = 1.098 005 24 ± 0.000 000 38 d, and have masses M pri = 0.3978 ± 0.0033 M⊙ and M sec = 0.2245 ± 0.0018 M⊙, radii R pri = 0.4037 ± 0.0048 R⊙ and R sec = 0.2759 ± 0.0055 R⊙, and effective temperatures |$T_{\rm pri}=\mbox{$3372\, ^{+44}_{-37}$}$| K and |$T_{\rm sec}=\mbox{$3231\, ^{+38}_{-31}$}$| K. We compare these properties to the predictions of seven stellar evolution models, which typically imply an inflated primary. The system joins a list of 19 well-characterized, low-mass, sub-Gyr, stellar-mass eclipsing binaries, which constitute some of the strongest observational tests of stellar evolution theory at low masses and young ages. [ABSTRACT FROM AUTHOR]

The reflectance of the Earth is a fundamental climate parameter that we measured from Big Bear Solar Observatory between 1998 and 2017 by observing the earthshine using modern photometric techniques to precisely determine daily, monthly, seasonal, yearly and decadal changes in terrestrial albedo from earthshine. We find the inter‐annual fluctuations in albedo to be global, while the large variations in albedo within individual nights and seasonal wanderings tend to average out over each year. We measure a gradual, but climatologically significant ∼0.5 W/m2 decline in the global albedo over the two decades of data. We found no correlation between the changes in the terrestrial albedo and measures of solar activity. The inter‐annual pattern of earthshine fluctuations are in good agreement with those measured by CERES (data began in 2001) even though the satellite observations are sensitive to retroflected light while earthshine is sensitive to wide‐angle reflectivity. The CERES decline is about twice that of earthshine. Plain Language Summary: The net sunlight reaching the Earth's climate system depends on the solar irradiance and the Earth's reflectance (albedo). We have observed earthshine from Big Bear Solar Observatory to measure the terrestrial albedo. For earthshine we measure the sunlight reflected from Earth to the dark part of the lunar face and back to the nighttime observer, yielding an instantaneous large‐scale reflectance of the Earth. In these relative measurements, we also observe the sunlit, bright part of the lunar face. We report here reflectance data (monthly, seasonal and annual) covering two decades, 1998–2017. The albedo shows a decline corresponding to a net climate forcing of about 0.5 W/m2. We find no correlation between measures of solar cycle variations and the albedo variations. The first precise satellite measures of terrestrial albedo came with CERES. CERES global albedo data (2001‐) show a decrease in forcing that is about twice that of earthshine measurements. The evolutionary changes in albedo motivate continuing earthshine observations as a complement to absolute satellite measurements, especially since earthshine and CERES measurements are sensitive to distinctly different parts of the angular reflectivity. The recent drop in albedo is attributed to a warming of the eastern pacific, which is measured to reduce low‐lying cloud cover and, thereby, the albedo. Key Points: We report on two decades of earthshine measurements of the earth's reflectance made from Big Bear Solar Observatory yielding a large‐scale terrestrial albedoWe find a decline in albedo between 1998 and 2017, corresponding to a radiative increase of 0.5 W/m2, which is climatologically significantThe CERES data show the same behavior, which is attributed to a reversal of the Pacific Decadal Oscillation reducing the Earth's albedo [ABSTRACT FROM AUTHOR]

We present results from a circular polarization survey for radio stars in the Rapid ASKAP Continuum Survey (RACS). RACS is a survey of the entire sky south of δ = +41○ being conducted with the Australian Square Kilometre Array Pathfinder telescope (ASKAP) over a 288 MHz wide band centred on 887.5 MHz. The data we analyse include Stokes I and V polarization products to an RMS sensitivity of 250  μ Jy PSF−1. We searched RACS for sources with fractional circular polarization above 6 per cent, and after excluding imaging artefacts, polarization leakage, and known pulsars we identified radio emission coincident with 33 known stars. These range from M-dwarfs through to magnetic, chemically peculiar A- and B-type stars. Some of these are well-known radio stars such as YZ CMi and CU Vir, but 23 have no previous radio detections. We report the flux density and derived brightness temperature of these detections and discuss the nature of the radio emission. We also discuss the implications of our results for the population statistics of radio stars in the context of future ASKAP and Square Kilometre Array surveys. [ABSTRACT FROM AUTHOR]

Binary and multiple stellar systems are numerous in our solar neighbourhood with 80 per cent of the solar-type stars being members of systems with high order multiplicity. The Contact   Binaries   Towards   Merging (CoBiToM) Project is a programme that focuses on contact binaries and multiple stellar systems, as a key for understanding stellar nature. The goal is to investigate stellar coalescence and merging processes, as the final state of stellar evolution of low-mass contact binary systems. Obtaining observational data of approximately 100 eclipsing binaries and multiple systems and more than 400 archival systems, the programme aspires to give insights for their physical and orbital parameters and their temporal variations, e.g. the orbital period modulation, spot activity etc. Gravitational phenomena in multiple-star environments will be linked with stellar evolution. A comprehensive analysis will be conducted, in order to investigate the possibility of contact binaries to host planets, as well as the link between inflated hot Jupiters and stellar mergers. The innovation of CoBiToM Project is based on a multimethod approach and a detailed investigation, that will shed light for the first time on the origin of stellar mergers and rapidly rotating stars. In this work, we describe the scientific rationale, the observing facilities to be used and the methods that will be followed to achieve the goals of CoBiToM Project and we present the first results as an example of the current research on evolution of contact binary systems. [ABSTRACT FROM AUTHOR]

This paper presents the results of a combined spectroscopic and photometric study of 20 contact binary systems: HV Aqr, OO Aql, FI Boo, TX Cnc, OT Cnc, EE Cet, RW Com, KR Com, V401 Cyg, V345 Gem, AK Her, V502 Oph, V566 Oph, V2612 Oph, V1363 Ori, V351 Peg, V357 Peg, Y Sex, V1123 Tau, and W UMa, which was conducted in the frame of the W UMa Project. Together with 51 already covered by the project and an additional 67 in the existing literature, these systems bring the total number of contact binaries with known combined spectroscopic and photometric solutions to 138. It was found that mass, radius, and luminosity of the components follow certain relations along the MS and new empirical power relations are extracted. We found that 30 per cent of the systems in the current sample show extreme values in their parameters, expressed in their mass ratio or fill-out factor. This study shows that, among the contact binary systems studied, some have an extremely low mass ratio (q < 0.1) or an ultrashort orbital period (P orb < 0.25 d), which are expected to show evidence of mass transfer progress. The evolutionary status of these components is discussed with the aid of correlation diagrams and their physical and orbital parameters compared to those in the entire sample of known contact binaries. The existence of very short orbital periods confirms the very slow nature of the merging process, which seems to explain why their components still exist as MS stars in contact configurations even after several Gyr of evolution. [ABSTRACT FROM AUTHOR]

We analysed the photometry of 20 038 cool stars from campaigns 12, 13, 14, and 15 of the K2 mission in order to detect, characterize, and validate new planetary candidates transiting low-mass stars. We present a catalogue of 25 new periodic transit-like signals in 22 stars, of which we computed the parameters of the stellar host for 19 stars and the planetary parameters for 21 signals. We acquired speckle and AO images, and also inspected archival Pan-STARRS1 images and Gaia DR2 to discard the presence of close stellar companions and to check possible transit dilutions due to nearby stars. False positive probability (FPP) was computed for 22 signals, obtaining FPP < |$1{{\ \rm per\ cent}}$| for 17. We consider 12 of them as statistically validated planets. One signal is a false positive and the remaining 12 signals are considered as planet candidates. 20 signals have an orbital period of P |$_{\rm orb} \lt 10\,\mathrm{ d}$|⁠ , 2 have |$10\, \mathrm{ d} \lt $|   P |$_{\rm orb} \lt 20\, \mathrm{ d}$|⁠ , and 3 have P |$_{\rm orb} \gt 20\, \mathrm{ d}$|⁠. Regarding radii, 11 candidates and validated planets have computed radius R < 2R⊕, 9 have 2R⊕ < R < 4R⊕, and 1 has R  > 4R⊕. Two validated planets and two candidates are located in moderately bright stars (⁠|$\rm \mathit{ m}_{kep}\lt 13$|⁠) and two validated planets and three candidates have derived orbital radius within the habitable zone according to optimistic models. Of special interest is the validated warm super-Earth K2-323 b (EPIC 248616368 b) with T |$_{\rm eq} = 318^{+24}_{-43} \, \mathrm{ K}$|⁠ , S |$_{\rm p} = 1.7\pm 0.2 \, \mathrm{ S}_{\oplus }$|⁠ , and R |$_{\rm p} = 2.1\pm 0.1 \, \mathrm{ R}_{\oplus }$|⁠ , located in an m |$\rm _{kep}$| = 14.13 star. [ABSTRACT FROM AUTHOR]

Magnetic fields play a fundamental role for interior and atmospheric properties of M dwarfs and greatly influence terrestrial planets orbiting in the habitable zones of these low-mass stars. Determination of the strength and topology of magnetic fields, both on stellar surfaces and throughout the extended stellar magnetospheres, is a key ingredient for advancing stellar and planetary science. Here, modern methods of magnetic field measurements applied to M-dwarf stars are reviewed, with an emphasis on direct diagnostics based on interpretation of the Zeeman effect signatures in high-resolution intensity and polarisation spectra. Results of the mean field strength measurements derived from Zeeman broadening analyses as well as information on the global magnetic geometries inferred by applying tomographic mapping methods to spectropolarimetric observations are summarised and critically evaluated. The emerging understanding of the complex, multi-scale nature of M-dwarf magnetic fields is discussed in the context of theoretical models of hydromagnetic dynamos and stellar interior structure altered by magnetic fields. [ABSTRACT FROM AUTHOR]

We present the characterization of CRTS J055255.7−004426 (=THOR 42), a young eclipsing binary comprising two pre-main sequence M dwarfs (combined spectral type M3.5). This nearby (103 pc), short-period (0.859 d) system was recently proposed as a member of the ∼24 Myr-old 32 Orionis Moving Group. Using ground- and space-based photometry in combination with medium- and high-resolution spectroscopy, we model the light and radial velocity curves to derive precise system parameters. The resulting component masses and radii are 0.497 ± 0.005 and 0.205 ± 0.002  |$\rm {M}_{\odot }$|⁠ , and 0.659 ± 0.003 and 0.424 ± 0.002  |$\rm {R}_{\odot }$|⁠ , respectively. With mass and radius uncertainties of ∼1 per cent and ∼0.5 per cent, respectively, THOR 42 is one of the most precisely characterized pre-main sequence eclipsing binaries known. Its systemic velocity, parallax, proper motion, colour–magnitude diagram placement, and enlarged radii are all consistent with membership in the 32 Ori Group. The system provides a unique opportunity to test pre-main sequence evolutionary models at an age and mass range not well constrained by observation. From the radius and mass measurements we derive ages of 22–26 Myr using standard (non-magnetic) models, in excellent agreement with the age of the group. However, none of the models can simultaneously reproduce the observed mass, radius, temperature, and luminosity of the coeval components. In particular, their H–R diagram ages are 2–4 times younger and we infer masses ∼50 per cent smaller than the dynamical values. [ABSTRACT FROM AUTHOR]

The 'Asteroid Terrestrial-impact Last Alert System' discovered hundreds of thousands of new candidate variable stars. Follow-up observations of three of these are reported in this paper. The targets were selected on the basis of having high probability of being periodic (false alarm probability for period detection smaller than 10−5), short periods (P < 0.2 d), and being relatively bright (g′ < 17). The targets were also chosen to be either very blue (g′ − i′ < −0.4, r′ − z′ < −0.4) or very red (g′ − i′ > 2.2, r′ − z′ > 1.5) as periodic variables with these colours are relatively rare. Two of the stars are hot subdwarfs, both of which are likely reflection effect binaries. In both cases simple models suggest that the companions may have masses very close to or below 0.1 |$\, \mathrm{M}_\odot$|⁠. The third star is also a binary, which appears to consist of two M dwarfs in a near contact configuration. At 0.12 d its period is one of the shortest known for M-type binaries. [ABSTRACT FROM AUTHOR]

We implement a steady, one-dimensional flow model for the X-ray jet of Centaurus A in which entrainment of stellar mass loss is the primary cause of dissipation. Using over 260 ks of new and archival Chandra /ACIS data, we have constrained the temperature, density and pressure distributions of gas in the central regions of the host galaxy of Centaurus A, and so the pressure throughout the length of its jet. The model is constrained by the observed profiles of pressure and jet width and conserves matter and energy, enabling us to estimate jet velocities and hence all the other flow properties. Invoking realistic stellar populations within the jet, we find that the increase in its momentum flux exceeds the net pressure force on the jet unless only about one half of the total stellar mass loss is entrained. For self-consistent models, the bulk speed only falls modestly, from ∼0.67 c to ∼0.52 c over the range of 0.25–5.94 kpc from the nucleus. The sonic Mach number varies between ∼5.3 and 3.6 over this range. [ABSTRACT FROM AUTHOR]

We present models for the magnetic support of the 'slingshot prominences' observed in the coronae of rapidly rotating stars. We calculate mechanical equilibria of loops in a spherical geometry. Prominence-forming loops are found first for dipolar and quadrupolar stellar fields that are fully closed. Equilibria are then found within the stellar wind for a dipolar field that becomes open beyond a given radius. We identify two physical processes that may produce gaps in the distribution of prominence heights: the location of this opening radius, and the behaviour of the buoyancy force. The buoyancy may differ from one prominence-bearing loop to another if they are at different temperatures, thus potentially smearing out any gap in observed height distributions. We produce synthetic prominence distributions and compare to the observations of two well-observed stars: AB Doradus and Speedy Mic. The model recovers the more compact prominence distribution observed for Speedy Mic and reproduces better the overall shape of the height distributions for both stars when the opening radius is beyond the co-rotation radius. [ABSTRACT FROM AUTHOR]

We consider donor evolution and mass transfer in the microquasar V404 Cyg/GS 2023+338. Based on X-ray observations of its two outbursts, its average mass accretion rate is substantially lower than our model mass-loss rate from its low-mass giant donor. A likely solution to this discrepancy is that a large fraction of the mass flowing from the donor leaves the binary in the form of outflows from the accretion disc around the accretor. The outflow can be parametrized by the fractions of the mass and angular momentum leaving the system. We calculate the latter as a function of the radius of the accretion disc from which the outflow takes place. We then perform detailed evolutionary calculations for V404 Cyg. Given our estimated average accretion rate, >70 per cent of the mass lost from the donor has to leave the binary. The allowed solutions for the actual donor mass-loss rate are parametrized by our two outflow parameters. Our results are in agreement with the observed outflows from the outer disc as well as with the variable near-Eddington accretion observed during the outbursts, compatible with outflows from the vicinity of the black hole. We then calculate the expected rate of the orbital period change. Its future measurements can constrain the presence of an outflow in V404 Cyg and its parameters. [ABSTRACT FROM AUTHOR]

We present two new planetary systems found around cool dwarf stars with data from the K2 mission. The first system was found in K2-239 (EPIC 248545986), characterized in this work as M3.0V and observed in the 14th campaign of K2. It consists of three Earth-sized transiting planets with radii of 1.1, 1.0, and 1.1 R⊕, showing a compact configuration with orbital periods of 5.24, 7.78, and 10.1 d, close to 2:3:4 resonances. The second was found in K2-240 (EPIC 249801827), characterized in this work as M0.5V and observed in the 15th campaign. It consists of two transiting super-Earths with radii 2.0 and 1.8 R⊕ and orbital periods of 6.03 and 20.5 d. The equilibrium temperatures of the atmospheres of these planets are estimated to be in the range of 380–600 K and the amplitudes of signals in transmission spectroscopy are estimated at ∼10 ppm. [ABSTRACT FROM AUTHOR]

We present optical photometry (i and Z band) and low-resolution spectroscopy (640-1015 nm) of very faint candidate members (J = 20.2-21.2 mag) of the Pleiades star cluster (120 Myr). The main goal is to address their cluster membership via photometric, astrometric, and spectroscopic studies, and to determine the properties of the least massive population of the cluster through the comparison of the data with younger and older spectral counterparts and state-of-the art model atmospheres.We confirm three bona fide Pleiades members that have extremely red optical and infrared colours, effective temperatures of ≈1150 and ≈1350 K, and masses in the interval 11-20 MJup, and one additional likely member that shares the same motion as the cluster but does not appear to be as red as the other members with similar brightness. This latter object requires further near-infrared spectroscopy to fully address its membership in the Pleiades. The optical spectra of two bona fide members were classified as L6-L7 and show features of KI, a tentative detection of Cs I, hydrides, and water vapour with an intensity similar to high-gravity dwarfs of related classification despite their young age. The properties of the Pleiades L6-L7 members clearly indicate that very red colours of L dwarfs are not a direct evidence of ages younger than ≈100 Myr. We also report on the determination of the bolometric corrections for the coolest Pleiades members. These data can be used to interpret the observations of the atmospheres of exoplanets orbiting stars. [ABSTRACT FROM AUTHOR]

The period variations of rotating, pulsating and eclipsing variable stars bear valuable astrophysical information about the presence of companions, evolutionary effects, and the inner structure of the stars. This talk described a universal method for de-trending and re-scaling precise photometric data (Kepler , MOST, CoRoT, OGLE, ...) appropriate for period-change diagnostics of periodic variables. We demonstrated the potential of the method by analysing the period variability of one of the newly-identified Kepler magnetic chemically-peculiar (mCP) stars. We showed that, surprisingly, our target star displays near-sinusoidal changes in its observed light-variations, with a period of 2.85(6) years, which is apparently the result of the presence of a nearby stellar companion. The expected long-term changes of the rotational period, as have been observed in several mCP stars, have not been identified among the sample of Kepler mCP stars. [ABSTRACT FROM AUTHOR]

In this paper we discuss the X-ray point source population of NGC 4472, an elliptical galaxy in the Virgo cluster. We used recent deep Chandra data combined with archival Chandra data to obtain a 380 ks exposure time. We find 238 X-ray point sources within 3.7 arcmin of the galaxy centre, with a completeness flux, FX, 0.5-2 keV = 6.3 × 10-16 erg s-1 cm-2. Most of these sources are expected to be low-mass X-ray binaries. We finding that, using data from a single galaxy which is both complete and has a large number of objects (~100) below 1038 erg s-1, the X-ray luminosity function is well fitted with a single power-law model. By cross matching our X-ray data with both space based and ground based optical data for NGC 4472, we find that 80 of the 238 sources are in globular clusters.We compare the red and blue globular cluster subpopulations and find red clusters are nearly six times more likely to host an X-ray source than blue clusters. We show that there is evidence that these two subpopulations have significantly different X-ray luminosity distributions. Source catalogues for all X-ray point sources, as well as any corresponding optical data for globular cluster sources, are also presented here [ABSTRACT FROM AUTHOR]

“Free-floating, non-deuterium-burning, substellar objects” are isolated bodies of a few Jupiter masses found in very young open clusters and associations, nearby young moving groups, and in the immediate vicinity of the Sun. They are neither brown dwarfs nor planets. In this paper, their nomenclature, history of discovery, sites of detection, formation mechanisms, and future directions of research are reviewed. Most free-floating, non-deuterium-burning, substellar objects share the same formation mechanism as low-mass stars and brown dwarfs, but there are still a few caveats, such as the value of the opacity mass limit, the minimum mass at which an isolated body can form via turbulent fragmentation from a cloud. The least massive free-floating substellar objects found to date have masses of about 0.004 Msol, but current and future surveys should aim at breaking this record. For that, we may need LSST, Euclid and WFIRST.

Carbon and oxygen abundances of eight late M dwarfs are determined based on the near-infrared spectra of medium resolution (R = λ/' λ ≈ 20000). In late M dwarfs, dust forms in their photospheres for Teff below about 2600 K, and this case applies to the M8.5 dwarf 2MASSI J1835379+325954 (hereafter 2MASS 1835+32) whose Teff is 2275 K. The other seven objects with Teff above 2600 K are analyzed with dust-free models. For the case of 2MASS 1835+32 analyzed with the dusty model, the surface temperature is higher by about 600 K due to the blanketing effect of the dust grains, mainly composed of iron grains, and the carbon and oxygen abundances are higher by 0.25 and 0.15 dex, respectively, compared to the analysis by the dust-free model. Once dust forms in the photosphere, the dust works as a kind of thermostat and the temperatures of the surface layers remain nearly the same as the condensation temperatures of the dust grains. For this reason, the temperatures of the surface layers of dusty dwarfs are relatively insensitive to the fundamental parameters, including Teff. In addition, it appears that 2MASS 1835+32 is a rapid rotator, for which its equivalent widths (EWs) are thought to remain unchanged by the rotational broadening. This is, however, true only when the true continuum is well defined. Otherwise, the pseudo-continuum level depends on the rotational velocity and hence the EWs as well. For this reason, the derived abundances depend on the rotational velocity assumed: for the values of Vrotsin i = 37.6 and 44.0kms−1 available in the literature, the derived carbon and oxygen abundances differ by 0.23 and 0.14 dex, respectively, and we find that the higher value provides a better account of the observed spectrum. The resulting carbon and oxygen abundances in the eight late M dwarfs show no systematic difference from our results for the early and middle M dwarfs, and confirm the higher AO/AC ratio at the lower metallicity. In late M dwarfs, CO and H2O remain as excellent abundance indicators of carbon and oxygen, respectively, except for additional uncertainty due to complexity associated with dust formation in the latest M dwarfs. [ABSTRACT FROM AUTHOR]

Inspired by Barnard 68, a Bok globule, that undergoes stable oscillations, we perform mul-tiphase hydrodynamic simulations to analyse the stability of Bok globules. We show that a high-density soft molecular core, with an adiabatic index Y = 0.7 embedded in a warm isothermal diffuse gas, must have a small density gradient to retain the stability. Despite being stable, the molecular core can still collapse spontaneously as it will relax to develop a suffi-ciently large density gradient after tens of oscillations, or a few 107 yr. However, during its relaxation, the core may abruptly collapse triggered by the impingement of small-amplitude, long-wavelength (~6-36 pc) sound waves in the warm gas. This triggered collapse mechanism is similar to a sonoluminescence phenomenon, where underwater ultrasounds can drive air bubble coalescence. The collapse configuration is found to be different from both inside-out and outside-in models of low-mass star formation; none the less the mass flux is close to the prediction of the inside-out model. The condition and the efficiency for this core collapse mechanism are identified. Generally speaking, a broad-band resonance condition must be met, where the core oscillation frequency and the wave frequency should match each other within a factor of several. A consequence of our findings predicts the possibility of propagating low-mass star formation, for which collapse of cores, within a mass range short of one order of magnitude, takes place sequentially tracing the wavefront across a region of few tens of pc over 107 yr. [ABSTRACT FROM AUTHOR]

We present 850 and 450 µm data from the James Clerk Maxwell Telescope (JCMT) Gould Belt Survey obtained with Submillimetre Common-User Bolometer Array 2 (SCUBA-2) and characterize the dust attributes of Class I, Class II and Class III disc sources in L 1495. We detect 23 percent of the sample at both wavelengths, with the detection rate decreasing through the Classes from I to III. The median disc mask is 1.6 × 10-3 M⊙, and only 7 percent of Class II sources have disc masses larger than 20 Jupiter masses. We detect a higher proportion of discs towards sources with stellar hosts of spectral type K than spectral type M. Class II discs with single stellar hosts of spectral type K have higher masses than those of spectral type M, supporting the hypothesis that higher mass stars have more massive discs. Variations in disc masses calculated at the two wavelengths suggest that there may be differences in dust opacity and/or dust temperature between discs with hosts of spectral types K to those with spectral type M. [ABSTRACT FROM AUTHOR]

We report on the search for new eclipsing white dwarf plus main-sequence (WDMS) binaries in the light curves of the Catalina surveys. We use a colour-selected list of almost 2000 candidate WDMS systems from the Sloan Digital Sky Survey, specifically designed to identify WDMS systems with cool white dwarfs and/or early M-type main-sequence stars. We identify a total of 17 eclipsing systems, 14 of which are new discoveries. We also find three candidate eclipsing systems, two main-sequence eclipsing binaries and 22 non-eclipsing close binaries. Our newly discovered systems generally have optical fluxes dominated by the main-sequence components, which have earlier spectral types than the majority of previously discovered eclipsing systems. We find a large number of ellipsoidally variable binaries with similar periods, near 4 h, and spectral types M2-3, which are very close to Roche lobe filling. We also find that the fraction of eclipsing systems is lower than found in previous studies and likely reflects a lower close binary fraction among WDMS binaries with early M-type main-sequence stars due to their enhanced angular momentum loss compared to fully convective late M-type stars, hence causing them to become cataclysmic variables quicker and disappear from the WDMS sample. Our systems bring the total number of known detached, eclipsing WDMS binaries to 71. [ABSTRACT FROM AUTHOR]

ABSTRACT We present time-resolved spectroscopy of the eclipsing, short-period cataclysmic variable CTCV J1300−3052. Using absorption features from the secondary star, we determine the radial velocity semi-amplitude of the secondary star to be K2= 378 ± 6 km s−1, and its projected rotational velocity to be v sin i= 125 ± 7 km s−1. Using these parameters and Monte Carlo techniques, we obtain masses of M1= 0.79 ± 0.05 M⊙ for the white dwarf primary and M2= 0.198 ± 0.029 M⊙ for the M-type secondary star. These parameters are found to be in good agreement with previous mass determinations found via photometric fitting techniques, supporting the accuracy and validity of photometric mass determinations in short-period CVs. [ABSTRACT FROM AUTHOR]

We report resolved near-infrared imaging and spectroscopic observations of the T8.5 binary WISEP J045853.90+643452.6AB obtained with Keck/NIRC2, Keck/OSIRIS, and the Keck Laser Guide Star Adaptive Optics system. These data confirm common proper and radial motion for the two components, and we see the first indications of orbital motion (mostly radial) for this system. H-band spectroscopy identifies both components as very late type brown dwarfs with strong H²0 and CH4 absorption. The spectrum of WISE J0458+6434B also exhibits a compelling signature of NH³ absorption over 1.52-1.54 µm when compared to the T9 dwarf UGPS J072227.51-054031.2. Comparison to T8-Y0 spectral standards and H-band spectral indices indicate classifications of T8.5 and T9.5 for these two components, approaching the boundary between the T dwarf and Y dwarf spectral classes. [ABSTRACT FROM AUTHOR]

TODCOR is a TwO-Dimensional CORrelation technique to measure radial velocities of the two components of a spectroscopic binary. Assuming the spectra of the two components are known, the technique correlates an observed binary spectrum against a combination of the two spectra with different shifts. TODCOR measures simultaneously the radial velocities of the two stars by finding the maximum correlation. The main use of the technique has been to turn single-lined binaries into double-lined systems. This helps to explore the binary mass-ratio distribution, especially the low-mass regime, where the secondaries are usually very faint and therefore hard to detect. The technique has been generalized to study multi-order spectra, and also triple- and quadruple-lined systems. It has several applications in studying extrasolar planets and in the future may even help to dynamically measure stellar masses of binaries through relativistc effects. [ABSTRACT FROM PUBLISHER]

We report the discovery of a very cool, isolated brown dwarf, UGPS 05, with the United Kingdom Infrared Telescope Deep Sky Survey (UKIDSS) Galactic Plane Survey. The near-infrared spectrum displays deeper H2O and CH4 troughs than the coolest known T dwarfs and an unidentified absorption feature at 1.275 μm. We provisionally classify the object as a T10 dwarf but note that it may in future come to be regarded as the first example of a new spectral type. The distance is measured by trigonometric parallax as , making it the closest known isolated brown dwarf. With the aid of Spitzer/Infrared Array Camera (IRAC) we measure = 4.71. It is the coolest brown dwarf presently known - the only known T dwarf that is redder in is the peculiar T7.5 dwarf SDSS J1416+13B, which is thought to be warmer and more luminous than UGPS 05. Our measurement of the luminosity, aided by Gemini/T-ReCS N-band photometry, is . Using a comparison with well-studied T8.5 and T9 dwarfs we deduce . This is supported by predictions of the Saumon & Marley models. With apparent magnitude , UGPS 05 is the brightest of the ∼90 T dwarfs discovered by UKIDSS so far. It offers opportunities for future study via high-resolution near-infrared spectroscopy and spectroscopy in the thermal infrared. [ABSTRACT FROM AUTHOR]

The aim of the Degenerate Objects around Degenerate Objects (DODO) survey is to search for very low-mass brown dwarfs and extrasolar planets in wide orbits around white dwarfs via direct imaging. The direct detection of such companions would allow the spectroscopic investigation of objects with temperatures much lower than the coolest brown dwarfs currently observed. These ultra-low-mass substellar objects would have spectral types >T8.5, and so could belong to the proposed Y dwarf spectral sequence. The detection of a planet around a white dwarf would prove that such objects can survive the final stages of stellar evolution and place constraints on the frequency of planetary systems around their progenitors (with masses between 1.5 and 8 , i.e. early B to mid-F). This paper presents the results of a multi epoch J band common proper motion survey of 23 nearby equatorial and Northern hemisphere white dwarfs. We rule out the presence of any common proper motion companions, with limiting masses determined from the completeness limit of each observation, to 18 white dwarfs. For the remaining five targets, the motion of the white dwarf is not sufficiently separated from the non-moving background objects in each field. These targets require additional observations to conclusively rule out the presence of any common proper motion companions. From our completeness limits, we tentatively suggest that of white dwarfs have substellar companions with between projected physical separations of 60–200 au. [ABSTRACT FROM AUTHOR]

I have re-visited the spatial distribution of stars and high-mass brown dwarfs in the σ Orionis (σ Ori) cluster (∼3 Ma, ∼360 pc). The input was a catalogue of 340 cluster members and candidates at separations less than 30 arcmin to σ Ori AB. Of them, 70 per cent have features of extreme youth. I fitted the normalized cumulative number of objects counting from the cluster centre to several power-law, exponential and King radial distributions. The cluster seems to have two components: a dense core that extends from the centre to 20 arcmin and a rarified halo at larger separations. The radial distribution in the core follows a power law proportional to r1, which corresponds to a volume density proportional to . This is consistent with the collapse of an isothermal spherical molecular cloud. The stars more massive than concentrate, however, towards the cluster centre, where there is also an apparent deficit of very low mass objects . Last, I demonstrated through Monte Carlo simulations that the cluster is azimuthally asymmetric, with a filamentary overdensity of objects that runs from the cluster centre to the Horsehead Nebula. [ABSTRACT FROM AUTHOR]

We present a new analysis of the light curve for the secondary star in the eclipsing binary millisecond pulsar system PSR B1957+20. Combining previous data and new data points at minimum from the Hubble Space Telescope, we have 100 per cent coverage in the R-band. We also have a number of new Ks-band data points, which we use to constrain the infrared magnitude of the system. We model this with the eclipsing light-curve (ELC) code. From the modelling with the ELC code we obtain colour information about the secondary at minimum light in BVRI and K. For our best-fitting model we are able to constrain the system inclination to 65°± 2° for pulsar masses ranging from 1.3 to . The pulsar mass is unconstrained. We also find that the secondary star is not filling its Roche lobe. The temperature of the unirradiated side of the companion is in agreement with previous estimates and we find that the observed temperature gradient across the secondary star is physically sustainable. [ABSTRACT FROM AUTHOR]

While numerous studies have been aimed at understanding the properties of young brown dwarfs relatively little exploration of their potential as drivers of outflows has occurred. Forbidden emission lines are important probes of outflows from young stellar objects, as they trace the shocks which form as an outflow interacts with the ambient medium of its driving source. While forbidden emission was identified in the spectra of young brown dwarfs, indicating the presence of outflows, these lines were weak and confined to the brown dwarf continuum position. Hence their origin in an outflow could not be confirmed. Our approach to this problem, is to analyse the forbidden line regions of brown dwarfs using spectro-astrometry. Spectro-astrometry is a novel technique which allows the user to recover spatial information from a spectrum beyond the limitations of the seeing of the observation. Using this technique we have found two brown dwarf outflows to date. In this chapter we outline this technique, describe our results for the brown dwarfs ρ-Oph 102 and 2MASS1207-3932 and discuss our future plans. [ABSTRACT FROM PUBLISHER]

The results of 27 h of time series photometry of SDSS 121209.31+013627.7 are presented. The binary period established from spectroscopy is confirmed and refined to 0.061 412 d (88.43 min). The photometric variations are dominated by a brightening of about 16 mmag, lasting a little less than half a binary cycle. The amplitude is approximately the same in V, R and white light. A secondary small brightness increase during each cycle may also be present. We speculate that SDSS 121209.31+013627.7 may be a polar in a low state. [ABSTRACT FROM AUTHOR]

In this paper we discuss the nature and the possible formation scenarios of the companion of the brown dwarf 2MASS 1207334-393254. We initially discuss the basic physical properties of this object and conclude that, although from its absolute mass it is a planetary object, in terms of its mass ratio q and of its separation a with respect to the primary brown dwarf, it is consistent with the statistical properties of binaries with higher primary mass. We then explore the possible formation mechanism for this object. We show that the standard planet formation mechanism of core accretion is far too slow to form this object within 10 Myr, the observed age of the system. On the other hand, the alternative mechanism of gravitational instability (proposed both in the context of planet and of binary formation) may, in principle, work and form a system with the observed properties. [ABSTRACT FROM AUTHOR]

The results of more than 100 h of CCD photometric monitoring of 10 L dwarfs and two T dwarfs are presented. Reasonable evidence is shown for IC band variability in DENIS 0255−4700, DENIS 1705−0516 and 2M 2104−1037. Substantial brightening of the T dwarf binaryε Indi Bab was observed over the course of 3.6 h; it is unlikely that this could have been caused by surface spots. No unequivocal variability could be detected in the RC band. [ABSTRACT FROM AUTHOR]