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Context. Accretion among planetary mass companions is a poorly understood phenomenon, due to the lack of both observational and theoretical studies. The detection of emission lines from accreting gas giants facilitates detailed investigations into this process. Aims. This work presents a detailed analysis of Balmer lines from one of the few known young, planetary-mass objects with observed emission, the isolated L2γ dwarf 2MASS J11151597+1937266 with a mass between 7 and 21 MJup and an age of 5–45 Myr, located at 45 ± 2 pc. Methods. We obtained the first high-resolution (R ~ 50 000) spectrum of the target with VLT/UVES, an echelle spectrograph operating in the near-ultraviolet to visible wavelengths (3200–6800 Å). Results. We report several resolved hydrogen (H I; H3–H6) and helium (He I; λ5875.6) emission lines in the spectrum. Based on the asymmetric line profiles of Hα and Hβ, the 10% width of Hα (199 ± 1 km s−1), tentative He Iλ6678 emission, and indications of a disk from mid-infrared excess, we confirm ongoing accretion at this object. Using the Gaia update of the parallax, we revise its temperature to 1816 ± 63 K and radius to 1.5 ± 0.1 RJup. Analysis of observed H I profiles using a 1D planet-surface shock model implies a pre-shock gas velocity, v0 = 120−40+ 80 km s−1, and a pre-shock density, log(n0/cm−3) = 14−5+ 0. The pre-shock velocity points to a mass, Mp = 6−4+ 8MJup, for the target. Combining H I line luminosities (Lline) and planetary Lline−Lacc (accretion luminosity) scaling relations, we derived a mass accretion rate, Ṁacc = 1.4−0.9+ 2.8 × 10−8MJup yr−1. Conclusions. The line-emitting area predicted from the planet-surface shock model is very small (~0.03%), and points to a shock at the base of a magnetospherically induced funnel. The Hα profile exhibits a much stronger flux than predicted by the model that best fits the rest of the H I profiles, indicating that another mechanism than shock emission contributes to the Hα emission. Comparison of line fluxes and Ṁacc from archival moderate-resolution SDSS spectra indicate variable accretion at 2MASS J11151597+1937266. [ABSTRACT FROM AUTHOR]

Context. Brown dwarfs in the spectral range L9–T3.5, within the so called L/T transition, have been shown to be variable at higher amplitudes and with greater frequency than other field dwarfs. This strong variability allows for the probing of their atmospheric structure in 3D through multi-wavelength observations for studying the underlying physical mechanisms responsible for the variability. The few known strongly variable dwarfs in this range have been extensively studied. Now, more variables such as these need to be discovered and studied to better constrain atmospheric models. This is also critical to better understand giant exoplanets and to shed light on a number of possible correlations between brown dwarf characteristics and variability. Aims. Previous studies suggest an occurrence rate for strong variability (peak-to-peak amplitudes > 2%) of up to ∼39% among brown dwarfs within the L/T transition. In this work, we aim to discover new strong variables in this spectral range by targeting ten previously unsurveyed brown dwarfs. Methods. We used the NOTCam at the Nordic Optical Telescope to observe 11 targets, with spectral types ranging from L9.5 to T3.5, in the J-band during October 2017 and September 2018. Using differential aperture photometry, we then analysed the light curves for significant variability using Lomb-Scargle periodogram algorithms and least squares fitting. Results. We report first discoveries of strong and significant variability in four out of the ten targets (false alarm probability < 0.1%), measuring peak-to-peak amplitudes up to 10.7 ± 0.4% in J for the T1 dwarf 2MASS J22153705+2110554, for which we observe significant light curve evolution between the 2017 and 2018 epochs. We also report a marginally significant detection of strong variability, and confirm that the well known 2MASS J01365662+0933473 is still strongly variable three years after the last reported epoch. Finally, we present an extensive multi-epoch catalogue of strong variables reported in the literature and discuss possible correlations that are identifiable from the catalogue. Conclusions. We significantly add to the number of known strong variables, and through Poisson statistics infer an occurrence rate for strong variability among L9–T3.5 brown dwarfs of 40 +32−19 40 − 19 + 32 $ {40}^{+32}_{-19} $ %, which is in agreement with previous estimates. The new variables identified in this work are also excellently suited for extensive multi-wavelength observations dedicated to probing the 3D structure of brown dwarf atmospheres. [ABSTRACT FROM AUTHOR]

We observe 14 young low-mass substellar objects using the VLT/SINFONI integral field spectrograph with laser guide star adaptive optics to detect and characterize three candidate binary systems. All three binary candidates show strong signs of youth, with two of them likely belonging to young moving groups. Together with the adopted young-moving-group ages we employ isochrones from the BT-Settle CIFIST substellar evolutionary models to estimate individual masses for the binary components. We find 2MASS J15104786–2818174 to be part of the ≈30−50 Myr Argus moving group and to be composed of a 34−48 MJup primary brown dwarf with spectral type M9γ and a fainter 15−22 MJup companion, separated by ≈100 mas. 2MASS J22025794–5605087 is identified as an almost equal-mass binary in the AB Dor moving group, with a projected separation of ≈60 mas. Both components share spectral type M9γ/β, which with the adopted age of 120−200 Myr yields masses in the range of 50−68 MJup for each component individually. The observations of 2MASS J15474719–2423493 are of lower quality and we obtain no spectral characterization for the target, but resolve two components separated by ≈170 mas which with the predicted young field age of 30−50 Myr yields individual masses below 20 MJup. Out of the three candidate binary systems, 2MASS J22025794–5605087 has unambiguous spectroscopic signs of being a bona-fide binary, while the other two will require second-epoch confirmation. The small projected separations between the binary components correspond to physical separations of ≈4−7 AU, meaning that astrometric monitoring of just a few years would be adequate to generate constrained orbital fits and dynamical masses for the systems. In combination with their young ages, these binaries will prove to be excellent benchmarks for calibrating substellar evolutionary models down to a very low-mass regime. [ABSTRACT FROM AUTHOR]

Wide low-mass substellar companions are known to be very rare among low-mass stars, but appear to become increasingly common with increasing stellar mass. However, B-type stars, which are the most massive stars within ~150 pc of the Sun, have not yet been examined to the same extent as AFGKM-type stars in that regard. In order to address this issue, we launched the ongoing B-star Exoplanet Abundance Study (BEAST) to examine the frequency and properties of planets, brown dwarfs, and disks around B-type stars in the Scorpius-Centaurus (Sco-Cen) association; we also analyzed archival data of B-type stars in Sco-Cen. During this process, we identified a candidate substellar companion to the B9-type spectroscopic binary HIP 79098 AB, which we refer to as HIP 79098 (AB)b. The candidate had been previously reported in the literature, but was classified as a background contaminant on the basis of its peculiar colors. Here we demonstrate that the colors of HIP 79098 (AB)b are consistent with several recently discovered young and low-mass brown dwarfs, including other companions to stars in Sco-Cen. Furthermore, we show unambiguous common proper motion over a 15-yr baseline, robustly identifying HIP 79098 (AB)b as a bona fide substellar circumbinary companion at a 345 ± 6 AU projected separation to the B9-type stellar pair. With a model-dependent mass of 16–25 MJup yielding a mass ratio of <1%, HIP 79098 (AB)b joins a growing number of substellar companions with planet-like mass ratios around massive stars. Our observations underline the importance of common proper motion analysis in the identification of physical companionship, and imply that additional companions could potentially remain hidden in the archives of purely photometric surveys. [ABSTRACT FROM AUTHOR]

We report the results of a J -band survey for photometric variability in a sample of young, low-gravity objects using the New Technology Telescope (NTT) and the United Kingdom Infrared Telescope (UKIRT). Surface gravity is a key parameter in the atmospheric properties of brown dwarfs and this is the first large survey that aims to test the gravity dependence of variability properties. We do a full analysis of the spectral signatures of youth and assess the group membership probability of each target using membership tools from the literature. This results in a 30 object sample of young low-gravity brown dwarfs. Since we are lacking in objects with spectral types later than L9, we focus our statistical analysis on the L0–L8.5 objects. We find that the variability occurrence rate of L0–L8.5 low-gravity brown dwarfs in this survey is |$30^{+16}_{-8}{{\ \rm per\ cent}}$|⁠. We reanalyse the results of Radigan (2014) and find that the field dwarfs with spectral types L0–L8.5 have a variability occurrence rate of |$11^{+13}_{-4}{{\ \rm per\ cent}}$|⁠. We determine a probability of |$98{{\ \rm per\ cent}}$| that the samples are drawn from different distributions. This is the first quantitative indication that the low-gravity objects are more likely to be variable than the field dwarf population. Furthermore, we present follow-up JS and KS observations of the young, planetary-mass variable object PSO 318.5–22 over three consecutive nights. We find no evidence of phase shifts between the JS and KS bands and find higher JS amplitudes. We use the JS light curves to measure a rotational period of 8.45 ± 0.05 h for PSO 318.5–22. [ABSTRACT FROM AUTHOR]

Stellar multiplicity properties have been studied for the lowest and the highest stellar masses, but intermediate-mass stars from F-type to late A-type have received relatively little attention. Here, we report on a Gemini/NICI snapshot imaging survey of 138 such stars in the young Scorpius–Centaurus (Sco–Cen) region, for the purpose of studying multiplicity with sensitivity down to planetary masses at wide separations. In addition to two brown dwarfs and a companion straddling the hydrogen-burning limit which we reported previously, here we present 26 new stellar companions and determine a multiplicity fraction within 0''.1–5''.0 of 21% ± 4%. Depending on the adopted semimajor axis distribution, our results imply a total multiplicity in the range of∼60%–80%, which further supports the known trend of a smooth continuous increase in the multiplicity fraction as a function of primary stellar mass. A surprising feature in the sample is a distinct lack of nearly equal-mass binaries, for which we discuss possible reasons. The survey yielded no additional companions below or near the deuterium-burning limit, implying that their frequency at >200 AU separations is not quite as high as might be inferred from previous detections of such objects within the Sco–Cen region. [ABSTRACT FROM AUTHOR]

Core accretion and disk instability have traditionally been regarded as the two competing possible paths of planet formation. In recent years, evidence has accumulated in favor of core accretion as the dominant mode, at least for close-in planets. However, it might be hypothesized that a significant population of wide planets formed by disk instabilities could exist at large separations, forming an invisible majority. In previous work, we addressed this issue through a direct imaging survey of B2-A0-type stars and concluded that <30% of such stars form and retain planets and brown dwarfs through disk instability, leaving core accretion as the likely dominant mechanism. In this paper, we extend this analysis to FGKM-type stars by applying a similar analysis to the Gemini Deep Planet Survey sample. The results strengthen the conclusion that substellar companions formed and retained around their parent stars by disk instabilities are rare. Specifically, we find that the frequency of such companions is <8% for FGKM-type stars under our most conservative assumptions, for an outer disk radius of 300 AU, at 99% confidence. Furthermore, we find that the frequency is always < 10% at 99% confidence independently of outer disk radius, for any radius from 5 to 500 AU. We also simulate migration at a wide range of rates and find that the conclusions hold even if the companions move substantially after formation. Hence, core accretion remains the likely dominant formation mechanism for the total planet population, for every type of star from M-type through B-type. [ABSTRACT FROM AUTHOR]

There has been a long-standing discussion in the literature as to whether core accretion or disk instability is the dominant mode of planet formation. Over the last decade, several lines of evidence have been presented showing that core accretion is most likely the dominant mechanism for the close-in population of planets probed by radial velocity and transits. However, this does not by itself prove that core accretion is the dominant mode for the total planet population, since disk instability might conceivably produce and retain large numbers of planets in the far-out regions of the disk. If this is a relevant scenario, then the outer massive disks of B-stars should be among the best places for massive planets and brown dwarfs to form and reside. In this study, we present high-contrast imaging of 18 nearby massive stars of which 15 are in the B2-A0 spectral-type range and provide excellent sensitivity to wide companions. By comparing our sensitivities to model predictions of disk instability based on physical criteria for fragmentation and cooling, and using Monte Carlo simulations for orbital distributions, we find that ~85% of such companions should have been detected in our images on average. Given this high degree of completeness, stringent statistical limits can be set from the null-detection result, even with the limited sample size. We find that <30% of massive stars form and retain disk instability planets, brown dwarfs, and very low mass stars of < 100 Mjup within 300 AU, at 99% confidence. These results, combined with previous findings in the literature, lead to the conclusion that core accretion is likely the dominant mode of planet formation. [ABSTRACT FROM AUTHOR]

Recent direct imaging discoveries of exoplanets have raised new questions about the formation of very low-mass objects in very wide orbits. Several explanations have been proposed, but all of them run into some difficulties, trying to explain all the properties of these objects at once. Here we present the results of a deep adaptive optics imaging survey of 85 stars in the Upper Scorpius young association with Gemini, reaching contrasts of up to 10 magnitudes. In addition to identifying numerous stellar binaries and a few triples, we also found several interesting sub-stellar companions. We discuss the implications of these discoveries, including the possibility of a second pathway to giant planet formation. [ABSTRACT FROM PUBLISHER]

Binary/multiple properties provide clues to the formation of stars. In the AstraLux binary survey, we use the Lucky Imaging technique to search for companions to a large sample of young, nearby M dwarfs. We present results from observations of the first sub-sample, consisting of 124 M dwarfs in the southern sky. [ABSTRACT FROM PUBLISHER]