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Context. The second Gaia data release (Gaia DR2) contains high-precision positions, parallaxes, and proper motions for 1.3 billion sources. The resulting Hertzsprung–Russel diagram reveals fine structures throughout the mass range. Aims. This paper aims to investigate the content of Gaia DR2 at the low-mass end and to characterize ultra-cool and brown dwarfs. Methods. We first retrieved the sample of spectroscopically confirmed ultra-cool and brown dwarfs in Gaia DR2. We used their locus in the precise Hertzsprung–Russel diagram to select new candidates and to investigate their properties. Results. The number of spectroscopically confirmed objects recovered in Gaia DR2 corresponds to 61% and 74% of the expected number of objects with an estimated Gaia magnitude Gest ≤ 21.5 and 20.3, respectively. This fills much of the gap to Gaia DR1. Furthermore, Gaia DR2 contains Ȉ13 000 ≥ M7 and 631 new L candidates. A tentative classification suggests that a few hundred of them are young or subdwarf candidates. Their distance distribution shows that the solar neighborhood census is still incomplete. Conclusions. Gaia DR2 offers a great wealth of information on low-mass objects. It provides a homogeneous and precise catalog of candidates that is worthwhile to be further characterized with spectroscopic observations. [ABSTRACT FROM AUTHOR]

The space density of the various classes of cataclysmic variables (CVs) has up to now only been weakly constrained, due to the small number of objects in complete X-ray flux-limited samples and the difficulty in deriving precise distances to CVs. The former limitation still exists. Here the impact of Gaia parallaxes and implied distances on the space density of X-ray-selected complete, flux-limited samples is studied. These samples have been described in the literature: Those of non-magnetic CVs are based on ROSAT (RBS – ROSAT Bright Survey & NEP – North Ecliptic Pole) and that of the intermediate polars (IPs) stems from Swift/BAT. All CVs appear to be rarer than previously thought, although the new values are all within the errors of past studies. Upper limits at 90% confidence for the space densities of non-magnetic CVs are ρRBS < 1.1 × 10−6 pc−3 and ρRBS+NEP < 5.1 × 10−6 pc−3 for an assumed scale height of h = 260 pc and ρIPs < 1.3 × 10−7 pc−3 for the long-period IPs at a scale height of 120 pc. Most of the distances to the IPs have previously been under-estimated. The upper limits to the space densities are only valid in cases where CVs do not have lower X-ray luminosities than the lowest-luminosity member of the sample. These results require confirmation using larger sample sizes, soon to be established through sensitive X-ray all-sky surveys to be performed with eROSITA on the Spektrum-X-Gamma mission. [ABSTRACT FROM AUTHOR]

The majority of searches for radio emission from exoplanets have to date focused on short period planets, i.e., the so-called hot Jupiter type planets. However, these planets are likely to be tidally locked to their host stars and may not generate sufficiently strong magnetic fields to emit electron cyclotron maser emission at the low frequencies used in observations (typically ≥150 MHz). In comparison, the large mass-loss rates of evolved stars could enable exoplanets at larger orbital distances to emit detectable radio emission. Here, we first show that the large ionized mass-loss rates of certain evolved stars relative to the solar value could make them detectable with the LOw Frequency ARray (LOFAR) at 150 MHz (λ = 2 m), provided they have surface magnetic field strengths >50 G. We then report radio observations of three long period (>1 au) planets that orbit the evolved stars β Gem, ι Dra, and β UMi using LOFAR at 150 MHz. We do not detect radio emission from any system but place tight 3σ upper limits of 0.98, 0.87, and 0.57 mJy on the flux density at 150 MHz for β Gem, ι Dra, and β UMi, respectively. Despite our non-detections these stringent upper limits highlight the potential of LOFAR as a tool to search for exoplanetary radio emission at meter wavelengths. [ABSTRACT FROM AUTHOR]

Context. As a probe of the metallicity of proto-planetary disks, stellar metallicity is an important ingredient for giant planet formation, most likely through its effect on the timescales in which rocky or icy planet cores can form. Giant planets have been found to be more frequent around metal-rich stars, in agreement with predictions based on the core-accretion theory. In the metal-poor regime, however, the frequency of planets, especially low-mass planets, and the way it depends on metallicity are still largely unknown. Aims. As part of a planet search programme focused on metal-poor stars, we study the targets from this survey that were observed with HARPS on more than 75 nights. The main goals are to assess the presence of low-mass planets and provide a first estimate of the frequency of Neptunes and super-Earths around metal-poor stars. Methods. We performed a systematic search for planetary companions, both by analysing the periodograms of the radial-velocities and by comparing, in a statistically meaningful way, models with an increasing number of Keplerians. Results. A first constraint on the frequency of planets in our metal-poor sample is calculated considering the previous detection (in our sample) of a Neptune-sized planet around HD 175607 and one candidate planet (with an orbital period of 68.42 d and minimum mass Mpsini = 11.14 ± 2.47 M⊕) for HD 87838, announced in the present study. This frequency is determined to be close to 13% and is compared with results for solar-metallicity stars. [ABSTRACT FROM AUTHOR]

Context. Passive galaxies at high redshift are much smaller than equally massive early types today. If this size evolution is caused by stochastic merging processes, then a small fraction of the compact galaxies should persist until today. Up to now it has not been possible to systematically identify the existence of such objects in Sloan Digital Sky Survey (SDSS). Aims. We aim at finding potential survivors of these compact galaxies in SDSS, as targets for more detailed follow-up observations. Methods. From the virial theorem, it is expected that for a given mass, compact galaxies have stellar velocity dispersion higher than the mean owing to their smaller sizes. Therefore velocity dispersion, coupled with size (or mass), is an appropriate method of selecting relics, independent of the stellar population properties. Based on these considerations, we designed a set of criteria the use the distribution of early-type galaxies from SDSS on the log 10(R0) - log10(σ0) plane to find the most extreme objects on it. We thus selected compact massive galaxy candidates by restricting them to high velocity dispersions σ0 > 323.2 km s-1 and small sizes R0 < 2.18 kpc. Results. We find 76 galaxies at 0.05

We present the first deep photometry of what may be a good candidate for a forming polar-ring galaxy at redshift z ≃ 0.05. The object FCSS J033710.0-354727 is a background galaxy in the VST Deep Field of the Fornax cluster. The deep exposures combined with the high angular resolution of the OmegaCam at VST allowed us to carry out the first detailed photometric analysis for this system in the g and i bands to derive the galaxy structure and colors. Results show that the central object resembles a disk galaxy, surrounded by a ring-like structure that is twice as extended as the central disk. The warped geometry and bright knots observed along the polar direction, as well as some debris detected on the NW side with colors similar to those of the galaxy, suggest that the polar structure is still forming. We argue that the wide polar ring or disk is the result of the ongoing disruption of a companion galaxy in the potential well of the central object, which is two to three times more massive than the accreting galaxy. [ABSTRACT FROM AUTHOR]

Context. Apparent period variations detected in several eclipsing, close-compact binaries are frequently interpreted as being caused by circumbinary giant planets. This interpretation raises the question of the origin of the potential planets that must have either formed in the primordial circumbinary disk, together with the host binary star, and survived its evolution into a close-compact binary or formed in a post-common-envelope circumbinary disk that remained bound to the post-common-envelope binary (PCEB). Aims. Here we combine current knowledge of planet formation and the statistics of giant planets around primordial and evolved binary stars with the theory of close-compact binary star evolution aiming to derive new constraints on possible formation scenarios. Methods. We compiled a comprehensive list of observed eclipsing PCEBs, estimated the fraction of systems showing apparent period variations, reconstructed the evolutionary history of the PCEBs, and performed binary population models of PCEBs to characterize their main sequence binary progenitors. We reviewed the currently available constraints on the fraction of PCEB progenitors that host circumbinary giant planets. Results. We find that the progenitors of PCEBs are very unlikely to be frequent hosts of giant planets (≲10 per cent), while the frequency of PCEBs with observed apparent period variations is very high (≲90 per cent). Conclusions. The variations in eclipse timings measured in eclipsing PCEBs are probably not caused by first-generation planets that survived common-envelope evolution. The remaining options for explaining the observed period variations are second-generation planet formation or perhaps variations in the shape of a magnetically active secondary star. We suggest observational tests for both options. [ABSTRACT FROM AUTHOR]

Aims. On the basis of the PPMXL star catalogue we performed a survey of star clusters in the second quadrant of the Milky Way. Methods. From the PPMXL catalogue of positions and proper motions we took the subset of stars with near-infrared photometry from 2MASS and added the remaining 2MASS stars without proper motions (called 2MAst, i.e. 2MASS with astrometry). We developed a data-processing pipeline including interactive human control of a standardised set of multi-dimensional diagrams to determine kinematic and photometric membership probabilities for stars in a cluster region. The pipeline simultaneously produced the astrophysical parameters of a cluster. From literature we compiled a target list of presently known open and globular clusters, cluster candidates, associations, and moving groups. From established member stars we derived spatial parameters (coordinates of centres and radii of the main morphological parts of clusters) and cluster kinematics (average proper motions and sometimes radial velocities). For distance, reddening, and age determination we used specific sets of theoretical isochrones. Tidal parameters were obtained by a fit of three-parameter King profiles to the observed density distributions of members. Results. We investigated all 871 objects in the 2nd Galactic quadrant, of which we successfully treated 642 open clusters, 2 globular clusters, and 8 stellar associations. The remaining 219 objects (24%) were recognised by us to be nonexistent clusters, duplicate entries, or clusters too faint for 2MAst. We found that our sample is complete in the 2nd quadrant up to a distance of 2 kpc, where the average surface density is 94 clusters per kpc². Compared with literature values we found good agreement in spatial and kinematic data, as well as for optical distances and reddening. Small, but systematic offsets were detected in the age determination. [ABSTRACT FROM AUTHOR]