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We present Keck I/OSIRIS and Keck II/NIRC2 adaptive optics imaging of two member candidates of the Praesepe stellar cluster (d=186.18$\pm$0.11 pc; 590-790 Myr), UGC J08451066+2148171 (L1.5$\pm$0.5) and UGCS J08301935$+$2003293 (no spectroscopic classification). We resolved UGCS J08451066$+$2148171 into a binary system in the near-infrared, with a $K$-band wavelength flux ratio of 0.89$\pm$0.04, a projected separation of 60.3$\pm$1.3 mas (11.2$\pm$0.7 au; 1$\sigma$). We also resolved UGCS J08301935$+$2003293 into a binary system with a flux ratio of 0.46$\pm$0.03 and a separation of 62.5$\pm$0.9 mas. Assuming zero eccentricity, we estimate minimum orbital periods of $\sim$100 years for both systems. According to theoretical evolutionary models, we derive masses in the range of 0.074-0.078 M$_{\odot}$ and 0.072-0.076 M$_{\odot}$ for the primary and secondary of UGCS J08451066$+$2148171 for an age of 700$\pm$100 Myr. In the case of UGCS J08301935$+$2003293, the primary is a low-mass star at the stellar/substellar boundary (0.070-0.078 M$_{\odot}$) while the companion candidate might be a brown dwarf (0.051-0.065 M$_{\odot}$). These are the first two binaries composed of L dwarfs in Praesepe. They are benchmark systems to derive the location of the substellar limit at the age and metallicity of Praesepe, determine the age of the cluster based on the lithium depletion boundary test, derive dynamical masses, and improve low-mass stellar and substellar evolutionary models at a well-known age and metallicity., Comment: 12 pages, 5 figures, 3 tables, accepted for publication in MNRAS

In the past years, some extremely red brown dwarfs were found. They were believed to have low surface gravity, but many of their spectral characteristics were similar to those of high surface gravity brown dwarfs, showing that youth (or low surface gravity) spectral characteristics are poorly understood. We aim to test surface gravity indicators in late-M and early-L brown dwarf spectra using data obtained with the X-shooter spectrograph at the Very Large Telescope. We selected a benchmark sample of brown dwarfs members of Chamaeleon\,I ($\sim$2 Myr), Upper Scorpius (5$-$10~Myr), Pleiades (132$\pm$27 Myr), and Praesepe (590$-$790 Myr) with well-constrained ages, and similar metallicities. We provided a consistent spectral classification of the sample in the optical and in the near-infrared. We measured the equivalent widths of their alkali lines, finding that they have a moderate correlation with age, especially for objects with spectral types M8 and later. We used spectral indices defined in the literature to estimate surface gravity, finding that their gravity assignment is accurate for 75\% of our sample. We investigated the correlation between red colours and age, finding that after $\sim$10~Myr, the colour does not change significantly for our sample {with spectral types M6.0-L3.0}. In this case, red colours might be associated with circumstellar disks, {ring structures, extinction, or viewing angle}. Finally, we calculated the {bolometric luminosity}, {and $J$ and $K$ bolometric corrections} for our sample. We found that six objects are overluminous compared to other members of the same association. Those objects are also flagged as binary candidates by the $Gaia$ survey., Comment: Accepted for publication in MNRAS. The spectra will be available in the publisher version of this paper and in GitHub:

This paper presents the specification, design, and development of the Radial Velocity Spectrometer (RVS) on the European Space Agency's Gaia mission. Starting with the rationale for the full six dimensions of phase space in the dynamical modelling of the Galaxy, the scientific goals and derived top-level instrument requirements are discussed, leading to a brief description of the initial concepts for the instrument. The main part of the paper is a description of the flight RVS, considering the optical design, the focal plane, the detection and acquisition chain, and the as-built performance drivers and critical technical areas. After presenting the pre-launch performance predictions, the paper concludes with the post-launch developments and mitigation strategies, together with a summary of the in-flight performance at the end of commissioning., Comment: 19 pages, 20 figures. Associated with the Gaia Data Release 2

For Gaia DR2 (GDR2), 280 million spectra, collected by the RVS instrument on-board Gaia, were processed and median radial velocities were derived for 9.8 million sources brighter than Grvs = 12 mag. This paper describes the validation and properties of the median radial velocities published in GDR2. Quality tests and filters are applied to select, from the 9.8 million radial velocities, those with the quality to be published in GDR2. The accuracy of the selected sample is assessed with respect to ground-based catalogues. Its precision is estimated using both ground-based catalogues and the distribution of the Gaia radial velocity uncertainties. GDR2 contains median radial velocities for 7 224 631 stars, with Teff in the range [3550, 6900] K, which passed succesfully the quality tests. The published median radial velocities provide a full sky-coverage and have a completness with respect to the astrometric data of 77.2\% (for $G \leq 12.5$ mag). The median radial velocity residuals with respect to the ground-based surveys vary from one catalogue to another, but do not exceed a few 100s m/s. In addition, the Gaia radial velocities show a positive trend as a function of magnitude, which starts around Grvs $\sim 9$ mag and reaches about $+500$ m/s at Grvs $= 11.75$ mag. The overall precision, estimated from the median of the Gaia radial velocity uncertainties, is 1.05 km/s. The radial velocity precision is function of many parameters, in particular the magnitude and effective temperature. For bright stars, Grvs in [4, 8] mag, the precision is in the range 200-350 m/s, which is about 3 to 5 times more precise than the pre-launch specification of 1 km/s. At the faint end, Grvs = 11.75 mag, the precisions for Teff = 5000 K and 6500 K are respectively 1.4 km/s and 3.7 km/s., Comment: Submitted to A&A

The Gaia Data Release 2 contains the 1st release of radial velocities complementing the kinematic data of a sample of about 7 million relatively bright, late-type stars. Aims: This paper provides a detailed description of the Gaia spectroscopic data processing pipeline, and of the approach adopted to derive the radial velocities presented in DR2. Methods: The pipeline must perform four main tasks: (i) clean and reduce the spectra observed with the Radial Velocity Spectrometer (RVS); (ii) calibrate the RVS instrument, including wavelength, straylight, line-spread function, bias non-uniformity, and photometric zeropoint; (iii) extract the radial velocities; and (iv) verify the accuracy and precision of the results. The radial velocity of a star is obtained through a fit of the RVS spectrum relative to an appropriate synthetic template spectrum. An additional task of the spectroscopic pipeline was to provide 1st-order estimates of the stellar atmospheric parameters required to select such template spectra. We describe the pipeline features and present the detailed calibration algorithms and software solutions we used to produce the radial velocities published in DR2. Results: The spectroscopic processing pipeline produced median radial velocities for Gaia stars with narrow-band near-IR magnitude Grvs < 12 (i.e. brighter than V~13). Stars identified as double-lined spectroscopic binaries were removed from the pipeline, while variable stars, single-lined, and non-detected double-lined spectroscopic binaries were treated as single stars. The scatter in radial velocity among different observations of a same star, also published in DR2, provides information about radial velocity variability. For the hottest (Teff > 7000 K) and coolest (Teff < 3500 K) stars, the accuracy and precision of the stellar parameter estimates are not sufficient to allow selection of appropriate templates. [Abridged], Comment: 25 pages, 17 figures, accepted for publication in A&A as part of the Gaia 2nd data release special issue

The European Space Agency Gaia satellite was launched into orbit around L2 in December 2013. This ambitious mission has strict requirements on residual systematic errors resulting from instrumental corrections in order to meet a design goal of sub-10 microarcsecond astrometry. During the design and build phase of the science instruments, various critical calibrations were studied in detail to ensure that this goal could be met in orbit. In particular, it was determined that the video-chain offsets on the analogue side of the analogue-to-digital conversion electronics exhibited instabilities that could not be mitigated fully by modifications to the flight hardware. We provide a detailed description of the behaviour of the electronic offset levels on microsecond timescales, identifying various systematic effects that are known collectively as offset non-uniformities. The effects manifest themselves as transient perturbations on the gross zero-point electronic offset level that is routinely monitored as part of the overall calibration process. Using in-orbit special calibration sequences along with simple parametric models, we show how the effects can be calibrated, and how these calibrations are applied to the science data. While the calibration part of the process is relatively straightforward, the application of the calibrations during science data processing requires a detailed on-ground reconstruction of the readout timing of each charge-coupled device (CCD) sample on each device in order to predict correctly the highly time-dependent nature of the corrections. We demonstrate the effectiveness of our offset non-uniformity models in mitigating the effects in Gaia data. We demonstrate for all CCDs and operating instrument and modes on board Gaia that the video-chain noise-limited performance is recovered in the vast majority of science samples., Comment: 20 pages, 30 figures, accepted for publication in Astronomy and Astrophysics

We directly measure twenty overhanging cliffs on the surface of comet 67P/Churyumov-Gerasimenko extracted from the latest shape model and estimate the minimum tensile strengths needed to support them against collapse under the comet's gravity. We find extremely low strengths of around one Pa or less (one to five Pa, when scaled to a metre length). The presence of eroded material at the base of most overhangs, as well as the observed collapse of two features and implied previous collapse of another, suggests that they are prone to failure and that true material strengths are close to these lower limits (although we only consider static stresses and not dynamic stress from, for example, cometary activity). Thus, a tensile strength of a few pascals is a good approximation for the tensile strength of 67P's nucleus material, which is in agreement with previous work. We find no particular trends in overhang properties with size, over the $\sim10-100$ m range studied here, or location on the nucleus. There are no obvious differences, in terms of strength, height or evidence of collapse, between the populations of overhangs on the two cometary lobes, suggesting that 67P is relatively homogenous in terms of tensile strength. Low material strengths are supportive of cometary formation as a primordial rubble pile or by collisional fragmentation of a small (tens of km) body., Comment: 13 pages, 11 figures. Accepted for publication in Astronomy & Astrophysics

On 3 July 2016, several instruments on board ESA's Rosetta spacecraft detected signs of an outburst event on comet 67P, at a heliocentric distance of 3.32 AU from the sun, outbound from perihelion. We here report on the inferred properties of the ejected dust and the surface change at the site of the outburst. The activity coincided with the local sunrise and continued over a time interval of 14 - 68 minutes. It left a 10m-sized icy patch on the surface. The ejected material comprised refractory grains of several hundred microns in size, and sub-micron-sized water ice grains. The high dust mass production rate is incompatible with the free sublimation of crystalline water ice under solar illumination as the only acceleration process. Additional energy stored near the surface must have increased the gas density. We suggest a pressurized sub-surface gas reservoir, or the crystallization of amorphous water ice as possible causes., Comment: 20 pages, 19 figures, 5 tables

We present a statistical analysis of the distribution of large scale topographic features on comet 67P/Churyumov-Gerasimenko. We observe that the cumulative cliff height distribution across the surface follows a power law with a slope equal to -1.69 +- 0.02. When this distribution is studied independently for each region, we find a good correlation between the slope of the power law and the orbital erosion rate of the surface. For instance, the northern hemisphere topography is dominated by structures on the 100~m scale while the southern hemisphere topography, illuminated at perihelion, is dominated by 10~m scale terrain features. Our study suggest that the current size of a cliff is controlled not only by material cohesion but by the dominant erosional process in each region. This observation can be generalized to other comets, where we argue that primitive nuclei are characterized by the presence of large cliffs with a cumulative height power index equal to or above -1.5, while older, eroded cometary surfaces have a power index equal to or below -2.3. In effect, our model shows that a measure of the topography provides a quantitative assessment of a comet's erosional history, i.e. its evolutionary age.

Images of the nucleus and the coma (gas and dust) of comet 67P/Churyumov- Gerasimenko have been acquired by the OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) cameras since March 2014 using both the Wide Angle Camera (WAC) and the Narrow Angle Camera (NAC). We use images from the NAC camera to study a bright outburst observed in the southern hemisphere on July 29, 2015. The high spatial resolution of the NAC is needed to localize the source point of the outburst on the surface of the nucleus. The heliocentric distance is 1.25 au and the spacecraft-comet distance is 186 km. Aiming to better understand the physics that led to the outgassing, we used the Direct Simulation Monte Carlo (DSMC) method to study the gas flow close to the nucleus and the dust trajectories. The goal is to understand the mechanisms producing the outburst. We reproduce the opening angle of the outburst in the model and constrain the outgassing ratio between the outburst source and the local region. The outburst is in fact a combination of both gas and dust, in which the active surface is approximately 10 times more active than the average rate found in the surrounding areas. We need a number of dust particles 7.83 $\times$ 10$^{11}$ - 6.90 $\times$ 10$^{15}$ (radius 1.97 - 185 {\mu}m), which corresponds to a mass of dust 220 - 21 $\times$ 10$^{3}$kg., Comment: 8 pages, 9 figures

Parallaxes for 331 classical Cepheids, 31 Type II Cepheids and 364 RR Lyrae stars in common between Gaia and the Hipparcos and Tycho-2 catalogues are published in Gaia Data Release 1 (DR1) as part of the Tycho-Gaia Astrometric Solution (TGAS). In order to test these first parallax measurements of the primary standard candles of the cosmological distance ladder, that involve astrometry collected by Gaia during the initial 14 months of science operation, we compared them with literature estimates and derived new period-luminosity ($PL$), period-Wesenheit ($PW$) relations for classical and Type II Cepheids and infrared $PL$, $PL$-metallicity ($PLZ$) and optical luminosity-metallicity ($M_V$-[Fe/H]) relations for the RR Lyrae stars, with zero points based on TGAS. The new relations were computed using multi-band ($V,I,J,K_{\mathrm{s}},W_{1}$) photometry and spectroscopic metal abundances available in the literature, and applying three alternative approaches: (i) by linear least squares fitting the absolute magnitudes inferred from direct transformation of the TGAS parallaxes, (ii) by adopting astrometric-based luminosities, and (iii) using a Bayesian fitting approach. TGAS parallaxes bring a significant added value to the previous Hipparcos estimates. The relations presented in this paper represent first Gaia-calibrated relations and form a "work-in-progress" milestone report in the wait for Gaia-only parallaxes of which a first solution will become available with Gaia's Data Release 2 (DR2) in 2018., Comment: 29 pages, 25 figures. Accepted for publication by A&A

Context. The first Gaia Data Release contains the Tycho-Gaia Astrometric Solution (TGAS). This is a subset of about 2 million stars for which, besides the position and photometry, the proper motion and parallax are calculated using Hipparcos and Tycho-2 positions in 1991.25 as prior information. Aims. We investigate the scientific potential and limitations of the TGAS component by means of the astrometric data for open clusters. Methods. Mean cluster parallax and proper motion values are derived taking into account the error correlations within the astrometric solutions for individual stars, an estimate of the internal velocity dispersion in the cluster, and, where relevant, the effects of the depth of the cluster along the line of sight. Internal consistency of the TGAS data is assessed. Results. Values given for standard uncertainties are still inaccurate and may lead to unrealistic unit-weight standard deviations of least squares solutions for cluster parameters. Reconstructed mean cluster parallax and proper motion values are generally in very good agreement with earlier Hipparcos-based determination, although the Gaia mean parallax for the Pleiades is a significant exception. We have no current explanation for that discrepancy. Most clusters are observed to extend to nearly 15 pc from the cluster centre, and it will be up to future Gaia releases to establish whether those potential cluster-member stars are still dynamically bound to the clusters. Conclusions. The Gaia DR1 provides the means to examine open clusters far beyond their more easily visible cores, and can provide membership assessments based on proper motions and parallaxes. A combined HR diagram shows the same features as observed before using the Hipparcos data, with clearly increased luminosities for older A and F dwarfs., Comment: Accepted for publication by A&A. 21 pages main text plus 46 pages appendices. 34 figures main text, 38 figures appendices. 8 table in main text, 19 tables in appendices

We present the results of the photometric and spectrophotometric properties of the 67P/Churyumov-Gerasimenko nucleus derived with the OSIRIS instrument during the closest fly-by over the comet, which took place on 14 th February 2015 at a distance of {\~} 6 km from the surface. Several images covering the 0{\deg}-33{\deg} phase angle range were acquired, and the spatial resolution achieved was 11 cm/pxl. The flown-by region is located on the big lobe of the comet, near the borders of the Ash, Apis and Imhotep regions. Our analysis shows that this region features local heterogeneities at the decimetre scale. We observed difference of reflectance up to 40{\%} between bright spots and sombre regions, and spectral slope variations up to 50{\%}. The spectral reddening effect observed globally on the comet surface by Fornasier et al. (2015) is also observed locally on this region, but with a less steep behaviour. We note that numerous metre-sized boulders, which exhibit a smaller opposition effect, also appear spectrally redder than their surroundings. In this region, we found no evidence linking observed bright spots to exposed water-ice-rich material. We fitted our dataset using the Hapke 2008 photometric model. The region overflown is globally as dark as the whole nucleus (geometric albedo of 6.8{\%}) and it has a high porosity value in the uppermost-layers (86{\%}). These results of the photometric analysis at a decimetre scale indicate that the photometric properties of the flown-by region are similar to those previously found for the whole nucleus., Comment: 19 pages, 16 figures, 5 tables, (also presented at DPS48/ESPC11: http://cdsads.u-strasbg.fr/abs/2016DPS....4830004F)

Gaia is a space mission currently measuring the five astrometric parameters as well as spectrophotometry of at least 1 billion stars to G = 20.7 mag with unprecedented precision. The sixth parameter in phase space (radial velocity) is also measured thanks to medium-resolution spectroscopy being obtained for the 150 million brightest stars. During the commissioning phase, two fields, one around each ecliptic pole, have been repeatedly observed to assess and to improve the overall satellite performances as well as the associated reduction and analysis software. A ground-based photometric and spectroscopic survey was therefore initiated in 2007, and is still running in order to gather as much information as possible about the stars in these fields. This work is of particular interest to the validation of the Radial Velocity Spectrometer (RVS) outputs. The paper presents the radial velocity measurements performed for the Southern targets in the 12 - 17 R magnitude range on high- to mid-resolution spectra obtained with the GIRAFFE and UVES spectrographs., Comment: 15 pages, 19 figures, 6 tables at CDS

We present OSIRIS/NAC observations of decimetre-sized, likely ice-containing aggregates ejected from a confined region on the surface of comet 67P/Churyumov-Gerasimenko. The images were obtained in January 2016 when the comet was at 2 AU from the Sun out-bound from perihelion. We measure the acceleration of individual aggregates through a two-hour image series. Approximately 50% of the aggregates are accelerated away from the nucleus, and 50% towards it, and likewise towards either horizontal direction. The accelerations are up to one order of magnitude stronger than local gravity, and are most simply explained by the combined effect of gas drag accelerating all aggregates upwards, and the recoil force from asymmetric outgassing, either from rotating aggregates with randomly oriented spin axes and sufficient thermal inertia to shift the temperature maximum away from an aggregate's subsolar region, or from aggregates with variable ice content. At least 10% of the aggregates will escape the gravity field of the nucleus and feed the comet's debris trail, while others may fall back to the surface and contribute to the deposits covering parts of the northern hemisphere. The rocket force plays a crucial role in pushing these aggregates back towards the surface. Our observations show the future back fall material in the process of ejection, and provide the first direct measurement of the acceleration of aggregates in the innermost coma (<2km) of a comet, where gas drag is still significant., Comment: 11 pages, 14 figures, accepted for publication in MNRAS

The Wide Angle Camera of the OSIRIS instrument on board the Rosetta spacecraft is equipped with several narrowband filters that are centered on the emission lines and bands of various fragment species. These are used to determine the evolution of the production and spatial distribution of the gas in the inner coma of comet 67P with time and heliocentric distance, here between 2.6 - 1.3 AU pre-perihelion. Our observations indicate that the emission observed in the OH, OI, CN, NH, and NH2 filters is mostly produced by dissociative electron impact excitation of different parent species. We conclude that CO2 rather than H2O is a significant source of the [OI] 630 nm emission. A strong plume-like feature observed in the in CN and [OI] filters is present throughout our observations. This plume is not present in OH emission and indicates a local enhancement of the CO2/H2O ratio by as much as a factor of 3. We observed a sudden decrease in intensity levels after March 2015, which we attribute to decreased electron temperatures in the first kilometers above the nucleus surface., Comment: 6 figures, 6 tables

We study the link between gravitational slopes and the surface morphology on the nucleus of comet 67P/Churyumov-Gerasimenko and provide constraints on the mechanical properties of the cometary material. We computed the gravitational slopes for five regions on the nucleus that are representative of the different morphologies observed on the surface, using two shape models computed from OSIRIS images by the stereo-photoclinometry (SPC) and stereo-photogrammetry (SPG) techniques. We estimated the tensile, shear, and compressive strengths using different surface morphologies and mechanical considerations. The different regions show a similar general pattern in terms of the relation between gravitational slopes and terrain morphology: i) low-slope terrains (0-20 deg) are covered by a fine material and contain a few large ($>$10 m) and isolated boulders, ii) intermediate-slope terrains (20-45 deg) are mainly fallen consolidated materials and debris fields, with numerous intermediate-size boulders from $<$1 m to 10 m for the majority of them, and iii) high-slope terrains (45-90 deg) are cliffs that expose a consolidated material and do not show boulders or fine materials. The best range for the tensile strength of overhangs is 3-15 Pa (upper limit of 150 Pa), 4-30 Pa for the shear strength of fine surface materials and boulders, and 30-150 Pa for the compressive strength of overhangs (upper limit of 1500 Pa). The strength-to-gravity ratio is similar for 67P and weak rocks on Earth. As a result of the low compressive strength, the interior of the nucleus may have been compressed sufficiently to initiate diagenesis, which could have contributed to the formation of layers. Our value for the tensile strength is comparable to that of dust aggregates formed by gravitational instability and tends to favor a formation of comets by the accrection of pebbles at low velocities., Comment: 18 pages, 15 figures, 1 table; Astronomy and Astrophysics, in press

(Abridged) We report on the near-infrared low-resolution spectroscopy and red optical (Z-band) photometry of seven proper-motion, very low-mass substellar member candidates of the Pleiades cluster with magnitudes in the interval J=17.5-20.8 and K=16.1-18.5 mag. Spectra were acquired for six objects with the LIRIS and NIRSPEC instruments mounted on the 4.2-m WHT and the 10-m Keck II telescopes. Z-band images of two of the faintest candidates were collected with ACAM/WHT. The new data confirm the low temperatures of all seven Pleiades candidates. From the imaging observations, we find extremely red Z-J and Z-K colors that suggest that the faintest target, Calar Pleiades 25, has a Galactic rather than extragalactic nature. We tentatively classify the spectroscopic targets from early-L to ~T0 and suggest that the L/T transition, which accounts for the onset of methane absorption at 2.1 micron, may take place at J,K ~ 20.3, 17.8 mag in the Pleiades (absolute values of M_J ~ 14.7 and M_K ~ 12.2 mag). We find evidence of likely low-gravity atmospheres based on the presence of triangular-shape H-band fluxes and the high flux ratio K/H (compatible with red H-K colors) of Calar Pleiades 20, 21, and 22, which is a feature also seen in field low-gravity dwarfs. Weak KI absorption lines at around 1.25 micron are probably seen in two targets. These observations add support to the cluster membership of all seven objects in the Pleiades. The trend delineated by the spectroscopic sequence of Pleiades late-M and L dwarfs resembles that of the field. With masses estimated at 0.012-0.015 Msol (solar metallicity and 120 Myr), Calar Pleiades 20 (L6 +/- 1), 21 (L7 +/- 1), and 22 (L/T) may become the coolest and least massive Pleiades members that are corroborated with photometry, astrometry, and spectroscopy. Calar Pleiades 25 (<0.012 Msol) is a firm free-floating planetary-mass candidate in the Pleiades., Comment: Accepted for publication in A&A Letters

We present the results of photometric, astrometric, and spectroscopic follow-up of L dwarf candidates identified in the Hyades cluster by Hogan et al. (2008). We obtained low-resolution optical spectroscopy with the OSIRIS spectrograph on the Gran Telescopio de Canarias for all 12 L dwarf candidates as well as new J-band imaging for a subsample of eight to confirm their proper motion. We also present mid-infrared photometry from the Wise Field Infrared Survey Explorer (WISE) for the Hyades L and T dwarf candidates and estimate their spectroscopic distances, effective temperatures, and masses. We confirm the cool nature of several L dwarf candidates and confirm astrometrically their membership, bridging the gap between the coolest M dwarfs and the two T dwarfs previously reported in the Hyades cluster. These members represent valuable spectral templates at an age of 625 Myr and slightly super solar metallicity (Fe/H=+0.13). We update the Hyades mass function across the hydrogen-burning limit and in the substellar regime. We confirm a small number numbers of very-low-mass members below ~0.1 Msun belonging to the Hyades cluster., Comment: 12 pages, 7 figures, 3 tables; accepted for publication in MNRAS

(Abridged) We aim at identifying the least massive population of the solar metallicity, young (120 Myr), nearby (133.5 pc) Pleiades star cluster with the ultimate goal of understanding the physical properties of intermediate-age, free-floating, low-mass brown dwarfs and giant planetary-mass objects, and deriving the cluster substellar mass function across the deuterium-burning mass limit at ~0.012 Msol. We performed a deep photometric and astrometric J- and H-band survey covering an area of ~0.8 deg^2. The images with completeness and limiting magnitudes of J,H ~ 20.2 and ~ 21.5 mag were acquired ~9 yr apart (proper motion precision of +/-6 mas/yr). J- and H-band data were complemented with Z, K, and mid-infrared magnitudes up to 4.6 micron coming from UKIDSS, WISE, and follow-up observations of our own. Pleiades member candidates were selected to have proper motions compatible with that of the cluster, and colors following the known Pleiades sequence in the interval J = 15.5-8.8 mag, and Z_UKIDSS - J > 2.3 mag or Z nondetections for J > 18.8 mag. We found a neat sequence of astrometric and photometric Pleiades substellar member candidates in the intervals J = 15.5-21.2 mag and ~0.072-0.008 Msol. The faintest objects show very red near- and mid-infrared colors exceeding those of field high-gravity dwarfs by >0.5 mag. The Pleiades photometric sequence does not show any color turn-over because of the presence of photospheric methane absorption down to J = 20.3 mag, which is about 1 mag fainter than predicted by the color-computed models. Pleiades brown dwarfs have a proper motion dispersion of 6.4-7.5 mas/yr and are dynamically relaxed at the age of the cluster. The Pleiades mass function extends down to the deuterium burning-mass threshold, with a slope fairly similar to that of other young star clusters and stellar associations., Comment: Accepted for publication in A&A. 16 pages

We analysed the wide-field near-infrared survey of the Praesepe cluster carried out by the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey (GCS) and released by the Data Release 9 (DR9). We compare our Praesepe mass function (MF) with the ones of the Pleiades, alpha Per, and the Hyades. We also present preliminary results of a spectroscopic follow-up for the low mass members (M=<0.1Msol) in Praesepe, alpha Per and Pleiades using the Optical System for Imaging and low Resolution Integrated Spectroscopy (OSIRIS) mounted on the 10.4m Gran Telescopio Canarias (GTC). We also present the optical spectrum of the first L dwarf in Praesepe., Comment: 3 pages, 4 figure, to appear in the proceedings of the Brown Dwarfs come of Age conference

We have obtained a low-resolution optical spectrum for one of the faintest cluster member candidates in Praesepe with the Optical System for Imaging and low Resolution Integrated Spectroscopy mounted on the 10.4m Gran Telescopio de Canarias. We confirm spectroscopically the first L dwarf member in Praesepe, UGCS J084510.66+214817.1. We derived an optical spectral type of L0.3+/-0.4 and estimated its effective temperature to Teff=2279+/-371 K and a mass of 71.1+/-23.0M_Jup, according to state-of-the-art models, placing it at the hydrogen-burning boundary. We measured the equivalent width of the gravity-sensitive sodium doublet at 8182/8194 A, which adds credit to the membership of this new L dwarf to Praesepe. We also derived a probability of ~20.5% that our candidate would be a field L0 dwarf. We conclude that this object is likely to be a true member of Praesepe, with evidence of being a binary system., Comment: 6 pages, 3 figures, accepted for publication in MNRAS

We present the spectroscopic characterization of the unusual high-amplitude very low mass pre-main-sequence periodic variable CHS7797. This study is based on optical medium-resolution (R=2200) spectroscopy in the 6450-8600 A range, carried out with GMOS-GEMINI-S in March 2011. Observations of CHS7797 have been carried out at two distinct phases of the 17.8d period, namely at maximum and four days before maximum. Four different spectral indices were used for the spectral classification at these two phases, all of them well-suited for spectral classification of young and obscured late M dwarfs. In addition, the gravity-sensitive NaI (8183/8195 A) and KI (7665/7699 A) doublet lines were used to confirm the young age of CHS7797. From the spectrum obtained at maximum light we derived a spectral type (SpT) of M6.05, while for the spectrum taken four days before maximum the derived SpT is M5.75. The derived SpTs confirm that CHS7797 has a mass in the stellar-substellar boundary mass range. In addition, the small differences in the derived SpTs at the two observed phases may provide indirect hints that CHS7797 is a binary system of similar mass components surrounded by a tilted circumbinary disk, a system similar to KH15D., Comment: 6 pages, accepted for publication A&A

The aim of the project is to improve our current knowledge of the density of T dwarfs and the shape of the substellar initial mass function by identifying a magnitude-limited sample of T dwarfs in the full southern sky. We present the results of a photometric search aimed at discovering cool brown dwarfs in the Southern sky imaged at infrared wavelengths by the Visible and Infrared Survey Telescope for Astronomy (VISTA) and the Wide Infrared Survey Explorer (WISE) satellite mission. We combined the first data release (DR1) of the VISTA Hemisphere Survey (VHS) and the WISE preliminary data release to extract candidates with red mid-infrared colours and near- to mid-infrared colours characteristics of cool brown dwarfs. The VHS DR1 vs WISE search returned tens of T dwarf candidates, 13 of which are presented here, including two previously published in the literature and five new ones confirmed spectroscopically with spectral types between T4.5 and T8. We estimate that the two T6 dwarfs lie within 16 pc and the T4.5 within 25 pc. The remaining three are 30-50 pc distant. The only T7 dwarf in our sample is the faintest of its spectral class with J=19.28 mag. The other six T dwarf candidates remain without spectroscopic follow-up. We also improve our knowledge on the proper motion accuracy for three bright T dwarfs by combining multi-epoch data from public databases (DENIS, 2MASS, VHS, WISE, Spitzer)., Comment: 13 pages, 2 tables, 4 figures, A&A in press

We report on the discovery of a fourth component in the HD 221356 star system, previously known to be formed by an F8V, slightly metal-poor primary ([Fe/H]=-0.26), and a distant M8V+L3V pair. In our ongoing common proper motion search based on VISTA Hemisphere Survey (VHS) and 2MASS catalogues, we have detected a faint (J=13.76+/-0.04 mag) co-moving companion of the F8 star located at angular separation of 12.13+/-0.18 arcsec (position angle of 221.8+/-1.7), corresponding to a projected distance of ~312 AU at 26 pc. Near-infrared spectroscopy of the new companion, covering the 1.5-2.4 micron wavelength range with a resolving power of R~600, indicates an L1+/-1 spectral type. Using evolutionary models the mass of the new companion is estimated at ~0.08 solar masses, which places the object close to the stellar-substellar borderline. This multiple system provides an interesting example of objects with masses slightly above and below the hydrogen burning mass limit. The low mass companions of HD 221356 have slightly bluer colours than field dwarfs with similar spectral type, which is likely a consequence of the sub-solar metallicity of the system., Comment: 7 pages, 4 figures, accepted for publication in MNRAS

Over the past decades open clusters have been the subject of many studies. Such studies are crucial considering that the universality of the Initial Mass Function is still a subject of current investigations. Praesepe is an interesting open cluster for the study of the stellar and substellar mass function (MF), considering its intermediate age and its nearby distance. Here we present the results of a wide field, near-infrared study of Praesepe using the Data Release 9 (DR9) of the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey (GCS). We obtained cluster candidates of Praesepe based on a 3sigma astrometric and 5 band photometric selection. We derived a binary frequency for Praesepe of 25.6+/-3.0% in the 0.2-0.45Msol mass range, 19.6+/-3.0% for 0.1-0.2Msol, and 23.2+/-5.6% for 0.07-0.1Msol. We also studied the variability of the cluster candidates of Praesepe and we conclude that seven objects could be variable. We inferred the luminosity function of Praesepe in the Z- and J- bands and derived its MF. We observe that our determination of the MF of Praesepe differs from previous studies: while previous MFs present an increase from 0.6 to 0.1Msol, our MF shows a decrease. We looked at the MF of Praesepe in two different regions of the cluster, i.e. within and beyond 1.25deg, and we observed that both regions present a MF which decrease to lower masses. We compared our results with the Hyades, the Pleiades and alpha Per MF in the mass range of 0.072-0.6Msol and showed that the Praesepe MF is more similar to alpha Per although they are respectively aged ~85 and ~600Myr. Even though of similar age, the Praesepe remains different than the Hyades, with a decrease in the MF of only ~0.2 dex from 0.6 down to 0.1Msol, compared to ~1 dex for the Hyades., Comment: 17 pages, 19 figures, accepted for publication in MNRAS. Higher resolution of Figures 1-2-3-5-6-8 in MNRAS published version

We present the results of a deep (J=19.1 mag) infrared (ZYJHK) survey over the full Alpha Persei open cluster extracted from the Data Release 9 of the UKIRT Infrared Deep Sky Survey Galactic Clusters Survey. We have selected ~700 cluster member candidates in ~56 square degrees in \APer{} by combining photometry in five near-infrared passbands and proper motions derived from the multiple epochs provided by the UKIDSS GCS DR9. We also provide revised membership for all previously published APer low-mass stars and brown dwarfs recovered in GCS based on the new photometry and astrometry provided by DR9. We find no evidence of $K$-band variability in members of APer with dispersion less than 0.06-0.09 mag. We employed two independent but complementary methods to derive the cluster luminosity and mass functions: a probabilistic analysis and a more standard approach consisting of stricter astrometric and photometric cuts. We find that the resulting luminosity and mass functions obtained from both methods are consistent. We find that the shape of the APer mass function is similar to that of the Pleiades although the characteristic mass may be higher after including higher mass data from earlier studies (the dispersion is comparable). We conclude that the mass functions of APer, the Pleiades, and Praesepe are best reproduced by a log-normal representation similar to the system field mass function although with some variation in the characteristic mass and dispersion values., Comment: 19 pages, 11 figures, 10 tables (5 in the main text, 5 in the appendix), accepted to MNRAS. arXiv admin note: substantial text overlap with arXiv:1204.2659

Here we present the results of a wide-field (~36 sq. deg.) near-infrared (ZYJHK) survey of the Praesepe cluster using the Data Release 9 of the UKIRT Infrared Deep Sky Survey Galactic Clusters Survey. We selected cluster candidates of Praesepe based on astrometry and photometry. With our candidate list, we have obtained the luminosity function of Praesepe in the Z and J bands, and we have derived the mass function of Praesepe from 0.6 down to 0.072 Msol. Moreover, we have estimated the binarity of the Praesepe members in the 0.45-0.07 Msol mass range and as well as their variability., Comment: 2 pages, 1 figure, to appear in the proceedings of the Labyrinth of Star Formation conference

Although the stellar and substellar populations have been studied in various young and old open clusters, additional studies in clusters in the age range from 5 to 100 Myr is crucial (e.g. to give more constrains on initial mass function variation with improved statistics). Among the open cluster candidates from recent studies, two clusters are best suited for photometric survey of very-low mass stars and brown dwarfs, considering their youth and relative proximity: Alessi 5 (t ~ 40 Myr, d ~ 400 pc) and beta Monocerotis (t ~ 9.1 Myr, d ~ 400 pc). For both clusters, we performed an optical and near-infrared photometric survey, and a virtual observatory survey. Our survey is predicted to be sensitive from the massive B main sequence stars down to brown dwarfs of 30 M_Jup. Here, we present and discuss preliminary results, including the mass function obtained for Alessi 5, which is surprisingly very similar to the mass function of the Hyades (t ~ 600 Myr), although they are of very different ages., Comment: 8 pages, 4 figures, to appear in the online proceeding of the Cool Stars 16 conference

Studies of the mass function (MF) of open clusters of different ages allow us to probe the efficiency with which brown dwarfs (BDs) are evaporated from clusters to populate the field. Surveys in old clusters (age > 100 Myr) do not suffer so severely from several problems encountered in young clusters, such as intra-cluster extinction and large uncertainties in BD models. Here we present the results of a deep photometric survey to study the MF of the old open cluster Praesepe (age 590 Myr and distance 190 pc), down to a 5 sigma detection limit at i~25.6 mag (~40M_Jup). We identify 62 cluster member candidates, of which 40 are substellar, from comparison with predictions from a dusty atmosphere model. The MF rises from the substellar boundary until ~60M_Jup and then declines. This is quite different from the form inferred for other open clusters older than 50 Myr, but seems to be similar to those found in very young open cluster, whose MFs peak at ~10M_Jup. Either Praesepe really does have a different MF from other clusters or they had similar initial MFs but have differed in their dynamical evolution. We further have identified six foreground T dwarf candidates towards Praesepe, which require follow-up spectroscopy to confirm their nature., Comment: 8 pages, 5 figures, to appear in the online proceedings of the Cool Stars 16 conference

[Abridged] In this paper, we present the results of a photometric survey to identify low mass and brown dwarf members of the old open cluster Praesepe (age of 590[+150][-120]Myr and distance of 190[+6.0][-5.8]pc) and use this to infer its mass function which we compare with that of other clusters. We have performed an optical (Ic-band) and near-infrared (J and Ks-band) photometric survey of Praesepe with a spatial coverage of 3.1deg^2. With 5sigma detection limits of Ic=23.4 and J=20.0, our survey is sensitive to objects with masses from about 0.6 to 0.05Msol. The mass function of Praesepe rises from 0.6Msol down to 0.1Msol and then turns-over at ~0.1Msol. The rise observed is in agreement with the mass function derived by previous studies, including a survey based on proper motion and photometry. Comparing our mass function with that for another open cluster with a similar age, the Hyades (age ~ 600Myr), we see a significant difference. Possible reasons are that dynamical evaporation has not influenced the Hyades and Praesepe in the same way, or that the clusters did not have the same initial mass function, or that dynamical interactions have modified the evolution of one or both clusters. Although a difference in the binary fractions of the clusters could cause the observed (i.e. system) mass functions to differ, measurements in the literature give no evidence for a significant difference in the binary fractions of the two clusters. Of our cluster candidates, six have masses predicted to be equal to or below the stellar/substellar boundary at 0.072Msol., Comment: 11 pages, 11 figures, accepted for publication in A&A. Higher resolution of Figures 2-3-4-5 in A&A published version. Revised version corrected for English

We present the stellar and substellar mass function of the open cluster IC2391, plus its radial dependence, and use this to put constraints on the formation mechanism of brown dwarfs. Our multiband optical and infrared photometric survey with spectroscopic follow-up covers 11 square degrees, making it the largest survey of this cluster to date. We observe a radial variation in the mass function over the range 0.072 to 0.3Msol, but no significant variation in the mass function below the substellar boundary at the three cluster radius intervals analyzed. This lack of radial variation for low masses is what we would expect with the ejection scenario for brown dwarf formation, although considering that IC2391 has an age about three times older than its crossing time, we expect that brown dwarfs with a velocity greater than the escape velocity have already escaped the cluster. Alternatively, the variation in the mass function of the stellar objects could be an indication that they have undergone mass segregation via dynamical evolution. We also observe a significant variation across the cluster in the colour of the (background) field star locus in colour-magnitude diagrams and conclude that this is due to variable background extinction in the Galactic plane. From our preliminary spectroscopic follow-up to confirm brown dwarf status and cluster membership, we find that all candidates are M dwarfs (in either the field or the cluster), demonstrating the efficiency of our photometric selection method in avoiding contaminants (e.g. red giants). About half of our photometric candidates for which we have spectra are spectroscopically-confirmed as cluster members; two are new spectroscopically-confirmed brown dwarf members of IC2391., Comment: 57 pages, 20 figures, accepted for publication in ApJ

Using the Wide Field Imager (WFI) at the ESO 2.2m telescope at La Silla and the CPAPIR camera at the CTIO 1.5m telescope at Cerro Tololo, we have performed an extensive, multiband photometric survey of the open cluster IC2391 (D~146pc, age~50Myr, solar metallicity). Here we present the results from our photometric survey and from a spectroscopic follow-up of the central part of the survey., Comment: 4 pages, 3 figures, to appear in the proceedings of the Cool Stars 15 conference

We discuss some of our current knowledge of the mass distribution of DA and non-DA stars using various methods for measuring white dwarf masses including spectroscopic, trigonometric parallax, and gravitational redshift measurements, with a particular emphasis on the problems encountered at the low end of the cooling sequence where energy transport by convection becomes important., Comment: 6 pages, 4 figures, to appear in the ASP Conference Series for the 15th European White Dwarf Workshop

White dwarf masses can be determined in several ways. Here we compare masses obtained from Balmer line spectroscopy with those derived from trigonometric parallax measurements for an ensemble of cool hydrogen-atmosphere white dwarfs., Comment: 4 pages, 5 figures, 14th European Workshop on White Dwarfs, ASP Conference Series, In press

We report the discovery of two new ZZ Ceti pulsators, LP 133-144 and HE 1258+0123, selected on the basis of model atmosphere fits to optical spectroscopic data. The atmospheric parameters for LP 133-144, Teff = 11,800 +/- 200 K and log g = 7.87 +/- 0.05, and for HE 1258+0123, Teff = 11,410 +/- 200 K and log g = 8.04 +/- 0.05, place them within the empirical boundaries of the ZZ Ceti instability strip. This brings the number of known ZZ Ceti stars to a total of 36, a quarter of which have now been discovered using the spectroscopic approach for estimating their atmospheric parameters. This method has had a 100% success rate so far in predicting the variability of candidate ZZ Ceti stars. We have also analyzed additional spectra of known nonvariable white dwarfs in the vicinity of the ZZ Ceti instability strip. Our study further strengthens the idea that ZZ Ceti stars occupy a pure region in the log g-Teff plane, a region where no nonvariable stars are found. This result supports the thesis that ZZ Ceti pulsators represent a phase through which all DA stars must evolve., Comment: 11 pages, 4 figures, accepted for publication in the Astrophysical Journal