E. L. Martín

GLANCING VIEWS OF THE EARTH: FROM A LUNAR ECLIPSE TO AN EXOPLANETARY TRANSIT

It has been posited that lunar eclipse observations may help predict the in-transit signature of Earth-like extrasolar planets. However, a comparative analysis of the two phenomena addressing in detail the transport of stellar light through the planets atmosphere has not yet been presented. Here, we proceed with the investigation of both phenomena by making use of a common formulation. Our starting point is a set of previously unpublished near-infrared spectra collected at various phases during the August 2008 lunar eclipse. We then take the formulation to the limit of an infinitely distant observer in order to investigate the in-transit signature of the Earth-Sun system as being observed from outside our Solar System. The refraction-bending of sunlight rays that pass through the Earths atmosphere is a critical factor in the illumination of the eclipsed Moon. Likewise, refraction will have an impact on the in-transit transmission spectrum for specific planet-star systems depending on the refractive properties of the planets atmosphere, the stellar size and the planets orbital distance. For the Earth-Sun system, at mid-transit, refraction prevents the remote observers access to the lower ~12-14 km of the atmosphere and, thus, also to the bulk of the spectroscopically-active atmospheric gases. We demonstrate that the effective optical radius of the Earth in transit is modulated by refraction and varies by ~12 km from mid-transit to 2nd contact. The refractive nature of atmospheres, a property which is rarely accounted for in published investigations, will pose additional challenges to the characterization of Earth-like extrasolar planets. Refraction may have a lesser impact for Earth-like extrasolar planets within the habitable zone of some M-type stars.

Chemical abundances of late-type pre-main sequence stars in the σ Orionis cluster

Context. The young σ Orionis cluster is an important location for studying the formation and evolution of stars, brown dwarfs, and planetary-mass objects. Its metallicity, although it is a fundamental parameter, has not been well determined yet. Aims. We present the first determination of the metallicity of nine young late-type stars in σ Orionis. Methods. Using the optical and near-infrared broadband photometry available in the literature we derive the effective temperatures for these nine cluster stars, which lie in the interval 4300–6500 K (1–3 M� ). These parameters are employed to compute a grid of synthetic spectra based on the code MOOG and Kurucz model atmospheres. We employ a χ 2 -minimization procedure to derive the stellar surface gravity and atmospheric abundances of Al, Ca, Si, Fe, Ni and Li, using multi-object optical spectroscopy taken with WYFFOS+AF2 at the William Herschel Telescope (λ/δλ ∼ 7500). Results. The average metallicity of the σ Orionis cluster is [Fe/H] = −0.02 ± 0.09 ± 0.13 (random and systematic errors). The abundances of the other elements, except lithium, seem to be consistent with solar values. Lithium abundances are in agreement with the “cosmic” 7 Li abundance, except for two stars which show a log � (Li) in the range 3.6–3.7 (although almost consistent within the error bars). There are also other two stars with log � (Li) ∼ 2.75. We derived an average radial velocity of the σ Orionis cluster of 28 ± 4k m s −1 . Conclusions. The σ Orionis metallicity is roughly solar.

Astrometric orbit of a low-mass companion to an ultracool dwarf

Little is known about the existence of extrasolar planets around ultracool dwarfs. Furthermore, binary stars with Sun-like primaries and very low-mass binaries composed of ultracool dwarfs show di erences in the distributions of mass ratio and orbital separation that can be indicative of distinct formation mechanisms. Using FORS2/VLT optical imaging for high precision astrometry we are searching for planets and substellar objects around ultracool dwarfs to investigate their multiplicity properties for very low companion masses. Here we report astrometric measurements with an accuracy of two tenths of a milli-arcsecond over two years that reveal orbital motion of the nearby L1.5 dwarf DENIS-P J082303.1-491201 located at 20:77 0:08 pc caused by an unseen companion that revolves about its host on an eccentric orbit in 246:4 1:4 days. We estimate the L1.5 dwarf to have 7:5 0:7% of the Sun’s mass that implies a companion mass of 28 2 Jupiter masses. This new system has the smallest mass ratio (0:36 0:02) of known very low-mass binaries with characterised orbits. With this discovery we demonstrate 200 micro-arcsecond astrometry over an arc-minute field and over several years that is su cient to discover sub-Jupiter mass planets around ultracool dwarfs. We also show that the achieved parallax accuracy of < 0.4 % makes it possible to remove distance as a dominant source of uncertainty in the modelling of ultracool dwarfs.

Narrow band Hα photometry of the super-Earth GJ 1214b with GTC/OSIRIS tunable filters

The super-earth planet GJ 1214b has recently been the focus of several studies, using the transit spectroscopy technique, trying to determine the nature of its atmosphere. Here we focus on the Halpha line as a tool to further restrict the nature of GJ1214s atmosphere. We used the Gran Telescopio Canarias (GTC) OSIRIS instrument to acquire narrow band photometry with tunable filters. With our observations, we were able to observe the primary transit of the super-Earth GJ 1214b in three bandpasses: two centered in the continuum around Halpha (653.5 nm and 662.0 nm) and one centered at the line core (656.3 nm). We measure the depth of the planetary transit at each wavelength interval.By fitting analytic models to the measured light curves we were able to compute the depth of the transit at the three bandpasses. Taking the difference in the computed planet to star radius ratio between the line and the comparison continuum filters, we find Delta (Rp/Rstar)_{Halpha-653.5} = (6.60 +/- 3.54) 10^-3 and Delta (Rp/Rstar)_{Halpha-662.0} = (3.30 +/- 3.61) 10^-3. Although the planet radius is found to be larger in the Halpha line than in the surrounding continuum, the quality of our observations and the sigma level of the differences (1.8 and 1.0, respectively) does not allow us to claim an Halpha excess in GJ1214s atmosphere. Further observations will be needed to resolve this issue.

Astrometric planet search around southern ultracool dwarfs - I. First results, including parallaxes of 20 M8–L2 dwarfs

Context. Extrasolar-planet searches that target very low-mass stars and brown dwarfs are hampered by intrinsic or instrumental limitations. Time series of astrometric measurements with precisions better than one milli-arcsecond can yield new evidence on the planet occurrence around these objects. Aims. We present first results of an astrometric search for planets around 20 nearby dwarf stars with spectral types M8‐L2. Methods. Over a time-span of two years, we obtained I-band images of the target fields with the FORS2 camera at the Very Large Telescope. Using background stars as references, we monitored the targets’ astrometric trajectories, which allowed us to measure parallax and proper motions, set limits on the presence of planets, and to discover the orbital motions of two binary systems. Results. We determined trigonometric parallaxes with an average accuracy of 0.09 mas (’ 0.2 %), which resulted in a reference sample for the study of ultracool dwarfs at the M/L transition, whose members are located at distances of 9.5‐40 pc. This sample contains two newly discovered tight binaries (DE0630 18 and DE0823 49) and one previously known wide binary (DE1520 44). Only one target shows I-band variability >5 mmag r.m.s. We derived planet exclusion limits that set an upper limit of 9 % on the occurrence of giant planets with masses & 5 MJ in intermediate-separation (0.01‐0.8 AU) orbits around M8‐L2 dwarfs. Conclusions. We demonstrate that astrometric observations with an accuracy of 120 as over two years are feasible from the ground and can be used for a planet-search survey. The detection of two tight very low-mass binaries shows that our search strategy is e cient and may lead to the detection of planetary-mass companions through follow-up observations.

New ultracool subdwarfs identified in large-scale surveys using Virtual Observatory tools (Corrigendum) - I. UKIDSS LAS DR5 vs. SDSS DR7

Aims. The aim of the project is to improve our knowledge of the low-mass and low-metallicity population to investigate the influence of metallicity on the stellar (and substellar) mass function. Methods. We present the results of a photometric and proper motion search aimed at discovering ultracool subdwarfs in large-scale surveys. We employed and combined the Fifth Data Release (DR5) of the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) and the Sloan Digital Sky Survey (SDSS) Data Release 7 complemented with ancillary data from the Two Micron All-Sky Survey (2MASS), the DEep Near-Infrared Survey (DENIS) and the SuperCOSMOS Sky Surveys (SSS). Results. The SDSS DR7 vs. UKIDSS LAS DR5 search returned a total of 32 ultracool subdwarf candidates, only two of which are recognised as a subdwarf in the literature. Twenty-seven candidates, including the two known ones, were followed-up spectroscopically in the optical between 600 and 1000 nm, thus covering strong spectral features indicative of low metallicity (e.g., CaH), 21 with the Very Large Telescope, one with the Nordic Optical Telescope, and five were extracted from the Sloan spectroscopic database to assess (or refute) their low-metal content. We confirm 20 candidates as subdwarfs, extreme subdwarfs, or ultra-subdwarfs with spectral types later than M5; this represents a success rate of ≥60%. Among those 20 new subdwarfs, we identify two early-L subdwarfs that are very likely located within 100 pc, which we propose as templates for future searches because they are the first examples of their subclass. Another seven sources are solar-metallicity M dwarfs with spectral types between M4 and M7 without Hα emission, suggesting that they are old M dwarfs. The remaining five candidates do not have spectroscopic follow-up yet; only one remains as a bona-fide ultracool subdwarf after revision of their proper motions. We assigned spectral types based on the current classification schemes and, when possible, we measured their radial velocities. Using the limited number of subdwarfs with trigonometric parallaxes, we estimated distances ranging from ∼95 to ∼600 pc for the new subdwarfs. We provide mid-infrared photometry extracted from the WISE satellite databases for two subdwarfs and discuss their colours. Finally, we estimate a lower limit of the surface density of ultracool subdwarfs about 5000–5700 times lower than that of solar-metallicity late-M dwarfs.

GTC/OSIRIS SPECTROSCOPIC IDENTIFICATION OF A FAINT L SUBDWARF IN THE UKIRT INFRARED DEEP SKY SURVEY

We present the discovery of an L subdwarf in 234?deg2 common to the UK InfraRed Telescope (UKIRT) Infrared Deep Sky Survey Large Area Survey Data Release 2 and the Sloan Digital Sky Survey Data Release 3. This is the fifth L subdwarf announced to date, the first one identified in the UKIRT Infrared Deep Sky Survey, and the faintest known. The blue optical and near-infrared colors of ULAS?J135058.86+081506.8 and its overall spectra energy distribution are similar to the known mid-L subdwarfs. Low-resolution optical (700-1000?nm) spectroscopy with the Optical System for Imaging and low Resolution Integrated Spectroscopy spectrograph on the 10.4?m Gran Telescopio de Canarias reveals that ULAS?J135058.86+081506.8 exhibits a strong K I pressure-broadened line at 770?nm and a red slope longward of 800?nm, features characteristics of L-type dwarfs. From direct comparison with the four known L subdwarfs, we estimate its spectral type to be sdL4-sdL6 and derive a distance in the interval 94-170?pc. We provide a rough estimate of the space density for mid-L subdwarfs of 1.5 ? 10?4?pc?3.

INTERMEDIATE RESOLUTION NEAR-INFRARED SPECTROSCOPY OF 36 LATE M DWARFS

We present observations of 36 late M dwarfs obtained with the Keck II/NIRSPEC in the J band at a resolution of {approx}20,000. We have measured projected rotational velocities, absolute radial velocities, and pseudo-equivalent widths of atomic lines. Twelve of our targets did not have previous measurements in the literature. For the other 24 targets, we confirm previously reported measurements. We find that 13 stars from our sample have v sin i below our measurement threshold (12 km s{sup -1}) whereas four of our targets are fast rotators (v sin i > 30 km s{sup -1}). As fast rotation causes spectral features to be washed out, stars with low projected rotational velocities are sought for radial velocity surveys. At our intermediate spectral resolution, we have confirmed the identification of neutral atomic lines reported in McLean et al. We also calculated pseudo-equivalent widths of 12 atomic lines. Our results confirm that the pseudo-equivalent width of K I lines is strongly dependent on spectral types. We observe that the pseudo-equivalent width of Fe I and Mn I lines remains fairly constant with later spectral type. We suggest that these lines are particularly suitable for deriving metallicities for late M dwarfs.

WISE/2MASS-SDSS brown dwarfs candidates using Virtual Observatory tools

Massive far-red and infrared imaging surveys in different bandpasses are the main contributors to the discovery of brown dwarfs. The Virtual Observatory represents an adequate framework to handle these vast datasets efficiently and filter them out according to specific requirements. A statistically significant number of BDs is mandatory for understanding their general properties better for identifing peculiar objects. WISE, an all-sky survey in the mid-infrared, provides an excellent opportunity to increase the number of BDs significantly, in particular those at the lower end of the temperature scale. We aim to demonstrate that VO tools are efficient in identifing and characterizing BDs by cross-correlating public catalogues released by large surveys. Using VO tools we performed a cross-match of the WISE Preliminary Release, the 2MASS Point Source and the SDSS Data Release 7 catalogues over the whole area of sky that they have in common (

Search for radial velocity variations in eight M-dwarfs with NIRSPEC/Keck II

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