Direct imaging and new technologies to search for substellar companions around MGs cool dwarfs

1. M.C. Gálvez-Ortiz (1) , J. R. A. Clarke (1), D. J. Pinfield (1), S..L.. Folkes (1) , J.S. Jenkins (2) , A.E. García-Perez (1) , B. Burninggham (1) , A.C. Day-Jones (1) & H. R. A. Jones (1) (1) Centre for Astrophysic, Science and technology Research Institute, University of Hertfordshire, (2) Department of Astronomy, Universidad de Chile, Casilla Postal 36D, Santiago, Chile Direct imaging and new technologies to search for substellar companions around MGs cool dwarfs I. SAMPLE SELECTION: The sample was selected from photometric and astrometric criteria in order to search for UCD in moving groups (MGs). We cross matched an extended version of the Liverpool-Edinburgh High Proper Motion survey (ELEHPM; Pokorny et al. 2004) and Southern Infrared Proper Motion Survey (SIPS; Deacon, Hambly & Cooke 2005) with the Two Micron All Sky Survey (2MASS) and applied colour cuts of J-K≥1.0 and R-K≥5.0 to select objects with a spectral type predominantly later than M6 (e.g. Kirkpatrick et al. 1999). We selected objects from both catalogues where µ/Δσµ>4 (µ is proper motion and σµ is the proper motion one sigma uncertainty) to ensure our desired level of proper motion accuracy. (See J.R.A. Clarke talk in Session 5.1 for details).Our final red object catalogue was made up of 817 objects. We could conclude that 133 of these objects were possible members of a MG (see Clarke et al. 2009). ABSTRACT: We describe here how current and future technology from the ground and space (high contrast imaging, high-precision optical and infrared Doppler determinations, ELT and JWST), will allow us to find sub-stellar companions (brown dwarfs and exo-planets) around young ultra-cool dwarfs (UCDs) and characterise their properties. Members of young moving groups have clear advantages in this field, and we compiled a catalogue of young UCD objects (selected via photometric and astrometric criteria) and studied their membership to five known young moving groups: Local Association (Pleiades moving group, 20 - 150 Myr), Ursa Mayor group (Sirius supercluster, 300 Myr), Hyades supercluster (600 Myr), IC 2391 supercluster (35 Myr) and Castor moving group (200 Myr).We assess them as members using different kinematic and spectroscopic criteria, and obtain a substantial sample of targets for AO imaging. II. Why AO Imaging in UCD in MGs? Nearby MG members will make ideal targets for AO searches seeking faint companions (e.g. Jenkins et al. 2006). Since substellar companions cool and fade with time, targeted systems must be young. It is also highly desirable, though not always the case, that AO targets have a well constrained age which will yield accurate theoretical companion mass measurements. Close-in planets could even produce measurable radial velocity signatures (e.g. a 2MJup planet 2AU from a 50MJup UCD would induce a reflex velocity of ~200m/s), and could yield a future dynamical mass measurement over a 5-10 year baseline. Our youngest and closest MG members (in the IC 2391 or Pleiades MGs) would allow AO to probe to 1MJup at separations of ~ 1AU.With known age, composition and distances, our MGs members would also provide a population of benchmark objects to test atmosphere and evolutionary UCD models. IV. ROTATIONAL VELOCITY: It is known that there is contamination of old objects that share kinematic properties with the genuine young MG members. Several studies have probed that after becoming a fully convective object (spectral types later than ~M3), when gravitational contraction is finished, rotational braking is present at least in all M type objects. The spin-down times are longer for Late M dwarfs than for earlier M types (e.g. Zapatero-Osorio et al. 2006; Reiners & Basri 2008, etc). Following these studies, we can use rotational velocity as an excellent way to differentiate between young and older M type UCD populations. We measure the rotational velocity of 55 of the total 69 sample, and of 38 of the total 45 YD objects and plot them (see figure) in a vsini-spectral type diagram from Reiners & Basri (2008). Young and old population occupy distinct regions in the diagram. In the figure, circles are from Reiners & Basri (2008), and open triangles from Zapatero-Osorio et al. (2006). Following Reiners & Basri (2008), blue circles are supposed to be young, red circles old and black circles have unknown age. Our data sample is plotted in different symbols depending on weather they have been classified as YD or old disk (OD). As in Reiners & Basri (2008), we mark the ages of 2, 5 and 10 Gyr (from upper left to lower right) in dashed lines. To obtain a criteria for separate different aged objects, we plotted the "apparent" separation between the considered "young" and "old" objects in a blue continuous line, and used it as a criterion for determining the possible membership of our targets to the MG, to which they are candidates. From the 38 kinematic YD candidates, we find that 27 present a projected rotational velocity in agreement with our criteria of youth and 5 that can not be dismissed as they have velocities in the limit between both populations. III. KINEMATIC: We determined the position of the targets in the velocity space to apply a simple kinematic criterion of membership. We computed the galactic space velocity components (U, V, W) of the sample taking into account their possible membership to one or several MGs, and plotted them (see figure) in the UV and WV planes including the boundaries (continuous line) that delimit the young disk population as defined by Eggen (1984, 1989). We then applied a simple criterion that consists on identifying a possible member of one of the five MGs by its relative position in the UV and VW diagrams with respect to the boxes (dashed lines) that marked the velocity ranges of different MGs. The boxes are defined by studies with higher mass member samples from the literature (Montes et al. 2001; Barrado Y Navacués1998). The centre of the five main MGs are marked (with large open symbols, squares for Sirius, diamonds for Hyades, triangles for IC 2391, circles for Pleiades and stars for Castor) and our kinematic members are shown as the corresponding filled symbols. If an object does not lie inside one of the MGs boxes but is inside the young disk area, it is classified as a YD. According to their kinematic behaviour, we have 45 object that belong to the young disk area and 32 of them are candidates to belong to one of the five MGs. VI. AO imaging and new technologies: The figure represents the potential mass-separation range of detectable companions to our MG candidates. The filled circles represent the minimum separation and mass of a companion that could be detected (assuming MG membership) with AO (we assume a minimum detectable separation of 0.15 arcsec). This is conservative for the estimated capabilities of NACO/SDI operating with a laser guide star, which could reach photon noise limits for a brightness ratio of 11 magnitudes in the H-band at 0.1'' separation. We derive minimum detectable companion masses by assuming a 10-σlimit of H=22.5 (cf. NACO 1hr sensitivities offset by 1 magnitude for narrow band imaging), accounting for the distance of each candidate, and using Lyon Group "COND" models to convert MH into mass for the relevant age. The total mass-separation domain of detectable companions is enclosed by a dashed line. New generation of telescopes, instruments and techniques will allow us to follow up the low-mass objects, brown dwarfs and exoplanets, found in this project and characterise them, as well as extend the study to fainter candidates. The European Extremely Large Telescope (E-ELT), with a diameter of 42 m, will have instruments like EPICS (Planet Imager and Spectrograph with Extreme Adaptive Optics) dedicated to the detection and characterization of exoplanets by direct imaging and spectroscopy, or METIS, (the Mid-infrared ELT Imager and Spectrograph) that will be dedicated to wavelengths longward of 3μm favouring the contrast ratio between planet and host star and improving the behaviour of the point spread function that increase the Strehl ratios. Also, space telescopes, like the The James Webb Space Telescope (JWST), will allow us to study the physical and chemical properties of the low-mass objects found in this project. These new technologies will give us the possibility of finding smaller object at closer distances and to fully characterize them. V. YOUTH CONSTRAINTS: Apart from the rotational velocities. to further assess the membership of our kinematic candidates, we will combine several methods (the Li I doublet at 6708 Å presence, the activity/age relation and the study of spectroscopic sensitivity to surface gravity features) to provide robust age constraints. The figure represents the spectral type (where M0=0, L0=10, etc) versus age, showing which of our dating criteria can be used for our various MG member candidates (from our complete sample). A dashed line represents the limit for which gravity feature dating criteria are useful (for objects younger than 200 Myr, see e.g. McGovern et al. 2004 and Gorlova et al. 2004). The position of the lithium edge is shown as a solid line (objects below this line would be expected to show lithium, (see e.g. Basri et al. 1997). A dotted line marks the limit where H emission criterion can be used following West et al. (2008), (objects with spectral types < M7). Due to the proximity between some MGs in UVW space, some of our candidates are possible members of more than one MG. Filled and open symbols are candidates that may belong to one or two different MG respectively. This plot helps us to discriminate which methods would be appropriate to not only constrain age but also to discriminate between different group candidatures. We slightly offset some overlapping points for clarity in this plot. 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