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E. Solano ¹ , N. Lodieu ² , M. Aberasturi ¹ , M.R. Zapatero-Osorio ¹ , E. Martín ¹

Discovery of ultracool subdwarfs using Virtual Observatory tools. E. Solano ¹ , N. Lodieu ² , M. Aberasturi ¹ , M.R. Zapatero-Osorio ¹ , E. Martín ¹ 1.- Centro de Astrobiología (INTA-CSIC), Madrid, Spain 2.- Instituto de Astrofísica de Canarias, Tenerife, Spain.

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E. Solano ¹ , N. Lodieu ² , M. Aberasturi ¹ , M.R. Zapatero-Osorio ¹ , E. Martín ¹

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  1. Discovery of ultracool subdwarfs using Virtual Observatory tools E. Solano¹, N. Lodieu², M. Aberasturi¹, M.R. Zapatero-Osorio¹, E. Martín¹ 1.- Centro de Astrobiología (INTA-CSIC), Madrid, Spain 2.- Instituto de Astrofísica de Canarias, Tenerife, Spain

  2. Ultracool subdwarfs: properties • Metal-poor dwarfs with spectral type later than M7. • Less luminous (actually hotter) than their solar-metallicity counterparts due to the dearth of metals in their atmospheres.

  3. Ultracool subdwarfs: Identification Henry et al. • Halo regions • No contamination by disk stars. • Low luminosity. • Trigonometrical parallaxes and spectra unobtainable. • Solar neighbourhood • Low luminosity. • Hard to find because of their local scarcity (4 out of 348 at d<10 pc, Henry et al. 2006, AJ, 132, 2360).

  4. Ultracool subdwarfs: Identification • Key parameter: Proper motion • The high space velocities of subdwarfs allow them to distinguish from the multitude of similarly faint but slowly moving disk stars.

  5. Ultracool subdwarfs: Identification (II) • Drawback • Identification typically requires the combination of attributes (colors, proper motions,...) available from different archives of large-area surveys (e.g. UKIDSS, SDSS, 2MASS, DENIS,...). • Very time-consuming task if performed by hand but, on the contrary, an approach that perfectly fits in the framework of the Virtual Observatory.

  6. The Virtual Observatory (VO) Science Tools Stds • VO is a consolidated initiative that represents the framework where to develop archive-based research. • By ensuring easy and transparent access to astronomical resources the Virtual Observatory is fostering new and/or more efficient science.

  7. This work: Objectives • Identification of a statistically significant number of bona-fide ultracool subdwarfs to: • Bridge the gap between M and L spectral types. • Help the discovery of cooler Population II brown dwarfs. • Establish optical and near-infrared spectral classification at cooler temperatures and lower metallicities. • Investigate the role of metallicity in the atmospheres of ultracool dwarfs. • Shed light on the star formation history of the Milky Way. • Good candidates for exoplanet searches (low chromospheric activity).

  8. This work: Methodology • Proper motion and photometric searches using VO tools (ALADIN, TOPCAT). • 8500 (SDSS-2MASS) + 1500 (SDSS-UKIDSS) square degrees explored.

  9. This work: Methodology (II) • Workflow: • All point sources in SDSS without 2MASS/UKIDSS counterparts within 5”. • Colours and reduced proper motion based on the M7-M9.5 subdwarfs identified by Lépine & Scholz (2008, AJ, 681, L33). • (J-K) ≤ 0.7 mag

  10. This work: Results • SDSS (Data Release 7) - 2MASS (Point Source Catalogue)‏ • 356 candidates after the initial search • Further refinement - Whenever possible, UKIDSS and SuperCosmos were used for a third epoch. - Visual inspection to discard false detections. • Final number of candidates: 30. Six were already known. • Optical spectroscopic follow-up of new candidates: - 3 + 1 + 2 = 6 candidates observed with NOT/ALFOSC in January, July and August 2009. - 6 out of 6 exhibit spectral features typical of subdwarfs.

  11. This work: Results (II) • SDSS (Data Release 7) – UKIDSS /LAS (Data Release 5)‏ • Aim: Detection of cooler (down to 1000K) and less massive (up to 60 Jupiter masses) subdwarfs in a radius of 25 pc assuming [Fe/H] = -1. • 31 new candidates (after removal of false detections and known subdwarfs) • Optical spectroscopic follow-up: • GTC / OSIRIS • 11 hours awarded in semester 09A. - 8 objects observed but S/N lower than expected. Very hard to confirm/discard candidates. • 16 hours awarded in 09B  19 objects sent to the queue. • VLT / FORS2 • 20 hours  22 objects sent to the queue for the next semester.

  12. Summary • The census of ultracool subdwarfs is largely incomplete. • 39 out of which only three show L-types. ‏ • We have developed a methodology to identify ultracool subdwarfs using Virtual Observatory tools. • Six confirmed subdwarfs out of six observed spectroscopically. Spectral types: M5-M6 (only the brightest targets – SDSS / 2MASS -have been analysed so far). • If the success rate remains so for fainter objects our procedure will double the number of subdwarfs with spectral types later than M7. • This methodology opens new prospects to further increase these numbers using upcoming large-scale surveys (VISTA, PanSTARRS,…).

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