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Science Results from the VISTA Survey of the Orion Star-Forming Region

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Science Results from the VISTA Survey of the Orion Star-Forming Region Astronomical Science Science Results from the VISTA Survey of the Orion Star-forming Region Monika Petr-Gotzens1 on the evolution of circumstellar discs. in the Orion molecular cloud (OMC) B and Juan Manuel Alcalá2 The data from the VISTA Orion Survey, which has an age of one million years or Cesar Briceño3 including catalogues, are available less. Nearly one degree southwest of ζ Eduardo González-Solares4 to the community. In this article we pre- Ori there is the well­known intermediate­ Loredana Spezzi5 sent an overview of selected science age cluster σ Ori (age ~ 3 Myr), while Paula Teixeira1 results that have emerged so far from members of the slightly older Ori OB 1b Maria Rosa Zapatero Osorio6 this survey. association (~ 5 Myr) populate a wide Fernando Comerón1 area around the Belt stars over a few Jim Emerson7 degrees on the sky and are found ‘’off’’ Simon Hodgkin4 In order to fully understand the star for­ the main molecular clouds (Briceño, Gaitee Hussain1 mation history of a star­forming region, it 2008). Only recently, yet another group of Mark McCaughrean5 is of the highest importance to obtain kinematically distinct young stars has Jorge Melnick1 a complete census of its entire stellar and been identified northwest of δ Ori, which Joanna Oliveira8 substellar populations since the earliest is the group of ~ 10-Myr-old stars around Suzanne Ramsay1 epoch of star formation. For the large the B­type star 25 Ori (Briceño et al., Thomas Stanke1 majority of stars in our Galaxy the com­ 2007). Elaine Winston9 plex process of star formation takes Hans Zinnecker10 place in giant molecular clouds (GMCs), Using VISTA, the world’s largest near­ from which typically several generations infrared survey facility (Emerson et al., of stars are formed sequentially. Compar­ 2006), which provides a field of view of 1 ESO ative observational studies across these ~ 1 by 1.5 degrees, we have carried out 2 INAF–Osservatorio di Capodimonte, populations of differing age are funda­ a deep multiwavelength survey of a large Napoli, Italy mental to elucidate issues such as the region around the Orion Belt stars. This 3 Centro de Investigaciones de timescales for circumstellar (protoplane­ survey has delivered a new census of Astronomía (CIDA), Mérida, Venezuela tary) disc evolution, environmental ef­ very low­mass stars and brown dwarfs 4 Institute of Astronomy, Cambridge, fects leading to possible non­universality and detected new brown dwarf candi­ United Kingdom of the low­mass end of the initial (sub­) dates down to ~ 3–4 Jupiter masses at 5 European Space Agency (ESTEC), stellar mass function (IMF) and the time­ an age of 3 Myr. The advantageous loca­ Noordwijk, the Netherlands scales for the dispersal of stellar ensem­ tion of Orion below the Galactic Plane 6 Centro de Astrobiología, Instituto bles. Wide-field imaging surveys in the and almost in the direction of the Galactic Nacional de Técnica Aerospacial infrared are the best means to accom­ anti­centre implies that the contamina­ (CSIC-INTA), Madrid, Spain plish such large­scale studies systemati­ tion by foreground and background stars 7 Astronomy Unit, Queen Mary, Univer­ cally and consistently homogeneously. is low. sity of London, United Kingdom 8 Astrophysics Group, Keele University, The Orion star­forming region is an ideal United Kingdom target for studying almost all aspects Survey strategy, observations and data 9 School of Physics, University of Exeter, related to the physics of star formation, reduction United Kingdom early stellar evolution or the interplay 10 SOFIA Science Center, NASA­Ames, between OB associations and the ISM The survey consists of Z, Y, J, H, Ks Moffett Field, USA (c.f., Bally, 2008). It is the closest GMC, images obtained during 14 nights between at an average distance of 400 parsecs 16 October and 2 November 2009. The (pc), and has been actively forming stars survey area is a mosaic of 20 VISTA fields As part of the VISTA Science Verifica- within at least the last ~ 10 Myr, which is covering a total of 30 square degrees tion programme, a large set of images also the approximate timescale on which around the Orion Belt stars; the surveyed in Orion was obtained at five near- giant planets are thought to be formed. area is shown in Figure 1a. Whenever infrared wavelength bands, from 0.9 to A particularly interesting region for the possible the observations in all filters 2.2 μm. The resulting multi-band cata- purpose of a wide-field survey, investigat­ were carried out sequentially for one field, logue contains approximately three ing stellar and substellar properties in before observing the next field. The one million sources, allowing investigation different evolutionary stages and environ­ VISTA field that included the young stellar of various issues concerning star ments, is the region around the Orion group 25 Ori was imaged up to 23 times and brown dwarf formation, such as Belt stars. at J­ and H­bands with the aim of de ­ a) the difference in the shape of the tecting the photometric variability among substellar mass function in a cluster vs. The Belt is a prominent part of the con­ the very low­mass stars and brown non-clustered environment, b) the influ- stellation of Orion and is marked by three dwarf members of the 25 Ori group. More ence of massive OB stars on the pro- luminous OB stars, namely ζ Ori (Alnitak), details on the observing strategy, the cess of brown dwarf formation, c) the ε Ori (Alnilam) and δ Ori (Mintaka), from exposure times per filter and particular size and morphology of dust envelopes east to west. To the east of ζ Ori there are observing patterns were described in around protostars, and d) the com- very young populous stellar clusters, like Arnaboldi et al. (2010). Figure 1b shows parative role of mass and environment NGC 2024, which is still partly embedded as an example of the data, a colour- The Messenger 145 – September 2011 29 Astronomical Science Petr-Gotzens M. et al., Science Results from the VISTA Survey R. Bernal Andreo Bernal R. Figure 1a. The boundary of the VISTA Orion Survey is indicated in white, overplotted on an optical image. The size of the image is 10.5 by 8.4 degrees and the orientation is north up, east left. tion of magnitude. In this way we find that Figure 1b. VISTA colour image of the young stellar composite image of the young cluster the survey should have detected, for a cluster NGC 2071 located in the Orion molecular cloud B (in the northeast of the surveyed area in NGC 2071. population as young as 1 Myr, essen- Figure 1a). The size of the image is ~ 16 × 16 arc­ tially all objects down to around six Jupiter minutes and shows a three colour composite (Z – blue, The amount of data collected for the masses in a region showing less than J – green, Ks – red). Orientation as Figure 1a. VISTA Orion Survey was 559 Gigabytes, 1 mag of visual interstellar extinction. not including calibration data, clearly making data handling and reduction a sequence that clearly separates from the challenge. The data reduction was per­ Selected science results older main sequence objects of the sur­ formed by a dedicated pipeline, devel­ rounding general field. In the same figure oped within the VISTA Data Flow System Young very low-mass stars and brown the position of the spectroscopically-­ (VDFS), and run by the Cambridge As ­ dwarfs close to ε Ori confirmed T Tauri stars, the higher mass tronomy Survey Unit (CASU). The pipeline The photometric identification of the low­ counterparts to the brown dwarfs, are delivers science­ready stacked images est mass objects has greatly benefitted plotted (green diamonds in Figure 2), and mosaics, as well as photometrically from the broad wavelength coverage of showing convincingly that the candidate and astrometrically calibrated source the survey. With inclusion of the Z and Y brown dwarf sequence is an extension catalogues. A total of 3.2 million sources filters, the lowest-mass objects stand out of the higher mass pre­main sequence was detected in the VISTA Orion Survey. in colour–magnitude diagrams involving stars. Note that the gap in between those bands, appearing in a region inac­ T Tauri stars and brown dwarfs is artificial The photometric calibration was deduced cessible to other objects regardless of and due to the selection criteria on one from 2MASS photometry. The photo- their reddening. This feature is caused by side and the spectroscopic sensitivity metric errors are usually below 5% and the appearance of small dust particles limit on the other side. A similar sequence the overall 5σ limiting magnitudes of the in the atmospheres of objects cooler than of pre­main sequence objects in the ε Ori survey are Z ~ 22.5 mag, Y ~ 21.2 mag, 2500 K, which causes a steep redden­ cluster has previously been reported by J ~ 20.4 mag, H ~ 19.4 mag, Ks ~ 18.6 ing of the colours involving wavelengths some authors from an analysis of optical mag; different parts of the survey can shorter than the J­band with decreas­ colour–magnitude diagrams (Caballero have slightly better or poorer limits due to ing tem perature, while the spectral & Solano, 2008; Scholz & Eislöffel, 2005). varying observing conditions. Additional energy distribution at longer wavelengths data taken at Z­ and J-bands for the field remains nearly unchanged. Employing the full wavelength range of centred on the cluster σ Ori significantly the survey, it can be further tested improved the sensitivity, and achieved 5σ The photometric analysis of a ~ 1 square whether the spectral energy distribution limits of Z ~ 22.9 mag and J ~ 21.4 mag.
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