Astronomical Science

Science Results from the VISTA Survey of the 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 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 well-known 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 star-forming 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 of star formation. For the large the B-type 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 low-mass stars and brown dwarfs 4 Institute of , Cambridge, fects leading to possible non-universality and detected new candi­ United Kingdom of the low-mass end of the initial (sub-) dates down to ~ 3–4 Jupiter masses at 5 European Space Agency (ESTEC), 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 anti-centre implies that the contamina- (CSIC-INTA), Madrid, Spain plish such large-scale 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 star-forming 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 or the interplay 10 SOFIA Science Center, NASA-Ames, 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 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 H-bands 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 low-mass 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 (), details on the observing strategy, the cess of brown dwarf formation, c) the ε Ori () and δ Ori (), 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 . 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 science-ready 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 pre-main 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 pre-main 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 J-band with decreas- colour–magnitude diagrams (Caballero have slightly better or poorer limits due to ing tem­perature, 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. degree region close to the B0 supergiant of the candidate brown dwarfs is indeed To further estimate the completeness, ar­ ε Ori, in a Z–J versus Z colour–magnitude consistent with brown dwarf atmosphere tificial stars of different magnitudes were diagram revealed a high number of models. One example is shown in Fig­ added to the images and the statistics of potentially young substellar objects (Fig­ ure 3, where the 0.9 to 2.2 μm fluxes of the re-detected stars were used to esti­ ure 2, shown as blue squares). These one of the brown dwarf candidates mate the completeness limits as a func­ candidate brown dwarfs populate a from Figure 2 is compared to the theo­

30 The Messenger 145 – September 2011 11 Figure 2. Colour–magni­ from the Z–J versus J colour–magnitude tude diagram of brown diagram (Figure 4). Out of 106 selected 12 CIDA + VISTA dwarf candidates (blue) VISTA BD cand. close to ε Ori. The red brown dwarf candidates, 37 are candi­ 13 CIDA TTS (25Ori) dots show field objects, date free-floating planetary mass objects mostly fore- and back­ (6–13 Jupiter masses). Of the latter, 14 ground dwarfs and 23 are new VISTA discoveries and have giants, in the Orion Belt 15 region, detected in the near-infrared colors compatible with VISTA Orion Survey and spectral types L and T. All the new detec­ 16 having optical counter­ tions are fainter than J ~ 18.5 mag, which 17 parts in the CIDA cata­ indicates they are on the planetary mass logue (Briceño et al., Z boundary or below, assuming they are 18 2005; 2011). Spectro­ scopically confirmed members of the 3-Myr-old σ Ori cluster. 19 T Tauri star (TTS) mem­ bers of the 10-Myr-old Scattered light images of protostellar 20 group around 25 Ori are plotted as green points; envelopes 21 the brown dwarf (BD) The youngest stellar objects are typically candidates may have a 22 surrounded by large dusty discs and similar age. infalling envelopes and are located at or 23 close to the cores of the molecular cloud. 24 They represent the earliest phase of –0.5 0.0 0.51.0 1.52.0 2.53.0 3.54.0 ­stellar evolution. About 350 such proto­ Z–J stars (class 0 to class I sources), dis­ tributed all over the Orion molecular Figure 3. Spectral clouds A and B, have been identified by energy distribution of Spitzer imaging (Megeath et al., 2005; a candidate brown –12.0 dwarf in the ε Ori clus­ Allen et al., 2007). For several of the ter. Black dots are the proto­stars located in parts of the Orion observed fluxes, while molecular cloud B which were covered by

) –12.5 the red dots are the –2 the VISTA Orion Survey, large ex­­tended

m fluxes dereddened with

c A = 1.5 mag. The spec­ scattered emission from protostellar enve­ –1 –13.0 v tral energy distribution lopes was detected in the images. g s model of a source with er ( λ Teff = 2500 K, L = 7.9 × F –13.5 –4 One of the best examples is the edge-on 10 LA, at a distance of g λ 420 pc is shown in cyan disc and associated bipolar envelope lo –14.0 (model from Allard et al., of the source HOPS333 shown in Figure 2001). 5. The diameter of the envelope, as seen in the deep VISTA images, is roughly –14.5 2500 astronomical units (AU). The central protostellar source was detected with 1234 Spitzer in all IRAC bands, i.e. from 3.6 to λ (μm) 8.0 μm, as well as with MIPS at 24 μm. The 24 μm flux of HOPS333 is 20 mJy, retical flux distribution of a source with The σ Ori cluster but it has not been detected in deep sub­ –4 Teff = 2500 K, L = 7.9 × 10 LA — the The σ Ori cluster is one of the most stud­ millimetre continuum maps, suggesting overall agreement is quite good, support­ ied clusters in Orion. It is essentially that HOPS333 is a slightly more evolved ing the brown dwarf nature of the can­ free of gas and dust, making the study of protostellar object. didate. A further step would be the its stellar and substellar content little assignment of masses to all the candi­ affected by interstellar extinction. Due to A serendipitous re-discovery: date brown dwarfs in order to extract the its youth (~ 3 Myr) substellar objects of Berkeley 20 substellar IMF. This, however, is difficult σ Ori are relatively easy to detect, because Northeast of δ Ori we detected a local from the colour–magnitude diagram they are brightest when they are young. stellar density enhancement, which alone, because the isochrones are very The VISTA Orion Survey detected the well turned out to be the Galactic old open tightly bunched together in the low-mass known T-dwarfs S Ori 70 and S Ori 73 cluster Berkeley 20. Most recently, regime and the age of the brown dwarf (Peña Ramírez et al., 2011) and has been ­Berkeley 20 has been studied at optical candidates are thus not well constrained. further explored to identify the lowest wavelengths by Andreuzzi et al. (2011) Hence assigning masses is very uncer­ mass brown dwarfs, the L- and T-dwarfs, who determined an age of ~ 5 Gyr and a tain although future spectroscopy may in a circular area of radius 30 arcmin distance of 8.7 kpc. In Figure 6 we plot help constrain them by probing surface around the bright, multiple star σ Ori. the VISTA colour–magnitude diagram of gravity sensitive features. Photometric candidates were selected Berkeley 20 for stars within one cluster

The Messenger 145 – September 2011 31 Astronomical Science Petr-Gotzens M. et al., Science Results from the VISTA Survey

13 Figure 4. Colour–magni­ tude diagram of a 0.78 square degree field cen­ 14 tred on σ Ori (Peña Ramírez et al. 2012, in prep.). Green dots are 15 previously known very low-mass stars and Mjup red diamonds indicate 16 60 new brown dwarf detec­ tions from the VISTA Orion Survey. The grey 30 ) 17 solid and dashed lines 20

ag are the 4σ and 10σ detection limits. Object (m

J 18 15 masses are labelled 12 on the right of the dia­ gram based on (sub-) Figure 5. Three colour composite image (Z, J, Ks) stellar evolutionary mod­ 19 centred on the protostar HOPS333. Clearly visible is els from the Lyon group a dark central obscuration, indicating an edge-on (Chabrier et al., 2000; 5 disc, and a large bipolar envelope structure showing 20 S Ori 70 Baraffe et al., 2003). 4 up in scattered near-infrared light. The full size of 3 this image is ~ 18 arcseconds, which corresponds to S Ori 73 almost 8000 AU at the distance of the Orion B 21 2 molecular cloud.

22 Access to the data products, including 012 34 Z–J (mag) images and source catalogues, will be available via the ESO Archive Query Inter­ radius (1.8 arcminutes) of the cluster cen­ sequence quite well. Spectroscopic ob­ faces for Phase 3 ingested data products tre; the cluster radius has been deter­ servations of two cluster members con­ at: http://archive.eso.org/wdb/wdb/adp/ mined based on the Ks-band radial stellar firm a of [M/H] = –0.45 (Sestito phase3_vircam/form density distribution. The 5 Gyr isochrones et al., 2008). From isochrone fitting we from the stellar evolutionary models of derive a distance of 8.4 kpc for Berkeley Girardi et al. (2002) are overplotted. Both 20 with an interstellar extinction of Acknowledgements isochrones, for metallicity [M/H] = –0.6 0.32 mag, in agreement with previously We gratefully thank Karla Y. Peña Ramírez and Victor as well as for [M/H] = –0.3, fit the stellar published values. J. Sanchez Bejar for providing Figure 4. We also very much appreciate the great work done by the VISTA 12 consortium who built and commissioned the VISTA [M/H] = –0.6 Outlook and data access telescope and camera. [M/H] = –0.3 Further analysis of the full census of can­ References 14 didate very low-mass stars and brown dwarfs over the whole survey area is Allard, F. et al. 2001, ApJ, 556, 357 underway. Detailed investigations of the Allen, L. et al. 2007, in Protostars and Planets V. Eds. stellar and substellar content of the B. Reipurth, D. Jewitt, & K. Keil, Univ. Arizona Press, Tucson, p.361 16 youngest stellar clusters imaged in the Andreuzzi, G. et al. 2011, MNRAS, 412, 1265

Z survey, e.g., NGC 2071, NGC 2068 and Arnaboldi, M. et al. 2010, The Messenger, 139, 6 NGC 2024, will discuss their substellar Bally, J. 2008, in Handbook of Star Forming Regions IMFs out to large cluster radii. Spectro­ Vol. I, ed. B. Reipurth, ASP, p. 459 Baraffe, I. et al. 2003, A&A, 402, 701 18 scopic follow-up of circumstellar discs Briceño, C. et al. 2005, AJ, 129, 907 surrounding cluster stars and brown Briceño, C. et al. 2007, ApJ, 661, 1119 dwarfs will allow the physical properties Briceño, C. 2008, in Handbook of Star Forming of these objects to be characterised in Regions Vol. I, ed. B. Reipurth, ASP, p. 838 Briceño, C. et al. 2011, in preparation 20 great detail. Complementary data from Caballero, J. A. & Solano, E. 2008, A&A, 485, 931 Spitzer and/or Chandra and XMM will Chabrier, G. et al. 2000, ApJ, 542, 464 help to ­distinguish young Orion sources Emerson, J., McPherson, A. & Sutherland, W. 2006, 0 0.5 1 1.5 2 from field stars. About one third of the The Messenger, 126, 41 (Z–K) Girardi, L. et al. 2002, A&A, 391, 195 sources, i.e. roughly one million objects, Megeath, S. T. et al. 2005, IAUS, 227, 383 Figure 6. Colour–magnitude diagram of the Galactic in the full area catalogue, have been Peña Ramírez, K. et al. 2011, A&A, 532, 42 old Berkeley 20, which was serendipi­ ­classified as ex­­tended, suggesting that Scholz, A. & Eislöffel, J. 2005, A&A, 429, 1007 tously re-discovered in the VISTA Orion Survey. Both Sestito, P. et al. 2008, A&A, 488, 943 isochrones represent a 5-Gyr population (Girardi et these are most likely background galaxies. al., 2002), but at different metallicity, as indicated. All of these await scientific exploitation.

32 The Messenger 145 – September 2011