Local Star Formation: the JCMT Gould's Belt Legacy Survey

FIGURE 1: Gould's Belt superimposed on to an IRAS 100 micron emission map.

INTRODUCTION

	FIGURE 2: A schematic of Gould's Belt

Most star formation within 0.5 kpc lies in Gould's Belt, a ring around the sky containing star-forming molecular clouds centred on a point 200 pc from the Sun and tilted at 20 degrees to the Galactic Plane (Figures 1 and 2).

The aim of the JCMT Gould's Belt Legacy Survey is to use SCUBA-2 and HARP-B at the JCMT to map the submillimetre continuum emission and CO line emission from as many clouds within 0.5 kpc as possible. The survey will include several well known Gould's Belt clouds: Orion, Taurus, Perseus, and Ophiuchus, as well as several objects outside of Gould's Belt, including nearby Bok Globules. To maximize the scientific return of the SCUBA-2 observations, molecular clouds will be preselected by visual extinction, Av, from the recent all-Galaxy extinction atlas of Dobashi (2005). Additional information on the identified star-forming cores will be obtained via molecular line observations with HARP-B, and with SCUBA-2 Polarimetric observations.

SCUBA-2

We will map almost all star-forming regions within 0.5 kpc that are accessible to the JCMT (outlined in the table at the bottom of the page). Most of the target regions are within Gould's belt. The sample, which includes many well-known regions, will provide a very significant snapshot of star formation near the Sun. The survey will provide a legacy of images, as well as point and extended-source catalogues, covering roughly 700 square degrees of sky.

The mapping will be divided into two layers, a wide survey of areas with Av = 1-3, and a deeper survey of area with Av > 3. These maps will be sensitive to every Class 0 and Class I protostar and every "L 1544-like" pre-stellar core (see e.g. Figures 3 and 4), yielding the first catalogue of such objects selected by submillimetre continuum emission and increasing the number of known sources from tens to thousands.

Within the field of low-mass star formation, many unresolved issues will be addressed, including; the duration of its various stages, the evolution of protostellar collapse, the origin of the initial mass function (IMF) from intermediate-mass stars to sub-stellar objects, and the connection between protostars and the molecular cloud structure from which they are formed. The SCUBA-2 observations will also provide finding charts for the other aspects of this survey (described below), and will be a detailed resource for users of future telescopes (ALMA, Herschel, JWST and beyond).

FIGURE 3: A SCUBA map of Ophiuchus in 850 micron continuum emission (Johnstone, Di Francesco, & Kirk 2004). This is currently the largest single map of a star-forming region made in the submillimetre. Note that the compact structures are confined to very specific regions.		FIGURE 4: A contour map of Ophiuchus, obtained from R-band extinction mapping by Cambresy (1999). The areas to be mapped with SCUBA-2 are highlighted in yellow (Av = 1-3) and red (Av > 3). Only the central region was mapped by Johnstone et al. with SCUBA (Figure 3); clearly a large portion of the cloud remains to be observed. The SCUBA-2 map will also be at least four times more sensitive than the SCUBA map.

HARP-B

The SCUBA-2 survey of the local molecular cloud population will provide a homogeneous catalogue of pre-stellar cores and protostars of unprecedented size. While this has by itself intrinsic scientific value, measurements of the kinematics of these cores and clusters of cores will allow us to address a large number of fundamental scientific problems in star formation, for the first time using samples that are large enough to be statistically significant.

In typical star-forming molecular cloud cores, like that shown in Figure 5, temperatures and densities are in the ranges 10-50 K and 10,000-100,000 particles per cubic centimetre. These are the conditions under which the CO and isotopic CO lines in the 350 GHz range are excited. The key goals of the HARP-B line observations are: to search for and map any high velocity outflows present in the cores to provide age estimates for the embedded objects; to measure the line widths and velocity profiles in the cores and filaments to help understand cloud support mechanisms and their evolution; to characterize the nature of cloud turbulence in a wide range of environments; to derive simple constraints on the column density and CO depletion in these cores; and to generate a large sample of objects for detailed follow-up with the eSMA and ALMA. These data will also complement the wide-field survey data being obtained at optical, near-IR and mid-IR wavelengths at the CFHT, UKIRT and with Spitzer.

FIGURE 5: A map of L 1448 in Perseus showing the CO outflows in red and blue contours, superposed on to a SCUBA submillimetre continuum map of the region (Hatchell et al., priv. communication)

POLARIMETRY

Attempts to characterise the magnetic field in star-forming regions are driven by the need to understand its significance to the formation of cloud structure and/or to the regulation of cloud core collapse. These factors are related to star formation rates and molecular cloud lifetimes, issues for which there is substantial debate in the literature (Myers & Goodman 1988; Hartmann et al. 2001; Elmegreen 2001).

Measurements of polarized emission from dust are the most effective means of probing the magnetic field within molecular clouds and cores, since absorption polarimetry is limited to the periphery of dense clouds and Zeeman splitting detections are relatively few (Crutcher 1999). In contrast, polarized emission from dust is detected from all objects on all scales observed (e.g. Matthews et al. 2001; Crutcher et al. 2003) and from all compact sources, regardless of evolutionary epoch (Ward-Thompson et al. 2000; Matthews & Wilson 2002). Sensitivity limitations have generally restricted observations to several dozen bright (i.e., > 1 Jy), compact objects (e.g. Figure 6). The key goals of the polarimetric mapping proposed as part of the Gould's Belt Legacy Survey are therefore: to obtain maps of polarization position angle and fractional polarization in a statistically meaningful sample of cores; to characterize the evidence for and the relevance of the field and turbulence (in conjunction with HARP-B observations) in cores and their surrounding environments; to test the predictions of the standard low-mass star formation theory (core, outflow, field geometry); and to generate a large sample of cores suitable for follow-up with forthcoming single-dish polarimeters and ALMA.

FIGURE 6: These images show the importance of mapping the ambient environment around cores and the advantage of high resolution. The SCUBA Serpens map (Davis et al. 2000) shows that the field structure can be more complex between the cores than their polarization patterns indicate. The same is true in Barnard 1 (centre; Matthews & Wilson 2002). The NGC 2024 map at right shows that SCUBA observations (red) of extended structure in Orion (Matthews, Fiege & Moriarty-Schieven 2002) reveal systematic variations missed in earlier lower resolution maps (green; from Dotson et al. 2000). The Orion B map reveals how critical resolution is to the effective mapping of magnetic field geometry.

SUMMARY

The science questions which this survey will address directly are of fundamental importance to star formation studies. What are the relative timescales of each of the protostellar stages? How does protostellar collapse proceed? What determines the stellar initial mass function (IMF)? How does the environment influence protostar formation? How do brown dwarfs form? What is the detailed spatial and velocity structure of the molecular clouds in which star formation occurs? What is the relative importance of magnetic fields and turbulence in providing cloud support? By answering or constraining these vital questions, the immediate impact of this survey will be immense.

NOTES:
¹Instrument to be used.
²Continuum wavelength or CO isotope.
³SCUBA-2 target regions will be defined by extinction maps; only the higher-extinction regions will be mapped at 450 microns.
⁴Approximate area to be mapped in each tracer.
⁵Sensitivity limit of each map.
⁶Spectral resolution of the CO maps.
⁷Weather conditions needed for each survey component, 1 being the best.
⁸Time dedicated to each survey component in the first two years.
⁹Approximate time required to complete each survey.

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Authors: Jane Buckle (Cambridge), James Di Francesco (NRC-HIA), Jane Greaves (St Andrews), Jennifer Hatchell (Exeter), Michiel Hogerheijde (Leiden), Doug Johnstone (NRC-HIA), Brenda Matthews (NRC-HIA), Dave Nutter (Cardiff), John Richer (Cambridge), Derek Ward-Thompson (Cardiff)

Team Members: Pierre Bastien, Chris Brunt, Jane Buckle, Harold Butner, Brad Cavanagh, Antonio Chrysostomou, Rachel Curran, Emily Curtis, Chris Davis, Bill Dent, James Di Francesco, Michel Fich, Jason Fiege, Laura Fissel, Per Friberg, Rachel Friesen, Gary Fuller, Sarah Graves, Jane Greaves, Andrew Gosling, Jennifer Hatchell, Michiel Hogerheijde, Martin Houde, Ray Jayawardhana, Doug Johnstone, Gilles Joncas, Helen Kirk, Jason Kirk, Lewis Knee, Brenda Matthews, Henry Matthews, Chris Matzner, Gerald Moriarty-Schieven, Dave Nutter, Russell Redman, Robert Simpson, Michael Reid, John Richer, Marco Spaans, Dmitri Stamatellos, Ewine Van Dishoeck, Serena Viti, Derek Ward-Thompson, Bernd Weferling, Glenn White, Ant Whitworth, Jan Wouterloot, Jeremy Yates, Ming Zhu

A restricted wiki page is available for team members at: http://wiki.astro.ex.ac.uk/bin/view/JCMTGouldBelt/WebHome.