First Results from BISTRO: A SCUBA-2 Polarimeter Survey of the Gould Belt

Our knowledge of the star formation process has increased dramatically as a result of the advent of satellites such as Spitzer and Herschel, and sensitive far-infrared and submillimeter detector arrays such as the Sub-millimetre Common-User Bolometer Array 2 (SCUBA-2). Following on from the highly successful first-generation James Clerk Maxwell Telescope (JCMT) Legacy Surveys, including the Gould Belt Legacy Survey (e.g., Ward-Thompson et al. 2007, 2016; Buckle et al. 2010; Graves et al. 2010; Sadavoy et al. 2013; Pattle et al. 2015, 2017; Rumble et al. 2015; Salji et al. 2015; Chen et al. 2016; Kirk et al. 2016; Mairs et al. 2016), the JCMT is currently undertaking a series of second-generation surveys, using the latest instruments to be commissioned on the telescope. These include POL-2, an imaging polarimeter for SCUBA-2. One of the surveys using POL-2 is the B-fields in STar-forming Region Observations (BISTRO) Survey that we report here. This is extremely timely because magnetic fields (hereafter referred to as B-fields) are still not well understood in star formation because of a paucity of observational evidence, despite widespread theoretical recognition of the significance of B-fields in the formation of cores (e.g., Basu et al. 2009 and references therein) and the evolution of protostars (e.g., Li et al. 2011 and references therein).

1.1. Observing Magnetic Fields

1.2. Theoretical Models

Previous surveys have either been piecemeal, been very restricted in sample size (e.g., Matthews et al. 2009; Vaillancourt & Matthews 2012; Hull et al. 2013, 2014; Matthews et al. 2014), or have too poor resolution to detect cores and protostars (e.g., Planck Collaboration et al. 2015). We here describe a project that aims to produce a large and unbiased survey of the B-fields in star-forming molecular material in the solar vicinity, simultaneously at 850 and 450 μm, and at relatively high resolution—14.1 and 9.6 arcsec, respectively (Dempsey et al. 2013), or ∼1000–2000 au at a typical Gould Belt cloud distance.

The BISTRO Survey is a large-scale survey of the Gould Belt clouds that we have previously mapped in continuum and spectral lines at JCMT (e.g., Ward-Thompson et al. 2007; Buckle et al. 2015; White et al. 2015), and in the far-infrared with Herschel (André et al. 2010).

The aims of the project are to obtain maps of polarization position angle and fractional polarization in a statistically meaningful sample of cores in numerous regions; to characterize the evidence for and relevance of the B-field and turbulence (in conjunction with previous and follow-up spectroscopic line observations) in cores and their surrounding environments; to test the predictions of low-mass star formation theories (core, filament, outflow, and field geometry) and grain alignment theories; to generate a large sample of objects that are suitable for follow-up with other instruments, such as ALMA, Nobeyama, SMA, and NOEMA (NOrthern Extended Millimeter Array); and to measure the B-field strength using the C-F method in as many clouds as possible within our sample.

The survey was granted an initial allocation of 224 hr of telescope time to observe 16 fields in 7 different Gould Belt clouds (Auriga, IC5146, Ophiuchus, Orion, Perseus, Serpens, and Taurus). The specific fields were chosen to match those previously mapped by SCUBA-2, HARP, and Herschel in the JCMT and Herschel Gould Belt Surveys (Ward-Thompson et al. 2007; André et al. 2010).

SCUBA-2 is an innovative 10,000-pixel submillimeter camera (Holland et al. 2006) that has revolutionized submillimeter astronomy in terms of its ability to carry out wide-field surveys to previously unprecedented depths (e.g., Buckle et al. 2015; Pattle et al. 2015). SCUBA-2 uses transition-edge superconducting bolometer arrays, which come complete with in-focal-plane superconducting quantum interference device amplifiers and multiplexed readouts, and are cooled to 100 mK by a liquid-cryogen-free dilution refrigerator (Holland et al. 2006). It has two arrays that operate simultaneously in parallel, one with filters centered at 850 μm and one at 450 μm. In this paper we discuss 850 μm data only.

The polarimeter POL-2 (Bastien et al. 2005a, 2005b; P. Bastien et al. 2017, in preparation; Friberg et al. 2016) has an achromatic continuously rotating half-wave plate in order to modulate the signal at a faster rate (2 Hz) than atmospheric transparency fluctuations. This modulation significantly improves the reliability and accuracy of submillimeter polarimetric measurements. The signal is analyzed by a wire-grid polarizer. For calibration, a removable polarizer is also available.

Figure 1 shows a schematic of a rotating half-wave plate polarimeter, such as the POL-2 instrument. POL-2 has three optical components, which are (in the order that the radiation encounters them): the calibration polarizer (not shown in Figure 1), the rotating half-wave plate, and the polarizer. The components are mounted in a box fixed in front of the entrance window of the main cryostat of SCUBA-2. All components are mounted so that they can be taken in and out of the beam remotely, making it very easy and fast to start polarimetry at the telescope (Bastien et al. 2005a, 2005b; P. Bastien et al. 2017, in preparation; Friberg et al. 2016).

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The BISTRO time was allocated to take place during Band 2 weather (0.05 < ${\tau }_{225\mathrm{GHz}}$ < 0.08), which is typical of moderately good weather conditions on Maunakea. The first data were taken with POL-2 on SCUBA-2 on 2016 January 11.

The POL-2 polarimeter fully samples circular regions with 12 arcmin diameter at a resolution of 14.1 arcsec in a version of the SCUBA-2 DAISY mapping mode (Holland et al. 2013) optimized for POL-2 observations (Friberg et al. 2016). The POL-2 DAISY scan pattern produces a central region of 3 arcmin diameter of approximately even coverage with high signal-to-noise ratio, with noise increasing to the edge of the map. The POL-2 DAISY scan pattern has a scan speed of 8 arcsec/s, with a half-wave plate rotation speed of 2 Hz (Friberg et al. 2016). Continuum observations are simultaneously taken at 450 μm with a resolution of 9.6 arcsec, but as the 450 μm POL-2 observing mode has not yet been fully commissioned, we do not use these data in this paper.

The data were reduced in a two-stage process. The raw bolometer time-streams were first converted into separate Stokes Q and Stokes U time-streams using the process calcqu in smurf (Chapin et al. 2013). The Q and U time-streams were then reduced separately using an iterative map-making technique, makemap in smurf (Chapin et al. 2013), and gridded to 4 arcsec pixels. The iterations were halted when the map pixels, on average, changed by $\leqslant 5$% of the estimated map rms noise. In order to correct for the instrumental polarization (IP), makemap is supplied with a total intensity image of the source, taken using SCUBA-2 while POL-2 is not in the beam. The IP correction is discussed in detail by P. Bastien et al. (2017, in preparation). The total intensity image of OMC 1 presented in this paper was taken using the standard SCUBA-2 DAISY observing mode, and reduced using makemap using the same convergence criterion and pixel size as the POL-2 data.

The reduced scans were combined in two stages: (1) each of the Stokes Q observations were coadded to form a mosaic Stokes Q image (the Stokes U maps were coadded similarly); (2) each of the Stokes Q and U observations was combined using the process pol2stack in smurf (Chapin et al. 2013) to produce an output half-vector catalog. We refer to data produced by these methods as BISTRO Internal Release 1 (IR1).

The data were calibrated in Jy/beam, using an aperture flux conversion factor (FCF) of 725 mJy/pW at 850 μm. When observing with POL-2, the standard SCUBA-2 850 μm FCF, of 537 Jy/beam, derived from average values of JCMT calibrators (Dempsey et al. 2013), is increased by a factor of 1.35 through additional losses introduced by POL-2 (Friberg et al. 2016; P. Bastien et al. 2017, in preparation).

The OMC 1 region was observed 21 times between 2016 January 11 and 25 in a mixture of very dry weather (Band 1; ${\tau }_{225\mathrm{GHz}}\leqslant 0.05$) and dry weather (Band 2; $0.05\leqslant {\tau }_{225\mathrm{GHz}}\,\leqslant 0.08$) under JCMT project reference numbers M16AL004 (BISTRO) and M15BEC02 (POL-2 commissioning).

In order to determine the behavior of rms noise in our observations as a function of integration time, we measured the standard deviation on the Stokes Q and Stokes U values in a region with relatively constant signal in both the Stokes Q and the Stokes U maps, located between OMC 1 and the Orion Bar. This region, centered at approximately ${05}^{{\rm{h}}}{35}^{{\rm{m}}}{21}^{{\rm{s}}}$ $-05^\circ 23^{\prime} {36}^{\prime\prime }$, was chosen because it was relatively flat, moderately unpolarized, low in emission, and away from the brightest sources, and because there was no region entirely without signal in the central part of the map. Figure 2 shows how the noise integrates down in this 21-repeat (∼14 hr) POL-2 observation.

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The polarization noise in Figure 2 is seen to integrate down close to ${t}^{-0.5}$, as in the ideal case. The scatter of individual measurements reduces satisfactorily as the data are subsequently combined. We find that there is no evidence of any "noise floor" in long integrations. From this plot we see that this data set has reached 2.1 mJy beam⁻¹ rms noise in 13.5 hr. An rms noise value of ∼2 mJy beam⁻¹ was set as the target value for the BISTRO survey. Appendices A and B list a series of tests that we carried out to confirm the repeatability of our measurements and to demonstrate consistency with previous data.

Figure 3 shows a polarization map taken with POL-2 of the OMC 1 region of the "integral filament" in the Orion A molecular cloud, with half-vectors rotated by 90° to trace the B-field direction. Only vectors with a signal-to-noise ratio of 3 or greater in polarization fraction are shown (i.e., $P/{DP}\geqslant 3$). The Orion A molecular cloud is a well-resolved and well-studied region of high-mass star formation (e.g., Bally 2008; O'Dell et al. 2008). It is the closest region of high-mass star formation, located at a distance of 388 ± 5 pc (Kounkel et al. 2017). The half-vector lengths show the percentage polarization, with a 5% scale bar in the corner to give the calibration. The underlying image is an 850 μm total intensity map of the same region taken using SCUBA-2.

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The "integral filament" (Bally et al. 1987) can be seen running roughly north-south through the region. The brightest part of the filament lies just south of the center of the image. The two brightest and most massive regions in the filament are the northern Becklin-Neugebauer Kleinmann-Low (BN/KL) object (Becklin & Neugebauer 1967; Kleinmann & Low 1967) and the southern Orion South clump (Batrla et al. 1983; Haschick & Baan 1989). Both are seen in Figure 3. In the southeast part of the map the Orion Bar photon-dominated region (PDR) extends from the center of the foot of the map in a roughly northeasterly direction.

In the brightest central part of the filament, the B-field direction, as indicated by the half-vectors, appears to lie roughly orthogonal to the main axis of the filament. This pattern continues on the main axis line of the filament over most of the length of the filament. More particularly, on the brightest part of the filament the orientation of the long axis of the filament is estimated to be +110 ± 15, while the calculated B-field direction is −642 ± 65 (both measured north through east; note that there is a 180° ambiguity on the B-field direction), yielding a difference of 752 ± 67. The filament direction was estimated by performing a linear regression on the coordinates of 12 bright peaks of submillimeter emission located along the linear portion of the integral filament, as observed in the JCMT GBS 850 μm SCUBA-2 data. The field direction was estimated by taking the mean of the position angles of the B-field half-vectors in the region of uniform field direction in the center of the OMC 1 region, between the Orion BN/KL and S clumps.

However, away from the central axis of the filament, the field appears to curve to either side. In the northern half of the filament, the field appears to curve northward, delineating a rough U-shape, centered on the filament. In the southern half of the filament, the field appears to curve to the south, forming an inverted U-shape. This so-called "hourglass" morphology was first noted by Schleuning (1998) at much lower resolution and signal-to-noise ratio, observing at 100 and 350 μm with the Kuiper Airborne Observatory (KAO) and the Caltech Submillimeter Observatory, respectively. However, we note a far higher degree of curvature of the field lines than was seen by Schleuning (1998).

There is a slight degree of depolarization visible toward the centers of the BN-KL and Orion-S clumps. This is a well-known effect resulting from tangled fields in the centers of very dense regions (e.g., Matthews & Wilson 2002). The pattern along the Orion Bar appears somewhat more complex. Furthermore, in the northeastern section of the map there is a region of half-vectors that appear to follow a different pattern. Here the half-vectors seem to be running along a different filament. All of the above is consistent with the much lower signal-to-noise ratio data of Houde et al. (2004) and Matthews et al. (2009). The interferometry data of Rao et al. (1998) on the peaks of OMC 1 are also consistent with our data. We now discuss all of these features.

Herschel has shown that the dominant formation mechanism for prestellar cores is core formation along filaments (André et al. 2014), revealing several examples of large-scale filaments lying perpendicular to the (plane-of-sky) B-field directions, as measured with large-scale absorption polarimetry (e.g., Palmeirim et al. 2013). This is consistent with findings from previous emission polarization measurements from SCUPOL on SCUBA (e.g., Matthews et al. 2001) and more recent large-scale polarization emission data from BLASTPol (e.g. Matthews et al. 2014). Based on these examples, a model has emerged whereby collapse occurs first along field lines to form filaments, and then along filaments to form cores (André et al. 2014). In the lower-density regions around the main filament, striations (or subfilaments) are typically seen parallel to the B-field (Palmeirim et al. 2013).

The polarization pattern we have observed in OMC 1 in Figure 3 follows this theoretical picture on-axis. The main part of the integral filament containing the BN-KL object and Orion South has a B-field direction apparently roughly orthogonal to the main filament direction, as mentioned above.

However, our wide-field data also allow us to trace the B-field direction off-axis, and it is here that even more interesting behavior is seen, as noted above, with a roughly "hourglass" morphology. If we follow this theoretical picture, then we would predict that the field lines started out roughly orthogonal to the filament in the lower density as well as the higher-density material, in a more uniform configuration, and was subsequently distorted into its current configuration.

There appear to be two possibilities as to how the hourglass morphology could have formed. One possibility is that the motion of the denser central material along the filament axis pulled the B-field lines into this configuration, as predicted by the model (see Figure 9(a) of André et al. 2014). Another possibility is that the well-known BN-KL outflow (Thaddeus et al. 1972) caused the field lines in the lower-density peripheral material to deviate from their original orientation. The effect of the highly collimated central part of the BN-KL outflow on the B-field on arcsecond scales is discussed by Tang et al. (2010).

We note that the outflow has a wide opening angle and high-velocity wings with multiple ejecta, often referred to as the "bullets of Orion" (Allen & Burton 1993). The central point of the outflow coincides with the position of the BN-KL object, the northern submillimeter-bright region in Figure 3. Consequently, the position and opening angle of the outflow roughly match the central part of the hourglass pattern, as well as the angle between the U-shape and the inverted U-shape fields, as if the outflow had pushed aside the field. Further work is required to decide which of these scenarios is correct.

A close-up of the Orion Bar region is shown in Figure 4. Here we see that the field follows a more complex morphology. At the southern end of the Bar, the field appears to be running north-south. In the middle of the Bar, the field runs roughly east-west. In the northern part of the Bar, the field appears to turn again to run in a northeasterly direction.

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This complex pattern clearly indicates a complex field structure. One possibility is a field that is simply twisting along the PDR front. Close examination of the Bar does appear to show that the Bar roughly twists in line with the field direction. Another possibility is that the field is running helically around the Orion Bar. In cases as complex as this, it is often difficult to determine which of a number of different three-dimensional scenarios is being projected onto our two-dimensional field of view (see, e.g., Franzmann & Fiege 2017). However, the simulations produced by Franzmann & Fiege (2017) show that a helical field could produce the polarization pattern that we are seeing.

Figure 5 shows a close-up of the northeastern filament that runs in a roughly east-west direction and is roughly orthogonal to the main integral filament. This is reminiscent of the subfilaments, or striations, seen in Taurus (Palmeirim et al. 2013), which lie perpendicular to the main filament. Figure 5 shows that the B-field lies roughly parallel to this subfilament, again as seen in Taurus (Palmeirim et al. 2013). Similar behavior is also seen in the low-density striations in the Polaris Flare region (Ward-Thompson et al. 2010; Panopoulou et al. 2016).

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Furthermore, the B-field pattern lying along the northeastern filament appears to lie in the foreground relative to the hourglass field. Both north and south of the northeastern filament the field lies in a direction running northeast-southwest, as if it continued behind the northeastern filament. Hence, we hypothesize that the northeastern filament is foreground to the rest of the cloud.

This behavior of parallel versus perpendicular field geometries is predicted theoretically. For example, numerous studies of non-self-gravitating (i.e., low-density) filaments see B-fields lying parallel to filaments—essentially by running simulations without gravity (e.g., Heitsch et al. 2001; Ostriker et al. 2001; Falceta-Gonçalves et al. 2008). Nakamura & Li (2008) include self-gravity and see "elongated condensations (i.e., dense filaments) that are generally perpendicular to the large-scale field".

More recently, Soler et al. (2013) studied in detail the effects of varying the B-field strength in a filament, as well as varying the density of the filament. They found that field lines are preferentially perpendicular to the filaments above a certain critical density and parallel to the filaments below this density.

This is exactly what we see here—the field is running parallel to the low-density northeastern filament, and perpendicular to the high-density integral filament (see Figure 1 of Soler et al. 2013). Incidentally, Soler et al. (2013) find field lines perpendicular to filaments only in intermediate-strength and high-strength field cases. This would tend to indicate that the field we are observing in Orion is relatively strong.

In this paper we have introduced the BISTRO survey, which will map the dense regions of many nearby star-forming clouds with the POL-2 polarimeter and SCUBA-2 on the JCMT. We have described the rationale behind the survey, and the scientific questions that the survey will answer. The most important of these is the role of B-fields in the star formation process on small scales and in dense regions, and its importance relative to other processes, such as turbulent or nonthermal motions of the gas.

We have described the data acquisition and reduction processes for POL-2, demonstrating that the rms noise on BISTRO POL-2 observations decreases as ${t}^{-0.5}$, as expected. We presented the first POL-2 polarization map from the BISTRO survey, which is of the OMC 1 region of Orion A, and showed compatibility with previous observations, as well as repeatability of the POL-2 results.

We saw that the field lies perpendicular to the integral filament in the densest regions of that filament. Furthermore, we saw an hourglass B-field morphology extending beyond the densest region of the integral filament into the less-dense surrounding material, and discussed possible causes for this. We observed a more complex morphology along the Orion Bar.

We examined the morphology of the field along the lower-density northeastern filament. We found consistency with previous theoretical models that predict B-fields lying parallel to low-density, non-self-gravitating filaments, and perpendicular to higher-density, self-gravitating filaments.

The James Clerk Maxwell Telescope is operated by the East Asian Observatory on behalf of The National Astronomical Observatory of Japan, Academia Sinica Institute of Astronomy and Astrophysics, the Korea Astronomy and Space Science Institute, the National Astronomical Observatories of China and the Chinese Academy of Sciences (grant No. XDB09000000), with additional funding support from the Science and Technology Facilities Council of the United Kingdom and participating universities in the United Kingdom and Canada. Additional funds for the construction of SCUBA-2 and POL-2 were provided by the Canada Foundation for Innovation. The data taken in this paper were observed under project codes M15BEC02 and M16AL004. D.W.T. and K.P. acknowledge Science and Technology Facilities Council (STFC) support under grant numbers ST/K002023/1 and ST/M000877/1. W.K., M.K., C.W.L. and S.S.L. were supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (W.K.: NRF-2016R1C1B2013642; M.K.: NRF-2015R1C1A1A01052160, S.S.L.: NRF-2016R1C1B2006697) and the Ministry of Education, Science and Technology (C.W.L.: NRF-2016R1A2B4012593). A.P. acknowledges the financial support provided by a Canadian Institute for Theoretical Astrophysics (CITA) National Fellowship. J.C.M. acknowledges support from the European Research Council under the European Community's Horizon 2020 framework program (2014–2020) via the ERC Consolidator grant "From Cloud to Star Formation (CSF)" (project number 648505). This research has made use of the NASA Astrophysics Data System. K.Q. acknowledges support from National Natural Science Foundation of China (NSFC) through grants NSFC 11473011 and NSFC 11590781. S.P.L., E.C., T.C.C., and J.W.W. are grateful for the support of the Ministry of Science and Technology (MOST) of Taiwan, through grants 102-2119-M-007-004-MY3, 104-2119-M-007-021, 105-2119-M-007-022-MY3, and 105-2119-M-007-024. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

Facility: James Clerk Maxwell Telescope (JCMT).

Software: Starlink (Currie et al. 2014), smurf (Berry et al. 2005; Chapin et al. 2013), Interactive Data Language (IDL).

In this appendix we present a demonstration of the repeatability of POL-2 observations of extended structure. These results are a subset of a larger study to be presented in the POL-2 commissioning paper (P. Bastien et al. 2017, in preparation), to which we refer the reader for further information.

In order to test the repeatability of our observations, we performed jackknife tests on our observations of OMC 1. We divided the data into odd- and even-numbered scans, the half-vector maps produced from which are shown in Figure 6. This division of scans is intentionally arbitrary, and is used to show the variation that might be expected between any two samples, uncorrelated in any observational property. We see excellent consistency between the two maps.

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In this appendix we compare the POL-2 map of OMC 1 to previous observations of OMC 1 made using the previous JCMT polarimeter, SCUPOL. There is no a priori reason to expect identical performance from SCUPOL and POL-2; the two instruments were/are mounted on different cameras (SCUBA and SCUBA-2 respectively; see Holland et al. 1999; Holland et al. 2006), and take data in different modes (see Greaves et al. 2003; Friberg et al. 2016; P. Bastien et al. 2017, in preparation). However, the two instruments take data at the same wavelength and resolution, and so the data taken ought to be directly comparable.

The SCUPOL observations of OMC 1 were published as part of the SCUPOL Legacy Catalog (Matthews et al. 2009). Figure 7 shows the SCUPOL data superposed on the POL-2 data. It can be seen that the POL-2 and SCUPOL half-vectors show a very similar morphology, but that the polarization fractions seen in the SCUPOL half-vectors are slightly larger than the POL-2 half-vectors. We believe that this is due to the lower signal-to-noise ratio of the older SCUPOL data.

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The similarity in the polarization angles of the POL-2 and SCUPOL half-vectors is shown quantitatively in Figure 8. The POL-2 and SCUPOL polarization angles are plotted at positions matched to within one JCMT beam (14.1 arcsec). The two half-vector sets show correlated polarization angles, and in fact the POL-2 and SCUPOL polarization angles are consistent with a 1:1 relationship.

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