Understanding the Origin of the Magnetic Field Morphology in the Wide-binary Protostellar System BHR 71

Early theories of magnetized star formation suggested that the formation of stars within molecular clouds should be regulated by a strong magnetic field (Mestel & Spitzer 1956; Shu et al. 1987; McKee et al. 1993; McKee & Ostriker 2007). In such a scenario, if one observed at small enough spatial scales (≲1000 au), it was thought that one should see an "hourglass" morphology in the magnetic field lines, where a strong, poloidal magnetic field is pinched by the gravitational infall very near the central source (Fiedler & Mouschovias 1993; Galli & Shu 1993a, 1993b; Allen et al. 2003). And, in fact, several examples of the fabled hourglass were seen in some of the first sources whose magnetic fields were observed at high resolution by the Berkeley–Illinois–Maryland Association (BIMA) millimeter array, the Submillimeter Array (SMA) and the Combined Array for Research in Millimeter-wave Astronomy (CARMA), including the bright, deeply embedded Class 0 protostellar sources NGC 1333-IRAS 4A (Girart et al. 1999, 2006), IRAS 16293A (Rao et al. 2009), and L1157 (Stephens et al. 2013). These sources exhibit other hallmarks of strong-field star formation, including powerful outflows (whose generation is intimately connected to the magnetic field; Frank et al. 2014) and high (inferred) magnetic field strengths, on the order of a few milli-Gauss.

Furthermore, the sources are not extremely fragmented, either being single sources (L1157, Tobin et al. 2013b), binaries (IRAS 4A, Looney et al. 2000; Girart et al. 2006), or possibly triple systems (IRAS 16293, Chandler et al. 2005). This is consistent with both theoretical work that found that a strong magnetic field strongly limits fragmentation on ∼1000–10,000 au scales (e.g., Hennebelle et al. 2011), as well as with a significant number of studies showing that a strong magnetic field could impede the formation of large disks, thus potentially reducing the frequency with which close-multiple systems form via disk fragmentation (Kratter et al. 2010; Tobin et al. 2016a) at the scales of a few ×100 au. See Wurster & Li (2018) for a recent review of the effect of the magnetic field on disk formation. However, other studies have found that a strong magnetic field can actually increase fragmentation under certain circumstances (Boss 2000; Offner et al. 2016; Offner & Chaban 2017). See Krumholz & Federrath (2019) for a recent review discussing this topic in more detail.

After more than a decade of high-resolution observations of magnetic fields in forming stars with BIMA, the SMA, CARMA, and now the Atacama Large Millimeter/submillimeter Array (ALMA), it has become clear that hourglass-shaped fields appear to be the exception rather than the rule (Hull & Zhang 2019). When observing at scales ≳100 au, these instruments probe polarization of dust grains that have been aligned with their minor axes parallel to the ambient magnetic field via the phenomenon of "Radiative Alignment Torques" (RATs; Draine & Weingartner 1996; Lazarian & Hoang 2007a; Hoang & Lazarian 2009; Andersson et al. 2015); this makes the polarized millimeter and submillimeter (hereafter, "(sub)millimeter") emission from the dust grains an excellent tracer of the magnetic field. Surveys of dust polarization toward both low- and high-mass young stellar objects found that outflows and magnetic fields are randomly aligned, calling into question the ability of magnetic fields to regulate star formation on small scales (Hull et al. 2013, 2014; Zhang et al. 2014; Hull & Zhang 2019). Furthermore, recent ALMA observations have revealed a source with chaotic magnetic fields whose structure is most likely dominated by turbulence and infall (Hull et al. 2017b), and other sources whose magnetic fields have been affected (and possibly shaped) by outflows (Hull et al. 2017a; Maury et al. 2018; Le Gouellec et al. 2019).

The fact that the BHR 71 binary system (Bourke 2001) is among the brightest Class 0 protostars known—similar to IRAS 4A, IRAS 16293, and L1157—might suggest that we should see an hourglass around one or both members of the binary. This is because strong, poloidal fields could plausibly help funnel infalling material onto the central sources more efficiently, thus increasing their brightness. However, in contrast to this well-ordered, quiescent formation scenario, recent evidence points to a turbulent origin for BHR 71, which comprises two binary components—IRS1 and IRS2—that are separated by ∼15'', or ∼3000 au at a distance of 200 pc (Seidensticker & Schmidt-Kaler 1989; Bourke et al. 1997). And indeed, in general, wide binaries (with separations ≳1000 au) like BHR 71 are suspected to be the result of turbulent fragmentation (e.g., Pineda et al. 2015; Lee et al. 2016; Tobin et al. 2016b).

The first piece of evidence pointing to a turbulent origin for BHR 71 is the misalignment of the outflows emanating from IRS1 and IRS2 (Bourke 2001; Parise et al. 2006). Previous studies have identified misaligned outflows (Tobin et al. 2015; Lee et al. 2016) and misaligned disks (Jensen & Akeson 2014; Lee et al. 2017) in wide-binary/multiple sources; these cases are consistent with simulations of multiple sources forming in turbulent environments (Offner et al. 2016). Second, there is tentative evidence that the envelopes around the two binary components are counter-rotating, as seen in C¹⁸O(J = 2 → 1) (Tobin et al. 2019, hereafter T19); these misaligned velocity gradients of the dense envelope material around IRS1 and IRS2 are also consistent with the turbulent formation of wide binaries. This is in contrast to close binaries (with separations ≲200 au), which are most likely the result of disk fragmentation, and whose components tend to have consistent angular momentum orientations that are roughly perpendicular to the binary/multiple's orbital plane (Tobin et al. 2018). Finally, the outflows from IRS1 and IRS2 are misaligned with the filamentary dust and ammonia structure in which they are embedded (T19), consistent with a turbulent origin for the sources, which have not obviously inherited their angular momentum orientation from their natal filament. This is consistent with work in the Perseus molecular cloud by Stephens et al. (2017a) showing that outflows and filamentary structure are randomly oriented with respect to one another.

Our understanding of the role of the magnetic field in the formation of binary/multiple systems is in its infancy. One of the first studies to touch on this topic is the recent work by Galametz et al. (2018), who found tentative evidence in their SMA data that there are large misalignments between the outflows and the magnetic field orientations in protostellar cores with higher rotational energies. They observed a ∼90°misalignment in some objects, which could be attributed to rotational winding of the magnetic field lines. Additionally, they found that several of the objects in this subsample of sources (with magnetic fields and outflows oriented perpendicular to one another) happen to be wide multiple sources and/or have large disks, whereas the sources in their sample with aligned outflows and magnetic fields tend to be single objects with small (or unresolved) disks. This is consistent with the tentative trend seen in the Karl G. Jansky Very Large Array (VLA) observations of Class 0 and I protostellar cores by Segura-Cox et al. (2018), where sources with larger disks tend to have misaligned magnetic fields and outflows. These results hint at a relationship between multiplicity and the magnetic field that can be further revealed by targeted ALMA studies such as the one we present here, as well as future surveys of dust polarization toward sources whose multiplicity has already been determined, such as those observed as part of the VLA Nascent Disk and Multiplicity (VANDAM) survey (Tobin et al. 2016b) using the VLA.

Here, we present full-polarization observations of BHR 71, with the goal of understanding the origin of the magnetic field morphologies toward the two components of this iconic wide-binary system. There have been many observations of BHR 71 over the last two decades (e.g., Bourke et al. 1997; Garay et al. 1998; Bourke 2001; Parise et al. 2006; Chen et al. 2008); these include recent ALMA observations of spectral lines by T19 (see Section 1.1) with spatial resolutions similar to those of the polarization observations we present here. As we describe below, our observations reveal an unexpected combination of what appears to be a natal hourglass magnetic field around BHR 71 IRS1, and a magnetic field that clearly has been affected by the outflow around IRS2.

We discuss our ALMA dust polarization observations toward BHR 71 in Sections 2 and 3. In Section 4, we compare our polarization observations with several ALMA spectral-line observations of outflows and dense-gas tracers published by T19, and we discuss the possible origins of the polarized emission in the two sources. We offer our conclusions and potential paths forward in Section 5.

1.1. Previous Observations by Tobin et al. (2019, or T19)

We present ALMA observations of dust polarization at 1.3 mm (Band 6) toward BHR 71 taken on 2018 May 03. The observations included 43 antennas. The precipitable water vapor (PWV) ranged from 0.9 to 1.7 mm during the observations. The pointing center was (α_J2000 = 12:01:36.514, δ_J2000 = −65:08:49.31) for IRS1 and (α_J2000 = 12:01:34.042, δ_J2000 = −65:08:47.870) for IRS2. The observations included ∼100 minutes of on-source time, taken during the local sidereal time range of ∼11:30–16:15 (i.e., ∼00:30 before transit to ∼4:15 after transit). With a Briggs weighting parameter of robust = 2.0 (see further discussion later in this section), the maps have a synthesized beam (or resolution element) of 110 × 089, equivalent to a linear resolution of ∼200 au at a distance of 200 pc. The baselines in the C-2 antenna configuration range from 15 to 500 m. The maximum spatial scale at which emission can be recovered by ALMA is approximately 106.

The polarization data include 8.0 GHz of wide-band dust continuum ranging in frequency from ∼223 to 227 GHz and ∼239 to 243 GHz, with a mean frequency of 233 GHz (1.3 mm). Each 2 GHz spectral window (with 1.875 GHz of usable bandwidth) was divided into 64 channels with widths of 31.25 MHz. The flux and bandpass calibrator was J1107-4449; the polarization calibrator was J1256-0547; and the phase calibrator was J1206-6138. These flux/bandpass and phase calibrators were chosen automatically by querying the ALMA source catalog when the project was executed. The polarization calibrator was chosen by hand due to its high polarization fraction. The observatory's flux monitoring program has determined that at Band 6, ALMA's systematic flux-calibration accuracy is ∼10%. See Nagai et al. (2016) for a detailed discussion of the ALMA polarization system. The systematic uncertainty in on-axis linear polarization observations with ALMA is 0.03% (corresponding to a minimum detectable polarization of 0.1%).

We produced the dust continuum images using the Common Astronomy Software Applications (CASA; McMullin et al. 2007) tasks TCLEAN and CLEAN. For all of the figures below that show both IRS1 and IRS2 in the same image, we constructed a two-point mosaic of the sources using TCLEAN, centered at (α_J2000 = 12:01:35.202, δ_J2000 = −65:08:48.64), half way between the two sources. While we mainly analyze the lower (∼10) resolution, robust = 2.0 images, we also made higher-resolution images using Briggs weighting parameters of robust = 0.5 and −0.5, yielding approximate resolutions of 075 and 053, respectively, in order to highlight higher-resolution features toward IRS1.

We also made images of IRS1 and IRS2 individually using CLEAN. Making non-mosaicked images was necessary to produce high-fidelity images of the Stokes I emission from IRS2, which is much fainter than IRS1. When IRS1 was observed in the center of the field of view, it was so bright that our ability to image faint Stokes I emission was limited by dynamic range. As a result, in the mosaicked images, the quality of the Stokes I image of IRS2 was degraded by the presence of IRS1 in the mosaic. However, when IRS2 was imaged alone, IRS1 was significantly removed from the phase center (and was thus fainter, due to the primary beam response of the ALMA antennas), which mitigated the dynamic-range effects and allowed us to produce higher-quality images of low-level Stokes I dust emission surrounding IRS2.

All images were improved by performing four rounds of phase-only self-calibration, where the total intensity (Stokes I) image was used as a model. The shortest interval for determining the gain solutions was 12 s. See Brogan et al. (2018) for a detailed discussion of and best practices for self-calibration. The Stokes I, Q, and U maps (where the Q and U maps show the polarized emission) were independently cleaned with an appropriate number of CLEAN or TCLEAN iterations after the final round of self-calibration. The rms noise level in the dynamic-range-limited Stokes I dust maps ranges from σ_I ≈ 300 μJy beam⁻¹ (robust = 2.0; mosaic) to 250 μJy beam⁻¹ (robust = 0.5, −0.5; mosaics) to 150 μJy beam⁻¹ (robust = 2.0; single pointing toward IRS2). The rms noise level σ_P in the maps of polarized intensity P (see Equation (1) below), which are not dynamic-range limited, ranges from 25 μJy beam⁻¹ (robust = 2.0, 0.5) to 40 μJy beam⁻¹ (robust = −0.5). The noise level in the P maps increases as the robust parameter decreases because weighting the uv-data to produce higher-resolution images tends to increase the noise level (Thompson et al. 2017).

The quantities we derive from the polarization maps include the polarized intensity P, the linear polarization fraction P_frac, and the polarization position angle χ:

$$\begin{eqnarray}&&P=\sqrt{{Q}^{2}+{U}^{2}}\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{P}_{\mathrm{frac}}=\displaystyle \frac{P}{I}\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&\chi =\displaystyle \frac{1}{2}\arctan \left(\displaystyle \frac{U}{Q}\right).\end{eqnarray} \tag{ 3 }$$

χ is defined to have a position angle of 0° when oriented N–S, and it increases to the east (Contopoulos & Jappel 1974). Note that P has a positive bias because it is always a positive quantity, whereas the Stokes parameters Q and U (from which P is derived) can be positive or negative. This bias is particularly significant in low-signal-to-noise (<5σ) measurements. We debias the polarized intensity map as described in Vaillancourt (2006) and Hull & Plambeck (2015); although, we note that it has only a very minor effect on our results.

As has been the case with several polarization results from ALMA, we detect a marginal circularly-polarized signal in the Stokes V map; however, the circular polarization fraction in the on-axis (single-pointing, non-mosaicked) observations of IRS1 is ∼0.08% of the total intensity, nearly an order of magnitude smaller than the current 0.6% systematic uncertainty in ALMA circular polarization observations, and thus is likely to be spurious. Furthermore, the behavior of the Stokes V signal when both IRS1 and IRS2 are observed off-axis is consistent with the known beam-squint profile (Chu & Turrin 1973; Adatia & Rudge 1975; Rudge & Adatia 1978) of the ALMA 12 m antennas at Band 6.

We show the results of our full-polarization, 1.3 mm dust continuum observations toward the BHR 71 binary in Figures 1 and 2. Later, we overlay these dust polarization results on images of the dense-gas tracers C¹⁸O(J = 2 → 1) and N₂D⁺(J = 3 → 2) (Figure 3) and on CO(J = 2 → 1) outflow emission (Figures 4 and 5). We also show dust polarization maps at multiple resolutions (Figure 6).

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In Figure 1, we show the Stokes Q, U, and polarized intensity P maps. By comparing by eye with the synthetic models of Q and U for an hourglass-shaped magnetic field morphology shown in Figure 4 of Frau et al. (2011), we can constrain the inclination of the magnetic field in BHR 71 IRS1 with respect to the the plane of the sky. The centers of the Q and U maps toward IRS1 look quite similar to the Frau et al. results for ω = 30°, which is equivalent to a ∼60° inclination of the source with respect to the plane of the sky. This is consistent with the inclination estimates by both T19 and Yang et al. (2017).

In Figure 2, we overlay the inferred magnetic field orientations (produced by rotating the polarization orientations by 90°) on the total intensity (Stokes I) dust continuum map. When one studies only the maps of dust polarization, the magnetic field morphologies of both sources are consistent with the hourglass shape discussed in Section 1. However, when the magnetic field maps are analyzed alongside spectral-line observations, it becomes clear that while the magnetic field toward IRS1 looks more like a "traditional" hourglass, in IRS2, the majority of the polarization detections are consistent with a magnetic field lying along the cavity walls of the bipolar outflow, traced by CO(J = 2 → 1) emission (see Figures 4 and 5). While the extended hourglass structure toward IRS1 has a symmetry axis that is well aligned with the CO outflow, the majority of the polarized emission is spatially extended far beyond the outflow cavity, and thus, in contrast to IRS2, the magnetic field toward IRS1 was most likely not shaped by the outflow. See Section 4.2 for further discussion.

In Figure 2, we also show maps of the polarization fraction P_frac toward IRS1 and IRS2. These maps exhibit the typical features of polarization maps of protostellar cores, including a "polarization hole," or a drop in P_frac to values ≲1% toward the Stokes I intensity peak (see Hull et al. 2014, and references therein). In spite of having polarization holes toward the very centers of the sources, the maps of IRS1 and IRS2 show polarization fraction levels >10% across much of both sources. These high levels of polarization can be reproduced by the models of grain alignment via RATs and have been seen in interferometric observations of polarization toward both low- and high-mass star-forming regions (e.g., Stephens et al. 2013; Hull et al. 2014; Cortes et al. 2016, 2019; Kwon et al. 2019). However, our ability to interpret P_frac is limited by the fact that the Stokes I image is much more strongly dynamic-range limited than the P image, resulting in a P map that extends beyond the limits of the I map. This has been seen in other high-sensitivity ALMA polarization maps (e.g., Hull et al. 2017a; Kwon et al. 2019; Le Gouellec et al. 2019).

We can take advantage of the excellent uv-coverage of ALMA to make images at different resolutions, varying the resolution by up to a factor of two by using different robust weighting parameters in the imaging process. When we make the polarization map of BHR 71 with three resolutions (10, 075, and 053), we clearly see an exceptionally sharp filamentary/stream-like structure extending to the NE of IRS1 (see Figure 6). These sharp structures, which are generally more apparent in P images than in I images, have been seen in other high-resolution, high-sensitivity ALMA polarization observations of protostellar cores (Hull et al. 2017a, 2017b; Maury et al. 2018; Sadavoy et al. 2018; Le Gouellec et al. 2019; Takahashi et al. 2019), and are discussed further in Section 4.4.

We begin this discussion by focusing on several plots, including the dust continuum, inferred magnetic field, and polarization fraction toward both binary components of BHR 71 (Figure 2); the magnetic field overlaid on maps of the dense-gas tracers C¹⁸O(J = 2 → 1) and N₂D⁺(J = 3 → 2) (Figure 3); and the magnetic field overlaid on the bipolar outflows from IRS1 and IRS2 traced by CO(J = 2 → 1) (Figures 4 and 5).

As first mentioned in Section 3 and discussed further in Sections 4.2 and 4.1, when the polarization maps are analyzed alongside maps of spectral-line emission, it becomes clear that the magnetic field morphologies in IRS1 and IRS2 have different origins. The brighter source IRS1 has a magnetic field configuration that resembles an hourglass, which is aligned with (but not significantly disturbed by) the bipolar outflow. In contrast, the fainter source IRS2 shows a magnetic field that has been shaped by the outflow. The main goal of this discussion is to understand why these differences arise in the two components of the same binary source.

4.1. Dust Polarization and Dense-gas Tracers

4.2. A Natal Hourglass in IRS1 versus an Outflow-shaped Magnetic Field in IRS2

4.3. Irradiation of the Outflow Cavity Walls in BHR 71 IRS2

Single-dish studies of high-J CO emission toward protostars have suggested that such energetic CO emission cannot be reproduced by models of passive envelope heating. Rather, this gas must be heated by UV radiation originating in the accretion shock around the central protostar as well as in shocks distributed throughout the outflow itself (Spaans et al. 1995; van Kempen et al. 2009a; Visser et al. 2012). More recent models explained the shape of the high-J CO ladder and the chemical signatures (e.g., the line ratios H₂O/CO and H₂O/OH) seen in Herschel observations of embedded protostars by invoking models of UV-irradiated shocks in the protostars' bipolar outflow cavities (Kristensen et al. 2017; Karska et al. 2018). These findings are consistent with many other observational and theoretical studies that have seen enhanced chemistry in the vicinity of shocked, irradiated gas (e.g., in HH objects: Girart et al. 1994; Taylor & Williams 1996; Viti & Williams 1999; Girart et al. 2002; Christie et al. 2011). CO(J = 6 → 5) was detected in observations of BHR 71 with the Atacama Pathfinder EXperiment telescope by van Kempen et al. (2009b) and Gusdorf et al. (2015); the observations by Gusdorf et al. resolve this high-J CO emission in both the IRS1 and IRS2 outflows. Here, we consider the scenario that the enhanced polarization along the outflow cavity walls of IRS2 may be the result of irradiation by photons generated in accretion and outflow shocks.

The polarized lane along the northern edge of the blue- and redshifted IRS2 outflow lobes is marginally resolved, with an average thickness of ∼300 au. Based on PDR models by Girart et al. (2005), UV radiation is fully extincted at an A_V of order unity, which is achieved at a molecular hydrogen column density of between 10²¹ and 10²² cm⁻² (Bohlin et al. 1978). To estimate the amount of material along the IRS2 outflow cavity wall, we use the standard conversion from flux density S_ν to gas mass M_gas (see, e.g., Hull et al. 2017a; Hull & Zhang 2019):

$$\begin{eqnarray}&&{M}_{\mathrm{gas}}=\displaystyle \frac{{S}_{\nu }{d}^{2}}{{\kappa }_{\nu }{B}_{\nu }\left({T}_{{\rm{d}}}\right)},\end{eqnarray} \tag{ 4 }$$

where ${B}_{\nu }\left({T}_{{\rm{d}}}\right)$ is the Planck function at the 233 GHz frequency of our observations, the distance d = 200 pc, and the dust opacity at 1.3 mm κ_ν = 2 cm² g⁻¹ (Ossenkopf & Henning 1994). We assume a gas-to-dust ratio of 100.

In a circle with a diameter equal to 300 au, which is the approximate width of the enhanced dust emission along the outflow cavity, the flux density along the entire northern edges of the blue- and redshifted outflow cavity walls is a roughly constant ∼2–3 mJy. The fact that the brightness of the cavity walls is constant—as opposed to decreasing with distance from the central protostar—suggests that the heating of the outflow cavities is indeed from distributed shocks and not purely from high-energy radiation from the central protostar, the latter of which would decrease with distance from the central source. This is perhaps unsurprising in the case of IRS2, which, in addition to having a bipolar CO outflow, is known to have a high-velocity (±100 km s⁻¹), bipolar SiO jet, whose internal shocks may contribute to the illumination of the cavity walls (T. L. Bourke et al. 2019, in preparation).

To calculate the column density along the IRS2 outflow cavity walls, we use the IRS2 temperature estimate of 20 K from T19, assume a mean molecular weight of 2.8 in the gas (Kauffmann et al. 2008), and assume an average flux of 2.5 mJy in a 300 au-diameter circle centered on the outflow cavity wall. The resulting column density is ∼2.6 × 10²² cm⁻². However, the dust along the outflow walls could be warmer than the overall dust temperature toward the IRS2 core. The gas temperature of the IRS2 outflow could be up to 300 K (Parise et al. 2006), which is reasonable based on the models of UV-irradiated outflow cavities by, e.g., Visser et al. (2012) and Drozdovskaya et al. (2015). If we take that same value as the dust temperature (which is perhaps unreasonably high, as the dust is usually significantly cooler than the gas: Yıldız et al. 2015), the column density we derive is ∼1.3 × 10²¹ cm⁻². Even considering this high value of 300 K for the dust temperature, the column density of the cavity walls is still too high for UV photons to penetrate to a depth of 300 au. In light of this, below we paint two pictures of the potential origin of the polarization we see in the outflow cavity walls in IRS2: the "thick" scenario, and the "thin" scenario.

We first consider the "thick" scenario, where the polarization originates in a layer of the cavity walls with a thickness of ∼300 au. The first question to be addressed is, Which photons can penetrate to that depth and align the dust grains? We thus calculate how far into the walls photons with wavelengths longer than those of UV photons can penetrate. We use the dust opacity values κ_ν from Ossenkopf & Henning (1994) that correspond to gas densities of 10⁶ cm⁻³ (and thus dust densities of 10⁴ cm⁻³, assuming a gas-to-dust ratio of 100) and grains without icy mantles.¹² Assuming a constant opacity κ_ν and mass density ρ of the dust grains throughout the thickness of the cavity wall, the path length s to the point where the optical depth τ = 1 can be calculated simply as s = 1/κ_ν ρ. The values of κ_ν in Ossenkopf & Henning (1994) are normalized by the dust mass density ρ; hence, we must multiply κ_ν by the value of ρ in the cavity walls in order to calculate s.

For 1 μm photons, the penetration depths assuming 20 and 300 K temperatures are ∼14 and ∼280 au, respectively. For 10 μm photons, the depths are ∼70 and ∼1400 au. For 100 μm photons, the depths are ∼2800 and ∼56,000 au. Finally, for 1.3 mm photons, the depths are ∼84,000 and ∼1,700,000 au. If indeed the polarization originates in a layer with a thickness of ∼300 au, then it appears that the dust grains have been aligned by mid- to far-infrared (MIR/FIR) photons and/or possibly by an anisotropic radiation field resulting from the temperature gradient between the inner part of the outflow cavity and the cold, unpolarized material traced by N₂D⁺(J = 3 → 2).

A major caveat of this "thick" scenario is grain growth. RAT theory suggests that photons can efficiently spin-up dust grains only if the grain size is comparable to the photon wavelength (Lazarian & Hoang 2007a). Therefore, in order for the MIR/FIR photons to be the cause of the enhanced polarization we see in IRS2, there would need to be a substantial population of ∼ 10 μm-sized dust grains in the cavity walls. While grains of that size are expected in circumstellar disks based on both SED-fitting and dust-scattering-polarization studies (e.g., Pérez et al. 2012; Testi et al. 2014; Stephens et al. 2017b; Hull et al. 2018), they are not expected in the regions where we see polarization toward IRS2 here, i.e., at distances that in some places exceed ∼1000 au from the central source.

We also consider a second, "thin" scenario, where the resolved layer of polarization that we see is caused by projection effects. In this case, the observed polarization actually originates from a very thin layer of material in the cavity wall, which comprises a negligible amount of mass, but which is distributed around a substantial fraction of the diameter of the cavity where the column density of dust is still high enough to detect polarized emission. This scenario would allow higher-energy photons (i.e., UV, optical, NIR), which cannot penetrate very deep into the cavity walls (see above), to align the smaller, micron-sized dust grains that are expected to dominate the grain population at large distances from the central source. This thin layer of aligned, small grains could then be co-located with molecular emission associated with UV-driven chemistry in outflow cavities (Karska et al. 2018, and references therein); emission from several UV tracers has been spatially resolved in ALMA observations (e.g., Imai et al. 2016; Le Gouellec et al. 2019), but is expected to originate mainly in the very thin layer of the cavity walls accessible to the UV radiation.

This "thin" scenario has its own caveats. If only an extremely thin layer of dust grains were aligned, it seems unlikely that the polarization fraction would reach the high (>10%) levels that we detect in IRS2, considering that the polarization fraction is derived from the polarized intensity from the aligned grains divided by the total intensity of the dust emission, the latter of which originates from the entire core. In addition, the high-energy radiation would be reprocessed by the thin layer of initially-heated dust grains in the cavity wall, being re-emitted as longer-wavelength photons that could penetrate deeper into the walls. However, it is unclear if there would be enough of these photons to align a significant number of grains to an appreciable depth beyond the initial heated layer.

Given the major caveat of the polarization fraction levels in the "thin" case, and the fact that recent studies have shown that grain growth in Class 0 protostellar cores may be necessary to explain high-resolution, interferometric polarization results (Maury et al. 2018; Le Gouellec et al. 2019; Valdivia et al. 2019), we find the "thick" scenario to be the more plausible of the two possibilities for the origin of the polarization in the IRS2 outflow cavities. However, the answer is far from clear. Future, multi-wavelength polarization observations of protostellar cores, coupled with synthetic observations of MHD simulations with next-generation radiative transfer codes like POLARIS (Reissl et al. 2016) that incorporate dust-grain alignment via RATs, will help us to better understand the origin of polarization in irradiated environments like cavity walls.

4.4. A Possible Magnetized Accretion Streamer in BHR 71 IRS1

4.5. Histograms of Relative Orientation (HRO) of the Magnetic Field versus Dust and Outflow Emission in BHR 71 IRS1 and IRS2

To better understand the differences in the magnetic field morphologies of IRS1 and IRS2, we analyze the HRO (Soler et al. 2013) of the magnetic field versus the gradients of both dust and the CO(J = 2 → 1) (outflow) emission toward IRS1 and IRS2 (see Figure 7). HROs have been used many times recently to shed light on the importance (or lack thereof) of the magnetic field in the formation of structure in star-forming regions, from the spatial scales of molecular clouds (Planck Collaboration et al. 2016a, 2016b; Soler et al. 2017; Fissel et al. 2019; Soler 2019) to individual protostellar cores (Hull et al. 2017b).

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When producing the histograms, both sources were isolated to clearly establish the corresponding distributions. We calculated the gradient in the zone of significant signal-to-noise: in the Stokes I continuum map where the signal I > 3 σ_I, and in moment 0 maps produced with a threshold at 10× the rms noise level in a channel of the CO image cube (the moment 0 maps we analyze are the same as those plotted in Figure 4). We then selected the zones with significant gradient values. In the outflow moment 0 maps this served to highlight the edges of the outflow cavities, whereas in the dust continuum maps the gradient picked out both the central cores of the protostars as well as (in the case of IRS2) regions of enhanced dust emission along the outflow cavity walls. Finally, for the locations where there is both a gradient value as well as a magnetic field orientation (i.e., where P > 3 σ_P), we derived ϕ, defined as the difference in angles between the magnetic field and the emission gradient, and added the points to the distributions. For further details of this HRO analysis, see Le Gouellec et al. (2019).

Toward IRS2, it is clear from Figure 5 that the magnetic field follows the northern edge of the outflow cavity. The HRO comparing the magnetic field and the redshifted CO emission does not exhibit this, as it is limited by statistics because of the physical offset between the redshifted outflow lobe and the polarized emission. However, we can clearly see that in the blueshifted lobe of IRS2, ϕ peaks near 90°, indicating that the magnetic field orientation tends to be perpendicular to the CO emission gradient (i.e., nearly parallel to the edges of the outflow). For IRS1, however, the HRO for both the blue- and redshifted lobes look similar to the blueshifted lobe of IRS2, simply because the hourglass symmetry axis and the outflow axis are aligned. The curvature of the hourglass-shaped magnetic field in IRS1 does yield a broader HRO compared with the HRO toward the blueshifted outflow lobe of IRS2; however, this is not a strong distinguishing factor. The fact that the hourglass magnetic field in IRS1 and the magnetic field along the edges of the outflow in IRS2 yield such similar distributions in the magnetic-field-versus-CO HROs highlights the difficulty of distinguishing between the "natal hourglass" versus "outflow-affected magnetic field" scenarios.

We can make a clearer distinction between the two sources by looking at the HRO of the magnetic field versus the dust emission (Figure 7, left panels). The HRO from IRS2 demonstrates that the magnetic field is perpendicular to the dust-emission gradient (i.e., the magnetic field is parallel to the outflow-cavity walls), indicating that the magnetic field has been affected by the outflow. In contrast, the HRO from IRS1 shows that the field is more parallel to the dust-intensity gradient, indicative of a magnetically regulated but gravity-dominated scenario in a centrally condensed protostellar core (see, e.g., Koch et al. 2012, 2018). These differences in the magnetic-field-versus-dust HROs can help us distinguish whether the dust morphology in a source has been more affected by gravity or by the outflow; however, it is still essential to find a robust way to use outflow tracers to determine quantitatively whether magnetic fields have been affected by outflows.

We have presented 1.3 mm ALMA observations of polarized dust emission toward the wide-binary protostellar system BHR 71. After analyzing the inferred magnetic field morphology toward both sources alongside maps of the bipolar outflows and dense-gas tracers, we come to the following conclusions:

• 1.  
  While the magnetic field morphologies of both BHR 71 IRS1 and IRS2 are consistent with hourglass shapes, analysis of the magnetic field maps alongside spectral-line observations reveals that IRS1 has what appears to be a natal, hourglass-shaped magnetic field. In contrast, its fainter, more embedded binary counterpart IRS2 exhibits a magnetic field that has been affected by its bipolar outflow.
• 2.  
  Toward IRS1, there is a strong correlation of polarized emission with C¹⁸O, which traces warm (≳25 K) material throughout the whole protostellar envelope. Toward IRS2, in contrast, the polarization is confined mainly to the outflow cavity walls.
• 3.  
  Along the northern edge of the redshifted outflow cavity in IRS2, the polarized emission is sandwiched between the outflowing material and a filament of cold, dense gas traced by N₂D⁺, toward which no dust polarization is detected. This suggests that the origin of the enhanced polarization in IRS2 is the irradiation of the outflow cavity walls, which enables the alignment of dust grains with respect to the magnetic field—but only to a depth of ∼300 au, beyond which the dust is cold and unpolarized. However, in order to align grains deep enough in the cavity walls, and to produce the high polarization fraction seen in IRS2, the aligning photons are likely to be in the mid-infrared to far-infrared range, which suggests a degree of grain growth beyond what is typically expected in very young, Class 0 sources.
• 4.  
  The anticorrelation of dust polarization from IRS1 and IRS2 and emission from N₂D⁺ suggests that this species (and its non-deuterated counterpart N₂H⁺) is an excellent tracer of unpolarized material because it is very sensitive to regions of cold, dense gas where RATs cannot efficiently align dust grains with the magnetic field.
• 5.  
  The difference in magnetic field morphologies toward the two binary components of BHR 71 may arise simply because of the higher temperature and ∼10× higher luminosity of IRS1 relative to IRS2. The higher temperature yields warmer dust and, thus, more easily detectable polarization. The higher luminosity yields a stronger radiation field and larger temperature gradients, which enable the alignment of dust grains with respect to the magnetic field throughout the majority of the envelope of IRS1. In contrast, toward less luminous IRS2, we are only able to detect polarization in regions with enhanced polarization, such as the irradiated outflow cavity walls. This same logic could explain why polarization is so widespread and easily detected in high-mass star-forming regions and in bright low-mass sources (like IRS1), which are warmer and have stronger radiation fields than their lower-luminosity counterparts (like IRS2).
• 6.  
  Recent ALMA observations have revealed narrow polarization features in sources such as BHR 71 IRS1 and IRS2 (shown here), Ser-emb 8 (Hull et al. 2017b), Serpens SMM1 (Hull et al. 2017a; Le Gouellec et al. 2019), B335 (Maury et al. 2018), IRAS 16293 (Sadavoy et al. 2018), OMC3–MMS6 (Takahashi et al. 2019), and Serpens Emb 8(N) (Le Gouellec et al. 2019). These features seem to fall into two categories:

  • (a)  
    Outflow-related features. These manifest themselves as features that have very high polarization fractions (sometimes ≳20%), and that are well aligned with the outflow cavity walls. Examples are the outflow cavities of BHR 71 IRS2 (shown here); the E–W wall of the redshifted, low-velocity outflow lobe of Serpens SMM1-a; and the outflow cavities toward both B335 and Ser-emb 8(N). The polarization in these regions is most likely enhanced by irradiation of the cavity walls.
  • (b)  
    Potentially accretion-related features. These are thin, approximately linear features with high polarization fractions (usually 10%–20%) and extremely well-ordered magnetic fields; however, these features do not appear to be associated with outflow cavities. Examples include the sharp feature to the NE of BHR 71 IRS1; the magnetized filamentary structure around Ser-emb 8; the N–S equatorial-plane feature in B335; the "arm-like" structure in OMC3–MMS6; the "bridge" and streamers observed toward the two binary components in IRAS 16293; and one of the two narrow filaments to the south of Serpens SMM1-a. We speculate that these features may be magnetized accretion streamers; however, this scenario has yet to be confirmed by kinematic observations of dense-gas tracers.

With the advent of ALMA, our ability to probe the structure of magnetic fields in protostellar cores has vastly improved. Upcoming surveys of large numbers of young, embedded sources will soon reveal how common is each of the scenarios that individual-source studies have recently unveiled: natal hourglass-shaped fields, magnetic fields affected by bipolar outflows, and possible magnetized accretion streamers. Furthermore, observations of large-scale magnetic fields using current and upcoming single-dish polarimeters on instruments such as the Stratospheric Observatory for Infrared Astronomy (SOFIA; Vaillancourt et al. 2007), the James-Clerk-Maxell Telescope (JCMT; e.g., the BISTRO survey: Ward-Thompson et al. 2017); the BLAST-TNG balloon-borne experiment (Galitzki et al. 2014), the IRAM 30 m telescope (Ritacco et al. 2017), and the Large Millimeter Telescope will allow us to understand how the larger-scale magnetic environment connects with the myriad small-scale magnetic field morphologies revealed by ALMA.

The authors thank the anonymous referee, whose insightful comments led to substantial improvements in the manuscript. The authors acknowledge the support of Gerald Schieven at the North American ALMA Science Center, and Walker Lu and Toshinobu Takagi at the East Asian ALMA Regional Center. C.L.H.H. and V.J.M.L.G. acknowledge helpful discussions with the ALMA Fellows, Ruud Visser, Lars Kristensen, Anaëlle Maury, Zhi-Yun Li, Phil Myers, Ian Stephens, Sarah Sadavoy, and Felipe Alves. C.L.H.H. acknowledges the support of both the NAOJ Fellowship as well as JSPS KAKENHI grant 18K13586. V.J.M.L.G. acknowledges the support of the ESO Studentship Program. J.M.G. acknowledges the support of the Spanish MINECO AYA2017-84390-C2-R grant, and the Joint ALMA Observatory Visitor Program. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2013.1.00518.S and ADS/JAO.ALMA#2017.1.00655.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST, and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Facility: ALMA. -

Software: APLpy, an open-source plotting package for Python hosted at http://aplpy.github.com (Robitaille & Bressert 2012). CASA (McMullin et al. 2007). Astropy (Astropy Collaboration et al. 2018).