Understanding the origin of the magnetic field morphology in the wide-binary protostellar system BHR 71
Charles L. H. Hull, Valentin J. M. Le Gouellec, Josep M. Girart, John J. Tobin, Tyler L. Bourke
SSubmitted to ApJ on 17 May 2019; accepted to ApJ on 15 Oct 2019
Preprint typeset using L A TEX style emulateapj v. 12/16/11
UNDERSTANDING THE ORIGIN OF THE MAGNETIC FIELD MORPHOLOGY IN THEWIDE-BINARY PROTOSTELLAR SYSTEM BHR 71
Charles L. H. Hull , Valentin J. M. Le Gouellec , Josep M. Girart , John J. Tobin , andTyler L. Bourke Submitted to ApJ on 17 May 2019; accepted to ApJ on 15 Oct 2019
ABSTRACTWe present 1.3 mm ALMA observations of polarized dust emission toward the wide-binary protostellarsystem BHR 71 IRS1 and IRS2. IRS1 features what appears to be a natal, hourglass-shaped magneticfield. In contrast, IRS2 exhibits a magnetic field that has been affected by its bipolar outflow. TowardIRS2, the polarization is confined mainly to the outflow cavity walls. Along the northern edge ofthe redshifted outflow cavity of IRS2, the polarized emission is sandwiched between the outflow anda filament of cold, dense gas traced by N D + , toward which no dust polarization is detected. Thissuggests that the origin of the enhanced polarization in IRS2 is the irradiation of the outflow cavitywalls, which enables the alignment of dust grains with respect to the magnetic field—but only to adepth of ∼
300 au, beyond which the dust is cold and unpolarized. However, in order to align grainsdeep enough in the cavity walls, and to produce the high polarization fraction seen in IRS2, thealigning photons are likely to be in the mid- to far-infrared range, which suggests a degree of graingrowth beyond what is typically expected in very young, Class 0 sources. Finally, toward IRS1 we see anarrow, linear feature with a high (10–20%) polarization fraction and a well ordered magnetic field thatis not associated with the bipolar outflow cavity. We speculate that this feature may be a magnetizedaccretion streamer; however, this has yet to be confirmed by kinematic observations of dense-gas tracers.
Keywords:
ISM: magnetic fields — polarization — ISM: jets and outflows — stars: protostars —binaries: general — radiation mechanisms: thermal INTRODUCTION
Early theories of magnetized star-formation suggestedthat the formation of stars within molecular clouds shouldbe regulated by a strong magnetic field (Mestel & Spitzer1956; Shu et al. 1987; McKee et al. 1993; McKee & Os-triker 2007). In such a scenario, if one observed at smallenough spatial scales ( (cid:46)
Electronic address: [email protected] National Astronomical Observatory of Japan, NAOJ Chile,Alonso de Córdova 3788, Office 61B, 7630422, Vitacura, Santiago,Chile Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura,Santiago, Chile European Southern Observatory, Alonso de Córdova 3107, Vi-tacura, Santiago, Chile AIM, CEA, CNRS, Université Paris-Saclay, Université ParisDiderot, Sorbonne Paris Cité, F-91191 Gif-sur-Yvette Institut de Ciències de l’Espai (ICE-CSIC), Campus UAB, Car-rer de Can Magrans S/N, E-08193 Cerdanyola del Vallès, Catalonia Institut d’Estudis Espacials de Catalunya, E-08030 Barcelona,Catalonia National Radio Astronomy Observatory, 520 Edgemont Road,Charlottesville, Virginia 22903, USA Homer L. Dodge Department of Physics and Astronomy, Uni-versity of Oklahoma, 440 W. Brooks Street, Norman, OK 73019,USA Leiden Observatory, Leiden University, P.O. Box 9513, NL-2300RA Leiden, The Netherlands Square Kilometre Array Organization, Jodrell Bank Observa-tory, Lower Withington, Macclesfield, Cheshire SK11 9DL, UK NAOJ Fellow fabled hourglass were seen in some of the first sourceswhose magnetic fields were observed at high resolutionby the Berkeley-Illinois-Maryland Association (BIMA)millimeter array, the Submillimeter Array (SMA) and theCombined Array for Research in Millimeter-wave Astron-omy (CARMA), including the bright, deeply embeddedClass 0 protostellar sources NGC 1333-IRAS 4A (Girartet al. 1999, 2006), IRAS 16293A (Rao et al. 2009), andL1157 (Stephens et al. 2013). These sources exhibit otherhallmarks of strong-field star formation, including pow-erful outflows (whose generation is intimately connectedto the magnetic field; Frank et al. 2014) and high (in-ferred) magnetic field strengths, on the order of a fewmilli-Gauss.Furthermore, the sources are not extremely fragmented,either being a single source (L1157, Tobin et al. 2013b),a binary (IRAS 4A, Looney et al. 2000; Girart et al.2006), or a triple system (IRAS 16293, Wootten 1989;Chandler et al. 2005). This is consistent with both the-oretical work that found that a strong magnetic fieldstrongly limits fragmentation on ∼ ×
100 au. See Wurster& Li (2018) for a recent review of the effect of the mag-netic field on disk formation. However, other studies havefound that a strong magnetic field can actually increasefragmentation under certain circumstances (Boss 2000;Offner et al. 2016; Offner & Chaban 2017). See Krumholz& Federrath (2019) for a recent review discussing this a r X i v : . [ a s t r o - ph . S R ] O c t Hull et al. topic in more detail.After more than a decade of high-resolution observa-tions of magnetic fields in forming stars with BIMA, theSMA, CARMA, and now the Atacama Large Millime-ter/submillimeter Array (ALMA), it has become clearthat hourglass-shaped fields appear to be the exceptionrather than the rule (Hull & Zhang 2019). When ob-serving at scales (cid:38)
100 au, these instruments probe polar-ization of dust grains that have been aligned with theirminor axes parallel to the ambient magnetic field via thephenomenon of “Radiative Alignment Torques” (RATs;Draine & Weingartner 1996; Lazarian & Hoang 2007a;Hoang & Lazarian 2009; Andersson et al. 2015); thismakes the polarized millimeter and submillimeter (here-after, “(sub)millimeter”) emission from the dust grains anexcellent tracer of the magnetic field. Surveys of dust po-larization toward both low- and high-mass young stellarobjects (YSOs) found that outflows and magnetic fieldsare randomly aligned, calling into question the abilityof magnetic fields to regulate star formation on smallscales (Hull et al. 2013, 2014; Zhang et al. 2014; Hull &Zhang 2019). Furthermore, recent ALMA observationshave revealed a source with chaotic magnetic fields whosestructure is most likely dominated by turbulence and in-fall (Hull et al. 2017b), and other sources whose magneticfields have been affected (and possibly shaped) by out-flows (Hull et al. 2017a; Maury et al. 2018; Le Gouellecet al. 2019).The fact that the BHR 71 binary system (Bourke 2001)is among the brightest Class 0 protostars known—similarto IRAS 4A, IRAS 16293, and L1157—might suggest thatwe should see an hourglass around one or both membersof the binary. This is because strong, poloidal fields couldplausibly help funnel infalling material onto the centralsources more efficiently, thus increasing their brightness.However, in contrast to this well ordered, quiescent forma-tion scenario, recent evidence points to a turbulent originfor BHR 71, which comprises two binary components—IRS1 and IRS2—that are separated by ∼ (cid:48)(cid:48) , or ∼ (cid:38) O ( J = 2 → (cid:46)
200 au), which are most likely the resultof disk fragmentation, and whose components tend tohave consistent angular momentum orientations that areroughly perpendicular to the binary/multiple’s orbitalplane (Tobin et al. 2018). Finally, the outflows from IRS1 and IRS2 are misaligned with the filamentary dust andammonia structure in which they are embedded (T19),consistent with a turbulent origin for the sources, whichhave not obviously inherited their angular momentum ori-entation from their natal filament. This is consistent withwork in the Perseus molecular cloud by Stephens et al.(2017a) showing that outflows and filamentary structureare randomly oriented with respect to one another.Our understanding of the role of the magnetic fieldin the formation of binary/multiple systems is in its in-fancy. One of the first studies to touch on this topic isthe recent work by Galametz et al. (2018), who foundtentative evidence in their SMA data that there are largemisalignments between the outflows and the magneticfield orientations in protostellar cores with higher rota-tional energies. They observed a ∼ ◦ misalignment insome objects, which could be attributed to rotationalwinding of the magnetic field lines. Additionally, theyfound that several of the objects in this subsample ofsources (with magnetic fields and outflows oriented per-pendicular to one another) happen to be wide multiplesources and/or have large disks, whereas the sources intheir sample with aligned outflows and magnetic fieldstend to be single objects with small (or unresolved) disks.This is consistent with the tentative trend seen in theVLA observations of Class 0 and I protostellar cores bySegura-Cox et al. (2018), where sources with larger diskstend to have misaligned magnetic fields and outflows.These results hint at a relationship between multiplic-ity and the magnetic field that can be further revealedby targeted ALMA studies such as the one we presenthere, as well as future surveys of dust polarization towardsources whose multiplicity has already been determined,such as those observed as part of the VLA Nascent Diskand Multiplicity (VANDAM) survey (Tobin et al. 2016b)using the Karl G. Jansky Very Large Array (VLA).Here we present full-polarization observations ofBHR 71, with the goal of understanding the origin ofthe magnetic field morphologies toward the two compo-nents of this iconic wide-binary system. There have beenmany 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 recentALMA observations of spectral lines by T19 (see Section1.1) with spatial resolutions similar to those of the po-larization observations we present here. As we describebelow, our observations reveal an unexpected combina-tion of what appears to be a natal hourglass magneticfield around BHR 71 IRS1, and a magnetic field thatclearly has been affected by the outflow around IRS2.We discuss our ALMA dust polarization observationstoward BHR 71 in § 2 and § 3. In § 4 we compare ourpolarization observations with several ALMA spectral-lineobservations of outflows and dense-gas tracers publishedby T19, and we discuss the possible origins of the polarizedemission in the two sources. We offer our conclusions andpotential paths forward in § 5. Previous observations by Tobin et al. (2019, or T19)
T19 discussed the formation conditions of BHR 71 atlength, using both new and archival data to analyze thekinematics and continuum properties across a wide rangeof spatial scales ranging from ∼ he magnetic field in BHR 71 Spitzer
IRAC, MIPS, and IRS observations; far-infrared
Herschel
PACS and SPIRE observations; millimeter-waveALMA observations of CO ( J = 2 → CO ( J = 2 → O ( J = 2 → D + ( J = 3 → (1,1) observations from the Parkes radio tele-scope; and NH (1,1), (2,2), (3,3) observations from theAustralian Telescope Compact Array (ATCA).While the Parkes and ATCA observations showed aclear (albeit small, ∼ − pc − ) velocity gradientacross the core in which both IRS1 and IRS2 are em-bedded, analysis by T19 of the suite of higher-resolutionspectral-line observations showed much more complexstructure, including tentative signatures in the ALMAC O ( J = 2 →
1) emission at < 1000 au scales that theenvelopes around IRS1 and IRS2 may be rotating in op-posite directions. These observations led the authors toconclude that the BHR 71 binary most likely formed fromturbulent fragmentation rather than from rotational/diskfragmentation.Assuming dust temperatures of 34 K for IRS1 and 20 Kfor IRS2, T19 calculated dust + gas masses of 0.59 and0.11 M (cid:12) for the two sources, respectively. The availablemulti-wavelength observations also enabled T19 to con-firm that both sources are protostars in the youngest(Class 0) stage of protostellar evolution, and to esti-mate their bolometric luminosities: 14.7 L (cid:12) for IRS1 and1.7 L (cid:12) for IRS2.Estimates by T19 of the inclination of the outflows fromIRS1 and IRS2 with respect to the plane of the sky yieldedvalues of ∼ ◦ for both sources (where 90 ◦ means theoutflow is fully in the plane of the sky). This is consistentwith an estimate from Yang et al. (2017), who estimateda 50 ◦ inclination angle for IRS1. The full opening angles(i.e., the angle between the two edges of the outflow) forIRS1 and IRS2 are 55 ◦ and 47 ◦ , respectively. OBSERVATIONS
We present ALMA observations of dust polarizationat 1.3 mm (Band 6) toward BHR 71 taken on 2018May 03. The observations included 43 antennas. Theprecipitable water vapor (PWV) ranged from 0.9 to1.7 mm during the observations. The pointing center was( α J2000 = 12:01:36.514, δ J2000 = –65:08:49.31) for IRS1and ( α J2000 = 12:01:34.042, δ J2000 = –65:08:47.870) forIRS2. The observations included ∼
100 min of on-sourcetime, taken during the LST range of ∼ ∼ ∼ robust = 2.0 (see fur-ther discussion later in this section), the maps have asynthesized beam (or resolution element) of 1 . (cid:48)(cid:48) × . (cid:48)(cid:48) ∼
200 au at a distanceof 200 pc. The baselines in the C-2 antenna configurationrange from 15–500 m. The maximum spatial scale atwhich emission can be recovered by ALMA is approxi-mately 10 . (cid:48)(cid:48) ∼ ∼ ∼ TCLEAN and
CLEAN . For all ofthe figures below that show both IRS1 and IRS2 in thesame image, we constructed a two-point mosaic of thesources 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 ( ∼ . (cid:48)(cid:48)
0) resolution, robust = 2.0 images, we also made higher resolution im-ages Briggs weighting parameters of robust = 0.5 and–0.5, yielding approximate resolutions of 0 . (cid:48)(cid:48)
75 and 0 . (cid:48)(cid:48) CLEAN . Making non-mosaicked images was necessaryto produce high fidelity images of the Stokes I emissionfrom IRS2, which is much fainter than IRS1. WhenIRS1 was observed in the center of the field of view, itwas so bright that our ability to image faint Stokes I emission was limited by dynamic range. As a result, inthe mosaicked images the quality of the Stokes I imageof IRS2 was degraded by the presence of IRS1 in themosaic. However, when IRS2 was imaged alone, IRS1was significantly removed from the phase center (and wasthus fainter, due to the primary beam response of theALMA antennas), which mitigated the dynamic rangeeffects and allowed us to produce higher quality imagesof low-level Stokes I dust emission surrounding IRS2.All images were improved by performing four roundsof phase-only self-calibration, where the total intensity(Stokes I ) image was used as a model. The shortest in-terval for determining the gain solutions was 12 s. SeeBrogan et al. (2018) for a detailed discussion of andbest practices for self calibration. The Stokes I , Q ,and U maps (where the Q and U maps show the po-larized emission) were independently cleaned with anappropriate number of CLEAN or TCLEAN iterations af-ter the final round of self-calibration. The rms noiselevel in the dynamic-range-limited Stokes I dust mapsranges from σ I ≈ µ Jy beam − ( robust = 2.0; mo-saic) to 250 µ Jy beam − ( robust = 0.5, –0.5; mosaics) to150 µ Jy beam − ( robust = 2.0; single pointing towardIRS2). The rms noise level σ P in the maps of po-larized intensity P (see Equation 1 below), which arenot dynamic-range limited, ranges from 25 µ Jy beam − ( robust = 2.0, 0.5) to 40 µ Jy beam − ( robust = –0.5).The noise level in the P maps increases as the robust parameter decreases because weighting the uv -data to Hull et al. produce higher resolution images tends to increase thenoise level (Thompson et al. 2017).The quantities we derive from the polarization mapsinclude the polarized intensity P , the linear polarizationfraction P frac , and the polarization position angle χ : P = (cid:112) Q + U (1) P frac = PI (2) χ = 12 arctan (cid:18) UQ (cid:19) . (3) χ is defined to have a position angle of 0 ◦ when orientedN–S, and increases to the east (Contopoulos & Jappel1974). Note that P has a positive bias because it isalways a positive quantity, whereas the Stokes parameters Q and U (from which P is derived) can be positive ornegative. This bias is particularly significant in low-signal-to-noise (< 5 σ ) measurements. We debias the polarizedintensity map as described in Vaillancourt (2006) andHull & Plambeck (2015), although we note that it hasonly a very minor effect on our results.As has been the case with several polarization resultsfrom ALMA, we detect a marginal circularly-polarizedsignal in the Stokes V map; however, the circular po-larization fraction in the on-axis (single-pointing, non-mosaicked) observations of IRS1 is ∼ V signalwhen both IRS1 and IRS2 are observed off-axis is consis-tent with the known beam-squint profile (Chu & Turrin1973; Adatia & Rudge 1975; Rudge & Adatia 1978) ofthe ALMA 12 m antennas at Band 6. RESULTS
We show the results of our full-polarization, 1.3 mmdust continuum observations toward the BHR 71 binary inFigures 1 and 2. Later, we overlay these dust polarizationresults on images of the dense-gas tracers C O ( J = 2 →
1) and N D + ( J = 3 →
2) (Figure 3) and on CO ( J = 2 → Q , U , and polarizedintensity P maps. By comparing by eye with the syntheticmodels of Q and U for an hourglass-shaped magnetic fieldmorphology shown in Frau et al. (2011, Figure 4), we canconstrain the inclination of the magnetic field in BHR 71IRS1 with respect to the the plane of the sky. The centersof the Q and U maps toward IRS1 look quite similar tothe Frau et al. results for ω = 30 ◦ , which is equivalentto a ∼ ◦ inclination of the source with respect to theplane of the sky. This is consistent with the inclinationestimates by both T19 and Yang et al. (2017).In Figure 2 we overlay the inferred magnetic field ori-entations (produced by rotating the polarization orien-tations by 90 ◦ ) on the total intensity (Stokes I ) dustcontinuum map. When one studies only the maps of dustpolarization, the magnetic field morphologies of bothsources are consistent with the hourglass shape discussedin Section 1. However, when the magnetic field maps areanalyzed alongside spectral-line observations, it becomes h m s s s s s s Right Ascension (J2000)05 − ◦ D ec li n a t i o n ( J ) IRS1 IRS2 Q − . − . . . . S t o k e s Q fl u x d e n s i t y ( m J y b e a m − ) h m s s s s s s Right Ascension (J2000)05 − ◦ D ec li n a t i o n ( J ) U − . − . − . . . . . S t o k e s U fl u x d e n s i t y ( m J y b e a m − ) h m s s s s s s Right Ascension (J2000)05 D ec li n a t i o n ( J ) P . . . . P o l a r i ze d fl u x d e n s i t y ( m J y b e a m ) Figure 1.
Maps of Stokes Q (top), U (middle), and polarizedintensity P (bottom) toward BHR 71. The maximum and theminimum of the Q and U color scales are symmetric around zero, andthe scale ranges are set by | Q | max = 1 . mJy beam − and | U | max =1 . mJy beam − . The peak value of P is 1.94 mJy beam − , andthe rms noise level σ P = 25 µ Jy beam − . The contour levels are5, 8, 12, 16, 20, 30, 40, 50, 60, 70 × σ P . The black ellipses in the lower-left corners of all panels represent the ALMA synthesized beam(resolution element), which measures 1 . (cid:48)(cid:48) × . (cid:48)(cid:48)
89, equivalent to alinear resolution of ∼
200 au at a distance of 200 pc. Crosses indicatethe continuum peaks of IRS1 and IRS2. Blue and red arrows in thebottom panel indicate the orientations of the blue- and redshiftedlobes of the bipolar outflows from IRS1 and IRS2.
The ALMA dataused to make this figure are available in the online version of thispublication. he magnetic field in BHR 71 h m s s s s Right Ascension (J2000)09 D ec li n a t i o n ( J ) IRS1 h m s s s s Right Ascension (J2000)09 − ◦ D ec li n a t i o n ( J ) . . . . P o l a r i z a t i o n f r a c t i o n ( P f r a c ) h m s s s Right Ascension (J2000)55 D ec li n a t i o n ( J ) IRS2 h m s s s Right Ascension (J2000)55 − ◦ D ec li n a t i o n ( J ) . . . . P o l a r i z a t i o n f r a c t i o n ( P f r a c ) Figure 2.
Magnetic field morphology (left) and linear polarization fraction (right) toward BHR 71 IRS1 and IRS2.
Left column: grayscaleis the total intensity (Stokes I ) thermal dust emission, plotted beginning at 3 σ I , where σ I is the rms noise in the Stokes I map. ForIRS1, σ I = 300 µ Jy beam − ; for IRS2, σ I = 150 µ Jy beam − . Line segments are the inferred magnetic field, rotated by 90 ◦ from thepolarization orientation, and are plotted starting at 3 σ P , where σ P is the rms noise in the polarized intensity P map. For both IRS1and IRS2, σ P = 25 µ Jy beam − . The line segments are all the same length, and do not represent any other quantity. Blue and redarrows indicate the orientations of the blue- and redshifted lobes of the bipolar outflows from IRS1 and IRS2. Right column: polarizationfraction ( P frac ) is in color scale, and is plotted where both Stokes I > 3 σ I and P > 3 σ P . Contours are the Stokes I emission, plotted at5, 8, 16, 32, 64, 128, 256, 512, 1024 × the σ I value in the respective map. The black ellipses in the lower-left corners of all panels represent thesynthesized beam of the dust emission, which measures 1 . (cid:48)(cid:48) × . (cid:48)(cid:48) The ALMA data used to make this figure are available in the onlineversion of this publication. clear that while the magnetic field toward IRS1 looksmore like a “traditional” hourglass, in IRS2 the major-ity of the polarization detections are consistent with amagnetic field lying along the cavity walls of the bipolaroutflow, traced by CO ( J = 2 →
1) emission (see Figures 4and 5). While the extended hourglass structure towardIRS1 has a symmetry axis that is well aligned with theCO outflow, the majority of the polarized emission isspatially extended far beyond the outflow cavity, and thus, in contrast to IRS2, the magnetic field toward IRS1was most likely not shaped by the outflow. See Section4.2 for further discussion.In Figure 2 we also show maps of the polarizationfraction P frac toward IRS1 and IRS2. These maps exhibitthe typical features of polarization maps of protostellarcores, including a “polarization hole,” or a drop in P frac tovalues (cid:46)
1% toward the Stokes I intensity peak (see Hullet al. 2014, and references therein). In spite of having Hull et al. polarization holes toward the very centers of the sources,the maps of IRS1 and IRS2 show polarization fractionlevels > 10% across much of both sources. These highlevels of polarization can be reproduced by the modelsof grain alignment via RATs, and have been seen ininterferometric observations of polarization toward bothlow- and high-mass star-forming regions (e.g., Stephenset al. 2013; Hull et al. 2014; Cortes et al. 2016; Kwonet al. 2019; Cortes et al. 2019). However, our ability tointerpret P frac is limited by the fact that the Stokes I image is much more strongly dynamic-range limited thanthe P image, resulting in a P map that extends beyondthe limits of the I map. This has been seen in otherhigh-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 ofALMA to make images at different resolutions, varyingthe resolution by up to a factor of 2 by using differ-ent robust weighting parameters in the imaging process.When we make the polarization map of BHR 71 withthree resolutions (1 . (cid:48)(cid:48)
0, 0 . (cid:48)(cid:48)
75, and 0 . (cid:48)(cid:48) P imagesthan in I images, have been seen in other high-resolution,high-sensitivity ALMA polarization observations of pro-tostellar cores (Hull et al. 2017b,a; Maury et al. 2018;Sadavoy et al. 2018; Takahashi et al. 2019; Le Gouellecet al. 2019), and are discussed further in Section 4.4. DISCUSSION
We begin this discussion by focusing on several plots,including the dust continuum, inferred magnetic field,and polarization fraction toward both binary compo-nents of BHR 71 (Figure 2); the magnetic field overlaidon maps of the dense-gas tracers C O ( J = 2 →
1) andN D + ( J = 3 →
2) (Figure 3); and the magnetic field over-laid on the bipolar outflows from IRS1 and IRS2 tracedby CO ( J = 2 →
1) (Figures 4 and 5).As first mentioned in Section 3 and discussed further inSections 4.1 and 4.2, when the polarization maps are ana-lyzed alongside maps of spectral-line emission, it becomesclear that the magnetic field morphologies in IRS1 andIRS2 have different origins. The brighter source IRS1 hasa magnetic field configuration that resembles an hourglass,which is aligned with (but not significantly disturbed by)the bipolar outflow. In contrast, the fainter source IRS2shows a magnetic field that has been shaped by the out-flow. The main goal of this discussion is to understandwhy these differences arise in the two components of thesame binary source.
Dust polarization and dense-gas tracers
In Figure 3 we show the inferred magnetic field towardBHR 71 overlaid on moment 0 maps of the dense-gastracers C O ( J = 2 →
1) and N D + ( J = 3 →
2) emissionfrom T19. The C O and N D + are roughly anticorre-lated, since the C O traces warm gas in the core and theN D + traces cold, pre-stellar gas (see below). Further-more, it is clear that the C O emission correlates wellwith the polarized emission, whereas the N D + emissionis anticorrelated with the polarization. These trends areparticularly strong in IRS1, which is brighter and warmerthan IRS2. h m s s s s s s Right Ascension (J2000)05 − ◦ D ec li n a t i o n ( J ) IRS1 IRS2C O h m s s s s s s Right Ascension (J2000)05 − ◦ D ec li n a t i o n ( J ) N D + Figure 3.
Maps of integrated (moment 0) C O ( J = 2 →
1) andN D + ( J = 3 →
2) emission toward BHR 71, from the data pre-sented in T19. The C O emission was integrated from –6.7 to–1.7 km s − ; the minimum and maximum values of the map are 0.06and 3.28 Jy beam − km s − , respectively. The N D + emission wasintegrated from –5.5 to –3.4 km s − ; the minimum and maximumvalues of the map are 0.04 and 0.44 Jy beam − km s − , respectively.BHR 71 has a systemic velocity of –4.5 km s − (Bourke et al. 1997).The line segments are the inferred magnetic field orientation, plot-ted as in Figure 2. The black ellipses in the lower-left corners ofboth panels represent the synthesized beam of the polarized dustemission, which measures 1 . (cid:48)(cid:48) × . (cid:48)(cid:48)
89. The green ellipses representthe beams of the spectral-line emission; the beam of the C O emis-sion measures 1 . (cid:48)(cid:48) × . (cid:48)(cid:48)
54, and the beam of the N D + emissionmeasures 1 . (cid:48)(cid:48) × . (cid:48)(cid:48)
32. Crosses indicate the continuum peaks ofIRS1 and IRS2.
It is well known that N D + , and its non-deuteratedisotopologue N H + , are excellent tracers of very cold,pre-stellar material where CO has been frozen out of thegas phase onto dust grains. CO tends to destroy bothN D + and N H + once the CO sublimates off of the dustgrains at a temperature of ∼
25 K (Aikawa et al. 2001;Vasyunina et al. 2012; Tobin et al. 2013a). Furthermore,above the same temperature, the formation pathway ofN D + is shut off because its precursor molecule H D + isno longer being formed, but is rather being converted back he magnetic field in BHR 71 +3 and HD (Herbst 1982). This explains the overallanticorrelation between C O and N D + in BHR 71. Italso explains the fact that while significant N D + emissionis still seen on the outskirts of cooler and less luminousIRS2, warmer and brighter IRS1 shows almost no N D + emission toward the central core. This type of drop inemission toward the centers of the protostellar envelopewhere CO is abundant has also been seen in N H + inBHR 71 (Chen et al. 2008) and other Class 0 sourcessuch as, e.g., L483 (Jørgensen 2004), L1157 (Kwon et al.2015; Anderl et al. 2016), NGC 1333-IRAS 4A and 4B,and L1448C (Anderl et al. 2016).This anticorrelation may prove critical for theoreticaland observational studies of dust-grain alignment. Forexample, a high-resolution survey of either C O or N D + (or N H + ) could be used to predict where dust is mostlikely to be polarized. The cold environment traced byN D + , far from the IRS1 and IRS2 protostellar radiationsources, is likely to be the perfect environment for adramatic decrease in grain-alignment efficiency. This sortof behavior has been seen in the center of starless cores(e.g., Alves et al. 2014; Andersson et al. 2015), wherethe decrease in polarization at the center of the sourceis most likely due to the lack of an anisotropic radiationfield (from either external interstellar UV photons orthe internal “lightbulb” of the protostar itself), which isrequired for RATs to align dust grains with respect tothe magnetic field.C O is a dense-gas tracer sensitive to high columndensities, but only in warm ( (cid:38)
25 K) regions where CO isin the gas phase. The fact that the polarization closelyfollows the C O ( J = 2 →
1) emission, particularly towardIRS1, may be because both polarization and C O can beassociated with warm regions: CO because it is no longerfrozen onto dust grains and has been sublimated into thegas phase; and polarization because the dust is warm,and thus bright, allowing the detection of polarization atthe few-percent level.Note that while C O ( J = 2 →
1) is spatially coincidentwith nearly all of the polarization toward IRS1, we onlysee strong C O ( J = 2 →
1) emission toward the verycenter of IRS2, and not extended along the outflow lobes.This suggests that the polarization in IRS1 originatesfrom warm material throughout the natal clump, allowingus to detect its well ordered, hourglass-shaped magneticfield. However, in IRS2, the polarization originates almostexclusively in the outflow cavity walls, which are stronglyirradiated (thus yielding strong polarization), but whichlie far away from the central heating source of IRS2(resulting in a lack of C O, which is frozen out of thegas phase onto the dust grains at large distances fromthe protostar).Higher temperatures and stronger irradiation couldexplain why polarization is so widespread and easily de-tected in high-mass star-forming regions (e.g., Zhang et al.2014) and in bright low-mass sources (e.g., BHR 71 IRS1,NGC 1333-IRAS 4A, IRAS 16293, and L1157), whichare warmer and have stronger radiation fields than theirlower-luminosity counterparts like IRS2. In Sections 4.2and 4.3, we discuss the differences in magnetic field be-tween BHR 71 IRS1 and IRS2 in this context, focusingin particular on the question of irradiation.
A natal hourglass in IRS1 versus an outflow-shapedmagnetic field in IRS2
Despite the powerful outflow emanating from IRS1, thepolarization toward IRS1 is not obviously shaped by theoutflow. Rather, the majority of the polarized emissiontoward the source lies outside of the region of influence ofthe outflow, which can be see in CO ( J = 2 →
1) emissionin Figure 4. On the eastern edge of IRS1, the two lanes ofpolarization extending NE and SE of the central sourcehave an orientation that is 30–45 ◦ different from that ofthe outflow edge. This situation is similar to observationsof hourglass-shaped magnetic fields in IRAS 4A, IRAS16293A, and L1157, mentioned in Section 1: all of themhave powerful bipolar outflows (or two outflows, in somecases), and yet none of them show significant shaping ofthe magnetic field by the outflows.This situation may arise simply because of the highluminosity of these iconic sources. All of them almostcertainly have irradiated outflow cavity walls; however,the clear morphology of the magnetic field along thecavity walls may be obscured because polarized emissionis emanating not just from the cavity walls, but fromthroughout the majority of the envelope, thanks to thestrong temperature gradients and to the high fluxes thatenable the alignment of dust grains with respect to themagnetic field. This abundance of aligned grains enablesus to detect the natal hourglass-shaped field, which hasbeen preserved from the sources’ earlier formation stages.In contrast, less luminous sources like BHR 71 IRS2,B335 (Maury et al. 2018), and Ser-emb 8(N) (Le Gouellecet al. 2019) all show polarization that clearly has been af-fected by the outflow. When we examine the polarizationmap of IRS2 in detail, we also see that the polarizationalong the northern edge of the redshifted outflow lobelies between the outflow and a cold, dense streamer ofN D + (see Figure 5). The fact that the polarization issandwiched between the outflowing gas and the unpolar-ized material traced by N D + suggests that the origin ofthe enhanced polarization is the irradiation of the outflowcavity walls—but only to a depth of ∼
300 au, beyondwhich the dust is cold and unpolarized. We suspect thatthis is the case for all of these fainter sources, which haveneither the strong temperature gradients nor the highfluxes of the brighter, aforementioned sources. Conse-quently, toward these faint sources we are only able todetect polarization in regions with enhanced polarization,such as in the outflow cavity walls.
Irradiation of the outflow cavity walls in BHR 71IRS2
Single-dish studies of high- J CO emission toward proto-stars have suggested that such energetic CO emission can-not be reproduced by models of passive envelope heating.Rather, this gas must be heated by UV radiation originat-ing in the accretion shock around the central protostaras well as in shocks distributed throughout the outflowitself (Spaans et al. 1995; van Kempen et al. 2009a; Visseret al. 2012). More recent models explained the shape ofthe high- J CO ladder and the chemical signatures (e.g.,the line ratios H O/CO and H O/OH) seen in
Herschel observations of embedded protostars by invoking modelsof UV-irradiated shocks in the protostars’ bipolar outflowcavities (Kristensen et al. 2017; Karska et al. 2018). These
Hull et al. h m s s s s s s s Right Ascension (J2000)10 − ◦ D ec li n a t i o n ( J ) IRS1 IRS2 h m s s s s s s s Right Ascension (J2000)10 − ◦ D ec li n a t i o n ( J ) Figure 4.
Maps of integrated (moment 0) CO ( J = 2 →
1) emission in the red- (left) and blueshifted (right) lobes of the bipolar outflowstoward BHR 71 IRS1 and IRS2, from the data presented in T19. The redshifted emission was integrated from –2.2 to 29.9 km s − ; theminimum and maximum values of the map are 0.6 and 13.05 Jy beam − km s − , respectively. The blueshifted emission was integrated from–40 to –6.0 km s − ; the minimum and maximum values of the map are 0.6 and 13.92 Jy beam − km s − , respectively. BHR 71 has a systemicvelocity of –4.5 km s − (Bourke et al. 1997). The line segments are the inferred magnetic field orientation, plotted as in Figure 2. The blackellipses in the lower-left corners of both panels represent the synthesized beam of the polarized dust emission, which measures 1 . (cid:48)(cid:48) × . (cid:48)(cid:48) . (cid:48)(cid:48) × . (cid:48)(cid:48)
33. Crosses indicate the continuum peaks of IRS1 and IRS2. findings are consistent with many other observational andtheoretical studies that have seen enhanced chemistryin the vicinity of shocked, irradiated gas (e.g., in HHobjects: 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 71with the Atacama Pathfinder EXperiment (APEX) tele-scope by van Kempen et al. (2009b) and Gusdorf et al.(2015); the observations by Gusdorf et al. resolve thishigh- J CO emission in both the IRS1 and IRS2 outflows.Here we consider the scenario that the enhanced polar-ization along the outflow cavity walls of IRS2 may be theresult of irradiation by photons generated in accretionand 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 PDRmodels by Girart et al. 2005, UV radiation is fully ex-tincted at an A V of order unity, which is achieved at amolecular hydrogen column density of between 10 and10 cm − (Bohlin et al. 1978). To estimate the amountof material along the IRS2 outflow cavity wall, we usethe standard conversion from flux density S ν to gas mass M gas (see, e.g., Hull et al. 2017a; Hull & Zhang 2019): M gas = S ν d κ ν B ν ( T d ) , (4)where B ν ( T d ) is the Planck function at the 233 GHzfrequency of our observations, the distance d = 200 pc, and the dust opacity at 1.3 mm κ ν = 2 cm / g (Ossenkopf& Henning 1994). We assume a gas-to-dust ratio of 100.In a circle with a diameter equal to 300 au, which isthe approximate width of the enhanced dust emissionalong the outflow cavity, the flux density along the entirenorthern edges of the blue- and redshifted outflow cavitywalls is a roughly constant ∼ ±
100 km s − ), bipolar SiO jet,whose internal shocks may contribute to the illuminationof the cavity walls (Bourke et al., in prep.).To calculate the column density along the IRS2 outflowcavity walls, we use the IRS2 temperature estimate of20 K from T19, assume a mean molecular weight of 2.8 inthe gas (Kauffmann et al. 2008), and assume an averageflux of 2.5 mJy in a 300 au-diameter circle centered onthe outflow cavity wall. The resulting column density is ∼ . × cm − . However, the dust along the outflowwalls could be warmer than the overall dust temperaturetoward the IRS2 core. The gas temperature of the IRS2outflow could be up to 300 K (Parise et al. 2006), which isreasonable based on the models of UV-irradiated outflowcavities by, e.g., Visser et al. (2012) and Drozdovskaya he magnetic field in BHR 71 h m s s s Right Ascension (J2000)55 − ◦ D ec li n a t i o n ( J ) IRS2
Figure 5.
Zoom-in on IRS2, showing the same CO ( J = 2 → D + ( J = 3 →
2) emission at levels of 3, 4, 6, 8, 10 × − km s − , which is the rms noise level in the moment0 map (see Figure 3, bottom panel). The black ellipse represents thesynthesized beam of the polarized dust emission, which measures1 . (cid:48)(cid:48) × . (cid:48)(cid:48)
89. The gray ellipse represents the beams of both the blue-and redshifted CO emission (and is nearly identical to the beam ofthe N D + emission), and measures 1 . (cid:48)(cid:48) × . (cid:48)(cid:48)
33. It is clear that onthe redshifted side of the IRS2 outflow, the polarized emission issandwiched between the outflow emission and the cold, unpolarizedmaterial traced by N D + . et al. (2015). If we take that same value as the dust temperature (which is perhaps unreasonably high, as thedust is usually significantly cooler than the gas: Yıldızet al. 2015), the column density we derive is ∼ . × cm − . Even considering this high value of 300 K forthe dust temperature, the column density of the cavitywalls is still too high for UV photons to penetrate toa depth of 300 au. In light of this, below we paint twopictures of the potential origin of the polarization we seein the outflow cavity walls in IRS2: the “thick” scenario,and the “thin” scenario.We first consider the “thick” scenario, where the po-larization originates in a layer of the cavity walls with athickness of ∼
300 au. The first question to be addressedis, Which photons can penetrate to that depth and alignthe dust grains? We thus calculate how far into thewalls photons with wavelengths longer than those of UVphotons can penetrate. We use the dust opacity values κ ν from Ossenkopf & Henning (1994) that correspondto gas densities of cm − (and thus dust densities of cm − , assuming a gas-to-dust ratio of 100) and grains without icy mantles. Assuming a constant opacity κ ν and mass density ρ of the dust grains throughout thethickness of the cavity wall, the path length s to thepoint where the optical depth τ = 1 can be calculatedsimply 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 thecavity walls in order to calculate s .For 1 µ m photons, the penetration depths assuming20 K and 300 K temperatures are ∼
14 au and ∼
280 au, re-spectively. For 10 µ m photons, the depths are ∼
70 au and ∼ µ m photons, the depths are ∼ ∼ ∼ ∼ ∼
300 au,then it appears that the dust grains have been alignedby mid- to far-infrared (MIR/FIR) photons and/or possi-bly by an anisotropic radiation field resulting from thetemperature gradient between the inner part of the out-flow cavity and the cold, unpolarized material traced byN D + ( J = 3 → ∼ µ m-sizeddust grains in the cavity walls. While grains of thatsize are expected in circumstellar disks based on bothSED-fitting and dust-scattering-polarization studies (e.g.,Pérez et al. 2012; Testi et al. 2014; Stephens et al. 2017b;Hull et al. 2018), grain growth in the regions where wesee polarization toward IRS2 here—at distances that insome places exceed ∼ The gas number density in the cavity walls of IRS2 is onthe order of cm − , assuming a dust temperature of 20 K. Wecalculated this value by dividing the mass from Equation 4 by thevolume of a sphere with a diameter of 300 au, the same as theapproximate thickness of the cavity walls. This density is betweenthe gas density values of cm − and cm − for which κ ν iscalculated by Ossenkopf & Henning (1994); however, the values of κ ν for the two densities differ by less than a factor of two at allrelevant frequencies. Hull et al. h m s s s s s Right Ascension (J2000)05 − ◦ D ec li n a t i o n ( J ) IRS1 IRS2 h m s s s s s Right Ascension (J2000)05 − ◦ D ec li n a t i o n ( J ) IRS1 IRS2 h m s s s s s Right Ascension (J2000)05 − ◦ D ec li n a t i o n ( J ) IRS1 IRS2
Figure 6.
Multi-resolution images of the polarized intensity P (color scale) made using robust = 2.0 (left), 0.5 (middle), and –0.5 (right),where the respective synthesized beam sizes are 1 . (cid:48)(cid:48) × . (cid:48)(cid:48)
89, 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
67, and 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
48. Line segments are the inferred magnetic fieldorientation toward BHR 71 IRS1 and IRS2, plotted as in Figure 2, where σ P = 25 µ Jy beam − for robust = 2.0 and 0.5, and 40 µ Jy beam − for robust = –0.5. Crosses indicate the continuum peaks of IRS1 and IRS2. The sharp, magnetized feature to the NE of IRS1 is enhancedwith increasing resolution. UV-driven chemistry in outflow cavities (Karska et al.2018, and references therein); emission from several UVtracers has been spatially resolved in ALMA observations(e.g., Imai et al. 2016; Le Gouellec et al. 2019), but isexpected to originate mainly in the very thin layer of thecavity walls accessible to the UV radiation.This “thin” scenario has its own caveats. If only anextremely thin layer of dust grains were aligned, it seemsunlikely that the polarization fraction would reach thehigh (> 10%) levels that we detect in IRS2, consideringthat the polarization fraction is derived from the po-larized intensity from the aligned grains divided by thetotal intensity of the dust emission, the latter of whichoriginates from the entire core. In addition, the high-energy radiation would be reprocessed by the thin layerof initially-heated dust grains in the cavity wall, beingre-emitted as longer-wavelength photons that could pene-trate deeper into the walls. However, it is unclear if therewould be enough of these photons to align a significantnumber of grains to an appreciable depth beyond theinitial heated layer.Given the major caveat of the polarization fractionlevels in the “thin” case, and the fact that recent studieshave shown that grain growth in Class 0 protostellar coresmay be necessary to explain high-resolution, interfero-metric polarization results (Maury et al. 2018; Valdiviaet al. 2019; Le Gouellec et al. 2019), we find the “thick”scenario to be the more plausible of the two possibilitiesfor the origin of the polarization in the IRS2 outflowcavities. However, the answer is far from clear. Future,multi-wavelength polarization observations of protostellarcores, coupled with synthetic observations of MHD simu-lations with next-generation radiative transfer codes likePOLARIS (Reissl et al. 2016) that incorporate dust-grainalignment via RATs, will help us to better understandthe origin of polarization in irradiated environments likecavity walls.
A possible magnetized accretion streamer in BHR 71IRS1
The most intriguing emission feature in the P map ofBHR 71 IRS1 is the strip oriented NE of the central source (Figure 1, bottom panel). This feature is most clearlyseen in the multi-resolution image shown in Figure 6,and is oriented at a very different angle from the easternedge of the redshifted outflow cavity wall (Figure 4).If it is indeed part of the natal hourglass structure inIRS1, then it is not clear why it is so much sharper andbrighter than the other “corners” of the hourglass, whichare resolved out (or are too faint to be detected) at higherresolution. This is a similar situation to Serpens SMM1,where the polarization along the E–W cavity wall seen inHull et al. (2017a) is not detectable at higher resolution,whereas extremely sharp, bright polarization structuresnear the N–S wall of the outflow cavity remain visible athigher resolution (Le Gouellec et al. 2019). These sharp,approximately linear structures south of SMM1-a and thesharp structure to the NE of BHR 71 IRS1 have similarwidths of ∼
50 au.Narrow structures have also been seen in other sources,including filamentary features surrounding the low-massprotostellar core Ser-emb 8 (Hull et al. 2017b); the N–Sequatorial-plane feature in B335 (Maury et al. 2018); the“bridge” and streamers observed toward the two binarycomponents in IRAS 16293 (Sadavoy et al. 2018); andthe “arm-like” structure in OMC3-MMS6 (Takahashi et al.2019). The existence of these features, which are alwaysmore prominent in P maps than in I maps, suggest thatthe production of polarized emission inside a protostellarcore is strongly dependent on the local environmentalconditions such as the optical depth and the anisotropy ofthe local radiation field. As discussed in Le Gouellec et al.(2019), the high polarization of these regions is surprising,considering that they are deeply embedded and far awayfrom any obvious source of strong irradiation.It is conceivable that these sharp, filamentary featuresare the result of grains aligned in accretion streamers; seeAlves et al. (2019) for the highest-resolution observationsto date of accretion streamers that are funneling materialonto the central sources in a low-mass protostellar binary.Theoretical work by Lazarian & Hoang (2007b) and Hoanget al. (2018) on grain alignment via mechanical alignmenttorques (MATs) has suggested that helical grains could he magnetic field in BHR 71 φ N u m b e r o f p o i n t s Dust Emission BHR71 IRS1 0 15 30 45 60 75 90 φ N u m b e r o f p o i n t s Blueshifted outflow lobeBHR71 IRS1 0 15 30 45 60 75 90 φ N u m b e r o f p o i n t s Redshifted outflow lobeBHR71 IRS10 15 30 45 60 75 90 φ N u m b e r o f p o i n t s Dust EmissionBHR71 IRS2 0 15 30 45 60 75 90 φ N u m b e r o f p o i n t s Blueshifted outflow lobeBHR71 IRS2 0 15 30 45 60 75 90 φ N u m b e r o f p o i n t s Redshifted outflow lobeBHR71 IRS2
Figure 7.
Histograms of relative orientation (HROs) of φ , defined as the difference between the magnetic field orientation and the Stokes I dust-intensity gradient (left); and the difference between the magnetic field orientation and the gradients in the blueshifted (center) andredshifted (right) outflow emission toward both IRS1 and IRS2. We calculate the gradient in the zone of significant signal-to-noise: in theStokes I continuum maps where the signal I > σ I , and in moment 0 maps produced with a threshold at 10 × the rms noise level in achannel of the CO image cube (the moment 0 maps we analyze are the same as those plotted in Figure 4). We then select high gradientvalues (see Le Gouellec et al. 2019 for details). The angles between the magnetic field and the gradients are calculated ∼ × per synthesizedbeam (Nyquist, or twice, in both RA and DEC), where the beam width is ∼ . (cid:48)(cid:48) ∼ . (cid:48)(cid:48) be mechanically aligned with the magnetic field by thedrift (both sub- and supersonic) of gas relative to dustgrains. These relative flows of gas and dust could occur inan outflow, an accretion inflow/streamer, or possibly ina region of strong ambipolar diffusion between the weaklycharged dust and the neutral gas. The MAT mechanismyields a polarization orientation consistent with what wesee toward the highly polarized filament to the NE ofIRS1. However, the kinematics of the N D + ( J = 3 → O ( J = 2 →
1) emission from T19 do not revealany such accretion features, and thus it is not possible to Note that this is different from the traditional “Gold alignment”version of mechanical alignment (Gold 1952). Gold alignmentproduces polarization oriented along the direction of the flow, andthus would yield an inferred magnetic field along the minor axisof the filament, whereas we see a magnetic field aligned with thefilament’s major axis. Note that while the polarization toward IRS2 is stronglyassociated with the outflow cavity walls, we find it unlikely that theaction of the outflow itself is producing the polarization via MATs.The main argument against MATs in this case is that the northernedge of the redshifted outflow lobe from IRS2 is significantly offsetfrom the observed polarized dust emission. We therefore find itmore likely that the enhanced polarization toward IRS2 is due toirradiation of the outflow cavity walls. confirm this scenario with our current suite of dense-gastracers.
Histograms of Relative Orientation of the magneticfield versus dust and outflow emission in BHR 71IRS1 and IRS2
To better understand the differences in the magneticfield morphologies of IRS1 and IRS2, we analyze theHistogram of Relative Orientation (HRO; Soler et al.2013) of the magnetic field versus the gradients of bothdust and the CO ( J = 2 →
1) (outflow) emission towardIRS1 and IRS2 (see Figure 7). HROs have been usedmany times recently to shed light on the importance (orlack thereof) of the magnetic field in the formation ofstructure in star-forming regions, from the spatial scalesof molecular clouds (Planck Collaboration et al. 2016a,b;Soler et al. 2017; Fissel et al. 2019; Soler, J. D. 2019) toindividual protostellar cores (Hull et al. 2017b).When producing the histograms, both sources wereisolated to clearly establish the corresponding distribu-tions. We calculated the gradient in the zone of significantsignal-to-noise: in the Stokes I continuum map where thesignal I > σ I , and in moment 0 maps produced with2 Hull et al. a threshold at 10 × the rms noise level in a channel ofthe CO image cube (the moment 0 maps we analyze arethe same as those plotted in Figure 4). We then selectedthe zones with significant gradient values. In the outflowmoment 0 maps this served to highlight the edges of theoutflow cavities, whereas in the dust continuum maps thegradient picked out both the central cores of the proto-stars as well as (in the case of IRS2) regions of enhanceddust emission along the outflow cavity walls. Finally, forthe locations where there is both a gradient value as wellas a magnetic field orientation (i.e., where P > σ P ),we derived φ , defined as the difference in angles betweenthe magnetic field and the emission gradient, and addedthe points to the distributions. For further details of thisHRO analysis, see Le Gouellec et al. (2019).Toward IRS2, it is clear from Figure 5 that the magneticfield follows the northern edge of the outflow cavity. TheHRO comparing the magnetic field and the redshifted COemission does not exhibit this, as it is limited by statis-tics because of the physical offset between the redshiftedoutflow lobe and the polarized emission. However, we canclearly see that in the blueshifted lobe of IRS2, φ peaksnear 90 ◦ , indicating that the magnetic field orientationtends 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 lobeslook similar to the blueshifted lobe of IRS2, simply be-cause the hourglass symmetry axis and the outflow axisare aligned. The curvature of the hourglass-shaped mag-netic field in IRS1 does yield a broader HRO comparedwith the HRO toward the blueshifted outflow lobe of IRS2;however, this is not a strong distinguishing factor. Thefact that the hourglass magnetic field in IRS1 and themagnetic field along the edges of the outflow in IRS2 yieldsuch similar distributions in the magnetic-field-versus-COHROs highlights the difficulty of distinguishing betweenthe “natal hourglass” versus “outflow-affected magneticfield” scenarios.We can make a clearer distinction between the twosources by looking at the HRO of the magnetic field ver-sus the dust emission (Figure 7, left-hand panels). TheHRO from IRS2 demonstrates that the magnetic fieldis perpendicular to the dust-emission gradient (i.e., themagnetic field is parallel to the outflow-cavity walls), in-dicating that the magnetic field has been affected by theoutflow. In contrast, the HRO from IRS1 shows that thefield is more parallel to the dust intensity gradient, indica-tive of a magnetically regulated but gravity-dominatedscenario in a centrally condensed protostellar core (see,e.g., Koch et al. 2012, 2018). These differences in themagnetic-field-versus-dust HROs can help us distinguishwhether the dust morphology in a source has been moreaffected by gravity or by the outflow; however, it is stillessential to find a robust way to use outflow tracers todetermine quantitatively whether magnetic fields havebeen affected by outflows. CONCLUSIONS
We have presented 1.3 mm ALMA observations of po-larized dust emission toward the wide-binary protostellarsystem BHR 71. After analyzing the inferred magneticfield morphology toward both sources alongside maps ofthe bipolar outflows and dense-gas tracers, we come tothe following conclusions: 1. While the magnetic field morphologies of bothBHR 71 IRS1 and IRS2 are consistent with hour-glass shapes, analysis of the magnetic field mapsalongside spectral-line observations reveals thatIRS1 has what appears to be a natal, hourglass-shaped magnetic field. In contrast, its fainter, moreembedded binary counterpart IRS2 exhibits a mag-netic field that has been affected by its bipolaroutflow.2. Toward IRS1, there is a strong correlation of po-larized emission with C O, which traces warm( (cid:38)
25 K) material throughout the whole protostellarenvelope. Toward IRS2, in contrast, the polariza-tion is confined mainly to the outflow cavity walls.3. Along the northern edge of the redshifted outflowcavity in IRS2, the polarized emission is sandwichedbetween the outflowing material and a filament ofcold, dense gas traced by N D + , toward which nodust polarization is detected. This suggests that theorigin of the enhanced polarization in IRS2 is the ir-radiation of the outflow cavity walls, which enablesthe alignment of dust grains with respect to the mag-netic field—but only to a depth of ∼
300 au, beyondwhich the dust is cold and unpolarized. However,in order to align grains deep enough in the cavitywalls, and to produce the high polarization fractionseen in IRS2, the aligning photons are likely to bein the mid- to far-infrared range, which suggestsa degree of grain growth beyond what is typicallyexpected in very young, Class 0 sources.4. The anticorrelation of dust polarization from IRS1and IRS2 and emission from N D + suggests thatthis species (and its non-deuterated counterpartN H + ) is an excellent tracer of unpolarized materialbecause it is very sensitive to regions of cold, densegas where Radiative Alignment Torques (RATs) can-not efficiently align dust grains with the magneticfield.5. The difference in magnetic field morphologies to-ward the two binary components of BHR 71 mayarise simply because of the higher temperatureand ∼ × higher luminosity of IRS1 relative toIRS2. The higher temperature yields warmer dust,and thus more easily detectible polarization. Thehigher luminosity yields a stronger radiation fieldand larger temperature gradients, which enable thealignment of dust grains with respect to the mag-netic field throughout the majority of the envelopeof IRS1. In contrast, toward less luminous IRS2, weare only able to detect polarization in regions withenhanced polarization, such as the irradiated out-flow cavity walls. This same logic could explain whypolarization is so widespread and easily detectedin high-mass star-forming regions and in brightlow-mass sources (like IRS1), which are warmerand have stronger radiation fields than their lower-luminosity counterparts (like IRS2).6. Recent ALMA observations have revealed narrowpolarization features in sources such as BHR 71IRS1 and IRS2 (shown here), Ser-emb 8 (Hull et al. he magnetic field in BHR 71 Outflow-related features.
These manifestthemselves as features that have very highpolarization fractions (sometimes (cid:38)
Potentially accretion-related features.
These are thin, approximately linear fea-tures with high polarization fractions (usu-ally 10–20%) and extremely well orderedmagnetic fields; however, these featuresdo not appear to be associated with out-flow cavities. Examples include the sharpfeature to the NE of BHR 71 IRS1; themagnetized filamentary structure aroundSer-emb 8; the N–S equatorial-plane fea-ture in B335; the “arm-like” structure inOMC3-MMS6; the “bridge” and streamersobserved toward the two binary compo-nents in IRAS 16293; and one of the twonarrow filaments to the south of SerpensSMM1-a. We speculate that these featuresmay be magnetized accretion streamers;however, this scenario has yet to be con-firmed by kinematic observations of dense-gas tracers.With the advent of ALMA, our ability to probe thestructure of magnetic fields in protostellar cores has vastlyimproved. Upcoming surveys of large numbers of young,embedded sources will soon reveal how common is eachof the scenarios that individual-source studies have re-cently unveiled: natal hourglass-shaped fields, magneticfields affected by bipolar outflows, and possible magne-tized accretion streamers. Furthermore, observations oflarge-scale magnetic fields using current and upcomingsingle-dish polarimeters on instruments such as the Strato-spheric Observatory for Infrared Astronomy (SOFIA; Vail-lancourt 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 (Gal-itzki et al. 2014), the IRAM 30 m telescope (Ritacco et al.2017), and the Large Millimeter Telescope (LMT) willallow us to understand how the larger-scale magnetic en-vironment connects with the myriad small-scale magneticfield morphologies revealed by ALMA.The authors thank the anonymous referee, whose in-sightful comments led to substantial improvements inthe manuscript. The authors acknowledge the support of Gerald Schieven at the North American ALMA Sci-ence Center, and Walker Lu and Toshinobu Takagi atthe East Asian ALMA Regional Center. C.L.H.H. andV.J.M.L.G. acknowledge helpful discussions with theALMA Fellows, Ruud Visser, Lars Kristensen, AnaëlleMaury, Zhi-Yun Li, Phil Myers, Ian Stephens, Sarah Sa-davoy, and Felipe Alves. C.L.H.H. acknowledges the sup-port of both the NAOJ Fellowship as well as JSPS KAK-ENHI grant 18K13586. V.J.M.L.G. acknowledges the sup-port of the ESO Studentship Program. J.M.G. acknowl-edges the support of the Spanish MINECO AYA2017-84390-C2-R grant, and the Joint ALMA ObservatoryVisitor Program. This paper makes use of the follow-ing ALMA data: ADS/JAO.ALMA
Facilities:
ALMA.
Software:
APLpy, an open-source plotting package forPython hosted at http://aplpy.github.com (Robitaille& Bressert 2012). CASA (McMullin et al. 2007). Astropy(Astropy Collaboration et al. 2018).REFERENCES
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