Magnetic fields in Bok globules: Multi-wavelength polarimetry as tracer across large spatial scales
AAstronomy & Astrophysics manuscript no. main c (cid:13)
ESO 2018August 2, 2018
Magnetic fields in Bok globules: Multi-wavelength polarimetry astracer across large spatial scales (cid:63)
S. Jorquera and G. H.-M. Bertrang , Universidad de Chile, Departamento de Astronomía, Casilla 36-D, Santiago, Chilee-mail: [email protected] Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germanye-mail: [email protected]
August 2, 2018
ABSTRACT
Context.
The role of magnetic fields in the process of star formation is a matter of continuous debate. Clear observational proof ofthe general influence of magnetic fields on the early phase of cloud collapse is still pending. First results on Bok globules with simplestructures indicate dominant magnetic fields across large spatial scales (Bertrang et al. 2014).
Aims.
The aim of this study is to test the magnetic field influence across Bok globules with more complex density structures.
Methods.
We apply near-infrared polarimetry to trace the magnetic field structure on scales of 10 − au ( ∼ . − . − au ( ∼ . − Results.
We present polarimetric data in the near-infrared wavelength range for the three Bok globules CB34, CB56, and[OMK2002]18, combined with archival polarimetric data in the optical wavelength range for CB34 and CB56, and in the sub-millimeter wavelength range for CB34 and [OMK2002]18. We find a strong polarization signal ( P ≥ − au ( ∼ . − . − au( ∼ . − − au ( ∼ . − . Conclusions.
We find strongly aligned polarization segments on large scales which indicate dominant magnetic fields across Bokglobules with complex density structures. To reconcile our findings in globules, the lowest mass clouds known, and the results onintermediate (e.g., Taurus) and more massive (e.g., Orion) clouds, we postulate a mass dependent role of magnetic fields, wherebymagnetic fields appear to be dominant on low and high mass but rather sub-dominant on intermediate mass clouds.
Key words.
Magnetic fields – Polarization – Stars: formation – Stars: low-mass – ISM: clouds – Instrumentation: polarimeters
1. Introduction
The significance of magnetic fields on the star formation processis subject of ongoing investigations (e.g., Kandori et al. 2007;Galli 2009; Franco et al. 2010; Federrath 2015; Lizano & Galli2015; Sadavoy et al. 2018). Recent studies show comparativelylong lifetimes of massive (e.g., Orion) clouds ( ∼
10 Myr; e.g.,Murray 2011; Meidt et al. 2015) and suggest "dynamically rel-evant" magnetic fields (e.g., Alves et al. 2008; Sugitani et al.2010; Commerçon et al. 2011; Palau et al. 2013; Stutz & Gould2016; Busquet et al. 2016). The structure of intermediate massclouds (e.g., Taurus), showing lifetimes of an order of magnitudeshorter ( ∼ ff ner et al. 2008; Kritsuk et al. 2013). In turn, recentstudies on Bok globules, the lowest mass clouds known at thistime, suggest that globules appear to be magnetically supported(Stutz et al. 2007; Alves et al. 2011, 2014; Bertrang et al. 2014;Das et al. 2016). These studies have been conducted on globuleswith rather simple density structure. In this study, we aim at test- (cid:63) Based on observations made with an ESO telescope at the La SillaObservatory under programme ID 096.C-0115. (PI: G. H.-M. Bertrang) ing the magnetic field influence on globules with more complexdensity structures.The structure of magnetic fields in the dusty envelopesaround young stellar objects can be derived from the polariza-tion of background starlight due to dichroic extinction and ther-mal emission by dust grains (e.g., Weintraub et al. 2000; Girartet al. 2008). In the case of polarization due to thermal emission,the polarization direction is perpendicular to the magnetic fieldlines, as the dust grains get partially aligned to the magnetic field,with their major axes perpendicular to the magnetic field lines,projected onto the plane-of-sky (POS; e.g., Hoang et al. 2007;Lazarian 2007; Bertrang & Wolf 2017). The light of the back-ground stars that runs through the star-forming region also be-comes polarized due to dichroic absorption by the dust grains.In this case, the polarization direction directly traces the mag-netic field lines, projected onto the POS (Weintraub et al. 2000).The influence of magnetic fields on low-mass star forma-tion is best verified when studied independently of other phe-nomena such as turbulence or stellar feedback. Bok globules(Bok & Reilly 1947) arise as the ideal candidates to study themagnetic field influence on low-mass star forming regions. Bokglobules are small, isolated, relative simple structured molecu-lar clouds (e.g; Clemens et al. 1991), and are often associated
Article number, page 1 of 9 a r X i v : . [ a s t r o - ph . S R ] A ug & A proofs: manuscript no. main with low-mass star formation (Clemens & Barvainis 1988; Yun& Clemens 1994; Launhardt & Henning 1997). Bok globulesare in particular ideal environments to study the correlation be-tween protostellar collapse, fragmentation, and magnetic fields,as these objects are less a ff ected by large-scale turbulence andother nearby star-forming events.The classical method to characterize magnetic fields is thelinearly polarized dust emission and dichroic absorption. Ther-mal emission of the dust grains, observable in the submillime-ter (sub-mm) wavelength range, traces the polarization of thedensest, innermost part of the globule, due to the sensitivityof millimeter / sub-mm telescopes. The outer, less dense partsof the globule are traced by the polarization of backgroundstarlight due to dichroic absorption, observable in the near in-frared (near-IR) / optical wavelength range. Thus, by applyingmulti-wavelength polarimetry, it is possible to trace the magneticfield on large scales across the globule.In the following, we will describe our target selection (Sec-tion 2), observations (Section 3), and data reduction (Section 4).We analyze the polarization maps in Section 5. In Section 6, wediscuss the magnetic field, its relation to outflows, as well as themagnetic field influence on star formation as a function of massbefore we conclude in Section 7.
2. Description of the sources
Our aim is to verify the large-scale magnetic field influence onlow-mass star formation. As stated in Section 1, Bok globules areideal candidates for this kind of study. Furthermore, we have torestrict us to globules with available archival sub-mm or opticalpolarization maps that trace the magnetic field structure at scalesnot observable in the near-IR wavelength range. We further aimat expanding our previous study of the magnetic field structurein low-mass star-forming regions (Bertrang et al. 2014) to morecomplex structured objects. We found that these criteria are sat-isfied by three Bok globules: CB34, CB56, and [OMK2002]18.
CB34 is a Bok globule located at a distance of ∼ . ∼ . (cid:12) (C1) ,
94 M (cid:12) (C2) , and 13 M (cid:12) (C3), respectively (Fig. 1;Codella & Scappini 2003). These cores exhibit ongoing star for-mation based on the presence of a water maser as well as sev-eral collimated outflows (Fig. 1; Gómez et al. 2006; Huard et al.2000; Yun & Clemens 1994). The chemical age of the globule isestimated to be > yr (Launhardt et al. 2010), with the pres-ence of a pre-main-sequence star with an age of ∼ yr lo-cated in the core C1 (Alves et al. 1997). This massive globuleencloses within its center (4 . × . × au ( ∼ × − M (cid:12) (Launhardt et al. 2010; Codella & Scappini2003; Launhardt et al. 1998). CB56 is a compact, irregular shaped cloud with a major andminor axis of 4 (cid:48)(cid:48) . (cid:48)(cid:48) . ◦ with respect to the galac-tic plane. CB56 is associated with two infrared point sources(IRAS 07125-2503 and IRAS 07125-2507; Clemens & Bar-vainis 1988). The distance to CB56 has yet to be determined.The optical polarization signal of this globule is significantlystronger than the interstellar polarization in the optical wave-length range within 200 pc (Chakraborty et al. 2014; Gontcharov2017, Gontcharov, priv. comm.). Together with the strong near-IR polarization, this indicates that CB56 is located within a dis- tance of 200 pc and the measured polarization signal is associ-ated with CB56. [OMK2002]18 is a low-mass star forming region at adistance of 140 pc (Maheswar et al. 2011). It consist ofthe two dense clouds [OMK2002]18a and [OMK2002]18b.Both clouds have a radius of ∼ . × au and enclosemasses of 2 . (cid:12) and 1 . (cid:12) respectively (Onishi et al. 2002).OMK[2002]18 is associated with one compact infrared source(IRAS 04191 +
3. Observations
The observations were performed in the near-IR with the instru-ment Son OF ISAAC (SOFI) at the 3 .
58 m New TechnologyTelescope (NTT) from November 11th to 15th, 2015. SOFI / NTTis mounted at the Nasmyth A focus, equipped with a 1024 × − . µ m.In SOFI / NTT, a single Wollaston prism is used for polarizationobservations. In this observing mode, the polarized flux is mea-sured simultaneously at two di ff erent angles that di ff er by 90 ◦ .To derive the linear polarization degree and orientation of an ob-ject, two observations must be performed at each pointing of thetelescope with di ff erent orientations of the Wollaston prism, typ-ically 0 ◦ and 45 ◦ . This is realized by a rotation of the completeinstrument. To avoid overlapping between di ff erent polarizationimages an aperture mask of three alternating opaque and trans-mitting strips of about 40 (cid:48)(cid:48) × (cid:48)(cid:48) for SOFI / NTT is used.We carried out Js-band polarization observations of three fieldsof CB34 and [OMK2002]18ab as well as one field of CB56.
4. Data Reduction
For the data reduction, we apply an specialized pipeline cre-ated to work with polarization data obtained with SOFI / NTT(Bertrang et al. 2014). This pipeline performs bias correction,flat-fielding, and instrumental polarization extraction as well asthe calculation of the Stokes parameters I , Q , and U via aper-ture photometry. Table 1 summarizes the information of polar-ized and unpolarized standard stars used to determine the instru-mental polarization.A correction factor for the Wollaston prism was computedfor the reduction of the instrumental polarization (see Ap-pendix). This correction factor was tested for the polarized stan-dard star, and as a successful test, the polarization data obtainedafter bias correction corresponds well to literature values.
5. Polarization Maps
In this section, we present the polarization maps of the threeBok globules CB34, CB56, and OMK[2002]18. We compare ournear-IR data to archival polarization data in the optical and sub-mm wavelength range. As stated in Section 1, the polarization atthese di ff erent wavelength ranges arises from di ff erent sources:while the polarization in the sub-mm range arises from thermalemission, in the near-IR and the optical it is caused by dichroicabsorption. Thus, the polarization of a dust grain observed in thesub-mm is oriented perpendicular to the polarization of the verysame dust grain observed in the near-IR or optical.For CB34, in addition to our new data in the near-IR wave-length range, archival data is available in both the optical andsub-mm wavelength ranges (Das et al. 2016). This enables ananalysis over the whole range of scales in this globule, from its Article number, page 2 of 9. Jorquera and G. H.-M. Bertrang: Magnetic fields in Bok globules across large spatial scales
Table 1.
Polarization standard stars.
Instrument Object α δ Type P γ Filter Ref.(hh:mm:ss.ss) (dd:mm:ss.ss) (%) ( ◦ )SOFI / NTT CMa R1 No.24 07:04:47.36 -10:56:17.44 polarized 2 . ± .
05 86 ± . ± .
032 B 2WD0310-688 03:10:31.02 -68:36:03.39 unpolarized 0 . ± .
09 V 3
References. (1) Whittet et al. (1992); (2) Turnshek et al. (1990); (3) Fossati et al. (2007).
Fig. 1. (a) Near-IR polarization segments (red) and optical polarization segments (blue) are plotted over a 15 (cid:48) × (cid:48) R-Band DSS image of thefield containing CB34. A 10% polarization segment is drawn as reference for both data sets into the lower left corner. Contours correspond toJCMT / SCUBA 850 µ m dust continuum emissions from 0.1 to 0.7 Jy beam − , with intervals of 0.1 Jy beam − (Das et al. 2016). (b) Zoom into thedense center of CB34. The near-IR polarization segments (red) and the archival sub-mm polarization segments (blue) are plotted over the totalcontinuum intensity map at 850 (Das et al. 2016). The sub-mm segments are binned over a 10 (cid:48)(cid:48) × (cid:48)(cid:48) grid. The solid (dashed) black (gray) contourlines correspond to blueshifted (redshifted) integrated CO emission. Contour levels are spaced at 0.1 K km s − intervals of 0.5 K km s − (black),and 0.2 K km s − intervals of 0.8 K km s − (white), respectively (from Yun & Clemens (1994), beam size of 48"). A 5% polarization segmentis drawn as reference for both data sets into the lower right corner. The well-ordered polarization pattern indicates dominant magnetic fields onscales of 10 − au ( ∼ . − most dense center (sub-mm) towards the less dense outer enve-lope (near-IR, optical).For CB56, the data set is restricted to archival data in the op-tical wavelength range (Paul et al. 2012; Chakraborty et al. 2014)in addition to the newly obtained near-IR data. However, thosetwo wavelength ranges cover this object almost completely. Forthis comparison, we make use of the optical polarization datapublished in Chakraborty et al. (2014), given that it is the morecomprehensive data set at this wavelength range.For [OMK2002]18, archival data only in the sub-mm wave-length range were available (Matthews et al. 2009), in additionto the newly obtained near-IR polarization data. With these, ananalysis of the polarization of the most dense center of the glob-ule is possible (sub-mm), as well as an analysis of the less denseouter regions (near-IR).In the following, we discuss the polarization maps separately,first focusing on the new near-IR data, and subsequently, com-paring it to the maps of the optical and sub-mm wavelengthranges. The polarization maps of the Bok globule CB34 are shown inFig. 1. CB34 disclose a strong polarization signal, P ≥ − au ( ∼ . − . ◦ ± ◦ .Most of the outer regions of this globule are traced in bothnear-IR and optical polarization. Both data sets fit well in ori-entation (within the errors). The mean value of the polarizationangle in the near-IR wavelength range in this regions is relativelyconsistent with the position angle of the galactic plane at the lat-itude of CB34, θ GP ≈ . ◦ , which suggests that the magneticfield is coupled to the Galactic magnetic field in the observedPOS. This is similar to the findings in the optical (Das et al.2016)A puzzling finding is the relation between the polarizationdegrees in the near-IR and the optical. As opposed to the em-pirical Serkowski relation (Serkowski et al. 1975), the near-IRsignal is stronger polarized than the optical. Similar discrepan-cies have been found also in very few other globules, tentatively Article number, page 3 of 9 & A proofs: manuscript no. main
Fig. 2. (a) Near-IR polarization segments plotted over a 15 (cid:48) × (cid:48) R-Band DSS image of the field containing CB56. A 10% polarization segment isdrawn for reference in the lower left corner. Only the segments with P /σ p ≥ (cid:48) × (cid:48) R-Band DSS image (Chakraborty et al. 2014). A 1% polarization segment is drawn for reference in the lower left corner. The near-IRand optical polarization is strong and well-ordered, indicating dominant magnetic fields on scales of 10 − au ( ∼ . − . Fig. 3. (a) Near-IR polarization segments plotted over a ∼ (cid:48) × (cid:48) R-Band DSS image of the field containing [OMK2002]18. A 10% polarizationsegment is drawn for reference in the lower left corner. Only the segments with P /σ p ≥ (cid:48)(cid:48) × (cid:48)(cid:48) grid. Contours correspond to JCMT / SCUBA 850 µ m dust continuum emissions from 0.05 to 0.25 Jybeam − , with intervals of 0.05 Jy beam − (Matthews et al. 2009, c (cid:13) AAS. Reproduced with permission). Only segments with P /σ p ≥ − au ( ∼ . − . explained by either a limited location of origin of polarization orthe presence of two di ff erent dust populations (Clemens 2012;Alves et al. 2014). Recently, it has been suggested that not onlythe grain size but also the chemical composition of grains alterthe extinction-dependency of polarization (Papoular 2018). Fu-ture theoretical work is needed to investigate this in more detail.In the sub-mm, the polarization segments can be associatedto the cores C1 and C2, while there is no detection on top of C3(Fig. 1). The region observed in the sub-mm overlaps with ournear-IR data. The polarization segments in the sub-mm wave-length range were plotted using the original data from Das et al.(2016). Thus, it has to be keep in mind that the segments must berotated 90 ◦ to be aligned with the magnetic field orientation. Inthis region, the orientation of the polarization segments obtainedin the near-IR wavelength range fit very well to the sub-mm po-larization segments associated with core C1, but deviate strongerfrom those of core C2. The polarization segments in the near-IR wavelength range are located just in between the cores C1 andC2. In this region, the mean polarization angle in the near-IRwavelength is ¯ γ near − IR ≈ . ◦ ± . ◦ , which suggest that inthis case the orientation of the magnetic field in the center of theglobule is di ff erent to the Galactic magnetic field. Similar find-ings where obtained in Das et al. (2016) using the data from thesub-mm wavelength range.The change in the polarization pattern from the outer to thecentral part of CB34 indicates that the magnetic field is coupledon large-scales to the galactic magnetic fields, while it discon-nects in the center where the globule is fragmented into severalcores. The scale of the transition is ∼ au ( ∼ . The polarization maps of the Bok Globule CB56 are shown inFig. 2. CB56 discloses a strong polarization signal with polar-
Article number, page 4 of 9. Jorquera and G. H.-M. Bertrang: Magnetic fields in Bok globules across large spatial scales
Fig. 4.
Distribution of polarization degree, P for P ≥ N ( P ), of CB34, CB56, and OMK[2002]18, observed in the near-IR. Fig. 5.
Distribution of polarization angle, γ , counts given by N ( γ ) of CB34, CB56, and OMK[2002]18, observed in the near-IR. The dashed linesrepresent the mean polarization angles, while dotted lines represent the corresponding 1 σ deviation. ization degrees of mostly 5% −
12% (Fig. 4). The polarizationstructure of this globule in the near-IR is well-ordered and pre-dominantly oriented towards North-North / West (Fig. 5).The region of CB56 which is covered by our near-IR obser-vations overlays with the archival optical data set (Chakrabortyet al. 2014). The globule discloses a weak polarization signal inthe optical wavelength range, with an average of P ≈ The polarization maps of OMK[2002]18 is shown in Fig. 3.OMK[2002]18 shows a strong polarization signal, 2 ≥ P ≥ / North-Westorientation with strong polarization degrees of P ≈ P ≈
3% indicative for de-polarization e ff ects (e.g., Brauer et al. 2016). Considering theseparation between these regions, uniformly ordered polariza-tion segments are not necessarily expected.A spatial gap is seen between the sub-mm an near-IR po-larization segments, which of about 3 . × au ( ∼ . / NTT, resp. SCUBA / JCMT(Bertrang et al. 2014). The near-IR polarization segments in thewestern part of this object are well-aligned with the most exter-nal polarization segments in the sub-mm wavelength range.
6. Magnetic Fields
In our analysis we assume that the magnetic field is oriented per-pendicular to the measured polarization segments in the sub-mmand parallel oriented to the measured polarization segments inthe near-IR and in the optical. This widely applied concept isbased on the finding that, independently of the alignment mech-anism, charged interstellar dust grains would have a substantialmagnetic moment leading to a rapid precession of the grain an-gular momentum around the magnetic field direction, implying anet alignment of the grains with the magnetic field (e.g., Draine& Weingartner 1997; Lazarian 2007). However, one has to keepin mind that polarization observations strongly su ff er from pro-jectional e ff ects along the line-of-sight (LOS). Thus, for a com-prehensive understanding of the magnetic field structure addi-tional 3D radiative transfer modeling is essential but beyond thescope of this study.In general, a high polarization signal is connected to a mag-netic field strong enough to align the dust grains along the LOS.In Section 5, we find well-ordered polarization segments onscales of 10 au to 10 au ( ∼ . − . | B POS | = α · (cid:114) π ρ gas υ turb σ γ , (1)where ρ gas is the gas density in units of g cm − , υ turb is therms turbulence velocity in units of cm s − , and σ γ the standarddeviation of the polarization angles in radians. It is assumed thatthe magnetic field is frozen in the cloud material. Article number, page 5 of 9 & A proofs: manuscript no. main
Originally, α = ff erent studies(e.g, Zweibel 1990; Myers & Goodman 1991) have suggesteda lower value for α . Ostriker et al. (2001) found that, for σ γ (cid:46) ◦ (0 .
44 radians), using α ∼ . ρ gas , is given by: ρ gas = . n H M H , (2)where M H = . molecule.The rms turbulence velocity, υ turb , is given by (Wang et al.1995): υ turb = (cid:52) υ . , (3)where (cid:52) υ is the FWHM line width, measured at quiescent posi-tions located away from the emission peaks.For the outer, less dense regions of CB34, the mean density, n H , is 3 . × cm − (Das et al. 2016) on average, while theFWHM line width, (cid:52) υ , corresponds to 1.37 km s − (Sepúlvedaet al. 2011). Considering the globule’s morphology, we splitCB34 into two regions: South-East (SE) and North-West (NW)of the center. Our near-IR observations trace the central regionitself too sparsely for a robust application of the CF method. Allparameters used for each region are listed in Table 2.We determine the dispersion of the polarization angles in theSE region to σ γ = ◦ and in the NW region to σ γ = ◦ .Since the correction found by Ostriker et al. (2001) is not appli-cable, the derived magnetic field strengths are upper limits. Ap-plying Equation (1) ( α = B NWNIR ≈ . µ G (North-West) and B SENIR ≈ . µ G (South-East), respectively. The similar magnetic field strengths in bothregions, along with the well-ordered polarization segments (Sec-tion 5), implies a uniform large-scale magnetic field across theglobule.Towards the globule’s center, the magnetic field strength in-creases to 34 µ G for core C1 and 70 µ G for core C2 (Das et al.2016). This is consistent with the theoretical picture of magnet-ically supported star formation (e.g., Peters et al. 2010; Hen-nebelle & Ciardi 2009; Federrath 2016).For CB56 and OMK[2002]18, necessary information on thegas densities and turbulence velocities in the regions traced bythe presented polarization data is not available.
Alongside the collapse of the dust in Bok globules, magneticfields are also presumed to influence the formation of circum-stellar disks and outflows (e.g., Matsumoto et al. 2006; Bertranget al. 2017; Bertrang & Wolf 2017). Both aligned and misalignedorientation of the outflow axes along the magnetic field direc-tions have been reported (e.g., Jones & Amini 2003; Bertranget al. 2014; Hull et al. 2014; Zhang et al. 2014). Based on MHDsimulations, Matsumoto et al. (2006) find that the alignment de-gree between outflow and magnetic field is directly correlated tothe magnetic field strength: the stronger the magnetic field, thebetter the alignment.In the following, we examine the relativeposition of the outflow axes and the magnetic field directions forCB34, using our near-IR data and compare our result to the find-ings in the optical and sub-mm wavelength range by Das et al.(2016).In discussing the relative orientation between the CO outflowand the magnetic field, one has to consider that only one com-ponent of the spatial orientation of the outflow ( v sin i) is known from velocity measurements, so projectional e ff ects have to beconsidered, as well as that polarization segments only trace themagnetic field structure projected on the plane of sky. We assumethat the magnetic field is oriented parallel to the polarization seg-ments in the near-IR wavelength range, and perpendicular to thepolarization segments in the sub-mm wavelength range.The orientation of the CO outflow of CB34 is roughly par-allel to the axis linking the cores C1 and C2 (Khanzadyan et al.2002), indicating a North-East to South-West orientation.The orientation of polarization segments in the near-IRwavelength range, tracing the less-dense outer parts of the glob-ule, is almost perpendicular to the direction of the outflow. Thisfits well to the findings in the optical (Das et al. 2016). However,the polarization segments in the sub-mm polarization range, trac-ing the dense center of the globule, are roughly aligned to theoutflow direction, especially for those related to core C1 (Daset al. 2016).Our new data is consistent with the finding of a correlationbetween the magnetic field orientation and the direction of theoutflow, depending on the density and distance to the protostellarcore. Here, we are presenting a sample of the lowest mass star-formingclouds that are known, located completely isolated. These low-mass clouds appear to be dominated by magnetic fields (see also,e.g., Alves et al. 2011; Bertrang et al. 2014). In larger but stillrelatively small clouds, such as Taurus, the cloud structure canbe explained at least proximately in terms of turbulent simula-tions (e.g., Ostriker et al. 2001; O ff ner et al. 2008; Kritsuk et al.2013). These intermediate mass clouds have comparatively shortlifetimes of ∼ ∼ M (cid:12) ), recent work seems toindicate that clouds may have an order of magnitude longer life-times ( ∼
30 Myr; e.g., Murray 2011; Meidt et al. 2015), andbe subject to the influence of “dynamically relevant” magneticfields (e.g., Commerçon et al. 2011; Palau et al. 2013; Stutz &Gould 2016; Busquet et al. 2016). In this picture, in order to rec-oncile the above results including globules, we postulate a massdependent role of magnetic fields in clouds, whereby magneticfields appear to dominate at low and high masses, but perhapsare rather sub-dominant at intermediate (e.g., Taurus) masses.Why do the globules present magnetic field dominated condi-tions? The answer to this question, in the absence of more rigor-ous age or timescale estimates, requires more detailed modelingof the origin of globules. However, it is consistent with Fig. 1 ofStutz & Gould (2016) which strongly suggests that at least in thecase of B 68 it originated as part of an undulating filamentarystructure which then fragmented into cores such as B 68. Thiscan only be explained by magnetic fields (and not gravity, dueto the undulation). The fact that a reasonable fraction of suchglobules present “filamentary tails” (e.g., Launhardt et al. 2013)in sub-mm data is consistent with this scenario. The fact that notall globules present such “vestigial tails” may also indicate thatglobules have comparatively long lifetimes, which would then beconsistent with both the fact that they are observed in the star-less phases (indicating slow evolution that would then allow forthe dissipation of the tails below detectable levels in the outerless dense regions), and the fact that the magnetic fields are ob-served to be dominant. This tentative scenario must be tested,but it should be noted that proposed reasons for why magneticfields maybe dominant in high mass clouds are di ff erent than Article number, page 6 of 9. Jorquera and G. H.-M. Bertrang: Magnetic fields in Bok globules across large spatial scales
Table 2.
Gas densities, gas velocities, polarization, and magnetic field strengths of the regions of CB34 traced in the near-IR.
Region n H ρ gas (cid:52) υ υ turb N vec ¯ γ σ γ B(cm − ) (g cm − ) (km s − ) (km s − ) (rad) (rad) ( µ G)North-West 3 . × . × − . × . × − ff erences between the ISF and L1641within Orion, filaments which both have the same large scalegravitational potential but di ff erent inner gravity and density pro-files). In any case, this ansatz should be tested, but for the timebeing is consistent with observations. Irrespective of the detailsof internal evolution of individual systems, the role of magneticfields in clouds spanning ∼
7. Conclusions
For the first time, we have obtained near-IR polarization dataand, in conjunction with archival optical and sub-mm data,compiled multi-wavelength polarization maps which allow forthe verification of the magnetic field influence on scales of10 − ( ∼ . − ≤
200 pc, forthe first time, by applying a comparison with measurementsof the interstellar polarization 200 pc (see Section 2).2. We find a strong polarization signal of several percents in allthree Bok globules in the near-IR.3. In CB34, the polarization segments in the near-IR are wellaligned with those obtained in the optical tracing scales of10 − au ( ∼ . − au ( ∼ . − au ( ∼ . − . ff ects in the east-ern region.6. We find a correlation between the magnetic field structureand the CO outflow of the Bok globule CB34, depending onthe density and distance to the protostellar core.7. For CB34, comparable magnetic field strengths are found inthe regions traced in the near-IR resp. sub-mm observationsby applying the CF method.8. We find strong indications for dominant magnetic fields inall of these three globules. Article number, page 7 of 9 & A proofs: manuscript no. main
Fig. .1.
Comparison between the correction factor that minimizes theinstrumental polarization and the air mass of the observed unpolarizedstandard stars. The dashed line correspond to the mean value of thecomputed correction factors. In contrast to ISAAC / VLT (Bertrang et al.2014), we do not find a correlation between the instrumental polariza-tion and the elevation angle of the telescope, respectively the airmass ofthe object.
Appendix A: Correction factor derivation forWollaston prism
As stated in Section 4, a correction factor, C λ for the Wollastonprism has to be calculated to correct for the instrumental polar-ization, as the transmission ratio of the prism is not ideal. Thiscorrection factor is applied to the intensities of the upper, i u , andlower, i l , beams created by the Wollaston prism in the followingway (Wolf et al. 2002): i u , = i u , ∗ C λ i l , = i l , (A.1)In our previous study, we find that the instrumental polarizationfor an instrument in Nasmyth focus has a strong dependency ofthe elevation angle of the telescope (ISAAC / VLT; Bertrang et al.2014). For the here presented study, we test SOFI / NTT for thisdependency as well. To determine the instrumental polarization, C λ has to be adjusted in such a way that the measured polariza-tion of the unpolarized standard star is minimal.From November 11 to November 15, 2015, we observed un-polarized standard stars (see Tab. 1) and derive C λ dependenton the elevation angle of the telescope, resp. the airmass of theobject (see Fig. .1). We find no correlation between the correc-tion factor, C λ , and the elevation angle of SOFI / NTT. Hence,we use the mean value of the correction factor to reduce ourdata. Please note that we observed several standard stars withSOFI / NTT over only a few nights. Observations of the same tar-get over a longer period, as performed in Bertrang et al. (2014),might result in a di ff erent finding. Acknowledgements.
The authors thank George A. Gontcharov for providing thedata on interstellar polarization in advance to publication. We thank Amelia M.Stutz for valuable discussions which improved this paper. We thank the anony-mous referee for a critical and helpful report. GHMB acknowledges financialsupport from CONICYT through FONDECYT grant 3170657. This project hasalso received funding from the European Research Council (ERC) under the Eu-ropean Union’s Horizon 2020 research and innovation programme (grant agree-ment No. 757957).
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