ALMA polarization of HL Tau - investigating planet formation -

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1 ALMA polarization of HL Tau - investigating planet formation - The Astrophysical Journal Letters, 844:L5 (5pp), 2017 July 20 ALMA Band 7 (870 µm) essential. The wavelength dependence of the polarization fraction is not strong in the case of the grain alignment, while it is strong in the case of the self-scattering because the scattering-induced polarization is efficient only when the maximum grain size is around l 2p where λ is the wavelengths (Kataoka et al. 2015). To obtain the wavelength-dependent polarimetric images, we observe the HL Tau disk with the Atacama Large Millimeter/ submillimeter Array (ALMA) using Band 3. HL Tau is a young star in the Taurus molecular cloud with a distance of 140 pc (Rebull et al. 2004). The circumstellar disk is around in 100 au scale (Kwon et al. 2011). The disk has several ring and gap structures with tens of au scales (ALMA Partnership et al. 2015). The observed band corresponds to wavelengths of 3.1 mm, which is sufficiently longer than the previous CARMA polarimetric observations at 1.3 mm (Stephens et al. 2014). ALMA Band 3 (3.1 mm) HL Tau Kataoka et al. 2. Observations Stephens et al Kataoka et al HL Tau was observed by ALMA on 2016 October 12, during its Cycle 4 operation ( S, PI: A. Kataoka). Scattering Alignment The antenna configuration was C40-6, and 41 antennas were operating. The correlator processed four spectral windows centered at 90.5, 92.5, 102.5, and GHz with a bandwidth of 1.75 GHz each. The bandpass, amplitude, and phase were calibrated by observationsakimasa of J , J , Kataoka and (NAOJ fellow, NAOJ) J , respectively, and the polarization calibration was performed T. bymuto observations (Kogakuin of J U.), M. Momose, The rawt. data Tsukagoshi were (Ibaraki U.), H.Nagai (NAOJ), M. Fukagawa (Nagoya U.), reduced H. Shibai by the(osaka EA-ARC U.), staff. T. Hanawa (Chiba U.), K. Murakawa (Osaka-S.), Kees Dullemond, Adriana Pohl (Heidelberg) We further perform the iterative CLEAN deconvolution

2 Today's talk Old and new theories for explaining millimeter-wave polarization 1. Alignment with magnetic fields 2. Self-scattering of thermal dust emission Kataoka et al Alignment with radiation fields gure 8. Same as Figure 5, but for model A in the case of the lopsided protoplanetary disk. The object is optically thin everywhere. Testing the theory with ALMA polarization observations HD morphology of pol. vectors HL Tau - wavelength dependence Kataoka et al AU

3 Star and disk formation Molecular cloud cores Protostar and envelope Protostar and protoplanetary disk Toward MS Timescale ~ years ~ years ~ years ~0.1 pc Spatial scale =20,000 AU ~1,000 AU ~100 AU ~ 200 arcsec ~10 arcsec ~1 arcsec Key physics magnetic fields or turbulence? jets, outflows grain growth

4 SED of a protoplanetary disk TW Hya (Mstar = 0.6 M, Teff = 4000 K) MIR Scattered Light (sub-)mm star a b d c Distance in AU 1 Turbulent Mixing (radial or vertical) disk Vertical Settling Radial Drift a) Sticking b) Bouncing c) Fragmentation with mass transfer d) Fragmentation mm 3.0 mm ALMA 10 µm VLTI/MATISSE 2 µm 10 µm EELT JWST/MIRI Fig. 1. Illustration of the structure, grain evolution processes and observational constraints for protoplanetary disks. On the left side we show the main grain transport and collision mechanism properties. The different lengths of the arrows illustrate the different velocities Menu of the different et al. grains On the right hand side, we show the Testi areas ofet theal. disk 2014 that can be probed by the various techniques. The axis shows the logarithmic radial distance from the central star. The horizontal bars show the highest angular resolutions (left edge of the bars) that can be achieved with a set of upcoming facilities and instruments for at the typical distance of the nearest star forming regions. The millimeter emission is thermal dust emission from the disk. How can we polarize the thermal dust emission? with respect to the gas. The force exerted on them depends not only on the relative Akimasa motion Kataoka between (NAOJ gas andfellow) dust, but a dimensionless number, which relates the stopping time to the orbital period K. The concept of the Stokes number is

5 Polarization mechanisms 1. Alignment of elongated dust grains with magnetic fields Magnetic Field Linear polarization e.g., Lazarian and Hoang The self-scattering of thermal dust emission Kataoka et al Alignment of elongated dust grains with radiation fields Tazaki, Lazarian et al. 2017

6 Dust is big in disks 0.1μm 1mm 1m 1km km κ abs,sca [cm 2 /g] dust opacity a max =1 µm, κ abs a max =1 µm, κ sca a max =100 µm, κ abs a max =100 µm, κ sca λ [µm] scattering > absorption

7 Light source of scattering IR scattered light example (face-on, PI) Infrared disk radio scattered light (self-scattering) Pohl et al millimeter?

8 Polarization due to scattering incident light (unpolarized) a dust grain an observer an observer an observer

9 Polarization due to scattering thermal dust emission of other dust grains a dust grain The observer is you. Horizontal Polarization (the line of sight is perpendicular to the plane of this slide)

10 Polarization due to scattering Horizontal Polarization

11 Polarization due to scattering Unpolarized

12 Polarization due to scattering Unpolarized

13 Polarization due to scattering Vertical Polarization

14 self-scattering in an inclined disk polarization reversal in the large grain case, which yields an intrinsic (or face-on) polarization direction in the radial (as opposed to azimuthal) direction and an inclination-induced polarization along the major (rather than minor) axis. The interplay between the intrinsic and inclination-induced polarization leads to polarization directions in the region of high polarized intensity (the most easily observable part) completely different from those observed in HL thermal dust emission of the disc closer to the observer (the right half) brighter. The polarization fraction is, however, higher on the far side (especially towards the outer part of the disc) because the polarization degree of the scattered light is higher for backward scattering than for forward scattering (see Fig. 6). The most striking difference between this case and the Rayleigh scattering case shown in Fig. 5 lies in the polarization direction. The difference comes from the polarization vector (disk, edge-on view) Figure 7. Scattering-induced polarization by large grains. As in Fig. 5, plotted are the polarized intensity (colour map) and polarization vectors (line segments, with length proportional to the polarization fraction). Note the strong asymmetry with respect to the major axis in both the polarized intensity and the polarization vectors. The polarization along the major axis in the central region is due to polarization reversal, which may be a robust indicator of scattering by large, mm/cm-sized, grains. The near side of the disc is on the right. thermal dust emission i=45 Yang, Li, et al See also Kataoka et al. 2016a

15 Conditions of dust grains for polarization For efficient scattering (grain size) >~ λ For efficient polarization (grain size) <~ λ There is a grain size which contributes most to the polarized emission P P 90 λ=870 µm (ALMA Band 7) Maximum grain size [cm] grain size [cm] P ω Albedo If (grain size) ~ λ/2π, the polarized emission due to dust scattering is the strongest

16 Grain size constraints by polarization Expected polarization degree (scalable) 0.87 mm (Band 7) 0.34 mm (Band 10) 3.1 mm (Band 3) 7 mm (Band 1) Maximum grain size [cm] Kataoka, et al., 2015 Multi-wave polarization constraints on the grain size

17 HL Tau - continuum The Astrophysical Journal Letters, 808:L3 (10pp), ALMA2015 Partnership July 20 et al. ALMA Partnership, 2015

18 HL Tau pol. - prediction λ=870µm i = 47 (ALMA Partnership 2015) The polarization vectors are parallel to the minor axis Kataoka, et al., 2016a (see also Yang et al. 2016)

19 HL Tau polarization with ALMA 100 AU 100 AU We find the azimuthal polarization vectors at 3.1 mm wavelength Kataoka, et al., 2017

20 HL Tau polarization 100 AU 100 AU data from Stephens et al., 2014 Kataoka, et al., 2017 The polarization vectors at 1.3 mm are parallel to the minor axis The polarization vectors at 3.1 mm are in the azimuthal direction wavelength-dependent polarization in mm range

21 Polarization mechanisms 1. Alignment of elongated dust grains with magnetic fields Magnetic Field Thermal emission Linear polarization e.g., Lazarian and Hoang The self-scattering of thermal dust emission Kataoka et al Alignment of elongated dust grains with radiation fields Tazaki, Lazarian et al. 2017

22 Alignment with radiation fields Incident light Front view Side view If dust grains have a helicity, they emit intrinsic polarization. Tazaki, Lazarian et al. 2017

23 Alignment with radiation fields a~100 µm azaki et al. aligned with mag. fields aligned with rad. fields Figure 5. Timescales relevant to RAT alignment. Top and bottom panels show the a n of E-vector is plotted as the white bar. Left and right panels represent mid-infrared panels correspond to the two different locations in the disk, (R, z) = (50 au, 0 au), a vely. The dust grains are assumed to be magnetically poor ( f p = 0.01 and φ sp = 0). the timescale of the gaseous damping (t represent gas ) and the RAT alignment times 3 and f -parameter are assumed to be α = 10 = 0.5, respectively. J high J precession timescale (t ). Green and blue lines show the Larmor precession ti r Tazaki, Lazarian et al. 2017rad, (see also Lazarian and Hoang 2007) p inclusions ( f p = 10%, φsp = 3%, Ncl = ), respectively. The dashed line in th grain size does not strongly affect on the resultant polarization

24 Polarization mechanisms alignment with B-fields self-scattering alignment with radiation Toroidal magnetic fields are assumed Inclination-induced scattering -> parallel to the minor axis Grain size is a ~λ/2π: strong wavelength dependence Grains are needed to be big (~>100um) Radiation gradient is in the radial direction.

25 Polarization mechanisms alignment with B-fields self-scattering alignment with radiation AASTEX wavelength-dependentpolarization AU 100 AU self-scattering alignment with radiation Figure 2. Comparison of the polarization images between = 1.3 mm(carma Stephens et al. 2014) and = 3.1 mm

26 espite these detections, the polarization morpholousually were not consistent with what would be exed from magnetically Wavelength aligned dust grains. In particudependence Stephens et al. (2014) used the Combined Array for arch in Millimeter-wave Astronomy (CARMA) to sure the 1.3 mm polarization morphology in HL Tau. 2 morphology was inconsistent with grains aligned the commonly-expected toroidal magnetic fields 2015), the Class 0 disk candidate of NGC 1333 IRAS 4A arization/e-field vectors distributed radially in the (Cox et al. 2015), the Herbig AE late-stage protoplanetary disk HD (Kataoka et al. 2016), and the disk ). Instead, the E-vectors were oriented more or less g the minor axis of the disk. Kataoka et al. (2015, candidate of the high-mass protostar Cepheus A HW2 ) and Yang et al. (2016) suggested that the polarizamorphology is actually consistent with that expected have also been detected at mid-infrared wavelengths of (Fernández-López et al. 2016). Polarization toward disks self-scattering (also see Pohl et al. 2016; Yang et 8.7, 10.3, and 12.5 µm (Li et al. 2016, 2017). However, 017). Indeed, several disks where polarization polarized is emission at mid-infrared wavelengths can occur cted show consistency with the polarization morlogy expected from self-scattering rather than grains causing di culty in interpreting the polarization mor- due to absorption, emission, and sometimes scattering, ed with the magnetic field. However, except for phology. ALMA observations of HD (Kataoka et al. Despite these detections, the polarization morphologies usually were not consistent with what would be ex- ) and HL Tau (Kataoka et al. 2017), the published rvations are too coarse to resolve more than a fewpected from magnetically aligned dust grains. In particular, Stephens et al. (2014) used the Combined Array for pendent beams across the disk, making it di cult istinguish between scattering and other polarization Research in Millimeter-wave Astronomy (CARMA) to hanisms. measure the 1.3 mm polarization morphology in HL Tau. he high-resolution ALMA observations of HD The morphology was inconsistent with grains aligned ataoka et al. (2016) resolved polarization for many with the commonly-expected toroidal magnetic fields Figure 1. ALMA polarimetric observations at 3 mm of independent resolution elements across the disk. (polarization/e-field (top, vectors distributed radially in the Kataoka et al. 2017), 1.3 mm (middle), and 870 µm (bottom), polarization was radial throughout most of the disk, disk). Instead, the E-vectors were oriented more or less where the red vectors show the >3 polarization morphology (i.e., h is vectors expected have not for been grains rotated). alignedvector withlengths a toroidal are linearly field, along proportionalthe to P edges.thecolorscaleshowsthepolarizedintensity,whichis morphology changed from ra- 2016) and Yang et al. (2016) suggested that the polariza- the minor axis of the disk. Kataoka et al. (2015, toward masked to only show 3 detections. Stokes I contours in each to azimuthal, which is more consistent with scatter- tion panel morphology is actually consistent with that expected are shown for [3, 10, 25, 50, 100, 200, 325, 500, 750, 1000] Models in Kataoka et al. (2016) found that scattercan broadly from I,where self-scattering (also see Pohl et al. 2016; Yang et selfscattering sharply and , respectively. but not everywhere. A complete phology expected from self-scattering rather than grains I is 44, 154, and 460 µjy bm 1 for 3 mm, 1.3 mm, and 870 µm, respectively. reproduce the features observed in parts al. 2017). Indeed, several disks where polarization is he disk especially where the polarization ~1 % orientas change of detected show consistency with ~0.8 the polarization % mor- ~0 % erstanding of this interesting case is still missing. aligned with the magnetic field. However, except for L Tau is one of the brightest Class I/II in the sky at the ALMA observations of HD (Kataoka et al. )millimeter wavelengths, and thus the polarization 2016) and HL Tau (Kataoka et al. 2017), the published phology can be determined at high resolution + with observations are too coarse to resolve more than a few + onable integration times. Kataoka et al. (2017) fold up on the Stephens et al. (2014) observations with to distinguish between scattering and other polarization independent beams across the disk, making it di cult + observations of HL Tau. Surprisingly, they found mechanisms. the polarization morphology was azimuthal, which The high-resolution ALMA observations of HD estsalignment grains aligned with their long axes perpendicuto the radiation field, as predicted~1 by Tazaki % et al. 10s of independent resolution elements across the disk. by Kataoka et al. (2016) resolved polarization for many Figure 1. ALMA polarimetric observations at 3 mm (top, Kataoka et al. 2017), 1.3 mm ~1 (middle), % and 870 µm (bottom), ~1 % 7). Henceforth, we will call this grain alignment The polarization where the red was vectors radial show throughout the >3 polarization most of the morphology disk, (i.e., with rad. hanism alignment with the radiation anisotropy. which is vectors expected have not forbeen grains rotated). aligned Vector withlengths a toroidal are linearly field, proportional to P.Thecolorscaleshowsthepolarizedintensity,whichis he very di erent polarization morphologies observed but toward the edges morphology changed from ra- to.3 mm (0. 00 masked to only show 3 detections. Stokes I contours in each panel 6 resolution, Stephens et al. 2014) and 3 mmdial are azimuthal, shown for which [3, 10, 25, is50, more 100, consistent 200, 325, 500, with 750, 1000] scattering. Models I,where resolution, Kataoka et al. 2017) suggest that the I is 44, in 154, Kataoka and 460 et µjy al. bm (2016) 1 for found 3 mm, that 1.3 mm, scattering canrespectively. broadly reproduce the features observed in parts and 870 µm, phology of the polarization emission is strongly deent on the wavelength. This Letter presents ALMAof the disk especially Akimasa where Kataoka the(naoj polarization fellow) orienta- = Stephens et al (see also Kataoka et al. 2017) = = *Numbers are based on very rough estimate

27 HL Tau polarization The maximum grain size is ~ 70 µm Kataoka, et al., 2017

28 Summary Theories 1. Alignment with (toroidal) magnetic fields. 2. Self-scattering of thermal dust emission 3. Alignment with radiation fields Observations HD (+1,3?) HL Tau 2+3 (1 is ruled out) 100 AU Implications to planet formation if (2) self-scattering works, the grain size is ~ λ/(2π) 100 AU 100 AU