Super-rotating protostellar jets

1 Protostellar jets are most striking phenomena in star-forming re- 2 gions and considered to be an essential ingredient in the star forma- 3 tion process. Stars form in gravitationally collapsing clouds. The 4 mass of protostar at its birth is equivalent to Jovian mass or 0.1 per- 5 cent of the solar mass 1,2 . After the birth, the protostar acquires its 6 mass by accreting material from a surrounding rotation disk em- 7 bedded in an infalling envelope that is a remnant of the natal cloud 8 of the star. Protostellar jets are believed to expel the excess angu- 9 lar momentum from the circumstellar region that allows accretion 10 on to the star. Here, we report the detection of super-rotating jets 11 driven from a protostar FIR 6b (HOPS 60) in Orion Molecular 12 Cloud-2 3,4,6 . The jet rotation velocity exceeds 20 km s − 1 and the 13 speciﬁc angular momentum of the jet is as large as ∼ 10 22 cm 2 s − 1 , 14 which hitherto are the largest that have been observed in proto- 15 stellar jets. The extraordinary large rotation velocity and speciﬁc 16 angular momentum can be explained by a magnetohydrodynamic 17 disk wind 5 . This is clear evidence that magnetic ﬁelds play a cru- 18 cial role for protostellar evolution and that angular momentum is 19 removed by protostellar jets. 20 The target (HOPS is located in Orion Molecular Cloud- 21 2 (hereafter we call this object and identiﬁed as a Class 0 in- 22 termediate mass protostar 4,6,7 . The distance to FIR is d ∼ 392 pc 8 23 and the systemic velocity is v sys ∼ 11 km s − 1 in the Local Standard 24 of Rest (LSR) 9 . FIR 6b has a bolometric ALMA observation and data reduction. Mosaicking observations in the millimeter CO ( J = 2–1; 230.538 GHz) molecular line and the 1.3 mm continuum 2 were carried out with the ALMA 12m array on 19 April 2018 and with the ACA 7m array (Morita array) on 7, 10, 11, and 17 January 2018. The data was 3 obtained through the Cycle 5 program 2017.1.01353.S (PI: S. Takahashi). The OMC-2/FIR 6 region covers 3 . 8 (cid:48) × 3 . 9 (cid:48) area centered at (R.A., Dec.) = 4 ( 05 h 35 m 21 s . 700 , − 05 ◦ 12 (cid:48) 51 (cid:48)(cid:48) . 000 ) with the Nyquist sampling. In the ALMA 12m array and ACA 7m array observations, the mosaic ﬁelds consist of 5 108 and 42 pointings with the total on-source duration per pointing of 20 and 260 seconds, respectively. The overview of the full survey at OMC-2/FIR 6 6 region will be presented in a separate paper (Matsushita et al. in prep 2020). In this paper, we presented the result in the area centered on OMC-2/FIR 7 6b. Two spectral windows with a 1875 MHz width are allocated to the continuum observations centered at 233.100 and 215.200 GHz. Two other spectral 8 windows are placed at the frequency of CO J = 2–1 and SiO J = 5–4 with a 938 MHz width and a 244 kHz frequency resolution (velocity resolution of 9 0.64 km s − 1 ) in the dual polarization mode. The SiO emission is not detected in FIR 6b. The arrays consisted of 44 and 11 antennas for the ALMA 12m 10 array and the ACA 7m array observations, respectively, with projected baseline coverage from 15.1 to 500.2 m and 8.9 to 48.9 m. The primary beam size 11 was 25 . 2 (cid:48)(cid:48) and 43 . 2 (cid:48)(cid:48) , and the system temperature was from 70 to 180 K and 60 to 210 K for the ALMA 12m array and the ACA 7m array, respectively.

| CO (J=2-1) high velocity integrated intensity map (color and black contour). High velocity components of the CO (J=2-1) emission are integrated over the LSR velocity ranges of 32.5 to 97.5 km s −1 (redshifted component, northeast side) and 17.5 to 0 km s −1 (blue-shifted component, southwest side). The 1.3 mm continuum emission is overlaid with gray contours, which shows the peak toward OMC-2/FIR 6b at (R.A., Dec.) = (05 h 35 m 23 s .34, −05 • 12 03 .970). The integrated CO emission shows a well-collimated structure that is distributed from the northeast to the southwest direction centered around FIR 6b. Several knots corresponding to the local emission peaks indicated by arrows are found within the jets. The synthesized beams are denoted in the left bottom with a black open ellipse for the CO emission and filled gray ellipse for the 1.3 mm continuum emission, respectively. The contour levels of the CO emission are 5σ, 10σ, 15σ, and 20σ, (1σ = 1.0 Jy beam −1 · km s −1 ), and those of the 1.3 mm continuum emission are 8σ, 40σ, 80σ, 120σ, 160σ, 240σ, and 320σ (1σ = 0.2 mJy beam −1 ). Fig. 2 shows maps of the mean velocity obtained from the CO 19 (J=2-1) emission. A clear velocity gradient is found along the jet 20 short-axis in the red-shifted jet (Fig. 2a). The northern part of the red-21 shifted jet is moving towards the front and the southern part is moving 22 toward the back. The velocity shifts along the minor axis by 25 km s −1 1 to 50 km s −1 . Thus, the red-shifted jet seems to rotate around its long 2 axis. Several alternative interpretations might be considered to explain 3 the velocity gradient such as jet precession, twin jets and asymmetrical 4 jet structure 13 . Thus, we need to carefully investigate the cause of the 5 velocity gradient. 6 When the jet precesses, the velocity gradient tends to appear along 7 the jet long-axis or the jet propagation direction. On the other hand, 8 the jet velocity would not be significantly changed along the jet short-9 axis in the precession model 14,15 . In addition, the precessing jet should 10 have a strong wave-like structure as a whole 15 , while the observed jet is 11 straight in both the red-shifted and blue-shifted lobes in the integrated 12 intensity map (Fig. 1). 13 If FIR 6b is a binary system, two protostars can drive two jets. In 14 such a case, the velocity gradient (Fig. 2a) can be potentially explained 15 by twin jets if the direction of jets is slightly different each other. It 16 seems that the red-shifted jet splits along its long-axis in the velocity 17 map (Fig. 2a). However, we cannot see any signature of the twin jets in 18 the integrated intensity map (Fig. 1). In addition, there is no evidence 19 for a protobinary system in the dust continuum within the ALMA at 20 1.3 mm data at 400 au spatial resolution ( Fig. 1) nor in ALMA at 0.87 21 mm and VLA at 9 mm observations at 40 au resolution 8 . Thus, it is not 22 plausible to consider that the jet are composed of twin flows.

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On the other hand, when the jet has an asymmetric or complex 24 velocity distribution, the velocity gradient would appear in the jet short-25 axis. However, the distribution of CO emission is rather smooth (Fig. 1) 26 and the velocity gradient is systematic. Thus, the velocity gradients 27 within the jets cannot be explained just by the asymmetric nature. 28 We conclude that rotation is most plausible to interpret the velocity 29 gradient of the jet seen in velocity map (Fig. 2a). It is difficult to explain  Using the position-velocity diagram, the jet rotation velocity v φ 52 and the distance from the jet axis (or radius in the cylindrical coordi-53 nate) rrot are estimated as and respectively 13 , where v blue and v red are the velocity at emission 56 peak of red-shifted and blue-shifted components in each PV diagram 57 (Fig. 3). The jet rotation axis was determined using the PV diagrams, in which the half distance of two emission peaks is adopted as the axis 1 (Fig. 3). In the position velocity diagrams, the jet width l shift was cal-2 culated from the distance between two peaks (Fig. 3). The disk incli-3 nation angle is estimated to i = 80 • with respect to the line of sight 4 in past observation 6 . We also estimated the jet inclination angle us-

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The measured rotation velocities are in the range of v φ = 11.2 − 20 28.4 km s −1 , while the jet radius (or the distance from the jet axis) is in 21 the range of rrot ∼ 220−800 au (   in the range of 9.2 × 10 21 cm 2 s −1 ≤ j ≤ 1.9 × 10 22 cm 2 s −1 ( Table   1 1). The maximum specific angular momentum estimated in this study 2 is one or two orders of magnitude larger than those reported in other 3 sources. The specific angular momenta calculated in previous studies 4 are j ∼ 3.5 × 10 20 cm 2 s −1 for DG Tau 19 , ∼ 1.5 × 10 20 cm 2 s −1 for 5 CB 26 20 and ∼ 7.5 × 10 20 cm 2 s −1 for Orion Source I 21 . Among the 6 jets where the rotation speed was measured, only the rotating jet bullet 7 SVS 13A has a comparable but slightly smaller value of the specific 8 angular momentum (∼ 9.8 × 10 21 cm 2 s −1 ) 13 .

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It is crucial to explore the launching radius r0 of the jet in order 10 to clarify the jet driving mechanism. Based on the Bernoulli's the-11 orem and the angular momentum conservation law, we can estimate 12 the jet launching radius 24 . Using the same method, recent ALMA where the poloidal velocity components of the jet are derived with The mass of the central protostar is simply assumed to be Mstar =  (Table 1). Thus, the jet is expected To investigate the effect of magnetic field on the angular momen-24 tum transfer, the Alfvén radius rA 25 at each position is estimated using where Ω0 = GMstar/r 3 0 is the Keplerian angular velocity at the jet  Table 1). The ratio of the Alfvén radius to the jet launching 28 radius λ is as large as λ ≡ rA/r0 = 25.8 − 33.9. Thus, the long 29 magnetic lever arm efficiently transports the angular momentum from 30 the circumstellar region for the super-rotating jet case 29 .

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The magnetic pressure dominates the ram pressure within the 32 Alfvén radius. Therefore, the large λ means that the magnetic field locity at foot points of the magnetic field lines (Fig. 4). As a result, the 7 fluid elements receive the angular momentum and are expelled from 8 the circumstellar region by magnetic effect 30 . On the other hand, a par-9 cel of gas in the circumstellar disk near the protostar loses the angular 10 momentum and falls onto the protostar 29 . Therefore, the excess angu-11 lar momentum is ejected from the circumstellar disk by the rotating jet, 12 and the gas whose angular momentum has been removed by the jet falls 13 onto the protostar and promotes the protostellar growth.
14 The protostellar jets were observed by CO emission using ALMA.     was 25.2 and 43.2 , and the system temperature was from 70 to 180 K and 60 to 210 K for the ALMA 12m array and the ACA 7m array, respectively.

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The flux, gain, and bandpass calibrators were J0423-0120, J0541-0211, and J0423-0120 for the ALMA 12m array, and J0522-3627 and J0607-0834, 13 J0542-0913, and J0423-0120 for the ACA 7m array. gradient depends on the distance from the position of FIR 6b, the emission peak is seen on both side of the jet axis in the channel map. In addition, we can 28 confirm three knots or relatively strong emission peaks in Fig. 5. We cannot confirm the emission peak on both sides of the jet axis in the blue-shifted jet 29 (Fig. 6). The channel maps presented in the LSR velocity range between 30 km s −1 and 95 km s −1 , which corresponds to the relative velocity range of 19km/s to 84km/s with respect to the system velocity (vsys = 11 km s −1 ). The center value of the LSR velocity is described in each panel. The plus symbol at the center corresponds to the position of protostar FIR 6b measured in the 1.3 mm continuum. The broken lines indicate the axes of the red-shifted (northeast side) and blue-shifted (southeast side) jets. The jet axis of the red-shifted components is determined on the map of 70.0 km s −1 . The contour levels are 3σ, 5σ, 10σ, 15σ, 20σ, 30σ, 40σ, 60σ, 80σ, and 100σ (1σ = 30 mJy beam −1 · km s −1 ). The black open ellipses in the bottom left corner is the synthesized beam size. CO (J=2-1) mean velocity map (color). As same as in Fig. 7a but for the mean velocity of the low velocity components. The contour represents the integrated intensity are shown in Fig. 7a. (c) CO (J=2-1) line profile. The blue-shifted emission is detected in the LSR velocity range of −20 − 10 km s −1 , while the red shifted emission is detected around 10 − 100 km s −1 .