# Unveiling a Few Astronomical Unit Scale Rotation Structure around the Protostar in B335

Muneaki Imai, Yoko Oya, Nami Sakai, Ana López-Sepulcre, Yoshimasa Watanabe, Satoshi Yamamoto

► To cite this version:

Muneaki Imai, Yoko Oya, Nami Sakai, Ana López-Sepulcre, Yoshimasa Watanabe, et al. Unveiling a Few Astronomical Unit Scale Rotation Structure around the Protostar in B335. The Astrophysical Journal Letters, 2019, 873, 10.3847/2041-8213/ab0c20. insu-03703891

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Submitted on 29 Aug 2025

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# Unveiling a Few Astronomical Unit Scale Rotation Structure around the Protostar in B335

Muneaki Imai<fnref n="1" />, Yoko Oya<fnref n="1" />, Nami Sakai<fnref n="2" />, Ana López-Sepulcre<fnref n="3" />,<fnref n="4" />, Yoshimasa Watanabe<fnref n="1" />,<fnref n="5" />,<fnref n="6" /> and Satoshi Yamamoto<fnref n="1" />,<fnref n="7" />

Received 2018 November 12; revised 2019 February 9; accepted 2019 March 4; published 2019 March 15

## Abstract

We report a kinematic structure in the innermost envelope of the low-mass Class 0 protostar IRAS 19347+0727 in the Bok globule B335 observed at the best angular resolution ever achieved for this source with ALMA. This is based on observations of complex organic molecule emission in the 1.2 mm band, which selectively traces a hot and dense area around the protostar. The distribution of the CH₃OH and HCOOH emission is resolved, and a clear velocity gradient is observed. Moreover, the direction of the gradient is found to be different between these two molecules. These features are well explained by the model of an infalling and rotating gas, but not by the model of a Keplerian motion. The protostellar mass and the radius of the centrifugal barrier are determined to be 0.02–0.06 $M_\odot$ and <5 au, respectively. The different directions of the gradients observed for CH₃OH and HCOOH are interpreted as the different sizes of their distributions. On the other hand, the SiO emission seems to trace a compact region at the closest vicinity of the protostar, which is a launching point of the outflow or a shocked region caused by the gas accretion onto the protostar. These results first reveal the transition zone from the infalling motion to the rotating motion in this representative isolated protostellar source, which has long been employed as a testbed for star formation studies.

**Key words:** ISM: individual objects (B335) – ISM: kinematics and dynamics – ISM: molecules – stars: formation – stars: low-mass – stars: protostars

## 1. Introduction

An isolated protostellar core without influences from nearby stars/protostars provides us with an ideal testbed of star formation theories. It also offers the best opportunity to trace chemical evolution from an interstellar cloud to a protostellar disk. In this context, B335 is the best-studied source, and its physical structure and kinematics have extensively been investigated (e.g., Hirano et al. 1988, 1992; Chandler & Sargent 1993; Zhou et al. 1993; Wilner et al. 2000; Harvey et al. 2001; Evans et al. 2015; Yen et al. 2015).

B335 is a representative Bok globule (Keene et al. 1980) that harbors the Class 0 low-mass protostar IRAS 19347+0727. Its distance, bolometric luminosity, and bolometric temperature are reported to be 90–120 pc (Olofsson & Olofsson 2009), 0.72 $L_\odot$ (Evans et al. 2015), and 37 K (Andre et al. 2000), respectively. We employ a distance of 100 pc for convenience. A beautiful bipolar outflow structure extending over a few thousand astronomical units along the east–west direction is identified with single-dish and interferometric observations by Hirano et al. (1988, 1992), whereas infalling motion of an inner part of the protostellar core was investigated with ALMA by Evans et al. (2015) and Yen et al. (2015). According to their results, the gas motion at a scale of 100 au around the protostar is dominated by infall, and rotation motion has not clearly been detected even at a spatial resolution of 0.′′3 (~30 au). Yen et al. (2015) reported the upper limit to the radius of the Keplerian disk to be 10 au and the protostellar mass of 0.05 $M_\odot$ with their observation of the C¹⁸O and SO emission. To determine the protostellar mass precisely, it is essential to resolve and

characterize the rotation motion (e.g., infall+rotation or Keplerian rotation).

To investigate the structure of such a compact region around the protostar, emission lines of complex organic molecules (COMs) can be a powerful tracer (Oya et al. 2016, 2017). Recently, Imai et al. (2016) investigated the chemical property of this source with ALMA with a 0.′′5 beam. They detected a number of COM lines in the vicinity of the protostar ($r \sim 10$ au), and revealed that B335 harbors a hot corino. This result means that we can study the innermost area around the protostar using the COM emission without contamination from the surrounding gas. We report the kinematic structure of the innermost envelope of this source observed in the COM lines at the highest angular resolution ever made (10 au). Hereafter, we will refer to organic molecules containing at least six atoms as COMs, and HCOOH as a COM related molecule.

## 2. Observation

We conducted the 1.2 mm (250 GHz) observation (Band 6) toward B335 with ALMA (Cycle 4) on 2017 July 29–August 17. In total, 46 antennas were used in the observation. The field center is $\alpha$(J2000) = 19h37m0s902, $\delta$(J2000) = 7°34′9″64. Minimum and maximum baselines are 18 kλ and 3100 kλ, respectively. The primary beam size is 23″6, and the synthesized beam size is 0″12 × 0″08 (P.A. = −58°6). We used 16 spectral windows whose bandwidths and channel spacings are 117 MHz and 122 kHz, respectively. This channel spacing corresponds to a velocity resolution of 0.28 km s⁻<fnref n="1" />. Some spectral windows overlap with those in our previous

<footnote n="1">¹Department of Physics, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan</footnote>

<footnote n="2">²RIKEN Cluster for Pioneering Research, 2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan</footnote>

<footnote n="3">³CNRS, IPAG, University Grenoble Alpes, F-38000 Grenoble, France</footnote>

<footnote n="4">⁴Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, F-38406 Saint-Martin-d'Hères, France</footnote>

<footnote n="5">⁵Division of Physics, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan</footnote>

<footnote n="6">⁶Tomonaga Center for the History of the Universe, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan</footnote>

<footnote n="7">⁷Research Center for the Early Universe, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Overview of the observed spectrum toward the continuum peak position ($\alpha_{2000}, \delta_{2000}$) = (19$^{h}$37$^{m}$0$^{s}$898, 7$^{\circ}$34'9"/53). The lines analyzed in this paper are shown in red.</figcaption></figure>

ALMA Cycle 2 observation (Imai et al. 2016). On-source integration time was 2 hr 39 minutes, which results in the rms noise of 1–2 mJy beam⁻¹ at a nominal channel width of 0.28 km s⁻¹. J2148+0657 was observed as a flux calibrator. J1928+0848 and J2025+3343 were observed as a phase calibrator and a bandpass calibrator, respectively. The flux calibration accuracy is about 5%.

We used CASA (Common Astronomy Software Applications) for data reduction and imaging. A continuum image was prepared by averaging line-free channels, resulting in the rms noise of 88 µJy beam⁻¹. Maps of the spectral line emission were obtained by CLEANing the dirty images (Briggs robust = 0.5) after subtracting the continuum directly from the visibility data.

# 3. Results and Discussion

## 3.1. Detection of 10 au Scale Velocity Gradient

Figure 1 shows the spectrum observed toward the protostellar position derived by the Gaussian fit of the continuum image: ($\alpha_{2000}, \delta_{2000}$) = (19$^{h}$37$^{m}$0$^{s}$898, 7$^{\circ}$34'9"/53). We detected the lines of various COMs and their related species such as CH$_3$OH, CH$_3$CHO, HCOOCH$_3$, HCOOH, CH$_2$(OH)CHO, and CH$_3$COCH$_3$ toward this position. Some lines of COMs and their related molecules show a double-peaked profile (Figure 1). Such a double-peaked profile can be a sign of rotation motion, because the double-peaked profile is

seen in various lines regardless of their optical depth. It should be noted that the effect of the optical depth of the continuum emission would not seriously affect the spectrum based on the following estimation. The peak intensity of the continuum emission is derived to be 25.8 mJy beam⁻¹ for the 0"/12 × 0"/08 beam by the Gaussian fit, which corresponds to 50.7 K in the brightness temperature. The gas kinetic temperature around this region should be higher than 100 K, because some high excitation lines such as the CH$_3$OH line in Table 1 ($E_u$ = 359.77 K) are detected. Hence, the optical depth of the continuum emission is roughly estimated to be lower than 0.7 at this resolution, where the dust temperature is assumed to be the same as the gas temperature.

We first focus on the emission line of CH$_3$OH (12$_{6,7}$$-$13$_{5,8}$ E) (Table 1) and investigate its velocity structure. CH$_3$OH is the most fundamental COM, and its emission shows the most extended distribution ($r \sim 15$ au) of all the detected COMs and their related species (Figure 2(a)). Its emission peak almost coincides with the protostellar position. Figure 2(b) also shows the velocity map of the CH$_3$OH line. A clear velocity gradient can be seen, which can be ascribed to rotation motion at the innermost envelope. The bipolar outflow of this source extends along the east-west direction with significant overlapping of the blueshifted and redshifted components (Hirano et al. 1988). Thus, the envelope/disk component lies along the south-north direction. Its inclination angle is reported to be 80°–87° (90° for edge-on; Hirano et al. 1988; Stutz et al. 2008). If the
<table><caption>**Table 1**
Parameters for Analyzed Lines</caption><thead><tr><th>Molecule</th><th>Transition</th><th>Frequency (GHz)</th><th>S<sub>μ</sub><sup>2</sup> (Debye<sup>2</sup>)</th><th>E<sub>u</sub> (cm<sup>-1</sup>)</th><th>t<sub>peak</sub> (mJy beam<sup>-1</sup>)</th><th>Catalog<sup>a</sup></th></tr></thead><tbody><tr><td>SiO</td><td>6–5</td><td>260.518009</td><td>57.6</td><td>30.42</td><td>18</td><td>CDMS</td></tr><tr><td>CH<sub>3</sub>CHO<sup>b</sup></td><td>14<sub>1,14</sub>–13<sub>1,13</sub> E</td><td>260.530403</td><td>13.9</td><td>67.00</td><td>23</td><td>JPL</td></tr><tr><td>CH<sub>3</sub>CHO<sup>b</sup></td><td>14<sub>1,14</sub>–13<sub>1,13</sub> A</td><td>260.544020</td><td>13.9</td><td>66.95</td><td>25</td><td>JPL</td></tr><tr><td>CH<sub>3</sub>OH</td><td>12<sub>6,7</sub>–13<sub>5,8</sub> E</td><td>261.704409</td><td>8.5</td><td>250.05</td><td>33</td><td>CDMS</td></tr><tr><td>HCOOH</td><td>12<sub>0,12</sub>–11<sub>0,11</sub></td><td>262.103481</td><td>24.2</td><td>57.53</td><td>22</td><td>CDMS</td></tr></tbody></table>

**Notes.**

<sup>a</sup> The catalog used for line identification. CDMS for the Cologne Database for Molecular Spectroscopy (Müller et al. 2005). JPL for the Jet Propulsion Laboratory (Pickett et al. 1998).

<sup>b</sup> These lines are contaminated with the SiO emission.

rotation motion around the protostar is completely Keplerian (i.e., without radial velocity), the velocity gradient should be along the south–north direction regardless of the inclination. However, the observed velocity gradient is seen approximately along the northwest–southeast direction.

More interestingly, the direction of the velocity gradient is different among molecular species. A distinct case is the HCOOH line (12<sub>0,12</sub>–11<sub>0,11</sub>) (Table 1) as shown in Figure 2(e). Its distribution is more compact than the CH<sub>3</sub>OH distribution (Figure 2(d)), and the velocity gradient is almost along the south–north direction, being consistent with the envelope/disk direction. Such a difference of the direction of the velocity gradient among molecules cannot be explained by the Keplerian motion. Rather, they can be interpreted as a different contribution of the infall motion in the infalling rotating envelope. Indeed, if the flattened envelope is slightly inclined, the velocity gradient perpendicular to the disk direction appears. This is because the gas falling from the front and back sides will be spatially separated. Slant velocity gradients due to the infall motion are indeed reported for other low-mass protostellar sources (e.g., Ohashi et al. 1996; Saito et al. 1996; Momose et al. 1998; Oya et al. 2016). Our model employed in Section 3.2 shows that such a feature can be seen even for the inclination angle of 80°, which is the B335 case (Hirano et al. 1988). The contribution of the infall motion is more conspicuous for CH<sub>3</sub>OH whose distribution is more extended than that of HCOOH, because the infalling motion is dominant in the outer region. Thus the difference of the direction of the velocity gradient likely reflects different distributions of molecules in the infalling and rotating envelope.

## 3.2. CH<sub>3</sub>OH and HCOOH in the Infalling Rotating Envelope

For more quantitative analyses of the kinematic structure, the position–velocity (PV) diagrams are prepared. Figures 2(c), (f) show the PV diagrams of the CH<sub>3</sub>OH and HCOOH emission along the south–north (disk) and east–west directions. A velocity gradient is seen both for the south–north (disk) and east–west directions for the CH<sub>3</sub>OH line. On the other hand, for the HCOOH line, the emission is more compact and a velocity gradient is seen only for the south–north direction. These features are consistent with the velocity maps (Section 3.1).

To investigate the above features, we employ the ballistic model of the infalling rotating envelope (IRE), where angular momentum and energy of the gas are conserved (see Oya et al. 2014 for details of the model). This model has successfully been applied to the kinematic structure of the innermost envelope gas ($r \sim 10–100$ au) for several low-mass protostellar

sources (e.g., Sakai et al. 2014; Oya et al. 2016, 2017). The beam size is considered in the model calculation. For simplicity, we assume that the emission intensity is proportional to $r^{-1.5}$ (e.g., Oya et al. 2016) and the envelope has constant thickness. We verify that the result does not change significantly for the $r^{-0.5}–r^{-2.5}$ dependence within the parameter space shown below. The result is not seriously affected by the envelope shape either: we confirm that the result does not significantly change, even if the envelope thickness has a slope angle less than 45°. Since it is difficult to determine the inclination angle of the disk/envelope system from the present observation, we employ the literature value of inclination angle of 80° (Hirano et al. 1988). We confirm that the results do not vary significantly for the inclination angle of 60°–80°. However, the inclination angle of 90° does not account for the velocity gradient along the east–west direction seen in CH<sub>3</sub>OH emission. Since infalling and rotating gas cannot fall inward of the centrifugal barrier (Oya et al. 2014), our model assumes no emission inside it.

We conduct the model calculation with the following four free parameters: (1) the radius of the centrifugal barrier ($r_{CB}$), (2) the mass of the central protostar ($M_*$), (3) the outer radius of the emitting region in the envelope ($R_0$), and (4) the full thickness of the emitting region of the envelope ($h_0$). $r_{CB}$ and $M_*$ are the physical parameters specific to the source and independent of the molecular distributions. On the other hand, $R_0$ and $h_0$ reflect the distribution of the molecular line emission (i.e., molecular distribution). The parameter ranges that we investigate by eye are as follows: 1–9 au at a step of 1 au for $r_{CB}$, 0.01–0.12 $M_*$ at a step of 0.01 $M_*$ for $M_*$, 6–24 au at a step of 3 au for $R_0$, and 5–20 au at a step of 5 au for $h_0$.

For CH<sub>3</sub>OH, the PV diagrams (Figure 2(c)) are well explained with $r_{CB}$ of 3 au, $M_*$ of 0.04 $M_*$, $R_0$ of 15 au, and $h_0$ of 15 au. The results are overlaid in black contours in Figure 2(c). We set these values of $r_{CB}$ and $M_*$ as the fiducial values. Here, $R_0$ and $h_0$ are mostly determined by the extent of the emitting region for the disk and outflow direction, respectively. We also simulate the PV diagrams of HCOOH. They are successfully reproduced with the same $r_{CB}$ and $M_*$ as the above fiducial ones, but with the $R_0$ of 6 au and $h_0$ of 10 au (Figure 2(f)). The smaller outer radius of the emitting region for HCOOH than for CH<sub>3</sub>OH is consistent with the integrated intensity maps (Figures 2(a), (d)). Thus, the different kinematic structure described in Section 3.1 is explained by the IRE model with different emitting regions of the molecules. Since the infall motion is more prominent in the outer region, the slant feature in the velocity maps (Figures 2(b), (e)) is more prominent for the molecules with extended distributions. Note
<figure><img src="image_4.png" /><figcaption>**Figure 2.** (Left two panels) The integrated intensity maps (left) and the velocity maps (right) of CH₃OH and HCOOH emission. The contours in each panel represent the 10%, 25%, 40%, 55%, 70%, and 85% of the intensity peak of the integrated intensity map. The gray cross marks show the continuum peak position. The synthesized beam size is shown in the bottom right. (Right two panels) Position-velocity diagrams of CH₃OH and HCOOH along the south-north (envelope/disk) and east-west (outflow) directions. The origin of the position axes is the continuum peak position. Black contours are the results of the IRE model with the parameters, *M*<sub>*</sub> = 0.04 M<sub>☉</sub> and *r*<sub>cb</sub> = 3 au. The emitting regions are *R*<sub>0</sub> of 15 au and *h*<sub>0</sub> of 15 au for CH₃OH, and *R*<sub>0</sub> of 6 au and *h*<sub>0</sub> of 10 au for HCOOH. The contour levels denote 10%, 20%, 40%, and 80% of the intensity peak. White contours in the south-north direction panels show the fit by the Keplerian model, where a flattened disk is assumed. In this case, the protostellar mass is 0.10 M<sub>☉</sub>, and the outer radius and the thickness of the disk are 18 and 20 au for CH₃OH and 9 and 10 au for HCOOH, respectively. The contour levels are the same as those in the IRE model case.</figcaption></figure>

that *R*₀ and *h*₀ reflect the size of the emitting region of the molecule. Hence, the result for HCOOH that *R*₀ is smaller than *h*₀ does not necessarily reflect the whole envelope structure. Namely, HCOOH is likely concentrated in a donut-like structure in front of the centrifugal barrier of the envelope.

On the other hand, a model of the Keplerian motion does not explain the kinematic structure of CH₃OH. The best result for the Keplerian model is shown in white contours in Figures 2(c), (f). In the Keplerian model, the inclination angle of the source and the emission profile are assumed to be the same as those for the IRE model. The flattened disk with a constant thickness is also assumed. We try to reproduce the PV diagrams for CH₃OH and HCOOH by optimizing the protostellar mass, the outer radius, and the thickness of the disk. Although the same parameter space as in the IRE model is examined, we cannot reproduce the observation well in contrast to the IRE model case. In particular, the counter velocity components (blue-shifted in the northern part and redshifted in the southern part) cannot be reproduced.

In the PV diagrams (Figures 2(c), (f)), the redshifted emission is weaker than the blueshifted emission. This feature is also confirmed in the velocity map, where the redshifted component seems weakened at the northeast region. This could be due to asymmetric distribution of molecules.

To constrain $r_{\text{CB}}$ and $M_*$, we change these values from the fiducial ones and compare the results with the PV diagrams of CH$_3$OH for the disk direction (Figure 3). $M_*$ mainly affects the velocity shift, because the velocity of the gas at a certain radius is roughly proportional to the $M_*^{1/2}$ in the IRE model. As shown in Figure 3(a), the protostellar mass is constrained to be in the range from 0.02 $M_\odot$ to 0.06 $M_\odot$. On the other hand, $r_{\text{CB}}$ affects the degree of the gradient in the PV diagram. The observed PV diagrams can be explained for $r_{\text{CB}}$ less than 5 au, as shown in Figure 3(b). Considering the synthesized beam size of 0.′′1 (10 au), we can only set 5 au as an upper limit to $r_{\text{CB}}$. In this optimization, $R_0$ and $h_0$ are fixed to 15 au and 15 au, respectively, which can independently be evaluated from the extent of the emission. Although they affect the slant feature in the velocity maps (Figures 2(b), (e)), the velocity gradient for the disk direction in the PV diagrams (Figures 2(c), (f)) is mostly determined by $r_{\text{CB}}$ and $M_*$. Hence, $R_0$ and $h_0$ have little effect on the evaluation of $r_{\text{CB}}$ and $M_*$. On the other hand, $r_{\text{CB}}$ and $M_*$ have positive correlation with each other. The variation of $M_*$ for the range of $r_{\text{CB}}$ from 1 to 5 au is less than 50%. The $R_0$ and $h_0$ are determined by the distributions of the molecules, and hence, they are not significantly affected by the other parameters. Although, they have a slight correlation with the inclination angle, the results do not significantly change for the
<figure><img src="image_5.png" /><figcaption>(a) Protostellar Mass</figcaption></figure>

**Figure 3.** Position–velocity diagrams of CH₃OH along the south–north (disk) direction. The origin of the position axes is the continuum peak position. (a) Contours are the results of the IRE model for the protostellar masses of 0.02, 0.04, 0.06, and 0.08 *M*⊙, where *r*CB is fixed to the fiducial value 3 au. The other parameters are the same as those of Figure 2. The contour levels denote 10%, 20%, 40%, and 80% of the intensity peak. The maximum velocity of the Keplerian motion is shown with white lines. Solid and dotted parts denote the inside and the outside of the centrifugal barrier radius. (b) Contours are the results of the IRE model for the *r*CB of 1, 3, 5, and 7 au, where *M*<sub>*</sub> is fixed to the fiducial value 0.04 *M*⊙. The other parameters are the same as those of Figure 2. The contour levels are the same as (a).

inclination angle from 60° to 80°. We use the CH₃OH emission for the above constraint, but we also confirm that the HCOOH emission is also explained by the above range.

An interesting feature of this protostellar source is the remarkably small radius of the centrifugal barrier (*r*<sub>CB</sub>) (<5 au). It is the smallest of those ever reported for low-mass sources, and smaller than the typical value by an order of magnitude (e.g., 50 au for IRAS 16293–2422; Oya et al. 2016, 100 au for L483; Oya et al. 2017). Since the gas motion outside the centrifugal barrier is explained by infall and rotation, the rotationally supported disk, if present, should be inside the centrifugal barrier at a scale of less than 5 au. This size is consistent with the previous estimate by Yen et al. (2015). Another explanation for the small radius of the centrifugal barrier is that the protostar is in a stage of the first core (Larson 1969; Masunaga & Inutsuka 2000) whose typical size is a few astronomical units.

### 3.3. Different Distribution among COMs

As described in Section 3.2, the distribution of CH₃OH is extended over the area of 15 au in radius. Compared with other hot corino sources (e.g., Oya et al. 2016), the distribution of CH₃OH in B335 is much smaller. However, CH₃OH is extended well beyond the centrifugal barrier, and exists in the innermost part of the IRE. On the other hand, HCOOH shows

even more compact distribution. Thus, the distributions would be different among molecular species. Since the upper state energy of the CH₃OH line is higher than that of HCOOH by a factor of 5, this difference cannot be explained by the excitation condition. Furthermore, the surface binding energy of HCOOH (5000 K) is almost comparable to that of CH₃OH (4930 K), and hence, the different distribution is not simply ascribed to the liberation process from grain mantle. Rather, it may reflect the difference of their formation and destruction processes.

In this observation, we detected more COM lines, and a clear rotation motion is found for the lines of CH₃CHO, HCOOCH₃, and NH₂CHO. A similar trend, i.e., the relation between the slant feature in the velocity map and the size of the distribution, is also confirmed for these lines. Of all the detected COMs with clear rotation motion, CH₃OH and HCOOH show the most extended and most compact distributions, respectively. Hence, we present the two distinct cases in this paper. We will report a more detailed comparison among COM species in a separate publication, which will be crucial in understanding the different distributions of COMs in terms of chemical processes.

### 3.4. SiO Emission Tracing the Inside of the Centrifugal Barrier

The SiO emission shows another interesting feature in B335. Its spectrum toward the protostellar position is shown in Figure 4. The SiO line emission is extended over a velocity
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Spectrum of SiO (left panel). The line emission is extended over a velocity range as large as 70 km s⁻¹. The integrated intensity maps of the blueshifted and redshifted components (contours) are overlaid on the continuum image (color) in the right panel. The contours denote 30%, 50%, 70%, and 90% of the integrated intensity peak. Since the blueshifted component is contaminated with the lines of CH₃CHO, the contaminated range is excluded in the integration. The integrated velocity range for the redshifted and blueshifted components are shown with red and blue in the spectrum, respectively. The blue and red arrows in the right panel denote the direction of blue- and redshifted outflow components, respectively (Yen et al. 2015).</figcaption></figure>

range as large as 70 km s⁻¹, which is about 10 times broader than those of the CH₃OH and HCOOH lines in B335. It has a compact distribution at the protostellar position without extended components. The integrated intensity maps of the redshifted and blueshifted components of SiO are overlaid in contours on the continuum image in Figure 4. Since the blueshifted component is heavily contaminated with the CH₃CHO emission, only noncontaminated channels are used for the integration. Both components are not spatially resolved with the synthesized beam. SiO usually traces outflow-shocked regions in low-mass protostellar sources (e.g., Mikami et al. 1992; Bachiller & Pérez Gutiérrez 1997). In this source, spatial separation between redshifted and blueshifted components are not definitely observed, although a marginal trend might be seen for the outflow direction; the east (blue)–west (red) direction. This trend is also confirmed for the integration of the only high velocity component. Furthermore, the SiO emission including its high velocity component is not elongated to the outflow direction. One possibility is that SiO traces the launching point of the outflow, which is too compact to be resolved with a 0.′′1 beam. Another possibility is that SiO traces the strong shock where the gas is accreting onto the protostar. If we simply assume the energy conservation of the infalling gas without influence of radiation pressure, the emitting region of the highest velocity component of SiO (velocity shift of 35 km s⁻¹) is estimated to be 4 R<sub>Sun</sub>, which is almost comparable to the size of the photosphere (3 R<sub>Sun</sub> reported by Yen et al. 2015). Here we employ the protostellar mass of 0.04 M<sub>☉</sub>. Whichever the case is, SiO seems to trace the closest vicinity of the protostar inside the centrifugal barrier. To reveal the origin of SiO emission in B335, higher spatial resolution observations are now in progress.

### 3.5. Summary

In the 1.2 mm band observation toward B335 with ALMA, a rotation structure at a scale of about 10 au is found in the CH₃OH and HCOOH lines, and its kinematic structure is discussed. The main results are summarized below.

1. The velocity gradient of HCOOH is almost along the disk direction, while that of CH₃OH is inclined to the outflow direction. This feature can be explained by the infalling and rotating envelope model with different molecular distributions. The different distribution may reflect the formation and destruction processes of these molecules.

2. The protostellar mass and the radius of the centrifugal barrier are derived to be 0.02–0.06 M<sub>☉</sub> and <5 au, respectively. The radius of the centrifugal barrier is the smallest of previously known values for low-mass protostars.

3. SiO shows very broad lines and has compact distribution at the protostellar position. It appears to reside in the closest vicinity of the protostar tracing the launching point of the outflow or gas accreting onto the protostar.

This paper makes use of the following ALMA data: ADS/JAO.ALMA#2016.1.01376.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. This study is supported by Grant-in-Aids from Ministry of Education, Culture, Sports, Science, and Technologies of Japan (25400223, 25108005, 18H05222, and JP18J11010). The authors acknowledge the financial support by JSPS and MAEE under the Japan-France integrated action programme (SAKURA: 25765VC).

### ORCID iDs

Muneaki Imai https://orcid.org/0000-0002-5342-6262

Yoko Oya https://orcid.org/0000-0002-0197-8751

Nami Sakai https://orcid.org/0000-0002-3297-4497

Ana López-Sepulcre https://orcid.org/0000-0002-6729-3640

Yoshimasa Watanabe https://orcid.org/0000-0002-9668-3592
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