L483

Abstract

Various molecular lines in the 1.2 mm band were observed toward the Class 0 protostellar source L483 at a subarcsecond resolution with ALMA. The CS emission traces an extended infalling-rotating envelope gas. Meanwhile, a compact component with a broad velocity width is detected in the CS, SO, HNCO, NH$_2$CHO, and HCOOCH$_3$ emissions. This means that L483 harbors a hot corino characterized by complex organic molecules such as NH$_2$CHO and HCOOCH$_3$ in the vicinity of its protostar, although it is regarded as the warm carbon-chain chemistry (WCCC) candidate source at a 1000 au scale. Thus, both hot corino chemistry and WCCC are seen in L483. Although such a hybrid chemical character source has been recognized as an intermediate source in previous single-dish observations, the two chemical characteristics are spatially resolved for the first by this study. The kinematic structure of the envelope gas is roughly explained by a simple ballistic model with the protostellar mass from 0.1 to 0.2 $M_\odot$ and the radius of the centrifugal barrier from 30 to 200 au by assuming a nearly edge-on geometry with the inclination angle of 80$^{\circ }$. The line emissions detected with broad velocity widths most likely come from the disk component inside the centrifugal barrier of the infalling-rotating envelope. Thus, a drastic chemical change is confirmed there. The outflow cavity wall of L483 is traced by the CS emission. Its kinematic structure is explained by a parabolic outflow model, and its inclination angle is constrained to be from 75$^{\circ }$ to 90$^{\circ }$. A hint of a rotation motion of the outflow is found. The specific angular momentum of the outflow is evaluated to be comparable to or twice that of the infalling-rotating envelope.

This chapter has been published in Oya et al., 2017, ApJ, 837, 174 and Oya et al., 2018, ApJ, 863, 72. © AAS. Reproduced with permission.

8.1 Introduction

So far, it has been demonstrated that the infalling-rotating envelope and its centrifugal barrier will exist in low-mass protostellar sources regardless of their chemical characteristics. However, the chemical change occurring across the centrifugal barrier was confirmed with different molecular species depending on the chemical composition of the source. In the previous chapters, sources with peculiar chemical compositions were dealt with; warm carbon-chain chemistry (WCCC) and hot corino sources. They show the exclusive chemical compositions with each other. Thus more general understandings of the chemical change requires such chemical diagnostics in other sources with different chemical characteristics. In this chapter, a hybrid chemical characteristics of L483 is confirmed. The kinematic structure of the gas in L483 is also investigated.

It is of fundamental importance to characterize the chemical composition in the vicinity of the protostar, because it will define chemical heritage from interstellar matters to protoplanetary disks. However, observational studies of chemical characterization have been performed at a 100 au scale only for a several sources so far, including four hot corinos (IRAS 16293$-$2422 Source A, Source B, IRAS 2A, B335; Chaps. 6, 7; [12, 18]) and three WCCC sources (L1527, IRAS 15398$-$3359, TMC−1A; Chaps. 4, 5; [27, 28, 30]). Therefore, observations for other protostellar sources, especially for ones with intermediate chemical compositions, are awaited. In this chapter, a well-studied Class 0 protostellar source L483 is focused on in this context.

L483 is a dark cloud located in the Aquila Rift ($d=200$ pc [15, 25]). It harbors the Class 0 protostar IRAS 18148−0440 [4, 7], and its bolometric luminosity is reported to be 13 $L_\odot$ [32]. The systemic velocity of 5.5 km s$^{-1}$ is employed in this study based on previous single-dish observations [10]. In this source, the C$_4$H abundance is relatively high, and it is regarded as a possible candidate for the WCCC source [10, 11, 29, 31]. The carbon-chain-molecule rich nature of L483 has been further supported by the detection of the carbon-chain radical HCCO [1].

The outflow of L483 has extensively been studied [7, 9, 14, 17, 23, 34, 35]. It is known to be extended along the east-west axis; the eastern and western lobes are red-shifted and blue-shifted, respectively. The position angle of the outflow axis is reported to be 95$^{\circ }$ by [23] based on the HCO$^+$ ($J=1-0$) observation and 105$^{\circ }$ by [4] based on the shocked H$_2$ emission reported by [7]. Its inclination angle is reported to be $\sim$50$^{\circ }$ (0$^{\circ }$ for a pole-on configuration) [7].

The gas on the northern and southern sides of L483 are blue-shifted and red-shifted, respectively, according to the CS ($J=2-1$, $7-6$) and HCN ($J=4-3$) observations [14, 35]. This velocity gradient is perpendicular to the outflow axis, and thus, it suggests a rotating motion of the envelope gas. The position angle of the pseudo-disk of L483 is reported to be 36$^{\circ }$ based on Spitzer 4.5 $\upmu$m observation [4].

In the previous studies described above, the disk/envelope system of L483 was not well spatially resolved. The chemical composition of the gas in the closest vicinity of the protostar is still highly unknown. In this study, the physical and chemical structures around the protostar are investigated at a 100 au scale with ALMA.

8.2 Observation

L483 was observed with ALMA during its Cycle 2 operation on 12 June 2014. Band 6 receiver was employed to observe the spectral lines of CCH, CS, SO, HNCO, t-HCOOH, CH$_3$CHO, NH$_2$CHO, HCOOCH$_3$, (CH$_3$)$_2$O, and SiO in the frequency range from 244 to 246 GHz. Their line properties are summarized in Table 8.1. Thirty-four antennas were used during the observation with the baseline length ranging from 18.5 to 644 m. The field center of the observation was ($\alpha _{2000}$, $\delta _{2000}$) = ($18^\textrm{h} 17^\textrm{m} 29.\!\!^{\mathrm s}910$, −04$^{\circ } 39^\prime 39.\!\!^{\prime \prime }60$), and the primary beam size (FWHM) is 23$.\!\!^{\prime \prime }$03. The total on-source time was 25.85 min. A typical system temperature was from 60 to 100 K. A backend correlator was tuned to a resolution of 61.030 kHz, corresponding to the velocity resolution of 0.073 km s$^{-1}$ at 250 GHz, and a bandwidth of each chunk of 58.5892 MHz. The bandpass calibrations were carried out with J1733−1304, as well the phase calibrations for every 7 min. Titan was observed to derive the absolute flux density scale. The data calibration was performed in the antenna-based manner, where the uncertainties are expected to be less than 9%.

Table 8.1 Parameters of the Observed Lines$^\mathrm{{a}}$

The continuum and line images were obtained with the CLEAN algorithm. The Brigg’s weighting with the robustness parameter of 0.5 was employed. Self-calibration was carried out for the phase and amplitude by using the continuum data and was applied to the continuum and line data. A continuum image was prepared by averaging line-free channels, and the line maps were obtained after subtracting the continuum component directly from the visibilities. Synthesized-beam sizes for the spectral lines are summarized in Table 8.1. An rms noise level for the continuum is 0.13 mJy beam$^{-1}$. Those for spectral line images are obtained in nearby line-free channels with the channel width of 61.030 kHz; 8.2, 7.6, 6.1, 6.5, 4.4, 5.8, and 3.5 mJy beam$^{-1}$ for the CCH, CS, SO, HNCO, NH$_2$CHO, HCOOCH$_3$, and SiO emissions, respectively.

8.3 Distribution

The 1.2 mm dust continuum map is shown in Fig. 8.1. Its distribution is fitted by a two-dimensional Gaussian profile. Its peak position is derived to be: ($\alpha _{2000}$, $\delta _{2000}$) = (18$^\textrm{h}$17$^\textrm{m}$29$.\!\!^{\mathrm s}$947, −04$^{\circ }$39$^\prime$39$.\!\!^{\prime \prime }$55). Its size deconvolved by the beam is derived to be $0.\!\!^{\prime \prime }23 \times 0.\!\!^{\prime \prime }16$ (P.A. 158$^{\circ }$), and thus, the continuum image is scarcely resolved in this observation. Its total flux is 28 mJy.

Various molecular lines are detected in L483: CCH, CS, SO, HNCO, NH$_2$CHO, HCOOCH$_3$, and SiO. This source is regarded as a WCCC candidate source [10, 11, 29, 31], as mentioned in Sect. 8.1. Hence, detections of complex organic molecules (COMs), such as NH$_2$CHO and HCOOCH$_3$, which are characteristic to hot corinos (e.g. [3, 31]), are notable. The integrated intensity maps of CCH, CS, SO, HNCO, NH$_2$CHO, HCOOCH$_3$, and SiO are shown in Figs. 8.2, 8.3 and 8.4.

Fig. 8.1

1.2 mm dust continuum map. Contour levels are 3, 5, 10, 20, 40, 80, and 160$\sigma$, where the rms noise level is 0.13 mJy beam$^{-1}$. The beam ($0.\!\!^{\prime \prime }46 \times 0.\!\!^{\prime \prime }42$; P.A. 11.$^{\circ }$76) is denoted in the bottom right corner

Fig. 8.2

Integrated intensity maps of the CCH ($N=3-2, J=7/2-5/2, F=4-3$ and $3-2$; a) and CS ($J=5-4$; b, d) lines, and the velocity map of the CS line (c). Black contours represent the 1.2 mm dust continuum map. Contour levels are 10, 20, 40, 80, and 160$\sigma$, where the rms noise level is 0.13 mJy beam$^{-1}$. The velocity-shift range for the integration is ±8 km s$^{-1}$ for panels (a) and (b) with respect to the systemic velocity (5.5 km s$^{-1}$ [10]). The CCH emission has the contribution from the two hyperfine components (see Table 8.1). Panel (d) shows the low velocity-shift components of the CS line with an outer taper of 1$^{\prime \prime }$, where the velocity-shift range for the integration is from 0 to $+2.5$ km s$^{-1}$ (red contours) and from $-2.5$ to 0 km s$^{-1}$ (blue contours). Contour levels for the CS emissions are every 5$\sigma$ from 3$\sigma$, where the rms noise level is 20 mJy beam$^{-1}$ km s$^{-1}$. The red arrow with a P.A. of 105$^{\circ }$ in panel (b) represents the outflow axis. The red arrow with a P.A. of 15$^{\circ }$ represents the position axis along which the PV diagrams in Fig. 8.8b, c are prepared. It is perpendicular to the outflow axis and centered at the position with an offset of 4$^{\prime \prime }$ to the southeast from the continuum peak along the outflow axis (P.A. 105$^{\circ }$)

Fig. 8.3

Integrated intensity maps of the SO ($J_N=6_7-5_6$; a), HNCO ($12_{0, 12}-11_{0, 11}$; b), NH$_2$CHO ($12_{0, 12}-11_{0, 11}$; c), and HCOOCH$_3$ ($20_{5, 16, 0}-19_{5, 15, 0}$; d) emissions. Contours represent the 1.2 mm continuum map, where the contour levels are the same as those in Fig. 8.2

Fig. 8.4

Integrated intensity map of the SiO ($J=6-5$) emission. White contours represent the 1.2 mm continuum map, where the contour levels are the same as those in Fig. 8.2

The CCH ($N=3-2, J=7/2-5/2, F=4-3$ and $3-2$) emission is extended over a 10$^{\prime \prime }$ scale (Fig. 8.2a). Thus, the outer taper of 1$^{\prime \prime }$ is applied to improve a signal-to-noise ratio of the image. The WCCC nature of L483 is confirmed by the existence of the carbon-chain molecule CCH around the protostar at a few 100s au scale. The CCH emission has an intensity dip near the protostellar position with a radius of $\sim$0$.\!\!^{\prime \prime }$5. This feature is similar to that found in the WCCC sources L1527 and IRAS 15398$-$3359 (Chap. 5; [28, 30]), which would originate from the gas-phase destruction and/or depletion onto dust grains of CCH. The hole of the intensity distribution seems to have a slight offset from the continuum peak position to the western side, implying asymmetry in the gas distribution in the vicinity of the protostar. This asymmetry may be related to inhomogeneities of the initial gas distribution. A part of the CCH emission seems to trace the outflow cavity walls extending along the southeast-northwest direction, as well as the envelope component. The direction of the outflow axis looks consistent with the previous reports (P.A. 95$^{\circ }$–105$^{\circ }$; e.g. [4, 23, 34]).

As well as CCH, the CS ($J=5-4$) emission traces the component extended over a 10$^{\prime \prime }$ (Fig. 8.2b). In addition, it shows a compact component concentrated to the continuum peak position. The deconvolved size of this compact component is derived to be $1.\!\!^{\prime \prime }26\times 0.\!\!^{\prime \prime }88$ by the two-dimensional Gaussian fitting; it is slightly more extended than the 1.2 mm dust continuum. This slightly extended component will be discussed in Sect. 8.4. Meanwhile, the distribution of the SO ($J_N=6_7-5_6$), HNCO ($12_{0, 12}-11_{0, 11}$), NH$_2$CHO ($12_{0, 12}-11_{0, 11}$), and HCOOCH$_3$ ($20_{5, 16, 0}-19_{5, 15, 0}$) emissions are highly concentrated to the continuum peak position (Fig. 8.3). Their deconvolved sizes are derived by the two-dimensional Gaussian fittings; $0.\!\!^{\prime \prime }56\times 0.\!\!^{\prime \prime }39$, and $0.\!\!^{\prime \prime }26\times 0.\!\!^{\prime \prime }16$ for SO and HNCO, respectively. The HNCO distribution is almost point-like. Similarly, the distributions of NH$_2$CHO and HCOOCH$_3$ are point-like with the angular resolution of $\sim$0$.\!\!^{\prime \prime }$5 ($\sim$100 au). In addition to these COM emissions, the t-HCOOH emission is tentatively detected concentrated near the protostellar position.

Fig. 8.5

Velocity (moment 1) maps of the SO ($J_N=6_7-5_6$; a) and HNCO ($12_{0, 12}-11_{0, 11}$; b) emissions. Black contours represent the 1.2 continuum map, where the contour levels are the same as those in Fig. 8.2. The black arrows in panel (a) represent the position axes of the PV diagrams in Figs. 8.7, 8.9, 8.11–8.14

Fig. 8.6

Schematic illustrations of the disk/envelope system in L483. In panel (a), the line of sight is perpendicular to the paper. The mid-plane of the disk/envelope is extended along the northeast-southwest direction with a P.A. of 15$^{\circ }$. Its western side faces the observer. Line emissions in the western side of the protostar are missing in the observation (Sect. 8.3). As shown in panel (b), the outflow blows nearly parallel to the plane of the sky so that both the red- and blue-shifted components are seen in each lobe. The PV diagrams in Fig. 8.8b, c are prepared for the positions offset from the protostellar position by 4$^{\prime \prime }$ as indicated by the ‘observer’

The SiO ($J=6-5$) emission has a different distribution from all the above molecular emissions. As shown in Fig. 8.4, the SiO distribution is slightly extended toward the northeastern direction from the continuum peak position. The extension is significant, considering the beam size of $\sim$0$.\!\!^{\prime \prime }$5 ($\sim$100 au). The deconvolved size of the SiO distribution is derived to be $0.\!\!^{\prime \prime }70\times 0.\!\!^{\prime \prime }43$ (P.A. 35$^{\circ }$) by the two-dimensional Gaussian fitting.

8.4 Kinematic Structure

8.4.1 Geometrical Configuration

This section focuses on the kinematic structure of the gas concentrated around the protostar. The velocity (moment 1) maps of the SO and HNCO emissions (Fig. 8.5) show velocity gradients perpendicular to the outflow axis (P.A. 95$^{\circ }$–105$^{\circ }$; e.g. [4, 23, 34]), which strongly suggests rotating motion of the gas; the gas on the northern side of the continuum peak position is blue-shifted, while that on the southern side is red-shifted. The direction of this velocity gradient is qualitatively consistent with that at a 20$^{\prime \prime }$ scale reported by [35] based on the CS ($J=7-6$) and HCN ($J=4-3$) observations. In this study, the position axis (P.A.) of 105$^{\circ }$ is employed as the direction of the outflow axis according to the report by [4], and the mid-plane of the disk/envelope system is assumed to extend along the P.A. of 15$^{\circ }$. The eastern and western lobes of the bipolar outflow are reported to be red- and blue-shifted, respectively (e.g. [7, 9, 14, 17, 23, 34, 35]), and thus, the northwestern side of the disk/envelope system is thought to face the observer. This geometrical configuration is schematically illustrated in Fig. 8.6. The following subsections describe the kinematic structures traced by the observed molecular line emissions, except for CCH because of the heavy blending of its hyperfine components and poor S/N ratio.

Fig. 8.7

Position-velocity diagrams of the CS ($J=5-4$) line. The position axes are indicated by the arrows in Fig. 8.5a, which are taken along the disk/envelope direction (P.A. 15$^{\circ }$; a, c) and perpendicular to it (P.A. 105$^{\circ }$; b, d). Panels (c) and (d) are the blow-ups of the central parts of panels (a) and (b). Contour levels are every 5$\sigma$, where the rms noise level is 7.6 mJy beam$^{-1}$. White dashed lines represent the systemic velocity (5.5 km s$^{-1}$). The rectangle in the top-right or top-left corner of each panel represents the spatial and velocity resolutions

8.4.2 CS

The position-velocity (PV) diagrams of the CS ($J=5-4$) line are shown in Fig. 8.7. Figure 8.7a, c are prepared along the mid-plane of the disk/envelope system (‘the disk/envelope direction’; P.A. 15$^{\circ }$). Spin-up features to the protostar are seen along the disk/envelope direction on an 8$^{\prime \prime }$ scale; the gas is red- and blue-shifted on the southwestern and northeastern side, respectively. Meanwhile, the counter velocity component is weakly seen, although the blue-shifted component on the southwestern side is marginal. This feature is specific to the infalling-rotating motion, as demonstrated in the previous chapters (e.g. Chaps. 3 and 4). Hence, CS likely exists in the infalling-rotating envelope. In addition, high velocity-shift components are detected concentrated to the protostar, whose maximum velocity-shift is as high as about 6 km s$^{-1}$.

Fig. 8.8

Position-velocity diagrams of the CS ($J=5-4$; a, b) and CCH ($N=3-2, J=7/2-5/2, F=4-3$ and $3-2$; c) lines. The position axes are indicated by the arrows in Fig. 8.2b. Panel (a) is a part of Fig. 8.7b, which is prepared along the outflow axis (P.A. 105$^{\circ }$). Panels (b) and (c) show the kinematic structure of the southeastern outflow lobe, which is prepared along the line across the outflow axis (P.A. 15$^{\circ }$). Their position axes are centered at the distance of 4$^{\prime \prime }$ from the protostellar position toward the southeastern direction. White lines represent the results of the parabolic outflow model. The parameters for the model are as follows; i = 80$^{\circ }$, C = $2.5 \times 10^{-3}$ au$^{-1}$, and $v_0$ = $1.5 \times 10^{-3}$ km s$^{-1}$. In panel (c), the two hyperfine components of CCH are detected with the velocity separation of 2.54 km s$^{-1}$, and the elliptic feature appears twice. Thus, the outflow model is prepared for each hyperfine component

An extended component over 15$^{\prime \prime }$ is seen along the line perpendicular to the disk/envelope direction (P.A. 105$^{\circ }$; Fig. 8.7b). Although this component looks complicated, it likely traces a part of the outflow cavity. The PV diagrams of the outflow are shown in Fig. 8.8. Figure 8.8a is prepared along the outflow axis, which is a part of Fig. 8.7b. Figure 8.8b shows the PV diagram of the southeastern outflow lobe prepared along the line across the outflow axis indicated by the arrow in Fig. 8.2b (P.A.15$^{\circ }$). In Fig. 8.8a, it is confirmed that the northwestern and southeastern lobes are blue- and red-shifted, respectively. Figure 8.8a also shows that the velocity of the outflow component accelerates as the distance from the protostar.

The elliptic feature of the PV diagram characteristic to the outflow cavity wall is clearly seen in Fig. 8.8b. Similar features are confirmed in the two hyperfine components of CCH (Fig. 8.8c). This elliptic feature has both the red- and blue-shifted velocities, where its central velocity is slightly red-shifted. This kinematic structure suggests the configuration of the outflow illustrated in Fig. 8.6b. It is quite similar to those observed in the nearly edge-on outflow system of IRAS 15398$-$3359 (Chap. 5), where both the red-shifted and blue-shifted components can be seen in each outflow lobe. Therefore, it is most likely that the outflow of L483 blows nearly perpendicular to the line of sight at least in the vicinity of the protostar in contrast to the previous reports (e.g. [7]). Hence, the disk/envelope system likely has a nearly edge-on geometry. The more detailed structure of the outflow is described in Sect. 8.6.1.

Assuming the configuration illustrated in Fig. 8.6a for the disk/envelope system, the northwestern side of the disk/envelope system will face the observer. If there is an infall motion in the envelope component, the southeastern and northwestern sides of the protostar would be red-shifted and blue-shifted, respectively. However, this feature is not clearly recognized in the PV diagram along the P.A. of 105$^{\circ }$ (Fig. 8.7d). This can be attributed to overwhelming contributions from the outflow component and the missing of the blue-shifted envelope component. The velocity gradient due to the infall motion will be verified for the low velocity-shift component ($\left| v_\textrm{shift} \right| < 2$ km s$^{-1}$) with the aid of the kinematic model in Sect. 8.5. On the other hand, the high velocity-shift components ($\left| v_\textrm{shift} \right| > 2$ km s$^{-1}$) do not show any velocity gradient along the P.A. of 105$^{\circ }$ around the protostellar position.

8.4.3 SO and HNCO

Figure 8.9 shows the PV diagrams of the SO ($J_N=6_7-5_6$) and HNCO ($12_{0, 12}-11_{0, 11}$) lines. The SO emission shows a slight velocity gradient along the disk/envelope direction (Fig. 8.9a). This feature is scarcely recognized in the HNCO emission (Fig. 8.9c). The velocity gradient in the SO emission is consistent with that found in the CS emission, and therefore, it likely originates from the rotating motion in the disk/envelope system. Meanwhile, no definitive velocity gradient is seen along the line perpendicular to the disk/envelope direction either in the SO and HNCO emissions (Fig. 8.9b, d). Hence, these emissions do not reveal a significant infall motion in the vicinity of the protostar.

Fig. 8.9

Position-velocity diagrams of the SO ($J_N=6_7-5_6$; a, b) and the HNCO ($12_{0, 12}-11_{0, 11}$; c, d) lines. The position axes are as the same as those in Fig. 8.7c, d. Contour levels are every 5 and 3$\sigma$, where the rms noise levels are 6.1 and 6.5 mJy beam$^{-1}$, for SO and HNCO, respectively

All the SO, HNCO, and CS emission have high velocity-shift components near the protostellar position in their PV diagrams (Figs. 8.7−8.9). These components are confirmed in the line profiles of these molecular lines prepared in a circular region with a diameter of 0$.\!\!^{\prime \prime }$5 centered at the continuum peak (Fig. 8.10). The SO emission has a line profile similar to the CS emission, except for the self-absorption feature in the CS emission. Their broad line-widths are recognized as the high velocity-shift components concentrated to the protostar, which are found in the PV diagrams.

The HNCO emission has a component with a velocity-shift larger than 5 km s$^{-1}$; the red-shifted component is brighter than the blue-shifted component. This feature can be confirmed in both its PV diagrams (Figs. 8.9c, d) and line profile (Fig. 8.10). It implies that the HNCO distribution is asymmetric in the vicinity of the protostar. Alternatively, the blue-shifted emission from the back side of the protostar may be attenuated by dust in the closest vicinity of the protostar. This effect is discussed for the similar feature found in the Class I low-mass protostellar source Elias 29 [21].

Fig. 8.10

Spectral line profiles of the CS ($J=5-4$), SO ($J_N=6_7-5_6$), HNCO ($12_{0, 12}-11_{0, 11}$), NH$_2$CHO ($12_{0, 12}-11_{0, 11}$), HCOOCH$_3$ ($20_{5, 16, 0}-19_{5, 15, 0}$), and t-HCOOH ($12_{0, 12}-11_{0, 11}$) lines toward the protostellar position. The line intensities are averaged in a circular region with a diameter of 0$.\!\!^{\prime \prime }$5 centered at the continuum peak position. The spectra are smoothed to improve the signal-to-noise ratio so that the velocity resolution is changed to be 0.5 km s$^{-1}$ for the CS, SO, HNCO, and NH$_2$CHO lines, and 1 km s$^{-1}$ for the HCOOCH$_3$ and t-HCOOH lines

Table 8.2 Physical Parameters of L483

8.4.4 NH$_2$CHO and HCOOCH$_3$

It is notable that the COM emissions, NH$_2$CHO and HCOOCH$_3$, are successfully detected in L483. Their distributions are concentrated to the protostar, as shown in their integrated intensity maps (Figs. 8.3c, d). Their spectral line profiles toward the protostellar position are dominated by a red-shifted component (Fig. 8.10), as well as the profile of the HNCO line. The consistency between the profiles of these molecular lines supports their secure detections rather than a possibility that they correspond to other molecular lines. As described in Sect. 8.4.3, the line profiles biased to the red-shifted components imply asymmetric distributions of these molecules in the vicinity of the protostar and/or the attenuation of the blue-shifted components by dust.

8.5 Analysis with the Models for the Disk/Envelope System

The molecular distributions described above show chemical differentiation in L483. To understand it in terms of the physical structure around the protostar, the observed kinematic structure of the disk/envelope system is analyzed in this section. In previous chapters, the kinematic structures of the infalling-rotating envelopes are successfully explained by a simple ballistic model in several sources. Hence, the same model is applied to the CS emission observed in L483. However, it is found that an infalling-rotating envelope model cannot reproduce all the observed velocity structure. Two physical components need to be considered as described below; the infalling-rotating envelope and the centrally concentrated component.

Fig. 8.11

Position-velocity diagrams of the CS ($J=5-4$) line and the fiducial models. Color maps are the same as those in Fig. 8.7c, d. Black contours in panels (a) and (b) represent the results of the infalling-rotating envelope model. The parameters of the model are as follows: M is 0.15 $M_\odot$, $r_\textrm{CB}$ is 100 au, and i is 80$^{\circ }$. Blue contours in panels (c) and (d) represent the results of the Keplerian disk model. The above M and i values are employed, and the emitting region is assumed to be inside the centrifugal barrier. Black contours in panels (e) and (f) represent the results combining the infalling-rotating envelope model and the Keplerian disk model. The intrinsic line width of 0.2 km s$^{-1}$ is employed for all the models, and the model images are convolved with the Gaussian beam for the CS line in the observation (Table 8.1). Contour levels are every 20% from 5% (a−d) or 3% (e, f) of the peak intensity in each panel

The infalling-rotating envelope model described in Chap. 3 is employed; a flat envelope with a constant thickness (30 au) is assumed. The inclination angle (i) of 80$^{\circ }$ is employed (Table 8.2), which is obtained based on the analysis of the outflow structure (Sect. 8.6). It is unfortunate that the key parameters for the model are loosely constrained based on these observations because of the contamination of the overwhelming centrally-concentrated component. Hence, models with various parameters are calculated to find the reasonable set of the parameters by eye. An example of the infalling-rotating envelope model is shown in Fig. 8.11a, b, which reproduces the observed PV diagrams as much as possible except for the centrally-concentrated high velocity-shift components. The parameters of this model are as follows: the protostellar mass (M) is 0.15 $M_\odot$ and the radius of the centrifugal barrier ($r_\textrm{CB}$) is 100 au. Then, the specific angular momentum ($j_\textrm{IRE}$) of the gas is calculated to be $7.9^{+4}_{-3} \times 10^{-4}$ km s$^{-1}$ pc. These values are summarized in Table 8.2. With the aid of the model result in Fig. 8.11b, the infall motion of the gas is marginally recognized. Its blue-shifted part is missing in the observation probably due to the asymmetric gas distribution described above (Fig. 8.6). A possible effect of the attenuation of blue-shifted components by dust is expected to be modester for positions slightly shifted from the protostar than toward the protostellar position.

The dependency of the infalling-rotating envelope model result on the protostellar mass (M) and the radius of the centrifugal barrier ($r_\textrm{CB}$) are examined by calculating the models with various sets of these two parameters. The results are shown in Figs. 8.12 and 8.13. The models with M of 0.05 $M_\odot$ or 0.5 $M_\odot$ do not reproduce the observed PV diagrams with any values of $r_\textrm{CB}$. In the model without any rotating motion, i.e. $r_\textrm{CB}$ is 0 au, no velocity gradient appears along the disk/envelope direction (Fig. 8.12). This contradicts with the observed feature. Meanwhile, the velocity shift of the counter-velocity component due to the infall motion is underestimated with $r_\textrm{CB}$ of 300 au. Based on the above results, a reasonable agreement between the observation and the model is obtained for the range of M from 0.1 $M_\odot$ to 0.2 $M_\odot$ and $r_\textrm{CB}$ from 30 au to 200 au, except for the centrally-concentrated high velocity-shift components in the observation. Hence, the infalling-rotating envelope model with M of 0.15 $M_\odot$ and $r_\textrm{CB}$ of 100 au is employed as the fiducial one, which is shown in Fig. 8.11. To obtain more stringent constraints on the parameters, further detailed analysis with a higher angular-resolution observation is required.

The infalling-rotating envelope model cannot reproduce both the high velocity-shift components concentrated to the protostar and the extended low velocity-shift components simultaneously; if the parameters are set to explain the high velocity-shift components, the infall motion in the model is calculated to be much larger than the observed emission. This justifies the two-component model consisting of the infalling-rotating envelope and the centrally concentrated component. Although the observed red-shifted emission shows some excess on the southwestern side to the protostar (Fig. 8.11a), the fiducial model roughly reproduce the infalling-rotating envelope component in the PV diagrams.

Fig. 8.12

Comparison of the position-velocity diagrams of the observed CS ($J=5-4$) line and the infalling-rotating envelope model. Color maps are the same as that in Fig. 8.7c, where the position axis is along the disk/envelope direction. Black contours represent the results of the infalling-rotating envelope model with the following parameters: M is 0.05, 0.15, and 0.5 $M_\odot$, $r_\textrm{CB}$ is 0, 100, and 300 au, and i is fixed to be 80$^{\circ }$. Blue contours in panels represent the results of the Keplerian disk model with the same parameters as those for the infalling-rotating envelope model in each panel. In the Keplerian disk model, the emission is simply assumed to come from the inside of the centrifugal barrier. Then, the Keplerian disk model is not prepared for panels (a), (d), and (g), where $r_\textrm{CB}$ is 0 au. Contour levels are every 20% from 5% of the peak intensity of each model. Dashed contours around the central position in panels (a), (b), (d), (f), (h), and (i) represent the dip in the modeled intensity

Fig. 8.13

Comparison of the position-velocity diagrams of the observed CS ($J=5-4$) line and the infalling-rotating envelope model. Color maps are the same as that in Fig. 8.7d, where the position axis is perpendicular to the disk/envelope direction. Black and blue contours represent the results of the infalling-rotating envelope model and the Keplerian disk model. Their details are described in the caption of Fig. 8.12

Then, the high velocity-shift components concentrated to the protostar are analyzed in a separated way. The Keplerian disk component is the most likely candidate for them, although the rotation signature is not spatially resolved. The results of the Keplerian disk model are superposed on the PV diagrams of the CS and SO lines in Figs. 8.11 and 8.14, repsectively. M of 0.15 $M_\odot$ and i of 80$^{\circ }$ are employed in this model as the fiducial values based on the analyses for the envelope structure described above and for the outflow structure in Sect. 8.6. The observed emission has the maximum velocity-shift as high as 6 km s$^{-1}$, which corresponds to the emitting region of the radius as small as 4 au in the Keplerian disk model. Moreover, the results combining the infalling-rotating envelope model and the Keplerian disk model are shown in Fig. 8.11e, f. The results of the Keplerian disk model with various parameter values are also shown in Figs. 8.12 and 8.13 for reference.

In the infalling-rotating envelope model and the Keplerian disk model, the emissivity is assumed to be proportional to $r^{-1.5}$ for simplicity, where r denotes the distance from the protostar. Then, the effects of the optical depth, excitation, and the temperature gradient are not considered, as described in Chap. 3. The optically thin condition can roughly be justified for the CS emission in L483 as discussed below. The observed intensity of the CS line is less than 0.2 Jy beam$^{-1}$ at the distance of 100 au ($r_\textrm{CB}$) or further from the protostar, which corresponds to the brightness temperature less than 17 K. Meanwhile, the desorption temperature of CS is evaluated to be 35 K (see Appendix A; [37]) by using its binding energy of 1900 K (UMIST Database for Astrochemistry [19]; http://udfa.ajmarkwick.net/index.php), and thus, the gas kinetic temperature is expected to be similar to or higher than this value. Therefore, the observed brightness temperature of the CS emission is expected to be lower than the gas kinetic temperature by a factor of 2 or more. That is, the CS emission can be assumed to be optically thin in the envelope component outside the centrifugal barrier. The peak intensity of the CS line is about 0.35 Jy beam$^{-1}$ inside the centrifugal barrier, corresponding to the brightness temperature of 30 K. The gas temperature (T) is roughly estimated to be 75 K at the distance (r) of 100 au from the protostar with the luminosity (L) of 13$L_\odot$ [32], by using the following equation: ${\displaystyle \frac{L}{4 \pi r^2} = \sigma T^4}$, where $\sigma$ denotes the Stefan-Boltzmann constant. Thus, the peak CS intensity is expected to be lower than the gas kinetic temperature. Therefore, the assumption of the optically thin condition for the CS emission seems reasonable. Nevertheless, the excitation effect and the temperature gradient may affect the intensity distribution in reality. These effects may have a larger contribution at positions nearer to the protostar, where the gas has a higher velocity-shift. To avoid seriously being affected by these effects, the above analyses based on the models are performed by mainly focusing on the velocity structures rather than the intensity profiles.

Fig. 8.14

Position-velocity diagrams of the SO ($J_N=6_7-5_6$) line and the Keplerian disk model. Color maps are the same as those in Fig. 8.9a, b. The model is the same as that in Fig. 8.11c, d, other than the emission is convolved with the Gaussian beam for the SO line in the observation (Table 8.1). Contour levels are every 20% from 3% of the peak intensity in each panel

The kinematic structure around the protostar traced by the CS emission in L483 is similar to that traced by the H$_2$CS line in IRAS 16293$-$2422 Source A (Chap. 6), which is recognized as a combination of the infalling-rotating envelope component and the possible Keplerian disk component inside the centrifugal barrier. Hence, the CS distribution in L483 is found to be different from those in L1527 and TMC−1A (Chap. 4; [27]), where the CS emission comes only from the infalling-rotating envelope. The distribution of CS in L483 seems to resemble to that in IRAS 16293$-$2422 Source A [22]; in IRAS 16293$-$2422 Source A, the C$^{34}$S ($J=2-1$) emission was detected by ALMA in both the circummultiple structure with the infalling-rotating motion and the circumstellar disk with the Keplerian rotation, although the latter component was not detected in the C$^{34}$S ($J=7-6$) emission with the Submillimeter Array possibly due to the limitation of the angular resolution and the sensitivity [6]. The difference of the behavior of the CS lines would originate from the difference of the bolometric luminosity of the above sources; L483 (13 $L_\odot$ [32]) and IRAS 16293$-$2422 Source A (22 $L_\odot$ [5]) has higher luminosities than L1527 (1.7 $L_\odot$ [8]) and TMC−1A (2.5 $L_\odot$ [8]). The higher bolometric luminosity would cause the higher temperature of the disk component inside the centrifugal barrier, which prevents the CS depletion in this region. As described above, the desorption temperature of CS is evaluated to be about 35 K (Appendix A). This value is just above the temperature of the disk mid-plane just inside the centrifugal barrier in L1527 (30 K [28]). On the contrary, CS is possibly prevented from freezing out in L483 if the gas and dust temperatures inside the centrifugal barrier is higher than the desorption temperature of CS due to the high bolometric luminosity of L483.

A rotation feature is clearly revealed in the PV diagram of the SO emission (Fig. 8.9a). A similar feature was reported for L1527 [30], where the SO emission mainly highlights the centrifugal barrier. With respect to the SO line observed in L1527, that in L483 shows much brighter emission with the high velocity-shift concentrated toward the protostar. Thus, the rotating motion traced by the SO emission in L483 is expected to mainly come from the disk component inside the centrifugal barrier (Fig. 8.14). The relatively high bolometric luminosity of L483 could contribute to prevent SO from depletion onto dust grains in the disk component, as mentioned above.

The disk radius can be estimated by assuming that the SO emission appears inside the centrifugal barrier. The distribution of the SO emission has the beam-deconvolved size (FWHM) of 0$.\!\!^{\prime \prime }$5 (100 au) along the disk/envelope direction (P.A. 15$^{\circ }$), and thus, the radius of the centrifugal barrier is estimated to be about 50 au. Since the size of the distribution is underestimated due to the strong emission concentrated to the protostar, it can be regarded as the lower limit. Meanwhile, the 5$\sigma$ contour in the PV diagram of the SO line is extended over a 2$.\!\!^{\prime \prime }$4 (480 au) area along the disk/envelope direction (Fig. 8.9a), whose radius is obtained to be about 1$^{\prime \prime }$ (200 au) by deconvolving from the beam size. Considering that the SO emission may trace the infalling-rotating envelope component just outside the centrifugal barrier [26], this radius is regarded as the upper limit for the radius of the centrifugal barrier. The above estimations will give rough estimates for the radius of the centrifugal barrier in L483, although it is difficult to be derived directly from the SO emission due to the contamination by the disk component. These values are consistent with the estimate based on the analysis of the PV diagrams of the CS emission.

8.6 Outflow Structure

8.6.1 Outflow Cavity Wall Traced by CS

The disk/envelope system of L483 is assumed to have a nearly edge-on configuration with the inclination angle (i) of 80$^{\circ }$ (0$^{\circ }$ for a face-on configuration) in the analysis of the infalling-rotating envelope described above. This value is derived from the kinematic structure of the outflow traced by the CS emission as described in this section.

The PV diagram of the CS emission (Fig. 8.8a) shows that the outflow component is accelerated as leaving from the protostar along the outflow axis. An elliptic feature is evident across the outflow axis in Fig. 8.8b. Since an extended component is often resolved out in interferometric observations, the elliptic feature likely corresponds to the outflow cavity wall. These features are quite similar to those found in a low-mass Class 0 source IRAS 15398$-$3359 (Chap. 5), where the kinematic structure of the outflow cavity wall was well explained by a parabolic outflow model [16]. Thus, the outflow structure in L483 is analyzed with this outflow model, whose details are described in Chap. 3.

8.6.2 Comparison with the Outflow Model

The results of the parabolic outflow model is shown by white lines in Fig. 8.8. This fiducial model reasonably explains the observed kinematic structure. In this model, the outflow cavity wall has a parabolic feature, and the velocity of the gas on the cavity wall is proportional to the distance from the protostar. The parameters for the fiducial model are as follows; i is 80$^{\circ }$, C is $2.5 \times 10^{-3}$ au$^{-1}$, and $v_0$ is $1.5 \times 10^{-3}$ km s$^{-1}$. They are summarized in Table 8.2. This model is also superposed on the velocity map of the CS emission (Fig. 8.2c).

The observed kinematic structure cannot be reproduced by the outflow model with the inclination angle less than 75$^{\circ }$ or larger than 90$^{\circ }$. Thus, the outflow is confirmed to blow almost parallel to the plane of the sky based on its kinematic structure. This is contrast to the previous report that L483 has the inclination angle of 50$^{\circ }$ [7], which was evaluated from the asymmetric brightness of the two outflow lobes in the near-infrared and submillimeter observations. This discrepancy in the inclination angle would come from the large uncertainty of the previous estimation. Thus, the inclination angle of 80$^{\circ }$ is employed as the fiducial value in this chapter.

Figures 8.15 and 8.16 show the PV diagrams of the CS ($J=5-4$) emission. They are prepared along the lines perpendicular to the outflow axis. The origin of each position axis is taken on the outflow axis with an offset from 0$^{\prime \prime }$ to 10$^{\prime \prime }$ from the protostellar position along the southeast or northwest direction in Figs. 8.15 and 8.16, respectively. Although the diagrams are heavily contaminated by the emission from the disk/envelope system in the panels with the offsets from 0$^{\prime \prime }$ to 1$^{\prime \prime }$ from the protostellar position, the elliptic feature characteristic to the outflow cavity wall is confirmed in the panels with larger offsets as well as that with an offset of 4$^{\prime \prime }$ (Fig. 8.8b). The radial size of the elliptic feature seems to increase as leaving from the protostellar position. This indicates the expansion of the outflow cavity wall. The velocity centroid of the elliptic feature is slightly red- and blue-shifted in the southeastern and northwestern outflow lobe, respectively. This is consistent with the configuration of L483 illustrated in Fig. 8.6. The PV diagrams of the two outflow lobes with the same distance from the protostellar position are not always similar to each other; for instance, the radial size of the elliptic feature at the distance of 8$^{\prime \prime }$ is more compact in the southeastern lobe (Fig. 8.15) than in the northwestern lobe (Fig. 8.16). This asymmetry of the outflow structure would be caused by an asymmetry in the ambient environment.

Fig. 8.15

Position-velocity diagrams of the CS ($J=5-4$) emission across the southeastern outflow lobe. The position axes are prepared along the lines with the P.A. of 15$^{\circ }$, which are perpendicular to the outflow axis (P.A. 105$^{\circ }$). Their origins are on the outflow axis with an offset of the distance from 0$^{\prime \prime }$ to 10$^{\prime \prime }$ from the protostellar postion toward the southeastern direction. The panel labeled as ‘SE offset 4$^{\prime \prime }$’ is the same as the color map in Fig. 8.8b. The contour levels are every 5$\sigma$, where the rms noise level is 7.6 mJy beam$^{-1}$

Fig. 8.16

Same as Fig. 8.15 for the northwestern outflow lobe

Fig. 8.17

Position-velocity diagrams of the CS ($J=5-4$) emission across the southeastern outflow lobe. The color maps and black contours are the same as those in Fig. 8.15. Elliptic lines in each panel represent the results of the outflow model, where the parameters are as follows; i = 80$^{\circ }$, C = $2.5 \times 10^{-3}$ au$^{-1}$, and $v_0$ = $1.5 \times 10^{-3}$ km s$^{-1}$. The specific angular momentum of the outflow model is 0, $j_\textrm{IRE}$ $\times 1$, $j_\textrm{IRE}$ $\times 2$, and $j_\textrm{IRE}$ $\times 4$ for the white, blue, green, and red lines, respectively, where the specific angular momentum of the infalling-rotating envelope ($j_\textrm{IRE}$) is $7.9 \times 10^{-4}$ km s$^{-1}$ pc

Fig. 8.18

Same as Fig. 8.17 for the northwestern outflow lobe

The result of the fiducial outflow model is superposed on the observed PV diagrams in white lines in Figs. 8.17 and 8.18. The outflow model reasonably explains the observed kinematic structure. In the southeastern outflow lobe, there is some excess of red-shifted velocity component in the panels with an offset less than 4$^{\prime \prime }$5. This possibly comes from a local shock on the outflow cavity wall. Such a feature is also detected in the outflow of IRAS 15398$-$3359 (Chap. 5).

The dynamical time scale ($t_\textrm{dyn}$) of the outflow of L483 is evaluated to be $(3 \pm 1) \times 10^3$ years by using the fiducial value of $v_0$ ($1.5 \times 10^{-3}$ km s$^{-1}$). [9] previously reported model calculations with $t_\textrm{dyn}$ of $(2-6) \times 10^3$ years for the CO ($J=4-3, 2-1$) observations, and [38] reported it to be $(4.4-6.2) \times 10^3$ years based on the CO ($J=6-5, 3-2$) observations. Meanwhile, [7] evaluated $13 \times 10^3$ years based on the CS ($J=3-2$) observations by assuming the inclination angle of 50$^{\circ }$. It is recalculated to be $2 \times 10^3$ years for the inclination angle of 80$^{\circ }$ derived from the model analysis described above. Thus, these estimations for $t_\textrm{dyn}$ are almost consistent with one another, and a few $10^3$s years is the most plausible value. It is notable that the $t_\textrm{dyn}$ derived based on the kinematic structure observed on a $10^3$ au scale in this study is consistent with the previous reports based on the observations on larger scales (e.g. $10^4$ au [38]).

8.6.3 Rotation Motion in the Outflow

Outflow launching is thought to be a potential mechanism to extract the angular momentum of the envelope gas, for instance at its centrifugal barrier. If this is the case, outflows are expected to have a rotating motion. There is indeed a hint that the outflow structure of L483 has a velocity gradient perpendicular to the outflow axis (Fig. 8.2c, d). Thus, a rotating motion on the outflow cavity wall is examined in this section with the aid of the outflow model.

The blue, green, and red lines in Figs. 8.17 and 8.18 represent the results of the outflow model with some rotation motion. In these models, the rotation velocity on the outflow cavity wall is simply calculated by assuming the angular momentum conservation. The specific angular momentum of the outflowing gas ($j_\textrm{out}$) is set to be 1 time (blue), 2 times (green), and 4 times (red) that of the infalling-rotating envelope ($j_\textrm{IRE}$; Table 8.2). The difference between the four outflow models are more evident in panels with a smaller offset from the protostellar position. The model results with rotation motion show a slant distortion in the elliptic features in the PV diagrams, which is more significant with a larger $j_\textrm{out}$. This is because the rotation motion is more prominent for a smaller radial size under the angular momentum conservation. The models with $j_\textrm{out}$ of 1 and 2 times $j_\textrm{IRE}$ would better reproduce the observations than the model without the rotation motion in the panels with an offset from 1$^{\prime \prime }$ to 4$^{\prime \prime }$ from the protostellar position. The rotation motion tends to be overestimated in the model with $j_\textrm{out}$ of 4 times $j_\textrm{IRE}$, and thus, this value can be recognized as the upper limit for $j_\textrm{out}$. Hence, the specific angular momentum of the gas on the outflow cavity wall is likely comparable to or slightly larger than that of the infalling-rotating envelope. This implies that the outflow may extract the angular momentum of the gas from the infalling-rotating envelope in L483 (see Chap. 10). Evaluation of the gas mass of these structures are required to assess the contribution of the outflow in the redistribution of the angular momentum more precisely.

8.6.4 SiO Emission

The SiO ($J=6-5$) emission shows a distribution extended toward the northeastern direction from the protostellar position, as mentioned in Sect. 8.3 (Fig. 8.4). This elongation has the position angle of about 35$^{\circ }$. The velocity channel maps of the SiO emission is shown in Fig. 8.19. While the red-shifted component weakly appears at the protostellar position, the blue-shifted component ($v_\textrm{LSR}$ $< 3.8$ km s$^{-1}$) is detected at an offset of about 0$.\!\!^{\prime \prime }$5 (100 au). The position of the SiO blue-shifted emission is close to the expected position of the centrifugal barrier ($r_\textrm{CB}$ of 100 au) or inside it. At this position, the velocity shift in the SiO emission is larger than that in the CS emission. Thus, the SiO emission likely comes from a possible shock caused by the outflow rather than a part of the infalling-rotating envelope. The existence of such a shocked gas near the centrifugal barrier is puzzling. It might be related to the launching mechanism of the outflow, although the association of the shocked gas with the centrifugal barrier has to be explored at a higher angular-resolution.

Fig. 8.19

Velocity channel maps of the SiO ($J=6-5$) emission. Contours represent the 1.2 mm continuum map, where the contour levels are the same as those in Fig. 8.2. Each map represents the averaged intensity with the velocity width of 1.4 km s$^{-1}$. The averaged velocity for each panel is denoted in its top left corner in a unit of km s$^{-1}$

8.7 Chemical Composition

L483 is proposed to have the WCCC character on the basis of the single-dish observations [10, 11, 29, 31]. It is indeed confirmed by the detection of CCH on a 1000 au scale in Fig. 8.2. Meanwhile, emission of COMs characteristic to hot corino chemistry are also detected; NH$_2$CHO and HCOOCH$_3$ are firmly detected, and t-HCOOH is tentatively detected. This is the first spatially-resolved detection of COMs in a WCCC source, and thus L483 can be recognized as a ‘hybrid’ source. Although such a chemical characteristic source can be interpreted as an intermediate source in previous studies [31], the present observation confirms its definitive existence at a high angular resolution.

The column densities of NH$_2$CHO and HCOOCH$_3$ are evaluated as summarized in Table 8.3; they are the beam averaged values at the protostellar position. The local thermodynamic equilibrium (LTE) condition is assumed with the temperature of 70, 100, and 130 K, which are typical values in hot corinos (e.g. Chap. 6). The column densities of NH$_2$CHO and HCOOCH$_3$ are obtained to be $(1.5 \pm 0.7) \times 10^{14}$ cm$^{-2}$ and $(7 \pm 4) \times 10^{15}$ cm$^{-2}$, respectively, at 100 K. The results for NH$_2$CHO and HCOOCH$_3$ change by 30% and 14%, respectively, with the change in the assumed temperature by $\pm 30$ K. The column densities of t-HCOOH, CH$_3$CHO, and (CH$_3$)$_2$O are also obtained as summarized in Table 8.3, where the values for CH$_3$CHO and (CH$_3$)$_2$O are their upper limits.

The beam averaged column density of H$_2$ is derived to be $6.5 \times 10^{23}$ cm$^{-2}$ from the dust continuum emission by by using the following relation [36]:

$$\begin{aligned} N (\textrm{H}_2)&= \frac{2 \ln 2 \cdot c^2}{\pi h \kappa _\nu m} \times \frac{F(\nu )}{\nu ^3 \theta _\textrm{major} \theta _\textrm{minor}} \times \left( \exp \left( \frac{h\nu }{kT} \right) - 1\right) . \end{aligned}$$

(8.1)

$\kappa _\nu$ the mass absorption coefficient with respect to the gas mass, m the averaged mass of a particle in the gas ($3.83 \times 10^{-24}$ g), $\nu$ the frequency, $F(\nu )$ the peak flux, $\theta _\textrm{major}$ and $\theta _\textrm{minor}$ the major and minor beam size, respectively, c the speed of light, h the Planck’s constant, k the Boltzmann constant, and T the dust temperature. The dust temperature is assumed to be 100 K. $\kappa _\nu$ is evaluated to be 0.008 cm$^2$ g$^{-1}$ at 1.2 mm with $\beta$ = 1.8 [33] by assuming that $\kappa _\nu$ depends on the wavelength $\lambda$ with the equation: $\kappa _\nu = 0.1 \times \left( 0.3 \mathrm{\ mm} / \lambda \right) ^\beta$ cm$^2$ g$^{-1}$ [2]. With the dust temperature of 70 K and 130 K, the H$_2$ column density is obtained to be $9.5 \times 10^{23}$ and $4.9 \times 10^{23}$ cm$^{-2}$, respectively. Then, the fractional abundances relative to H$_2$ are evaluated to be $(1.3-3.9) \times 10^{-10}$ and $(7.3-16.2) \times 10^{-9}$ for NH$_2$CHO and HCOOCH$_3$, respectively, by assuming that the gas temperature is the same as the dust temperature (Table 8.3). These fractional abundances of NH$_2$CHO and HCOOCH$_3$ are comparable to those reported for the hot corinos: $6 \times 10^{-10}$ and $9 \times 10^{-9}$ in IRAS 16293$-$2422 [13], and $(1-9) \times \ 10^{-10}$ and $(2-8) \times \ 10^{-9}$ in B335 [12]. Therefore, L483 indeed harbors a hot corino activity in the closest vicinity of the protostar.

As mentioned in Sect. 8.4, different molecular emission traces different parts in this source. The extended infalling-rotating envelope is traced by the CS emission, and possibly the CCH emission. On the contrary, the compact component concentrated in the vicinity of the protostar, which is likely to be a disk component inside the centrifugal barrier, is traced by the CS, SO, HNCO, NH$_2$CHO, and HCOOCH$_3$ emissions. Such a chemical change across the centrifugal barrier is reported for some other sources as described in the previous chapters; L1527, TMC−1A, IRAS 16293$-$2422 Source A, and Source B (Chaps. 4, 6, 7; [27, 28]). In contrast to the L1527 and TMC−1A cases, the CS and SO emissions in L483 are found to be quite abundant in the compact component concentrated to the protostar. This situation is rather similar to the IRAS 16293$-$2422 Source A case (Sect. 8.5). Thus, the chemical change would be highly dependent on sources. Hence, it is still essential to investigate chemical structures of various protostellar sources at a sub-arcsecond resolution.

Table 8.3 Column densities and fractional abundances of the molecules observed toward the protostar position$^\mathrm{{a}}$

8.8 Summary of This Chapter

The Class 0 protostellar core L483 is observed with ALMA in various molecular lines. The major results are summarized as follows:

• (1) A chemical differentiation at a 100 au scale is found in L483. The CCH emission traces the outflow cavity wall, and shows a hole around the protostellar position with a radius of 0$.\!\!^{\prime \prime }$5 (100 au) in its distribution. The CS emission traces a part of the outflow cavity wall, the extended envelope component, and the compact component concentrated to the protostar. The SO and HNCO emissions are detected only in the compact component.

• (2) In spite of the WCCC character of this source, the COMs, NH$_2$CHO and HCOOCH$_3$, are firmly detected. Their emission is highly concentrated near the protostar. This is the first spatially-resolved observation of the ‘hybrid’ chemical character of WCCC and hot corino chemistry.

• (3) The kinematic structure of the disk/envelope system is analyzed by the models of an infalling-rotating envelope and a Keplerian disk. The infalling-rotating envelope model with the protostellar mass from 0.1 to 0.2 $M_\odot$ and the radius of the centrifugal barrier from 30 to 200 au reasonably explains the extended part of the observed PV diagrams of the CS emission. The compact component likely traces the Keplerian disk component inside the centrifugal barrier, although the rotation curve is not resolved well. Hence, the above chemical change seems to be occurring across the centrifugal barrier.

• (4) The CS emission traces the disk component in addition to the envelope component in L483, although it is thought to be a good tracer of the infalling-rotating envelope in other sources. As well, the SO emission traces the disk component in L483, which highlights the ring-like structure around the centrifugal barrier in L1527 and TMC−1A. These results would originate from the relatively high luminosity of L483.

• (5) The outflow structure of L483 is extended along the southeast-northwest direction. Its kinematic structure traced by the CS emission is well explained by a parabolic outflow model. There is a hint that the outflow has a rotation motion.

• (6) The SiO emission shows an extension from the protostellar position. It may be related to a possible local outflow-shock occurring near the centrifugal barrier.