A STUDY OF DYNAMICAL PROCESSES IN THE ORION KL REGION USING ALMA—PROBING MOLECULAR OUTFLOW AND INFLOW

The continuum image of the 1.3 mm emission is presented in Figure 1(a). Seven millimeter cores are identified and denoted with MM followed by a number. MM1 is the strongest and largest, and corresponds to the hot core (Wilson et al. 2000). The others are located in the compact ridge. MM7 is too far away from the phase center and thus excluded from further analysis. The known infrared sources were plotted on the map and are shown by crosses (Shuping et al. 2004). Two-dimensional Gaussian fits were made toward the continuum cores. The peak position is listed in Column 2 of Table 1. Column 3 is the core size; here 414 pc was adopted as the distance (Menten et al. 2007). The peak emission intensity and the total flux of the cores are given in Columns 4 and 5 of Table 1.

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Table 1. Parameters of the Continuum Sources

Name	Offset	R = $\sqrt{ab}$	Ipeak	Sν	Vlsr	Δ V	Trot	Mvirial
	('', '')	(10−3 pc)	(Jy beam−1)	(Jy)	(km s−1)	(km s−1)	(K)	(M☉)
MM1/hot core	(2.90 ± 0.05, 3.61 ± 0.08)	6.6	1.34 (0.06)	6.18	5.4 (0.3)	6.5 (0.3)	198$^{+36}_{-26}$	59
MM2	(−1.07 ± 0.11, −2.04 ± 0.16)	2.3	0.15 (0.02)	0.38	7.5 (0.1)	3.1 (0.1)	156$^{+51}_{-31}$	5
MM3	(−4.49 ± 0.12, −1.97±0.12)	3.8	0.28 (0.03)	1.02	8.0 (0.2)	2.4 (0.2)	100?	5
MM4	(−1.58 ± 0.14, 4.09 ± 0.13)	2.4	0.22 (0.03)	0.43	5.5 (0.2)	5.9 (0.8)	182$^{+81}_{-43}$	18
MM5	(−3.61 ± 0.07, 6.09 ± 0.15)	6.7	0.28 (0.02)	1.27	10.1 (0.1)	5.4 (0.2)	157$^{+43}_{-28}$	41
MM6	(−3.72 ± 0.09, 7.58 ± 0.12)	2.2	0.28 (0.02)	0.61	7.5 (0.3)	5.7 (0.3)	119$^{+18}_{-14}$	15

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¹³CH₃CN (12₃–11₃) and (13₃–12₃) transitions were extracted from each of the cores. The systematic velocities (V_lsr), and the mean line widths (Δ V) of the ¹³CH₃CN lines by Gaussian fitting are given in Columns 6 and 7 of Table 1. We also calculated the rotational temperatures of the cores with the rotational temperature diagram (RTD) method (Araya et al. 2005; Goldsmith & Langer 1999; Liu et al. 2002; Qin et al. 2010). Only the lines not blended and with high S/N levels (>5σ) are used for RTD analysis. Assuming that the local thermodynamic equilibrium (LTE) holds and the lines are optically thin, T_rot values are obtained and are listed in the eighth column of Table 1. Figure 1(b) shows the rotational temperature diagram of MM6 as an example. We estimated the virial mass of the MM cores with the relation (MacLaren et al. 1988): M_vir = 210(R/pc)(ΔV/km s⁻¹)²(M_☉) which is given in the last column of Table 1.

Figure 2 shows the spectral lines of CH₃CN (12₄–11₄) and CH₃OH (8_{−1, 8}–7_{0, 7}) from the peak position of each dust core observed with ALMA. There are two kinds of line profiles from these cores. One has the emission distributed in both sides of the systemic velocity (e.g., the lines at MM1 and MM2 positions). Both CH₃CN and CH₃OH lines at MM1 have double peaks, with the blue peak stronger than the red one, and the absorption dip is red shifted with respect to the systemic velocity. These absorption dips are unlikely to be caused by missing flux, as the emission of the KL region falls well inside the primary beam, especially the high excitation density CH₃CN (12₄–11₄) and CH₃OH (8_{−1, 8}–7_{0, 7}) lines we have used. Additionally both lines from the core MM2 are symmetric, indicating that the instrument effect cannot account for the absorption, which can be interpreted as gas inflow motion toward the core (see more in Section 3.4).

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The other kind of line profiles shows that most of the gas emission is located in one side of the systemic velocity. The spectral line at MM4 has stronger emission on the blue side, and on the red side the intensity decreases sharply from the peak. In contrast to the MM4, the spectral emission of MM3, MM5, and MM6 is mainly on the red side. These profiles show characteristics of high-velocity gas moving toward us or away from us, which are similar to the fast moving molecular bullets near YSOs as shown in Figures 7 and 9 of Bachiller et al. (1990), but there are apparent differences. The high-velocity gases observed here are not molecular bullets ejected from the immediate vicinity of the YSOs, but are jet propelled cores (hereafter JPCs). First, the JPCs in Orion KL show strong dust emission with total flux ranging from 0.43 to 1.27 Jy, which is very rare among the bullets in bipolar molecular outflows or HH objects. Their virialization masses range between those of the hot core and core MM2. The total masses of bullets near low- and high-mass YSOs are ⩽0.07 M_☉ in L1448 (Bachiller et al. 1990; Richer et al. 1992; Hatchell et al. 1999). Second, the molecular bullets near YSOs are always highly collimated, and the blue and red bullets in high- or low-mass star formation regions are located along an outflow axis (Richer et al. 1992; Hatchell et al. 1999). In the Orion KL region the JPCs have no such bipolar structure, but appear to distribute within the inner edge of the gas region. Third, the JPCs are hot with T_rot of 100 to 182 K. Line profiles of bullets in bipolar molecular outflows have usually been observed in the low transition lines of CO J = 1–0, 2–1 (Bachiller et al. 1990; Hatchell et al. 1999). However, for the JPCs in Orion KL, no such profile was found in the lines of CO J = 1–0, 3–2, HCO⁺ J = 1–0, and SO (3₂–2₁) (Erickson et al. 1982; Olofsson et al. 1982; Friberg 1984). In addition, the spectral profiles of the JPCs are different from those of molecular bullets in bipolar outflows near the YSOs. The former have a single component, whereas the latter are usually accompanied by a line from the host molecular core (Bachiller et al. 1990; Richer et al. 1992; Hatchell et al. 1999), indicating that the JPCs in the Orion KL region were not ejected by a YSO. Their systemic velocity is not separated from the high-velocity emission, meaning that the original gas in the KL region was impacted and propelled by the explosive jets (Taylor et al. 1984; Bally et al. 2011; Zapata et al. 2011). These cores are not explosive jets, but jet-propelled cores.

The spectral features can also be seen from channel maps of the CH₃CN (12₄–11₄) line illuminated in Figure 3, where the white contours denote the emission intensity averaging over 3 km s⁻¹ velocity range. The gas emission at velocity ≲(−13) km s⁻¹ is contaminated by the CH₃CH₂CN (25_{2, 24}–24_{2, 23}) line. The major bulk of the gas is distributed within the velocity range of −5 to 16 km s⁻¹. A comparison with Figure 1(a) shows that as the velocity increases, the emission first arises from the west of MM1, and then from MM4 and southeast of MM5. From 1 to 16 km s⁻¹, the morphology of the emission region looks like a semi-wedge ring. A complete wedge ring appears with a velocity ranging from 6 to 11 km s⁻¹, which is similar to an angle open to the north. These results show that the molecular gas emissions of the KL region are physically related to different velocity components, including molecular JPCs and cores.

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In the blue-shifted, high-velocity range of −14 to −2 km s⁻¹, gas emission arises from MM4, the southeast of MM5, and the west of MM1, and then extends to the northeast and south. The red-shifted, high-velocity gas emissions arise from MM6 to MM5, the southwest of MM1, and near MM2/MM3, with the velocity range of ∼19 to 22 km s⁻¹. The components of the high-velocity gas also consist of the emissions of JPCs and cores.

From Figure 2, the blue- and red-shifted, high-velocity gases range from −13 to −3 km s⁻¹ and 15 to 24 km s⁻¹, respectively. The contours of the velocity integration intensities of the high- velocity gas measured with the CH₃CN (12₄–11₄) line are presented in Figure 4, in which the crosses represent the infrared sources.

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The CH₃CN (12₄–11₄) outflow has two blue lobes and five red lobes. The blue-shifted, high-velocity gas peaks near MM4 and to the west of MM1. The red-shifted, high-velocity gas reaches maximum near MM5. The spectra of MM1 and Source I have a core emission profile with blue and red high-velocity wings, but the lines of the JPCs have much more obvious blue- or red-shifted emissions (Figures 2 and 5; see also Section 3.4). The distribution of the high-velocity gas suggests that the outflow mainly overlaps or connects with JPCs. The dissimilarities between the spectra of the cores and the JPCs also demonstrate that the molecular outflow is not from MM1 or Source I.

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The red outflow has five lobes and looks like a projection of a bracelet, whereas the blue outflow has two lobes. The outflow covers JPCs MM4–MM6. The southwest part of MM1 overlaps the outflow, and Source I is located outside the outflow. Neither MM1 nor Source I can be the origin or the driving source of the outflow. The other YSOs in this region are located in different lobes and cannot serve as the driving source of the outflow either (see Figure 4). These results demonstrate that the high-velocity gas in the KL region is not caused by a usual bipolar molecular outflow derived by a YSO, but is mainly from distinct molecular cores propelled by bullets from a burst. That is why blue and red lobes cannot be separated, and the bipolar outflow detected in the KL region with CO at the early period has poor collimation (Kwan & Scoville 1976; Wu et al. 2004).

To examine the line emissions of MM1 (the hot core) and Source I, we plot the CH₃OH (8_{−1, 8}–7_{0, 7}) and ¹³CH₃CN (13₃–12₃ lines at the boundaries of MM1 and Source I in Figure 5. The CH₃OH spectral profile from Source I shows blue-shifted emission and red-shifted absorption with respect to the systematic velocity of 5.5 km s⁻¹. It presents an inverse P Cygni profile, indicating that both the gases behind and in front of the continuum source are moving toward the continuum source.

The spectrum of CH₃OH at MM1 has a gas self-absorption dip that is red-shifted relative to the systematic velocity. The blue peak of the CH₃OH line is stronger than the red one with a ratio of T_B/T_R = 2.16. For the line profile, the asymmetry parameter δV (=V_thick − V_thin)/δV_thin) defined by Mardones et al. (1997) was calculated, giving δV ⩽ −0.25 for the blue profile, where V_thick is the peak velocity of the optical thick line, and V_thin and δV_thin are the systemic velocity and the line width of the optically thin line, respectively. For the line pairs of the center of MM1, δV = (V_thick − V_thin)/δV_thin = −0.26 was obtained. The two parameters of T_B/T_R and δ both satisfy the criteria of the blue profile.

The optical thick and thin spectral lines from position (d), east of MM1, have similar intensity and velocity partner to those from the center of MM1. The ratio of T_B/T_R = 2.13 and the δV = −0.25 satisfy the criteria of a blue profile. The signature is slightly weaker than that of the center one, suggesting that inflow motion in MM1 is moving toward the core center.

The line of the position (e), from MM1, has a profile that belongs to those of JPCs and has blue-shifted, high-velocity gas. This suggests that the west side of the core MM1 was impacted by the explosive jets. The profiles of the lines from north and south of MM1 seem to be affected by the explosive outflow, but not as significantly as the one at position (e).

As stated before, the molecular gas of the KL region actually consists of different velocity components. Beside the cores, the JPCs are mainly distributed in the compact ridge. There are two kinds of line profiles produced from the JPCs: one has blue wings and the other has red wings. The high-velocity wings in the two spectra are from −13 to −3 km s⁻¹ and 15 to 24 km s⁻¹, respectively, which are similar to those from SiO (8–7, v = 0) (Zapata et al. 2012). Such high velocities may be excited by C-type shock because they are below the critical velocities of J-type shocks, which range from 25 to 45 km s⁻¹. J-type shocks are also found with emissions primarily in the UV and optical, whereas C-type shocks mainly excite the vibrational and high rotational levels of molecules in infrared and radio emissions (Wang & Scoville 1992). Unlike the spectra of molecular bullets near the YSOs, the emissions of ambient and high-velocity gas of the JPCs are not separated. From Table 1 and Figure 5, one can see that the V_lsr of the JPCs are different. The spectra of JPCs with blue wings have smaller V_lsr, whereas those with red wings have larger V_lsr. If we take the average value of the peak velocities of all the JPCs and cores in the KL region, 7.7 km s⁻¹ as the systemic velocity of the KL region gas, then the shifts of the peak velocities of the JPCs range from −2.4 to 2.3 km s⁻¹. The JPCs are moving away or toward us at a slow speed. The slow, systemic motions of the JPCs may be caused by a rocket effect when the JPCs were propelled by the jets.

4.2.1. High-velocity Molecular Outflow Driven by Explosive Jets

The high-velocity molecular outflow has multiple lobes (see Figure 4). The outflow gas mainly distributes in the ridge and only a small part overlaps with the southeast of MM1 and the north of MM2, indicating that the high-velocity gas comes mainly from the JPCs. From Figures 2 and 5, one can see that the blue-shifted JPCs are located east of the ridge and the red ones distribute more widely. Unlike bipolar molecular outflows in which a YSO is usually driving the high-velocity gas emission, every blue or red lobe in the KL region harbors YSOs. Sources I and IRn are located far from the outflow center. These results suggest that the outflow is unlikely to be driven by some young stellar objects in the outflow region.

To examine the origin of the high-velocity gas and shock effects on the JPCs, we investigated the distribution of the IRAC [4.5/3.6] flux ratio in the Orion KL region. The 4.5 μm band emissions detected with IRAC of Spitzer contain lines excited by shocks (Cyganowski et al. 2009). A ratio >1.5 represents shocks or jets (Takami et al. 2010). The ratio map, with a color scale overlaid with the continuum map, is displayed in Figure 6. Four infrared bubbles (IRBs) were identified, and the largest and brightest one, IRB1, is centered at BN and the emissions are saturated at the inner region. IRB2 is the second largest and is connected with IRB1 and IRB3. IRB4 is located south of the region. Among the ALMA-detected dust cores, MM6 and MM5 are located at the outer part of IRB1 and IRB2, respectively. MM4 is located at the outer part of IRB 3. The northeast of MM3 is connected with IRB4. These bubbles have the flux ratio between ≲1.0 and 1.4, and seem to be swept out by wind and jets. The JPCs are located outside the bubbles, or partially overlap with bubbles. It is notable that the strongest and largest shocked area in the KL region is a large wedge ring, and surrounds the KL gas wedge. This ring is well associated with the H₂ 2.12 and 2.15 μm clumps (Stolovy et al. 1998), suggesting shocked effects in this region.

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To further probe the excitation mechanism of the molecular outflow, we also plot the JPCs and cores of the KL region, overlaid with the 2.12 μm H₂ jets (Bally et al. 2011 and the references therein) in Figure 7. Clearly the JPCs do not overlap with the finger-like jets. The western side of the core MM1, where the shocked gas (Figure 5) was detected, partially overlaps with a finger-like jet. These results indicate that the jets break into the region and the cores are propelled or impacted. The gas is accelerated and entrained, which results in the multi-lobe molecular outflow.

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We identify the BN object as the explosive center in the next section. Here we estimate the dynamical age of the expanding bubble. Using the distance from BN to the project center of the high-velocity gas as the average length of the jets (89), and using the average velocity of the outflow, the dynamic time is estimated to be 1280 yr, which is consistent with the numerical experiments of gradational slingshot (Chatterjee & Tan 2012).

4.2.2. The Explosive Center

The inverse P Cygni profile of line (b) shown in Figure 5 indicates gravitational collapse toward Source I. The material infall velocity can be estimated as 1.5 km s⁻¹ from the V_sys and V_obs of the absorption peak (Welch et al. 1987). Here we take the V_sys of Source I as same as that of the core MM1. Assuming the inflow is free fall (Pineda et al. 2014), the mass accretion rate of 1.2 × 10⁻³ M_☉/Yr can be obtained with dM/dt = 3V$_{{\rm in}}^3$/2G, which is much larger than those of low mass cores (Myers et al. 1996). The result is consistent with the mass of Source I, which is about 10 M_☉ (Testi et al. 2010; Liu et al. 2013).

For the core MM1, the lines at the center and position (d) have blue profiles showing inflow motion. The infall velocity is calculated with the model of Myers et al. (1996):

$$\begin{equation} V_{{\rm in}}\approx \frac{\sigma ^{2}}{v_{{\rm red}}-v_{{\rm blue}}}\hbox{ln}\left(\frac{1+eT_{{\rm BD}}/T_{D}}{1+eT_{{\rm RD}}/T_{D}}\right) \end{equation} \tag{ 1 }$$

where T_D is the brightness temperature of the dip (assumed to be optically thick), T_BD and T_RD are the height of the blue and red peaks above the dip, respectively. The velocity dispersion σ ≈ 2.7 km s⁻¹ is obtained from the optically thin ¹³CH₃CN(13₃–12₃) line. Thus the infall velocities inferred from CH₃CN (12₄–11₄) and CH₃OH (8_{−1, 8}–7_{0, 7}) lines are ∼0.4 and ∼0.8 km s⁻¹, respectively. Taking the average value ∼0.6 km s⁻¹ as V_in, the mass accretion rate of 8.0 × 10⁻⁵ M_☉/Yr was obtained. Both the infall velocity and the mass accretion rate are smaller than those of Source I, but still larger than those of low mass cores (Myers et al. 1996).

Signatures of gravitational collapse or inflow motion were detected in the Orion KL region for the first time (see Section 1). For Source I, the material infall velocity and mass accretion rate are computable with those of high mass sources (see Liu et al. 2013 and the references therein). Early, an outflow from the Source I was detected with v = 0, J = 2–1 line of SiO (Plambeck et al. 2009), which is from northeast to southwest and is mainly distributed within the hot core. Around Source I, an ionized disk was imaged with radio and 7 mm continuum emission (Reid et al. 2007). Recently a hot and neutral circumstellar disk around Source I was detected with ALMA (Hirota et al. 2014). The collapse signature, disk, and outflow suggest that the high-mass YSO in Source I forms similarly to those of low-mass stars.

Blue profiles were detected in the hot core, indicating that the inflow motion is weaker than that in Source I. Previous observations showed that inflows occur more frequently and more strongly in UCHii regions than UCHii precursors (see Wu et al. 2007 and the reference therein). The inflow difference of Source I and MM1 is attributed to the fact that Source I is a radio source, while no YSO has been found in the hot core yet.