ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP): How Do Dense Core Properties Affect the Multiplicity of Protostars?

ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP): How Do Dense Core Properties Affect the Multiplicity of Protostars? Qiu-yi Luo (罗秋怡) 1,2 , Tie Liu (刘铁) 1 , Ken’ichi Tatematsu 3,4 , Sheng-Yuan Liu 5 , Pak Shing Li 6 , James di Francesco 7,8 , Doug Johnstone 7,8 , Paul F. Goldsmith 9 , Somnath Dutta 5 , Naomi Hirano 5 , Chin-Fei Lee 5 , Di Li 10 , Kee-Tae Kim 11,12 , Chang Won Lee 11,12 , Jeong-Eun Lee 13 , Xun-chuan Liu 1 , Mika Juvela 14 , Jinhua He 15,16,17 , Sheng-Li Qin 18 , Hong-Li Liu 18 , David Eden 19,20 , Woojin Kwon 21,22 , Dipen Sahu 5 , Shanghuo Li 11 , Feng-Wei Xu 23,24 , Si-ju Zhang 24 , Shih-Ying Hsu 5,25 , Leonardo Bronfman 17 , Patricio Sanhueza 4,26 , Veli-Matti Pelkonen 27 , Jian-wen Zhou 2,10 , Rong Liu 2,10 , Qi-lao Gu 1 , Yue-fang Wu 23,24 , Xiao-feng Mai 1 , Edith Falgarone 28 , and Zhi-Qiang Shen 1 1 Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, Peopleʼs Republic of China; [email protected], [email protected] 2 School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, Peopleʼs Republic of China 3 Nobeyama Radio Observatory, National Astronomical Observatory of Japan, National Institutes of Natural Sciences, Nobeyama, Minamimaki, Minamisaku, Nagano 384-1305, Japan 4 Department of Astronomical Science, The Graduate University for Advanced Studies, SOKENDAI, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan 5 Academia Sinica Institute of Astronomy and Astrophysics, 11F AS/NTU Astronomy-Mathematics Building, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan, R.O.C. 6 Astronomy Department, University of California, Berkeley, CA 94720-3411, USA 7 NRC Herzberg Astronomy and Astrophysics, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada 8 Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Elliot Building, Victoria, BC, V8P 5C2, Canada 9 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA 10 National Astronomical Observatories of China, Chinese Academy of Sciences, Beijing 100012, Peopleʼs Republic of China 11 Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea 12 University of Science and Technology, Korea (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea 13 The School of Space Research, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do, Republic of Korea 14 Department of Physics, P.O. Box 64, FI-00014, University of Helsinki, Finland 15 Yunnan Observatories, Chinese Academy of Sciences, 396 Yangfangwang, Guandu District, Kunming 650216, Peopleʼs Republic of China 16 Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, Peopleʼs Republic of China 17 Departamento de Astronomía, Universidad de Chile, Las Condes, 7591245 Santiago, Chile 18 Department of Astronomy, Yunnan University, Kunming 650091, Peopleʼs Republic of China 19 Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool, L3 5RF, UK 20 Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DB, UK 21 Department of Earth Science Education, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea 22 SNU Astronomy Research Center, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea 23 Department of Astronomy, School of Physics, Peking University, Beijing 100871, Peopleʼs Republic of China 24 Kavli Institute for Astronomy and Astrophysics, Peking University, Haidian District, Beijing 100871, Peopleʼs Republic of China 25 National Taiwan University (NTU), No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan (R.O.C.) 26 National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan 27 Institut de Ciències del Cosmos, Universitat de Barcelona, IEEC-UB, Martí i Franquès 1, E-08028 Barcelona, Spain 28 LPENS, Ecole Normale Supérieure, Université PSL, CNRS, Sorbonne Université, Université de Paris, 75005 Paris, France Received 2022 March 7; revised 2022 April 12; accepted 2022 April 12; published 2022 June 6 Abstract During the transition phase from a prestellar to a protostellar cloud core, one or several protostars can form within a single gas core. The detailed physical processes of this transition, however, remain unclear. We present 1.3 mm dust continuum and molecular line observations with the Atacama Large Millimeter/submillimeter Array toward 43 protostellar cores in the Orion molecular cloud complex (λOrionis, Orion B, and Orion A)with an angular resolution of ∼035 (∼140 au). In total, we detect 13 binary/multiple systems. We derive an overall multiplicity frequency (MF)of 28% ±4% and a companion star fraction (CSF)of 51% ±6%, over a separation range of 300–8900 au. The median separation of companions is about 2100 au. The occurrence of stellar multiplicity may depend on the physical characteristics of the dense cores. Notably, those containing binary/multiple systems tend to show a higher gas density and Mach number than cores forming single stars. The integral-shaped filament of the Orion A giant molecular cloud (GMC), which has the highest gas density and hosts high-mass star formation in its central region (the Orion Nebula cluster), shows the highest MF and CSF among the Orion GMCs. In contrast, the λOrionis GMC has a lower MF and CSF than the Orion B and Orion A GMCs, indicating that feedback from H II regions may suppress the formation of multiple systems. We also find that the protostars comprising a binary/multiple system are usually at different evolutionary stages. The Astrophysical Journal, 931:158 (22pp), 2022 June 1 https://doi.org/10.3847/1538-4357/ac66d9 © 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s)and the title of the work, journal citation and DOI. 1 Key words: Star formation –(1569); Early stellar evolution –(434); Protostars –(1302); Low mass stars –(2050); Multiple stars –(1081); Dense interstellar clouds –(371); Interstellar medium (847) 1. Introduction Molecular clouds exhibit hierarchical structures at different levels from large to small scales. Dense cores lie at the terminus, where stars are born through gravitational fragmentation (Heggie 1975; Cohen & Kuhi 1979; Shu et al. 1987; Bergin & Tafalla 2007; Kraus & Hillenbrand 2012; Duchêne & Kraus 2013; Reipurth et al. 2014). Isolated star formation has been investigated for decades (di Francesco et al. 2007). The formation of binary/multiple-star systems, however, has been not as well studied, though these systems are commonly seen in star associations and clusters (Kraus & Hillenbrand 2012; Reipurth et al. 2014; Lomax et al. 2015; Dutta et al. 2020; Lee et al. 2020). Binary/multiple systems in the main sequence have been studied for decades. Indeed, star systems that have more than one star are as common as single-star systems in the Milky Way (Lippincott 1978; Marcy & Benitz 1989; Duquennoy & Mayor 1991; Connelley et al. 2009; Raghavan et al. 2010). Previous near-infrared observational studies have concentrated on the multiplicity statistics for young stellar objects (YSOs) (Duchêne et al. 2004,2007; Haisch et al. 2004; Kraus & Hillenbrand 2007; Connelley et al. 2008; Kraus et al. 2011; Daemgen et al. 2015; Kounkel et al. 2016; Ma et al. 2019). The high occurrence rates of multiple YSO systems revealed in these studies imply that stellar multiplicity is determined in the star formation process. In these surveys, the fraction of stars having companions (companion star fraction (CSF)) ranges from 16% ±4% to 62% ±14%, and the fraction of systems having multiple YSOs (multiplicity frequency (MF)) ranges from 18% ±4% to ∼75%. In the past 10 years, high-resolution interferometric observations with the Submillimeter Array, Very Large Array (VLA), and Atacama Large Millimeter/ submillimeter Array (ALMA)of nearby clouds have systematically revealed the multiplicity of low-mass protostars in still earlier phases, notably the Class 0/I phase (Chen et al. 2013; Lee et al. 2015; Hatziminaoglou et al. 2018). These observations indicate that the CSF ranges from ∼71% to 91% ±5% while the MF ranges from 40% to 60% for young protostars with separations below 10,000 au. Recently, higher angular resolution ALMA and VLA observations by Tobin et al. (2022) revealed the multiplicity of Orion protostars that were previously identified in the Herschel Orion Protostar Survey (HOPS): the CSF is 44% ±3% and the MF is 30% ±3%. Although different theoretical models of binary/multiple systems at various evolutionary stages have been proposed, e.g., turbulent fragmentation (Fisher 2004; Goodwin et al. 2004)and disk fragmentation (Adams et al. 1989; Bonnell & Bate 1994), the most relevant scenario remains unclear. Nevertheless, some predictions can be tested. For example, turbulent fragmentation suggests a nonlinear gravitational collapse, resulting in wider-separation binary/multiple systems (above 1000 au). On the other hand, disk fragmentation caused by gravitational instability is more likely to generate closer (below 600 au)binary/multiple systems (Beichman et al. 1986; Raghavan et al. 2010; Reipurth et al. 2007; Cohen & Kuhi 1979; Reipurth et al. 2014). In a multiplicity study of all known protostars (94)in the Perseus molecular cloud, Tobin et al. (2016)found a bimodal distribution of separation among binary/multiple systems, with peaks at ∼75 au and ∼3000 au. The bimodal distribution was recently confirmed by a more detailed analysis of protostar separations in the Orion and Perseus molecular clouds (Tobin et al. 2022). Based on these results, Tobin et al. (2022)suggest that multiples with small separations (<500 au)are likely produced by both disk fragmentation and turbulent fragmentation with migration, and those with wide separation (>10 3 au) result primarily from turbulent fragmentation. Lee et al. (2017) found a low-mass binary system in the earliest stages of formation with the rotation axes of its disks misaligned, suggesting that this binary system is likely formed due to turbulent fragmentation. This system, however, has small separation (∼860 au), which has probably been decreased by the migration of the protostars (Lee et al. 2020). Further, Tobin et al. (2016)detected a close triple system that was likely formed out of a protostellar disk undergoing gravitational instability. However, previous investigations have rarely addressed how the properties of the host dense cores affect stellar multiplicity, even though they may play an important role. In this work, we explore the relation between the physical characteristics of dense cores and the formation of single or multiple stellar systems. This paper is structured as follows: Section 2 introduces our sample, and Section 3describes the observational data from the surveys we use in the paper. In Section 4, we report the results of multiplicity analysis in binary/multiple systems. Section 5discusses the origin of multiplicity, environmental effects, and physical and chemical differences among member protostars. Section 6provides a summary. 2. The Sample The Planck survey detected 13,188 Planck Galactic cold clumps (PGCCs)across the whole sky that exhibit extremely low temperatures (T∼14 K)(Planck Collaboration et al. 2016). PGCCs are therefore excellent targets for studying the very initial conditions of star formation (Juvela et al. 2010; Planck Collaboration et al. 2011; Wu et al. 2012; Montillaud et al. 2015; Planck Collaboration et al. 2016; Liu et al. 2018; Eden et al. 2019; Xu et al. 2020,2021). In follow-up observations, the James Clerk Maxwell Telescope (JCMT)large program SCOPE observed ∼1300 PGCCs in the 850 μm continuum to study the early evolution of dense cores, which targets high-column-density (>5× 10 20 cm −2 for a 5′beam of the Planck telescope)clumps (Liu et al. 2018; Eden et al. 2019). The SCOPE sample is important for statistically investigating evolution between the starless and protostellar star formation phases. The Orion Molecular Cloud Complex is about 380–420 pc away from us, and it has been extensively studied with many observations (Kim et al. 2020; Sahu et al. 2021a). As a part of SCOPE, Yi et al. (2018)turned to the Orion giant molecular cloud (GMC)complex, and observed 58 PGCCs in the three GMCs (Orion A, Orion B, and λOrionis)with the JCMT in the 850 μm continuum. Beyond those they included 38 other PGCCs for which archival 850 μm continuum data were available in the JCMT Science Archive. The sample of Yi et al. (2018)is complete for PGCCs with column densities higher 2 The Astrophysical Journal, 931:158 (22pp), 2022 June 1 Luo et al. than 5 ×10 20 cm −2 (for a 5′beam of the Planck telescope)in the Orion GMCs. In total, 119 dense cores were identified from JCMT SCUBA-2 observations of these Orion PGCCs, forming a unique sample of cold cores at similar distances for further studies. As a follow-up observation of these Orion PGCCs, the ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP)subsequently observed 72 of the 119 dense Orion PGCCs at high resolution (∼140 au). This resolution is high enough to resolve close binary/multiple systems with separations of a few hundred astronomical units. These 72 cores are among the densest in these clouds and are arguably the closest to the onset of star formation (Dutta et al. 2020), i.e., those that have started to collapse or have already formed protostars. In previous work with ALMASOP data, we focused on the chemical evolution of hot corino sources (Hsu et al. 2020,2022), the fragmentation of prestellar cores (Sahu et al. 2021b), and outflow jets (Dutta et al. 2020). Dutta et al. (2020)presented the 1.3 mm continuum emission maps in ALMASOP observations and identified protostars in 43 cores. Multiple protostars were frequently seen within a single core, but the multiplicity of these protostellar systems was not discussed in Dutta et al. (2020). The present paper investigates binary/multiple systems in ALMASOP, and focuses on how dense core properties relate to the multiplicity of protostars. The major limitation of this work is that very close binary systems with separation 140 au are not resolvable due to the angular resolution limits of the ALMASOP observation. 3. Observations and Data 3.1. ALMA Observations ALMASOP (project ID: 2018.1.00302.S; PI: Tie Liu) observed 72 extremely cold young dense cores in the Orion molecular clouds, including 23 starless core candidates and 49 protostellar core candidates, with ALMA Band 6 from 2018 October to 2019 January. The observations were performed in four spectral windows centered at 216.6, 218.9, 231.0, and 233.0 GHz each with a 1.875 GHz bandwidth, with a velocity resolution of ∼1.4 km s −1 . Three array configurations were used for the observations: 12 m C43-5 (TM1), 12 m C43-2 (TM2), and 7 m Atacama Compact Array (ACA). Additional details of the observations are presented in Dutta et al. (2020). Several molecular line transitions were observed: CO(J=2–1), C 18 O(J=2–1),N 2 D + (J=3–2), DCO + (J=3–2), DCN (J=3–2), and SiO(J=5–4). In this paper, we utilize the results from the 1.3 mm continuum and CO(J=2–1)data. The acquired visibilities were calibrated with the standard pipeline in CASA (McMullin et al. 2007). The 1.3 mm continuum images were generated using the TCLEAN task of CASA with a threshold of 3σtheoretical sensitivity in three sets of the continuum images with the ACA (beam size ∼58) FWHM, the TM2+ACA (beam size ∼12)FWHM combination, and the TM1+TM2+ACA (beam size ∼035)FWHM combination. We applied the “hogbom”deconvolver, and Briggs weighting with a robust value of +0.5 to obtain highresolution maps, which allow us to identify multiple components more precisely. The TM1+TM2+ACA combination provides the best resolution to distinguish multiple components. The typical sensitivity of 1.3 mm continuum emission ranges from 0.01 to 0.2 mJy beam −1 in the TM1+TM2+ACA data (Dutta et al. 2020). The corresponding 3σmass sensitivity is better than 0.002 M ☉ , assuming a dust temperature of 25 K and a distance of 400 pc. 3.2. JCMT SCUBA-2 Observations The natal cores of the ALMASOP sources were observed in the 850 μm continuum with JCMT/SCUBA-2, as a part of the JCMT legacy survey SCOPE, “SCUBA-2 Continuum Observations of Pre-protostellar Evolution”(project ID: M16AL003; PI: Tie Liu). The beam size of JCMT at 850 μm is about 14″. An analysis of the 850 μm continuum emission of these dense cores was presented in Yi et al. (2018). The dense core parameters including the sizes, masses, and H 2 mean densities derived by Yi et al. (2018)are listed in Tables 1and 2.Itis worth mentioning that we have updated these parameters following new distance measurements as used in Kim et al. (2020), which are 380 pc for λOrionis, 390 pc and 430 pc for Orion A, and 390 pc and 420 pc for Orion B. 3.3. No:45m Observations The ALMASOP sources were also observed in nine molecular lines (CCS(J N =8 7 –7 6 ), CCS(J N =7 6 –6 5 ), HC 3 N(J=9–8),N 2 H + (J=1–0), DNC(J=1–0), HN 13 C(J=1–0),N 2 D + (J=1–0), c-C 3 H 2 (JKK ac=2 12 –101), and NH 3 (J,K)=(1,1)) toward the 850 μm intensity peak positions of the SCUBA-2 cores with the 45 m radio telescope at Nobeyama Radio Observatory (No:45m; project IDs: CG161004, LP177001; PI: K. Tatematsu)from 2015 December to 2019 May (Tatematsu et al. 2017; Kim et al. 2020; Tatematsu et al. 2021). The observations were conducted with the receiver TZ1, T70, and FOREST which are dualpolarization and two-sideband superconductor–insulator– superconductor receivers. The velocity resolution is 0.05–0.06 km s −1 for both TZ1 and T70, 0.1 km s −1 for FOREST, and 0.05 km s −1 for H22. The details of the No:45m observations are described in Kim et al. (2020). Among the observed molecular lines, the N 2 H + line emission exhibits a spatial distribution similar to that of the SCUBA-2 850 μm dust continuum emission (Tatematsu et al. 2017,2021). Therefore, we adopt the Mach number, system velocity, line width (FWHM)of N 2 H + (J=1–0), dust temperature, and H 2 column density from Kim et al. (2020)for further analysis in this work, which are compiled in Table 1and Table 2. In addition, there are 16 dense cores in ALMASOP that were also mapped in N 2 H + (J=1–0)line emission (Tatematsu et al. 2017,2021). We also present the N 2 H + (J=1–0)maps for these cores in this work. 3.4. Infrared Data We used the Spitzer Enhanced Imaging Products (SEIP) from the Spitzer Heritage Archive (Megeath et al. 2012)and Wide-field Infrared Survey Explorer (WISE)data (Wright et al. 2010)to help classify protostars distributed throughout the region of the cores. The SEIP includes data from the four channels of the IRAC instrument (3.6, 4.5, 5.8, and 8 μm)and the 24 μm channel of the MIPS instrument. The WISE data were also observed in 3.4, 4.6, 12, and 22 μm. In this paper, we mainly use the highest-resolution 3.6/4.5/8μm data in the SEIP observations or the highest-resolution 3.4/4.6/12 μm data in the WISE observations. 3 The Astrophysical Journal, 931:158 (22pp), 2022 June 1 Luo et al. Table 1 Physics and Chemical Properties of Binary/Multiple Systems N 2 H + Source R.A. (J2000)Decl.(J2000)Size T d N(H 2 )nH2V lsr ΔV HFS ΔV GA M enve+disk Mcore CGF L jeans (h:m:s)(d:m:s)(pc)(K) (×10 23 cm −2 ) (×10 5 cm −3 ) (km s −1 ) (km s −1 ) (km s −1 )(M e )(M e )(au) (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15) G196.92-10.37 05:44:29.56 +09:08:50.20 0.22 14.8 ±0.4 1.8 ±0.1 5.6 ±0.1 11.68 0.84 1.02 1.5 ±0.2 0.116 ±0.034 5.69 ±0.32 0.02 7000 G205.46-14.56M1 a 05:46:08.06 −00:10:43.71 0.09 12.5 ±0.9 8.7 ±0.8 35.9 ±1.7 9.99 0.73 1.18 1.4 ±0.1 2.309 ±0.96 6.36 ±1.33 0.36 2500 G205.46-14.56M2 a 05:46:07.89 −00:10:01.82 0.06 12.5 ±0.9 5.5 ±0.5 22.6 ±1.5 10.05 0.88 1.26 1.7 ±0.1 0.291 ±0.063 1.83 ±0.38 0.16 3200 G205.46-14.56S1 05:46:07.05 −00:13:37.78 0.13 12.5 ±0.9 6.7 ±0.6 27.4 ±1.7 10.31 1.18 1.23 2.4 ±0.1 0.542 ±0.179 11.64 ±2.18 0.04 2900 G206.93-16.61E2 05:41:37.31 −02:17:18.13 0.10 16.8 ±5.3 4.5 ±1.5 18.8 ±2.4 9.76 0.60 0.58 1.0 ±0.1 0.863 ±0.195 4.08 ±0.16 0.21 4000 G207.36-19.82N1 05:30:50.94 −04:10:35.60 0.06 11.9 ±1.4 3.5 ±0.5 5.7 ±1.1 10.52 1.13 1.48 2.3 ±0.1 0.124 ±0.048 1.15 ±0.46 0.10 6200 G208.68-19.20N2 05:35:20.45 −05:00:50.39 0.05 19.7 ±3.8 11.6 ±3.0 18.9 ±3.6 11.14 0.44 0.67 0.6 ±0.1 4.786 ±2.046 2.27 ±1.15 4400 G208.68-19.20N3 05:35:18.02 −05:00:20.70 0.05 19.7 ±3.8 11.7 ±2.5 18.9 ±3.7 11.12 0.69 0.96 1.1 ±0.1 0.607 ±0.195 2.65 ±1.50 0.23 4400 G208.68-19.20S 05:35:26.32 −05:03:54.39 0.09 19.7 ±3.8 4.6 ±1.0 7.6 ±1.4 10.35 0.95 0.94 1.5 ±0.1 0.464 ±0.190 3.55 ±1.03 0.13 6900 G209.55-19.68N1 05:35:08.90 −05:55:54.40 0.16 14.1 ±4.0 0.4 ±0.1 0.7 ±0.1 7.20 0.84 1.30 1.6 ±0.1 0.334 ±0.087 0.75 ±0.04 0.44 19300 G210.49-19.79W 05:36:18.86 −06:45:28.03 0.11 11.8 ±2.7 11.3 ±2.9 18.4 ±0.2 9.01 0.60 0.77 1.2 ±0.1 0.209 ±0.086 6.62 ±0.25 0.03 3400 G210.97-19.33S2 05:38:45.30 −07:01:04.41 0.07 12.8 ±1.1 1.2 ±0.3 2.0 ±0.1 0.86 0.038 ±0.012 0.37 ±0.09 0.10 10900 G211.47-19.27N 05:39:57.18 −07:29:36.07 0.07 12.4 ±1.1 4.1 ±0.4 6.6 ±0.3 3.99 0.52 0.97 1.0 ±0.2 0.105 ±0.035 1.54 ±0.43 0.06 5900 G212.10-19.15N2 05:41:24.03 −07:53:47.51 0.12 10.8 ±1.4 1.8 ±0.3 2.9 ±0.3 4.45 0.69 1.04 1.5 ±0.1 0.048 ±0.016 1.53 ±0.72 0.03 8300 Notes. Column(1): ALMASOP core name. Note that the marked a is different from the JCMT dense core name and those marked with * are dense core detected only in the ALMASOP survey. (2)-(3): coordinates in equatorial system (J2000)from (Yi et al. 2018)and (Dutta et al. 2020).(4): Core size from Yi et al. (2018).(5): Dust temperature comes from Kounkel et al. (2017).(6):H 2 column density cited from Kim et al. (2020). (7):H 2 number density from Yi et al. (2018).(8)-(10):N 2 H + (J=1-0)of systemic velocity, FWHM, and FWHM inferred by Gaussian fitting from Kim et al. (2020).(11): Mach number cited from Kim et al. (2020).(12): Gas mass of envelope and disk from Dutta et al. (2020). 4 The Astrophysical Journal, 931:158 (22pp), 2022 June 1 Luo et al. Table 2 Physics and Chemical Properties of Single Systems N 2 H + Source R.A.(J2000)Decl.(J2000)Size T d N(H 2 )nH2V lsr ΔV HFS ΔV GA  M enve+disk Mcore CGF L jeans (h:m:s)(d:m:s)(pc)(K)(×10 23 cm −2 )(×10 5 cm −3 )(km s −1 )(km s −1 )(km s −1 )(M e )(M e )(au) (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15) G191.90-11.21S 05:31:31.73 +12:56:14.99 0.10 14.7 ±4.1 0.7 ±0.2 2.2 ±0.5 10.53 0.41 0.38 0.7 ±0.1 0.079 ±0.034 0.93 ±0.09 0.085 11100 G192.12-11.10 05:32:19.54 +12:49:40.19 0.12 13.3 ±3.0 1.8 ±0.4 5.4 ±0.2 10.02 0.73 0.35 1.4 ±0.2 0.340 ±0.145 2.32 ±0.33 0.147 6800 G192.32-11.88N 05:29:54.47 +12:16:56.00 0.06 17.3 ±6.0 1.1 ±0.4 3.4 ±0.4 12.12 0.66 0.59 1.1 ±0.2 0.408 ±0.174 0.51 ±0.05 0.800 9700 G192.32-11.88S 05:29:54.74 +12:16:32.00 0.05 17.3 ±6.0 0.9 ±0.4 2.9 ±0.2 12.08 0.54 0.58 0.9 ±0.1 0.100 ±0.043 0.23 ±0.02 0.435 10500 G200.34-10.97N 05:49:03.71 +05:57:55.74 0.09 13.5 ±0.9 0.8 ±0.1 2.5 ±0.6 13.36 0.45 0.51 0.8 ±0.1 0.068 ±0.029 0.81 ±0.06 0.084 10000 G201.52-11.08 05:50:59.01 +04:53:53.10 0.05 13.6 ±1.3 0.8 ±0.1 3.3 ±0.8 0.060 ±0.026 0.14 ±0.04 0.429 8800 G203.21-11.20W1 05:53:42.83 +03:22:32.90 0.12 11.2 ±0.7 2.7 ±0.3 11.1 ±1.9 10.70 0.50 1.02 1.0 ±0.1 0.091 ±0.039 2.88 ±0.13 0.032 4300 G203.21-11.20W2 05:53:39.62 +03:22:24.90 0.10 11.2 ±0.7 3.2 ±0.4 13.4 ±1.5 10.11 0.50 0.72 1.0 ±0.1 0.034 ±0.015 2.57 ±0.19 0.013 3900 G205.46-14.56N1 a 05:46:09.65 −00:12:16.45 0.05 12.5 ±0.9 5.5 ±0.5 22.5 ±1.1 9.92 1.34 0.475 ±0.203 1.06 ±0.16 0.448 3200 G205.46-14.56N2 a 05:46:07.49 −00:12:22.42 0.03 12.5 ±0.9 4.7 ±0.4 19.2 ±1.9 10.29 0.70 0.75 1.4 ±0.1 0.223 ±0.095 0.49 ±0.09 0.475 3500 G205.46-14.56S2 05:46:04.49 −00:14:18.87 0.03 12.5 ±0.9 4.8 ±0.5 19.8 ±2.9 10.44 0.46 0.54 0.9 ±0.1 0.069 ±0.029 0.47 ±0.10 0.147 3400 G205.46-14.56S3 05:46:03.54 −00:14:49.34 0.04 12.5 ±0.9 5.0 ±0.5 20.4 ±1.9 10.36 0.61 0.81 1.2 ±0.1 0.167 ±0.072 0.88 ±0.16 0.190 3300 G206.12-15.76 05:42:45.26 −01:16:11.37 0.14 11.9 ±1.6 2.6 ±0.4 10.8 ±1.3 1.035 ±0.442 3.95 ±1.73 0.262 4500 G206.93-16.61W2 a 05:41:25.04 −02:18:08.11 0.06 16.8 ±5.3 9.9 ±3.1 40.08 ±4.7 9.25 0.65 1.02 1.1 ±0.1 0.771 ±0.333 3.27 ±0.15 0.235 2800 G208.68-19.20N1 05:35:23.37 −05:01:28.70 0.09 19.7 ±3.8 10.9 ±2.4 17.7 ±3.4 11.13 0.72 0.96 1.1 ±0.1 2.312 ±0.988 7.79 ±2.53 0.297 4500 G208.68-20.04E 05:32:48.40 −05:34:47.14 0.13 12.8 ±4.2 2.6 ±0.9 4.3 ±0.3 8.74 0.38 0.52 0.7 ±0.1 0.073 ±0.031 3.95 ±0.29 0.018 7400 G208.89-20.04Walma * 05:32:28.03 −05:34:26.69 0.028 ±0.012 G209.55-19.68S1 05:35:13.25 −05:57:58.65 0.23 14.1 ±4.0 0.8 ±0.2 1.2 ±0.2 7.35 0.68 0.94 1.3 ±0.1 0.264 ±0.113 2.31 ±0.97 0.114 14800 G209.55-19.68S2 a 05:35:08.96 −05:58:26.38 0.16 14.1 ±4.0 1.0 ±0.3 1.6 ±0.5 8.11 0.45 0.51 0.8 ±0.1 0.084 ±0.036 1.52 ±0.26 0.054 12800 G210.37-19.53S 05:37:00.55 −06:37:10.16 0.15 14.0 ±4.1 1.1 ±0.3 1.8 ±0.7 5.61 0.72 0.69 1.3 ±0.1 0.133 ±0.050 2.27 ±0.23 0.059 12000 G210.82-19.47S 05:38:03.67 −06:58:24.141 0.08 0.4 ±0.1 0.7 ±0.8 0.010 ±0.004 0.22 ±0.125 0.600 G211.01-19.54N 05:37:57.23 −07:06:56.72 0.10 14.7 ±8.4 2.5 ±1.4 4.1 ±0.2 6.03 0.83 0.79 1.5 ±0.1 0.130 ±0.056 2.16 ±0.36 0.060 8200 G211.01-19.54S 05:37:59.04 −07:07:24.14 0.07 14.7 ±8.4 2.5 ±1.4 4.0 ±0.1 5.72 0.70 1.01 1.3 ±0.1 0.021 ±0.009 1.05 ±0.32 0.020 8300 G211.16-19.33N2 05:39:05.83 −07:10:41.52 0.10 12.5 ±1.8 0.8 ±0.1 1.2 ±0.1 3.49 0.46 0.70 0.9 ±0.1 0.016 ±0.007 0.48 ±0.20 0.033 13900 G211.47-19.27S 05:39:56.10 −07:30:28.40 0.01 12.4 ±1.1 7.2 ±0.8 11.7 ±0.3 5.52 1.03 0.93 2.1 ±0.1 0.990 ±0.424 11.39 ±3.18 0.087 4500 G212.10-19.15S 05:41:26.39 −07:56:51.81 0.15 10.8 ±1.4 1.9 ±0.3 3.0 ±0.4 3.78 0.38 0.74 0.6 ±0.1 0.237 ±0.101 2.41 ±1.00 0.098 8200 G212.84-19.45N 05:41:32.07 −08:40:10.94 0.10 11.7 ±1.1 2.0 ±0.2 3.2 ±0.5 4.31 0.34 0.42 0.8 ±0.1 0.273 ±0.117 1.26 ±0.50 0.217 8300 G215.87-17.62M 05:53:32.52 −10:25:05.99 0.24 12.2 ±1.2 0.5 ±0.1 0.8 ±0.1 8.96 0.35 0.46 0.7 ±0.1 0.065 ±0.028 2.50 ±0.64 0.026 16800 G215.87-17.62N 05:53:41.91 −10:24:02.00 0.26 12.2 ±1.2 0.6 ±0.1 1.0 ±0.2 9.28 0.46 0.89 0.9 ±0.4 0.0090 ±004 3.96 ±0.84 0.002 15000 Notes. Column(1): ALMASOP core name. Note that the marked a is different from the JCMT dense core name and those marked with * are dense core detected only in the ALMASOP survey. (2)-(3): coordinates in equatorial system (J2000)from (Yi et al. 2018)and Dutta et al. (2020).(4): Core size from Yi et al. (2018).(5): Dust temperature comes from Kounkel et al. (2017).(6):H 2 column density cited from Kim et al. (2020). (7):H 2 number density from Yi et al. (2018).(8)-(10):N 2 H + (J=1-0)of systemic velocity, FWHM, and FWHM inferred by Gaussian fitting from Kim et al. (2020).(11): Mach number cited from Kim et al. (2020).(12): Gas mass of envelope and disk from Dutta et al. (2020). 5 The Astrophysical Journal, 931:158 (22pp), 2022 June 1 Luo et al. 4. Results 4.1. Identification of Binary/Multiple Systems We focus on the 43 protostellar cores among 72 cores that were detected in 1.3 mm continuum emission in ALMASOP (Dutta et al. 2020). The 1.3 mm continuum emission from these cores is relatively bright, and indicates that one or more compact objects, previously identified as young protostars (Dutta et al. 2020), have formed within them. Figure 1presents the infrared and (sub)millimeter continuum images obtained with various instruments for an exemplar core G196.92-10.37. Multiple point sources are seen in the Spitzer image but some may be contamination from background/foreground stars or very evolved protostars because they do not show strong 1.3 mm continuum emission (Tobin et al. 2022). We will only consider young protostars with disks and envelopes, which should have strong 1.3 mm continuum emission, in further multiplicity analysis. From the highest-resolution ALMA 1.3 mm continuum image shown in the right panel, one can identify a triple protostellar system formed in the exemplar core G196.92-10.37. The image descriptions of the other cores are compiled in Appendix. We identify binary/multiple systems within the SCUBA-2 cores, whose sizes are smaller than or comparable to the FOV of the ALMA 12 m array, following the same strategy used in previous works (e.g., Tobin et al. 2016,2022). The core sizes are listed in Tables 1and 2. In total, we identify 13 binary/ multiple systems and 29 single systems, and one binary-system candidate (one of its members is a prestellar core). Here we note again that our observations cannot resolve systems with below 140 au separation; thus some single-star systems could also be very close binary/multiple systems. Some ALMASOP sources were also observed in the VANDAM survey (see Table 6in the Appendix; Tobin et al. 2022).Wefind that two single-protostar systems (G206.12-15.76 and G211.47-19.27S) in ALMASOP were further resolved to very close binary/ multiple systems in the VANDAM survey. However, the very close binary/multiple systems in G206.12-15.76 and G211.4719.27S are apparently formed due to disk fragmentation, and hence their properties are not determined by the fragmentation of their natal dense cores. Therefore, they are treated as single stars in the below analysis of the relations between core properties and protostellar multiplicity. The other single stars in ALMASOP remain to be single in the VANDAM survey. To quantify the multiplicity in each Orion GMC, we adopt the statistical parameters of MF and CSF. They give a good indication of the proportion of binary/multiple systems and the average number of companions. The two quantities associated with the number of star systems and companions are given by () BTQ SBTQ MF ... ... 1=++ + +++ () BTQ SBTQ CSF 2 3 ... ... 2=++ + +++ where S,B,T, and Qare the numbers of single, binary, triple, and quadruple systems in the sample, respectively. Figure 2 shows the MF and CSF of each of the three Orion GMCs, and the actual quantities are listed in Table 3. As the two subregions (separated by a −6°decl.)of Orion A, the integral-shaped filament (ISF)and L1641, have very different physical environments and levels of star formation activity (Megeath et al. 2016), we also calculate their MF and CSF separately. The overall MF and CSF for the ALMASOP sample are 28% ±4% and 51% ±6%, respectively. For comparison, Tobin et al. (2022)found that the overall MF and CF for the HOPS protostars are 30% ±3% and 44% ±3%, respectively, which are broadly consistent with our statistical results. However, we note that their sample contains very close Figure 1. Images of an exemplar core G196.92-10.37. (a)JCMT SCUBA-2 850 μm contours superimposed with RGB image of Spitzer 3.6/4.5/8μm data. The yellow circle represents the field of view (FOV)of the ALMA ACA image shown in panel (b). The contour levels are from 6σto 30σwith steps of 6σ(1σis 13.8 mJy beam −1 ).(b)Zoom into the infrared data, with contours of the ALMA ACA 1.3 mm dust continuum data. The contour levels are 3σ,6σ,9σ,12σ, and 30σ(1σis 5 mJy beam −1 ).(c)Color image showing ALMA 1.3 mm dust continuum emission from combined TM1+TM2+ACA data, and contours showing the 1.3 mm dust continuum emission from lower-resolution TM2+ACA data. The contours correspond to 5σ,10σ,15σ,20σ,25σ,30σ,50σ,70σ, and 90σ(1σis 4 mJy beam −1 ). Figure 2. MF (blue hexagons)and CSF (black hexagons)in three GMCs (λ Orionis, Orion A, and Orion B)in the Orion region, and in two subregions of Orion A, L1641 and ISF. 6 The Astrophysical Journal, 931:158 (22pp), 2022 June 1 Luo et al. binary/multiple systems with separations smaller than 100 au, which cannot be resolved in our observations. In our sample, the λOrionis cloud shows the lowest MF and CSF, and the ISF region in Orion A shows the highest MF and CSF. When comparing the two subregions of Orion A, the ISF region, which contains the Orion Nebula cluster (ONC), has much higher multiplicity than L1641. The different occurrences of stellar multiplicity in the Orion GMCs may be caused by environmental effects such as stellar feedback, gas density, and turbulence levels, which we explore later in Section 5. 4.2. Properties of the Natal Dense Cores We are interested in how the properties of dense cores affect the multiplicity of the protostars that form within them. To explore this possible connection, we summarize detailed information of the 43 protostellar cores from previous studies and divide the cores into two groups: dense cores forming single stars and dense cores forming more than one protostar. As the assumptions and calculation procedures for many of the parameters have already been described in detail in previous works (Yi et al. 2018; Kim et al. 2020; Dutta et al. 2020),we simply describe the core parameters here. The core parameters of these two groups are listed in Tables 1and 2. Yi et al. (2018)used the ClumpFind task in the STARLINK package to identify dense cores. The method was to fit the SCUBA-2 850 μm continuum emission maps in 2D Gaussian profiles with a threshold of 5σ. ClumpFind limited the edge of the core according to the threshold, and extracted the sizes and fluxes relying on the FWHM brightness and the integrated fluxes of the core. The flux density produced by the SCUBA-2 850 μm dust emission can be expressed as () () ()SBT mBT.3 dust H dust tmk=W =W nln nn ¢ The H 2 column density of the cores can therefore be derived from the flux density. The calculation of the following parameters, with flux density involved, follows the optically thin assumption: () () ()NS mBT H4 2 H dust mk =W n nn ¢ where S¢ nis the beam-averaged flux density, τ λ is the dust optical depth, Ωis the solid angle of the source, μis the mean molecular weight and its value is 2.8, m H is the mass unit of atomic hydrogen, and B ν (T dust )is the Planck function of dust temperature T dust , which is from the PGCC catalog. The dust opacity per gram of gas is calculated following the equation from Beckwith et al. (1990):κ ν =() 0 .1 10 Hz 12 n b cm 2 g −1 , where βis the dust emissivity spectral index from Planck Collaboration et al. (2016). The column densities of the 43 protostellar cores in the ALMASOP sample range from 0.4 ±0.1 ×10 23 cm −2 to 11.7 ±2.5 ×10 23 cm −2 , with a median value of 2.5 ×10 23 cm −2 . The mean H 2 number densities of the cores are estimated from the H 2 column densities based on the SCUBA-2 850 μm dust continuum: () ()nN R H5 H2 2= where Ris the core diameter and is calculated using · R ab=,aand bbeing the major and minor axes of the FWHM obtained with ClumpFind and converted to linear distances assuming the source distances of Kim et al. (2020). The mean density of the cores ranges from 0.7 ×10 5 cm −3 to 40.1 ×10 5 cm −3 , with a median value of 5.4 ×10 5 cm −3 . The core masses are inferred from the SCUBA-2 850 μm continuum fluxes compiled in Yi et al. (2018): () ()MSD BT 6 core 2 dust k =¢ n nn where S¢ nis the flux density of cores from the SCUBA-2 850 μm observations, and Dis the distance adopted by Kim et al. (2020). The dust opacity κ ν is adopted from Beckwith et al. (1990). As listed in Table 1and Table 2, the mass of the cores ranges from 0.14 ±0.04 M e to 11.64 ±2.18 M e , with a median mass of 2.27 M e . The high-resolution ALMA 1.3 mm continuum emission traces the total gas mass of the envelope and disk, M enve+disk , for each protostar. M enve+disk can be roughly estimated based on the integrated 1.3 mm continuum flux density under an optically thin assumption, which was already calculated in Dutta et al. (2020). The smallest gas mass of protostars in the whole sample is 0.009 M e and the largest mass is 2.312 M e , with a median mass of 0.3 M e . First, we estimate the mass of the envelope+disk for individual protostars (Menve disk *+). Second, we estimate the total envelope+disk masses (M enve+disk )of all protostars within each core. We then calculate a “compact gas fraction”(CGF)to describe the fraction of gas that has accumulated from the dense core into the protostellar envelope and disk: ()MMCGF . 7 enve disk core =+ The CGF is a ratio calculated after collecting all the M enve+disk in each core. The CGF is shown in Column (14)of Tables 1 and 2. The minimum and maximum values of the CGF for whole cores are 0.2% and 80%, and the median value is 11%. Once the perturbations caused by self-gravity inside a molecular cloud are larger than the Jeans length, they can lead Table 3 Stellar Multiplicity in Orion Molecular Cloud Complex Subregion MF CSF Sample Number Sample Number Sample Number Sample Number Sample Number (Single Systems)(Binary Systems)(Triple Systems)(Quadruple Systems)(Five-star Systems) (1)(2)(3)(4)(5)(6)(7)(8) λOrionis 20% ±8% 40% ±15% 4 0 1 0 0 Orion B 29% ±7% 64% ±13% 10 2 0 1 1 Orion A 35% ±7% 48% ±8% 15 5 2 1 0 L1641 28% ±7% 42% ±11% 10 3 0 1 0 ISF 44% ±14% 66% ±22% 5 2 2 0 0 7 The Astrophysical Journal, 931:158 (22pp), 2022 June 1 Luo et al. the region to become unstable. Hence, Jeans fragmentation may lead to substructures that become binary/multiple systems. The Jeans lengths of dense cores can be calculated via the following equations from Wang et al. (2014): ⎜⎟ ⎛ ⎝⎞ ⎠⎛ ⎝⎞ ⎠⎛ ⎝⎞ ⎠ () Lc G Tn 0.066 10 K 10 cm pc 8 sJeans 12 12 53 12 p r == - - where Tis the dust temperature of the dense core from Kounkel et al. (2017), and nis the mean column density of H 2 from Yi et al. (2018). The Jeans lengths of dense cores in our sample range from 2500 au to 19,300 au with a median value of 7000 au. The Mach number ()is a measurement of turbulence within dense cores. We adopt the Mach numbers from Kim et al. (2020), which are calculated using () c9 s NT s = where σ NT is the nonthermal velocity dispersion derived from σ NT =vkT m8ln2 kB -. Here, k B is the Boltzmann constant, T k the kinetic temperature, !vthe N 2 H + (J=1–0)line width (FWHM),mthe mass of the N 2 H + molecule, and c s the sound speed, which depends on the temperature assumed. The Mach number of the cores ranges from 0.6 ±0.1 to 2.4±0.2, with a median value of 1.1. In order to examine whether the statistical properties of groups of dense cores harboring different types of protostellar systems differ from one another, we calculate the median and mean values of the above-derived core parameters in the two groups of cores, and list them in Table 4. Statistically, the N(H 2 ),n(H 2 ),,and M core of cores containing binary/multiple systems are higher than those of cores forming single stars. The L Jeans and Menve disk *+of individual protostars in those multiple systems are smaller than those of single systems. The core sizes of the two groups, however, do not show any noticeable difference. We present the cumulative distribution functions of the core parameters of the two groups of cores in Figure 3. In addition, we use the Kolmogorov–Smirnov (KS)test to gauge whether these two groups of cores have the same underlying core parameter distributions. Overall, the distributions of number density, H 2 column density and Mach number of the two groups appear to differ substantially with p-values smaller than 10%. In particular, the two groups show significant differences in H 2 column density and Mach number with very low p-values (<5%)in the KS test of their distributions. In contrast, the distributions in M core , size, and M enve+disk of the two groups of cores are statistically similar, as indicated by the very high pvalues (>28%)in the KS test. We find that the proportion of envelope+disk masses below 1 M e for protostars in binary/ multiple systems is larger than that in single systems. We discuss these results more thoroughly in Section 5below. 4.3. N 2 H + Maps of 16 Protostellar Cores There are 16 protostellar cores in ALMASOP that have been mapped in N 2 H + (J=1–0)line emission with the No:45m telescope (Tatematsu et al. 2021). Among the 16 cores, 5 are forming binary/multiple systems and the other 11 contain single protostars. We note that the fraction of binary/multiple systems in this small sample is similar to that of the whole ALMASOP sample. With these data, we can investigate whether there are significant differences in gas kinematics between the two groups of cores. We fit the hyperfine spectra to all N 2 H + emission lines with signal-to-noise ratios higher than 3, and finally derive the centroid velocity field maps of these cores. The velocity field maps of dense cores that are forming single systems are shown in Figure 4, while the maps of cores forming binary/multiple systems are shown in Figure 5. Most of the cores show velocity gradients in the N 2 H + emission maps, no matter how many protostars have formed within them. To evaluate the gas kinematics quantitatively, we derive local velocity gradients across these maps in steps of 10″. The local velocity gradients are shown as arrows on the velocity field maps. In Table 5, we present the maximum, minimum, mean, and standard deviation of the local velocity gradients in these 16 dense cores. The median velocity gradient of the single-system cores ranges from 1.92 to 7.21 km s −1 pc −1 , with a mean value of 3.94 km s −1 pc −1 . For comparison, the median velocity gradient of the binary/multiple cores ranges from 1.90 to 7.08 km s −1 pc −1 , with a mean value of 4.05 km s −1 pc −1 . We also list the statistics of the velocity gradients in Table 4. These statistics indicate that there is no significant difference in velocity gradient between the two groups of cores with a pvalue in the KS test of 92%. These results, however, need to be tested with a much larger sample of cores from higher angular resolution observations, which may separate more clearly the core kinematics from those of the surroundings. The ALMASOP data, unfortunately, do not have high enough spectral resolution or suitable molecular line tracers for gas kinematics studies on the core scale. Table 4 Comparison of Physical Parameters of Cores Parameter Core(Single system)Core(Binary/Multiple system)KS-test Number Mean Median sigma Number Mean Median sigma statistic p-value N(H 2 )(×10 23 cm −2 )28 2.76 1.90 2.69 13 5.06 4.50 3.52 0.45 4.3% n H2 (×10 5 cm −3 )28 8.33 3.70 9.19 13 13.31 7.60 10.64 0.41 7.0% 24 1.06 1.00 0.34 12 1.51 1.50 0.43 0.50 3.0% L jeans (10 −2 pc)27 3.95 3.97 1.98 13 3.16 2.86 2.11 0.36 15.0% M cor e (M e )28 2.27 1.84 2.39 13 3.67 2.65 3.08 0.31 28.0% M enve+disk (M e )28 0.30 0.15 0.47 13 0.47 0.29 0.58 0.35 18.9% M enve disk *+(M e )28 0.30 0.15 0.47 34 0.18 0.11 0.38 0.22 40.1% Size(×10 −1 pc)28 1.02 1.00 0.60 13 1.02 0.90 0.45 0.22 89.7% Velocity Gradient (km/s)11 3.93 3.60 1.55 5 4.05 3.62 1.70 0.25 92.6% Note. G208.68-19.20N2 is not included in the statistics because it is a protobinary system candidate. 8 The Astrophysical Journal, 931:158 (22pp), 2022 June 1 Luo et al. 4.4. Separation of Protostars in Binary/Multiple Systems We derive the projected separations among member protostars, for 36 protostars in the 13 binary/multiple-system cores. For high-order systems, we use the MiSTree (minimum spanning tree)package 29 to get the distance or separation between protostars. Figure 6shows the distribution of separation in the binary/multiple systems. The projected separation between companion protostars of the whole sample ranges from 300 au to 8900 au, and the median projected separation is about 2100 au. Our result highlights a bimodal behavior in the projected separation distribution with one peak around 500 au and another peak around 3500 au, similar in character to the bimodal distribution of projected separations presented in Tobin et al. (2016)and Tobin et al. (2022). 5. Discussion 5.1. Origin of the Multiplicity of Protostars in Dense Cores In Section 4.2, we statistically compare the physical properties of dense cores that form different systems. We find that dense cores forming binary/multiple systems are Figure 3. Cumulative distribution functions of the physical parameters in the two groups of dense cores. The compared core parameters are listed in Table 4:H 2 column density, mean density, Mach number, core mass, Jeans length, mass of envelope and disk, and size of core. In each panel, the blue line stands for single-star systems and the red line stands for binary/multiple systems. M enve+disk stands for the total envelope+disk mass of a core. M enve disk *+stands for each protostar’s envelope+disk mass. 29 https://joss.theoj.org/papers/10.21105/joss.01721.pdf 9 The Astrophysical Journal, 931:158 (22pp), 2022 June 1 Luo et al. Figure 8. (Continued.) 16 The Astrophysical Journal, 931:158 (22pp), 2022 June 1 Luo et al. Figure 8. (Continued.) 17 The Astrophysical Journal, 931:158 (22pp), 2022 June 1 Luo et al. Figure 9. CO outflows. 18 The Astrophysical Journal, 931:158 (22pp), 2022 June 1 Luo et al. G205.46-14.56M2_C and G205.46-14.56M2_D are relatively close, and their separation is ∼1000 au. Their envelope+disk masses are 0.091 ±0.039 M e and 0.028 ±0.012 M e . G205.46-14.56M2_D of the two protostars is certified to be in the Class I phase. The distance between these two pairs is ∼3800 au. In our analysis, we find that G205.46-14.56M2_E, located ∼5000 au away from the other four protostars, has an unknown evolutionary stage with an envelope+disk mass of 0.011 ±0.005 M e . As shown in Figure 9, G205.46-14.56M2_B and G205.4614.56M2_D drive weak but well-collimated outflows. G205.46-14.56M2_A and G205.46-14.56M2_C are also associated with weak outflows. This multiple system will be discussed in a detailed paper by T. Liu et al. (2022, in preparation). A.4. G205.46-14.56S1 G205.46-14.56S1 is located in the HH 24–26 region of Orion B. HH 25MMS, a Class 0 source studied extensively by the JCMT, VLA, and IRAM, is situated near this dense core (Gibb & Davis 1998; Bontemps et al. 1995; Lis et al. 1999; Phillips et al. 2001; Gibb et al. 2004; Chen et al. 2013).In addition, several YSOs were discovered in the vicinity by Spitzer, AKARI, and JCMT SCUBA surveys (Megeath et al. 2012; Noble et al. 2013; Kang et al. 2015; Furlan et al. 2016; Kirk et al. 2016). As shown in Figure 8(a), no infrared sources are found at the center of the 850 μm continuum emission map. Two protostars, however, are identified in our ALMA observation. G205.4614.56S1_A and G205.46-14.56S1_B are in the Class 0 phase with a separation of ∼5200 au and their envelope+disk masses are 0.151 ±0.065 M e and 0.391 ±0.167 M e , respectively. Figure 9shows a very collimated outflow traced by 12 CO that is driven by G205.46-14.56S1_B. On the other hand, G205.46-14.56S1_A drives a much weaker outflow, which seems to overlay the outflow from G205.46-14.56S1_B. A.5. G206.93-16.61E2 G206.93-16.61E2 in Orion B is found close to the reflection nebula NGC 2023. Several YSOs have been discovered in JCMT SCUBA-2, Spitzer, and 2MASS all-sky surveys near G206.93-16.61E2 (Cutri et al. 2003; Megeath et al. 2012; Kirk et al. 2016). As shown in Figure 8(a), a bright infrared source is detected in the ALMA FOV. In the ALMA image, as shown in Figure 8(c), the infrared source is resolved into four protostars in this system. Since these four protostars cannot be separated in the infrared band, their evolutionary stages are not well determined. Their envelope+disk masses are 0.280 ±0.120 M e , 0.112 ±0.048 M e , 0.219 ±0.097 M e , and 0.252 ±0.110 M e . Three protostars (A, C, and D)are close to one another with an average separation of ∼450 au, while the other protostar G206.93-16.61E2_B is approximately 1000 au away from them. From Figure 9, one can see that the outflows of this core are very complicated, which seems to be caused by the orbital rotation of the member protostars in the multiple system. More details of this system will be presented in a following paper (Q. Luo et al. 2022, in preparation). A.6. G207.36-19.82N1 G207.36-19.82N1 is located in Orion A. In the VISTA Orion A survey, a protostar candidate named 2MASS J053051290410322 was identified, which we regard as G207.3619.82N1_B in our ALMA data observation (Meingast et al. 2018). An HH 58 object was also discovered near the core (Tatematsu et al. 2017). Figure 8(a)shows an infrared source slightly offset from the center of the 850 μm continuum, implying that the evolutionary stage of G207.36-19.82N1_B may be similar to that of G207.36-19.82N1_A. In our ALMA observation, we find a binary system in this dense core. The two protostars have envelope+disk masses of 0.113 ±0.048 M e and Table 6 ALMASOP Sources Observed in the VANDAM Survey ALMASOP Sources HOPS Class of HOPS (1)(2)(3) Binary/Multiple Systems G205.46-14.56M1_A HOPS-317-A 0 G205.46-14.56M1_B HOPS-317-B 0 G205.46-14.56M2_A HOPS-387-B I G205.46-14.56M2_B HOPS-387-A I G205.46-14.56M2_C HOPS-386-A I G205.46-14.56M2_D HOPS-386-B I G205.46-14.56M2_E HOPS-386-C I G205.46-14.56S1_A HOPS-358-A 0 G205.46-14.56S1_B HOPS-358-B 0 G206.93-16.61E2_A HOPS-298-A I G206.93-16.61E2_B HOPS-298-B I G208.68-19.20N2_B HOPS-89 Flat G208.68-19.20N3_B HOPS-92-A-A Flat G208.68-19.20N3_C HOPS-92-B Flat G208.68-19.20S_A HOPS-84-A I G208.68-19.20S_B HOPS-84-B I G209.55-19.68N1_B HOPS-12-B-A 0 G209.55-19.68N1_C HOPS-12-A 0 G210.49-19.79W_A HOPS-168-A 0 G210.97-19.33S2_A HOPS-377 0 G210.97-19.33S2_B HOPS-144 I G211.47-19.27N_A HOPS-290-B 0 G211.47-19.27N_B HOPS-290-A 0 G212.10-19.15N2_A HOPS-263 I G212.10-19.15N2_B HOPS-262 Flat Single-star Systems G205.46-14.56N1 HOPS-402 0 G205.46-14.56N2 HOPS-401 0 G205.46-14.56S2 HOPS-385 Flat G205.46-14.56S3 HOPS-315 I G206.12-15.76 * HOPS-400-B 0 G206.93-16.61W2 HOPS-399 0 G209.55-19.68S1 HOPS-11 0 G209.55-19.68S2 HOPS-10 0 G210.37-19.53S HOPS-164 0 G210.83-19.47S HOPS-156 I G211.01-19.54N HOPS-153 0 G211.01-19.54S HOPS-152 0 G211.16-19.33N2 HOPS-133 I G211.47-19.27S * HOPS-288-A-A 0 G212.10-19.15S HOPS-247 0 G212.84-19.45N HOPS-244 0 Note. The two sources marked with * are identified in the VANDAM survey as binary/multiple systems. HOPS-400 contains two protostars, A and B, separated by 180 au. HOPS-288 contains three protostars; the remaining two protostars are 60 au and 250 au away from HOPS-288-A-A. 19 The Astrophysical Journal, 931:158 (22pp), 2022 June 1 Luo et al. 0.011 ±0.005 M e and the separation between them is ∼5400 au. As shown in Figure 8(c), G207.36-19.82N1_A shows a flattened disklike structure. Figure 9reveals a high-velocity outflow with asymmetric structures in the vicinity of G207.36-19.82N1_A, while G207.36-19.82N1_B is not associated with an outflow. Both of them are likely at the Class II phase or an even later phase. A.7. G208.68-19.20N2 G208.68-19.20N2 is located in Orion A and some YSOs were discovered to be associated with it in JCMT SCUBA and Green Bank Telescope 3.3 mm continuum emission observations (Di Francesco et al. 2008; Schnee et al. 2014). A protobinary system candidate containing a starless core and a protostar is found in this dense core based on our ALMA 1.3 mm dust continuum observations. The starless core G208.68-19.20N2_A is very dense and will collapse to form a new protostar (Sahu et al. 2021b). The protostar G208.6819.20N2_B is not associated with an outflow, indicating that it is likely an evolved Class I protostar. The envelope+disk mass of this protostar is just 0.009 ±0.001 M e . A.8. G208.68-19.20N3 G208.68-19.20N3 is located in a filament in Orion A. As shown in Figure 8(b), our ACA 1.3 mm continuum observations resolve the core into two subcores. The southern subcore is associated with an infrared object, which is further resolved into a close binary in our high-resolution ALMA observations shown in Figure 8(c). In total, the ALMA 1.3 mm emission continuum reveals three protostars in this core. G208.6819.20N3_A is a Class 0 protostar and its distance from the other two protostars is ∼6000 au. G208.68-19.20N3_B and G208.68-19.20N3_C are only ∼600 au apart. Both of them are Class I protostars, and are located offset from the center of the SCUBA-2 core. These three protostars have envelope+disk masses of 0.436 ±0.189 M e , 0.078 ±0.033 M e , and 0.093 ±0.040 M e , respectively. Figure 9shows a collimated outflow from G208.68-19.20N3_A that is perpendicular to its extended disklike structure. The two remaining protostars are also associated with outflows but most of their outflows are located beyond our FOV. A.9. G208.68-19.20S G208.68-19.20S is in Orion A, and several protostars have been discovered there in the AzTEC 1.1 mm survey (Shimajiri et al. 2015). As shown in Figure 8(c), two Class I protostars are found in our ALMA observations, and they are close to each other with a separation of only ∼300 au. Their envelope+disk masses are 0.421 ±0.180 M e for protostar A and 0.043 ±0.061 M e for protostar B. As shown in Figure 9, the binary system drives an outflow along the east–west direction and the outflow is much weaker than other Class 0 outflows in the ALMASOP sample. Moreover, there is no obvious redshift emission of the highspeed component of 12 CO around the protostars. A.10. G209.55-19.68N1 G209.55-19.68N1 is in Orion A, and a young star named HOPS-12 has been discovered near this dense core (Furlan et al. 2016). From ALMA data, three protostars are detected in this dense core. Figure 8(c)shows two substructures encasing the three protostars. G209.55-19.68N1_B and G209.55-19.68N1_C reside in the same substructure and are about 400 au apart. G209.5519.68N1_A lies in another substructure and is separated from G209.55-19.68N1_B by ∼4800 au. Their envelope+disk masses are 0.141 ±0.060 M e ,0.061±0.027 M e , and 0.132 ±0.057 M e for components A, B, and C, respectively. As shown in Figure 9, G209.55-19.68N1_A drives a very collimated outflow. The red lobe of outflow from G209.5519.68N1_B, however, is distinctly distorted. Moreover, the CO emission near G209.55-19.68N1_C is weak and may be contaminated by the outflow from G209.55-19.68N1_B. A.11. G210.49-19.79W G210.49-19.79W is in Orion A. In the vicinity of the dense core, past VLA observations have detected the VLA 4 source with an H 2 O maser (Rodríguez et al. 2000). There are two protostars in different evolutionary stages in the dense core. G210.49-19.79W_A is proved to be in Class 0 and its envelope+disk mass is 0.201 ±0.086 M e . G210.49-19.79W_B is at a more evolved stage and its envelope+disk mass is 0.008 ±0.003 M e . The separation of one from the other is ∼2800 au. The younger protostar G210.49-19.79W_A is located at the center of the SCUBA-2 core, while the older protostar is clearly offset from the core center. As shown in Figure 9, G210.49-19.79W_A drives a collimated wide-angle outflow. We do not detect 12 CO emission in the surroundings of G210.49-19.79W_B, which may imply that G210.49-19.79W_B has stopped accretion. A.12. G210.97-19.33S2 G210.97-19.33S2 is located in Orion A, and we have identified a protobinary system in its center. Its envelope+disk masses are 0.020 ±0.009 M e and 0.018 ±0.008 M e . The separation between the two protostars (AandB)is ∼3400 au. Furthermore, we have discovered two additional protostars in the outer part of the core: G210.97-19.33S2_C and G210.97-19.33S2_D. Figure 8(c)shows that G210.97-19.33S2_C and G210.97-19.33S2_D are more widely separated from the core center. Their position is near the edge of the FOV, and their emission is weak. According to the high-velocity 12 CO emission map in Figure 9, we have only detected outflows near G210.9719.33S2_A. The CO emission around G210.97-19.33S2_B is very faint. G210.97-19.33S2_C and G210.97-19.33S2_D do not exhibit any line emission. A.13. G211.47-19.27N G211.47-19.27N is a faint SCUBA-2 850 μm source to the north of a star cluster in Orion A. Several YSOs have been found in its surroundings by Spitzer observations and the Bolocam Galactic Plane Survey (Megeath et al. 2012; Merello et al. 2015). The infrared source associated with G211.4719.27N is very faint and diffuse, as shown in Figures 8(a)and (b). As shown in Figure 8(c), two protostars are identified in the ALMA data, and they share a common envelope. The distance between the two protostars is ∼300 au, indicating that they are forming a close binary system. These two protostars are in the Class 0 phase, and their envelope+disk masses are 0.067 ±0.030 M e and 0.039 ±0.019 M e , respectively. 20 The Astrophysical Journal, 931:158 (22pp), 2022 June 1 Luo et al. From Figure 9, we can see that the two Class 0 protostars drive powerful outflows, but it is impossible to identify the outflow-driving source. A.14. G212.10-19.15N2 G212.10-19.15N2 is in Orion A. Two YSOs were detected in this core in Spitzer and 2MASS observations (Cutri et al. 2003; Megeath et al. 2012). We have identified two Class I protostars in this dense core. G212.10-19.15N2_A has an envelope+disk mass of 0.034 ±0.014 M e , and G212.10-19.15N2_B has an envelope +disk mass of 0.014 ±0.006 M e . They are ∼2800 au apart. According to Figure 9, neither of these two protostars exhibits a clear outflow. ORCID iDs Qiu-yi Luo (罗秋怡)https://orcid.org/0000-0003-4506-3171 Tie Liu (刘铁)https://orcid.org/0000-0002-5286-2564 Ken’ichi Tatematsu https://orcid.org/0000-0002-8149-8546 Sheng-Yuan Liu https://orcid.org/0000-0003-4603-7119 Pak Shing Li https://orcid.org/0000-0001-8077-7095 James di Francesco https://orcid.org/0000-0002-9289-2450 Doug Johnstone https://orcid.org/0000-0002-6773-459X Paul F. Goldsmith https://orcid.org/0000-0002-6622-8396 Somnath Dutta https://orcid.org/0000-0002-2338-4583 Naomi Hirano https://orcid.org/0000-0001-9304-7884 Chin-Fei Lee https://orcid.org/0000-0002-3024-5864 Di Li https://orcid.org/0000-0003-3010-7661 Kee-Tae Kim https://orcid.org/0000-0003-2412-7092 Chang Won Lee https://orcid.org/0000-0002-3179-6334 Jeong-Eun Lee https://orcid.org/0000-0003-3119-2087 Mika Juvela https://orcid.org/0000-0002-5809-4834 Jinhua He https://orcid.org/0000-0002-3938-4393 Sheng-Li Qin https://orcid.org/0000-0003-2302-0613 Hong-Li Liu https://orcid.org/0000-0003-3343-9645 David Eden https://orcid.org/0000-0002-5881-3229 Woojin Kwon https://orcid.org/0000-0003-4022-4132 Dipen Sahu https://orcid.org/0000-0002-4393-3463 Shanghuo Li https://orcid.org/0000-0003-1275-5251 Feng-Wei Xu https://orcid.org/0000-0001-5950-1932 Shih-Ying Hsu https://orcid.org/0000-0002-1369-1563 Leonardo Bronfman https://orcid.org/0000-0002-9574-8454 Patricio Sanhueza https://orcid.org/0000-0002-7125-7685 Veli-Matti Pelkonen https://orcid.org/0000-0002-8898-1047 Yue-fang Wu https://orcid.org/0000-0002-5076-7520 Xiao-feng Mai https://orcid.org/0000-0001-7573-0145 Edith Falgarone https://orcid.org/0000-0003-0693-2477 Zhi-Qiang Shen https://orcid.org/0000-0003-3540-8746 References Adams, F. 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