Molecular and Atomic Clouds toward the Wolf-Rayet Nebula NGC 2359: Possible Evidence for Isolated High-Mass Star Formation Triggered by a Cloud-Cloud Collision Hidetoshi SANO1,2, Katsuhiro HAYASHI2 , Rei ENOKIYA2 , Kazufumi TORII3, Shun SAEKI2 , Kazuki OKAWA2 , Kisetsu TSUGE2 , Daichi TSUTSUMI2 , Mikito KOHNO2, Yusuke HATTORI2 , Shinji FUJITA2 , Satoshi YOSHIIKE2 , Ryuji OKAMOTO2 , Atsushi NISHIMURA4 , Akio OHAMA2 , Takahiro HAYAKAWA2 , Hiroaki YAMAMOTO2 , Kengo TACHIHARA2 , Cristina Elisabet CAPPA5,6 and Yasuo FUKUI1,2 1Institute for Advanced Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan 2Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan 3Nobeyama Radio Observatory, Minamimaki-mura, Minamisaku-gun, Nagano 384-1305, Japan 4Department of Physical Science, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan 5Instituto Argentino de Radioastronom´ıa, CONICET, Argentina. arXiv:1708.08149v2 [astro-ph.GA] 17 Mar 2020 6Facultad de Ciencias Astronomicas´ y Geof´ısicas, Universidad Nacional de la Plata, Paseo del Bosque s/n, 1900, La Plata, Argentina. ∗E-mail: [email protected] Received ; Accepted Abstract NGC 2359 is an HII region located in the outer Galaxy that contains the isolated Wolf-Rayet (WR) star HD 56925. We present millimeter/submillimeter observations of 12CO(J = 1–0, 3–2) 1 line emission toward the entire nebula. We identified that there are three molecular clouds −1 at VLSR ∼37, ∼54, and ∼67 km s , and three HI clouds: two of them are at VLSR ∼54 km s−1 and the other is at ∼63 km s−1. These clouds except for the CO cloud at 67 km s−1 are limb-brightened in the radio continuum, suggesting part of each cloud has been ionized. We newly found an expanding gas motion of CO/HI, whose center and expansion velocities are ∼51 and ∼4.5 km s−1, respectively. This is consistent with large line widths of the CO − − and HI clouds at 54 km s 1. The kinematic temperature of CO clouds at 37 and 54 km s 1 are derived to be 17 and 61 K, respectively, whereas that of the CO cloud at 67 km s−1 is only 6 K, indicating that the former two clouds have been heated by strong UV radiation. We − concluded that the 37 and 54 km s 1 CO clouds and three HI clouds are associated with NGC 2359, even if these clouds have different velocities. Although the velocity difference including the expanding motion are typical signatures of the stellar feedback from the exciting star, our analysis revealed that the observed large momentum for the 37 km s−1 CO cloud cannot be explained only by the total wind momentum of the WR star and its progenitor. We therefore propose an alternative scenario that the isolated high-mass progenitor of HD 56925 was formed by a collision between the CO clouds at 37 and 54 km s−1. If we apply the collision scenario, NGC 2359 corresponds to the final phase of the cloud-cloud collision. Key words: ISM: HII regions—Stars: formation—ISM: individual objects (NGC 2359) 1 Introduction It is a longstanding question how isolated (or so-called “field”) high-mass stars are formed in inter- stellar space, because high-mass stars are thought to be formed as a cluster (e.g., Motte et al. 2018 and references therein). Recently, the cloud-cloud collision model has received considerable attention as a formation mechanism not only for high-mass stellar clusters but also for isolated high-mass stars. Torii et al. (2015) showed that the isolated O-type star associated with the HII region RCW 120 was formed by a collision between two molecular clouds, that have a velocity separation of ∼20 km s−1. The high intensity ratios (> 0.8)ofCO J = 3–2 / 1–0 toward both clouds correspond to a kinetic tem- perature of ∼> 20–40 K, indicating radiative heating of the two clouds by the O-type star. They also found that the two clouds have complementary spatial distributions and that a bridging feature of HI physically connects the two CO clouds in velocity space. The authors concluded that the complemen- tary spatial distributions and the bridging feature can be interpreted as typical cloud-cloud collision 2 signatures. Therefore, the isolated O-type star in RCW 120 was formed by strong gas compression during the cloud-cloud collision. Theoretical studies also support this idea of triggered star formation because a collision be- tween two dense clouds increases the effective sound speed and gas density in the shocked layer (e.g., Habe & Ohta 1992; Anathpindika 2010; Inoue & Fukui 2013; Takahira et al. 2014; Inoue et al. 2018). According to Inoue et al. (2018), isothermal magnetohydrodynamic simulations of cloud-cloud colli- −4 −1 sions can achieve high mass accretion rates Macc > 10 M⊙ yr , which is high enough to allow the formation of O-type stars (e.g., Wolfire & Cassinelli 1987). They also found that the most massive 5 sink particle, which has a mass of 50 M⊙ or higher, was created within a few 10 yr after the collision. As of early 2020, observational evidence for cloud-cloud collisions as a formation mechanism for O-type (or early B-type) stars has been obtained from 34 sources associated with stellar clusters containing more than two high-mass stars (DR 21, Dobashi et al. 2019; Sgr B2, Hasegawa et al. 1994; W 49 N, Serabyn et al. 1993; Arches cluster, Stolte et al. 2008; Westerlund 2, Furukawa et al. 2009, Ohama et al. 2010; NGC 3603, Fukui et al. 2014; 50 km s−1 cloud in Sgr A, Tsuboi et al. 2015; RCW 38, Fukui et al. 2016; R 136, Fukui et al. 2017a; Sh2-237, Dewangan et al. 2017a; N37 & G35.20−0.74, Dewangan 2017; E-S235ABC, Dewangan & Ojha 2017; N49, Dewangan et al. 2017b; M42, Fukui et al. 2018a; NGC 6334 & NGC 6357, Fukui et al. 2018b; NGC 6618, Nishimura et al. 2018; RCW 36, Sano et al. 2018; RCW 79, Ohama et al. 2018a; S116, S117 & S118, Fukui et al. 2018d; N21 & N22, Ohama et al. 2018b; W 33, Kohno et al. 2018a; NGC 604, Tachihara et al. 2018; G24.80+0.10, Dewangan et al. 2018b; LHA 120-N 44, Tsuge et al. 2019a; AFGL 5142, Dewangan et al. 2019a; G49.5−0.4 & G49.4−0.3, Fujita et al. 2019b; M33GMC 37, Sano et al. 2019b; Firecracker in Antennae Galaxies, Tsuge et al. 2019b), and with 27 sources associated with an isolated high- mass star (IRAS 04000+5052, Wang et al. 2004; BD +40 4124, Looney et al. 2006; S87, Xue & Wu 2008; M20, Torii et al. 2011, 2017a; RCW 120, Torii et al. 2015; N 159 W-South, Fukui et al. 2015, Tokuda et al. 2019; N 159 E-Papillon, Saigo et al. 2017, Fukui et al. 2019; E-S235 main East1 & Southwest, Dewangan & Ojha 2017; M43, Fukui et al. 2018a; GM 24, Fukui et al. 2018c; G018.149−0.283, Ohama et al. 2018b; RCW 34, Hayashi et al. 2018; RCW 32, Enokiya et al. 2018; N35, G024.392+00.072 & G024.510−00.060, Torii et al. 2018a; Sh2-48, Torii et al. 2018b; S44, Kohno et al. 2018b; N4, Fujita et al. 2019a; G49.57−0.27, Fujita et al. 2019b; IRAS 18223−1243, Dewangan et al. 2018a; G24.85+0.09, G24.74+0.08, G24.71−0.13, and G24.68−0.16, Dewangan et al. 2018b; G8.14+0.23, Dewangan et al. 2019b). However, isolated Wolf-Rayet (WR) stars have not previously been studied as products of star formation triggered by a cloud-cloud collision. A WR star is thought to be a late evolutionary stage of an O-type star with a mass of ∼25 M⊙ or higher. WR stars have lost their hydrogen envelopes via 3 strong stellar winds, with typical velocities and mass-loss rates of ∼1,000 km s−1 to 3,000 km s−1 −5 −1 and ∼10 M⊙ yr , respectively (e.g., Crowther 2007 and references therein). To explore the origin of an isolated high-mass star that can become a WR star, we need to clarify whether the formation of the progenitor of isolated WR star may have been also triggered by a cloud-cloud collisions. NGC 2359 (also known as Sh 2–298 or Thor’s Helmet) is an optical ring nebula associated with the isolated WR star HD 56925 (= WR 7, van der Hucht et al. 1981). Figure 1(a) shows an optical image of NGC 2359 (courtesy of Robert Franke) obtained at the Focal Pointe Observatory. The WR star is categorized as spectral type WN4b (Smith et al. 1996) and is located at (αJ2000, h m s ◦ ′ ′′ ′ δJ2000) = (07 18 29.13, −13 13 01. 5). It has formed a wind-blown bubble (∼4.5 in diameter) with bright optical lobes in the directions of northwest, southwest, and northeast (as shown in Figure 1a). By using the interferometric profiles of Hα, [OIII], and [NII], the expansion velocity of the bubble was found to range from ∼15 to 30 km s−1 (Schneps et al. 1981; Treffers & Chu 1982; Goudis et al. 1983; Goudis et al. 1994; Meaburn et al. 1994) up to 55 ± 25 km s−1 (Lozinskaya 1973). h m s ◦ ′ ′′ Two prominent optical dark lanes are located at (αJ2000, δJ2000) ∼ (07 18 36.84, −13 16 46 ) and (07h18m44s.15, −13◦12′26′′). The nebula is also bright in radio continuum. Figure 1(b) shows the radio continuum image with the Very Large Array (VLA; Cappa et al.