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تسجيل مستخدم جديدHigh-mass star formation in Orion B triggered by cloud-cloud collision: Merging molecular clouds in NGC 2024
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We performed new comprehensive $^{13}$CO($J$=2--1) observations toward NGC 2024, the most active star forming region in Orion B, with an angular resolution of $sim$100 obtained with NANTEN2. We found that the associated cloud consists of two independent velocity components. The components are physically connected to the H{sc ii} region as evidenced by their close correlation with the dark lanes and the emission nebulosity. The two components show complementary distribution with a displacement of $sim$0.6 pc. Such complementary distribution is typical to colliding clouds discovered in regions of high-mass star formation. We hypothesize that a cloud-cloud collision between the two components triggered the formation of the late O-type stars and early B stars localized within 0.3 pc of the cloud peak. The duration time of the collision is estimated to be 0.3 million years from a ratio of the displacement and the relative velocity $sim$3 km s$^{-1}$ corrected for probable projection. The high column density of the colliding cloud $sim$10$^{23}$ cm$^{-2}$ is similar to those in the other high-mass star clusters in RCW 38, Westerlund 2, NGC 3603, and M42, which are likely formed under trigger by cloud-cloud collision. The present results provide an additional piece of evidence favorable to high-mass star formation by a major cloud-cloud collision in Orion.
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We analyzed the NANTEN2 13CO (J=2-1 and 1-0) datasets in NGC 2024. We found that the cloud consists of two velocity components, whereas the cloud shows mostly single-peaked CO profiles. The two components are physically connected to the HII region as
evidenced by their close correlation with the dark lanes and the emission nebulosity. The two components show complementary distribution with a displacement of 0.4 pc. Such complementary distribution is typical to colliding clouds discovered in regions of high-mass star formation. We hypothesize that cloud-cloud collision between the two components triggered the formation of the late O stars and early B stars localized within 0.3 pc of the cloud peak. The collision timescale is estimated to be ~ 10^5 yrs from a ratio of the displacement and the relative velocity 3-4 km s-1 corrected for probable projection. The high column density of the colliding cloud 1023 cm-2 is similar to those in the other massive star clusters in RCW 38, Westerlund 2, NGC 3603, and M42, which are likely formed under trigger by cloud-cloud collision. The present results provide an additional piece of evidence favorable to high-mass star formation by a major cloud-cloud collision in Orion.
Using the NANTEN2 Observatory, we carried out a molecular line study of high-mass star forming regions with reflection nebulae, NGC 2068 and NGC 2071, in Orion in the 13CO(J=2-1) transition. The 13CO distribution shows that there are two velocity com
ponents at 9.0 and 10.5 km/s . The blue-shifted component is in the northeast associated with NGC 2071, whereas the red-shifted component is in the southwest associated with NGC 2068. The total intensity distribution of the two clouds shows a gap of ~1 pc, suggesting that they are detached at present. A detailed spatial comparison indicates that the two show complementary distributions. The blue-shifted component lies toward an intensity depression to the northwest of the red-shifted component, where we find that a displacement of 0.8 pc makes the two clouds fit well with each other. Furthermore, a new simulation of non-frontal collisions shows that observations from 60 degrees off the collisional axis agreed well with the velocity structure in this region. On the basis of these results, we hypothesize that the two components collided with each other at a projected relative velocity 3.0 km/s estimated to be 0.3 Myr for an assumed axis of the relative motion 60 degrees off the line of sight. We assume that the two most massive early B-type stars in the cloud, illuminating stars of the two reflection nebulae, were formed by collisional triggering at the interfaces between the two clouds. Given the other young high-mass star forming regions, namely, M42, M43, and NGC 2024 (Fukui et al. 2018b; Ohama et al. 2017a), it seems possible that collisional triggering has been independently working to form O-type and early B-type stars in Orion in the last Myr over a projected distance of ~80 pc.
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 $^{12}$CO($J$ = 1-0, 3-2) line emission toward the entire nebula. We identified that t
here are three molecular clouds at VLSR $sim$37, $sim$54, and $sim$67 km s$^{-1}$, and three HI clouds: two of them are at VLSR $sim$54 km s$^{-1}$ and the other is at $sim$63 km s$^{-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 $sim$51 and $sim$4.5 km s$^{-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.
We report a possibility that the high-mass star located in the HII region RCW 34 was formed by a triggering induced by a collision of molecular clouds. Molecular gas distributions of the $^{12}$CO and $^{13}$CO $J=$2-1, and $^{12}$CO $J=$3-2 lines to
ward RCW 34 were measured by using the NANTEN2 and ASTE telescopes. We found two clouds with the velocity ranges of 0-10 km s$^{-1}$ and 10-14 km s$^{-1}$. Whereas the former cloud as massive as ~2.7 x 10$^{4}$ Msun has a morphology similar to the ring-like structure observed in the infrared wavelengths, the latter cloud with the mass of ~10$^{3}$ Msun, which has not been recognized by previous observations, distributes just likely to cover the bubble enclosed by the other cloud. The high-mass star with the spectral types of O8.5V is located near the boundary of the two clouds. The line intensity ratio of $^{12}$CO $J=$3-2 / $J=$2-1 yields high values (~1.5) in the neighborhood of the high-mass star, suggesting that these clouds are associated with the massive star. We also confirmed that the obtained position-velocity diagram shows a similar distribution with that derived by a numerical simulation of the supersonic collision of two clouds. Using the relative velocity between the two clouds (~5 km s$^{-1}$), the collisional time scale is estimated to be $sim$0.2 Myr with the assumption of the distance of 2.5 kpc. These results suggest that the high-mass star in RCW 34 was formed rapidly within a time scale of ~0.2 Myr via a triggering of cloud-cloud collision.
A collision between two molecular clouds is one possible candidate for high-mass star formation. The HII region RCW 36, located in the Vela molecular ridge, contains a young star cluster with two O-type stars. We present new CO observations of RCW 36
with NANTEN2, Mopra, and ASTE using $^{12}$CO($J$ = 1-0, 2-1, 3-2) and $^{13}$CO($J$ = 2-1) line emissions. We have discovered two molecular clouds lying at the velocities $V_mathrm{LSR} sim$5.5 and 9 km s$^{-1}$. Both clouds are likely to be physically associated with the star cluster, as verified by the good spatial correspondence among the two clouds, infrared filaments, and the star cluster. We also found a high intensity ratio of $sim$0.6-1.2 for CO $J$ = 3-2 / 1-0 toward both clouds, indicating that the gas temperature has been increased due to heating by the O-type stars. We propose that the O-type stars in RCW 36 were formed by a collision between the two clouds, with a relative velocity separation of 5 km s$^{-1}$. The complementary spatial distributions and the velocity separation of the two clouds are in good agreement with observational signatures expected for O-type star formation triggered by a cloud-cloud collision. We also found a displacement between the complementary spatial distributions of the two clouds, which we estimate to be 0.3 pc assuming the collision angle to be 45$^{circ}$ relative to the line-of-sight. We estimate the collision timescale to be $sim$10$^5$ yr. It is probable that the cluster age by Ellerbroek et al. (2013b) is dominated by the low-mass members which were not formed under the triggering by cloud-cloud collision, and that the O-type stars in the center of the cluster are explained by the collisional triggering independently from the low-mass star formation.
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