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تسجيل مستخدم جديدDiscovery of Star Formation in the Extreme Outer Galaxy Possibly Induced by a High-velocity Cloud Impact
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We report the discovery of star formation activity in perhaps the most distant molecular cloud in the extreme outer galaxy. We performed deep near infrared imaging with the Subaru 8.2 m telescope, and found two young embedded clusters at two CO peaks of Digel Cloud 1 at the kinematic distance of D = 16 kpc (Galactocentric radius RG = 22 kpc). We identified 18 and 45 cluster members in the two peaks, and the estimated stellar density are ~ 5 and ~ 3 pc^-2, respectively. The observed K-band luminosity function suggests that the age of the clusters is less than 1 Myr and also the distance to the clusters is consistent with the kinematic distance. On the sky, Cloud 1 is located very close to the H I peak of high-velocity cloud (HVC) Complex H, and there are some H I intermediate velocity structures between the Complex H and the Galactic disk, which could indicate an interaction between them. We suggest possibility that Complex H impacting on the Galactic disk has triggered star formation in Cloud 1 as well as the formation of Cloud 1 molecular cloud.
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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.
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.
The extreme outer Galaxy (EOG) has a very different environment from that in the solar neighborhood, with low metallicity (less than -0.5 dex), much lower gas density, and small or no perturbation from spiral arms. The EOG is an excellent laboratory
for the study of the star formation processes that happened during the formation period of the Galaxy. In particular, the study of the EOG may shed light on the origin and role of the thick disk, whose metallicity range matches well with that of the EOG. We show an example of a molecular cloud in the EOG (Digels Cloud 2), which is located at R_g ~ 20 kpc beyond the Outer arm. Based on our NIR and 12CO data as well as HI, radio continuum, and IRAS data in the archives, we examined the detailed star formation processes in this unique environment, especially the supernova triggered star formation, which should have been the major star formation mode during the halo and thick disk formation.
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