Understanding high-mass star formation is one of the top-priority issues in astrophysics. Recent observational studies have revealed that cloud-cloud collisions may play a role in high-mass star ...formation in several places in the Milky Way and the Large Magellanic Cloud. The Trifid Nebula M20 is a well-known Galactic H ii region ionized by a single O7.5 star. In 2011, based on the CO observations with NANTEN2, we reported that the O star was formed by the collision between two molecular clouds ∼0.3 Myr ago. Those observations identified two molecular clouds toward M20, traveling at a relative velocity of . This velocity separation implies that the clouds cannot be gravitationally bound to M20, but since the clouds show signs of heating by the stars there they must be spatially coincident with it. A collision is therefore highly possible. In this paper we present the new CO J = 1-0 and J = 3-2 observations of the colliding clouds in M20 performed with the Mopra and ASTE telescopes. The high-resolution observations revealed that the two molecular clouds have peculiar spatial and velocity structures, i.e., a spatially complementary distribution between the two clouds and a bridge feature that connects the two clouds in velocity space. Based on a new comparison with numerical models, we find that this complementary distribution is an expected outcome of cloud-cloud collisions, and that the bridge feature can be interpreted as the turbulent gas excited at the interface of the collision. Our results reinforce the cloud-cloud collision scenario in M20.
Collisions between giant molecular clouds are a potential mechanism for triggering the formation of massive stars, or even super star clusters. The trouble is identifying this process observationally ...and distinguishing it from other mechanisms. We produce synthetic position–velocity diagrams from models of cloud–cloud collisions, non-interacting clouds along the line of sight, clouds with internal radiative feedback and a more complex cloud evolving in a galactic disc, to try and identify unique signatures of collision. We find that a broad bridge feature connecting two intensity peaks, spatially correlated but separated in velocity, is a signature of a high-velocity cloud–cloud collision. We show that the broad bridge feature is resilient to the effects of radiative feedback, at least to around 2.5 Myr after the formation of the first massive (ionizing) star. However for a head-on 10 km s−1 collision, we find that this will only be observable from 20 to 30 per cent of viewing angles. Such broad–bridge features have been identified towards M20, a very young region of massive star formation that was concluded to be a site of cloud–cloud collision by Torii et al., and also towards star formation in the outer Milky Way by Izumi et al.
We investigate the longevity of broad bridge features in position-velocity diagrams that appear as a result of cloud-cloud collisions. Broad bridges will have a finite lifetime due to the action of ...feedback, conversion of gas into stars and the time-scale of the collision. We make a series of analytic arguments with which to estimate these lifetimes. Our simple analytic arguments suggest that for collisions between clouds larger than R ... 10 pc the lifetime of the broad bridge is more likely to be determined by the lifetime of the collision rather than the radiative or wind feedback disruption time-scale. However, for smaller clouds feedback becomes much more effective. This is because the radiative feedback time-scale scales with the ionizing flux Nly as ... so a reduction in cloud size requires a relatively large decrease in ionizing photons to maintain a given time-scale. We find that our analytic arguments are consistent with new synthetic observations of numerical simulations of cloud-cloud collisions (including star formation and radiative feedback). We also argue that if the number of observable broad bridges remains ... constant, then the disruption time-scale must be roughly equivalent to the collision rate. If this is the case, our analytic arguments also provide collision rate estimates, which we find are readily consistent with previous theoretical models at the scales they consider (clouds larger than about 10 pc) but are much higher for smaller clouds. (ProQuest: ... denotes formulae/symbols omitted.)
ABSTRACT We present distributions of two molecular clouds having velocities of 2 and 14 km s−1 toward RCW 38, the youngest super star cluster in the Milky Way, in the 12CO J = 1-0 and 3-2 and 13CO J ...= 1-0 transitions. The two clouds are likely physically associated with the cluster as verified by the high intensity ratio of the J = 3-2 emission to the J = 1-0 emission, the bridging feature connecting the two clouds in velocity, and their morphological correspondence with the infrared dust emission. The velocity difference is too large for the clouds to be gravitationally bound. We frame a hypothesis that the two clouds are colliding with each other by chance to trigger formation of the ∼20 O stars that are localized within ∼0.5 pc of the cluster center in the 2 km s−1 cloud. We suggest that the collision is currently continuing toward part of the 2 km s−1 cloud where the bridging feature is localized. This is the third super star cluster alongside Westerlund 2 and NGC 3603 where cloud-cloud collision has triggered the cluster formation. RCW 38 is the youngest super star cluster in the Milky Way, holding a possible sign of on-going O star formation, and is a promising site where we may be able to witness the moment of O star formation.
We present new large field observations of molecular clouds with NANTEN2 toward the super star cluster NGC 3603 in the transitions super(12)CO(J = 2-1, J = 1-0) and super(13)CO(J = 2-1, J = 1-0). We ...suggest that two molecular clouds at 13 km s super(-1) and 28 km s super(-1) are associated with NGC 3603 as evidenced by higher temperatures toward the H II region, as well as morphological correspondence. The mass of the clouds is too small to gravitationally bind them, given their relative motion of ~20 km s super(-1). We suggest that the two clouds collided with each other 1 Myr ago to trigger the formation of the super star cluster. This scenario is able to explain the origin of the highest mass stellar population in the cluster, which is as young as 1 Myr and is segregated within the central sub-pc of the cluster. This is the second super star cluster along with Westerlund 2 where formation may have been triggered by a cloud-cloud collision.
Context. Three dimensional interstellar extinction maps provide a powerful tool for stellar population analysis. However, until now, these 3D maps were rather limited by sensitivity and spatial ...resolution. Aims. We use data from the VISTA Variables in the Via Lactea survey together with the Besançon stellar population synthesis model of the Galaxy to determine interstellar extinction as a function of distance in the Galactic bulge covering −10°<l< 10° and −10°<b< 5°. Methods. We adopted a recently developed method to calculate the colour excess. First we constructed the H − Ks vs. Ks and J − Ks vs. Ks colour−magnitude diagrams based on the VVV catalogues that matched 2MASS. Then, based on the temperature−colour relation for M giants and the distance-colour relations, we derived the extinction as a function of distance. The observed colours were shifted to match the intrinsic colours in the Besançon model as a function of distance iteratively. This created an extinction map with three dimensions: two spatial and one distance dimension along each line of sight towards the bulge. Results. We present a 3D extinction map that covers the whole VVV area with a resolution of 6′× 6′ for J − Ks and H − Ks using distance bins of 0.5 kpc. The high resolution and depth of the photometry allows us to derive extinction maps for a range of distances up to 10 kpc and up to 30 mag of extinction in AV (3.0 mag in AKs). Integrated maps show the same dust features and consistent values as other 2D maps. We discuss the spatial distribution of dust features in the line of sight, which suggests that there is much material in front of the Galactic bar, specifically between 5−7 kpc. We compare our dust extinction map with the high-resolution 12CO maps (NANTEN2) towards the Galactic bulge, where we find a good correlation between 12CO and AV. We determine the X factor by combining the CO map and our dust extinction map. Our derived average value X = 2.5 ± 0.47 × 1020 cm-2 K-1 km-1s is consistent with the canonical value of the Milky Way. The X-factor decreases with increasing extinction.
ABSTRACT RCW 120 is a Galactic H ii region that has a beautiful ring shape that is bright in the infrared. Our new CO J = 1-0 and J = 3-2 observations performed with the NANTEN2, Mopra, and ASTE ...telescopes have revealed that two molecular clouds with a velocity separation of 20 km s−1 are both physically associated with RCW 120. The cloud at −8 km s−1 apparently traces the infrared ring, while the other cloud at −28 km s−1 is distributed just outside the opening of the infrared ring, interacting with the H ii region as suggested by the high kinetic temperature of the molecular gas and by the complementary distribution with the ionized gas. A spherically expanding shell driven by the H ii region is usually considered to be the origin of the observed ring structure in RCW 120. Our observations, however, indicate no evidence of the expanding motion in the velocity space, which is inconsistent with the expanding shell model. We postulate an alternative that, by applying the model introduced by Habe & Ohta, the exciting O star in RCW 120 was formed by a collision between the present two clouds at a collision velocity of ∼30 km s−1. In the model, the observed infrared ring can be interpreted as the cavity created in the larger cloud by the collision, whose inner surface is illuminated by the strong ultraviolet radiation after the birth of the O star. We discuss that the present cloud-cloud collision scenario explains the observed signatures of RCW 120, i.e., its ring morphology, coexistence of the two clouds and their large velocity separation, and absence of the expanding motion.
Consistent with recent reports indicating that neurons differentiated in vitro from human-induced pluripotent stem cells (hiPSCs) are immature relative to those in the human brain, gene expression ...comparisons of our hiPSC-derived neurons to the Allen BrainSpan Atlas indicate that they most resemble fetal brain tissue. This finding suggests that, rather than modeling the late features of schizophrenia (SZ), hiPSC-based models may be better suited for the study of disease predisposition. We now report that a significant fraction of the gene signature of SZ hiPSC-derived neurons is conserved in SZ hiPSC neural progenitor cells (NPCs). We used two independent discovery-based approaches-microarray gene expression and stable isotope labeling by amino acids in cell culture (SILAC) quantitative proteomic mass spectrometry analyses-to identify cellular phenotypes in SZ hiPSC NPCs from four SZ patients. From our findings that SZ hiPSC NPCs show abnormal gene expression and protein levels related to cytoskeletal remodeling and oxidative stress, we predicted, and subsequently observed, aberrant migration and increased oxidative stress in SZ hiPSC NPCs. These reproducible NPC phenotypes were identified through scalable assays that can be applied to expanded cohorts of SZ patients, making them a potentially valuable tool with which to study the developmental mechanisms contributing to SZ.
A large-scale study of the molecular clouds toward the Trifid Nebula, M20, has been made in the J = 2-1 and J = 1-0 transitions of 12CO and 13CO. M20 is ionized predominantly by an O7.5 star ...HD164492. The study has revealed that there are two molecular components at separate velocities peaked toward the center of M20 and that their temperatures--30-50 K as derived by a large velocity gradient analysis--are significantly higher than the 10 K of their surroundings. We identify the two clouds as the parent clouds of the first generation stars in M20. The mass of each cloud is estimated to be ~103 M and their separation velocity is ~8 km s--1 over ~1-2 pc. We find that the total mass of stars and molecular gas in M20 is less than ~3.2 X 103 M , which is too small by an order of magnitude to gravitationally bind the system. We argue that the formation of the first generation stars, including the main ionizing O7.5 star, was triggered by the collision between the two clouds in a short timescale of ~1 Myr, a second example alongside Westerlund 2, where a super-star cluster may have been formed due to cloud-cloud collision triggering.
Dark gas in the interstellar medium (ISM) is believed to not be detectable either in CO or H I radio emission, but it is detectable by other means including gamma rays, dust emission, and extinction ...traced outside the Galactic plane at b > 5degrees. In these analyses, the 21 cm H I emission is usually assumed to be completely optically thin. We have reanalyzed the H I emission from the whole sky at b > 15degrees by considering temperature stratification in the ISM inferred from the Planck/IRAS analysis of the dust properties. The results indicate that the H I emission is saturated with an optical depth ranging from 0.5 to 3 for 85% of the local H I gas. This optically thick H I is characterized by spin temperature in the range 10K-60K, significantly lower than previously postulated in the literature, whereas such low temperature is consistent with emission/absorption measurements of the cool H I toward radio continuum sources. The distribution and the column density of the H I are consistent with those of the dark gas suggested by gamma rays, and it is possible that the dark gas in the Galaxy is dominated by optically thick cold H I gas. This result implies that the average density of H I is 2-2.5 times higher than that derived on the optically thin assumption in the local ISM.
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