Context.
The mass segregation of stellar clusters could be primordial rather than dynamical. Despite the abundance of studies of mass segregation for stellar clusters, those for stellar progenitors ...are still scarce, so the question concerning the origin and evolution of mass segregation is still open.
Aims.
Our goal is to characterize the structure of the NGC 2264 molecular cloud and compare the populations of clumps and young stellar objects (YSOs) in this region whose rich YSO population has shown evidence of sequential star formation.
Methods.
We separated the
Herschel
column density map of NGC 2264 into three subregions and compared their cloud power spectra using a multiscale segmentation technique. We extracted compact cloud fragments from the column density image, measured their basic properties, and studied their spatial and mass distributions.
Results.
In the whole NGC 2264 cloud, we identified a population of 256 clumps with typical sizes of ~0.1 pc and masses ranging from 0.08
M
⊙
to 53
M
⊙
. Although clumps have been detected all over the cloud, most of the massive, bound clumps are concentrated in the central subregion of NGC 2264. The local surface density and the mass segregation ratio indicate a strong degree of mass segregation for the 15 most massive clumps, with a median Σ
6
three times that of the whole clumps population and Λ
MSR
≃ 8. We show that this cluster of massive clumps is forming within a high-density cloud ridge, which is formed and probably still fed by the high concentration of gas observed on larger scales in the central subregion. The time sequence obtained from the combined study of the clump and YSO populations in NGC 2264 suggests that the star formation started in the northern subregion, that it is now actively developing at the center, and will soon start in the southern subregion.
Conclusions.
Taken together, the cloud structure and the clump and YSO populations in NGC 2264 argue for a dynamical scenario of star formation. The cloud could first undergo global collapse, driving most clumps to centrally concentrated ridges. After their main accretion phase, some YSOs, and probably the most massive, would stay clustered while others would be dispersed from their birth sites. We propose that the mass segregation observed in some star clusters is inherited from that of clumps, originating from the mass assembly phase of molecular clouds.
Context.
The spatial properties of small star clusters suggest that they may originate from a fragmentation cascade starting from molecular cloud, of which there might be traces found at spatial ...scales up to a few tens of thousands of astronomical units (kAU).
Aims.
Our goal is to investigate the multi-scale spatial structure of gas clumps, to probe the existence of a hierarchical cascade over a range of characteristic spatial scales, and to evaluate its possible link with star production in terms of multiplicity.
Methods.
From the
Berschel
emission maps of NGC 2264 at 70, 160, 250, 350, 500 μm, clumps are extracted using getsf software at each of the associated spatial resolutions (respectively 8.4,13.5,18.2, 24.9,36.3″). Using the spatial distribution of these clumps and the class 0/I young stellar object (YSO) from
Spitzer
data, we developed a graph-theoretic analysis to represent the multi-scale structure of the cloud as a connected network. This network is organised in levels, and each level represents a characteristic scale among the available spatial scales. A link is created between two nodes which could be either a clump or a YSO from two different levels if their footprints overlap with each other. A parent node is then associated with a child node from a lower scale. The way in which the network subdivides scale after scale is compared with a geometric model that we have developed. This model generates extended objects that have a particularity in that they are geometrically constrained and subdivide along the scales following a fractal law. This graph-theoretic representation allows us to develop new statistical metrics and tools aiming at characterising, in a quantitative way, the multi-scale nature of molecular clouds.
Results.
We obtain three classes of multi-scale structure in NGC 2264 according to the number of nodes produced at the deepest level (called graph-sinks): hierarchical (several graph-sinks), linear (a single graph-sink with at most a single parent at each level), and isolated (no connection to any other node). The class of structure is strongly correlated with the column density
N
H2
of NGC 2264. The hierarchical structures dominate the regions whose column density exceeds
N
H2
= 6 × 10
22
cm
−2
. Although the latter are in the minority, namely 23% of the total number of structures, they contain half of the class 0/I YSOs, proving that they are highly efficient in producing stars. We define a novel statistical metric, the fractality coefficient
F
, corresponding to the fractal index that an equivalent population of clumps would have if they were generated by an ideal fractal cascade. For NGC 2264, over the whole range of spatial scales (1.4–26 kAU), this coefficient is globally estimated to be
F
= 1.45 ± 0.12 and its dispersion suggests that the cascade may depend on local physical conditions. However, a single fractal index is not the best fit for the NGC 2264 data because the hierarchical cascade starts at a 13 kAU characteristic spatial scale.
Conclusions.
Our novel methodology allows us to correlate YSOs with their gaseous environment which displays some degree of hierarchy for spatial scales below 13 kAU. We identify hierarchical multi-scale structures, which we associate with a hierarchical fragmentation process, and linear structures, which we associate with a monolithic fragmentation process. Hierarchical structures are observed as the main vectors of star formation. This cascade, which drives efficient star formation, is then suspected of being both hierarchical and rooted by the larger scale gas environment up to 13 kAU. We do not see evidence for any hierarchical structural signature of the cloud within the 13–26 kAU range, implying that the structure of the cloud does not follow a simple fractal law along the scales but instead might be submitted to a multi-fractal process.
ALMA-IMF Motte, F.; Bontemps, S.; Csengeri, T. ...
Astronomy and astrophysics (Berlin),
06/2022, Letnik:
662
Journal Article
Recenzirano
Odprti dostop
Aims.
Thanks to the high angular resolution, sensitivity, image fidelity, and frequency coverage of ALMA, we aim to improve our understanding of star formation. One of the breakthroughs expected from ...ALMA, which is the basis of our Cycle 5 ALMA-IMF Large Program, is the question of the origin of the initial mass function (IMF) of stars. Here we present the ALMA-IMF protocluster selection, first results, and scientific prospects.
Methods.
ALMA-IMF imaged a total noncontiguous area of ~53 pc
2
, covering extreme, nearby protoclusters of the Milky Way. We observed 15 massive (2.5 −33 × 10
3
M
⊙
), nearby (2−5.5 kpc) protoclusters that were selected to span relevant early protocluster evolutionary stages. Our 1.3 and 3 mm observations provide continuum images that are homogeneously sensitive to point-like cores with masses of ~0.2
M
⊙
and ~0.6
M
⊙
, respectively, with a matched spatial resolution of ~2000 au across the sample at both wavelengths. Moreover, with the broad spectral coverage provided by ALMA, we detect lines that probe the ionized and molecular gas, as well as complex molecules. Taken together, these data probe the protocluster structure, kinematics, chemistry, and feedback over scales from clouds to filaments to cores.
Results.
We classify ALMA-IMF protoclusters as Young (six protoclusters), Intermediate (five protoclusters), or Evolved (four proto-clusters) based on the amount of dense gas in the cloud that has potentially been impacted by H
II
region(s). The ALMA-IMF catalog contains ~700 cores that span a mass range of ~0.15
M
⊙
to ~250
M
⊙
at a typical size of ~2100 au. We show that this core sample has no significant distance bias and can be used to build core mass functions (CMFs) at similar physical scales. Significant gas motions, which we highlight here in the G353.41 region, are traced down to core scales and can be used to look for inflowing gas streamers and to quantify the impact of the possible associated core mass growth on the shape of the CMF with time. Our first analysis does not reveal any significant evolution of the matter concentration from clouds to cores (i.e., from 1 pc to 0.01 pc scales) or from the youngest to more evolved protoclusters, indicating that cloud dynamical evolution and stellar feedback have for the moment only had a slight effect on the structure of high-density gas in our sample. Furthermore, the first-look analysis of the line richness toward bright cores indicates that the survey encompasses several tens of hot cores, of which we highlight the most massive in the G351.77 cloud. Their homogeneous characterization can be used to constrain the emerging molecular complexity in protostars of high to intermediate masses.
Conclusions.
The ALMA-IMF Large Program is uniquely designed to transform our understanding of the IMF origin, taking the effects of cloud characteristics and evolution into account. It will provide the community with an unprecedented database with a high legacy value for protocluster clouds, filaments, cores, hot cores, outflows, inflows, and stellar clusters studies.
Judging by results, the methods undertaken to teach software development to large classes of students are flawed; too many students are failing to grasp any real understanding of programming and ...software design. To address this problem the University of Wales, Aberystwyth has developed VorteX, an interactive collaborative design tool that captures the design processes of novice students, provides a diagnosis system capable of interpreting the students’ work, and advises on their design process.
This paper provides an overview of VorteX, its capabilities and use, and explains how the case-based system identifies redundancies in the storage of student designs and reduces data volume. The paper describes how equivalence maps merge similar classes to reduce the design structure possibilities, how snippets eliminate the replication of components and how abstract snippets represent the design intent of students in a minimalist form. Finally it concludes with comments on the student experience of the VorteX case-based reasoning assistant.
BACKGROUND A work force based case-control study of lung cancer was performed in non-silicotic subjects exposed to crystalline silica to investigate the association between silica dust and lung ...cancer excluding the influence of silicosis. METHODS Two hundred and forty seven patients with lung cancer and 795 control subjects were enrolled, all of whom had been employed in the German stone, quarrying, or ceramics industries. Smoking was used as a matching criterion. Exposure to silica was quantified by measurements, if available, or otherwise by industrial hygienists. Several indices (peak, average and cumulative exposure) were used to analyse the relationship between the level of exposure and risk of lung cancer as odds ratios (OR). RESULTS The risk of lung cancer is associated with the year of and age at first exposure to silica, duration of exposure, and latency. All odds ratios were adjusted for these factors. Considering the peak exposure, the OR for workers exposed to high levels (⩾0.15 mg/m3 respirable silica dust which is the current occupational threshold value for Germany) compared with those exposed to low levels (<0.15 mg/m3) was 0.85 (95% CI 0.58 to 1.25). For the time weighted average exposure the OR was 0.91 (95% CI 0.57 to 1.46). The OR for the cumulative exposure was 1.02 (95% CI 0.67 to 1.55). No increase in risk was evident with increasing exposure. CONCLUSIONS This study shows no association between exposure to crystalline silica and lung cancer. The exclusion of subjects with silicosis may have led to dilution with respect to the level of exposure and therefore reduced the power to detect a small risk. Alternatively, the risk of getting lung cancer may be restricted to subjects with silicosis and is not directly linked to silica dust.
A gene encoding a homologue of the Escherichia coli GidA protein (glucose‐inhibited division protein A) lies immediately upstream of aglU, a gene encoding a WD‐repeat protein required for motility ...and development in Myxococcus xanthus. The GidA protein of M. xanthus shares about 48% identity overall with the small (≈ 450 amino acid) form of GidA from eubacteria and about 24% identity overall with the large (≈ 620 amino acid) form of GidA from eubacteria and eukaryotes. Each of these proteins has a conserved dinucleotide‐binding motif at the N‐terminus. To determine if GidA binds dinucleotide, the M. xanthus gene was expressed with a His6 tag in E. coli cells. Purified rGidA is a yellow protein that absorbs maximally at 374 and 450 nm, consistent with FAD or FMN. Thin‐layer chromatography (TLC) showed that rGidA contains an FAD cofactor. Fractionation and immunocytochemical localization show that full length GidA protein is present in the cytoplasm and transported to the periplasm of vegetative‐grown M. xanthus cells. In cells that have been starved for nutrients, GidA is found in the cytoplasm. Although GidA lacks an obvious signal sequence, it contains a twin arginine transport (Tat) motif, which is conserved among proteins that bind cofactors in the cytoplasm and are transported to the periplasm as folded proteins. To determine if GidA, like AglU, is involved in motility and development, the gidA gene was disrupted. The gidA– mutant has wild‐type gliding motility and initially is able to form fruiting bodies like the wild type when starved for nutrients. However, after several generations, a stable derivative arises, gidA*, which is indistinguishable from the gidA– parent on vegetative medium, but is no longer able to form fruiting bodies. The gidA* mutant releases a heat‐stable, protease‐resistant, small molecular weight molecule that acts in trans to inhibit aggregation and gene expression of wild‐type cells during development.
A previous cohort-study in a cardboard factory demonstrated that high and prolonged occupational exposure to trichloroethene (C2HCl3) is associated with an increased incidence of renal cell cancer. ...The present hospital-based case/control study investigates occupational exposure in 58 patients with renal cell cancer with special emphasis on C2HCl3 and the structurally and toxicologically closely related compound tetrachloroethene (C2Cl4). A group of 84 patients from the accident wards of three general hospitals in the same area served as controls. Of the 58 cases, 19 had histories of occupational C2HCl3 exposure for at least 2 years and none had been exposed to C2Cl4; of the 84 controls, 5 had been occupationally exposed to C2HCl3 and 2 to C2Cl4. After adjustment for other risk factors, such as age, obesity, high blood pressure, smoking and chronic intake of diuretics, the study demonstrates an association of renal cell cancer with long-term exposure to C2HCl3 (odds ratio 10.80; 95% CI: 3.36-34.75).
Since its invention in 1991, premixed combustion synthesis of fullerenic materials has been established as the major industrial process for manufacturing of these materials. Large-scale production of ...fullerenes such as C60, C70 and C84 has been implemented. More recently, combustion technology has been extended to the targeted synthesis of single-walled carbon nanotubes (SWCNT). Addition of catalyst precursor and operation at well-controlled fuel-rich but non-sooting conditions are required. Extensive parametric studies have allowed for the optimization of the formation of high-quality SWCNT. Purification techniques previously reported in the literature have been adjusted and used successfully for the nearly complete removal of metal and metal oxide. Material has been characterized using Raman spectroscopy, scanning (SEM) and transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Correlations between process conditions and nanotube properties such as length have been established. Product reproducibility and process scalability of the combustion process have been demonstrated. Sample preparation was found to affect significantly the apparent characteristics of nanotubes as seen in electron microscopy images.
We demonstrate a new self-aligned TFT process for hydrogenated amorphous silicon thin-film transistors (a-Si:H TFTs). Two backside exposure photolithography steps are used to fabricate fully ...self-aligned tri-layer TFTs with deposited n/sup +/ contacts. Since no critical data alignment is required, this simple process is well suited to fabrication of short channel TFTs. We have fabricated fully self-aligned tri-layer a-Si:H TFTs with excellent device performance, and contact overlaps <1 μm. For a 20-μm channel length TFT with an a-Si:H thickness of 13 nm, the linear region (V/sub DS/=0.1 V) and saturation region (V/sub DS/=25 V) extrinsic mobility values are both 1.2 cm 2 /V-s, the off currents are <1 pA, and the on/off current ratio is >10/sup 7/.
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