269 research outputs found
Detection of a methanol megamaser in a major-merger galaxy
We have detected emission from both the 4_{-1}-3_{0} E (36.2~GHz) class I and
7_{-2}-8_{-1} E (37.7~GHz) class II methanol transitions towards the centre of
the closest ultra-luminous infrared galaxy Arp 220. The emission in both the
methanol transitions show narrow spectral features and have luminosities
approximately 8 orders of magnitude stronger than that observed from typical
class I methanol masers observed in Galactic star formation regions. The
emission is also orders of magnitude stronger than the expected intensity of
thermal emission from these transitions and based on these findings we suggest
that the emission from the two transitions are masers. These observations
provides the first detection of a methanol megamaser in the 36.2 and 37.7 GHz
transitions and represents only the second detection of a methanol megamaser,
following the recent report of an 84 GHz methanol megamaser in NGC1068. We find
the methanol megamasers are significantly offset from the nuclear region and
arise towards regions where there is Ha emission, suggesting that it is
associated with starburst activity. The high degree of correlation between the
spatial distribution of the 36.2 GHz methanol and X-ray plume emission suggests
that the production of strong extragalactic class I methanol masers is related
to galactic outflow driven shocks and perhaps cosmic rays. In contrast to OH
and H2O megamasers which originate close to the nucleus, methanol megamasers
provide a new probe of feedback (e.g. outflows) processes on larger-scales and
of star formation beyond the circumnuclear starburst regions of active
galaxies.Comment: Accepted for publication in ApJ
Breaking the icosahedra in boron carbide
Findings of laser-assisted atom probe tomography experiments on boron carbide elucidate an approach for characterizing the atomic structure and interatomic bonding of molecules associated with extraordinary structural stability. The discovery of crystallographic planes in these boron carbide datasets substantiates that crystallinity is maintained to the point of field evaporation, and characterization of individual ionization events gives unexpected evidence of the destruction of individual icosahedra. Statistical analyses of the ions created during the field evaporation process have been used to deduce relative atomic bond strengths and show that the icosahedra in boron carbide are not as stable as anticipated. Combined with quantum mechanics simulations, this result provides insight into the structural instability and amorphization of boron carbide. The temporal, spatial, and compositional information provided by atom probe tomography makes it a unique platform for elucidating the relative stability and interactions of primary building blocks in hierarchically crystalline materials
Growth and cellular differentiation: a physical-biochemical conundrum? The example of the hand
Currently, the predominant hypothesis explains cellular differentiation as an essentially genetic intracellular process. The goal of this paper is to suggest that cell growth and differentiation may be, simply, the result of physical and chemical constraints.Bone growth occurs at the level of cartilage conjunction (growth plate) in a zone of lesser constrain. It appears that this growth also induces muscle, tendon, nerve and skin elongation. This cartilage growth by itself seems to explain the elongation of the hand. Growth stops at puberty likely because of feed-back from an increasing muscle load. The ossification (that is differentiation of cartilage into bone) appears to result from the shear stress induced. The study of bone age, obtained by X-ray picture of the hand, shows that ossification of epiphyses is very precise both in time and space. Computer modelization suggests that this ossification occurs where shear stress is greatest. The cartilage which does not ossify (joint, nose, larynx, ear, bronchus, etc.) is not exposed to high shear.Shear stress induces the secretion of extracellular matrix and a change of the biochemical environment of the cell. Precipitation of calcium phosphate, as in ossification, seems related to the alkalosis induced by shear stress.To speak in more general terms, loss of cellular differentiation, as occurs with cancer, can result from a change in the physical–chemical environments
Selectivity control in Pt-catalyzed cinnamaldehyde hydrogenation
Chemoselectivity is a cornerstone of catalysis, permitting the targeted modification of specific functional groups within complex starting materials. Here we elucidate key structural and electronic factors controlling the liquid phase hydrogenation of cinnamaldehyde and related benzylic aldehydes over Pt nanoparticles. Mechanistic insight from kinetic mapping reveals cinnamaldehyde hydrogenation is structure-insensitive over metallic platinum, proceeding with a common Turnover Frequency independent of precursor, particle size or support architecture. In contrast, selectivity to the desired cinnamyl alcohol product is highly structure sensitive, with large nanoparticles and high hydrogen pressures favoring C=O over C=C hydrogenation, attributed to molecular surface crowding and suppression of sterically-demanding adsorption modes. In situ vibrational spectroscopies highlight the role of support polarity in enhancing C=O hydrogenation (through cinnamaldehyde reorientation), a general phenomenon extending to alkyl-substituted benzaldehydes. Tuning nanoparticle size and support polarity affords a flexible means to control the chemoselective hydrogenation of aromatic aldehydes
A Spitzer Space Telescope survey of massive young stellar objects in the G333.2-0.4 giant molecular cloud
The G333 giant molecular cloud contains a few star clusters and H II regions,
plus a number of condensations currently forming stars. We have mapped 13 of
these sources with the appearance of young stellar objects (YSOs) with the
Spitzer Infrared Spectrograph in the SL, SH, and LH modules (5-36 micron). We
use these spectra plus available photometry and images to characterize the
YSOs. The spectral energy distributions (SEDs) of all sources peak between 35
and 110 micron, thereby showing their young age. The objects are divided into
two groups: YSOs associated with extended emission in IRAC band 2 at 4.5 micron
(`outflow sources') and YSOs that have extended emission in all IRAC bands
peaking at the longest wavelengths (`red sources'). The two groups of objects
have distinctly different spectra: All the YSOs associated with outflows show
evidence of massive envelopes surrounding the protostar because the spectra
show deep silicate absorption features and absorption by ices at 6.0, 6.8, and
15.2 micron. We identify these YSOs with massive envelopes cool enough to
contain ice-coated grains as the `bloated' protostars in the models of Hosokawa
et al. All spectral maps show ionized forbidden lines and PAH emission
features. For four of the red sources, these lines are concentrated to the
centres of the maps, from which we infer that these YSOs are the source of
ionizing photons. Both types of objects show evidence of shocks, with most of
the outflow sources showing a line of [S I] in the outflows and two of the red
sources showing the more highly excited [Ne III] and [S IV] lines in outflow
regions at some distance from the YSOs. The 4.5 micron emission seen in the
IRAC band 2 images of the outflow sources is not due to H2 lines, which are too
faint in the 5-10 micron wavelength region to be as strong as is needed to
account for the IRAC band 2 emission.Comment: 31 pages and 30 figures in the paper plus 11 figures from the online
Supporting Information. To be published in the MNRAS. Version 2 has many
small changes (typos, spelling, punctuation) and reordering of the Supporting
Information figures to make this version conform to the paper that will be
printed in MNRA
Effects of Increased Nitrogen Deposition and Precipitation on Seed and Seedling Production of Potentilla tanacetifolia in a Temperate Steppe Ecosystem
The responses of plant seeds and seedlings to changing atmospheric nitrogen (N) deposition and precipitation regimes determine plant population dynamics and community composition under global change.In a temperate steppe in northern China, seeds of P. tanacetifolia were collected from a field-based experiment with N addition and increased precipitation to measure changes in their traits (production, mass, germination). Seedlings germinated from those seeds were grown in a greenhouse to examine the effects of improved N and water availability in maternal and offspring environments on seedling growth. Maternal N-addition stimulated seed production, but it suppressed seed mass, germination rate and seedling biomass of P. tanacetifolia. Maternal N-addition also enhanced responses of seedlings to N and water addition in the offspring environment. Maternal increased-precipitation stimulated seed production, but it had no effect on seed mass and germination rate. Maternal increased-precipitation enhanced seedling growth when grown under similar conditions, whereas seedling responses to offspring N- and water-addition were suppressed by maternal increased-precipitation. Both offspring N-addition and increased-precipitation stimulated growth of seedlings germinated from seeds collected from the maternal control environment without either N or water addition. Our observations indicate that both maternal and offspring environments can influence seedling growth of P. tanacetifolia with consequent impacts on the future population dynamics of this species in the study area.The findings highlight the importance of the maternal effects on seed and seedling production as well as responses of offspring to changing environmental drivers in mechanistic understanding and projecting of plant population dynamics under global change
Teachers and didacticians: key stakeholders in the processes of developing mathematics teaching
This paper sets the scene for a special issue of ZDM-The International Journal on Mathematics Education-by tracing key elements of the fields of teacher and didactician/teacher-educator learning related to the development of opportunities for learners of mathematics in classrooms. It starts from the perspective that joint activity of these two groups (teachers and didacticians), in creation of classroom mathematics, leads to learning for both. We trace development through key areas of research, looking at forms of knowledge of teachers and didacticians in mathematics; ways in which teachers or didacticians in mathematics develop their professional knowledge and skill; and the use of theoretical perspectives relating to studying these areas of development. Reflective practice emerges as a principal goal for effective development and is linked to teachers' and didacticians' engagement with inquiry and research. While neither reflection nor inquiry are developmental panaceas, we see collaborative critical inquiry between teachers and didacticians emerging as a significant force for teaching development. We include a summary of the papers of the special issue which offer a state of the art perspective on developmental practice. © 2014 FIZ Karlsruhe
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