193 research outputs found
Formaldehyde Densitometry of Galactic Star-Forming Regions Using the H2CO 3(12)-3(13) and 4(13)-4(14) Transitions
We present Green Bank Telescope (GBT) observations of the 3(12)-3(13) (29
GHz) and 4(13)-4(14) (48 GHz) transitions of the H2CO molecule toward a sample
of 23 well-studied star-forming regions. Analysis of the relative intensities
of these transitions can be used to reliably measure the densities of molecular
cores. Adopting kinetic temperatures from the literature, we have employed a
Large Velocity Gradient (LVG) model to derive the average hydrogen number
density [n(H2)] within a 16 arcsecond beam toward each source. Densities in the
range of 10^{5.5}--10^{6.5} cm^{-3} and ortho-formaldehyde column densities per
unit line width between 10^{13.5} and 10^{14.5} cm^{-2} (km s^{-1})^{-1} are
found for most objects, in general agreement with existing measurements. A
detailed analysis of the advantages and limitations to this densitometry
technique is also presented. We find that H2CO 3(12)-3(13)/4(13)-4(14)
densitometry proves to be best suited to objects with T_K >~ 100 K, above which
the H2CO LVG models become relatively independent of kinetic temperature. This
study represents the first detection of these H2CO K-doublet transitions in all
but one object in our sample. The ease with which these transitions were
detected, coupled with their unique sensitivity to spatial density, make them
excellent monitors of density in molecular clouds for future experiments. We
also report the detection of the 9_2--8_1 A^- (29 GHz) transition of CH3OH
toward 6 sources.Comment: 17 pages; 6 figures; Accepted by Ap
Neurodevelopment Genes in Lampreys Reveal Trends for Forebrain Evolution in Craniates
The forebrain is the brain region which has undergone the most dramatic changes through vertebrate evolution. Analyses conducted in lampreys are essential to gain insight into the broad ancestral characteristics of the forebrain at the dawn of vertebrates, and to understand the molecular basis for the diversifications that have taken place in cyclostomes and gnathostomes following their splitting. Here, we report the embryonic expression patterns of 43 lamprey genes, coding for transcription factors or signaling molecules known to be involved in cell proliferation, stemcellness, neurogenesis, patterning and regionalization in the developing forebrain. Systematic expression patterns comparisons with model organisms highlight conservations likely to reflect shared features present in the vertebrate ancestors. They also point to changes in signaling systems –pathways which control the growth and patterning of the neuroepithelium-, which may have been crucial in the evolution of forebrain anatomy at the origin of vertebrates
The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets
This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics
Measurement of the Forward-Backward Asymmetry in the B -> K(*) mu+ mu- Decay and First Observation of the Bs -> phi mu+ mu- Decay
We reconstruct the rare decays , , and in a data sample
corresponding to collected in collisions at
by the CDF II detector at the Fermilab Tevatron
Collider. Using and decays we report the branching ratios. In addition, we report
the measurement of the differential branching ratio and the muon
forward-backward asymmetry in the and decay modes, and the
longitudinal polarization in the decay mode with respect to the squared
dimuon mass. These are consistent with the theoretical prediction from the
standard model, and most recent determinations from other experiments and of
comparable accuracy. We also report the first observation of the {\mathcal{B}}(B^0_s \to
\phi\mu^+\mu^-) = [1.44 \pm 0.33 \pm 0.46] \times 10^{-6}27 \pm 6B^0_s$ decay observed.Comment: 7 pages, 2 figures, 3 tables. Submitted to Phys. Rev. Let
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