1,721,000 research outputs found
Modelling star formation in Ophiuchus
No full textWe perform simulations of prestellar core collapse with initial conditions inferred from observations of Ophiuchus. The cores in Ophiuchus are mostly isolated, hence we run multiple simulations of individual cores. We statistically model the intrinsic shapes of cores as a population of triaxial ellipsoids with a single free parameter which is fitted to aspect ratio data. We assume a turbulent velocity field with modifications which add ordered radial and rotational motion. Mass, size and non-thermal velocity dispersion are drawn randomly from observational data. Preliminary results show a largely realistic IMF. Future work will explore solenoidal to compressive velocity mode ratios and the effects of accretion luminosity. Results will lead to inferences on quantities such as star formation efficiency in the region
The theory of the formation of brown dwarfs and low-mass stars
No full textMore than half of all stars (including brown dwarfs) have masses below 0.2 Msun. The formation mechanism of these objects is uncertain. I will review the four main theories for the formation of low-mass objects: turbulent fragmentation, ejection of protostellar embryos, disc fragmentation, and photo-erosion of prestellar cores. I will discuss the observational predictions of these models regarding the low-mass initial mass function, the brown dwarf desert, and the binary statistics of low-mass stars and brown dwarfs. I will further discuss whether observations may be used to distinguish between different formation mechanisms, and give a few examples of systems that strongly favour a specific formation scenario. Finally, I will argue that it is likely that all mechanisms may play a role in the formation of brown dwarfs and low-mass stars
Estimating the Galactic Coronal Density via Ram-Pressure Stripping from Dwarf Satellites
No full textCosmological simulations and theories of galaxy formation predict that the Milky Way should be embedded in an extended hot gaseous halo or corona. To date, a definitive detection of such a corona in the Milky Way remains elusive. We have attempted to estimate the density of the Milky Way's cosmological corona using the effect that it has on the surrounding population of dwarf galaxies. We have considered two dSphs close to the Galaxy: Sextans and Carina. Assuming that they have lost all their gas during the last pericentric passage via ram-pressure stripping, we were able to estimate the average density (n ˜ 2 ṡ 10-4 cm-3) of the corona at a distance of ˜ 70 kpc from the Milky Way. If we consider an isothermal profile and extrapolate it at large radii, the corona could contain a significant fraction of the missing baryons associated to the Milky Way
GLIMPSE Extended Green Objects and the Early Stages of Massive Star Formation
No full textLarge-scale Spitzer surveys of the Galactic plane have yielded a new tracer of massive young stellar objects (MYSOs) that are actively accreting and driving outflows: extended emission in the IRAC 4.5 μmband, believed to arise from shocked molecular gas. The GLIMPSE catalog of extended 4.5 μm sources (called EGOs, Extended Green Objects, forthe common coding of 3-color IRAC images) is a unique sample,well-suited for studying the initial stages of massive star formation and establishing an observation-based MYSO evolutionary sequence. In this proceeding, we present results from extensive recent high-resolution Submillimeter Array (SMA) and Very Large Array (VLA)observations of EGOs at cm-mm wavelengths, including maser, molecular line, and continuum studies. These observations often reveal (proto)clusters that exhibit chemical and evolutionary diversity
The migration of gas giant planets in gravitationally unstable disks
No full textPlanets form in the disks of gas and dust that surround young stars. It is not known whether or not gas giant planets on wide orbits form the same way as Jupiter or form by the fragmentation of gravitationally unstable disks. Here we show that a giant planet that has formed in the outer regions of a protostellar disk initially migrates quickly toward the central star (migration timescale ~104 years) while accreting gas from the disk. However, in contrast with previous studies, we find that the planet eventually opens up a gap in the disk and the migration is essentially halted. At the same time, accretion-powered radiative feedback from the planet significantly limits its mass growth, keeping it within the planetary-mass regime, (i.e., below the deuterium burning limit) at least for the initial stages of disk evolution. Giant planets may therefore be able to survive on wide orbits despite their initial fast inward migration, consequently shaping the environment in which terrestrial planets that may harbor life can form
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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