1,721,111 research outputs found
The molecular environment of H_2_O masers: VLA ammonia observations
No full textWe present the results of single dish and interferometric observations of ammonia towards 5 sources selected from a sample of H_2_O and OH masers associated with star forming regions. The Medicina telescope was used to observe the NH_3_(1,1), (2,2), and (3,3) inversion transitions. High resolution maps in the NH_3_(2,2) and (3,3) lines and in the 1.3 cm continuum were then obtained with the Very Large Array. The main result of this research is to confirm the belief that H_2_O masers form in hot dense molecular cores which are sites of massive star formation. We also find evidence for the H_2_O maser phase to be prior to the appearance of an ultracompact HII region around the embedded high mass star(s)
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
Anatomy of a high-mass star forming cloud: The G24.78+0.08 (proto)stellar cluster
No full textWe present the results of an interferometric and single-dish study of G24.78+0.08, a region associated with high-mass star formation. Observations have been carried out in several molecular species, which are suitable to trace environments with different densities and temperatures. Evidence for this region to contain a cluster of very young massive stellar objects has been presented in a previous paper (Furuya et al. \cite{furu}). We suggest that the embedded stars might be too young to have affected the surrounding molecular cloud significantly on a large scale. This gives us the opportunity to investigate the configuration of the cloud as it was prior to the star formation episode. We assess that the (proto)stellar cluster lies at the center of a molecular clump with diameter of ~ 2 pc: to a good approximation this may be described as a spherically symmetric clump with density profile of the type nH_2~ R-1.8. Inside 0.5 pc from the center, instead, the gas is much more inhomogeneous and concentrated in a few high-density cores surrounding the (proto)stars. Our findings indicate that a self-regulating formation mechanism for the high-mass stars in G24.78 is plausible: in the proposed scenario star formation would occur from inside-out collapse of the parsec-scale clump, followed by infall reversal due to outflows powered by the newly formed massive stars. We also find that one of the two bipolar outflows powered by the embedded YSOs is more extended and hence older than the other, thus confirming the evolutionary sequence proposed in our previous article
IRAS 23385+6053: A Prototype Massive Class 0 Object
No full textIRAS 23385+6053 is a young stellar object with a luminosity of ~1.6×104 Lsolar at a kinematic distance of 4.9 kpc. This candidate precursor of an ultracompact H II region is associated with a millimeter source detected at the James Clerk Maxwell Telescope but is undetected at centimeter wavelengths with the VLA. We observed this source with the Owens Valley Radio Observatory millimeter array at 3.4 mm in the continuum, HCO+ (1-->0), H13CO+ (1-->0), and SiO (v=0, 2-->1) line emission and with CAM aboard the Infrared Space Observatory at 6.75 and 15 μm. The IRAS source is coincident with a 3.4 mm compact (rcore~=0.048 pc) and massive (M~=370 Msolar) core, which is undetected at 15 μm to a 3 σ level of 6 mJy; this is compatible with the derived H2 column density of ~2×1024 cm-2 and the estimated visual extinction AV~2000 mag. We find Lsubmm/Lbol~3×10-3 and Menv/M*>>1, which is typical of class 0 objects. The source is also associated with a compact outflow characterized by a size <~rcore, a dynamical timescale of <~7×103 yr, and a mass-loss rate Ṁ>~10-3 Msolar yr-1. The axis of the outflow is oriented nearly perpendicular to the plane of the sky, ruling out the possibility that the nondetection at 15 μm is the result of a geometric effect. All these properties suggest that IRAS 23385+6053 is the first example of a bona fide massive class 0 object
The disk-outflow system around the high-mass (proto)star IRAS 20126+4104
No full textWe have used the Pico Veleta 30-m telescope and the Plateau de Bure interferometer to obtain maps of the molecular gas associated with IRAS 20126+4104 in 11 rotational transitions. We also present near infrared images of the same region taken with the TIRGO and NOT telescopes. A molecular clump centred on the H_2_O masers associated with the IRAS source is found. A molecular bipolar outflow is clearly evidenced by the interferometer maps in the HCO^+^(1-0) line, originating at the center of the clump; also, the images in the K-band continuum and in the H_2_ v=1->0 S(1) vibrational line trace the same bipolar structure seen in HCO^+^. A flattened structure elongated in the direction perpendicular to the outflow axis is seen in the CH_3_CN(5-4) line: we interpret this as a rotating disk with a very young massive early type star at its centre. We conclude that IRAS 20126+4104 represents a beautiful example of a disk-outflow system associated with an early type (proto)star, prior to the development of an ultracompact HII region
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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