173 research outputs found
Factors affecting social interaction on Social Network Sites: The Facebook Case
Purpose - Our paper focuses on understanding which factors affect the social interaction in the case of Facebook. Many authors point out the great potential of these networks for social interaction and as conduits of information. However, studies show that the topology of the network is disconnected, consisting of small sub-networks that make Facebook unsuitable for disseminating information. This situation has created the need to introduce exogenous factors, aimed at boosting and providing cohesion to the network structure. In this context, we test the following question: how exogenous and endogenous factors contribute to encouraging social interaction on Facebook. Design/methodology/approach - For the analysis of social interaction on Facebook, a population consisting of all the followers of the walls of 10 corporate social networks was used. From the total 269,424 users analysed, a stratified sample of 132 followers was obtained and networks were built for each of them. We then proceeded to search for each follower's friends and friends of friends to build the social network up to the fourth level, obtaining a total of 132 subnets with 1,628,074 links between them. To determine the impact of both exogenous and endogenous factors in the interaction of the network we performed a causal analysis. Findings - The results obtained from this study provide empirical evidence on the adequacy of companies? dynamization measures used and how exogenous and endogenous factors influence the social interaction on Facebook. Thus, our results show that exogenous factors, such as the activity of the community manager and the digital marketing investment in the network, do not have a significant effect on the interaction. On the other hand, endogenous factors, such as network density and clustering, have a positive effect on the trigger of social interaction between the followers. Therefore, companies must consider the importance of the structural factors that characterize network followers, such as density or clustering coefficient, to be able to interpret and optimize them to obtain higher levels of social interaction. Originality/value - This is one of a few papers that examine interactions in social network sites, particularly in corporate network sites in Facebook. The results expose the importance for organizations to have reliable information on the patterns of interaction to properly manage the resources allocated for this purpose in SNS
Tip of the Red Giant Branch Distances to NGC 4214, UGC 685, and UGC 5456
We have used WFPC2 VRI observations to calculate the distances to three
nearby galaxies, NGC 4214, UGC 685, and UGC 5456 using the tip of the red giant
branch method. Our values for NGC 4214 (2.94 +/- 0.18 Mpc) and UGC 685 (4.79
+/- 0.30 Mpc) are the most precise measurementes of the distances to these
objects ever made. For UGC 5456 the data do not allow us to reach a decisive
conclusion since there are two possible solutions, one leading towards a short
distance around 3.8 Mpc and another one towards a long distance of 5.6 Mpc or
more.Comment: To appear in the March 2002 issue of the Astronomical Journal. 19
pages, including 4 tables and 8 figures. Due to the size limitations of the
astro-ph service, a reduced resolution version of figures 1-3 is provided
here. For a full resolution version, please go to http://www.stsci.edu/~jmai
The young stellar population of NGC 4214 as observed with HST. II. Results
We present the results of a detailed UV-optical study of the nearby dwarf
starburst galaxy NGC 4214 using multifilter HST/WFPC2+STIS photometry. The
stellar extinction is found to be quite patchy, with some areas having values
of E(4405-5495)< 0.1 mag and others, associated with star forming regions, much
more heavily obscured, a result which is consistent with previous studies of
the nebular extinction. We determined the ratio of blue-to-red supergiants and
found it to be consistent with theoretical models for the metallicity of the
SMC. The stellar IMF of the field in the range 20-100 solar masses is found to
be steeper than Salpeter. A number of massive clusters and associations with
ages between a few and 200 million years are detected and their properties are
discussed.Comment: 49 pages, 12 figures, 6 table
The Interaction between the ISM and Star Formation in the Dwarf Starburst Galaxy NGC 4214
We present the first interferometric study of the molecular gas in the
metal-poor dwarf starburst galaxy NGC 4214. Our map of the 12CO(1-0) emission,
obtained at the OVRO millimeter array, reveals an unexpected structural wealth.
We detected three regions of molecular emission in the north-west (NW),
south-east (SE) and centre of NGC 4214 which are in very different and distinct
evolutionary stages (total molecular mass: 5.1 x 10^6 M_sun). These differences
are apparent most dramatically when the CO morphologies are compared to optical
ground based and HST imaging: massive star formation has not started yet in the
NW region; the well-known starburst in the centre is the most evolved and star
formation in the SE complex started more recently. We derive a star formation
efficiency of 8% for the SE complex. Using high--resolution VLA observations of
neutral hydrogen HI and our CO data we generated a total gas column density map
for NGC 4214 (HI + H_2). No clear correlation is seen between the peaks of HI,
CO and the sites of ongoing star formation. This emphasizes the irregular
nature of dwarf galaxies. The HI and CO velocities agree well, so do the
H-alpha velocities. In total, we cataloged 14 molecular clumps in NGC 4214. Our
results from a virial mass analysis are compatible with a Galactic CO-to-H_2
conversion factor for NGC 4214 (lower than what is usually found in metal-poor
dwarf galaxies).Comment: accepted for publication in the AJ (February 2001), full ps file at:
ftp://ftp.astro.caltech.edu/users/fw/ngc4214/walter_prep.p
Pulsed current-voltage electrodeposition of stoichiometric Bi2Te3 nanowires and their crystallographic characterization by transmission electron backscatter diffraction
Bi2Te3 nanowires with diameters ranging from 25 to 270 nm, ultra-high aspect ratio, and uniform growth front were fabricated by electrodeposition, pulsing between zero current density during the off time and constant potential during the on time (pulsed-current-voltage method, p-IV). The use of zero current density during the off time is to ensure no electrodeposition is carried out and the system is totally relaxed. By this procedure, stoichiometric nanowires oriented perpendicular to the c-axis is obtained for the different diameters of porous alumina templates. In addition, the samples show a uniform growth front with ultra-high aspect ratio single crystal nanowires. The high degree of crystallinity was verified by transmission electron backscatter diffraction. This characterization revealed that the nanowires present both large single crystalline areas and areas with alternating twin configurations
Nucleation and Crystallization of PA6 Composites Prepared by T-RTM: Effects of Carbon and Glass Fiber Loading
Thermoplastic resin transfer molding (T-RTM) is attracting much attention due to the need for recyclable alternatives to thermoset materials. In this work, we have prepared polyamide-6 (PA6) and PA6/fiber composites by T-RTM of caprolactam. Glass and carbon fibers were employed in a fixed amount of 60 and 47 wt.%, respectively. Neat PA6 and PA6 matrices (of PA6-GF and PA6-CF) of approximately 200 kg/mol were obtained with conversion ratios exceeding 95%. Both carbon fibers (CF) and glass fibers (GF) were able to nucleate PA6, with efficiencies of 44% and 26%, respectively. The α crystal polymorph of PA6 was present in all samples. The lamellar spacing, lamellar thickness and crystallinity degree did not show significant variations in the samples with or without fibers as result of the slow cooling process applied during T-RTM. The overall isothermal crystallization rate decreased in the order: PA6-CF > PA6-GF > neat PA6, as a consequence of the different nucleation efficiencies. The overall crystallization kinetics data were successfully described by the Avrami equation. The lamellar stack morphology observed by atomic force microscopy (AFM) is consistent with 2D superstructural aggregates (n = 2) for all samples. Finally, the reinforcement effect of fibers was larger than one order of magnitude in the values of elastic modulus and tensile strength.We gratefully acknowledge the Basque Government by the funding received through the ELKARTEK 2017 Cooperative Fundamental Research project: ”Automotion Composites fabricated by RTM adapted to the 4.0 industrial philosophy (RTM4.0). The authors thank the ALBA Synchrotron Light facility for supporting the X-rays experiments at beamline BL11-NCD-SWEET and the funding by ALBA project nº 2017092338. The UPV/EHU team gratefully acknowledges the financial contribution of the Basque Government through grant IT1309-19
The R136 star cluster dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of HeII 1640 in young star clusters
We introduce a HST/STIS stellar census of R136a, the central ionizing star
cluster of 30 Doradus. We present low resolution far-ultraviolet STIS/MAMA
spectroscopy of R136 using 17 contiguous 52x0.2 arcsec slits which together
provide complete coverage of the central 0.85 parsec (3.4 arcsec). We provide
spectral types of 90% of the 57 sources brighter than m_F555W = 16.0 mag within
a radius of 0.5 parsec of R136a1, plus 8 additional nearby sources including
R136b (O4\,If/WN8). We measure wind velocities for 52 early-type stars from CIV
1548-51, including 16 O2-3 stars. For the first time we spectroscopically
classify all Weigelt & Baier members of R136a, which comprise three WN5 stars
(a1-a3), two O supergiants (a5-a6) and three early O dwarfs (a4, a7, a8). A
complete Hertzsprung-Russell diagram for the most massive O stars in R136 is
provided, from which we obtain a cluster age of 1.5+0.3_-0.7 Myr. In addition,
we discuss the integrated ultraviolet spectrum of R136, and highlight the
central role played by the most luminous stars in producing the prominent HeII
1640 emission line. This emission is totally dominated by very massive stars
with initial masses above ~100 Msun. The presence of strong HeII 1640 emission
in the integrated light of very young star clusters (e.g A1 in NGC 3125)
favours an initial mass function extending well beyond a conventional upper
limit of 100 Msun. We include montages of ultraviolet spectroscopy for LMC O
stars in the Appendix. Future studies in this series will focus on optical
STIS/CCD medium resolution observations.Comment: 20 pages plus four Appendices providing LMC UV O spectral templates,
UV spectral atlas in R136, wind velocities of LMC O stars and photometry of
additional R136 source
HST/WFPC2 and VLA Observations of the Ionized Gas in the Dwarf Starburst Galaxy NGC 4214
We present new H alpha and [O III] 5007 narrow band images of the
starbursting dwarf galaxy NGC 4214, obtained with the WFPC2 onboard HST,
together with VLA observations of the same galaxy. The HST images resolve
features down to physical scales of 2-5 pc, revealing several young (<10 Myr)
star forming complexes of various ionized gas morphologies (compact knots,
complete or fragmentary shells) and sizes (10-200 pc). Our results are
consistent with a uniform set of evolutionary trends: The youngest, smaller,
filled regions that presumably are those just emerging from dense star forming
clouds, tend to be of high excitation and are highly obscured. Evolved, larger
shell-like regions have lower excitation and are less extincted due of the
action of stellar winds and supernovae. In at least one case we find evidence
for induced star formation which has led to a two-stage starburst. Age
estimates based on W(H alpha) measurements do not agree with those inferred
from wind-driven shell models of expanding H II regions. The most likely
explanation for this effect is the existence of a 2 Myr delay in the formation
of superbubbles caused by the pressure exerted by the high density medium in
which massive stars are born. We report the detection of a supernova remnant
embedded in one of the two large H II complexes of NGC 4214. The dust in NGC
4214 is not located in a foreground screen but is physically associated with
the warm ionized gas.Comment: 41 pages, including 9 figures and 7 tables. To appear in the November
issue of the Astronomical Journa
A Galactic O-Star Catalog
We have produced a catalog of 378 Galactic O stars with accurate spectral
classifications which is complete for V<8 but includes many fainter stars. The
catalog provides cross-identifications with other sources; coordinates
(obtained in most cases from Tycho-2 data); astrometric distances for 24 of the
nearest stars; optical (Tycho-2, Johnson, and Stromgren) and NIR photometry;
group membership, runaway character, and multiplicity information; and a
web-based version with links to online services.Comment: 76 pages, 13 tables, and 3 figures. Accepted for publication in
Astrophysical Journal. Online version of the catalog available at
http://www.stsci.edu/~jmaiz/GOSmain.htm
The Dwarf Irregular/Wolf-Rayet Galaxy NGC4214: I. A New Distance, Stellar Content, and Global Parameters
We present the results of a detailed optical and near-IR study of the nearby
star-forming dwarf galaxy NGC4214. We discuss the stellar content, drawing
particular attention to the intermediate-age and/or old field stars, which are
used as a distance indicator. On images obtained with the Hubble Space
Telescope WFPC2 and NICMOS instruments in the equivalents of the V, R, I, J and
H bands, the galaxy is well resolved into stars. We achieve limiting magnitudes
of F814W ~27 in the WF chips and F110W ~25 in the NIC2. The optical and
near-infrared color-magnitude diagrams confirm a core-halo galaxy morphology:
an inner high surface-brightness young population within ~1.5' (~1 kpc) from
the center of the galaxy, where the stars are concentrated in bright complexes
along a bar-like structure; and a relatively low-surface-brightness, field-star
population extending out to at least 8' (7 kpc). The color-magnitude diagrams
of the core region show evidence of blue and red supergiants, main-sequence
stars, asymptotic giant branch stars and blue loop stars. We identify some
candidate carbon stars from their extreme near-IR color. The field-star
population is dominated by the "red tangle", which contains the red giant
branch. We use the I-band luminosity function to determine the distance based
on the tip-of-the-red-giant-branch method: 2.7\pm0.3 Mpc. This is much closer
than the values usually assumed in the literature, and we provide revised
distance dependent parameters such as physical size, luminosity, HI mass and
star-formation rate.Comment: 24 pages, 19 figures, accepted for publication in the July 2002 issue
of AJ. Version with high resolution figures is available at
http://www.astro.spbu.ru/staff/dio/preprints.htm
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