64,413 research outputs found
A photometric and spectroscopic survey of solar twin stars within 50 parsecs of the Sun: I. Atmospheric parameters and color similarity to the Sun
Solar twins and analogs are fundamental in the characterization of the Sun's
place in the context of stellar measurements, as they are in understanding how
typical the solar properties are in its neighborhood. They are also important
for representing sunlight observable in the night sky for diverse photometric
and spectroscopic tasks, besides being natural candidates for harboring
planetary systems similar to ours and possibly even life-bearing environments.
We report a photometric and spectroscopic survey of solar twin stars within 50
pc of the Sun. Hipparcos absolute magnitudes and (B-V)_Tycho colors were used
to define a 2 sigma box around the solar values, where 133 stars were
considered. Additional stars resembling the solar UBV colors in a broad sense,
plus stars present in the lists of Hardorp, were also selected. All objects
were ranked by a color-similarity index with respect to the Sun, defined by
uvby and BV photometry. Moderately high-resolution, high-S/N spectra were used
for a subsample of equatorial-southern stars to derive Teff, log g, and [Fe/H]
with average internal errors better than 50 K, 0.20 dex, and 0.08 dex,
respectively. Ages and masses were estimated from theoretical HR diagrams. The
color-similarity index proved very successful. We identify and rank new
excellent solar analogs, which are fit to represent the Sun in the night sky.
Some of them are faint enough to be of interest for moderately large
telescopes. We also identify two stars with near-UV spectra indistinguishable
from the Sun's. We present five new "probable" solar twin stars, besides five
new "possible" twins. Masses and ages for the best solar twin candidates lie
very close to the solar values, but chromospheric activity levels range
somewhat. We propose that the solar twins be emphasized in the ongoing searches
for extra-solar planets and SETI searches.Comment: 25 pages, 15 figures, 14 table
The Effect of Shill Bidding upon Prices: Experimental Evidence
This paper explores, through a series of experiments, the effect of shill bidding upon revenues and prices in auctions. We study the practice of shill bidding in a common value framework. Our findings are consistent with the theoretical prediction that, if bidders are aware of the possibility of seller participation in an auction, shill bidding lowers profits on average. Shill bidding can alleviate the problem of the winner's curse by lowering the price and it can, thus, provide benefits to the bidders. Finally, even though there were too many bidders that submitted bids in these auctions, the number of entrants was not affected by the possibility of seller participation, which is also consistent with the theory.Auctions, Experiment, Shill Bidding, Entry
Geographic concentration and firm survival
If localization economies are present, firms within denser industry concentrations should exhibit higher levels of performance than more isolated firms. Nevertheless, research in industrial organization that has focused on the influences on firm survival has largely ignored the potential effects from agglomeration. Recent studies in urban and regional economics suggests that agglomeration effects may be very localized. Analyses of industry concentration at the MSA or county-level may fail to detect important elements of intra-industry firm interaction that occur at the sub-MSA level. Using a highly detailed dataset on firm locations and characteristics for Texas, this paper analyses agglomeration effects on firm survival over geographic areas as small as a single mile radius. We find that greater firm density within very close proximity (within 1 mile) of firms in the same industry increases mortality rates while greater concentration over larger distances reduces mortality rates.Firm Survival, Agglomeration, Localization, and Knowledge Externalities
An evolutionary model for GHz Peaked Spectrum Sources. Predictions for high frequency surveys
We have explored, in the general framework of the ``young source'' scenario,
evolutionary models for GHz Peaked Spectrum (GPS) galaxies and quasars which
reproduce the observed counts, redshift and peak frequency distributions of
currently available samples. Substantially different cosmological evolution
properties are found for the two populations: the quasar luminosity function
must evolve strongly up to , while the data on galaxies may be
consistent with no evolution. The models show that GPS sources (mostly quasars)
may comprise quite a significant fraction of bright (Jy) radio sources
at GHz if the intrinsic distribution of peak frequencies extends
up to GHz. In any case, however, their fraction decreases rapidly
with decreasing flux and their contribution to small scale fluctuations in the
frequency range covered by the forthcoming space missions MAP and Planck
Surveyor is expected to be minor.Comment: 7 pages, 4 figures, A&A accepte
Research universities and regional high-tech firm start-ups and exit
If localized knowledge spillovers are present in the university setting, higher rates of both start-ups and/or survival than in the broader economy would be observed in areas that are geographically proximate to the university. Using a fully-disclosed Quarterly Census of Employment and Wages for Texas for the years 1999:3-2006:2, this paper analyzes start-ups and exit rates for high-tech firms in Texas. We find that there is evidence that the presence of a research institution will affect the likelihood of technology start-ups. However, results suggest that geographic proximity to knowledge centers does not reduce hazard rates.Entry and Survival, R & D, Regional, Urban, and Rural Analyses
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