15,668 research outputs found
Measuring the Virial Masses of Disk Galaxies
I present detailed models for the formation of disk galaxies, and investigate
which observables are best suited as virial mass estimators. Contrary to naive
expectations, the luminosities and circular velocities of disk galaxies are
extremely poor indicators of total virial mass. Instead, I show that the
product of disk scale length and rotation velocity squared yields a much more
robust estimate. Finally, I show how this estimator may be used to put limits
on the efficiencies of cooling and feedback during the process of galaxy
formation.Comment: 6 pages, 2 fig. To appear in proceedings of "The Mass of Galaxies at
Low and High Redshift", eds. R. Bender and A. Renzini (ESO Astrophysics
Symposia, Springer-Verlag
Do not forget: Full memory in memory-based learning of word pronunciation
Memory-based learning, keeping full memory of learning material, appears a
viable approach to learning NLP tasks, and is often superior in generalisation
accuracy to eager learning approaches that abstract from learning material.
Here we investigate three partial memory-based learning approaches which remove
from memory specific task instance types estimated to be exceptional. The three
approaches each implement one heuristic function for estimating exceptionality
of instance types: (i) typicality, (ii) class prediction strength, and (iii)
friendly-neighbourhood size. Experiments are performed with the memory-based
learning algorithm IB1-IG trained on English word pronunciation. We find that
removing instance types with low prediction strength (ii) is the only tested
method which does not seriously harm generalisation accuracy. We conclude that
keeping full memory of types rather than tokens, and excluding minority
ambiguities appear to be the only performance-preserving optimisations of
memory-based learning.Comment: uses conll98, epsf, and ipamacs (WSU IPA
A model of growth and development in copepods
We present a model for the growth and development of copepods based on the simple assumption that a constant fraction of assimilated carbon is invested in the formation of a new carapace. The individual molts into the next stage when the weight of the accumulated carapace building bricks is a fixed fraction of the body mass at the beginning of the stage. This simple stage-transition coordinating system is built into an individual growth model.
The model predicts decreasing stage duration and increasing body weight with increasing food concentrations. These trends closely agree with published data. Anisochronality, as found in many copepod species, agrees with model predictions. Acartia species usually show isochronal development. The model predicts isochronal development for one particular value of a parameter of the feeding process. Literature data show that this parameter value is realistic for Acartia spp. ''Equiproportionality'' is predicted by the model. The model thus shows close agreement with observed growth and development pattern
Closed-cycle gas flow system for cooling of high Tc d.c. SQUID magnetometers
A high Tc.d.c SQUID based magnetometer for magnetocardiography is currently under development at the University of Twente. Since such a magnetometer should be simple to use, the cooling of the system can be realized most practically by means of a cryocooler. A closed-cycle gas flow cooling system incorporating such a cooler has been designed, constructed and tested. The aimed resolution of the magnetometer is 0.1 pT Hz−1/2. The required operating temperature for the SQUIDs is 30 to about 77 K with a stability of 2 × 10−4 K Hz−1/2. After a cool-down time of 1–2 h, a stationary cooling power of at least 0.2 W is required. In the design, helium gas is cooled by a Leybold Heraeus RG 210 cryocooler, transported through a gas line, and subsequently passed through a heat exchanger on which SQUIDs can be installed. The lowest obtainable SQUID heat exchanger temperature is 31 ± 2 K. This can be reached in roughly 2–3 h with an optimal mass flow with respect to the cooling power of 6 × 10−6 kg s−1. At this mass flow the cooling power at the SQUID heat exchanger is 0.2 W at 42 K and roughly 1.2 W at 77 K. A temperature stability of 0.05 K was measured at a SQUID heat exchanger temperature of 54 K and a mass flow of 3 × 10kg s−5. The experience gained with this large cooling system will be used in the design of a smaller configuration cooling system, incorporating miniature Stirling cryocoolers. In this paper the design and the construction of the present closed-cycle system are described and test results are presented.\ud
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