2,034 research outputs found
Formalism and quality of a proper motion link with extragalactic objects for astrometric satellite missions
The accuracy of the link of the proper motion system of astrometric satellite
missions like AMEX and GAIA is discussed. Monte-Carlo methods were used to
simulate catalogues of positions and proper motions of quasars and galaxies to
test the link. The main conclusion is, that future satellite missions like GAIA
may be ``self-calibrated'' by their measurements of QSOs, while additional
measurements from radio stars or HST-data are needed to calibrate the less deep
reaching astrometric satellite missions of AMEX type.Comment: Accepted for publication in A&A, LaTeX A&A style, 7 pages, 4 figure
Translation from Classical Two-Way Automata to Pebble Two-Way Automata
We study the relation between the standard two-way automata and more powerful
devices, namely, two-way finite automata with an additional "pebble" movable
along the input tape. Similarly as in the case of the classical two-way
machines, it is not known whether there exists a polynomial trade-off, in the
number of states, between the nondeterministic and deterministic pebble two-way
automata. However, we show that these two machine models are not independent:
if there exists a polynomial trade-off for the classical two-way automata, then
there must also exist a polynomial trade-off for the pebble two-way automata.
Thus, we have an upward collapse (or a downward separation) from the classical
two-way automata to more powerful pebble automata, still staying within the
class of regular languages. The same upward collapse holds for complementation
of nondeterministic two-way machines.
These results are obtained by showing that each pebble machine can be, by
using suitable inputs, simulated by a classical two-way automaton with a linear
number of states (and vice versa), despite the existing exponential blow-up
between the classical and pebble two-way machines
Abundances in Stars from the Red Giant Branch Tip to the Near the Main Sequence in M71: I. Sample Selection, Observing Strategy and Stellar Parameters
We present the sample for an abundance analysis of 25 members of M71 with
luminosities ranging from the red giant branch tip to the upper main sequence.
The spectra are of high dispersion and of high precision. We describe the
observing strategy and determine the stellar parameters for the sample stars
using both broad band colors and fits of H profiles. The derived
stellar parameters agree with those from the Yale stellar evolutionary
tracks to within 50 -- 100K for a fixed log g, which is within the level of the
uncertainties.Comment: Minor changes to conform to version accepted for publication, with
several new figures (Paper 1 of a pair
Remarks on separating words
The separating words problem asks for the size of the smallest DFA needed to
distinguish between two words of length <= n (by accepting one and rejecting
the other). In this paper we survey what is known and unknown about the
problem, consider some variations, and prove several new results
Photometric and kinematic studies of open star clusters. III. NGC 4103, NGC 5281, and NGC 4755
We present CCD photometry and proper motion studies of the three open star
clusters NGC 4103, NGC 5281, and NGC 4755 (kappa Cru). By fitting isochrones to
the colour magnitude diagrams, we found that all three objects are young open
star clusters with ages of at most t=45 Myr. They are located at distances from
approx. 1600 pc to 2200 pc, derived from distance moduli (m-M)_0 ranging from
11 mag to 12 mag. We combined membership determinations based on proper motions
and statistical field star subtraction to derive the initial mass function
(IMF) of the clusters. The shape of the IMFs could be represented by power laws
with exponents of Gamma=-1.46 +/- 0.22 for NGC 4103, Gamma=-1.60 +/- 0.50 for
NGC 5281, and Gamma=-1.68 +/- 0.14 for NGC 4755, when - as a reference -
Salpeter's (1955) value would be Gamma=-1.35. These results agree well with
other IMF studies of open star clusters.Comment: 16 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
A proper motion study of the globular cluster M10
We present the first proper motion study of M10 (NGC6254). Absolute proper
motions of about 532 stars in the field of the globular cluster M10 were
determined with respect to Hipparcos and ACT reference stars. In addition to
photographic plates of Bonn and Shanghai also wide field CCD observations as
second epoch plates were used. The wide field CCD observations show an accuracy
comparable to that of the photographic plates. A good coincidence of the
solutions based on reference stars from Hipparcos and from ACT was found. Our
final proper motions allow a sufficient separation of cluster and field stars.
Two population II Cepheids were confirmed to be members of M10. The absolute
proper motion of M10 was determined and combined with its distance from the Sun
and its radial velocity. The space motion and metallicity of M10 indicates the
characteristics of a halo object with an orbit reaching to a maximal z-distance
of less than 3kpc.Comment: 5 pages, LaTeX, incl. 2 PostScript figures. Accepted for publication
by Astronomy & Astrophysics (main journal
Coherence resonance in a network of FitzHugh-Nagumo systems: interplay of noise, time-delay and topology
We systematically investigate the phenomena of coherence resonance in
time-delay coupled networks of FitzHugh-Nagumo elements in the excitable
regime. Using numerical simulations, we examine the interplay of noise,
time-delayed coupling and network topology in the generation of coherence
resonance. In the deterministic case, we show that the delay-induced dynamics
is independent of the number of nearest neighbors and the system size. In the
presence of noise, we demonstrate the possibility of controlling coherence
resonance by varying the time-delay and the number of nearest neighbors. For a
locally coupled ring, we show that the time-delay weakens coherence resonance.
For nonlocal coupling with appropriate time-delays, both enhancement and
weakening of coherence resonance are possible
The Magic Number Problem for Subregular Language Families
We investigate the magic number problem, that is, the question whether there
exists a minimal n-state nondeterministic finite automaton (NFA) whose
equivalent minimal deterministic finite automaton (DFA) has alpha states, for
all n and alpha satisfying n less or equal to alpha less or equal to exp(2,n).
A number alpha not satisfying this condition is called a magic number (for n).
It was shown in [11] that no magic numbers exist for general regular languages,
while in [5] trivial and non-trivial magic numbers for unary regular languages
were identified. We obtain similar results for automata accepting subregular
languages like, for example, combinational languages, star-free, prefix-,
suffix-, and infix-closed languages, and prefix-, suffix-, and infix-free
languages, showing that there are only trivial magic numbers, when they exist.
For finite languages we obtain some partial results showing that certain
numbers are non-magic.Comment: In Proceedings DCFS 2010, arXiv:1008.127
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