174 research outputs found
A discrete slug population model determined by egg production
Slugs are significant pests in agriculture (as well as a nuisance to gardeners), and it is therefore important to understand their population dynamics for the construction of efficient and effective control measures. Differential equation models of slug populations require the inclusion of large (variable) temporal delays, and strong seasonal forcing results in a non-autonomous system. This renders such models open to only a limited amount of rigorous analysis. In this paper, we derive a novel batch model based purely upon the quantity of eggs produced at different times of the year. This model is open to considerable reduction; from the resulting two variable discrete-time system it is possible to reconstruct the dynamics of the full population across the year and give conditions for extinction or global stability and persistence. Furthermore, the steady state temporal population distribution displays qualitatively different behavior with only small changes in the survival probability of slugs. The model demonstrates how small variations in the favorability of different years may result in widely different slug population fluctuations between consecutive years, and is in good agreement with field data
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 QUEST RR Lyrae Survey: Confirmation of the Clump at 50 kpc and Other Over-Densities in the Outer Halo
We have measured the periods and light curves of 148 RR Lyrae variables from
V=13.5 to 19.7 from the first 100 sq. degrees of the QUEST RR Lyrae survey.
Approximately 55% of these stars belong to the clump of stars detected earlier
by the Sloan Digital Sky Survey. According to our measurements, this feature
has ~10 times the background density of halo stars, spans at least 37.5 deg by
3.5 deg in right ascension and declination (>=30 by >=3 kpc), lies ~50 kpc from
the Sun, and has a depth along the line of sight of ~5 kpc (1 sigma). These
properties are consistent with the recent models that suggest it is a tidal
stream from the Sgr dSph galaxy. The mean period of the type ab variables, 0.58
d, is also consistent. In addition, we have found two smaller over-densities in
the halo, one of which may be related to the globular cluster Pal 5.Comment: 12 pages (including 4 figures). Accepted for publication in the ApJ
Letter
Astrometry with Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator RR Lyrae
We present an absolute parallax and relative proper motion for the
fundamental distance scale calibrator, RR Lyr. We obtain these with astrometric
data from FGS 3, a white-light interferometer on HST. We find mas. Spectral classifications and VRIJHKTM and DDO51 photometry of
the astrometric reference frame surrounding RR Lyr indicate that field
extinction is low along this line of sight. We estimate =0.07\pm0.03 for
these reference stars. The extinction suffered by RR Lyr becomes one of the
dominant contributors to the uncertainty in its absolute magnitude. Adopting
the average field absorption, =0.07 \pm 0.03, we obtain M_V^{RR} = 0.61
^{-0.11}_{+0.10}. This provides a distance modulus for the LMC, m-M = 18.38 -
18.53^{-0.11}_{+0.10} with the average extinction-corrected magnitude of RR Lyr
variables in the LMC, , remaining a significant uncertainty. We compare
this result to more than 80 other determinations of the distance modulus of the
LMC.Comment: Several typos corrected. To appear in The Astronomical Journal,
January 200
The quest RR lyrae survey. I. The first catalog
With the 1 m Schmidt telescope of the Llano del Hato Observatory and the QUEST CCD camera, 380 deg 2 of the sky have been surveyed for RR Lyrae variables in a band 2.°3 wide in declination (centered at ÎŽ = -1°) and covering right ascensions from 4. h1 to 6. h1 and from 8. h0 to 17. h0. The bright limit (due to CCD saturation) and the faint limit are VâŒ13.5 and âŒ19.7, respectively, which correspond to âŒ4 and âŒ60 kpc from the Sun. We present a catalog of the positions, amplitudes, mean magnitudes, periods, and light curves of the 498 RR Lyrae variables that have been identified in this region of the sky. The majority of these stars (86%) are new discoveries. The completeness of the survey has been estimated from simulations that model the periods and light curves of real RR Lyrae variables and take into account the pattern of epochs of observation. While the completeness of the survey varies with apparent magnitude and with position, almost everywhere in the surveyed region it is quite high (\u3e80%) for the type ab RR Lyrae variables and between 30% and 90% for the low-amplitude type c variables
A Large Area CCD Camera for the Schmidt Telescope at the Venezuelan National Astronomical Observatory
We have designed, constructed and put into operation a large area CCD camera
that covers a large fraction of the image plane of the 1 meter Schmidt
telescope at Llano del Hato in Venezuela. The camera consists of 16 CCD devices
arranged in a 4 x 4 mosaic covering 2.3 degrees x 3.5 degrees of sky. The CCDs
are 2048 x 2048 LORAL devices with 15 micron pixels. The camera is optimized
for drift scan photometry and objective prism spectroscopy. The design
considerations, construction features and performance parameters are described
in the following article.Comment: 34 pages, 18 figures, accepted for publication in PAS
The Distance to the Hyades Cluster Based on HST Fine Guidance Sensor Parallaxes
Trigonometric parallax observations made with the Hubble Space Telescope's
Fine Guidance Sensor #3 (HST FGS) of seven Hyades Cluster members in six fields
of view have been analyzed along with their proper motions to determine the
distance to the cluster. Knowledge of the Cluster's convergent point and mean
proper motion are critical to the derivation of the distance to the center of
the cluster. Depending on the choice of the proper-motion system, the derived
cluster center distance varies by 9%. Adopting a reference distance of 46.1 pc
or m-M=3.32, which is derived from the ground-based parallaxes in the General
Catalogue of Trigonometric Stellar Parallaxes (1995 edition), the FK5/PPM
proper-motion system yields a distance 4% larger, while the Hanson (1975)
system yields a distance 2% smaller. The HST FGS parallaxes reported here yield
either a 14% or 5% larger distance depending on the choice of the proper-motion
system. Orbital parallaxes (Torres et al. 1997a, 1997b, 1997c) yield an average
distance 4% larger than the reference distance. The variation in the distance
derived from the HST data illustrates the importance of the proper-motion
system and the individual proper motions to the derivation of the distance to
the Hyades Cluster center, therefore a full utilization of the HST FGS
parallaxes awaits the establishment of an accurate and consistent proper-motion
system.Comment: 7 pages; This study is collaborated with 8 institution
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