4,315 research outputs found
A computer program for hydrostatic bearings including the effects of non-uniform film thickness and relative velocity for various methods of lubricant supply final technical report
Computer program for hydrostatic bearing - effects of nonuniform film thickness and lubricant suppl
Theoretical Models for Classical Cepheids: IV. Mean Magnitudes and Colors and the Evaluation of Distance, Reddening and Metallicity
We discuss the metallicity effect on the theoretical visual and near-infrared
PL and PLC relations of classical Cepheids, as based on nonlinear, nonlocal and
time--dependent convective pulsating models at varying chemical composition. In
view of the two usual methods of averaging (magnitude-weighted and
intensity-weighted) observed magnitudes and colors over the full pulsation
cycle, we briefly discuss the differences between static and mean quantities.
We show that the behavior of the synthetic mean magnitudes and colors fully
reproduces the observed trend of Galactic Cepheids, supporting the validity of
the model predictions. In the second part of the paper we show how the estimate
of the mean reddening and true distance modulus of a galaxy from Cepheid VK
photometry depend on the adopted metal content, in the sense that larger
metallicities drive the host galaxy to lower extinctions and distances.
Conversely, self-consistent estimates of the Cepheid mean reddening, distance
and metallicity may be derived if three-filter data are taken into account. By
applying the theoretical PL and PLC relations to available BVK data of Cepheids
in the Magellanic Clouds we eventually obtain Z \sim 0.008, E(B-V) \sim 0.02
mag, DM \sim 18.63 mag for LMC and Z \sim 0.004, E(B-V) \sim 0.01 mag., DM \sim
19.16 mag. for SMC. The discrepancy between such reddenings and the current
values based on BVI data is briefly discussed.Comment: 16 pages, 11 postscript figures, accepted for publication on Ap
Differential Radial Velocities and Stellar Parameters of Nearby Young Stars
Radial velocity searches for substellar mass companions have focused
primarily on stars older than 1 Gyr. Increased levels of stellar activity in
young stars hinders the detection of solar system analogs and therefore there
has been a prejudice against inclusion of young stars in radial velocity
surveys until recently. Adaptive optics surveys of young stars have given us
insight into the multiplicity of young stars but only for massive, distant
companions. Understanding the limit of the radial velocity technique,
restricted to high-mass, close-orbiting planets and brown dwarfs, we began a
survey of young stars of various ages. While the number of stars needed to
carry out full analysis of the problems of planetary and brown dwarf population
and evolution is large, the beginning of such a sample is included here. We
report on 61 young stars ranging in age from beta Pic association (~12 Myr) to
the Ursa Majoris association (~300 Myr). This initial search resulted in no
stars showing evidence for companions greater than ~1-2 M_Jup in short period
orbits at the 3 sigma-level. Additionally, we present derived stellar
parameters, as most have unpublished values. The chemical homogeneity of a
cluster, and presumably of an association, may help to constrain true
membership. As such, we present [Fe/H] abundances for the stars in our sample.Comment: Accepted for publication in the PAS
Searching for star-forming dwarf galaxies in the Antlia cluster
The formation and evolution of dwarf galaxies in clusters need to be
understood, and this requires large aperture telescopes. In this sense, we
selected the Antlia cluster to continue our previous work in the Virgo, Fornax,
and Hydra clusters and in the Local Volume (LV). Because of the scarce
available literature data, we selected a small sample of five blue compact
dwarf (BCD) candidates in Antlia for observation. Using the Gemini South and
GMOS camera, we acquired the Halpha imaging needed to detect star-forming
regions in this sample. With the long-slit spectroscopic data of the brightest
seven knots detected in three BCD candidates, we derived their basic chemical
properties. Using archival VISTA VHS survey images, we derived K_S magnitudes
and surface brightness profile fits for the whole sample to assess basic
physical properties. FS90-98, FS90-106, and FS90-147 are confirmed as BCDs and
cluster members, based on their morphology, K_S surface photometry, oxygen
abundance, and velocity redshift. FS90-155 and FS90-319 did not show any
H{\alpha} emission, and they could not be confirmed as dwarf cluster
star-forming galaxies. Based on our data, we studied some fundamental relations
to compare star forming dwarfs (BCDs and dIs) in the LV and in the Virgo,
Fornax, Hydra, and Antlia clusters. Star-forming dwarfs in nearby clusters
appear to follow same fundamental relations in the near infrared with similar
objects in the LV, specifically the size-luminosity and the
metallicity-luminosity, while other more fundamental relations could not be
checked in Antlia due to lack of data.Comment: Accepted for publication in A&A (early 2014
The Distance of the First Overtone RR Lyrae Variables in the MACHO LMC Database: A New Method to Correct for the Effects of Crowding
Previous studies have indicated that many of the RR Lyrae variables in the
LMC have properties similar to the ones in the Galactic globular cluster M3.
Assuming that the M3 RR Lyrae variables follow the same relationships among
period, temperature, amplitude and Fourier phase parameter phi31 as their LMC
counterparts, we have used the M3 phi31-logP relation to identify the M3-like
unevolved first overtone RR Lyrae variables in 16 fields near the LMC bar. The
temperatures of these variables were calculated from the M3 logP-logTe relation
so that the extinction could be derived for each star separately. Since blended
stars have lower amplitudes for a given period, the period amplitude relation
should be a useful tool for identifying which stars are affected by crowding.
We find that the low amplitude stars are brighter. We remove them from the
sample and derive an LMC distance modulus 18.49+/-0.11.Comment: 30 pages, 7 figures, accepted for publication in the Astronomical
Journa
MOS CCDs for the wide field imager on the XEUS spacecraft
In recent years the XEUS mission concept has evolved and has been the subject of several industrial studies. The mission concept has now matured to the point that it could be proposed for a Phase A study and subsequent flight programme. The key feature of XEUS will be its X-ray optic with collecting area ~30-100x that of XMM. The mission is envisaged at an orbit around the L2 point in space, and is formed from two spacecraft; one for the mirrors, and the other for the focal plane detectors. With a focal length of 50m, the plate scale of the optic is 6.5x that of XMM, which using existing focal plane technology will reduce the effective field of view to a few arc minutes. Cryogenic instrumentation, with detector sizes of a few mm can only be used for narrow field studies of target objects, and a wide field instrument is under consideration using a DEPFET pixel array to image out to a diameter of 5 arcminutes, requiring an array of dimension 70mm. It is envisaged to extend this field of view possibly out to 15 arcminutes through the use of an outer detection ring comprised of MOS CCD
Construct, Merge, Solve and Adapt: Application to the repetition-free longest common subsequence problem
In this paper we present the application of a recently proposed, general, algorithm for combinatorial optimization to the repetition-free longest common subsequence problem. The applied algorithm, which is labelled Construct, Merge, Solve & Adapt, generates sub-instances based on merging the solution components found in randomly constructed solutions. These sub-instances are subsequently solved by means of an exact solver. Moreover, the considered sub-instances are dynamically changing due to adding new solution components at each iteration, and removing existing solution components on the basis of indicators about their usefulness. The results of applying this algorithm to the repetition-free longest common subsequence problem show that the algorithm generally outperforms competing approaches from the literature. Moreover, they show that the algorithm is competitive with CPLEX for small and medium size problem instances, whereas it outperforms CPLEX for larger problem instances.Peer ReviewedPostprint (author's final draft
A multi-color and Fourier study of RR Lyrae variables in the globular cluster NGC 5272 (M3)
We have performed a detailed study of the pulsational and evolutionary
characteristics of 133 RR Lyrae stars in the globular cluster NGC5272 (M3)
using highly accurate BVI data taken on 5 separate epochs. M3 seems to contain
no less than ~32% of Blazhko stars, and the occurrence and characteristics of
the Blazhko effect have been analyzed in detail. We have identified a good
number (~ 14%) of overluminous RR Lyrae stars that are likely in a more
advanced evolutionary stage off the Zero Age Horizontal Branch (ZAHB). Physical
parameters (i.e. temperature, luminosity, mass) have been derived from (B--V)
colors and accurate color-temperature calibration, and compared with Horizontal
Branch evolutionary models and with the requirements of stellar pulsation
theory. Additional analysis by means of Fourier decomposition of the V light
curves confirms, as expected, that no metallicity spread is present in M3.
Evolution off the ZAHB does not affect [Fe/H] determinations, whereas Blazhko
stars at low amplitude phase do affect [Fe/H] distributions as they appear more
metal-rich. Absolute magnitudes derived from Fourier coefficients might provide
useful average estimates for groups of stars, if applicable, but do not give
reliable {\em individual} values. Intrinsic colors derived from Fourier
coefficients show significant discrepancies with the observed ones, hence the
resulting temperatures and temperature-related parameters are unreliable.Comment: 86 pages, 19 figures, 13 tables, in press A
Globular Clusters: DNA of Early-Type galaxies?
This paper explores if the mean properties of Early-Type Galaxies (ETG) can
be reconstructed from "genetic" information stored in their GCs (i.e., in their
chemical abundances, spatial distributions and ages). This approach implies
that the formation of each globular occurs in very massive stellar
environments, as suggested by some models that aim at explaining the presence
of multi-populations in these systems. The assumption that the relative number
of globular clusters to diffuse stellar mass depends exponentially on chemical
abundance, [Z/H], and the presence of two dominant GC sub-populations blue and
red, allows the mapping of low metallicity halos and of higher metallicity (and
more heterogeneous) bulges. In particular, the masses of the low-metallicity
halos seem to scale up with dark matter mass through a constant. We also find a
dependence of the globular cluster formation efficiency with the mean projected
stellar mass density of the galaxies within their effective radii. The analysis
is based on a selected sub-sample of galaxies observed within the ACS Virgo
Cluster Survey of the {\it Hubble Space Telescope}. These systems were grouped,
according to their absolute magnitudes, in order to define composite fiducial
galaxies and look for a quantitative connection with their (also composite)
globular clusters systems. The results strengthen the idea that globular
clusters are good quantitative tracers of both baryonic and dark matter in
ETGs.Comment: 20 pages, 28 figures and 5 table
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