1,644 research outputs found
Redshift-Independent Distances to Type Ia Supernovae
We describe a procedure for accurately determining luminosity distances to
Type Ia supernovae (SNe Ia) without knowledge of redshift. This procedure,
which may be used as an extension of any of the various distance determination
methods currently in use, is based on marginalizing over redshift, removing the
requirement of knowing a priori. We demonstrate that the Hubble diagram
scatter of distances measured with this technique is approximately equal to
that of distances derived from conventional redshift-specific methods for a set
of 60 nearby SNe Ia. This indicates that accurate distances for cosmological
SNe Ia may be determined without the requirement of spectroscopic redshifts,
which are typically the limiting factor for the number of SNe that modern
surveys can collect. Removing this limitation would greatly increase the number
of SNe for which current and future SN surveys will be able to accurately
measure distance. The method may also be able to be used for high- SNe Ia to
determine cosmological density parameters without redshift information.Comment: 12 pages, 3 figures, accepted for publication in Astrophysical
Journal Letter
Extragalactic Large-Scale Structures behind the Southern Milky Way. IV. Redshifts Obtained with MEFOS
Abbreviated: As part of our efforts to unveil extragalactic large-scale
structures behind the southern Milky Way, we here present redshifts for 764
galaxies in the Hydra/Antlia, Crux and Great Attractor region (266deg < l <
338deg, |b| < 10deg), obtained with the Meudon-ESO Fibre Object Spectrograph
(MEFOS) at the 3.6-m telescope of ESO. The observations are part of a redshift
survey of partially obscured galaxies recorded in the course of a deep optical
galaxy search behind the southern Milky Way. A total of 947 galaxies have been
observed, a small percentage of the spectra (N=109, 11.5%) were contaminated by
foreground stars, and 74 galaxies (7.8%) were too faint to allow a reliable
redshift determination. With MEFOS we obtained spectra down to the faintest
galaxies of our optical galaxy survey, and hence probe large-scale structures
out to larger distances (v <~ 30000 km/s) than our other redshift follow-ups.
The most distinct large-scale structures revealed in the southern Zone of
Avoidance are discussed in context to known structures adjacent to the Milky
Way.Comment: 11 pages, 7 figures; accepted for publication in A&A. Tables 1 and 2
will be available in electronic format at the CDS. Figure 1 at full
resolution, and both tables are available at
http://mensa.ast.uct.ac.za/~pwoudt/data/H4462/
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Study of thermoelectric magnetohydrodynamic convection on solute redistribution during laser additive manufacturing
Melt pools formed in laser additive manufacturing (AM) are subject to large thermal gradients, resulting in the formation of thermoelectric currents due to the Seebeck effect. When in the presence of an external magnetic field, a Lorentz force is formed which drives fluid flow in the melt pool. This Thermoelectric Magnetohydrodynamics (TEMHD) phenomenon, can have a significant impact on the melt pool morphology and can alter the microstructural evolution of the solidification process. By coupling steady-state mesoscopic melt pool calculations to a microscopic solidification model, predictions of the resulting microstructure for multiple deposited layers have been obtained. The results indicate that the magnetic field can have a transformative effect on the microstructure and solute redistribution. This study highlights the theoretical potential for using magnetic fields as an additional control system to tailor AM microstructures
Thermoelectric magnetohydrodynamic control of melt pool dynamics and microstructure evolution in additive manufacturing
Large thermal gradients in the melt pool from rapid heating followed by rapid cooling in metal additive manufacturing generate large thermoelectric currents. Applying an external magnetic field to the process introduces fluid flow through thermoelectric magnetohydrodynamics. Convective transport of heat and mass can then modify the melt pool dynamics and alter microstructural evolution. As a novel technique, this shows great promise in controlling the process to improve quality and mitigate defect formation. However, there is very little knowledge within the scientific community on the fundamental principles of this physical phenomenon to support practical implementation. To address this multiphysics problem that couples the key phenomena of melting/solidification, electromagnetism, hydrodynamics, heat and mass transport, the lattice Boltzmann method for fluid dynamics was combined with a purpose-built code addressing solidification modelling and electromagnetics. The theoretical study presented here investigates the hydrodynamic mechanisms introduced by the magnetic field. The resulting steady-state solutions of modified melt pool shapes and thermal fields are then used to predict the microstructure evolution using a cellular automata based grain growth model. The results clearly demonstrate that the hydrodynamic mechanisms and, therefore, microstructure characteristics are strongly dependent on magnetic field orientation
New distances of unresolved dwarf elliptical galaxies in the vicinity of the Local Group
We present Surface Brightness Fluctuation distances of nine early-type dwarf
galaxies and the S0 galaxy NGC 4150 in the Local Volume based on deep B- and
R-band CCD images obtained with the 2.56 metre Nordic Optical Telescope.
Typically, six stellar fields at various galactocentric distances have been
chosen for each galaxy as appropriately free of foreground stars and other
contaminants, and Fourier analysed to determine the distances, which are found
to lie in the range of 3 to 16 Mpc. The SBF method is thus demonstrated to
efficiently measure distances from the ground with mid-aperture telescopes for
galaxies for which only the tip of the red giant branch method in combination
with the Hubble Space Telescope has been available until now. We obtained the
following distance moduli: 28.11 +/- 0.15 mag (or 4.2 +/- 0.3 Mpc) for UGC
1703, 27.61 +/- 0.17 mag (or 3.3 +/- 0.3 Mpc) for KDG 61, 29.00 +/- 0.27 mag
(or 6.3 +/- 0.8 Mpc) for UGCA 200, 27.74 +/- 0.18 mag (or 3.5 +/- 0.3 Mpc) for
UGC 5442, 30.22 +/- 0.17 mag (or 11.1 +/- 0.9 Mpc) for UGC 5944, 30.79 +/- 0.11
mag (or 14.4 +/- 0.7 Mpc) for NGC 4150, 31.02 +/- 0.25 mag (or 16.0 +/- 1.9
Mpc) for BTS 128, 29.27 +/- 0.16 mag (or 7.1 +/- 0.6 Mpc) for UGC 7639, 30.19
+/- 0.23 mag (or 10.9 +/- 1.2 Mpc) for UGC 8799 with an alternative distance of
30.61 +/- 0.26 mag (or 13.2 +/- 1.7 Mpc), and 29.60 +/- 0.20 mag (or 8.3 +/-
0.8 Mpc) for UGC 8882.Comment: 14 pages, 13 figures (figures 1, 2 and 10 are of reduced quality),
accepted for publication in A&
Multiwavelength optical observations of chromospherically active binary systems V. FF UMa (2RE J0933+624): a system with orbital period variation
This is the fifth paper in a series aimed at studying the chromospheres of
active binary systems using several optical spectroscopic indicators to obtain
or improve orbital solution and fundamental stellar parameters. We present here
the study of FF UMa (2RE J0933+624), a recently discovered, X-ray/EUV selected,
active binary with strong H_alpha emission. The objectives of this work are, to
find orbital solutions and define stellar parameters from precise radial
velocities and carry out an extensive study of the optical indicators of
chromospheric activity. We obtained high resolution echelle spectroscopic
observations during five observing runs from 1998 to 2004. We found radial
velocities by cross correlation with radial velocity standard stars to achieve
the best orbital solution. We also measured rotational velocity by
cross-correlation techniques and have studied the kinematic by galactic space-
velocity components (U, V, W) and Eggen criteria. Finally, we have determined
the chromospheric contribution in optical spectroscopic indicators, from Ca II
H & K to Ca II IRT lines, using the spectral subtraction technique. We have
found that this system presents an orbital period variation, higher than
previously detected in other RS CVn systems. We determined an improved orbital
solution, finding a circular orbit with a period of 3.274 days. We derived the
stellar parameters, confirming the subgiant nature of the primary component and
obtained rotational velocities (vsini), of 33.57 km/s and 32.38 km/s for the
primary and secondary components respectively. From our kinematic study, we can
deduce its membership to the Castor moving group. Finally, the activity study
has given us a better understanding of the possible mechanisms that produce the
orbital period variation.Comment: Latex file with 16 pages, 18 figures. Available at
http://www.ucm.es/info/Astrof/invest/actividad/actividad_pub.html Accepted
for publication in: Astronomy & Astrophysics (A&A
RJK Observations of the Optical Afterglow of GRB 991216
We present near-infrared and optical observations of the afterglow to the
Gamma-Ray Burst (GRB) 991216 obtained with the F. L. Whipple Observatory 1.2-m
telescope and the University of Hawaii 2.2-m telescope. The observations range
from 15 hours to 3.8 days after the burst. The temporal behavior of the data is
well described by a single power-law decay with index -1.36 +/-0.04,
independent of wavelength. The optical spectral energy distribution, corrected
for significant Galactic reddening of E(B-V)=0.626, is well fitted by a single
power-law with index -0.58 +/- 0.08. Combining the IR/optical observations with
a Chandra X-ray measurement gives a spectral index of -0.8 +/- 0.1 in the
synchrotron cooling regime. A comparison between the spectral and temporal
power-law indices suggest that a jet is a better match to the observations than
a simple spherical shock.Comment: Accepted to the Astrophysical Journal, 12 pages, 4 postscript figure
The Revival of the Unified Dark Energy-Dark Matter Model ?
We consider the generalized Chaplygin gas (GCG) proposal for unification of
dark energy and dark matter and show that it admits an unique decomposition
into dark energy and dark matter components once phantom-like dark energy is
excluded. Within this framework, we study structure formation and show that
difficulties associated to unphysical oscillations or blow-up in the matter
power spectrum can be circumvented. Furthermore, we show that the dominance of
dark energy is related to the time when energy density fluctuations start
deviating from the linear behaviour.Comment: 6 pages, 4 eps figures, Revtex4 style. New References are added. Some
typos are corrected. Conclusions remain the sam
The NN2 Flux Difference Method for Constructing Variable Object Light Curves
We present a new method for optimally extracting point-source time
variability information from a series of images. Differential photometry is
generally best accomplished by subtracting two images separated in time, since
this removes all constant objects in the field. By removing background sources
such as the host galaxies of supernovae, such subtractions make possible the
measurement of the proper flux of point-source objects superimposed on extended
sources. In traditional difference photometry, a single image is designated as
the ``template'' image and subtracted from all other observations. This
procedure does not take all the available information into account and for
sub-optimal template images may produce poor results. Given N total
observations of an object, we show how to obtain an estimate of the vector of
fluxes from the individual images using the antisymmetric matrix of flux
differences formed from the N(N-1)/2 distinct possible subtractions and provide
a prescription for estimating the associated uncertainties. We then demonstrate
how this method improves results over the standard procedure of designating one
image as a ``template'' and differencing against only that image.Comment: Accepted to AJ. To be published in November 2005 issue. 16 page, 2
figures, 2 tables. Source code available at
http://www.ctio.noao.edu/essence/nn2
Brown Dwarfs in Young Moving Groups from Pan-STARRS1. I. AB Doradus
Substellar members of young (150 Myr) moving groups are valuable
benchmarks to empirically define brown dwarf evolution with age and to study
the low-mass end of the initial mass function. We have combined Pan-STARRS1
(PS1) proper motions with opticalIR photometry from PS1, 2MASS and
to search for substellar members of the AB Dor Moving Group
within 50 pc and with spectral types of late-M to early-L,
corresponding to masses down to 30 M at the age of the group
(125 Myr). Including both photometry and proper motions allows us to
better select candidates by excluding field dwarfs whose colors are similar to
young AB~Dor Moving Group members. Our near-IR spectroscopy has identified six
ultracool dwarfs (M6L4; 30100 M) with intermediate
surface gravities (INT-G) as candidate members of the AB Dor Moving Group. We
find another two candidate members with spectra showing hints of youth but
consistent with field gravities. We also find four field brown dwarfs
unassociated with the AB Dor Moving Group, three of which have INT-G gravity
classification. While signatures of youth are present in the spectra of our
125 Myr objects, neither their nor colors are
significantly redder than field dwarfs with the same spectral types, unlike
younger ultracool dwarfs. We also determined PS1 parallaxes for eight of our
candidates and one previously identified AB Dor Moving Group candidate.
Although radial velocities (and parallaxes, for some) are still needed to fully
assess membership, these new objects provide valuable insight into the spectral
characteristics and evolution of young brown dwarfs.Comment: ApJ, accepte
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