544 research outputs found
Constraint violation in free evolution schemes: comparing BSSNOK with a conformal decomposition of Z4
We compare numerical evolutions performed with the BSSNOK formulation and a
conformal decomposition of a Z4-like formulation of General Relativity. The
important difference between the two formulations is that the Z4 formulation
has a propagating Hamiltonian constraint, whereas BSSNOK has a zero-speed
characteristic variable in the constraint subsystem. In spherical symmetry we
evolve both puncture and neutron star initial data. We demonstrate that the
propagating nature of the Z4 constraints leads to results that compare
favorably with BSSNOK evolutions, especially when matter is present in the
spacetime. From the point of view of implementation the new system is a simple
modification of BSSNOK.Comment: Published in PR
Search for Variable Stars in the Globular Cluster M3
We describe here results of a photometric time-sequence survey of the
globular cluster M3 (NGC 5272), in a search for contact and detached eclipsing
binary stars. We have discovered only one likely eclipsing binary and one SX
Phe type star in spite of monitoring 4077 stars with and observing 25
blue stragglers. The newly identified SX Phe star, V237, shows a light curve
with a variable amplitude. Variable V238 shows variability either with a period
of 0.49 d or with a period of 0.25 d. On the cluster colour-magnitude diagram,
the variable occupies a position a few hundredths of magnitude to the blue of
the base of the red giant branch. V238 is a likely descendent of a binary blue
straggler. As a side result we obtained high quality data for 42 of the
previously known RR Lyrae variables, including 33 of Bailey type ab, 7 type c
and 2 double-mode pulsators. We used equations that relate the physical
properties of RRc stars to their pulsation periods and Fourier parameters to
derive masses, luminosities, temperatures and helium parameters for five of the
RRc stars. One of the RRd stars (V79) has switched modes. In previous studies,
it was classified as RRab, but our observations show that it is an RRd star
with the first overtone mode dominating. This indicates blueward evolution on
the horizontal branch.Comment: 21 pages including 14 figures, Latex, requires mn.sty, psfig.sty.
Submitted, MNRA
Post-forming heat treatment of dual phase steel components for automotive applications using a fluid bed furnace
Improving student engagement through content and assessment choice in a common first year
The mass and radius of the M dwarf companion to GD 448
We present spectroscopy and photometry of GD 448, a detached white dwarf - M
dwarf binary with a period of 2.47h. We find that the NaI 8200A feature is
composed of narrow emission lines due to irradiation of the M dwarf by the
white dwarf within broad absorption lines that are essentially unaffected by
heating. Combined with an improved spectroscopic orbit and gravitational red
shift measurement from spectra of the H-alpha line, we are able to derive
masses for the white dwarf and M dwarf directly (0.41 +/- 0.01 solar masses and
0.096 +/- 0.004 solar masses, respectively). We use a simple model of the CaII
emission lines to establish the radius of the M dwarf assuming the emission
from its surface to be proportional to the incident flux per unit area from the
white dwarf. The radius derived is 0.125 +/- 0.020 solar radii. The M dwarf
appears to be a normal main-sequence star in terms of its mass and radius and
is less than half the size of its Roche lobe. The thermal timescale of the M
dwarf is much longer than the cooling age of the white dwarf so we conclude
that the M dwarf was unaffected by the common-envelope phase. The anomalous
width of the H-alpha emission from the M dwarf remains to be explained, but the
strengh of the line may be due to X-ray heating of the M dwarf due to accretion
onto the white dwarf from the M dwarf wind.Comment: 8 pages, 8 figure
A Search for Distant Galactic Cepheids Toward l=60
We present results of a survey of a 6-square-degree region near l=60, b=0 to
search for distant Milky Way Cepheids. Few MW Cepheids are known at distances
>~ R_0, limiting large-scale MW disk models derived from Cepheid kinematics;
this work was designed to find a sample of distant Cepheids for use in such
models. The survey was conducted in the V and I bands over 8 epochs, to a
limiting I~=18, with a total of ~ 5 million photometric observations of ~ 1
million stars. We present a catalog of 578 high-amplitude variables discovered
in this field. Cepheid candidates were selected from this catalog on the basis
of variability and color change, and observed again the following season. We
confirm 10 of these candidates as Cepheids with periods from 4 to 8 days, most
at distances > 3 kpc. Many of the Cepheids are heavily reddened by intervening
dust, some with implied extinction A_V > 10 mag. With a future addition of
infrared photometry and radial velocities, these stars alone can provide a
constraint on R_0 to 8%, and in conjunction with other known Cepheids should
provide good estimates of the global disk potential ellipticity.Comment: 18 pages, 4 tables, 13 figures (LaTeX / AASTeX
Evolution of Horizontal Branch Stars in Globular Clusters: The Interesting Case of V79 in M3
New observations of variable stars in the globular cluster M3 reveal that the
RR Lyrae variable V79 is a double-mode (RRd) variable with the first overtone
mode dominating. In all previous studies, V79 was found to be a fundamental
mode (RRab) pulsator with an irregular light curve. This is the first observed
mode switch for an RR Lyrae variable and it is direct observational evidence
for blueward evolution of horizontal branch stars in the Oosterhoff type I
cluster M3. It also demonstrates that there is a connection between the Blazhko
effect and pulsational mode mixing in RR Lyrae variables. These new
observations also show that the strength of the overtone oscillations in the
RRd star V68 in M3 may have increased in the last 70 years, thus indicating
blueward evolution for V68 as well. A survey of previously published
investigations of RRd stars in Oosterhoff type II systems indicates that there
is marginal evidence for an increase in the strength of fundamental mode
oscillations in two stars: V30 in M15 and AQ Leo. If these increases are
confirmed by future observations, it will indicate redward evolution for RRd
stars in type II systems.Comment: 10 pages including 3 figures, Latex, requires aaspp4.sty. Accepted by
ApJ
NGC 602 Environment, Kinematics and Origins
The young star cluster NGC 602 and its associated HII region, N90, formed in
a relatively isolated and diffuse environment in the Wing of the Small
Magellanic Cloud. Its isolation from other regions of massive star formation
and the relatively simple surrounding HI shell structure allows us to constrain
the processes that may have led to its formation and to study conditions
leading to massive star formation. We use images from Hubble Space Telescope
and high resolution echelle spectrographic data from the Anglo-Australian
Telescope along with 21-cm neutral hydrogen (HI) spectrum survey data and the
shell catalogue derived from it to establish a likely evolutionary scenario
leading to the formation of NGC 602. We identify a distinct HI cloud component
that is likely the progenitor cloud of the cluster and HII region which
probably formed in blister fashion from the cloud's periphery. We also find
that the past interaction of HI shells can explain the current location and
radial velocity of the nebula. The surrounding Interstellar Medium is diffuse
and dust-poor as demonstrated by a low visual optical depth throughout the
nebula and an average HI density of the progenitor cloud estimated at 1 cm^-3.
These conditions suggest that the NGC 602 star formation event was produced by
compression and turbulence associated with HI shell interactions. It therefore
represents a single star forming event in a low gas density region.Comment: Accepted for publication in PASP. 25 pages, 10 figure
Normal modes of Proca fields in AdS spacetime
A normal mode analysis for Proca fields in the anti-de Sitter (AdS) spacetime
is given. It is found that the equations for the Proca field can be decoupled
analytically. This is performed by changing the basis of the vector spherical
harmonics (VSH) decomposition. The normal modes and the normal mode frequencies
of the Proca equation in the AdS spacetime are then analytically determined. It
is also shown that the Maxwell field can be recovered by taking the massless
limit of the Proca field with care so that the nonphysical gauge modes are
eliminated.Comment: 6 page
On the sdOB primary of the post common-envelope binary AA Doradus (LB 3459)
AA Dor is an eclipsing, post common-envelope binary with an sdOB-type primary
and a low-mass secondary. Eleven years ago, an NLTE spectral analysis showed a
discrepancy in the surface gravity that was derived by radial-velocity and
light-curve analysis, log g = 5.21 +/- 0.1 (cm/sec^2) and log g = 5.53 +/-
0.03, respectively.
We aim to determine both the effective temperature and surface gravity of AA
Dor precisely from high-resolution, high-S/N observations taken during the
occultation of the secondary.
We calculated an extended grid of metal-line blanketed, state-of-the-art,
non-LTE model atmospheres in the parameter range of the primary of AA Dor.
Synthetic spectra calculated from this grid were compared to optical
observations.
We verify Teff = 42000 +/- 1000 K from our former analyses and determine a
higher log g = 5.46 +/- 0.05. The main reason are new Stark-broadening tables
that were used for calculating of the theoretical Balmer-line profiles.
Our result for the surface gravity agrees with the value from light-curve
analysis within the error limits, thereby solving the so-called gravity problem
in AA Dor.Comment: 6 pages, 7 figure
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