8,098 research outputs found
Radial Velocities and Pulsation Ephemerides of 11 Field RR Lyrae Stars
We present new radial velocities, improved pulsation periods and reference
epoch s of 11 field RR Lyrae ab-type variables: AS Vir, BS Aps, CD Vel, DT Hya,
RV Oct, TY Gru, UV Oct, V1645 Sgr, WY Ant, XZ Aps and Z Mic. This study is
based on high resolution spectra obtained with the echelle spectro graph of the
2.5-m du Pont telescope at Las Campanas Observatory. We obtained ~200 spectra
per star (i.e, total of ~2300 spectra) distributed more or less uniformly
throughout their pulsation cycles. Radial velocity curves and photometric
lightcurves phased to our new ephemerides are presented for all program stars.
In a subsequent paper, we will use these spectra to derive stellar atmospheric
parameters and chemical compositions throughout the pulsational cycles, based
purely on spectroscopic constraints.Comment: 28 pages, 14 figures, 5 tables. Accepted for publication in ApJ
Cosmological backreaction in higher-derivative gravity expansions
We calculate a general effective stress-energy tensor induced by cosmological
inhomogeneity in effective theories of gravity where the action is
Taylor-expandable in the Riemann tensor and covariant derivatives of the
Riemann tensor. This is of interest as an effective fluid that might provide an
alternative to the cosmological constant, but it also applies to gravitational
waves. We use an adaptation of Green and Wald's weak-averaging framework, which
averages over perturbations in the field equation where the perturbation length
scales are small compared to the averaging scale. In this adaptation, the
length scale of the effective theory, , is also taken to be small compared
with the averaging scale. This ensures that the perturbation length scales
remain in fixed proportion to the length scale of the effective theory as the
cosmological averaging scale is taken to be large. We find that backreaction
from higher-derivative terms in the effective action can continue to be
important in the late universe, given a source of sufficiently high-frequency
metric perturbations. This backreaction might also provide a window on exotic
particle physics in the far ultraviolet.Comment: 27 pages, 2 references added, minor clarifications made, comments
added to introduction and discussion, some details moved to appendice
Axial rotation and turbulence of RR ab stars: the Peterson Conundrum revisited
We calibrate and then use the relation between equivalent width (EW) and
full-width-half-maximum (FWHM) of metallic absorption lines in the spectra of
RR Lyrae stars to estimate a new upper limit of Vrot sini less than or equal to
6 km/s on their axial equatorial rotational velocities, and to derive the
variations of macroturbulent velocities in their atmospheres during pulsation
cycles. Finally, we present a simple way to estimate macroturbulent/rotational
velocity from FWHM of the cross-correlation function.Comment: 15 pages, 7 figures, 1 table. EAS Publications Series.: "New advances
in stellar physics: from microscopic to macroscopic processes", 27-31 May
2013, Roscoff, Franc
A light-cone gauge for black-hole perturbation theory
The geometrical meaning of the Eddington-Finkelstein coordinates of
Schwarzschild spacetime is well understood: (i) the advanced-time coordinate v
is constant on incoming light cones that converge toward r=0, (ii) the angles
theta and phi are constant on the null generators of each light cone, (iii) the
radial coordinate r is an affine-parameter distance along each generator, and
(iv) r is an areal radius, in the sense that 4 pi r^2 is the area of each
two-surface (v,r) = constant. The light-cone gauge of black-hole perturbation
theory, which is formulated in this paper, places conditions on a perturbation
of the Schwarzschild metric that ensure that properties (i)--(iii) of the
coordinates are preserved in the perturbed spacetime. Property (iv) is lost in
general, but it is retained in exceptional situations that are identified in
this paper. Unlike other popular choices of gauge, the light-cone gauge
produces a perturbed metric that is expressed in a meaningful coordinate
system; this is a considerable asset that greatly facilitates the task of
extracting physical consequences. We illustrate the use of the light-cone gauge
by calculating the metric of a black hole immersed in a uniform magnetic field.
We construct a three-parameter family of solutions to the perturbative
Einstein-Maxwell equations and argue that it is applicable to a broader range
of physical situations than the exact, two-parameter Schwarzschild-Melvin
family.Comment: 12 page
Light-cone coordinates based at a geodesic world line
Continuing work initiated in an earlier publication [Phys. Rev. D 69, 084007
(2004)], we construct a system of light-cone coordinates based at a geodesic
world line of an arbitrary curved spacetime. The construction involves (i) an
advanced-time or a retarded-time coordinate that labels past or future light
cones centered on the world line, (ii) a radial coordinate that is an affine
parameter on the null generators of these light cones, and (iii) angular
coordinates that are constant on each generator. The spacetime metric is
calculated in the light-cone coordinates, and it is expressed as an expansion
in powers of the radial coordinate in terms of the irreducible components of
the Riemann tensor evaluated on the world line. The formalism is illustrated in
two simple applications, the first involving a comoving world line of a
spatially-flat cosmology, the other featuring an observer placed on the axis of
symmetry of Melvin's magnetic universe.Comment: 11 pages, 1 figur
The Chemical Compositions Of RR Lyrae Type C Variable Stars
We present a detailed chemical abundance study of eight RR Lyrae variable stars of subclass c (RRc). The target RRc stars chosen for study exhibit "Blazhko-effect" period and amplitude modulations to their pulsational cycles. Data for this study were gathered with the echelle spectrograph of the 100 inch du Pont telescope at Las Campanas Observatory. Spectra were obtained throughout each star's pulsation cycle. Atmospheric parameters-effective temperature, surface gravity, microturbulent velocity, and metallicity-were derived at multiple phase points. We found metallicities and element abundance ratios to be constant within observational uncertainties over the pulsational cycles of all stars. Moreover, the alpha-element and Fe-group abundance ratios with respect to iron are consistent with other horizontal-branch members (RRab, blue and red non-variables). Finally, we have used the [Fe/H] values of these eight RRc stars to anchor the metallicity estimates of a large-sample RRc snapshot spectroscopic study being conducted with the same telescope and instrument combination employed here.NSF AST-0908978, AST-1211585Baker Centennial Research EndowmentJohn W. Cox Endowment for the Advanced Studies in AstronomyMcDonald Observator
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