6,647 research outputs found
Theoretical insights into the RR Lyrae K-band Period-Luminosity relation
Based on updated nonlinear, convective pulsation models computed for several
values of stellar mass, luminosity and metallicity, theoretical constraints on
the K-band Period-Luminosity (PLK) relation of RR Lyrae stars are presented. We
show that for each given metal content the predicted PLK is marginally
dependent on uncertainties of the stellar mass and/or luminosity. Then, by
considering the RR Lyrae masses suggested by evolutionary computations for the
various metallicities, we obtain that the predicted infrared magnitude M_K over
the range 0.0001< Z <0.02 is given by the relation
MK=0.568-2.071logP+0.087logZ-0.778logL/Lo, with a rms scatter of 0.032 mag.
Therefore, by allowing the luminosities of RR Lyrae stars to vary within the
range covered by current evolutionary predictions for metal-deficient (0.0001<
Z <0.006) horizontal branch models, we eventually find that the infrared
Period-Luminosity- Metallicity (PLZK) relation is
MK=0.139-2.071(logP+0.30)+0.167logZ, with a total intrinsic dispersion of 0.037
mag. As a consequence, the use of such a PLZK relation should constrain within
+-0.04 mag the infrared distance modulus of field and cluster RR Lyrae
variables, provided that accurate observations and reliable estimates of the
metal content are available. Moreover, we show that the combination of K and V
measurements can supply independent information on the average luminosity of RR
Lyrae stars, thus yielding tight constraints on the input physics of stellar
evolution computations. Finally, for globular clusters with a sizable sample of
first overtone variables, the reddening can be estimated by using the PLZK
relation together with the predicted MV-logP relation at the blue edge of the
instability strip (Caputo et al. 2000).Comment: 8 pages, including 5 postscript figures, accepted for publication on
MNRA
Viscous wing theory development. Volume 1: Analysis, method and results
Viscous transonic flows at large Reynolds numbers over 3-D wings were analyzed using a zonal viscid-inviscid interaction approach. A new numerical AFZ scheme was developed in conjunction with the finite volume formulation for the solution of the inviscid full-potential equation. A special far-field asymptotic boundary condition was developed and a second-order artificial viscosity included for an improved inviscid solution methodology. The integral method was used for the laminar/turbulent boundary layer and 3-D viscous wake calculation. The interaction calculation included the coupling conditions of the source flux due to the wing surface boundary layer, the flux jump due to the viscous wake, and the wake curvature effect. A method was also devised incorporating the 2-D trailing edge strong interaction solution for the normal pressure correction near the trailing edge region. A fully automated computer program was developed to perform the proposed method with one scalar version to be used on an IBM-3081 and two vectorized versions on Cray-1 and Cyber-205 computers
The M_BH-M_star relation of obscured AGNs at high redshift
We report the detection of broad Halpha emission in three X-ray selected
obscured AGNs at z=1-2. By exploiting the Halpha width and the intrinsic X-ray
luminosity, we estimate their black hole masses, which are in the range
0.1-3x10^9 Msun. By means of multi-band photometric data, we measure the
stellar mass of their host galaxy and, therefore, infer their M_BH/M_star
ratio. These are the first obscured AGNs at high-z, selected based on their
black hole accretion (i.e. on the basis of their X-ray luminosity), that can be
located on the M_BH-M_star relation at high-z. All of these obscured high-z
AGNs are fully consistent with the local M_BH-M_star relation. This result
conflicts with those for other samples of AGNs in the same redshift range,
whose M_BH/M_star ratio departs significantly from the value observed in local
galaxies. We suggest that the obscured AGNs in our sample are in an advanced
evolutionary stage, have already settled onto the local M_BH-M_star relation,
and whose nuclear activity has been temporarily revived by recent galaxy
interactions.Comment: 4 pages, 2 figures, accepted for publication in A&A Letters, slightly
revised discussion on SMG
A theoretical approach for the interpretation of pulsating PMS intermediate-mass stars
The investigation of the pulsation properties of pre-main-sequence
intermediate-mass stars is a promising tool to evaluate the intrinsic
properties of these stars and to constrain current evolutionary models. Many
new candidates of this class have been discovered during the last decade and
very accurate data are expected from space observations obtained for example
with the CoRoT satellite. In this context we aim at developing a theoretical
approach for the interpretation of observed frequencies, both from the already
available ground-based observations and from the future more accurate and
extensive CoRoT results. To this purpose we have started a project devoted to
the computations of fine and extensive grids of asteroseismic models of
intermediate mass pre-main-sequence stars. The obtained frequencies are used to
derive an analytical relation between the large frequency separation and the
stellar luminosity and effective temperature and to develop a tool to compare
theory and observations in the echelle diagram. The predictive capabilities of
the proposed method are verified through the application to two test stars. As
a second step, we apply the procedure to two true observations from multisite
campaigns and we are able to constrain their stellar parameters, in particular
the mass, in spite of the small number of frequencies. We expect that with a
significantly higher number of frequencies both the stellar mass and age could
be constrained and, at the same time, the physics of the models could be
tested.Comment: Accepted for publication on A&
Detection of delta Scuti-like pulsation in H254, a pre-main sequence F-type star in IC 348
We present time series observations of intermediate mass PMS stars belonging
to the young star cluster IC 348. The new data reveal that a young member of
the cluster, H254, undergoes periodic light variations with delta Scuti-like
characteristics. This occurrence provides an unambiguous evidence confirming
the prediction that intermediate-mass pre-main sequence (PMS) stars should
experience this transient instability during their approach to the
main-sequence. On the basis of the measured frequency f=7.406 c/d, we are able
to constrain the intrinsic stellar parameters of H254 by means of linear, non
adiabatic, radial pulsation models. The range of the resulting luminosity and
effective temperature permitted by the models is narrower than the
observational values. In particular, the pulsation analysis allows to derive an
independent estimate of the distance to IC 348 of about 320 pc. Further
observations could either confirm the monoperiodic nature of H254 or reveal the
presence of other frequencies.Comment: 7 pages, including 7 postscript figures, accepted for publication on
A&
Non-Reciprocal MEMS Periodic Structure
In recent years, active periodic structures with in-time modulated parameters have drawn ever-increasing attention due to their peculiar (and sometimes exotic) wave propagation properties. Although many experimental works have shown the efficacy of time-modulation strategies, the benchmarks proposed until now have been mostly proof-of-concept demonstrators, with little attention to the feasibility of the solution for practical purposes. In this work, we propose a micro electro-mechanical system (MEMS) periodic structure with modulated electromechanical stiffness featuring non-reciprocal band-gaps that are frequency bands where elastic waves are allowed to travel only in one direction. To this aim, we derive a simplified analytical lumped-parameter model, which is then verified through numerical simulations of both the lumped-parameter system and the high-fidelity multiphysics finite element model including electrostatic effects. We envision that this system, which can easily be manufactured through standard MEMS production processes, may be used as a directional filter in MEMS devices such as insulators and circulators
The Elusive Active Nucleus of NGC 4945
We present new HST NICMOS observations of NGC 4945, a starburst galaxy
hosting a highly obscured active nucleus that is one of the brightest
extragalactic sources at 100 keV. The HST data are complemented with ground
based [FeII] line and mid--IR observations. A 100pc-scale starburst ring is
detected in Pa alpha, while H_2 traces the walls of a super bubble opened by
supernova-driven winds. The conically shaped cavity is particularly prominent
in Pa alpha equivalent width and in the Pa alpha/H_2 ratio. Continuum images
are heavily affected by dust extinction and the nucleus of the galaxy is
located in a highly reddened region with an elongated, disk-like morphology. No
manifestation of the active nucleus is found, neither a strong point source nor
dilution in CO stellar features, which are expected tracers of AGN activity.
Even if no AGN traces are detected in the near-IR, with the currently available
data it is still not possible to establish whether the bolometric luminosity of
the object is powered by the AGN or by the starburst: we demonstrate that the
two scenarios constitute equally viable alternatives. However, the absence of
any signature other than in the hard X-rays implies that, in both scenarios,
the AGN is non-standard: if it dominates, it must be obscured in all
directions, conversely, if the starburst dominates, the AGN must lack UV
photons with respect to X-rays. An important conclusion is that powerful AGNs
can be hidden even at mid-infrared wavelengths and, therefore, the nature of
luminous dusty galaxies cannot be always characterized by long-wavelength data
alone but must be complemented with sensitive hard X-ray observations.Comment: Accepted for publication in A&A, high quality color pictures
available at http://www.arcetri.astro.it/~marconi/colpic.htm
Interface pinning and slow ordering kinetics on infinitely ramified fractal structures
We investigate the time dependent Ginzburg-Landau (TDGL) equation for a non
conserved order parameter on an infinitely ramified (deterministic) fractal
lattice employing two alternative methods: the auxiliary field approach and a
numerical method of integration of the equations of evolution. In the first
case the domain size evolves with time as , where is
the anomalous random walk exponent associated with the fractal and differs from
the normal value 2, which characterizes all Euclidean lattices. Such a power
law growth is identical to the one observed in the study of the spherical model
on the same lattice, but fails to describe the asymptotic behavior of the
numerical solutions of the TDGL equation for a scalar order parameter. In fact,
the simulations performed on a two dimensional Sierpinski Carpet indicate that,
after an initial stage dominated by a curvature reduction mechanism \`a la
Allen-Cahn, the system enters in a regime where the domain walls between
competing phases are pinned by lattice defects.
The lack of translational invariance determines a rough free energy
landscape, the existence of many metastable minima and the suppression of the
marginally stable modes, which in translationally invariant systems lead to
power law growth and self similar patterns. On fractal structures as the
temperature vanishes the evolution is frozen, since only thermally activated
processes can sustain the growth of pinned domains.Comment: 16 pages+14 figure
Light collection and wavelength calibration for an extreme ultraviolet diode array spectrograph
Includes bibliographical references.We describe the use of a simple spherical aluminum-coated mirror to image the radiation of a distant XUV-emitting plasma in the slit of a vacuum spectrograph. Calculations to determine the optimum position and curvature radius of the mirror as a function of the divergence of the emitted plasma radiation are described, Efficient light collection by the grazing incidence mirror allows calibration of an intensified diode array used as a detector with a low-pressure (10-4 Torr) dc capillary discharge that does not require a complex differential pumping system.This work was supported by the National Science Foundation Grant No. ECS 8606226. M. C. Marconi was supported by a fellowship from Universidad Nacional de Buenos Aires and Consejo Nacional de Investigaciones Cientificas y Tecnicas de la Republica Argentina
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