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Holzlohner R.,European Southern Observatory | Rakich A.,European Southern Observatory | Noethe L.,European Southern Observatory | Kuijken K.,Leiden Observatory | Schipani P.,Osservatorio Astronomico di Capodimonte
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2014

We study a novel active optics control scheme at the VST on Cerro Paranal, an f/5:5 survey telescope with a 1x1 degree field of view and a 2.6m primary mirror. This scheme analyzes the elongation pattern of the star PSFs across the full science image (267 Mpixels) and compares their second moments with an analytical model based on 5th-order geometrical optics, comprising 9 degrees of freedom in mirror misalignments and deformations. Using a numerical optimization method, we can complete the star extraction and fitting process in under one minute, fast enough for effective closed-loop active optics control in survey observing cadences. © 2014 SPIE.

Pizzuti L.,University of Trieste | Sartoris B.,University of Trieste | Sartoris B.,National institute for astrophysics | Borgani S.,University of Trieste | And 16 more authors.
Journal of Cosmology and Astroparticle Physics | Year: 2016

We use high-precision kinematic and lensing measurements of the total mass profile of the dynamically relaxed galaxy cluster MACS J1206.2-0847 at z=0.44 to estimate the value of the ratio η=Ψ/Φ between the two scalar potentials in the linear perturbed Friedmann-Lemaitre-Robertson-Walker metric. An accurate measurement of this ratio, called anisotropic stress, could show possible, interesting deviations from the predictions of the theory of General Relativity, according to which Ψ should be equal to Φ. Complementary kinematic and lensing mass profiles were derived from exhaustive analyses using the data from the Cluster Lensing And Supernova survey with Hubble (CLASH) and the spectroscopic follow-up with the Very Large Telescope (CLASH-VLT). Whereas the kinematic mass profile tracks only the time-time part of the perturbed metric (i.e. only Φ), the lensing mass profile reflects the contribution of both time-time and space-space components (i.e. the sum Φ+Ψ). We thus express η as a function of the mass profiles and perform our analysis over the radial range 0.5 Mpc≤ r≤ r200=1.96 Mpc. Using a spherical Navarro-Frenk-White mass profile, which well fits the data, we obtain η(r200)=1.01 -0.28 +0.31 at the 68% C.L. We discuss the effect of assuming different functional forms for mass profiles and of the orbit anisotropy in the kinematic reconstruction. Interpreting this result within the well-studied f(R) modified gravity model, the constraint on η translates into an upper bound to the interaction length (inverse of the scalaron mass) smaller than 2 Mpc. This tight constraint on the f(R) interaction range is however substantially relaxed when systematic uncertainties in the analysis are considered. Our analysis highlights the potential of this method to detect deviations from general relativity, while calling for the need of further high-quality data on the total mass distribution of clusters and improved control on systematic effects. © 2016 IOP Publishing Ltd and Sissa Medialab srl .

Ferrario L.,Australian National University | de Martino D.,Osservatorio Astronomico di Capodimonte | Gansicke B.T.,University of Warwick
Space Science Reviews | Year: 2015

In this paper we review the current status of research on the observational and theoretical characteristics of isolated and binary magnetic white dwarfs (MWDs). Magnetic fields of isolated MWDs are observed to lie in the range 103–109 G. While the upper limit cutoff near 109 G appears to be real, the lower limit is more difficult to investigate. The incidence of magnetism below a few 103 G still needs to be established by sensitive spectropolarimetric surveys-conducted on 8 m class telescopes. Highly magnetic WDs tend to exhibit a complex and non-dipolar field structure with some objects showing the presence of higher order multipoles. There is no evidence that fields of highly magnetic WDs decay over time, which is consistent with the estimated Ohmic decay times scales of ∼1011 yrs. The slow rotation periods (∼100 yrs) inferred for a large number of isolated MWDs in comparison to those of non-magnetic WDs (a few days) suggest that strong magnetic fields augment the braking of the stellar core. MWDs, as a class, also appear to be more massive (0.784±0.047 M) than their weakly or non-magnetic counterparts (0.663±0.136 M). MWDs are also found in binary systems where they accrete matter from a low-mass donor star. These binaries, called magnetic Cataclysmic Variables (MCVs), comprise about 20–25 % of all known CVs. Zeeman and cyclotron spectroscopy of MCVs have revealed the presence of fields in the range ∼7–230 MG. Complex field geometries have been inferred in the high field MCVs (the polars) whilst magnetic field strength and structure in the lower field group (intermediate polars, IPs) are much harder to establish. The MCVs exhibit an orbital period distribution which is similar to that of non magnetic CVs. Polars dominate the distribution at orbital periods ≲4 h and IPs at longer periods. It has been argued that IPs above the 2–3 hr CV period gap with magnetic moments ≳ 5×1033 G cm3 may eventually evolve into polars. It is vital to enlarge the still incomplete sample of MCVs to understand not only their accretion processes but also their evolution. The origin of fields in MWDs is still being debated. While the fossil field hypothesis remains an attractive possibility, field generation within the common envelope of a binary system has been gaining momentum, since it would explain the absence of MWDs paired with non-degenerate companions and also the lack of relatively wide pre-MCVs. © 2015 Springer Science+Business Media Dordrecht

Kanbur S.M.,State University of New York at Oswego | Marconi M.,Osservatorio Astronomico di Capodimonte | Ngeow C.,National Central University | Musella I.,Osservatorio Astronomico di Capodimonte | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

We present multiphase period-colour/amplitude-colour/period-luminosity relations using the Optical Gravitational Lensing Experiment III (OGLE III) and Galactic Cepheid data and compare with state of the art theoretical pulsation models. Using this new way to compare models and observations, we find convincing evidence that both period-colour and period-luminosity relations as a function of phase are dynamic and highly non-linear at certain pulsation phases. We extend this to a multiphase Wesenheit function and find the same result. Hence our results cannot be due to reddening errors. We present statistical tests and movies depicting the period-colour/period-luminosity and Wesenheit relations as a function of phase for the Large Magellanic Cloud (LMC) OGLE III Cepheid data: these tests and movies clearly demonstrate non-linearity as a function of phase and offer a new window towards a deeper understanding of stellar pulsation. When comparing with models, we find that the models also predict this non-linearity in both period-colour and period-luminosity planes. The models with (Z= 0.004, Y= 0.25) fare better in mimicking the LMC Cepheid relations, particularly at longer periods, though the models predict systematically higher amplitudes than the observations. © 2010 The Authors. Journal compilation © 2010 RAS.

Ngeow C.-C.,National Central University | Marconi M.,Osservatorio Astronomico di Capodimonte | Musella I.,Osservatorio Astronomico di Capodimonte | Cignoni M.,University of Bologna | Kanbur S.M.,State University of New York at Oswego
Astrophysical Journal | Year: 2012

In this paper, the synthetic period-luminosity (P-L) relations in Spitzer's IRAC bands, based on a series of theoretical pulsation models with varying metal and helium abundance, were investigated. Selected sets of these synthetic P-L relations were compared to the empirical IRAC band P-L relations recently determined from Galactic and Magellanic Clouds Cepheids. For the Galactic case, synthetic P-L relations from model sets with (Y = 0.26, Z = 0.01), (Y = 0.26, Z = 0.02), and (Y = 0.28, Z = 0.02) agree with the empirical Galactic P-L relations derived from the Hubble Space Telescope parallaxes. For Magellanic Cloud Cepheids, the synthetic P-L relations from model sets with (Y = 0.25, Z = 0.008) agree with both of the empirical Large Magellanic Cloud (LMC) and Small Magellanic Cloud P-L relations. Analysis of the synthetic P-L relations from all model sets suggested that the IRAC band P-L relations may not be independent of metallicity, as the P-L slopes and intercepts could be affected by the metallicity and/or helium abundance. We also derive the synthetic period-color (P-C) relations in the IRAC bands. Non-vanishing synthetic P-C relations were found for certain combinations of IRAC band filters and metallicity. However, the synthetic P-C relations disagreed with the [3.6]-[8.0] P-C relation recently found for the Galactic Cepheids. The synthetic [3.6]-[4.5] P-C slope from the (Y = 0.25, Z = 0.008) model set, on the other hand, is in excellent agreement to the empirical LMC P-C counterpart, if a period range 1.0 < log (P) < 1.8 is adopted. © 2012. The American Astronomical Society. All rights reserved.

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