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

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 $z$ 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-$z$ SNe Ia to determine cosmological density parameters without redshift information.Comment: 12 pages, 3 figures, accepted for publication in Astrophysical Journal Letter

Cylindrical, periodic surface lattice — theory, dispersion analysis, and experiment

A two-dimensional surface lattice of cylindrical topology obtained via perturbing the inner surface of a cylinder is considered. Periodic perturbations of the surface lead to observation of high-impedance, dielectric-like media and resonant coupling of surface and non-propagating volume fields. This allows synthesis of tailored-for-purpose "coating" material with dispersion suitable, for instance, to mediate a Cherenkov type interaction. An analytical model of the lattice is discussed and coupled-wave equations are derived. Variations of the lattice dispersive properties with variation of parameters are shown, illustrating the tailoring of the structure's electromagnetic properties. Experimental results are presented showing agreement with the theoretical model

Testing non-standard cosmological models with supernovae

In this work we study the magnitude-redshift relation of a non-standard cosmological model. The model under consideration was firstly investigated within a special case of metric-affine gravity (MAG) and was recently recovered via different approaches by two other groups. Apart from the usual cosmological parameters for pressure-less matter $\Omega_{\rm m}$, cosmological constant/dark energy $\Omega_{\lambda}$, and radiation $\Omega_{\rm r}$ a new density parameter $\Omega_\psi$ emerges. The field equations of the model reduce to a system which is effectively given by the usual Friedmann equations of general relativity, supplied by a correction to the energy density and pressure in form of $\Omega_\psi$, which is related to the non-Riemannian structure of the underlying spacetime. We search for the best-fit parameters by using recent SN Ia data sets and constrain the possible contribution of a new dark-energy like component at low redshifts, thereby we put an upper limit on the presence of non-Riemannian quantities in the late stages of the universe. In addition the impact of placing the data in redshift bins of variable size is studied. The numerical results of this work also apply to several anisotropic cosmological models which, on the level of the field equations, exhibit a similar scaling behavior of the density parameters like our non-Riemannian model.Comment: 21 pages, 10 figures, uses IOP preprint style, submitted to Class. Quantum Gra

Hubble Space Telescope Observations of Nine High-Redshift ESSENCE Supernovae

We present broad-band light curves of nine supernovae ranging in redshift from 0.5 to 0.8. The supernovae were discovered as part of the ESSENCE project, and the light curves are a combination of Cerro Tololo 4-m and Hubble Space Telescope (HST) photometry. On the basis of spectra and/or light-curve fitting, eight of these objects are definitely Type Ia supernovae, while the classification of one is problematic. The ESSENCE project is a five-year endeavor to discover about 200 high-redshift Type Ia supernovae, with the goal of tightly constraining the time average of the equation-of-state parameter [w = p/(rho c^2)] of the "dark energy." To help minimize our systematic errors, all of our ground-based photometry is obtained with the same telescope and instrument. In 2003 the highest-redshift subset of ESSENCE supernovae was selected for detailed study with HST. Here we present the first photometric results of the survey. We find that all but one of the ESSENCE SNe have slowly declining light curves, and the sample is not representative of the low-redshift set of ESSENCE Type Ia supernovae. This is unlikely to be a sign of evolution in the population. We attribute the decline-rate distribution of HST events to a selection bias at the high-redshift edge of our sample and find that such a bias will infect other magnitude-limited SN Ia searches unless appropriate precautions are taken.Comment: 62 pages, 18 numbered figures, accepted for publication in the Astronomical Journa

Naked Singularity in a Modified Gravity Theory

The cosmological constant induced by quantum fluctuation of the graviton on a given background is considered as a tool for building a spectrum of different geometries. In particular, we apply the method to the Schwarzschild background with positive and negative mass parameter. In this way, we put on the same level of comparison the related naked singularity (-M) and the positive mass wormhole. We discuss how to extract information in the context of a f(R) theory. We use the Wheeler-De Witt equation as a basic equation to perform such an analysis regarded as a Sturm-Liouville problem . The application of the same procedure used for the ordinary theory, namely f(R)=R, reveals that to this approximation level, it is not possible to classify the Schwarzschild and its naked partner into a geometry spectrum.Comment: 8 Pages. Contribution given to DICE 2008. To appear in the proceeding

Transmission spectroscopy and Rossiter-McLaughlin measurements of the young Neptune orbiting AU Mic

AU Mic~b is a Neptune size planet on a 8.47-day orbit around the nearest pre-main sequence ($\sim$20 Myr) star to the Sun, the bright (V=8.81) M dwarf AU Mic. The planet was preliminary detected in Doppler radial velocity time series and recently confirmed to be transiting with data from the TESS mission. AU Mic~b is likely to be cooling and contracting and might be accompanied by a second, more massive planet, in an outer orbit. Here, we present the observations of the transit of AU Mic~b using ESPRESSO on the VLT. We obtained a high-resolution time series of spectra to measure the Rossiter-McLaughlin effect and constrain the spin-orbit alignment of the star and planet, and simultaneously attempt to retrieve the planet's atmospheric transmission spectrum. These observations allow us to study for the first time the early phases of the dynamical evolution of young systems. We apply different methodologies to derive the spin-orbit angle of AU Mic~b, and all of them retrieve values consistent with the planet being aligned with the rotation plane of the star. We determine a conservative spin-orbit angle $\lambda$ value of $-2.96^{+10.44}_{-10.30}$, indicative that the formation and migration of the planets of the AU Mic system occurred within the disk. Unfortunately, and despite the large SNR of our measurements, the degree of stellar activity prevented us from detecting any features from the planetary atmosphere. In fact, our results suggest that transmission spectroscopy for recently formed planets around active young stars is going to remain very challenging, if at all possible, for the near future.Comment: Submitted to A&A, under second revie

Spectroscopy of High-Redshift Supernovae from the ESSENCE Project: The First Four Years

We present the results of spectroscopic observations from the ESSENCE high-redshift supernova (SN) survey during its first four years of operation. This sample includes spectra of all SNe Ia whose light curves were presented by Miknaitis et al. (2007) and used in the cosmological analyses of Davis et al. (2007) and Wood-Vasey et al. (2007). The sample represents 273 hours of spectroscopic observations with 6.5 - 10-m-class telescopes of objects detected and selected for spectroscopy by the ESSENCE team. We present 174 spectra of 156 objects. Combining this sample with that of Matheson et al. (2005), we have a total sample of 329 spectra of 274 objects. From this, we are able to spectroscopically classify 118 Type Ia SNe. As the survey has matured, the efficiency of classifying SNe Ia has remained constant while we have observed both higher-redshift SNe Ia and SNe Ia farther from maximum brightness. Examining the subsample of SNe Ia with host-galaxy redshifts shows that redshifts derived from only the SN Ia spectra are consistent with redshifts found from host-galaxy spectra. Moreover, the phases derived from only the SN Ia spectra are consistent with those derived from light-curve fits. By comparing our spectra to local templates, we find that the rate of objects similar to the overluminous SN 1991T and the underluminous SN 1991bg in our sample are consistent with that of the local sample. We do note, however, that we detect no object spectroscopically or photometrically similar to SN 1991bg. Although systematic effects could reduce the high-redshift rate we expect based on the low-redshift surveys, it is possible that SN 1991bg-like SNe Ia are less prevalent at high redshift.Comment: 21 pages, 17 figures, accepted to A

Stars of extragalactic origin in the solar neighborhood

We computed the spatial velocities and the galactic orbital elements using Hipparcos data for 77 nearest main-sequence F-G-stars with published the iron, magnesium, and europium abundances determined from high dispersion spectra and with the ages estimated from theoretical isochrones. A comparison with the orbital elements of the globular clusters that are known was accreted by our Galaxy in the past reveals stars of extragalactic origin. We show that the relative elemental abundance ratios of r- and \alpha- elements in all the accreted stars differ sharply from those in the stars that are genetically associated with the Galaxy. According to current theoretical models, europium is produced mainly in low mass Type II supernovae (SNe II), while magnesium is synthesized in larger amounts in high mass SN II progenitors. Since all the old accreted stars of our sample exhibit a significant Eu overabundance relative to Mg, we conclude that the maximum masses of the SNII progenitors outside the Galaxy were much lower than those inside it are. On the other hand, only a small number of young accreted stars exhibit low negative ratios $[Eu/Mg] < 0$. The delay of primordial star formation burst and the explosions of high mass SNe II in a relatively small part of extragalactic space can explain this situation. We provide evidence that the interstellar medium was weakly mixed at the early evolutionary stages of the Galaxy formed from a single proto-galactic cloud and that the maximum mass of the SN II progenitors increased in it with time simultaneously with the increase in mean metallicity.Comment: Accepted for 2004, Astronomy Letters, Vol. 30, No. 3, P.148-158 15 pages, 3 figure