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Nishiyama S.,Japan National Astronomical Observatory | Schodel R.,Institute Astrofisica Of Andalucia Csic
Astronomy and Astrophysics | Year: 2012

Context. Nuclear star clusters (NSCs) are ubiquitous at the centers of galaxies. They show mixed stellar populations and the spectra of many NSCs indicate recent events of star formation. However, it is impossible to resolve external NSCs in order to examine the relevant processes. The Milky Way NSC, on the other hand, is close enough to be resolved into its individual stars and presents therefore a unique template for NSCs in general. Aims. Young, massive stars have been found by systematic spectroscopic studies at projected distances R≲0.5pc from the supermassive black hole, SagittariusA* (SgrA*). In recent years, increasing evidence has been found for the presence of young, massive stars also at R>0.5pc. Our goal in this work is a systematic search for young, massive star candidates throughout the entire region within R∼2.5pc of the black hole. Methods. The main criterion for the photometric identification of young, massive early-type stars is the lack of CO-absorption in the spectra. We used narrow-band imaging with the near-infrared camera ISAAC at the ESO VLT under excellent seeing conditions to search for young, massive stars within ∼2.5pc of SgrA*. Results. We have found 63 early-type star candidates at R≲2.5pc, with an estimated erroneous identification rate of only about 20%. Considering their K-band magnitudes and interstellar extinction, they are candidates for Wolf-Rayet stars, supergiants, or early O-type stars. Of these, 31 stars are so far unknown young, massive star candidates, all of which lie at R>0.5pc. The surface number density profile of the young, massive star candidates can be well fit by a single power-law (∝R-Γ), with Γ=1.6±0.17 at R<2.5pc, which is significantly steeper than that of the late-type giants that make up the bulk of the observable stars in the NSC. Intriguingly, this power-law is consistent with the power-law that describes the surface density of young, massive stars in the same brightness range at R≲0.5pc. Conclusions. The finding of a significant number of newly identified early-type star candidates at the Galactic center suggests that young, massive stars can be found throughout the entire cluster which may require us to modify existing theories for star formation at the Galactic center. Follow-up studies are needed to improve the existing data and lay the foundations for a unified theory of star formation in the Milky Way's NSC. © © ESO, 2012.

Maiz Apellaniz J.,Institute Astrofisica Of Andalucia Csic
Astronomy and Astrophysics | Year: 2010

Context. Massive stars have high-multiplicity fractions, and many of them have still undetected components, thus hampering the study of their properties. Aims. I study a sample of massive stars with high angular resolution to better characterize their multiplicity. Methods. I observed 138 fields that include at least one massive star with AstraLux, a lucky imaging camera at the 2.2 m Calar Alto telescope. I also used observations of 3 of those fields with ACS/HRC on HST to obtain complementary information and to calibrate the AstraLux data. The results were compared with existing information from the Washington Double Star Catalog, Tycho-2, 2MASS, and other literature results. Results. I discover 16 new optical companions of massive stars, the majority of which are likely to be physically bound to their primaries. I also improve the accuracy for the separation and magnitude difference of many previously known systems. In a few cases the orbital motion is detected when comparing the new data with existing ones and constraints on the orbits are provided. Conclusions. The analysis indicate that the majority of the AstraLux detections are bound pairs. For a range of separations of 0′1-14′ and magnitude differences lower than 8, I find that the multiplicity fraction for massive stars is close to 50%. When objects outside those ranges are included, the multiplicity fraction should be considerably higher. © 2010 ESO.

Context: Anisoplanatic effects can cause significant systematic photometric uncertainty in the analysis of dense stellar fields observed with adaptive optics. Program packages have been developed for a spatially variable PSF, but they require that a sufficient number of bright, isolated stars in the image are present to adequately sample the PSF. Aims: Imaging the Galactic center is particularly challenging. We present two ways of dealing with spatially variable PSFs when only one or very few suitable PSF reference stars are present in the field. Methods: Local PSF fitting with the StarFinder algorithm is applied to the data. Satisfying results can be found in two ways: (a) creating local PSFs by merging locally extracted PSF cores with the PSF wings estimated from the brightest star in the field; (b) Wiener deconvolution of the image with the PSF estimated from the brightest star in the field and subsequent estimation of local PSFs on the deconvolved image. The methodology is tested on real, and on artificial images. Results: The method involving Wiener deconvolution of the image prior to local PSF extraction and fitting gives excellent results. It limits systematic effects to ∼2-5% in point source photometry and ∼10% in diffuse emission on fields-of-view as large as 28″ × 28″ and observed through the H-band filter. Particular attention is given to how deconvolution changes the noise properties of the image. It is shown that mean positions and fluxes of the stars are conserved by the deconvolution. However, the estimated uncertainties of the PSF fitting algorithm are too small if the presence of covariances is ignored in the PSF fitting as has been done here. An appropriate scaling factor can, however, be determined from simulated images or by comparing measurements on independent data sets. Conclusions: We present ways of obtaining reliable photometry and astrometry from images with a spatially variable, but poorly sampled PSF, where standard techniques may not work. © 2010 ESO.

Cid Fernandes R.,Federal University of Santa Catarina | Gonzalez Delgado R.M.,Institute Astrofisica Of Andalucia Csic
Monthly Notices of the Royal Astronomical Society | Year: 2010

High-resolution spectral models for simple stellar populations (SSP) developed in the past few years have become a standard ingredient in studies of stellar population of galaxies. As more such models become available, it becomes increasingly important to test them. In this and a companion paper, we test a suite of publicly available evolutionary synthesis models using integrated optical spectra in the blue-near-UV range of 27 well-studied star clusters from the work of Leonardi and Rose spanning a wide range of ages and metallicities. Most (23) of the clusters are from the Magellanic Clouds. This paper concentrates on the methodological aspects of spectral fitting. The data are fitted with SSP spectral models from Vazdekis and collaborators, based on the Medium-resolution INT Library of Empirical Spectra. Best-fitting and Bayesian estimates of age, metallicity and extinction are presented, and degeneracies between these parameters are mapped. We find that these models can match the observed spectra very well in most cases, with small formal uncertainties in t, Z and AV. In some cases, the spectral fits indicate that the models lack a blue old population, probably associated with the horizontal branch. This methodology, which is mostly based on the publicly available code starlight, is extended to other sets of models in Paper II, where a comparison with properties derived from spatially resolved data (colour-magnitude diagrams) is presented. The global aim of these two papers is to provide guidance to users of evolutionary synthesis models and empirical feedback to model makers. © 2010 The Authors. Journal compilation © 2010 RAS.

Barbado L.C.,Institute Astrofisica Of Andalucia Csic | Visser M.,Victoria University of Wellington
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

We analyze the response function of an Unruh-DeWitt detector moving with time-dependent acceleration along a one-dimensional trajectory in Minkowski spacetime. To extract the physics of the process, we propose an adiabatic expansion of this response function. This expansion is also a useful tool for computing the click rate of detectors in general trajectories. The expansion is done in powers of the time derivatives of the acceleration (jerk, snap, and higher derivatives). At the lowest order, we recover a Planckian spectrum with temperature proportional to the acceleration of the detector at each instant of the trajectory. Higher orders in the expansion involve powers of the derivatives of the acceleration, with well-behaved spectral coefficients with different shapes. Finally, we illustrate this analysis in the case of an initially inertial trajectory that acquires a given constant acceleration in a finite time. © 2012 American Physical Society.

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