Publications (24)14.07 Total impact
-
Article: International observational campaigns of the last two eclipses in EE Cephei: 2003 and 2008/9
[show abstract] [hide abstract]
ABSTRACT: Two observational campaigns were carried out during the eclipses of EE Cep in 2003 and 2008/9 to verify whether the eclipsing body in the system is indeed a dark disk and to understand the observed changes in the depth and durations of the eclipses. Multicolour photometric data and spectroscopic observations at both low and high resolution were collected. We numerically modelled the variations in brightness and colour during the eclipses. We tested models with different disk structure. We considered the possibility of disk precession. The complete set of observational data collected during the last three eclipses are made available to the astronomical community. Two blue maxima in the colour indices were detected during these two eclipses, one before and one after the photometric minimum. The first (stronger) blue maximum is simultaneous with a "bump" that is very clear in all the UBVRI light curves. Variations in the spectral line profiles seem to be recurrent during each cycle. NaI lines always show at least three absorption components during the eclipse minimum and strong absorption is superimposed on the H_alpha emission. These observations confirm that the eclipsing object in EE Cep system is indeed a dark, dusty disk around a low luminosity object. The primary appears to be a rapidly rotating Be star that is strongly darkened at the equator and brightened at the poles. Some of the conclusions of this work require verification in future studies: (i) a complex, possibly multi-ring structure of the disk in EE Cep; (ii) our explanation of the "bump" observed during the last two eclipses in terms of the different times of obscuration of the hot polar regions of the Be star by the disk; and (iii) our suggested period of the disk precession (~11-12 P_orb) and predicted depth of about 2 mag the forthcoming eclipse in 2014.04/2012; -
Article: A semi-empirical library of galaxy spectra for Gaia classification based on SDSS data and PEGASE models
[show abstract] [hide abstract]
ABSTRACT: Aims:This paper is the third in a series implementing a classification system for Gaia observations of unresolved galaxies. The system makes use of template galaxy spectra in order to determine spectral classes and estimate intrinsic astrophysical parameters. In previous work we used synthetic galaxy spectra produced by PEGASE.2 code to simulate Gaia observations and to test the performance of Support Vector Machine (SVM) classifiers and parametrizers. Here we produce a semi-empirical library of galaxy spectra by fitting SDSS spectra with the previously produced synthetic libraries. We present (1) the semi-empirical library of galaxy spectra, (2) a comparison between the observed and synthetic spectra, and (3) first results of claassification and parametrization experiments with simulated Gaia spectrophotometry of this library. Methods: We use chi2-fitting to fit SDSS galaxy spectra with the synthetic library in order to construct a semi-empirical library of galaxy spectra in which (1) the real spectra are extended by the synthetic ones in order to cover the full wavelength range of Gaia, and (2) astrophysical parameters are assigned to the SDSS spectra by the best fitting synthetic spectrum. The SVM models were trained with and applied to semi-empirical spectra. Tests were performed for the classification of spectral types and the estimation of the most significant galaxy parameters (in particular redshift, mass to light ratio and star formation history). Results: We produce a semi-empirical library of 33670 galaxy spectra covering the wavelength range 250 to 1050 nm at a sampling of 1 nm or less. Using the results of the fitting of the SDSS spectra with our synthetic library, we investigate the range of the input model parameters that produces spectra which are in good agreement with observations. (abridged)10/2011; -
Article: Towards an Optimized Library of Synthetic Galaxy Spectra for the Gaia Mission
[show abstract] [hide abstract]
ABSTRACT: ESA’s cornerstone Gaia mission is going to observe millions of unresolved galaxies during the next few years. A corresponding software package (Unresolved Galaxy Classifier - UGG) is being built in order to classify these galaxies and predict some of their astrophysical parameters. For that reason, there is a need of a library of synthetic galaxy spectra. We present the first results on processing and optimizing the existing synthetic library.06/2010; 424:252. -
Article: Multi-ring structure of the eclipsing disk in EE Cep - possible planets?
[show abstract] [hide abstract]
ABSTRACT: The photometric and spectroscopic observational campaign organized for the 2008/9 eclipse of EE Cep revealed features, which indicate that the eclipsing disk in the EE Cep system has a multi-ring structure. We suggest that the gaps in the disk can be related to the possible planet formation. Comment: 6 pages, 6 figures, contribution to the conference proceedings: Binaries - Key to Comprehension of the Universe, Brno, Czech Republic, 8-12 Jul. 200910/2009; -
Article: Towards a library of synthetic galaxy spectra and preliminary results of classification and parametrization of unresolved galaxies for Gaia - II
[show abstract] [hide abstract]
ABSTRACT: This paper is the second in a series, implementing a classification system for Gaia observations of unresolved galaxies. Our goals are to determine spectral classes and estimate intrinsic astrophysical parameters via synthetic templates. Here we describe (1) a new extended library of synthetic galaxy spectra, (2) its comparison with various observations, and (3) first results of classification and parametrization experiments using simulated Gaia spectrophotometry of this library. Using the PEGASE.2 code, based on galaxy evolution models that take account of metallicity evolution, extinction correction, and emission lines (with stellar spectra based on the BaSeL library), we improved our first library and extended it to cover the domain of most of the SDSS catalogue. We produce an extended library of 28885 synthetic galaxy spectra at zero redshift covering four general Hubble types of galaxies, over the wavelength range between 250 and 1050 nm at a sampling of 1 nm or less. The library is also produced for 4 random values of redshift in the range of 0-0.2. It is computed on a random grid of four key astrophysical parameters (infall timescale and 3 parameters defining the SFR) and, depending on the galaxy type, on two values of the age of the galaxy. The synthetic library was compared and found to be in good agreement with various observations. The first results from the SVM classifiers and parametrizers are promising, indicating that Hubble types can be reliably predicted and several parameters estimated with low bias and variance. Comment: 14 pages, accepted for publication in A&A07/2009; -
Article: Star complexes and stellar populations in NGC 6822 - Comparison with the Magellanic Clouds
[show abstract] [hide abstract]
ABSTRACT: The star complexes (large scale star forming regions) of NGC 6822 were traced and mapped and their size distribution was compared with the size distribution of star complexes in the Magellanic Clouds (MCs). Furthermore, the spatial distributions of different age stellar populations were compared with each other. The star complexes of NGC 6822 were determined by using the isopleths, based on star counts, of the young stars of the galaxy, using a statistical cutoff limit in density. In order to map them and determine their geometrical properties, an ellipse was fitted to every distinct region satisfying this minimum limit. The Kolmogorov-Smirnov statistical test was used to study possible patterns in their size distribution. Isopleths were also used to study the stellar populations of NGC 6822. The star complexes of NGC 6822 were detected and a list of their positions and sizes was produced. Indications of hierarchical star formation, in terms of spatial distribution, time evolution and preferable sizes were found in NGC 6822 and the MCs. The spatial distribution of the various age stellar populations has indicated traces of an interaction in NGC 6822, dated before 350 +/- 50 Myr. Comment: 10 pages, 7 figures, accepted by A&A; minor typeface corrections02/2009; -
Article: Classification and Parametrization of Unresolved Galaxies with Gaia
[show abstract] [hide abstract]
ABSTRACT: The ESA satellite mission Gaia will acquire spectrophotometric observations of several million unresolved galaxies during its five years of operation. Our objective is to design and implement a classification system for these data. For this purpose we need to build a new library of galaxy spectra which covers the necessary parameter space. Using the evolutionary code PÉGASE.2 we have produced a library of 28885 synthetic galaxy spectra at zero redshift covering four general spectral types of galaxies over the wavelength range from 250 to 1050 nm, at a sampling of 1 nm or less. The library was also reproduced for 4 random values of redshift in the range of 0–0.2 and it is computed on a random grid of four key astrophysical parameters (3 for SFR and 1 for timescale of the infall of gas). The synthetic library was compared with various photometric and spectroscopic observations (e.g. from SDSS) and found in good agreement with them. Using simulated Gaia photometry of this library we train and test the performance of Support Vector Machine (SVM) classifiers and parametrizers. The first results are promising, indicating that galaxy types can be reliably predicted and several parameters (e.g. redshift, mass to light ratio, present SFR) can be estimated with low bias and variance from Gaia observations.AIP Conference Proceedings. 12/2008; 1082(1):111-118. -
Article: Towards a library of synthetic galaxy spectra and preliminary results of classification and parametrization of unresolved galaxies for Gaia
[show abstract] [hide abstract]
ABSTRACT: Aims:The Gaia astrometric survey mission will, as a consequence of its scanning law, obtain low resolution optical (330-1000 nm) spectrophotometry of several million unresolved galaxies brighter than V=22. We present the first steps in a project to design and implement a classification system for these data. The goal is both to determine morphological classes and to estimate intrinsic astrophysical parameters via synthetic templates. Here we describe (1) a new library of synthetic galaxy spectra, and (2) first results of classification and parametrization experiments using simulated Gaia spectrophotometry of this library. Methods:We have created a large grid of synthetic galaxy spectra using the PEGASE.2 code, which is based on galaxy evolution models that take into account metallicity evolution, extinction correction, emission lines (with stellar spectra based on the BaSeL library). Our classification and regression models are Support Vector Machines (SVMs), which are kernel-based nonlinear estimators. Results:We produce a basic library of about 4000 zero redshift galaxy spectra covering the main Hubble types over wavelength range 250 to 1050 nm at a sampling of 1 nm or less. It is computed on a regular grid of four key astrophysical parameters for each type and for intermediate random values of the same parameters. An extended library reproduces this at a series of redshifts. Initial results from the SVM classifiers and parametrizers are promising, indicating that Hubble types can be reliably predicted and several parameters estimated with low bias and variance. Comparing the colours of our synthetic library with Sloan Digital Sky Survey (SDSS) spectra we find good agreement over the full range of Hubble types and parameters. Comment: 10 pages, 12 figures, accepted in A&A05/2007; -
Chapter: Automatic Detection-Extraction-Classification for a Large Number of Stellar Spectra from Slitless Wide Field Spectroscopy. A Project for AVO
[show abstract] [hide abstract]
ABSTRACT: The distribution of spectral types in extended stellar systems in galaxies can be a powerful mine of information regarding: • 1. not only the properties of the stellar content (such as age, composition) but also • 2. the spatial distribution of certain types in order to trace specific structures (such as star forming regions and their geometry) Objective prism plates are ideal data for this analysis, since thousands of spectra have been reported simultaneously. Even more so, when using low dispersion prism plates we are dealing with a pool of stellar spectra. The full exploitation of digitized spectra in 1.2m UK Schmidt plates for a large number of stars involves three main automatic stages. • Detection of the spectra • Extraction of their images • Classification Automated methods for detection, extraction and classification published previously (Bratsolis et al. 1998, 2000a) have been implemented in a small SMC area very successfully.11/2006: pages 220-225; -
Article: Searching for flickering variability in several symbiotic stars and related objects: BX Mon, V471 Per, RS Oph, V627 Cas, CI Cam V886 Her, Z And, T CrB, MWC 560, V407 Cyg
[show abstract] [hide abstract]
ABSTRACT: UBVRI photometry observations of 10 symbiotic stars and related objects obtained in the period 2002-2003 are presented. Analysing differential light curves we found rapid light variations with timescales of tens of minutes and significant amplitudes in the well-known flickerers MWC 560, RS Oph, V407 Cyg and T CrB. MWC 560 and V407 Cyg demonstrate quasi periodic oscillations (QPO) with similar amplitudes and timescales. Flickering and unusual flare in V627 Cas as well as some indications of flickering presence in BX Mon are detected. The existence of 29 minutes oscillations in Z And with an amplitude about 0.02 mag in U band is confirmed. Only one symbiotic star, V471 Per, and both non symbiotic, CI Cam and V886 Her seem to be constant on flickering timescales. Nevertheless, small night to night changes in the brightness of V886 Her were observed as well.05/2006; -
Article: Mass Segregation in Young Magellanic Clouds Star Clusters: Four Clusters observed with HST
[show abstract] [hide abstract]
ABSTRACT: We present the results of our investigation on the phenomenon of mass segregation in young star clusters in the Magellanic Clouds. HST/WFPC2 observations on NGC 1818, NGC 2004 & NGC 2100 in the Large Magellanic Cloud and NGC 330 in the Small Magellanic Cloud have been used for the application of diagnostic tools for mass segregation: i) the radial density profiles of the clusters for various mass groups and ii) their mass functions (MFs) at various radii around their centres. All four clusters are found to be mass segregated, but each one in a different manner. Specifically not all the clusters in the sample show the same dependence of their density profiles on the selected magnitude range, with NGC 1818 giving evidence of a strong such relation and NGC 330 showing only a hint of the phenomenon. NGC 2004 did not also show any significant signature of mass segregation in its density profiles. The MFs radial dependence provides clear proof of the phenomenon for NGC 1818, NGC 2100 and NGC 2004, while for NGC 330 it gives only indications. An investigation on the constraints introduced by the application of both diagnostic tools is presented. We also discuss the problems related to the construction of a reliable MF for a cluster and their impact on the investigation of the phenomenon of mass segregation. We find that the MFs of these clusters as they were constructed with two methods, are comparable to Salpeter's IMF. A discussion is given on the dynamical status of the clusters and a test is applied on the equipartition among several mass groups in them. Both showed that the observed mass segregation in the clusters is of primordial nature. Comment: A&A Accepted, 20 pages, 9 Figures, Version with language errors and typos corrected11/2003; -
Article: The role of central density in the evolution and formation of LMC clusters. observational evidence
Astronomical and Astrophysical Transactions 06/2001; 20(1):65-72. -
Article: The European Large Area ISO Survey – I. Goals, definition and observations
[show abstract] [hide abstract]
ABSTRACT: We describe the European Large Area ISO Survey (ELAIS). ELAIS was the largest single Open Time project conducted by ISO, mapping an area of 12 deg2 at 15 μm with ISOCAM and at 90 μm with ISOPHOT. Secondary surveys in other ISO bands were undertaken by the ELAIS team within the fields of the primary survey, with 6 deg2 being covered at 6.7 μm and 1 deg2 at 175 μm.This paper discusses the goals of the project and the techniques employed in its construction, as well as presenting details of the observations carried out, the data from which are now in the public domain. We outline the ELAIS ‘preliminary analysis’ which led to the detection of over 1000 sources from the 15 and 90-μm surveys (the majority selected at 15 μm with a flux limit of ∼3 mJy), to be fed into a ground-based follow-up campaign, as well as a programme of photometric observations of detected sources using both ISOCAM and ISOPHOT.We detail how the ELAIS survey complements other ISO surveys in terms of depth and areal coverage, and show that the extensive multi-wavelength coverage of the ELAIS fields resulting from our concerted and on-going follow-up programme has made these regions amongst the best studied areas of their size in the entire sky, and, therefore, natural targets for future surveys. This paper accompanies the release of extremely reliable subsets of the ‘preliminary analysis’ products. Subsequent papers in this series will give further details of our data reduction techniques, reliability and completeness estimates and present the 15- and 90-μm number counts from the ‘preliminary analysis’, while a further series of papers will discuss in detail the results from the ELAIS ‘final analysis’, as well as from the follow-up programme.Monthly Notices of the Royal Astronomical Society 07/2000; 316(4):749 - 767. · 4.90 Impact Factor -
Article: Automated detection and classification of objective prism stellar spectra
Astronomical and Astrophysical Transactions 12/1999; 18(3):413-424. -
Article: Mass segregation in two young clusters in the Large Magellanic Cloud: SL 666 and NGC 2098
[show abstract] [hide abstract]
ABSTRACT: The age and dynamics of the young LMC clusters SL 666 and NGC 2098 were investigated using CCD observations obtained with the 3.9m Anglo-Australian Telescope. SL 666 was found to have an age of 1 - 1.25 x 10(8) yr, while NGC 2098 is somewhat younger, with an age of 6.3 - 7.9*E(7) yr. In the case of SL 666 the good quality of the acquired data allowed the construction of the main sequence luminosity function as a function of radius, which revealed mass segregation, also confirmed from star counts. Since the age of SL 666 is significantly lower than the mean relaxation time of at least 5 x 10(8) yr calculated for this cluster and since the mass range of the cluster members does not permit us to assume faster energy equipartition, the observed mass segregation seems to be due to the way stars form in the cluster. Indications on a star formation scenario are examined. Disruption time is also discussed and there is strong evidence that the cluster is bound. NGC 2098 shows strong evidence of mass segregation, but further observations are needed in this case.Astronomy and Astrophysics 07/1998; 336:503-517. · 4.59 Impact Factor -
Article: Morphology and stellar content of complexes in the LMC.
[show abstract] [hide abstract]
ABSTRACT: Three LMC stellar aggregates and two LMC stellar complexes located inside the constellations Shapley I, IV, IX, and X have been examined in order to study their morphology and properties. Star counts were performed on excellent quality film copies of direct plates taken with the 1.2m U.K. Schmidt telescope. They have been used for deriving isodensity contour mapping of the four studied regions. Low dispersion objective prism plates taken with the same telescope were also used to classify the spectra of the stars down to M_V_~0.0mag. Combination of the two sets of data was used to define the boundaries of these regions, their age, the density and the spatial distribution of their OB stars. It is therefore found that the bright and massive OB type stars are the predominant stellar component of the four studied regions. They are embedded in a fainter and less massive stellar component within the boundaries of a region, revealed by the isopleths, where the limiting detection magnitude is down to M_V_~1.5mag. Thus it appears that the stellar content of the complexes and aggregates is made not only by the stars as massive as ~40Msun_, but they also contain lower mass stars, at least ~3Msun_. The spatial distribution of early type stars (down to M_V_~0.0mag) shows a gradient which reveals a region coinciding with the one, defined by the isopleths, where fainter stars are also located. For the near future, we plan to study whether the gradient of the radial distribution of the early type stars represents the mass distribution of the molecular cloud from which these stellar structures are formed, or it is due to sequential star formation process and/or expansion because of the high stellar winds of the very massive stars.Astronomy and Astrophysics 03/1996; 308:40-48. · 4.59 Impact Factor -
Article: A Search for Mass Segregation in Four Young Magellanic Cloud Clusters
228:473. -
Article: Triggered star formation in the inner wing of the SMC. Two possible supernova explosions in the N83-84-85 region
[show abstract] [hide abstract]
ABSTRACT: In this article we study the N83-84-85 region of the inner wing of the SMC. Direct and low-dispersion objective prism plates taken with the 1.2 m UK Schmidt Telescope have been digitized by the SuperCOSMOS machine. Star counts have been performed for our region in selected luminosity slices in the $U$ filter and isodensity contours have been used to identify the structures with enhanced stellar number density. We find evidence of triggered star formation from massive stars of older to more recent OB associations. Circular arcs constructed by O and B stars have been detected. A study of the population places stars with more recent ages in the groups of the arcs than of their centers. These effects can be explained by supernova explosions. A catalogue of the non-saturated detected OB stars in this region is given.http://dx.doi.org/10.1051/0004-6361:20035770. -
Article: A library of galaxy spectra for Gaia
[show abstract] [hide abstract]
ABSTRACT: The ESA satellite mission Gaia will acquire spectrophotometric observations of several million unresolved galaxies during its five years of operation. In order to implement a classification scheme for these observations we need to build a new library of galaxy spectra which covers the necessary parameter space. Using the evolutionary code PÃGASE.2 we have produced a library of 28885 synthetic galaxy spectra at zero redshift covering four general spectral types of galaxies over the wavelength range from 250 to 1050 nm, at a sampling of 1 nm or less. The library was also reproduced for four random values of redshift in the range of 0-0.2 and it is computed on a random grid of four key astrophysical parameters (3 for SFR and 1 for timescale of the infall of gas). The synthetic library was compared with various photometric and spectroscopic observations (e.g. from SDSS) and found in good agreement with them.Giobbi, G.; Tornambe, A.; Raimondo, G.; Limongi, M.; Antonelli, L.A.; Menci, N.; Brocato, E.: Probing Stellar Populations Out to the Distant Universe, AIP, Springer, 79-82 (2009). -
Article: Automatic extraction and classification of low-dispersion objective prism stellar spectra
[show abstract] [hide abstract]
ABSTRACT: The observing material used for this task is prism spectral plates taken with Schmidt-class telescopes. Such a plate generally contains thousands of spectra, and there are prism-plate libraries and digitized databases in several astronomical centers that can be exploited for this analysis. After a successive detection from the prism plate image, the spectra are automaticaly extracted in one-dimensional streams containing all the basic information. These spectra require automated classification methods to be analyzed in an objective form. In this article we compare two classification methods directly applied to stellar spectra: a linear correlation and a minimum distance method.http://dx.doi.org/10.1051/aas:2000153.
Top co-authors
Top Journals
Institutions
-
1998–2010
-
National Observatory of Athens
Athens, Attiki, Greece
-
