Home About us Contact
|
Home About us Contact | ||
|
|
||
Variable Stars (variable + star)
Selected AbstractsPeriod analysis of variable stars by robust smoothingJOURNAL OF THE ROYAL STATISTICAL SOCIETY: SERIES C (APPLIED STATISTICS), Issue 1 2004Hee-Seok Oh Summary., The objective is to estimate the period and the light curve (or periodic function) of a variable star. Previously, several methods have been proposed to estimate the period of a variable star, but they are inaccurate especially when a data set contains outliers. We use a smoothing spline regression to estimate the light curve given a period and then find the period which minimizes the generalized cross-validation (GCV). The GCV method works well, matching an intensive visual examination of a few hundred stars, but the GCV score is still sensitive to outliers. Handling outliers in an automatic way is important when this method is applied in a ,data mining' context to a vary large star survey. Therefore, we suggest a robust method which minimizes a robust cross-validation criterion induced by a robust smoothing spline regression. Once the period has been determined, a nonparametric method is used to estimate the light curve. A real example and a simulation study suggest that the robust cross-validation and GCV methods are superior to existing methods. [source] Spectroscopic and photometric observations of the selected Algol-type binaries , II.MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008V2080 Cygni, V2365 Ophiuchi ABSTRACT This paper is the second in the planned series of investigations. We present new radial velocities and photometric observations of V2080 Cyg and V2365 Oph. New UBV photometric data and radial velocities were analysed for the systems' parameters. While V2080 Cyg consists of two nearly equal F-type main-sequence stars, V2365 Oph has two different components, namely an early A-type primary and a G-type secondary star. New ephemerides are calculated for both systems. The masses of the component stars have been derived as 1.19 ± 0.02 and 1.16 ± 0.02 M, for V2080 Cyg and 1.97 ± 0.02 and 1.06 ± 0.01 M, for V2365 Oph. The effective temperatures and reddening of the systems have been estimated from Johnson wide-band UBV photometric calibrations. The radii have been measured by simultaneous fitting the UBV light curves using Wilson,Devinney code and are 1.60 ± 0.01 R, for both components of V2080 Cyg and 2.19 ± 0.01 and 0.934 ± 0.004 R, for V2365 Oph. The absolute parameters of the stars in both systems lie within the same ranges in the mass,radius, mass,effective temperature, mass,luminosity and luminosity,effective temperature planes as in detached Algol systems. A comparison between the properties of the systems of interest and the predictions of theoretical evolutionary models is undertaken in the log g,logTeff, log R, log M and radius,log age diagrams. The model predictions match the measured properties of V2080 Cyg for an age of about 5.6 Gyr and a ,solar' metal abundance (Z= 0.019), indicating the components near the end of their core hydrogen-burning phases. However, the position of the components of V2365 Oph on the Hertzsprung,Russell (HR) diagram is best reproduced with evolutionary models for somewhat metal-deficient (Z= 0.004) stars. We found an age of about 700 Myr, with the primary component slightly evolved off the zero-age main-sequence and the secondary one still very close to it. From the basic stellar parameters we have also redetermined the distances to V2080 Cyg and V2365 Oph as 78 ± 1 and 251 ± 8 pc, which are in agreement with, and more accurate than, Hipparcos values. The observations show that at least one of the components of V2365 Oph is an intrinsic variable with a period and peak-to-peak amplitude of 0.07 d and 0.05 mag, respectively. In accordance with its position in the HR diagram, the primary component should be considered as a , Scuti star and it is believed to be the variable star in the system. [source] Surface trapping and leakage of low-frequency g modes in rotating early-type stars , I. Qualitative analysisMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2000R. H. D. Townsend A qualitative study of the surface trapping of low-frequency non-radial g modes in rotating early-type stars is undertaken within the Cowling, adiabatic and traditional approximations. A dispersion relation describing the local character of waves in a rotating star is derived; this dispersion relation is then used to construct propagation diagrams for a 7-M, stellar model, which show the location and extent of wave trapping zones inside the star. It is demonstrated that, at frequencies below a cut-off, waves cannot be fully trapped within the star, and will leak through the surface. Expressions for the cut-off frequency are derived in both the non-rotating and rotating cases; it is found from these expressions that the cut-off frequency increases with the rotation rate for all but prograde sectoral modes. While waves below the cut-off cannot be reflected at the stellar surface, the presence of a sub-surface convective region in the stellar model, owing to He ii ionization, means that they can become partially trapped within the star. The energy leakage associated with such waves, which are assigned the moniker virtual modes owing to their discrete eigenfrequencies, means that stability analyses which disregard their existence (by assuming perfect reflection at the stellar surface) may be in error. The results are of possible relevance to the 53 Per and SPB classes of variable star, which exhibit pulsation frequencies of the same order of magnitude as the cut-off frequencies found for the stellar model. It is suggested that observations either of an upper limit on variability periods (corresponding to the cut-off), or of line-profile variations owing to virtual modes, may permit asteroseismological studies of the outer layers of these systems. [source] Magnetic survey of emission line B-type stars with FORS 1 at the VLT,ASTRONOMISCHE NACHRICHTEN, Issue 7 2009S. Hubrig Abstract We report the results of our search for magnetic fields in a sample of 16 field Be stars, the binary emission-line B-type star , Sgr, and in a sample of fourteen members of the open young cluster NGC3766 in the Carina spiral arm. The sample of cluster members includes Be stars, normal B-type stars and He-strong/He-weak stars. Nine Be stars have been studied with magnetic field time series obtained over ,1 hour to get an insight into the temporal behaviour and the correlation of magnetic field properties with dynamical phenomena taking place in Be star atmospheres. The spectropolarimetric data were obtained at the European Southern Observatory with the multi-mode instrument FORS1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four field Be stars, HD 62367, , Cen, o Aqr, and , Tuc. The strongest longitudinal magnetic field, ,Bz, = 117 ± 38 G, was detected in the Be star HD 62367. Among the Be stars studied with time series, one Be star, , Eri, displays cyclic variability of the magnetic field with a period of 21.12 min. The binary star , Sgr, in the initial rapid phase of mass exchange between the two components with strong emission lines in the visible spectrum, is a magnetic variable star, probably on a timescale of a few months. The maximum longitudinal magnetic field ,Bz, = ,102 ± 10 G at MJD 54333.018 was measured using hydrogen lines. The cluster NGC3766 seems to be extremely interesting, where we find evidence for the presence of a magnetic field in seven early B-type stars out of the observed fourteen cluster members (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Planetary transit observations at the University Observatory Jena: TrES-2,ASTRONOMISCHE NACHRICHTEN, Issue 5 2009St. Raetz Abstract We report on observations of several transit events of the transiting planet TrES-2 obtained with the Cassegrain-Teleskop-Kamera at the University Observatory Jena. Between March 2007 and November 2008 ten different transits and almost a complete orbital period were observed. Overall, in 40 nights of observation 4291 exposures (in total 71.52 h of observation) of the TrES-2 parent star were taken. With the transit timings for TrES-2 from the 34 events published by the TrES-network, the Transit Light Curve project and the Exoplanet Transit Database plus our own ten transits, we find that the orbital period is P = (2.470614 ± 0.000001) d, a slight change by ,0.6 s compared to the previously published period. We present new ephemeris for this transiting planet. Furthermore, we found a second dip after the transit which could either be due to a blended variable star or occultation of a second star or even an additional object in the system. Our observations will be useful for future investigations of timing variations caused by additional perturbing planets and/or stellar spots and/or moons (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Living with an active starASTRONOMY & GEOPHYSICS, Issue 3 2001Andy Breen Observations have shown that the Sun is far from being a stable, unchanging object. Rather, we live within the outer atmosphere of a variable star. Andy Breen presents a summary of a meeting that drew together new results from observations and computational modelling of the Sun, interplanetary space and the upper atmosphere of the Earth. [source] Period analysis of variable stars by robust smoothingJOURNAL OF THE ROYAL STATISTICAL SOCIETY: SERIES C (APPLIED STATISTICS), Issue 1 2004Hee-Seok Oh Summary., The objective is to estimate the period and the light curve (or periodic function) of a variable star. Previously, several methods have been proposed to estimate the period of a variable star, but they are inaccurate especially when a data set contains outliers. We use a smoothing spline regression to estimate the light curve given a period and then find the period which minimizes the generalized cross-validation (GCV). The GCV method works well, matching an intensive visual examination of a few hundred stars, but the GCV score is still sensitive to outliers. Handling outliers in an automatic way is important when this method is applied in a ,data mining' context to a vary large star survey. Therefore, we suggest a robust method which minimizes a robust cross-validation criterion induced by a robust smoothing spline regression. Once the period has been determined, a nonparametric method is used to estimate the light curve. A real example and a simulation study suggest that the robust cross-validation and GCV methods are superior to existing methods. [source] Variability in red supergiant stars: pulsations, long secondary periods and convection noiseMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006L. L. Kiss ABSTRACT We study the brightness variations of galactic red supergiant stars using long-term visual light curves collected by the American Association of Variable Star Observers over the last century. The full sample contains 48 red semiregular or irregular variable stars, with a mean time-span of observations of 61 yr. We determine periods and period variability from analyses of power density spectra and time,frequency distributions. We find two significant periods in 18 stars. Most of these periods fall into two distinct groups, ranging from a few hundred to a few thousand days. Theoretical models imply fundamental, first and possibly second overtone mode pulsations for the shorter periods. Periods greater than 1000 d form a parallel period,luminosity relation that is similar to the long secondary periods of the asymptotic giant branch stars. A number of individual power spectra shows a single mode resolved into multiple peaks under a Lorentzian envelope, which we interpret as evidence for stochastic oscillations, presumably caused by the interplay of convection and pulsations. We find a strong 1/f noise component in the power spectra that is remarkably similar in almost all stars of the sample. This behaviour fits the picture of irregular photometric variability caused by large convection cells, analogous to the granulation background seen in the Sun. [source] Finding outlier light curves in catalogues of periodic variable starsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006P. Protopapas ABSTRACT We present a methodology to discover outliers in catalogues of periodic light curves. We use a cross-correlation as the measure of ,similarity' between two individual light curves, and then classify light curves with lowest average ,similarity' as outliers. We performed the analysis on catalogues of periodic variable stars of known type from the MACHO and OGLE projects. This analysis was carried out in Fourier space and we established that our method correctly identifies light curves that do not belong to those catalogues as outliers. We show how an approximation to this method, carried out in real space, can scale to large data sets that will be available in the near future such as those anticipated from the Panoramic Survey Telescope & Rapid Response System (Pan-STARRS) and Large Synoptic Survey Telescope (LSST). [source] The analysis of indexed astronomical time series , X. Significance testing of O,C dataMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006Chris Koen ABSTRACT It is assumed that O,C (,observed minus calculated') values of periodic variable stars are determined by three processes, namely measurement errors, random cycle-to-cycle jitter in the period, and possibly long-term changes in the mean period. By modelling the latter as a random walk, the covariances of all O,C values can be calculated. The covariances can then be used to estimate unknown model parameters, and to choose between alternative models. Pseudo-residuals which could be used in model fit assessment are also defined. The theory is illustrated by four applications to spotted stars in eclipsing binaries. [source] The use of frequency-separation ratios for asteroseismologyMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2005H. Otí Floranes ABSTRACT The systematic patterns of separations between frequencies of modes of different degree and order are a characteristic of p-mode oscillations of stars. The frequency separations depend on the internal structure of the star and so measuring them in the observed oscillation spectra of variable stars gives valuable diagnostics of the interior of a star. Roxburgh & Vorontsov proposed using the ratio of the so-called small frequency separation to the large frequency separation as a diagnostic of the stellar interior, and demonstrated that this ratio was less sensitive than the individual frequency separations themselves to uncertain details of the near-surface structure. Here we derive kernels relating the frequency separation ratio to structure, and show why the ratio is relatively insensitive to the near-surface structure in terms of the very small amplitude of the kernels in the near-surface layers. We also investigate the behaviour of the separation ratio for stars of different masses and ages, and demonstrate the usefulness of the ratio in the so-called asteroseismic Hertzsprung,Russell diagram. [source] Finding the most variable stars in the Orion Belt with the All Sky Automated SurveyASTRONOMISCHE NACHRICHTEN, Issue 3 2010J.A. Caballero Abstract We look for high-amplitude variable young stars in the open clusters and associations of the Orion Belt. We use public data from the ASAS-3 Photometric V -band Catalogue of the All Sky Automated Survey, infrared photometry from the 2MASS and IRAS catalogues, proper motions, and the Aladin sky atlas to obtain a list of the most variable stars in a survey area of side 5° centred on the bright star Alnilam (, Ori) in the centre of the Orion Belt. We identify 32 highly variable stars, of which 16 had not been reported to vary before. They are mostly variable young stars and candidates (16) and background giants (8), but there are also field cataclysmic variables, contact binaries, and eclipsing binary candidates. Of the young stars, which typically are active Herbig Ae/Be and T Tauri stars with H, emission and infrared flux excess, we discover four new variables and confirm the variability status of another two. Some of them belong to the well known , Orionis cluster. Besides, six of the eight giants are new variables, and three are new periodic variables (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The young open cluster Trumpler 3ASTRONOMISCHE NACHRICHTEN, Issue 3 2010G. Maciejewski Abstract We present a photometric and spectroscopic study of the poorly investigated open cluster Trumpler 3. Basic parameters such as the age of 70 ± 10 Myr, the color excess E (B , V) = 0.30 ± 0.02 mag, the distance of 0.69 ± 0.03 kpc and the limiting radius of 12, were redetermined and compared with previous preliminary studies. The distance of 0.65 ± 0.09 kpc was determined independently by spectral parallaxes. Simultaneously, our analysis allowed us to estimate a total number of members to be Ntot = 570 ± 90 and a total mass of the cluster to be Mtot = 270 ± 40 M,. We also determined a state of cluster's dynamical evolution. We conclude that Trumpler 3 is a young low-massive stellar ensemble with a typical mass function slope, located near to the outer edge of the Galaxy's Orion Spur. As a result of a wide-field search for short period variable stars, 24 variables were discovered in the cluster's area. Only one of them , a variable of the , -Dor type , was found to be a likely cluster member (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] FIASCO: A new spectrograph at the University Observatory Jena,ASTRONOMISCHE NACHRICHTEN, Issue 5 2009M. Mugrauer Abstract A new spectrograph (FIASCO) is in operation at the 0.9 m telescope of the University Observatory Jena. This article describes the characterization of the instrument and reports its first astronomical observations, among those lithium (6708 Å) detection in the atmosphere of young stars, and the simultaneous photometric and spectroscopic monitoring of variable stars (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] TAROT: Robotic observatories for gamma-ray bursts and other sources,ASTRONOMISCHE NACHRICHTEN, Issue 3 2008A. Klotz Abstract TAROT (Télescopes à Action Rapide pour les Objets Transitoires) are two robotic observatories designed to observe the early optical counterpart of gamma-ray bursts (GRBs) sources. As GRBs do not occur often, we use TAROT to observe other celestial targets: variable stars, minor-planets, supernovae, etc. In this paper we present the software tools presently used to manage the observations and to process and archive the data. We present also CADOR (Coordination et Analyse des Données des Observatoires Robotiques) which will centralize virtual observatory services associated to both TAROT telescopes. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The WASP project in the era of robotic telescope networksASTRONOMISCHE NACHRICHTEN, Issue 8 2006D. J. Christian Abstract We present the current status of the WASP project, a pair of wide angle photometric telescopes, individually called Super-WASP. SuperWASP-I is located in La Palma, and SuperWASP-II at Sutherland in South Africa. SW-I began operations in April 2004. SW-II is expected to be operational in early 2006. Each SuperWASP instrument consists of up to 8 individual cameras using ultra-wide field lenses backed by high-quality passively cooled CCDs. Each camera covers 7.8 × 7.8 sq degrees of sky, for nearly 500 sq degrees of total sky coverage. One of the current aims of the WASP project is the search for extra-solar planet transits with a focus on brighter stars in the magnitude range ,8 to 13. Additionally, WASP will search for optical transients, track Near-Earth Objects, and study many types of variable stars and extragalactic objects. The collaboration has developed a custom-built reduction pipeline that achieves better than 1 percent photometric precision. We discuss future goals, which include: nightly on-mountain reductions that could be used to automatically drive alerts via a small robotic telescope network, and possible roles of the WASP telescopes as providers in such a network. Additional technical details of the telescopes, data reduction, and consortium members and institutions can be found on the web site at: http://www.superwasp.org/. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Not just variable starsASTRONOMY & GEOPHYSICS, Issue 3 2009Article first published online: 29 MAY 200 From: Bill Leatherbarrow, Director, Lunar Section, British Astronomical Association [source] Complexity in close binary evolutionCOMPLEXITY, Issue 4 2008Michael Politano Abstract The complex nature of close binary evolution is briefly reviewed. Population synthesis calculations of zero-age cataclysmic variable stars (ZACVs) are presented using a common envelope (CE) efficiency parameter, ,CE, that is a function of the core mass of the primary. Specifically, we investigate the possibility that ,CE is different for CE evolution involving red giant branch (RGB) primaries than for asymptotic giant branch (AGB) primaries. We calculate a sequence of model orbital period distributions in present-day ZACVs for 11 different combinations of efficiency parameters (,RGB, ,AGB). We find that if CE evolution is much less efficient for RGB primaries than for AGB primaries (,RGB , 0.1 or less), the number of CVs that form below 2 h is significantly decreased compared with standard constant ,CE models. © 2008 Wiley Periodicals, Inc. Complexity, 2008. [source] | |||||||||||||