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מאמרים קשורים
On Machine-learned Classification of Variable Stars with Sparse and Noisy Time-series Data
With the coming data deluge from synoptic surveys, there is a need forframeworks that can quickly and automatically produce calibratedclassification probabilities for newly observed variables based on smallnumbers of time-series measurements. In this paper, we introduce amethodology for variable-star classification, drawing from modernmachine-learning techniques. We describe how to homogenize theinformation gleaned from light curves by selection and computation ofreal-numbered metrics (features), detail methods to robustly estimateperiodic features, introduce tree-ensemble methods for accuratevariable-star classification, and show how to rigorously evaluate aclassifier using cross validation. On a 25-class data set of 1542well-studied variable stars, we achieve a 22.8% error rate using therandom forest (RF) classifier; this represents a 24% improvement overthe best previous classifier on these data. This methodology iseffective for identifying samples of specific science classes: forpulsational variables used in Milky Way tomography we obtain a discoveryefficiency of 98.2% and for eclipsing systems we find an efficiency of99.1%, both at 95% purity. The RF classifier is superior to othermethods in terms of accuracy, speed, and relative immunity to irrelevantfeatures; the RF can also be used to estimate the importance of eachfeature in classification. Additionally, we present the firstastronomical use of hierarchical classification methods to incorporate aknown class taxonomy in the classifier, which reduces the catastrophicerror rate from 8% to 7.8%. Excluding low-amplitude sources, the overallerror rate improves to 14%, with a catastrophic error rate of 3.5%.
| A List of Minima and Maxima Timings
Not Available
| The luminosities and distance scales of type II Cepheid and RR Lyrae variables
Infrared and optical absolute magnitudes are derived for the type IICepheids κ Pav and VY Pyx using revised Hipparcos parallaxes andfor κ Pav, V553 Cen and SW Tau from pulsational parallaxes.Revised Hipparcos and HST parallaxes for RR Lyrae agree satisfactorilyand are combined in deriving absolute magnitudes. Phase-corrected J, Hand Ks mags are given for 142 Hipparcos RR Lyraes based onTwo-Micron All-Sky Survey observations. Pulsation and trigonometricalparallaxes for classical Cepheids are compared to establish the bestvalue for the projection factor (p) used in pulsational analyses.The MV of RR Lyrae itself is 0.16 +/- 0.12 mag brighter thanpredicted from an MV-[Fe/H] relation based on RR Lyrae starsin the Large Magellanic Cloud (LMC) at a modulus of 18.39 +/- 0.05 asfound from classical Cepheids. This is consistent with the prediction ofCatelan & Cortés that it is overluminous for its metallicity.The results for the metal- and carbon-rich Galactic disc stars, V553 Cenand SW Tau, each with small internal errors (+/-0.08 mag) have a meandeviation of only 0.02 mag from the period-luminosity (PL) relationestablished by Matsunaga et al. for type II Cepheids in globularclusters and with a zero-point based on the same LMC-scale. Comparingdirectly the luminosities of these two stars with published data on typeII Cepheids in the LMC and in the Galactic bulge leads to an LMC modulusof 18.37 +/- 0.09 and a distance to the Galactic Centre of R0= 7.64 +/- 0.21kpc. The data for VY Pyx agree with these results withinthe uncertainties set by its parallax. Evidence is presented thatκ Pav may have a close companion and possible implications of thisare discussed. If the pulsational parallax of this star is incorporatedin the analyses, the distance scales just discussed will be increased by~0.15 +/- 0.15 mag. V553 Cen and SW Tau show that at optical wavelengthsPL relations are wider for field stars than for those in globularclusters. This is probably due to a narrower range of masses in thelatter case.
| The Chemical Compositions of the Type II Cepheids-The BL Herculis and W Virginis Variables
Abundance analyses from high-resolution optical spectra are presentedfor 19 type II Cepheids in the Galactic field. The sample includes bothshort-period (BL Her) and long-period (W Vir) stars. This is the firstextensive abundance analysis of these variables. The C, N, and Oabundances with similar spreads for the BL Her and W Vir show evidencefor an atmosphere contaminated with 3α process and CN-cyclingproducts. A notable anomaly of the BL Her stars is an overabundance ofNa by a factor of about 5 relative to their presumed initial abundances.This overabundance is not seen in the W Vir stars. The abundanceanomalies running from mild to extreme in W Vir stars but not seen inthe BL Her stars are attributed to dust-gas separation that provides anatmosphere deficient in elements of high condensation temperature,notably, Al, Ca, Sc, Ti, and s-process elements. Such anomalies havepreviously been seen among RV Tau stars which represent a long-periodextension of the variability enjoyed by the type II Cepheids. Commentsare offered on how the contrasting abundance anomalies of BL Her and WVir stars may be explained in terms of the stars' evolution from theblue horizontal branch.
| The GEOS RR Lyr Survey
Not Available
| Multiperiodic Galactic field RR Lyrae stars in the ASAS catalogue
The All Sky Automated Survey (ASAS) monitors bright stars (8 < V <14 mag) south of declination +28°. The ASAS Catalogue of VariableStars (ACVS) presently contains 50099 objects; among them are 2212objects classified as RR Lyrae pulsating variables. We use ASASphotometric V-band data to search for multiperiodicity in those stars.We find that 73 of 1435 RRab stars and 49 of 756 RRc stars exhibit theBlazhko effect. We observe a deficiency of RRab Blazhko variables withmain pulsation periods greater than 0.65 d. The Blazhko periods of RRcstars exhibit a strongly bimodal distribution. During our study wediscovered the Blazhko effect with multiple periods in object ASAS050747-3351.9 = SU Col. Blazhko periods of 89.3 and 65.8 d and acandidate of 29.5 d were identified with periodogram peaks near thefirst three harmonics of the main pulsation. These observations mayinspire new models of the Blazhko effect, which has eluded a consistenttheory since its discovery about one hundred years ago. Long-term lightcurve changes were found in 29 stars. We also found 19 Galactic doublemode pulsators (RRd), of which four are new discoveries, raising thenumber of ASAS discoveries of such objects to 16, out of 27 known in thefield of our Galaxy.
| Southern RR Lyrae Stars Exhibiting the Blazhko Effect
Blazhko periods are given for 43 RRab stars based on data from ASAS3.
| Proper identification of RR Lyrae stars brighter than 12.5 mag
RR Lyrae stars are of great importance for investigations of Galacticstructure. However, a complete compendium of all RR-Lyraes in the solarneighbourhood with accurate classifications and coordinates does notexist to this day. Here we present a catalogue of 561 local RR-Lyraestars (V_max ≤ 12.5 mag) according to the magnitudes given in theCombined General Catalogue of Variable Stars (GCVS) and 16 fainter ones.The Tycho2 catalogue contains ≃100 RR Lyr stars. However, manyobjects have inaccurate coordinates in the GCVS, the primary source ofvariable star information, so that a reliable cross-identification isdifficult. We identified RR Lyrae from both catalogues based on anintensive literature search. In dubious cases we carried out photometryof fields to identify the variable. Mennessier & Colome (2002,A&A, 390, 173) have published a paper with Tyc2-GCVSidentifications, but we found that many of their identifications arewrong.
| RR Lyrae stars: kinematics, orbits and z-distribution
RR Lyrae stars in the Milky Way are good tracers to study the kinematicbehaviour and spatial distribution of older stellar populations. Arecently established well documented sample of 217 RR Lyr stars withV<12.5 mag, for which accurate distances and radial velocities aswell as proper motions from the Hipparcos and Tycho-2 catalogues areavailable, has been used to reinvestigate these structural parameters.The kinematic parameters allowed to calculate the orbits of the stars.Nearly 1/3 of the stars of our sample have orbits staying near the MilkyWay plane. Of the 217 stars, 163 have halo-like orbits fulfilling one ofthe following criteria: Θ < 100 km s-1, orbiteccentricity >0.4, and normalized maximum orbital z-distance>0.45. Of these stars roughly half have retrograde orbits. Thez-distance probability distribution of this sample shows scale heightsof 1.3±0.1 kpc for the disk component and 4.6±0.3 kpc forthe halo component. With our orbit statistics method we found a(vertical) spatial distribution which, out to z=20 kpc, is similar tothat found with other methods. This distribution is also compatible withthe ones found for blue (HBA and sdB) halo stars. The circular velocityΘ, the orbit eccentricity, orbit z-extent and [Fe/H] are employedto look for possible correlations. If any, it is that the metal poorstars with [Fe/H] <1.0 have a wide symmetric distribution aboutΘ=0, thus for this subsample on average a motion independent ofdisk rotation. We conclude that the Milky Way possesses a halo componentof old and metal poor stars with a scale height of 4-5 kpc having randomorbits. The presence in our sample of a few metal poor stars (thus partof the halo population) with thin disk-like orbits is statistically notsurprising. The midplane density ratio of halo to disk stars is found tobe 0.16, a value very dependent on proper sample statistics.
| Absolute Magnitudes and Kinematic Parameters of the Subsystem of RR Lyrae Variables
The statistical parallax technique is applied to a sample of 262 RRabLyrae variables with published photoelectric photometry, metallicities,and radial velocities and with measured absolute proper motions.Hipparcos, PPM, NPM, and the Four-Million Star Catalog (Volchkov et al.1992) were used as the sources of proper motions; the proper motionsfrom the last three catalogs were reduced to the Hipparcos system. Wedetermine parameters of the velocity distribution for halo [(U_0, V_0,W_0) = (-9 +/- 12, -214 +/- 10, -16 +/- 7) km/s and (sigma_U, sigma_V,sigma_W) = (164 +/- 11, 105 +/- 7, 95 +/- 7) km/s] and thick-disk [(U_0,V_0, W_0) = (-16 +/- 8, -41 +/- 7, -18 +/- 5) km/s and (sigma_U,sigma_V, sigma_W) = (53 +/- 9, 42 +/- 8, 26 +/- 5) km/s] RR Lyrae, aswell as the intensity-averaged absolute magnitude for RR Lyrae of thesepopulations: = 0.77 +/- 0.10 and = +1.11 +/-0.28 for the halo and thick-disk objects, respectively. The metallicitydependence of the absolute magnitude of RR Lyrae is analyzed(=(0.76 +/- 0.12) + (0.26 +/- 0.26) x ([Fe/H] + 1.6) = 1.17 +0.26 x [Fe/H]). Our results are in satisfactory agreement with the_(RR)-[Fe/H] relation from Carney et al. (1992)(_(RR) = 1.01 + 0.15 x [Fe/H]) obtained by Baade-Wesselink'smethod. They provide evidence for a short distance scale: the LMCdistance modulus and the distance to the Galactic center are 18.22 +/-0.11 and 7.4 +/-±0.5 kpc, respectively. The zero point ofthe distance scale and the kinematic parameters of the RR Lyraepopulations are shown to be virtually independent of the source ofabsolute proper motions used and of whether they are reduced to theHipparcos system or not.
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| Kinematics of Metal-poor Stars in the Galaxy. II. Proper Motions for a Large Nonkinematically Selected Sample
We present a revised catalog of 2106 Galactic stars, selected withoutkinematic bias and with available radial velocities, distance estimates,and metal abundances in the range -4.0<=[Fe/H]<=0.0. This updateof the 1995 Beers & Sommer-Larsen catalog includes newly derivedhomogeneous photometric distance estimates, revised radial velocitiesfor a number of stars with recently obtained high-resolution spectra,and refined metallicities for stars originally identified in the HKobjective-prism survey (which account for nearly half of the catalog)based on a recent recalibration. A subset of 1258 stars in this cataloghave available proper motions based on measurements obtained with theHipparcos astrometry satellite or taken from the updated AstrographicCatalogue (second epoch positions from either the Hubble Space TelescopeGuide Star Catalog or the Tycho Catalogue), the Yale/San Juan SouthernProper Motion Catalog 2.0, and the Lick Northern Proper Motion Catalog.Our present catalog includes 388 RR Lyrae variables (182 of which arenewly added), 38 variables of other types, and 1680 nonvariables, withdistances in the range 0.1 to 40 kpc.
| The absolute magnitudes of RR Lyraes from HIPPARCOS parallaxes and proper motions
We have used HIPPARCOS proper motions and the method of StatisticalParallax to estimate the absolute magnitude of RR Lyrae stars. Inaddition we used the HIPPARCOS parallax of RR Lyrae itself to determineit's absolute magnitude. These two results are in excellent agreementwith each other and give a zero-point for the RR Lyrae M_v,[Fe/H]relation of 0.77+/-0.15 at [Fe/H]=-1.53. This zero-point is in goodagreement with that obtained recently by several groups usingBaade-Wesselink methods which, averaged over the results from thedifferent groups, gives M_v = 0.73+/-0.14 at [Fe/H]=-1.53. Taking theHIPPARCOS based zero-point and a value of 0.18+/-0.03 for the slope ofthe M_v,[Fe/H] relation from the literature we find firstly, thedistance modulus of the LMC is 18.26+/-0.15 and secondly, the mean ageof the Globular Clusters is 17.4+/-3.0 GYrs. These values are comparedwith recent estimates based on other "standard candles" that have alsobeen calibrated with HIPPARCOS data. It is clear that, in addition toastrophysical problems, there are also problems in the application ofHIPPARCOS data that are not yet fully understood. Table 1, whichcontains the basic data for the RR Lyraes, is available only at CDS. Itmay be retrieved via anonymous FTP at cdsarc.u-strasbg.fr (130.79.128.5)or via the Web at http://cdsweb.u-strasbg.fr/Abstract.html
| Radial velocities and iron abundances of field RR Lyraes. II.
This is the second of the papers devoted to derive radial velocities andiron abundances of field RR Lyraes observed by HIPPARCOS. Our abundancesshow good agreement with those in the literature obtained both fromphotometric (Delta S index) and spectroscopic methods. Binary candidatesand stars misclassified as RR Lyraes in the original HIPPARCOS list havebeen also identified. Appendix is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u- strasbg.fr/Abstract.html
| Detection of a galactic color gradient for blue horizontal-branch stars of the halo field and implications for the halo age and density distributions
Abstract image available at:http://adsabs.harvard.edu/abs/1991ApJ...375..121P
| Physical parameters of pulsating variables with periods between one and three days. II - Fundamental parameters
On the basis of the Walraven VBLUW photometry data reported by Diethelm(1986) on 57 pulsating variables with periods between one and threedays, primary parameters (metallicity, log g, Teff, and the amount ofreddening) were deduced for 11 of these stars, using a semiempiricalapproach based on model atmosphere calculations. Results indicate thatboth the C-delta and the SA-class of the pulsating variables are youngstars of the disc-population pulsating either in the fundamental(C-delta) or in the first overtone mode (SA). The AHB1 stars, whichshowed a marked deficiency in metal abundance, are members of the halopopulaton. The stars of the AHB2 and AHB3 groups, which show anintermediate metallicity, must be considered as members of anintermediate stellar population.
| Physical parameters of pulsating variables with periods between one and three days. I - Photometry and metallicities
Based on a new set of observations in the Walraven VBLUW system,obtained at the European Southern Observatory, La Silla, Chile,Fe/H(VBLU) values of 57 pulsating variables, mostly with periods betweenone and three days,. are determined. The distribution of the metalabundances is shown to be essentially bimodal. All the variables withFe/H ratios of less than -0.4 were classified by Diethelm (1983) as RRdstars. They are identified as the galactic field counterpart of theCepheids in the same period range found in globular clusters.
| A catalogue of field Type II Cepheids
A catalogue of field Type II Cepheids is presented. The primary listconsists of 152 Cepheids sufficiently far from the galactic plane to bevery probably Type II stars. A second list contains 56 additional starsthat are likely, but less certain, Type II Cepheids, including bothstars estimated to be at large distances from the galactic plane butwith uncertain distances and stars close to the galactic plane believedto be Type II for independent reasons.
| Southern stars of high radial velocity
First, it has been shown that the Stock Velocity Survey is a usefulsource of velocity information on 8-13th mag southern stars. Theparticular use has been to identify stars of very large velocity.Thackeray (1975) has tabulated those galactic objects with heliocentricradial velocities over 250 km/s; his list contains 69 objects, of which59 are stellar. The authors have found twelve more stars. Second, theslit spectra of a sample of high-velocity stars in the Stock Surveyrevealed several extremely metal-weak stars. Abundance analysis forthese objects would be worthwhile. Finally, among the high-velocityobjects are a surprising number of late B and early A stars. In mostinstances, these objects appear to be spectroscopically normal,main-sequence stars, but three are definite A + G spectrum binaries.High-velocity binaries are rare.
| Astrophysical Parameters from Four-Colour Photometry
Not Available
| BVI Photometry of Southern Cepheids
Not Available
| 58th Name-List of Variable Stars
Not Available
| Discoveries on Southern Objective-Prism Plates. II. New Probable Field Horizontal-Branch Stars
Abstract image available at:http://adsabs.harvard.edu/abs/1971PASP...83...98M
| Bright Southern BV-Stars
Not Available
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הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | נחש מים |
| התרוממות ימנית: | 12h04m47.27s |
| סירוב: | -27°40'43.3" |
| גודל גלוי: | 10.213 |
| תנועה נכונה: | -63.9 |
| תנועה נכונה: | -50.2 |
| B-T magnitude: | 10.511 |
| V-T magnitude: | 10.238 |
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