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Random forest automated supervised classification of Hipparcos periodic variable stars
We present an evaluation of the performance of an automatedclassification of the Hipparcos periodic variable stars into 26 types.The sub-sample with the most reliable variability types available in theliterature is used to train supervised algorithms to characterize thetype dependencies on a number of attributes. The most useful attributesevaluated with the random forest methodology include, in decreasingorder of importance, the period, the amplitude, the V-I colour index,the absolute magnitude, the residual around the folded light-curvemodel, the magnitude distribution skewness and the amplitude of thesecond harmonic of the Fourier series model relative to that of thefundamental frequency. Random forests and a multi-stage scheme involvingBayesian network and Gaussian mixture methods lead to statisticallyequivalent results. In standard 10-fold cross-validation (CV)experiments, the rate of correct classification is between 90 and 100per cent, depending on the variability type. The main mis-classificationcases, up to a rate of about 10 per cent, arise due to confusion betweenSPB and ACV blue variables and between eclipsing binaries, ellipsoidalvariables and other variability types. Our training set and thepredicted types for the other Hipparcos periodic stars are availableonline.
| Spectra of Type II Cepheid Candidates and Related Stars
We present low-resolution spectra for variable stars in the Cepheidperiod range from the ROTSE-I Demonstration Project and the All SkyAutomated Survey, some of which were previously identified as type IICepheid candidates. We have derived effective temperatures, gravities,and metallicities from the spectra. Based on this, three types ofvariables were identified: Cepheid strip stars, cool stars that liealong the red subgiant and giant branch, and cool main-sequence stars.Many fewer type II Cepheids were found than expected and most haveamplitudes less than 0.4 mag. The cool variables include many likelybinaries as well as intrinsic variables. Variation among themain-sequence stars is likely to be mostly due to binarity or stellaractivity.
| V371 Per - A thick-disc, short-period F/1O Cepheid
V371 Per was found to be a double-mode Cepheid with a fundamental modeperiod of 1.738 days, the shortest among Galactic beat Cepheids, and anunusually high period ratio of 0.731, while the other Galactic beatCepheids have period ratios between 0.697 and 0.713. The latter suggeststhat the star has a metallicity [Fe/H] between -1 and -0.7. The deriveddistance from the Galactic plane places it in the thick disc or theHalo, while all other Galactic beat Cepheids belong to the thin disc.There are indications from historical data that both the fundamental andfirst overtone periods have lengthened.
| Automated Variable Star Classification Using the Northern Sky Variability Survey
We have identified 4659 variable objects in the Northern Sky VariabilitySurvey. We have classified each of these objects into one of the fivevariable star classes: (1) Algol/β Lyr systems includingsemidetached, and detached eclipsing binaries, (2) W Ursae Majorisovercontact and ellipsoidal variables, (3) long-period variables such asCepheid and Mira-type objects, (4) RR Lyr pulsating variables, and (5)short-period variables including δ Scuti stars. All the candidateshave outside of eclipse magnitudes of ~10-13. The primary classificationtool is the use of Fourier coefficients combined with period informationand light-curve properties to make the initial classification. Briefmanual inspection was done on all light curves to remove nonperiodicvariables that happened to slip through the process and to quantify anyerrors in the classification pipeline. We list the coordinates, period,Two Micron All Sky Survey colors, total amplitude variation, and anyprevious classification of the object. 548 objects previously identifiedas Algols in our previous paper are not included here.
| 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.
| Photometry of Type II Cepheids. II. The Short-Period Stars
We present 668 new photometric observations for 24 Cepheids with periodsless than 3 days. Most of the stars are probable type II Cepheids, butwe have included some type I Cepheids for comparison. A discussion ofthe Fourier parameters of the light curves leads to the conclusion thatthey can be used to distinguish among the several types of light curvesfound among the type II stars but are of limited usefulness fordistinguishing between type I and type II Cepheids. The pulsationalstability is investigated by searching for long-term changes in thelight-curve shapes, period changes, and light-curve scatter. In terms ofthese parameters, the pulsation is more stable than those that werefound for the long-period stars, but for each there are several starsthat show unusually large effects. However, there does not seem to beany obvious difference in light-curve stability of type II Cepheidscompared with type I Cepheids.
| The Spectra of Type II Cepheids. I. The Hα Line in Short-Period Stars
We present 88 Hα profiles for 24 pulsating variable stars withperiods between 1 and 3 days in order to explore the behavior of thisline in type II as compared with classical Cepheids. Surprisingly, largevelocity differences were found between Hα and the metal lines insome type II Cepheids. Strong emission was observed in three stars, VZAql, NW Lyr, and V439 Oph, while line filling by incipient emission ispresent in seven others. All of the stars with emission and most withincipient emission belong to Diethelm's AHB2 class, and the emission isassociated with the secondary bump on the rising branch of the lightcurve. Two stars, BF Ser and MQ Aql, show doubling of the core nearmaximum light, and asymmetry of the line is noted in some spectra.Based in part on observations obtained with the Apache Point Observatory3.5 m telescope, which is owned and operated by the AstrophysicalResearch Consortium.
| Period Changes in Galactic Classical Cepheids. Slow Evolution of Long-Period Cepheids
We compared period changes derived from O-C diagrams for 63 classicalCepheids from our Galaxy with model calculations. We found that forCepheids with log P > 1.0 the observed changes are smaller thanpredicted values, except variable SZ Cas. However some of the firstovertone Cepheids, particularly EU Tau and Polaris, change its periodmuch faster than it follows from theory. Summary of the known data onperiod changes in Cepheids from the Galaxy and from the MagellanicClouds (previous papers) leads to conclusion that none of the 999Cepheids is undergoing the first crossing of the instability strip. Alsothe observed period changes for long-period Cepheids are a few timesslower than predicted by the models. These results imply that muchlarger fraction of helium is burned in the Cepheid stage than it ispredicted by models.
| 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.
| Using BV Photometry to Distinguish between Type I and Type II Cepheids
We use Johnson BV photometry of Galactic type I and type II Cepheids ina variety of diagrams to see whether the two populations can beadequately separated. The diagrams include the Fourier parametersphi_21, phi_31, phi_41, R_21, R_31, R_41, the loop width in the V-(B-V)plane, the relative phasing of V and B-V extrema, and the amplituderatios of B-V to V, all versus period. We found the R_i1 diagramsuseless in this regard, while a plot of the phasing parameter versusamplitude ratio was best, closely followed by the phi_i1 diagrams. Amajor difficulty was arriving at a sample of Cepheids that could bereliably taken to be definitely type II. We found it necessary to useonly stars that would have been more than 2 kpc from the Galactic planehad they been type I for this sample, and even then it is uncertainwhether such overlap between types as we find in our diagrams is due toinadequacy of the tests or is due to type I stars' having contaminatedour type II sample. We point out evidence strongly suggesting that typeI stars can be found at least up to 2 kpc from the plane. Selectioneffects in our sample of type II Cepheids resulted in only one BLHerculis star (short-period type II) being included, so our conclusionsbasically apply only to W Virginis stars.
| Baade-Wesselink Radius Determination of Type II Cepheids
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....113.1833B&db_key=AST
| Period changes of AHB1 variables.
In order to investigate the period variations of AHB1 pulsatingvariables due to evolutionary effects predicted by Sandage, Diethelm andTammann (1994), the photographic Sky Survey of the SonnebergObservatory, Germany, for the years 1928-1994 was searched. Byconstructing seasonal mean light curves and using the rise-time method,it is shown that the periods of four of six field AHB1 variablesexamined (EK Del, UY Eri, CE Her, V716 Oph) have been increasing overthe last 70 years, while two stars, BF Ser and XX Vir, shows no changeof period in this interval. The rate of the period change is in goodagreement with the theoretically expected values.
| Nonlinear stellar pulsation. III. Fourier decomposition of theoretical RRab models.
The fundamental mode RR-Lyrae light and radial velocity curves presentedin Feuchtinger & Dorfi (1994) are Fourier analysed and compared toobservational data and other theoretical investigations. For the periodrange up to 0.8days the Fourier decomposition of the light curvesreproduces the observed values for R_21_, {PHI}_21_ and {PHI}_31_. Thecomparison to Lagrangean investigations shows excellent agreement forthe radial velocity Fourier parameters. In addition we are able toresolve the phase discrepancy problem at least for the short periodrange. For periods higher than 0.8days the Fourier phases {PHI}_21_ and{PHI}_31_ lie above the observed ranges. We discuss this problem in thecontext of a lack of observational data and convective energy transport.
| New method to recognize s-Cepheids
A new method of delineating sinusoidal or s-Cepheids is presented. Themethos uses the values of (V) (the mean intensity), V - Bar (the averagemagnitude), and Vmean (the value of the mean magnitude).Fourier coefficient data from galactic Cepheids is used to derive theseterms in the V band and the differences between the various terms showsystematic trends with increasing period. The Cepheids can be easilygrouped into 3 divisions-short period s-Cepheids, intermediate periodCepheids (P less than 9 days), and long period Cepheids (P greater than9 days). Cepheids previously designated as s-Cepheids by others arecompared to those found using the method outlined here. The method isalso applied to Cepheids in the Small Magellanic Cloud to examine itssuitability as a pulsation mode discriminator.
| Kinematics of metal-poor stars in the galaxy
We discuss the kinematic properties of a sample of 1936 Galactic stars,selected without kinematic bias, and with abundances (Fe/H) is less thanor equal to -0.6. The stars selected for this study all have measuredradial velocities, and the majority have abundances determined fromspectroscopic or narrow-/intermediate-band photometric techniques. Incontrast to previous examinations of the kinematics of the metal-poorstars in the Galaxy, our sample contains large numbers of stars that arelocated at distances in excess of 1 kpc from the Galactic plane. Thus, amuch clearer picture of the nature of the metal-deficient populations inthe Galaxy can now be drawn.
| The P-L relation for RR Lyrae-like stars with 0.8d less than P less than 3d
Data are summarized for 52 of the rare pulsating variables, here calledAHB1 variables for 'above horizontal branch', that have RRab Lyrae-likelight curves but periods in the range of 0.8 less than P less than 3d.The absolute magnitudes, calculated either from the pulsation equationor determined relative to the level of the globular cluster horizontalbranch, range from Mv = +0.2 to Mv = -1.4. The P-Lrelation is well defined, with a ridge-line equation of Mbol= -2.00 log P - 0.10. The standard deviation of the present sample aboutthe rigid-line is only 0.29 mag. Theoretical evolutionary tracks on theway from the ZAHB to the Asymptotic Giant Branch (AGB) conform well withthe data and show that the P-L relation for AHB1 stars is independent ofmetallicity in this period range. This is in contrast to the absolutemagnitudes of normal RR Lyraes near the horizontal branch in the periodrange from 0.4 to 0.8d, where there is a strong dependence ofMv on metallicity, also predicted by the models shown here.The rate of transversal through the instability strip is of the order of100 times faster for these stars than for the ordinary RR Lyrae starscloser to the horizontal branch, making it likely that their observedperiod changes are due to secular evolution, well above pulsation'noise'.
| Pulsational study of BL Herculis models. II - Light curves
A survey of the nonlinear pulsational properties of nine sequences oflow-luminosity Population II Cepheids has been made. The resulting lightcurves are discussed and confronted with the observational data. Despitethe fact that the 2:1 resonance with the second overtone dominates thedynamical behavior of the models, the Fourier phase Phi(21) exhibits amonotone, nearly featureless progression as the period ratio P2/P0 isvaried. This progression is very different from the one displayed by theradial velocity curves. It is also dramatically different from thePhi(21) progression found in the classical Cepheid models. On the otherhand, it is in a qualitative agreement with the observed behavior of theBL Her stars. Also, in contrast to the classical Cepheids, we find theremaining Fourier parameters of the models to be extremely sensitive tostellar mass and luminosity. It is shown that a small, observationallyacceptable dispersion of M and L can account for a very large variety ofshapes among the BL Her light curves.
| 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.
| Color Excesses on a Uniform Scale for 328 Cepheids
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1990ApJS...72..153F&db_key=AST
| Studies of Cepheid-type variability. V - The Fourier phases of type II Cepheids with periods of 1-3 days
Fourier phases of type II Cepheids with periods 1-3 d are reanalyzed andcompared with similar data for classical Cepheids, using a new phasedefinition recently proposed by Stellingwerf and Donohoe (1986). It isfound that this definition allows an improved analysis, mainly due tothe possibility of comparison with a standard case. It is shown that theavailable data for the type II Cepheids are in good agreement with theassumption of a resonance at a period of about 1.5 d.
| On the short-period type II Cepheid field stars
The most accurate recent photoelectric light curve observations of theshort-period type II Cepheids are used here to plot Fourier diagrams andmake comparisons with both the classical type I Cepheids and with themodels of Hodson, Cox, and King (1982), also subjected to Fourierdecomposition. It is found that the resonance sequence among the type IIstars differs in significant ways from its Population I counterpart, butthat hydrodynamic models of the type II Cepheids are amazinglysuccessful in depicting the light curve progression for these stars. Theproblems posed for theories of stellar and galactic evolution by theexistence of disk, halo, and cluster pulsators with very similar lightcurves are discussed.
| Population I pulsating stars. II - Period-age (-colour) relations
Ages corresponding to various evolutionary phases of population Ipulsating stars (89 Delta Scuti variables and 155 classical cepheids)are interpolated in the evolutionary track systems of Iben (1967) andPaczynski (1970). The stellar ages are considerably less in the lattersystem than in the former one. The undertainty of the age of a star isestimated when various evolutionary phases are possible for this star (agreater age corresponds to a later phase). Semiempiricalperiod-age-color (P-t-C) and period-age (P-1) relations are derived forvarious modes, groups of stars, color indices (and effectivetemperature), and evolutionary phases. For Delta Scuti stars, theuncertainty of ages calculated from the P-t relations for differentmodes, is estimated. Theoretical P-t-C and P-t relations for Delta Scutistars are obtained and compared with semiempirical relations (such acomparison of P-t relations is performed for classical cepheids too).The improvement of the age accuracy is estimated when a P-t-C relationis used instead of the corresponding P-t relation. The theoretical andsemiempirical period ratios of radial pulsations, derived from the P-trelations for Delta Scuti stars, are compared. There is relatively goodagreement between the P-t relations for the two types of population Ipulsating stars, but a 'gap' exists between them.
| Population studies. II - Kinematics as a function of abundance and galactocentric position for (Fe/H) of -0.6 or less
A catalog is presented of some 1200 Galactic objects which have radialvelocities and (Fe/H) abundances of -0.6 or less. These data areanalyzed to yield information on the kinematic properties of the olderpopulations of the Galaxy and on the interdependence between kinematicsand abundance. It is found that the kinematics of the availablekinematically selected stars differ from those of the nonkinematicallyselected objects. No evidence is found for any significant difference inthe kinematic properties of the various halo subgroups, nor for anydependence of kinematics on abundance. While the rotation of the halo issmall at about 37 km/s for (Fe/H) of -1.2 or less, it rises quickly forhigher abundances to a value of about 160 km/s at (Fe/H) = 0.6. Objectsin the abundance range -0.9 to -0.6 appear to belong predominantly to apopulation possessing the kinematic characteristics of a thick disk. Theimplications of these findings for the suggestion that globular clustersbelong to the same population as the noncluster objects, for the originof the thick disk, and for the mass of the Galaxy are discussed.
| 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.
| Studies of Cepheid-type variability. III - Fourier description of light curves of Type II Cepheids of periods 1-3 days
Abstract image available at:http://adsabs.harvard.edu/abs/1986A&A...156..337P
| Population I pulsating stars. I - Period-luminosity (-colour) relations
Luminosities of Population I pulsating stars (Delta Scuti variables andclassical cepheids) are investigated. From data for 80 Delta Scutistars, semiempirical period-luminosity-color (P-L-C) relations andperiod-luminosity (P-L) relations are obtained for the four lowest modesof radial pulsations. The improvement of the accuracy of the stellarluminosity is determined when a P-L-C relation is used instead of thecorresponding P-L relation. From data for 155 classical cepheids,empirical P-L relations are derived for short-period stars, long-periodstars, and s-cepheids. The comparison of the P-L relations for the twotypes of variable stars shows good agreement, but between them there isa 'gap' with a dim nature.
| 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.
| Kinematics of field Type II Cepheid variables
The spectra of 49 field Type II Cepheids have been assembled andcombined with 14 additional stars with previously published radialvelocities in order to investigate the kinematics and spectralcharacteristics of these stars. Many of the stars have low peculiarvelocities consistent with circular rotation around the galaxy, whilesome are high-velocity stars. Outside the nuclear bulge, the velocitiesare correlated with photometric metallicities in the sense thatmetal-rich stars show a nearly normal, disklike rotation velocity and alow velocity dispersion. This correlation shows that the low-velocity,metal-rich stars have an old-disk origin. Fewer Cepheids are found inthe field than expected relative to clusters, suggesting a differencebetween the populations of the halo field and clusters. The kinematicproperties of Cepheids in the nuclear bulge appear to be different inthat both metal-poor and metal-rich Cepheids have a high velocitydispersion and show little systematic galactic rotation.
| A photometric classification of pulsating variables with periods between one and three days
Available photometric (2000 UBV measurements) data were employed toestablish standard classifications of Cepheid variables. The datacovered pulsating variables in the Galaxy with periods between 1-3 days.The V lightcurves revealed that the 28 variables in the surveys could beclassified into four groups. The RR Lyrae stars have a smooth Vlightcurve, a short rise time, and a large amplitude in B(AB). The WVirginis stars exhibit a pronounced brightness hump on the ascendingbranch 0.2 h before brightness maximum, while BL Herculis stars have ahump on the descending branch, with rise times decreasing withincreasing periods. Finally, classical Cepheids have a smooth variationwith a gradual rise to maximum. All lie within 3 deg of the galacticequator. The UBV measurements are concluded to be suitable for a coarseseparation of populations of stars.
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