Binary Stars. Selected Topics on Observations and Physical Processes: Lectures Held at the Astrophysics School XII Organized by the European Astrophysics Doctoral Network (EADN) in La Laguna, Tenerife, Spain, 6–17 September 1999 PDF
Preview Binary Stars. Selected Topics on Observations and Physical Processes: Lectures Held at the Astrophysics School XII Organized by the European Astrophysics Doctoral Network (EADN) in La Laguna, Tenerife, Spain, 6–17 September 1999
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OnlineVersion/LNPHomepage LNPhomepage(listofavailabletitles,aimsandscope,editorialcontactsetc.): http://www.springer.de/phys/books/lnpp/ LNPonline(abstracts,full-texts,subscriptionsetc.): http://link.springer.de/series/lnpp/ F.C. La´zaro M.J. Are´valo (Eds.) Binary Stars. Selected Topics on Observations and Physical Processes Lectures Held at the Astrophysics School XII Organized by the European Astrophysics Doctoral Network (EADN) in La Laguna, Tenerife, Spain, 6-17 September 1999 1 3 Editors F.CarlosLa´zaro MariaJ.Are´valo DepartmentofAstrophysics UniversidaddelaLaguna Tenerife,Spain Coverpicture:Theinter-binaryx-rayemissionintheorbitalplaneofARLac.Theobser- vationsweremadewiththeSolid-StateImagingSpectrograph(SIS)intheenergyrange 0.4--1.5keV.(seeStrassmeieretalinthisvolume) LibraryofCongressCataloging-in-PublicationData. DieDeutscheBibliothek-CIP-Einheitsaufnahme Binarystars:selectedtopicsonobservationsandphysicalprocesses ;lecturesheldattheAstrophysicsSchoolXIIinLaLaguna,Tenerife, Spain,6-17September1999/F.C.L=E1zaro;M.J.Ar=E9valo(ed.). OrganizedbytheEuropeanAstrophysicsDoctoralNetwork(EADN).- Berlin;Heidelberg;NewYork;Barcelona;HongKong;London; Milan;Paris;Singapore;Tokyo:Springer,2001 (Lecturenotesinphysics;Vol.563) ISBN3-540-41256-5 ISSN0075-8450 ISBN3-540-41256-5Springer-VerlagBerlinHeidelbergNewYork Thisworkissubjecttocopyright.Allrightsarereserved,whetherthewholeorpartofthe materialisconcerned,specificallytherightsoftranslation,reprinting,reuseofillustra- tions, recitation, broadcasting, reproduction on microfilm or in any other way, and storageindatabanks.Duplicationofthispublicationorpartsthereofispermittedonly undertheprovisionsoftheGermanCopyrightLawofSeptember9,1965,initscurrent version,andpermissionforusemustalwaysbeobtainedfromSpringer-Verlag.Violations areliableforprosecutionundertheGermanCopyrightLaw.Springer-VerlagBerlinHei- delbergNewYork amemberofBertelsmannSpringerScience+BusinessMediaGmbHhttp://www.springer.de (cid:1)c Springer-VerlagBerlinHeidelberg2001 PrintedinGermanyTheuseofgeneraldescriptivenames,registerednames,trademarks, etc.inthispublicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuch namesareexemptfromtherelevantprotectivelawsandregulationsandthereforefreefor generaluse. Typesetting:Camera-readybytheauthors/editors Camera-dataconversionbySteingraeberSatztechnikGmbHHeidelberg Coverdesign:design&production,Heidelberg Printedonacid-freepaper SPIN:10787507 55/3141/du-543210 Preface The XII Predoctoral School of Astrophysics, organized by the European As- trophysics Doctoral Network (EADN) took place on 6–17 September 1999, and was hosted by the Faculty of Physics of the University of La Laguna (Tenerife, Spain).ThetopicoftheSchoolwaschosenbecauseoftheimportantroleplayed by binary stars in modern astrophysics. Today, it is thought that at least half of all known stars in our galaxy form and evolve in binary or multiple systems, andthismakesthestudyofbinarystarsessentialtoourunderstandingofstellar evolution.Thephysicsofbinarystarsisaveryactivearea,inwhichrecentmajor advanceshavebeenmade,bothintheoreticalmodellingandobservations.While the study of binary stars is a field with too many branches to be fully covered in a two-week course, for the 1999 EADN School we selected a few topics that gave a rather wide view of the present research. Theschoolwasattendedby41studentsfrom12differentEuropeancountries (Belgium, Croatia, Estonia, France, Germany, Greece, Italy, Ireland, Poland, Portugal, Spain and the United Kingdom). The lectures were delivered in the morningsovertwoweeks,withtwoafternoonsessionsinwhichthestudentshad the possibility of presenting their research work in the form of a short talk or poster paper. The students’ accommodation and other organizational costs were partially coveredbytheEuropeanUnionTrainingandMobilityofResearchersProgramme, Contract No. ERBFMMACT960173, and by a grant from the Gramholm Foun- dation of Sweden. We acknowledge the support received from the following in- stitutions: the University of La Laguna, the Instituto de Astrof´ısica de Canarias and the island’s local authorities (Cabildo de Tenerife) and La Laguna Town Hall). The travel costs of the invited lecturers were partially covered by Iberia airlines, while the Disa Oil Corporation supported the cost of the coffee breaks offered to attendants at the School. We are much indebted to Prof. Tom Ray, from the Dublin Institute for Advanced Studies (Ireland), Coordinator of the EADNSchools,forhisadviceintheorganizationoftheXIIPredoctoralSchool. La Laguna F. Carlos La´zaro February 2001 Maria J. Ar´evalo List of Contributors Dr. Antonio Claret Prof. J´ozef I. Smak Instituto de Astrof´ısica de Andalucia, N. Copernicus Astronomical Center CSIC Bartycka 18 C/ Sancho Panza s/n. Apartado 3004 00-716 Warszawa, Poland E-18080 Granada, Spain [email protected] [email protected] Dr. Tom R. Marsh Dr. A´lvaro Gim´enez University of Southampton Laboratorio de Astrof´ısica Espacial y DepartmentofPhysicsandAstronomy F´ısica Fundamental, INTA Highfield SO17 1BJ Southampton, Apartado 50727 United Kingdom E-28080 Madrid, Spain [email protected] [email protected] Prof. Bert C. de Loore Dr. Klaus G. Strassmeier Astrophysical Institute. Institut fu¨r Astronomie Vrije Universiteit Brussel Universita¨t Wien Pleinlaan 2, B-1050 Brussels, Belgium Tu¨rkenschanzstr. 17 [email protected] A-1180 Vienna, Austria [email protected] Dr. Jorge Casares Instituto de Astrof´ısica de Canarias C/ V´ıa La´ctea s/n. 38200 La Laguna, Tenerife, Spain [email protected] Contents 1 Physical Processes in Close Binary Systems Antonio Claret, Alvaro Gim´enez ..................................... 1 2 Magnetic Activity in Binary Stars Klaus G. Strassmeier............................................... 48 3 Cataclysmic Variables J´ozef Smak ....................................................... 110 4 Observations of Cataclysmic Variable and Double Degenerate Stars Tom R. Marsh .................................................... 151 5 Evolution of Close Binaries Bert C. De Loore .................................................. 203 6 X-Ray Binaries and Black Hole Candidates: A Review of Optical Properties Jorge Casares ..................................................... 277 1 Physical Processes in Close Binary Systems Antonio Claret1 and Alvaro Gim´enez1,2 1 Instituto de Astrof´ısica de Andaluc´ıa, CSIC, Apartado 3004, E-18080 Granada, Spain [email protected] 2 Laboratorio de Astrof´ısica Espacial y F´ısica Fundamental, INTA, Apartado 50727, E-28080 Madrid, Spain ag@laeff.esa.es Abstract. Wereviewbrieflysomebasicaspectsofthestructureandevolutionofastar distorted by tides and rotation. Proximity effects like, for example gravity-darkening, are discussed using the properties of stellar internal structure. On the other hand, the differential equations of the dynamics of close binary systems are discussed with particularattentiontoapsidalmotion.Thetidalevolutionequationsforlateandearly- type stars are also included and theoretical predictions are compared, when possible, with observations. Concerning stellar atmospheres, limb-darkening laws are revised using recent models. Furthermore, the influence of an external radiation field on the distribution of the intensities and on the irradiated spectrum is also investigated. 1.1 Introduction The theory of stellar structure and evolution is certainly one of the most im- portant achievements of modern Astrophysics. It allows us to understand the observational HR diagram, the stellar content of clusters and galaxies, their chemical evolution, the estimation of ages and distances for single stars, or the evolution of close binary systems. A quite good description, both qualitative and quantitative, of stellar structure and evolution is now-a-days available and the comparison of theoretical models with observations has provided important constraints to our knowledge of the Universe. Among the most important ones is the possibility to look to stars into the past and compare stellar structure for differentages.Theseresultshavetobecarefullyevaluatedunderthelightofthe adopted input physics for the models. Inordertotestthetheory,orderiveempiricalvaluesforadoptedparameters, observational data are needed. But the main parameter driving the structure and evolution of a star is its mass, which can only be determined accurately in binary systems. The study of astrophysical processes in detached eclipsing binaries is thus a well-known method to empirically explore stellar structure for conditions different than those in the Sun. The need for double-lined eclipsing binariescomesfromtherequirementofaccuratedimensionsforavarietyofstars, allowingareliablecomparisonofobservationaldatawiththeoreticalpredictions. Inouraimofderivingnewinformationfromthecomparisonofobservationswith theoreticalmodels,anotherconditionistorestrictthestarsampletomembersof F.C.L´azaro,M.J.Ar´evalo(Eds.):LNP563,pp.1–47,2001. (cid:1)c Springer-VerlagBerlinHeidelberg2001 2 A. Claret and A. Gim´enez well-detached binaries. We consider a double-lined eclipsing binary to be “well- detached” when the two components behave like “normal” single stars in all aspects except their dynamical evolution, i.e. they are well inside their Roche critical limits, there is no mass transfer, and previous evolution has not been affected by binarity. The problem from the observational point of view is of coursethatmostwell-detachedbinarieswillbecomeinteractingwhentheyevolve and their radii become close to their Roche lobe. It is therefore difficult to find evolved stars in binaries still in a well-detached configuration. Inthefollowingpages,weintendtointroducebrieflyhowtheoreticalmodels areconstructed,whicharetheirmainproblemsanduncertainties,andhowthey canbebetterunderstoodundertheilluminatinginformationprovidedbydouble- lined eclipsing binaries. For this purpose, we have separated this presentation in five sections, including the Introduction. The second section set the scene for theoretical developments indicating the needs for additional information. In the third section, we explore the modifications introduced in stellar structure for binary stars, while their atmospheres are treated in Sect. 4. The dynamical behavior of detached binary systems is discussed in the fifth section. The com- parison between theoretical predictions and actual observations are performed in each section, when possible. Finally, on the word close in the title of this Chapter. We refer to close binaries as double systems for which the proximity effects caused by deformations are important; not those which interact strongly, exchanging mass, for example. 1.2 Basics of Stellar Structure 1.2.1 Equations of Hydrodynamics Because of the prevailing physical conditions in their interiors, stars can gen- erally be studied as self-gravitating fluids. We therefore use standard equations of hydrodynamics to specify local physical quantities (like density, ρ, pressure, p, and instantaneous velocity, v, or any other thermodynamical quantity that may be needed) as a function of position, x, and time, t. For a given point, the position in space corresponds to what is seen by a stationary observer in the Eulerian description while, if it corresponds to that seen by an observer that follows the motion of a reference element, we use the Lagrangian description of the fluid. Both are connected through the Lagrangian derivative, D ∂ ≡ +v·∇ (1.1) Dt ∂t which can be applied to any studied quantity of the fluid. For the study of stellar structure, three well-known equations of hydrody- namics are found to be useful: 1. The equation of continuity, ∂ρ +div(ρv)=0 (1.2) ∂t 1 Physical Processes in Close Binary Systems 3 which gives the rate of change of mass-density in a given element of flowing gas. If we furthermore define the specific volume as V =1/ρ, measuring the volume taken up by a unit of mass, and use the Lagrangian description of the equation of continuity, we can write, 1 DV =divv (1.3) V Dt anddivv istherateofexpansionofagivenvolumeofgasduringitsmotion. 2. The equation of motion ∂v 1 +(v·∇)v+2ω×v =− ∇p−ω×(ω×r)+f (1.4) ∂t ρ for a frame of reference rotating at the angular rate ω, where f denotes theexternalforcesperunitmass(whichhaveyettobespecified)otherthan thosecorrespondingtorotationorpressure(eitherfromthegasorradiation). Let us now consider three possible contributions to f: • The force per unit mass due to gravity, which is nothing else than the gravitational acceleration g and can be expressed through the gradient of a potential Φ , G g =∇Φ (1.5) G such that the mass-density obeys Poisson’s equation, ∇2Φ =−4πGρ (1.6) G where G is the gravitational constant and Φ (x,t) is the full gravita- G tional potential, i.e, taking into account self-gravitation terms as well as external contributions. Moreover, we can express the term of the cen- trifugal force by means of a rotational potential, 1 Φ =− (ω×r)2 (1.7) R 2 such that ∇Φ is the centrifugal term and we can combine it with the R gravitational term using Φ = Φ +Φ . Of course, Poisson’s equation G R should be re-written as, ∇2Φ=−4πGρ+2ω2 (1.8) Intheabsenceofanyfurthercontributiontotheforceactingonthefluid element, Euler’s equation of motion takes the form, ∂v 1 +(v·∇)v+2ω×v =− ∇p+∇Φ (1.9) ∂t ρ • The force due to the presence of a magnetic field with strength B, 1 1 f =− ∇B2+ (B·∇)B (1.10) 8πc 4πc under the assumption of no displacement currents and setting the mag- netic permeability µ=1.
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