1956ApJ. . .123 . . .44C -13 25 lines, theKstarisfoundtoejectmaterial,someofwhichcollectedbybluecompanionTheorder grangian pointofthesystem.Onthishypothesis,supportedbypresencenarrowCa11emission brought ittothepointoffillingonelobezero-velocitysurfacepassingthroughinnerLa- value of10forthemassstar,indicatesthatarapidevolutionaryexpansionmayalreadyhave retical Sandage-Schwarzschildrateofevolutionaryexpansion;(2)theobservedluminositybluestar; tory, givingadispersionof130AmmatH7.Thelate-typecomponentthisdoublestar,classifiedonthe accordingly, ^10g/cc.Itissuggestedthattheaccretionofmassbybluestarrelatedto magnitude oftheratethistransfermassisestimatedbythreeindependentmethods:(1)theo- Morgan-Johnson system,isfoundtobeK5IV-V.Thisclassification,takentogetherwithJoy’sminimum pointed out. a rateof~10g/year.Thedensitymaterialintheregionsurroundingbluestarisfoundtobe, and (3)theintensitiesofnarrowbroademissionfeaturesinHô.Theseestimatesagree,yielding possibility ofbinarymotion.ThusitisnotimpossiblethattheexplosiveUGeminorum continuous spectrumwithsuperimposedbroademissionlinesofH,He1,andCa11.Two outbursts of2mag.foundbyZinner,andapossibleconnectionwiththeUGeminorumtypevariablesis system. JoyhasfoundthatAEAquariiconsistsofapairdwarfstars,onespectral a spectroscopicbinaryofshortperiodhasledustoattemptaninterpretationthis to showsimilarspectralfeaturesandlargeradial-velocityvariations,suggestingthe stars oftheUGeminorumclass,SSCygniandRUPegasi,werereportedbyhim(1954a) year. Henize(1949),Lenouvel(1952),andGolay(1954)observedastrong phenomenon requires,insomeunexplainedfashion,twostarsashort-periodorbitas type K,havingrelativelynarrowemissionlinesofHandCa11,theotherpossessinga Zinner (1938)foundoutburstsof2mag.(photographic)withapossibleperiodabout1 does notappeartobeatypicalUGeminorumstarwhenjudgedbyitslight-variability. year shouldbe5or6mag.Wewouldthenconcludethateithertheperiodismorenearly tendency for“flickering”atminimum,withoccasionalflaresupto1mag.The a necessary,thoughnotsufficient,condition. close relativeofsuchvariablesasSSCygniandRU Pegasi. given heremaybethoughtpfasanalternative to thatadvancedbyJoy.Briefly,the signed toexplainthemajorspectroscopicfeatures ofAEAquarii.Theinterpretation bursts. IneithercasethespectrumfoundbyJoycertainlysuggeststhatAEAquariiisa A.A.V.S.O. detectednomajoroutburstsduring1953,its“AEAquariiyear”(Mayall of theorder5-10daysorthatAEAquariidoesnothavebonafideUGeminorumout- nago (1933),asmodifiedbyKopylov(1954),applies,theoutburstcorrespondingto1 main sequenceatK5bysuchanamountthatitfills onelobeoftheinnerzero-velocity argument isasfollows:ItproposedinSectionII thatthelate-typestarliesabove 1954). Ifwesupposethattheperiod-amplituderelationfoundbyKukarkinandPare- surface surroundingthesystem.InSectionsIIIand IVwefollowthemotionofmaterial © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The spectrumofAEAquariiisdiscussedonthebasisspectrogramstakenatLickObserva- Despite thespectralsimilarityofthesethreestars,itshouldbenotedthatAEAquarii The recentdiscoverybyA.H.Joy(1954a,b)thattheflaringvariableAEAquariiis The presentdiscussionconsistsofanattemptto obtainaself-consistentmodelde- * NationalScienceFoundationpredoctorialfellow. AN INTERPRETATIONOFAEAQUARII Berkeley AstronomicalDepartment,UniversityofCalifornia John A.CrawfordandRobertP.Kraft* Received August8,1955;revisedSeptember19,1955 I. INTRODUCTION ABSTRACT 44 1956ApJ. . .123 . . .44C © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem > o o X O o QC CD O m < — < K) — rO 1 J M M »-^H «.i ■5 ü '’O o 'i ^ « g 'tíí tí < ^ M-H H o fe ^ ttí o _ *4-H ■g = ^Os- rH tí 4^ ^ Oí ^ CN 4- J ^ '*" a ¿I £ o ^ o cfi (U +-> cfi 'XD en tu a ^ aQí en '. o . Cfi H biD O) star veiling,however,canhavelittleeffectonthe ratioofthe K5. Moreover,theweakness ofCriandCaisinsupportaclassification ofabout weak featuresastheGband, sothatthelate-typecomponentisactually earlierthan fication isK5IV-V,attempttoanticipatesomeobjections thatmightberaisedagainst spectrum iscomposite;thefilling-inbybluestar tendstowipeoutsuchincipiently it, andshowhowtheymaybeanswered.Inthefirst place,itmightbearguedthatthe correct classification.ThentheabsenceofGband wouldagainsuggestthatthestar be furnishedbyspectrogramsreachingX4077of Sr n. cate aluminositysomewhatgreaterthanclassV. Further evidenceonthispointwould the intervalK0-K3orK4-K5,properties ofthespectrumseemtoindi- should beplacedabovethemainsequenceinthat range.Thus,whetherthestarliesin at K4orK5.Onemight,ofcourse,adoptthepoint ofviewthatK0-K3isthebasically can, however,beremovedbyassumingthatthestar isslightlyabovethemainsequence K2 V,propertieswhichseeminconsistentwithcriteria1and2.Thesecontradictions shows thattheratioX4254:X4260andweaknessofX4226areclosely Mount WilsonsystemtendstoclassifyKstarslaterthantheMKsystem,e.g.,HD tral classificationdonotapparentlyaccountfortheinconsistency,since,ingeneral, are tooweakforK4V-K5V. ent resultsfromdifferentlineratios.ItcanmostnearlybedescribedasK4IV-VorK5 placed ontheMKsystem.Thespectrumseemstobeslightlyanomalous,yieldingdiffer- is themostsuitableplateforclassificationoflate-typestar.Joyassignsitto sufficiently denseinthevicinityofGbandtopermitspectralclassification.The violet, thespectraofAEAquariiareallveryweaktoshortwardX4150but dKO; wedonot,however,findthecorrespondingclassificationKOVwhenstaris spectrum reproducedinFigure1,havingtheweakestcontinuousemissionofseries, IV-V forthefollowingreasons:(1)Gbandisaltogetherabsent;(2)ratioX4290: not reproduced.)Owingtotheincreasingopacityof36-inchobjectivetoward including thebroad,unsymmetricalemissionlinesofhydrogenaswelllate-type absorption spectrum,areclearlyseen.(TheCanemissionalsoisfaintlypresentbut 154363 isMountWilsondMO(Wilson1953);MK:K5V.AninspectionofFigure1 X 4299isthatofdK4ordK5;(3)theresonancelinesCri,4254,andCa4226, stars, isreproducedinFigure1.ThegeneralspectroscopicfeaturespointedoutbyJoy, with thesameprisms,camera,andslit-width. Johnson andHarris1954)weretakenforcomparisonpurposesbyoneofus(R.P.K.) attached tothe36-inchrefractor.SuitableMKstandards(JohnsonandMorgan1953; persion ofabout130A/mmatHy,wereobtainedwithtwoprismsanda3j-inchcamera ously suppliedthemtousforuseinthisinvestigation.Thesespectrograms,givingadis- also fromspectrogramstakenattheLickObservatorybyG.H.Herbig,whohasgener- briefly discussedthroughwhichtheaccretionofionizedmaterialontobluecom- velope, identifiablewiththeoutburstsofUGeminorumstars. ponent mightengender,undersuitableconditions,suddenreleasesofenergyintheen- infall. AnadditionalcheckonthedensityisobtainedfromSandage-Schwarzschild star, theluminosityofwhichisfoundtobelargelysuppliedbykineticenergy Hô emissionandindependentlyfromtherateofaccretionthismaterialbyblue lobe (or,equivalently,therateofaccretion)isderivedfromintensitybroad flowing intothelobesurroundingbluestar.Thedensityofgaseousmaterialinthis (1952) rateofevolutionaryexpansiontheKstar.InSectionVapossibleprocessis © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem We shallproceed,forillustrativepurposes,onthe assumption thatthespectralclassi- Possible systematicdifferencesbetweentheMountWilsonandMKsystemsofspec- One ofthesespectrogramsAEAquarii,togetherwithcertainstandardcomparison The observationalmaterialisderivedinpartfromthestudymadebyJoy(1954Z>)and II. THESPECTRUM AE AQUARII45 1956ApJ. . .123 . . .44C 46 JOHNA.CRAWFORDANDROBERTP.KRAFT field atthesubstellarsurfaceofKcompanionand,inparticular,shouldberesponsible radiation ofthebluecompanion.Weshallnowconsiderrefutationthisobjection. extended atmosphereassociatedwiththeKcomponent,presumablyexcitedby role playedbytheionizingradiationofbluestar.Shouldfieldat of neutralmetals,owingtoionization,mightbethoughtoccur.Thisargumentappears late asdK4. moderately strongfeaturesdescribedin2,andtheseleadtoaclassificationatleastas having beentakenjustpriortothatphase,Kstarreceding.Thenetresultwillbe exposed tothebluestarisaboutone-fifthatelongation,plateillustratedinFigure1 for ahighdegreeofionizationmetals.ThefractionalprojectedareatheK-stardisk substellar surfaceoftheKstarbecontrolledbybluestar,aweakeninglines to bestrengthenedbytheobservedpresenceofhydrogenemissionlinesarisinginan luminosity, andradius.Itisreasonabletosupposethatthemaximumattainablevolume, lead tothebeliefthatsuchastarisinstateofrapidexpansion,proceedingessentially seems clearthatthespectralclassificationobtainedmustbeanintegralpropertyof erties arealtogetherdifferentfromthoseofthemajorportionsurface.Ittherefore highly turbulentgascloud. Itisinterestingtonotethatastrong,broadCa nemission between theinnerandouterzero-velocitysurfacescanbereadilyestimatedfrom features are2-5Awide.Thevelocityofgasesmovingintherotatingframereference pansion throughaninnerzero-velocitysurface.Thisevidenceisthepresence,inJoy’s we shallgoaheadonthisassumption,whichfurtherresultsseemtojustify. namely, thatofonelobetheinnerzero-velocitysurface,hasalreadybeenreached,and order of1Oandremainonthemainsequence.Presentideasstellarevolutionwould mum masses).ItdoesnotappearlikelythataK5starcanhaveminimummassofthe tively, usingthesecondofhistwosolutions(thefirstsolutionleadstoevenlargermini- work, theminimummassesofKstarandblueare0.94O0.97O,respec- dius of0.58Rq,usingKuiper’s(1938)empiricalbolometriccorrection.FromJoy’s gion asproducingaperturbationonthespectrumofstarwhole:itsspectralprop- a smalloneoftotallyforeigncharacter.Innocasecanwethinkthisionizedre- the K-starsurfaceisdividedintotwoparts,onelargeregioncontrolledbyKstarand on thesametimescaleasthatofagravitationallycontractingstarmass, star. be proposedinthisarticle thatthebluecomponentofAEAquariiissurrounded bya periods lessthan1day(WUMa,RWCom,RTAnd, FGHya)(Bidelman1954).Itwill man 1954).Severalothereclipsingbinarieswithsubgiant secondariesareknowntoshow emission lines,associatedwiththesubgiantcomponent, onaYerkesspectrogram(Bidel- mechanism fortheproductionofsuchemissionlines. Itisalsoofinteresttoconsiderthe difference ofpotentialthesesurfaces,usingthemassesandseparationstars spectra, ofnarrowHandKemissionlinessharingtheradialvelocitystar.These surrounded byaturbulentgasenvelope,aneventtobeexpectedifitisinstateofex- case ofTWDraconis.Thisisaneclipsingbinarywhose secondaryisasubgiantlistedby certain long-periodvariables,cepheidvariables). Itseemsthatturbulenceisalikely in whichconsiderableturbulenceisbelievedtoexist (supergiantsoflatespectraltype, Joy’s observedwidthforthenarrowemissionfeatures. adopted inthisarticle.Thisgivesavalueofabout200km/sec,goodagreementwith Ca iiemission(RZEri,WWDra,RTLac,ARLac), asarealsoseveralbinarieswith Crawford (1955)asfillingtheinnerzero-velocity surface. MissRomanhasfoundCan 4 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem A differentkindofobjectiontotheclassificationK5IV-Vmightbelookedforin Despite thedilutionfactoroforder10~,bluestarwillcontrolradiation A K5VstarhasT=4400°K,M+7.8(KeenanandMorgan1951),ara- There is,however,somedirectobservationalevidencesuggestingthattheKstaris Generally speaking,Canemissionischaracteristic ofSSCygnistarsandobjects ev 1956ApJ. . .123 . . .44C 9 11 11 produced inFigure1israther narrow.Thereisalsosomequestionregarding theuseof velocity surfacesareindicated.Turbulentgasstreamsrepresentedbyarrows. choice ofthemassininterval1.0-1.5O.The^physicalradius”(Kuiper1941) hand, themassofKstarcanhardlyexceed1.5OifitistobeclassifiedasK5IV-V. between theinnerandouterzero-velocitysurfaces can,however,bynomeansberuled with hisinterpretationofßLyrae.Thesimultaneous ejectionofmaterialintotheregion manner analogoustothe“ejectionoftypeA”described byKuiper(1941)inconnection main sequence.Theparallaxisthen0'i013,andwehave,accordingtoJoy,M=+6.5, star becomes+4.8ifT=4300°K,andM+5.5,i.e.,theis2.3mag.above sequence starofthesamespectraltype),separationbetweencentersmass from CrntoiandcalciumCa will bedelayedasonepassesfromK0 when comparedwithasinglestarofthesamemass andradius.Suchareductionwill out andisprobablytobeexpected.(2)TheKstar will sufferadecreaseinsurfacegravity of gaswillbeejectedfromtheKstarintoregion surroundingthebluestar,ina with R=6.6X10cmforthebluestar. 3.2 X10cm=4.5R®,andi64°.TheabsolutebolometricmagnitudeoftheK5 K5 starwouldthenbe1.2X10cm=1.7Rq(aboutthreetimestheradiusofamain- phasized thatnoneoftheconclusionsreachedinthisarticleareaffectedbyexact flickering, oftheorder0.3mag.(Henize1949;Lenouvel1952;andGolay tainly earlierthanK5.In thisconnectionitshouldbepointedoutthatthe spectrumre- tend toincreasethedegreeofionization,and,inparticular, theconversionofchromium The systemisnotknowntoeclipse,butthismayverywellbedisguisedbythepersistent to K5.ThusCri,X4254,andCaI,4226,areweakened, aswehavealreadyindicated. two consequencesofthefillingonelobebythé K stararethefollowing:(1)Astream by Kopal(1954)permitanavoidanceofeclipseifthemassesstarsarenotlessthan gives amassratioofunity(1.0)andminimum1Oforeachstar.Ontheother line showsthesameradialvelocityasbluestar,inagreementwithgeneralidea that Caiiemissionisacharacteristicofturbulence. 1954). Foramassratioof1.0,thedimensionsinnerzero-velocitysurfacegiven 1.3 O.Weshallthereforeadoptthisvalueinthediscussiontofollow.Itshouldbeem- v ev © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Fig. 2—ThesystemofAEAquarii(schematic).Theviewispole-on,andtheinnerouterzero- Dr. Joy(privatecommunication)hasexpressed the opinionthatKstariscer- The modelasviewed“pole-on”isshowninFigure2.Ashasalreadybeensuggested, In ordertoproceedfurther,weshallarbitrarilyadoptJoy’ssecondsolution,which AE AQUARII47 1956ApJ. . .123 . . .44C 26 25 estimate ofthecorrespondingratemasslossa of our(double-star)componentfilling readily interpolatedforintermediatemasses.This wasdonefortheadoptedvalue, where pisthemeandensity ofthemodel.Thiscanbewritten radial expansionofthemodelsSandageandSchwarzschild (1952).Althoughgiven the innerLagrangiansurfaceisgivenroughlyby only formasses10,20,and4O,thephysical characteristics ofthesemodelscanbe in thepresentcase.Ifthiswereso,accretionratewouldbecome0.2X10g/year, luminosity. Itisnotpossibletostatewhetherorafactorofthisorderapplicable a valuewhichissmallerthan,butcloserto,the twoindependentestimatesofvob- the totalrateofincreasepotentialenergyisaboutthirtytimessmallerthan magnitude relationshipexpressedbyequation(1).Thispresumablygives face andissignificantlysmallerinmagnitudethanthemeanpotentialenergyperunit star, theresultsofSandageandSchwarzschild(1952)indicatethat,foramass2O, mass ofthestar.Wearethereforejustifiedinneglectingtoincludeitorder-of- will thenbeacceleratedinthegravitationalfieldofcompanionstar(hereblue velocity surface,theconsequenceoftendencytoexpandmustbeoutpouring fraction ofthetotalenergyreleasedisconvertedintopotentialouterregions tained later. the adoptedvaluesofL,9W,andRforKstar.However,anisolatedexpanding 1.3 O.IfÄmistherateatwhichradiusof the(single-star)modelincreases,an The potentialoftheejectedmaterialisthatassociatedwithinnerLagrangiansur- luminosity thus: can equatetheorderofmagnituderateincreasepotentialenergyto occurs atarategivenessentiallybythegravitationaltimescale,meaningthatcertain the orderofmagnitude

The rate of radiative recombination of ionized hydrogen into different energy levels has been calculated by Gillie (1932) for several assumed temperatures. The rate of re- combination per cubic centimeter at 10000° K into the sixth level is given by him as 1.45 X 10_1W2, where N is the density of hydrogen nuclei, assuming that all electrons come from the ionization of hydrogen. The probability that the excited atom will emit a quantum in Hô is 0.221. The density N is then given by the expression

0.221 X 1.45 X 10-14X^S*N*= I ^■ where T is the effective temperature, R the radius of the blue star, and X the wave length of Hô. We obtain in this way N2SZ = 1.3 X 1056 . (2)

This figure is overestimated because of the neglect of the effect of captures in levels higher than the sixth, followed by cascading into the sixth level. If, at the other extreme, we assume that all captures in levels above the sixth result in cascades to this level, we obtain the value 1.9 X 1055 for N2SZ. Strömgren’s theory (1939) gives a relation between the density of hydrogen and the radius of the largest sphere, Ss, ionized by the blue-star radiation: N2S% = 1.0 X 1055 . (3)

As this value is perhaps significantly smaller than the one given by equation (2), there is an indication that another mechanism besides blue-star radiation is at work keeping the gas ionized, in fact, all the way to the neighborhood of the K star, where the narrow hydrogen emission arises. Turbulence may very well provide such a mechanism. The fact that H5 is observed and shows no self-absorption makes it clear that no H i region envelops the ionized zone, so that S should be taken as the effective radius of the outer zero-velocity surface, beyond which the density can be assumed to decrease abruptly. This radius is about 1.6 X 1011 cm. Substituting this value in equation (2), we get for N the value 1.8 X 1011 atoms/cc, yielding the value 4.0 X 10-13 g/cc for the density p on the assumption of a hydrogen abundance of 75 per cent. The velocity v given by the Hô line at half-maximum is 400 km/sec, corresponding to a free-fall distance r from the blue star of 8 X 1010 cm. The order of magnitude of the accretion rate is given by the relation (T — 4:'irr2p v .

In this way we obtain for radial velocity was not measured by Joy but is apparently similar to that of the K star. Its intensity on the spectrogram in Figure 1 seems about one-fifth that of the broad line. This is in agreement with the mass-transfer picture of AE Aquarii, according to which the region between the inner and outer zero-velocity surfaces surrounding the K star is expected to be filled with material of about the same density as the gas in the lobe surrounding the blue com- panion. This is, of course, because of the condition for the continuity of gas flow, since the velocities characteristic of the two regions are of the same order of magnitude. It seems possible that turbulence keeps hydrogen ionized even in the shadow cone be- hind the K star. If not, we would expect the radial velocity of the narrow hydrogen- emission features to be systematically smaller than that of the K component. Spectro- grams taken with a higher dispersion than was available to us are needed to settle this question.

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1956ApJ. . .123 . . .44C lines ofapproach:(1)thetheoryexpansion;(2)luminositybltiestar;and observed byLenouvel(1952)inthephotographicregionareprobablyassociatedwith relating tosourcesofstellarenergyandhasconcludedthatthermonuclearprocessesin 52 JOHNA.CRAWFORDANDROBERTP.KRAFT identifiable withtheflaresobservedinthisTTauriobject.Asimilarmechanismmaybe dark cloudintheimmediatevicinityofthisstarhavevaluesquiteclosetothosefound with theTTaurivariableBD—6°1253isofinterest.Thedensityandvelocity result fromtheaccretionofmatter,asuggestionbyGreenstein(1950)inconnection the bluestar;and,finally,thereisnoevidencethatothersystemsinwhichonecom- blue star:thespectrumissimilartothatofanoldnova;magnitudefluctuations ence ofoutbursts.Thereisevidencethattheoutburstsareprobablyassociatedwith lation ofthatfortheUGeminorumvariables.Itwouldthereforeseempossible operating inAEAquarii. period-amplitude relationforcertainrecurrentnovaeisnotnecessarilyasimpleextrapo- gas willitselfbeaccreted,withthepossibilityofresultingintenseelectricaldischarges Out thatthemagneticfieldwhippedupbyturbulentmotionofaccretedionized by usintheregionsurroundingbluecomponentofAEAquarii.Greensteinpoints ponent fillstheinnerzero-velocitysurfaceandothercomponentisamain-sequence the precedingmodelisaconnectionbetweenprocessofmasstransferandexist- comet-like nebulae,certainirregularvariablesoflowluminosity,etc.).Hehassuggested stellar interiorsareinsufficienttoexplainawidevarietyofunusualstars(TTauristars, model ofAEAquarii. nitude. Thisagreementseemstoconstituteagoodargumentinfavorofthisproposed Ambarzumian, V.A.1954,Contr.BurakanObs,No.13. mechanism responsiblefortheoutburstsofAEAquariiandstarsUGeminorum tionship betweenthisstarandtheUGeminorumgroup.Whatisfurthersuggestedby of materialontothesurface. This generalpointofviewseemstoreceivesomefurthersupportfromourconclusion that additionalenergysourcesshouldbelookedforintheouterregionsofsuchobjects. Henize, K.G.1949,A.J.,54, 89. novae. star orasubgiantexhibitoutbursts. that theluminosityofbluecomponentAEAquariiismostlyresultinfall Dadaev, A.N.1954,Izvestia, Central Obs.Poulkovo,Vol.19,Part5,No.152. cussion. WeareindebtedalsotoDr.A.H.Joy, Dr. J.L.Greenstein,andtoA.R. spectrograms ofAEAquariiandalsotothankDr. W.P.Bidelmanforavaluabledis- class neednotbeidenticalinkindwiththatresponsiblefortheexplosionsofrepeating Cillié, M.1932,M.N.,92,820. Bidelman, W.P.1954,Ap.J.Suppl.,1,No.7,223. to ProfessorO.Struveforhiscriticism. Sandage forreadingthemanuscriptandcommenting onit.Weareespeciallygrateful (3) therecombinationofhydrogenandhavefoundthemtosameordermag- Greenstein, J.L.1950,Pub.A.S.P., 62,156. Crawford, J.A.1955,Ap.121,71. © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem To summarize,wehaveobtainedvaluesoftheaccretionratefromthreeindependent In arecentpublication(1954)Ambarzumianhasreviewedtheobservationalevidence With regardtotheprocessthroughwhichoutburstsinluminosityofbluestar The discoveryofoutburstsinAEAquariibyZinner(1938)hintsatapossiblerela- In thisconnectiontheworkofKopylov(1954)isinterest.Hehasshownthat We wishtorecordourappreciationDr.G.H.Herbig formakingavailabletoushis V. FURTHERDISCUSSION REFERENCES AE AQUARII S3 Johnson, H. L , and Harris, D. L. 1954, Ap 120, 196 Johnson, H. L., and Morgan, W. W 1953, Ap / , 117, 313 Joy, A. H. 1954a, Pub. A.S.P., 66, 5. . 19546, Ap. 120, 377. Keenan, P. C., and Morgan, W. W 1951, Astrophysics, ed. J. A. Hynek (New York: McGraw-Hill Book Co., Inc ), pp. 20 and 23. Kopal, Z. 1954, Jodreil Bank Ann , 1, 37. . 1955, “Trans I.A.U. Report of Commission 42” (unpublished) Kopylov, I. 1954, Doklady Akad. Nauk. U S S.R., Nov. ser., 99, 515. Kuiper, G P. 1938, Ap. /., 88, 429. 1941, ibid., 93, 133. Kukarkin, B , and Parenago, P. 1933, Akad. Nauk. S S.S R., Astr. Sovet Peremmennye Zvezdy, 4, 251. Lenouvel, F. 1952, C.R , 235, 1282. Lenouvel, F., and Golay, M¿ 1954, C.R., 237, 1215. Mayall, M. 1954, J.R.A.S , Canada, 48, 22. Sandage, A , and Schwarzschild, M. 1952, Ap J., 116, 475. Schönberg, M., and Chandrasekhar, S. 1942, Ap. J., 96, 161. Strömgren, B. 1939, Ap J., 89, 533 Wilson, R. E. 1953, General Catalogue of Stellar Radial Velocities (“Carnegie Institution of Washington Publications,” No. 601 [Washington: Carnegie Institution]), p. 197. Wood, F. B. 1950, / , 112, 196. Zinner, E 1938, A.N , 265, 345.

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