19 6 9ApJ. . .155. .4 3 9E 3680 isslightlyyoungerthanM67,theclumpofredgiants,characteristicoldclusters,redderin possibly somewhatlower)metalabundanceasthemembersofM67,and{d)showsapronouncedgapin that ofM67(9.5mag),(6)isprobablyonlyslightlyyoungerthanM67,(c)hasstarswiththesame(or cluster NGC3680.The(a)has,withintheobservationaluncertainty,amodulusidenticalwith the distributionofevolvedstarsnearmainsequenceatMp=+3.1.Early-typecluster The AstrophysicalJournal,Vol.155,February1969 have onlybeencorrectedforreddening,thelittle-evolved main-sequencestarsoccupythesame plane ofFigure3asdotsaftercorrectiononly for E(B—F)=+0.06(Eggenand in thisdirectionandthevalueofE(B—F)=+0.04isobtainedbyinterpolationfor position ofthe(V^(B— V)^)-plane withintheobservationaluncertainty.That is,both for whichmorethanoneobservationwasavailable areshowninthe(V¿(B—F)o)- Sandage 1964);thetightclumpofredgiants, horizontal subgiantsequence,andthe crosses. Fivestars,indicatedbyasterisksinTable1,areprobablynotclustermembers in Figure1.TheseobservationsarelistedTable1togetherwithanadditionaltwenty- stars. Onehundredandfiftyobservations,withthe40-inchreflectoratSidingSpring south of—45°(HoggandHunt1965)inasearchforoldgalacticclusters.Onesuch with thesamesymbols.Theage-zeromainsequence (Eggen1965)withablanketingof may alsobeclustermembers. one observationareshownasopencircles,andthosewithonly indicated inFigure1andcorrectedonlyforE(B—F)=+0.04.Starswithmorethan six observationsofeighteenstarslyingoutsidetheseboundaries.Theclusterhasa cluster isNGC3680,whichappearstobeverysimilarM67,althoughnotasrichin give E{B—V)+004.Althoughseveralfeaturesofthecolor-luminosityarrayindicatethatNGC clusters haveamodulus near 9.5mag. 9.5 isshownasacontinuouscurveinFigure3.Although theobservationsineachcluster k(B —V)=0.03appliedtostarswithBVredder than0.3andwithVq—My^ Sandage 1964).ThemembersofNGC3680from Figure2arealsoshownin3 obvious gapinthesequenceofslightlyevolvedmain-sequence stars.ThestarsinM67 and arerepresentedinFigure2byfilledtriangles.Twoorthreeoftheoutlyingstars the distanceofNGC3680derivedbelow. Mountain, havebeenmadeofsixty-onestarslyingwithintheclusterboundariesdrawn NGC 3680. galactic latitudeof+17°. © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Photoelectric (UBV)observationshavebeenobtainedforseventy-ninestarsinandnearthegalactic Three early-typestarsneartheclusterarelistedinTable2.Thereddeningissmall Photoelectric (UBV)surveyobservationshavebeenmadeofstarsinseveralclusters The (Vo'(B—F)o)-relationisshowninFigure2forthestarswithinboundaries Three featuresofFigure2arereminiscenttheold galacticclusterM67(Eggenand Mount StromloandSidingSpringObservatories,ResearchSchoolofPhysicalSciences, THE OLDGALACTICCLUSTERNGC3680 Australian NationalUniversity,Canberra II. REDDENINGANDDISTANCE Received July1,1968 I. INTRODUCTION O. J.Eggen ABSTRACT 439 19 6 9ApJ. . .155. .4 3 9E © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem O u- 440

Fig. 1.—The cluster NGC 3680. Taken from a 10-min exposure with 103aO plate at the Newtonian focus of the 74-inch reflector 19 6 9ApJ. . .155. .4 3 9E HR 4402. HD 110119 HR 4423.. NGC 3680. 38 39 37 36 35 34 33 32 31 30 29 28 27 26 23 22 20 25A 25 24 21A 21 12 11 10 19 18 17 16 15 14 13 6. 9. 4 8* 5 3 2 7* 1 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem * Probablefieldstar, t Closedouble. Name No. (mag) 6 12 5 18 7 21 Ve 12 41 12 47 13 13 44 13 17 10 69 11 88 12 95 13 50 13 22 13 63 12 14 10 20 12 54 15 06 10 96 12 91 14 20 12 80 12 48 12 86 12 95 11 60 14 75 15 24 10 97 11 04 13 92 14 06 13 47 14 69 14 57 12 49 12 10 12 05 13 35 12 40 10 90 11 98 13 84 13 89 (mag) Ve -Ó Í75 -0 05 -0 03 (mag) B-V +0 52 +0 475 B-V (mag) 0 53 0 46 0 56 0 50 0 87 0 55 0 52 0 49 0 46 0 52 0 54 0 49 0 64 0 545 0 51 0 48 0 75 0 445 0 55 0 49 0 685 0 88 0 48 0 55 0 56 0 55 0 73 0 65 0 515 0 78 0 50 1 085 1 60 1 165 1 13 1 01 1 15 1 32 1 015 Early-Type StarsNearNGC3680 (mag) -0 195 U-B 0 79 0 105 +0 01 +0 07 +0 05 +0 01 +0 03 +0 50 +0 06 +0 02 +0 04 +0 82 +0 025 +0 055 +0 08 +0 02 -0 02 +0 53 +1 03 +0 97 +0 02 +0 725 +0 06 +0 33 +0 045 +0 01 +1 06 +0 26 +1 3: +0 04 +0 20 -0 03 +0 26 +0 16 +0 81 -0 015 -0 01 -0 06 -0 02 -0 02 (mag) U-B 0 00 0 00 Stars inNGC3680 TABLE 2 TABLE 1 441 (+0 04) +0 06 +0 04 +0 02 (mag) L*. P 49* 48. 45 44 42 N J* F* 471 46A 46 43 41 40* Q M* K* I* H* A* O* 58. 54* 53. 52 51 50 E. B* 61 60 59* 57 56 55 D* R C* G* E No -5 +0 5 +1 R (pc) 1550 12 88 13 49 12 49 12.72 12 43 13 81 15 16 12 74 12 58 13 87 11 97 13 08 11 83 13 75 12 04 10 06 12 50 13 04 11 01 13 24 14 32 12 86 12 56 12 96 12 56 10 87 12 02 10 91 15 10 14 70 13 50 12 55 13 30 11 81 13 09 12 41 11 64 14 46 14 36 (mag) 220 9 75 8 96 795 v E 67 +17 5 +17 +17?5 +18 5 + 1 +0 47 B-V (mag) 0 61 0 50 0 01 0 44 0 79 0 71 0 24 0 855 0 45 0 86 0 50 0 56 0 47 0 53 0 49 0 77 0 58 0 62 0 565 0 495 0 44 0 87 0 775 0 68 0 83 0 495 0 45 0 59 0 90 0 495 0 79 0 63 1 24 1 18 1 14 1 23 1 1 19 1 28 15 12 c 286 287 287 287 +0 05 +1 16 +0 07 +0 15 +1 21 +0 355 +0 09 +0 39 +0 03 +0 05 +0 04 +0 97 +0 06 +0 02 +0 03 +1 27 +0 02 +0 04 +0 42 +0 16 +0 23 +0 09 +0 075 +0 50 +0 01 +0 34 +0 08 +0 35 +1 00 +0 08 +0 005 +0 01 -j-0 31 +0 10 +1 13 +0 14 -0 05 -0 05 +0 57 +1 30 (mag) U-B 0 00 B0 III B8 B9 Sp 442 O. J. EGGEN

III. LINE BLANKETING When gravity effects were taken into account for the evolved stars, the observations of the members of M67 indicated a blanketing effect measured by A(B — F) = +0.03 (Eggen and Sandage 1964). The subgiant and giant members of M67 are shown as dots in the ((U — — F)o)-plane of Figure 4. The continuous curve represents the main-sequence relation for Hyades stars. The open circles in the figure represent the subgiants and giant members of NGC 3680. For values of {B — F)o of about 0.2-0.6

Fig. 2.—The (Fo' {B — F)o)-diagram for stars within the boundary marked in Fig. 1. Open circles represent two or more observations, crosses, single observations, and triangles, stars considered to be non-members. Magnitude and colors are corrected for E(B — V) = +0 04 mag. the color-color relation is gravity-sensitive, and the stars with surface gravities lower than that of main-sequence stars will lie below those indicated by the continuous curve in Figure 4 (Eggen 1963a, 1966). On the other hand, the presence of differential line blanketing, with respect to Hyades stars, will place the stars above the relation shown in the figure (cf. Sandage and Eggen 1959). The two effects nearly cancel each other for the M67 stars in Figure 4 (Eggen and Sandage 1964), and the same is apparently true for the members of NGC 3680. The only systematic difference between M67 and NGC 3680 members in Figure 4 is the possibility that the red giants in the latter cluster are displaced from those of the former in the sense that at a given B — V the values of U — B are about 0.05 mag smaller in NGC 3680.

© American Astronomical Society • Provided by the NASA Astrophysics Data System 19 6 9ApJ. . .155. .4 3 9E Continuous curverepresents the Hyadesmain-sequencestars. mag, for5—Vgreaterthan+03andFoM=9.5mag. +0.06 mag.Continuouscurverepresentstheage-zeromainsequencecorrectedforA(B—V)=0.03 v Fig. 5.—Sameas4butfor themain-sequenceornear-main-sequenceclusterstars. Fig. 4.—Thecolor-colorrelation forthesubgiantsinM67andNGC3680,correctedonly reddening. © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Fig. 3.—-Sameas2,butthemembersofM67havebeenaddedaftercorrectingforE(B—F)= Fig. 4 Fig. 5 (B-V) (B-VX, 0 444 O. J. EGGEN Vol. 155

Figure 5 contains the little-evolved main-sequence stars in both clusters. Here the differential line blanketing with respect to the Hyades stars is the only effect responsible for the displacements from the continuous curve which represents Hyades main-sequence stars. The observations of members of M67 led to A (5 — V) = 0.03 as the mean value for this blanketing effect (Eggen and Sandage 1964), and from Figure 5 it appears that within the observational uncertainties the same value holds for the members of NGC 368°- In summary, aside from a small apparent ultraviolet excess in the red giants of NGC 3680, the color-color relation for the other stars of the two clusters is identical within the observational uncertainty. IV. DISCUSSION If the result that the moduli and the differential line blanketing (and therefore, pre- sumably, the metal contents) of the two clusters, M67 and NGC 3680, are identical is correct, then the differences between the two color-luminosity arrays in Figure 3 are caused only by age and/or an unknown parameter. The outstanding features of the ar- rays can then be compared directly and may be of major importance in considerations of stellar models. Some of these features are as follows. a) The break in the evolved sequence near the main sequence.—This break, first noted in NGC 2477 (Eggen and Stoy 1960) and later in NGC 752 (Eggen 1963Ö), M67 (Eggen and Sandage 1964), NGC 188 (Eggen and Sandage 1968), and NGC 2360 (Eggen 1968) presumably exists because of the short time scale of the gravitational contraction that follows the depletion of core hydrogen and precedes the hydrogen burning in the shell. The relation between the luminosity and color of this break in the arrays of the two clusters, indicated by parallel lines in Figure 3, compares with those for NGC 752 and 2360 as in the accompanying tabulation, where {B — V)t is the observed value, cor-

Name Mv M67.. .. +3 3 +0 49 +0 03 NGC 3680 3 1 40 .03 NGC 752. 2 8 30 03-0 04 NGC 2360. +1 9 +0 27 +0 03-0.04 rected for reddening only, of the blue boundary of the breakaway from the main se- quence. The values of A(B — F), the differential line blanketing with respect to the Hyades stars, obtained from the main-sequence objects in each cluster are also listed. For a given chemical composition it is likely that the values oí Mv and (B — V)t listed above also order the clusters by age. b) The luminosities of the nearly horizontal subgiant sequences.—The fact that the sub- giant sequence of NGC 3680 is a half-magnitude brighter than that of M67 and the difference, noted above, in {B — V)t both lead to approximately a factor of 2 in age difference and a 20 per cent difference in maximum mass of the stars in the two clusters (cf. Iben 1967). c) The clump of red giants near (B — F)0 = -\-l.05 in M 67 and +1-1 m NGC 3680.— The fact that these stars in the presumably younger NGC 3680 are redder than in M67 may be in contradiction to the age and mass difference discussed above. Faulkner (1966) suggests that this clumping in M67 may represent the stars which have not been af- fected by mass loss during their late evolution, whereas the pseudohorizontal branch stars, brighter than F0 = 11.5 and bluer than about B — V — +0.8 in Figure 3, may represent those that have lost mass. If this is the case, Faulkner’s results would indicate that the metal content of the giants in NGC 3680 is considerably lower than that of

© American Astronomical Society • Provided by the NASA Astrophysics Data System 19 6 9ApJ. . .155. .4 3 9E metal content,undetectableinthemain-sequenceornear-main-sequencestars,does .1966,No.120. are brighterintheultraviolet.Thepossibilitythereforeremainsthatadifference No. 2,1969GALACTICCLUSTERNGC3680445 .1965,Ann.Reo.Astr.andAp.,3,235. 19636,Ap.138,356. show itselfinthecolorsofredgiants. the colorsofmemberstwoclustersisfactthatgiantsinNGC3680 the membersofM67.In§IIitwasfoundthatonlydetectabledifferencebetween .1968,ibid,(inpress). 1968Ap.J,152,83. Eggen, O.J.1963a,A./.,68,483. Eggen, O.J.,andStoy,R.H.1960,RO.B.,No.24. Eggen, OJ.,andSandage,A.R.1964,Ap.J.,140,130. Iben, I.1967,Ann.Rev.Astr.andAp.,5,571. Hogg, A.R.,andHunt,V.O.1965,CatalogueofOpenClustersSouthDeclination—45°,Canberra, Faulkner, J.1966,Ap.J.,144,978. Sandage, A.R.,andEggen,O.J.1959,M.N.R.AS,119,278. © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Australia. (c) 1969.TheUniversityofChicagoAllrightsreserved.PrintedinUSA. REFERENCES 19 6 9ApJ. . .155. .4 3 9E © American Astronomical Society Provided bytheNASA Astrophysics DataSystem