CCD Strömgren photometry of young reddened clusters

1997AJ 113. . 713D THE ASTRONOMICAL JOURNAL VOLUME 113, NUMBER 2 FEBRUARY 1997 CCD STRÖMGREN PHOTOMETRY OF YOUNG REDDENED CLUSTERS An onio J. Delgado, 1 and Emilio J. Al a o 1 Ins i u o de As o ísica de Andalucía, CSIC. P.O. Box 3004, E-18080, G anada, Spain Elec onic mail:[email p o ec ed], [email p o ec ed] Jesús Cab e a-Caño Facul ad de Física, Uni e sidad de Se illa, P.O. Box 1045, E-41080, Se illa, Spain Elec onic mail: [email p o ec ed] Recei ed 1996 Augus 15; e ised 1996 Oc obe 31 abs ac The capabili ies o CCD u by pho ome y o he s udy o eddened young clus e s a e in es iga ed. Obse a ions o ou ields in he di ec ion o he clus e s Basel 1, Be keley 86, NGC 6704, and NGC 6756, wi h a o al o 1665 s a s measu ed in a leas he y and b bands, p oduce new es ima es o he clus e s pa ame e s. The esul s a e compa ed o hose om p e iously published s udies in di e en pho ome ic sys ems showing good ag eemen in he alues o colo excess and dis ance modulus, wi h he excep ion o NGC 6756, o which an essen ially la ge dis ance is ound. Fu he mo e, he ages es ima ed o his clus e , Basel 1, and NGC 6704 a e ound o be highe han p e iously assumed, in pa icula o he las one. The p esence o ed gian s as clus e membe s is sugges ed in Basel 1, NGC 6704, and NGC 6756. © 1997 Ame ican As onomical Socie y. [80004-6256(97)01702-0] 1. INTRODUCTION A p ojec on CCD u by pho ome y o eddened young clus e s has been unning du ing he pas h ee yea s, wi h he aims o gaining u he insigh in he peculia i ies o eddened u by colo s o OB- ype s a s. P e ious wo k (Man oid & S e ken 1987; Delgado & Al a o 1989; Al a o & Delgado 1991; Delgado e al 1992) has e ealed and dis- cussed he exis ence o sys ema ic e o s in he in insic u by colo s, possibly dependen on spec al ype, and/o colo excess. The u by obse a ion o young eddened clus e s wi h a CCD o e s some ad an ages in his con ex . Wi h elescopes o mode a e size, and easonable in eg a ion imes, i is pos- sible o secu e measu emen s o s a s in a wide ange o spec al ypes, om he mos luminous OB- ype clus e membe s, down o F-, and e en ea ly G- ype main sequence s a s. This allows a mo e eliable de e mina ion o he clus e pa ame e s, and in pa icula , he analysis o he eddening a ec ing he clus e . When complemen ed wi h measu e- men s in he Hyß sys em, indi idual de e mina ions o he colo excess a e possible o he whole spec al ype ange co e ed, allowing in u n a measu emen o he clus e me- allici y. Finally, hese obse a ions help in e ining he knowledge o he quan i y and quali y o he in e s ella ed- dening in he di ec ion o he clus e . In pa icula , he spe- ci ic ea u es o he ex inc ion due o in aclus e ma e , as di e enced om he di use in e s ella ex inc ion (Tu ne 1994, and e e ences he ein), can be in es iga ed. A selec ion o no he n objec s om he ca alog o Lyngà, Visi ing As onome , Ge man-Spanish As onomical Cen e , Cala Al o, ope a ed by he Max Planck Ins i u e ü As onomie, Heidelbe g, join ly wi h he Spanish Comisión Nacional de As onomía. 713 As on. J. 113 (2), Feb ua y 1997 wi h a colo excess E(B — V) la ge han 0.5, and ages equal o less han 10 8 yea s, p o ides a sample o abou 50 clus- e s. The amoun o published da a o he clus e s in his sample is inhomogeneous. Only a ew ha e published mea- su emen s in he S ömg en sys em, and o many o hem he ca alogued alues s em om old pho ome ic s udies in he RGU pho og aphic sys em. In his pape , we p esen he esul s o he i s CCD obse ing campaigns, o he clus e s Basel 1, Be keley 86, NGC 6704, and NGC 6756. S ömg en pho ome y is usually complemen ed wi h measu emen s in he C aw o d Hß-sys em. In ac , o s a s o spec al ype la e han A, he Hß index is commonly used as he inde- penden empe a u e pa ame e , and mos calib a ions, in- cluding hose o me allici y, [Fe/H], in e ms o Smi, make use o i . Addi ional wo k is unde way, o comple e he p esen da a wi h measu emen s o he Hß index, which, ob iously enough, a e mo e ime consuming by an impo an ac o , i one wan s o achie e measu emen s wi h p ecision compa able o he one in u by bands, o he s a s in he lowe main sequence. In he nex sec ion we desc ibe he educ ion p ocedu e and he ans o ma ion o he s anda d sys em. Sec ion 3 con- ains he analysis o he pho ome ic diag ams, calcula ion o he es ima es o he di e en clus e s pa ame e s, and a compa ison wi h esul s p e iously published. The las sec- ion esumes he main conclusions o ou s udy. 2. OBSERVAHONS AND DATA REDUCTION The obse a ions we e ob ained a he 1.5 m, and 1.23 m elescopes in he Cala Al o Obse a o y, in 1993 June. Fig- u es 1(a)-1(d), show plo s o he obse ed ields, wi h he s a s measu ed in a leas y and b bands. The do sizes a e indica i e o he ela i e ins umen al y magni ude in each © 1997 Am. As on. Soc. 713 0004-6256/97/113 (2)/713/9/$ 10.00 © Ame ican As onomical Socie y • P o ided by he NASA As ophysics Da a Sys em 1997AJ 113. . 713D 714 DELGADO ET AL. : YOUNG REDDENED CLUSTERS 714 Basel 1 Be keley 86 NGC 6704 NGC 6756 Fig. 1. (a)-(d). Schema ic maps o he ou clus e ields obse ed. Only s a s measu ed in a leas y and b a e plo ed. The size o he do s ep esen s ela i e b igh ness o s a s in he ield. No h is up and Eas le in all ields, wi h he excep ion o he ield in Basel 1, whe e he No h-Sou h axis is clockwise o a ed abou 30 deg ees. ame. The da es, and de ec o s used in each elescope a e, 1.5 m elescope: 1993 June 14-19, de ec o THX 1024 X 1024, wi h 19 jilpx, and 0.33%*. 1.23 m elescope: 1993 June 13-17, de ec o Tex4 520X520, wi h lljjilpx, and 0.57%*. In Table 1 we include he schedule o he obse a- ions in bo h ins umen s, along wi h in eg a ion imes, ai - mass a he middle o obse a ion, and ull wid h a hal maximum in each ame. The s anda d s a s we e selec ed om h ee sou ces, y- ing o co e he la ges possible ange in {b — y) alues. P ima y s anda ds we e selec ed om he ca alog by Olsen (1983). Some s a s we e also aken om he measu emen s o B- ype s a s by Knude (1992) and om he compila ion o Schus e & Nissen (1988). Howe e , he ans o ma ion o s anda d colo s equi es ex apola ion o he calib a ion ela- ions. In he absence o signi ican ly eddened B- ype s an- da ds, he e ec s o his ex apola ion can be diminished by including la e ype s a s in he s anda d sample. This can howe e in oduce new unce ain ies, since he a ia ions o colo indices due o eddening migh di e om hose due o di e ences in spec al ype (Delgado & Al a o 1989). Table © Ame ican As onomical Socie y • P o ided by he NASA As ophysics Da a Sys em 1997AJ 113. . 713D 715 DELGADO ET AL. : YOUNG REDDENED CLUSTERS 715 Table 1. Schedule o he obse a ions. Table 3. Numbe s o measu ed s a s. Clus e (da e, Tl.) Band T in (s) Ai mass FWHM (px) Basel 1 16.6.93, 1.5 300 800 1200 3000 1.373 1.401 1.441 1.558 5.1 7.0 5.7 6.6 Be keley 86 17.6.93, 1.5 20 300 50 500 1200 120 3000 1.191 1.157 1.150 1.132 1.083 1.062 1.030 3.5 4.4 4.0 5.3 5.3 3.8 5.0 NGC 6704 14.6.93, 1.23 620 1180 1500 3600 1.356 1.360 1.383 1.484 3.2 3.4 4.4 4.5 15.6.93, 1.23 NGC 6756 16.6.93, 1.23 960 1360 1500 3600 1080 2000 3000 3600 1.425 1.361 1.355 1.400 1.345 1.226 1.191 1.209 3.5 3.5 3.8 4.0 4.0 4.9 4.6 5.5 2 lis s he s anda d s a s measu ed in bo h elescopes, wi h numbe o measu emen s and e e ences. The educ ion o all ames was done wi h he IRAF ou- ines o he bias, da k and la - ield co ec ions, and he eg- is e ing and combina ion o ames. The ames lis ed in Table 1 a e hose ac ually used o pho ome y. The in eg a- ion imes in his able a e in some cases (Basel 1, y; NGC 6704, NGC 6756 all bands) he esul o adding se e al con- secu i e ames, a e p e ious egis e ing o all ames o Table 2. S anda d s a s obse ed. HD b-y Sp 1.23 1.50 Re 133769 134248 135200 135262 161848 165503 166179 166600 166710 166928 164760 165343 160233 170783 171491 175544 177549 180126 8.541 8.372 8.122 8.563 8.911 7.844 8.328 8.639 8.353 8.532 7.959 8.490 9.102 7.717 7.952 7.395 9.257 8.012 0.181 0.402 0.271 0.457 0.489 0.160 0.188 0.277 0.497 0.255 0.163 0.151 0.025 0.187 0.541 0.144 0.300 0.207 0.270 0.205 0.143 0.199 0.340 0.220 0.168 0.185 0.246 0.121 0.038 0.048 0.032 0.016 -0.056 0.002 -0.092 -0.029 0.778 0.343 0.616 0.434 0.283 0.749 0.693 0.748 0.428 1.100 0.958 0.648 0.071 0.593 0.201 0.171 0.325 0.361 F0 GO F0 GO K0 F0 F0 GO GO A5 B9 B8 B5 B5 B5 B3 B3 B3 2 5 6 4 0 7 0 4 3 11 3 3 0 1 0 1 0 0 Re : 1. Olsen 1983, 2. Knude 1992, 3. Schus e & Nissen 1989. Bands Bas 1 Be 86 NGC 6704 NGC 6756 To al y,b 604 160 533 368 1665 u, ,b,y 121 59 a 165 139 484 a The 5 b igh es s a s in Be 86 a e sa u a ed in he «- ame. he same ield wi h espec o he b igh es ame in he y band. The sho and long ames o Be keley 86 we e e- duced sepa a ely, and he inal ins umen al magni udes in each band we e ob ained om weigh ed a e ages o bo h ames. The IRAF/DAOPHOT package (upda e 1994 Janua y) was used o ob ain ape u e pho ome y, and PSF i ing and ap- e u e co ec ions o he p oblem s a s. The e o s o he ape u e co ec ion, de ined as he ms de ia ion o he co - ec ions o abou 10 s a s pe ame, a e be ween 0.01 and 0.02 o he 1.5 m elescope ames, and 0.05 o he 1.23 elescope ames. In Table 3 we p esen he numbe o s a s measu ed in a leas wo bands (y,b) and in he ou u by bands, in he espec i e clus e ields. A mosphe ic ex inc ion co ec ions we e pe o med ol- lowing a mul inigh me hod (see Young 1974), whe eby ex- inc ion coe icien s a e calcula ed e e y nigh , as he mean o he coe icien s o all he s anda d s a s measu ed. The e o s o he coe icien s ( ms de ia ion) amoun o 0.01, whe eas hose o he ex inc ion co ec ed magni udes a e mos equen ly below 0.01. Wi h he ex inc ion co ec ed magni udes, he ollowing se o ans o ma ions a e ob- ained o each elescope: 1.5 m elescope: y¿ = 4.175+1.036 V-0.074 (b~y) 0.03 —y) ¿ = 0.138 + 1001 {b-y) 0.02 -0.458+1.018^-0.185 {b-y) 0.05 {u ¿ — 0.569+ 1.024 {u — ¿7) + 0.171 {b-y) 0.03 1.23 m elescope: y¿= 6.100+ 1.027V—0.108 {b-y) 0.04 (¿> — y)- = 0.180+ 1.107 {b-y) 0.01 q ¿= —0.473 + 0.577C! — 0.316 {b-y) 0.02 (w b)} — 0.084+ 0.814 (m-¿7) + 0.259 {b-y) 0.06 m { = [{u — b) - 2 (¿7 —y)-cJ/2, whe e he subindex i s ands o ins umen al magni udes. The co esponding s anda d e o s, de ined as he ms de ia- ion o he esiduals, a e also quo ed. Conce ning he V ans o ma ion, i has been poin ed ou ha a coe icien o he y magni ude sligh ly bu signi ican ly smalle han 1 migh be a ea u e o ins umen al sys ems wi h a CCD (Jónch-Só ensen & Knude 1990; Jónch- Só ensen 1993). This possibili y is es ed o ou bo h ins u- men al sys ems. As o he m! index, i is compu ed om he ans o med {b-y),Ci, and {u — b) indices. This p ocedu e is ad isable o diminish as much as possible he sys ema ic e o s in he s anda d alues caused by he una oidable ex- apola ion o he ans o ma ions (Delgado & Al a o 1989). © Ame ican As onomical Socie y • P o ided by he NASA As ophysics Da a Sys em 1997AJ 113. . 713D 716 DELGADO ETAL: YOUNG REDDENED CLUSTERS 10 V 15 10 V 15 Fig. 2. To al es ima ed c ’s o he s anda d V,{b ~y),c x ,(u — b) plo ed s V. C osses, 1.23 m elescope; ci cles, 1.5 m elescope. Only s a s selec ed as membe s om hei loca ions in a leas wo CM diag ams a e plo ed (Sec. 3.1). An es ima e o he o al unce ain y a ec ing ou s anda d indices can be gained om Fig. 2, whe e he o al <x’s (squa e oo o he added squa ed e o s men ioned abo e: PSF- i ing, ape u e co ec ion, ex inc ion, and ans o ma- ion) a e plo ed e sus V, o V,(b — y),c l , ( u — b). These plo s show only hose s a s conside ed clus e membe s om he loca ion o hei ep esen a i e poin s in a leas wo colo -magni ude (CM) diag ams (see Sec. 3.1 below). Fo he selec ed B- ype membe s, which a e he ones used o compu e he colo excess (see also Sec. 3), he la ges e o s in y,(& —y),^ ,(w —Z?) amoun o 0.06, 0.08, 0.12, 0.11, and 0.03, 0.04, 0.07, 0.05 o he 1.23 and 1.5 obse a ions e- spec i ely. A able wi h he comple e pho ome y o he 1665 s a s measu ed a leas in y and b bands, oge he wi h coo dina es X,Y in he ields ske ched in Fig. 1, can be ob- ained om he au ho s on eques . 3. PHOTOMETRIC DIAGRAMS 3.1 Membe S a s and Reddening Slopes The i s s ep in he analysis is he es ima e o membe - ship o he s a s measu ed in each ield. This can be done wi h he use o se e al CM diag ams, and he equi emen ha he s a s lie close o he main sequence concen a ion in all hem. He e we use he h ee CM diag ams V—(b — y), V—(u — b), and V—( —y). S a s selec ed as membe s ha e o lie a dis ances no la ge han 0.2 om eye-es ima ed lines along he main sequence concen a ion in each diag am. In Fig. 3 we show as example he CM diag ams V—(b —y) o he ou clus e s, wi h hese lines d awn. The dis ance chosen o selec ion akes in o accoun no only he e o s in V and he di e en colo indices, bu also he ex- pec ed p esence o a ia ions in colo excess, which can each se e al en hs o a magni ude. In addi ion, we selec as membe s he s a s lying in simila posi ions in he gian e- gion in all diag ams. Ob iously, he only c i e ion a ailable he e o decide he membe ship o gian s a s o he clus e s is based on hei pho ome ic colo s. The loca ion o hese s a s in he diag ams o he h ee olde clus e s is howe e a 716 (b-y) (b-y) Fig. 3. V-(b-y) diag am o he ou clus e s. The hues used o membe selec ion a e plo ed as example. Fo commen s on he wo ci cled do s in Be keley 86 (Sec. 3.1) s ong indica ion in a o o hei membe ship. As we shall see la e , he h ee clus e s Basel 1, NGC 6704, and NGC 6756 ha e ages simila o he Hyades, and he p esence o gian membe s should be expec ed. The i s membe s selec ion om his p ocedu e, is now e ined wi h he use o he diag ams Ci — (b — y), (u — b) — (b — y), and Ci — (u — b). These diag ams help also in es ablishing an spec al classi ica ion be ween s a s ea lie and la e han spec al ype A0, and es ima ing o mean ed- dening slopes, app op ia ed o he pa icula egion. Fo hese es ima es, we use he e e ence lines o un eddened s a s gi en by C aw o d (1978, 1979, and 1975, o B-, A-, and F- ype s a s, espec i ely). The e e ence line o in e - media e A- ype s a s by Hildi ch e al. (1983), linking he B and A anges o he C aw o d’s ines is also used. Figu e 4 shows as example he men ioned wo colo dia- g ams o he clus e Basel 1, wi h e e ence ines shi ed acco ding o he quo ed alues o colo excess, and edden- ing slopes. Ci cled do s indica e he s a s selec ed as B- ype membe s, acco ding o hei loca ions in all h ee diag ams. The eddening slopes E(ci)/E(b — y),E(u — b)/E(b—y) es- ima ed wi h his p ocedu e a e 0.20,1.60;0.20,1.40;0.30, 1.50 o Basel 1, NGC 6704, and NGC 6756. Fo Be keley 86 he a ailable A- and F- ype s a s a e e y sca ce. He e we a e helped by published spec al ypes o some clus e membe s (Fo bes e al. 1992, he ea e F92). Un o una ely, he i e b igh es s a s in ou ield a e sa u a ed in he u ame. Only o s a No. 6 in he pho ome y o F92 we ha e © Ame ican As onomical Socie y • P o ided by he NASA As ophysics Da a Sys em 1997AJ 113. . 713D 717 DELGADO ET AL. : YOUNG REDDENED CLUSTERS Fig. 4. Two colo diag ams o Basel 1. The e e ence lines a e shi ed wi h he alues: E(¿> —y) = 0.33,E(c])/£(£ —;y) = 0.20,E(w — ¿?)/E(¿> — ;y) = 1.60. The ci cled do s a e he selec ed B- ype membe s. spec al ype (B2 V) and measu ed (u — b) and Cj indices. The slopes used, 0.18,1.52, ha e been compu ed o his s a , using he calib a ion o in insic colo s in e ms o spec al ype gi en by Wa en (1976). Fu he mo e, wo o he s a s gi en as membe s by F92 (Nos. 24, 26), i s excluded by ou c i e ion, ha e also been included as B- ype membe s in Be - keley 86. They a e indica ed wi h ci cled do s in Fig. 3. In all cases, he slope o he m 1 index is compu ed as E(mi)/ E(b — y) = [E(u — b)/E(b — y) — 2 — E(ci)/E(b — y)]/2. A quan i a i e es ima e o he unce ain ies o hese slopes is di icul o ob ain, since hey a e based on isual i s. How- e e , hey will be ob iously a ec ed by a leas he same e o s han hose o he co esponding pho ome ic indices o he selec ed B- ype membe s (see Sec. 2). In iew o hese e o s, he di e ences o he slopes used he e wi h hose ob ained by C aw o d & Mandwewala (1976: E(ci)/ E(b-y) = 0.20,E(u-b)/E(b-y) = l.54) canno be consid- e ed signi ican . 3.2 Pho ome ic Pa ame e s 3.2.1 Reddening and me allici y Once he eddening slopes a e es ima ed, we p oceed o de e mine he in insic colo s o indi idual s a s. We can do his only o s a s ea lie han A0, as he calib a ion o (£ —y)o o la e s a s depends on he H/3 index. The s a s selec ed as B- ype membe s a e de eddened wi h he o mu- las gi en by Shobb ook (1984). Fo he emaining s a s se- lec ed as membe s, he colo excess is compu ed as he mean o he alues ob ained o indi idual B- ype membe s. The alues o colo excess ob ained amoun o 717 0.33±0.04,0.71 ±0.10,0.54±0.03,0.73±0.04 o Basel 1, Be keley 86, NGC 6704, and NGC 6756, espec i ely. O h- e wise, he in insic la ges e o o he colo excess o one s a can be es ima ed om he pho ome ic e o s gi en in Sec. 2 (see Phelps & Janes 1994, o an explana ion o he quan i a i e es ima es). They a e 0.07 and 0.13 o he 1.5 (Basel 1, Be keley 86) and 1.23 (NGC 6704, NGC 6756) measu emen s, espec i ely, which a e o be compa ed wi h he ms de ia ions o he mean, gi en abo e. The e o e, no signs o di e en ial o a iable eddening abo e he pho o- me ic e o s can be guessed om ou da a, wi h he excep- ion o Be keley 86. An es ima e o he me allici y could be a emp ed, a leas o Basel 1 and NGC 6704, whe e we ha e a la ge numbe o obse ed F- ype s a s. The compu a ion wi h he a ailable calib a ions o [Fe/H] in e ms o £ a 0 ( Z ? —y) = a 0 / / ne — m 0 ^ (C aw o d 1975; C aw o d & Pe y 1976), p oduces he mean [Fe/H] alues —0.3±0.7, 0.6± 1.3 o Basel 1 and NGC 6704, espec i ely. The la ge unce ain ies e lec he la ge e o s o ou pho ome ic indices o he weakes mem- be s. Measu emen s o H/3, and longe in eg a ion imes also in he and u bands, should imp o e his de e mina ion h ough a de e mina ion o indi idual eddening o AF s a s, use o Sm 0 ( ß ) , mo e p ecise membe ship de e mina ion, and, ob iously, mo e p ecise alues o he magni udes in he di e en bands. 3.2.2 Dis ances and ages As has been also exempli ied by Tu ne (1994), local a ia ions o he eddening slopes a e in some cases associ- a ed wi h a ia ions o R, he a io o abso p ion in V o colo excess E(B — V). How hese a ia ions migh beha e in he S ömg en sys em is unknown. On he o he hand, he eddening slopes used he e a e no signi ican ly di e en om he a e age alues commonly used (C aw o d & Mand- wewala 1976). We assume R = 3, and compu e he abso p- ion coe icien o each clus e wi h he ela ion be ween E(B—V) and E(b~y) gi en by Penp ase (1992). Dis ance moduli a e hen ob ained by shi ing he obse a ional main sequence o i he de eddened CM diag am V 0 - ( b - y ) 0 . The esul s a e 11.0±0.3,11.1±0.3,11.3±0.4,12.6±0.4 o Basel 1, Be keley 86, NGC 6704, and NGC 6756, espec- i ely. The unce ain ies o hese alues ha e been es ima ed om he pho ome ic e o s, ollowing he easoning by Phelps & Janes (1994). To es ima e he ages we dispose o ecen model isoch- ones, p o ided o speci ic ages by Cla e (1996, p i a e). All he isoch ones used he e a e o sola chemical compo- si ion (Z= 0.02,A=0.7). The models include he conside - a ion o newes opaci ies and ene gy gene a ion a es. A mode a e amoun o o e shoo ing om he con ec i e co e is pa ame ically included in he compu a ions (see Cla e 1995, and F. Pé ez-Hemández e al. 1995, o a desc ip ion o he models, and he calib a ions used o empe a u e and bolome ic co ec ion in e ms oí UBV colo s). The isoch- ones a e ansla ed o he S ömg en colo -magni ude dia- g ams, F 0 - ( ¿ ?-y) 0 , and V 0 - ( w - Z ?) 0 , wi h he well cali- b a ed o mulas gi en by Penp ase (1992) o ( -y)o and Tu ne (1990) o («-¿>) 0 . In Fig. 5 we show he in insic © Ame ican As onomical Socie y • P o ided by he NASA As ophysics Da a Sys em 718 DELGADO ET AL : YOUNG REDDENED CLUSTERS 718 Fig. 5. (a)-(d). Isoch one i s o he de eddened V-(b-y) 0 , and V—(u — b) 0 diag ams: (a) Basel 1, (b) Be keley 86, (c) NGC 6704, (d) NGC 6756. The co esponding ages and dis ance moduli a e lis ed in Table 4. Empi ical ZAMS lines a e plo ed as hicke lines. La ge do s indica e membe s in all h ee CM diag ams, acco ding o ou c i e ion (Sec. 3.1). colo magni ude diag ams o he ou clus e s, wi h he bes isoch one i s supe imposed. The e o s o hese age- es ima es can only be guessed quali a i ely, on he basis o he e o s o he pho ome ic poin s, and he a ia ions o he i quali y wi h a ying age o he isoch one, in bo h CM diag ams simul aneously. We es ima e ages o 2.3 + 0.3X10 8 y ,8±5X 10 6 y ,2.0±0.6X 10 8 y ,1.3±0.6 X 10 8 y , o Basel 1, Be keley 86, NGC 6704, NGC 6756, espec i ely. The bes guesses, based on hese i s, a e in- cluded in Table 4, oge he wi h ou es ima es o he dis- ance moduli and colo excess, compa ed o alues gi en in he li e a u e. We wish o s ess he e he adequacy o his isoch one calib a ion, ia colo ans o ma ion, ha can be app ecia ed in he good ag eemen be ween he ans o med isoch ones, and he obse a ional u by main sequence lines, also plo ed in Fig. 5 as hicke con inuous ines. Fu he mo e, we con- side ou age es ima es as eliable, in iew o he sa is ac o y simul aneous i s o heo e ical in insic colo s, om wo in- dependen calib a ions, o ou wo CM diag ams, in a ela- i ely la ge ange o spec al ypes along he main sequence. 3.3 Compa ison wi h P e ious S udies Pho ome ic s udies ha e been published o he ou clus- e s in he p esen epo . The amoun o in o ma ion is a i- able, bu all hem ha e a leas one de e mina ion o he basic pho ome ic pa ame e s, colo excess, age, and dis ance. O h- e wise, none o hem has published u by pho ome y, and only some s a s in Be keley 86 ha e published spec al ypes (Me milliod, Ga ison: p i a e). The h ee clus e s Basel 1, NGC 6704, and NGC 6756 we e measu ed in he pho o- g aphic RGU sys em some decades ago (Fenka 1965; G ubbisich 1965; Slo opoulos 1965). Excep o NGC 6704, © Ame ican As onomical Socie y • P o ided by he NASA As ophysics Da a Sys em 1997AJ 113. . 713D 719 DELGADO ET AL. : YOUNG REDDENED CLUSTERS 719 Table 4. Clus e s pa ame e s compa ed o published es ima es. Clus e E(b-y) Log Age (yea s) DM, E(B-V) Bas 1 0.33 8.35 7.76(0) 11.00 10.67 (0) 11.00 (1) 10.62 (2) 11.02 (3) 0.43 0.57(0) Be 86 0.71 6.94 6.78(0) 6.75(4) 6.78(5) 6.40(6) 6.70(7) 11.10 10.23 (0) 11.18 (5) 11.39 (6) 11.01 (7) 0.89 0.99(0) 0.96(5) 0.80(6) 1.01(7) NGC 6704 0.54 8.30 7.30(0) 11.30 11.44 (0) 11.29 (2) 11.40 (8) 0.69 0.72(0) 0.71(8) NGC 6756 0.73 8.12 7.67(0) 8.48(9) 12.60 10.89 (0) 11.09(10) 0.91 1.18(0) REFERENCES.—(0) Lyngá, Open Clus e s Ca alog. (1) Becke & Fenka 1970. (2) G ubissich 1965. (3) Fenka 1965. (4)Smi h e al. 1988. (5) Fo bes 1981. (6) Massey e al. 1995. (7) Fo bes e al. 1992. (8) Fo bes & DuPuy 1978. (9) San os e al. 1993. (10) Slo opoulos 1965. o which U BV pho ome y has been published mo e e- cen ly (Fo bes & DuPuy 1978, he ea e e e ed o as F78), hese RGU wo ks con ain he only a ailable es ima es o he clus e pa ame e s. Fo Be keley 86, UBV pho ome y has been published by Fo bes (1981), F92, and Massey e al. (1995). 3.3.1 Compa ison o indi idual s a s The compa ison wi h p e iously published pho ome ic indices can be pe o med o he clus e s Be keley 86 and NGC 6704, a e ans o ma ion o ou u by indices (b — y) and (u — b) wi h o mulas gi en by Tu ne (1990). The a e age di e ences o he s a s in common wi h he UBV pho ome ic s udy by F92, in he sense F92-ou s, a e AV=0.10±0.15,A(5-y) = 0.01±0.03,A(U-5)=-0.00 ±0.07. The ag eemen is qui e sa is ac o y, bu a sys ema ic di - e ence appea s in he V alues, whe eas he di e ences in (B — V) and (U-B) a e smalle and no sys ema ic. The di e ence in V is signi ican ly dec eased using he classical ans o ma ion o ob ain he V s anda d magni udes (C aw- o d & Ba nes 1970; see Sec. 2 abo e): y i - V = 4 A T8-0.11T(b-y) S .e o = 0.03 which gi es a di e ence A —0.06±0.13. Figu e 6 shows he wo se s o magni udes compa ed o hose in F92. Two o he s a s in common, (s a s 3, 4 in F92), deno ed wi h squa es in he igu e, s ill show la ge di e ences wi h ou alues. On he o he hand, he mean di e ence wi h he CCD alues ob ained by Massey e al. (1995) o h ee s a s in common (1, 2, and 4 in F92) a e much smalle , AV=— 0.009±0.009 in he sense Massey-ou s. We no e ha Fig. 6. Di e ences be ween ou V magni udes and hose in F92, o he s a s in common in Be keley 86. The esul s o wo ans o ma ions o he s an- da d sys em a e shown. Ci cles: V ans o ma ion wi h a iable y coe i- cien . Do s: V ans o ma ion wi h y coe icien equal o 1. Fo commen s on he wo squa ed do s (Sec. 3.3.1). s a s F92-3 and F92-4 a e double, and p obably e y ha dly sepa able in pho oelec ic pho ome y. Rejec ing hem, he a e age di e ence F92-ou s educes o AV=-0.02±0.07. On he basis o he good ag eemen wi h he pho oelec ic alues in F92, and hose o Massey e al. as well, we use he ans o ma ion equa ion wi h he y-coe icien equal o 1 o he measu emen s in he 1.5 m elescope ins umen al sys- em. Fo NGC 6704, we compa e wi h he esul s in F78, he only UBV pho ome ic s udy known o us. The in o ma ion con ained in he ables o F78 is somewha con using, mak- ing unce ain he compa ison wi h hei pho og aphic alues. O he 41 s a s in common wi h us, 5 ha e pho oelec ic measu emen s in F78 (s a s A, B, F, H, J, whe eas A, F, and H, also ha e pho og aphic measu emen s). Res ic ing he compa ison o hese 5 s a s, we ob ain mean alues, A -0.01 ±0.07, A(B-V)=- 0.01 ± 0.27, A(U-B) = 0.07±0.27. The di e ences a e no e y big, bu he dis- pe sions a e oo high, well abo e he e o s o ei he ou pho ome y, o he one in F78. We canno hink o a eason- able explana ion o ha . We jus no e ha he mean di e - ences in F78, be ween hei own pho og aphic and pho o- elec ic alues o he h ee s a s wi h bo h kind o measu emen s, a e al eady e y high, A V= 0.04±0.11, A(B-V) = 0.10±0.07, A(U-B) = 0.28±0.17. 3.3.2 Membe ship Membe ship assignmen s a e gi en in he pho ome ic s udies ci ed abo e o all ou clus e s, using he loca ion o he ep esen a i e poin s in he espec i e pho ome ic dia- g ams. Some di e ences wi h ou assignmen s a e p esen , mainly o s a s o spec al ypes a ound BA, and la e . In gene al we keep ou es ima es, conside ing hem mo e con- iden on he basis o he wide ange o spec al ypes mea- su ed, and he disposal o 3 independen colo magni ude diag ams. We jus commen on wo di e ences he e. Fi s , s a s 24, 26, in Be keley 86 (F92 numbe s), we e i s ejec ed by us because o hei {b — y) alues, al hough hei loca ions in he o he CM diag ams we e inside he dis ance chosen o selec ion. The s a s a e conside ed membe s by F92, based on hei pho oelec ic colo s, and we ha e indeed included © Ame ican As onomical Socie y • P o ided by he NASA As ophysics Da a Sys em 1997AJ 113. . 713D 720 DELGADO ET AL : YOUNG REDDENED CLUSTERS 720 400 x 200 0 400 7/9 x ^ 200 0 400 7/9 x 200 0 400 200 0 X(pxs) e o s es ima ed o ou pho ome y. We conside ou alues mo e ealis ic, in iew o he accep able i s o bo h obse a- ional e e ence lines, and isoch ones, o a wide ange along he main sequence. The h ee clus e s seem in ac o be he same objec , jus obse ed a di e en dis ances. In pa icu- la , we poin ou he imp o emen o he age es ima e o NGC 6704, which u ns ou o be signi ican ly olde han p e iously es ima ed. Be keley 86 is a much younge objec . The age ob ained he e is in good ag eemen wi h p e iously published alues, bu ou absolu e dis ance modulus is smalle han o he e- cen de e mina ions by 0.1 o 0.3. We ha e seen in he p e- ious subsec ion ha he ag eemen wi h he alues o ed- dening, and o colo indices o he s a s in common, in pa icula in V, is accep able. On he o he hand, he alue o dis ance modulus ob ained by Massey e al. (1995) is la ge han ou s by 0.3, and is based on spec al ypes. This di e - ence is mos p obably due o he egion co e ed by he ob- se a ions o Massey e al, o abou hal a deg ee, while ou obse a ions a e es ic ed o he mo e eddened clus e co e. In ac , hey ob ain a lowe colo excess, E(£ —V) = 0.8, compa ed o ou alue E(Z?-y) = 0.71 which esul s in E(5 — V) = 0.89 wi h he ela ion by Penp ase (1992). Wi h a alue o he abso p ion coe icien R = 3, also used in hei wo k, his causes a di e ence in dis ance modulus o 0.3, which accoun s o he disc epancy. Fig. 7. Densi y con ou s o membe s a s in he ield o NGC 6756 shown in Fig. 1(d). (a) All membe s, (b) F^18. (c)F^16.5. hem as B- ype membe s in he compu a ion o he colo excess o he clus e . Second, in none o he published s ud- ies on he clus e s Basel 1, NGC 6704, and NGC 6706 is he e a men ion o he membe ship o gian s a s. In hese h ee clus e s, we sugges some gian s a s as membe s, on he basis o hei loca ion in all CM diag ams. In Fig. 5, showing he de eddened CM diag ams, we obse e he ac- cep able loca ion o hese s a s, in spi e o he ough es i- ma es o hei colo excesses. As has been commen ed abo e, wi h he ages ob ained o hese clus e s, hey a e expec ed o ha e an incipien ed gian b anch. The spa ial dis ibu ion o membe s has been only ana- lyzed o NGC 6756, he a hes clus e in ou sample, whe e he obse ed ield is expec ed o comp ise mo e com- ple ely he clus e dimensions. Some signs o mass seg ega- ion can be obse ed in ha case. In Fig. 7 we show densi y con ou s, o membe s a s in h ee V magni ude anges: All s a s, U^18, and U^16.5. This plo s e eal s onge con- cen a ion o b igh e membe s. 3.3.3 Compa ison wi h published ages and dis ances In Table 4 we ha e also included he a ailable published alues o he clus e pa ame e s. As can be seen, ou es i- ma es o he ages a e e y simila , and la ge han 10 8 yea s o all h ee clus e s Basel 1, NGC 6704, and NGC 6756. In he ca alog o Lyngà ( e e ence 0 in Table 4), hese objec s ha e quo ed ages sensibly below his igu e, and in pa icula o NGC 6756, he exis ing es ima es o dis ance a e sensi- bly di e en om ou esul s, he di e ence being abo e he 4. SUMMARY AND CONCLUSIONS We ha e ob ained imp o ed measu emen s o colo ex- cess, dis ance and age o ou galac ic clus e s. The new es ima es di e om p e iously published alues in some issues. We poin ou he ages ob ained o he h ee clus e s Basel 1, NGC 6704, and NGC 6756, which u n ou o be olde han p e iously assumed, in pa icula o he wo i s ones. As o NGC 6756, i s dis ance is dis inc ly la ge han he alues es ima ed by o he au ho s. We indeed conside ou esul s as an imp o emen , as hey a e based on i s o bo h e e ence lines, and upda ed model isoch ones on a wide ange o spec al ypes om B o F. The membe ship o gian s a s is sugges ed in he h ee clus e s Basel 1, NGC 6704, and NGC 6756, om hei lo- ca ions in h ee colo magni ude diag ams. In his las clus- e , signs o mass seg ega ion a e de ec ed, in he sense o he mos massi e membe s being mo e concen a ed owa ds he clus e co e. We a e g a e ul o Eloy Rod iguez, o allowing us he use o he obse ing ime g an ed o him in he 1.23 m ele- scope a he CAHA. P i a e communica ions by R. F. Ga - ison, on spec al ypes, and J. C. Me milliod, who p o ided us wi h his Open Clus e s Da a Base, a e acknowledged. This wo k has been inancially suppo ed by he Spanish DGICYT, h ough g an s numbe s PB91-0053, PB94-0007, and by he Resea ch and Educa ion Council o he Au ono- mous Go e nmen o Andalucía (Spain). The obse a ions ha e been aken a he Ge man-Spanish Obse a o y o Cala Al o (Spain). © Ame ican As onomical Socie y • P o ided by he NASA As ophysics Da a Sys em 721 DELGADO ETAL: YOUNG REDDENED CLUSTERS 721 REFERENCES Al a o, E. L, & Delgado, A. J. 1991, A&A, 241, 69 Becke , W., & Fenka , R. 1971, A&AS, 4, 241 Cla e , A. 1995, A&AS, 109, 441 C aw o d, D. L. 1975, AJ, 80, 955 C aw o d, D. L. 1978, AJ, 83, 48 C aw o d, D. L. 1979, AJ, 84, 1858 C aw o d, D. L., & Ba nes, J. V. 1970, AJ, 75, 978 C aw o d, D. L„ & Mandwewala, N. 1976, PASP, 88, 917 C aw o d, D. L., & Pe y, 1976, PASP, 88, 454 Delgado, A. J., & Al a o, E. J. 1989, A&A, 219, 121 Delgado, A. J., Al a o, E. J., Ga cia Pelayo, J. M., & Ga ido, R. 1992, AJ, 103, 891 Fenka , R. P. 1965, Z A, 62, 90 Fo bes, D. 1981, PASP, 93, 441 Fo bes, D., & DuPuy, D. L. 1978, AJ, 83, 266 (F78) Fo bes, D., English, D., DeRobe is, M. M., & Dawson, P. C. 1992, AJ, 103, 916 (F92) G ubissich, Cl. 1965, Z A, 60, 249 Hildi ch, R. W„ Hill, G., & Ba nes, J. V. 1983, MNRAS, 204, 241 Jónch-Só ensen, H. 1993, A&A, 267, 54 J iinch-Só ensen, H., & Knude, J. 1990, A&A, 238, 75 Knude, J. 1992, A&AS, 92, 841 Lyngá, G. 1987, Ca alogue o Open Clus e Da a (Lund Obse a o y) Man oid, J., & S e ken, C. 1987, A&AS, 71, 539 Massey, P., Johnson, K. E., & DeGioia-Eas wood, K. 1995, ApJ, 454, 151 Olsen, E. H. 1983, A&AS, 54, 55 Penp ase, B. E. 1992, ApJS, 83, 273 Pé ez-Hemández, F., Cla e , A., & Belmon e, J. A. 1995, A&A, 295, 113 Phelps, R. L, & Janes, K. A. 1994, ApJS, 90, 31 San os, J. F. C, & Bica, E. 1993, MNRAS, 260, 915 Schus e , W. J., & Nissen, P. E. 1988, A&AS, 73, 225 Shobb ook, R. R. 1984, MNRAS, 211, 659 Slo opoulos, S 1965, Z A, 61, 105 Smi h, L. F., Meyne , G., & Me milliod, J. C. 1988, A&A, 287, 835 Tu ne , D. G. 1990, PASP, 102, 1331 Tu ne , D. G. 1994, RMxA&A, 29, 163 Wa en, W. H. 1976, MNRAS, 174, 111 Young, A. T. 1974, Me hods Exp. Phys., 12A, 123 © Ame ican As onomical Socie y • P o ided by he NASA As ophysics Da a Sys em