CCD photometric search for peculiar stars in open clusters

Transcription

1 A&A 385, (2002) DOI:.51/ : c ESO 2002 Astronomy & Astrophysics CCD photometric serch for peculir strs in open clusters III. NGC 2439, NGC 3960, NGC 64, NGC 6192 nd NGC 6451 E. Punzen 1,2 nd H. M. Mitzen 1 1 Institut für Astronomie der Universität Wien, Türkenschnzstr., 80 Wien, Austri 2 Zentrler Informtikdienst der Universität Wien, Universitätsstr. 7, Wien, Austri Received 9 October 2001 / Accepted Jnury 2002 Abstrct. In totl, more thn 550 objects within the fields of five open clusters were investigted in order to find clssicl chemiclly peculir strs. One bon-fide CP cndidte ws detected in ech of the open clusters NGC 3960, NGC 6192 nd NGC All three objects seem to be members of the corresponding cluster, tking the results from our photometry s well s from the literture. For NGC 6192 nd NGC 6451 we found widely different interstellr reddening vlues nd therefore distnces nd ges in the literture. From n nlysis of published Johnson UB RI photometry, we were ble to rule out rther old ge for NGC This is lso supported by the finding of one peculir object in this cluster with spectrl type of bout A2. Five vrible objects within the field of NGC 64 were detected. The vribility exceeds times (or typiclly 0.03 mg) the stndrd devition of the weighted individul mesurements. But since our observtions were obtined mostly within one hour (typicl integrtion times of five minutes per filter) on severl nights within 45 dys, no light curves could be derived. For this cluster, some Am cndidtes were reported for which we were not ble to detect significnt positive -vlue. Key words. strs: chemiclly peculir strs: erly-type techniques: photometric open clusters nd ssocitions: generl 1. Introduction Five open clusters of our Milky Wy were investigted in order to find chemiclly peculir (CP) objects using CCD -photometry. Two previous ppers (Byer et l. 2000; Punzen & Mitzen 2001) hve lredy shown the high efficiency of this system. The trget clusters (NGC 2439, NGC 3960, NGC 64, NGC 6192 nd NGC 6451) hve distnces from 00 to 4200 pc from the Sun nd hve ges from 0.02 to 0.8 Gyr. Two clusters (NGC 2439 nd NGC 3960) re only poorly investigted wheres the other three objects re well studied, fct tht mkes clibrtion nd comprison of our photometry possible. For ll clusters, Johnson UB photometry is vilble from the literture. In ddition, Strömgren uvbyβ mesurements were published for NGC 64. This cluster ws lso investigted for δ Scuti type pulstors by Kjeldsen & Frndsen (1989). Send offprint requests to: E. Punzen, e-mil: Ernst.Punzen@univie.c.t Bsed on observtions t ESO-L Sill nd UTSO-Ls Cmpns. We discuss the widely different reddening nd thus distnces nd ges found for NGC 6192 nd NGC 6451 in the literture. In totl, we report the detection of one bon-fide CP str in ech of the open clusters NGC 3960, NGC 6192 nd NGC Observtions, reduction nd results Observtions of the five open clusters were performed with the Bochum 61 cm (ESO-L Sill) s well s the Helen- Swyer-Hogg 61 cm telescope (UTSO-Ls Cmpns Observtory) in The instrumenttion nd filters used re described in Byer et l. (2000). The observing log is listed in Tble 1. In totl, 9 frmes for ll five clusters in three filters were observed nd used for the further nlysis. The bsic reductions (bis-subtrction, drkcorrection, flt-fielding) were crried out within stndrd IRAF routines. For ll frmes we hve pplied pointspred-function-fitting within the IRAF tsk DAOPHOT (Stetson 1987). Photometry of ech frme ws performed seprtely nd the mesurements were then verged nd weighted by their individul photometric error. Article published by EDP Sciences nd vilble t or

2 868 E. Punzen nd H. M. Mitzen: CCD photometric serch for peculir strs in open clusters. III Y [pixel] NGC 2439 NGC 3960 Y [pixel] NGC 64 NGC X [pixel] Y [pixel] NGC X [pixel] Fig. 1. Finding chrts for the progrm clusters. North is to the right nd west is upwrds; 1 pixel = 0.5.Thesizes(byre) of the open circles re inversely proportionl to the -mgnitudes tken from Tbles 4 to 8 in the sense tht lrger open circles denote brighter objects.

3 E. Punzen nd H. M. Mitzen: CCD photometric serch for peculir strs in open clusters. III NGC NGC NGC = = = NGC 6451 NGC = = Fig. 2. versus y) nd versus y) for our progrm clusters. Filled circles indicte pprent peculir strs wheres open circles re non peculir objects. The solid line is the normlity line wheres the dotted lines re the confidence intervls corresponding to 99.9%. The error brs for ech individul objects re the men errors. The mesurement errors of re much smller thn the symbols nd hve been omitted.

4 870 E. Punzen nd H. M. Mitzen: CCD photometric serch for peculir strs in open clusters. III. Tble 1. Observing log. Cluster Site Nights # g1 # g2 # y NGC 2439 ESO NGC 3960 ESO NGC 64 ESO UTSO NGC 6192 ESO 5 UTSO NGC 6451 ESO The normlity lines were independently derived for y), (B )nd(b y), respectively. As in the other ppers of this series, we did not trnsform the y) vlues into stndrd (B )or(b y) ones. Only for the bon-fide CP str in NGC 6451, trnsformtion s described in Punzen & Mitzen (2001) ws pplied to hve n estimte of its spectrl type. The wy of clculting the normlity line, deriving the errors nd clibrtion of our y mesurements, re the sme s in Punzen & Mitzen (2001). Tble 2 lists our observed open clusters nd their chrcteristics from the literture (s listed in the corresponding sections). Furthermore, the number of observed strs, the number of individul frmes (Tble 1), the regression coefficients for ll trnsformtions nd normlity lines re listed. In ddition, the described peculir objects with their -vlues nd (B ) 0 vlues s well s the 3σ detection limit re given. The finding chrts of our open clusters re shown in Fig. 1. The size of the symbols (by re) is inversely proportionl to the pprent visul mgnitudes of the objects in the sense tht lrger symbols denote brighter objects. The complete tbles with ll dt for the individul cluster strs re vilble both from SIMBAD (vi nonymous ftp) or upon request from the first uthor. These tbles include the cross identifiction of objects from the literture, the observed y) nd vlues with their corresponding errors, mgnitudes, the (B ) s well s (b y) vlues from the literture nd -vlues derived from the normlity lines of y), (B )nd(b y), respectively. Tble 3 lists the photometriclly peculir strs. The individul objects re discussed in more detil in the following subsections. The versus y)nd versus y) digrms for ll five open clusters re shown in Fig. 2. Furthermore, the normlity lines nd the confidence intervls corresponding to 99.9% re plotted. Filled circles indicte the peculir objects NGC 2439 This distnt young open cluster hs been poorly investigted now. White (1975) presented photoelectric nd photogrphic Johnson UB mesurements of 183 objects wheres Rmsy & Pollcco (1992) nlyzed CCD UBI c colors of 0 strs within the field of NGC The results of both references gree firly well, yielding reddening of E(B ) = 0.37 mg, distnce of 3.98 < d < 4.45 kpc from the Sun nd very young ge (White 1975 lists 20 Myr wheres Rmsy & Pollcco 1992 give only n upper limit of 300 Myr). We find well-defined min sequence (Fig. 2) in excellent greement with White (1975; see Fig. 3 therein). There re probble non-members present which re clerly distinct within the versus y) digrm. These objects coincide with the list of non-members given in Rmsy & Pollcco (1992). Although there re minly cluster strs within the temperture rnge of the clssicl CP strs (B5 to F0) nd the detection limit is quite low (0.0 mg), no photometriclly peculir object ws detected NGC 3960 An extensive investigtion of NGC 3960 ws done by Jnes (1981) who obtined photoelectric Johnson B nd DDO dt. The ltter llowed to derive metllicity of [Fe/H] = 0.30(6) dex which is significnt sub-solr. He estimted n ge between 0.5 nd 1 Gyr (older thn the Hydes but younger thn NGC 752) nd reddening of E(B )= 0.29 mg. All of our observed objects of this open cluster re lso included in the work of Jnes (1981). He defined n inner region (of rdius 2. 75) nd n outer ring concluding tht ll strs finter thn th mgnitude re probble non-members. We find one photometric CP str (No. 2; No. 1 ccording to Jnes 1981) mong smple of 93 objects observed. This object exhibits very lrge -index ( mg). It lies well within the inner region ( =.22 mg) nd is very probbly true member of this cluster. Its dereddened color (B ) 0 =+0.19 mg suggests spectrl type of bout A6 (Schmidt-Kler 1982). Another object (No. 89; No. 5 ccording to Jnes 1981) hs very lrge -index ( mg) by its (B ) color (bsed on only one mesurement) wheres our y) index yields rther norml one ( mg). According to our photometry (bsed on eight individul y) mesurements) this coolest object within the cluster re (see Fig. 2) is very probbly non-member NGC 64 Kjeldsen & Frndsen (1989) nd Frndsen et l. (1996) performed n extensive serch for δ Scuti type pulstors in the field of NGC 64. They found t lest six vribles mking this cluster n interesting subject for further studies. An overview of ll results cn be found in Bruntt et l. (1999) who lso presented severl bon-fide Am cndidtes nd Strömgren uvbyβ photometry. This rther old cluster with n ge of bout 690 Myr shows well defined turn-off point, severl gints nd

5 E. Punzen nd H. M. Mitzen: CCD photometric serch for peculir strs in open clusters. III. 871 Tble 2. Summry of results. In prenthesis re the errors in the finl digits of the corresponding quntity. Nme NGC 2439 NGC 3960 NGC 64 NGC 6192 NGC 6451 C C C C C l/b 246/ 4 294/+6 335/ 0 341/+2 360/ 2 E(B ) d [pc] log t Tr-type II3r I2m II3m I2r I2r n(obj) /(B ) = + b (y) 8.21(8) 1.000(6) 8.28() 1.009(7) 8.(7) 1.000(6) 8.(8) 0.997(3) 8.27() 1.001(7) = + b y) 0.869(4) 0.375(7) 0.804(4) 0.274() 0.809(2) 0.279(6) 6(2) 0.251(5) 0.798(2) 0.343(6) = + b (b y) 0.618(3) 0.5(5) = + b (B ) 0.619(2) 0.6(3) 0.645(5) 0.086(6) 0.603(2) 0.4(3) 0.560(6) 0.6(7) 3 σ [mg] n(frmes) blue-strgglers (Fig. 2). The turn-off color is t bout y) mg which corresponds to (b y) mg or spectrl type of bout A8. Only two or three non-members re indicted from the colormgnitude digrm. No peculir object ws found bsed on very low detection limit (0.009 mg). Our field of view includes the following bon-fide Am cndidtes: No. 598 (in the nottion of Bruntt et l. 1999), 619, 832, 919, 23; nd vrible strs from the literture: No. 66 (number ccording to Lindoff 1972), 647, 679, 853 nd 906. None of these objects show ny peculir -vlue in ccordnce with Mitzen (1976) nd ogt et l. (1998). We found lso no cler evidence for vribility of the known vrible strs. This is due to the very smll mplitudes (below 0.0 mg; Frndsen et l. 1996) nd the uneven distribution of our mesurements. However, from our dt set we re ble to conclude tht the objects No. 3 (number ccording to Lindoff 1972), 735, 46, nd 54 re definitely not constnt on level which exceeds more thn σ of the rms error or typiclly 0.03 mg. Becuse of the rther high mplitude, these objects were not included in Fig. 2. Due to the distribution of the observtions, no conclusion bout the nture of vribility cn be drwn. In the relevnt temperture rnge one might think of δ Scuti nd γ Dordus pulstion s well s vribility due to eclipsing binries. Especilly interesting is the cse of No. 54 which is lso bon-fide Am cndidte. Explntions of the detected vribility s pulstion (only few Am strs show such behviour) or s eclipses (Am strs tend to be found in spectroscopic binry systems) re both very interesting NGC 6192 A remrkble disgreement on the bsic properties for NGC 6192 ws found in the literture. Kilmbi & Fitzgerld (1983) nd King (1987) reported rther old ge (00 Myr) with reddening of E(B )=0.26 mg nd distnce of bout 00 pc from the Sun. These results re bsed on photogrphic (Kilmbi & Fitzgerld 1983) s well s CCD photometry (King 1987). Kjeldsen & Frndsen (1991) contrdicted these vlues nd list reddening of E(B )=0.68 mg, n ge of 89 Myr nd distnce of bout 00 pc from the Sun. They find very good greement with the individul mesurements of King (1987). We hve noticed tht the photogrphic mesurements of Kilmbi & Fitzgerld (1983) only poorly correlte with the CCD ones of King (1987) nd Kjeldsen & Frndsen (1991) who used (B ) 0 versus (U B) 0 digrm wheres King (1987) fitted isochrones to versus (B ) in order to derive the reddening. The Q-method (Goly 1974) for the hotter strs does strongly support the findings of Kjeldsen & Frndsen (1991). We find No. 50 (No. 54 ccording to King 1987; No. 86, Kjeldsen & Frndsen 1991) s being peculir with = mg. According to ll references nd our colormgnitude digrm, this object is member of NGC Following the mesurements nd results from Kjeldsen & Frndsen (1991) we find (B ) 0 = 0.06 mg nd n bsolute mgnitude of M = +0.0(2) mg. This is typicl for B8 type, luminosity clss str suggesting tht this object might be lte B-type Si str. Note tht the vlues of King (1987) would result in (B ) 0 = mg which is typicl for F5 type str nd therefore much too cool for clssicl CP str NGC 6451 This represents puzzling cse of widely different reddening vlues nd therefore ges nd distnces found in the literture. Svolopoulos (1966) lists E(B )=0.08 mg wheres Kjeldsen & Frndsen (1991) give E(B )= 0.70 mg s best vlue. This results in difference of 560 pc versus 20 pc nd 5 Gyr versus 0.2 Gyr, respectively. Kjeldsen & Frndsen (1991) clim tht due to the well populted color-mgnitude digrm we find it difficult to understnd the lrge discrepncy. These re the only two references found in the literture which included n independent estimtion of the reddening.

6 872 E. Punzen nd H. M. Mitzen: CCD photometric serch for peculir strs in open clusters. III. Tble 3. Strs of the investigted open clusters with peculir -vlues. Cluster No 1 No 2 X Y σ() y) σ y) (B ) NGC NGC NGC Col. 1: Cluster nme. Col. 2: Nottion sorted fter X nd Y, respectively (Fig. 1). Col. 3: Nottion ccording to Jnes (1981; NGC 3960), Kjeldsen & Frndsen (1991; NGC 6192). Col. 4, 5: X nd Y coordintes in the finding chrts (Fig. 1). Col. 6: Johnson mgnitude. Cols.7,8:Men-index nd its stndrd devition. Cols. 9, : Men y) vlue nd its stndrd devition. Col. : Devition from cluster line = + b y) using the corresponding constnts s listed in Tble 2. Col. : (B ) from the literture. Col. : Devition from cluster line = + b (B ) using the corresponding constnts s listed in Tble 2. Pitti et l. (1998; nd other references which cn be found therein) used the vlues of Svolopoulos (1966) to derive M versus ( I) 0 digrm for NGC 6451 together with nine other open clusters. Before we try to disentngle these discrepncies, n overview of the results from the -photometry is presented. Figure 2 shows well defined min sequence with bout 25 probble non-members. Two strs devite significntly from the normlity line: No. 30 nd 34. The ltter seems to be bright foreground str with =. wheres No. 30 seems to be true member of NGC None of these objects ws mesured by Kjeldsen & Frndsen (1991) nd Pitti et l. (1998). Depending on the reddening we get (B ) 0 = mg or mg, respectively. These vlues correspond either to spectrl type of A2 or G5. The ltter is clerly too cool for clssicl CP str. We re not ble to decide from our photometry which reddening vlue fits best ll photometric dt from our work nd the literture. However, the cse of NGC 6451 hs initilized study determining the reddening nd ges using -photometry (minly the or y versus g 1 y digrm) in collbortion with A. Clret. We hve renlyzed the dt from Kjeldsen & Frndsen (1991) nd Pitti et l. (1998) since both re bsed on CCD mesurements. The conclusions of Svolopoulos (1966) nd Lyngå (1987) re bsed on estimtes from photogrphic pltes which do not tke into ccount ny effects by the crowding of the field nd result normlly in much lrger errors thn CCD photometry. Note tht Kjeldsen & Frndsen (1991) reported problems in clibrting their U nd B vlues to stndrd ones becuse the combintion of the chrcteristics for the used filter nd the CCD response function ws fr from the originl ones. We hve lso detected n pprent inconsistency of the mgnitudes from both references. The vlues of Pitti et l. (1998) re on the verge one mgnitude brighter thn those of Kjeldsen & Frndsen (1991). For n independent check, the RGU mesurements from Becker et l. (1978) for NGC 6451 were used. To trnsform these mgnitudes in Johnson ones, the clibrtions from Crwford & Mnder (1966) nd Buser (1978) were tken = G +0.05(U G) 0.82(G R) αe(g R) G = +0.63(B ) 0.01(U B)+αE(B ) with α (depending on the color) being in the rnge of 0.1 to 0.3 in the relevnt spectrl domin nd E(G R) = 1.39E(B ). These trnsformtions gurntee tht the influence of wrong mesurements either in the RGU or UB system will be minimized. Figure 3 shows the comprison of the dt from ll three references. We re definitely ble to conclude tht the vlues from Pitti et l. (1998) re too bright by bout one mgnitude. This finding is bsed on 0 nd 40 strs in common by Pitti et l. (1998) with Kjeldsen & Frndsen (1991) nd Becker et l. (1976), respectively. Another independent check vi the Guide-Str-Ctlogue gives the sme consistent result. From our nlysis of ll photometric dt we believe tht vlue of 0.70 mg for the reddening of NGC 6451 is probbly correct becuse of dditionl rguments: The Q vlues (Goly 1974) for the hottest member re between 0.6 nd +0.1 which re only comptible with spectrl types erlier thn A0. This result is independent of the reddening. According to Neckel et l. (1980), the interstellr bsorption (A ) towrds NGC 6451 is between 1.2 nd 1.9 mg kpc 1 close to the edge of region with 2.6 A < 3.3 mg kpc 1. Such vlues only support distnce for NGC 6451 which is lrger thn 2000 pc. Tking the newest isochrones from Lejeune & Scherer (2001) nd fitting the photometric dt from Pitti et l. (1998; the mesurements were corrected by one mgnitude) we get E(B )=0.60 mg, ( M )=.90 nd n ge of bout 0.4 Gyr. This is very close to the the results of Kjeldsen & Frndsen (1991) nd gives further confidence tht the peculir object detected by our -photometry hs spectrl type of bout A2.

7 E. Punzen nd H. M. Mitzen: CCD photometric serch for peculir strs in open clusters. III of five minutes) on severl nights within 45 dys, no conclusion bout the kind of vribility cn be mde. clib (Becker et l.) [mg] N = 40 Acknowledgements. We would like to thnk Drs. Frndsen, Kjeldsen, Pitti nd Sterken for stimulting discussions. This work benefitted from the finncil contributions of the City of ienn (Hochschuljubiläumsstiftung project: Wiener Zweiknlphotometer). EP cknowledges prtly support by the Fonds zur Förderung der wissenschftlichen Forschung, project P984. Use ws mde of the SIMBAD dtbse, operted t CDS, Strsbourg, Frnce nd the WEBDA dtbse, operted t the Institute of Astronomy of the University of Lusnne. (Kjeldsen & Frndsen) [mg] N = (Pitti et l.) [mg] Fig. 3. Comprison of the Johnson mgnitudes for members of NGC 6451 from Pitti et l. (1998) nd those listed by Kjeldsen & Frndsen (1991, lower pnel) nd the trnsformed ones from the RGU photometry by Becker et l. (1978, upper pnel). The stright line corresponds to zero offset. 3. Conclusions In totl, more thn 550 objects within the fields of five open clusters were investigted. Three bon-fide CP strs were detected ltogether in the open clusters NGC 3960, NGC 6192 nd NGC All three objects seem to be members of the corresponding cluster. For NGC 6192 nd NGC 6451 we found widely different reddening vlues nd therefore distnces nd ges in the literture. From n nlysis of published photometric dt, we were ble to rule out rther old ge for NGC This is lso supported by the finding of one photometric peculir str in this cluster with spectrl type of bout A2. Five vrible objects within the field of NGC 64 were detected. Since our observtions re unfvorbly distributed s they spn one hour (typicl integrtion times References Byer, C., Mitzen, H. M., Punzen, E., Rode-Punzen, M., & Sperl, M. 2000, A&AS, 7, 99 Becker, W., Svolopoulos, S. N., & Fng, Ch. 1976, Ktloge photogrphischer und photolektrischer Helligkeiten von 25 glktischen Sternhufen im RGU und UB-System, Astron. Inst. Univ. Bsel Bruntt, H., Frndsen, S., Kjeldsen, S., & Andersen, M. I. 1999, A&AS, 0, 5 Buser, R. 1978, A&A, 62, 425 Crwford, D. L., & Mnder, J. 1966, AJ, 71, 1 Frndsen, S., Blon, L. A., iskum, M., Koen, C., & Kjeldsen, H. 1996, A&A, 308, 2 Goly, M. 1974, Introduction to stronomicl photometry (D. Reidel Publishing Co., Dordrecht) Jnes, K. A. 1981, AJ, 86, Kilmbi, G. C., & Fitzgerld, M. P. 1983, Bull. Astr. Soc. Indi,, 226 King, D. J. 1987, Observtory, 7, 7 Kjeldsen, H., & Frndsen, S. 1989, The Messenger, 57, 48 Kjeldsen, H., & Frndsen, S. 1991, A&AS, 87, 9 Lejeune, T., & Scherer, D. 2001, A&A, 366, 538 Lindoff, U. 1972, A&AS, 7, 23 Lyngå, G. 1987, Ctlogue of Open Cluster Dt, 5th edition (CDS, Strsbourg) Mitzen, H. M. 1976, A&A, 51, 223 Neckel, Th., Klre, G., & Srcnder, M. 1980, A&AS, 42, 251 Punzen, E., & Mitzen, H. M. 2001, A&A, 373, 3 Pitti, A. E., Clriá, J. J., & Bic, E. 1998, ApJS, 6, 263 Rmsy, G., & Pollcco, D. L. 1992, A&AS, 94, 73 Schmidt-Kler, Th. 1982, In Lndolt-Börnstein New Ser., group I, vol. 2b, Stetson, P. B. 1987, PASP, 99, 191 Svolopoulos, S. N. 1966, Z. Astrophys., 64, 67 ogt, N., Kerschbum, F., Mitzen, H. M., & Fúndez-Abns, M. 1998, A&AS, 0, 455 White, S. D. M. 1975, ApJ, 197, 67