New Color-Magnitude Diagrams and Basic Parameters of the Open Cluster NGC 5749

ACTA ASTRONOMICA Vol. 42 (1992) pp. 343±353

Juan J. Clari aÂ 1 and E. Lapasset2

Observatorio Astronomico, Laprida 854, 5000 Cordoba, Argentina

Received August 7, 1992

ABSTRACT

New photoelectric UBV magnitudes and colors for 112 stars and DDO photometry of two red stars in the ®eld of the open cluster NGC 5749 are presented. Using photometric membership criteria only 30% of the observed stars appear to be cluster members, including one suspected variable star.

The cluster does not probably contain any red giant. The reddening is variable across the cluster,

(B V ) = ( ) the mean value being E 0 42 0 04 (s.d.). A distance of 1280 118 pc, an age of

7 27 10 yr and other basic cluster parameters are derived. Key words:

1. Introduction

= b = NGC 5749 (IAU designation C1445-543, l 319. 5, 4. 5) is a group of stars with little central concentration lying near the south-western edge of the Lupus constellation. This is a poorly populated open cluster with stars in a narrow range of brightness; hence Ruprecht (1966) described it as belonging to class IV1p. NGC 5749 lies in a region of the galactic plane where comparatively large color excessesat modest distances are not unusual (Neckel 1967, Moffat and Vogt 1973). Old distance determinations by Trumpler (1930), Shapley (1930), Collinder (1931), and Barkhatova (1950) place the cluster as lying between 0.9 and 2.7 kpc from the

Sun. The most recent reference is from Lynga (1964a), who derived a color excess

(B V ) = E 0 35 and a distance of 0.9 kpc. The latter results, however, are rather uncertain since they were derived from UBV photometry of only 15 relatively bright stars in the cluster ®eld (Lynga 1964b).

1 VisitingastronomeratCerroTololoInter-AmericanObservatorysupportedbytheNationalScience Foundation under contract No.AST 74-04128. 2 Visiting astronomer, University of Toronto (David Dunlap Observatory) 24-inch telescope at Las Campanas.

344 A. A.

= In this study new photoelectric UBV data of 112 stars brighter than V 14 4, located within about15 0 from the assumedclustercenter, are presented. In addition, DDO photometry of two comparatively bright red stars in the cluster ®eld is also given. These data not only allow the construction of well-de®ned color-magnitude (CM) and color-color (CC) diagrams, but also permit a more accurate determination of the basic cluster parameters, particularly its distance.

2. UBV Photoelectric Observations

The UBV measurements were carried out during three observing runs between 1985 and 1987 with the 91 cm telescope of the Cerro Tololo Inter-American Obser- vatory (CTIO) and the 60 cm Canadian telescope of the David Dunlap Observatory located in Las Campanas Observatory (LCO). Dry-ice cooled RCA 31034 and 1P21 photomultipliers in single channel photometers with pulse-counting electron- ics were used at CTIO and LCO, respectively. About 12±15 E-region standards (Cousins 1973, 1974) were observed each night to insure accurate transformations

to the standard system. No evidence of systematic differences either in the V magnitude or in the colors among the stars measured in CTIO and LCO was detected. Consequently, mean UBV values were computed and listed in Table 1.

Table1 UBV photometry for stars in the ®eld of NGC 5749

Star HD/CPD/CD V B-V U-B n Remarks

1 11.895 0.460 0.177 1 nm 2 12.346 0.750 0.172 1 nm 3 13.011 0.509 0.123 2 nm 4 13.132 0.553 0.413 2 pm

5 a 13.394 0.551 0.293 2 pm 6 13.643 0.511 0.351 1 m 7 130348 9.720 1.025 0.777 2 G8III/IV,nm 8 12.564 0.468 0.284 2 pm 9 12.019 0.386 0.184 2 m 10 13.160 0.712 0.256 2 nm 11 13.198 0.599 0.421 2 nm 12 13.329 0.453 0.335 2 m 13 12.336 0.375 0.105 3 m 14 13.847 0.498 0.330 2 m 15 12.142 0.320 -0.006 3 m 16 13.714 0.622 0.098 3 nm 17 12.144 0.490 0.304 2 nm 18 13.915 0.731 0.137 2 nm 19 13.282 0.620 0.044 3 nm 20 14.015 0.692 0.080 2 nm Vol. 42 345

Table1 continued

Star HD/CPD/CD V B-V U-B n Remarks

21 14.159 0.798 0.200 2 nm

22 -54 6170 11.273 0.266 -0.276 3 m 23 -54 6169 10.852 0.260 -0.206 3 m 24 13.314 0.602 0.211 3 nm

25 -53 6114 11.143 0.308 -0.113 2 m 26 -53 6117 10.891 0.216 -0.268 2 m 27 12.535 0.329 0.195 3 m 28 11.941 1.428 1.292 2 nm 29 -53 6112 10.285 1.260 1.325 2 nm 30 12.304 1.345 1.091 1 nm 31 11.644 2.037 2.363 2 nm

32 b 13.373 0.634 0.024 2 nm 33 13.043 0.421 0.200 2 m 34 13.031 0.361 0.240 4 m 35 14.184 0.667 0.080 1 nm 36 12.763 0.430 0.250 3 m 37 13.643 0.637 0.097 3 nm 38 -53 6113 11.594 0.259 -0.060 4 m 39 130244 10.398 0.112 0.059 4 A0V,nm

40 -53 6119 11.496 0.256 -0.035 4 m 41 13.284 0.455 0.291 1 m 42 13.592 0.681 0.095 2 nm 43 13.945 0.536 0.338 1 m 44 13.134 0.573 0.226 1 nm 45 13.208 0.643 0.352 1 nm 46 11.873 1.289 0.988 3 nm 47 13.710 0.683 0.130 2 nm 48 14.336 0.713 0.093 2 nm 49 12.179 1.454 1.299 2 nm 50 c 12.566 0.510 0.027 2 nm 51 13.690 0.778 0.257 3 nm 52 13.652 0.660 0.168 1 nm 53 13.629 0.750 0.339 2 nm

54 b 12.767 0.407 0.210 1 m 55 12.444 0.658 0.053 2 nm 56 11.660 0.278 0.129 2 m 57 10.924 1.328 1.200 1 nm

58 12.444 0.658 0.053 2 nm 59 d -53 6110 10.701 0.313 0.166 2 nm 60 11.776 0.928 0.527 1 nm 61 11.390 0.438 0.129 2 nm 62 12.143 0.455 0.082 2 nm 63 12.840 0.415 0.263 2 m 64 13.445 0.613 0.341 2 pm 65 12.429 1.381 1.156 2 nm 66 -53 5709 11.162 1.015 0.646 4 nm 67 12.042 0.647 0.184 3 nm 68 12.844 0.574 0.083 3 nm 69 11.896 0.377 0.285 2 pm

70 -53 6105 10.171 0.170 -0.478 4 m

71 d 13.018 0.700 0.318 2 nm 346 A. A.

Table1 concluded.

Star HD/CPD/CD V B-V U-B n Remarks

72 130152 9.676 0.167 -0.332 4 B6Ib-II,m 73 13.919 0.894 0.462 1 nm 74 e 13.016 0.417 0.252 3 m 75 12.390 0.352 0.074 2 m 76 14.082 0.642 0.199 1 nm 77 -54 6168 10.861 0.230 -0.259 3 m 78 13.949 0.666 0.250 2 nm 79 12.558 0.289 0.130 3 m 80 12.960 0.380 0.213 3 m 81 11.776 0.483 0.020 2 nm 82 11.843 0.502 0.160 3 nm 83 13.079 0.709 0.265 1 nm 84 13.506 0.623 0.078 2 nm 85 13.908 0.643 0.053 2 nm 86 12.521 0.550 0.012 1 nm 87 13.070 1.125 0.928 2 nm 88 12.738 0.496 0.196 2 nm 89 13.183 1.138 1.036 1 nm 90 -54 6165 10.937 0.619 0.074 1 nm

91 f 12.569 1.457 1.684 2 nm 92 12.961 0.382 0.201 3 m 93 11.971 1.273 0.947 2 nm

94 d 11.815 1.826 2.178 1 nm 95 13.729 0.486 0.090 2 nm 96 12.659 0.683 0.126 2 nm 97 13.506 0.577 0.051 3 nm 98 13.447 0.828 0.216 2 nm 99 13.522 0.722 0.152 3 nm 100 13.423 0.483 0.319 3 m 101 13.601 0.649 0.038 2 nm 102 10.701 1.173 0.968 3 nm 103 13.448 0.602 0.090 1 nm 104 11.874 0.352 0.136 1 m 105 12.268 0.733 0.248 2 nm 106 12.772 0.444 0.250 1 m 107 11.916 0.246 0.004 2 m 108 13.098 0.496 0.002 1 nm 109 12.284 0.235 -0.273 1 nm 110 10.017 2.023 1.606 1 nm 111 -53 6101 9.575 1.089 0.963 2 nm 112 129793 8.500 0.112 0.108 2 A0IV,nm a: Variable star No. 82 of Lapasset et al. (1991).

b: Double star, only A component was measured.

V = c: Double star, 1 6. Only A component was measured. d: Double star; photometry refers to the combined light of both components. e: Variable star No. 64 of Lapasset et al. (1991). f: Variable star No. 40 of Lapasset et al. (1991).

Column (6) of this Table lists the number of nights on which each star was observed

in both observatories. The star numbers are identi®ed in Fig. 1. The mean standard

deviations of a single observation are, respectively, 015, 011, and 019 in

(B V ) (U B ) V V , , and , independently of the magnitude. Vol. 42 347

Fig. 1. Finding chart for stars in the open cluster NGC 5749 (C1445-543) enlarged from that of Hogg (1965).

A comparison of the present photometry with that of Lynga (1964b) shows

close agreement. In fact, the mean differences (Lynga minus present study) from

V = (B V ) =

15 stars in common are: 0 03 0 03, 0 01 0 02, and

U B ) = ( 0 02 0 04. Stars 5, 74, and 91 have been previously reported as suspected variables (Lapasset et al. 1991), their variations being greater than 0.1 mag.

3. The UBV Diagrams: Cluster Membership

Figs. 2 and 3 show the observed CM and CC diagrams constructed from the photoelectric results of Table 1. Well-de®ned sequences are seen in both diagrams, which demonstrates that NGC 5749 is really an open cluster. The criteria here adopted for discriminating between cluster members (m), probable members (pm), and ®eld stars (nm) are those described in previous papers (see, e.g., ClariaÂ and 348 A. A.

Fig. 2. Observed color-magnitude diagrams for stars in the ®eld of NGC 5749. Cluster members, prob-

able members, and ®eld stars are represented by ®lled circles, triangles, and open circles, respectively.

(B V ) = E (U B ) =

The ZAMS of Schmidt-Kaler (1982) has been adjusted to E 0 42, 0 29,

V M = and V 11 80 .

Lapasset 1986, ClariaÂ et al. 1991). Column (7) of Table 1 indicates the individual membership status assigned to the observed stars. MK spectral types available from Houk and Cowley (1975) for only four stars are listed in the same column. We ®nd that scarcely 30% of the stars observed exhibit a high probability of being cluster members. The 33 objects satisfying the above photometric membership criteria are represented by ®lled circles in the UBV diagrams, while 5 probable members and 74 ®eld stars are shown as triangles and open circles, respectively.

It is seen that the cluster main sequence consists essentially of B and A-type

= stars and extends from V 9 7 to about 14.0. Three stars (7, 29, and 111) could be red giant members according to their positions in Fig. 2. Star 29 has

a radial velocity of 39 km/s, while star 7 ± identical to star a from Lynga (1964b) ± is a long period, low amplitude binary with a mean radial velocity close

to +7 km/s (Mermilliod 1992). Both stars were observed in the DDO system

( ) = C ( ) =

at CTIO yielding: C 45 48 1 169 0 006, 42 45 0 875 0 030,

( ) =

and C 41 42 0 105 0 026 for star 7, and 1 292 0 014, 1 070 0 028,

and 0359 0 042 for star 29. Application of the iterative method described by

(B V ) = Janes (1977) to derive reddening leads to E 0 06 and 0.0 for stars 7 and 29, respectively. These values are consistent with the positions of both stars in the CC diagram. Therefore, they are very likely foreground ®eld stars. Star 111, althoughnotobservedintheDDOsystem,alsoseemstobeapractically unreddened star in the CC diagram. We conclude that NGC 5749 does not probably contain any red giant. Vol. 42 349

Fig. 3. Color-color diagram for stars observed in the ®eld of NGC 5749. The solid lines are the intrinsic color-color lines of Schmidt-Kaler (1982) for classes III and V stars. Symbols are as in Fig. 2.

4. Cluster Basic Parameters

The CC diagram of Fig. 3 suggests possible variable reddening among the clus-

ter members since the spread of the observed values largely exceeds the quoted er-

B V ) (U B ) (B V ) rors for the ( and colors. Cluster members with 0 40 were corrected individually for reddening using the algorithm of Garcia et al. (1988). The results are shown in Table 2. The mean color excesses and standard

deviations from 21 members of Table 2 are:

E (B V )i =

h 0 42 0 04 (s.d.)

E (U B )i = h 0 29 0 03 (s.d.)

As seen in Table 2, the full width of the observed CC diagram for stars earlier

E (B V ) = E (B V ) E (B V ) = min than about A0V is max 0 13 mag, 350 A. A. which exceeds the lower limit of 0.11 mag estimated by Burki (1975) for clusters

with differential reddening. We conclude that the reddening across NGC 5749 is

A = E (B V )

variable, the mean total visual absorption v 3 0 in front of it being 1.26 mag.

Table2

B V ) Color excesses derived for cluster members of NGC 5749 with ( 0 40

Star HD/CPD E(B-V) E(U-B)

9 0.47 0.33 13 0.48 0.33 15 0.44 0.31 22 -5406170 0.45 0.31 23 -5406164 0.42 0.30 25 -5306114 0.46 0.32 26 -5306117 0.39 0.27 27 0.40 0.28 34 0.43 0.30 38 -5306113 0.38 0.27 40 -5306119 0.37 0.26 56 0.36 0.25 70 -5306105 0.39 0.27 72 130152 0.35 0.24 75 0.46 0.32 77 -5406168 0.40 0.28 79 0.37 0.26 80 0.46 0.32 92 0.46 0.32 104 0.44 0.31 107 0.35 0.25

Fig. 4 shows two CM diagrams of NGC 5749 after correction for absorption

[= V E (B V )]

and interstellar reddening. We note that the intrinsic V0 3 ,

B V ) (U B ) B

( 0 , and 0 parameters for the -type members, i.e., , those with

B V ) ( 0 40, were determined from individual reddening determinations,while those of the remaining members and probable members were computed adopting

the above mean values for the cluster.

(U B ) The observed scatter in Fig. 4, particularly in the V0 vs. 0 diagram, appears to be larger than expected even allowing for differential reddening effects in the cluster ®eld. As it is well known, duplicity among memberstarsin a clusteris one of the reasons, and probably the principal one, which causes scatter in the UBV diagrams. Another source of scatter comes from stellar rotations of individual cluster members. Theory suggests (e.g.,, Maeder and Peytremann 1970) and Vol. 42 351

Fig. 4. The unreddened color-magnitude diagrams of NGC 5749. Stars individually corrected for differential reddening are represented by circles, whereas crosses denote stars corrected for reddening by using the mean cluster values. Empirical isochrones de®ned by Mermilliod (1981)

for the NGC 3766 and IC 4665 age groups are plotted using the adopted distance modulus of

V M = 0 V 10 54.

observations of several clusters indicate that the dispersion of the main sequence

B V ) (U B )

by stellar rotation is at most 0.2 mag for a ®xed ( or value.

(U B ) Looking at the CM diagrams, in particular the V0 vs. 0 diagram of Fig. 4, we might suspect that a sequence of binaries does probably exist in NGC 5749. If we call binaries those stars falling 0.50 mag above the cluster main sequence, then a lower limit to the binary frequency of about 27 per cent is found. A ®t of the zero-age main sequence (ZAMS) of Schmidt±Kaler (1982) to the

cluster sequencein the two CM diagrams of Fig. 4 leads to a true distance modulus

V M =

0 v 10 54, corresponding to a distance of 1280 pc from the Sun and 100 pc

(U B ) above the galactic plane. Due to the observed scatter in the V0 vs. 0

diagram, we have assigned more weight to the distance derived from the V0 vs.

B V ) ( 0 diagram. The uncertainty in the ®tting procedure is estimated to be about 0.20 mag, which implies an error of less than 10% in the adopted distance. Our distance appears to be somewhat larger than that of Lynga (1964a) ± based on a much smaller sample of stars ± but considerably smaller than some earlier estimates (e.g.,Collinder 1931, 2.7 kpc). A more extensive photometric (or CCD) study of this cluster would be desirable for a better de®nition of the lower main sequence. 352 A. A.

If we assume spherical symmetry and adopt for NGC 5749 an angular diameter of 12 0 (see Fig. 1), a linear diameter of 4.5 pc results, so that the cluster stellar

density turns out to be about 0.7 stars per cubic parsec.

B V ) (U B )

The bluest ( 0 and 0 color indices on the cluster main sequence

are, respectively, 0 22 and 0 75, placing NGC 5749 in an intermediate group

betweentheNGC3766andIC4665agegroupsofMermilliod(1981).The empirical

V M = isochrones of Mermilliod for these two age groups, adjusted for 0 v 10 54,

7 are shownin Fig. 4. We have®nally adoptedanage of 2 7 10 yrfor NGC 5749. Table 3 summarizes the results obtained in the present study. We note that the total mass listed in the table was computed from the mass-luminosity relations given by Schmidt±Kaler (1982) for main sequence stars.

Table3 Summary of results obtained for NGC 5749

Position 0

h m

= =

14 45 54 19 (1950.0)

0 0

l = = 319 30 b 4 30 (1950.0)

Distance

V M =

V : apparent distance modulus 11 80

(B V ) =

E : selective absorption 0 42 0 04 (sd)

V M =

0 V : true distance modulus 10 54

= ( )

d : distance from the Sun 1280 118 pc = z : distance from the galactic plane 100 pc

Dimensions

0 =

D : estimated angular diameter 12

d : linear diameter = 4 5 pc

Membership

(m) =

N : number of members 33

(pm) =

N : number of probable members 5

(r g ) =

N : number of red giants 0

(v ) = N : number of variables 1

Main sequence structure

B V ) (B V ) =

( 0 : bluest 0 color index 0 22

U B ) (U B ) = ( 0 : bluest 0 color index 0 75

MK : earliest spectral type = B3

= : age 2 7 10 yr

Integrated properties

V0 : apparent visual magnitude 6 6

B V ) (B V ) =

( 0 : intrinsic color 0 0 13

T T

U B ) (U B ) =

( 0 : intrinsic color 0 0 42

M =

V : absolute magnitude 3 9

m = M

: total mass 116

= M

: mean space density 2 4 /pc Vol. 42 353

Acknowledgements. We thank the staff and night assistants of CTIO for their kind hospitality and Dr. Robert F. Garrison for permission to use the facili- ties of the University of Toronto at the Las Campanas Observatory. We wish to thank J.C. Mermilliod for very generously providing unpublished radial velocity information. Thanks are also due to J. Laborde for the preparation of the diagrams. This work was partially supported by the Argentinian institutions CONICET and CONICOR.

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