Membership, binarity and metallicity of red giants in the southern open cluster NGC 2354

Astronomy and Astrophysics Supplement Series, Jul 2018

We present new Coravel radial-velocity observations and photoelectric photometry in the UBV, DDO and Washington systems for a sample of red giant candidates in the field of the intermediate-age open cluster NGC 2354. Photometric membership probabilities show very good agreement with those obtained from Coravel radial velocities. The analysis of the photometric and kinematical data allow us to confirm cluster membership for 9 red giants, one of them being a spectroscopic binary, while 4 confirmed spectroscopic binaries appear to be probable members. We have also discovered 4 spectroscopic binaries not belonging to the cluster. A mean radial velocity of (33.40 ± 0.27) km s-1 and a mean reddening ± 0.03 were derived for the cluster giants. NGC 2354 has a mean ultraviolet excess = -0.03 ± 0.01, relative to the field K giants, and a mean new cyanogen anomaly ± 0.007, both implying [Fe/H] ≈ -0.3. The moderately metal-poor character of NGC 2354 is confirmed using five different metal abundance indicators of the Washington system. The cluster giant branch is formed by a well defined clump of 7 stars and 4 stars with high membership probabilities seem to define an ascending giant branch. The whole red giant locus cannot be reproduced by any theoretical track.

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Membership, binarity and metallicity of red giants in the southern open cluster NGC 2354

Astron. Astrophys. Suppl. Ser. Membership, binarity and metallicity of red giants in the southern open cluster NGC 2354? J.J. Claria 1 J.-C. Mermilliod 0 A.E. Piatti 1 0 Institut d'Astronomie de Lausanne , CH-1290, Chavannes-des-Bois , Switzerland 1 Observatorio Astronomico , Laprida 854, 5000 Cordoba , Argentina We present new Coravel radial-velocity observations and photoelectric photometry in the U BV , DDO and Washington systems for a sample of red giant candidates in the eld of the intermediate-age open cluster NGC 2354. Photometric membership probabilities show very good agreement with those obtained from Coravel radial velocities. The analysis of the photometric and kinematical data allow us to con rm cluster membership for 9 red giants, one of them being a spectroscopic binary, while 4 con rmed spectroscopic binaries appear to be probable members. We have also discovered 4 spectroscopic binaries not belonging to the cluster. A mean radial velocity of (33.40 0.27) km s−1 and a mean reddening E(B−V ) = 0:13 0.03 were derived for the cluster giants. NGC 2354 has a mean ultraviolet excess < (U − B) > = −0:03 0.01, relative to the eld K giants, and a mean new cyanogen anomaly CN = −0:035 0.007, both implying [Fe/H] −0:3. The moderately metal-poor character of NGC 2354 is con rmed using ve di erent metal abundance indicators of the Washington system. The cluster giant branch is formed by a well de ned clump of 7 stars and 4 stars with high membership probabilities seem to de ne an ascending giant branch. The whole red giant locus cannot be reproduced by any theoretical track. open clusters; individual; NGC 2354 | stars; HR diagram; abundances - Send o print requests to: J.J. Claria, e-mail: ? Based on observations collected with the Danish 1.54-m telescope at the European Southern Observatory, La Silla (Chile); at Complejo Astronomico El Leoncito, which is operated under agreement between the Consejo Nacional de Investigaciones Cient cas y Tecnicas de la Republica Argentina and the National Universities of La Plata, Cordoba, and San Juan, Argentina, and at Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. 1. Introduction Our actual knowledge of the red giant phase in the evolutionary history of the stars is still rather uncertain and incomplete. This is mainly due to the lack of numerous and su ciently accurate observational data of red giants in open clusters to be compared with the theoretical predictions. In particular, intermediate-age open clusters are in an evolutive stage which is well suited to study the morphology of the red giant branch as well as to check the issue of convective overshooting (see, e.g. Bertelli et al. 1985) . NGC 2354 (C0712−256; Trumpler class III-2m) is an intermediate-age open cluster located in Canis Major at (B1950) = 7h 12m:2, = −25 390; l = 238: 42, b = −6: 80. Although it is interesting for the number of red giant candidates it contains, this cluster has so far received little attention. The only photometric study was published by Du¨rbeck (1960) who derived a reddening E(B − V ) = 0.14, a distance of 1850 pc and an age of 7.0 108 years. These values, however, are rather uncertain as they have been determined from U BV photographic data. More recently, Ahumada & Lapasset (1996) reported preliminary results of a CCD U BV R photometric study in an area of about 100 centered on NGC 2354. They obtained a mean reddening E(B − V ) = 0.15, a distance of 1445 pc, and a somewhat larger age of about 1 Gyr. A photometric search for variable stars among the blue stragglers (BS) in this cluster led them to the discovery of one eclipsing binary BS (Lapasset & Ahumada 1996) . As part of a long-term project to determine abundances and astrophysical properties of red evolved stars in southern open clusters, we present here Coravel radialvelocity observations and accurate U BV , DDO and Washington photometry of red giant candidates in the eld of NGC 2354. The present data are used to discuss cluster membership, to detect spectroscopic binaries among the red giants, to determine individual E(B − V ) colour excesses as well as to derive the cluster metal content. Preliminary results on the NGC 2354 red giants were published by Claria & Piatti (1994) . 2. The observational material 2.1. Photometric observations A total of 27 stars in the eld of NGC 2354 brighter than V = 13:1 and redder than B − V = 0:78, were selected as red giant candidates from the colour-magnitude diagram published by Du¨rbeck (1960) . With the exception of one star (#152), all the red giant candidates were observed in the Washington photometric system (Canterna 1976) , while nineteen of them with V 11:9 were observed in the U BV and DDO systems with the purpose of obtaining additional information about their reddening and metallicity. Three additional stars fainter than V = 11:9 were also observed in the U BV system. The photometric data reported here were obtained at Cerro Tololo Inter-American Observatory (CTIO) using the 0.6- and 1.0-m telescopes in January 1992 and January 1993. Single-channel pulse-counting photometers were used at CTIO in conjunction with dry-ice cooled Hamamatsu R943-02 and EMI 9781A photomultipliers for the U BV and DDO observations, while Hamamatsu R943-02 and RCA 31034A phototubes were used for the CM T1T2 observations. Only the four primary lters of the DDO system were used, since they provide adequate information for the present purposes. Mean U BV , DDO and CM T1T2 extinction coe cients for CTIO were used to reduce the photometric data, and nightly observations of about 13-15 standard stars from the lists of Cousins (1973 , 1974), Graham (1982) , McClure (1976) , Dean (1981) and Canterna (1976) , were used to place the observations in the standard U BV , DDO and Washington systems. Some U BV measurements were also done in March 1992 at the Complejo Astronomico El Leoncito (CASLEO) using the 2.15-m telescope and the photopolarimeter VATPOL (Magalh~aes et al. 1986), which has two Ga-As RCA 31034 phototubes refrigerated with dry ice. Mean extinction coe cients derived by Minniti et al. (1989) for CASLEO were used. A comparison of the observed mean values with the published ones for the standard stars yields the external mean errors of a single U BV , DDO and CM T1T2 observation, an indication of how closely the standard systems have been reproduced. External mean errors of the U BV and Washington photometries range between 0.01 and 0.02 mag, while those of the DDO photometry are typically lower than 0.01 mag. The observed U BV , DDO and Washington photometric data together with their mean internal errors , in units of 0.001 mag, are given in Tables 1 and 2 (using the numbering system of Du¨rbeck 1960) , where n1, n2, and n3 indicate the number of nights on which each star was observed in the U BV , DDO, and CM T1T2 systems, respectively. Stars #292, 293 and 298 in the tables are those denoted as B, C, and H, respectively, by Du¨rbeck. A comparison of the previous photographic V and B − V data of Du¨rbeck (1960) and the present photoelectric ones is rather poor, the mean di erences (previous minus present value) being: V = −0:05 0.20 (s.d.) and (B − V ) = 0.04 0.19 (s.d.). The large standard deviations are almost entirely due to the low accuracy of Du¨rbeck's photographic data. However, if the four most discrepant stars #47, 161, 200 and 248 are omitted, the mean di erences are: V = 0.04 0.10 and (B − V ) = 0.04 0.07. 2.2. Coravel radial velocities Radial velocity observations have been obtained in February 1989, March 1990 and January 1994 and 1995 with the radial velocity scanner CORAVEL (Baranne et al. 1979) installed on the 1.54 m Danish telescope at the ESO observatory at La Silla (Chile). The candidate giants were selected from the list of Du¨rbeck (1960) . It appears that star #157 (V = 11.04, B − V = 1.31) has been inadvertently dropped from the list. We shall try to get some observations for this object because it could be a probable member lying on the ascending giant branch. Usually three observations were obtained for non variable, member stars, while binaries received more attention. The radial velocity system is that de ned by Mayor & Maurice (1985) which is a natural system for the southern CORAVEL. It corresponds to the IAU standard system de ned by the faint list (V > 4:3). Columns (2) to (7) of Table 3 give in succession the mean radial velocity and its uncertainty in [km s−1], the number of radial-velocity observations, the ratio of the external to internal errors (E=I), the probability P ( 2) that the scatter is due to random noise, the time interval covered by the observations of each star, and remarks on membership and duplicity The star designations are from Du¨rbek (1960). Although most stars have only three observations, the interval of time, nearly six years and the good agreement between the observations mean that binaries with periods less than about 5000 days have been detected. The probability to obtain three radial velocities at the same values over six years is quite low, although, of course, not zero. V sin i for the red giant members is small: either V sin i is smaller than 1.8 km s−1 or only an upper limit has been determined. The two di erent cases are #179 and 219, with V sin i equal to 3.8 1.8 and 5.6 1.7 km s−1 respectively. Individual observations, including the Julian dates, are listed in Table 7, which is only available in electronic form. 3. Analysis and discussion of the data 3.1. Cluster membership and interstellar reddening The observed colour-magnitude (CM) and colour-colour (CC) diagrams of NGC 2354 with the stars listed in n1 219 179 113 Fig. 1. The colour-magnitude diagram for red giant stars in NGC 2354. Con rmed or possible cluster members and red eld stars are represented by lled and open circles, respectively. Spectroscopic binaries (underlined) are also indicated. The distance between the apparent ascending giant branch and the clump is much too large when compared to theoretical models Table 2 are shown in Figs. 1 and 2. The selection process for cluster membership is here primarily based on the radial velocity data. Fourteen stars with mean radial velocities larger than 40 km s−1 or lower than 20 km s−1 are undoubtedly non-members, four of them most probably spectroscopic binaries (see Table 3). The radial velocities of six non-SB obvious members from Table 4 (stars #66, 91, 125, 152, 183 and 205) fall within an interval of only 1.7 km s−1. The mean radial velocity of these stars is 33.40 0.27 km s−1 (s.e. of the mean) and has been adopted for NGC 2354. When the mean radial velocities of some stars di er by some 2 − 2:5 km s−1 from the cluster mean velocity, i.e. di erences well larger than 3 , it is more di cult to derive the membership from the radial velocity only. The examination of the colour-magnitude diagram may provide further help to the decision. Generally, there is a very good agreement between the kinematic and photometric membership estimates, but in the case of NGC 2354, there appear to be some contradictions. Although it is formally possible to compute membership probabilities, practically the results do not bring much insight in the membership determination. The six stars listed above would have high membership probabilities, and all other would be close to 0. In fact it does not properly take into account the case of the binaries, because the rough mean velocities are not fully representative of the true mean values. Further observations, and not only radial velocities, but also proper motions, are needed to settle the point. Stars #59 and 219 di er by 2.1 and 2.2 km s−1 from this mean velocity so that they are possible cluster members. They could be long period, low amplitude binaries with an eccentric orbit. The position of both stars in the colour-magnitude diagram (Fig. 1) may also indicate that they are non-member. However, star #59 falls very close to star #152, which has a radial velocity (33.84 km s−1) very close to the cluster mean. Star #219 has a high V sin i, like #179 which is a binary. The status of star #184, a possible spectroscopic binary, is di cult to decide because it has a radial velocity close to the cluster mean but its position in Figs. 1 and 2 falls also too red, if one uses available isochrones to analyse the distribution of the red giants in the colour-magnitude diagram of NGC 2354. Finally, #200, the brightest red giant contributes to the de nition of a plausible ascending giant branch. The mean velocities of the con rmed spectroscopic binaries #113, 179 and 269 are based on observations not well distributed with respect to the mean cluster velocity as a result of the telescope time allocation. Accordingly, they are not yet representative of the real mean velocities of these stars. Although these mean values di er by about 5.3, 4.7 and 5.8 km s−1 from the cluster mean, these stars are considered as probable members, because the observed individual radial velocities for each star do scatter around the cluster mean. A de nitive statement about their membership will await more observations and the Table 4. Red giant membership results determination of the orbits. With the exception of stars #59, 152, 184 and 200, all Star E(B − V ) E LC MK Membership the remaining members form an elongated clump of stars Pred. DDO P(Ah)ot(oBm) . Vr Fin. near V ' 11.5 in the CM diagram. The position of stars #179 and 219 in the CM diagram is due to their binary 11 0.20 0.02 II/III G9 III nm m nm nm character, certain for star #179 and probable for 219. The 47 0.00 0.06 II/III K1 III nm m nm nm (sb) morphology of the CM diagram will be discussed later. 59a m m Cluster membership was also examined by applying 66 0.14 0.03 III K0 III/IV m m m m the photometric criteria A and B described by Claria & 19113 00..1172 00..0036 IIII/IIII GG55 IIII/IIII mm mm mm? mm? (sb) Lapasset (1983). Taking into account the di erent com- 125 0.09 0.06 III G5 II/III m m m m binations that might result from the application of both 152a m m criteria, we decided to consider a star to have a high prob- 161 0.07 0.03 II G8 III nm pm nm nm ability of being a cluster member if one (or both) of the 166 0.08 0.02 II/III K0 III nm m nm nm criteria implies membership, while the other indicates that 179b m? m?(sb) the star is a probable member. If one criterion (or both) 183 0.16 0.08 III K0/1 III m m m m suggests non-membership, the star is rejected as a cluster 184 0.31 0.05 II/III K1 III nm m m m? (sb?) member. Finally, if both criteria simultaneously indicate 200 0.17 0.04 II K4/5 III m pm m m (sb) probable membership, the star is then considered to be a 205 0.09 0.03 III K0/1 III pm m m m probable member of the cluster. 217 0.02 0.03 II G5/8 III nm pm nm nm To apply criteria A and B, the colour excess E(B − 224189b 0.00 0.05 II K1/2 III nm pm nmm nmm (sb) V )MS = 0.15 mag and true distance modulus (m − M )0 = 269 0.12 0.05 III K0 III m m m? m? (sb) 10.80 both derived by Ahumada & Lapasset (1996) were 292 0.02 0.02 II/III K1/2 III nm m nm nm adopted. The DDO colours were dereddened according 293 0.04 0.02 II/III G9 III nm m nm nm to the reddening coe cients of McClure (1973) and the predicted luminosity class for each observed star was de- a Not observed in the DDO system. termined from the Schmidt-Kaler (1982) calibration as- b Star outside the range of the DDO calibration. suming R = Av/E(B − V ) = 3.0. Columns (6)-(9) of Table 4 contain the results from applying the photometric and kinematic criteria and the The interstellar reddening derived from Janes's (1977 ) membership status nally adopted for each star (sb = method average to <E(B − V )GK> = 0.13 0.03 mag, spectroscopic binary, m = member, nm = non-member). in very good agreement with the previous values derived Column (2) of Table 4 lists the E(B−V )GK colour excesses by Du¨rbeck (1960) and Ahumada & Lapasset (1996) . derived from Janes's (1977 ) iterative procedure, which is However, the individual E(B −V )GK values listed in Table abundance independent and valid over a wide range of 4 were used to correct the DDO photometry for interstelluminosities for Population I stars. The standard devia- lar reddening. tion E, calculated from Claria's (1985 ) Eq. (10) is given in Col. (3), while Cols. (4) and (5) include the predicted luminosity class (LC) and the MK spectral type inferred 3.2. Metal content from the dereddened DDO colours. Although the results obtained from criterion A should 3.2.1. DDO and U BV abundance parameters be taken with caution because of the probable nonuniform reddening in the cluster eld (Ahumada & As a rst abundance indicator, we have used the intrinsic Lapasset 1996) , the agreement between the photometric DDO colour index C0(41 − 42), which is an excellent analysis and the kinematic data is really excellent. This abundance indicator measuring the strength of the demonstrates once again that the photometric criteria A 4216 cyanogen band, such that the larger the index and B lead to reliable membership results provided the the greater the absorption by this band. Using this BV and DDO photometric data are of high quality. The parameter we have computed for each cluster red giant only discrepant star (#184), an apparent radial velocity the new cyanogen anomaly, CN , de ned by Piatti member, has a reddening signi cantly larger than those of et al. (1993) as the di erence between the dereddened the cluster giants (see Table 4), compatible with its posi- C0(41 − 42) and the standard value of this index cortion in the CM and CC diagrams. Since this star is located responding to a star with the same temperature and in an apparently obscured region in the cluster eld and surface gravity, but not with the same C0(42 − 45) has a metal content nearly similar to that of the remain- and C0(45 − 48) as the star in question. Column (3) of ing red giants (see Sect. 3.2.1), we have retained it as a Table 5 lists the cyanogen anomaly CN obtained for possible cluster member. nine cluster giants. No value could be determined for 59a 66 91 113 125 179b 183 184c 200c 205 219b 269 stars #59, 179 and 219 which fall outside the range of Piatti et al. (1993) calibration. The mean cyanogen anomaly is < CN > = −0.035 0.007 (m.e.), the negative sign indicating a weak cyanogen band compared with the mean for solar neigbourhood K giants. The cluster metallicity derived from the [Fe/H] versus CN relation given by Piatti et al. (1993) is then [Fe/H] = −0.29 0.10. We note that the DDO abundance derived for star #184 ([Fe/H] = −0.3) suggests again that this is a cluster giant. We also examine the cluster abundance by determining the ultraviolet excesses (U − B) with respect to the eld K giants. These quantities were derived using Janes's (1979) Eq. (7) and comparing the (U − B)0 and (B − V )0 intrinsic colours of the cluster giants with the standard class III two-colour line of Fitzgerald (1970) (see Fig. 2). The derived U V excesses are then directly comparable to CN , which is also based on typical eld stars. The computed (U − B) excesses are given in Table 5. The mean value < (U − B)> = −0.03 0.01 (m.e.) derived from 10 cluster giants implies [Fe/H] ' −0.3, if Janes's (1979) Eq. (8) and Janes's (1975 ) Eq. (2) are used. We note that this value practically does not change if the three spectroscopic binaries #113, 179 and 269 are omitted. The resulting metallicity is then in excellent agreement with that found from the DDO data. Therefore, both CN and (U −B) values support the conclusion that NGC 2354 is a moderately metal-poor open cluster. 3.2.2. Washington abundance parameters The Washington photometric system provides several independent metallicity indicators. Geisler et al. (1991) have de ned ducial lines for solar abundance giants in the Washington colour-colour diagrams, including the (C − T1)0 = (T1 − T2)0, (C − M )0 = (M − T2)0 and (C − T1)0 = (M − T2)0 relations. The abundance-sensitive index is the di erence between the observed colour and the solar-abundance colour at the observed (T1 − T2) [or (M − T2)], where all colours refer to dereddened values. Geisler et al. (1991) described a method for correcting the decrease in abundance sensitivity with temperature and established new empirical calibrations of the abundance indices 01 − 05 with [Fe/H], where 01 − 05 refer respectively to 0(C − M )T1−T2 , 0(M − T1)T1−T2 , 0(C − T1)T1−T2 , 0(C − M )M−T2 and 0(C − T1)M−T2 . These 0 indices can be calculated from the indices using Eq. (2) of Geisler et al. (1991) . The derived Washington abundance indices for NGC 2354 giants are given in Cols. (4)-(8) of Table 5. Stars #184 and 200 fall outside the range of the Washington calibration. The resulting mean values and standard deviations of the mean from ten cluster giants are: < 01> = < 0(C − M )T1−T2 > = −0.14 < 02> = < 0(M − T1)T1−T2 > = −0.04 < 03> = < 0(C − T1)T1−T2 > = −0.18 < 04> = < 0(C − M )M−T2 > = −0.09 < 05> = < 0(C − T1)M−T2 > = −0.10 These values practically do not change if the three spectroscopic binaries #113, 179 and 269 are omitted. Using the abundance calibration of Geisler et al. (1991) , the above mean indices yield [Fe/H]1 = −0.37 0.06, [Fe/H]2 = −0.33 0.06, [Fe/H]3 = −0.37 0.06, [Fe/H]4 = −0.32 0.07 and [Fe/H]5 = −0.32 0.07. The average of the ve Washington abundance estimates, [Fe/H] = −0.34 0.02 (s.d.), is in very good agreement with the values derived from both the DDO data and the U V excesses. NGC 2354 is therefore on the metal-poor side of the distribution of the intermediate-age open clusters. Since this cluster is located about 1.4 kpc from the Sun at l = 238 , its adopted metallicity ([Fe/H] = −0.30) is consistent with the existence of a radial metallicity gradient in the Galactic disk (see, e.g., Janes 1979; Piatti et al. 1995) . 4. The colour-magnitude diagram The MV vs. (B − V )0 diagram for the giant members or probable members of NGC 2354 is plotted in Fig. 3. By adopting the true distance modulus (m − M )0 = 10.80 from Ahumada & Lapasset (1996) and our mean E(B −V ) colour excess, the age of NGC 2354 may be estimated by tting theoretical isochrones computed by Bertelli et al. (1994) , which include mass loss and moderate overshooting. In Fig. 3 we plotted the isochrones for Y = 0:25, Z = 0:008, equivalent to [Fe/H] = −0.4, and log t = 8.8, 9.0 and 9.2. A reasonable t has been found for the isochrone of log t = 9.0 both for the Bertelli et al. (1994) and Schaller et al. (1992) isochrones. A better age determination is awaiting more precise CCD data to replace the old, photographic data of Du¨rbeck (1960) and the determination of membership of main-sequence stars. Neither set is able to reproduce correctly the distribution of all red giants, the Italian isochrones being too blue and the Swiss ones too red. The peculiar appearance of the red giant clump is partly due to the position of the binaries: binarity has a tendency to move points towards bluer B−V colours. A photometric decomposition of the two most pronounced binaries (#113, 179) has been attempted. The primary of #179 falls with the other single clump stars, while that for #113 falls close to #183 and would de ne the ascending giant branch. From this, we could conclude that the shape of the isochrone is in good agreement with the overall distribution of the member red giants when the e ect of binarity on the joined colours is properly taken into account. However, the positions of the four stars (#59, 152, 184 and 200) is quite puzzling. If the case of #184 may be explained by di erential reddening as mentioned above, no convincing explanation have been found for the three others. The agreement of the radial velocities with the cluster mean (Vr = 33:4 km s−1), in particular that for #152 (33.84 km s−1) and #200 (33.08 km s−1), cannot be fortuitous. Red giants with colours redder that those of the bottom of the ascending red giant branch have been found in NGC 6940 (Mermilliod & Mayor 1989) and in NGC 2360 (Mermilliod & Mayor 1990) , but both stars are binaries, which does not seem to be the case of stars #59 and 152. Five stars in an anomalous position with respect to the isochrone have been identi ed in the somewhat older cluster NGC 752 (Mermilliod et al. 1998) , but they are bluer than the isochrone, not redder. In many photometric analysis of the colour-magnitude diagrams, stars located outside the regular sequences are generally rejected, which gives the impression that sequences are clean. Independent kinematical data sometimes support the membership of stars located in an anomalous position. Clearly, additional observations are necessary before one can conclude that the four anomalous stars in NGC 2354 are really non-members. Meanwhile, the observed distribution of the red giants cannot be reproduced by any available model. Acknowledgements. We are grateful to the referee, Dr J. Andersen, for his valuable remarks. We would like to thank ESO, CTIO and CASLEO for the generous allocation of observing time. Thanks are also due to M.A. Nicotra for his help with the software support. This work was partially supported by grants from the Argentinian institutions CONICET and CONICOR. 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J. J. Clariá, J.-C. Mermilliod, A. E. Piatti. Membership, binarity and metallicity of red giants in the southern open cluster NGC 2354, Astronomy and Astrophysics Supplement Series, 301-308, DOI: 10.1051/aas:1999142