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Chemically Peculiar Stars in the Large Magellanic Cloud⋆

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Chemically Peculiar Stars in the Large Magellanic Cloud⋆ 1 The mean percentage of CP2 stars in the Milky Way is of the order of 5% for the spectral range from early B- to F-type, luminosity class V objects. The origin of the CP2 phenomenon seems to be closely connected to the overall metallicity and global magnetic field environment. The theoretical models are still only tested by observa- tions in the Milky Way. It is therefore essential to pro- vide high quality observations in rather different global environments. The young clusters NGC 2136/7 were ob- served in the ∆a photometric system. This intermediate band photometric system samples the depth of the 520 nm flux depression by comparing the flux at the center with the adjacent regions with bandwidths of 11 nm to 23 nm. The ∆a photometric system is most suitable for detect- ing CP2 stars with high efficiency, but is also capable of detecting a small percentage of non-magnetic CP objects. Furthermore, the groups of (metal-weak) λ Bootis, as well as classical Be/shell stars, can be successfully investigated. We present high precision photometric ∆a observations of 417 objects in NGC 2136/7 and its surrounding field, of which five turned out to be bona fide magnetic CP stars. In addition, we discovered two Be/Ae stars. From our in- vestigations of NGC 1711, NGC 1866, NGC 2136/7, their surroundings, and one independent field of the LMC pop- ulation, we derive an occurrence of classical chemically peculiar stars of 2.2(6)% in the LMC, which is only half the value found in the Milky Way. The mass and age dis- tribution of the photometrically detected CP stars is not different from that of similar objects in galactic open clus- ters. Key words. Stars: chemically peculiar – stars: early-type – techniques: photometric – Magellanic Clouds – open clus- ters and associations: individual: NGC 2136 arXiv:astro-ph/0608200v1 9 Aug 2006 Astronomy & Astrophysics manuscript no. ms5333rev November 6, 2018 (DOI: will be inserted by hand later) Chemically peculiar stars in the Large Magellanic Cloud⋆ E. Paunzen1, H.M. Maitzen1, O.I. Pintado2,⋆⋆, A. Claret3, I.Kh. Iliev4, and M. Netopil1 1 Institut f¨ur Astronomie der Universit¨at Wien, T¨urkenschanzstr. 17, A-1180 Wien, Austria 2 Departamento de F´isica, Facultad de Ciencias Exactas y Tecnolog´ia, Universidad Nacional de Tucum´an, Argentina - Consejo Nacional de Investigaciones Cient´ificas y T´ecnicas de la Rep´ublica Argentina 3 Instituto de Astrof´ısica de Andaluc´ıa CSIC, Apartado 3004, 18080 Granada, Spain 4 Institute of Astronomy, National Astronomical Observatory, P.O. Box 136, BG-4700 Smolyan, Bulgaria Received 31 March 2006 / Accepted 08 July 2006 Abstract. The detection of magnetic chemically peculiar (CP2) stars in open clusters of extragalactic systems can give observational answers to many unsolved questions. For example, one can study the influence of different global as well local environments on the lack of and presence of peculiarities. 1. Introduction The intrinsic peculiarity index ∆a is defined as the dif- ference between the individual a-values and those (= a0) The classical chemically peculiar (CP) stars of the upper of non-peculiar stars of the same color. The locus of the main sequence (luminosity classes V and IV) are targets a0-values has been called, the normality line. of detailed investigations since their first description by Virtually all peculiar objects with magnetic fields (CP2 Maury (1897). They provide excellent test objects for as- and CP4 stars, Preston 1974) have positive ∆a values trophysical processes like diffusion, convection, and strat- in excess of +100mmag whereas Be/Ae/shell and metal- ification in stellar atmospheres in the presence of rather weak (e.g., λ Bootis group) stars exhibit significantly neg- strong magnetic fields. These mechanisms for stars, both ative ones up to −35mmag (Paunzen et al. 2005a). In in the Milky Way and its surrounding stellar systems, con- general, some spectroscopic binary systems can mimic a tribute important knowledge to stellar evolution under dif- positive ∆a value of up to +16 mmag, which is well below ferent local circumstances, e.g., different metallicities. the observed values for the stars presented in this paper. CP stars can be detected very efficiently by applying Later type, evolved objects of luminosity classes III to the ∆a photometric system (cf. Paunzen et al. 2005a), I might also show the same behavior, but can be easily which measures the characteristic broadband absorption identified within the color-magnitude diagram and do not feature located around 520 nm, which is most certainly influence the overall statistics. a consequence of the non-solar elemental abundance dis- In this paper we present our efforts to detect chemi- tribution of CP and related objects in the presence of a cally peculiar stars in the field of NGC 2136/7, a binary strong stellar magnetic field (Kupka et al. 2004). It sam- cluster system, located in the Large Magellanic Cloud ples its depth, comparing the flux at the center (521 nm, (LMC). In total, we observed 417 objects of which five g2), with the adjacent regions (503 nm, g1 and 551 nm, y), turned out to be bona fide magnetic CP stars. These ob- using bandwidths from 11 nm to 23 nm. The respective servations, added to our statistical analysis in the LMC index a was introduced as presented in Paunzen et al. (2005b), show an overall oc- currence of 2.2(6)% for chemically peculiar stars in the a = g2 − (g1 + y)/2. LMC. ⋆ Based on observations at CASLEO and ESO-La Silla ⋆⋆ Member of Carrera del Investigador del Consejo Nacional 2. Observations and reduction de Investigaciones Cient´ificas y T´ecnicas de la Rep´ublica Argentina and Visiting Astronomer at Complejo Astron´omico The observations were done on twelve nights at two dif- El Leoncito operated under agreement between the Consejo ferent telescopes with the identical ∆a filter set: Nacional de Investigaciones Cient´ificas y T´ecnicas de la Rep´ublica Argentina and the National Universities of La Plata, – CASLEO: 215cm telescope, TEK-1024 CCD, field of ′ C´ordoba y San Juan. view of about 9.5 , August 2001 and January 2003, Correspondence to: [email protected] observer: O.I. Pintado E. Paunzen et al.: Chemically peculiar stars in the LMC 3 Table 1. Peculiar objects found in the field of NGC2136/7 and δ(2000.0) = −69◦29′30′′. The estimated reddening of (upper part) and its surrounding area (lower part). The E(B − V ) = 0.1 mag is typical for other clusters in the objects No. 127 and 228 are most certainly not classical LMC. chemically peculiar stars, whereas No. 219 is a red super- Especially interesting is their result of the surround- giant with high mass-loss and emission. The errors in the ing field population. They found a younger age and lower final digits of the corresponding quantity are in parenthe- metallicity than for the clusters. However, the error of sis. their metallicity determination is 0.59 dex. No X Y (B − V )0 ∆a MV Unfortunately it was not possible to use the Str¨omgren 165 +47.79 −87.73 +0.164(7) +0.095 −0.449(5) photometry from Dirsch et al. (2000) for the calibration 204 +98.00 −16.22 −0.255(8) −0.062 −1.480(6) 219 +110.54 −57.90 +0.479(14) −0.056 −4.153(9) of our photometric values and the identification of objects 228 +117.09 −30.35 +0.443(8) +0.074 −4.264(6) because it has been available neither in electronic form 233 +120.22 −214.24 −0.220(2) +0.044 −1.051(2) 283 +167.19 −96.05 −0.084(3) +0.049 −1.663(2) nor upon request from the authors of the given reference. 91 −91.72 −416.28 −0.205(7) +0.041 −1.483(7) Furthermore, no printed tables are available. 110 −54.52 −157.76 +0.005(8) +0.060 +0.051(7) 127 −13.32 −146.05 +0.701(10) +0.053 +0.330(6) We have therefore used the Johnson-Kron-Cousins 392 +377.70 −202.34 +0.033(3) −0.053 −0.709(2) photometric UBVI data published by Zaritsky et al. (2004). They provide no consecutive numbering system but equatorial coordinates (see their Table 1). The identi- – ESO-LaSilla: Bochum 61 cm telescope, Thompson fication of stars in common was done in two steps. First of ′ ′ 7882 CCD, 384x576 pixels, 3 x4 , April 1995, ob- all, we identified the brightest objects by eye and derived servers: H.M. Maitzen and E. Paunzen a first calibration of the instrumental magnitude yinst ver- sus V and the coordinates within our frames (X, Y ) versus The filters have the following characteristics: g 1 (α, δ). With these preliminary values, a second automatic (λc =5027A,˚ FWHM = 222A,˚ Peak transmission = 66%), iteration was done limiting the magnitude difference to g (5205/107/50), and y (5509/120/54). In total, 69 2 0.5 mag and a varying radius limit. It is difficult to de- frames for the three filters (20/23/26) were obtained. cide which objects coincide in the cluster areas of NGC The bias subtraction, dark correction, flat-fielding, and 2136/7. If more than one object from the catalogue by a point-spread function fitting were carried out within Zaritsky et al. (2004) matches at the same level of signif- standard IRAF V2.12.2 routines on a Personal Computer icance, we did not include its UBVI data in the analysis. with a Linux distribution. In total we identified 248 objects that were used for the One of the advantages of the ∆a photometric system is final calibration of the instrumental magnitudes yinst and that the “standard” as well as program stars are always on (g −y)inst.
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