A&A462, 621-626(2007) Astronomy DOI: 10.1051/0004-6361:20042124 © ESO 2007 Astrophysics Optical multicolor polarization observations in the region of the open cluster NGC 5749* ** M. M. Vergne,*** C. ,***Feinstein and R. Martinez Facultad de Ciencias Astronómicas y Geofísicas. Observatorio Astronómico. Paseo del Bosque. 1900 La Plata. Argentina e-mail: [email protected] Instituto de Astrofísica de La Plata. CONICET Received 6 October 2004 / Accepted 21 September 2006 ABSTRACT We present (UBVRI) multicolor linear polarimetric data for 31 of the brightest stars in the area of the open cluster NGC 5749 considered to study the properties of the interstellar medium (ISM) towards the cluster. Our data yield a mean polarization percentage of Pv ~ 1.1%. close to the polarization value produced by the ISM with normal efficiency (P(%) = 3.5E{B_V°'S) for a mean color excess of E = 0.42. The mean angle of polarization vectors. 9 = 74°. agrees quite well with the expected angle produced by dust particles aligned in the direction of the galactic disk (and the magnetic field) in the region. Our analysis indicates that the visual absorption affecting the stars in NGC 5749 is partially produced by a dust layer located up to 300 pc from the Sun and also by a second layer of dust closer to the cluster (located at least at 700 pc). The observed photometry and our polarization data are consistent with the existence of dust within the cluster. We also show in this work how polarimetry could be an excellent technique for identifying nonmember stars. Key words. ISM: dust, extinction - open clusters and associations: individual: NGC 5749 1. Introduction the structure of both the macroscopic field in our Galaxy (Mathewson & Ford 1970; Axon & Ellis 1976) and the local field NGC 5749 (C1445-543) is a poorly populated open cluster with associated with the individual clouds (Goodman et al. 1990). little central concentration located at I = 319)5 and b = 4°5, The polarimetric technique is a very useful tool to obtain sig­ near the south-western edge of the Lupus constellation. Lynga nificant information (magnetic field direction, Amax, Pmax, etc.) (1964a,b) carried out a photometric study (in three colors) of from the dust located in front of a luminous object. In particular, 15 bright stars in the field of NGC 5749, deriving a color ex­ young open clusters are very good candidates to carry out po­ cess Eb-v = 0.35 and a distance of 0.9 kpc. The most thor­ larimetric observations because previous photometric and spec­ ough study published is by Claria and Lapasset (1992, hereafter troscopic studies of these clusters have provided detailed infor­ CL), who got photoelectric UBV data of 112 stars brighter than mation on the color and luminosity of the main sequence stars of V = 14.4, located within about 15' from the assumed cluster the cluster. Thus, we can compute the physical parameters of the center. Their color-color diagram suggests possible variable red­ clusters (age, distance, extinction, membership, etc.) and then, dening among the cluster members with an average excess of with the polarimetric data, we can study the location, size, and Eb-v = 0.42 ± 0.04, a distance of 1280 ± 118 pc, and an age of efficiency of the dust grains to polarize the starlight and the dif­ 2.7 x 107 yr. ferent directions of the galactic magnetic field in the line of sight Studies of interstellar polarization are important for two rea­ to the cluster. As the open clusters are also spread within an area, sons: they provide information on the dust itself and a means the evolution of the physical parameters of the dust all over the to trace the galactic magnetic field. Comparison of polarization region can be analyzed. Additionally, the polarimetry data can and extinction data in the same lines of the sight provides tests be used as a powerful criterion for determining the membership for models of extinction and alignment of the grains. As they in a galactic cluster (e.g., Feinstein et al. 2003a) and for detect­ are thought to align so that their longest axes tend to become ing the location of an energetic phenomenon that occurred in the orthogonal to the direction of the local magnetic field, the ob­ history of a cluster (Feinstein et al. 2003b) served polarization vectors map the mean field direction pro­ For some years now, we have been carrying out systematic jected on the plane of the sky. This allows us to investigate polarimetric observations in a large number of galactic open clusters. The aim of this survey is to provide clues about the ★ Based on observations obtained at Complejo Astronómico characteristics of the dust responsible for the extinction and po­ El Leoncito (CASLEO), operated under agreement between the CONICET and the National Universities of La Plata. Córdoba, and San larization in the light from cluster members, about its spatial dis­ Juan. Argentina. tribution, and also about possible memberships. Following our ★★ Table 1 is only available in electronic form at the CDS via ongoing program, we have performed polarimetry in the open anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via cluster NGC 5749. In this paper we report the results of the mul­ http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/462/621 ticolor (UBVRI) measurements of polarization in NGC 5749. We Member of the Catrera del Investigador Científico. CONICET. studied the characteristics (Pmax, 4max, polarization efficiency, Argentina. etc.) of the dust located along the line of sight towards the region Article published by EDP Sciences and available at http://www.aanda.org http://dx.doi.Org/10.1051 /0004-6361:20042124 622 M. M. Vergne et al.: Optical multicolor polarization in the region of NGC 5749 Fig. 1. Projection on the sky of the polarization vectors (Johnson V filter) of the stars observed in the region of NGC 5749. The dot-dashed line is the galactic parallel b = 4.45 where the cluster is situated. In the next sections, we will discuss According to the photometric membership criteria from Claria the observations in the region, the data calibrations, and the re­ & Lapasset (1986), 18 of the 31 stars reported in this paper are sults of both the individual stars and the cluster as a whole. cluster members, one (#69) being a probable member, and 12 stars, nonmembers. 2. Observations and data reduction 3. Results Data on linear optical polarimetry were obtained during two observing runs at the Complejo Astronomico El Leoncito The sky projection of the V-band polarization vectors for the ob­ (CASLEO) in San Juan, Argentina, in 2003 (April 29 to May 5 served stars in NGC 5749 are shown in Fig. 1. The dot-dashed and August 29 to September 2). The observations were car­ line superimposed on the figure is the galactic parallel b = 4°45 ried out using the Torino five-channel photopolarimeter (Scaltriti denoting the close alignment of the polarization vectors with 1994) attached to the 2.15-m telescope. Each star was observed the projection of the Galactic Plane. This indicates that the dust simultaneously through the Johnson-Cousins broad band UBVRI along the line of sight is aligned by a magnetic field also close filters (Atfeff = 0.360 //m , ABeff = 0.440 //m, AVeff = 0.530 //m, to the direction of the Galactic Disk. This result means that the 'Ifieff = 0.690 jum, A/eff = 0.830 /mi). Standard stars for null po­ dust layer responsible for the observed polarization is located in larization and for the zero point of the polarization position angle a rather undisturbed place in our Galaxy. were observed several times each night for calibration purposes. Figure 2 displays the relation that exists between Pv and 0y Each observation in each filter was corrected for sky polariza­ ( the different symbols are the member and nonmember stars ac­ tion and calibrated in the angle in the equatorial system with the cording to CL). Whereas star #68 is a nonmember, stars 31 and proper correction obtained by the data of the angle standards. 25 are probably located far away behind the cluster. This figure No correction was done by intrinsic polarization because it was shows an interesting behavior: stars with larger percent values of found to be insignificant in the observations of null polarization polarization have a lower angle than stars with low polarization. stars. For further information on the instrument, data acquisition, Most objects with polarization larger than 1% (and a polariza­ and data reduction, see Scaltriti (1994). tion angle 0 lower than 80°) are confirmed member stars (CL), The polarimetric observations are listed in Table 1, which while stars with polarization lower than 1% (and 0 > 80°) are shows, in self explanatory format, the stellar identification as mostly nonmembers. given by CL, the polarization percentage average (P.i), and the This result may also be displayed by the plotting of the run position angle of the electric vector (0A) through each filter, of the polarization vector P through its components in the equa­ along with their respective mean errors computed as described torial system (the Stokes parameters Q and U). The plot (Fig. 3) by Maronna et al. (1992). Since the Torino photopolarimeter provides useful information on variations in interstellar environ­ collects photons simultaneously in all the filters ( UBVRI), the ments: if the light from individual stars of a region of an open final data from each filter may be of different quality, especially cluster has gone through a common sheet of dust, their represen­ those in the C-band.