The Astrophysical Journal, 598:349–356, 2003 November 20 # 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A. OPTICAL POLARIZATION OBSERVATIONS OF NGC 6231: EVIDENCE FOR A PAST SUPERNOVA FINGERPRINT1 Carlos Feinstein,2 Ruben Martı´nez, M. Marcela Vergne,2 Gustavo Baume,2 and Rube´nVa´zquez2 Facultad de Ciencias Astrono´micas y Geofı´sicas, Paseo del Bosque, 1900 La Plata, Argentina Received 2003 May 5; accepted 2003 July 30 ABSTRACT We present the first linear multicolor polarization observations for a sample of 35 stars in the direction of the Galactic cluster NGC 6231. We have found a complex pattern in the angles of the polarimetric vectors. Near the core of this cluster the structure shows a semicircular pattern that we have interpreted as a reorientation of the dust particles showing the morphology of the magnetic field. We propose that a supernova event occurred some time ago and produced a shock on the local ISM. We discuss in this paper independent confirmations of this event, both from the studies on the diffuse interstellar absorptions and the results of the pre–main-sequence stars. We also show that a supernova is supported by the evolutionary status of the cluster. Subject headings: dust, extinction — open clusters and associations: individual (NGC 6231) — polarization — supernova remnants 1. INTRODUCTION polarization angles of the cluster members show some scat- ter ( 10 or less) over this mean angle, mainly because of Polarimetric techniques are very useful tools for finding important information (P , , magnetic field density inhomogeneities in the dust along the line of sight to max max the cluster and inhomogeneities of the dust inside the cluster direction, etc.) from the dust located in front of a star (or a to the intrinsic polarimetric properties of the stars and also luminous object) or from dust located inside a stellar cluster. because of more complex magnetic field structures. For Young open clusters are very good candidates on which to example, in Tr 27, the polarimetric data show a mean angle carry out polarimetric observations, because previous of 30 for the cluster with a FWHM of 10 (Feinstein photometric and spectroscopic studies of these clusters give et al. 2000); in Ara OB1 the polarimetric observations give a detailed information of the main sequence, so we can FWHM of 9 9 for NGC 6167, 13 1 for NGC 6193, and characterize the physical parameters of the member stars = = 9 5 for NGC 6204 (Waldhausen, Martı´nez, & Feinstein (age, distance, memberships, etc.) in order to study the = 1999). In the case of Stock 16 (Feinstein et al. 2002), the extinction of the dust in the direction of the cluster and within the cluster. FWHM is 6=7, which seems to be slightly smaller than the other clusters. For example, from the study of the P versus the E max BÀV NGC 6231 (l 345 5, b 1 2) is known in the literature of each star of a cluster, it is possible to find the polarimetric ¼ = ¼ = components of the extinction produced by Galactic dust to be one of the youngest open clusters in the Galaxy and one of the brightest in the southern hemisphere. The cluster located between the Sun and the cluster (e.g., Tr 27; is embedded in an extended system of O and early B stars, Feinstein et al. 2000). By subtracting the effect of this dust namely, the very young association Sco OB1, which con- on the line of sight from the data, it is possible to study the tains the ring-shaped H ii region IC 4678 centered on NGC component associated with the internal extinction of the 6231. Usually, NGC 6231 is considered the nucleus of Sco cluster. In addition to this valuable information, stars with OB1 (Perry, Hill, & Christodoulou 1991). Its age determina- intrinsic polarimetric properties can be isolated. The angle tions range between 3 106 yr (Mermilliot & Maeder 1986) of the polarization vector is an estimator of the direction of  and 6:9 106 yr (Perry et al. 1991). It is known as one of the the Galactic magnetic field. By observing member and non-  richest open clusters, containing WR stars, Cepheid varia- member stars (foreground and background), the direction of the magnetic field toward the cluster at several distances bles, and more than 100 O and B stars (Shobbrook 1983). According to photometric CCD observations of Sung, can be inferred. Bessell, & Lee (1998) and of Baume, Va´zquez, & Feinstein In a typical open cluster the polarimetric data show, for (1999), there is evidence of a population of pre–main- ‘‘ normal ’’ stars (that is, not considering stars with intrinsic sequence stars (PMS). The distance obtained by deep CCD polarization), a distribution of angles that can be described photometry by Baume et al. (1999) locates the cluster NGC by a mean fixed value that characterizes the cluster. The 6231 at 1990 Æ 200 pc from the Sun. The average visual extinction toward the cluster is about AV 1:4 (Balona & Laney 1995; Baume et al. 1999). 1 Based on observations obtained at Complejo Astrono´mico El According to Raboud, Cramer, & Bernasconi (1997), in the Leoncito (CASLEO), operated under agreement between the CONICET northern part of the cluster, the absorption is constant, but and the National Universities of La Plata, Co´rdoba, and San Juan, Argentina. it increases from the central part toward the south. A similar 2 Member of Carrera del Investigador Cientı´fico, CONICET, trend, but less pronounced, is observed from west to east in Argentina. the southern part. Part of the absorption suffered by the 349 350 FEINSTEIN ET AL. light from the cluster stars is caused by matter within the TABLE 1 distance interval 100–1300 pc from the Sun (Perry et al. Polarimetric Observations of Stars 1991), but the field shows a relatively good transparency in NGC 6231 between about 400 and 1000 pc at a constant value averaging A 0:75 mag (Raboud et al. 1997). P Æ P Æ V ¼ Filter (%) (deg) Star 1 2. OBSERVATIONS U........................... 1.24 Æ 0.42 58.4 Æ 9.3 The bulk of the observations were obtained on two B ........................... 0.84 Æ 0.19 47.9 Æ 6.5 observing runs. The first one was performed during 1995 V........................... 0.66 Æ 0.17 25.5 Æ 7.1 (May 30 to June 3). The second one was on three nights in R........................... 0.94 Æ 0.22 26.3 Æ 6.6 2001 (August 17–20). Two more runs in 2002 May and 2002 I ............................ 0.99 Æ 0.47 32.7 Æ 12.8 August were used for reobserving and checking some stars. Star 6 All runs were carried out using the five-channel photo- polarimeter of the Torino Astronomical Observatory B ........................... 0.33 Æ 0.07 31.6 Æ 6.2 (Scaltriti et al. 1989) attached to the 2.15 m telescope at the V........................... 0.34 Æ 0.06 15.5 Æ 5.3 Complejo Astrono´mico El Leoncito (San Juan, Argentina). R........................... 0.35 Æ 0.05 10.0 Æ 3.9 I ............................ 0.34 0.10 17.0 8.3 Polarimetric measurements have been made for 36 stars to Æ Æ the cluster NGC 6231. Most of them were observed through Star 16 the Johnson broadband UBVRI filters V........................... 0.36 Æ 0.07 20.8 Æ 5.7 R........................... 0.37 Æ 0.07 15.6 Æ 5.0 ðU; eff ¼ 0:360 lm ;B; eff ¼ 0:440 lm ; I ............................ 0.31 Æ 0.07 24.4 Æ 6.3 V; eff ¼ 0:530 lm ;R; eff ¼ 0:690 lm ; Star 34 I; eff ¼ 0:830 lmÞ : B ........................... 1.36 Æ 0.12 37.4 Æ 2.5 Our results are listed in Table 1, which shows, in a self- V........................... 1.37 Æ 0.14 28.4 Æ 2.8 explanatory format, the stellar identification as given by R........................... 1.45 Æ 0.17 21.7 Æ 3.3 Seggewiss (1968), the average of the percentage of polariza- I ............................ 1.60 Æ 0.24 24.8 Æ 4.2 tion (P), and the position angle of the electric vector () Star 70 observed through each filter, with their respective mean errors. U........................... 1.77 Æ 0.09 130.1 Æ 1.4 The Torino polarimeter collects data for all the filters B ........................... 0.94 Æ 0.08 157.3 Æ 2.4 simultaneously (UVBRI ), so all the wavebands have the V........................... 1.68 Æ 0.07 103.4 Æ 1.2 R........................... 1.48 Æ 0.07 146.9 Æ 1.3 same exposure times, and the S/N varies between filters. I ............................ 1.90 Æ 0.10 167.8 Æ 1.4 Therefore, the values from different filters may be of differ- ent quality. Observations whose values are not over the 3 Star 73 error level were discarded, and they are neither reported in U........................... 1.82 Æ 0.09 140.6 Æ 1.4 Table 1 nor used in this work. B ........................... 1.59 Æ 0.07 164.7 Æ 1.2 Several standard stars for null polarization and for the V........................... 1.08 Æ 0.08 101.6 Æ 2.1 zero point of the polarization position angle were measured R........................... 1.87 Æ 0.05 154.5 Æ 0.8 for calibration purposes. At least three standard angle stars I ............................ 2.53 Æ 0.07 169.5 Æ 0.8 were measured every night at different times. Errors were Star 80 handled as described by Maronna, Feinstein, & Clocchiatti (1992). In order to verify the lack of systematic differences U..........................