Page 1 of 16 AUTHOR SUBMITTED MANUSCRIPT - PASP-100333.R1 1 2 3 Draft version July 9, 2017 4 Typeset using LATEX manuscript style in AASTeX61 5 6 7 8 INTERSTELLAR POLARIZATION AND EXTINCTION TOWARDS THE OPEN CLUSTER 9 10 NGC 457 11 12 13 14 Gregory A. Topasna,1 Eryn A. Daman,1 and Nadia T. Kaltcheva2 15 16 17 1Department of Physics and Astronomy, Virginia Military Institute, Lexington, VA 24450, USA 18 2Department of Physics and Astronomy, University of Wisconsin Oshkosh, 800 Algoma Blvd., Oshkosh, WI 54901, 19 USA 20 21 22 (Received ...; Revised ...; Accepted ...) 23 24 25 Submitted to PASP 26 27 28 ABSTRACT 29 30 NGC 457 is a moderately reddened and relatively young open cluster, very rich in variable stars and 31 32 in Be stars in particular, but not associated with any prominent star-forming field. We combine new 33 34 multi-wavelength polarization measurements with existing UBV and uvby photometries to obtain 35 36 precise estimates of the total-to-selective extinction in the field of the cluster and reevaluate its 37 38 distance and age. The polarization measurements show a tight alignment of the polarization vectors 39 40 with the Galactic plane and yield an average value of the total-to-selective extinction of 3.05 ± 0.17 41 42 for the cluster’s field. Using this value and the confirmed color excess E(B − V )=0.500 ± 0.030 43 44 . ± . ± . 45 mag, we obtain a distance modulus of 12 20 0 39, corresponding to 2 75 0 49 kpc and, assuming 46 47 slightly sub-solar metallicity, an age of 15.8 Myr. 48 49 50 Keywords: stars: early-type, stars: fundamental parameters, stars: general, techniques: 51 52 polarimetry, open clusters and associations: individual: NGC 457 53 54 55 Corresponding author: Nadia Kaltcheva 56 [email protected] 57 58 59 60 AUTHOR SUBMITTED MANUSCRIPT - PASP-100333.R1 Page 2 of 16 1 2 3 Draft version July 9, 2017 A 4 Typeset using L TEX manuscript style in AASTeX61 5 6 7 8 INTERSTELLAR POLARIZATION AND EXTINCTION TOWARDS THE OPEN CLUSTER 9 10 NGC 457 11 12 13 1 1 2 14 Gregory A. Topasna, Eryn A. Daman, and Nadia T. Kaltcheva 15 1Department of Physics and Astronomy, Virginia Military Institute, Lexington, VA 24450, USA 16 2Department of Physics and Astronomy, University of Wisconsin Oshkosh, 800 Algoma Blvd., Oshkosh, WI 54901, 17 USA 18 19 20 (Received ...; Revised ...; Accepted ...) 21 22 23 Submitted to PASP 24 25 26 27 28 Keywords: stars: early-type, stars: fundamental parameters, stars: general, techniques: 29 30 polarimetry, open clusters and associations: individual: NGC 457 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 3 of 16 AUTHOR SUBMITTED MANUSCRIPT - PASP-100333.R1 1 NGC 457 3 2 3 4 1. INTRODUCTION 5 6 NGC 457 is a young open cluster located at l = 126.6348◦, b =-04.3826◦, somehow below the star- 7 8 forming fields associated with Cas OB1 and Cas OB8. It appears isolated from the Cassiopeia-Perseus 9 10 complex of open clusters and not associated with any prominent nebulosities. The most extensive 11 12 photoelectric UBV photometry of cluster’s stars is obtained by Pesch (1959) (78 stars) and Hoag 13 14 et al. (1961) (34 stars). CCD intermediate-band uvby photometry of nearly 60 stars in the direction 15 16 of the cluster is provided by Fitzsimmons (1993). Hβ photometry is available only for several stars 17 18 of the cluster (Eggen 1982). The yellow supergiant φ Cassiopeiae (F0 Ia) is projected within the 19 20 boundaries of the cluster. As a part of their CCD variability survey, Mo´zdzierski et al. (2014) found 21 22 that NGC 457 is very rich in Be stars and in variable stars in general. 23 24 High resolution spectroscopy of main-sequence B-type stars has been performed by Dufton et al. 25 26 Fe/H − . 27 (1994) and yielded normal chemical composition. The works by Tadross (2003)([ ]= 0 46) 28 M/H − . 29 and Fitzpatrick & Massa (2007)([ ]= 0 43) suggest, however, that the metallicity of the 30 E B − V . 31 cluster might be lower than solar. The cluster is moderately reddened ( ( )=05 mag) with 32 33 distance estimates varying between 2.4 kpc and 3.3 kpc (see also Mo´zdzierski et al. (2014) for a recent 34 35 review). 36 37 In order to reevaluate the existing estimates of the NGC 457 parameters, we present new multi- 38 39 wavelength polarization measurements of 12 stars in this direction. We use the wavelength of maxi- 40 41 mum polarization to calculate the total-to-selective extinction ratio for the cluster and further com- 42 43 bine this result with existing UBV and uvby photometric data to provide new estimates of the 44 45 cluster’s distance and age. 46 47 In general, a combination between multi-wavelength polarimetry and precise photometry provides 48 49 more constraints on the derivation of the four free clusters’ parameters: reddening, distance, metal- 50 51 licity and age. Often, especially when fitting isochrones to the cluster sequence, we introduce bias 52 53 because of an assumed (average for the Galaxy) value of the total-to-selective extinction ratio, or 54 55 assumed solar metallicity. These uncertainties affect various aspects of the analysis of the cluster 56 57 populations in the Galaxy (see for example Paunzen et al. (2010) for a thorough discussion). A com- 58 59 60 AUTHOR SUBMITTED MANUSCRIPT - PASP-100333.R1 Page 4 of 16 1 4 Topasna et al. 2 3 uvbyβ 4 bination between polarization and intermediate-band photometry should be especially useful 5 6 because it can provide metallicity independent reddening and distance and mutually independent 7 8 estimates of metallicity and age. 9 10 2. POLARIZATION OBSERVATIONS AND ANALYSIS 11 12 13 Polarimetric observations of NGC 457 were performed during the night of September 22, 2016 using 14 15 the optical polarimeter on the 0.5-m f/13.5 Cassegrain telescope at the Virginia Military Institute 16 17 (VMI) Observatory located at McKethan Park in Lexington, VA. The design, construction, and 18 19 operation of the polarimeter are described by Topasna et al. (2013). The polarimeter uses a rotatable 20 ◦ ◦ ◦ ◦ 21 achromatic half-wave plate at positions of 0 ,22.5 ,45,and67.5 with respect to the fiducial setting 22 ◦ 23 at 0 along a line of constant right ascension. The star’s light then passes through a Wollaston prism 24 25 and filter wheel and is imaged on an Alta U6 CCD. The CCD records a dual image of each star, its 26 27 ordinary Io and extraordinary Ie rays. Due to vignetting by the optics, the usable field-of-view is 28 29 approximately ten arcminutes in diameter. 30 31 Aperture photometry is used to measure the ordinary and extraordinary fluxes Io and Ie. Following 32 33 R2 ≡ Io Ie /Ie Io the method described by di Serego Alighieri (1997) we compute the ratios q 0◦ 45◦ 0◦ 45◦ and 34 35 R2 ≡ Io Ie /Ie Io q R − / R u 22.5◦ 67.5◦ 22.5◦ 67.5◦ and then normalized Stokes parameters =( q 1) ( q +1)and 36 37 u =(Ru − 1)/(Ru + 1). While this method removes the detector’s polarization, the telescope’s 38 39 instrumental polarization is eliminated by imagining an unpolarized standard star, calculating its 40 41 normalized Stokes parameters, and then vectorially subtracting these from the Stokes parameters of 42 43 the program stars. Using the list of standard stars (both polarized and unpolarized) from Serkowski 44 45 (1974), we imaged the unpolarized standard HD 185395 (θ Cyg) as well as the polarized standard 46 47 HD 7927 (φ Cas). 48 49 p 50 The degree of polarization and position angle are computed from the Stokes parameters using 0 = 51 q2 + u2 and θ = 1 tan u with standard errors given by [q2σ2 + u2σ2]1/2/p and [q2σ2 + u2σ2]1/2/2p2. 52 2 q q u u q 53 p p2 − σ2 54 Finally, the inherent bias is removed using the Wardle & Kronberg (1974) estimator = 0 p. 55 56 From our UBVRI observations, we measured the wavelength dependence of the polarization. The 57 58 observed polarization of starlight has been well studied and is described by the empirical relationship 59 60 Page 5 of 16 AUTHOR SUBMITTED MANUSCRIPT - PASP-100333.R1 1 NGC 457 5 2 3 ! ! ! & #!$%! & #!$%! & 4 " " "#!$%!& #!$%! #!&$%!& " " ! "#!$%! 5 ! #&!$%! ! #!$%! #&!$%!& 6 #! #! ! #! 7 ! ! 8 ! ! 9 ! ! 10 ! 11 ! ! 12 ! 13 &! &! ! 14 15 ! ! ! #!$%! 16 "#!$%! " "#!$%! #!$%!& ! #!$%! #!$%! 17 ! " " " #!$%!& #!$%! ! #!&&$%! 18 ! #! #! #! ! 19 ! 20 ! 21 ! ! ! ! 22 ! 23 ! ! ! 24 ! 25 ! ! ! 26 27 28 ! ! ! 29 "#!&$%! #!&$%!& " & "#!$%!& ! 30 "#!$%! ! #!$%! " "#!$%! #&!$%!& #&!$%! #!&$%&! 31 ! #! ! ! #! #! 32 !& 33 ! ! ! 34 ! ! 35 ! ! 36 ! 37 ! ! 38 ! ! ! 39 40 41 ! ! ! 42 && #!$%! ) #!$%! #!$%! " & " " #!$%! #!$%!& #!$%! 43 " " " ! ' ( ! #!$%!& *#!$%! #!$%! 44 ! #! #! #! ! 45 ! 46 ! 47 ! ! ! 48 ! ! 49 ! 50 !& 51 ! ! ! 52 53 54 55 56 Figure 1. Serkowski’s curves for the measured stars in the field of NGC 457. 57 58 59 60 AUTHOR SUBMITTED MANUSCRIPT - PASP-100333.R1 Page 6 of 16 1 6 Topasna et al. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Figure 2. Polarization vectors map for the observed stars in the field of NGC 457. 25 26 27 28 29 2 30 p(λ)/pmax =exp[−K ln (λmax/λ)] (1) 31 32 33 where pmax is the maximum degree of polarization at the wavelength λmax. The parameter K was 34 35 shown by Wilking et al.