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J/AJ/107/1433                         Cygnus OB2 polarimetry (Kobulnicky+, 1994)

R band polarimetry of Cygnus OB2: implications for the magnetic field geometry and polarization models KOBULNICKY H.A., MOLNAR L.A., JONES T.J. <Astron. J. 107, 1433 (1994)> =1994AJ....107.1433K (SIMBAD/NED Reference)
Abstract: We present new R band polarimetry of 132 members of the Cygnus OB2 association. From these data we have determined the angular coherence length of the polarization position angle and tested two models for interstellar polarization toward a region where the Galactic magnetic field is expected to be primarily along the line of sight. The polarization magnitudes and position angles in our sample decorrelate on size scales of ∼0.5deg, compared to 0.2deg for the optical extinction. This constrains the variations in the magnetic field orientation to sizes less than 8pc. The distribution of polarization magnitudes and position angles is consistent with a model which describes the polarization in terms of preferential extinction in small dark clouds threaded by a two-component magnetic field. One component is constant in direction, while the second is random in orientation and is characterized by the amplitude of Alfven waves in the interstellar medium. A similar model, in which the magnitude of the random component has a Gaussian distribution and an orientation evenly distributed through 4π steradians, is also consistent with the data. A previously proposed geometrical model involving two slabs of obscuring material with different magnetic field orientations does not seem adequate given these new data. We explore the predictions of each model for the polarization magnitude and position angle as a function of the orientation of the constant component and point out several key differences that might be exploited to further test their potential for describing interstellar polarization. We find the polarization position angles of stars in the vicinity of Cygnus X-3 are approximately orthogonal to the position angle of the elliptical radio scattering reports by Molnar et al. [BAAS, 21, 1124 (1989)]. If the scattering is taking place on the near side of Cyg OB2, the data are consistent with mechanisms that require the major axis of the scattering ellipse to lie perpendicular to the projected magnetic field. Addresses: Kobulnicky H.A. Department of Astronomy, University of Minnesota, 116 Church St. SE, Minneapolis, Minnesota 55455 Molnar L.A. Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242 Jones T.J. Department of Astronomy, University of Minnesota, 116 Church St. SE, Minneapolis, Minnesota 55455 File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file table1.dat 80 168 Polarization data table1.tex 138 222 LaTeX version of table1.dat
Byte-by-byte Description of file: table1.dat
Bytes Format Units Label Explanations
1- 6 F6.3 deg GLON Galactic longitude 8- 12 F5.3 deg GLAT Galactic latitude 14- 15 I2 h RAh Right ascension J2000.0 17- 18 I2 min RAm Right ascension J2000.0 20- 23 F4.1 s RAs Right ascension J2000.0 25 A1 --- DE- Declination sign 26- 27 I2 deg DEd Declination J2000.0 29- 30 I2 arcmin DEm Declination J2000.0 32- 33 I2 arcsec DEs Declination J2000.0 36 I1 --- Code [1-6] Coded notes (1) 38- 42 F5.2 % Pol Percent polarization 43 A1 --- n_Pol [ ab] Note for Pol (2) 45- 48 F4.2 % e_Pol rms uncertainty in Pol 50- 54 F5.1 deg PosAng Position angle of polarization 56- 60 F5.2 mag AV Amount of visual absorption 61- 65 A5 --- RLP Other name in Reddish et al (1966) (3) 66- 70 A5 --- MT Other name in Massey & Thompson (1991) (3) 71- 75 A5 --- Sch Other name in Schulte (1956) (3) 77- 80 A4 --- KMJ Other name in this work (3)
Note (1): The coded numbers designate the following: 1) The star appears in Table 7 of Massey & Thompson (1991) and is an association member based on its early type spectra. 2) The star is an early type star based on the UBV photometry by Massey & Thompson (1991); the authors observed 46 such stars tabulated by Molnar & Halas (1994). 3) The star is a member of Cyg OB22 based on UBV photometry by Reddish et al (1966). 4) The star has no published photometry, but is a probable member of the association based on its high polarization. A histogram of stars without photometric measurements reveals they fall into two distinct groups; those with P>2.8% (14 stars) and those with P<1.8% (32 stars). It is improbable that the former are foreground stars, so they are included as likely association members. 5) The star has no published photometry and a low polarization. This does not necessarily preclude membership in Cyg OB2, but makes membership unlikely, and these stars were removed from further analysis in the paper. 6) The star is not a member of Cyg OB2 based on UBV photometry from Massey & Thompson (1991) or Reddish et al (1966) and is not included in any of the analyses in the paper. Note (2): a) The star has previous photometry results in the literature by either McMillan & Tapia (1977) or Serkowski (1965). b) The star has a close companion, making measurement uncertain. Published photometry and polarimetry should be treated with caution. Note (3): Stars numbered "0" indicate no catalog entry for that object. Stars numbered "-1" have multiple catalog entries for the same star. Stars with other negative values are actually a blend of two or more stars and hence the measurements should be regarded as uncertain.
References: Massey, P., & Thompson, A. B. 1991, AJ, 101, 1408 McMillan, R. S., & Tapia, S. 1977, ApJ, 212, 714 Molnar, L. A., & Halas, C. D. 1994, in preparation Reddish, V. C., Lawrence, L. C., & Pratt, N. M. 1966, Publ. Obs. Edinburgh, 5 111 Schulte, D. H. 1956, ApJ, 124, 530 Serkowski, K. 1965, ApJ, 141, 1340 Origin: AAS CD-ROM series, Volume 2, 1994
(End) Lee Brotzman [ADS] 23-Apr-1994, Patricia Bauer [CDS] 19-Oct-1994
The document above follows the rules of the Standard Description for Astronomical Catalogues.From this documentation it is possible to generate f77 program to load files into arrays or line by line

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