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 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:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 80 168 Polarization data
table1.tex 138 222 LaTeX version of table1.dat
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Byte-per-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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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)
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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.
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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
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Origin: AAS CD-ROM series, Volume 2, 1994
(End) Lee Brotzman [ADS] 23-Apr-1994, Patricia Bauer [CDS] 19-Oct-1994