CDROM/AJ/V107/P1433
Cygun OB2 (Kobulnicky+ 1994)
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R Band Polarimetry of Cygnus OB2: Implications for the Magnetic Field
Geometry
and Polarization Models
Henry A. Kobulnicky, Lawrence A. Molnar, Terry Jay Jones
<1994, AJ, 107, 1433>
=1994AJ....107.1433K
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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 ~0.5 deg, compared to 0.2 deg for the optical
extinction. This constrains the variations in the magnetic field
orientation to sizes less than 8 pc. 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 4 pi 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.
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
File Summary:
------------------------------------------------------------------------------File Name
Lrecl
Records
Explanations
------------------------------------------------------------------------------table1.dat
80
168
Polarization Data
table1.tex
138
222
AASTeX version of table 1
------------------------------------------------------------------------------Byte-by-byte Description of file: table1.dat
------------------------------------------------------------------------------Bytes Format Units
Label
Explanations
------------------------------------------------------------------------------1- 6
F6.3
deg
GLon
Galactic longitude, l
7-12
F6.3
deg
GLat
Galactic latitude, b
13-15
F3.0
h
RAh
Right ascension, J2000.0
16-18
F3.0
min
RAm
R.A.
19-23
F5.1
s
RAs
R.A.
241X
----Blank
25
A1
--DESign declination, J2000.0
26-27
F2.0
deg
DEd
Declination
28-30
F3.0
arcmin
DEm
Dec.
31-33
F3.0
arcsec
DEs
Dec.
34-35
2X
----Blank
36
I1
--Code
*[1,6] Coded notes
37-42
F6.2
--Pol
Percent polarization
43
A1
--n_Pol
*[ab] Note for Pol
44-48
F5.2
--e_Pol
Uncertainty in Pol
49-54
F6.1
deg
PosAng
Position angle of polarization
55-60
F6.2
mag
AV
Amount of visual absorption
61-65
A5
--RLP
*Other name in Reddish et al (1966)
66-70
A5
--MT
*Other name in Massey & Thompson
(1991)
71-75
A5
--Sch
*Other name in Schulte (1956)
76-80
A5
--KMJ
*Other name in this work
------------------------------------------------------------------------------Notes for file: table4.dat
------------------------------------------------------------------------------Code:
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.
n_Pol:
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.
RLP, MT, Sch, KMJ:
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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(End)
Lee Brotzman [ADS] 23Apr-1994