J/ApJ/793/126 L204 - Cloud 3 polarimetry and photometry (Cashman+, 2014)
The magnetic field of Cloud 3 in L204.
Cashman L.R., Clemens D.P.
<Astrophys. J., 793, 126 (2014)>
=2014ApJ...793..126C 2014ApJ...793..126C (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; Interstellar medium ; Molecular clouds ;
Polarization ; Photometry, infrared ; Extinction
Keywords: dust, extinction - ISM: clouds -
ISM: individual objects (L204) - ISM: magnetic fields -
magnetic fields - polarization
Abstract:
The L204 dark cloud complex is a nearby filamentary structure in
Ophiuchus North that has no signs of active star formation. Past
studies show that L204 is interacting with the nearby runaway O star,
ζ Oph, and hosts a magnetic field that is coherent across
parsec-length scales. Near-infrared H-band (1.6 µm) linear
polarization measurements were obtained for 3896 background stars
across a 1°x1.5° region centered on the dense Cloud 3 in L204,
using the Mimir near-infrared instrument on the 1.8 m Perkins
Telescope. Analysis of these observations reveals both large-scale
properties and small-scale changes in the magnetic field direction in
Cloud 3. In the northern and western ζ Oph facing regions of the
cloud, the magnetic field appears to be pushed up against the face of
the cloud. This may indicate that the UV flux from ζ Oph has
compressed the magnetic field on the western edge of L204. The
plane-of-sky magnetic field strength is estimated to be ∼11-26 µG
using the Chandrasekhar-Fermi method. The polarimetry data also reveal
that the polarization efficiency (PE ≡ PH/AV) steadily
decreases with distance from ζ Oph (-0.09%±0.03%/mag/pc).
Additionally, power-law fits of PE versus AV for localized samples
of probe stars show steeper negative indices with distance from ζ
Oph. Both findings highlight the importance of external illumination,
here from ζ Oph, in aligning dust grains to embedded magnetic
fields.
Description:
Linear polarimetry measurements were obtained in the NIR H band
(1.6 um) using the Mimir instrument (Clemens et al.
2007PASP..119.1385C 2007PASP..119.1385C) on the 1.8 m Perkins telescope outside of
Flagstaff, AZ. Mimir used a 10242 Aladdin III InSb detector array,
operated at 33.5 K. The instrument had a field of view (FOV) of
10'x10' and pixel scale of 0.58''. Polarimetric analysis was
performed with a cold, stepping, H-band, compound half-wave plate
(HWP) plus a fixed, cold wire-grid analyzer. Sky observations were
planned for a grid of 54 abutting fields (a 6x9 field grid)
covering a 1°x1.5° region of L204 centered on Cloud 3. They were
conducted over several nights between 2010 May and 2012 May. To obtain
polarimetry measurements, exposures of five second durations were
obtained for 16 unique HWP position angles at six sky-dither
positions, yielding a total of 96 images per field. The total on-sky
integration time of the entire map (eight minutes per FOV) was 7.2 hr.
Polarization flat-fields for each HWP position were taken using a
lights-on/lights-off method against a uniformly illuminated flat-field
screen inside the closed telescope dome.
Objects:
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RA (ICRS) DE Designation(s)
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16 47 47.3 -12 05 16 L204 = LDN 204
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 123 7150 Polarimetry and Photometry of 7150 Stars
Towards L204 - Cloud 3
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See also:
J/A+AS/141/175 : Dark clouds imaging polarimetry (Sen+, 2000)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 4 I4 --- ID [1/7150] Star index number
6- 7 I2 h RAh Hour of Right Ascension (J2000)
9- 10 I2 min RAm Minute of Right Ascension (J2000)
12- 15 F4.1 s RAs Second of Right Ascension (J2000)
17 A1 --- DE- Sign of the Declination (J2000)
18- 19 I2 deg DEd Degree of Declination (J2000)
21- 22 I2 arcmin DEm Arcminute of Declination (J2000)
24- 25 I2 arcsec DEs Arcsecond of Declination (J2000)
27- 33 F7.2 % U Stokes U Mimir measurement
35- 39 F5.2 % e_U Uncertainty in U
41- 46 F6.2 % Q Stokes Q Mimir measurement
48- 52 F5.2 % e_Q Uncertainty in Q
54- 59 F6.2 % P Degree of polarization (1)
61- 65 F5.2 % e_P Uncertainty in P
67- 69 I3 deg PA Equatorial position angle; measured East of North
71- 73 I3 deg e_PA Uncertainty in PA
75 I1 --- UFlag [1/3] Usage flag (2)
77 A1 --- l_Jmag [<] Upper limit flag on Jmag
78- 82 F5.2 mag Jmag ?=99.99 2MASS J band magnitude (3)
84- 87 F4.2 mag e_Jmag ? Uncertainty in Jmag (3)
89- 93 F5.2 mag Hmag ?=99.99 2MASS H band magnitude (3)
95- 98 F4.2 mag e_Hmag ? Uncertainty in Hmag (3)
100 A1 --- l_Ksmag [<] Upper limit flag on Ksmag
101-105 F5.2 mag Ksmag ?=99.99 2MASS KS band magnitude (3)
107-110 F4.2 mag e_Ksmag ? Uncertainty in Ksmag (3)
112 A1 --- BFlag [0/2] Background flag (4)
114-118 F5.2 mag AV ?=-9.99 The V band extinction (5)
120-123 F4.2 mag e_AV ? Uncertainty in AV
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Note (1): P values have been corrected for positive bias. The correction was
also propagated into the PA uncertainties.
Note (2): Flag as follows:
1 = highest quality;
2 = moderate;
3 = poor.
Note (3): J, H, and K values of 99.99 indicate that no matching 2MASS star was
found. No values in uncertainties indicate measurements which are upper
limits.
Note (4): Flag as follows:
1 = foreground;
2 = background;
0 = unknown.
Note (5): From NICER for BFlag=1 and 2. BFlag=0 set to AV=-9.99±0.00.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Tiphaine Pouvreau [CDS] 20-Apr-2017