Transcript HSDAS_poster_70CMx100CMx - Israel Institute for Advanced

33rd Winter School in Theoretical Physics on: Exoplanets (28 December, 2015 – 08 January, 2016) at the Israel Institute of Advanced Studies (IIAS), the Hebrew University of Jerusalem.
Magnetic field structure of a large Bok Globule CB34
Himadri Sekhar
1
1
Das ,
A.
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Bok globules are small,
opaque, and isolated nearby
molecular clouds.
Their cores usually have a
quite simple structure that
makes them ideally suited
for studies of isolated lowmass star formation (Bok
1947, ApJ, 105, 255).
& B. J.
2
Medhi
Department of Physics, Assam University, Silchar 788011 (INDIA)
2 ARIES, Manora Peak, Nainital 263129 (INDIA)
OPTICAL POLARIZATION
BOK GLOBULES
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1
Das
Optical
Optical polarimetric
observations of CB34
were performed with
the 1.04-metre
Sampurnanand
telescope of he
Aryabhatta Research
Institute of
observational sciencES
(ARIES) near Nainital
in India on 11-13
March, 2013 and on
20 Feb, 2014.
NIR
Barnard 68
Fig: The stellar polarization vectors and position angles are superimposed on a 15×15
arcmin2 R-band DSS image of the field containing CB34. A vector with a polarization of
1% is drawn for reference; the length of all polarization vectors is proportional to it.
The ‘×’ symbol represents the globule centre RA = 05h47m01s, DEC=+21d01m11s.
SUBMILLIMETER POLARIZATION
Ref: Launhardt et al. (2010, ApJS, 188, 139)
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LARGE BOK GLOBULE CB34
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CB34 is rather atypical and more massive than other Bok globules.
This globule is situated at a distance of ~1.5 kpc.
It has three dense cores and is associated with numerous young stars and that
seem to have formed from this cloud (Huard et al. 2000, A&A, 362, 635;
Khanzadyan et al. 2002, A&A, 383, 502; Launhardt et al. 2010, ApJS, 188,
139).
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MAGNETIC FIELD IN BOK GLOBULES
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Magnetic fields play a major role on the evolution of dark clouds and may
control the fragmentation of clouds to form stars (Mouschovias & Morton
1991, ApJ, 371, 296; Li & Nakamura 2004, ApJ, 609, L83).
In general, they can influence the contraction timescale, the gas-dust coupling,
and the shape of cloud fragments, and they host jets and outflows (McKee &
Ostriker 2007, ARA&A, 45, 565).
POLARIMETRY
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Fig: 850 μm polarization vectors are sampled on a
10’’ grid. Contours range from 0.1 Jy beam−1 in
steps of 0.05 Jy beam−1
The submm observation of CB34
were made at the James Clerk
Maxwell Telescope (JCMT),
Mauna Kea, Hawaii, using the
Submillimeter Common User
Bolometer Array (SCUBA).
The submm polarization data
presented here is taken from the
Matthews et al. (2009, ApJS, 182,
143) legacy data set. Their analysis
restricted to only six data points.
To increase the number of data
points, we have reanalyzed the data
set and calculated polarization
vector at each pixel and use the
criteria I > 0, p/Ep > 2 and Ep < 6%.
MAGNETIC FIELD STRENGTH
The magnetic field strength (in Gauss) in the plane-of-sky is (Chandrasekhar & Fermi 1953,
ApJ, 118, 113):
Polarization arises from alignment of interstellar dust grains in the presence of
a magnetic field (Davis & Greenstein 1951, ApJ, 114, 206).
When the light from the background stars passes through the clouds,
extinction and reddening are caused due to absorption and scattering by the
dust grains present in the clouds. This phenomenon also introduces linear
polarization in the starlight, if the dust grains are aligned and dichroic.
Optical polarimetric observations
of dark cloud can give information
about the magnetic field orientation
in the low-density edge regions of
clouds
Polarimetric observations in the
infrared and submillimetre range
can map the field orientation in the
higher density central regions of the
clouds.
Thus, the resulting polarization map
outlines the geometry of the
magnetic field lines projected on to
the plane of the sky, which in turn
helps us to study the relationship
between structure kinematics and
the embedded magnetic field during
various stages of star formation
RESEARCH POSTER PRESENTATION DESIGN © 2015
www.PosterPresentations.com
CONCLUSIONS
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The mean value of polarization (optical) and position angle for the 35 field stars is
<popt>= 2.14 % and <θopt>= 143.2o with a standard deviation of σp = 0.84% and σθ =
7.8o respectively.
The peripheral magnetic field of the cloud CB34 (optical polarization study) is almost
aligned with galactic magnetic field having position angle of 146o.
The mean polarization and position angle obtained from sub-mm observations are given
by <psub>= 12.5 % and <θsub>= 74.5o.
The angular offset between peripheral magnetic field and inner magnetic field is ~ 69o.
The mangetic field strength at peripheral region is higher (~ 205μG) as compared to
inner magnetic field strength (~ 28μG). The inner magnetic field is also not aligned
among themselves due to high turbulence of gas at center of the cloud (~ 0.64 km s-1).
ACKNOWLEDGEMENTS
Ref: Goodman et al. (1995), ApJ, 448, 748
This work is supported by the Science and Engineering Research Board (SERB), a
statutory body under Department of Science and Technology (DST), Government
of India, under Fast Track scheme for Young Scientist (SR/FTP/PS-092/2011).