The far-infrared polarization spectrum of Rho Ophiuchi A from HAWC+/SOFIA observations. (arXiv:1905.00705v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Santos_F/0/1/0/all/0/1">Fabio P. Santos</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chuss_D/0/1/0/all/0/1">David T. Chuss</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dowell_C/0/1/0/all/0/1">C. Darren Dowell</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Houde_M/0/1/0/all/0/1">Martin Houde</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Looney_L/0/1/0/all/0/1">Leslie W. Looney</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rodriguez_E/0/1/0/all/0/1">Enrique Lopez Rodriguez</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Novak_G/0/1/0/all/0/1">Giles Novak</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ward_Thompson_D/0/1/0/all/0/1">Derek Ward-Thompson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Berthoud_M/0/1/0/all/0/1">Marc Berthoud</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dale_D/0/1/0/all/0/1">Daniel A. Dale</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Guerra_J/0/1/0/all/0/1">Jordan A. Guerra</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hamilton_R/0/1/0/all/0/1">Ryan T. Hamilton</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hanany_S/0/1/0/all/0/1">Shaul Hanany</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Harper_D/0/1/0/all/0/1">Doyal A. Harper</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Henning_T/0/1/0/all/0/1">Thomas K. Henning</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jones_T/0/1/0/all/0/1">Terry Jay Jones</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lazarian_A/0/1/0/all/0/1">Alex Lazarian</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Michail_J/0/1/0/all/0/1">Joseph M. Michail</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Morris_M/0/1/0/all/0/1">Mark R. Morris</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Staguhn_J/0/1/0/all/0/1">Johannes Staguhn</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Stephens_I/0/1/0/all/0/1">Ian W. Stephens</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tassis_K/0/1/0/all/0/1">Konstantinos Tassis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Trinh_C/0/1/0/all/0/1">Christopher Q. Trinh</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Camp_E/0/1/0/all/0/1">Eric Van Camp</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Volpert_C/0/1/0/all/0/1">C. G. Volpert</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wollack_E/0/1/0/all/0/1">Edward J. Wollack</a>

We report on polarimetric maps made with HAWC+/SOFIA toward Rho Oph A, the
densest portion of the Rho Ophiuchi molecular complex. We employed HAWC+ bands
C (89 $mu$m) and D (154 $mu$m). The slope of the polarization spectrum was
investigated by defining the quantity R_DC = p_D/p_C, where p_C and p_D
represent polarization degrees in bands C and D, respectively. We find a clear
correlation between R_DC and the molecular hydrogen column density across the
cloud. A positive slope (R_DC > 1) dominates the lower density and well
illuminated portions of the cloud, that are heated by the high mass star Oph
S1, whereas a transition to a negative slope (R_DC < 1) is observed toward the denser and less evenly illuminated cloud core. We interpret the trends as due to a combination of: (1) Warm grains at the cloud outskirts, which are efficiently aligned by the abundant exposure to radiation from Oph S1, as proposed in the radiative torques theory; and (2) Cold grains deep in the cloud core, which are poorly aligned due to shielding from external radiation. To assess this interpretation, we developed a very simple toy model using a spherically symmetric cloud core based on Herschel data, and verified that the predicted variation of R_DC is consistent with the observations. This result introduces a new method that can be used to probe the grain alignment efficiency in molecular clouds, based on the analysis of trends in the far-infrared polarization spectrum.

We report on polarimetric maps made with HAWC+/SOFIA toward Rho Oph A, the
densest portion of the Rho Ophiuchi molecular complex. We employed HAWC+ bands
C (89 $mu$m) and D (154 $mu$m). The slope of the polarization spectrum was
investigated by defining the quantity R_DC = p_D/p_C, where p_C and p_D
represent polarization degrees in bands C and D, respectively. We find a clear
correlation between R_DC and the molecular hydrogen column density across the
cloud. A positive slope (R_DC > 1) dominates the lower density and well
illuminated portions of the cloud, that are heated by the high mass star Oph
S1, whereas a transition to a negative slope (R_DC < 1) is observed toward the
denser and less evenly illuminated cloud core. We interpret the trends as due
to a combination of: (1) Warm grains at the cloud outskirts, which are
efficiently aligned by the abundant exposure to radiation from Oph S1, as
proposed in the radiative torques theory; and (2) Cold grains deep in the cloud
core, which are poorly aligned due to shielding from external radiation. To
assess this interpretation, we developed a very simple toy model using a
spherically symmetric cloud core based on Herschel data, and verified that the
predicted variation of R_DC is consistent with the observations. This result
introduces a new method that can be used to probe the grain alignment
efficiency in molecular clouds, based on the analysis of trends in the
far-infrared polarization spectrum.

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