The far-infrared polarization spectrum of Rho Ophiuchi A from HAWC+/SOFIA observations

• Published in: arXiv.org 2019-07
• Main Authors: Santos, Fabio P, Chuss, David T, Dowell, C Darren, Houde, Martin, Looney, Leslie W, Enrique Lopez Rodriguez, Novak, Giles, Ward-Thompson, Derek, Berthoud, Marc, Dale, Daniel A, Guerra, Jordan A, Hamilton, Ryan T, Hanany, Shaul, Harper, Doyal A, Henning, Thomas K, Jones, Terry Jay, Lazarian, Alex, Michail, Joseph M, Morris, Mark R, Staguhn, Johannes, Stephens, Ian W, Tassis, Konstantinos, Trinh, Christopher Q, Eric Van Camp, Volpert, C G, Wollack, Edward J
• Format: Article
• Language: English
• Subjects: Alignment; Astrochemistry; Density; Far infrared radiation; Grains; Infrared analysis; Massive stars; Molecular clouds; Polarimetry; Polarization; Radiation shielding; Space telescopes; Trends
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1905.00705

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.