Spectropolarimetry of the Massive post-Red Supergiants IRC +10420 and HD 179821

M. Patel$^1,2$, R.D. Oudmaijer$^1$, J.S. Vink$^3,4$, J.E. Bjorkman$^5$, B. Davies$^1,6$, M.A.T. Groenewegen$^7$, A.S. Miroshnichenko$^8$, J.C. Mottram$^1$

1 - University of Leeds; 2 - Imperial College London; 3 - Armagh Observatory; 4 - Keele University; 5 - Ritter Observatory; 6 - Rochester Institute of Technology; 7 - Instituut voor Sterrenkunde; 8 - University of North Carolina at Greensboro

We present medium resolution spectropolarimetry and long term
photo-polarimetry of two massive post-red supergiants, IRC +10420 and
HD 179821. The data provide new information on their circumstellar
material as well as their evolution. In IRC +10420, the polarization
of the H alpha line is different to that of the continuum, which
indicates that the electron-scattering region is not spherically
symmetric. The observed long term changes in the polarimetry can be
associated with an axi-symmetric structure, along the short axis of
the extended reflection nebulosity. Long term photometry reveals that
the star increased in temperature until the mid-nineties, after which
the photospheric flux in the optical levelled off. As the photometric
changes are mostly probed in the red, they do not trace high stellar
temperatures sensitively. And so, it is not obvious whether the star
has halted its increase in temperature or not. For HD 179821 we find
no polarization effects across any absorption or emission lines, but
observe very large polarization changes of order 5% over 15
years. During the same period, the optical photometry displayed modest
variability at the 0.2 magnitude level. This is unexpected, because
large polarization changes are generally accompanied by strong
photometric changes. Several explanations for this puzzling fact are
discussed. Most of which, involving asymmetries in the circumstellar
material, seem to fail as there is no evidence for the presence
of hot, dusty material close to the star. A caveat is that the
sparsely available near-infrared photometry could have missed periods
of strong polarization activity. Alternatively, the variations can be
explained by the presence of a non-radially pulsating photosphere.
Changes in the photometry hint at an increase in temperature
corresponding to a change through two spectral subclasses over the
past ten years.

Reference: MNRAS, accepted
Status: Manuscript has been accepted



Email: m.patel06@imperial.ac.uk