Near-infrared polarimetric study of the
bipolar nebula IRAS 19312+1950
Murakawa, K., Nakashima, J., Ohnaka,
K., Deguchi, S.
A&A, 470, pg.957(2007)
Abstract
Aims. We have investigated the properties of the central star
and dust in the bipolar nebula IRAS 19312+1950, which is an unusual
object showing characteristics of a supergiant, a young stellar object,
and an asymptotic giant branch (AGB) star.
Methods. We obtained H-band polarimetric data of IRAS 19312+1950
using the near-infrared camera (CIAO) on the 8 m Subaru telescope. In
order to investigate the physical properties of the central star and
the nebula, we performed dust radiative transfer modeling and compared
the model results with the observed spectral energy distributions
(SEDs), the radial profiles of the total intensity image and the
fraction of linear polarization map.
Results. The total intensity image shows a nearly spherical core
with ~3" radius, an S-shaped arm ex tending ~10" in the northwest to
southeast direction, and an extended lobe towards the southwest. The
polarization map shows a centrosymmetric vector alignment in almost
the entire nebula, and low polar izations along the S-shaped arm.
These results suggest that the nebula is accompanied by a central star,
and the S-shaped arm has a physically ring-like structure. From our
radiative transfer modeling, we estimated the stellar temperature, the
bolometric luminosity, and the current mass-loss rate to be 2 800 K,
7000 Lsun , and 5.3 × 10 -6 Msun yr
-1 ,
respectively.
Conclusions. Taking into account previous observational results
such as the detection of SiO maser emissions and silicate absorption
feature in the 10 µm spectrum, our dust radiative trans fer analysis
based on our near-infrared imaging polarimetry suggests that (1) the
central star of IRAS 19312+1950 is likely to be an oxygen-rich,
dust-enshrouded AGB star and (2) most of the circumstellar material
originates from other sources (e.g. ambient dark clouds) rather than as
a result of mass loss from the central star.
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