Hubble imaging of V1331 Cygni: Proper motion study of its circumstellar
  structures
  The young star V1331 Cyg received previous attention because it is surrounded
by an optical, arc-like reflection nebula. V1331 Cyg is commonly considered to
be a candidate for an object that has undergone an FU-Ori (FUOR) the outbreak
in the past. This in turn could lead to a time-varying appearance of the dusty
arcs that may be revealed by multi-epoch imaging. In particular, a radial
colour analysis of the dust arcs can then be attempted to check whether radial
grain size distribution was modified by a previous FUOR wind. Second-epoch
imaging of V1331 Cyg was obtained by us in 2009 using the Hubble Space
Telescope (HST). By comparing this to archival HST data from 2000, we studied
the time evolution of the circumstellar nebulae. After a point spread function
subtraction using model point spread functions, we used customised routines to
perform a proper motion analysis. The nebula expansion was first derived by
deconvolving and correlating the two-epoch radial brightness profiles.
Additional data from other facilities TLS, UKIDSS, SPITZER, and HERSCHEL were
also incorporated to improve our understanding of the star in terms of
environment, viewing angle, bipolar outflow length, and the FUOR phenomenon.
The derived radial colour profiles do not indicate a spatial separation of the
dust grain sizes. The HERSCHEL 160 micron images show for the time thermal
emission from dust probably residing in the outer arc. By viewing V1331 Cyg
almost pole-on, the length of the bipolar outflow exceeds previous estimates by
far. The outer arc expansion timescale is consistent with the implantation time
of the CO torus, which supports the hypothesis of an outburst that occurred a
few thousand years ago. The azimuthal colour variation of the outer arc is
probably due to changes of the scattering angle, imposed by a tilt or helical
geometry of the dust configuration.
