Short time scale spectral variability in the A0 supergiant HD92207 and
the importance of line profile variations
for the interpretation of FORS2 spectropolarimetric observations

S. Hubrig (1), M. Schoeller (2), A.F. Kholtygin (3,4)

1 Leibniz-Institut fuer Astrophysik Potsdam
3 Saint-Petersburg State University
4 Isaac Newton Institute of Chile, Saint-Petersburg Branch

Our recent search for the presence of a magnetic field in the bright early
A-type supergiant HD92207 using FORS2 in spectropolarimetric mode indicated the
presence of a longitudinal magnetic field of the order of a few hundred Gauss.
Assuming the ideal case of a non-variable star, this discovery has recently been
questioned in one work trying to demonstrate the importance of non-photon noise
in FORS2 observations. The assumption of non-variability of HD92207 can,
however, not be held since substantial profile variations of diverse lines on a
time scale of minutes or maybe even a fraction of a minute are detected in
FORS2 spectra. The presence of short-term spectral variability in blue
supergiants, which are considered as type II supernova progenitors, has not been
a subject of systematic studies before and is critical for the current
theoretical understanding of their physics. Given the detected short term
variability, the question of the presence of a magnetic field cannot be answered
without proper modeling of the impact of such a variability on the measurements
of the magnetic field. Since the short-term periodicity does not fit into the
currently known domain of non-radially pulsating supergiants, its confirmation
is of great importance for models of stellar evolution.

Reference: MNRAS
Status: Manuscript has been accepted