Download OH26.5+0.6 : Insight into the circumstellar structure of

OH26.5+0.6 :
Insight into the circumstellar structure
of an extreme OH/IR star
Sandra Etoka
In collaboration with
Philip Diamond
Outline
• Introduction
• Maser emission extent
• Geometric and polarimetric structure
• Discussion/Summary
Introduction
• OH26.5+0.6 is: an extreme 8M OH/IR star with a massloss rate on the order of 5 x 10-4 M/yr. It is one of the
brightest OH maser emitters in our Galaxy and is suspected
of experiencing a superwind which emerged less than 150
years ago (Justtanont et al. 1996).
Introduction
• OH26.5+0.6 is: an extreme 8M OH/IR star with a massloss rate on the order of 5 x 10-4 M/yr. It is one of the
brightest OH maser emitters in our Galaxy and is suspected
of experiencing a superwind which emerged less than 150
years ago (Justtanont et al. 1996).
• MERLIN OH observations at 18 cm in the 1667, 1665 and
1612 MHz maser line allowed the first combined insight into
the geometric and polarimetric structure of the circumstellar
material surrounding this object.
Introduction
• OH26.5+0.6 is: an extreme 8M OH/IR star with a massloss rate on the order of 5 x 10-4 M/yr. It is one of the
brightest OH maser emitters in our Galaxy and is suspected
of experiencing a superwind which emerged less than 150
years ago (Justtanont et al. 1996).
• MERLIN OH observations at 18 cm in the 1667, 1665 and
1612 MHz maser line allowed the first combined insight into
the geometric and polarimetric structure of the circumstellar
material surrounding this object.
• This work intends to bring a new insight into the structural
changes undergone by the circumstellar material of an
evolved star when it enters the OH/IR stage.
Maser emission
extent
• The spot located at the
centre of the maser
distribution at 1612 at
a velocity of 41 km/s
is quite likely to be on
the stellar position.
1612 MHz velocity integrated image in
Stokes I
Maser emission
extent
• The total extent of the
OH masing shell at
1612 MHz is ~ 5 arcsec
corresponding to a
linear extent of ~7000
au at 1.4 kpc
• It is an ellipsoid
elongated in the E-W
direction
1612 MHz velocity integrated image in
Stokes I without the strong central
maser spot
Maser emission
extent
• The 1667 MHz extends ~1
arcsec further than the 1612
MHz. This is surprising
since maser theory predict
the 1665/67 MHz emission
to be internal to that at 1612
MHz
• Also, the orientation of the
long axis at 1612 and 1667
MHz maser distribution
does not seem to coincide
• Why is that so ?
1667 MHz velocity integrated image in
Stokes I
Maser emission
extent
• the 1665 MHz maser
emission is totally
asymmetric:
• a strong central maser spot
+ a filament oriented NW
• the total extent is less than 2
arcsec
1665 MHz velocity integrated image
in Stokes I
1665 MHz maser
spot distribution
• Maser spot
distribution
criteria:
• 3 consecutive
channels
• 1665 MHz: 3σ
• 1667 & 1612
MHz: 4σ
• P only shown if
≥3σ
1665 MHz maser
spot distribution
• Maser spot
distribution
criteria:
• 3 consecutive
channels
• 1665 MHz: 3σ
• 1667 & 1612
MHz: 4σ
• P only shown if
≥3σ
• The 2 blue-shifted
central spots are
quite likely an
amplification of
the stellar radiation
1665 MHz maser
spot distribution
• Maser spot
distribution
criteria:
• 3 consecutive
channels
• 1665 MHz: 3σ
• 1667 & 1612
MHz: 4σ
• P only shown if
≥3σ
• The 2 blue-shifted
central spots are
quite likely an
amplification of
the stellar radiation
• a clear filament is
traced by the red
shifted maser spots
1667 MHz maser
spot distribution
• 81 maser spots
• Only the 2
strongest red
shifted maser spots
show P≥ 3σ
• The south part of
the shell is
dominated by blue
shifted maser
emission while the
N-NE part by the
red shifted
emission
1612 MHz maser
spot distribution
• Out of the 277
maser spots at 4σ
106 show P ≥ 3σ
• The southern part
of the maser
distribution is
clearly dominated
by blue-shifted
emission
• B=3-4 mG inferred
by Zeeman splitting
1612 MHz maser
spot distribution
• Out of the 277
maser spots at 4σ
106 show P ≥ 3σ
• The southern part
of the maser
distribution is
clearly dominated
by blue-shifted
emission
• B=3-4 mG inferred
by Zeeman splitting
• The distribution of
the vectors of
polarization is a
mixture of radial
(N) and tangential
(S).
1612 MHz maser
spot distribution
• Out of the 277
maser spots at 4σ
106 show P ≥ 3σ
• The southern part
of the maser
distribution is
clearly dominated
by blue-shifted
emission
• B=3-4 mG inferred
by Zeeman splitting
• The distribution of
the vectors of
polarization is a
mixture of radial
40-60º
(N) and tangential
(S). This is
consistent with a
poloidal magnetic
field titled ~40-60º
to the LOS (Elitzur 1992; Kemball & Diamond 1997)
1667 MHz
radial velocity
distribution
• V=f(θ) allow us to
infer the thickness
of the OH masing
shell (~1.5-2
arcsec) and that
acceleration takes
place
1612 MHz
radial velocity
distribution
1612 MHz
radial velocity
distribution
1612 MHz
radial velocity
distribution
• V=f(θ) for both the
1667 and 1612 MHz
is well explained by a
prolate shell titled
about 45º - 65º to the
line of sight (Bower’s
model 1991)
1612 MHz
radial velocity
distribution
• V=f(θ) for both the
1667 and 1612 MHz
is well explained by a
prolate shell titled
about 45º - 65º to the
line of sight (Bower’s
model 1991)
Discussion/Summary
• Polarization structure consistent with a poloidal magnetic
field inclined ~ 40-60º to the line of sight
Discussion/Summary
• Polarization structure consistent with a poloidal magnetic
field inclined ~ 40-60º to the line of sight
• This coincides with the geometrical major-axis of the
ellipsoidal maser distribution inferred from V=f(θ)
Discussion/Summary
• Polarization structure consistent with a poloidal magnetic
field inclined ~ 40-60º to the line of sight
• This coincides with the geometrical major-axis of the
ellipsoidal maser distribution inferred from V=f(θ)
• Magnetic field strength inferred from Zeeman splitting: 3-4
mG at 7000 au. Similar to what was found for the super
massive evolved star NML Cyg at similar distance (Etoka &
Diamond 2004)
Discussion/Summary
• Polarization structure consistent with a poloidal magnetic
field inclined ~ 40-60º to the line of sight
• This coincides with the geometrical major-axis of the
ellipsoidal maser distribution inferred from V=f(θ)
• Magnetic field strength inferred from Zeeman splitting: 3-4
mG at 7000 au. Similar to what was found for the super
massive evolved star NML Cyg at similar distance (Etoka &
Diamond 2004)
The end: Thanks !