Transcript Circumstellar

The Circumstellar Environments of the
Cool Hypergiants:
Implications for the Mass Loss Mechanism
Roberta M Humphreys
University of Minnesota
VY CMa
NML Cyg
IRC +10420
The Cool Hypergiants -- lie just below the upper luminosity envelope
with spectral types A to M, high mass loss
rates, photometric and spectroscopic
variability, large infrared excess,
circumstellar ejecta
Evolutionary state?
M supergiants
Intermediate – type or
“yellow” hypergiants
Intermediate –Type
M Supergiants
IRC +10420 (A – F Ia)
VY CMa (M5e Ia)
rho Cas (F8 p Ia)
NML Cyg (M6 I)
HR 8752 (G0 – 5 Ia)
S Per (M3 – 4 e Ia)
HR 5171a (G8 Ia)
VX Sgr ( M4e Ia – M9.5 I)
mu Cep (M2e Ia)
HST WFPC2 multi-wavelength imaging
No detections – rho Cas, HR 8752, HR 5171a, mu Cep
VX Sgr and S Per (both OH/IR sources) – both marginally resolved
NML Cyg (OH/IR source) -- ejecta shaped by its environment
VY CMa and IRC+10420 (OH/IR sources) – extensive CS nebula
IRC+10420
Strong IR excess
High mass loss rate 3-6 x 10-4
Warmest maser source
Spectroscopic variation late F  mid A
Complex CS Environment
One or more distant reflection shells
Within 2 “ – jet-like structures, rays,
small nearly spherical shells or arcs
Evidence for high mass loss ejections in
the past few hundred years
IRC +10420 – excellent candidate for post– red supergiant
evolution
In post RSG evolution, these stars will enter a region (6000-9000 K) of
increased dynamical instability, high mass loss, increasing opacity –
deJager’s “yellow void”
HST/STIS spatially resolved
spectroscopy –
Halpha – uniform outflow, nearly
spherical distribution
Wind is optically thick  variations
in apparent temperature not due to
evolution
Mass loss history
Origin of ejecta – stellar activity,
convection and magnetic fields?
rho Cas and the yellow hypergiants
rho Cas – famous shell ejection episodes,
1945-47, 1985-87, 2002
develops TiO bands, very high but
temporary mass loss (~ 10-2), quickly
returns to its normal F-type supergiant
spectrum  formation of an optically thick
wind
Most recent 2002 – variability indicative of
pulsational instability. Lobel et al (2003)
demonstrated the outburst was due to the
recombination of H as atmosphere cooled
due to expansion.
Variable A in M33 – a cool hypergiant ~ 45 years in eruption!
One of the original Hubble – Sandage
variables.
“Eruption” or shell ejection began in 1950.
faded 3 mags or more becoming very red.
Spectrum 1985-86 -- M supergiant, CS IR
excess
Spectrum and energy distribution 2004 -05
F-type, bluer, but stayed faint.
NML Cyg – Circumstellar Ejecta Interacting with Its Environment
Optically obscured star embedded in a
small asymmetric bean-shaped nebula,
strong OH/IR source, 6 x 10-5
Similar in shape to HII contours (30” away)
due to interaction of RSG wind with ionizing
photons hot stars in Cyg OB2
Schuster, Humphreys &
Marengo (2006) showed this is
the molecular photodissociation
boundary
VY CMa -- the extreme red supergiant, powerful OH/IR source
mass loss rate 4 x 10-4
Famous asymmetric red nebula, > 10”
across, visible in small ground based
telescopes.
HST/WFPC2 images revealed complex
environment – numerous knots, filamentary
arcs, prominent nebulous arc
Due to multiple, asymmetric
ejection episodes -- analogous to
solar/stellar activity
High Resolution, Long-Slit Spectroscopy --Keck HIRES Spectrograph
Reflected absorption lines -A strong velocity gradient
across the NW arc
Moving mirror effect -> expanding at 50 km/s
K I emission lines
Across the NW arc
K I emission -- across Arcs 1 and 2
2nd epoch images > 6 yrs later  transverse motions
Combined with radial or Doppler velocities  total space motion
and direction of motion
Results: Expansion velocities relative to star and ejection times
NW Arc + 46 km/s at ~ +22o towards WNW , ~ 500 yrs ago
Arc 1
- 68 km/s at -33o towards SW, ~ 800 yrs ago
Arc 2
- 64 km/s at -17o towards SSE, ~ 460 yrs ago
SW knots -36 km/s at -25o towards W, ~ 250 yrs ago
S knots -17 km/s at -27o towards SE, ~ 160 yrs ago
VY CMa -- Work in Progress, polarimetry with HST/ACS
VY CMa is highly polarized -- up to 70%
Polarization combined with color 
relative distance along line of sight
Together with space motion and
vector
3D Morphology of VY CMa
What drives the mass loss of red supergiants and cool hypergiants ??
Pulsational instability? -- rho Cas, irregular and LPVs
Non-radial pulsations and/or convective activity (starspots)?
VY CMa – no general spherical outflow,
looplike structures, clumps of knots,
ejected at different times and from
different positions on star.
Magnetic field ~ 1 – 2 G SiO masers
IRC +10420 – outermost arcs ~ 3000 yrs
ago  RSG, but looplike structures, arcs
and knots close to star, high mass loss
episode < 600 yrs ago ended ~ 90 yrs
ago.
Magnetic field 1 mG OH masers  few G
at star
Supernova Impostors -Recent examples
= SN1954j
V1 in N 2363
SN 1997bs in N 3627
SN 1999bw in N 3198
SN 2000ch in N 3432
SN 2001ac in N 3504
SN 2002kg = V74 in
N 2403
SN 2003gm in N 5334
SN 2003hy in IC 5145