The Circumstellar Environments of the Cool Hypergiants: Implications for the Mass Loss Mechanism...
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The Circumstellar Environments of the Cool Hypergiants:
Implications for the Mass Loss Mechanism
Roberta M Humphreys University of Minnesota
VY CMa
NML CygIRC +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 supergiantsIntermediate – type or “yellow” hypergiants
Intermediate –Type
IRC +10420 (A – F Ia)
rho Cas (F8 p Ia)
HR 8752 (G0 – 5 Ia)
HR 5171a (G8 Ia)
M Supergiants
VY CMa (M5e Ia)
NML Cyg (M6 I)
S Per (M3 – 4 e 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 excessHigh mass loss rate 3-6 x 10-4 Warmest maser sourceSpectroscopic 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 arcsEvidence 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
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
mass loss rate 4 x 10-4
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%
3D Morphology of VY CMa
Polarization combined with color relative distance along line of sight
Together with space motion and vector
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 --
= SN1954j
Recent examples
V1 in N 2363SN 1997bs in N 3627SN 1999bw in N 3198SN 2000ch in N 3432SN 2001ac in N 3504 SN 2002kg = V74 in N 2403
SN 2003gm in N 5334SN 2003hy in IC 5145