the circumstellar environments of the cool hypergiants: implications for the mass loss mechanism...

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