Mass Loss from Red Giant Branch (and AGB) Stars in Globular Clusters

Andrea DupreeHarvard-Smithsonian Center for Astrophysics

AGB Workshop: 20 May 2010

Stellar Evolution

Outline is solid

Important aspects unresolved: mass loss,

Second parameter….

Globular clusters a classical testing ground to confront these issues

Problems remainMass loss necessary for evolution, but not detected directlyMass lost not found in globular clustersMetallicity [Fe/H] creates differences (‘first parameter’), but…something else is at work…(‘second parameter’)

Candidates for 2nd parameter: total cluster mass; age; environment; free-floating planets; primordial He abundance; post-mixing surface He abundance; CNO abundance; stellar rotation; mass loss; more than one… Here focus on mass loss from the cool stars …

How to detect mass lossCool stars with 2 relevant attributes (based on the Sun,mostly)

1.Chromosphere and coronas T >Tphotosphere

2. Dynamics controlled bymagnetic field configuration: not spherically symmetric.

3. Dust (historic mass loss)

Thus, need to choose diagnosticswisely, and expect variability

Yohkoh x-ray image of Sun

Model of metal-poor giant

Hectochelle at MMT

Meszaros, S. et al. 2008, 2009, AJ

Fibre (240)-fed echelle(A. Szentgyorgyi &D. Fabricant)

R~34,000; 1 degree FOV

Outstanding for clusterstudies

Dynamics from H-α

Wing emission present in luminous stars

Asymmetry varies

But H-alpha core shift/asymmetry always indicates outflow (or static)

Suggests pulsation in lower layers createssteady outflow at top of chromosphere

Meszaros et al. 2009, AJ

Red giants in M92

Dynamics from H-α H-α core shift largest for luminous starsVelocities 0-~15 km s-1; variability in AGB starsOutflow speeds may distinguish AGB from RGB

Meszaros et al. 2008

Meszaros et al. 2009

Flows are outflows; not escape speeds signaling a wind.

Higher up… Ca II K

Ca K3 (●) indicates higher outflow velocity than H-α (×)

Meszaros et al. 2009, AJ

K3

Magellan spectra ofOmega Cen giants

So, what about the mass loss rate?Only reliable way for M_dot from H-α uses non-LTE spherical models with mass flow

Semi-empirical models of the atmospheres constructed for ~20 globular cluster red giants (M15, M92, M13) to match profiles

Meszaros et al 2009

Current ‘laws’ overestimate mass loss

Mass loss increases with L and with decreasing TEFF

Suggestion of metallicity dependenceRates are ~order magnitude less than ‘Reimers’ and IR results

Meszaros et al 2009

‘Dust’ rate(to be corrected;Boyer et al. 2010)

“Reimer’s rate”

AGB Stars with ‘dust’

Filled squares mark dusty AGB stars id’d by Spitzer (Boyer et al. 2006)

H-α bisector velocity similar tostars with no dust.

H-α mass loss rate similar tostars with no dust.

K479

K421

K479

K421

K825 in M15 (RGB/AGB at tip)

Direct evidence for pulsation

McDonald, I. et al. 2010, MNRAS

ΔT [K]Δ

L [s

ola

r lu

min

ositi

es]

NO dust in K825: pulsations do not lead to dust productionPulsation period: ~350 days; LPV; [Fe/H]=-2.5SED validates change in T and Luminosity

Accelerating outflows

Abundances returned to the ism…

A lesson from the Sun…

The helium abundance

found in the solar wind

varies depending onsolar activity and

wind speed….(Kasper et al. 2007)

YEAR

HE

LIU

M

AB

UN

DA

NC

E

Conclusions

Developing a consistent picture of mass loss in metal poor stars H-alpha, Ca K give evidence for accelerating fast outflows from the majority of metal-poor field RGB and AGB starsAGB objects show faster outflows and more variability than RGB starsNo difference in dynamics between ‘dusty’ stars and stars with no IR excessPulsation can drive outflow without dustInferred mass loss provides confirmation needed for stellar evolution calculations