Large grains in 7 classical RV Tauri stars

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Large grains in 7 classical RV Tauri stars
Stephanie De Ruyter (UGent)Hans Van Winckel (KU
Leuven)Carsten Dominik (Amsterdam)Herwig
Dejonghe (UGent)
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Introduction
?
AGB star Spherically symmetric
HST Planetary Nebulae All kind of forms
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Introduction

• Post-AGB stars
• RV Tauri stars
• Archetypical example AC Her
• A sample of 7 classical RV Tauri stars
• Star Kuruczmodel
• IR excess optically thin dust model
• Importance of 850 micron data
• Characteristics of dust
• SCUBA 850 micron observations
• TIMMI2 10 micron observations
• Conclusions

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Post-AGB stars

• High luminosity
• Possess a large IR excess
• Associated with a circumstellar dust shell
• High mass-loss rate in previous phase
• Typical dust grain size 0.1 ?m
• Example HD161796

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RV Tauri stars

• classified by 3 parameters
• (1) distinct light curve
• uncommon type of variable stars
• light variation with an alternating pattern of
  deep and shallow minima
• (also in the colours)
• (2) period 30-150 days
• (3) spectral type F to K

Example AC Her Period 75 days
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RV Tauri stars as pulsating stars
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RV Tauri stars as post-AGB stars
Example AC Her

• Large IR excess circumstellar dust
• Difference
• Genuine post-AGB stars peak around 30 ?m
• ? Td 100 K
• RV Tauri objects shallow slope between 12 and 25
  micron
• ? Td 1000 K

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RV Tauri stars as binaries

• Searching for new RV Tauri like objects using the
  SED characteristics of the rare RV Tauri stars
• ? Sample of newly identified objects
• with RV Tauri type IR colours,
• but with wider spread in
• spectral range
• Systematic radial velocity
• monitoring program
• ? Thomas Maas found a lot of binaries
• ? BUT not for classical RV Tauri stars
• reason very large pulsation amplitude
• (typical 20 km/s)

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SED Large IR excess
Dust processing Crystalline features
CO profiles
Weak and narrow emission Typical rotation
velocities instead of outflows
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Archetypical RV Tauri star AC Her
Binarity Orbital period P 1200 days
Variability Pulsation period P 75 days
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A sample of 7 classical RV Tauri stars
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Starmodel E(B-V)
Starmodel Kuruczmodel with Teff and log g ?
Colour excess E(B-V)
UY CMa E(B-V) 0 ? no reddening ? not
spherically symmetric ? disc Other stars small
E(B-V) as well
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Optically thin dust model

• Assumptions
• Dust in thermal equilibrium with stellar
  radiation field
• Radius of dust particles constant
• Consider
• (spherically symmetric) dust cloud around a star
• that extends from inner radius to outer
  radius
• with a number density distribution
• with an emission efficiency of the dust grains
• with the distance to the star
• Then the flux observed at earth is

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Optically thin dust model
Thermal equilibrium gives with some
normalization temperature The parameters of the
optically thin dust model are
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Optically thin dust model
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Importance of 850 micron data
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Characteristics of dustJCMT SCUBA 850 micron
observations

• To extend the SED to submm continuum emission
• ? to constrain disc characteristics
• ? to model circumstellar emission
• ? to constrain grain size
• ? to obtain a good estimate of the dust mass
• To gain insight into the structure, physical
  properties and evolution of circumstellar
  material around RV Tauri stars

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Spectral index p

• p 0
• large grains ( 1 ?m)
• gray particles

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Distance D and Dust Mass M
P-L relation (MACHO project LMC variable
inventory) (for P gt 12 days) ? distance D
Dust mass M
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Ratio of LIR to Lstar
Thick disc large angle necessary to produce an
IR excess as large as found here
LIR amount of energy absorbed Lstar
bolometric stellar flux
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Characteristics of dust 3p6 TIMMI2 10 micron
observations

• IRAS 12067-4508 (RU Cen)
• TIMMI2 spectrum March 10 , 2004
• Only preliminary reduction
• Clear 10 micron feature
• presence of small warm silicates
• Spectral features (e.g. forsterite)
• dust processing

• IRAS 10174-5704
• Spectrum as seen in outflow
• Similar to ISM silicate feature

• HD179218
• ISO SWS
• spectrum
• of a young
• stellar object
• Very similar

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Conclusions

• Models are compatible with the assumption of a
  disc - small colour index E(B-V)
• - spectral index p ? 0 ? large grains
• - large LIR/Lstar
• - clear 10 micron feature
• - crystalline silicates
• These discs resemble very much protoplanetary
  discs around young stars while their formation
  history has been very different.
• A disc may be helpful in explaining the bipolair
  outflows seen in lots of PNe.