Transcript Molecular Cooling Rates (Neufeld, Lepp and Melnick 1995)

Studying circumstellar
envelopes with ALMA
David Neufeld
Johns Hopkins University
The 0.1 – 104 mm background
Since t = 105 yr, most of the photons generated
in the Universe were emitted by interstellar dust
Ned Wright, UCLA
Evolved stars as dust factories
Where does this dust come from?
Here’s an inventory of sources from Gehrz (1989)
90% of Galactic dust comes from AGB stars
‘O-rich’ (C/O < 1)
‘O-rich’ (C/O < 1)
‘C-rich’ (C/O > 1)
Most AGB stars are oxygen-rich, and
produce silicate dusts: prominent 9.7 micron
emission features
Toward the end of the AGB phase, these
stars may become carbon-rich and produce
carbonaceous dust
Canonical picture
• AGB stars are intermediate mass stars which
are burning H or He in a shell
• Stellar radius ~ 1 a.u.
• Stellar luminosity ~ 104 L
• Photospheric temperature ~ 2000 – 3000 K
• Long period pulsational variables (P ~ 1 yr)
• Outflowing envelopes (v ~ 10 km/s) driven by
radiation pressure on newly formed dust
• Mass loss rates ~10–6 – 10–4 M/yr and
variable
The envelopes of AGB stars show
a very rich molecular inventory
Recent 345 GHz line survey of the carbon –rich AGB
star IRC+10216, obtained with the SMA (Patel et al.
2011)
442 spectral lines
in a 60 GHz
bandpass, 149 of
which are
unassigned
The envelopes of AGB stars show
a very rich molecular inventory
Recent 345 GHz line survey of the carbon –rich AGB
star IRC+10216, obtained with the SMA (Patel et al.
2011)
The spectral line profiles are readily resolved
The envelopes of AGB stars show
a very rich molecular inventory
List of detected 63
molecules from Olofsson
(2008, ApSS)
Note the 1st astrophysical
detection of a molecular
anion, C6H–
(and C4H–, C8H– and C3N–
have since been
detected)
SMA maps can be obtained for each
spectral line
Molecules resulting from photochemical
processing appear in shells (e.g. the C4H
radical in the above example)
Open questions
• Where exactly does dust form?
• How quickly is the outflowing material accelerated?
• How does the envelope composition (dust and gas) relate to the
photospheric abundances (C-rich or O-rich)?
• What roles do photochemistry and shock chemistry play in the
circumstellar envelopes of evolved stars?
• What is the thermal structure of circumstellar envelopes?
• Which transitions show maser action, and how is a population
inversion established
• What are the elemental and isotopic abundances of the material
injected by AGB stars?
• What is the fate of orbiting planets?
• After the AGB phase, how do these stars evolve further?
Open questions
• Where exactly does dust form?
• How quickly is the outflowing material accelerated?
• How does the envelope composition (dust and gas) relate to the
photospheric abundances (C-rich or O-rich)?
• What roles do photochemistry and shock chemistry play in the
circumstellar envelopes of evolved stars?
• What is the thermal structure of circumstellar envelopes?
• Which transitions show maser action, and how is a population
inversion established
• What are the elemental and isotopic abundances of the material
injected by AGB stars?
• What is the fate of orbiting planets?
• After the AGB phase, how do these stars evolve further?
The outflow velocity profile provides
a clue to the location of dust formation
Based on Herschel/HIFI observations, Decin et al. (2010)
compared the linewidths of various molecular transitions in IK
Tau: acceleration more gradual than predicted in simple
models
The outflow velocity profile provides
a clue to the location of dust formation
Interferometric observations
measure narrower line profiles
in the inner envelope, probing
the acceleration zone
SMA results from Patel et al.
(2009), show narrow and
compact SiS v=1-1 J=19-18
emission
Open questions
• Where exactly does dust form?
• How quickly is the outflowing material accelerated?
• How does the envelope composition (dust and gas) relate to the
photospheric abundances (C-rich or O-rich)?
• What roles do photochemistry and shock chemistry play in the
circumstellar envelopes of evolved stars?
• What is the thermal structure of circumstellar envelopes?
• Which transitions show maser action, and how is a population
inversion established
• What are the elemental and isotopic abundances of the material
injected by AGB stars?
• What is the fate of orbiting planets?
• After the AGB phase, how do these stars evolve further?
Circumstellar chemistry is expected
to depend upon C/O ratio
• Oxygen-rich stars: expect CO and H2O
• Carbon-rich stars: expect CO, C2H2, HCN
To ZEROTH order, this is the observed behavior, but
IRC+10216 has much higher than expected H2O, OH,
H2CO, C3O, and SiO abundances
Indeed, water is widely observed in C-rich stars
Herschel/HIFI indicates that water is
widely detectable in carbon stars
Neufeld et al. 2011, ApJ – could be shock chemistry
(Cherchneff 2011) or photochemistry (Decin et al. 2010)
Open questions
• Where exactly does dust form?
• How quickly is the outflowing material accelerated?
• How does the envelope composition (dust and gas) relate to the
photospheric abundances (C-rich or O-rich)?
• What roles do photochemistry and shock chemistry play in the
circumstellar envelopes of evolved stars?
• What is the thermal structure of circumstellar envelopes?
• Which transitions show maser action, and how is a population
inversion established
• What are the elemental and isotopic abundances of the material
injected by AGB stars?
• What is the fate of orbiting planets?
• After the AGB phase, how do these stars evolve further?
Circumstellar envelopes radiate strongly at
submillimeter wavelengths
Royer et al. 2010, A&A – low resolution SPIRE spectrum
of the O-rich red supergiant VY CMa
Open questions
• Where exactly does dust form?
• How quickly is the outflowing material accelerated?
• How does the envelope composition (dust and gas) relate to the
photospheric abundances (C-rich or O-rich)?
• What roles do photochemistry and shock chemistry play in the
circumstellar envelopes of evolved stars?
• What is the thermal structure of circumstellar envelopes?
• Which transitions show maser action, and how is a population
inversion established
• What are the elemental and isotopic abundances of the material
injected by AGB stars?
• What is the fate of orbiting planets?
• After the AGB phase, how do these stars evolve further?
Open questions
• Where exactly does dust form?
• How quickly is the outflowing material accelerated?
• How does the envelope composition (dust and gas) relate to the
photospheric abundances (C-rich or O-rich)?
• What roles do photochemistry and shock chemistry play in the
circumstellar envelopes of evolved stars?
• What is the thermal structure of circumstellar envelopes?
• Which transitions show maser action, and how is a population
inversion established
• What are the elemental and isotopic abundances of the material
injected by AGB stars into the ISM?
• What is the fate of orbiting planets?
• After the AGB phase, how do these stars evolve further?
Many molecular isotopologues are observed
in IRC+10216
Isotopic ratios from
Olofsson (2008, ApSS)
Some are quite different
from the solar system
values
No detection yet of
radioactive isotopes 14C
and 26Al
Open questions
• Where exactly does dust form?
• How quickly is the outflowing material accelerated?
• How does the envelope composition (dust and gas) relate to the
photospheric abundances (C-rich or O-rich)?
• What roles do photochemistry and shock chemistry play in the
circumstellar envelopes of evolved stars?
• What is the thermal structure of circumstellar envelopes?
• Which transitions show maser action, and how is a population
inversion established
• What are the elemental and isotopic abundances of the material
injected by AGB stars into the ISM?
• What is the fate of orbiting planets?
• After the AGB phase, how do these stars evolve further?
The role of ALMA
Several key capabilities are well suited to
addressing these questions:
• high spatial resolution
• high spectral resolution
• access to high frequency transitions
• high sensitivity
The role of ALMA
Several key capabilities are well suited to
addressing these questions:
• high spatial resolution
• high spectral resolution
• access to high frequency transitions
• high sensitivity
AGB stars are fairly rare,
and therefore distant
For example, the closest known C-rich AGB star is
IRC+10216, at a distance of ~ 150 pc*
• Angular radius of photosphere ~ 0.03 arcsec
• Angular radius of dust formation zone ~ 0.1 arcsec
* We are pretty lucky: there are no other known
C-rich AGB stars within 500 pc
The role of ALMA
Several key capabilities are well suited to
addressing these questions:
• high spatial resolution
• high spectral resolution
• access to high frequency transitions
• high sensitivity
The role of ALMA
Several key capabilities are well suited to
addressing these questions:
• high spatial resolution
• high spectral resolution
• access to high frequency transitions
• high sensitivity
The role of ALMA
Several key capabilities are well suited to
addressing these questions:
• high spatial resolution
• high spectral resolution
• access to high frequency transitions
• high sensitivity
High sensitivity will allow imaging of
thermal emission from the photosphere
The very nearest Mira
variables (d ~ 100 pc) can
be imaged with the VLA
(Reid and Menten 2007 –
observations at 43 GHz)
The radio photospheres are
roughly twice the size of the
optical photosphere (H– and
H2– free-free opacity)
Open questions
• Where exactly does dust form?
• How quickly is the outflowing material accelerated?
• How does the envelope composition (dust and gas) relate to the
photospheric abundances (C-rich or O-rich)?
• What roles do photochemistry and shock chemistry play in the
circumstellar envelopes of evolved stars?
• What is the thermal structure of circumstellar envelopes?
• Which transitions show maser action, and how is a population
inversion established
• What are the elemental and isotopic abundances of the material
injected by AGB stars?
• What is the fate of orbiting planets?
• After the AGB phase, how do these stars evolve further?