Studying circumstellarenvelopes with ALMA

David Neufeld

Johns Hopkins University

The 0.1 – 104 m backgroundSince t = 105 yr, most of the photons generated in the Universe were emitted by interstellar dust

Ned Wright, UCLA

Where does this dust come from?Here’s an inventory of sources from Gehrz (1989)

Evolved stars as dust factories

‘O-rich’ (C/O < 1)

‘O-rich’ (C/O < 1)

‘C-rich’ (C/O > 1)

90% of Galactic dust comes from AGB stars

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?