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Studying circumstellar envelopes with ALMA David Neufeld Johns Hopkins University.

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Presentation on theme: "Studying circumstellar envelopes with ALMA David Neufeld Johns Hopkins University."— Presentation transcript:

1 Studying circumstellar envelopes with ALMA David Neufeld Johns Hopkins University

2 The 0.1 – 10 4  m background Since t = 10 5 yr, most of the photons generated in the Universe were emitted by interstellar dust Ned Wright, UCLA

3 Where does this dust come from? Here’s an inventory of sources from Gehrz (1989) Evolved stars as dust factories

4 ‘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

5 Canonical picture AGB stars are intermediate mass stars which are burning H or He in a shell Stellar radius ~ 1 a.u. Stellar luminosity ~ 10 4 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

6 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

7 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

8 The envelopes of AGB stars show a very rich molecular inventory List of detected 63 molecules from Olofsson (2008, ApSS) Note the 1 st astrophysical detection of a molecular anion, C 6 H – (and C 4 H –, C 8 H – and C 3 N – have since been detected)

9 SMA maps can be obtained for each spectral line Molecules resulting from photochemical processing appear in shells (e.g. the C 4 H radical in the above example)

10 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?

11 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?

12 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

13 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

14 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?

15 Circumstellar chemistry is expected to depend upon C/O ratio Oxygen-rich stars: expect CO and H 2 O Carbon-rich stars: expect CO, C 2 H 2, HCN To ZEROTH order, this is the observed behavior, but IRC has much higher than expected H 2 O, OH, H 2 CO, C 3 O, and SiO abundances Indeed, water is widely observed in C-rich stars

16 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)

17 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?

18 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

19 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?

20 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?

21 Many molecular isotopologues are observed in IRC Isotopic ratios from Olofsson (2008, ApSS) Some are quite different from the solar system values No detection yet of radioactive isotopes 14 C and 26 Al

22 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?

23 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

24 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

25 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

26 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

27 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

28 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

29 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 H 2 – free-free opacity)

30 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?


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