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Interstellar molecules in the protostellar environment Recent results from Spitzer

Interstellar molecules in the protostellar environment Recent results from Spitzer. David Neufeld Johns Hopkins University. Molecules in the protostellar environment. Introduction The astrophysical environment of protostars Molecular astrophysics with the Spitzer Space Telescope

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Interstellar molecules in the protostellar environment Recent results from Spitzer

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  1. Interstellar molecules in the protostellar environment Recent results from Spitzer David Neufeld Johns Hopkins University

  2. Molecules in the protostellar environment • Introduction • The astrophysical environment of protostars • Molecular astrophysics with the Spitzer Space Telescope • New results from Spitzer • Interstellar carbon dioxide: probing grain destruction • Molecular hydrogen: a fossil record of the thermal history of the interstellar gas • Hydrogen deuteride: measuring the gas density and the deuterium abundance

  3. Molecules in the protostellar environment • Introduction • The astrophysical environment of protostars • Molecular astrophysics with the Spitzer Space Telescope • New results from Spitzer • Interstellar carbon dioxide: probing grain destruction • Molecular hydrogen: a fossil record of the thermal history of the interstellar gas • Hydrogen deuteride: measuring the gas density and the deuterium abundance

  4. Protostars and the interstellar gas • Protostars have two principal effects on the interstellar material that surrounds them • They are sources of luminosity that heat the surrounding gas and dust • They emit supersonic jets that drive shock waves into the ambient medium • Protostars wreak profound physical and chemical changes on the interstellar medium

  5. Radiative heating by protostars • Protostars can be prodigious sources of luminosity, with L = few – few x 104 L, depending upon the mass • In the early stages of evolution, the radiation is typically absorbed by interstellar dust and reradiated in the infrared • Yields “hot cores” with gas and dust temperatures of 100 – few x 100 K and unusual chemical composition

  6. Chemistry in hot cores • Hot core regions  enhanced abundances of water vapor, methanol, deuterated species, sulphur-containing species, large saturated molecules Attributed to non-equilibrium chemistry, driven by ice mantle evaporation (recall yesterday’s talks by Malcolm, Cecilia, Xander)

  7. Absorption line spectroscopy of gaseous and solid H2O toward massive protostars(Boonman & van Dishoeck 2003) • Observed water vapor abundances in hot cores n(H2O)/n(H2) ~ 5 x 10–6 to 6 x 10–5 …. a factor of 102 – 104 greater than those measured in cold molecular cloud cores where protostars are absent

  8. Analogous results for CO2(Boonman et al. 2003) Observed carbon dioxide abundances in hot cores n(CO2)/n(H2) ~ 7 x 10–8 to 3 x 10–7 (from absorption line observations of n2 Q-branch at 14.97 mm)

  9. Effects of shocks • Physical effects: shocks heat, accelerate and compress the interstellar medium, elevating gas temperatures to a few x 102 to a few x 103 K at substantial distances from the protostar • Chemical effects: • Shocks can drive chemical reactions with activation energy barriers • Shocks can sputter grain mantles, releasing material into the gas phase • Fast shocks can dissociate molecules, ionize the resultant atoms, and even destroy refractory grain cores

  10. The Herbig-Haro objects HH1–2 HH2 HH1 Source VLA1 HST/WFPC2 image from Hester et al. 1998, AJ (red = [SII], green = Ha, blue = [OIII])

  11. The Herbig-Haro objects HH7–11 Khanzadyan et al. 2003, MNRAS H2 v = 1– 0 S(1) 2.12 mm

  12. Molecules in the protostellar environment • Introduction • The astrophysical environment of protostars • Molecular astrophysics with the Spitzer Space Telescope • New results from Spitzer • Interstellar carbon dioxide: probing grain destruction • Molecular hydrogen: a fossil record of the thermal history of the interstellar gas • Hydrogen deuteride: measuring the gas density and the deuterium abundance

  13. Mid- and far-infrared spectroscopy • Mid and far-IR spectroscopy is a key tool for studying of the molecular environment of protostars • Infrared Space Observatory (1995 – 1998) provided complete coverage of the 2.5 – 197 micron region: Results reviewed in 2004 ARA&A article by van Dishoeck, “ISO SPECTROSCOPY OF GAS AND DUST: From Molecular Clouds to Protoplanetary Disks”

  14. Mid- and far-infrared spectroscopy • Spitzer Space Telescope (launched 2003). IRS instrument provides coverage of the 5.2 – 37 micron region with much higher sensitivity and better spatial resolution than ISO • Provides access to • Fine structure emissions from atoms and atomic ions (e.g. Fe+, Si+, S, Ne+) • vibrational bands of gas-phase molecules (e.g. CO2, C2H2) and ices (e.g. H2O, CO2) • pure rotational transitions of H2 and HD

  15. Molecules in the protostellar environment • Introduction • The astrophysical environment of protostars • Molecular astrophysics with the Spitzer Space Telescope • New results from Spitzer • Interstellar carbon dioxide: probing grain mantle vaporization • Molecular hydrogen: a fossil record of the thermal history of the interstellar gas • Hydrogen deuteride: measuring the gas density and the deuterium abundance

  16. Gaseous carbon dioxide • CO2 was one of three interstellar molecules detected for the first time with ISO • CO2, in both the solid and gas phases, has been observed toward several massive protostars (e.g. Boonman et al. 2003), mainly in absorption toward the protostar • Spitzer allows us to map weak CO2emission in the vicinity of massive protostars • Study of Cepheus A East led by Paule Sonnentrucker

  17. Spitzer observations of Cepheus A East (Sonnentrucker et al. 2006): warm H2 (in J=4)

  18. Spitzer observations of Cepheus A East (Sonnentrucker et al. 2006): ionized neon, Ne+

  19. Spitzer observations of Cepheus A East (Sonnentrucker et al. 2006): gaseous CO2

  20. Carbon dioxide in Cepheus A East CO2 is fluorescently pumped by 15 mm continuum radiation from the protostar HW2. Column density estimates (from Eduardo Gonzalez-Alfonso) reach N(CO2 gas) ~ 1016 cm–3and appear to be correlated with those of warm H2 Derived CO2 column density

  21. Origin of the CO2 mapped in Cepheus A East • Spatial distribution of gaseous CO2 suggests that it associated with slow shocks (like warm H2) • Probably the result of grain mantle sputtering in shocks of velocity 15 – 30 km/s • Absorption by solid CO2 also widely detected against the extended IR continuum with N(CO2 ice)/nH ~ 10–5 • N(CO2 gas)/N(CO2 ice) reaches a maximum of roughly 0.04  up to 4% of the material along a given sight-line is subject to grain mantle sputtering in shocks

  22. Molecules in the protostellar environment • Introduction • The astrophysical environment of protostars • Molecular astrophysics with the Spitzer Space Telescope • New results from Spitzer • Interstellar carbon dioxide: probing grain destruction • Molecular hydrogen: a fossil record of the thermal history of the interstellar gas • Hydrogen deuteride: measuring the gas density and the deuterium abundance

  23. Molecular hydrogen • The most abundant molecule in the Universe, discovered by UV spectroscopy in 1970 • Transitions • Electronic: Dipole-allowed Lyman and Werner bands observable in the far-UV in absorption and fluorescent emission • Vibrational: quadrupole transitions in near-IR • Rotational: quadrupole transitions in mid-IR

  24. H2 rotational structure Absence of dipole moment  DJ = 2 selection rule J = 4 S(2) J = 3 S(1) J = 2 S(0) J = 1 J = 0 Molecular hydrogen

  25. H2 rotational structure The hydrogen nuclei are two identical spin-1/2 fermions  wavefunction must be antisymmetric with respect to interchange of those nuclei For the total spin = 1 state (ortho-H2): the rotational part of the wavefunction must be antisymmetric  J is odd For the total spin = 0 state (para-H2): the rotational part of the wavefunction must be symmetric  J is even

  26. H2 rotational structure Absence of dipole moment  DJ = 2 selection rule J = 4 S(2) J = 3 S(1) J = 2 S(0) J = 1 J = 0 Molecular hydrogen

  27. H2 rotational structure Absence of dipole moment  DJ = 2 selection rule J = 4 S(2) J = 3 S(1) J = 2 S(0) J = 1 J = 0 Para-hydrogen Nuclear spin, I = 0 Ortho-hydrogen Nuclear spin, I = 1

  28. Ortho-to-para ratio in equilibrium High temperature: Ortho-H2/para-H2 = 3, the ratio of the nuclear spin degeneracies Low temperature: Ortho-H2/para-H2 = nJ=1 / nJ=0 = 9 exp (171 K /T )

  29. Ortho-para conversion is extremely slow Not only are DJ = ±1 transitions radiatively forbidden, they are negligible in non-reactive inelastic collisions Reason: a change from even  odd J must be accompanied by a change in nuclear spin Implication: ortho-to-para conversion is extremely slow

  30. A well known effect in the industrial production and storage of LH2 Straightforward refrigeration of H2 leads to liquid H2 with the ortho/para ratio initially “frozen in” at 3

  31. Ortho-to-para ratio in equilibrium Room temperature: Ortho-H2/para-H2 = 3 LH2 temperature: Ortho-H2/para-H2 = 0.01 Room temperature Boiling point

  32. A well known effect in the industrial production and storage of LH2 Straightforward refrigeration of H2 leads to liquid H2 with the ortho/para ratio initially “frozen in” at 3 Ortho-para conversion proceeds slowly, with a timescale ~ 6.5 days, releasing heat as it occurs

  33. A similar effect has also been observed with astrophysical H2 Infrared Space Observatory (ISO/SWS) observations of HH54, a Herbig-Haro object in which molecular gas is shocked by a protostellar outflow  detection of S(1) through S(5) pure rotational lines (Neufeld et al. 1998, ApJL)

  34. HH54 rotational diagram Zigzag behavior  ortho-para ratio out of equilibrium Slope  Tgas = 650 K Ortho-para ratio = 1.2  Top = 90 K

  35. Interpretation of the non-equilibrium ortho/para ratio The gas is currently warm, T ~ 650 K The gas was previously cold, T  90 K The gas not been warm long enough to establish an equilibrium ortho/para ratio

  36. Para-to-ortho conversion • Possible conversion processes • Reactive collisions: • para-H2 + H ↔ H + ortho-H2 • para-H2 + H+ ↔ H+ + ortho-H2 • para-H2 + H3+ ↔ H3+ + ortho-H2 • Grain-surface catalysis • Destruction followed by grain-catalysed reformation

  37. Para-to-ortho conversion • Possible conversion processes • Reactive collisions: • para-H2 + H ↔ H + ortho-H2 • para-H2 + H+ ↔ H+ + ortho-H2 • para-H2 + H3+ ↔ H3+ + ortho-H2 • Grain-surface catalysis • Destruction followed by grain-catalysed reformation • HH54 conditions suggest reaction with H will dominate: barrier ~ 4000 K Para-to-ortho conversion timescale is ~ 5000 yr at 650 K

  38. Shock heating suggested • Shock waves heat gas temporarily: cooling time is smaller than the ortho/para conversion time • Compared results with predictions based on detailed calculations of Timmerman (1996, ApJ) • Ortho/para ratio in HH54 consistent with models for shocks with velocity 10 – 20 km/s

  39. Spitzer observations • The great sensitivity of Spitzer allows the S(0) to S(7) pure rotational transitions of H2 to be mapped at 3 – 10′′ resolution • Results on HH7 –11 and HH54 reported by Neufeld et al. (2006, ApJ, in press; also astro-ph/0606232) • Team members: Joel Green, Kyounghee Kim, Dan Watson, Judy Pipher, Bill Forrest (University of Rochester); Paule Sonnentrucker (Johns Hopkins), Gary Melnick (CfA), Ted Bergin (Michigan)

  40. Spitzer observations of H2in HH54

  41. Map of column density of warm H2(states with J = 4 – 9)

  42. Maps of physical conditions:Gas temperature

  43. Maps of physical conditions:H2 ortho-to-para ratio

  44. Results from mapping • Mean gas temperature shows little variation from one sightline to the next: typical values are 600 – 1000 K • Mean ortho-to-para ratio varies substantially from ~ 0.5 to ~ 3

  45. Similar results obtained for HH7 – 11 Gas temperature Column density Ortho/para ratio

  46. H2 rotational diagrams for HH54 H2 S(3) [NeII]

  47. Fits to rotational diagrams • Curvature  multiple temperatures present • We typically get good agreement with a two-component fit that invokes a warm component at TW ~ 400 K and a hot component at TH ~ 1000 K • Consistent with admixture of shock velocities in the range 10 – 20 km/s • Zigzag behavior • H2 ortho-to-para ratio is smaller than 3 • Departures from LTE are greater for the lower temperature components

  48. H2 rotational diagrams for HH54 Fit parameters for position HH54E Warm component T = 424K N(H2) = 1019.84 OPR = 0.51 Hot component T = 1029 K N(H2) = 1019.20 OPR = 1.89

  49. Correlations between temperature and ortho-to-para ratio Orange curve: LTE ratio Green squares: warm Red squares: hot Black curve: 150 yr age with initial o/p ratio of 0.4 Cyan curves: detailed Wilgenbus et al. models for initial o/p ratio of 0.01

  50. Correlations between temperature and ortho-to-para ratio Orange curve: LTE ratio Green squares: warm Red squares: hot Red and green crosses: HH8/9 Black curve: 150 yr age with initial o/p ratio of 0.25 Cyan curves: detailed Wilgenbus et al. models for initial o/p ratio of 0.01

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