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Molecular Cooling Rates _Neufeld_ Lepp and Melnick 1995_

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Molecular Cooling Rates _Neufeld_ Lepp and Melnick 1995_ Powered By Docstoc
					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
• 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
       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
         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
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
     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)
  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
          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)
            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
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])
The Herbig-Haro objects HH7–11




       Khanzadyan et al. 2003, MNRAS
       H2 v = 1– 0 S(1) 2.12 mm
       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
        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”
        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
       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
    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 CO2 emission
  in the vicinity of massive protostars
• Study of Cepheus A East led by Paule
  Sonnentrucker
  Spitzer observations of Cepheus A East
(Sonnentrucker et al. 2006): warm H2 (in J=4)
   Spitzer observations of Cepheus A East
(Sonnentrucker et al. 2006): ionized neon, Ne+
 Spitzer observations of Cepheus A East
(Sonnentrucker et al. 2006): gaseous CO2
Carbon dioxide in Cepheus A East
                              CO2 is fluorescently pumped by 15
 Derived CO2 column density   mm continuum radiation from the
                              protostar HW2. Column density
                              estimates (from Eduardo Gonzalez-
                              Alfonso) reach N(CO2 gas) ~ 1016
                              cm–3 and appear to be correlated
                              with those of warm H2
     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
       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
        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
        H2 rotational structure
Absence of dipole moment  DJ = 2 selection rule

                  J=4

                               S(2)


                  J=3
                          S(1)
                  J=2
                  J=1   S(0)
                  J=0

                        Molecular
                        hydrogen
        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
        H2 rotational structure
Absence of dipole moment  DJ = 2 selection rule

                  J=4

                               S(2)


                  J=3
                          S(1)
                  J=2
                  J=1   S(0)
                  J=0

                        Molecular
                        hydrogen
         H2 rotational structure
Absence of dipole moment  DJ = 2 selection rule

                                J=4

                                          S(2)


   J=3
          S(1)
                                J=2
   J=1                                 S(0)
                                J=0

    Ortho-hydrogen                Para-hydrogen
    Nuclear spin, I = 1           Nuclear spin, I = 0
    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 )
     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
    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-to-para ratio in equilibrium

Room temperature:
Ortho-H2/para-H2 = 3




                                          Room temperature
LH2 temperature:


                          Boiling point
Ortho-H2/para-H2 = 0.01
    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
     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)
        HH54 rotational diagram

Zigzag behavior
 ortho-para ratio out
of equilibrium

Slope  Tgas = 650 K

Ortho-para ratio = 1.2
       Top = 90 K
      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
    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
    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
     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
           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)
Spitzer observations of H2
         in HH54
 Map of column density of
warm H2 (states with J = 4 – 9)
Maps of physical conditions:
    Gas temperature
Maps of physical conditions:
  H2 ortho-to-para ratio
       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
Similar results obtained for HH7 – 11
                           Gas temperature


         Column density




                            Ortho/para ratio
H2 rotational diagrams for HH54




                           H2 S(3)
                           [NeII]
     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
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
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
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
Correlations between temperature
     and ortho-to-para ratio
• For both sources, the points are
  confined to a specific “allowed” region


                    Forbidden (o/p ratio above LTE)
 Ortho/para ratio




                    Allowed       Forbidden: para-to-ortho conversion
                                  puts all points above cyan line


                    Forbidden (below minimum initial o/p ratio)
                                            Rotational temperature
  Correlations between temperature
       and ortho-to-para ratio
• Conclusions:
  – Amongst the best evidence we have for a non-steady-
    state chemistry in a shocked gas region
  – Lower envelope to allowed region is consistent with para-
    to-ortho conversion via H + p-H2  o-H2 + H with an
    energy barrier of ~ 4000 K
  – Minimum ortho-to-para ratios of 0.25 (HH7) to 0.4 (HH54)
    correspond to LTE at temperatures of 50 – 60 K
  – Higher initial ortho-to-para ratios in HH8 (0.8 – 3.0) may
    indicate previous episode of shock heating to above
    1000 K
       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
Interstellar hydrogen deuteride

• We have recently obtained a
  serendipitous detection of hydrogen
  deuteride in the molecular cloud IC443
  (ApJL, submitted)

• Collaborators: Joel Green, David Hollenbach,
  Paule Sonnentrucker, Gary Melnick, Ted
  Bergin, Ronald Snell, Bill Forrest, Dan Watson,
  and Michael Kaufman
Interstellar hydrogen deuteride

• In our observations of HH54 and HH7–11, we
  discovered weak features at the wavelengths of
  the R(3) and R(4) transitions of hydrogen
  deuteride
  – Here, a weak dipole moment (and the fact that we
    have two non-identical nuclei) permits transitions with
    DJ = 1
• Stronger features were evident in the spectra of
  Cepheus A West and IC443, a molecular cloud
  subject a supernova-driven shock
Previous detection of mid-IR HD emission
in Orion, claimed by Bertoldi et al. (1999)




                          ISO short wavelength
                          spectrometer
  Observed spectra


Cep A W

IC443
                     Cep A W
 HH7
                      IC443
 HH54C

 HH54E+K

 HH54FS
        Mapping observations are
           possible in IC443
• Very clear
  morphological
  similarities between
  the emission from
  warm H2 and the
  features we attribute
  to warm HD
HD abundance determination
• The HD/H2 abundance ratio can be
  determined reliably because
  – The H2 temperature and column density is
    very well constrained by observations of
    the H2 S(0) through S(7) rotational lines
  – The HD emission is optically thin
  – The gas density is well constrained by the
    HD R(4)/R(3) line ratio
HD R(4)/R(3) is an increasing
  function of gas pressure
              Results for IC443
N(HD)/N(H2) = 1.23 0.26 105
                   0 .20

                      0.11
log10(n[H2]T) = 7.20 0.09

(68.3% confidence limits)
      Deuterium abundance
• If all the gas-phase deuterium is in HD,
  the implied [D]/[H] ratio in the gas-phase
  is ~ 5.1 – 7.5 x 10–6
  – a factor 2.5 below that in the local bubble
    (Moos et al. 2002), but comparable to
    n(D)/n(H) measured in atomic clouds at
    distances > 500 pc from the Sun
           Deuterium abundance
(plotted on atomic D/H compilation of Friedman et al. 2006)




                                              IC443
      Deuterium abundance
 Ongoing debate:
• Does depletion play a role in determining the
  gas-phase deuterium abundance, as suggested
  by Jura (1982) and Draine (1994, who
  suggested deuterated PAHs as an important
  reservoir)?
  (Also, might additional depletion in deuterated
  ices – e.g. CH2DOH – occur in dense clouds?)

• Or has the abundance been reduced below the
  primordial/high-z value of 2.5 – 3 x 10–5 entirely
  by astration?
     Upcoming observations
• An upcoming Cycle 4 Spitzer program
  will increase the signal-to-noise ratio in
  the sources HH54, HH7, IC443, and
  Cepheus A West
  – Promises to yield a larger sample of HD/H2
    abundance ratios in shocked molecular
    gas throughout the Galaxy.
                 Summary

• Protostars profoundly effect molecules within
  the interstellar gas clouds in which they are
  born
• Spitzer’s Infrared Spectrometer provides a
  sensitive probe of emission from these
  molecules
  – 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

				
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