Ultraviolet Observations of Diffuse Cloud Chemistry

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Ultraviolet Observations of Diffuse Cloud
Chemistry

• Why study IS chemistry this way
• Technical considerations - why it is difficult
• Notable UV spectroscopic missions
• The atomic gas
• Molecular observations
• Future opportunities

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Why UV Observations are Useful for Studies of
Diffuse IS Chemistry

• Most of the abundant elements have ground
  state transitions in the UV, allowing the
  derivation of diffuse cloud physical parameters
• Many molecular species have electronic
  transitions from the ground state at UV
  wavelengths
• These include the two most abundant identified
  molecules, H2 and CO
• The UV may offer the best hope for specific
  identification of large organic species suspected
  to be present

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Why UV Observations are Useful for Studies of
Diffuse IS Chemistry
Most ground-state electronic transitions lie at
UV wavelengths
(Courtesy E. B. Jenkins)
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Why UV Studies of Interstellar Chemistry are
Difficult

• Observations must be made from space
• While normal-incidence optics work in the UV,
  special coatings are needed for UV reflectance -
  and some are tricky to use
• Interstellar extinction due to dust works
  against you, especially for lines of sight with
  significant column densities
• The hot (O and B) stars used as background
  light sources have complex spectra, making it
  difficult to detect weak IS lines
• Molecular hydrogen bands get in the way

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Why UV Studies of Interstellar Chemistry are
Difficult
Observations must be made from space
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Why UV Studies of Interstellar Chemistry are
Difficult
Extinction due to interstellar dust
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Why UV Studies of Interstellar Chemistry are
Difficult
Hot stars have complex spectra
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Why UV Studies of Interstellar Chemistry are
Difficult
Molecular hydrogen gets in the way
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Notable UV Spectroscopic Missions
Many spectacular sounding rocket
experiments (e.g., Stecher, Carruthers,
Morton) Copernicus (1972 - 1980) IUE
(1978 - 1996)
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Notable UV Spectroscopic Missions
HST/GHRS (1990 - 1997) HST/STIS (1997 -
2004) FUSE (1999 - present) HST/COS?
(2007?)
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UV Studies of Atomic Gas in Diffuse Clouds

• Overall composition of the diffuse IS gas
• Depletions - the raw material for molecule
  formation
• Physical conditions in diffuse clouds
• Cloud kinematics the role of shocks

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The Astronomers Periodic Table(Courtesy B. J.
McCall)
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The Depletions of Elements in Diffuse
Interstellar Clouds
Taken from Savage and Sembach 1995
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Cloud Physical Conditions from Atomic Observations
Ionization equilibrium provides electron
densities Lines arising from fine-structure
excited states reveal cloud densities Line
widths (velocity dispersions) can indicate cloud
thermal temperatures - but more often show the
spread of macroscopic velocities Line shifts
reveal cloud kinematics, distribution of
components
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UV Observations of Interstellar Molecules

• List of species detected and upper limits
  (zeta Oph)
• Molecular hydrogen
• Deuterated molecular hydrogen (HD)
• Carbon monoxide (CO)
• Other species

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Molecules Detected Through UV Spectra(for zeta
Oph)

• H2 logN(H2) 20.62 H2/H 0.31 Morton 1975
• H2 logN(H2) 11.81 H2/H
  4.8(-10) Federfman et al. 1995
• HD logN(HD) 14.20 HD/H 1.2(-7) Morton 1975
• CO logN(CO) 15.40 CO/H 1.9(-6) Lambert et
  al. 1994
• 13CO logN(13CO) 13.17 13CO/H 1.1(-8) Lambert
  et al. 1994
• HCl loN(HCl) 11.43 HCl/H 2.0(-10) Federman
  et al. 1995
• OH logN(OH) 13.70 OH/H 3.7(-8) Crutcher and
  Watson 1976
• H2O logN(H2O) Smith 1981

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Results of UV H2 Observations
Analysis of the observed lines Abundance,
self shielding, and molecular fraction
Rotational excitation Vibrational
excitation Major survey papers on H2
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FUSE Spectra of H2 in Reddened Lines of
Sight Rachford et al. 2002
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Analysis of Saturated Lines from J 0, 1 By
Profile fitting
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H2 Column densities for J 1 by Curve of Growth
Method
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H2 Abundances, Self Shielding, and Molecular
fraction
Column densities range from 1015 to 1021
cm-2 H2 becomes self-shielding for N(H2)
1019 cm-2 Molecular fractions range from 0.01
to 0.8 No translucent clouds found!
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Rotational Excitation in H2
Rotational excitation is due to a combination
of collisional and radiative effects
Excitation depends on optical thickness of the
low-J lines For lines of sight with self
shielding core, J 0 and J 1 are controlled by
collisions, yielding the local gas kinetic
temperature Higher-J states are governed by
UV pumping
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Vibrational Excitation of H2
High density or, more commonly, high UV
radiatiative flux, can excite vibrationally
excited levels of the ground electronic state
Such excited states are rarely seen in diffuse
clouds But they can be highly populated in H
II regions
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Interstellar HD
Important indicator of chemical
fractionation May help constrain cosmic D/H
ratio If HD becomes self-shielding, then
HD/H2 approaches D/H
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UV Observations of Interstellar CO
CO is second only to H2 in molecular
abundance, in reddened diffuse clouds CO
becomes the dominant form of carbon in dense
molecular clouds UV observations can help
locate the threshold where carbon becomes
molecular CO has several electronic bands in
the UV
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Analysis of CO UV Absorption Bands
Allowed electronic transitions are often saturated
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UV Observations of Interstellar CO
Intersystem lines can be weak enough to be
unsaturated
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UV Observations of Interstellar 13CO
13CO lines can be compared with intersystem lines
of 12CO
12CO/13CO is greatly enhanced over 12C/13C due to
chemical fractionation (Lambert et al
1994) (Note 16C17O and 16C18O are also
underabundant)
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Other Molecules Observed in the UV
OH Pair of lines near 3078A First detected
from the ground Easily seen with HST Abundance
consistent with models (OH/H 10-8)
HCl Predicted by Jura (1976) due to rapid
reaction of H2 with Cl II Detected by Federman
et al. (1995) N2 First sought with
Copernicus (Lutz et al. 1979) Detected with FUSE
by Knauth et al. (2004)
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Interstellar N2
N2 has a ground electronic state transition at
958 A Upper limit for by Lutz, Owen, and
Snow 1979 (delta Sco, Epsilon Per N2/H 10(-9) Detection of 958 A feature claimed
by Knauth et al. 2004, for HD 124314 (Av 1.5)
N2/H 3.1(-8) Measured abundance is low for
dense cloud but high for gas-phase diffuse cloud
chemistry - grain surface reactions?
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Molecules Not Yet Seen in UV Absorption

• CS N 1976
• CH2 N
• CO2 N
• NO N et al. 1973
• NO N Oph Federman et al. 1995
• O2 N
• NaH N Smith 1977
• MgH N
• AlH N
• SiO N

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Future Possibilities for UV Studies of
Interstellar Chemistry
Current outlook is grim STIS is
dead FUSE is dying COS is sitting on the
ground There are no plans for future UV
instruments
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Future Possibilities for UV Studies of
Interstellar Chemistry
The Cosmic Origins Spectrograph (COS)
Slated for installation aboard the HST in 2007
- IF the Shuttle returns to regular operations
UV spectrograph covering 1175 - 3200 A, in two
resolution modes (R 5,000 and R 20,000)
Design and calibration tests show a factor of 10
- 20 greater throughput than previous HST UV
instruments, for comparable resolving power
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Observations of the Cold Interstellar Medium with
the COS
Atomic Gas Atomic abundances and depletions
up to Av 5 New constraints on gas-dust
interaction and grain composition Molecular
observations Look for the transition of
carbon from atomic to molecular form Search
for undetected species (such as C2H and H2
others) Search for electronic transitions of
PAHs and other large organic Dust observations
Extend measures of UV extinction curves to Av
10 Look for weak structure in the extinction
curve
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Acknowledgements
Research colleagues Brian Rachford Mike
Shull John Black Ewine van Dishoeck Ben
McCall Veronica Bierbaum Don York (et al. - the
entire DIBs consortium) Students Nick
Betts Meredith Drosback Adam Jensen Ralph
Shuping Josh Destree Lynsi Aldridge Grants and
Contracts Several from NASA (FUSE, Lab Astro,
Exobiology, HST, COS)