Transcript osu2006.ysheffer.ppt

VUV Survey of 12CO/13CO in the
Solar Neighborhood with the
Hubble
Space
Telescope
Y. Sheffer, M. Rogers, S. R. Federman
Department of Physics and Astronomy
University of Toledo, Toledo, OH 43606
D. L. Lambert
Astronomy Department
University of Texas, Austin, TX 78712
R. Gredel
Max Planck Institute for Astronomy
Konigstuhl 17, 69117 Heidelberg, Germany
VUV Survey of 12CO/13CO in the Solar
Neighborhood with HST
• VUV = Vacuum Ultra-Violet.
• Survey = An archival sample of 23 lines of sight.
•
13CO
= 12CO + n
• Isotopologues, not isotopomers…
• Solar Neighborhood = Stellar targets are typically less than 1-2
kpc from the Sun, a fraction of the Galactic Radius.
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Why? (1)
•
12C
and 13C isotopes have different nucleosynthesis production
avenues.
• Their ratio provides a check on the history of chemical
enrichment in the Galaxy.
• Radio studies of mm-wave CO and H2CO emission have
provided a mean of 12C/13C = 70 ± 7 based on high-density
molecular clouds (Wilson and Rood 1994, Wilson 1999).
Isotopologues are proxies for isotopes.
• VUV absorption probes diffuse clouds. This is a sub-sample
from our larger CO & H2 project.
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Why? (2)
• CO is susceptible to chemical fractionation via
12CO + 13C+ → 13CO + 12C+ + 35K. This makes 12/13
lower at lower Tkin (Watson et al. 1976).
• Physical fractionation occurs via selective
photodissociation (PD): 12CO self-shields first, thus
driving 12/13 to higher values (van Dishoeck &
Black 1988).
• Fractionation provides information about physical
and chemical processes along the line of sight.
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Observations
• Twenty-three lines of sight from HST archival data: 18 from STIS
and 5 from GHRS.
• Instrumental resolution: λ/Δλ = 46,000 to 160,000 (STIS) and
18,500 (GHRS).
• Need to obtain exposures of both very strong and very weak
absorption bands in order to derive the correct optical depth along
sightline. STIS has full UV coverage in Echelle mode.
• Up to 14 simultaneous A-X (4th positive) bands: τ range ~1000.
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Method
• ISMOD is a simplex code that models Voigt profiles of
IS absorption lines, using equations from Black & van
Dishoeck (1988).
• It can handle single atomic transitions or a multitude of
lines from molecules (H2, CO). Here, the input is fvalues for A 1Π - X 1Σ bands (Chan et al. 1993,
Eidelsberg et al. 1999) and λ (Morton & Noreau 1994).
• Spectrum synthesis returns radial velocity, column
density, excitation temperature, and cloud structure
(velocity differences, fractions, Doppler widths).
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P.S. Cloud structure
• The number of components, their velocities and widths affect the
optical depth.
• For lo-res GHRS spectra, structure is imported from our hi-res
optical data (McDonald, ESO) or from the literature.
• For hi-res STIS spectra, ISMOD can find the structure directly
from the CO bands.
• There is a very good agreement b/w the hi-res, UV, CO-derived
structure and the optical, CH- and/or CN-derived structures.
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16 cm-2
N(12
CO)
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21 10
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21 1014 cm-2
N(13
CO)
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Analysis
• We compute Ntot(12CO)/Ntot(13CO) and compare the result with
Ntot(12C)/Ntot(13C).
• Determine the fraction of sight lines that significantly show
fractionated ratios.
• Higher ratios = self-shielding of 12CO.
• Lower ratios = CE with C+ enhances 13CO.
• Check for possible correlations of the CO isotopologic(?) ratio
with other observables.
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Results
• 39% of sight lines show CO isotopologic ratios that
are significantly fractionated (> 2 σ).
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Sheffer et al. (2002)
Lambert et al. (1994)
Federman et al. (2003)
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Results
• 39% of sight lines show CO isotopologic ratios that
are significantly fractionated (> 2 σ).
• When plotted vs. Ntot(12CO), one sees 3 regimes of
12/13: selective photo-dissociation, charge exchange
with C+, and no fractionation (≈ 70).
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Selective PD
Un-fractionated
13C+
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 13CO
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Results
• 39% of sight lines show CO isotopologic ratios that
are significantly fractionated (> 2 σ).
• When plotted vs. Ntot(12CO), one sees 3 regimes of
12/13: selective photodissociation, charge exchange
with C+, and no fractionation (≈ 70).
• There are no obvious correlations with other
observables, including IUV, nCN.
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To be continued…
• This is “work in progress”.
• The two primary CO isotopologues do show that fractionation
is not uncommon in the diffuse ISM, by up to a factor of 2.
• Intrinsic vs. extrinsic effects need to be sorted out for each
sight line.
• Additional diffuse avenues: 12C18O (Lambert et al. 1994) and
12C17O (Sheffer et al. 2002).
The End
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