Testing
Interstellar Medium Chemical Complexity: Line Surveys of the Complex
Molecular Zone at the CSO 
Nearly 160 molecules have
been detected in interstellar and circumstellar environments
(Muller et al. 2005), and a great fraction of these are complex organic
molecules
(COMs, or molecules with 5 or more atoms). Recent chemical modeling
suggests that
reactions of simple radicals on interstellar grain surfaces may be
integral to COM formation
(Garrod et al. 2008), and that grain surface reactions are the dominant
formation
mechanism for many of the most abundant COMs under typical interstellar
cloud conditions.
Modeling studies must be compared with observations to test the
robustness of
the model. Unfortunately, the subset of sources for which extensive COM
inventories
have been compiled is quite small. Modeling studies have initially
focused on comparisons
with the Sagittarius B2(N) hot molecular core, which has the highest
number of
detected molecules of any sightline studied to date. Sgr B2(N) may not
represent an
average source, but instead may be an extraordinary case of complex
interstellar chemistry.
Other types of sources besides high-mass hot cores have been shown to
contain molecules
once considered to be present only in hot core environments. The
research group led by Professor
Susanna L. Widicus Weaver at Emory University is surveying other types
of interstellar
molecular clouds, where the physical and chemical conditions are
different than those seen
in typical hot cores. Their aim is to probe the infuence of physical
environment on molecular
complexity in the interstellar medium. They conducted observations in
July, 2010, to investigate
the molecular complexity in the Complex Molecular Zone (CMZ) of the
Galactic Center
(GC) using the CSO's 1.3 mm broadband receiver. The CMZ has high gas
kinetic temperatures
(>100 K), but low excitation temperatures (~10-20 K) and dust
temperatures (<20K)
(Rodriguez-Fernandez et al. 2000), in stark contrast to those found in
hot cores.
Despite the different physical conditions, gas-phase abundances for
COMs comparable
to those found in hot cores are observed for methanol (CH3OH),
dimethyl ether
(CH3OCH3), and ethylene glycol (HOCH2CH2OH)
toward multiple lines of sight in
the CMZ (Martin-Pintado et al. 2001 & references therein). These
molecules should not
be present in these cold regions if they form through gas-phase
ion-molecule reactions
(Horn et al. 2004, Geppert et al. 2006), and their presence in the CMZ
points to grain
surface formation. It has been proposed that these molecules were
released from grain
surfaces through widespread shocks in the CMZ (Martin-Pintado et al.
2001). Comprehensive
chemical and physical characterization of these sources could provide
insight into the
mechanisms that drive COM formation in the ISM.
Brett A. McGuire, Jay Kroll, and Susanna L. Widicus Weaver have
conducted
deep spectral snapshots of five CMZ sources previously shown to
have high abundances
of COMs at the CSO. They then chose the source that displays the
richest molecular
complexity and the brightest line intensity and completed a full
spectral line survey.
An example spectrum from the G+0.693-0.027 source is shown in Figure 1.
Several of the lines identifed in this spectrum are attributed to
molecules that
have not previously been detected in this source. The variety of
species seen in just
this one window demonstrates the chemical complexity of this source.
Full spectral
deconvolution is underway, and they will soon have a more comprehensive
picture of the
chemistry in this source.
Figure 1: Double sideband spectrum of G+0.693-0.027 from the CSO 1.3
mm-band line survey.
References
Garrod R. T., Widicus Weaver S. L., & Herbst E. (2008) ApJ 682, 283.
Geppert W. D. et al. (2006) Farad. Discuss. 133, 177.
Herbst E. & van Dishoeck E., 2009, Ann. Rev. of Astron. &
Astrophys., 47, 427.
Horn A. et al. (2008) ApJ 611, 605.
Martin-Pintado J. et al. (2001) ApJL 548, L65.
Muller H. S. P. et al. (2005), J. Mol. Struct., 742, 215.
Rodriguez-Fernandez N. J., et al. (2000) A & A, 356, 695.
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