G11.92-0.61-MM2: A Bonafide Massive Prestellar Core?
  Core accretion models of massive star formation require the existence of
stable massive starless cores, but robust observational examples of such
objects have proven elusive. We report subarcsecond-resolution SMA 1.3 mm, 1.1
mm, and 0.88 mm and VLA 1.3 cm observations of an excellent massive starless
core candidate, G11.92-0.61-MM2, initially identified in the course of studies
of GLIMPSE Extended Green Objects (EGOs). Separated by ~7.2" from the nearby
MM1 protostellar hot core, MM2 is a strong, compact dust continuum source
(submillimeter spectral index alpha=2.6+/-0.1), but is devoid of star formation
indicators. In contrast to MM1, MM2 has no masers, no centimeter continuum, and
no (sub)millimeter wavelength line emission in ~24 GHz of bandwidth observed
with the SMA, including N2H+(3-2), HCO+(3-2), and HCN(3-2). Additionally, there
is no evidence for an outflow driven by MM2. The (sub)millimeter spectral
energy distribution (SED) of MM2 is best fit with a dust temperature of ~17-19
K and luminosity of ~5-7 L_sun. The combined physical properties of MM2, as
inferred from its dust continuum emission, are extreme: M>30 M_sun within a
radius<1000 AU, N(H2)>10^25 cm^-2 and n(H2)>10^9 cm^-3. Comparison of the
molecular abundance limits derived from our SMA observations with gas-grain
chemical models indicates that extremely dense (n(H)>>10^8 cm^-3), cold (<20 K)
conditions are required to explain the lack of observed (sub)millimeter line
emission, consistent with the dust continuum results. Our data suggest that
G11.92-0.61-MM2 is the best candidate for a bonafide massive prestellar core
found to date, and a promising target for future, higher-sensitivity
observations.
