Abstract
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.
Highlights
Do massive starless cores exist in nature? The answer to this question is a key discriminant between the two major classes of models for massive star formation: “core accretion” and “competitive accretion”
In studying GLIMPSE Extended Green Objects (EGOs; Cyganowski et al 2008), we have identified an excellent candidate for a massive starless core: G11.92−0.61-MM2
The South Pole Telescope Sunyaev–Zel’dovich (SPT-SZ) survey is multi-wavelength (λ = 3.2, 2.0, 1.4 mm), allowing dusty sources to be distinguished from those dominated by synchrotron emission
Summary
Do massive starless cores exist in nature? The answer to this question is a key discriminant between the two major classes of models for massive star formation: “core accretion” and “competitive accretion” (recently reviewed by Tan et al 2014). In addition to the candidates disqualified by sensitive mid-infrared surveys with Spitzer and Herschel, centimeter-submillimeter interferometers have revealed molecular outflows and/or masers—indisputable signs of active star formation—in past “starless” core candidates (e.g., Bontemps et al 2010, Duarte-Cabral et al 2013 in Cygnus-X). In this context, the best chances for identifying robust massive starless core candidates lie in massive star-forming regions for which comprehensive, high-resolution multiwavelength data sets are available. Our initial Submillimeter Array (SMA) 1.3 mm observations of the EGO G11.92−0.61 revealed a massive (proto)cluster, containing three compact cores (Cyganowski et al 2011b, resolution ∼2.4). Primary beam size (FWHP) Frequency coverage: LSB USB Channel widtha
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