Compact substructure is expected to arise in a starless core as mass becomes concentrated in the central region likely to form a protostar. Additionally, multiple peaks may form if fragmentation occurs. We present ALMA Cycle 2 observations of 60 starless and protostellar cores in the Ophiuchus molecular cloud. We detect eight compact substructures which are >15 arcsec from the nearest Spitzer YSO. Only one of these has strong evidence for being truly starless after considering ancillary data, e.g., from Herschel and X-ray telescopes. An additional extended emission structure has tentative evidence for starlessness. The number of our detections is consistent with estimates from a combination of synthetic observations of numerical simulations and analytical arguments. This result suggests that a similar ALMA study in the Chamaeleon I cloud, which detected no compact substructure in starless cores, may be due to the peculiar evolutionary state of cores in that cloud.

We present new measurements of the dust emissivity index, beta, for the high-mass, star-forming OMC 2/3 filament. We combined 160-500 mu m data from Herschel with long-wavelength observations at 2 mm and fit the spectral energy distributions across a similar or equal to 2 pc long, continuous section of OMC 2/3 at 15 000 AU (0.08 pc) resolution. With these data, we measured beta and reconstructed simultaneously the filtered-out large-scale emission at 2 mm. We implemented both variable and fixed values of beta, finding that beta = 1.7-1.8 provides the best fit across most of OMC 2/3. These beta values are consistent with a similar analysis carried out with filtered Herschel data. Thus, we show that beta values derived from spatial filtered emission maps agree well with those values from unfiltered data at the same resolution. Our results contradict the very low beta values (similar to 0.9) previously measured in OMC 2/3 between 1.2 mm and 3.3 mm data, which we attribute to elevated fluxes in the 3.3 mm observations. Therefore, we find no evidence of rapid, extensive dust grain growth in OMC 2/3. Future studies with Herschel data and complementary ground-based long-wavelength data can apply our technique to obtain reliable determinations of beta in nearby cold molecular clouds.

We have obtained new Jansky Very Large Array (VLA) observations at X, K, and Ka bands (3 cm, 1.3 cm, and 0.9 cm) which have resolved the continuum emission from the most promising candidate for a massive pre-stellar core discovered to date: 011.92-0.61 MM2. As described in Cyganowski et al. (2014), this bright dust continuum source (190 mJy at 1.1 ram) exhibits no spectral line emission in sub-arcsecond-resolution Submillimeter Array (SMA) images across 24 GHz of bandwidth, including the typical tracers CO, HCN, HCO, and N2H+. Astrochemical models require high density (>109 cm(-3)) and low temperature (<20 K) to explain the rare chemistry of this massive (M >= 30 M-circle dot) object, which may exist in a fleeting evolutionary state. This source is well detected and elongated in VLA Ka-band (9 mm) continuum image with a 0.25 '' beam (800 AU), is marginally detected in poorer resolution (1) K-band (1.3 cm) data, and is undetected at X-band (3 cm) with 0.25 '' resolution. In combination with existing SMA millimeter wavelength data, our results provide an accurate spectral energy distribution of this source, constraining the dust grain emissivity index to 1.0-1.6 and the luminosity to 3-37 L-circle dot. Preliminary results from ALMA Band 7 images confirm that the dust emission from MM2 is resolved in an east-west direction.

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.

We present single-dish observations of the L1689-SMM16 core in the Ophiuchus molecular cloud in NH3 (1, 1) and (2, 2) emission using the Green Bank Telescope, in N2H+ (1-0) emission using the Nobeyama Radio Observatory, and in NH2D (1(1,1)(a)-1(0,1)(s)), HCN (1-0), HNC (1-0), (HCO+)-C-13 (1-0), and HCO+ (1-0) emission using the Mopra telescope. The morphologies of the integrated NH3 (1, 1) and N2H+ (1-0) emission well match that of 250 mu m continuum emission. Line widths of NH3 (1, 1) and N2H+ (1-0) show the presence of transonic turbulence across the core. Jeans and virial analyses made using updated measurements of core mass and size confirm that L1689-SMM16 is prestellar, i.e., gravitationally bound. It also has accumulated more mass compared to its corresponding Jeans mass in the absence of magnetic fields and therefore is a "super-Jeans" core. The high levels of X(NH3)/X(N2H+) and deuterium fractionation reinforce the idea that the core has not yet formed a protostar. Comparing the physical parameters of the core with those of a Bonnor-Ebert sphere reveals the advanced evolutionary stage of L1689-SMM16 and shows that it might be unstable to collapse. We do not detect any evidence of infall motions toward the core. Instead, red asymmetry in the line profiles of HCN (1-0) and HNC (1-0) indicates the expansion of the outer layers of the core at a speed of similar to 0.2 km s(-1) to 0.3 km s(-1). For a gravitationally bound core, expansion in the outer layers might indicate that the core is experiencing oscillations.

We have detected bright HC7N J = 21-20 emission toward multiple locations in the Serpens South cluster-forming region using the K-Band Focal Plane Array at the Robert C. Byrd Green Bank Telescope. HC7N is seen primarily toward cold filamentary structures that have yet to form stars, largely avoiding the dense gas associated with small protostellar groups and the main central cluster of Serpens South. Where detected, the HC7N abundances are similar to those found in other nearby star forming regions. Toward some HC7N `clumps', we find consistent variations in the line centroids relative to NH3 (1,1) emission, as well as systematic increases in the HC7N non-thermal line widths, which we argue reveal infall motions onto dense filaments within Serpens South with minimum mass accretion rates of M 2-5 M_sun Myr^-1. The relative abundance of NH3 to HC7N suggests that the HC7N is tracing gas that has been at densities n 10^4 cm^-3, for timescales t < 1-2 x 10^5 yr. Since HC7N emission peaks are rarely co-located with those of either NH3 or continuum, it is likely that Serpens South is not particularly remarkable in its abundance of HC7N, but instead the serendipitous mapping of HC7N simultaneously with NH3 has allowed us to detect HC7N at low abundances in regions where it otherwise may not have been looked for. This result extends the known star-forming regions containing significant HC7N emission from typically quiescent regions, like the Taurus molecular cloud, to more complex, active environments.

We report the results of a pilot multiwavelength survey in the radio continuum (X, Ka, and Q bands, i.e., from 3.6 cm to 7 mm) carried out with the Expanded Very Large Array (EVLA) in order to confirm the presence of very large dust grains in dusty disks and torii around the central stars in a small sample of post-asymptotic giant branch (pAGB) objects, as inferred from millimeter (mm) and submillimeter (submm) observations. Supporting mm-wave observations were also obtained with the Combined Array for Research in Millimeter-wave Astronomy toward three of our sources. Our EVLA survey has resulted in a robust detection of our most prominent submm emission source, the pre-planetary nebula (PPN) IRAS 22036+5306, in all three bands, and the disk-prominent pAGB object, RV Tau, in one band. The observed fluxes are consistent with optically thin free-free emission, and since they are insignificant compared to their submm/mm fluxes, we conclude that the latter must come from substantial masses of cool, large (mm-sized) grains. We find that the power-law emissivity in the cm-to-submm range for the large grains in IRAS22036 is nu(beta), with beta = 1-1.3. Furthermore, the value of beta in the 3-0.85 mm range for the three disk-prominent pAGB sources (beta <= 0.4) is significantly lower than that of IRAS22036, suggesting that the grains in pAGB objects with circumbinary disks are likely larger than those in the dusty waists of pre-planetary nebulae.

In this paper we study the determinants of starless core temperatures in the Perseus molecular cloud. We use NH3 (1,1) and (2,2) observations to derive core temperatures (T_kin) and data from the COMPLETE Survey of Star Forming Regions and the c2d Spitzer Legacy Survey for observations of the other core and molecular cloud properties. The kinetic temperature distribution probed by NH3 is in the fairly narrow range of 9 - 15 K. We find that cores within the clusters IC348 and NGC1333 are significantly warmer than "field" starless cores, and T_kin is higher within regions of larger extinction-derived column density. Starless cores in the field are warmer when they are closer to class O/I protostars, but this effect is not seen for those cores in clusters. For field starless cores, T_kin is higher in regions in which the 13CO linewidth and the 1.1mm flux from the core are larger, and T_kin is lower when the the peak column density within the core and average volume density of the core are larger. There is no correlation between T_kin and 13CO linewidth, 1.1mm flux, density or peak column density for those cores in clusters. The temperature of the cloud material along the line of sight to the core, as measured by CO or far-infrared emission from dust, is positively correlated with core temperature when considering the collection of cores in the field and in clusters, but this effect is not apparent when the two subsamples of cores are considered separately.

Using far-infrared emission maps taken by IRAS and Spitzer and a near-infrared extinction map derived from 2MASS data, we have made dust temperature and column density maps of the Perseus molecular cloud. We show that the emission from transiently heated very small grains (VSGs) and the big grain dust emissivity vary as a function of extinction and dust temperature, with higher dust emissivities for colder grains. This variable emissivity cannot be explained by temperature gradients along the line of sight or by noise in the emission maps, but it is consistent with grain growth in the higher density and lower temperature regions. By accounting for the variations in the dust emissivity and VSG emission, we are able to map the temperature and column density of a nearbymolecular cloud with better accuracy than has previously been possible.

We have mapped the central 10' x 10' of the dense core TMC-1C at 450, 850, and 1200 mu m using SCUBA on the James Clerk Maxwell Telescope and MAMBO on the IRAM 30 m telescope. We show that although one can, in principle, use images at these wavelengths to map the emissivity spectral index, temperature, and column density independently, noise and calibration errors would have to be less than similar to 2% to accurately derive these three quantities from a set of three emission maps. Because our data are not this free of errors, we use our emission maps to fit the dust temperature and column density assuming a constant value of the emissivity spectral index and explore the effects of noise on the derived physical parameters. We find that the derived extinction values for TMC-1C are large for a starless core (similar to 80 mag A(V)) and the derived temperatures are low (similar to 6 K) in the densest regions of the core, using our derived value of beta = 1.8.

We have mapped the starless core TMC-1C in a variety of molecular lines with the IRAM 30 m telescope. High-density tracers show clear signs of self-absorption, and subsonic infall asymmetries are present in N2H+(1-0) and DCO+(2-1) lines. The inward velocity profile in N2H+(1-0) is extended over a region about 7000 AU in radius around the dust continuum peak, which is the most extended “infalling” region observed in a starless core with this tracer. The kinetic temperature (~12 K) measured from C17O and C18O suggests that their emission comes from a shell outside the colder interior traced by the millimeter continuum dust. The C18O(2-1) excitation temperature drops from 12 to ≃10 K away from the center. This is consistent with a volume density drop of the gas traced by the C18O lines, from ≃4 × 104 cm−3 toward the dust peak to ≃6 × 103 cm−3 at a projected distance from the dust peak of 80″ (or 11,000 AU). The column density implied by the gas and dust show similar N2H+ and CO depletion factors (fD ⩽ 6). This can be explained with a simple scenario in which: (1) the TMC-1C core is embedded in a relatively dense environment [n(H2) ≃ 104 cm−3], where CO is mostly in the gas phase and the N2H+ abundance had time to reach equilibrium values; (2) the surrounding material (rich in CO and N2H+) is accreting onto the dense core nucleus; (3) TMC-1C is older than 3 × 105 yr, to account for the observed abundance of N2H+ across the core (≃10−10 with respect to H2); and (4) the core nucleus is either much younger (≃104 yr) or “undepleted” material from the surrounding envelope has fallen toward it in the past 10,000 yr.

We have used a numerical simulation of a turbulent cloud to synthesize maps of the thermal emission from dust at a variety of far-IR and submillimeter wavelengths. The average column density and external radiation field in the simulation is well matched to clouds such as Perseus and Ophiuchus. We use pairs of single-wavelength emission maps to derive the dust color temperature and column density, and we compare the derived column densities with the true column density. We demonstrate that longer wavelength emission maps yield less biased estimates of column density than maps made toward the peak of the dust emission spectrum. We compare the scatter in the derived column density with the observed scatter in Perseus and Ophiuchus. We find that while in Perseus all of the observed scatter in the emission-derived versus the extinction-derived column density can be attributed to the flawed assumption of isothermal dust along each line of sight, in Ophiuchus there is additional scatter above what can be explained by the isothermal assumption. Our results imply that variations in dust emission properties within a molecular cloud are not necessarily a major source of uncertainty in column density measurements.

A simple predictive model was developed for three-body abrasive wear of ductile metals assuming that mass loss resulted from repeated indentation from the corners and asperities of angular particles rolling across the surface of the wear specimen. The model assumed that material removal occurred through a ratcheting/fatigue mechanism at a rate given by a modified Coffin–Manson equation. The validity of the model was examined for the three-body wear of a mild steel and stainless steel under dry and wet conditions with silicon carbide particles (180 and 1000Grit) as the abrasive. Wear tests for both dry and wet conditions were carried out on the same tester, a commercial lapping and polishing machine. Under most conditions the model proved to be unsatisfactory, as it does not take into account wear through sliding and cutting by the particles. Examination of wear surfaces suggested that individual particles traversed the wear surface in a mixed slide (scratch)—roll manner. Under most of the test conditions examined, sliding/cutting was the dominant metal removal mechanism, although wear by indentation was generally not insignificant. Wet conditions, for tests with the larger (180Grit) particles, further promoted a sliding and cutting mode of wear. However, for tests with smaller (1000Grit) particles the hydrodynamic lift provided by the lubricant may in some circumstances facilitate rolling of the particles and an indentation wear mechanism.

We have mapped the central 10 ′ ×10 ′ of the dense core TMC-1C at 450 and 850 µm using SCUBA on the James Clerk Maxwell Telescope. The unusally high quality of the 450 µm map allows us to make a detailed analysis of the temperature and column density profiles of the core. We find that the dust temperature at the center of TMC-1C is ∼ 7 K, rising to ∼ 11 K at the edges. We discuss the possibility and effects of a variable emissivity spectral index on the derived mass profile. The low dust temperature of TMC-1C results in a high derived mass for the core, significantly larger than the virial mass estimated from the linewidth of the N 2 H + (1-0) transition. This result is valid within a wide range of dust properties and ellipticities of the core. The N 2 H + (1-0) spectra, taken with the IRAM 30m telescope, show signs of self-absorption, which provide evidence of sub-sonic infall motions. The derived density profile and infall velocity is compared to the predictions of several popular star formation models, and the Bonnor-Ebert model is the best fit analytic model.