-On the basis of near-infrared imaging observations, we derived visual extinction (A V ) distribution toward ten Bok globules through measurements of both the color excess (E H−K ) and the stellar density at J, H, and K s (star count). Radial column density profiles for each globule were analyzed with the Bonnor-Ebert sphere model. Using the data of our ten globules and four globules in the literature, we investigated the stability of globules on the basis of ξ max , which characterizes the Bonnor-Ebert sphere as well as the stability of the equilibrium state against the gravitational collapse. We found that more than half of starless globules are located near the critical state (ξ max = 6.5 ± 2). Thus, we suggest that a nearly critical Bonnor-Ebert sphere characterizes the typical density structure of starless globules. Remaining starless globules show clearly unstable states (ξ max > 10). Since unstable equilibrium states are not long maintained, we expect that these globules are on the way to gravitational collapse or that they are stabilized by non-thermal support. It was also found that all the starforming globules show unstable solutions of ξ max > 10, which is consistent with the fact that they have started gravitational collapse. We investigated the evolution of a collapsing gas sphere whose initial condition is a nearly critical Bonnor-Ebert sphere. We found that the column density profiles of the collapsing sphere mimic those of the static Bonnor-Ebert spheres in unstable equilibrium. The collapsing gas sphere resembles marginally unstable Bonnor-Ebert spheres for a long time. We found that the frequency distribution of ξ max for the observed starless globules is consistent with that from model calculations of the collapsing sphere. In addition to the near-infrared observations, we carried out radio molecular line observations (C 18 O and N 2 H + ) toward the same ten globules. We confirmed that most of the globules are dominated by thermal support. The line width of each globule was used to estimate the cloud temperature including the contribution from turbulence, with which we estimated the distance to the globules from the Bonnor-Ebert model fitting.Subject headings: ISM: clouds -dust, extinction -ISM: globules -stars: formation * Recently, a low luminosity protostar was discovered toward the globule Lynds 1014, which is known to accompany no IRAS point sources, using the Spitzer Space Telescope (Young et al. 2004). Though Spitzer data are useful to clarify whether globules contain protostars or not, Spitzer data is available only for one source, CB 131, among the "IRAS-less" globules in Table 1. We briefly checked mid-infrared data of the Spitzer telescope toward CB 131 and found that there is no protostar candidates near the center of the globule. Though a source is located near the globule †

The Infrared Dark Cloud (IRDC) G028.23-00.19 hosts a massive (1,500 M ), cold (12 K), and 3.6-70 µm IR dark clump (MM1) that has the potential to form high-mass stars. We observed this prestellar clump candidate with the SMA (∼3. 5 resolution) and JVLA (∼2. 1 resolution) in order to characterize the early stages of high-mass star formation and to constrain theoretical models. Dust emission at 1.3 mm wavelength reveals 5 cores with masses ≤15 M . None of the cores currently have the mass reservoir to form a high-mass star in the prestellar phase. If the MM1 clump will ultimately form high-mass stars, its embedded cores must gather a significant amount of additional mass over time. No molecular outflows are detected in the CO (2-1) and SiO (5-4) transitions, suggesting that the SMA cores are starless. By using the NH 3 (1,1) line, the velocity dispersion of the gas is determined to be transonic or mildly supersonic (∆V nt /∆V th ∼1.1-1.8). The cores are not highly supersonic as some theories of high-mass star formation predict. The embedded cores are 4 to 7 times more massive than the clump thermal Jeans mass and the most massive core (SMA1) is 9 times less massive than the clump turbulent Jeans mass. These values indicate that neither thermal pressure nor turbulent pressure dominates the fragmentation of MM1. The low virial parameters of the cores (0.1-0.5) suggest that they are not in virial equilibrium, unless strong magnetic fields of ∼1-2 mG are present. We discuss high-mass star formation scenarios in a context based on IRDC G028.23-00.19, a study case believed to represent the initial fragmentation of molecular clouds that will form high-mass stars.

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We have compared molecular line emission to dust continuum emission and modeled molecular lines using Monte Carlo simulations in order to study the depletion of molecules and the ionization fraction in three preprotostellar cores, L1512, L1544, and L1689B. L1512 is much less dense than L1544 and L1689B, which have similar density structures. L1689B has a different environment from those of L1512 and L1544. We used density and temperature profiles, calculated by modeling dust continuum emission in the submillimeter, for modeling molecular line profiles. In addition, we have used molecular line profiles and maps observed in several different molecules toward the three cores. We find a considerable diversity in chemical state among the three cores. The molecules include those sensitive to different timescales of chemical evolution such as CCS, the isotopes of CO and HCO + , DCO + , and N 2 H + . The CO molecule is significantly depleted in L1512 and L1544, but not in L1689B. CCS may be in the second enhancement of its abundance in L1512 and L1544 because of the significant depletion of CO molecules. N 2 H + might already start to be depleted in L1512, but it traces very well the distribution of dust emission in L1544. On the other hand, L1689B may be so young that N 2 H + has not reached its maximum yet. The ionization fraction has been calculated using H 13 CO + and DCO + . The result shows that the ionization fraction is similar toward the centers of the three cores. This study suggests that chemical evolution depends on the absolute timescale during which a core stays in a given environment as well as its density structure.

We report on mapping observations of the CO J = 3-2 and CO J = 1–0 lines toward supernova remnant (SNR) W28, which is supposed to be an EGRET 7-ray source. A broad CO line emission (maximum linewidth reaches 70 km s−1), which suggests an interaction between the molecular cloud and W28 SNR, was detected. Moreover, the distribution of the unshocked and shocked gas is clearly resolved. The distribution of the shocked gas is similar to that of the radio-continuum emission, and tends to be stronger along the radio-continuum ridge. The unshocked gas is displaced by 0.4–1.0 pc outward with respect to the shocked gas. The spatial relationship between shocked and unshocked gas has been clarified for the first time for the SNR-cloud interaction. All of the known OH maser spots are located along the filament of the shocked gas. These facts convincingly indicate that W28 SNR interacts with the molecular cloud.

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Pre-protostellar cores likely represent the incipient stages of low-mass (≈ 1M ⊙ ) star formation. Lynds 1498 is a pre-protostellar core (PPC) and was one of the initial objects toward which molecular depletion and differentiation was detected. Despite the considerable scrutiny of L1498, there has not been a extensive study of the density and temperature structure as derived from radiative transfer modeling of dust continuum observations. We present deep SCUBA observations of L1498 at 850 and 450 µm, high resolution BEARS maps of the N 2 H + 1 → 0 transition, CSO observations of the N 2 H + 3 → 2 transition, and GBT observations of the C 3 S 4 → 3 transition. We also present a comparison of derived properties between L1498 and nearby PPCs that have been observed at far-infrared and submillimeter wavelengths. The L1498 continuum emission is modeled using a one-dimensional radiative transfer code that self-consistently calculates the temperature distribution and calculates the SED and intensity profiles at 850 and 450 µm. We present a more realistic treatment of PPC heating which varies the strength of the ISRF, s isrf , and includes attenuation of the ISRF due to dust grains at the outer radius of the core, A V . The best-fitted model consists of a Bonner-Ebert sphere with a central density of 1 − 3 × 10 4 cm −3 , R o ≈ 0.29 pc, 0.5 ≤ s isrf ≤ 1, A v ≈ 1 mag, and a nearly isothermal temperature profile of ≈ 10.5 K for OH8 opacities. C 3 S emission shows a central depletion hole while N 2 H + emission is centrally peaked. We derive a mean N 2 H + abundance of 4.0 × 10 −10 relative to H 2 that is consistent with chemical models for a dynamically young yet chemically evolved source. The observed depletions of C 3 S and H 2 CO, the modest N 2 H + abundance, and a central density that is an order of magnitude lower than other modeled PPCs suggests that L1498 may be a forming PPC. Our derived temperature and density profile will improve modeling of molecular line observations that will explicate the core's kinematical and chemical state.