Keywords

We present Spitzer observations of a sample of 12 starless cores selected to have prominent 24 μm shadows. The Spitzer images show 8 μm and 24 μm shadows and in some cases 70 μm shadows; these spatially resolved absorption features trace the densest regions of the cores. We have carried out a ¹²CO (2–1) and ¹³CO (2–1) mapping survey of these cores with the Heinrich Hertz Telescope (HHT). We use the shadow features to derive optical depth maps. We derive molecular masses for the cores and the surrounding environment; we find that the 24 μm shadow masses are always greater than or equal to the molecular masses derived in the same region, a discrepancy likely caused by CO freezeout onto dust grains. We combine this sample with two additional cores that we studied previously to bring the total sample to 14 cores. Using a simple Jeans mass criterion, we find that ∼2/3 of the cores selected to have prominent 24 μm shadows are collapsing or near collapse, a result that is supported by millimeter line observations. Of this subset at least half have indications of 70 μm shadows. All cores observed to produce absorption features at 70 μm are close to collapse. We conclude that 24 μm shadows, and even more so the 70 μm ones, are useful markers of cloud cores that are approaching collapse.

We present our analysis of a fully sampled, high resolution dust extinction map of the Barnard 59 complex in the Pipe Nebula. The map was constructed with the infrared color excess technique applied to a photometric catalog that combines data from both ground and space based observations. The map resolves for the first time the high density center of the main core in the complex, which is associated with the formation of a small cluster of stars. We found that the central core in Barnard 59 shows an unexpected lack of significant substructure consisting of only two significant fragments. Overall, the material appears to be consistent with being a single, large core with a density profile that can be well fit by a King model. A series of NH₃ pointed observations toward the high column density center of the core appear to show that the core is still thermally dominated, with subsonic non-thermal motions. The stars in the cluster could be providing feedback to support the core against collapse, but the relatively narrow radio lines suggest that an additional source of support, for example, a magnetic field, may be required to stabilize the core. Outside the central core our observations reveal the structure of peripheral cores and resolve an extended filament into a handful of significant substructures whose spacing and masses appear to be consistent with Jeans fragmentation.

We used near-infrared Two Micron All Sky Survey data to construct visual extinction maps of a sample of Southern Bok globules utilizing the NICE method. We derived radial extinction profiles of dense cores identified in the globules and analyzed their stability against gravitational collapse with isothermal Bonnor–Ebert spheres. The frequency distribution of the stability parameter (ξ_max) of these cores shows that a large number of them are located in stable states, followed by an abrupt decrease of cores in unstable states. This decrease is steeper for globules with associated IRAS point sources than for starless globules. Moreover, globules in stable states have a Bonnor–Ebert temperature of T = 15 ± 6 K, while the group of critical plus unstable globules has a different temperature of T = 10 ± 3 K. Distances were estimated to all the globules studied in this work and the spectral class of the IRAS sources was calculated. No variations were found in the stability parameters of the cores and the spectral class of their associated IRAS sources. On the basis of ¹³CO J = 1 − 0 molecular line observations, we identified and modeled a blue-asymmetric line profile toward a globule of the sample, obtaining an upper limit infall speed of 0.25 km s⁻¹.

We present a deep and wide field-of-view (4' × 7') image of the planetary nebula (PN) NGC 7293 (the Helix Nebula) in the 2.12 μm H₂ v = 1 → 0 S(1) line. The excellent seeing (04) at the Subaru Telescope, allows the details of cometary knots to be examined. The knots are found at distances of 22–64 from the central star (CS). At the inner edge and in the inner ring (up to 45 from the CS), the knot often show a "tadpole" shape, an elliptical head with a bright crescent inside and a long tail opposite to the CS. In detail, there are variations in the tadpole shapes, such as narrowing tails, widening tails, meandering tails, or multipeaks within a tail. In the outer ring (45–64 from the CS), the shapes are more fractured, and the tails do not collimate into a single direction. The transition in knot morphology from the inner edge to the outer ring is clearly seen. The number density of knots governs the H₂ surface brightness in the inner ring: H₂ exists only within the knots. Possible mechanisms which contribute to the shaping of the knots are discussed, including photoionization and streaming motions. A plausible interpretation of our images is that inner knots are being overrun by a faster wind, but that this has not (yet) reached the outer knots. Based on H₂ formation and destruction rates, H₂ gas can survive in knots from formation during the late asymptotic giant branch phase throughout the PN phase. These observations provide new constraints on the formation and evolution of knots, and on the physics of molecular gas embedded within ionized gas.

We present observations in eight wavebands from 1.25 to 24 μm of four dense cores: L204C-2, L1152, L1155C-2, and L1228. Our goals are to study the young stellar object (YSO) population of these cores and to measure the mid-infrared extinction law. With our combined near-infrared and Spitzer photometry, we classify each source in the cores as, among other things, background stars, galaxies, or embedded YSOs. L1152 contains three YSOs and L1228 has seven, but neither L204C-2 nor L1155C-2 appear to contain any YSOs. We estimate an upper limit of 7 × 10⁻⁵ to 5 × 10⁻⁴L_☉ for any undiscovered YSOs in our cores. We also compute the line-of-sight extinction law toward each background star. These measurements are averaged spatially, to create χ² maps of the changes in the mid-infrared extinction law throughout our cores, and also in different ranges of extinction. From the χ² maps, we identify two small regions in L1152 and L1228 where the outflows in those cores appear to be destroying the larger dust grains, thus altering the extinction law in those regions. On average, however, our extinction law is relatively flat from 3.6 to 24 μm for all ranges of extinction and in all four cores. From 3.6 to 8 μm, this law is consistent with a dust model that includes larger dust grains than the diffuse interstellar medium, which suggests grain growth has occurred in our cores. At 24 μm, our extinction law is two to four times higher than predicted by dust models. However, it is similar to other empirical measurements.

In this paper, we derive an improved core mass function (CMF) for the Pipe Nebula from a detailed comparison between measurements of visual extinction and molecular-line emission. We have compiled a refined sample of 201 dense cores toward the Pipe Nebula using a two-dimensional threshold identification algorithm informed by recent simulations of dense core populations. Measurements of radial velocities using complimentary C¹⁸O (1–0) observations enable us to cull out from this sample those 43 extinction peaks that are either not associated with dense gas or are not physically associated with the Pipe Nebula. Moreover, we use the derived C¹⁸O central velocities to differentiate between single cores with internal structure and blends of two or more physically distinct cores, superposed along the same line of sight. We then are able to produce a more robust dense core sample for future follow-up studies and a more reliable CMF than was possible previously. We confirm earlier indications that the CMF for the Pipe Nebula departs from a single power-law-like form with a break or knee at M ∼ 2.7 ± 1.3 M_☉. Moreover, we also confirm that the CMF exhibits a similar shape to the stellar initial mass function (IMF), but is scaled to higher masses by a factor of ∼4.5. We interpret this difference in scaling to be a measure of the star formation efficiency (22% ± 8%). This supports earlier suggestions that the stellar IMF may originate more or less directly from the CMF.

We present a unified model for molecular core formation and evolution, based on numerical simulations of converging, supersonic flows. Our model applies to star formation in giant molecular clouds dominated by large-scale turbulence, and contains four main stages: core building, core collapse, envelope infall, and late accretion. During the building stage, cores form out of dense, post-shock gas, and become increasingly centrally stratified as the mass grows over time. Even for highly supersonic converging flows, the dense gas is subsonic, consistent with observations showing quiescent cores. When the shock radius defining the core boundary exceeds R ≈ 4a(4πGρ_mean)^−1/2, where a is the isothermal sound speed, a wave of collapse propagates from the edge to the center. During the building and collapse stages, density profiles can be fitted by Bonnor–Ebert profiles with temperature 1.2–2.9 times the true value, similar to many observed cores. As found previously for initially static equilibria, outside-in collapse leads to a Larson–Penston density profile ρ ≈ 8.86a²/(4πGr²). The third stage, consisting of an inside-out wave of gravitational rarefaction leading to ρ ∝ r^−3/2, v ∝ r^−1/2, is also similar to that for initially static spheres, as originally described by Shu. We find that the collapse and infall stages have comparable duration, ∼t_ff, consistent with estimates for observed prestellar and protostellar (Class 0/I) cores. Core building takes longer, but does not produce high-contrast objects until shortly before collapse. The time to reach core collapse, and the core mass at collapse, decrease with increasing inflow Mach number. For all cases, the accretion rate is ≫ a³/G early on but sharply drops off; the final system mass depends on the duration of late-stage accretion, set by large-scale conditions in a cloud.

We present high angular resolution observations of water masers at 1.3 cm and radio continuum emission at 1.3, 3.6, and 6 cm toward the Bok globule CB 54 using the Very Large Array. At 1.3 cm, with subarcsecond angular resolution, we detect a radio continuum compact source located to the southwest of the globule and spatially coincident with a mid-infrared (mid-IR) embedded object (MIR-b). The spectral index derived between 6 and 1.3 cm (α = 0.3 ± 0.4) is flat, consistent with optically thin free–free emission from ionized gas. We propose the shock-ionization scenario as a viable mechanism for producing the radio continuum emission observed at cm frequencies. Water masers are detected at two different positions separated by 23, and coincide spatially with two mid-IR sources: MIR-b and MIR-c. The association of these mid-IR sources with water masers confirms that they are likely protostars undergoing mass loss, and they are the best candidate as driving sources of the molecular outflows in the region.

Dense, small molecular cloud cores have been identified as the direct progenitors of stars. One of the best studied examples is Barnard 68 which is considered a prototype stable, spherical gas core, confined by a diffuse high-pressure environment. Observations of its radial density structure, however, indicate that Barnard 68 should be gravitationally unstable and collapsing, which appears to be inconsistent with its inferred long lifetime and stability. We argue that Barnard 68 is currently experiencing a fatal collision with another small core which will lead to gravitational collapse. Despite the fact that this system is still in an early phase of interaction, our numerical simulations imply that the future gravitational collapse is already detectable in the outer surface density structure of the globule which mimics the profile of a gravitationally unstable Bonnor–Ebert sphere. Within the next 2 × 10⁵ years, Barnard 68 will condense into a low-mass solar-type star(s), formed in isolation, and surrounded by diffuse, hot interstellar gas. As witnessed in situ for Barnard 68, core mergers might in general play an important role in triggering star formation and shaping the molecular core mass distribution and by that also the stellar initial mass function.

At ∼ 400 pc, the Horsehead Nebula (B33) is the closest radiatively sculpted pillar to the Sun, but the state and extent of star formation in this structure is not well understood. We present deep near-infrared (IRSF/SIRIUS JHK_S) and mid-infrared (Spitzer/IRAC) observations of the Horsehead Nebula to characterize the star-forming properties of this region and to assess the likelihood of triggered star formation. Infrared color–color and color–magnitude diagrams are used to identify young stars based on infrared excess emission and positions to the right of the zero-age main sequence, respectively. Of the 45 sources detected at both near- and mid-infrared wavelengths, three bona fide and five candidate young stars are identified in this 7' × 7' region. Two bona fide young stars have flat infrared spectral energy distributions and are located at the western irradiated tip of the pillar. The spatial coincidence of the protostars at the leading edge of this elephant trunk is consistent with the radiation-driven implosion model of triggered star formation. There is no evidence, however, for sequential star formation within the immediate ∼ 15 (0.17 pc) region from the cloud/H ii region interface.