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1.
We present a fully sampled C18O (1–0) map towards the southern giant molecular cloud (GMC) associated with the H  ii region RCW 106, and use it in combination with previous 13CO (1–0) mapping to estimate the gas column density as a function of position and velocity. We find localized regions of significant 13CO optical depth in the northern part of the cloud, with several of the high-opacity clouds in this region likely associated with a limb-brightened shell around the H  ii region G333.6−0.2. Optical depth corrections broaden the distribution of column densities in the cloud, yielding a lognormal distribution as predicted by simulations of turbulence. Decomposing the 13CO and C18O data cubes into clumps, we find relatively weak correlations between size and linewidth, and a more sensitive dependence of luminosity on size than would be predicted by a constant average column density. The clump mass spectrum has a slope near −1.7, consistent with previous studies. The most massive clumps appear to have gravitational binding energies well in excess of virial equilibrium; we discuss possible explanations, which include magnetic support and neglect of time-varying surface terms in the virial theorem. Unlike molecular clouds as a whole, the clumps within the RCW 106 GMC, while elongated, appear to show random orientations with respect to the Galactic plane.  相似文献   

2.
We present Monte Carlo radiative-transfer simulations for spiral galaxies modelled as a stellar disc and a two-phase clumpy dust distribution. We divide the volume occupied by the dust into a three-dimensional grid and assign each cell a clump or smooth medium status. Cell dimension, clump dust mass and spatial distribution are derived from the observed properties of giant molecular clouds and molecular gas in the Galaxy. We produce models for several values of the optical depth and fraction of the interstellar medium residing in clumps. As a general result, clumpy models are less opaque than the corresponding homogeneous models. For the adopted parameters, the increase in the fraction of energy that escapes the disc is moderate, resulting in surface-brightness profiles that are less than one magnitude brighter than those of the homogeneous models. The effects of clumping are larger for edge-on views of the disc. This is in contrast with previous preliminary results for clumping in the literature. We show how differences arise from the different parametrization and clump distribution adopted. We also consider models in which a fraction of the stellar radiation is emitted within the clumps. In this case, galaxies are less transparent than in the case when only dust is clumped. The opacity can be even higher than in the homogeneous case, depending on the fraction of embedded stellar emission. We point out the implications of the results for the determination of the opacity and dust mass of spiral galaxies.  相似文献   

3.
There is now compelling evidence that dark molecular clouds are clumpy. Much of the clumpiness is unresolved by single-dish telescopes but is apparent in the data from array telescopes. Molecular clumps may also be observed close to Herbig-Haro (HH) objects. These clumps are easily observable because they are `illuminated' due to the UV radiation from the shock front of the HH jet. A detailed observational and theoretical study of one HH clump has been performed and it indicates that this clump must be transient and has a similar density and temperature to those clumps detected in the cloud interior. Thus, HH clumps may be used as an independent method of determining physical parameters of the clumpiness of molecular clouds.  相似文献   

4.
X-rays from massive OB stars: thermal emission from radiative shocks   总被引：1，自引：0，他引：1  
Chandra grating spectra of a sample of 15 massive OB stars were analysed under the basic assumption that the X-ray emission is produced in an ensemble of shocks formed in the winds driven by these objects. Shocks develop either as a result of radiation-driven instabilities or due to confinement of the wind by a relatively strong magnetic field, and since they are radiative, a simple model of their X-ray emission was developed that allows a direct comparison with observations. According to our model, the shock structures (clumps, complete or fractional shells) eventually become 'cold' clouds in the X-ray sky of the star. As a result, it is expected that for large covering factors of the hot clumps, there is a high probability for X-ray absorption by the 'cold' clouds, resulting in blueshifted spectral lines. Our analysis has revealed that such a correlation indeed exists for the considered sample of OB stars. As to the temperature characteristics of the X-ray emission plasma, the studied OB stars fall in two groups: (i) one with plasma temperature limited to ∼0.1–0.4 keV and (ii) the other with X-rays produced in plasmas at considerably higher temperatures. We argue that the two groups correspond to different mechanisms for the origin of X-rays: in radiation-driven instability shocks and in magnetically confined wind shocks, respectively.  相似文献   

5.
The multi-transitional observations of CS molecules towards the NGC 2071 core have been re-analysed by using a tri-dimensional Monte Carlo radiative transfer code. Better agreement with the observations is made by an introduction of clumpiness to this model than by smoothly varying density to the 1D microturbulent one. The best-fitting model shows that, when a unique density is assumed for clumps, the volume filling factor of the clumps varies as r −2 with an average of ∼5 per cent over the entire core, and that the H2 number density and the CS abundance of the clump relative to H2 are ∼ 2 × 106 cm−3 and ∼ 6 × 10−10, respectively. The radial density gradient ∝ r −2 obtained from our clumpy core model is steeper than that (∝ r −1.3) obtained from the microturbulent model. Since all clumps are subject to random bulk motions in this 3D clumpy macroturbulent model, synthesized line profiles do not show self-absorption dips even for opaque transitions and the resulting linewidth is in good accordance with the observations.  相似文献   

6.
We investigate, by means of numerical simulations, the phenomenology of star formation triggered by low-velocity collisions between low-mass molecular clumps. The simulations are performed using a smoothed particle hydrodynamics code which satisfies the Jeans condition by invoking on-the-fly particle splitting.
Clumps are modelled as stable truncated (non-singular) isothermal, i.e. Bonnor–Ebert, spheres. Collisions are characterized by M 0 (clump mass), b (offset parameter, i.e. ratio of impact parameter to clump radius) and     (Mach number, i.e. ratio of collision velocity to effective post-shock sound speed). The gas subscribes to a barotropic equation of state, which is intended to capture (i) the scaling of pre-collision internal velocity dispersion with clump mass, (ii) post-shock radiative cooling and (iii) adiabatic heating in optically thick protostellar fragments.
The efficiency of star formation is found to vary between 10 and 30 per cent in the different collisions studied and it appears to increase with decreasing M 0, and/or decreasing b , and/or increasing     . For   b < 0.5  collisions produce shock-compressed layers which fragment into filaments. Protostellar objects then condense out of the filaments and accrete from them. The resulting accretion rates are high,     , for the first     . The densities in the filaments,     , are sufficient that they could be mapped in NH3 or CS line radiation, in nearby star formation regions.  相似文献   

7.
The evolution of substructure embedded in non-dissipative dark haloes is studied through N -body simulations of isolated systems, both in and out of initial equilibrium, complementing cosmological simulations of the growth of structure. We determine by both analytic calculations and direct analysis of the N -body simulations the relative importance of various dynamical processes acting on the clumps, such as the removal of material by global tides, clump–clump heating, clump–clump merging and dynamical friction. The ratio of the internal clump velocity dispersion to that of the dark halo is an important parameter; as this ratio approaches a value of unity, heating by close encounters between clumps becomes less important, while the other dynamical processes continue to increase in importance. Our comparison between merging and disruption processes implies that spiral galaxies cannot be formed in a protosystem that contains a few large clumps, but can be formed through the accretion of many small clumps; elliptical galaxies form in a more clumpy environment than do spiral galaxies. Our results support the idea that the central cusp in the density profiles of dark haloes is the consequence of self-limiting merging of small, dense haloes. This implies that the collapse of a system of clumps/substructure is not sufficient to form a cD galaxy, with an extended envelope; plausibly, subsequent accretion of large galaxies is required. The post-collapse system is in general triaxial, with rounder systems resulting from fewer, but more massive, clumps. Persistent streams of material from disrupted clumps can be found in the outer regions of the final system, and at an overdensity of around 0.75, can cover 10 to 30 per cent of the sky.  相似文献   

8.
Recent observations have revealed that damped Lyα clouds (DLAs) host star formation activity. In order to examine if such star formation activity can be triggered by ionization fronts, we perform high-resolution hydrodynamics and radiative transfer simulations of the effect of radiative feedback from propagating ionization fronts on high-density clumps. We examine two sources of ultraviolet (UV) radiation field to which high-redshift ( z ∼ 3) galaxies could be exposed: one corresponding to the UV radiation originating from stars within the DLA, itself, and the other corresponding to the UV background radiation. We find that, for larger clouds, the propagating I-fronts created by local stellar sources can trigger cooling instability and collapse of significant part, up to 85 per cent, of the cloud, creating conditions for star formation in a time-scale of a few Myr. The passage of the I-front also triggers collapse of smaller clumps (with radii below ∼4 pc), but in these cases the resulting cold and dense gas does not reach conditions conducive to star formation. Assuming that 85 per cent of the gas initially in the clump is converted into stars, we obtain a star formation rate of  ∼0.25 M⊙ yr−1 kpc−2  . This is somewhat higher than the value derived from recent observations. On the other hand, the background UV radiation which has harder spectrum fails to trigger cooling and collapse. Instead, the hard photons which have long mean free-path heat the dense clumps, which as a result expand and essentially dissolve in the ambient medium. Therefore, the star formation activity in DLAs is strongly regulated by the radiative feedback, both from the external UV background and internal stellar sources and we predict quiescent evolution of DLAs (not starburst-like evolution).  相似文献   

9.
A numerical scheme is proposed for the solution of the three-dimensional (3D) radiative transfer equation with variable optical depth. We show that time-dependent ray tracing is an attractive choice for simulations of astrophysical ionization fronts, particularly when one is interested in covering a wide range of optical depths within a three-dimensional clumpy medium. Our approach combines the explicit advection of radiation variables with the implicit solution of local rate equations given the radiation field at each point. Our scheme is well suited to the solution of problems for which line transfer is not important, and could, in principle, be extended to those situations also. This scheme allows us to calculate the propagation of supersonic ionization fronts into an inhomogeneous medium. The approach can be easily implemented on a single workstation and should also be fully parallelizable.  相似文献   

10.
Some features of the envelopes of WR stars have not yet been explained in terms of the standard stellar wind. One of these features is the presence of gas condensations (clumps) in the envelopes. In this paper the possible reasons for clump formation are examined along with their role in the structure of envelopes. Clumps can be formed in the transitional zone between a star and its envelope because of instabilities in the gas. This zone lies much deeper than the "optical radus" of the star, so it cannot be observed. A clump expands as it moves under the action of radiation pressure from the star and its density decreases at the same time. The clump mixes with the surrounding gas if its mass is low. Large clumps can reach the visible region of the envelope.  相似文献   

11.
We have surveyed a ∼0.9 square degree area of the W3 giant molecular cloud (GMC) and star-forming region in the 850-μm continuum, using the Submillimetre Common-User Bolometer Array on the James Clerk Maxwell Telescope. A complete sample of 316 dense clumps were detected with a mass range from around 13 to  2500 M⊙  . Part of the W3 GMC is subject to an interaction with the H  ii region and fast stellar winds generated by the nearby W4 OB association. We find that the fraction of total gas mass in dense, 850-μm traced structures is significantly altered by this interaction, being around 5–13 per cent in the undisturbed cloud but ∼25–37 per cent in the feedback-affected region. The mass distribution in the detected clump sample depends somewhat on assumptions of dust temperature and is not a simple, single power law but contains significant structure at intermediate masses. This structure is likely to be due to crowding of sources near or below the spatial resolution of the observations. There is little evidence of any difference between the index of the high-mass end of the clump mass function in the compressed region and in the unaffected cloud. The consequences of these results are discussed in terms of current models of triggered star formation.  相似文献   

12.
We investigate the effects of magnetic fields and radiative protostellar feedback on the star formation process using self-gravitating radiation magnetohydrodynamical calculations. We present results from a series of calculations of the collapse of  50 M⊙  molecular clouds with various magnetic field strengths and with and without radiative transfer. We find that both magnetic fields and radiation have a dramatic impact on star formation, though the two effects are in many ways complementary. Magnetic fields primarily provide support on large scales to low-density gas, whereas radiation is found to strongly suppress small-scale fragmentation by increasing the temperature in the high-density material near the protostars. With strong magnetic fields and radiative feedback, the net result is an inefficient star formation process with a star formation rate of  ≲10  per cent per free-fall time that approaches the observed rate, although we have only been able to follow the calculations for 1/3 of a free-fall time beyond the onset of star formation.  相似文献   

13.
In this article, some aspects of the clumpy nature of molecular clouds are reviewed. In particular the observational evidence for small-scale structures both in low and high mass star forming regions will be discussed. I will review some examples of `clumpiness' such as: i) the molecular clumps ahead of HH objects and how the study of the physical and chemical nature of these clumps is important for the understanding of the clumpiness of the Interstellar Medium; and ii)hot cores and their use as a tool to study the early phases of massive star formation. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

14.
In order to investigate the differences between the molecular clouds which are associated with the massive star forming regions and those which are not, we have performed the single-dish simultaneous observations of 12CO J=2-1 and J=3-2 lines toward a sample of 59 Spitzer Extended Green Objects (EGOs) as the massive star forming regions in the northern sky. Combining our results with the data of the 12CO J=1-0 observations toward the same sample EGOs in the literature, we have made the statistical comparisons on the intensities and linewidths of multiple 12CO lines between the molecular clouds associated with EGOs (EGO molecular clouds, in brief) and other non-EGO molecular clouds. On this basis, we have discussed the effects of the gas temperature, density, and velocity field distributions on the statistical characteristics of the two kinds of molecular clouds. It is found that both the EGO molecular clouds and non-EGO molecular clouds have similar mass ranges, hence we conclude that for the formation of massive stars, the key-important factor is probably not the total mass of a giant molecular cloud (GMC), but the volume filling factor of the molecular clumps in the GMC (or the compression extent of the molecular gas in the cloud).  相似文献   

15.
We have obtained wide-field thermal infrared (IR) images of the Carina nebula, using the SPIREX/Abu telescope at the South Pole. Emission from polycyclic aromatic hydrocarbons (PAHs) at 3.29 μm, a tracer of photodissociation regions (PDRs), reveals many interesting well-defined clumps and diffuse regions throughout the complex. Near-IR images  (1–2 μm)  , along with images from the Midcourse Space Experiment ( MSX ) satellite  (8–21 μm)  have been incorporated to study the interactions between the young stars and the surrounding molecular cloud in more detail. Two new PAH emission clumps have been identified in the Keyhole nebula, and have been mapped in  12CO(2–1)  and  (1–0)  using the Swedish–ESO Submillimetre Telescope (SEST). Analysis of their physical properties reveals that they are dense molecular clumps, externally heated with PDRs on their surfaces and supported by external pressure in a similar manner to the other clumps in the region. A previously identified externally heated globule containing IRAS 10430−5931 in the southern molecular cloud shows strong 3.29-, 8- and 21-μm emission, the spectral energy distribution (SED) revealing the location of an ultracompact (UC) H  ii region. The northern part of the nebula is complicated, with PAH emission intermixed with mid-IR dust continuum emission. Several point sources are located here, and through a two-component blackbody fit to their SEDs we have identified three possible UC H  ii regions as well as a young star surrounded by a circumstellar disc. This implies that star formation in this region is ongoing and not halted by the intense radiation from the surrounding young massive stars.  相似文献   

16.
We present a picture of star formation around the H  ii region Sh2-235 (S235) based upon data on the spatial distribution of young stellar clusters and the distribution and kinematics of molecular gas around S235. We observed 13CO (1–0) and CS (2–1) emission toward S235 with the Onsala Space Observatory 20-m telescope and analysed the star density distribution with archival data from the Two Micron All-Sky Survey (2MASS). Dense molecular gas forms a shell-like structure at the southeastern part of S235. The young clusters found with 2MASS data are embedded in this shell. The positional relationship of the clusters, the molecular shell and the H  ii region indicates that expansion of S235 is responsible for the formation of the clusters. The gas distribution in the S235 molecular complex is clumpy, which hampers interpretation exclusively on the basis of the morphology of the star-forming region. We use data on kinematics of molecular gas to support the hypothesis of induced star formation, and distinguish three basic types of molecular gas components. The first type is primordial undisturbed gas of the giant molecular cloud, the second type is gas entrained in motion by expansion of the H  ii region (this is where the embedded clusters were formed) and the third type is a fast-moving gas, which might have been accelerated by winds from the newly formed clusters. The clumpy distribution of molecular gas and its kinematics around the H  ii region implies that the picture of triggered star formation around S235 can be a mixture of at least two possibilities: the 'collect-and-collapse' scenario and the compression of pre-existing dense clumps by the shock wave.  相似文献   

17.
The fate of the cooling gas in the central regions of rich clusters of galaxies is not well understood. In one plausible scenario clouds of atomic or molecular gas are formed. However the mass of the cold gas, inferred from measurements of low-energy X-ray absorption, is hardly consistent with the absence of powerful CO or 21-cm emission lines from the cooling flow region. Among the factors which may affect the detectability of the cold clouds are their optical depth, shape and covering fraction. Thus, alternative methods to determine the mass in cold clouds, which are less sensitive to these parameters, are important.   For the inner region of the cooling flow (e.g. within a radius of ∼50–100 kpc) the Thomson optical depth of the hot gas in a massive cooling flow can be as large as ∼ 0.01. Assuming that the cooling time in the inner region is few times shorter than the lifetime of the cluster, the Thomson depth of the accumulated cold gas can be accordingly higher (if most of the gas remains in the form of clouds). The illumination of the cold clouds by the X-ray emission of the hot gas should lead to the appearance of a 6.4-keV iron fluorescent line, with an equivalent width proportional to τT. The equivalent width only weakly depends on the detailed properties of the clouds, e.g. on the column density of individual clouds, as long as the column density is less than a few 1023 cm−2. Another effect also associated exclusively with the cold gas is a flux in the Compton shoulder of bright X-ray emission lines. It also scales linearly with the Thomson optical depth of the cold gas. With the new generation of X-ray telescopes, combining large effective area and high spectral resolution, the mass of the cold gas in cooling flows (and its distribution) can be measured.  相似文献   

18.
We present an analysis of star-forming gas cores in a smooth particle hydrodynamics simulation of a giant molecular cloud. We identify cores using their deep potential wells. This yields a smoother distribution with clearer boundaries than density. Additionally, this gives an indication of future collapse, as bound potential cores (p-cores) represent the earliest stages of fragmentation in molecular clouds. We find that the mass function of the p-cores resembles the stellar initial mass function and the observed clump mass function, although p-core masses  (∼0.7 M⊙)  are smaller than typical density clumps. The bound p-cores are generally subsonic, have internal substructure and are only quasi-spherical. We see no evidence of massive bound cores supported by turbulence. We trace the evolution of the p-cores forward in time, and investigate the connection between the original p-core mass and the stellar mass that formed from it. We find that there is a poor correlation, with considerable scatter suggesting accretion on to the core is dependent on more factors than just the initial core mass. During the accretion process the p-cores accrete from beyond the region first bound, highlighting the importance of the core environment to its subsequent evolution.  相似文献   

19.
The 2.2–200 μm spectrum of OH32.8–0.3 has been modelled. Mie theory and radiative transfer models of the 3-μm band of H2O ice are consistent with a strongly crystalline structural phase. This is also confirmed by the presence of a 44/62-μm band complex analogous to that of laboratory crystalline H2O ice analogues. The highly ordered phase may be the result of direct crystallization upon deposition as has been theorized by Kouchi et al. At the large total optical depths typical of the radiative transfer models for this object (τ9.7∼ 40), we find no significant difference between the Mie theory and radiative transfer models of the 3-μm band. On the other hand, large differences are found for the 9.7-μm silicate band. In contrast to Mie theory extinction profiles, those computed via radiative transfer modelling indicate that the 12-μm H2O ice band (the so-called librational band) is substantially attenuated. This, in addition to the inherent broadness and weakness of the 12-μm ice band, may explain why this band has not been clearly identified in observational spectra of oxygen–rich evolved objects.  相似文献   

20.
We quantitatively scrutinize the effects of the radiation drag arising from the radiation fields in a galactic bulge in order to examine the possibility that the radiation drag could be an effective mechanism to extract angular momentum in a spheroidal system like a bulge and allow plenty of gas to accrete on to the galactic centre. For this purpose, we numerically solve the relativistic radiation hydrodynamical equation coupled with accurate radiative transfer, and quantitatively assess the radiation drag efficiency. As a result, we find that in an optically thick regime the radiation drag efficiency is sensitively dependent on the density distributions of the interstellar medium (ISM). The efficiency drops according to     in an optically thick uniform ISM, where τ T is the total optical depth of the dusty ISM , whereas the efficiency remains almost constant at a high level if the ISM is clumpy . Hence, if bulge formation begins with a star formation event in a clumpy ISM, the radiation drag will effectively work to remove the angular momentum and the accreted gas may form a supermassive black hole. As a natural consequence, this mechanism reproduces a putative linear relation between the mass of a supermassive black hole and the mass of a galactic bulge, although further detailed modelling for stellar evolution is required for a more precise prediction.  相似文献   

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