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1.
We have used observations of the rare isotopes of HCN and HNC to determine the relative abundance of these two chemical isomers along the central ridge of the Orion molecular cloud. The abundance ratio [HCN]/[HNC] decreases by more than an order of magnitude from the relatively warm plateau and hot core sources toward the KL nebula to the colder, more quiescent clouds to the north and south. Even in the cooler regions, however, the ratio is an order of magnitude larger than that found in previous investigations of cold dark clouds. We determine the kinetic temperature in the regions we have studied from new observations of methylacetylene (CH3CCH), together with other recent estimates of the gas temperature near KL. The results suggest that the warmer portions of the cloud are dominated by different chemical pathways than those in the general interstellar cloud material. 相似文献
2.
E. Churazov R. Sunyaev M. Gilfanov W. Forman & C. Jones 《Monthly notices of the Royal Astronomical Society》1998,297(4):1274-1278
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
3.
We have computed a set of multiphase chemical evolution models in which the radial mass distributions of each theoretical
galaxy is calculated using the universal rotation curve from Persic, Salucci and Steel (1996). We obtain the chemical evolution
for galaxies of different masses and morphological types by changing the efficiencies to form molecular clouds and stars according
with these types. We obtain the radial distributions of diffuse and molecular gas densities, the star formation rate and abundances
for 15 elements for each galaxy.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
4.
Peter Berczik Gerhard Hensler Christian Theis Rainer Spurzem 《Astrophysics and Space Science》2002,281(1-2):297-300
We present our recently developed 3-dimensional chemodynamical code for galaxy evolution. This code follows the evolution
of different galactic components like stars, dark matter and different components of the interstellar medium (ISM), i.e. a
diffuse gaseous phase and the molecular clouds. Stars and dark matter are treated as collisionless N-body systems. The ISM
is numerically described by a smoothed particle hydrodynamics (SPH) approach for the diffuse gas and a sticky particle scheme
for the molecular clouds. Additionally, the galactic components are coupled by several phase transitions like star formation,
stellar death or condensation and evaporation processes within the ISM. As an example we show the dynamical and chemical evolution
of a star forming dwarf galaxy with a total baryonic mass of 2 ċ 109 M⊙. After a moderate collapse phase the stars and the molecular clouds follow an exponential radial distribution, whereas the
diffuse gas shows a central depression as a result of stellar feedback. The metallicities of the galactic components behave
quite differently with respect to their temporal evolution as well as their radial distribution. Especially, the ISM is at
no stage well mixed.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
5.
V. A. Dogiel A. V. Gurevich Ya. N. Istomin K. P. Zybin 《Astrophysics and Space Science》2005,297(1-4):201-211
Acceleration of charged particles by neutral gas turbulence in giant molecular clouds is considered. The gamma-ray emission
from these clouds is estimated. It is shown that molecular clouds can be the counterparts of some of unidentified sources. 相似文献
6.
C. Melioli F. Brighenti A. D'Ercole E. M. de Gouveia Dal Pino 《Monthly notices of the Royal Astronomical Society》2008,388(2):573-586
The ejection of the gas out of the disc in late-type galaxies is related to star formation and is due mainly to Type II supernovae. In this paper, we studied in detail the development of the Galactic fountains in order to understand their dynamical evolution and their influence on the redistribution of the freshly delivered metals over the disc. To this aim, we performed a number of 3D hydrodynamical radiative cooling simulations of the gas in the Milky Way where the whole Galaxy structure, the Galactic differential rotation and the supernova explosions generated by a single OB association are considered. A typical fountain powered by 100 Type II supernovae may eject material up to ∼2 kpc which than collapses back mostly in the form of dense, cold clouds and filaments. The majority of the gas lifted up by the fountains falls back on the disc remaining within a radial distance Δ R = 0.5 kpc from the place where the fountain originated. This localized circulation of disc gas does not influence the radial chemical gradients on large scale, as required by the chemical models of the Milky Way which reproduce the metallicity distribution without invoking large fluxes of metals. Simulations of multiple fountains fuelled by Type II supernovae of different OB associations will be presented in a companion paper. 相似文献
7.
Gas phases of the interstellar medium (ISM) coexist locally, penetrate each other and mix by means of dynamical and plasmaphysical
processes. E.g. heat conduction from the hot to the cooler gas leads to energy and mass exchange between the gas phases. Analytical
solutions exist under which evaporation of cloudy material or condensation of hot gas onto the clouds' surface dominate. Since
these results are derived for stationary and static conditions and under ideal assumptions, they do not necessarily hold for
a dynamical ISM. On the other hand, the mass and energy exchange between the gas phases is of great importance for the energy
budget of the ISM and by this influences the evolution of galaxies. This led us to investigate the evolution of interstellar
clouds in a hot gas by means of numerical simulations. At first, we compare static models with the analytical results and
found that interstellar clouds with parameters requiring analytically evaporation are, in contrast, accreting surrounding
material if self-gravitation and cooling are implied. For the more realistic case, where clouds are embedded in a streaming
hot gas, the models show that Kelvin-Helmholtz instability which leads to the disruption of the clouds is suppressed by heat
conduction so that the clouds are stabilized to survive.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
8.
W. F. Wall 《Monthly notices of the Royal Astronomical Society》2007,375(1):278-300
Determining temperatures in molecular clouds from ratios of CO rotational lines or from ratios of continuum emission in different wavelength bands suffers from reduced temperature sensitivity in the high-temperature limit. In theory, the ratio of far-infrared (FIR), submillimetre or millimetre continuum to that of a 13 CO (or C18 O) rotational line can place reliable upper limits on the temperature of the dust and molecular gas. Consequently, FIR continuum data from the COBE /Diffuse Infrared Background Experiment (DIRBE) instrument and Nagoya 4-m 13 CO J = 1 → 0 spectral line data were used to plot 240 μm/13 CO J = 1 → 0 intensity ratios against 140/240 μm dust colour temperatures, allowing us to constrain the multiparsec-scale physical conditions in the Orion A and B molecular clouds.
The best-fitting models to the Orion clouds consist of two components: a component near the surface of the clouds that is heated primarily by a very large scale (i.e. ∼1 kpc) interstellar radiation field and a component deeper within the clouds. The former has a fixed temperature and the latter has a range of temperatures that vary from one sightline to another. The models require a dust–gas temperature difference of 0 ± 2 K and suggest that 40–50 per cent of the Orion clouds are in the form of dust and gas with temperatures between 3 and 10 K. The implications are discussed in detail in later papers and include stronger dust–gas thermal coupling and higher Galactic-scale molecular gas temperatures than are usually accepted, and an improved explanation for the N (H2 )/ I (CO) conversion factor. It is emphasized that these results are preliminary and require confirmation by independent observations and methods. 相似文献
The best-fitting models to the Orion clouds consist of two components: a component near the surface of the clouds that is heated primarily by a very large scale (i.e. ∼1 kpc) interstellar radiation field and a component deeper within the clouds. The former has a fixed temperature and the latter has a range of temperatures that vary from one sightline to another. The models require a dust–gas temperature difference of 0 ± 2 K and suggest that 40–50 per cent of the Orion clouds are in the form of dust and gas with temperatures between 3 and 10 K. The implications are discussed in detail in later papers and include stronger dust–gas thermal coupling and higher Galactic-scale molecular gas temperatures than are usually accepted, and an improved explanation for the N (H
9.
K. Tainaka S. Fukazawa H. Nishimori M. Yokosawa S. Mineshige 《Astrophysics and Space Science》1993,210(1-2):117-120
Population dynamics of multi-phased interstellar medium (ISM) is investigated by using the lattice model in position-fixed reaction. Interactions between three distinct phases of gas, cold clouds, warm gas, and hot gas give rise to cyclic phase changes in ISM. Such local phase changes are propagated in space, and stochastic steady-state spatial pattern is finally achieved. We obtain the following two characteristic patterns:
- When the sweeping rate of a warm gas into a cold component is relatively high, cold clouds associated with warm gas form small-scale clumps and are dispersively distributed, whereas hot gas covers large fraction of space.
- When the sweeping rate is relatively low, in contrast, warm gas and cold clouds are diffusively and equally distributed, while hot gas component is substantially localized.
10.
A model of supernova feedback in galaxy formation 总被引：3，自引：0，他引：3
G. Efstathiou 《Monthly notices of the Royal Astronomical Society》2000,317(3):697-719
A model of supernova feedback during disc galaxy formation is developed. The model incorporates infall of cooling gas from a halo, and outflow of hot gas from a multiphase interstellar medium (ISM). The star formation rate is determined by balancing the energy dissipated in collisions between cold gas clouds with that supplied by supernovae in a disc marginally unstable to axisymmetric instabilities. Hot gas is created by thermal evaporation of cold gas clouds in supernova remnants, and criteria are derived to estimate the characteristic temperature and density of the hot component and hence the net mass outflow rate. A number of refinements of the model are investigated, including a simple model of a galactic fountain, the response of the cold component to the pressure of the hot gas, pressure-induced star formation and chemical evolution. The main conclusion of this paper is that low rates of star formation can expel a large fraction of the gas from a dwarf galaxy. For example, a galaxy with circular speed 50 km s1 can expel 6080 per cent of its gas over a time-scale of 1 Gyr, with a star formation rate that never exceeds 0.1 M1 . Effective feedback can therefore take place in a quiescent mode and does not require strong bursts of star formation. Even a large galaxy, such as the Milky Way, might have lost as much as 20 per cent of its mass in a supernova-driven wind. The models developed here suggest that dwarf galaxies at high redshifts will have low average star formation rates and may contain extended gaseous discs of largely unprocessed gas. Such extended gaseous discs might explain the numbers, metallicities and metallicity dispersions of damped Lyman systems. 相似文献
11.
J. M. Greenberg 《Astrophysics and Space Science》1986,128(1):17-31
The observations of dust gas in diffuse and molecular clouds are shown to reflect not only their current state but their past history. The interpretation of infrared spectra of dust in molecular clouds using appropriate core-mantle grains shows that: (1) the kinds and amounts of ices, (2) the relative proportion of such important interstellar molecules as H2O and CO, (3) the evidence for the less abundant solid species X–CN, COS, H2S, and (4) the thermal history of the dust may all be demonstrated quantitatively from laboratory analog studies of ultraviolet photoprocessing of relevant ices and from theoretical studies of gas-dust interactions. In diffuse clouds the dust is shown to consist predominantly of refractory organic compounds which originate as residues of the photoprocessing of volatile ices in molecular clouds and which undergo further physical and chemical evolution in the diffuse clouds.Review paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986. 相似文献
12.
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
13.
M.A. de Avillez 《Astrophysics and Space Science》2000,272(1-3):23-30
Three-dimensional simulations of the disk-halo interaction show the formation of a thick HI and HII gas disk with different
scale heights. The thick HI disk prevents the disk gas from expanding freely upwards, unless some highly energetic event such
as chimneys occurs, whereas the thick HII disk acts as a disk-halo interaction region from where the hot ionized gas flows
freely into the halo. The upflowing gas reaches the maximum height at z ∼ 9.3 ± 1 kpc becoming thermally unstable due to radiative losses, and condenses into HI clouds. Because the major fraction
of the gas is gravitationally bound to the Galaxy, the cold gas returns to the disk. The descending clouds will have at some
height high velocities. In a period of 200 Myr of fountain evolution, some 10 percent of the total number of clouds are HVCs.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
14.
G. P. Horedt 《Astrophysics and Space Science》1978,54(2):253-261
We discuss the rotation of interstellar clouds which are in a stage immediately before star formation. Cloud collisions seem to be the principal cause of the observed rotation of interstellar clouds. The rotational motion of the clouds is strongly influenced by turbulence.Theories dealing with the resolution of the angular momentum problem in star formation are classified into five major groups. We develop the old idea that the angular momentum of an interstellar cloud passes during star formation into the angular momentum of double star systems and/or circumstellar clouds.It is suggested that a rotating gas cloud contracts into a ring-like structure which fragments into self-gravitating subcondensations. By collisions and gas accretion these subcondensations accrete into binary systems surrounded by circumstellar clouds. Using some rough approximations we find analytical expressions for the semi-major axis of the binary system and for the density of the circumstellar clouds as a function of the initial density and of the initial angular velocity of an interstellar cloud. The obtained values are well within the observational limits. 相似文献
15.
《天文和天体物理学研究(英文版)》2021,(8)
By means of identical cubic elements, we generate a partition of a volume in which a particle-based cosmological simulation is carried out. In each cubic element, we determine the gas particles with a normalized density greater than an arbitrarily chosen density threshold. By using a proximity parameter, we calculate the neighboring cubic elements and generate a list of neighbors. By imposing dynamic conditions on the gas particles, we identify gas clumps and their neighbors, so that we calculate and fit some properties of the groups so identified, including the mass, size and velocity dispersion, in terms of their multiplicity(here defined simply as the number of member galaxies). Finally, we report the value of the ratio of kinetic energy to gravitational energy of such dense gas clumps, which will be useful as initial conditions in simulations of gravitational collapse of gas clouds and clusters of gas clouds. 相似文献
16.
17.
Warrick A. Lawson Lisa A. Crause Eric E. Mamajek Eric D. Feigelson 《Monthly notices of the Royal Astronomical Society》2002,329(2):L29-L33
Cold, dense clouds of gas have been proposed to explain the dark matter in Galactic haloes, and have also been invoked in the Galactic disc as an explanation for the excess faint submillimetre sources detected by SCUBA. Even if their dust-to-gas ratio is only a small percentage of that in conventional gas clouds, these dense systems would be opaque to visible radiation. We examine the possibility that the data sets of microlensing experiments searching for massive compact halo objects can also be used to search for occultation signatures by such clouds. We compute the rate and time-scale distribution of stellar transits by clouds in the Galactic disc and halo. We find that, for cloud parameters typically advocated by theoretical models, thousands of transit events should already exist within microlensing survey data sets. We examine the seasonal modulation in the rate caused by the Earth's orbital motion and find it provides an excellent probe of whether detected clouds are of disc or halo origin. 相似文献
18.
L. Binette 《Monthly notices of the Royal Astronomical Society》1998,294(3):L49-L51
The line ratios from coronal gas in Seyferts can be successfully fitted with photoionized clouds of high densities and low volume filling factor. The ionization parameter implied is sufficiently high that models must consider the effect of radiation pressure from the active nucleus. In spite of the gravitational force of the nucleus, radiation pressure is sufficiently strong to compress and radially accelerate the internally stratified gas clouds provided that these contain small amounts of dust (≃ 10 per cent of the solar neighbourhood value). This radial acceleration could explain the blueshift of the coronal lines relative to the systemic velocity without the need to invoke an ambient 'pushing' wind. Embedded dust has the interesting effect of making the photoionized clouds marginally ionization-bounded instead of matter-bounded. 相似文献
19.
Akiko Kawamura 《Astrophysics and Space Science》2008,313(1-3):145-151
Star formation is a fundamental process that dominates the life-cycle of various matters in galaxies: Stars are formed in
molecular clouds, and the formed stars often affect the surrounding materials strongly via their UV photons, stellar winds,
and supernova explosions. It is therefore revealing the distribution and properties of molecular gas in a galaxy is crucial
to investigate the star formation history and galaxy evolution. Recent progress in developing millimeter and sub-millimeter
wave receiver systems has enabled us to rapidly increase our knowledge on molecular clouds. In this proceedings, the recent
results from the surveys of the molecular clouds in the Milky Way and the Magellanic Clouds as well as the Galactic center
as the most active regions in the Milky Way are presented. The high sensitivity with unrivaled high resolution of ALMA will
play a key role in detecting denser gas that is tightly connected to star formation. 相似文献
20.
We suggest a model that explains the stratification peculiarities of the [O III] and Hα line emission from some of the ring nebulae around Wolf-Rayet stars. These peculiarities lie in the fact that the [O III] line emission regions are farther from the central star than the Hα regions, with the distance between them reaching several tenths of a parsec. We show that the radiative shock produced by a Wolf-Rayet stellar wind and propagating with a velocity of ～100 km s?1 cannot explain such large distances between these regions due to the low velocity of the gas outflow from the shock front. The suggested model takes into account the fact that the shock produced by a Wolf-Rayet stellar wind propagates in a two-phase medium: a rarefied medium and dense compact clouds. The gas downstream of a fast shock traveling in a rarefied gas compresses the clouds. Slow radiative shocks are generated in the clouds; these shocks heat the latter to temperatures at which ions of doubly ionized oxygen are formed. The clouds cool down, radiating in the lines of this ion, to temperatures at which Balmer line emission begins. The distance between the [O III] and Hα line emission regions is determined by the cooling time of the clouds downstream of the slow shock and by the velocity of the fast shock. Using the ring nebula NGC 6888 as an example, we show that the gas downstream of the fast shock must be at the phase of adiabatic expansion rather than deceleration with radiative cooling, as assumed previously. 相似文献