首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 787 毫秒
1.
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   

2.
Here we make a new study of the behaviour of the Na  i /K  i column density ratio in the interstellar medium, using a sample of new observations of 28 stars obtained at the Nordic Optical Telescope (NOT) in 1996 and 1997, and previously published observations (obtained by some of the authors) of 21 stars. The sightlines cover a range of distances and directions, including into the Galactic halo. We make use of new observations of the Na  i ultraviolet (UV) doublet for some 18 stars. This doublet is much weaker than the Na  i D doublet and so is less susceptible to saturation effects, and it is well known that it can be used to obtain more accurate Na  i column densities with a smaller error range. We find an average N (Na  i )/ N (K  i ) ratio from the Na  i UV data of about 90, which is rather higher than that found previously by Hobbs and Lequeux. The Na  UV–K  i   ratio shows a small increase in value with increasing column density, while we also find a sample of low N (Na  i )/ N (K  i ) ratio clouds generally seen towards distant objects on high-latitude sightlines that reach into the halo, so that the ratio decreases more sharply at lower column densities. As the values of the ratio for these halo clouds  (10–20)  bracket the cosmic Na/K abundance ratio, we suggest that these ratios result from a harder radiation field in the lower halo, such that the ionized fractions of Na  i and K  i become similar. Clearly caution needs to be applied in using any kind of 'standard value' for the Na  i /K  i column density ratio.  相似文献   

3.
To explain the effects of the ultraviolet (UV) background radiation on the collapse of pre-galactic clouds, we implement a radiation–hydrodynamical calculation, combining one-dimensional spherical hydrodynamics with an accurate treatment of the radiative transfer of ionizing photons. Both absorption and scattering of UV photons are explicitly taken into account. It turns out that a gas cloud contracting within the dark matter potential does not settle into hydrostatic equilibrium, but undergoes run-away collapse even under the presence of the external UV field. The cloud centre is shown to become self-shielded against ionizing photons by radiative transfer effects before shrinking to the rotation barrier. Based on our simulation results, we further discuss the possibility of H2 cooling and subsequent star formation in a run-away collapsing core. The present results are closely relevant to the survival of subgalactic Population III objects as well as to metal injection into intergalactic space.  相似文献   

4.
The expected lifetimes for molecular clouds has become a topic of considerable debate as numerical simulations have shown that MHD turbulence, the nominal means of support for clouds against self-gravity, will decay on short timescales. Thus it appears that either molecular clouds are transient features or they are resupplied with turbulent energy through some means. Jets and molecular outflows are recognized as a ubiquitous phenomena associated with star formation. Stars however form not isolation but in clusters of different density and composion. The ubiquity and high density of outflows from young stars in clusters make them an intriguing candidate for the source of turbulence energy in molecular clouds. In this contribution we present new studies, both observational and theoretical, which address the issue of jet/outflow interactions and their abilityto drive turbulent flows in molecular clouds. Our studies focus on scales associated with young star forming clusters. In particular we first show that direct collisions between active outflows are not effective at stirring the ambient medium. We then show that fossil cavities from “extinct” outflows may provide the missing link in terms of transferring momentum and energy to the cloud.  相似文献   

5.
We study the inhomogeneous reionization in a critical density CDM universe resulting from stellar sources, including Population III objects. The spatial distribution of the sources is obtained from high-resolution numerical N -body simulations. We calculate the source properties, taking into account a self-consistent treatment of both radiative (i.e. ionizing and H2-photodissociating photons) and stellar (i.e. SN explosions) feedbacks regulated by massive stars. This allows us to describe the topology of the ionized and dissociated regions at various cosmic epochs, and to derive the evolution of H, He and H2 filling factors, soft UV background, cosmic star formation rate and the final fate of ionizing objects. The main results are: (i) galaxies reionize the intergalactic medium by z ≈10 (with some uncertainty related to the gas clumping factor), whereas H2 is completely dissociated already by z ≈25; (ii) reionization is mostly caused by the relatively massive objects which collapse via H line cooling, while objects the formation of which relies on H2 cooling alone are insufficient for this purpose; (iii) the diffuse soft UV background is the major source of radiative feedback effects for z ≤15; at higher z direct flux from neighbouring objects dominates; (iv) the match of the calculated cosmic star formation history with that observed at lower redshifts suggests that the conversion efficiency of baryons into stars is ≈1 per cent; (v) we find that a very large population of dark objects which failed to form stars is present by z ≈8. We discuss and compare our results with similar previous studies.  相似文献   

6.
We present calculations of the dynamics of highly ionized gas (HIG) clouds that are confined by external pressure, and are photoionized by AGN continuum. We focus on the gas that is seen, in absorption, in the X-ray spectrum of many AGN and show that such gas can reach hydrostatic equilibrium under various conditions. The principal conclusion is that the clouds can be accelerated to high velocities by the central X-ray source. The dynamical problem can be reduced to the calculation of a single parameter, the average force multiplier, 〈 M 〉. The typical value of 〈 M 〉 is ∼10 suggesting that radiation pressure acceleration by X-rays is efficient for L / L Edd≳0.1 . The terminal velocity scales with the escape velocity at the base of the flow and can exceed it by a large factor. The typical velocity for a HIG flow that originates at R =1017 cm in a source with L x =1044 erg s−1 is ∼1000 km s−1, i.e. similar to the velocities observed in several X-ray and UV absorption systems.
Highly ionized AGN clouds are driven mainly by bound–free absorption, and bound–bound processes are less important unless the lines are significantly broadened or the column density is very small. Pressure laws that result in constant or outward decreasing ionization parameters are most effective in accelerating the flow.  相似文献   

7.
We performed an observational study of the dark filaments Lupus 1 and Lupus 4 using both polarimetric observations of 190 stars and a sample of 72 12CO profiles towards these clouds. We have estimated lower limits to the distances of Lupus 1 and Lupus 4 (≳ 140 and ≳ 125 pc, respectively). The observational strategy of the survey allows us to compare the projected magnetic field in an extended area around each cloud with the magnetic field direction observed to prevail along the clouds. Lupus 4 could have collapsed along the magnetic field lines, while in Lupus 1 the magnetic field appears to be less ordered, having the major axis of the filaments parallel to the large-scale projected magnetic field. These differences would imply that both filaments have different pattern evolutions. From the CO observations we have probed the velocity fields of the filaments and the spatial extension of the molecular gas with respect to the dust.  相似文献   

8.
  相似文献   

9.
We analyse spectra of slightly reddened, early-type stars in which the major diffuse interstellar bands at 5870 and 5797 Å are either strong or weak in relation to E(B – V) . It is demonstrated that among the low E(B – V) objects one can find stars obscured by 'sigma' as well as 'zeta' type clouds. The profiles of the diffuse bands seem not to be dependent on the total opacities of the interstellar clouds. We also discuss the physical conditions leading to the formation of the diffuse band carriers in low-opacity interstellar clouds; the carriers are apparently formed in media in which one can observe complex velocity fields.  相似文献   

10.
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   

11.
We present ultra-high-resolution (0.35 km s−1 FWHM) observations of the interstellar Ca K line towards seven nearby stars. The spectral resolution was sufficient to resolve the line profiles fully, thereby enabling us to detect hitherto unresolved velocity components, and to obtain accurate measurements of the velocity dispersions ( b values). Absorption components with velocities similar to those expected for the Local Interstellar Cloud (LIC) and the closely associated 'G cloud' were identified towards six of the seven stars. However, in most cases the b values deduced for these components were significantly larger than the b  ≈ 2.2 km s−1 (i.e. T k ≈ 7000 K, v t ≈ 1 km s−1) expected for the LIC, and it is argued that this results from the presence of additional, spectrally unresolved, components having similar velocities and physical conditions. For two stars (δ Vel and α Pav) we detect interstellar components with much smaller b values (1.1 ± 0.3 and 0.8 ± 0.1 km s−1, respectively) than are expected for low-density clouds within the Local Bubble. In the case of the narrow α Pav component, we also find an anomalously large Na  i /Ca  ii column density ratio, which is indicative of a relatively high density. Thus it is possible that, in addition to LIC-type clouds, the local interstellar medium contains a population of previously undetected cooler and denser interstellar clouds.  相似文献   

12.
Our ongoing investigation of how ‘Pillars’ and other structure form in molecular clouds irradiated by ultraviolet (UV) stars has revealed that the Rayleigh–Taylor instability is strongly suppressed by recombination in the photoevaporated outflow, that clumps and filaments may be key, that the evolution of structure is well-modeled by compressible hydrodynamics, and that directionality of the UV radiation may have significant effects. We discuss a generic, flexible set of laboratory experiments that can test these results.  相似文献   

13.
The distribution of proper motions of stars in the Pup-CMa association is presented. The stars' velocities are approximately parallel to each other, which indicates that the stars are close together in space. The mutual distribution of stars and molecular clouds in the association is interpreted as proof that the stars emerged from a single gigantic primordial molecular cloud (or several large clouds), destroyed by radiation and/or stellar wind coming from those stars. It is assumed that part of that cloud is being dissipated, while part is being broken into several small clouds, which we are observing at present.  相似文献   

14.
We present NH3(1,1) and (2,2) observations of MBM 12, the closest known molecular cloud (65-pc distance), aimed at finding evidence for on-going star formation processes. No local temperature (with a T rot upper limit of 12 K) or linewidth enhancement is found, which suggests that the area of the cloud that we have mapped (15-arcmin size) is not currently forming stars. Therefore this nearby 'starless' molecular gas region is an ideal laboratory to study the physical conditions preceding new star formation.
A radio continuum source has been found in Very Large Array archive data, close to but outside the NH3 emission. This source is likely to be a background object.  相似文献   

15.
Low mass star formation may be triggered by the dynamical effects of radiation fields and winds from massive stars on nearby molecular material. The columns of neutral material observed at the edges of many Hii regions may be the tracers of this process. Magnetic fields are dynamically important in the molecular clouds from which new stars form, but their effect on the development of molecular columns has not been studied in detail. In this paper, I present initial MHD simulations of this process.  相似文献   

16.
Observations of the diffuse emission in the 8–22 keV energy range, elongated parallel to the Galactic plane, and detection of the strong 6.4-keV fluorescent line with ∼ 1 keV equivalent width from some giant molecular clouds (e.g. Sgr B2) in the Galactic Centre region suggest that the neutral matter of these clouds is (or was) illuminated by powerful X-ray radiation, which gave rise to the reprocessed radiation. The source of this radiation remains unknown. A transient source close to the Sgr B2 cloud, or a short outburst of the X-ray emission from a supermassive black hole at the Galactic Centre are the two prime candidates under consideration. We argue that a new generation of X-ray telescopes combining very high sensitivity and excellent energy and angular resolutions would be able to discriminate between these two possibilities when studying time-dependent changes of the morphology of the surface brightness distribution, the equivalent width and the shape of the fluorescent line in Sgr B2 and other molecular clouds in the region. We note also that detection of broad and complex structures near the 6.4-keV line in the spectra of distant AGNs, which are X-ray weak now, may prove the presence of violent activity in the central engines of these objects in the past. Accurate measurements of the line shape may provide information on the time elapsed since the outburst. Proper motion (super- or subluminal) of the fluorescent radiation wave front can give additional information on the location of the source. Observations of the described effects can provide unique information on the matter distribution inside Sgr B2 and other giant molecular clouds.  相似文献   

17.
We present high-resolution optical echelle spectroscopy for a large fraction of the Li-rich late-type stars recently discovered in the vicinity of the Lupus dark clouds. Our results confirm the high Li  i   λ 6708 equivalent widths previously estimated from medium-resolution spectra, thus adding strength to the conclusion that the large majority of these stars are still in the pre-main-sequence phase of their evolution, contrary to claims from other authors that many of them might be zero-age main-sequence stars. We present a statistical approach to derive a mean distance for the sample, and find that it is consistent with, or slightly lower than, the Hipparcos distance of the Lupus star-forming region. The radial velocities measured for part of these stars are consistent with those observed for the Lupus star-forming region, while stars outside the dark clouds show a mean difference of the order of 3 km s−1. The projected rotational velocities show a lack of slow rotators, which is interpreted as a consequence of the X-ray selection of the sample. The Li-rich stars in Lupus studied in this work yield a fairly 'clean' sample of very young stars, while in other star-forming regions a larger fraction of older zero-age main-sequence stars has been found among ROSAT -discovered Li-rich stars. We argue that this fact reflects the relation of these stars with the Gould Belt.  相似文献   

18.
To better understand the conditions under which ice mantles form on grains in molecular clouds, three globules in the Southern Coalsack have been searched for the presence of H2O ice. Given the total lack of star formation in the Coalsack, it is an ideal site for studying unprocessed icy molecular mantles. In our sample of eight field stars lying behind the Coalsack we detect strong H2O ice absorption in the lines of sight to two stars and possible weak absorption in four others. We estimate H2O ice column densities or upper limits for these lines of sight. Compared to dark clouds such as Taurus, the Coalsack H2O ice column densities are lower than expected given the quiescent nature of the Coalsack region. It is possible that the chemical evolution of the Coalsack may simply be at too early a stage for significant ice mantles to appear on the grains, except perhaps in the densest parts of some of the globules. Alternatively, the presence or absence of ice absorption may be related to the distribution of dust along each line of sight, specifically, the relative contributions of dense globules and a more extended diffuse component. For example, our observations are consistent with an ice threshold extinction similar to that observed in the Taurus dark cloud if extinction amounting to   A V∼5  towards Globules 2 and 3 arises in the extended component. Globule 1 appears to have no extended component.  相似文献   

19.
We consider sulphur depletion in dense molecular clouds, and suggest hydrated sulphuric acid, H2SO4 ·  n H2O, as a component of interstellar dust in icy mantles. We discuss the formation of hydrated sulphuric acid in collapsing clouds and its instability in heated regions in terms of the existing hot core models and observations. We also show that some features of the infrared spectrum of hydrated sulphuric acid have correspondence in the observed spectra of young stellar objects.  相似文献   

20.
We use long-slit spectroscopic optical data to derive the properties of the extended emitting gas and the nuclear luminosity of a sample of 18 Seyfert 2 galaxies. From the emission-line luminosities and ratios we derive the density, reddening and mass of the ionized gas as a function of distance up to 2–4 kpc from the nucleus. Taking into account the geometric dilution of the nuclear radiation, we derive the radial distribution of covering factors and the minimum rate of ionizing photons emitted by the nuclear source. This number is an order of magnitude larger than that obtained from the rate of ionizing photons 'intercepted' by the gas and measured from the Hα luminosity. A calibration is proposed to recover this number from the observed luminosity. The He  ii λ4686/Hβ line ratio was used to calculate the slope of the ionizing spectral energy distribution (SED), which in combination with the number of ionizing photons allows the calculation of the hard X-ray luminosities. These luminosities are consistent with those derived from X-ray spectra in the eight cases for which such data are available and recover the intrinsic X-ray emission in Compton-thick cases. Our method can thus provide reliable estimates of the X-ray fluxes in Seyfert 2 galaxies for the cases where it is not readily available. We also use the ionizing SED and luminosity to predict the infrared luminosity under the assumption that it is dominated by reprocessed radiation from a dusty torus, and find a good agreement with the observed IRAS luminosities.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号