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1.
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2.
A reliable estimate of the molecular gas content in galaxies plays a crucial role in determining their dynamical and star-forming properties. However, H2, the dominant molecular species, is difficult to observe directly, particularly in the regions where most molecular gas is thought to reside. Its mass is therefore commonly inferred by assuming a direct proportionality with the integrated intensity of the  12CO( J = 1 → 0)  emission line, using a CO-to-H2 conversion factor, X . Although a canonical value for X is used extensively in such estimates, there is increasing evidence, both theoretical and observational, that the conversion factor may vary by over an order of magnitude under conditions different from those of the local neighbourhood. In an effort to understand the influence of changing environmental conditions on the conversion factor, we derive theoretical estimates of X for a wide range of physical parameters using a photon-dominated region (PDR) time-dependent chemical model, benchmarking key results against those of an independent PDR code to ensure reliability. Based on these results, the sensitivity of the X factor to change in each physical parameter is interpreted in terms of the chemistry and physical processes within the cloud. In addition to confirming previous observationally derived trends, we find that the time-dependence of the chemistry, often neglected in such models, has a considerable influence on the value of the conversion factor.  相似文献   

3.
Results are presented from a model of molecular line formation in collapsing star-forming cores. The study includes, for the first time, a self-consistent chemical and dynamical model which is then directly coupled to an appropriate radiative transfer model. The assumptions of chemical uniformity or simple monotonic variations within such cores are shown to be unacceptable. The results show that the abundance variations and the line profiles are highly sensitive to the assumed values of the free parameters in the chemical model. Extreme caution is therefore advised in the quantitative analysis of emission-line profiles from infall sources. The implied degeneracy can be overcome by multiple line-of-sight observations of many species and transitions.  相似文献   

4.
We present a densely sampled map of visual polarimetry of stars in the direction of the Southern Coalsack dark cloud. Our sample consists of new polarimetric observations of 225 stars drawn from the spectrophotometric survey of Seidensticker, and an additional 173 stars, covering the surrounding areas of the cloud, taken from the literature. Because all the target stars have at least spectroscopic parallaxes, we can reliably investigate the spatial origins of the polarization, in three dimensions. We decompose the polarization into three components, due to (i) the wall of the local hot bubble, (ii) the Coalsack cloud and (iii) material in the Carina spiral arm. The polarization due to the Coalsack varies, both in alignment efficiency  ( p / AV )  and in the dispersion in polarization angle, across the cloud. Using a simplified radiative transfer treatment we show that the measured polarization in background gas is significantly affected by foreground polarization, and specifically that the analysis of the Coalsack polarization must take the effects of the local hot bubble wall into consideration. Correcting for this effect as well as for the internal line-of-sight averaging in the Coalsack, we find, based on a Chandrasekhar–Fermi analysis, a plane-of-the-sky magnetic field for the Coalsack cloud of  〈 B ⊥〉= 93 ± 23 μG  . A systematic error, best described by a multiplicative factor between 0.5 and 1.5, additionally arises from radiative transfer effect uncertainties. We propose that this high value for the magnetic field in the cloud envelope is due to the fact that the Coalsack cloud is embedded in the hot interior of the Upper Centaurus–Lupus superbubble.  相似文献   

5.
Using numerical techniques we study the global stability of cooling flows in X-ray luminous giant elliptical galaxies. As an unperturbed equilibrium state we choose the hydrostatic gas recycling model. Non-equilibrium radiative cooling, stellar mass loss, heating by type Ia supernovae, distributed mass deposition and thermal conductivity are included. Although the recycling model reproduces the basic X-ray observables, it appears to be unstable with respect to the development of inflow or outflow. In spherical symmetry the inflows are subject to a central cooling catastrophe, while the outflows saturate in a form of a subsonic galactic wind. Two-dimensional axisymmetric random velocity perturbations of the equilibrium model trigger the onset of a cooling catastrophe, which develops in an essentially non-spherical way. The simulations show a patchy pattern of mass deposition and the formation of hollow gas jets, which penetrate through the outflow down to the galaxy core. The X-ray observables of such a hybrid gas flow mimic those of the equilibrium recycling model, but the gas temperature exhibits a central depression. The mass deposition rate M ˙ consists of two contributions of similar size: (i) a hydrostatic one resembling that of the equilibrium model, and (ii) a dynamical one which is related to the jets and is more concentrated towards the centre. For a model galaxy, like NGC 4472, our 2D simulations predict M ˙ ≈ 2 M⊙ yr−1 within the cooling radius for the advanced non-linear stage of the instability. We discuss the implications of these results to Hα nebulae and star formation in cooling flow galaxies and emphasize the need for high-resolution 3D simulations.  相似文献   

6.
Theoretical predictions by Farebrother et al. and Meijer et al. of rovibrational excitation probabilities in H2 arising from formation by Eley-Rideal processes on a graphite surface are incorporated into a model of the chemistry and excitation of interstellar H2. The model includes the usual radiative and collisional pumping of H2 rotational and vibrational states, in addition to the formation processes. Predictions are made for HH2 rovibrational emission line intensities for representative points in diffuse and in dark interstellar clouds. We find that – if all the interstellar HH2 is formed by this Eley-Rideal process – then the consequences of formation pumping, as distinct from collisional and radiative pumping, should be clearly evident in both cases. In particular, we predict a clear spectral signature of this direct HH2 formation process on graphite, distinct from radiative and collisional pumping; this signature should be evident in both diffuse and dark clouds; but the emissivity for dark clouds is predicted to be some 500 times greater than that in diffuse clouds in which the dense material may be embedded. An observational search for this signature in two dark cloud sources was made, but a preliminary analysis of the data did not yield a detection. The implications of and possible reasons for this preliminary conclusion are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

7.
We present a mini-survey of ultrahigh-resolution spectroscopy (UHRS) of CH towards three southern molecular cloud envelopes. The sightlines are selected to probe physically similar gas in different Galactic environments. With a velocity resolution of ∼0.5 km s−1  ( R =575 000)  these observations resolve most kinematic components of the absorption lines. We do, however, detect one line component in the Lupus region, which is not resolved and for which an upper limit of   b <0.3 km s-1  is found. We find a correlation between distance of the absorbing gas from the Galactic mid-plane and the fractional abundance of CH. We show that this correlation can be explained as being a result of a fall-off in the ultraviolet radiation field intensity and propose that CH observations in carefully selected sightlines might allow a mapping of the variations in the interstellar radiation field.  相似文献   

8.
We present single-dish Arecibo 21-cm H  i observations, covering a 0675×0625 RA–Dec. grid, of the intermediate-velocity cloud (IVC) centred upon the M15 globular cluster. The velocity and positional structure of the IVC gas at V LSR=70 km s−1 are investigated; it is found to be clumpy and has a peak surface density N H  i ∼8×1019 cm−2. Additionally, we have performed a long H  i integration towards HD 203664, a Galactic halo star some 31 from M15, in which optical IVC absorption has previously been detected. No H  i with a velocity exceeding 60 km s−1 was found to a brightness temperature limit of 0.05 K. However, additional pointings did detect IVC gas approximately mid-way between HD 203664 and M15. Finally, we present both Arecibo H  i pointings and low-resolution spectra in the Ca  ii H and K lines towards 15 field stars in the general field towards M15, in an attempt to obtain the distance to the IVC. Intermediate-velocity H  i is detected towards seven sightlines. Stellar spectral types are derived for 12 of the sample. Assuming that these stars lie on the main sequence, their distances are estimated to lie in the range 150≤ d ≤1350 pc. No Ca  ii absorption is observed, either because the IVC is further away than ∼1350 pc or more likely because the gas along these sightlines is of too low a density to be detected by the current observations.  相似文献   

9.
We show that in a quiescent, dense pre-stellar core, exposed to the average interstellar radiation field, radiation pressure can cause the dust to migrate inwards, relative to the gas, on a time-scale of a few megayears – and faster if the radiation field is stronger than average. This has two potentially important effects.
First, there is an increase in the abundance of dust relative to gas in the inner parts of the core, and hence also in the efficiency of gas-cooling by dust. The increased cooling efficiency predisposes these regions to dynamical collapse and star formation. Additionally, it predisposes them to fragmentation, particularly if – as seems likely – the dust enhancements are stochastic and inhomogeneous, due to anisotropy of the incident radiation field and/or to directing of the migration by the local magnetic field. It also increases the metallicities of the resulting stars, and hence presumably the likelihood of planet formation in their accretion discs.
Secondly, there is a steepening of the optical-depth profile, especially at those impact parameters b where the visual optical depth through the core   τ t∼1  . Since the observational evidence for steep optical-depth profiles in the outer envelopes of some pre-stellar cores (specifically   τ t∝ b - β   , with   β ≳2)  constrains only the dust column density, this leaves open the possibility that the gas has a shallower column-density profile.  相似文献   

10.
The coupled set of equations of hydrodynamics and radiative transfer is derived for small disturbances in a plane, grey atmosphere. Only radiative transfer is taken into account in the energy equation; dynamical effects of radiation are ignored. A mean stationary radiative flux through the photosphere is taken into account. The radiative transfer equation is used by assuming the Eddington approximation, moreover, an exponential height profile of the temperature and an analytical opacity formula are supposed. For this model we obtained an asymptotic solution for plane nonadiabatic acoustic waves and radiation waves. The approach provides a detailed discussion of the interaction of nonadiabatic p‐modes and radiation waves in a realistic model of the photosphere of a solar‐like star.  相似文献   

11.
We introduce a differential equation for star formation in galaxies that incorporates negative feedback with a delay. When the feedback is instantaneous, solutions approach a self-limiting equilibrium state. When there is a delay, even though the feedback is negative, the solutions can exhibit cyclic and episodic solutions. We find that periodic or episodic star formation only occurs when two conditions are satisfied. First the delay time-scale must exceed a cloud consumption time-scale. Secondly, the feedback must be strong. This statement is quantitatively equivalent to requiring that the time-scale to approach equilibrium be greater than approximately twice the cloud consumption time-scale. The period of oscillations predicted is approximately four times the delay time-scale. The amplitude of the oscillations increases with both feedback strength and delay time.
We discuss applications of the delay differential equation (DDE) model to star formation in galaxies using the cloud density as a variable. The DDE model is most applicable to systems that recycle gas and only slowly remove gas from the system. We propose likely delay mechanisms based on the requirement that the delay time is related to the observationally estimated time between episodic events. The proposed delay time-scale accounting for episodic star formation in galaxy centres on periods similar to   P ∼ 10 Myr  , irregular galaxies with   P ∼ 100 Myr  , and the Milky Way disc with   P ∼ 2  Gyr, could be that for exciting turbulence following creation of massive stars, that for gas pushed into the halo to return and interact with the disc and that for spiral density wave evolution, respectively.  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.
The evolution of star-forming core analogues undergoing inside-out collapse is studied with a multipoint chemodynamical model which self-consistently computes the abundance distribution of chemical species in the core. For several collapse periods the output chemistry of infalling tracer species such as HCO+, CS and N2H+ is then coupled to an accelerated Λ-iteration radiative transfer code, which predicts the emerging molecular line profiles using two different input gas/dust temperature distributions. We investigate the sensitivity of the predicted spectral line profiles and line asymmetry ratios to the core temperature distribution, the time-dependent model chemistry, as well as to ad hoc abundance distributions. The line asymmetry is found to be strongly dependent on the adopted chemical abundance distribution. In general, models with a warm central region show higher values of blue asymmetry in optically thick HCO+ and CS lines than models with a starless core temperature profile. We find that in the formal context of Shu-type inside-out infall, and in the absence of rotation or outflows, the relative blue asymmetry of certain HCO+ and CS transitions is a function of time and, subject to the foregoing caveats, can act as a collapse chronometer. The sensitivity of simulated HCO+ line profiles to linear radial variations, subsonic or supersonic, of the internal turbulence field is investigated in the separate case of static cores.  相似文献   

14.
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   

15.
We present the first C-shock and radiative transfer model that calculates the evolution of the line profiles of neutral and ion species like SiO, H13CO+ and HN13C for different flow times along the propagation of the shock through the unperturbed gas. We find that the line profiles of SiO characteristic of the magnetic precursor stage have very narrow linewidths and are centered at velocities close to the ambient cloud velocity, as observed toward the young shocks in the L1448-mm outflow. Consistently with previous works, our model also reproduces the broad SiO emission detected in the high velocity gas in this outflow, for the downstream postshock gas in the shock. This implies that the different velocity components observed in L1448-mm are due to the coexistence of different shocks at different evolutionary stages.  相似文献   

16.
We numerically follow the nonlinear evolution of the Parker instability in the presence of phase transitions from a warm to a cold H  i interstellar medium in two spatial dimensions. The nonlinear evolution of the system favours modes that allow the magnetic field lines to cross the galactic plane. Cold H  i clouds form with typical masses  ≃105 M⊙  , mean densities  ≃20 cm−3  , mean magnetic-field strengths  ≃4.3 μG  (rms field strengths  ≃6.4 μG  ), mass-to-flux ratios  ≃0.1–0.3  relative to critical, temperatures  ≃50 K  , (two-dimensional) turbulent velocity dispersions  ≃1.6 km s−1  and separations  ≃500 pc  , in agreement with observations. The maximum density and magnetic-field strength are  ≃103 cm−3  and  ≃20 μG  , respectively. Approximately 60 per cent of all H  i mass is in the warm neutral medium. The cold neutral medium is arranged into sheet-like structures both perpendicular and parallel to the galactic plane, but it is also found almost everywhere in the galactic plane, with the density being highest in valleys of the magnetic field lines. 'Cloudlets' also form whose physical properties are in quantitative agreement with those observed for such objects by Heiles. The nonlinear phase of the evolution takes ≲30 Myr, so that, if the instability is triggered by a nonlinear perturbation such as a spiral density shock wave, interstellar clouds can form within a time suggested by observations.  相似文献   

17.
Cygnus X-2 appears to be the descendant of an intermediate-mass X-ray binary (IMXB). Using Mazzitelli's stellar code we compute detailed evolutionary sequences for the system and find that its prehistory is sensitive to stellar input parameters, in particular the amount of core overshooting during the main-sequence phase. With standard assumptions for convective overshooting a case B mass transfer starting with a 3.5-M⊙ donor star is the most likely evolutionary solution for Cygnus X-2. This makes the currently observed state rather short-lived, of order 3 Myr, and requires a formation rate > 10−7–10−6 yr−1 of such systems in the Galaxy. Our calculations show that neutron star IMXBs with initially more massive donors (≳4 M⊙) encounter a delayed dynamical instability; they are unlikely to survive this rapid mass transfer phase. We determine limits for the age and initial parameters of Cygnus X-2 and calculate possible dynamical orbits of the system in a realistic Galactic potential, given its observed radial velocity. We find trajectories which are consistent with a progenitor binary on a circular orbit in the Galactic plane inside the solar circle that received a kick velocity ≤200 km s−1 at the birth of the neutron star. The simulations suggest that about 7 per cent of IMXBs receiving an arbitrary kick velocity from a standard kick velocity spectrum would end up in an orbit similar to Cygnus X-2, while about 10 per cent of them reach yet larger Galactocentric distances.  相似文献   

18.
A new self-similar solution describing the dynamical condensation of a radiative gas is investigated under a plane-parallel geometry. The dynamical condensation is caused by thermal instability. The solution is applicable to generic flow with a net cooling rate per unit volume and time  ∝ρ2 T α  , where  ρ,  T   and α are the density, temperature and a free parameter, respectively. Given α, a family of self-similar solutions with one parameter η is found in which the central density and pressure evolve as follows:  ρ( x = 0, t ) ∝ ( t c− t )−η/(2−α)  and   P ( x = 0, t ) ∝ ( t c− t )(1−η)/(1−α)  , where t c is the epoch at which the central density becomes infinite. For  η∼ 0  the solution describes the isochoric mode, whereas for  η∼ 1  the solution describes the isobaric mode. The self-similar solutions exist in the range between the two limits; that is, for  0 < η < 1  . No self-similar solution is found for  α > 1  . We compare the obtained self-similar solutions with the results of one-dimensional hydrodynamical simulations. In a converging flow, the results of the numerical simulations agree well with the self-similar solutions in the high-density limit. Our self-similar solutions are applicable to the formation of interstellar clouds (H  i clouds and molecular clouds) by thermal instability.  相似文献   

19.
We present measurements of the distribution of the OH masers at 1665 and 1667 MHz towards the cometary ultracompact H  ii region in the complex G34.3+0.2. The results are based on observations made in both senses of circular polarization with a very long baseline interferometry (VLBI) array having an angular resolution of 5×20 mas2. 38 maser features are identified in the region. 33 of these lie on an arc at the edge of the cometary H  ii region. Five are located in a cluster offset toward the north-east by 3 arcsec, and are probably associated with an independent ultracompact H  ii region. There is a velocity gradient of 30 km s−1 pc−1 across the arc. We identify five Zeeman pairs and determine that the magnetic field varies between 1 and 7 mG, but is always directed away from the Earth.
The OH masers may arise in clumps in a shell of gas in a bow shock caused by the motion of the exciting star through the molecular cloud. The stand-off distance and the thickness of the shocked shell are roughly consistent with those predicted by such a bow-shock model. Also, the position of the exciting star(s), as estimated from the focus of the parabolic bow shock, closely matches that of the peak emission from the cometary H  ii region. However, the north–south velocity gradient in the ionized material remains difficult to explain in the context of the bow-shock model.  相似文献   

20.
The maser site OH 323.459−0.079 has been studied using the Long Baseline Array of the Australia Telescope National Facility. Simultaneous observations of the 1665- and 1667-MHz hydroxyl ground-state transitions yielded a series of maps at a velocity spacing of 0.18 km s−1, in both senses of circular polarization, with tenth-arcsec spatial resolution. Many small-diameter maser spots were detected within a 2-arcsec region. Pairs of spots with the same position, but with right- and left-hand circular polarization offset in frequency, reveal Zeeman splitting. Six pairs were found, and in four cases, the pairs at 1667 and 1665 MHz mutually corroborate the derived values of magnetic field and (central) kinematic velocity. Over the whole site, magnetic field estimates range from +1.47 to +4.13 mG with a median value of +2.5 mG. The excited state of OH at 6035 MHz also displays Zeeman pairs revealing a similar magnetic field, and we show that the most prominent of these pairs coincides with the most prominent pair at 1665 and 1667 MHz.
We also compared the morphology and kinematics at 1665 and 1667 MHz with those of maser emission from the excited state of OH at 6035 MHz and from methanol at 6668 MHz. All three varieties of masers appear intermingled, and associated with an ultracompact H  ii region. In many respects we find that OH 323.459−0.079 is similar to W3(OH), one of the few other maser sites yet studied in comparable detail.  相似文献   

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