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Molecular Clouds (molecular + cloud)

Distribution by Scientific Domains

Physics and Astronomy	97%

Kinds of Molecular Clouds

dense molecular cloud

giant molecular cloud

Selected Abstracts

Interferometric Insights into the Cycle of Matter

ASTRONOMISCHE NACHRICHTEN, Issue S2 2003
Katharina Schreyer
C01 A Multiplicity Survey of the , Oph Molecular Cloud , Preliminary Results C02 Bispectrum Speckle Interferometry of Young Jet- and Outflow-sources [source]

X-rays from the HII Regions and Molecular Clouds near the Galactic Center

ASTRONOMISCHE NACHRICHTEN, Issue S1 2003
Katsuji Koyama
Abstract We report measurements by Chandra of a variety of X-ray sources in the molecular clouds and HII regions of the Sgr B2, Arches, Quintuplet and the Galactic center clusters. Moderately bright X-ray sources are present in the Sgr B2, Quintuplet and the Galactic center clusters at the positions of ultra compact HII regions and bright infrared sources. Their X-ray spectra are fitted with models of a thin thermal plasma with 2,10 keV temperatures and luminosities of ,1032,33erg s,1. The X-ray properties are typical of those of high-mass young stellar objects or clusters of such objects. The Arches Cluster has three bright X-ray sources, at the positions of bright IR and radio stars, with X-ray luminosities of a few ×1033 erg s,1 each, which may indicate an unusual X-ray emission mechanism from high mass YSOs. A unique X-ray feature of molecular clouds and HII regions near the Galactic center is the presence of diffuse emission with a strong 6.4 keV line; in Sgr B2 this is attributable to the fluorescence of gas irradiated by external sources in the Galactic center, while the diffuse emission from Arches is puzzling. [source]

The star formation efficiency and its relation to variations in the initial mass function

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
Paul C. Clark
ABSTRACT We investigate how the dynamical state of a turbulently supported, 1000 M,, molecular cloud affects the properties of the cluster it forms, focusing our discussion on the star formation efficiency (SFE) and the initial mass function (IMF). A variety of initial energy states are examined in this paper, ranging from clouds with |Egrav| = 0.1 Ekin to clouds with |Egrav| = 10 Ekin, and for both isothermal and piece-wise polytropic equations of state (similar to that suggested by Larson). It is found that arbitrary SFEs are possible, with strongly unbound clouds yielding very low SFEs. We suggest that the low SFE in the Maddelena cloud may be a consequence of the relatively unbound state of its internal structure. It is also found that competitive accretion results in the observed IMF when the clouds have initial energy states of |Egrav| ,Ekin. We show that under such conditions the shape of the IMF is independent of time in the calculations. This demonstrates that the global accretion process can be terminated at any stage in the cluster's evolution, while still yielding a distribution of stellar masses that is consistent with the observed IMF. As the clouds become progressively more unbound, competitive accretion is less important and the protostellar mass function flattens. These results predict that molecular clouds should be permeated with a distributed population of stars that follow a flatter than Salpeter IMF. [source]

Ionization-induced star formation , I. The collect-and-collapse model

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
J. E. Dale
ABSTRACT We conduct smoothed particle hydrodynamics simulations of the ,collect-and-collapse' scenario for star formation triggered by an expanding H ii region. We simulate the evolution of a spherical uniform molecular cloud with an ionizing source at its centre. The gas in the cloud is self-gravitating, although the cloud is prevented from globally collapsing. We find that the shell driven by the H ii region fragments to form numerous self-gravitating objects. We repeat our calculations at four numerical resolutions to ensure that they are converged. We compare our results to the analytical model of Whitworth et al. and show that our simulations and the predictions of Whitworth et al. are in good agreement in the sense that the shell fragments at the time and radius predicted by Whitworth et al. to within 20 and 25 per cent, respectively. Most of the fragments produced in our two highest resolution calculations are approximately half the mass of those predicted by Whitworth et al., but this conclusion is robust against both numerical resolution and the presence of random noise (local fluctuations in density of a factor of ,2) in the initial gas distribution. We conclude that such noise has little impact on the fragmentation process. [source]

Star formation triggered by SN explosions: an application to the stellar association of , Pictoris

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006
C. Melioli
ABSTRACT In the present study, considering the physical conditions that are relevant in interactions between supernova remnants (SNRs) and dense molecular clouds for triggering star formation we have built a diagram of SNR radius versus cloud density in which the constraints above delineate a shaded zone where star formation is allowed. We have also performed fully 3D radiatively cooling numerical simulations of the impact between SNRs and clouds under different initial conditions in order to follow the initial steps of these interactions. We determine the conditions that may lead either to cloud collapse and star formation or to complete cloud destruction and find that the numerical results are consistent with those of the SNR,cloud density diagram. Finally, we have applied the results above to the , Pictoris stellar association which is composed of low-mass post-T Tauri stars with an age of 11 Myr. It has been recently suggested that its formation could have been triggered by the shock wave produced by an SN explosion localized at a distance of about 62 pc that may have occurred either in the Lower Centaurus Crux or in the Upper Centaurus Lupus which are both nearby older subgroups of that association (Ortega and co-workers). Using the results of the analysis above we have shown that the suggested origin for the young association at the proposed distance is plausible only for a very restricted range of initial conditions for the parent molecular cloud, that is, a cloud with a radius of the order of 10 pc and density of the order of 20 cm,3 and a temperature of the order of 50,100 K. [source]

Triggered star formation in bright-rimmed clouds: the Eagle nebula revisited

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2006
J. Miao
ABSTRACT A three-dimensional smoothed particle hydrodynamics model has been extended to study the radiation-driven implosion effect of massive stars on the dynamical evolution of surrounding molecular clouds. The new elements in the upgraded code are the inclusion of Lyman continuum in the incident radiation flux and the treatment of hydrogen ionization process; the introduction of ionization heating and recombination cooling effects; and the addition of a proper description of the magnetic and turbulent pressures to the internal pressure of the molecular cloud. This extended code not only provides a realistic model to trace the dynamical evolution of a molecular cloud, but also can be used to model the kinematics of the ionization and shock fronts and the photoevaporating gas surrounding the molecular cloud, which the previous code is unable to handle. The application of this newly developed model to the structure of the middle Eagle nebula finger suggests that the shock induced by the ionizing radiation at the front side of the head precedes an ionization front moving towards the centre of the core, and that the core at the fingertip is at a transition stage evolving toward a state of induced star formation. The dynamical evolution of the velocity field of the simulated cloud structure is discussed to illustrate the role of the self-gravity and the different cloud morphologies which appear at different stages in the evolutionary process of the cloud. The motion of the ionization front and the evaporating gas are also investigated. The modelled gas evaporation rate is consistent with that of other current models and the density, temperature and chemical profiles are in agreement with the observed values. The relative lifetimes of different simulated cloud morphologies suggest a possible answer to the question of why more bright-rimmed clouds are observed to possess a flat-core than an elongated-core morphology. [source]

Simulations of the heating of the Galactic stellar disc

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2002
Jyrki Hänninen
ABSTRACT The velocity dispersion of nearby stars in the Galactic disc is well known to increase substantially with age; this is the so-called age,velocity relation, and is interpreted as a ,heating' of the disc as a function of time. We have studied the heating of the Galactic stellar disc caused by giant molecular clouds and halo black holes, via simulations of the orbits of tracer stars embedded in a patch of the local Galactic disc. We examine a range of masses and number densities of the giant molecular cloud and halo black hole perturbers. The heating of the stellar disc in the simulations is fitted with a simple power law of the form ,,t,, where , is the velocity dispersion of the tracer stars as a function of time, t. We also fit this form to the best determinations of the increase in the velocity dispersion as a function of time as derived from stars in the solar neighbourhood for which ages can be reliably assigned. Observationally, , is found to lie in the range 0.3,0.6, i.e. it remains poorly constrained and its determination is probably still dominated by systematic errors. Better constrained observationally is the ratio of the velocity dispersions of the stars in the vertical z and horizontal x directions (i.e. towards the Galactic Centre), ,z/,x= 0.5 ± 0.1. For the heating of the stellar disc caused by giant molecular clouds (GMCs) we derive a heating ,,t0.21, which differs somewhat from early (analytic) studies in which ,,t1/4. This confirms the well-known results that there are insufficient GMCs to heat the Galactic disc appropriately. A range of dark halo black hole scenarios are verified to heat the stellar disc as ,,t1/2 (as expected from analytical studies), and give ,z/,x in the range 0.5,0.6, which is consistent with observations. Black holes with a mass of 107 M, are our favoured disc heaters, although they are only marginally consistent with observations. Simulations featuring a combination of giant molecular clouds and halo black holes can explain the observed heating of the stellar disc, but since other perturbing mechanisms, such as spiral arms, are yet to be included, we regard this solution as being ad hoc. [source]

Self-consistent simulations of star cluster formation from gas clouds under the influence of galaxy-scale tidal fields

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2008
Jarrod R. Hurley
ABSTRACT We present the first results of a project aimed at following the formation and long-term dynamical evolution of star clusters within the potential of a host galaxy. Here, we focus on a model evolved within a simplified potential representing the Large Magellanic Cloud. This demonstrates for the first time the self-consistent formation of a bound star cluster from a giant molecular cloud. The model cluster reproduces the density profiles and structural characteristics of observed star clusters. [source]

Modelling CO formation in the turbulent interstellar medium

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2010
S. C. O. Glover
ABSTRACT We present results from high-resolution three-dimensional simulations of turbulent interstellar gas that self-consistently follow its coupled thermal, chemical and dynamical evolution, with a particular focus on the formation and destruction of H2 and CO. We quantify the formation time-scales for H2 and CO in physical conditions corresponding to those found in nearby giant molecular clouds, and show that both species form rapidly, with chemical time-scales that are comparable to the dynamical time-scale of the gas. We also investigate the spatial distributions of H2 and CO, and how they relate to the underlying gas distribution. We show that H2 is a good tracer of the gas distribution, but that the relationship between CO abundance and gas density is more complex. The CO abundance is not well-correlated with either the gas number density n or the visual extinction AV: both have a large influence on the CO abundance, but the inhomogeneous nature of the density field produced by the turbulence means that n and AV are only poorly correlated. There is a large scatter in AV, and hence CO abundance, for gas with any particular density, and similarly a large scatter in density and CO abundance for gas with any particular visual extinction. This will have important consequences for the interpretation of the CO emission observed from real molecular clouds. Finally, we also examine the temperature structure of the simulated gas. We show that the molecular gas is not isothermal. Most of it has a temperature in the range of 10,20 K, but there is also a significant fraction of warmer gas, located in low-extinction regions where photoelectric heating remains effective. [source]

A method for reconstructing the variance of a 3D physical field from 2D observations: application to turbulence in the interstellar medium

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2010
C. M. Brunt
ABSTRACT We introduce and test an expression for calculating the variance of a physical field in three dimensions using only information contained in the two-dimensional projection of the field. The method is general but assumes statistical isotropy. To test the method we apply it to numerical simulations of hydrodynamic and magnetohydrodynamic turbulence in molecular clouds, and demonstrate that it can recover the three-dimensional (3D) normalized density variance with ,10 per cent accuracy if the assumption of isotropy is valid. We show that the assumption of isotropy breaks down at low sonic Mach number if the turbulence is sub-Alfvénic. Theoretical predictions suggest that the 3D density variance should increase proportionally to the square of the Mach number of the turbulence. Application of our method will allow this prediction to be tested observationally and therefore constrain a large body of analytic models of star formation that rely on it. [source]

The star formation efficiency and its relation to variations in the initial mass function

A model of cloud fragmentation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
George B. Field
ABSTRACT We present a model in which the supersonic motions observed in molecular clouds are driven by gravitational energy released as large structures fragment into smaller ones. The fragmentation process begins in large molecular clouds, and continues down to fragments of a critical mass defined as the mass at which gravitational confinement may be replaced by pressure confinement. The power laws that describe the scaling of density, mass, and number spectra of the fragments are given in terms of the observed velocity dispersion of the fragments. The results agree with observations over the range from several to about a third of a million solar masses. [source]

A census of the Carina Nebula , II.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
Energy budget, global properties of the nebulosity
ABSTRACT The first paper in this series took a direct census of energy input from the known OB stars in the Carina Nebula, and in this paper we study the global properties of the surrounding nebulosity. This detailed comparison may prove useful for interpreting observations of extragalactic giant H ii regions and ultraluminous infrared (IR) galaxies. We find that the total IR luminosity of Carina is about 1.2 × 107 L,, accounting for only about 50,60 per cent of the known stellar luminosity from Paper I. Similarly, the ionizing photon luminosity derived from the integrated radio continuum is about 7 × 1050 s,1, accounting for ,75 per cent of the expected Lyman continuum from known OB stars. The total kinetic energy of the nebula is about 8 × 1051 erg, or ,30 per cent of the mechanical energy from stellar winds over the lifetime of the nebula, so there is no need to invoke a supernova (SN) explosion based on energetics. Warm dust grains residing in the H ii region interior dominate emission at 10,30 ,m, but cooler grains at 30,40 K dominate the IR luminosity and indicate a likely gas mass of ,106 M,. We find an excellent correlation between the radio continuum and 20,25 ,m emission, consistent with the idea that the ,80-K grain population is heated by trapped Ly, photons. Similarly, we find a near perfect correlation between the far-IR optical depth map of cool grains and 8.6-,m hydrocarbon emission, indicating that most of the nebular mass resides as atomic gas in photodissociation regions and not in dense molecular clouds. Synchronized star formation around the periphery of Carina provides a strong case that star formation here was indeed triggered by stellar winds and ultraviolet radiation. This second generation appears to involve a cascade toward preferentially intermediate- and low-mass stars, but this may soon change when , Carinae and its siblings explode. If the current reservoir of atomic and molecular gas can be tapped at that time, massive star formation may be rejuvenated around the periphery of Carina much as if it were a young version of Gould's Belt. Furthermore, when these multiple SNe occur, the triggered second generation will be pelted repeatedly with SN ejecta bearing short-lived radioactive nuclides. Carina may therefore represent the most observable analogue to the cradle of our own Solar system. [source]

A large-scale extinction map of the Galactic Anticentre from 2MASS

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
D. Froebrich
ABSTRACT We present a 127 × 63 -deg2 extinction map of the Anticentre of the Galaxy, based on ,J,H, and ,H,K, colour excess maps from the Two-Micron All-Sky Survey. This 8001-deg2 map with a resolution of 4 arcmin is provided as online material. The colour excess ratio ,J,H,/,H,K, is used to determine the power-law index of the reddening law (,) for individual regions contained in the area (e.g. Orion, Perseus, Taurus, Auriga, Monoceros, Camelopardalis, Cassiopeia). On average we find a dominant value of ,= 1.8 ± 0.2 for the individual clouds, in agreement with the canonical value for the interstellar medium. We also show that there is an internal scatter of , values in these regions, and that in some areas more than one dominant , values are present. This indicates large-scale variations in the dust properties. The analysis of the AV values within individual regions shows a change in the slope of the column density distribution with distance. This can be attributed either to a change in the governing physical processes in molecular clouds on spatial scales of about 1 pc or to an AV dilution with distance in our map. [source]

On the relative motions of dense cores and envelopes in star-forming molecular clouds

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
Ben A. Ayliffe
ABSTRACT Hydrodynamical simulations of star formation indicate that the motions of protostars through their natal molecular clouds may be crucial in determining the properties of stars through competitive accretion and dynamical interactions. Walsh, Myers & Burton recently investigated whether such motions might be observable in the earliest stages of star formation by measuring the relative shifts of line-centre velocities of low- and high-density tracers of low-mass star-forming cores. They found very small (,0.1 km s,1) relative motions. In this paper, we analyse the hydrodynamical simulation of Bate, Bonnell & Bromm and find that it also gives small relative velocities between high-density cores and low-density envelopes, despite the fact that competitive accretion and dynamical interactions occur between protostars in the simulation. Thus, the simulation is consistent with the observations in this respect. However, we also find some differences between the simulation and the observations. Overall, we find that the high-density gas has a higher velocity dispersion than that observed by Walsh et al. We explore this by examining the dependence of the gas velocity dispersion on density and its evolution with time during the simulation. We find that early in the simulation the gas velocity dispersion decreases monotonically with increasing density, while later in the simulation, when the dense cores have formed multiple objects, the velocity dispersion of the high-density gas increases. Thus, the simulation is in best agreement with the observations early on, before many objects have formed in each dense core. [source]

A systematic survey for infrared star clusters with |b| < 20° using 2MASS

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007
D. Froebrich
ABSTRACT We used star density maps obtained from the Two-Micron All-Sky Survey (2MASS) to obtain a sample of star clusters in the entire Galactic Plane with |b| < 20°. A total of 1788 star cluster candidates are identified in this survey. Among those are 681 previously known open clusters and 86 globular clusters. A statistical analysis indicates that our sample of 1021 new cluster candidates has a contamination of about 50 per cent. Star cluster parameters are obtained by fitting a King profile to the star density. These parameters are used to statistically identify probable new globular cluster candidates in our sample. A detailed investigation of the projected distribution of star clusters in the Galaxy demonstrates that they show a clear tendency to cluster on spatial scales in the order of 12,25 pc, a typical size for molecular clouds. [source]

Star formation triggered by SN explosions: an application to the stellar association of , Pictoris

The formation of molecular clouds in spiral galaxies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
C. L. Dobbs
ABSTRACT We present smoothed particle hydrodynamics simulations of molecular cloud formation in spiral galaxies. These simulations model the response of a non-self-gravitating gaseous disc to a galactic potential. The spiral shock induces high densities in the gas, and considerable structure in the spiral arms, which we identify as molecular clouds. We regard the formation of these structures as due to the dynamics of clumpy shocks, which perturb the flow of gas through the spiral arms. In addition, the spiral shocks induce a large velocity dispersion in the spiral arms, comparable with the magnitude of the velocity dispersion observed in molecular clouds. We estimate the formation of molecular hydrogen, by post-processing our results and assuming the gas is isothermal. Provided the gas is cold (T, 100 K), the gas is compressed sufficiently in the spiral shock for molecular hydrogen formation to occur in the dense spiral arm clumps. These molecular clouds are largely confined to the spiral arms, since most molecular gas is photodissociated to atomic hydrogen upon leaving the arms. [source]

Triggered star formation in bright-rimmed clouds: the Eagle nebula revisited

Clumpy shocks and the clump mass function

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
Paul C. Clark
ABSTRACT One possible mechanism for the formation of molecular clouds is large-scale colliding flows. In this paper, we examine whether clumpy, colliding, flows could be responsible for the clump mass functions that have been observed in several regions of embedded star formation, which have been shown to be described by a Salpeter-type slope. The flows presented here, which comprise a population of initially identical clumps, are modelled using smoothed particle hydrodynamics (SPH) and calculations are performed with and without the inclusion of self-gravity. When the shock region is at its densest, we find that the clump mass spectrum is always well modelled by a Salpeter-type slope. This is true regardless of whether the self-gravity is included in the simulations or not, and for our choice of filling factors for the clumpy flows (10, 20 and 40 per cent), and Mach number (5, 10 and 20). In the non-self-gravitating simulations, this slope is retained at lower Mach numbers as the simulations progress past the densest phase. In the simulations which include self-gravity, we find that low Mach number runs yield a flatter mass function after the densest phase. This is simply a result of increased coagulation due to gravitational collapse of the flows. In the high Mach number runs the Salpeter slope is always lost. The self-gravitating calculations also show that the subgroup of gravitationally bound clumps in which star formation occurs, always contain the most massive clumps in the population. Typically these clumps have a mass of order of the Jeans mass of the initial clumps. The mass function of these bound star-forming clumps is not at all similar to the Salpeter-type mass function observed for stars in the field. We conclude that the clump mass function may not only have nothing to do with gravity, but also nothing to do with the star formation process and the resulting mass distribution of stars. This raises doubt over the claims that the clump mass function is the origin of the stellar initial mass function (IMF), for regions such as , Oph, Serpens and the Orion B cloud. [source]

The origin of the initial mass function and its dependence on the mean Jeans mass in molecular clouds

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2005
Matthew R. Bate
ABSTRACT We investigate the dependence of stellar properties on the mean thermal Jeans mass in molecular clouds. We compare the results from the two largest hydrodynamical simulations of star formation to resolve the fragmentation process down to the opacity limit, the first of which was reported by Bate, Bonnell & Bromm. The initial conditions of the two calculations are identical except for the radii of the clouds, which are chosen so that the mean densities and mean thermal Jeans masses of the clouds differ by factors of 9 and 3, respectively. We find that the denser cloud, with the lower mean thermal Jeans mass, produces a higher proportion of brown dwarfs and has a lower characteristic (median) mass of the stars and brown dwarfs. This dependence of the initial mass function (IMF) on the density of the cloud may explain the observation that the Taurus star-forming region appears to be deficient in brown dwarfs when compared with the Orion Trapezium cluster. The new calculation also produces wide binaries (separations >20 au), one of which is a wide binary brown dwarf system. Based on the hydrodynamical calculations, we develop a simple accretion/ejection model for the origin of the IMF. In the model, all stars and brown dwarfs begin with the same mass (set by the opacity limit for fragmentation) and grow in mass until their accretion is terminated stochastically by their ejection from the cloud through dynamically interactions. The model predicts that the main variation of the IMF in different star-forming environments should be in the location of the peak (due to variations in the mean thermal Jeans mass of the cloud) and in the substellar regime. However, the slope of the IMF at high masses may depend on the dispersion in the accretion rates of protostars. [source]

On the properties of young multiple stars

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004
E. J. Delgado-Donate
ABSTRACT We present numerical results on the properties of young binary and multiple stellar systems. Our analysis is based on a series of smoothed particle hydrodynamics (SPH) +N -body simulations of the fragmentation of small molecular clouds, which fully resolve the opacity limit for fragmentation. These simulations demonstrate that multiple star formation is a major channel for star formation in turbulent flows. We have produced a statistically significant number of stable multiple systems, with component separations in the range ,1,103 au. At the end of the hydrodynamic stage (0.5 Myr), we find that ,60 per cent of stars and brown dwarfs are members of multiples systems, with about a third of these being low-mass, weakly bound outliers in wide eccentric orbits. Our results imply that in the stellar regime most stars are in multiples (,80 per cent) and that this fraction is an increasing function of primary mass. After N -body integration to 10.5 Myr, the percentage of bound objects has dropped to about 40 per cent, this decrease arising mostly from very low-mass stars and brown dwarfs that have been released into the field. Brown dwarfs are never found to be very close companions to stars (the brown dwarf desert at very small separations), but one case exists of a brown dwarf companion at intermediate separations (10 au). Our simulations can accommodate the existence of brown dwarf companions at large separations, but only if the primaries of these systems are themselves multiples. We have compared the outcome of our simulations with the properties of real stellar systems as deduced from the infrared colour,magnitude diagram of the Praesepe cluster and from spectroscopic and high-resolution imaging surveys of young clusters and the field. We find that the spread of the observed main sequence of Praesepe in the 0.4,1 M, range appears to require that stars are indeed commonly assembled into high-order multiple systems. Similarly, observational results from Taurus and , Ophiuchus, or moving groups such as TW Hydrae and MBM 12, suggest that companion frequencies in young systems can indeed be as high as we predict. The comparison with observational data also illustrates two problems with the simulation results. First, low mass ratio (q < 0.2) binaries are not produced by our models, in conflict with both the Praesepe colour,magnitude diagram and independent evidence from field binary surveys. Secondly, very low-mass stars and brown dwarf binaries appear to be considerably underproduced by our simulations. [source]

Hydrodynamical simulations of the decay of high-speed molecular turbulence , I. Dense molecular regions

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2002
Georgi Pavlovski
ABSTRACT We present the results from three-dimensional hydrodynamical simulations of decaying high-speed turbulence in dense molecular clouds. We compare our results, which include a detailed cooling function, molecular hydrogen chemistry and a limited C and O chemistry, with those previously obtained for decaying isothermal turbulence. After an initial phase of shock formation, power-law decay regimes are uncovered, as in the isothermal case. We find that the turbulence decays faster than in the isothermal case because the average Mach number remains higher, owing to the radiative cooling. The total thermal energy, initially raised by the introduction of turbulence, decays only a little more slowly than the kinetic energy. We discover that molecule reformation, as the fast turbulence decays, is several times faster than that predicted for a non-turbulent medium. This is caused by moderate speed shocks which sweep through a large fraction of the volume, compressing the gas and dust. Through reformation, the molecular density and molecular column appear as complex patterns of filaments, clumps and some diffuse structure. In contrast, the molecular fraction has a wider distribution of highly distorted clumps and copious diffuse structure, so that density and molecular density are almost identically distributed during the reformation phase. We conclude that molecules form in swept-up clumps but effectively mix throughout via subsequent expansions and compressions. [source]

Simulations of the heating of the Galactic stellar disc

Spectropolarimetry of the 3-,m water-ice feature towards young stellar objects

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2002
R. P. Holloway
Abstract We present spectropolarimetry of the 3-,m water-ice feature towards five young stellar objects embedded in molecular clouds, including the Becklin,Neugebauer object, with wavelength range and spectral resolution much improved over previous studies. There is ice-feature polarization excess in four of the five sources and our observations indicate that the polarization is caused by the dichroic absorption of aligned grains in at least three of these. The ice-feature polarization excess is always accompanied by a systematic variation in the position angle of polarization, indicating that the ice-mantled grains are fractionated in the line of sight through a changing magnetic-field orientation. The results are compared with a recently published mid-infrared survey and we find good correlations between the polarization of the 3-,m ice feature and the 10-,m silicate feature, compelling evidence for the presence of water-ice mantled silicate grains, and which suggests that the core/mantle ratio does not differ widely between objects, an important result for grain models. [source]

A turbulent MHD model for molecular clouds and a new method of accretion on to star-forming cores

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2001
D. Balsara
We describe the results of a sequence of simulations of gravitational collapse in a turbulent magnetized region. The parameters are chosen to be representative of molecular cloud material. We find that several protostellar cores and filamentary structures of higher than average density form. The filaments inter connect the high-density cores. Furthermore, the magnetic field strengths are found to correlate positively with the density, in agreement with recent observations. We make synthetic channel maps of the simulations, and show that material accreting on to the cores is channelled along the magnetized filamentary structures. This is compared with recent observations of S106, and shown to be consistent with these data. We postulate that this mechanism of accretion along filaments may provide a means for molecular cloud cores to grow to the point where they become gravitationally unstable and collapse to form stars. [source]

Star formation in the LMC: Comparative CCD observations of young stellar populations in two giant molecular clouds

ASTRONOMISCHE NACHRICHTEN, Issue 8 2009
J. Ruppert
Abstract This work deals with a CCD imaging study at optical and near-infrared wavelength oftwo giant molecular clouds (plus a control field) in the southern region of the Large Magellanic Cloud, one ofwhich shows multiple signs of star formation, whereas the other does not. The observational data from VLT FORS2 (R band) and NTT SOFI (Ks band) have been analyzed to derive luminosity functions and color-magnitude diagrams. The young stellar content of these two giant molecular clouds is compared and confirmed to be different, in the sense that the apparently "starless" cloud has so far formed only low-luminosity, low-mass stars (fainter than mKs , 16.5 mag, not seen by 2MASS), while the other cloud has formed both faint low-mass and luminous high-mass stars. The surface density excess oflow-luminosity stars (,2 per square arcmin) in the "starless" cloud with respect to the control field is about 20% whereas the excess is about a factor of 3 in the known star-forming cloud. The difference may be explained theoretically by the gravo-turbulent evolution of giant molecular clouds, one being younger and less centrally concentrated than the other (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Spectroscopy of Hydrocarbon Grains toward the Galactic Center and Quintuplet Cluster

ASTRONOMISCHE NACHRICHTEN, Issue S1 2003
J.E. Chiar
Abstract Our view of the Galactic center (GC) is affected by extinction from both diffuse interstellar medium (ISM) dust and dense molecular clouds along the line of sight. The enormous visual extinction present toward the center of our Galaxy (,31 magnitudes) necessitates a study of the interstellar dust properties as well as an investigation into the distribution of the different dust components. We have built upon the historic spectroscopy of Willner et al. (1979), Butchart et al. (1986), and McFadzean et al. (1989) in order to investigate the distribution of these dust components across the GC field. Specifically, we employ spectroscopy in the 3 ,m region to investigate absorption features at 3.0 ,m and 3.4 ,m in lines of sight toward the GC central cluster and the Quintuplet cluster to the northeast. The 3.4 ,m feature is one of the primary spectral signatures of the organic component of interstellar dust and is, to date, only observed in the cold diffuse interstellar medium. The 3.0 ,m ice feature is carried by dense molecular cloud material, and can therefore be used to loosely trace the distribution of such material across the GC field. By obtaining spectra for multiple sightlines we have been able to deconvolve the diffuse ISM and dense molecular cloud components. Our study shows that differences exist in the spectra of relatively nearby lines of sight in the Galactic center central cluster. The depth of the 3.0 ,m water-ice feature varies by a factor of almost 5 across a 2 parsec (in projection) region, perhaps re.ecting the clumpy nature of the dense clouds. In addition, we found that the 3.4 ,m hydrocarbon feature varies in depth across the areas studied toward the central cluster, whereas the depth is relatively constant toward the Quintuplet cluster. This is likely a reflection of the distribution of extinction from the foreground diffuse ISM. Our ground-based and space-based spectroscopy reveals differences in absorption features in the 3 and 6 ,m regions between sightlines toward the GC central cluster and those toward the Quintuplet cluster. While the 3 ,m spectra of both regions show a broad absorption feature blueward of the 3.4 ,m absorption, only the Quintuplet spectra show a distinct absorption feature at 3.28 ,m. This feature is indicative of the presence of polycyclic aromatic hydrocarbons (PAHs) along the line of sight. The Quintuplet-proper sources have 6 ,m spectra that are markedly different than that of GC IRS 7 in the central cluster, and instead strongly resemble the spectra seen toward dusty late-type carbon-class (WC)Wolf-Rayet stars. This is the first hint of some spectroscopic similarity between the Quintuplet sources and dusty WC stars. [source]

X-rays from the HII Regions and Molecular Clouds near the Galactic Center

Experimental and Theoretical Study of the Broadening and Shifting of N2H+ Rotational Lines by Helium

CHEMPHYSCHEM, Issue 14 2010
Dr. Giovanni Buffa
Abstract Pressure broadening and pressure shift of N2H+ rotational lines perturbed by collisions with He are studied for the first time using experiment and theory. Results are reported from measurements at 88 K for the rotational transitions , , and with frequencies ranging from 0.28 to 0.56 THz. The agreement between experiment and theoretical data derived from close coupling calculations confirms the reliability of a theoretical framework used for state-to-state transition rates of interest in the interpretation of spectroscopic data from interstellar molecular clouds. The influence of hyperfine effects on shifts and widths of the rotational lines is discussed in detail. Although in principle possible, experiment and theoretical considerations lead to the conclusion that hyperfine effects only play a minor role. [source]