Fragmentation Of Molecular Clouds Research Articles

Spectral signatures of disk eccentricity in young binary systems

Star formation occurs via fragmentation of molecular clouds, which means that the majority of stars born are a members of binaries. There is growing evidence that planets might form in circumprimary disks of medium-separation binaries. The tidal forces caused by the secondary generally act to distort the originally circular disk to an eccentric one. To infer the disk eccentricity from high-res NIR spectroscopy, we calculate the fundamental band emission lines of the CO molecule emerging from the atmosphere of the disk. We model circumprimary disk evolution under the gravitational perturbation of the orbiting secondary using a 2D grid-based hydrodynamical code, assuming alpha-type viscosity. The hydrodynamical results are combined with our spectral code based on the double-layer disk model to calculate the CO molecular line profiles. We find that the orbital velocity distribution of the gas parcels differs significantly from the circular Keplerian fashion, thus the line profiles are asymmetric in shape. The magnitude of asymmetry is insensitive to the binary mass ratio, the magnitude of viscosity, and the disk mass. In contrast, the disk eccentricity, thus the level of the line profile asymmetry, is influenced significantly by the binary eccentricity and the disk geometrical thickness. We demonstrate that the disk eccentricity profile in the planet-forming region can be determined by fitting the high-resolution CO line profile asymmetry using a simple 2D spectral model that accounts for the velocity distortions caused by the disk eccentricity. Thus, with our novel approach the disk eccentricity can be inferred with high-resolution near-IR spectroscopy prior to the era of high angular resolution optical or radio direct-imaging. By determining the disk eccentricity in medium-separation young binaries, we might be able to constrain the planet formation theories.

FORMATION OF COLLAPSING CORES IN SUBCRITICAL MAGNETIC CLOUDS: THREE-DIMENSIONAL MAGNETOHYDRODYNAMIC SIMULATIONS WITH AMBIPOLAR DIFFUSION

We employ the three-dimensional magnetohydrodynamic simulation including ambipolar diffusion to study the gravitationally-driven fragmentation of subcritical molecular clouds, in which the gravitational fragmentation is stabilized as long as magnetic flux-freezing applies. The simulations show that the cores in an initially subcritical cloud generally develop gradually over an ambipolar diffusion time, which is about a few $\times 10^7$ years in a typical molecular cloud. On the other hand, the formation of collapsing cores in subcritical clouds is accelerated by supersonic nonlinear flows. Our parameter study demonstrates that core formation occurs faster as the strength of the initial flow speed in the cloud increases. We found that the core formation time is roughly proportional to the inverse of the square root of the enhanced density created by the supersonic nonlinear flows. The density dependence is similar to that derived in quasistatically contracting magnetically supported clouds, although the core formation conditions are created by the nonlinear flows in our simulations. We have also found that the accelerated formation time is not strongly dependent on the initial strength of the magnetic field if the cloud is highly subcritical. Our simulation shows that the core formation time in our model subcritical clouds is several $\times 10^6$ years, due to the presence of large-scale supersonic flows ($\sim 3$ times sound speed). Once a collapsing core forms, the density, velocity, and magnetic field structure of the core does not strongly depend on the initial strength of the velocity fluctuation. The infall velocities of the cores are subsonic and the magnetic field lines show weak hourglass shapes.

The impact of X-rays on molecular cloud fragmentation and the inital mass function

Star formation is regulated through a variety of feedback processes. In this study, we treat feedback by X-rays and discuss its implications. Our aim is to investigate whether star formation is significantly affected when a star forming cloud resides in the vicinity of a strong X-ray source. We perform an Eulerian grid simulation with embedded Lagrangian sink particles of a collapsing molecular cloud near a massive, 107 M⊙ black hole. The chemical and thermal changes caused by radiation are incorporated into the FLASH code. When there is strong X-ray feedback the star forming cloud fragments into larger clumps whereby fewer but more massive protostellar cores are formed. Competitive accretion has a strong impact on the mass function and a near-flat, non-Salpeter IMF results.

The fate of the pre-main sequence-rich clusters Collinder 197 and vdB 92: dissolution?

We investigate the nature and possible evolution of the young Galactic star clusters Collinder 197 (Cr 197) and vdB 92. The colour-magnitude diagrams (CMDs) are basically characterised by a poorly-populated MS and a dominant fraction ($\ga75%$) of PMS stars, and the combined MS and PMS CMD morphology in both clusters consistently constrains the age to within $5\pm4$ Myr, with a $\sim10$ Myr spread in the star formation process. The MS$ + $PMS stellar masses are $\approx660^{+102}_{-59} \ms$ (Cr 197) and $\approx750^{+101}_{-51} \ms$ (vdB 92). Cr 197 and vdB 92 appear to be abnormally large, when compared to clusters within the same age range. They have irregular stellar radial density distributions (RDPs) with a marked excess in the innermost region, a feature that, at less than 10 Myr, is more likely related to the star formation and/or molecular cloud fragmentation than to age-dependent dynamical effects. The velocity dispersion of both clusters, derived from proper motions, is in the range $\sim15 - 22 \kms$. Both clusters appear to be in a super-virial state, with velocity dispersions higher than those expected of nearly-virialised clusters of similar mass and size. A possible interpretation is that Cr 197 and vdB 92 deviate critically from dynamical equilibrium, and may dissolve into the field. We also conclude that early cluster dissolution leaves detectable imprints on RDPs of clusters as massive as several $10^2 \ms$. Cr 197 and vdB 92 may be the link between embedded clusters and young stellar associations.

On the evolution of a star cluster and its multiple stellar systems following gas dispersal

We investigate the evolution, following gas dispersal, of a star cluster produced from a hydrodynamical calculation of the collapse and fragmentation of a turbulent molecular cloud. We find that when the gas, initially comprising ≈60 per cent of the mass, is removed, the system settles into a bound cluster containing ≈30–40 per cent of the stellar mass surrounding by an expanding halo of ejected stars. The bound cluster expands from an initial radius of <0.05 to 1–2 pc over ≈4–10 Myr, depending on how quickly the gas is removed, implying that stellar clusters may begin with far higher stellar densities than usually assumed. With rapid gas dispersal, the most massive stars are found to be mass segregated for the first ∼1 Myr of evolution, but classical mass segregation only develops for cases with long gas removal time-scales. Eventually, many of the most massive stars are expelled from the bound cluster. Despite the high initial stellar density and the extensive dynamical evolution of the system, we find that the stellar multiplicity is almost constant during the 10 Myr of evolution. This is because the primordial multiple systems are formed in a clustered environment and, thus, by their nature are already resistant to further evolution. The majority of multiple system evolution is confined to the decay of high-order systems (particularly quadruple systems) and the formation of a significant population of very wide (104–105 au) multiple systems in the expanding halo. This formation mechanism for wide binaries potentially solves the problem of how most stars apparently form in clusters and yet a substantial population of wide binaries exist in the field. We also find that many of these wide binaries and the binaries produced by the decay of high-order multiple systems have unequal mass components, potentially solving the problem that hydrodynamical simulations of star formation are found to underproduce unequal mass solar-type binaries.

THE MASS-SIZE RELATION FROM CLOUDS TO CORES. I. A NEW PROBE OF STRUCTURE IN MOLECULAR CLOUDS

We use a new contour-based map analysis technique to measure the mass and size of molecular cloud fragments continuously over a wide range of spatial scales (0.05 < r / pc < 10), i.e., from the scale of dense cores to those of entire clouds. The present paper presents the method via a detailed exploration of the Perseus Molecular Cloud. Dust extinction and emission data are combined to yield reliable scale-dependent measurements of mass. This scale-independent analysis approach is useful for several reasons. First, it provides a more comprehensive characterization of a map (i.e., not biased towards a particular spatial scale). Such a lack of bias is extremely useful for the joint analysis of many data sets taken with different spatial resolution. This includes comparisons between different cloud complexes. Second, the multi-scale mass-size data constitutes a unique resource to derive slopes of mass-size laws (via power-law fits). Such slopes provide singular constraints on large-scale density gradients in clouds.

The thermodynamics of molecular cloud fragmentation

Properties of candidate stars, forming out of molecular clouds, depend on the ambient conditions of the parent cloud. We present a series of 2D and 3D simulations of fragmentation of molecular clouds in starburst regions as well as clouds under conditions in dwarf galaxies, leading to the formation of protostellar cores. We explore in particular the metallicity dependence of molecular cloud fragmentation and the possible variations in the dense core mass function, as the expression of a multi-phase ISM, due to dynamic and thermodynamic effects in starburst and metal-poor environments. To study the level of fragmentation during the collapse, the adaptive mesh refinement code FLASH is used. With this code, including self-gravity, thermal balance, turbulence and shocks, collapse is simulated with four different metallicity dependent cooling functions. Turbulent and rotational energies are considered as well. During the simulations, number densities of 10^8-10^9 cm^-3 are reached. The influences of dust and cosmic ray heating are investigated and a comparison to isothermal cases is made. The presented results indicate that fragmentation increases with metallicity, while cosmic ray and gas-grain collisional heating counteract this. We also find that modest rotation and turbulence can impact the cloud evolution as far as fragmentation is concerned. In this light, we conclude that radiative feedback, in starburst regions, will inhibit fragmentation, while low-metallicity dwarfs also should enjoy a star formation mode in which fragmentation is suppressed.

Testing molecular-cloud fragmentation theories: self-consistent analysis of OH Zeeman observations

Abstract The ambipolar-diffusion theory of star formation predicts the formation of fragments in molecular clouds with mass-to-flux ratios greater than that of the parent-cloud envelope. By contrast, scenarios of turbulence-induced fragmentation do not yield such a robust prediction. Based on this property, Crutcher et al. recently proposed an observational test that could potentially discriminate between fragmentation theories. However, the analysis applied to the data severely restricts the discriminative power of the test: the authors conclude that they can only constrain what they refer to as the ‘idealized’ ambipolar-diffusion theory that assumes initially straight–parallel magnetic field lines in the parent cloud. We present an original, self-consistent analysis of the same data taking into account the non-uniformity of the magnetic field in the cloud envelopes, which is suggested by the data themselves, and we discuss important geometrical effects that must be accounted for in using this test. We show quantitatively that the quality of current data does not allow for a strong conclusion about any fragmentation theory. Given the discriminative potential of the test, we urge for more and better-quality data.

The initial core mass function due to ambipolar diffusion in molecular clouds

We show that the ambipolar-diffusion–initiated fragmentation of molecular clouds leads simply and naturally to an initial core mass function (CMF) which is very similar to the initial stellar mass function (IMF) and is in excellent agreement with existing observations. This agreement is robust provided that the three (input) free parameters remain within their range of values suggested by observations. Other, observationally testable, predictions are made.

The nature of the young and low-mass open clusters Pismis 5, vdB 80, NGC 1931 and BDSB 96

We investigate the nature of 4 young and low-mass open clusters (OCs) located in the $2^{nd}$ and $3^{rd}$ quadrants with near-IR 2MASS photometry (errors $<0.1$ mag). After field decontamination, the colour-magnitude diagrams (CMDs) display similar morphologies: a poorly-populated main sequence (MS) and a dominant fraction of pre-MS (PMS) stars somewhat affected by differential reddening. Pismis 5, vdB 80 and BDSB 96 have MS ages within $5\pm4$ Myr, while the MS of NGC 1931 is $10\pm3$ Myr old. However, non-instantaneous star formation is implied by the wider ($\sim20$ Myr) PMS age spread. The cluster masses derived from MS + PMS stars are low, within $\sim60-180 \ms$, with mass functions (MFs) significantly flatter than Salpeter's initial mass function (IMF). Distances from the Sun are within $1.0-2.4$ kpc, and the visual absorptions are in the range $\aV=1.0-2.0$. From the stellar radial density profiles (RDPs), we find that they are small ($\rc\la0.48$ pc, $\rl\la5.8$ pc), especially Pismis 5 with $\rc\approx0.2$ pc and $\rl\approx1.8$ pc. Except for the irregular and cuspy inner regions of NGC 1931 and Pismis 5, the stellar RDPs follow a King-like profile. At $\sim10$ Myr, central cusps - which in old clusters appear to be related to advanced dynamical evolution - are probably associated with a star-formation and/or molecular cloud fragmentation effect. Despite the flat MFs, vdB 80 and BDSB 96 appear to be typical young, low-mass OCs. NGC 1931 and especially Pismis 5, with irregular RDPs, low cluster mass and flat MFs, do not appear to be in dynamical equilibrium. Both may be evolving into OB associations and/or doomed to dissolution in a few $10^7$ yr.

Dust coagulation and fragmentation in molecular clouds

In dense molecular clouds collisions between dust grains alter the ISM-dust size distribution. We study this process by inserting the results from detailed numerical simulations of two colliding dust aggregates into a coagulation model that computes the dust size distribution with time. All collisional outcomes -- sticking, fragmentation (shattering, breakage, and erosion) -- are included and the effects on the internal structure of the aggregates are also tabulated. The dust aggregate evolution model is applied to an homogeneous and static cloud of temperature 10 K and gas densities between 10^3 and 10^7 cm^-3. The coagulation is followed locally on timescales of ~10^7 yr. We find that the growth can be divided into two stages: a growth dominated phase and a fragmentation dominated phase. Initially, the mass distribution is relatively narrow and shifts to larger sizes with time. At a certain point, dependent on the material properties of the grains as well as on the gas density, collision velocities will become sufficiently energetic to fragment particles, halting the growth and replenishing particles of lower mass. Eventually, a steady state is reached, where the mass distribution is characterized by a mass spectrum of approximately equal amount of mass per logarithmic size bin. The amount of growth that is achieved depends on the cloud's lifetime. If clouds exist on free-fall timescales the effects of coagulation on the dust size distribution are very minor. On the other hand, if clouds have long-term support mechanisms, the impact of coagulation is important, resulting in a significant decrease of the opacity on timescales longer than the initial collision timescale between big grains.

Fragmentation in molecular clouds and its connection to the IMF

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

The minimum Jeans mass, brown dwarf companion IMF, and predictions for detection of Y-type dwarfs

Cool L- and T-type objects were discovered first as companions to stars in 1988 and 1995, respectively. A certain example of the yet cooler Y-type spectral class (Teff <~ 500K?) has not been seen. Recent infrared imaging observations of stars and brown dwarfs indicate that substellar companions with large semi-major axes and with masses less than the brown dwarf/giant planet dividing line (~13.5 Mj) are rare. Theoretical considerations of Jeans mass fragmentation of molecular clouds are consistent with this minimum mass cutoff and also with the semi-major axis (hundreds of AU) characteristic of the lowest mass imaged companions. As a consequence, Y-class companions with large semi-major axes should be scarce around stars <2Gyr old, and also around substellar primaries of all ages. By focusing on brown dwarf companions to young stellar primaries, it is possible to derive a first estimate of the brown dwarf IMF over the entire range of brown dwarf masses (13 Mj to 79 Mj) -- the number of companion brown dwarfs is proportional to mass to the -1.2+-0.2 power.

Three-dimensional MHD simulations of molecular cloud fragmentation regulated by gravity, ambipolar diffusion, and turbulence

AbstractWe find that the star formation is accelerated by the supersonic turbulence in the magnetically dominated (subcritical) clouds. We employ a fully three-dimensional simulation to study the role of magnetic fields and ion-neutral friction in regulating gravitationally driven fragmentation of molecular clouds. The time-scale of collapsing core formation in subcritical clouds is a few ×107 years when starting with small subsonic perturbations. However, it is shortened to approximately several ×106 years by the supersonic flows in the clouds. We confirm that higher-spacial resolution simulations also show the same result.

Submillimeter Observations of the Young Low‐Mass Object IRAS 04158+2805

We present high spatial resolution Submillimeter Array observations and supplementary single-dish photometry of the molecular gas and dust around IRAS 04158+2805, a young source with spectral type M5-M6 in the Taurus star-forming region. A bright, highly elongated dust structure that extends 8'' (~1120 AU) in diameter is revealed in a 883 μm thermal continuum image. This emission geometry is in good agreement with optical observations that show a similar structure in absorption, aligned perpendicular to bipolar scattered light nebulae. However, the interferometric data also clearly demonstrate that the submillimeter continuum emission is not centrally concentrated, but rather appears to have a toroidal geometry with substantially lower intensities inside a radius of ~250-300 AU. Spatially resolved emission from the CO J = 3–2 transition exhibits a velocity gradient along the major axis of the dust structure. If this kinematic pattern is interpreted as the signature of rotation around a central object, then a relatively low mass is inferred (M* ~ 0.3 M☉, with a ~50% uncertainty). We discuss several possible explanations for the observed gas and dust environment around IRAS 04158+2805, including a flattened envelope with an outflow cavity and a large circumbinary ring. This source offers unique views of the gas and dust environment surrounding a young low-mass stellar system. Its properties are generally not commensurate with formation scenarios for such low-mass objects that rely on dynamical ejection, but rather confirms that a single mechanism—molecular cloud core collapse and fragmentation—can produce stars over a wide range of stellar masses (at least an order of magnitude).

Time resolved star formation in the SMC: the youngest star clusters

AbstractThe two young clusters NGC 346 and NGC 602 in the Small Magellanic Cloud provide us with the opportunity to study and the efficiency of feedback mechanism at low metallicity, as well as the impact of local and global conditions in cluster formation and evolution. I describe the latest results from a multi-wavelength, large-scale study of these two clusters. HST/ACS images reveal that the clusters have very different structures: NGC 346 is composed by a number of sub-clusters which appear coeval with ages of 3 ± 1 Myr, strongly suggesting formation by the hierarchical fragmentation of a turbulent molecular cloud (Nota et al. 2006; Sabbi et al. 2007a). NGC 602, on the contrary, appears as a single small cluster of OB stars surrounded by pre-main sequence stars. For both clusters high-resolution spectroscopy of the ionized gas shows little evidence for gas motions. This suggests that at the low SMC metallicity, the winds from the hottest stars are not powerful enough to sweep away the residual gas. Instead we find that stellar radiation is the dominant process shaping the interstellar environment of NGC 346 and NGC 602.

Deconstructing the High‐Mass Star‐Forming Region IRAS 23033+5951

We report interferometric observations of the high-mass star-forming object IRAS 23033+5951. Our observations reveal two massive molecular cloud cores, designated IRAS 23033+5951-MMS1 and IRAS 23033+5951-MMS2. MMS1 has already formed a massive protostar and MMS2 appears to be on the verge of doing so. The latter core may be an example of a massive analog to a Class 0 star-forming object. The more evolved core shows some evidence of N2H+ destruction near the protostar, consistent with similar findings in low-mass star-forming objects. In addition to the already-known prominent HCO+ outflow, our SiO 2-1 and CH3OH 2-1 maps show evidence for two more candidate outflows, both presumably less powerful than the main one. Both cores are embedded in an elongated feature whose major axis is oriented almost exactly perpendicular to the axis of the most prominent outflow in the region. Although it has many of the characteristics of a disk, the 87,000 AU (0.42 pc) diameter of this structure suggests that it is more likely to be the flattened, rotating remnant of the natal molecular cloud fragment from which the star-forming cores condensed. We conclude that IRAS 23033+5951 is an excellent example of massive star formation proceeding in relative isolation, perhaps by the method of monolithic collapse and disk accretion.

The embedded cluster DBSB 48 in the nebula Hoffleit 18: Comparison with Trumpler 14

We derive fundamental parameters of the embedded cluster DBSB 48 in the southern nebula Hoffleit 18 and the very young open cluster Trumpler 14, by means of deep JHK s infrared photometry. We build colour–magnitude and colour–colour diagrams to derive reddening and age, based on main sequence and pre-main sequence distributions. Radial stellar density profiles are used to study cluster structure and guide photometric diagram extractions. Field-star decontamination is applied to uncover the intrinsic cluster sequences in the diagrams. Ages are inferred from K-excess fractions. A prominent pre-main sequence population is present in DBSB 48, and the K-excess fraction f K = 55 ± 6% gives an age of 1.1 ± 0.5 Myr. A mean reddening of A K s = 0.9 ± 0.03 was found, corresponding to A V = 8.2 ± 0.3. The cluster CMD is consistent with the far kinematic distance of 5 kpc for Hoffleit 18. For Trumpler 14 we derived similar parameters as in previous studies in the optical, in particular an age of 1.7 ± 0.7 Myr. The fraction of stars with infrared excess in Trumpler 14 is f K = 28 ± 4%. Despite the young ages, both clusters are described by a King profile with core radii R core = 0.46 ± 0.05 pc and R core = 0.35 ± 0.04 pc, respectively, for DBSB 48 and Trumpler 14. Such cores are smaller than those of typical open clusters. Small cores are probably related to the cluster formation and/or parent molecular cloud fragmentation. In DBSB 48, the magnitude extent of the upper main sequence is Δ K s ≈ 2 mag, while in Trumpler 14 it is Δ K s ≈ 5 mag, consistent with the estimated ages.

NGC 346 in the Small Magellanic Cloud. III. Recent Star Formation and Stellar Clustering Properties in the Bright HiiRegion N66

In the third part of our photometric study of the star-forming region NGC 346/N66 and its surrounding field in the Small Magellanic Cloud (SMC), we focus on the large number of low-mass pre-main-sequence (PMS) stars revealed by the Hubble Space Telescope observations with the Advanced Camera for Surveys. We investigate the origin of the observed broadening of the PMS population in the V − I, V color-magnitude diagram. The most likely explanations are either the presence of differential reddening or an age spread among the young stars. Assuming the latter, simulations indicate that we cannot exclude the possibility that stars in NGC 346 might have formed in two distinct events occurring about 10 and 5 Myr ago, respectively. We find that the PMS stars are not homogeneously distributed across NGC 346, but instead are grouped in at least five different clusters. On spatial scales from 0.8'' to 8'' (0.24-2.4 pc at the distance of the SMC) the clustering of the PMS stars as computed by a two-point angular correlation function is self-similar with a power-law slope γ ≈ − 0.3. The clustering properties are quite similar to Milky Way star-forming regions like Orion OB or ρ Oph. Thus molecular cloud fragmentation in the SMC seems to proceed on the same spatial scales as in the Milky Way. This is remarkable given the differences in metallicity and hence dust content between SMC and Milky Way star-forming regions.

The Formation and Dynamics of the SMC Cluster NGC 346

AbstractThe young resolved cluster NGC 346 in the SMC provides us with the opportunity to study the details of cluster formation and the efficiency of feedback mechanisms at low metallicity. I describe the latest results from a large-scale study of this cluster and its H II region N66. HST/ACS images reveal that NGC 346 is composed of a number of sub-clusters which appear to be coeval with ages of 3±1 Myr, strongly suggesting formation by the hierarchical fragmentation of a giant molecular cloud (Nota et al. 2006; Sabbi et al. 2007a). HST Hα images show that the central cluster and the sub-clusters still contain some of their residual gas. We present high resolution spectroscopy of the ionized gas, and find that it shows little evidence for gas motions. This suggests that, at the low SMC metallicity, the cluster O star winds are not powerful enough to sweep away the residual gas. Instead, we find that stellar radiation is the dominant process shaping the interstellar environment of NGC 346.