Filament Fragmentation

We show three-dimensional numerical simulations in which stars form sequentially in a filamentary molecular cloud. The star formation is triggered by expansion of an H II region. The H II region is distant from the filamentary cloud at the initial stage. As it expands, it interacts with the filamentary cloud. The cloud is pinched and separated into two parts. Subsequently, the gravitational instability is induced to form two cores of the first generation along the filament axis in a typical model. The separation of the two cores is several times larger than the filament diameter. It is comparable to the wavelength of the fastest growing fragmentation mode. The first-generation cores become isolated, and filamentary clouds shorten to widen the separation. New cores of second generation form at the edges of the shortened filamentary clouds. This core formation is recursive, and our model shows sequential star formation triggered by an expanding H II region. The age difference is several times that of the dynamical timescale between the first- and second-generation cores. This sequential star formation is similar to that observed in the filamentary cloud associated with the H II region NGC 2024. Our first-generation cores correspond to FIR 4 and FIR 5, while the second-generation cores correspond to FIR 3 and FIR 6.

We have constructed two types of analytical models for an isothermal filamentary cloud supported mainly by magnetic tension. The first one describes an isolated cloud while the second considers filamentary clouds spaced periodically. Both the models assume that the filamentary clouds are highly flattened. The former is proved to be the asymptotic limit of the latter in which each filamentary cloud is much thinner than the distance to the neighboring filaments. We show that these models reproduce main features of the 2D equilibrium model of Tomisaka (2014) for filamentary cloud threaded by perpendicular magnetic field. It is also shown that the critical mass to flux ratio is $ M /\Phi = (2 \pi \sqrt{G}) ^{-1} $, where $ M $, $ \Phi $ and $ G $ denote the cloud mass, the total magnetic flux of the cloud, and the gravitational constant, respectively. This upper bound coincides with that for an axisymmetric cloud supported by poloidal magnetic fields. We applied the variational principle for studying the Jeans instability of the first model. Our model cloud is unstable against fragmentation as well as the filamentary clouds threaded by longitudinal magnetic field. The fastest growing mode has a wavelength several times longer than the cloud diameter. The second model describes quasi-static evolution of filamentary molecular cloud by ambipolar diffusion.

The extinction curve of interstellar dust in the dense molecular cloud cores is crucial for understanding dust properties, particularly size distribution and composition. We investigate the infrared extinction law in four nearby isolated molecular cloud cores—L429, L483, L673, and L1165—across the 1.2–8.0 μm wavelength range, using deep near-infrared and mid-infrared photometric data from UKIRT Infrared Deep Sky Survey and Spitzer Space Telescope. These observations probe an unprecedented extinction depth, reaching A V ∼ 40–60 mag in these dense cloud cores. We derive color-excess ratios E(K − λ)/E(H − K) by fitting color–color diagrams of (K − λ) versus (H − K), which are subsequently used to calculate the extinction law A λ /A K . Our analysis reveals remarkably similar and exceptionally flat infrared extinction curves for all four cloud cores, exhibiting the most pronounced flattening reported in the literature to date. This flatness is consistent with the presence of large dust grains, suggesting significant grain growth in dense environments. Intriguingly, our findings align closely with the Astrodust model for a diffuse interstellar environment proposed by Hensley and Draine. This agreement between dense core observations and a diffuse medium model highlights the complexity of dust evolution and the need for further investigation into the processes governing dust properties in different interstellar environments.

We investigate the condition for the formation of micron-sized grains in dense cores of molecular clouds. This is motivated by the detection of mid-infrared emission from deep inside a number of dense cores, the so-called ‘coreshine,’ which is thought to come from scattering by micron (μm)-sized grains. Based on numerical calculations of coagulation starting from the typical grain-size distribution in the diffuse interstellar medium, we obtain a conservative lower limit to the time t to form μm-sized grains: t/tff > 3(5/S)(nH/105 cm−3)−1/4 (where tff is the free-fall time at hydrogen number density nH in the core and S the enhancement factor of the grain–grain collision cross-section to account for non-compact aggregates). At the typical core density nH = 105 cm−3, it takes at least a few free-fall times to form the μm-sized grains responsible for coreshine. The implication is that those dense cores observed in coreshine are relatively long-lived entities in molecular clouds, rather than dynamically transient objects that last for one free-fall time or less.

The age of dense interstellar cloud cores, where stars and planets form, is a crucial parameter in star formation and difficult to measure. Some models predict rapid collapse, whereas others predict timescales of more than one million years (ref. 3). One possible approach to determining the age is through chemical changes as cloud contraction occurs, in particular through indirect measurements of the ratio of the two spin isomers (ortho/para) of molecular hydrogen, H2, which decreases monotonically with age. This has been done for the dense cloud core L183, for which the deuterium fractionation of diazenylium (N2H(+)) was used as a chemical clock to infer that the core has contracted rapidly (on a timescale of less than 700,000 years). Among astronomically observable molecules, the spin isomers of the deuterated trihydrogen cation, ortho-H2D(+) and para-H2D(+), have the most direct chemical connections to H2 (refs 8, 9, 10, 11, 12) and their abundance ratio provides a chemical clock that is sensitive to greater cloud core ages. So far this ratio has not been determined because para-H2D(+) is very difficult to observe. The detection of its rotational ground-state line has only now become possible thanks to accurate measurements of its transition frequency in the laboratory, and recent progress in instrumentation technology. Here we report observations of ortho- and para-H2D(+) emission and absorption, respectively, from the dense cloud core hosting IRAS 16293-2422 A/B, a group of nascent solar-type stars (with ages of less than 100,000 years). Using the ortho/para ratio in conjunction with chemical models, we find that the dense core has been chemically processed for at least one million years. The apparent discrepancy with the earlier N2H(+) work arises because that chemical clock turns off sooner than the H2D(+) clock, but both results imply that star-forming dense cores have ages of about one million years, rather than 100,000 years.

Context. How the diffuse medium of molecular clouds condenses in dense cores and how many of these cores will evolve in protostars is still a poorly understood step of the star formation process. Much progress is being made in this field, thanks to the extensive imaging of star-forming regions carried out with the Herschel Space Observatory. Aims. The Herschel Gould Belt Survey key project mapped the bulk of nearby star-forming molecular clouds in five far-infrared bands with the aim of compiling complete census of prestellar cores and young, embedded protostars. From the complete sample of prestellar cores, we aim at defining the core mass function and studying its relationship with the stellar initial mass function. Young stellar objects (YSOs) with a residual circumstellar envelope are also detected. Methods. In this paper, we present the catalogue of the dense cores and YSOs/protostars extracted from the Herschel maps of the Lupus I, III, and IV molecular clouds. The physical properties of the detected objects were derived by fitting their spectral energy distributions. Results. A total of 532 dense cores, out of which 103 are presumably prestellar in nature, and 38 YSOs/protostars have been detected in the three clouds. Almost all the prestellar cores are associated with filaments against only about one third of the unbound cores and YSOs/protostars. Prestellar core candidates are found even in filaments that are on average thermally subcritical and over a background column density lower than that measured in other star-forming regions so far. The core mass function of the prestellar cores peaks between 0.2 and 0.3 M⊙, and it is compatible with the log-normal shape found in other regions. Herschel data reveal several, previously undetected, protostars and new candidates of Class 0 and Class II with transitional disks. We estimate the evolutionary status of the YSOs/protostars using two independent indicators: the α index and the fitting of the spectral energy distribution from near- to far-infrared wavelengths. For 70% of the objects, the evolutionary stages derived with the two methods are in agreement. Conclusions. Lupus is confirmed to be a very low-mass star-forming region, in terms of both the prestellar condensations and the diffuse medium. Noticeably, in the Lupus clouds we have found star formation activity associated with interstellar medium at low column density, usually quiescent in other (more massive) star-forming regions.

We present the first results of high-spectral resolution (0.023 km s−1) N2H+ observations of dense gas dynamics at core scales (∼0.01 pc) using the recently commissioned Argus instrument on the Green Bank Telescope (GBT). While the fitted linear velocity gradients across the cores measured in our targets nicely agree with the well-known power-law correlation between the specific angular momentum and core size, it is unclear if the observed gradients represent core-scale rotation. In addition, our Argus data reveal detailed and intriguing gas structures in position–velocity (PV) space for all five targets studied in this project, which could suggest that the velocity gradients previously observed in many dense cores actually originate from large-scale turbulence or convergent flow compression instead of rigid-body rotation. We also note that there are targets in this study with their star-forming discs nearly perpendicular to the local velocity gradients, which, assuming the velocity gradient represents the direction of rotation, is opposite to what is described by the classical theory of star formation. This provides important insight on the transport of angular momentum within star-forming cores, which is a critical topic on studying protostellar disc formation.

The formation process of giant molecular clouds is investigated from the standpoint of the coagulation theory of molecular clouds. Small clouds collide with one another and grow to massive ones. Ultimately they form giant molecular clouds with a finite lifetime. The occurrence of star formation in a giant cloud destroys it and small clouds are formed again consequently. We study the time evolution of the clouds which move through a spiral gravitational potential by an N-body simulation. Then the ensemble of clouds with an internal mass circulation responds to the spiral potential and forms a structure similar to the gas galactic shock. It is shown that giant molecular clouds are formed in the spiral arm region by mutual collisions between clouds. The mass fraction of giant molecular clouds reaches 0.15–0.4 in the solar neighborhood even if their lifetime is as short as 4 × 107 yr. By comparison with the observation, we indicate that a giant molecular cloud is formed based on the coagulation theory.

In this paper we derive an improved core mass function (CMF) for the Pipe Nebula from a detailed comparison between measurements of visual extinction and molecular-line emission. We have compiled a refined sample of 201 dense cores toward the Pipe Nebula using a 2-dimensional threshold identification algorithm informed by recent simulations of dense core populations. Measurements of radial velocities using complimentary C18O (1-0) observations enable us to cull out from this sample those 43 extinction peaks that are either not associated with dense gas or are not physically associated with the Pipe Nebula. Moreover, we use the derived C18O, central velocities to differentiate between single cores with internal structure and blends of two or more physically distinct cores, superposed along the same line-of-sight. We then are able to produce a more robust dense core sample for future follow-up studies and a more reliable CMF than was possible previously. We confirm earlier indications that the CMF for the Pipe Nebula departs from a single power-law like form with a break or knee at M ~ 2.7 +/- 1.3 Msun. Moreover, we also confirm that the CMF exhibits a similar shape to the stellar IMF, but is scaled to higher masses by a factor of ~4.5. We interpret this difference in scaling to be a measure of the star formation efficiency (22 +/- 8%). This supports earlier suggestions that the stellar IMF may originate more or less directly from the CMF.

We show data from the SCUBA2 camera on JCMT, of molecular clouds. We focus on starless cores within the clouds. We present data of the Taurus region and show how the environment is affecting some cores' appearance in this region. We compare the SCUBA2 data with Herschel data and discuss the sensitivity of SCUBA2 to surface brightness in the sub-millimetre. We show how this leads to its ability to pick out the densest cores at a given temperature. Hence SCUBA2 preferentially picks out gravitationally bound pre-stellar cores. We discuss the effects of the magnetic field, and how this lends support to a model for the formation and evolution of cores in filamentary molecular clouds.

Context. Regarding the evolution of dust grains from diffuse regions of space to dense molecular cloud cores, many questions remain open. Scattering at near-infrared wavelengths, or “cloudshine”, can provide information on cloud structure, dust properties, and the radiation field that is complementary to mid-infrared “coreshine” and observations of dust emission at longer wavelengths. Aims. We examine the possibility of using near-infrared scattering to constrain the local radiation field and the dust properties, the scattering and absorption efficiency, the size distribution of the grains, and the maximum grain size. Methods. We use radiative transfer modelling to examine the constraints provided by the J, H, and K bands in combination with mid-infrared surface brightness at 3.6 μm. We use spherical one-dimensional and elliptical three-dimensional cloud models to study the observable effects of different grain size distributions with varying absorption and scattering properties. As an example, we analyse observations of a molecular cloud in Taurus, TMC-1N. Results. The observed surface brightness ratios of the bands change when the dust properties are changed. However, even a change of ±10% in the surface brightness of one band changes the estimated power-law exponent of the size distribution γ by up to ~30% and the estimated strength of the radiation field KISRF by up to ~60%. The maximum grain size Amax and γ are always strongly anti-correlated. For example, overestimating the surface brightness by 10% changes the estimated radiation field strength by ~20% and the exponent of the size distribution by ~15%. The analysis of our synthetic observations indicates that the relative uncertainty of the parameter distributions are on average Amax, γ ~ 25%, and the deviation between the estimated and correct values ΔQ < 15%. For the TMC-1N observations, a maximum grain size Amax > 1.5μm and a size distribution with γ > 4.0 have high probability. The mass weighted average grain size is ⟨am⟩ = 0.113μm. Conclusions. We show that scattered infrared light can be used to derive meaningful limits for the dust parameters. However, errors in the surface brightness data can result in considerable uncertainties on the derived parameters.

Magnetized, cold, dense molecular cloud cores provide the birth environment for stars, discs, and planets. The multi-scale scenario of low-mass star formation occurs via the formation of two quasi-hydrostatic cores. Furthermore, the conservation of angular momentum can lead to the formation of a disc around the second core (i.e.~the forming protostar). During these early stages of star formation, magnetically driven outflows and jets can be launched from the first and second cores, respectively. Star, disc, and outflow formation involve complex physical processes, which require a robust, self-consistent numerical treatment. In this thesis, we use numerical simulations to probe the gravitational collapse scenario that involves the transition of an isolated molecular cloud core to a hydrostatic core with a surrounding disc. We use the \emph{PLUTO} code to perform radiation (magneto-)hydrodynamic (MHD) collapse simulations, using one- and two-dimensional (2D) grids. We include the effects of self-gravity and a grey flux-limited diffusion approximation for the radiative transfer. Additionally, we use for the gas equation of state density- and temperature-dependent thermodynamic quantities to account for the dissociation, ionisation, and molecular vibrations and rotations. Our spherically symmetric simulations span seven orders of magnitude in spatial scale. We survey a wide range of initial low- to high-mass (0.5 -- 100~$M_{\odot}$) molecular cloud cores, yielding the largest parameter scan so far. Our results highlight the dependence of the first and second hydrostatic core properties on the initial cloud core properties. These simulations indicate that in the high-mass regime, the first hydrostatic cores do not have enough time to form due to large accretion rates. We further expand our studies to three different sets of 2D simulations using axial and midplane symmetry. First, we perform 2D simulations for non-rotating molecular cloud cores with masses of 1~$M_{\odot}$, 5~$M_{\odot}$, 10~$M_{\odot}$, and 20~$M_{\odot}$. For each of these cases, we use an unprecedented resolution to model the evolution of the second core for $\geq$~100 years after its formation. For the first time, we demonstrate that convection is generated in the outer layers of the second core. This supports the intriguing possibility that dynamo-driven magnetic fields may be generated during the earliest phases of star formation. Following which, for the 1~$M_{\odot}$ case, we analyse the effects of solid-body rotation on the properties of the hydrostatic cores and disc formation. In this model, the first hydrostatic core evolves into a more oblate, pseudo-disc like structure and a sub-au disc starts forming after the formation of the second core. Finally, we explore the effects of ideal and non-ideal (including Ohmic resistivity) MHD during the collapse of rotating molecular cloud cores. We investigate the dependence of molecular outflows and disc formation on the initial cloud core mass, rotation, resistivity, and magnetic field strength. We find the presence of magnetically driven outflows launched from both first and second cores in the resistive models. We also reveal ongoing disc formation in some of our resistive simulations. In conclusion, we use detailed thermodynamical modelling to quantify the properties of the hydrostatic cores, outflows, and discs for collapse scenarios with a wide range of initial cloud core properties. The models presented herein will serve as the foundation for follow-up studies that link these theoretical insights with observational signatures.

We study the shape of the gas-phase mass-metallicity relation (MZR) of a combined sample of present-day dwarf and high-mass star-forming galaxies using IZI, a Bayesian formalism for measuring chemical abundances presented in Blanc et al. 2015. We observe a characteristic stellar mass scale at $M_* \simeq 10^{9.5}$M$_{\odot}$, above which the ISM undergoes a sharp increase in its level of chemical enrichment. In the $10^{6}-10^{9.5}$M$_{\odot}$ range the MZR follows a shallow power-law ($Z\propto M^{\alpha}_*$) with slope $\alpha=0.14\pm0.08$. At approaching $M_* \simeq 10^{9.5}$M$_{\odot}$ the MZR steepens significantly, showing a slope of $\alpha=0.37\pm0.08$ in the $10^{9.5}-10^{10.5}$M$_{\odot}$ range, and a flattening towards a constant metallicity at higher stellar masses. This behavior is qualitatively different from results in the literature that show a single power-law MZR towards the low mass end. We thoroughly explore systematic uncertainties in our measurement, and show that the shape of the MZR is not induced by sample selection, aperture effects, a changing N/O abundance, the adopted methodology used to construct the MZR, secondary dependencies on star formation activity, nor diffuse ionized gas (DIG) contamination, but rather on differences in the method used to measure abundances. High resolution hydrodynamical simulations can qualitatively reproduce our result, and suggest a transition in the ability of galaxies to retain their metals for stellar masses above this threshold. The MZR characteristic mass scale also coincides with a transition in the scale height and clumpiness of cold gas disks, and a typical gas fraction below which the efficiency of star formation feedback for driving outflows is expected to decrease sharply.

We observe the dense gas in the L1204/S140 molecular complex using CS(J=1-0) and NH 3 . The large-scale CS(J=1-0) maps show that L1204 is formed by three filamentary clouds, each being fragmented into cores of a few hundred solar masses and surrounded by low-level emission. The most prominent core is associated with S140, a well-known high-mass star-forming region. The star-forming activity, however, is not restricted to the vicinity of S140, but extends throughout the complex; very red IRAS sources lie close to most of the cores and molecular outflows have been detected in half of them

The dynamical instability of a self-gravitating magnetized filamentary cloud was investigated by means of a normal-mode analysis. The density distribution in equilibrium was assumed to be a function of the radial distance from the axis, ρ0(r) = ρc(1 + r2/8H2)–2, where ρc and Hare model parameters specifying the density on the axis and the length scale, respectively; the magnetic field was assumed to have both longitudinal (z-) and azimuthal (φ-) components with a strength of B0r∝ρ0r. Our model filamentary cloud is unstable against both axisymmetric and non-axisymmetric perturbations. The growth rates and eigenfunctions of the unstable perturbations were obtained numerically as a function of the strength and configuration of the magnetic field. The most unstable perturbation is axisymmetric and its wavelength in units of H is shorter when the ratio of the magnetic pressure to the gas pressure is higher. This means that the filamentary cloud fragments into pieces with an apparently shorter interval when the magnetic field is stronger. The motion induced by the most unstable perturbation also depends on the magnetic field. When the z-component of the magnetic field is stronger, the fragment produced by the instability collapses faster in the z-direction than in the r-direction. We discuss the late stage of fragmentation.