Keywords

We perform a suite of 3D radiation hydrodynamics simulations of photoevaporation of molecular gas clumps illuminated by external massive stars. We study the fate of solar-mass clumps and derive their lifetimes by varying the gas metallicity over a range of ${10}^{-3}\,{Z}_{\odot }\leqslant Z\leqslant 1{Z}_{\odot }$. Our simulations incorporate radiation transfer of far- and extreme-ultraviolet photons and follow atomic/molecular line cooling and dust–gas collisional cooling. Nonequilibrium chemistry is coupled with the radiative transfer and hydrodynamics in a self-consistent manner. We show that radiation-driven shocks compress gas clumps to have a volume that is set by the pressure equilibrium with the hot ambient gas. Radiative cooling enables metal-rich clumps to condense and have small surface areas where photoevaporative flows are launched. For our fiducial setup with an O-type star at a distance of 0.1 pc, the resulting photoevaporation rate is as small as $\sim {10}^{-5}\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$ for metal-rich clumps, but it is larger for metal-poor clumps that have larger surface areas. The clumps are continuously accelerated away from the radiation source by the so-called rocket effect and can travel over ∼1 pc within the lifetime. We also study the photoevaporation of clumps in a photodissociation region. Photoelectric heating is inefficient for metal-poor clumps that contain a smaller amount of grains, and thus they survive for over 10⁵ yr. We conclude that the gas metallicity strongly affects the clump lifetime and thus determines the strength of feedback from massive stars in star-forming regions.

During star formation, the accretion disk drives fast MHD winds, which usually contain two components, a collimated jet and a radially distributed wide-angle wind. These winds entrain the surrounding ambient gas producing molecular outflows. We report a recent observation of ¹²CO (2–1) emission of the HH 46/47 molecular outflow by the Atacama Large Millimeter/submillimeter Array, in which we identify multiple wide-angle outflowing shell structures in both the blueshifted and redshifted outflow lobes. These shells are highly coherent in position–position–velocity space, extending to ≳40–50 km s⁻¹ in velocity and 10⁴ au in space, with well-defined morphology and kinematics. We suggest these outflowing shells are the result of the entrainment of ambient gas by a series of outbursts from an intermittent wide-angle wind. Episodic outbursts in collimated jets are commonly observed, yet detection of a similar behavior in wide-angle winds has been elusive. Here we show clear evidence that the wide-angle component of the HH 46/47 protostellar outflows experiences variability similar to that seen in the collimated component.

We investigate how the dynamical state of molecular clouds relates to host galaxy environment and how this impacts the star formation efficiency (SFE) in the Milky Way and seven nearby galaxies. We compile measurements of molecular cloud and host galaxy properties, and determine mass-weighted mean cloud properties for entire galaxies and distinct subregions within. We find molecular clouds to be in ambient pressure-balanced virial equilibrium, where clouds in gas-rich, molecular-dominated, high-pressure regions are close to self-virialization, whereas clouds in gas-poor, atomic-dominated, low-pressure environments achieve a balance between their internal kinetic pressure and external pressure from the ambient medium. The SFE per free-fall time of molecular clouds is low, ∼0.1%–1%, and shows systematic variations of 2 dex as a function of the virial parameter and host galactic environment. The trend observed for clouds in low-pressure environments—as the solar neighborhood—is well matched by state-of-the-art turbulence-regulated models of star formation. However, these models substantially overpredict the low observed SFEs of clouds in high-pressure environments, which suggest the importance of additional physical parameters not yet considered by these models.

Molecular cloud and protosolar nebula chemistry involves a strong interaction between the gas phase and the surface of icy grains. The exchanges between the gas phase and the solid phase depend not only on the adsorption and desorption rates but also on the geometry of the surface of the grains. Indeed, for sufficient levels of surface roughness, atoms and molecules have a significant probability to collide with the grain icy mantle several times before being potentially captured. In consequence, their net sticking probability may differ from their sticking probability for a single collision with the grain surface. We estimate the effectiveness of the recapture on uneven surfaces for the various desorption processes at play in astrophysical environments. We show that surface roughness has a significant effect on the desorption rates. We focus in particular on the production of O₂ since unexpectedly large amounts of it, probably incorporated in the comet when it formed, have been detected in the coma of comet 67P by the Rosetta probe. Our results suggest that the higher escape probability of hydrogen compared to heavier species on rough surfaces can contribute to enhancing the production of O₂ in the icy mantles of grains while keeping its abundance low in the gas phase and may significantly decrease the desorption probability of molecules involved in the O₂ chemical network.

Interaction of the solar wind with interstellar matter involves, among other processes, charge exchange between interstellar neutral atoms and plasma, which results in the creation of a secondary population of interstellar neutral (ISN) atoms. The secondary population of interstellar He was detected by Interstellar Boundary Explorer (IBEX), but interpretation of these measurements was mostly based on an approximation that the primary interstellar neutral population and the secondary population were non-interacting homogeneous Maxwell–Boltzmann functions in the outer heliosheath. We simulate the distribution function in the outer heliosheath and inside the heliopause using the "method of characteristics" with statistical weights obtained from solutions of the production and loss equations for the secondary atoms due to charge-exchange collisions in the outer heliosheath. We show that the two-Maxwellian approximation for the distribution function of neutral He is not a good approximation within the outer heliosheath but a reasonable one inside the termination shock. This is due to a strong selection effect: the He atoms able to penetrate inside the termination shock are a small, peculiar subset of the entire secondary He population. Nevertheless, the two-Maxwellian approximation reproduces the density distribution of ISN He inside the termination shock well and enables a realistic reproduction of the orientation of the plane defined by the Sun's velocity vector through the local interstellar matter and the vector of the unperturbed interstellar magnetic field.

The determination of the specific angular momentum radial profile, j(r), in the early stages of star formation is crucial to constrain star and circumstellar disk formation theories. The specific angular momentum is directly related to the largest Keplerian disk possible, and it could constrain the angular momentum removal mechanism. We determine j(r) toward two Class 0 objects and a first hydrostatic core candidate in the Perseus cloud, which is consistent across all three sources and well fit with a single power-law relation between 800 and 10,000 au: ${j}_{\mathrm{fit}}(r)={10}^{-3.60\pm 0.15}{\left(r/1000\mathrm{au}\right)}^{1.80\pm 0.04}\,\mathrm{km}\,{{\rm{s}}}^{-1}\,\mathrm{pc}$. This power-law relation is in between solid body rotation (∝r²) and pure turbulence (∝r^1.5). This strongly suggests that even at 1000 au, the influence of the dense core's initial level of turbulence or the connection between core and the molecular cloud is still present. The specific angular momentum at 10,000 au is ≈3× higher than previously estimated, while at 1000 au, it is lower by 2×. We do not find a region of conserved specific angular momentum, although it could still be present at a smaller radius. We estimate an upper limit to the largest Keplerian disk radius of 60 au, which is small but consistent with published upper limits. Finally, these results suggest that more realistic initial conditions for numerical simulations of disk formation are needed. Some possible solutions include: (a) using a larger simulation box to include some level of driven turbulence or connection to the parental cloud or (b) incorporating the observed j(r) to set up the dense core kinematics initial conditions.

We present the first results from the Small Magellanic Cloud portion of a new Australia Telescope Compact Array H i absorption survey of both of the Magellanic Clouds, comprising over 800 hr of observations. Our new H i absorption line data allow us to measure the temperature and fraction of cold neutral gas in a low-metallicity environment. We observed 22 separate fields, targeting a total of 55 continuum sources, against 37 of which we detected H i absorption; from this we measure a column-density-weighted mean average spin temperature of $\langle {T}_{{\rm{s}}}\rangle$ = 150 K. Splitting the spectra into individual absorption line features, we estimate the temperatures of different gas components and find an average cold gas temperature of ∼30 K for this sample, lower than the average of ∼40 K in the Milky Way. The H i appears to be evenly distributed throughout the SMC, and we detect absorption in 67% of the lines of sight in our sample, including some outside the main body of the galaxy (N_{H i} $\gt 2\times {10}^{21}$ cm⁻²). The optical depth and temperature of the cold neutral atomic gas show no strong trend with location spatially or in velocity. Despite the low-metallicity environment, we find an average cold gas fraction of ∼20%, not dissimilar from that of the Milky Way.

Interstellar neutral gas atoms penetrate the heliopause and reach 1 au, where they are detected by Interstellar Boundary Explorer (IBEX). The flow of neutral interstellar helium through the perturbed interstellar plasma in the outer heliosheath (OHS) results in the creation of a secondary population of interstellar He atoms, the so-called Warm Breeze, due to charge exchange with perturbed ions. The secondary population brings the imprint of the OHS conditions to the IBEX-Lo instrument. Based on a global simulation of the heliosphere with measurement-based parameters and detailed kinetic simulation of the filtration of He in the OHS, we find the number density of the interstellar He⁺ population to be (8.98 ± 0.12) × 10⁻³ cm⁻³. With this, we obtain the absolute density of interstellar H⁺ as 5.4 × 10⁻² cm⁻³ and that of electrons as 6.3 × 10⁻² cm⁻³, with ionization degrees of 0.26 for H and 0.37 for He. The results agree with estimates of the parameters of the Very Local Interstellar Matter obtained from fitting the observed spectra of diffuse interstellar EUV and the soft X-ray background.

We have investigated the formation and kinematics of submillimeter (submm) continuum cores in the Orion A molecular cloud. A comparison between submm continuum and near-infrared extinction shows a continuum core detection threshold of A_V ∼ 5–10 mag. The threshold is similar to the star formation extinction threshold of A_V ∼ 7 mag proposed by recent work, suggesting a universal star formation extinction threshold among clouds within 500 pc to the Sun. A comparison between the Orion A cloud and a massive infrared dark cloud G28.37+0.07 indicates that Orion A produces more dense gas within the extinction range 15 mag ≲ A_V ≲ 60 mag. Using data from the CARMA-NRO Orion Survey, we find that dense cores in the integral-shaped filament (ISF) show subsonic core-to-envelope velocity dispersion that is significantly less than the local envelope line dispersion, similar to what has been found in nearby clouds. Dynamical analysis indicates that the cores are bound to the ISF. An oscillatory core-to-envelope motion is detected along the ISF. Its origin is to be further explored.

We examine the linear stability of a filamentary cloud permeated by a perpendicular magnetic field. The initial magnetic field is assumed to be uniform and perpendicular to the cloud axis. The model cloud is assumed to have a Plummer-like density profile and to be supported against self-gravity by turbulence. The effects of turbulence are taken into account by enhancing the effective pressure of a low-density gas. We derive the effective pressure as a function of density from the condition of hydrostatic balance. It is shown that the model cloud is more unstable against radial collapse when the radial density slope is shallower. When the magnetic field is relatively weak, radial collapse is suppressed. If the displacement vanishes in a region very far from the cloud axis, the model cloud is stabilized completely by a relatively weak magnetic field. If rearrangement of the magnetic flux tubes is permitted, the model cloud is unstable even when the magnetic field is extremely strong. The stability depends on the outer boundary condition as in the case of an isothermal cloud. The growth rate of the rearrangement mode is smaller when the radial density slope is shallower.