Keywords

The collisional evolution of solid material in protoplanetary disks is a crucial step in the formation of planetesimals, comets, and planets. Although dense protoplanetary environments favor fast dust coagulation, there are several factors that limit the straightforward pathway from interstellar micron-size grains to pebble-size aggregates. Apart from the grain bouncing, fragmentation, and fast drift to the central star, a notable limiting factor is the electrostatic repulsion of like-charged grains. In this study we aim at theoretical modeling of the dust coagulation coupled with the dust charging and disk ionization calculations. We show that the electrostatic barrier is a strong restraining factor to the coagulation of micrometer-size dust in dead zones of the disk (where the turbulence is suppressed). While the sustained turbulence helps to overcome the electrostatic barrier, low fractal dimensions of dust aggregates can potentially block their further coagulation even in this case. Coulomb repulsion may keep a significant fraction of small dust in the disk atmosphere and outer regions.

Winds from asymptotic giant branch (AGB) stars not only provide mass and energy return, but also produce dust grains in massive elliptical galaxies. Due to the fast stellar velocity, the wind is thought to form a comet-like tail, similar to Mira in the Local Bubble. Many massive elliptical galaxies and cluster centrals host extended dusty cold filaments. We carry out both analytical and numerical studies of the interaction between an AGB wind and the surrounding hot gas. We find that the cooling time of the tail is inversely proportional to the ambient pressure. In the absence of cooling, or in low-pressure environments (e.g., the outskirts of elliptical galaxies), AGB winds are quickly mixed into the hot gas, and all the AGB winds have a similar appearance and head-to-tail ratio. In high-pressure environments, such as the Local Bubble and the central regions of massive elliptical galaxies, some of the gas in the mixing layer between the stellar wind and the surrounding hot gas can cool efficiently and cause the tail to become longer. Our simulated tail of Mira itself has a similar length and velocity to that observed, and appears similar to the simulated AGB tail in the central regions of massive galaxies. While confirmation awaits future studies, we speculate that instead of thermal instability, the induced condensation at the mixing layer of AGB winds may be the origin of cold filaments in massive galaxies and galaxy clusters. This naturally explains the existence of dust and polycyclic aromatic hydrocarbon in the filaments.

In a previous work, Hoang and Tram discovered a new mechanism for destruction of nanoparticles due to suprathermal rotation of grains in stationary C-shocks, which is termed rotational disruption. In this paper, we extend our previous study for nonstationary shocks driven by outflows and young supernova remnants that have dynamical ages shorter than the time required to establish a stationary C-shock, which is composed of a C-shock and a J-shock tail (referred to as CJ-shock). For the C-shock component, we find that the smallest nanoparticles (size ≲1 nm) of weak materials (i.e., tensile strength S_max ≲ 10⁹ erg cm⁻³) can be rotationally disrupted owing to suprathermal rotation induced by supersonic neutral drift. For the J-shock component, although nanoparticles are rotating thermally, the smallest ones can still be disrupted because the gas is heated to higher temperatures by J-shocks. We then model microwave emission from rapidly spinning nanoparticles where the grain size distribution has the lower cutoff determined by rotational disruption for the different shock models. We also calculate the spectral flux of microwave emission from a shocked region at a distance of 100 pc from the observer for the different gas density, shock age, and shock velocities. We suggest that microwave emission from spinning dust can be used to trace nanoparticles and shock velocities in dense molecular outflows. Finally, we discuss a new way that can release molecules from the nanoparticle surface into the gas in the shocked regions, which we name rotational desorption.

We present a systematic study of the most luminous (M_IR [Vega magnitudes] brighter than −14) infrared (IR) transients discovered by the SPitzer InfraRed Intensive Transients Survey (SPIRITS) between 2014 and 2018 in nearby galaxies (D < 35 Mpc). The sample consists of nine events that span peak IR luminosities of M_[4.5],peak between −14 and −18.2, show IR colors between 0.2 < ([3.6]–[4.5]) < 3.0, and fade on timescales between 55 days < t_fade < 480 days. The two reddest events (A_V > 12) show multiple, luminous IR outbursts over several years and have directly detected, massive progenitors in archival imaging. With analyses of extensive, multiwavelength follow-up, we suggest the following possible classifications: five obscured core-collapse supernovae (CCSNe), two erupting massive stars, one luminous red nova, and one intermediate-luminosity red transient. We define a control sample of all optically discovered transients recovered in SPIRITS galaxies and satisfying the same selection criteria. The control sample consists of eight CCSNe and one Type Iax SN. We find that 7 of the 13 CCSNe in the SPIRITS sample have lower bounds on their extinction of 2 < A_V < 8. We estimate a nominal fraction of CCSNe in nearby galaxies that are missed by optical surveys as high as ${38.5}_{-21.9}^{+26.0} \%$ (90% confidence). This study suggests that a significant fraction of CCSNe may be heavily obscured by dust and therefore undercounted in the census of nearby CCSNe from optical searches.

We investigate the nature of dense gas in the 3–10 pc circumnuclear ring (CNR) in the galactic center of the Milky Way, which is a structure that may be dynamically connecting the supermassive black hole Sgr A* with the central molecular zone at the 100 pc scale, and is the closest reservoir of molecular gas to the massive stars located within the central cluster. In the first of several papers addressing open issues with the CNR, we use far-infrared (FIR) diagnostic emission lines to probe the hot and dense phase of the photodissociation region (PDR) exposed to the radiation field of the central population of massive stars. We use the Far Infrared Field-Imaging Line Spectrometer (FIFI-LS) instrument on board the Stratospheric Observatory For Infrared Astronomy airborne observatory to obtain spatially resolved maps of FIR emission lines of the region with an angular resolution approximately 4 times higher than previous published data. We complement our data with archival continuum images at 19.7, 31.5 and 37.1 μm obtained with FORCAST and 70, 100 and 160 μm archival continuum images from PACS. We use the FIFI-LS emission line flux maps from ionized ([C ii] 157.7 μm), atomic ([O i] 63.2 μm, [O i] 145.5 μm), and molecular (CO J = 14–13 186.0 μm) species for a comparison with model predictions for PDRs. We present a method that dissects emission from the low and from the high excitation phase of the PDR and that also accounts for, e.g., absorption especially in the [O i] 63.2 μm transition. We present spatially resolved maps of dust temperature, atomic hydrogen column density, and FIR flux. The derived atomic hydrogen column density map is aligned with the galactic plane and extends spatially beyond previous near-infrared and radio based A_v determinations. The atomic hydrogen column densities range from 10^22.5 to 10^23.1 cm⁻² resulting in a total enclosed mass of the order of 10^3.5 M_⊙. We derive a [O i] 63.2 μm absorption map that is aligned with the galactic plane with no or little absorption in the northern lobe of the CNR but moderate absorption in the southern lobe of the CNR, which is consistent with the picture where the illuminated front surfaces of gas clouds in the northern lobe are directly visible to us, while in the southern lobe the illuminated surfaces are hidden by the clouds within the lobe itself. Local gas densities in the CNR are generally below the Roche limit.

Infrared observations of active galactic nuclei (AGNs) reveal emission from the putative dusty circumnuclear "torus" invoked by AGN unification, which is heated up by radiation from the central accreting black hole (BH). The strong 9.7 and 18 μm silicate features observed in the AGN spectra, in both emission and absorption, further indicate the presence of such dusty environments. We present detailed calculations of the chemistry of silicate dust formation in AGN accretion disk winds. The winds considered herein are magnetohydrodynamic winds driven off the entire accretion disk domain that extends from the BH vicinity to the radius of BH influence, of order ∼1–100 pc depending on the mass of the resident BH. Our results indicate that these winds provide conditions conducive to the formation of significant amounts of dust, especially for objects accreting close to their Eddington limit, making AGNs a significant source of dust in the universe, especially for luminous quasars. Our models justify the importance of an r⁻¹ density law in the winds for efficient formation and survival of dust grains. The dust production rate scales linearly with the mass of the central BH and varies as a power law of index between 2 and 2.5 with the dimensionless mass accretion rate. The resultant distribution of the dense dusty gas resembles a toroidal shape, with high column density and optical depths along the equatorial viewing angles, in agreement with the AGN unification picture.

We apply novel survival analysis techniques to investigate the relationship between a number of the properties of galaxies and their atomic (M_{H i}) and molecular (${M}_{{{\rm{H}}}_{2}}$) gas mass, with the aim of devising efficient, effective empirical estimators of the cold gas content in galaxies that can be applied to large optical galaxy surveys. We find that dust attenuation, A_V, of both the continuum and nebular emission, shows significant partial correlations with ${M}_{{{\rm{H}}}_{2}}$, after controlling for the effect of star formation rate (SFR). The partial correlation between A_V and M_{H i}, however, is weak. This is expected because in poorly dust-shielded regions molecular hydrogen is dissociated by far-ultraviolet photons. We also find that the stellar half-light radius, R₅₀, shows significant partial correlations with both ${M}_{{{\rm{H}}}_{2}}$ and M_{H i}. This hints at the importance of environment (e.g., galactocentric distance) on the gas content of galaxies and the interplay between gas and SFR. We fit multiple regression to summarize the median, mean, and the 0.15/0.85 quantile multivariate relationships among ${M}_{{{\rm{H}}}_{2}}$, A_V, metallicity, and/or R₅₀. A linear combination of A_V and metallicity (inferred from stellar mass) or A_V and R₅₀, can estimate molecular gas masses within ∼2.5–3 times the observed masses. If SFR is used in addition, ${M}_{{{\rm{H}}}_{2}}$ can be predicted to within a factor ≲2. In this case, A_V and R₅₀ are the two best secondary parameters that improve the primary relation between ${M}_{{{\rm{H}}}_{2}}$ and SFR. Likewise, M_{H i} can be predicted to within a factor ≲3 using R₅₀ and SFR.

Polycyclic aromatic hydrocarbon (PAH) emission has long been proposed to be a potential star formation rate indicator, as it arises from the photodissociation region bordering the Strömgren sphere of young, massive stars. We apply a recently developed technique of mid-infrared spectral decomposition to obtain a uniform set of PAH measurements from Spitzer low-resolution spectra of a large sample of star-forming galaxies spanning a wide range in stellar mass (M_⋆ ≈ 10⁶–10^11.4 M_⊙) and star formation rate (∼0.1–2000 M_⊙ yr⁻¹). High-resolution spectra are also analyzed to measure [Ne ii] 12.8 μm and [Ne iii] 15.6 μm, which effectively trace the Lyman continuum. We present a new relation between PAH luminosity and star formation rate based on the [Ne ii] and [Ne iii] lines. Calibrations are given for the integrated 5–15 μm PAH emission, the individual features at 6.2, 7.7, 8.6, and 11.3 μm, as well as several mid-infrared bandpasses sensitive to PAH. We confirm that PAH emission is suppressed in low-mass dwarf galaxies, and we discuss the possible physical origin of this effect.

We observationally investigate star formation process occurring in AFGL 5157 (area ∼13.5 pc × 13.5 pc) using a multi-wavelength approach. Embedded filaments are seen in the Herschel column density map, and one of them is identified as an elongated filamentary feature (FF) (length ∼8.3 pc; mass ∼1170 M_⊙). Five Herschel clumps (M_clump ∼45–300 M_⊙) are traced in the central part of the FF, where an extended temperature structure (T_d ∼13.5–26.5 K) is observed. In the direction of the central part of the FF, the warmer region at T_d ∼20–26.5 K spatially coincides with a mid-infrared shell surrounding a previously known evolved infrared cluster. Diffuse Hα emission is traced inside the infrared shell, suggesting the presence of massive stars in the evolved cluster. Based on the surface density analysis of young stellar objects (YSOs), embedded clusters of YSOs are traced toward the central part of the FF, and are distributed around the infrared shell. Previously detected H₂O masers, H₂ knots, massive protostar candidates, and an H ii region are also seen toward the embedded clusters. Using the ¹²CO and ¹³CO line data, the central part of the FF is observed at the overlapping zones of two filamentary molecular clouds (length ∼12.5 pc) around −20 and −17 km s⁻¹, which are also connected in velocity. Our observational results suggest that the formation of massive stars appears to be triggered by a collision of two filamentary molecular clouds, which might have also influenced the birth of YSOs in AFGL 5157.

Alignment of dust grains in astrophysical environments results in the polarization of starlight as well as the polarization of radiation emitted by dust. We demonstrate the advances in grain alignment theory that allow the use of linear and circular polarization to probe not only the magnetic field, but also dust composition, the dust environment, etc. We revisit the process of grain alignment by Radiative Torques (RATs) and focus on constraining magnetic susceptibility of grains via observations. We discuss the possibility of observational testing of the magnetic properties of grains as the alignment changes from being in respect to the magnetic field to being in respect to the radiation direction. This both opens a possibility of constraining the uncertain parameters of the RATs theory and provides a new way of measuring magnetic fields in the interstellar medium and circumstellar regions. We provide a detailed discussion of the precession induced both by the magnetic field and the anisotropic radiation and revisit a number of key processes related to magnetic response of the grains. We consider various effects that increase the rate of magnetic relaxation both in silicate and carbonaceous grains. In particular, we find a new relaxation process related to the change of the amplitude of internal magnetization within a wobbling triaxial grain and identify a range of grain sizes in which this effect can dominate the internal alignment of angular momentum within grain axes. We show that these relaxation processes significantly change the dynamics of grains in the presence of RATs. We apply our analysis for observed grain alignment in special environments to put constraints on the enhanced magnetic properties of dust grains in the cloud near supernovae, in cometary coma, and protoplanetary disks.