A Generalist Looks Back

The formation of molecules in the interstellar medium is an important topic of research nowadays because molecules play important roles in astrophysics and astrochemistry. Molecular hydrogen, the most abundant molecule, helps the cooling of collapsing clouds and through the study of its transitions, it gives information on the conditions of the ISM (Interstellar Medium) environment it finds itself in. While neutral or ionized, it intervenes in reactions to form most molecules that have been detected in the ISM. Water is the main component of ices coating dust grain; ices are the place of the formation and storage of molecules of biogenic interest. Furthermore, how water interacts with grains is of importance in understanding the delivery of water to planetary bodies. The formation of molecular hydrogen and water occurs largely on dust grains. Here we review the most important experimental discoveries about the formation of molecular hydrogen, and we present new work on the formation of water on ices and on grain analogs.

We construct a protostellar disk model that takes into account the combined effect of viscous evolution, photoevaporation, and the differential radial motion of dust grains and gas. For T Tauri disks, the lifetimes of dust disks that are mainly composed of millimeter-sized grains are always shorter than the gas disks' lifetimes and become similar only when the grains are fluffy (density 0.1 g cm-3). If grain growth during the classical T Tauri phase produces plenty of millimeter-sized grains, such grains completely accrete onto the star in 107 yr, before photoevaporation begins to drain the inner gas disk and the star evolves to the weak-line T Tauri phase. In the weak-line phase, only dust-poor gas disks remain at large radii (10 AU), without strong signs of gas accretion or of millimeter thermal emission from the dust. For Herbig Ae/Be stars, the strong photoevaporation clears the inner disks in 106 yr, before the dust grains in the outer disk migrate to the inner region. In this case, the grains left behind in the outer gas disk accumulate at the disk inner edge (at 10-100 AU from the star). The dust grains remain there even after the entire gas disk has been photoevaporated and form a gas-poor dust ring similar to that observed around HR 4796A. Hence, depending on the strength of the stellar ionizing flux, our model predicts opposite types of products around young stars. For low-mass stars with a low photoevaporation rate, dust-poor gas disks with an inner hole would form, whereas for high-mass stars with a high photoevaporation rate, gas-poor dust rings would form. This prediction should be examined by observations of gas and dust around weak-line T Tauri stars and evolved Herbig Ae/Be stars.

Fred Hoyle, who passed away on 20 August 2001, was one of the most important figures in 20th century physics and astronomy. He is most famous for coining the term 'Big Bang' in a BBC radio broadcast in 1950, even though the fact that his comment was an insult seems to have been lost in time. He left behind a lasting body of work, foremost of which is his work with Willy Fowler and the Burbidges on the origin of the chemical elements in the 1950s. Sir Martin Rees, Astronomer Royal and Professor at the Institute of Astronomy in Cambridge, speaking to Physics Education said 'Hoyle was a great astrophysicist, who from 1945 to 1970 contributed more good creative ideas than anyone else in the world.'

Planets, now known to be common companions of nearby stars, are believed to originate in circumstellar disks - disks of gas and dust that form around collapsing protostars. Because of their small angular size, disks in nearby star-forming regions are difficult to spatially resolve. However, since radial temperature gradients are set up by the luminous central star, the peak wavelength of blackbody emission is a proxy for disk location, and spectroscopic observations can thus be used to study disk structure. In this thesis, I present spectroscopic observations of circumstellar disks obtained with the Keck Near Infrared Spectrograph (NIRSPEC) and the Spitzer Space Telescope Infrared Spectrograph (IRS). High-resolution (R ~ 25000) M-band spectra have been obtained with NIRSPEC for over 100 circumstellar disks. CO vibrational emission in the M-band is nearly ubiquitous from classical, optically thick disks, and can be used to examine the disks' inner structure and gas content. The emission is consistent with high temperatures and densities, and small emitting areas, suggesting that it originates at small disk radii - a few AU or less. An analysis of CO lineshapes shows that the CO gas is truncated at or near the dust sublimation radius for low and mid-mass stars but at larger radii for more luminous stars. CO emission is also present from many transitional disks - disks in which the inner regions have been depleted of small dust grains -demonstrating that many of their inner disks have significant quantities of gas. Although the formation mechanism for transitional disks is not yet known, gas-rich inner disks are consistent with formation via tidal clearing by a giant planet. Spitzer-IRS spectra have been obtained for 82 disks, as part of the Cores to Disks (c2d) legacy program and other archived programs, and as part of a targeted IRS GO-5 survey at high S/N. Water vapor and other gaseous molecules, including OH, HCN, C2H2, and CO2, are detected from ~ 40% of disks. Excitation temperatures for the molecular species of ~ 500 K or higher and emitting radii of a few AU suggest an origin in the planet-forming regions of the disks. There are no strong correlations between detection rates or line equivalent widths and stellar or disk parameters. However, molecular detection rates are correlated with each other, and with disk color, suggesting a dependence on disk structure. Unlike for classical disks, transitional disks show no evidence for H2O, OH, or any organic molecules, perhaps because these molecules are easily photodissociated in the optically thin inner disk.

In typical astrophysical environments, where temperatures and densities are very low and radiation fluxes may be high, the transformation of simple molecules in the gas phase is difficult. Consequently, it is widely accepted that the formation of interstellar complex organic molecules (iCOMs) occurs through barrierless reactions or on the surface of dust grains, which are present in all stages of the evolution of a planetary system. Those grains can act as third bodies, absorbing excess energy from reactions, and are also covered in ices (mainly made of water, CO, N2 and other simple organics, such as methanol) which act to concentrate reactants, increase the chances of reactive collisions, and/or protect from radiation newly formed molecules.However, models generally assume that reactions occur on the ice phase covering the silicate cores, and tend to minimise the chemical role played bythe dust grains themselves. When grains are not covered in ice, interactions between the solid phase and the gas phase are also important. Indeed, dust grains can be a source of reactants and are also rich in metallic components, such as Fe and Ni. These metals are well known on Earth to act as catalysts for the synthesis of organic compounds, such as the Fischer-Tropsch synthesis (FT) to produce hydrocarbons, the Haber-Bosch (HB) for the synthesis of ammonia, and the cyclation of small hydrocarbons in Diels-Alder (DA) type reactions and further formation of aromatics and nanostructures. They also contain FeS phases, such as troilite or pyrrothite, which are also known to be reactive and could be important to the incorporation of S in complex iCOMs.Different works have already pointed to the importance of bare grain chemistry (Cazaux et al., 2010; Frankland et al., 2016), and in particular, of the catalytic activity of metallic inclusions (Llorca and Casanova, 2000; Ferrante et al., 2000; Tucker et al., 2018; Peters et al., 2023) and their potential rolein the chemical evolution of different astrophysical environments. In this talk, I will discuss the results of our last experiments which are part of our Astrocatalysis project, which aims at investigating relevant catalytic processes that could occur in different astrophysical environments, such as primeval planetary surfaces and atmospheres. I will present our experiments on the reactivity of chondritic material under relevant protoplanetary conditions toward FT synthesis (Cabedo et al., 2021) and towards the formation of H2S (Cabedo et al., 2024). I will also present future experiments regarding the reactivity of the material after ablation with plasma (Cabedo et. al., 2025, in prep.), simulating the potential reactivity of entering material during early stages after planetary formation. Understanding dust grains solid chemistry is important to completely interpret observational data and to have a complete model of the evolution of iCOMS at different stages of star and planetary formation and towards complex organic chemistry.

We perform high resolution direct <em>N</em>-body simulations to study the effect of an accretion disc on stellar dynamics in an active galactic nucleus (AGN). We show that the interaction of the nuclear stellar cluster (NSC) with the gaseous disc (AD) leads to formation of a stellar disc in the central part of the NSC. The accretion of stars from the stellar disc onto the super-massive black hole is balanced by the capture of stars from the NSC into the stellar disc, yielding a stationary density profile. We derive the migration time through the AD to be 3\% of the half-mass relaxation time of the NSC. The mass and size of the stellar disc are 0.7\% of the mass and 5\% of the influence radius of the super-massive black hole. An AD lifetime shorter than the migration time would result in a less massive nuclear stellar disc. The detection of such a stellar disc could point to past activity of the hosting galactic nucleus.

We present a study of the vertical structure of the gaseous and stellar disks in a sample of edge-on galaxies (NGC 4157, 4565, and 5907) using BIMA/CARMA 12CO (J = 1 --> 0), VLA H I, and Spitzer 3.6 micron data. In order to take into account projection effects when we measure the disk thickness as a function of radius, we first obtain the inclination by modeling the radio data. Using the measurement of the disk thicknesses and the derived radial profiles of gas and stars, we estimate the corresponding volume densities and vertical velocity dispersions. Both stellar and gas disks have smoothly varying scale heights and velocity dispersions, contrary to assumptions of previous studies. Using the velocity dispersions, we find that the gravitational instability parameter Q follows a fairly uniform profile with radius and is greater than or equal to 1 across the star forming disk. The star formation law has a slope that is significantly different from those found in more face-on galaxy studies, both in deprojected and pixel-by-pixel plots. Midplane gas pressure based on the varying scale heights and velocity dispersions appears to roughly hold a power-law correlation with the midplane volume density ratio.

We present a method to derive the dynamical mass of face-on galaxy disks using their neutral hydrogen (H i) velocity dispersion (σ H i ). We have applied the method to nearby, gas-rich galaxies that have extended H i gas disks and have low inclinations. The galaxy sample includes four large disk galaxies, NGC 628, NGC 6496, NGC 3184, and NGC 4214, and three dwarf galaxies, DDO 46, DDO 63, and DDO 187. We have used archival H i data from The H i Nearby Galaxy Survey (THINGS) and the LITTLE THINGS survey to derive the H i gas distributions and Spitzer mid-infrared images to determine the stellar disk mass distributions. We examine the disk dynamical and baryonic mass ratios in the extreme outer disks where there is H i gas but no visible stellar disk. We find that for the large galaxies, the disk dynamical and H i gas mass surface densities are comparable in the outer disks. But in the smaller dwarf galaxies, for which the total H i gas mass dominates the baryonic mass, i.e., M(H i) ≥ M(stars), the disk dynamical mass is much larger than the baryonic mass. For these galaxies, there must either be a very low-luminosity stellar disk which provides the vertical support for the H i gas disk or there is halo dark matter associated with their disks, which is possible if the halo has an oblate shape so that the inner part of the dark matter halo is concentrated around the disk. Our results are important for explaining the equilibrium of H i disks in the absence of stellar disks and is especially important for gas-rich, dwarf galaxies that appear to have significant dark matter masses associated with their disks.

By solving analytically the various types of Lane-Emden equations with rotation, we have discovered two new coupled fundamental properties of rotating, self-gravitating, gaseous disks in equilibrium: Isothermal disks must, on average, exhibit strict power-law density profiles in radius $x$ on their equatorial planes of the form $A x^{k-1}$, where $A$ and $k-1$ are the integration constants, and "flat" rotation curves precisely such as those observed in spiral galaxy disks. Polytropic disks must, on average, exhibit strict density profiles of the form $\left[\ln(A x^k)\right]^n$, where $n$ is the polytropic index, and "flat" rotation curves described by square roots of upper incomplete gamma functions. By "on average," we mean that, irrespective of the chosen boundary conditions, the actual profiles must oscillate around and remain close to the strict mean profiles of the analytic singular equilibrium solutions. We call such singular solutions the "intrinsic" solutions of the differential equations because they are demanded by the second-order equations themselves with no regard to the Cauchy problem. The results are directly applicable to gaseous galaxy disks that have long been known to be isothermal and to protoplanetary disks during the extended isothermal and adiabatic phases of their evolution. In galactic gas dynamics, they have the potential to resolve the dark matter--modified gravity controversy in a sweeping manner, as they render both of these hypotheses unnecessary. In protoplanetary disk research, they provide observers with powerful new probing tool, as they predict a clear and simple connection between the radial density profiles and the rotation curves of self-gravitating disks in their very early (pre-Class 0 and perhaps the youngest Class Young Stellar Objects) phases of evolution.

In the standard scenario of planet formation, the planetesimals grow from primordial dust grains of interstellar origin embedded in a gas disk, the so-called protoplanetary nebula. Large-scale gaseous structures, such as spiral waves and vortices, are likely to survive for a large number of rotation periods and can strongly influence the dynamics and growth of the dust grains. This has been stressed for the first time by P. Barge & J. Sommeria (1995, A&A, 295, L1) in the case of the vortices with a new scenario for the formation of the planetesimals and the rapid growth of the giant planet cores. This dissertation is concerned with the action of the mean flow of the gas on the dynamics of the solid dust particles. To reach this goal, we implemented a direct time-dependent integration code that models the particle dynamics under gravitational and viscous forces in a zero-thickness disk. A parallel version of this highorder accuracy particle-mesh code has been performed to carry out the potential role of large-scale vortices and spiral perturbations in the protoplanetary disks. The results show that the particles do not drift inward to the star as occurs in a standard symmetrical nebula. On the one hand, vortices tend to capture a large number of the particles. The effectiveness of this size-selective concentration mechanism depends on the value of the drag coefficient and on the distance from the Sun but also on the elongation of the vortex and its characteristic lifetime. We also found analytical expressions for the capture time as well as capture constraints as a function of the friction parameter, the elongation of the vortex, and the impact parameter. On the other hand, in the case of spiral perturbations, the velocity field can be described by a set of concentric streamlines as is usual in the kinematical description of a spiral wave. Moreover, since protoplanetary disks are nearly Keplerian, the spiral mode is likely the most probable and the most m p 1 robust one (E. Lee & J. Goodman 1999, MNRAS, 308, 684). In such a model, the streamlines are confocal ellipses and the particle trajectories become nearly coincident with them. With such a velocity field, we find that the particle trajectories end up in stable elliptic orbits and that the resulting dust surface density is significantly increased in the vicinity of the spiral pattern, incidentally facilitating the growth of the particles. We find also that a ringlike swarm of particles, with a given size distribution, tends to spread into a spiral-like pattern. This pattern results from the size segregation of the particles under the friction drag force. In summary, our results show that small disk asymmetries, due to giant vortices or spiral patterns, are able to confine the solid material within some small regions of the protoplanetary nebula. By increasing the lifetime and the surface density of the solid particles, these confining mechanisms offer new possibilities for the formation of the planetesimals and the giant planet cores and could help also in explaining a rapid formation of the extrasolar giant planets.

Molecular clouds (MCs) in space are the birthplace of various molecular species. Chemical reactions occurring on the cryogenic surfaces of cosmic icy dust grains have been considered to play important roles in the formation of these species. Radical reactions are crucial because they often have low barriers and thus proceed even at low temperatures such as ∼10 K. Since the 2000s, laboratory experiments conducted under low-temperature, high-vacuum conditions that mimic MC environments have revealed the elementary physicochemical processes on icy dust grains. In this review, experiments conducted by our group in this context are explored, with a focus on radical reactions on the surface of icy dust analogues, leading to the formation of astronomically abundant molecules such as H2, H2O, H2CO, and CH3OH and deuterium fractionation processes. The development of highly sensitive, non-destructive methods for detecting adsorbates and their utilization for clarifying the behavior of free radicals on ice, which contribute to the formation of complex organic molecules, are also described.

: Phenomenological computer models were constructed to interpret observations of cosmic infrared sources and to make predictions which would stimulate further studies. The research was divided into four areas: (1) Detailed radiation transport models, incorporating both the equilibrium heating of large dust grains and the transient heating of very small grains, were constructed for interstellar dust clouds and circumstellar dust shells of evolved stars to parameterize the observed radiation characteristics in terms of their physical source properties. (2) Existing methods of analysis (e.g., semi- analytical methods for determining the dust mass, dust temperature, and grain emissivity laws) in infrared astronomy were critically evaluated to determine the conditions under which these approaches were valid and reliable. Semi- empirical relationships were then developed to facilitate the rapid interpretation of infrared observations. (3) Theoretical models were developed to study various physical phenomena involving dust grains, e.g., molecule formation on grains, grain formation in expanding circumstellar envelopes. (4) Several computational techniques were investigated which could automate and improve the efficiency of existing radiation transport codes for modeling infrared sources.

By means of 2D spectroscopy we have found a lot of compact inner stellar and gaseous disks in morphologically normal galaxies. The evolution and spatial orientation of these inner structures are often decoupled from the evolution and spatial orientation of the large-scale galactic disks and bulges. We argue that this finding requires substantial change of common approaches to investigations of the central parts of galaxies, including central black hole studies.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We estimate the pattern speeds of 38 barred galaxies by simulation modeling. We construct the gravitational potentials of the galaxies from near-IR photometry by assuming that the mass-to-light ratio ( M/L) is constant in the H band and a single pattern speed dominates in the stellar disk. We use the response of gaseous and stellar particle disks to a rigidly rotating potential to determine the pattern speed. If our assumptions are correct, then the pattern speed depends on the morphological type: the average value of the ratio of the corotation resonance radius to the bar radius, R, increases from about 1.1 in type SB0/a to 1.4 in SBb and 1.7 in SBc. Within the error estimates, all the bars in galaxies of type SBab or earlier are fast rotators, having R ≤ 1.4, whereas latetype galaxies include both fast and slow rotators. Subject headings: galaxies: fundamental parameters — galaxies: kinematics and dynamics — galaxies: structure

Silicon carbide (SiC) is a major component of interstellar dust in carbon-rich environments, but its catalytic potential in space has remained largely unexplored. In this work, we investigate how single iron atoms supported on SiC (Fe0@SiC) can drive Fischer Tropsch-type (FTT) reactions, transforming the two most abundant gas-phase species in the interstellar medium (H2 and CO) into more complex organic compounds, i.e., formaldehyde (H2CO) and methanol (CH3OH). Using density functional theory (DFT), we model the catalytic cycle on the most stable β-SiC (110) surface, revealing that H2CO forms efficiently with relatively low activation barriers (up to 18.3 kcal mol−1), while, in contrast, CH3OH formation faces a significant energy barrier (32.6 kcal mol−1) in space. Atomistic mechanistic study highlights the role of Fe0@SiC in stabilizing reaction intermediates through Fe-H-Si bridging interactions, which facilitate H2 activation and CO hydrogenation. Kinetic analysis suggests that H2CO and CH3OH formation is viable in regions with temperatures above 200 and 350 K, respectively, aligning with observations of formaldehyde and methanol in protoplanetary disks and comets. The findings also suggest that FTT processes could contribute to the formation of other organic molecules, such as acetaldehyde and short-chain hydrocarbons, in space. This work offers new insights into how cosmic dust grains might drive the formation of complex molecules during the planetary system formation.