Interstellar Objects Follow the Collapse of Molecular Clouds

The detections of 1I/‘Oumuamua and 2I/Borisov within just two years of each other impressively demonstrate that interstellar objects (ISOs) must be common in the Milky Way. Once released from their parent system, these ISOs travel for billions of years through interstellar space. While often imagined as empty, interstellar space contains gas and dust most prominent in the form of molecular clouds. Performing numerical simulations, we test how often ISOs cross such molecular clouds (MCs). We find that the ISOs pass through MCs amazingly often. In the solar neighborhood, ISOs typically spend 0.1%–0.2% of their journey inside MCs, for relatively slow ISOs (<5 km s−1) this can increase to 1%–2%, equivalent to 10–20 Myr per Gyr. Thus the dynamically youngest ISOs spend the longest time in MCs. In other words, MCs must mainly contain relatively young ISOs (<1–2 Gyr). Thus the half-life of the seeding process by ISOs is substantially shorter than a stellar lifetime. The actual amount of time spent in MCs decreases with distance to the Galactic center. We find that ISOs pass through MCs so often that backtracing their path to find their parent star beyond 250 Myr seems pointless. Besides, we give a first estimate of the ISO density depending on the distance to the Galactic center based on the stellar distribution.

In this paper, we investigate the influence of the outer boundary condition on the collapse of dense molecular clouds. Observational data confirm subsonic inward contraction both before and after the formation of a central protostar. Here, we study the problem of steady, spherical accretion of gas onto a protostar with the polytropic equation of state. Our model include self-gravity of the cloud and has an open outer boundary condition in which the velocity is no longer zero there. Thus, matter continuously drifts across this outer boundary. Since we study the early protostellar cloud evolution, the central protostar is highly less massive than the surrounding cloud. We assume the cloud radius is very large and impose a finite, subsonic velocity at the cloud’s outer boundary and ignore magnetic field effects entirely. Our assumptions, while highly idealized, show supersonic infall is confined to the small central region of cloud.

In this project, all available databases of molecular and gas-dust clouds in the Galaxy were cross-identi ed by taking into account available properties, including position, angular dimensions, velocity,density, temperature and mass. An initial list of about 7000 entries was condensed into a cross-identi edall-sky catalogue containing molecular and gas-dust clouds. Some relationships were studied between themain physical features of clouds. Finally, we prepared a complex observing program and address futurework for lling in the gaps.Key words: Galaxy: structure{Galaxy: disk { Galaxy: kinematics and dynamics - ISM: structure {ISM: clouds - radio lines: ISM - Catalogs1. INTRODUCTIONAt the present time the rapid progress both in in-strumentation and observing technology creates con-ditions for in-depth study of the interstellar medium(ISM) in the Galaxy as well as in other galaxies such asLMC, SMC, M31, M33, M51, M77, M83, M110, IC10,NGC185, NGC1569, NGC2976, NGC3077, NGC4038/9,NGC4214, NGC4449, NGC4605, etc. A signi cant frac-tion of the baryon mass of the ISM in the Galaxy andobserved galaxies is concentrated in the form of nebulaor clouds with molecular content in the densest parts(see, e.g., Draine 2011). Naturally, the molecular clouds(MoC) should be closely related to cold dust-gas clouds,particularly HI ones. They have to play a key-role inthe star forming processes as well as in the kinematicsand dynamics of the Galaxy on the whole. The abovereasons prove the importance of the census, systematicstudy and survey of MoC populations.2. CENSUS AND CATALOGTo attempt to nd solutions for at least some issues re-garding the physics and evolution of the MoC systemin our Galaxy and their impact on the dynamics andevolution of the Galaxy generally, and to extend theresults to MoC systems in other galaxies, we drafteda consolidated and uni ed composite all-sky catalog ofmolecular and dust-gas clouds that are observable fromthe Earth based on recent data. The preliminary re-sults were reported in Hojaev et al. (2013). Electronicdata bases and webservices such as VizieR, SIMBAD atCDS, 2MASS (Ks Atlas) and DSS as well as originalhttp://pkas.kas.orgpapers, reports and other publications were used. Thegeneral catalog has been divided into 3 sub-catalogs: 1)large and giant MoCs; 2) MoCs with moderate massesand sizes; 3) small MoCs including clumps and cores.All main catalogs and subcatalogs contain the coordi-nates, sizes, distances, masses and other physical pa-rameters (such as density, temperature, radial velocity,etc.) that are available for the di erent clouds. In ourGalaxy there are about 200 large and giant molecularclouds, more than 2500 smaller cold dark clouds (includ-ing clumps and cores this value exceeds approximately6000 objects) observed in the Solar vicinity and neigh-borhood up to 11 kpc away.3. ANALYSIS AND DISCUSSIONBased on the data in the combined catalog, we analysedphysical conditions in the MoCs and searched for re-lations between the physical parameters of the MoCsobserved. Due to the space limitations, we restrictourselves to discussing only a few. In Figure 1, wepresent a plot of the column density of molecular hy-drogen as a function of the cloud virial mass. The lin-ear t to the data is: N(H

Among the many mysteries of our Universe, one still unanswered question is how globular clusters form. Globular clusters are very dense agglomerates of hundreds of thousands of stars and they host some of the oldest known stars in our Universe. Since they are luminous, old and found in all massive galaxies, they are a fundamental piece of the puzzle to understand galaxy formation and evolution processes. Traditionally, globular clusters were thought to be simple stellar systems, in which all stars were born at the same time and have the same chemical composition. %Therefore, globular clusters have been considered the perfect laboratory to study how stars evolve. However, in the last few decades, it has been shown that stars within a given globular cluster display inhomogeneities in their chemistry. Every massive old globular cluster located in the Milky Way, for which high precision and deep observations were obtained, was found to host several different stellar populations, i.e. multiple populations. Each stellar population is characterized by specific chemical patterns observed in the atmospheres of individual stars. Only certain elements are found to vary, and they do not do so randomly, but rather the variations are observed to correlate between the elements. The stellar population that has enhanced nitrogen (N) content, also has enhanced sodium and helium abundances but has a depletion in carbon and oxygen, to cite a few examples. At the same time, the iron content is found to be constant among the different populations. Such chemical patterns are often called anomalies. More interestingly, it seems like such chemical anomalies are unique to globular cluster systems, i.e. dense stellar systems, since they are basically not found in other stars located in the field. Knowing how such multiple populations form and how they impact the evolution of globular clusters is crucial to understand the formation of stars and clusters themselves and, more broadly, the formation and evolution of galaxies. Many theoretical scenarios have been proposed to explain the origin of the chemical anomalies in globular clusters. Most models treat the origin of this phenomenon as multiple events of star formation. In such models, a first generation of stars forms from the collapse of a giant molecular cloud which is homogeneous in its chemical composition. The winds of the massive stars from this first generation sink in the centre of the cluster to collapse and provide material for a second generation of stars, which then forms with a different chemical composition. While theoretically straightforward, such scenarios (which involve many types of massive stars) fail in reproducing many of the observed properties of multiple populations in globular clusters. Hence, the formation mechanism for the origin of multiple populations remains an open question. Most studies of multiple populations focused only on ancient globular clusters, aged up to $\sim$13 Gyr. However, many dense and massive younger star clusters are observed in nearby galaxies. Is the multiple populations phenomenon limited to the ancient globular clusters, i.e. could this be a cosmological effect? The goal of this thesis has been expanding the search for multiple populations to star clusters that are significantly younger than the old globular clusters, i.e. up to 10 times younger. Indeed, a compelling line of investigation is to look for multiple populations depending on certain global properties of the clusters, such as age, mass, metallicity. The first major result presented in this work is that multiple populations are found also in the young clusters, down to $\sim$2 Gyr old objects, showing that the phenomenon of multiple populations is not only restricted to the early Universe. Another interesting result I report is that the extent of the multiple populations (in chemical abundance spread) is a strong function of age, with older clusters having larger chemical variations. Additionally, I show that there is no difference in age between the populations in a young star cluster. Such results represent fundamental constraints for the origin of multiple populations and might point towards a new and fresh direction into the onset of this complex phenomenon. An important and related question is whether the young massive star clusters are the same type of stellar systems as the ancient globular clusters, just observed at a different stage of their lifetimes. If confirmed, this could provide important constraints on star cluster formation studies. Therefore, in this thesis I explored clusters at younger ages in order to address the fundamental question whether the star (and cluster) formation conditions were different in the early Universe. The results presented here represent an important hint that ancient and young clusters share the same origin and are only separated in age. I show that star clusters do not require special conditions in which to form, so that they can be used as tracers for the formation and evolution of galaxies.

One of the stumbling blocks for studying the evolution of interstellar molecules is the lack of adequate knowledge about the rate coefficients of various reactions which take place in the interstellar medium and molecular clouds. Some theoretical models of rate coefficients do exist in the literature for computing abundances of complex pre-biotic molecules. So far these have been used to study the abundances of these molecules in space. However, in order to obtain more accurate final compositions in these media, we have calculated the rate coefficients for the formation of some of the most important interstellar pre-biotic molecules by using quantum chemical theory. We use these rates inside our hydro-chemical model to examine the chemical evolution and final abundances of pre-biotic species during the collapsing phase of a proto-star. We find that a significant amount of various pre-biotic molecules could be produced during the collapse phase of a proto-star. We thoroughly study the formation of these molecules via successive neutral-neutral and radical-radical/radical-molecular reactions. We present the time evolution of the chemical species with an emphasis on how the production of these molecules varies with the depth of a cloud. We compare the formation of adenine in interstellar space using our rate-coefficients and using those obtained from existing theoretical models. Formation routes of the pre-biotic molecules are found to be highly dependent on the abundances of the reactive species and the rate coefficients involved in the reactions. The presence of grains strongly affects the abundances of the gas phase species. We also carry out a comparative study between different pathways available for the synthesis of adenine, alanine, glycine and other molecules considered in our network. Despite the huge abundances of the neutral reactive species, production of adenine is found to be strongly dominated by the radical-radical/radical-molecular reaction pathways. If all the reactions considered here contribute to the production of alanine and glycine, then neutral-neutral and radical-radical/radical-molecular pathways are both found to have a significant part in the production of alanine. Moreover, radical-radical/radical-molecular pathways also play a major role in the production of glycine.

The impact of galactic cosmic rays (CRs) in terms of CR pressure and parallel CR diffusion has been investigated on the low-frequency magnetohydrodynamic (MHD) waves and linear gravitational instability in the typical dusty plasma environment of molecular clouds (MCs). The dusty fluid model is formulated by combining the equations of the magnetized electrons/ions and dust particles, including the CR effects. The interactions between CR fluid and gravitating magnetized dusty plasma have been studied with the help of modified dispersion properties of the MHD waves and instabilities using the hydrodynamic fluid–fluid (CR–plasma) approach. CR diffusion affects the coupling of CR pressure-driven mode with dust-Alfvén MHD mode and causes damping in the MHD waves. It persists in its effect along the direction of the magnetic field and is diminished across the magnetic field. The phase-speed diagram shows that for super-Alfvénic wave, the slow mode becomes the intermediate Alfvén mode. The fundamental Jeans instability criterion remains unaffected due to CR effects, but in the absence of CR diffusion, the effects of dust-acoustic speed and CR pressure-driven wave speed are observed in the instability criterion. It is found that CR pressure stabilizes while CR diffusion destabilizes the growth rates of Jeans instability and significantly affects the gravitational collapse of dusty MCs. The charged dust grains play a dominant role in the sub-Alfvénic and super-Alfvénic MHD waves and the collapse of MCs, triggering gravitational instability. The consequences have been discussed to understand the gravitational instability in the dense photodissociation regions of dusty MCs.

Stars form within molecular clouds in galaxies. Therefore, understanding the formation, evolution and collapse of molecular clouds is critical for understanding galactic evolution. We present a systematic series of numerical simulations of a kiloparsec-scale size, elongated box, with sub-parsec resolution, developed to study the dynamics of molecular clouds in a galactic environment. We explore the origin of empirically observed relations such as the velocity dispersion-size relation in molecular clouds (Chapter 4), where we find that supernova explosions appear to be ineffcient at driving strong turbulent motions inside the clouds, where instead gravity appears to be the dominant process driving the observed fast motions. However, supernova explosions do play an important secondary role in the mass accretion histories of molecular clouds, simultaneously enhancing and suppressing inflow of gas onto the clouds by compressing and disrupting their mass reservoirs, (Chapter 5). We complete our analysis by studying the relative importance of magnetic fields in the evolution of molecular clouds and their envelopes. We find that, although we recover magnetic field strengths comparable to the observed values, they appear unable to prevent clouds from collapsing but capable of maintaining the diffuse envelopes supported, while restricting the gas flows in the diffuse ISM along field lines (Chapter 6). Together these results strongly support a picture of molecular clouds as highly dynamical objects that collapse quickly, and shortly after begin forming stars. However the subsequent evolution of these clouds must be strongly influenced by the newborn stars to avoid star formation effciencies higher than those observed.

Simple though admittedly speculative considerations explain why most of our galaxy’s stellar nurseries are highly fragile but a few survive for a remarkably long time.

view Abstract Citations (848) References (69) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Composite CO Survey of the Entire Milky Way Dame, T. M. ; Ungerechts, H. ; Cohen, R. S. ; de Geus, E. J. ; Grenier, I. A. ; May, J. ; Murphy, D. C. ; Nyman, L. -A. ; Thaddeus, P. Abstract Large-scale CO surveys of the entire Galactic plane and specific nearby clouds have been combined to produce a panorama of the entire Milky Way in molecular clouds at an angular resolution of 1/2°. Covering 10° - 20° in latitude at all longitudes and all or nearly all large, nearby clouds at higher latitude, the composite survey is the only molecular line survey to date with sky coverage and resolution comparable to that of the early 21 cm surveys. The inner Galaxy spiral arms produce, as expected, a thin, intense ridge of emission along the Galactic plane within ≡60° of the Galactic center. The local emission shows the same large-scale features as the distribution of dark clouds. The survey provides a thorough inventory of large molecular clouds near the Sun. The overall distribution of clouds within 1 kpc is consistent with the Sun lying near the inner edge of a local spiral arm or spur. The half-thickness at half-intensity of the local molecular cloud layer is 87 pc. Publication: The Astrophysical Journal Pub Date: November 1987 DOI: 10.1086/165766 Bibcode: 1987ApJ...322..706D Keywords: Abundance; Carbon Monoxide; Infrared Astronomy Satellite; Milky Way Galaxy; Molecular Clouds; Interstellar Gas; Nebulae; Normal Density Functions; Astrophysics; GALAXIES: THE GALAXY; GALAXIES: STRUCTURE; INTERSTELLAR: MOLECULES full text sources ADS | data products SIMBAD (36) CDS (2) Astroverse (2) NED (1) Related Materials (1) Catalog: 1996yCat.8039....0D

We present Berkeley-Illinois-Maryland Association (BIMA) millimeter interferometer observations of giant molecular clouds (GMCs) along a spiral arm in M31. The observations consist of a survey using the compact configuration of the interferometer and follow-up, higher resolution observations on a subset of the detections in the survey. The data are processed using an analysis algorithm designed to extract GMCs and correct their derived properties for observational biases, thereby facilitating comparison with Milky Way data. The algorithm identifies 67 GMCs, of which 19 have a sufficient signal-to-noise ratio to accurately measure their properties. The GMCs in this portion of M31 are indistinguishable from those found in the Milky Way, having a similar size-line width relationship and distribution of virial parameters, confirming the results of previous, smaller studies. The velocity gradients and angular momenta of the GMCs are comparable to the values measured in M33 and the Milky Way, and in all cases are below expected values based on the local galactic shear. The studied region of M31 has an interstellar radiation field, metallicity, Toomre Q parameter, and midplane volume density similar to those of the inner Milky Way, so the similarity of GMC populations between the two systems is not surprising.

Stellar systems are often formed through the collapse of dense molecular clouds which, in turn, return copious amounts of atomic and molecular material to the interstellar medium. An in-depth understanding of chemical evolution during this cyclic interaction between the stars and the interstellar medium is at the heart of astrochemistry. Systematic chemical composition changes as interstellar clouds evolve from the diffuse stage to dense, quiescent molecular clouds to star-forming regions and proto-planetary disks further enrich the molecular diversity leading to the evolution of ever more complex molecules. In particular, the icy mantles formed on interstellar dust grains and their irradiation are thought to be the origin of many of the observed molecules, including those that are deemed to be “prebiotic”; that is those molecules necessary for the origin of life. This review will discuss both observational (e.g., ALMA, SOFIA, Herschel) and laboratory investigations using terahertz and far-IR (THz/F-IR) spectroscopy, as well as centimeter and millimeter spectroscopies, and the role that they play in contributing to our understanding of the formation of prebiotic molecules. Mid-IR spectroscopy has typically been the primary tool used in laboratory studies, particularly those concerned with interstellar ice analogues. However, THz/F-IR spectroscopy offers an additional and complementary approach in that it provides the ability to investigate intermolecular interactions compared to the intramolecular modes available in the mid-IR. THz/F-IR spectroscopy is still somewhat under-utilized, but with the additional capability it brings, its popularity is likely to significantly increase in the near future. This review will discuss the strengths and limitations of such methods, and will also provide some suggestions on future research areas that should be pursued in the coming decade exploiting both space-borne and laboratory facilities.

They hover in the haloes and bulges of galaxies: globular clusters, gravitationally bound collections of 100,000 to 1 million stars following highly eccentric, elliptical orbits that drift across the galactic plane with very long periods. Among the first to recognize that these enigmatic objects were made of stars was the German-born astronomer William Herschel, who coined the term “globular cluster” in 1789. For the next three centuries, globular clusters remained fuzzy in detail but bright enough to be used as standard candles in cosmology. In the past three decades, major advances in observing tools and computing prowess have led to significant improvements in our understanding of their evolution, structure, dynamics, and perhaps most critically, ages. Age is central to globulars' fascination. Thought to be among the oldest stellar systems, they can provide valuable information about the first population of stars and the age of the universe. That status caused some vexation in the past, when astronomers pegged cluster ages between 15 billion and 20 billion years at a time when cosmologists calculated that the universe itself was only 10 billion to 15 billion years old. Nowadays, however, harmony reigns. In their review of age estimates for globulars in the Milky Way, Krauss and Chaboyer (p. 65) suggest that astronomers now have the lower limit about right, making the universe at least 11.2 billion years old. That cosmic youthfulness, coupled with evidence that the universe is geometrically flat and with current estimates of the Hubble constant, implies that the universe must be dominated by the mysterious antigravity-producing stuff called dark energy. In a new twist, astronomers have recently started estimating the ages of globular clusters from the decay of long-lived radioactive elements such as thorium (Th) and uranium (U) inside their stars. They have calculated stellar ages at 11 billion to 15 billion years. Although those numbers are consistent with estimates of the lower age limit of the universe, the “old” end of the range makes theorists uncomfortable, and more work is needed. To get more reliable ages, astronomers need to know how much U and Th the stars started with. That requires modeling the sites of nucleosynthesis and the masses of the neutron-rich nuclei that create the heavier elements. Sneden and Cowan (p. [70][1]) review these issues as well as other discoveries about the nucleosynthesis of heavy elements in the Milky Way. Globular clusters are also interesting for the light they shed on stellar processes and dynamics and, of course, as objects in their own right. Recent work has turned up some surprises: for example, evidence that hitherto unexpected star formation in old clusters may be confusing age estimates, and that a black hole or a concentration of neutron stars lurks in the core of some clusters. Irion (p. 60) summarizes these observations and describes how interactions between binary stars can turn placid-seeming clusters into stellar combat zones. Globulars also flourish in other galaxies. Schilling (p. 63) describes evidence that some extragalactic clusters, unlike those in the Milky Way, harbor two different generations of stars, one younger, redder, and more metal-rich than the other. Some theorists suggest that the younger clusters form in starbursts that ignite when galaxies collide—a challenge to the standard view that all clusters form in the collapse of molecular clouds during galaxy formation. We've come a long way since Herschel's insight that globulars consist of stars. A little more time should resolve what kinds of stars they are: a bit too old, a bit too young, or just right. [1]: /lookup/doi/10.1126/science.1077506

view Abstract Citations (30) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Pleiades Cluster. IV. The Visit of a Molecular CO Cloud Breger, Michel Abstract The location, size, and mass of the CO molecular cloud seen in the direction of the Pleiades cluster is determined from a study of the polarization and reddening of cluster members and nonmembers. Arguments are presented against both a foreground and background location of the molecular cloud, so that the cloud should be presently situated inside the cluster. Stellar reddening determinations with the appropriate value of R = 3.3 for the region, as well as star counts, lead to a determination of a total extinction of A(V) in the range of 1.0-1.6 mag for the central region of the CO cloud. The extinction determinations for cluster members and background stars indicate a mass of 20 solar masses for the CO cloud visiting the Pleiades cluster. Publication: The Astrophysical Journal Pub Date: August 1987 DOI: 10.1086/165495 Bibcode: 1987ApJ...319..754B Keywords: Carbon Monoxide; Interstellar Gas; Molecular Clouds; Plasma Interactions; Polarization Characteristics; Star Clusters; Astronomical Models; Astronomical Spectroscopy; Mass Distribution; Astrophysics; CLUSTERS: OPEN; INTERSTELLAR: MATTER; POLARIZATION; INTERSTELLAR: MOLECULES full text sources ADS | data products SIMBAD (26) Related Materials (3) Part 1: 1984A&A...137..145B Part 2: 1985A&A...143..455B Part 3: 1986ApJ...309..311B

The star formation efficiency (SFE) measures the proportion of molecular gas converted into stars, while the star formation rate (SFR) indicates the rate at which gas is transformed into stars. Here we propose such a model in the framework of a turbulence-regulated global radial collapse in molecular clouds being in quasi-virial equilibrium, where the collapse velocity depends on the density profile and the initial mass-to-radius ratio of molecular clouds, with the collapse velocity accelerating during the collapse process. This simplified analytical model allows us to estimate a lifetime of giant molecular clouds of approximately 0.44−7.36 × 107 yr, and a star formation timescale of approximately 0.5–5.88 × 106 yr. Additionally, we can predict an SFE of approximately 1.59%, and an SFR of roughly 1.85 M ⊙ yr−1 for the Milky Way in agreement with observations.

Using hydrodynamical simulations of entire galactic discs similar to the Milky Way, reaching 4.6pc resolution, we study the origins of observed physical properties of giant molecular clouds (GMCs). We find that efficient stellar feedback is a necessary ingredient in order to develop a realistic interstellar medium (ISM), leading to molecular cloud masses, sizes, velocity dispersions and virial parameters in excellent agreement with Milky Way observations. GMC scaling relations observed in the Milky Way, such as the mass-size ($M$--$R$), velocity dispersion-size ($\sigma$--$R$), and the $\sigma$--$R\Sigma$ relations, are reproduced in a feedback driven ISM when observed in projection, with $M\propto R^{2.3}$ and $\sigma\propto R^{0.56}$. When analysed in 3D, GMC scaling relations steepen significantly, indicating potential limitations of our understanding of molecular cloud 3D structure from observations. Furthermore, we demonstrate how a GMC population's underlying distribution of virial parameters can strongly influence the scatter in derived scaling relations. Finally, we show that GMCs with nearly identical global properties exist in different evolutionary stages, where a majority of clouds being either gravitationally bound or expanding, but with a significant fraction being compressed by external ISM pressure, at all times.