Collisional Scenario Research Articles

Binary Black Hole Mergers and Intermediate-mass Black Holes in Dense Star Clusters with Collisional Runaways

Intermediate-mass black holes (IMBHs) are believed to be the missing link between the supermassive black holes (BHs) found at the centers of massive galaxies and BHs formed through stellar core collapse. One of the proposed mechanisms for their formation is a collisional runaway process in high-density young star clusters, where an unusually massive object forms through repeated stellar collisions and mergers, eventually collapsing to form an IMBH. This seed IMBH could then grow further through binary mergers with other stellar-mass BHs. Here we investigate the gravitational-wave (GW) signals produced during these later IMBH–BH mergers. We use a state-of-the-art semi-analytic approach to study the stellar dynamics and to characterize the rates and properties of IMBH–BH mergers. We also study the prospects for detection of these mergers by current and future GW observatories, both space-based (LISA) and ground-based (LIGO Voyager, Einstein Telescope, and Cosmic Explorer). We find that most of the merger signals could be detected, with some of them being multiband sources. Therefore, GWs represent a unique tool to test the collisional runaway scenario and to constrain the population of dynamically assembled IMBHs.

Buildup of the Magnetic Flux Ropes in Homologous Solar Eruptions

Abstract Homologous coronal mass ejections (CMEs) are an interesting phenomenon, and it is possible to investigate the formation of CMEs by comparing multi-CMEs under a homologous physical condition. AR 11283 had been present on the solar surface for several days when a bipole emerged on 2011 September 4. Its positive polarity collided with the preexisting negative polarity belonging to a different bipole, producing recurrent solar activities along the polarity inversion line (PIL) between the colliding polarities, namely the so-called collisional PIL (cPIL). Our results show that a large amount of energy and helicity were built up in the form of magnetic flux ropes (MFRs), with recurrent release and accumulation processes. These MFRs were built up along the cPIL. A flux deficit method is adopted and shows that magnetic cancellation happens along the cPIL due to the collisional shearing scenario proposed by Chintzoglou et al. The total amount of canceled flux was ∼0.7 × 1021 Mx with an uncertainty of ∼13.2% within the confidence region of the 30° Sun-center distance. The canceled flux amounts to 24% of the total unsigned flux of the bipolar magnetic region. The results show that the magnetic fields beside the cPIL are very sheared, and the average shear angle is above 70° after the collision. The fast expansion of the twist kernels of the MFRs and the continuous eruptive activities are both driven by the collisional shearing process. These results are important for better understanding the buildup process of the MFRs associated with homologous solar eruptions.

Late Cretaceous to Paleocene magmatic record of the transition between collision and subduction in the Western and Central Cordillera of northern Colombia

Deformational field relations, compositional and U–Pb geochronological data of magmatic rocks of the northwestern Colombian Andes provide insights to recognize the different pre to post-collisional stages in an arc-arc collisional scenario until the re-initiation of a new subduction zone. A western belt of undeformed to mylonitized volcanic rocks with flat REE patterns and a weak LREE depletion present similar geochemical features of adjacent oceanic plateau basalts that were intruded by arc-related dykes at ca. 82 Ma, before their accretion to the continental margin. A sequence of greenschist and quartz + muscovite + graphite schist, and their associated mylonitized dikes, are part of the eastern continental arc, and present ages between 69 and 71 Ma, show tholeiitic affinity, flat rare-earth patterns, and Nb–Ti anomalies. These characteristics mark a decrease in the mantle wedge and older crustal signatures during magma genesis, as the oceanic-continent collision was advancing. Both suites were metamorphosed in a high geothermal gradient with temperatures between 490 and 400 °C and pressures between 2 and 4 Kbar. After this metamorphic event associated to the end of the collision a series of dykes intrude the different units at ca. 58 Ma, including basalts and andesites with Nb–Ti anomalies, adakite-like, and high niobium compositional signatures, suggesting that they were probably formed associated with slab melting in a young subduction environment that evolved after 62 Ma, following a ≤7 Ma magmatic hiatus. This magmatic and metamorphic record can be related to an arc-arc collision scenario in which mantle involvement in the late stages of collision influence both the magmatic and thermal regimes.

A chemical study of M67 candidate blue stragglers and evolved blue stragglers observed with APOGEE DR14

Within the variety of objects populating stellar clusters, blue straggler stars (BSSs) are among the most puzzling ones. BSSs are commonly found in globular clusters, but they are also known to populate open clusters of the Milky Way. Two main theoretical scenarios (collisions and mass transfer) have been suggested to explain their formation, although finding observational evidence in support of either scenario represents a challenging task. Among the APOGEE observations of the old open cluster M67, we found 8 BSS candidates known from the literature and two known evolved BSSs. We carried out a chemical analysis of 3 BSS candidates and of the 2 evolved BSSs out of the sample and found that the BSS candidates have surface abundances similar to those of stars on the main-sequence turn-off of M67. Especially the absence of any anomaly in their carbon abundances seems to support a collisional formation scenario for these stars. Furthermore, we note that the abundances of the evolved BSSs S1040 and S1237 are consistent with the abundances of the red clump stars of M67. In particular, they show a depletion in carbon by $\sim0.25$ dex, which could be either interpreted as the signature of mass transfer or as the product of stellar evolutionary processes. Finally, we summarise the properties of the individual BSS stars observed by APOGEE, as derived from their APOGEE spectra and/or from information available in the literature.

Anticlockwise pressure–temperature paths record Variscan upper‐plate exhumation: Example from micaschists of the Porto Vecchio region, Corsica

ABSTRACTTo better understand the evolution of deep‐seated crust of the Variscan orogen in the Sardinia‐Corsica region, we studied garnet‐bearing micaschists which were sampled 3 km east and 15 km northeast of Porto Vecchio, south‐eastern Corsica. After a careful investigation of the textural relations and compositions of minerals, especially of zoned garnet, a P–T path was reconstructed using contoured P–T pseudosections. U–Th–Pb dating of monazite in the micaschists was undertaken with the electron microprobe. The micaschists from both localities were formed along similar anticlockwise P–T paths. The prograde branch of these paths starts at 3 kbar close to 600°C in the P–T field of sillimanite and reaches peak conditions at 7 kbar and 600 (15 km NE of Porto Vecchio) to 630°C (3 km E of Porto Vecchio). The metamorphism at peak P–T conditions happened c. 340 Ma based on low‐Y (<0.65 wt% Y2O3) monazite. Ages of monazite with high‐Y contents (>2 wt% Y2O3), which probably have formed before garnet, scatter around 362 Ma. The retrograde branch of the P–T paths passes through 4 kbar at ~550°C. We conclude that the micaschists belong to a common metasedimentary sequence, which extends over the Porto Vecchio region and is separated from other metamorphic rock sequences in the north and the south by major tectonic boundaries. This sequence had experienced peak pressures which are lower than those determined for metamorphic rocks, such as micaschist and gneiss, from north‐eastern Sardinia. At present, we favour a continent–continent collisional scenario with the studied metasedimentary sequence buried during the collisional event as part of the upper plate. The contemporaneous high‐P metamorphic rocks from NE Sardinia were part of the upper portion of the lower plate. The addressed rocks from both plates were exhumed in an exhumation channel.

Three new stable L5 Mars Trojans

Abstract Mars was second to Jupiter in being recognized as the host of a population of Trojan minor bodies. Since 1990, five asteroids – 5261 Eureka, (101429) 1998 VF31, (121514) 1999 UJ7, 2001 DH47 and (311999) 2007 NS2 – have been identified as Mars Trojans, one L4 and four L5. Dynamical and spectroscopic evidence suggests that some Mars Trojans may be remnants of the original planetesimal population that formed in the terrestrial planets region. Here, we revisit the long-term dynamical evolution of the previously known Mars Trojans and show that 2011 SC191, 2011 SL25 and 2011 UN63 are also trailing (L5) Mars Trojans. They appear to be as stable as Eureka and may have been Trojans over the age of the Solar system. The fact that five Trojans move in similar orbits and one of them is a binary may point to the disruption of a larger body early in the history of the Solar system. Such a catastrophic event may also explain the apparently strong asymmetry in terms of number of objects (one versus seven) between the L4 and L5 regions. Future spectroscopic observations should be able to reject or confirm a putative common chemical signature that may lend further support to a collisional scenario.

Orbital clustering of martian Trojans: An asteroid family in the inner Solar System?

We report on the discovery of new martian Trojans within the Minor Planet Center list of asteroids. Their orbital evolution over 108yr shows characteristic signatures of dynamical longevity (Scholl, H., Marzari, F., Tricarico, P. [2005]. Icarus 175, 397–408) while their average orbits resemble that of the largest known martian Trojan, 5261 Eureka. The group forms a cluster within the region where the most stable Trojans should reside. Based on a combinatorial analysis and a comparison with the jovian Trojan population, we argue that both this feature and the apparent paucity of km-sized martian Trojans (Trilling, D.E., Spahr, T.B., Rivkin, A.S., Hergenrother, C.W., Kortenkamp, S.J. [2006]. ID 2006A-0251) as compared to expectations from earlier work (Tabachnik, S., Evans, N.W. [1999]. Astrophys. J. 517, L63–L66) is not due to observational bias but instead a natural end result of the collisional comminution (Jutzi, M., Michel, P., Benz, W., Richardson, D.C. [2010]. Icarus 207, 54–65) or, alternatively, the rotational fission (Pravec, P. et al. [2010]. Nature 466, 1085–1088) of a progenitor L5 Trojan of Mars. Under the collisional scenario in particular, the new martian Trojans are dynamically young, in agreement with our age estimate of this “cluster” of <2Gyr based on the earlier work of Scholl et al. (Scholl, H., Marzari, F., Tricarico, P. [2005]. Icarus 175, 397–408). This work highlights the Trojan regions of the terrestrial planets as natural laboratories to study processes important for small body evolution in the Solar System and provides the first direct evidence for an orbital cluster of asteroids close to the Earth.

Pre-Mesozoic Alpine basements—Their place in the European Paleozoic framework

Prior to their Alpine overprinting, most of the pre-Mesozoic basement areas in Alpine orogenic structures shared a complex evolution, starting with Neoproterozoic sediments that are thought to have received detrital input from both West and East Gondwanan cratonic sources. A subsequent Neoproterozoic–Cambrian active margin setting at the Gondwana margin was followed by a Cambrian–Ordovician rifting period, including an Ordovician cordillera-like active margin setting. During the Late Ordovician and Silurian periods, the future Alpine domains recorded crustal extension along the Gondwana margin, announcing the future opening of the Paleotethys oceanic domain. Most areas then underwent Variscan orogenic events, including continental subduction and collisions with Avalonian-type basement areas along Laurussia and the juxtaposition and the duplication of terrane assemblages during strike slip, accompanied by contemporaneous crustal shortening and the subduction of Paleotethys under Laurussia. Thereafter, the final Pangea assemblage underwent Triassic and Jurassic extension, followed by Tertiary shortening, and leading to the buildup of the Alpine mountain chain. Recent plate-tectonic reconstructions place the Alpine domains in their supposed initial Cambrian–Ordovician positions in the eastern part of the Gondwana margin, where a stronger interference with the Chinese blocks is proposed, at least from the Ordovician onward. For the Visean time of the Variscan continental collision, the distinction of the former tectonic lower-plate situation is traceable but becomes blurred through the subsequent oblique subduction of Paleotethys under Laurussia accompanied by large-scale strike slip. Since the Pennsylvanian, this global collisional scenario has been replaced by subsequent and ongoing shortening and strike slip under rising geothermal conditions, and all of this occurred before all these puzzle elements underwent the complex Alpine reorganization.

Metamorphism and kinematics of the early deformation in the Variscan suture of SW Iberia

AbstractThe early (Devonian) collisional stage in SW Iberia has been investigated through the analysis of deformation in the Cubito‐Moura schists, the main lithology of an Allochthonous Complex putatively rooted in the suture between the Ossa‐Morena and South Portuguese zones. The first deformation in these schists (D1) is recorded as a S1‐L1 mylonitic fabric well preserved in early quartz veins. Subsequent D2 deformation caused the main folds and the main (S2) foliation. After restoration, the stretching lineation (L1) trends at a small angle with the Ossa‐Morena/South Portuguese suture. This trend, together with the top‐to‐the‐east kinematics determined from quartz microfabric is indicative of an oblique left‐lateral collisional scenario in SW Iberia. Chlorite–white K‐mica–quartz ± chloritoid multi‐equilibrium calculations yield P–T conditions in the range 0.9–1.2 GPa and 300–400 °C, during the first collisional stage. P–T conditions during D2 were 0.3–0.8 GPa and 400–450 °C, thus indicating an important stage of exhumation of the Allochthonous Complex during these two collisional events, after subduction of the Ossa‐Morena Zone margin under the South Portuguese Zone continental crust. In the general context of the Variscan orogen, dominated by dextral collision, the left‐lateral convergence in SW Iberia can be explained in terms of the Avalonian salient represented by the South Portuguese Zone, which would impinge between Iberia and Morocco.

THE DEBRIS DISK OF VEGA: A STEADY-STATE COLLISIONAL CASCADE, NATURALLY

It has been argued that the photometric data and images of the archetypical debris disk around Vega may be in contradiction with the standard, steady-state collisional scenario of the disk evolution. Here we perform physical modeling of the Vega disk "from the sources". We assume that dust is maintained by a "Kuiper belt" of parent planetesimals at ~ 100 AU and employ our collisional and radiative transfer codes to consistently model the size and radial distribution of the disk material and then thermal emission of dust. In doing so, we vary a broad set of parameters, including the stellar properties, the exact location, extension, and dynamical excitation of the planetesimal belt, chemical composition of solids, and the collisional prescription. We are able to reproduce the spectral energy distribution in the entire wavelength range from the near-infrared to millimeter, as well as the mid-IR and sub-millimeter radial brightness profiles of the Vega disk. Thus our results suggest that the Vega disk observations are compatible with a steady-state collisional dust production, and we put important constraints on the disk parameters and physical processes that sustain it. The total disk mass in < 100 km-sized bodies is estimated to be ~ 10 Earth masses. Provided that collisional cascade has been operating over much of the Vega age of ~ 350 Myr, the disk must have lost a few Earth masses of solids during that time. We also demonstrate that using an intermediate luminosity of the star between the pole and the equator, as derived from its fast rotation, is required to reproduce the debris disk observations. Finally, we show that including cratering collisions into the model is mandatory.

A new perspective on the irregular satellites of Saturn - II. Dynamical and physical origin

The origin of the irregular satellites of the giant planets has been long debated since their discovery. Their dynamical features argue against an in-situ formation suggesting they are captured bodies, yet there is no global consensus on the physical process at the basis of their capture. In this paper we explore the collisional capture scenario, where the actual satellites originated from impacts occurred within Saturn's influence sphere. By modeling the inverse capture problem, we estimated the families of orbits of the possible parent bodies and the specific impulse needed for their capture. The orbits of these putative parent bodies are compared to those of the minor bodies of the outer Solar System to outline their possible region of formation. Finally, we tested the collisional capture hypothesis on Phoebe by taking advantage of the data supplied by Cassini on its major crater, Jason. Our results presented a realistic range of solutions matching the observational and dynamical data.

STELLAR DISKS OF COLLISIONAL RING GALAXIES. I. NEW MULTIBAND IMAGES, RADIAL INTENSITY AND COLOR PROFILES, AND CONFRONTATION WITHN-BODY SIMULATIONS

We present new multi-band imaging data in the optical (BV RI and Hα) and near infrared bands (JHK) of 15 candidate ring galaxies from the sample of Appleton & Struck-Marcell (1987). We use these data to obtain color composite images, global magnitudes and colors of both the ring galaxy and its companion(s), and radial profiles of intensity and colors. We find that only nine of the observed galaxies have multi-band morphologies expected for the classical collisional scenario of ring formation, indicating the high degree of contamination of the ring galaxy sample by galaxies without a clear ring morphology. The radial intensity profiles, obtained by masking the off-centered nucleus, peak at the position of the ring, with the profiles in the continuum bands broader than that in the Hα line. The images as well as the radial intensity and color profiles clearly demonstrate the existence of the pre-collisional stellar disk outside the star-forming ring, which is in general bluer than the disk internal to the ring. The stellar disk seems to have retained its size, with the disk outside the ring having a shorter exponential scale length as compared to the values expected in normal spiral galaxies of comparable masses. The rings in our sample of galaxies are found to be located preferentially at around half-way through the stellar disk. The most likely reason for this preference is bias against detecting rings when they are close to the center (they would be confused with the resonant rings), and at the edge of the disk the gas surface density may be below the critical density required for star formation. Most of the observed characteristics point to relatively recent collisions (< 80 Myr ago) according to the N-body simulations of Gerber et al. (1996). Subject headings: galaxies: photometry — galaxies: interactions

The Sloan Lens ACS Survey. III. The Structure and Formation of Early‐Type Galaxies and Their Evolution since z ≈ 1

We present a joint gravitational lensing and stellar-dynamical analysis of 15 massive field early-type galaxies selected from the Sloan Lens ACS (SLACS) Survey. The following numerical results are found: (1) A joint likelihood gives an average logarithmic density slope for the total mass density of ⟨γ'⟩ = 2.01 (68% CL; ρtot ∝ r) inside ⟨REinst⟩ = 4.2 ± 0.4 kpc (rms of 1.6 kpc) for isotropic models. The inferred intrinsic rms spread in logarithmic density slopes is σ = 0.12. (2) The average position-angle difference between the light distribution and the total mass distribution is found to be ⟨Δθ⟩ = 0° ± 3° (rms of 10°), setting an upper limit of ⟨γext⟩ ≲ 0.035 on the average external shear. The total mass has an average ellipticity ⟨qSIE⟩ = 0.78 ± 0.03 (rms of 0.12), which correlates extremely well with the stellar ellipticity, q*, resulting in ⟨qSIE/q*⟩ = 0.99 ± 0.03 (rms of 0.11) for σ ≳ 225 km s-1 omitting three S0 lens galaxies. (3) The average projected dark matter mass fraction is inferred to be ⟨fDM⟩ = 0.25 ± 0.06 (rms of 0.22) inside ⟨REinst⟩, using the stellar mass-to-light ratios derived from the fundamental plane as priors. (4) Combined with results from the Lenses Structure and Dynamics (LSD) Survey at z ≳ 0.3, we find no significant evolution of the total density slope inside one effective radius for galaxies with σap ≥ 200 km s-1: a linear fit gives α ≡ d⟨γ'⟩/dz = 0.23 ± 0.16 (1 σ) for the range z = 0.08-1.01. The small scatter and absence of significant evolution in the inner density slopes suggest a collisional scenario in which gas and dark matter strongly couple during galaxy formation, leading to a total mass distribution that rapidly converges to dynamical isothermality.

Observing IMBH-IMBH Binary Coalescences via Gravitational Radiation

Recent numerical simulations have suggested the possibility of forming double intermediate mass black holes (IMBHs) via the collisional runaway scenario in young dense star clusters. The two IMBHs formed would exchange into a common binary shortly after their birth, and quickly inspiral and merge. Since space-borne gravitational wave (GW) observatories such as LISA will be able to see the late phases of their inspiral out to several Gpc, and LIGO will be able to see the merger and ringdown out to similar distances, they represent potentially significant GW sources. In this Letter we estimate the rate at which LISA and LIGO will see their inspiral and merger in young star clusters, and discuss the information that can be extracted from the observations. We find that LISA will likely see tens of IMBH--IMBH inspirals per year, while advanced LIGO could see ~10 merger and ringdown events per year, with both rates strongly dependent on the distribution of cluster masses and densities.

Massive Black Hole Binaries from Collisional Runaways

Recent theoretical work has solidified the viability of the collisional runaway scenario in young dense star clusters for the formation of very massive stars (VMSs), which may be precursors to intermediate-mass black holes (IMBHs). We present first results from a numerical study of the collisional runaway process in dense star clusters containing primordial binaries. Stellar collisions during binary scattering encounters offer an alternate channel for runaway growth, somewhat independent of direct collisions between single stars. We find that clusters with binary fractions >~10% yield two VMSs via collisional runaways, presenting the exotic possibility of forming IMBH--IMBH binaries in star clusters. We discuss the implications for gravitational wave observations, and the impact on cluster structure.

Runaway collisions in young star clusters - II. Numerical results

We present a new study of the collisional runaway scenario to form an intermediate-mass black hole (IMBH, M-BH greater than or similar to 100M(circle dot)) at the centre of a young, compact stellar cluster. The first phase is the formation of a very dense central core of massive stars (M-* similar or equal to 30-120M(circle dot)) through mass segregation and gravothermal collapse. Previous work established the conditions for this to happen before the massive stars evolve off the main sequence ( MS). In this and a companion paper, we investigate the next stage by implementing direct collisions between stars. Using a Monte Carlo stellar dynamics code, we follow the core collapse and subsequent collisional phase in more than 100 models with varying cluster mass, size, and initial concentration. Collisions are treated either as ideal, 'sticky-sphere' mergers or using realistic prescriptions derived from 3D hydrodynamics computations. In all cases for which the core collapse happens in less than the MS lifetime of massive stars (similar or equal to 3 Myr), we obtain the growth of a single very massive star (VMS, M-* similar or equal to 400-4000M(circle dot)) through a runaway sequence of mergers. Mass loss from collisions, even for velocity dispersions as high as sigma(v) similar to 1000 km s(-1), does not prevent the runaway. The region of cluster parameter space leading to runaway is even more extended than predicted in previous work because, in clusters with sigma(v) > 300 km s(-1), collisions accelerate (and, in extreme cases, drive) core collapse. Although the VMS grows rapidly to greater than or similar to 1000M(circle dot) in models exhibiting runaway, we cannot predict accurately its final mass. This is because the termination of the runaway process must eventually be determined by a complex interplay between stellar dynamics, hydrodynamics, and the stellar evolution of the VMS. In the vast majority of cases, we find that the time between successive collisions becomes much shorter than the thermal time-scale of the VMS. Therefore, our assumption that all stars return quickly to the MS after a collision must eventually break down for the runaway product, and the stellar evolution of the VMS becomes very uncertain. For the same reason, the final fate of the VMS, including its possible collapse to an IMBH, remains unclear.

Runaway collisions in young star clusters - I. Methods and tests

We present the methods and preparatory work for our study of the collisional runaway scenario to form a very massive star (VMS, M∗ > 400 M� ) at the centre of a young, compact stellar cluster. In the first phase of the process, a very dense central core of massive stars (M∗ � 30‐120 M � ) forms through mass segregation and gravothermal collapse. This leads to a collisional stage, likely to result in the formation of a VMS (itself a possible progenitor for an intermediate-mass black hole) through a runaway sequence of mergers between the massive stars. In this paper, we present the runaway scenario in a general astrophysical context. We then explain the numerical method used to investigate it. Our approach is based on a Monte Carlo code to simulate the stellar dynamics of spherical star clusters, using a very large number of particles (a few 10 5 to several 10 6 ). Finally, we report on test computations carried out to ensure that our implementation of the important physics is sound. In a second paper, we present results from more than 100 cluster simulations realized to determine the conditions leading to the collisional formation of a VMS and the characteristics of the runaway sequences.

Carbon monoxide fluorescence from Titan's atmosphere

We report on the discovery of emissions due to carbon monoxide from Titan's atmosphere, from mid-infrared observations with the ISAAC spectrometer at the Very Large Telescope and covering the 4.50–4.85 μm range. We detected about 45 emission lines coinciding with CO ro-vibrational lines, including CO(1–0) (P18 to R11) and CO(2–1) (P11 to R11). We show that these emissions cannot be generated thermally but occur in non-LTE conditions, due to radiative de-excitation from the v = 1 and v = 2 CO levels after excitation at 4.7 and 2.3 μm by solar radiation. A complete fluorescence model is then developed, allowing to compute the state populations of the two most abundant CO isotopes and N 2(1). It includes absorption by CO and CH 4, and vibrational–thermal and vibrational–vibrational collisional exchanges with CO, N 2, CH 4, and H 2. Emerging radiances at the top of the atmosphere are evaluated with a line-by-line code and compared to observations. Contribution functions show that the CO emissions sound Titan's stratosphere: while the (1–0) lines generally probe two layers, located respectively at 100–250 km and 300–550 km, the (2–1) lines are sensitive to the intermediate layer at 150–300 km. A sensitivity study is performed to establish the effect of the main model parameters (temperature profile, collisional scenario, and CO stratospheric abundance) on the results. Models reproduce the essential structure of the observed emissions. The (1–0) fundamental band is generally well fit with a nominal CO mixing ratio of 32 ppm—as inferred in the troposphere from observations at 4.80–5.10 μm (Lellouch et al., 2003, Icarus 162, 126–143). However, this band is only weakly dependent on the CO abundance, and given temperature and collisional scenario uncertainties, it constrains the CO stratospheric mixing ratio only to within a factor of ∼3. In addition, the nominal model with 32 ppm CO underestimates the first hot (2–1) transition by approximately a factor of 2. This discrepancy can be resolved by a combined adjustment of collisional rates and an increased CO stratospheric ratio of 60 ppm, consistent with the determination of Gurwell and Muhleman (2000, Icarus 145, 653–656). In contrast, the CO vertical profile suggested by Hidayat et al. (1998, Icarus 133, 109–133), strongly depleted above 200 km, cannot match the data for any realistic collisional scenario, and is therefore not supported by our results.

Yarkovsky depletion and asteroid collisional evolution

The orbital parameters of small asteroids change with time, as a consequence of the so-called Yarkovsky effect. This leads to a steady removal of objects from the Main Belt, which takes place when the objects reach one of the major resonant regions in the orbital elements space. The process may influence the evolution of the inventory and size distribution of Main Belt asteroids, but it has not been taken into account by classical models of the collisional evolution of the asteroid population. In this paper we discuss the role of the Yarkovsky effect in producing the current observed size distribution. We show that adding Yarkovsky effect to purely collisional mechanisms may increase the removal of objects at sizes around 1 km by a factor of about 2 with respect to a purely collisional scenario. Moreover, waves in the size distribution may also be produced. However, taking also into account current uncertainties in the efficiency of purely collisional mechanisms, the role of the Yarkovsky effect seems not dominant, and cannot be unambiguously determined.

The Contribution of Primordial Binaries to the Blue Straggler Population in 47 Tucanae

The recent observation (Ferraro et al. 2003b) of the blue str aggler population in 47 Tucanae gives the first detailed characterization of their spatial distribution i n the cluster over its entire volume. Relative to the light distribution, blue stragglers appear to be overabundant in the core and at large radii. The observed surface density profile shows a central peak, a zone of avoidance and a rise bey ond twenty core radii. In light of these findings we explored the evolution of blue stragglers mimicking their dynamics in a multi-mass King model for 47 Tucanae. We find that the observed spatial distribution can not be expl ained within a purely collisional scenario in which blue stragglers are generated exclusively in the core throu gh direct mergers. An excellent fit is obtained if we require that a sizable fraction of blue stragglers is generate d in the peripheral regions of the cluster inside primordial binaries that evolve in isolation experiencing mass-trans fer. Subject headings: stars: blue stragglers - binaries: general - globular clust ers: individual (47 Tuc)