Tens Of Myr Research Articles

Gas evolution in protoplanetary disks

We present the results of a study of the evolution of the gas and dust in disks around T Tauri, Herbig Ae and Vega-like stars. We observed the two lowest rotational lines of H2 with the ISO-SWS as well as 12CO 3–2 and 13CO 3–2 with the JCMT, and CO 6–5 with the CSO. The H2 lines trace the warm (∽ 100 K) inner region whereas the CO lines probe the colder outer disks. Substantial amounts of H2 have been detected toward T Tauri and Herbig Ae stars, and, surprisingly, also around three Vega-like objects (49 Cet, HD 135344 and β Pictoris). In contrast with previous conclusions derived from CO data, a significant mass of warm gas is found to persist up to ages of several tens of Myrs, suggesting that slow formation of gas-rich giant planets is possible.

26Al in galaxy regions: massive-star interactions with the ISM

Galactic 26Al is believed to be predominantly injected into the ISM from winds and supernovae of massive stars. These stars are typically born in groups and associations, which evolve over tens of Myrs, and shape the interstellar medium in their surroundings. 26Al contained in stellar outflows produces a γ-ray line at 1.809 MeV, which is potentially observable from regions of active star formation on time scales of a few Myrs, i.e., comparable to the mean life of 26Al. This relatively long time scale may lead to spatial offsets between the observed 26Al γ-ray emission and tracers of 26Al sources, such as locations of massive-stars, HII regions, or IR emission from dust heated by starlight. We find evidence for such an offset in star forming regions towards Orion.

Mantle-circulation models with sequential data assimilation: inferring present-day mantle structure from plate-motion histories.

Data assimilation is an approach to studying geodynamic models consistent simultaneously with observables and the governing equations of mantle flow. Such an approach is essential in mantle circulation models, where we seek to constrain an unknown initial condition some time in the past, and thus cannot hope to use first-principles convection calculations to infer the flow history of the mantle. One of the most important observables for mantle-flow history comes from models of Mesozoic and Cenozoic plate motion that provide constraints not only on the surface velocity of the mantle but also on the evolution of internal mantle-buoyancy forces due to subducted oceanic slabs. Here we present five mantle circulation models with an assimilated plate-motion history spanning the past 120 Myr, a time period for which reliable plate-motion reconstructions are available. All models agree well with upper- and mid-mantle heterogeneity imaged by seismic tomography. A simple standard model of whole-mantle convection, including a factor 40 viscosity increase from the upper to the lower mantle and predominantly internal heat generation, reveals downwellings related to Farallon and Tethys subduction. Adding 35% bottom heating from the core has the predictable effect of producing prominent high-temperature anomalies and a strong thermal boundary layer at the base of the mantle. Significantly delaying mantle flow through the transition zone either by modelling the dynamic effects of an endothermic phase reaction or by including a steep, factor 100, viscosity rise from the upper to the lower mantle results in substantial transition-zone heterogeneity, enhanced by the effects of trench migration implicit in the assimilated plate-motion history. An expected result is the failure to account for heterogeneity structure in the deepest mantle below 1500 km, which is influenced by Jurassic plate motions and thus cannot be modelled from sequential assimilation of plate motion histories limited in age to the Cretaceous. This result implies that sequential assimilation of past plate-motion models is ineffective in studying the temporal evolution of core-mantle-boundary heterogeneity, and that a method for extrapolating present-day information backwards in time is required. For short time periods (of the order of perhaps a few tens of Myr) such a method exists in the form of crude 'backward' convection calculations. For longer time periods (of the order of a mantle overturn), a rigorous approach to extrapolating information back in time exists in the form of iterative nonlinear optimization methods that carry assimilated information into the past through the use of an adjoint mantle convection model.

Far infrared and radio emission in dusty starburst galaxies

We revisit the nature of the far infrared (FIR)/radio correlation by means of the most recent models of star forming galaxies, focusing in particular on the case of obscured starbursts. We model the IR emission with our population synthesis code, GRASIL (Silva et al. [CITE]). For the radio emission, we revisit the simple model of Condon & Yin ([CITE]). We find that a tight FIR/radio correlation is natural when the synchrotron mechanism dominates over the inverse Compton, and the electron cooling time is shorter than the fading time of the supernova (SN) rate. Observations indicate that both these conditions are met in star forming galaxies, from normal spirals to obscured starbursts. However, since the radio non–thermal (NT) emission is delayed, deviations are expected both in the early phases of a starburst, when the radio thermal component dominates, and in the post-starburst phase, when the bulk of the NT component originates from less massive stars. We show that this delay allows the analysis of obscured starbursts with a time resolution of a few tens of Myrs, unreachable with other star formation (SF) indicators. We suggest a strategy to complement the analysis of the deviations from the FIR/radio correlation with the radio slope (q–radio slope diagram) to obtain characteristic parameters of the burst, e.g. its intensity, age and fading time scale. The analysis of a sample of compact ULIRGs shows that they are intense but transient starbursts, to which one should not apply usual SF indicators devised for constant SF rates. We also discuss the possibility of using the q–radio slope diagram to assess the presence of obscured AGN. A firm prediction of the models is an apparent radio excess during the post-starburst phase, which seems to be typical of a class of star forming galaxies in rich cluster cores. Finally we discuss how deviations from the correlation, due to the evolutionary status of the starburst, affect the technique of photometric redshift determination widely used for high-z sources.

Dynamical evolution of the Prometheus--Pandora system

The system of Saturn's inner satellites is saturated with many resonances. Its structure should be strongly affected by tidal forces driving the satellites through several orbit–orbit resonances. The evolution of these satellites is investigated using analytic and numerical methods. We show that the pair of satellites Prometheus and Pandora has a particularly short lifetime (<20 Myr) if the orbits of the satellites converge without capture into a resonance. The capture of Pandora into a resonance with Prometheus increases the lifetime of the couple by a few tens of Myr. However, resonances of the system are not well separated, and capture results in a chaotic motion. Secondary resonances also disrupt the resonant configurations. In all cases, the converging orbits of these two satellites result in a close encounter. The implications for the origin of Saturn's rings are discussed.

Evidence for episodicity in the magma supply to the large Tharsis volcanoes

The volumes and estimated total lifetimes of the Tharsis shield volcanoes on Mars imply that they were built with an average magma supply rate within a factor of 2–3 of ∼0.05 m3 s−1. The morphologies and positions of the summit calderas of these volcanoes, specifically the overlapping relationships between the calderas, are evidence that multiple magma reservoirs existed within them, centered on distinctly different locations at various times. The requirement that the magma in an older reservoir must have cooled below its solidus temperature in order to change the local stresses in such a way that a new reservoir grew at a different location argues for long time gaps (probably tens of Myr) during which the supply rate of magma from the mantle was much smaller than at other times. Conversely, the need to offset conductive cooling to the surface once an active magma reservoir was established implies that the magma supply rate was large, ∼1–10 m3 s−1, for times of order 0.1 Myr. An initial pulse involving rates of more than 150 m3 s−1 acting for at least a few weeks was needed to initiate each new reservoir. The lengths of numerous long lava flows on the flanks of the Tharsis Montes imply that lava must have been erupted at rates of ∼100–300 m3 s−1 for periods of a few years to a few tens of years. The volumes of many of these flows exceed the amounts of magma that could be released from overpressured magma reservoirs of the sizes implied by the dimensions of the summit calderas with the surrounding rocks behaving elastically. A maintained high magma supply rate from the mantle (leading to buffered eruption conditions) is almost certainly required for such flows, whereas inelastic caldera collapse events (unbuffered eruption conditions) can explain the emplacement of other flows. Taken together, the various lines of evidence suggest that the Tharsis volcanoes were built episodically with active phases lasting less than 1 Myr alternating with ∼100 Myr quiet phases. We comment on the implications of these issues for the episodicity of the generation of magma in the mantle and for the mode of transport of the magma in dikes or diapirs.

Multiwavelength Study of the Starburst Galaxy NGC 7714. I. Ultraviolet‐Optical Spectroscopy

We have studied the physical conditions in the central 300 pc of the prototypical starburst galaxy NGC 7714. Our analysis is based on ultraviolet spectroscopy with the HST/GHRS and ground-based optical observations, and it also covers X-ray and radio data taken from the literature. The data are interpreted using evolutionary models optimized for young starburst regions. The massive stellar population is derived in a self-consistent way using the continuum and stellar absorption lines in the ultraviolet and the nebular emission-line optical spectrum. The central starburst has an age of about 4.5 Myr, with little evidence for an age spread. Wolf-Rayet features at the ultraviolet indicate a stellar population of ~2000 Wolf-Rayet stars. The overall properties of the newly formed stars are quite similar to those derived, e.g., in 30 Doradus. A standard Salpeter initial mass function is consistent with all observational constraints. The nucleus of NGC 7714 has a bolometric luminosity of (0.5-1)×1010 L☉ and a mass of (5-10)×106 M☉ (if low-mass stars have formed). We find evidence for spatial structure within the central 300 pc sampled. Therefore it is unlikely that the nucleus of NGC 7714 hosts a single star cluster exceeding the properties of other known clusters. Contrary to previous suggestions, we find no evidence for a nuclear supernova rate that would significantly exceed the total disk-integrated rate. About one supernova event per century is predicted. Most of these events are associated with the core collapse of a hydrogen-free or -poor progenitor. An older stellar generation, with ages of tens of Myr and older, is suggested as well. This population is less concentrated toward the nucleus and extends over kiloparsec scales.

The flux of Tunguska-sized fragments from the main asteroid belt

The numerical model described in Menichella et al. (Earth, Moon and Planets 72, 133–149, 1996) is used to investigate the flux of Tunguska-sized asteroid fragments into chaotic resonant orbits leading them to attain an Earth-crossing status. The assumed main-belt size distribution is derived from that of known asteroids, extrapolated down to sizes ≈ 1 m and modified in such a way as to yield a quasi-stationary fragment production rate over times ≈ 100 Myr. Collisional physics consistent with the results of laboratory hypervelocity impact experiments and evidence from asteroid families are used, and the sensitivity of the results to the most critical poorly known parameters is analysed. The results of simulations show that the main asteroid belt on average can inject into the resonant escape hatches about one Tunguska-sized fragment per year, with an uncertainty of about a factor of ∓3. Owing to their limited dynamical and collisional lifetimes (as inferred from the better-known behaviour of km-sized near-Earth asteroids), only a fraction ≈ 1% of the Tunguska-sized near-Earth fragments are likely to hit the Earth, yielding an average flux of the order of one impact per century, consistent with observations (within the existing uncertainties). Large-scale stochastic collisions in the main belt can enhance this fragment flux by a factor of up to 6 over intervals ≈ 1 Myr, assuming that this corresponds to the typical dynamical timescale in the resonances. Such enhanced-flux episodes are expected to occur every several tens of Myr.