UV Capabilities to Probe the Formation of Planetary Systems: From the ISM to Planets

Planetary systems are angular momentum reservoirs generated during star formation. Solutions to three of the most important problems in contemporary astrophysics are needed to understand the entire process of planetary system formation: The physics of the ISM. Stars form from dense molecular clouds that contain ∼ 30% of the total interstellar medium (ISM) mass. The structure, properties and lifetimes of molecular clouds are determined by the overall dynamics and evolution of a very complex system – the ISM. Understanding the physics of the ISM is of prime importance not only for Galactic but also for extragalactic and cosmological studies. Most of the ISM volume (∼ 65%) is filled with diffuse gas at temperatures between 3000 and 300 000 K, representing about 50% of the ISM mass. The physics of accretion and outflow. Powerful outflows are known to regulate angular momentum transport during star formation, the so-called accretion–outflow engine. Elementary physical considerations show that, to be efficient, the acceleration region for the outflows must be located close to the star (within 1 AU) where the gravitational field is strong. According to recent numerical simulations, this is also the region where terrestrial planets could form after 1 Myr. One should keep in mind that today the only evidence for life in the Universe comes from a planet located in this inner disk region (at 1 AU) from its parent star. The temperature of the accretion–outflow engine is between 3000 and 10 ⁷ K. After 1 Myr, during the classical T Tauri stage, extinction is small and the engine becomes naked and can be observed at ultraviolet wavelengths. The physics of planet formation. Observations of volatiles released by dust, planetesimals and comets provide an extremely powerful tool for determining the relative abundances of the vaporizing species and for studying the photochemical and physical processes acting in the inner parts of young planetary systems. This region is illuminated by the strong UV radiation field produced by the star and the accretion–outflow engine. Absorption spectroscopy provides the most sensitive tool for determining the properties of the circumstellar gas as well as the characteristics of the atmospheres of the inner planets transiting the stellar disk. UV radiation also pumps the electronic transitions of the most abundant molecules (H ₂, CO, etc.) that are observed in the UV.Here we argue that access to the UV spectral range is essential for making progress in this field, since the resonance lines of the most abundant atoms and ions at temperatures between 3000 and 300 000 K, together with the electronic transitions of the most abundant molecules (H ₂, CO, OH, CS, S ₂, CO ₂ ⁺, C ₂, O ₂, O₃, etc.) are at UV wavelengths. A powerful UV-optical instrument would provide an efficient mean for measuring the abundance of ozone in the atmosphere of the thousands of transiting planets expected to be detected by the next space missions (GAIA, Corot, Kepler, etc.). Thus, a follow-up UV mission would be optimal for identifying Earth-like candidates.     

Observations of Stellar Wind Instabilities and Variability on Small and Large Scales

I review various observations which suggest that the winds of hot stars are inhomogeneous because of instabilities in the wind flow. On large scales, local wind overdensities are indirectly detected in the form of excess in the infra-red (IR) and radio free-free continuum. The X-ray detection of a hot (T ∼ 10⁶) wind component suggests that the wind is pervaded with strong shocks. The small-scale density structure of the wind can be studied from observations of Line-Profile Variations (LPVs) in optical and UV spectral lines, which are formed close to the stellar surface. LPVs in lines of the P Cygni type consist of blue-edge variations in saturated profiles, and Discrete Absorption Components (DACs) and Periodic Absorption Modulations (PAMs) in unsaturated profiles. These LPVs are shown to be recurrent, and thought to result from instabilities propagating through the wind and generated at the stellar surface. LPVs in recombination lines appear as stochastic subpeaks, which suggest that wind instabilities have a clump-like, rather than shell-like, structure. The kinematics of LPVs in both line types is consistent with wind propagating shocks generated from radiative instabilities. This revised version was published online in July 2006 with corrections to the Cover Date.     

Magnetic Reconnection and Turbulent Mixing: From ISM to Clusters of Galaxies

Magnetic reconnection, or the ability of the magnetic field lines that are frozen in plasma to change their topology, is a fundamental problem of magnetohydrodynamics (MHD). Webriefly examine the problem starting with the well-known Sweet-Parker scheme, discuss effectsof tearing modes, anomalous resistivity and the concept of hyperresistivity. We show that the field stochasticity by itself provides a way toenable fast reconnection even if, at the scale of individual turbulent wiggles,the reconnection happens at the slow Sweet-Parker rate. We show that fast reconnectionallows efficient mixing of magnetic field in the direction perpendicular tothe local direction of magnetic field. While the idea of stochastic reconnection still requiresnumerical confirmation,our numerical simulations testify that mixing motions perpendicular to the local magnetic field are upto high degree hydrodynamical. This suggests that the turbulent heattransport should be similar to that in non-magnetized turbulent fluid, namely,should have a diffusion coefficient ～V _L L, whereV _L is the amplitude of the turbulent velocity and L is the scale of the turbulent motions. We present numericalsimulations which support this conclusion. The applicationof this idea to thermal conductivity in clusters of galaxies shows that thismechanism may dominate the diffusion of heat and may be efficient enoughto prevent cooling flow formation.     

Eagle Nebula Pillars: From Models to Observations

Over the past few years, our group has been developing hydrodynamic models to simulate formation of the Eagle Nebula pillars. The true test of any model is, of course, how well it can reproduce the observations. Here, we discuss how we go about testing our models against observations. We describe the process by which we “observe” the model data to create synthetic maps. We show an example of this technique using one of our model runs and compare the resultant synthetic map to the real one.     

A Large Scale Energy Source For Feeding: ISM Turbulence in Spiral Galaxies

One of the important consequences of a newly discovered secular dynamical evolution process of spiral galaxies (Zhang, 1996, 1998, 1999) is that the orbiting disk matter receives energy injection each time it crosses the spiral density wave crest. This energy injection has been shown to be able to quantitatively explain the observed age-velocity-dispersion relation of the solar neighborhood stars. We demostrate in this paper that similar energy injection into the interstellar medium could serve as the large-scale energy source to continuously power the observed interstellar turbulence and to offset its downward cascade tendency. This revised version was published online in September 2006 with corrections to the Cover Date.     

From Local Star Formation to Global Star Formation

In this brief report we summarise the most important points raised in the course of a two-hour evening discussion session on the above topic, organised by the author. Major questions that were debated included the universality of the IMF, the history of the star formation rate in the solar neighorhood, the star formation efficiency in molecular clouds, and the role of triggered star formation. The issue of a threshold gas surface density for star formation to occur was also critically discussed. This revised version was published online in September 2006 with corrections to the Cover Date.     

MHD Simulations of the ISM: The Importance of the Galactic Magnetic Field on the ISM “Phases”

We have carried out 1.25 pc resolution MHD simulations of the ISM, on a Cartesian grid of 0 ≤ (x, y) ≤ 1 kpc size in the galactic plane and ?10 ≤ z ≤ 10 kpc into the halo, thus being able to fully trace the time-dependent evolution of the galactic fountain. The simulations show that large scale gas streams emerge, driven by SN explosions, which are responsible for the formation and destruction of shocked compressed layers. The shocked gas can have densities as high as 800 cm^?3 and lifetimes up to 15 Myr. The cold gas is distributed into filaments which tend to show a preferred orientation due to the anisotropy of the flow induced by the galactic magnetic field. Ram pressure dominates the flow in the unstable branch 10² < T ≤ 10^3.9 K, whereas for T ≤ 100 K (stable branch) magnetic pressure takes over. Near supernovae thermal and ram pressures determine the dynamics of the flow. Up to 80% of the mass in the disk is concentrated in the thermally unstable regime 10² < T ≤ 10^3.9 K with ～30% of the disk mass enclosed in the T ≤ 10³ K gas. The hot gas in contrast is controlled by the thermal pressure, since magnetic field lines are swept towards the dense compressed walls.     

A Flare Due to the Interaction of a Small Loop and a Large Loop

A C6.0 GOES X-ray-class flare which occurred at 05:32 UT on 5 September 1994 in the active region NOAA 7773 is analyzed in this paper. We found that this flare was triggered by the interaction of a small loop and a large loop. The small loop connected a decaying magnetic flux, not an emergence of magnetic flux as usually shown. These two loops were matched well by the extrapolated force-free field lines based on the Boundary Element Method and the boundary condition of the observed photospheric vector magnetic field. Soft X-ray observation showed that these two loops gradually merged to become one, and the soft X-ray intensity increased during the merging process, agreeing with the simulated results of I-type coalescence derived by Fushiki and Sakai (1995) and Sakai and Fushiki (1995).     

Global Dynamics in Self-Consistent Models of Elliptical Galaxies

We compare two different N-body models simulating elliptical galaxies. Namely, the first model is a non-rotating triaxial N-body equilibrium model with smooth center, called SC model. The second model, called CM model, is derived from the SC by inserting a central mass in it, so that all possible differences between the two models are due to the effect of the central mass. The central mass is assumed to be mainly due to a massive central black hole of mass about 1% of the total mass of the galaxy. By using the fundamental frequency analysis, the two systems are thoroughly investigated as regards the types of orbits described either by test particles, or by the real particles of the systems at all the energy levels. A comparison between the orbits of test particles and the orbits of real particles at various energy levels is made on the rotation number plane. We find that extensive stable regions of phase space, detected by test particles remain empty, i.e. these regions are not occupied by real particles, while many real particles move in unstable regions of phase space describing chaotic orbits. We run self-consistently the two models for more than a Hubble time. During this run, in spite of the noise due to small variations of the potential, the SC model maintains (within a small uncertainly) the number of particles moving on orbits of each particular type. In contrast, the CM model is unstable, due to the large amount of mass in chaotic motion caused by the central mass. This system undergoes a secular evolution towards an equilibrium state. During this evolution it is gradually self-organized by converting chaotic orbits to ordered orbits mainly of the short axis tube type approaching an oblate spheroidal equilibrium. This is clearly demonstrated in terms of the fundamental frequencies of the orbits on the rotation number plane and the time evolution of the triaxiality index.     

Chemistry in the ISM: the ALMA (r)evolution

Increases in knowledge in various subfields of galactic astrochemistry that will ensue as ALMA becomes operational are discussed. A distinction is made between evolutionary and revolutionary changes. It is proposed that the most revolutionary enhancements will occur in our knowledge of small-scale structure and the attendant need for more complex chemical models that contain spatial inhomogeneities and dynamics as well as chemical processes.     

Chemo-dynamical Evolution of the ISM in Galaxies

Chemo-dynamical models have been introduced in the late eighties andare a generally accepted tool for understanding galaxy evolution. Theyhave been successfully applied to one-dimensional problems, e.g. theevolution of non-rotating galaxies, and two-dimensional problems,e.g. the evolution of disk galaxies. Recently, also three-dimensionalchemo-dynamical models have become available. In these models thedynamics of different components, i.e. dark matter, stars and amulti-phase interstellar medium, are treated in a self-consistent wayand several processes allow for an exchange of matter, energy andmomentum between the components or different gas phases. Some resultsof chemo-dynamical models and their comparison with observations ofchemical abundances or star formation histories will be reviewed.     

Global Detection of Infrasonic Signals from Three Large Bolides

We present the infrasonic observations of three large bolides that were observed at numerous International Monitoring System (IMS) infrasound arrays on a global scale. First, a simple procedure for the global association of infrasound detections from large infrasound events is outlined. Infrasound signals are associated with large events based on arrival time, backazimuth and uniqueness at a given IMS array. Next, we apply the algorithm to three bolides and investigate some of the factors affecting the detectability of infrasound from large events. Our findings suggest that site-noise effects significantly degrade the capability of the IMS infrasound network, suggesting that more effort is required to reduce ambient site noise. These results have implications for the use of infrasound measurements (in particular those from IMS stations) as a tool for evaluating the global flux of near-Earth objects.     

Global spectral energy distributions of the Large Magellanic Cloud with interstellar dust

The effects of dust on infrared emission vary among galaxies of different morphological types. We investigated integrated spectral energy distributions (SEDs) in infrared and submillimeter/millimeter emissions from the Large Magellanic Cloud (LMC) based on observations from the Herschel Space Observatory (HSO) and near- to mid-infrared observations from the Spitzer Space Telescope (SST). We also used IRAS and WMAP observations to constrain the SEDs and present the results of radiative transfer calculations using the spectrophotometric galaxy model. We explain the observations by using dust models with different grain size distributions in the interstellar medium of the LMC, noting that the LMC has undergone processes that differ from those in the Milky Way. We determined a spectral index and a normalization factor in the range of ?3.5 to ?3.45 with grain radii in the range of 1 nm–300 nm for the silicate grain and 2 nm–1 μm for the graphite grain. The best fit to the observed SED was obtained with a spectral index of ?3.47, similar to the value derived by Piovan et al. (Mon. Not. R. Astron. Soc. 366(3):923, 2006a). The grain size distribution is described using a power law but with a break that is introduced below a _b , where a larger exponent is used. Changing the graphite grain size distribution significantly changed the SED pattern within the observational uncertainties. Based on the SED fits to the observations from submillimeter wavelengths to infrared radiation from the LMC using GRASIL (Silva et al., Astrophys. J. 509(1):103, 1998), we obtained a reasonable set of parameter values in chemical and geometric space together with the grain size distributions (Weingartner and Draine, Astrophys. J. 548(1):296, 2001) and a modified MRN model with the LMC extinction curve (Piovan et al., Mon. Not. R. Astron. Soc. 366(3):923, 2006a). For a given set of parameters including the disc scale height, synthesis of the starlight spectrum, optical depth, escape time scale, dust model, and star formation efficiency, the adopted dust-to-gas ratio for modeling the observed SEDs, ～1/300 (from the literature) yields a reasonable fit to the observed SEDs and similar results with the metallicity of the LMC as those reported in Russell and Bessell (Astrophys. J. Suppl. Ser. 70:865, 1989). The dust-to-gas ratios that are given as the metallicity caused the variation in the model fits. The difference mainly appears at the wavelengths near 100 μm.     

The local interstellar medium: A test-bed for the galactic ISM

We outline a method to explore the column density of the Local Interstellar Medium (LISM) using absorptions in the resonance H and K lines of Mgii. The intrinsic strengths of these lines in the temperature and density conditions prevailing in warm clouds (T _eff<10⁴ K) in the LISM allows them to be used to explore many lines of sight where lines such a NaD and Caii H and K are too weak, but where L is saturated. The number of measurable lines-of-sight is greatly enhanced by using cool stars as the background emitters, but this implies reliable separation of the LISM components from stellar chromospheric selfabsorptions. We explain how to do this, and how to use a combination of column density and radial velocity data to measure the spatial extent and the physical parameters of the single cloud in which the Sun is embedded. This proves to be an oblate spheroid, of characteristic diameter 8 pc, withT _eff 10⁴ K,n(Hi) of 0.1 cm^–3 and a mass <5M , streaming in the LSR from a point 1=4°,B=+16° with velocity equal to 16 km s^–1, and is surrounded by the much hotter lower density ionized gas of the local supernova bubble.     

Filamentary structure of the ISM at high latitudes

Different mechanisms for filament formation are studied with a view to account for the results of Verschuur (1991a, b). It is shown that pinch phenomena can produce a filamentary structure of H I without any pressure contribution from the warm component of the interstellar medium (ISM). The mechanisms of filament formation involving the Parker instability and magnetoconvection are also discussed. It is shown that instabilities corresponding to all these mechanisms may result in a picture similar to that observed by Verschuur.     

Pulsar Observations and Structure of the Local ISM

Results from new observations of pulsars using the Ooty Radio Telescope(ORT) are used for investigating the structure of the Local InterstellarMedium (LISM) and the nature of the plasma turbulence spectrum in theInterstellar Medium (ISM). The observations show anomalous scintillationtowards several nearby pulsars, and these are modelled in terms oflarge-scale spatial inhomogeneities in the distribution of plasma densityfluctuations in the LISM. A 3-component model, where the Solar neighborhoodis surrounded by a shell of enhanced plasma turbulence, is proposed for theLISM. The inferred scattering structure is strikingly similar to the LocalBubble. The nature of the plasma turbulence spectrum is found to be Kolmogorov-like in the spatial scale range 10⁶ m to 10¹¹ m,and there is evidence for excess power at larger spatial scales.     

两点相关函数、关联分维及IRAS星系大尺度分布的典型尺度

本文对两点相关函数及关联分维间的关系进行了讨论，以ＩＲＡＳ星系红移巡天样本作为例子进行分析，分析结果表明，在大尺度（≥１５─２０ｈＭｐｃ）上，ＩＲＡＳ星系的分布既不能用简单幂律形式的两点相关函数，也不能用简单分形来描写。它可以用多级分形来更好地描写。多级分形结构的主要特征之一是存在典型尺度，即相邻分形级间的转变尺度，用非归一计数方法可以有效而准确地确定这些典型尺度，存在典型尺度对目前已有的结构形成模型提出了挑战。