High-resolution spectroscopic view of planet formation sites

Star formation occurs via fragmentation of molecular clouds, which means that\nthe majority of stars born are a members of binaries. There is growing evidence\nthat planets might form in circumprimary disks of medium-separation binaries.\nThe tidal forces caused by the secondary generally act to distort the\noriginally circular disk to an eccentric one. To infer the disk eccentricity\nfrom high-res NIR spectroscopy, we calculate the fundamental band emission\nlines of the CO molecule emerging from the atmosphere of the disk. We model\ncircumprimary disk evolution under the gravitational perturbation of the\norbiting secondary using a 2D grid-based hydrodynamical code, assuming\nalpha-type viscosity. The hydrodynamical results are combined with our spectral\ncode based on the double-layer disk model to calculate the CO molecular line\nprofiles. We find that the orbital velocity distribution of the gas parcels\ndiffers significantly from the circular Keplerian fashion, thus the line\nprofiles are asymmetric in shape. The magnitude of asymmetry is insensitive to\nthe binary mass ratio, the magnitude of viscosity, and the disk mass. In\ncontrast, the disk eccentricity, thus the level of the line profile asymmetry,\nis influenced significantly by the binary eccentricity and the disk geometrical\nthickness. We demonstrate that the disk eccentricity profile in the\nplanet-forming region can be determined by fitting the high-resolution CO line\nprofile asymmetry using a simple 2D spectral model that accounts for the\nvelocity distortions caused by the disk eccentricity. Thus, with our novel\napproach the disk eccentricity can be inferred with high-resolution near-IR\nspectroscopy prior to the era of high angular resolution optical or radio\ndirect-imaging. By determining the disk eccentricity in medium-separation young\nbinaries, we might be able to constrain the planet formation theories.\n

Spectral observations of star-forming molecular clouds sometimes reveal distinct red asymmetric double-peaked molecular line profiles with weaker blue peaks and stronger red peaks. For some star-forming molecular clouds, molecular line transitions with red asymmetric line profiles coexist with those with blue asymmetric line profiles (i.e. blue asymmetric doublepeaked molecular line profiles with weaker red peaks and stronger blue peaks) in spatially resolved spectral observations from different lines of sight, whereas for others molecular transitions with red asymmetric line profiles dominate. Blue asymmetric line profiles are usually interpreted as signals of central core collapses, whereas red asymmetric line profiles remain unexplained. In this paper, we advance a spherically symmetric self-similar hydrodynamic model framework for envelope expansions with core collapses (EECCs) of a general polytropic molecular gas cloud under self-gravity. Based on such EECC hydrodynamic cloud models, we perform tracer molecular line profile calculations using the publicly available RATRAN code for star-forming clouds with spectroscopic signatures of red asymmetric line profiles. The observation of red asymmetric line profiles in molecular cloud cores indicates that EECC processes are probably essential hydrodynamic processes in star formation. Using spatial distributions, we explore various profiles of molecular lines for several tracer molecules in various settings of EECC dynamic models with and without shocks.

\n Context. Planets are thought to form in protoplanetary accretion disks around young stars. Detecting a giant planet still embedded in a protoplanetary disk would be very important and give observational constraints on the planet-formation process. However, detecting these planets with the radial velocity technique is problematic owing to the strong stellar activity of these young objects.\n Aims. We intend to provide an indirect method to detect Jovian planets by studying near infrared emission spectra originating in the protoplanetary disks around T Tauri stars. Our idea is to investigate whether a massive planet could induce any observable effect on the spectral lines emerging in the disks atmosphere. As a tracer molecule we propose CO, which is excited in the ro-vibrational fundamental band in the disk atmosphere to a distance of \n ~2−3 AU (depending on the stellar mass) where terrestrial planets are thought to form.\n Methods. We developed a semi-analytical model to calculate synthetic molecular spectral line profiles in a protoplanetary disk using a double layer disk model heated on the outside by irradiation by the central star and in the midplane by viscous dissipation due to accretion. 2D gas dynamics were incorporated in the calculation of synthetic spectral lines. The motions of gas parcels were calculated by the publicly available hydrodynamical code FARGO which was developed to study planet-disk interactions.\n Results. We demonstrate that a massive planet embedded in a protoplanetary disk strongly influences the originally circular Keplerian gas dynamics. The perturbed motion of the gas can be detected by comparing the CO line profiles in emission, which emerge from planet-bearing to those of planet-free disk models. The planet signal has two major characteristics: a permanent line profile asymmetry, and short timescale variability correlated with the orbital phase of the giant planet. We have found that the strength of the asymmetry depends on the physical parameters of the star-planet-disk system, such as the disk inclination angle, the planetary and stellar masses, the orbital distance, and the size of the disk inner cavity. The permanent line profile asymmetry is caused by a disk in an eccentric state in the gap opened by the giant planet. However, the variable component is a consequence of the local dynamical perturbation by the orbiting giant planet. We show that a forming giant planet, still embedded in the protoplanetary disk, can be detected using contemporary or future high-resolution near-IR spectrographs like VLT/CRIRES and ELT/METIS. \n

Emission line profiles of tracer molecule H$_2$CO 140 GHz transition from gravitational core collapsing clouds in the dynamic process of forming protostars are calculated, using a simple ray-tracing radiative transfer model. Three self-similar dynamic inside-out core collapse models -- the conventional polytropic model, the empirical hybrid model and the isothermal model -- for star-forming molecular clouds are examined and compared. The isothermal model cannot produce observed asymmetric double-peak molecular line profiles. The conventional polytropic model, which gives flow velocity, mass density and temperature profiles self-consistently, can produce asymmetric double-peak line profiles for a core collapsing cloud. In particular, the blue peak is stronger than the red peak, consistent with a broad class of molecular line profile observations. The relative strengths of the blue and red peaks within a molecular line profile are determined by the cloud temperature gradient. The conventional polytropic model can be utilized to produce molecular line-profile templates, for extracting dynamical information from line spectra of molecular globules undergoing a gravitational core collapse. We show a sample fit using the 140 GHz H$_2$CO emission line from the central region of the molecular globule B335 by our model with $\gamma=1.2$. The calculation of line profiles and fitting processes also offer a scenario to estimate the protostellar mass, the kernel mass accretion rate, and the evolution time scale of a core collapsing cloud. Our model can be readily adapted to other tracer molecules with more or less constant abundances in star-forming clouds.

The presence of a cavity in a protoplanetary disk revealed by dust continuum emissions is sometimes postulated as a signpost of an embedded gas giant planet. More peculiarly, dust emissions exterior to the cavity are often observed to be asymmetric. We explore the possibility of the asymmetry as a result of the asymmetric distribution of dust in an eccentric protoplanetary disk under the secular gravitational perturbation of an embedded massive gas giant planet. We find that the surface density of the dust well coupled to the disk gas is enhanced around the apocenter of the disk. In addition, the azimuthal distributions of particles of various sizes can deviate significantly due to different coupling to the gas. Overall, the asymmetric structure exhibits a phase correlation between the gas velocity field and dust density distribution. A Doppler map for an eccentric disk is also presented based on Cycle 1 ALMA observations. Our study potentially provides a reality check as to whether an asymmetric disk gap detected at sub-mm and cm wavelengths is a signpost of a massive gas giant planet.

Exoplanets

We investigate the formation of double-peaked asymmetric line profiles of CO in the fundamental band spectra emitted by young (1-5Myr) protoplanetary disks hosted by a 0.5-2 Solar mass star. Distortions of the line profiles can be caused by the gravitational perturbation of an embedded giant planet with q=4.7 10^-3 stellar-to-planet mass ratio. Locally isothermal, 2D hydrodynamic simulations show that the disk becomes globally eccentric inside the planetary orbit with stationary ~0.2-0.25 average eccentricity after ~2000 orbital periods. For orbital distances 1-10 AU, the disk eccentricity is peaked inside the region where the fundamental band of CO is thermal excitated. Hence, these lines become a sensitive indicators of the embedded planet via their asymmetries (both in flux and wavelength). We find that the line shape distortions (e.g. distance, central dip, asymmetry and positions of peaks) of a given transition depend on the excitation energy (i.e. on the rotational quantum number J). The magnitude of line asymmetry is increasing/decreasing with J if the planet orbits inside/outside the CO excitation zone (R_CO<=3, 5 and 7 AU for a 0.5,1 and 2 Solar mass star, respectively), thus one can constrain the orbital distance of a giant planet by determining the slope of peak asymmetry-J profile. We conclude that the presented spectroscopic phenomenon can be used to test the predictions of planet formation theories by pushing the age limits for detecting the youngest planetary systems.

The gas giant planets’ formation processes in a viscously evolved protoplanetary disk are studied in the context of the core accretion model. In this paper, we follow the entire formation process of the core accretion model (the three stages). We find that the gas giant planets’ final masses and formation regions have strong dependence on the molecular cloud core’s properties (angular velocity $\omega $ and mass $M _{c d}$ ) and the $\alpha _{ \mathit{min} }$ parameter. We find and build the relationship between gas giant planets’ properties and molecular cloud core’s properties. In contrast to the previous works, we find that the formation process can be finished within the protoplanetary disk’s lifetime (4×106 yr) in our disk model. This is because the mass influx produced by the molecular cloud core can provide enough material to the protoplanetary disk. We also find that the gas giant planets’ final masses increase generally with the viscosity coefficient $\alpha $ . This is because most of the gas giant planet’s mass is captured during the rapid gas accretion phase (the third stage of the core accretion model), and furthermore the accretion of gas in this phase is dominated by the “gap limiting case”. And our numerical results can also be compared with the observed data of exoplanet systems.

Context. The birth of giant planets in protoplanetary discs is known to alter the structure and evolution of the disc environment, however most of our knowledge is focussed on its effects on the observable gas and dust. The impact on the evolution of the invisible planetesimal population remains insufficiently studied, yet mounting evidence from the Solar System shows how the appearance of its giant planets played a key role in shaping the habitability of the terrestrial planets. Aims. We investigate the dynamical and collisional transport processes of volatile elements by planetesimals in protoplanetary discs that host young giant planets using the HD 163296 system as our case study. HD 163296 is one of the best-characterised protoplanetary discs and has been proposed to host at least four giant planets on wide orbits as well as a massive planetesimal disc. The goal of this study is to assess the impact of the dynamical and collisional transport on the disc as well as on existing and forming planetary bodies. Methods. We performed high-resolution n-body simulations of the dynamical evolution of planetesimals embedded in HD 163296’s protoplanetary disc across and after the formation of its giant planets, accounting for the uncertainty on both the disc and planetary masses as well as for the effects of aerodynamic drag of the disc gas and the gas gravity. To quantify the impact probabilities with existing and possible undiscovered planetary bodies, we processed the output of the n-body simulations with well-tested statistical collisional algorithms from studies of the asteroid belt. Results. In our simulations the formation of giant planets in the HD 163296 system creates a large population of dynamically excited planetesimals, the majority of which originate from beyond the CO snowline. The excited planetesimals are then transported to the inner disc regions as well as scattered outward beyond the protoplanetary disc and into interstellar space. In the inner disc, potential solid planets can be enriched in volatile elements to levels that are comparable or larger than those of the Earth, while giant planets can be enriched to the levels of Jupiter and Saturn. Conclusions. The formation of giant planets on wide orbits impacts the compositional evolution of protoplanetary discs and young planetary bodies on a global scale. The collisional enrichment of the atmospheres of giant planets can alter or mask the signatures of their formation environments; this process can also provide independent constraints on the disc mass. In our simulations protoplanetary discs with giant planets on wide orbits prove efficient factories of interstellar objects.

Context. Orbital mean motion resonances in planetary systems originate from dissipative processes in disk-planet interactions that lead to orbital migration. In multi-planet systems that host giant planets, the perturbation of the protoplanetary disk strongly affects the migration of companion planets. Aims. By studying the well-characterized resonant planetary system around GJ 876 we aim to explore which effects shape disk-driven migration in such a multi-planet system to form resonant chains. Methods. We modelled the orbital migration of three planets embedded in a protoplanetary disk using two-dimensional locally isothermal hydrodynamical simulations. In order to explore the effect of several disk characteristics, we performed a parameter study by varying the disk thickness, α viscosity, mass as well as the initial position of the planets. Moreover, we have carefully analysed and compared simulations with various boundary conditions at the disk’s inner rim. Results. We find that due to the high masses of the giant planets in this system, substantial eccentricity can be excited in the disk. This results in large variations of the torque acting on the outer lower mass planet, which we attribute to a shift of Lindblad and corotation resonances as it approaches the eccentric gap that the giants create. Depending on disk parameters, the migration of the outer planet can be stopped at the gap edge in a non-resonant state. In other models, the outer planet is able to open a partial gap and to circularize the disk again, later entering a 2:1 resonance with the most massive planet in the system to complete the observed 4:2:1 Laplace resonance. Conclusions. Disk-mediated interactions between planets due to spiral waves and excitation of disk eccentricity by massive planets cause deviations from smooth inward migration of exterior lower mass planets. Self-consistent modelling of the disk-driven migration of multi-planet systems is thus mandatory. Constraints can be placed on the properties of the disk during the migration phase, based on the observed resonant state of the system. Our results are compatible with a late migration of the outermost planet into the resonant chain, when the giant planet pair already is in resonance.

Double-peaked broad emission lines in Active Galactic Nuclei (AGNs) may indicate the existence of a bound supermassive black hole (SMBH) binary where two distinct broad line regions (BLRs) contribute together to the line profile. An alternative interpretation is a disk emitter origin for the double-peaked line profile. Using simple BLR models, we calculate the expected broad line profile for a SMBH binary at different separations. Under reasonable assumptions that both BLRs are illuminated by the two active SMBHs and that the ionizing flux at the BLR location is roughly constant, we confirm the emergence of double-peaked features and radial velocity drifts of the two peaks due to the binary orbital motion. However, such a clear double-peaked feature only arises in a particular stage of the binary evolution when the two BHs are close enough such that the line-of-sight orbital velocity difference is larger than the FWHM of the individual broad components, while the two BLRs are still mostly distinct. Prior to this stage, the velocity splitting due to the orbit motion of the binary is too small to separate the emission from the two BLRs, leading to asymmetric broad line profiles in general. When the two BHs are even closer such that the two BLRs can no longer be distinct, the line profile becomes more complex and the splitting of the peaks does not correspond to the orbital motion of the binary. In this regime there are no coherent radial velocity drifts in the peaks with time. Asymmetric line profiles are probably a far more common signature of binary SMBHs than are double-peaked profiles. We discuss the temporal variations of the broad line profile for binary SMBHs and highlight the different behaviors of reverberation mapping in the binary and disk emitter cases, which may serve as a feasible tool to disentangle these two scenarios.

The location of surface brightness maxima (e.g. apocentre and pericentre glow) in eccentric debris discs are often used to infer the underlying orbits of the dust and planetesimals that comprise the disc. However, there is a misconception that eccentric discs have higher surface densities at apocentre and thus necessarily exhibit apocentre glow at long wavelengths. This arises from the expectation that the slower velocities at apocentre lead to a ‘pile up’ of dust, which fails to account for the greater area over which dust is spread at apocentre. Instead we show with theory and by modelling three different regimes that the morphology and surface brightness distributions of face-on debris discs are strongly dependent on their eccentricity profile (i.e. whether this is constant, rising, or falling with distance). We demonstrate that at shorter wavelengths the classical pericentre glow effect remains true, whereas at longer wavelengths discs can either demonstrate apocentre glow or pericentre glow. We additionally show that at long wavelengths the same disc morphology can produce either apocentre glow or pericentre glow depending on the observational resolution. Finally, we show that the classical approach of interpreting eccentric debris discs using line densities is only valid under an extremely limited set of circumstances, which are unlikely to be met as debris disc observations become increasingly better resolved.

The apparent regularity of the motion of the giant planets of our solar system suggested for decades that said planets formed onto orbits similar to the current onesand that nothing dramatic ever happened during their lifetime. The discovery of extrasolar planets showed astonishingly that the orbital structure of our planetary system is not typical. Many giant extrasolar planets have orbits with semimajor axes of ∼ 1 AU,and some have even smaller orbital radii, sometimes with orbital periods of just a few days. Moreover, most extrasolar planets have large eccentricities, up to values that only comets have in our solar system. Why is there such a great diversitybetween our solar system and the extrasolar systems, as well as among the extrasolar systems themselves? This chapter aims to give a partial answer to this fundamental question. Its guideline is a discussion of the evolution of our solarsystem, certainly biased by a view that emerges, in part, from a series of works comprising the “Nice model.” According to this view, the giant planets of the solar system migrated radially while they were still embedded in a protoplanetary disk of gas and presumably achieved a multi-resonant orbital configuration, characterized by smaller interorbital spacings and smaller eccentricities and inclinations with respect to the current configuration.The current orbits of the giant planets may have been achieved during a phase of orbital instability, during which the planets acquired temporarily large-eccentricity orbits and all experienced close encounters with at least oneother planet. This instability phase occurred presumably during the putative “Late Heavy Bombardment” of the terrestrial planets, approximately ∼ 3.9 Gy ago (Tera et al. 1974). The interaction with a massive, distant planetesimal disk (the ancestor of the current Kuiper belt) eventually damped the eccentricities of the planets, ending the phase of mutual planetary encounters and parking the planets onto their current, stable orbits. This new view of the evolution of the solar system makes our system not very different from the extrasolar ones. In fact, the best explanation for the large orbital eccentricities of extrasolar planets is that the planets that are observed are the survivors of strong instability phases of original multi-planet systems on quasi-circular orbits. The main difference between the solar system and the extrasolar systems is in the magnitude of such an instability. In the extrasolar systems, encounters among giant planets had to be the norm. In our case, the two major planets (Jupiter and Saturn) never had close encounters with each other: They only encountered “minor” planets like Uranus and/or Neptune. This was probably just mere luck, as simulations show that Jupiter-Saturn encounters in principle could have occurred. Another relevant difference with the extrasolar planets is that, during the gas-disk phase, our giant planets avoided migrating permanently into the inner solar system, thanks to the specific mass ratio of the Jupiter/Saturn pair and the rapid disappearance of the disk soon after the formation of the giant planets. This chapter ends on a note on terrestrial planets. The structure of a terrestrial-planet system depends sensitively on the dynamical evolution of the giant planets and on their final orbits. It appears clear that habitable terrestrial planets, with moderate eccentricity orbits, cannot exist in systems where the giant planets became violently unstable and developed very elliptic orbits. Thus, our very existence is possible only because the instability phase experienced by the giant planets of our solar system was of “moderate” strength.

This thesis presents a numerical study on the interaction between planets and circumstellar disks. We use the hydrodynamics/magnetohydrodynamics code PLUTO(Mignone et al, 2007) to simulate the circustellar accretion disk. A module to include embedded planets was incorporated into the code. We study two critical aspects for planet formation theory: the migration of planets due to gravitational disk torques and the accretion of gas onto planets from the surrounding disk. These two aspects are critical in any planet formation model as they will determine the final mass and the orbital separation. We first investigate aspects for massive planets in the evolutionary phase when a gap has been cleared in the disk. It is found that when a gap has been opened, the migration and gas accretion rate is linearly dependent on the surface density inside the gap. The torques exerted on the planet depend strongly on the material inside the Hill sphere when the local disk mass exceeds the planet mass. The depletion of the Hill sphere due to an accreting planet can increase migration timescales up to an order of magnitude of the linear estimate. Secondly, we investigate migration and gas accretion in turbulent disks, where turbulence is generated by the magnetorotational instability (MRI). In weakly magnetized turbulent disks, low-mass planet migration is dominated by stochastic density perturbations that can be characterized with a given amplitude and correlation time. More massive planets can undergo slower or reversed migration due to unsaturation of the corotation torque by turbulent advection and diffusion of gas into the horseshoe region. Magnetic turbulence is greatly supressed by giant planets that open a gap in the disk. Additionally, Jupiter-mass planets in turbulent disks are found to accrete less than expected from the global-averaged interal stresses in the disk. Our results can be directly implemented in planet population synthesis studies in order to better understand the nature of the observed population of extrasolar planets.

\n Contains fulltext :\n 122600.pdf (Author’s version preprint ) (Open Access)\n