Search

Planetary systems form in the discs of gas and dust that orbit young stars. In the past few years, observations of these discs at (sub-)millimetre wavelengths with very fine angular resolution have started to uncover the hallmarks of small-scale substructures in the spatial distributions of their pebble-sized particles. These are some of the fundamental signatures of the planet formation epoch, since they trace localised concentrations of material that facilitate the formation of much larger planetary building blocks, and may themselves be created by young planets interacting with their birth environments., Published in The Messenger vol. 174, pp. 19-23, December 2018.

We investigate how the X factor, the ratio of H_2 column density (NH2) to velocity-integrated CO intensity (W), is determined by the physical properties of gas in model molecular clouds (MCs). We perform radiative transfer calculations on chemical-MHD models to compute X. Using integrated NH2 and W reproduces the limited range in X found in observations, resulting in a mean value X=2\times10^20 s/cm^2/K^1/km^1 from the Galactic MC model. However, in limited velocity intervals, X can take on a much larger range due to CO line saturation. Thus, X strongly depends on both the range in gas velocities and volume densities. The temperature (T) variations within individual MCs do not strongly affect X, as dense gas contributes most to setting X. For fixed velocity and density structure, gas with higher T has higher W, yielding X ~ T^-1/2 for T~20-100 K. We demonstrate that the linewidth-size scaling relation does not influence the X factor - only the range in velocities is important. Clouds with larger linewidths, regardless of the linewidth-size relation, have a higher W, corresponding to a lower value of X, scaling roughly as X ~ sigma^-1/2. The "mist" model, consisting of optically thick cloudlets with well-separated velocities, does not accurately reflect the conditions in a turbulent MC. We propose that the observed cloud-average values of X ~ XGal is simply a result of the limited range in NH2, temperatures, and velocities found in Galactic MCs - a ~constant value of X therefore does not require any linewidth-size relation, or that MCs are virialized objects. Since gas properties likely differ (slightly) between clouds, masses derived through a standard X should only be considered as a rough first estimate. For temperatures T~10-20 K, velocity dispersions ~1-6 km/s, and NH2~2-20\times10^21 cm^-2, we find cloud-averaged X ~ 2-4\times10^20 s/cm^2/K^1/km^1 for Solar-metallicity models., Comment: 24 pages, including 21 Figures, Accepted to MNRAS

Determining the sequence of events in the formation of stars and planetary systems and their timescales is essential for understanding those processes, yet establishing ages is fundamentally difficult because we lack direct indicators. In this review we discuss the age challenge for young stars, specifically those less than ~ 100 m.y. old. Most age determination methods that we discuss are primarily applicable to groups of stars but can be used to estimate the age of individual objects. A reliable age scale is established above 20 m.y. from measurement of the lithium depletion boundary (LDB) in young clusters, and consistency is shown between these ages and those from the upper-main-sequence (UMS) and main-sequence (MS) tum-off - if modest core convection and rotation is included in the models of higher-mass stars. Other available methods for age estimation include the kinematics of young groups, placing stars in Hertzsprung-Russell diagrams (HRDs), pulsations and seismology, surface gravity measurement, rotation and activity, and Li abundance. We review each of these methods and present known strengths and weaknesses. Below ~20 m.y., both model-dependent and observational uncertainties grow, the situation is confused by the possibility of age spreads, and no reliable absolute ages yet exist. The lack of absolute age calibration below 20 m.y. should be born in mind when considering the lifetimes of protostellar phases and circumstellar material.

We are still in the early days of exoplanet discovery. Astronomers are beginning to model the atmospheres and interiors of exoplanets and have developed a deeper understanding of processes of planet formation and evolution. However, we have yet to map out the full complexity of multi-planet architectures or to detect Earth analogs around nearby stars. Reaching these ambitious goals will require further improvements in instrumentation and new analysis tools. In this chapter, we provide an overview of five observational techniques that are currently employed in the detection of exoplanets: optical and infrared (IR) Doppler measurements, transit photometry, direct imaging, microlensing, and astrometry. We provide a basic description of how each of these techniques works and discuss forefront developments that will result in new discoveries. We also highlight the observational limitations and synergies of each method and their connections to future space missions.

Results from recent space missions, in particular Spitzer and Herschel, have led to significant progress in our understanding of the formation and transport of water from clouds to disks, planetesimals, and planets. [n this review, we provide the underpinnings for the basic molecular physics and chemistry of water and outline these advances in the context of water formation in space, its transport to a forming disk, its evolution in the disk, and finally the delivery to forming terrestrial worlds and accretion by gas giants. Throughout, we pay close attention to the disposition of water as vapor or solid and whether it might be subject to processing at any stage. The context of the water in the solar system and the isotopic ratios (D/H) in various bodies are discussed as grounding data points for this evolution. Additional advances include growing knowledge of the composition of atmospheres of extrasolar gas giants, which may be influenced by the variable phases of water in the protoplanetary disk. Furthermore, the architecture of extrasolar systems leaves strong hints of dynamical interactions, which are important for the delivery of water and subsequent evolution of planetary systems. We conclude with an exploration of water on Earth and note that all the processes and key parameters identified here should also hold for exoplanetary systems.