We present a global study of low‐mass, young stellar object (YSO) surface densities (Σ) in nearby (<500 pc) star‐forming regions based on a comprehensive collection of Spitzer Space Telescope surveys. We show that the distribution of YSO surface densities in the solar neighbourhood is a smooth distribution, being adequately described by a lognormal function from a few to 103 YSOs pc−2, with a peak at ∼22 stars pc−2 and a dispersion of . We do not find evidence for multiple discrete modes of star formation (e.g. clustered and distributed). Comparing the observed surface density distribution to previously reported surface density threshold definitions of clusters, we find that the fraction of stars in clusters is crucially dependent on the adopted definitions, ranging from 40 to 90 per cent. However, we find that only a low fraction (<26 per cent) of stars are formed in dense environments where their formation/evolution (along with their circumstellar discs and/or planets) may be affected by the close proximity of their low‐mass neighbours.

We have carried out a kinematical, high angular resolution (∼ 0. ′′ 1) study of the optical blueshifted flow from DG Tau within 0. ′′ 5 from the source (i.e. 110 AU when de-projected along this flow). We analysed optical emission line profiles extracted from a set of seven long-slit spectra taken with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST), obtained by maintaining the slit parallel to the outflow axis while at the same time moving it transversely in steps of 0. ′′ 07. For the spatially resolved flow of moderate velocity (peaking at -70 km s −1 ), we have found systematic differences in the radial velocities of lines from opposing slit positions i.e. on alternate sides of the jet axis. The results, obtained using two independent techniques, are corrected for the spurious wavelength shift due to the uneven illumination of the STIS slit. Other instrumental effects are shown to be either absent or unimportant. The derived relative Doppler shifts range from 5 to 20 km s −1 . Assuming the flow is axially symmetric, the velocity shifts are consistent with the southeastern side of the flow moving towards the observer faster than the corresponding northwestern side. If this finding is interpreted as rotation, the flow is then rotating clockwise looking from the jet towards the source and the derived toroidal velocities are in 1 Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association -2the range 6 to 15 km s −1 , depending on position. Combining these values with recent estimates of the mass loss rate, one would obtain an angular momentum flux, for the low to moderate velocity regime of the flow, ofJ w,lm ∼ 3.8 10 −5 M ⊙ yr −1 AU km s −1 . Our findings may constitute the first detection of rotation in the initial channel of a jet flow. The derived values appear to be consistent with the predictions of popular magneto-centrifugal jet-launching models, although we cannot exclude the possibility that the observed velocity differences are due to some transverse outflow asymmetry other than rotation.

We have carried out a spatio-kinematic study of the outflow from the classical T Tauri star DG Tau using the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). A series of seven spatially offset long-slit spectra spaced by 0.07 ′′ were obtained along the axis of the outflow to build up a 3-D intensity-velocity "cube" in various forbidden emission lines (FELs) and Hα. Here we present high spatial resolution synthetic line images close to the star in distinct radial velocity intervals (from ∼ +50 km s −1 to ∼ -450 km s −1 in four bins, each ∼ 125 km s −1 wide). The lowest velocity emission is also examined in finer detail (from +60 km s −1 to -70 km s −1 in five bins ∼ 25 km s −1 wide). We have found that the highest velocity and most highly collimated component, i.e. the jet, can be traced from DG Tau to a distance D∼0.7 ′′ . The jet is on the axis of a pear-shaped limb-brightened bubble which extends between 0.4 ′′ and 1.5 ′′ from the source and which we interpret as a bow shock. Other condensations are seen close to the star indicating ongoing temporal variations in the flow. The low-velocity component of the outflow is found to be spatially wide close to the source (∼0.2 ′′ at D=0.2 ′′ ), in contrast to the high velocity jet (width < ∼ 0.1 ′′ ). We have also found evidence to suggest that not only does the density in the outflow increase longitudinally with proximity to the source but that it also increases laterally towards the flow axis. Thus, at least in the case of DG Tau, the flow becomes gradually denser as it increases in velocity and becomes more collimated. Our observations show a continous bracketing of the higher speed central flow within the lower speed, less collimated, broader flow, down to the lowest velocity scales. This suggests that the low and high velocity FELs in the highly active T Tauri star DG Tau are intimately related. Implications of these observations for FEL models will be considered in a future paper (Bacciotti et al. 2000).

We examine the conditions of the plasma along a sample of "classical" Herbig-Haro (HH) jets located in the Orion and Vela star forming regions, through combined optical-infrared spectral diagnostics. Our sample includes HH 111, HH 34, HH 83, HH 73, HH 24 C/E, HH 24 J, observed quasi-simultaneously and in the same manner at moderate spatial/spectral resolution. Once intercalibrated, the obtained spectra cover a wide wavelength range from 0.6−2.5 µm, including many transitions from regions of different excitation conditions. This allows us to probe the density and temperature stratification which characterises the cooling zones behind the shock fronts along the jet. From the line ratios we derive the variation of the visual extinction along the flow, the electron density and temperature (n e and T e ), the hydrogen ionisation fraction x e , and the total density n H in the emission region of different lines. The knowledge of such parameters is essential for testing existing jet models and for planning follow-up high-angular resolution observations. From the diagnostics of optical forbidden lines we find, on average, that in the examined jets, in the region of optical emission, n e varies between 50 cm −3 and 3 × 10 3 cm −3 , x e ranges between 0.03 and 0.6, and the electron temperature T e is ∼1.3 × 10 4 K in the HH 111 and HH 34 jets, while it appears to be higher (1.8 × 10 4 K on average) in the other examined jets. The electron density and temperature derived from [Fe ii] lines, turn out to be, respectively, higher and lower in comparison to those determined from optical lines, in agreement with the fact that the [Fe ii] lines arise in the more compressed gas located further from the shock front. An even denser component in the jets, with values of n e up to 10 6 cm −3 is detected using the ratio of calcium lines. The derived physical parameters are used to estimate the depletion onto dust grains of calcium and iron with respect to solar abundances. This turns out to be quite substantial, being between 70% and 0% for Ca and ∼90% for Fe. This leads us to suggest that the weak shocks present in the beams are not capable of completely destroying the ambient dust grains, confirming previous theoretical studies. We then derive the mass flux rates,Ṁ jet , in the flows using two independent methods. Taking into account the filling factor of the emitting gas,Ṁ jet is on average 5 × 10 −8 M yr −1 . The associated linear momentum fluxes (Ṗ jet = v jetṀjet ) are higher than, or of the same order as, those measured in the coaxial molecular flows, where present, suggesting that the flows are jet driven. Finally, we discuss differences between jets in our sample. In general, we find that higher ionisation and electron temperatures are associated with less dense jets. The comparison suggests that the shock mechanism exciting the knots along the flows has the same efficiency in all the examined objects, and the observed differences are consistent with the different densities, and hence cooling rates, found in the vario...

Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet's birth, and evolution. ARIEL was conceived to observe a large number (~1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25-7.8 μm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and welldefined planet sample within its 4-year mission lifetime. Transit, eclipse and phasecurve spectroscopy methods, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allow us to measure atmospheric signals from the planet at levels of 10-100 part per million (ppm) relative to the star and, given the bright nature of targets, also allows more sophisticated techniques, such as eclipse mapping, to give a deeper insight into the nature of the atmosphere. These types of observations require a stable payload and satellite platform with broad, instantaneous wavelength coverage to detect many molecular species, probe the thermal structure, identify clouds and monitor the stellar activity. The wavelength range proposed covers all the expected major atmospheric gases from e.g. H 2 O, CO 2 , CH 4 NH 3 , HCN, H 2 S through to the more exotic metallic compounds, such as TiO, VO, and condensed species. Simulations of ARIEL performance in conducting exoplanet surveys have been performedusing conservative estimates of mission performance and a