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Interferometry in the near-infrared: a very high angular resolution insight into the accretion/ejection process in young stars

Final Report Summary - ACCRETION/EJECTION (Interferometry in the near-infrared: a very high angular resolution insight into the accretion/ejection process in young stars)

Scientific Context: Young stars form via the gravitational collapse of denser regions present in the molecular clouds, called prestellar cores. The collapse of the slowly rotating core is characterized by the formation of a circumstellar disk from which the matter accretes onto the central star. In parallel, a part of the infalling gas is ejected by the system and forms collimated bipolar flows, so-called stellar jets (Shu et al. 1987). Jets are believed to play a key role in the overall star formation process, since they may remove excess angular momentum from the disk, thus allowing accretion to occur. However, the mechanism that generates jets and the nature of the connection between accretion and ejection is not clear.

Project Objectives: The goal of the proposed MC project was to investigate the accretion/ejection interplay in young stars and its effects on the structure and evolution of protoplanetary disks, hence on the formation of planets.

Methodologies: To this aim I analyzed high angular and spectral resolution observations of young stars driving jets taken over a wide spectral range, from ultraviolet to millimeter wavelengths by using both telescopes from the ground, such as the Very Large Telescope and the IRAM-30m, and from space, such as Herschel. These observations have been compared with the predictions by ejection, accretion, and disk models to derive stringent constraints on those phenomena.

Results: The main obtained results are summarized below:

1) The accretion/ejection interplay in young stars across all masses and ages
I have been studying the evolution of young sources from their infancy (~ thousands years) to the proto-planetary stage (a few million years).
The emission lines detected in the optical to far-infrared spectra are effective tracers of the accreting and outflowing gas, thus providing a very useful tool to investigate the gas physical conditions (velocity, density, temperature, ionization) and to infer a measurement of the accretion and ejection activity.
Interestingly we have found that:
(i) the line cooling and hence the efficiency of the ejection activity decreases as the source evolves and approaches its final mass (Podio et al. 2012);
(ii) mass ejection and mass accretion rates are in a fixed ratio dMjet/dMacc ~ 0.1 confirming the predictions by magneto-centrifugal models proposed to explain the jet launch (Ellerbroek et al. 2013, Podio et al. 2012, Ellerbroek et al. submitted to ApJ, Whelan et al., Maurri et al., submitted to A&A);
(iii) the jets associated with sources of all masses, from very low- (Giannini et al. 2013, Whelan et al. submitted to A&A), to solar- (Howard et al. 2013, Podio et al. 2012), and intermediate- mass stars (Ellerbroek et al. 2013, Ellerbroek et al., submitted to A&A) show similar properties, i.e. knotty structure, velocity, collimation, gas physical conditions. This points towards a universal mechanism for accretion and ejection in young stars.

2) Water in protoplanetary disks: understanding the origin of Earth's oceans
Water is key in the evolution of protoplanetary disks and the formation of comets and icy/water planets. While high excitation water lines originating in the hot inner disk have been detected in several T Tauri stars (e.g. Riviere-Marichalar et al. 2012), water vapor from the outer disk, where most of water ice reservoir is stored, was only recently reported in the closeby TTS TW Hya. I analyzed spectrally resolved Herschel/HIFI observations of the young TTS DG Tau in the ortho- and para- water ground-state transitions at 557, 1113 GHz. The lines show a narrow double-peaked profile, consistent with an origin in the outer disk. In contrast, CO and [C II] lines are dominated by emission from the envelope/outflow, which makes H2O lines a unique tracer of the disk of DG Tau. Disk modeling with the thermo-chemical code ProDiMo indicates that the strong UV field, due to the young age and strong accretion of DG Tau, irradiates a disk upper layer at 10-90 AU from the star, heating it up to temperatures of 600 K and producing the observed bright water lines. The models suggest a disk mass of 0.015-0.1 Msun, consistent with the estimated minimum mass of the solar nebula before planet formation, and a water reservoir of ~100-1000 Earth oceans in vapor, and ~100 times larger in the form of ice. Hence, this detection supports the scenario of ocean delivery on terrestrial planets by impact of icy bodies forming in the outer disk (Podio et al. 2013).

3) The chemical enrichment in protostellar shocks
The shocks produced when supersonic jets from young stars impacts on the high-density surrounding medium rapidly heat and compress the gas triggering several microscopic processes such as molecular dissociation, gas ionization, endothermic chemical reactions, sublimation of the icy mantles of dust grains and disruption of their cores. These processes produce a significant enhancement of the abundance of molecules which are evaporated or sputtered from dust grains, such as H2CO, CH3OH, NH3, SiO.
Of particular interest are molecular ions as they are key to understand the ionization and chemistry of shocks. As part of the CHESS and ASAI large programs I worked on an unbiased high-sensitivity survey of the protostellar shock B1 along the chemical rich outflow L1157. The observations obtained with the IRAM-30m and the Herschel/HIFI telescopes allowed searching for molecular ions emission in the shock. From the analysis of the detected emission lines I derived the gas physical conditions and fractional abundances of molecular ions. The latter are compared with estimates of steady-state abundances in the cloud and their evolution in the shock calculated with the chemical model Astrochem.
The performed analysis indicates that HCO+ and N2H+ are a fossil record of pre-shock gas in the outflow cavity, while HOCO+, SO+, and HCS+ are effective shock tracers and can be used to infer the amount of CO2 and sulphur-bearing species released from dust mantles in the shock. The observed HCS+ (and CS) abundance indicates that OCS should be one of the main sulphur carrier on grain mantles. These results have been published in Codella et al. 2013, Podio et al. (submitted to A&A).

The above summarized results are of fundamental importance for the understanding of star and planet formation. On the one hand, since the accretion/ejection mechanism is at work in a large number of astrophysical objects from active galactic nuclei to brown dwarfs, the obtained results will have repercussions in several fields of astrophysics. On the other hand, the search for water in proto-planetary disks is clearly a front line in astrophysics, as it may help to comprehend the formation of our solar system as well as of extra-solar planetary systems and the origin of life. Finally, the obtained results are crucial to fully exploit the potential of new observational facilities such as ALMA (Atacama Large Millimeter Array) and JWST (James Webb Space Telescope).