Jets and outflows from young stars

The formation of a new star from the gravitational collapse of a gas condensation is accompanied by ejection of part of the infalling matter through powerful winds. This ejection activity spans all the phases of the process and occurs for all stellar masses. Highly supersonic collimated jets flow with velocities up to 100-500 km/s and shine through the emission from internal shocks. The jets are often embedded in massive outflows, moving at 1-30 km/s, ambient gas swept-up by the jet bow-shocks. To complete the picture, slow wide-angle winds ejected from the disk carve a conic cavity around the base of the jet in the envelope still surrounding the newly-born star.

Figure 1. Jet from RY Tau imaged in emission lines with VLT/SPHERE (Garufi et al. 2019, A&A 628,68)

The beauty of these flows is outstanding, but they are not just amazing fireworks in the sky: actually they are believed to have a fundamental role in the formation process. In the most widely accepted scenario, the outflow ejected off the central regions by a gigantic magnetic sling carries away some angular momentum, thus allowing the material in the system to slow down its rotation rate and slide toward the young star, even for negligible disk viscosity. The mechanism allows to justify quantitatively the observed accretion rates.

In addition, the outflow can influence the disk chemistry by shielding the magneto-spheric activity and/or irradiating the disk with energetic photons and cosmic rays produced in the jet shocks. Note that the flows are launched from the inner 10-20 au of the disk, thus they may affect significantly the formation of planets.

The Arcetri group has been investigating the outflow phenomena since many years, through observations from UV to cm wavelengths taken with ground-based as well as space telescopes and interferometers (e.g. VLT, HST, Herschel, ALMA, IRAM-30m and IRAM-NOEMA, VLA). The group is involved in a number of observational campaigns and large programmes (LP) devoted to the study jets and outflows such as the IRAM-30m LP ASAI the IRAM-PdBI LP CALYPSO, the IRAM-NOEMA LP SOLIS, the ALMA LP FAUST the observational campaign ALMA-DOT, and the HST large programme ULLYSES and related archival programme ODYSSEUS, for the relationship with accretion in T Tauri stars.

The group participates to the national collaborations JEDI (Jet and Disks @ INAF), FRONTIERA, GENESIS-SKA, and has important partnerships with international teams based in Europe, US and Asia. We also collaborate to the development of new instruments as VLT/MAVIS and VLT/ERIS.

In the following the main lines of research followed by the group are described.

Jet diagnostics

The rich emission line spectra at UV, optical ,NIR and mm wavelengths allow us to derive many fundamental jet morphological, physical, and kinematical properties, e.g. the jet velocity and collimation, the gas density and temperature, the molecular column density and abundance, and finally the mass and momentum transported by the jet. The Arcetri group is a long-standing authority in such analyses, exploiting observations carried out from the ground with VLT, GEMINI and LBT and from space with HST and Herschel. In the radio domain the NOEMA and ALMA interferometers are used. In the future, JWST and ELT/HIRES will be instrumental in improving our knowledge of jet physics.

Jet rotation

If the excess angular momentum of the disk is removed by the jet, then jets will rotate. Astronomers of the Arcetri team discovered the first signatures of rotation observing with HST (fig. 2, left). This not only confirmed the magneto-centrifugal launch scenario, but provided us with an unique tool to infer the size of the launch region on the disk. Since then, rotation signatures have routinely been searched for in many jets, and they have recently been confirmed with ALMA observations (fig. 2, right).

Figure 2. Left: first signature of jet rotation from HST observations of the DG Tauri system (Bacciotti et al. 2002, ApJ 576,222). Right: ALMA observations of the SiO jet HH 212 (Lee et al. 2017, NatAs 1,152). In both cases the blue/red velocity shift indicates rotation around the axis.

Jets from Sun-like early stage Class 0 protostars

Our team leads observational campaigns to investigate the role of jets and outflows at the earliest stages of the formation of a Sun-like star, i.e. targeting Class 0 protostars of age 10e4 years. At these stages the star is very embedded and the jet emits in rotational transitions of molecules which probe the shock, such as CO, SO, and SiO, in the millimetre range. These campaigns are executed using millimetre interferometer such as IRAM-NOEMA and ALMA, which offer an unprecedented combination of angular resolution and sensitivity. An example is the large programme CALYPSO. At NIR wavelengths, the group participates to a survey conducted with HST, propedeutic to the studies with JWST.

These surveys demonstrate that outflows and jets are ubiquitous and appear very early at the protostellar stage, confirming that they have a key role in the star formation process. The mass ejection rate is much higher in the early phases than what is measured for pre-main sequence stars, which suggests that ejection decreases as the star evolves and accretion fades.

Jets from high mass protostars

Jets and outflows are associated with the formation process of stars of all masses. Our interest is focused on outflows from early-type stars and on their relationship with circumstellar disks. We study both the radio continuum emission from the thermal jets and the molecular emission from the outflows through species such as CO and its isotopologues and shock tracers like SiO. We also image the H_2 line and continuum emission from jets/outflows at near-IR wavelengths with 8-m class telescopes, using AO techniques when possible. One of the outcomes of our study is to establish the relationship between the accretion rate (through the disk) and the mass loss rate (through the jet/outflow) in newly formed massive stars.

An intense (maser) emission of the water molecule is often associated with the outflows from high-mass (M > 7 M_sun) forming .This water line is a powerful diagnostic tool, which allows us to measure the three-dimensional velocities of the outflowing gas. A recent water maser  survey performed by the Arcetri team (POETS) has revealed that the jets from massive protostars are also rotating, suggesting that the processes of mass accretion-ejetion are similar across the whole range of protostellar mass.

The chemistry of shocks in protostellar jets

High-velocity shocks caused by protostellar jets impacting the surrounding medium are perfect astrochemical laboratories. In the shocks the ice mantles covering dust grains is sputtered, releasing molecules into the gas phase. A large number of molecules, including deuterated molecules, O-, S-, N- and Si-bearing molecules, as well as the so-called interstellar complex organic molecules (iCOMs) are observed towards shocked regions. These are the building blocks of prebiotic chemistry. Our team is deeply involved in the study of shock chemistry, in the context of the IRAM-30m LP ASAI (Astrochemical Surveys At IRAM) and the IRAM-NOEMA LP SOLIS (Seeds Of LIfe in Space). The comparison between the observed iCOMs abundances and those predicted by astrochemical models can shed light on the formation routes of key complex organic molecules, such as acetaldehyde (CH₃CHO), and formamide (NH₂CHO).

Figure 3 The jet and outflow driven by the Sun-like protostar L1157-mm located in Cepheus at a distance of ~352 parsec. The precessing large scale molecular outflow extending out to ~0.2 pc distance from its driving source (left panels) is driven by a collimated high-velocity (~100 km/s) molecular jet recently revealed in the inner ~100au by IRAM-PdBI observations obtained within the CALYPSO large programme (right panel)