Key Words: Jets;Herbig-Haro Objects;Young Stars and Protostellar Objects; MHD Winds;Outflows;Forbidden (original) (raw)

Structure and Physical Conditions in MHD Jets from Young Stars

Journal of the Korean Astronomical Society

We have constructed the foundations to a series of theoretical diagnostic methods to probe the jet phenomenon in young stars as observed at various optical forbidden lines. We calculate and model in a self-consistent manner the physical and radiative processes which arise within an inner disk-wind driven magnetocentrifugally from the circumstellar accretion disk of a young sun-like star. Comparing with real data taken at high angular resolution, our approach will provide the basis of systematic diagnostics for jets and their related young stellar objects, to attest the emission mechanisms of such phenomena. This work can help bring first-principle theoretical predictions to confront actual multi-wavelength observations, and will bridge the link between many very sophiscated numerical simulations and observational data. Analysis methods discussed here are immediately applicable to new high-resolution data obtained with HST and Adaptic Optics.

The Origin of Jets from Young Stars: MHD Disk Wind Models Confronted to Observations

Astrophysics and Space Science, 2000

We discuss in this contribution constraints on the origin of mass-loss from young stars brought by recent observations at high angular resolution (0.1 = 14 AU) of the inner regions of winds from T Tauri stars. Jet widths and collimation scales, the large extent of the velocity profile as well as the detection of rotation signatures agree with predictions from magneto-centrifugal disk wind ejection models. However dynamically cold disk wind solutions predict too large terminal velocities and too low jet densities and ionisation fractions, suggesting that thermal gradients (originating in an accretion heated disk corona for example) may play an important role in accelerating the flow.

Jets and bipolar outflows from young stars: Theory and observational tests

Protostars and Planets V, 2007

Jets and outflows from young stars are an integral part of the star formation process. A particular framework for explaining these phenomena is the X-wind theory. Since PPIV, we have made good progress in modeling the jet phenomena and their associated fundamental physical processes, in both deeply embedded Class I objects and more revealed classical T Tauri stars. In particular, we have improved the treatment of the atomic physics and chemistry for modeling jet emission, including reaction rates and interaction cross-sections, as well as ambipolar diffusion between ions and neutrals. We have broadened the original X-wind picture to include the winds driven magnetocentrifugally from the innermost disk regions. We have carried numerical simulations that follow the wind evolution from the launching surface to large, observable distances. The interaction between the magnetocentrifugal wind and a realistic ambient medium was also investigated. It allows us to generalize the shell model of Shu et al. (1991) to unify the the jet-driven and wind-driven scenarios for molecular outflow production. In addition, we review related theoretical works on jets and outflows from young stars, and make connection between theory and recent observations, particularly those from HST/STIS, VLA and SMA.

Young stellar object jet models: From theory to synthetic observations

Astronomy & Astrophysics, 2014

Context. Astronomical observations, analytical solutions and numerical simulations have provided the building blocks to formulate the current theory of young stellar object jets. Although each approach has made great progress independently, it is only during the last decade that significant efforts are being made to bring the separate pieces together.

Radio observations of jets from massive young stars

Proceedings of the International Astronomical Union, 2010

The formation of low mass stars takes place with the assistance of an accretion disk that transports gas and dust from the envelope of the system to the star, and a jet that removes angular momentum and allows accretion to proceed. In the radio, these ionized jets can be studied very close to the star via the thermal (free-free) emission they produce and at larger scales by the molecular outflows that result from their interaction with the surrounding medium. Is the same disk-jet process responsible for the formation of massive stars? I will review recent evidence for the presence of collimated jets and accretion disks in association with forming massive stars. The jets in massive protostars have large velocities that could produce a synchrotron component and I discuss the evidence for the presence of this non-thermal process in the jet associated with the HH 80-81 system.

An Infrared Jet from a High-Mass Young Star

The Astrophysical Journal, 2001

Jets are a direct consequence of accretion in the inner regions of circumstellar disks. They trace the structure of the accretion disk and, indirectly, the star formation mechanism. Here we report on the discovery of a nearinfrared jet from a young B1 star, one of the most luminous young stars known to exhibit such a structure. The jet is seen in LЈ images of IRAS 18556ϩ0136 in the G35.2Ϫ0.74N region; a significant fraction of the emission is due to Bra line emission. At shorter wavelengths, the jet is obscured: the colors of the bipolar nebula are consistent with 25 mag visual extinctions to the near lobe and 40 mag to the far lobe. A previously detected radio continuum source and an elongated clump of OH maser spots coincide with a break in the jet. This is interpreted as the location of the circumstellar disk surrounding the embedded young star. A lower limit of ∼170 mag is derived for the extinction to the exciting source. This provides an estimate of 0.15 M for the mass , of the circumstellar disk within ∼1500 AU of the central source. Emission knots on either side of a second, weaker radio continuum source offset ∼3Љ from the jet source suggests the presence of a second outflow source in the region. The lack of known jets in high-mass protostars, in contrast to their prevalence among low-mass systems, points to a difference in the star formation process, e.g., mergers. The jet from this B1 star suggests that the mechanisms that form low-mass stars can operate up to at least 10 M stars. , Subject headings: accretion, accretion disks -H ii regions -ISM: individual (G35.2Ϫ0.74N) -ISM: jets and outflows -stars: formation

Outflows from Young Stars: Theory and Observation

Symposium - International Astronomical Union, 2004

Recent observations have revealed that young stellar objects are associated with jet-like structures and Herbig-Haro objects emitting at wavelengths ranging from optical lines to radio continua. These phenomena are similar in morphologies, and have mostly comparable energetics, dynamics, and kinematics. Probing such phenomena observed at various wavelengths with self-consistent physical and radiative processes arising within an inner disk-wind driven magnetocentrifugally from the circumstellar accretion disk is a challenge for confronting theory and observation of outflows. How such early outflow phase may play a role in forming planetary materials may help solve puzzles posed by meteorites. We will discuss the relevant observations, theoretical foundations for modelling approaches, magnetic structures and dynamical effects, and the connection to the early solar system.

Observations of Jets and Outflows from Young Stars

This review concentrates on observations of outflows from young stars during the last 6 years. Recent developments include detections of an increasing number of Herbig-Haro flows at X-rays and UV wavelengths, high resolution studies of irradiated jets with HST, wide-field imaging of parsec-scale outflows with ground-based CCDs and near-IR imagers, complete surveys of visual and near-IR emission from shocks in the vicinity of entire molecular clouds with wide-field imagers, far infrared studies with ISO and the Spitzer Space Telescope, and high angular sub-mm, mm, and cm wavelength aperture synthesis array data-cubes showing both the spatial and velocity structure of jets and outflows.

Emission lines from rotating proto-stellar jets with variable velocity profiles

Astronomy and Astrophysics, 2006

Using the Yguazú-a three-dimensional hydrodynamic code, we have computed a set of numerical simulations of heavy, supersonic, radiatively cooling jets including variabilities in both the ejection direction (precession) and the jet velocity (intermittence). In order to investigate the effects of jet rotation on the shape of the line profiles, we also introduce an initial toroidal rotation velocity profile, in agreement with some recent observational evidence found in jets from T Tauri stars which seems to support the presence of a rotation velocity pattern inside the jet beam, near the jet production region. Since the Yguazúa code includes an atomic/ionic network, we are able to compute the emission coefficients for several emission lines, and we generate line profiles for the Hα, [O I]λ6300, [S II]λ6716 and [N II]λ6548 lines. Using initial parameters that are suitable for the DG Tau microjet, we show that the computed radial velocity shift for the medium-velocity component of the line profile as a function of distance from the jet axis is strikingly similar for rotating and non-rotating jet models. These findings lead us to put forward some caveats on the interpretation of the observed radial velocity distribution from a few outflows from young stellar objects, and we claim that these data should not be directly used as a doubtless confirmation of the magnetocentrifugal wind acceleration models.

A Unified Model for Bipolar Outflows from Young Stars: Apparent Magnetic Jet Acceleration

The astrophysical journal, 2023

We develop a unified model for molecular outflows in star formation. The model incorporates essential features expected of the primary wind, which is thought to be driven magnetocentrifugally from close to the central stellar object, and the ambient core material shaped by anisotropic magnetic support. The primary wind is modeled as a toroidally magnetized fast outflow moving radially away from the origin, with an angle-dependent density distribution: a dense axial jet surrounded by a more tenuous wide-angle wind, as expected in the X-wind model. If dynamically significant magnetic fields are present, the star-forming core will settle faster along the field lines than across, forming a toroid-like structure. We approximate the structure with a singular isothermal toroid whose density distribution can be obtained analytically. The interaction of the laterally stratified wind and the ambient toroid is followed using the Zeus2D magnetohydrodynamics (MHD) code. We find that the lobes produced by the interaction resemble many systematics observed in molecular outflows from very young stars, ranging from Class 0 to I sources. In particular, both the dense axial jet and the wide-angle wind participate in the wind-ambient interaction. In our model, the jet-and wind-driven pictures of molecular outflows are unified. We discuss the observational implications of the unified picture, including the possibility of detecting the primary jet /wind directly.