ABSTRACT
Synchrotron radio emission from non-relativistic jets powered by massive protostars has been reported, indicating the presence of relativistic electrons and magnetic fields of strength ...∼0.3–5 mG. We study diffusive shock acceleration and magnetic field amplification in protostellar jets with speeds between 300 and 1500 km s−1. We show that the magnetic field in the synchrotron emitter can be amplified by the non-resonant hybrid (Bell) instability excited by the cosmic ray streaming. By combining the synchrotron data with basic theory of Bell instability we estimate the magnetic field in the synchrotron emitter and the maximum energy of protons. Protons can achieve maximum energies in the range 0.04–0.65 TeV and emit γ rays in their interaction with matter fields. We predict detectable levels of γ rays in IRAS 16547−5247 and IRAS 16848−4603. The γ ray flux can be significantly enhanced by the gas mixing due to Rayleigh–Taylor instability. The detection of this radiation by the Fermi satellite in the GeV domain and the forthcoming Cherenkov Telescope Array at higher energies may open a new window to study the formation of massive stars, as well as diffusive acceleration and magnetic field amplification in shocks with velocities of about 1000 km s−1.
Context.
Studies of Class 0 objects allow to characterize the dynamical processes taking place at the onset of the star formation process and to determine the physical mechanisms responsible for the ...outcome of the collapse. Observations of dense gas tracers allow for the characterization of key kinematics of the gas that are directly involved in the star formation process, such as infall, outflow, and rotation.
Aims.
This work is aimed at investigating the molecular line velocity profiles of the Class 0 protostellar object B335 and attempts to place constraints on the infall motions happening in the circumstellar gas of the object.
Methods.
We present observations of C
17
O (1–0), C
18
O (1–0), and
12
CO (2–1) transitions along with an analysis of spectral profiles at envelope radii between 100 and 860 au.
Results.
C
17
O emission presents a double-peaked line profile distributed in a complex velocity field. Both peaks present an offset of 0.2–1 km s
−1
from the systemic velocity of the source in the probed area. The optical depth of the C
17
O emission has been estimated and found to be less than 1, suggesting that the two velocity peaks trace two distinct velocity components of the gas in the inner envelope.
Conclusions.
After discarding possible motions that could produce the complex velocity pattern, such as rotation and outflow, we conclude that infall motions are responsible for producing the velocity field. Because inside-out symmetric collapse cannot explain those observed profiles, it is suggested that these are produced by non-isotropic accretion from the envelope into the central source along the outflow cavity walls.
Recent observations have suggested that circumstellar disks may commonly form around young stellar objects. Although the formation of circumstellar disks can be a natural result of the conservation ...of angular momentum in the parent cloud, theoretical studies instead show disk formation to be difficult from dense molecular cores magnetized to a realistic level, owing to efficient magnetic braking that transports a large fraction of the angular momentum away from the circumstellar region. We review recent progress in the formation and early evolution of disks around young stellar objects of both low-mass and high-mass, with an emphasis on mechanisms that may bridge the gap between observation and theory, including non-ideal MHD effects and asymmetric perturbations in the collapsing core (e.g., magnetic field misalignment and turbulence). We also address the associated processes of outflow launching and the formation of multiple systems, and discuss possible implications in properties of protoplanetary disks.
In recent years, exciting developments have taken place in the identification of the role of cosmic rays in star-forming environments. Observations from radio to infrared wavelengths and theoretical ...modelling have shown that low-energy cosmic rays (
<
1
TeV
) play a fundamental role in shaping the chemical richness of the interstellar medium, determining the dynamical evolution of molecular clouds. In this review we summarise in a coherent picture the main results obtained by observations and by theoretical models of propagation and generation of cosmic rays, from the smallest scales of protostars and circumstellar discs, to young stellar clusters, up to Galactic and extragalactic scales. We also discuss the new fields that will be explored in the near future thanks to new generation instruments, such as: CTA, for the
γ
-ray emission from high-mass protostars; SKA and precursors, for the synchrotron emission at different scales; and ELT/HIRES, JWST, and ARIEL, for the impact of cosmic rays on exoplanetary atmospheres and habitability.
We present polarimetric data of CW Tau and DG Tau, two well-known Class II disk/jet systems, obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) at 870 m and 0 2 average resolution. ...In CW Tau, the total and polarized emission are both smooth and symmetric, with polarization angles almost parallel to the minor axis of the projected disk. In contrast, DG Tau displays a structured polarized emission, with an elongated brighter region in the disk's near side and a belt-like feature beyond about 0 3 from the source. At the same time, the total intensity is spatially smooth, with no features. The polarization pattern, almost parallel to the minor axis in the inner region, becomes azimuthal in the outer belt, possibly because of a drop in optical depth. The polarization fraction has average values of 1.2% in CW Tau and 0.4% in DG Tau. Our results are consistent with polarization from self-scattering of the dust thermal emission. In this hypothesis, the maximum size of the grains contributing to polarization is in the range 100-150 m for CW Tau and 50-70 m for DG Tau. The polarization maps combined with dust opacity estimates indicate that these grains are distributed in a geometrically thin layer in CW Tau, representing a settling in the disk midplane. Meanwhile, such settling is not yet apparent for DG Tau. These results advocate polarization studies as a fundamental complement to total emission observations, in investigations of the structure and the evolution of protoplanetary disks.
Rigorous Theory for Secondary Cosmic-Ray Ionization Ivlev, Alexei V.; Silsbee, Kedron; Padovani, Marco ...
Astrophysical journal/The Astrophysical journal,
03/2021, Letnik:
909, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract
The energy spectrum of electrons produced in molecular gas by interstellar cosmic rays (CRs) is rigorously calculated as a function of gas column density
N
traversed by the CRs. This allows ...us to accurately compute the local value of the secondary ionization rate of molecular hydrogen,
, as a function of the local primary ionization rate,
ζ
p
(
N
). The ratio
increases monotonically with
N
, and can considerably exceed the value of ≈0.67 commonly adopted in the literature. For sufficiently soft interstellar spectra, the dependence
versus
N
is practically insensitive to their particular shape and thus is a general characteristic of the secondary CR ionization in dense gas.
Abstract
A full understanding of high-mass star formation requires the study of one of the most elusive components of the energy balance in the interstellar medium: magnetic fields. We report Atacama ...Large Millimeter/submillimeter Array (ALMA) 1.2 mm, high-resolution (700 au) dust polarization and molecular line observations of the rotating hot molecular core embedded in the high-mass star-forming region IRAS 18089−1732. The dust continuum emission and magnetic field morphology present spiral-like features resembling a whirlpool. The velocity field traced by the H
13
CO
+
(
J
= 3−2) transition line reveals a complex structure with spiral filaments that are likely infalling and rotating, dragging the field with them. We have modeled the magnetic field and find that the best model corresponds to a weakly magnetized core with a mass-to-magnetic-flux ratio (
λ
) of 8.38. The modeled magnetic field is dominated by a poloidal component, but with an important contribution from the toroidal component that has a magnitude of 30% of the poloidal component. Using the Davis–Chandrasekhar–Fermi method, we estimate a magnetic field strength of 3.5 mG. At the spatial scales accessible to ALMA, an analysis of the energy balance of the system indicates that gravity overwhelms turbulence, rotation, and the magnetic field. We show that high-mass star formation can occur in weakly magnetized environments, with gravity taking the dominant role.
Faraday tomography of radio polarimetric data below 200 MHz from the LOw Frequency ARray (LOFAR) has been providing new perspectives on the diffuse and magnetized interstellar medium (ISM). One ...aspect of particular interest is the unexpected discovery of Faraday-rotated synchrotron polarization associated with structures of neutral gas, as traced by atomic hydrogen (HI) and dust. Here, we present the first in-depth numerical study of these LOFAR results. We produced and analyzed comprehensive synthetic observations of low-frequency synchrotron polarization from magneto-hydrodynamical (MHD) simulations of colliding super shells in the multiphase ISM from the literature. Using an analytical approach to derive the ionization state of the multiphase gas, we defined five distinct gas phases over more than four orders of magnitude in gas temperature and density, ranging from hot, and warm fully ionized gas to a cold neutral medium. We focused on establishing the contribution of each gas phase to synthetic observations of both rotation measure and synchrotron polarized intensity below 200 MHz. We also investigated the link between the latter and synthetic observations of optically thin HI gas. We find that it is not only the fully ionized gas, but also the warm partially ionized and neutral phases that strongly contribute to the total rotation measure and polarized intensity. However, the contribution of each phase to the observables strongly depends on the choice of the integration axis and the orientation of the mean magnetic field with respect to the shell collision axis. A strong correlation between the HI synthetic data and synchrotron polarized intensity, reminiscent of the LOFAR results, is obtained with lines of sight perpendicular to the mean magnetic field direction. Our study suggests that multiphase modeling of MHD processes is needed in order to interpret observations of the radio sky at low frequencies. This work is a first step toward understanding the complexity of low-frequency synchrotron emission that will be soon revolutionized thanks to large-scale surveys with LOFAR and the Square Kilometre Array.
Submillimeter dust polarization measurements of a sample of 50 star-forming regions, observed with the Submillimeter Array (SMA) and the Caltech Submillimeter Observatory (CSO) covering parsec-scale ...clouds to milliparsec-scale cores, are analyzed in order to quantify the magnetic field importance. The magnetic field misalignment delta - the local angle between magnetic field and dust emission gradient - is found to be a prime observable, revealing distinct distributions for sources where the magnetic field is preferentially aligned with or perpendicular to the source minor axis. Source-averaged misalignment angles absolute value of delta fall into systematically different ranges, reflecting the different source-magnetic field configurations. Possible bimodal absolute value of delta distributions are found for the separate SMA and CSO samples. Across the sample, a transition from magnetically super-critical to sub-critical is observed with growing misalignment absolute value of delta .
The contribution of the magnetic field to the formation of high-mass stars is poorly understood. We report the high angular resolution (∼0.″3, 870 au) map of the magnetic field projected on the plane ...of the sky (BPOS) toward the high-mass star-forming region G333.46−0.16 (G333), obtained with the Atacama Large Millimeter/submillimeter Array at 1.2 mm as part of the Magnetic fields in Massive star-forming Regions survey. The BPOS morphology found in this region is consistent with a canonical “hourglass” with an embedded flattened envelope in a perpendicular direction, which suggests a dynamically important field. This region is fragmented into two protostars that appear to be gravitationally bound in a stable binary system with a separation of ∼1740 au. Interestingly, by analyzing H13CO+ (J = 3–2) line emission, we find no velocity gradient over the extent of the continuum, which is consistent with a strong field. We model the BPOS, obtaining a marginally supercritical mass-to-flux ratio of 1.43, suggesting an initially strongly magnetized environment. Based on the Davis–Chandrasekhar–Fermi method, the magnetic field strength toward G333 is estimated to be 5.7 mG. The absence of strong rotation and outflows toward the central region of G333 suggests strong magnetic braking, consistent with a highly magnetized environment. Our study shows that despite being a strong regulator, the magnetic energy fails to prevent the process of fragmentation, as revealed by the formation of the two protostars in the central region.
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