scholarly journals Magnetic Effects in Clouds

1992 ◽  
Vol 45 (4) ◽  
pp. 531 ◽  
Author(s):  
L Mestel
Keyword(s):  
Flux Ratio ◽  
High Mass ◽  
Interstellar Gas ◽  
Magnetic Forces ◽  
T Tauri ◽  
Low Degree ◽  
Magnetic Buoyancy ◽  
Gas Cloud ◽  
Flux Leakage ◽  
Maxwell Stresses

A realistic study of the structure and evolution of an interstellar gas cloud must take cognisance of the flux from the galactic magnetic field threading the cloud. If the non-dimensional mass-to-flux ratio is below a critical value, the forces exerted by the locally distorted field can balance gravity in the two trans-field dimensions, while Alfvenic turbulent motions yield support along the field. A super-critical cloud, collapsing with its flux virtually frozen in, may fragment into sub-condensations following spontaneous flattening along the field. Within a sub-critical molecular cloud, the very low degree of ionisation allows the magnetic forces to redistribute flux through the cloud, so that locally denser regions may become super-critical and condense out of the cloud. The Maxwell stresses also transport angular momentum efficiently from a slowly contracting condensation to the surroundings. If flux leakage remains slow throughout all the pre-opaque phases, the magnetic forces and the associated turbulent motions may shift the ultimate mass spectrum towards the high mass end. Most of the remnant flux may be lost by magnetic buoyancy during the pre-main sequence epoch, so possibly supplying a power source for the T Tauri phenomenon.

scholarly journals ALMA resolves the hourglass magnetic field in G31.41+0.31

2019 ◽  
Vol 630 ◽  
pp. A54 ◽  
Author(s):  
M. T. Beltrán ◽  
M. Padovani ◽  
J. M. Girart ◽  
D. Galli ◽  
R. Cesaroni ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Velocity Gradient ◽  
Flux Ratio ◽  
High Mass ◽  
Continuum Emission ◽  
Mass Star ◽  
Star Forming ◽  
The Core ◽  
Molecular Core ◽  
The Magnetic Field

Context. Submillimeter Array (SMA) 870 μm polarization observations of the hot molecular core G31.41+0.31 revealed one of the clearest examples up to date of an hourglass-shaped magnetic field morphology in a high-mass star-forming region. Aims. To better establish the role that the magnetic field plays in the collapse of G31.41+0.31, we carried out Atacama Large Millimeter/ submillimeter Array (ALMA) observations of the polarized dust continuum emission at 1.3 mm with an angular resolution four times higher than that of the previous (sub)millimeter observations to achieve an unprecedented image of the magnetic field morphology. Methods. We used ALMA to perform full polarization observations at 233 GHz (Band 6). The resulting synthesized beam is 0′′.28×0′′.20 which, at the distance of the source, corresponds to a spatial resolution of ~875 au. Results. The observations resolve the structure of the magnetic field in G31.41+0.31 and allow us to study the field in detail. The polarized emission in the Main core of G31.41+0.41is successfully fit with a semi-analytical magnetostatic model of a toroid supported by magnetic fields. The best fit model suggests that the magnetic field is well represented by a poloidal field with a possible contribution of a toroidal component of ~10% of the poloidal component, oriented southeast to northwest at approximately −44° and with an inclination of approximately −45°. The magnetic field is oriented perpendicular to the northeast to southwest velocity gradient detected in this core on scales from 103 to 104 au. This supports the hypothesis that the velocity gradient is due to rotation of the core and suggests that such a rotation has little effect on the magnetic field. The strength of the magnetic field estimated in the central region of the core with the Davis–Chandrasekhar-Fermi method is ~8–13 mG and implies that the mass-to-flux ratio in this region is slightly supercritical. Conclusions. The magnetic field in G31.41+0.31 maintains an hourglass-shaped morphology down to scales of <1000 au. Despite the magnetic field being important in G31.41+0.31, it is not enough to prevent fragmentation and collapse of the core, as demonstrated by the presence of at least four sources embedded in the center of the core.

scholarly journals The accretion rates and mechanisms of Herbig Ae/Be stars

2020 ◽  
Vol 493 (1) ◽  
pp. 234-249 ◽  
Author(s):  
C Wichittanakom ◽  
R D Oudmaijer ◽  
J R Fairlamb ◽  
I Mendigutía ◽  
M Vioque ◽  
...  
Keyword(s):  
Accretion Rate ◽  
High Mass ◽  
Be Stars ◽  
Alpha Emission ◽  
T Tauri ◽  
Accretion Model ◽  
Accretion Rates ◽  
Theoretical Considerations ◽  
Gaia Dr2 ◽  
Magnetospheric Accretion

ABSTRACT This work presents a spectroscopic study of 163 Herbig Ae/Be stars. Amongst these, we present new data for 30 objects. Stellar parameters such as temperature, reddening, mass, luminosity, and age are homogeneously determined. Mass accretion rates are determined from $\rm H\alpha$ emission line measurements. Our data is complemented with the X-Shooter sample from previous studies and we update results using Gaia DR2 parallaxes giving a total of 78 objects with homogeneously determined stellar parameters and mass accretion rates. In addition, mass accretion rates of an additional 85 HAeBes are determined. We confirm previous findings that the mass accretion rate increases as a function of stellar mass, and the existence of a different slope for lower and higher mass stars, respectively. The mass where the slope changes is determined tobe $3.98^{+1.37}_{-0.94}\, \rm M_{\odot }$. We discuss this break in the context of different modes of disc accretion for low- and high-mass stars. Because of their similarities with T Tauri stars, we identify the accretion mechanism for the late-type Herbig stars with the Magnetospheric Accretion. The possibilities for the earlier-type stars are still open, we suggest the Boundary Layer accretion model may be a viable alternative. Finally, we investigated themass accretion–age relationship. Even using the superior Gaia based data, it proved hard to select a large enough sub-sample to remove the mass dependence in this relationship. Yet, it would appear that the mass accretion does decline with age as expected from basic theoretical considerations.

The Main Sequence Evolution of Inhomogeneous Stars

1982 ◽  
Vol 4 (4) ◽  
pp. 396-400 ◽  
Author(s):  
J. Lattanzio
Keyword(s):  
Gravitational Collapse ◽  
Heavy Element ◽  
Main Sequence ◽  
Element Abundance ◽  
Sequence Evolution ◽  
Interstellar Gas ◽  
Convective Mixing ◽  
The Core ◽  
Gas Cloud

Duley (1974) has shown that, at the temperatures usually associated with interstellar gas clouds, we would expect CNO grains to be present. During gravitational collapse these grains migrate to the centre of the gas cloud, leading to an enhancement of the heavy-element abundance in the core (Prentice 1976, 1978). It was Krautschneider (1977) who verified such a scenario, by considering the dynamical collapse of gas and grain clouds. If such an initial radial abundance inhomogeneity existed, Prentice (1976a) showed that this configuration may well survive the later convective mixing phase and thus approach the zero-age main-sequence (ZAMS) with a small (-v 3% by mass) metal enhanced core.

scholarly journals Star Formation in Molecular Cloud Cores

1987 ◽  
Vol 115 ◽  
pp. 417-434 ◽  
Author(s):  
Frank H. Shu ◽  
Susana Lizano ◽  
Fred C. Adams
Keyword(s):  
Star Formation ◽  
Molecular Cloud ◽  
Theoretical Models ◽  
High Mass ◽  
Spectral Energy ◽  
Dark Cloud ◽  
T Tauri ◽  
Analogous Process ◽  
Static Contraction ◽  
Cloud Cores

The problem of gravitational collapse and star formation is entirely different when the ratio of the mass of a molecular cloud Mcl to its magnetic flux Φ is high than when it is low. Magnetically-diluted overall collapse of a large dense core and the formation of an OB association or a bound cluster are the likely outcomes in the former case; quasi-static contraction of many small cores and their ultimate collapse to form a T association, in the latter. In our picture, the birth of a T association in a dark cloud like Taurus proceeds by ambipolar diffusion on a time-scale of ∼ 107 years. As magnetic and turbulent support is gradually lost from a small condensing core, it approaches a state resembling a slowly rotating singular isothermal sphere which, when it passes the brink of instability, collapses from “inside-out,” building up a central protostar and nebular disk. The emergent spectral energy distributions of theoretical models in this stage of protostellar evolution resemble closely those of recently found sources with steep spectra in the infrared. The protostellar phase is ended by the reversal of the infall by an intense stellar wind, whose ultimate source of energy derived from the differential rotation of the star. We argue that the initial breakout is likely to occur along the rotational poles, leading to collimated jets and bipolar outflows. The stellar jet eventually widens to sweep out gas in nearly all 4π steradian, revealing at the center a T Tauri star and a remnant nebular disk. We give rough scaling relations which must apply if an analogous process is to succeed for producing high mass stars.

scholarly journals The collapse of a fast rotating interstellar gas cloud

1983 ◽  
Vol 203 (2) ◽  
pp. 491-515 ◽  
Author(s):  
S. Narita ◽  
D. McNally ◽  
G. L. Pearce ◽  
S. A. Sorensen
Keyword(s):  
Interstellar Gas ◽  
Gas Cloud ◽  
Fast Rotating

scholarly journals Planet-induced spirals in the circumbinary disk of GG Tauri A

2020 ◽  
Vol 635 ◽  
pp. L9 ◽  
Author(s):  
N. T. Phuong ◽  
A. Dutrey ◽  
E. Di Folco ◽  
S. Guilloteau ◽  
A. Pierens ◽  
...  
Keyword(s):  
Binary Stars ◽  
Hot Spot ◽  
Density Wave ◽  
Indirect Evidence ◽  
T Tauri Stars ◽  
High Mass ◽  
Linear Perturbation ◽  
Stable Configuration ◽  
High Angular Resolution ◽  
T Tauri

Context. ALMA high angular resolution observations of the dust and CO emission have already revealed signatures of protoplanets embedded in protoplanetary disks. These detections are around single T Tauri stars, while exoplanet surveys reveal that planets can also form in binary (or multiple) systems, either in circumstellar or circumbinary orbits. Aims. We searched for indirect evidence for planet formation in the multiple system GG Tau A, which harbors the most massive circumbinary disk among T Tauri stars. Methods. We performed CO(2–1) ALMA Cycle 6 observations of GG Tau A at 0.3″ resolution. The images confirm the “hot spot” detected at higher frequencies, but also reveal prominent spiral-like features. We modeled these features using the analytic prescription for the linear perturbation regime induced by low-mass planets. Results. The brightest spiral is well reproduced by a density wave excited by a protoplanet (GG Tau Ac) at the hot-spot location (290 au), just outside the dust ring. The absence of a clear gap (in gas or dust) at the planet location implies that its mass is significantly lower than that of Jupiter, i.e., of about the mass of Neptune or lower. Furthermore, other prominent (trailing) spiral patterns can be represented by adding one (or more) planet(s) at larger orbital radii, with the most obvious candidate located near the 2:1 mean-motion resonance with GG Tau Ac. Conclusions. The (proto-)planet GG Tau Ac appears to externally confine the ring in a stable configuration, explaining its high mass. Our results also suggest that planets similar in mass to Neptune may form in dense circumbinary disks orbiting (wide) binary stars. In the GG Tau case, orbital resonances appear to play an important role in shaping this multiple circumbinary planet system.

scholarly journals Isotopic imprint of the Solar system encounter with interstellar gas cloud around 660 BC (2610 BP)

2019 ◽  
Vol 1400 ◽  
pp. 022034
Author(s):  
A K Pavlov ◽  
A V Blinov ◽  
D A Frolov ◽  
A N Konstantinov ◽  
I V Koudriavtsev ◽  
...  
Keyword(s):  
Solar System ◽  
Interstellar Gas ◽  
Gas Cloud

scholarly journals The nature of stellar winds in the star-disk interaction

2007 ◽  
Vol 3 (S243) ◽  
pp. 299-306 ◽  
Author(s):  
Sean Matt ◽  
Ralph E. Pudritz
Keyword(s):  
Angular Momentum ◽  
Mass Loss ◽  
Loss Rate ◽  
Mass Loss Rate ◽  
T Tauri Stars ◽  
High Mass ◽  
Stellar Winds ◽  
Accretion Process ◽  
T Tauri ◽  
High Mass Loss

AbstractStellar winds may be important for angular momentum transport from accreting T Tauri stars, but the nature of these winds is still not well-constrained. We present some simulation results for hypothetical, hot (∼ 106 K) coronal winds from T Tauri stars, and we calculate the expected emission properties. For the high mass loss rates required to solve the angular momentum problem, we find that the radiative losses will be much greater than can be powered by the accretion process. We place an upper limit to the mass loss rate from accretion-powered coronal winds of ∼ 10−11M yr−1. We conclude that accretion powered stellar winds are still a promising scenario for solving the stellar angular momentum problem, but the winds must be cool (e.g., 104 K) and thus are not driven by thermal pressure.

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