scholarly journals A Mechanism for Variability of Cometary Nebulae

1987 ◽  
Vol 115 ◽  
pp. 398-400
Author(s):  
G. M. Rudnitskij
Keyword(s):  
Tilt Angle ◽  
Stellar Wind ◽  
Large Scale ◽  
Central Star ◽  
Similar Model ◽  
Conical Cavity ◽  
Bipolar Outflow ◽  
Circumstellar Gas ◽  
Small Disc ◽  
Optical Structure

A model is suggested to explain the variability of the optical structure and the integral brightness of cometary nebulae (CN) occurring at timescales of several years and tens of years (Gyulbudaghian et al. 1977, Cohen et al. 1977, 1981, Magakyan 1981, Gyulbudaghian 1982). A CN is assumed to be a reflection nebula; it is a wall of a conical cavity in the circumstellar gas-and-dust torus illuminated by the central star (Cohen 1974). I explain the CN's variability by the presence of small tilted circumstellar disc of gas-and-dust, located inside the internal channel of the large circumstellar torus (see Figure 1). A similar model was put forward by Ward-Thompson et al. (1985) to account for a tilt angle of about 30° between the direction of short optical jets (stellar wind, channelled by the small disc) and the large-scale bipolar outflow (focused by the large torus) in the CN NGC 6729 associated with the star R CrA. Tilt angles of about 30° between optical and radio structures exist in CN NGC 2261 (Cantó et al. 1981) and GM 1-29 (Levreault 1984).

scholarly journals Circumstellar environment of 55 Cancri

2020 ◽  
Vol 633 ◽  
pp. A48 ◽  
Author(s):  
C. P. Folsom ◽  
D. Ó Fionnagáin ◽  
L. Fossati ◽  
A. A. Vidotto ◽  
C. Moutou ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Stellar Wind ◽  
Large Scale ◽  
Rotation Axis ◽  
Doppler Imaging ◽  
Magnetic Pole ◽  
Average Strength ◽  
Detailed Understanding ◽  
The Magnetic Field ◽  
Circumstellar Environment

Context. 55 Cancri hosts five known exoplanets, most notably the hot super-Earth 55 Cnc e, which is one of the hottest known transiting super-Earths. Aims. Because of the short orbital separation and host star brightness, 55 Cnc e provides one of the best opportunities for studying star-planet interactions (SPIs). We aim to understand possible SPIs in this system, which requires a detailed understanding of the stellar magnetic field and wind impinging on the planet. Methods. Using spectropolarimetric observations and Zeeman Doppler Imaging, we derived a map of the large-scale stellar magnetic field. We then simulated the stellar wind starting from the magnetic field map, using a 3D magneto-hydrodynamic model. Results. The map of the large-scale stellar magnetic field we derive has an average strength of 3.4 G. The field has a mostly dipolar geometry; the dipole is tilted by 90° with respect to the rotation axis and the dipolar strength is 5.8 G at the magnetic pole. The wind simulations based on this magnetic geometry lead us to conclude that 55 Cnc e orbits inside the Alfvén surface of the stellar wind, implying that effects from the planet on the wind can propagate back to the stellar surface and result in SPI.

scholarly journals The Dust in the Hydrogen-Poor Ejecta of Abell 30

1993 ◽  
Vol 155 ◽  
pp. 386-386
Author(s):  
J.P. Harrington ◽  
K.J. Borkowski ◽  
W.P. Blair ◽  
J. Bregman
Keyword(s):  
High Resolution ◽  
Amorphous Carbon ◽  
Ir Spectrum ◽  
Stellar Wind ◽  
Thermal Emission ◽  
Central Star ◽  
Near Ir ◽  
High Resolution Images ◽  
Small Grains ◽  
Ir Emission

High-resolution images in [O III] λ5007 of the hydrogen-poor knots of Abell 30 reveal comet-like structures which may be indicative of interaction with the stellar wind. In the near IR, new, higher-resolution, K-band images show an equatorial ring of hot dust that corresponds closely to optical knots 2 and 4 of Jacoby and Ford, while their polar knots 1 and 3 show no comparable IR emission. Both the thermal IR emission and the heavy internal extinction of the central star demands an extremely dusty ejecta. Greenstein showed that the UV extinction curve is fit by amorphous carbon. Our comprehensive dust models consider both the UV extinction and the IR emission from a population of carbon grains. The thermal emission from larger grains produces the far IR emission, while the stochastic heating of very small grains to high temperatures is essential to explain the near IR flux. We are able to reproduce the shape of the near IR spectrum with an a−3.0 distribution of grain radii which extends down to a minimum grain radius of 8 Å.

Rotationally Modulated Winds of O Stars

1999 ◽  
Vol 169 ◽  
pp. 3-10
Author(s):  
Alex W. Fullerton
Keyword(s):  
Case Studies ◽  
Stellar Wind ◽  
Large Scale ◽  
Stellar Surface ◽  
Rotational Period ◽  
Rotational Modulation ◽  
Open Issues

AbstractThe stellar wind diagnostics of some well-studied O stars exhibit cyclical variations with periods that are probably related to the rotational period of the underlying star. This rotational modulation is usually attributed to large scale, persistent structures in the wind, which are thought to be generated and maintained by photospheric processes that alter the emergence of the wind from different regions of the stellar surface. In this review, three case studies are used to illustrate the patterns of variability that are attributed to rotational modulation and to highlight some open issues connected with this hypothesis. The problems associated with establishing the occurrence of rotational modulation rigorously are also discussed.

scholarly journals Atomic Processes in Planetary Nebulae

2003 ◽  
Vol 209 ◽  
pp. 325-334
Author(s):  
Sultana N. Nahar
Keyword(s):  
Cross Sections ◽  
Large Scale ◽  
Transition Probabilities ◽  
Central Star ◽  
Dielectronic Recombination ◽  
Impact Excitation ◽  
Electron Impact Excitation ◽  
Theoretical Treatment ◽  
Atomic Processes ◽  
Ion Recombination

A hot central star illuminating the surrounding ionized H II region usually produces very rich atomic spectra resulting from basic atomic processes: photoionization, electron-ion recombination, bound-bound radiative transitions, and collisional excitation of ions. Precise diagnostics of nebular spectra depend on accurate atomic parameters for these processes. Latest developments in theoretical computations are described, especially under two international collaborations known as the Opacity Project (OP) and the Iron Project (IP), that have yielded accurate and large-scale data for photoionization cross sections, transition probabilities, and collision strengths for electron impact excitation of most astrophysically abundant ions. As an extension of the two projects, a self-consistent and unified theoretical treatment of photoionization and electron-ion recombination has been developed where both the radiative and the dielectronic recombination processes are considered in an unified manner. Results from the Ohio State atomic-astrophysics group, and from the OP and IP collaborations, are presented. A description of the electronic web-interactive database, TIPTOPBASE, with the OP and the IP data, and a compilation of recommended data for effective collision strengths, is given.

scholarly journals Wiggling Structures Along the NGC 1333 IRAS 2A Outflow

2012 ◽  
Vol 8 (S292) ◽  
pp. 60-60
Author(s):  
Cheng-Hung Tsai ◽  
Huei-Ru Chen ◽  
Chin-Fei Lee ◽  
Naomi Hirano ◽  
Hsien Shang
Keyword(s):  
Binary System ◽  
Accretion Disk ◽  
Mirror Symmetry ◽  
Large Scale ◽  
Angular Resolution ◽  
Orbital Motion ◽  
Lateral Displacement ◽  
Binary Separation ◽  
Bipolar Outflow ◽  
Small Separation

AbstractWiggling structures in a bipolar outflow may be attributed to orbital motion of a binary system or precession of an accretion disk perturbed by a companion. The shocked knots along the outflow axis display a morphology with either mirror symmetry due to the orbital motion or point symmetry resulted from disk precession. Using the Submillimeter Array (SMA), our CO (2-1) and SiO (5-4) observations show wiggling structures in the collimated bipolar outflow driven by the NGC 1333 IRAS 2A Class 0 protostar (d ~ 200 pc). By fitting the peak positions of emission knots, we can examine the lateral displacement of the molecular jet to constrain parameters of the unresolved binary system, such as the binary separation and total binary mass. With an angular resolution of ~3″, we have determined the knot positions in SiO (5–4)(Fig. 1) and CO (2–1). As a first attempt, we consider the scenario of orbital motion in a binary system and estimate a total binary mass of ~ 1M⊙ and a binary separation of roughly ~ 20 AU, corresponding to ~ 0.1″. Such a small separation makes it challenging to resolve this hypothesized proto-binary system, which is thought to be responsible for the large-scale quadrupolar outflow nearly perpendicular with each other in CO (1–0).

scholarly journals Ablation and Wind Mass-Loading in the Born-Again Planetary Nebula A 30

2011 ◽  
Vol 7 (S283) ◽  
pp. 378-379
Author(s):  
Martín A. Guerrero ◽  
You-Hua Chu ◽  
Wolf-Rainer Hamann ◽  
Lidia Oskinova ◽  
Detlef Schönberner ◽  
...  
Keyword(s):  
Stellar Wind ◽  
Planetary Nebula ◽  
Central Star ◽  
Mass Loading ◽  
Diffuse Emission ◽  
X Ray ◽  
Thermal Pulse ◽  
Born Again

AbstractWe present XMM-Newton and Chandra observations of the born-again planetary nebula A 30. These X-ray observations reveal a bright unresolved source at the position of the central star whose X-ray luminosity exceeds by far the model expectations for photospheric emission and for shocks within the stellar wind. We suggest that a “born-again hot bubble” may be responsible for this X-ray emission. Diffuse X-ray emission associated with the petal-like features and one of the H-poor knots seen in the optical is also found. The weakened emission of carbon lines in the spectrum of the diffuse emission can be interpreted as the dilution of stellar wind by mass-loading or as the detection of material ejected during a very late thermal pulse.

scholarly journals MHD simulations of star-disk magnetospheres and the formation of outflows and jets

2007 ◽  
Vol 3 (S243) ◽  
pp. 265-276
Author(s):  
Christian Fendt
Keyword(s):  
Numerical Simulation ◽  
Stellar Wind ◽  
Large Scale ◽  
Disk Surface ◽  
Time Dependent ◽  
Mhd Simulations ◽  
Tremendous Progress ◽  
Stellar Magnetospheres ◽  
Scale Evolution

AbstractIn this review the recent development concerning the large-scale evolution of stellar magnetospheres in interaction with the accretion disk is discussed. I put emphasis on the generation of outflows and jets from the disk and/or the star. In fact, tremendous progress has occurred over the last decade in the numerical simulation of the star-disk interaction. The role of numerical simulations is essential in this area because the processes involved are complex, strongly interrelated, and often highly time-dependent. Recent MHD simulations suggest that outflows launched from a very concentrated region tend to be un-collimated. I present preliminary results of simulations of large-scale star-disk magnetospheres loaded with matter from the stellar, resp. the disk surface demonstrating how a disk jet collimates the wind from the star and also how the stellar wind lowers the collimation degree of the disk outflow.

Modeling of stellar wind from fast rotating stars

2018 ◽  
Vol 27 (10) ◽  
pp. 1844004 ◽  
Author(s):  
S. V. Bogovalov ◽  
S. M. Romanikhin
Keyword(s):  
Stellar Wind ◽  
Large Scale ◽  
Reynolds Numbers ◽  
Nonuniform Distribution ◽  
Polar Regions ◽  
Rotating Stars ◽  
Efficient Diffusion ◽  
Rotating Star ◽  
High Level ◽  
Fast Rotating

Two-dimensional (2D) axisymmetric hydrodynamic model for the stellar wind outflow from a fast rotating star is created. Nonuniform distribution of the pressure over the surface of the star corresponding to the condition of equilibrium in every point of plasma in the gravitational and centrifugal fields is taken into account. We also take into account the turbulence of the flow excited in the wind at Reynolds numbers [Formula: see text]. Calculations show the formation of the disklike outflow from the surface of the star at the equator which rather quickly (on the scale of the order of the radius of the star) expands into polar regions. Huge vertex is formed at high latitudes where high level of turbulence makes possible efficient diffusion of the angular moment. At the same time the turbulence is not excited at the equator. This prevents formation of the large scale disklike outflow at the equatorial plane. Dependence of the mass flux outflow on the velocity of the star rotation is obtained.

Mechanisms for magnetic field reversals

2010 ◽  
Vol 368 (1916) ◽  
pp. 1595-1605 ◽  
Author(s):  
F. Pétrélis ◽  
S. Fauve
Keyword(s):  
Magnetic Field ◽  
Turbulent Flow ◽  
Numerical Simulations ◽  
Large Scale ◽  
Conducting Fluid ◽  
Similar Model ◽  
Electrically Conducting ◽  
Electrically Conducting Fluid ◽  
Simple Mechanism ◽  
The Magnetic Field

We present a review of the different models that have been proposed to explain reversals of the magnetic field generated by a turbulent flow of an electrically conducting fluid (fluid dynamos). We then describe a simple mechanism that explains several features observed in palaeomagnetic records of the Earth’s magnetic field, in numerical simulations and in a recent dynamo experiment. A similar model can also be used to understand reversals of large-scale flows that often develop on a turbulent background.

scholarly journals The Stellar Wind From the Central Star of NGC 7009

2006 ◽  
Vol 2 (S234) ◽  
pp. 513
Author(s):  
G. Sonneborn ◽  
R. C. Iping ◽  
D. L. Massa ◽  
Y-H Chu
Keyword(s):  
Stellar Wind ◽  
Central Star
Sign in / Sign up

Export Citation Format

Share Document