scholarly journals Effects of radiation in accretion regions of classical T Tauri stars

2019 ◽  
Vol 629 ◽  
pp. L9 ◽  
Author(s):  
S. Colombo ◽  
L. Ibgui ◽  
S. Orlando ◽  
R. Rodriguez ◽  
G. Espinosa ◽  
...  
Keyword(s):  
Thermal Conduction ◽  
Thermal Structure ◽  
Radiation Hydrodynamics ◽  
Shock Heating ◽  
T Tauri ◽  
Shock Region ◽  
Accretion Rates ◽  
Post Shock ◽  
Effects Of Radiation ◽  
The Impact

Context. Models and observations indicate that the impact of matter accreting onto the surface of young stars produces regions at the base of accretion columns where optically thin and thick plasma components coexist. Thus, an accurate description of these impacts is necessary to account for the effects of absorption and emission of radiation. Aims. We study the effects of radiation emerging from shock-heated plasma in impact regions on the structure of the pre-shock down-falling material. We investigate whether a significant absorption of radiation occurs and if it leads to a pre-shock heating of the accreting gas. Methods. We developed a radiation hydrodynamics model describing an accretion column impacting onto the surface of a classical T Tauri star. The model takes into account the stellar gravity, the thermal conduction, and the effects of radiative losses and of absorption of radiation by matter in the nonlocal thermodynamic equilibrium regime. Results. After the impact, a hot slab of post-shock plasma develops at the base of the accretion column. Part of the radiation emerging from the slab is absorbed by the pre-shock accreting material. As a result, the pre-shock accretion column gradually heats up to temperatures of 105 K, forming a radiative precursor of the shock. The precursor has a thermal structure with the hottest part at T ≈ 105 K, with a size comparable to that of the hot slab, above the post-shock region. At larger distances the temperature gradually decreases to T ≈ 104 K. Conclusions. Our model predicts that ≈70% of the radiation emitted by the post-shock plasma is absorbed by the pre-shock accretion column immediately above the slab and is re-emitted in the UV band. This may explain why accretion rates derived from UV observations are systematically higher than rates inferred from X-ray observations.

scholarly journals Analysis of microscopic properties of radiative shock experiments performed at the Orion laser facility

2018 ◽  
Vol 6 ◽  
Author(s):  
R. Rodríguez ◽  
G. Espinosa ◽  
J. M. Gil ◽  
F. Suzuki-Vidal ◽  
T. Clayson ◽  
...  
Keyword(s):  
Spectroscopic Properties ◽  
Radiation Hydrodynamics ◽  
Post Processing ◽  
Radiative Shock ◽  
Hydrodynamic Simulations ◽  
Shock Region ◽  
Laser Facility ◽  
Post Shock ◽  
Charge State Distributions ◽  
Thermodynamic Regime

In this work we have conducted a study on the radiative and spectroscopic properties of the radiative precursor and the post-shock region from experiments with radiative shocks in xenon performed at the Orion laser facility. The study is based on post-processing of radiation-hydrodynamics simulations of the experiment. In particular, we have analyzed the thermodynamic regime of the plasma, the charge state distributions, the monochromatic opacities and emissivities, and the specific intensities for plasma conditions of both regions. The study of the intensities is a useful tool to estimate ranges of electron temperatures present in the xenon plasma in these experiments and the analysis performed of the microscopic properties commented above helps to better understand the intensity spectra. Finally, a theoretical analysis of the possibility of the onset of isobaric thermal instabilities in the post-shock has been made, concluding that the instabilities obtained in the radiative-hydrodynamic simulations could be thermal ones due to strong radiative cooling.

scholarly journals X-rays from accretion shocks in classical T Tauri stars: 2D MHD modeling and the role of local absorption

2013 ◽  
Vol 9 (S302) ◽  
pp. 48-49
Author(s):  
C. Argiroffi ◽  
R. Bonito ◽  
S. Orlando ◽  
M. Miceli ◽  
F. Reale ◽  
...  
Keyword(s):  
Magnetic Field ◽  
T Tauri Stars ◽  
X Rays ◽  
X Ray ◽  
T Tauri ◽  
Shock Region ◽  
Post Shock ◽  
The Magnetic Field ◽  
Accretion Shocks

AbstractIn classical T Tauri stars (CTTS) strong shocks are formed where the accretion funnel impacts with the denser stellar chromosphere. Although current models of accretion provide a plausible global picture of this process, some fundamental aspects are still unclear: the observed X-ray luminosity in accretion shocks is order of magnitudes lower than predicted; the observed density and temperature structures of the hot post-shock region are puzzling and still unexplained by models.To address these issues we performed 2D MHD simulations describing an accretion stream impacting onto the chromosphere of a CTTS, exploring different configurations and strengths of the magnetic field. From the model results we then synthesized the X-ray emission emerging from the hot post-shock, taking into account the local absorption due to the pre-shock stream and surrounding atmosphere.We find that the different configurations and strengths of the magnetic field profoundly affect the hot post-shock properties. Moreover the emerging X-ray emission strongly depends also on the viewing angle under which accretion is observed. Some of the explored configuration are able to reproduce the observed features of X-ray spectra of CTTS.

scholarly journals The impact of planet wakes on the location and shape of the water ice line in a protoplanetary disk

2020 ◽  
Vol 633 ◽  
pp. A29 ◽  
Author(s):  
Alexandros Ziampras ◽  
Sareh Ataiee ◽  
Wilhelm Kley ◽  
Cornelis P. Dullemond ◽  
Clément Baruteau
Keyword(s):  
Thermal Structure ◽  
Parameter Study ◽  
Water Ice ◽  
Numerical Hydrodynamics ◽  
Shock Heating ◽  
Gap Opening ◽  
Radiative Diffusion ◽  
Thick Disks ◽  
And Migration ◽  
The Impact

Context. Planets in accretion disks can excite spiral shocks and if these planets are massive enough, they can even open gaps in their vicinity. Both of these effects can influence the overall thermal structure of the disk. Aims. We model planets of different masses and semimajor axes in disks of various viscosities and accretion rates to examine their impact on disk thermodynamics and to highlight the mutable, non-axisymmetric nature of ice lines in systems with massive planets. Methods. We conducted a parameter study using numerical hydrodynamics simulations where we treated viscous heating, thermal cooling, and stellar irradiation as additional source terms in the energy equation, with some runs including radiative diffusion. Our parameter space consists of a grid containing different combinations of planet and disk parameters. Results. Both gap opening and shock heating can displace the ice line, with the effects amplified for massive planets in optically thick disks. The gap region can split an initially hot (T > 170 K) disk into a hot inner disk and a hot ring just outside of the planet’s location, while shock heating can reshape the originally axisymmetric ice line into water-poor islands along spirals. We also find that radiative diffusion does not alter the picture significantly in this context. Conclusions. Shock heating and gap opening by a planet can effectively heat up optically thick disks and, in general, they can move or reshape the water ice line. This can affect the gap structure and migration torques. It can also produce azimuthal features that follow the trajectory of spiral arms, creating hot zones which lead to “islands” of vapor and ice around spirals that could affect the accretion or growth of icy aggregates.

scholarly journals 2D numerical study for magnetic field dependence of neutrino-driven core-collapse supernova models

2020 ◽  
Vol 499 (3) ◽  
pp. 4174-4194
Author(s):  
J Matsumoto ◽  
T Takiwaki ◽  
K Kotake ◽  
Y Asahina ◽  
H R Takahashi
Keyword(s):  
Magnetic Field ◽  
Numerical Study ◽  
Small Scale ◽  
Radiation Hydrodynamics ◽  
Initial Strength ◽  
Shock Region ◽  
Main Driver ◽  
Post Shock ◽  
The Magnetic Field ◽  
Initial Magnetic Field

ABSTRACT We study the effects of the magnetic field on the dynamics of non-rotating stellar cores by performing 2D, magnetohydrodynamic (MHD) simulations. To this end, we have updated our neutrino-radiation-hydrodynamics supernova code to include MHD employing a divergence cleaning method with both careful treatments of finite volume and area reconstructions. By changing the initial strength of the magnetic field, the evolution of 15.0, 18.4, and $27.0\,\rm M_\odot$ pre-supernova progenitors is investigated. An intriguing finding in our study is that the neutrino-driven explosion occurs regardless of the strength of the initial magnetic field. For the 2D models presented in this work, the neutrino heating is the main driver for the explosion, whereas the magnetic field secondary contributes to the pre-explosion dynamics. Our results show that the strong magnetic field weakens the growth of the neutrino-driven turbulence in the small scale compared to the weak magnetic field. This results in the slower increase of the turbulent kinetic energy in the post-shock region, leading to the slightly delayed onset of the shock revival for models with the stronger initial magnetic field.

scholarly journals The Observational Evidence for Accretion

1997 ◽  
Vol 182 ◽  
pp. 391-405 ◽  
Author(s):  
Lee Hartmann
Keyword(s):  
Large Range ◽  
Observational Evidence ◽  
Young Stars ◽  
Young Stellar Objects ◽  
Stellar Objects ◽  
T Tauri ◽  
Accretion Rates ◽  
Low Mass ◽  
Stellar Magnetospheres ◽  
Mass Ejection

Outflows from low-mass young stellar objects are thought to draw upon the energy released by accretion onto T Tauri stars. I briefly summarize the evidence for this accretion and outline present estimates of mass accretion rates. Young stars show a very large range of accretion rates, and this has important implications for both mass ejection and for the structure of stellar magnetospheres which may truncate T Tauri disks.

scholarly journals The impact of bars and interactions on optically selected AGNs in spiral galaxies

2018 ◽  
Vol 618 ◽  
pp. A149 ◽  
Author(s):  
Sol Alonso ◽  
Georgina Coldwell ◽  
Fernanda Duplancic ◽  
Valeria Mesa ◽  
Diego G. Lambas
Keyword(s):  
Spiral Galaxies ◽  
Control Sample ◽  
Sloan Digital Sky Survey ◽  
Blue Color ◽  
Nuclear Activity ◽  
Internal Process ◽  
Two Samples ◽  
Accretion Rates ◽  
The Galaxy ◽  
The Impact

Aims. With the aim of performing a suitable comparison of the internal process of galactic bars with respect to the external effect of interactions on driving gas toward the inner most region of the galaxies, we explored and compared the efficiency of both mechanisms on central nuclear activity in optically selected active galactic nuclei (AGNs) in spiral galaxies. Methods. We selected homogeneous samples of barred AGNs and active objects residing in pair systems, derived from the Sloan Digital Sky Survey (SDSS). In order to carry out a reliable comparison of both samples (AGNs in barred hosts in isolation and in galaxy pairs), we selected spiral AGN galaxies with similar distributions of redshift, magnitude, stellar mass, color and stellar age population from both catalogs. With the goal of providing an appropriate quantification of the influence of strong bars and interactions on nuclear activity, we also constructed a suitable control sample of unbarred spiral AGNs without a companion and with similar host properties to the other two samples. Results. We found that barred optically selected AGNs show an excess of nuclear activity (as derived from the Lum[OIII]) and accretion rate onto a central black hole (ℛ) with respect to AGNs in pairs. In addition, both samples show an excess of high values of Lum[OIII] and ℛ with respect to unbarred AGNs in the control sample. We also found that the fractions of AGNs with powerful nuclear activity and high accretion rates increase toward more massive hosts with bluer colors and younger stellar populations. Moreover, AGNs with bars exhibit a higher fraction of galaxies with powerful Lum[OIII] and efficient ℛ with respect to AGN galaxies inhabiting pair systems, in bins of different galaxy properties. Regarding AGNs belonging to pair systems, we found that the central nuclear activity is remarkably dependent on the galaxy pair companion features. The Lum[OIII] for AGNs in pairs is clearly enhanced when the galaxy companion exhibits a bright and more massive host with high metallicity, blue color, efficient star formation activity and young stellar population. The results of this work reveal an important capacity of both mechanisms, bars and interactions, to transport material towards the galaxy central regions. In this context, it should also be noted that the internal process of the bar is more efficient at improving the central nuclear activity in AGN objects than that corresponding to the external mechanism of the galaxy–galaxy interactions.

scholarly journals Constraining the stellar energetic particle flux in young solar-like stars

2018 ◽  
Vol 14 (S345) ◽  
pp. 310-311
Author(s):  
Ch. Rab ◽  
M. Padovani ◽  
M. Güdel ◽  
I. Kamp ◽  
W-F. Thi ◽  
...  
Keyword(s):  
Particle Flux ◽  
High Energy ◽  
Young Stars ◽  
Solar Cosmic Rays ◽  
Spatially Resolved ◽  
T Tauri ◽  
Ionization Sources ◽  
Hydrogen Ionization ◽  
The Individual ◽  
The Impact

AbstractAnomalies in the abundance measurements of short lived radionuclides in meteorites indicate that the protosolar nebulae was irradiated by a large number of energetic particles (E≳ 10 MeV), often called solar cosmic rays. The particle flux of the contemporary Sun cannot explain these anomalies, but, similar to T Tauri stars, the young Sun was more active and probably produced enough high energy particles. However, the stellar particle (SP) flux of young stars is essentially unknown. We model the impact of high-energy ionization sources on the chemistry of the circumstellar environment (disks and envelopes). The model includes X-ray radiative transfer and makes use of particle transport models to calculate the individual molecular hydrogen ionization rates. We study the impact on the chemistry via the ionization tracers HCO+ and N2H+. We argue that spatially resolved observations of those molecules combined with detailed models allow for disentangling the contribution of the individual high-energy ionization sources and to put constraints on the SP flux in young stars.

scholarly journals Balmer-dominated shocks in Tycho’s SNR: omnipresence of CRs

2017 ◽  
Vol 12 (S331) ◽  
pp. 248-253
Author(s):  
Sladjana Knežević ◽  
Ronald Läsker ◽  
Glenn van de Ven ◽  
Joan Font ◽  
John C. Raymond ◽  
...  
Keyword(s):  
Line Width ◽  
Cosmic Ray ◽  
Wide Field ◽  
Hydrogen Atoms ◽  
Narrow Line Width ◽  
Shock Region ◽  
Bayesian Evidence ◽  
Post Shock ◽  
Spurious Result ◽  
First Time

AbstractWe present wide-field, spatially and highly resolved spectroscopic observations of Balmer filaments in the northeastern rim of Tycho’s supernova remnant in order to investigate the signal of cosmic-ray (CR) acceleration. The spectra of Balmer-dominated shocks (BDSs) have characteristic narrow (FWHM ~ 10 km s−1) and broad (FWHM ~ 1000 km s−1) Hα components. CRs affect the Hα-line parameters: heating the cold neutrals in the interstellar medium results in broadening of the narrow Hα-line width beyond 20 km s−1, but also in reduction of the broad Hα-line width due to energy being removed from the protons in the post-shock region. For the first time we show that the width of the narrow Hα line, much larger than 20 km s−1, is not a resolution or geometric effect nor a spurious result of a neglected intermediate (FWHM ~ 100 km s−1) component resulting from hydrogen atoms undergoing charge exchange with warm protons in the broad-neutral precursor. Moreover, we show that a narrow line width ≫ 20 km s−1extends across the entire NE rim, implying CR acceleration is ubiquitous, and making it possible to relate its strength to locally varying shock conditions. Finally, we find several locations along the rim, where spectra are significantly better explained (based on Bayesian evidence) by inclusion of the intermediate component, with a width of 180 km s−1on average.

scholarly journals The influence of line opacity treatment in stella on supernova light curves

2020 ◽  
Vol 499 (3) ◽  
pp. 4312-4324
Author(s):  
Alexandra Kozyreva ◽  
Luke Shingles ◽  
Alexey Mironov ◽  
Petr Baklanov ◽  
Sergey Blinnikov
Keyword(s):  
Radiative Transfer ◽  
Spectral Synthesis ◽  
Broad Band ◽  
Light Curves ◽  
Radiation Hydrodynamics ◽  
Type Ii ◽  
Type Ia ◽  
The Impact ◽  
Supernovae Type Ia

ABSTRACT We systematically explore the effect of the treatment of line opacity on supernova light curves. We find that it is important to consider line opacity for both scattering and absorption (i.e. thermalization, which mimics the effect of fluorescence). We explore the impact of the degree of thermalization on three major types of supernovae: Type Ia, Type II-peculiar, and Type II-plateau. For this we use the radiative transfer code stella and analyse broad-band light curves in the context of simulations done with the spectral synthesis code artis and in the context of a few examples of observed supernovae of each type. We found that the plausible range for the ratio between absorption and scattering in the radiation hydrodynamics code stella is (0.8–1):(0.2–0), i.e. the recommended thermalization parameter is 0.9.

scholarly journals Physical Interpretation of Dwarf Nova Primary Effective Temperatures

2004 ◽  
Vol 194 ◽  
pp. 192-193
Author(s):  
Dean M. Townsley ◽  
Lars Bildsten
Keyword(s):  
Gravitational Radiation ◽  
Accretion Rate ◽  
Thermal State ◽  
Cataclysmic Variables ◽  
Theoretical Work ◽  
Dwarf Novae ◽  
Radiation Losses ◽  
Accretion Rates ◽  
The Masses ◽  
The Impact

AbstractWe have undertaken a theoretical study of the impact of the accumulating envelopes on the thermal state of the underlying white dwarf (WD). This has allowed us to find the equilibrium WD core temperatures, the classical nova ignition masses and the thermal luminosities for WDs accreting at rates of 10–11 – 10–8M⊙ yr–1. These accretion rates are most, appropriate to WDs in cataclysmic variables (CVs) of (Porb ≲ 7 hr), many of which accrete sporadically as Dwarf Novae. Over twenty Dwarf Novae have been observed in quiescence, when the accretion rate is low and the WD photosphere is detected and Teff measured. Comparing our theoretical work to these observations allows us to constrain the WD mass and the time averaged accretion rate, ⟨Ṁ⟩. If ⟨Ṁ⟩ is that given by gravitational radiation losses alone, then the WD masses are > 0.8 M⊙. An alternative conclusion is that the masses are closer to 0.6M⊙ and ⟨Ṁ⟩ is 3-4 times larger than that expected from gravitational radiation losses.

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