Time-dependent accretion onto magnetized white dwarfs

Langer, Steven H.; Chanmugam, C.; Shaviv, G.

doi:10.1086/160079

Cited by 68 publications

(43 citation statements)

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“…These diverse treatments produced some different results. Langer et al (1981Langer et al ( , 1982 found a fundamental mode that is unstable to oscillations, whereas other studies (e.g. Imamura 1985;Imamura et al 1984;Wolff, Wood, & Imamura 1991;Wood, Imamura, & Wolff 1992) found that the first unstable mode is the first overtone, and the fundamental mode is damped, consistent with stability analyses.…”

Section: Introductionsupporting

confidence: 75%

Effects of Lower Boundary Conditions on the Stability of Radiative Shocks

Saxton

2002

Publ. – Astron. Soc. Aust

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Thermal instabilities can cause a radiative shock to oscillate, thereby modulating the emission from the post-shock region. The mode frequencies are approximately quantised in analogy to those of a vibrating pipe. The stability properties depend on the cooling processes, the electron–ion energy exchange, and the boundary conditions. This paper considers the effects of the lower boundary condition on the post-shock flow, both ideally and for some specific physical models. Specific cases include constant perturbed velocity, pressure, density, flow rate, or temperature at the lower boundary, and the situation with nonzero stationary flow velocity at the lower boundary. It is found that for cases with zero terminal stationary velocity, the stability properties are insensitive to the perturbed hydrodynamic variables at the lower boundary. The luminosity responses are generally dependent on the lower boundary condition.

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Section: Introductionsupporting

confidence: 75%

Effects of Lower Boundary Conditions on the Stability of Radiative Shocks

Saxton

2002

Publ. – Astron. Soc. Aust

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“…Thermal conduction does not alleviate the problem. A possible explanation is the expected thermal instability of the cooling flow (Imamura & Chevalier 1984;Langer et al 1982), so that the shock height and shock speed vary with time. Hydrodynamical simulations will be needed to determine whether such instabilities can change the H-like to He-like intensity ratios by the observed amount, but they should change the ratios in the correct sense.…”

Section: Discussionmentioning

confidence: 99%

Testing the cooling flow model in the intermediate polar EX Hydrae

Luna

Raymond

Brickhouse

et al. 2015

A&A

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We use the best available X-ray data from the intermediate polar EX Hydrae to study the cooling-flow model often applied to interpret the X-ray spectra of these accreting magnetic white dwarf binaries. First, we resolve a long-standing discrepancy between the X-ray and optical determinations of the mass of the white dwarf in EX Hya by applying new models of the inner disk truncation radius. Our fits to the X-ray spectrum now agree with the white dwarf mass of 0.79 M determined using dynamical methods through spectroscopic observations of the secondary. We use a simple isobaric cooling flow model to derive the emission line fluxes, emission measure distribution, and H-like to He-like line ratios for comparison with the 496 ks Chandra High Energy Transmission Grating observation of EX Hydrae. We find that the H/He ratios are not well reproduced by this simple isobaric cooling flow model and show that while H-like line fluxes can be accurately predicted, fluxes of lower-Z He-like lines are significantly underestimated. This discrepancy suggests that an extra heating mechanism plays an important role at the base of the accretion column, where cooler ions form. We thus explored more complex cooling models, including the change of gravitational potential with height in the accretion column and a magnetic dipole geometry. None of these modifications to the standard cooling flow model are able to reproduce the observed line ratios. While a cooling flow model with subsolar (0.1 ) abundances is able to reproduce the line ratios by reducing the cooling rate at temperatures lower than ∼10 7.3 K, the predicted line-to-continuum ratios are much lower than observed. We discuss and discard mechanisms, such as photoionization, departures from constant pressure, resonant scattering, different electronion temperatures, and Compton cooling. Thermal conduction transfers energy from the region above 10 7 K, where the H-like lines are mostly formed, to the cooler regions where the He-like ions of the lower-Z elements are formed, hence in principle it could help resolve the problem. However, simple models indicate that the energy is deposited below 10 6 K, which is too cool to increase the emission of the He-like lines we observe. We conclude that some other effect, such as thermally unstable cooling, modifies the temperature distribution.

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“…Following Langer et al (1982), we model the accretion column with a variable cross-section according to the dipole geometry and freefall velocities along the column, yielding a density varying with the distance r to the white dwarf, as n ∼ n 0 (r/R wd ) −2.5 . The density, n 0 , at the basis of the column, is computed from the accretion luminosity L X as n 0 =1.8 × 10 16 cm −3 (L X /10 34 erg s −1 ), assuming a typical value of 10 16 cm 2 for the polar cap surface, and a 0.8 M white dwarf.…”

Section: Model Of the Accretion Columnmentioning

confidence: 99%

The CNO problem in Magnetic Cataclysmic Variables

Bonnet‐Bidaud

Mouchet

2004

International Astronomical Union Colloquium

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Abstract. Some polars like BY Cam are characterized by unusual CNO line ratios compared to other polars and non-solar abundances have been suggested to explain this anomaly. We present here a first attempt to constrain the elemental abundances in these systems by applying a specific ionisation model combined with a geometrical description of the accretion column where these lines are thought to be formed. The line luminosities have been computed using the CLOUDY plasma code for different ionisation spectra and column extension. We show here selected results and compare to the values observed in "peculiar" magnetic CVs. The model applied to BY Cam confirms that ionization models with solar abundances fail to reproduce the observed line intensity ratios. Assuming the model to be valid, the induced best abundances imply an overabundance of N (x25), underabundance of C (:8) and nearly solar O (:2), in line with CNO reprocessing.

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Time-dependent accretion onto magnetized white dwarfs

Cited by 68 publications

References 0 publications

Effects of Lower Boundary Conditions on the Stability of Radiative Shocks

Effects of Lower Boundary Conditions on the Stability of Radiative Shocks

Testing the cooling flow model in the intermediate polar EX Hydrae

The CNO problem in Magnetic Cataclysmic Variables

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