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Early results from ChanPLaNS: Mystery of hard X-ray emitting CSPNe†
- Rodolfo Montez, Jr., J. H. Kastner, B. Balick, E. Behar, E. Blackman, V. Bujarrabal, Y.-H. Chu, R. Corradi, O. De Marco, A. Frank, D. Frew, M. Guerrero, S. Kwok, J. A. Lopez, B. Miszalski, J. Nordhaus, Q. Parker, R. Sahai, C. Sandin, D. Schoenberner, N. Soker, J. Sokoloski, W. Steffen, T. Ueta, E. Villaver, A. Zijlstra
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- Journal:
- Proceedings of the International Astronomical Union / Volume 7 / Issue S283 / July 2011
- Published online by Cambridge University Press:
- 30 August 2012, pp. 450-451
- Print publication:
- July 2011
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We are presently using the Chandra X-ray Observatory to conduct the first systematic X-ray survey of planetary nebulae (PNe) in the solar neighborhood. The Chandra Planetary Nebula Survey (ChanPlaNS) is a 570 ks Chandra Cycle 12 Large Program targeting 21 high-excitation PNe within ~1.5 kpc of Earth. When complete, this survey will provide a suite of new X-ray diagnostics that will inform the study of late stellar evolution, binary star astrophysics, and wind interactions. Among the early results of ChanPlaNS (when combined with archival Chandra data) is a surprisingly high detection rate of relatively hard X-ray emission from CSPNe. Specifically, X-ray point sources are clearly detected in roughly half of the ~30 high-excitation PNe observed thus far by Chandra, and all but one of these X-ray-emitting CSPNe display evidence for a hard (few MK) component in their Chandra spectra. Only the central star of the Dumbbell appears to display “pure” hot blackbody emission from a ~200 kK hot white dwarf photosphere in the X-ray band. Potential explanations for the“excess” hard X-ray emission detected from the other CSPNe include late-type companions (heretofore undetected, in most cases) whose coronae have been rejuvenated by recent interactions with the mass-losing WD progenitor, non-LTE effects in hot white dwarf photospheres, self-shocking variable winds from the central star, and slow (re-)accretion of previously ejected red giant envelope mass.
Evaporation of acoustically levitated droplets
- A. L. YARIN, G. BRENN, O. KASTNER, D. RENSINK, C. TROPEA
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- Journal:
- Journal of Fluid Mechanics / Volume 399 / 25 November 1999
- Published online by Cambridge University Press:
- 25 November 1999, pp. 151-204
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The rate of heat and mass transfer at the surface of acoustically levitated pure liquid droplets is predicted theoretically for the case where an acoustic boundary layer appears near the droplet surface resulting in an acoustic streaming. The theory is based on the computation of the acoustic field and squeezed droplet shape by means of the boundary element method developed in Yarin, Pfaffenlehner & Tropea (1998). Given the acoustic field around the levitated droplet, the acoustic streaming near the droplet surface was calculated. This allowed calculation of the Sherwood and Nusselt number distributions over the droplet surface, as well as their average values. Then, the mass balance was used to calculate the evolution of the equivalent droplet radius in time.
The theory is applicable to droplets of arbitrary size relative to the sound wavelength λ, including those of the order of λ, when the compressible character of the gas flow is important. Also, the deformation of the droplets by the acoustic field is accounted for, as well as a displacement of the droplet centre from the pressure node. The effect of the internal circulation of liquid in the droplet sustained by the acoustic streaming in the gas is estimated. The distribution of the time-average heat and mass transfer rate over the droplet surface is found to have a maximum at the droplet equator and minima at its poles. The time and surface average of the Sherwood number was shown to be described by the expression Sh = KB/√ω[Dscr ]0, where B = A0e/(ρ0c0) is a scale of the velocity in the sound wave (A0e is the amplitude of the incident sound wave, ρ0 is the unperturbed air density, c0 is the sound velocity in air, ω is the angular frequency in the ultrasonic range, [Dscr ]0 is the mass diffusion coefficient of liquid vapour in air, which should be replaced by the thermal diffusivity of air in the computation of the Nusselt number). The coefficient K depends on the governing parameters (the acoustic field, the liquid properties), as well as on the current equivalent droplet radius a.
For small spherical droplets with a[Lt ]λ, K = (45/4π)1/2 = 1.89, if A0e is found from the sound pressure level (SPL) defined using A0e. On the other hand, if A0e is found from the same value of the SPL, but defined using the root-mean-square pressure amplitude (prms = A0e/√2), then Sh = KrmsBrms/ √ω[Dscr ]0, with Brms = √2B and Krms = K/√2 = 1.336. For large droplets squeezed significantly by the acoustic field, K appears always to be greater than 1.89. The evolution of an evaporating droplet in time is predicted and compared with the present experiments and existing data from the literature. The agreement is found to be rather good.
We also study and discuss the effect of an additional blowing (a gas jet impinging on a droplet) on the evaporation rate, as well as the enrichment of gas at the outer boundary of the acoustic bondary layer by liquid vapour. We show that, even at relatively high rates of blowing, the droplet evaporation is still governed by the acoustic streaming in the relatively strong acoustic fields we use. This makes it impossible to study forced convective heat and mass transfer under the present conditions using droplets levitated in strong acoustic fields.