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Stanley P Owocki - One of the best experts on this subject based on the ideXlab platform.
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mass loss from inhomogeneous hot star winds iii an effective opacity formalism for line radiative transfer in accelerating clumped two component media and first results on theory and diagnostics
Astronomy and Astrophysics, 2014Co-Authors: J O Sundqvist, J Puls, Stanley P OwockiAbstract:Aims. We provide a fast and easy-to-use formalism for treating the reduction in effective opacity associated with optically thick clumps in an accelerating two-component medium. Methods. We develop and benchmark effective-opacity laws for continuum and line radiative transfer that bridge the limits of optically thin and thick clumps. We then use this formalism to i) design a simple method for modeling and analyzing UV wind resonance lines in hot, massive stars, and ii) derive simple correction factors to the line force driving the outflows of such stars. Results. Using a vorosity-modified Sobolev with exact integration (vmSEI) method, we show that, for a given ionization factor, UV resonance doublets may be used to analytically predict the upward corrections in empirically inferred mass-loss rates associated with porosity in velocity space (a.k.a. velocity-porosity, or vorosity). However, we also show the presence of a solution degeneracy: in a two-component clumped wind with given inter-clump medium density, there are always two different solutions producing the same synthetic doublet profile. We demonstrate this by application to SiIV and PV in B and O supergiants and derive, for an inter-clump density set to 1% of the mean density, upward empirical mass-loss corrections of typically factors of either ∼ 5o r∼50, depending on which of the two solutions is chosen. Overall, our results indicate that this solution dichotomy severely limits the use of UV resonance lines as direct mass-loss indicators in current diagnostic models of clumped hot stellar winds. We next apply the effective line-opacity formalism to the standard CAK theory of line-driven winds. A simple vorosity correction factor to the CAK line force is derived, which for normalized velocity filling factor fvel simply scales as f α vel ,w hereα is the slope of the CAK line-strength distribution function. By analytic and numerical hydrodynamics calculations, we further show that in cases where vorosity is important at the critical point setting the mass-loss rate, the reduced line force leads to a lower theoretical mass loss, by simply a factor fvel .O n the other hand, if vorosity is important only above this critical point, the predicted mass loss is not affected, but the wind Terminal Speed is reduced, by a factor scaling as f α/(2−2α) vel . This shows that porosity in velocity space can have a significant impact not only on the diagnostics, but also on the dynamics and theory of radiatively driven winds.
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mass loss from inhomogeneous hot star winds iii an effective opacity formalism for line radiative transfer in accelerating clumped two component media and first results on theory and diagnostics
arXiv: Solar and Stellar Astrophysics, 2014Co-Authors: J O Sundqvist, J Puls, Stanley P OwockiAbstract:[Abridged] We develop and benchmark a fast and easy-to-use effective-opacity formalism for line and continuum radiative transfer in an accelerating two-component clumpy medium. The formalism bridges the limits of optically thin and thick clumps, and is here used to i) design a simple vorosity-modified Sobolev with exact integration (vmSEI) method for analyzing UV wind resonance lines in hot, massive stars, and ii) derive simple correction factors to the line force driving the outflows of such stars. We show that (for a given ionization factor) UV resonance doublets may be used to analytically predict the upward corrections in empirically inferred mass-loss rates associated with porosity in velocity space (a.k.a. velocity-porosity, or vorosity), but that severe solution degeneracies exist. For an inter-clump density set to 1 % of the mean density, we for O and B supergiants derive upward empirical mass-loss corrections of typically factors of either ~5 or ~50, depending on which of the two applicable solutions is chosen. Overall, our results indicate this solution dichotomy severely limits the use of UV resonance lines as direct mass-loss indicators of clumped hot stellar winds. We next apply the effective-opacity formalism to the standard CAK theory of line-driven winds. By analytic and numerical hydrodynamics calculations, we show that in cases where vorosity is important at the critical point setting the mass-loss rate, the reduced line-force leads to a lower theoretical mass loss, by a factor scaling with the normalized velocity filling factor fvel. On the other hand, if vorosity is important only above this critical point, the predicted mass loss is not affected, but the wind Terminal Speed is reduced. This shows that porosity in velocity space can have a significant impact not only on the diagnostics, but also on the dynamics and theory of radiatively driven winds.
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x ray line profiles from parameterized emission within an accelerating stellar wind
arXiv: Astrophysics, 2001Co-Authors: Stanley P Owocki, David H CohenAbstract:Motivated by recent detections by the XMM and Chandra satellites of X-ray line emission from hot, luminous stars, we present synthetic line profiles for X-rays emitted within parameterized models of a hot-star wind. The X-ray line emission is taken to occur at a sharply defined co-moving-frame resonance wavelength, which is Doppler-shifted by a stellar wind outflow parameterized by a `beta' velocity law, $v(r)=v_{\infty} (1-\R_{\ast}/r)^\beta$. Above some initial onset radius $R_o$ for X-ray emission, the radial variation of the emission filling factor is assumed to decline as a power-law in radius, $f(r) \sim r^{-q}$. The computed emission profiles also account for continuum absorption within the wind, with the overall strength characterized by a cumulative optical depth $\tau_\ast$. In terms of a wavelength shift from line-center scaled in units of the wind Terminal Speed $v_{\infty}$, we present normalized X-ray line profiles for various combinations of the parameters $\beta$, $\tau_\ast$, $q$ and $R_o$, and including also the effect of instrumental broadening as characterized by a Gaussian with a parameterized width $\sigma$. We discuss the implications for interpreting observed hot-star X-ray spectra, with emphasis on signatures for discriminating between ``coronal'' and ``wind-shock'' scenarios. In particular, we note that in profiles observed so far the substantial amount of emission longward of line center will be difficult to reconcile with the expected attenuation by the wind and stellar core in either a wind-shock or coronal model.
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x ray line profiles from parameterized emission within an accelerating stellar wind
The Astrophysical Journal, 2001Co-Authors: Stanley P Owocki, David H CohenAbstract:Motivated by recent detections by the XMM and Chandra satellites of X-ray line emission from hot, luminous stars, we present synthetic line pro—les for X-rays emitted within parameterized models of a hot-star wind. The X-ray line emission is taken to occur at a sharply de—ned comoving-frame resonance wavelength, which is Doppler-shifted by a stellar wind out—ow parameterized by a ii b ˇˇ velocity law, Above some initial onset radius for X-ray emission, the radial variation of the v(r) \ v = (1 [ R * /r)b. R o emission —lling factor is assumed to decline as a power law in radius, f (r) D r~q. The computed emission pro—les also account for continuum absorption within the wind, with the overall strength characterized by a cumulative optical depth In terms of a wavelength shift from line center scaled in units of the q * . wind Terminal Speed we present normalized X-ray line pro—les for various combinations of the pav = , rameters b, q, and and also including the eUect of instrumental and/or macroturbulent broadening q * , R o as characterized by a Gaussian with a parameterized width p. We discuss the implications for interpreting observed hot-star X-ray spectra, with emphasis on signatures for discriminating between ii coronal ˇˇ and ii wind-shock ˇˇ scenarios. In particular, we note that in pro—les observed so far the substantial amount of emission longward of line center will be difficult to reconcile with the expected attenuation by the wind and stellar core in either a wind-shock or coronal model.
David H Cohen - One of the best experts on this subject based on the ideXlab platform.
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x ray line profiles from parameterized emission within an accelerating stellar wind
arXiv: Astrophysics, 2001Co-Authors: Stanley P Owocki, David H CohenAbstract:Motivated by recent detections by the XMM and Chandra satellites of X-ray line emission from hot, luminous stars, we present synthetic line profiles for X-rays emitted within parameterized models of a hot-star wind. The X-ray line emission is taken to occur at a sharply defined co-moving-frame resonance wavelength, which is Doppler-shifted by a stellar wind outflow parameterized by a `beta' velocity law, $v(r)=v_{\infty} (1-\R_{\ast}/r)^\beta$. Above some initial onset radius $R_o$ for X-ray emission, the radial variation of the emission filling factor is assumed to decline as a power-law in radius, $f(r) \sim r^{-q}$. The computed emission profiles also account for continuum absorption within the wind, with the overall strength characterized by a cumulative optical depth $\tau_\ast$. In terms of a wavelength shift from line-center scaled in units of the wind Terminal Speed $v_{\infty}$, we present normalized X-ray line profiles for various combinations of the parameters $\beta$, $\tau_\ast$, $q$ and $R_o$, and including also the effect of instrumental broadening as characterized by a Gaussian with a parameterized width $\sigma$. We discuss the implications for interpreting observed hot-star X-ray spectra, with emphasis on signatures for discriminating between ``coronal'' and ``wind-shock'' scenarios. In particular, we note that in profiles observed so far the substantial amount of emission longward of line center will be difficult to reconcile with the expected attenuation by the wind and stellar core in either a wind-shock or coronal model.
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x ray line profiles from parameterized emission within an accelerating stellar wind
The Astrophysical Journal, 2001Co-Authors: Stanley P Owocki, David H CohenAbstract:Motivated by recent detections by the XMM and Chandra satellites of X-ray line emission from hot, luminous stars, we present synthetic line pro—les for X-rays emitted within parameterized models of a hot-star wind. The X-ray line emission is taken to occur at a sharply de—ned comoving-frame resonance wavelength, which is Doppler-shifted by a stellar wind out—ow parameterized by a ii b ˇˇ velocity law, Above some initial onset radius for X-ray emission, the radial variation of the v(r) \ v = (1 [ R * /r)b. R o emission —lling factor is assumed to decline as a power law in radius, f (r) D r~q. The computed emission pro—les also account for continuum absorption within the wind, with the overall strength characterized by a cumulative optical depth In terms of a wavelength shift from line center scaled in units of the q * . wind Terminal Speed we present normalized X-ray line pro—les for various combinations of the pav = , rameters b, q, and and also including the eUect of instrumental and/or macroturbulent broadening q * , R o as characterized by a Gaussian with a parameterized width p. We discuss the implications for interpreting observed hot-star X-ray spectra, with emphasis on signatures for discriminating between ii coronal ˇˇ and ii wind-shock ˇˇ scenarios. In particular, we note that in pro—les observed so far the substantial amount of emission longward of line center will be difficult to reconcile with the expected attenuation by the wind and stellar core in either a wind-shock or coronal model.
J O Sundqvist - One of the best experts on this subject based on the ideXlab platform.
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mass loss from inhomogeneous hot star winds iii an effective opacity formalism for line radiative transfer in accelerating clumped two component media and first results on theory and diagnostics
Astronomy and Astrophysics, 2014Co-Authors: J O Sundqvist, J Puls, Stanley P OwockiAbstract:Aims. We provide a fast and easy-to-use formalism for treating the reduction in effective opacity associated with optically thick clumps in an accelerating two-component medium. Methods. We develop and benchmark effective-opacity laws for continuum and line radiative transfer that bridge the limits of optically thin and thick clumps. We then use this formalism to i) design a simple method for modeling and analyzing UV wind resonance lines in hot, massive stars, and ii) derive simple correction factors to the line force driving the outflows of such stars. Results. Using a vorosity-modified Sobolev with exact integration (vmSEI) method, we show that, for a given ionization factor, UV resonance doublets may be used to analytically predict the upward corrections in empirically inferred mass-loss rates associated with porosity in velocity space (a.k.a. velocity-porosity, or vorosity). However, we also show the presence of a solution degeneracy: in a two-component clumped wind with given inter-clump medium density, there are always two different solutions producing the same synthetic doublet profile. We demonstrate this by application to SiIV and PV in B and O supergiants and derive, for an inter-clump density set to 1% of the mean density, upward empirical mass-loss corrections of typically factors of either ∼ 5o r∼50, depending on which of the two solutions is chosen. Overall, our results indicate that this solution dichotomy severely limits the use of UV resonance lines as direct mass-loss indicators in current diagnostic models of clumped hot stellar winds. We next apply the effective line-opacity formalism to the standard CAK theory of line-driven winds. A simple vorosity correction factor to the CAK line force is derived, which for normalized velocity filling factor fvel simply scales as f α vel ,w hereα is the slope of the CAK line-strength distribution function. By analytic and numerical hydrodynamics calculations, we further show that in cases where vorosity is important at the critical point setting the mass-loss rate, the reduced line force leads to a lower theoretical mass loss, by simply a factor fvel .O n the other hand, if vorosity is important only above this critical point, the predicted mass loss is not affected, but the wind Terminal Speed is reduced, by a factor scaling as f α/(2−2α) vel . This shows that porosity in velocity space can have a significant impact not only on the diagnostics, but also on the dynamics and theory of radiatively driven winds.
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mass loss from inhomogeneous hot star winds iii an effective opacity formalism for line radiative transfer in accelerating clumped two component media and first results on theory and diagnostics
arXiv: Solar and Stellar Astrophysics, 2014Co-Authors: J O Sundqvist, J Puls, Stanley P OwockiAbstract:[Abridged] We develop and benchmark a fast and easy-to-use effective-opacity formalism for line and continuum radiative transfer in an accelerating two-component clumpy medium. The formalism bridges the limits of optically thin and thick clumps, and is here used to i) design a simple vorosity-modified Sobolev with exact integration (vmSEI) method for analyzing UV wind resonance lines in hot, massive stars, and ii) derive simple correction factors to the line force driving the outflows of such stars. We show that (for a given ionization factor) UV resonance doublets may be used to analytically predict the upward corrections in empirically inferred mass-loss rates associated with porosity in velocity space (a.k.a. velocity-porosity, or vorosity), but that severe solution degeneracies exist. For an inter-clump density set to 1 % of the mean density, we for O and B supergiants derive upward empirical mass-loss corrections of typically factors of either ~5 or ~50, depending on which of the two applicable solutions is chosen. Overall, our results indicate this solution dichotomy severely limits the use of UV resonance lines as direct mass-loss indicators of clumped hot stellar winds. We next apply the effective-opacity formalism to the standard CAK theory of line-driven winds. By analytic and numerical hydrodynamics calculations, we show that in cases where vorosity is important at the critical point setting the mass-loss rate, the reduced line-force leads to a lower theoretical mass loss, by a factor scaling with the normalized velocity filling factor fvel. On the other hand, if vorosity is important only above this critical point, the predicted mass loss is not affected, but the wind Terminal Speed is reduced. This shows that porosity in velocity space can have a significant impact not only on the diagnostics, but also on the dynamics and theory of radiatively driven winds.
Fumiyuki Adachi - One of the best experts on this subject based on the ideXlab platform.
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On Channel Estimation for Analog Network Coding in a Frequency-Selective Fading Channel
EURASIP Journal on Wireless Communications and Networking, 2011Co-Authors: Haris Gacanin, Tomas Sjodin, Fumiyuki AdachiAbstract:Recently, broadband analog network coding (ANC) was introduced for high-Speed transmission over the wireless (frequency-selective fading) channel. However, ANC requires the knowledge of channel state information (CSI) for self-information removal and coherent signal detection. In ANC, the users' pilot signals interfere during the first slot, which renders the relay unable to estimate CSIs of different users, and, consequently, four time-slot pilot-assisted channel estimation (CE) is required to avoid interference. Naturally, this will reduce the capacity of ANC scheme. In this paper, we theoretically analyze the bit error rate (BER) performance of bi-directional broadband ANC communication based on orthogonal frequency division multiplexing (OFDM) radio access. We also theoretically analyze the performance of the channel estimator's mean square error (MSE). The analysis is based on the assumption of perfect timing and frequency synchronization. The achievable BER performance and the estimator's MSE for broadband ANC is evaluated by numerical and computer simulation. We discuss how, and by how much, the imperfect knowledge of CSI affects the BER performance of broadband ANC. It is shown that the CE scheme achieves a slightly higher BER in comparison with ideal CE case for a low and moderate mobile Terminal Speed in a frequency-selective fading channel.
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two slot channel estimation for analog network coding based on ofdm in a frequency selective fading channel
Vehicular Technology Conference, 2010Co-Authors: Tomas Sjodin, Haris Gacanin, Fumiyuki AdachiAbstract:Recently, broadband analog network coding (ANC) was introduced to utilize high-data rate transmission over the wireless - frequency selective fading - channel. However, ANC requires the knowledge of channel state information (CSI) for self-information removal and coherent signal detection. In this paper, we propose a two-slot pilot-assisted CE for bi-directional broadband ANC. In the first slot, two users transmit their respective pilots to the relay, where the users' pilot signals are designed to avoid the interference and consequently, allow the relay to estimate the CSIs from both users. During the second slot the relay broadcast its pilot signal to both users that estimate the corresponding CSIs. It was shown by computer simulation that, even with imperfect CSI, the BER performance of broadband ANC gives a satisfactory performance for a low and moderate mobile Terminal Speed in a frequency-selective fading channel.
J Puls - One of the best experts on this subject based on the ideXlab platform.
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mass loss from inhomogeneous hot star winds iii an effective opacity formalism for line radiative transfer in accelerating clumped two component media and first results on theory and diagnostics
Astronomy and Astrophysics, 2014Co-Authors: J O Sundqvist, J Puls, Stanley P OwockiAbstract:Aims. We provide a fast and easy-to-use formalism for treating the reduction in effective opacity associated with optically thick clumps in an accelerating two-component medium. Methods. We develop and benchmark effective-opacity laws for continuum and line radiative transfer that bridge the limits of optically thin and thick clumps. We then use this formalism to i) design a simple method for modeling and analyzing UV wind resonance lines in hot, massive stars, and ii) derive simple correction factors to the line force driving the outflows of such stars. Results. Using a vorosity-modified Sobolev with exact integration (vmSEI) method, we show that, for a given ionization factor, UV resonance doublets may be used to analytically predict the upward corrections in empirically inferred mass-loss rates associated with porosity in velocity space (a.k.a. velocity-porosity, or vorosity). However, we also show the presence of a solution degeneracy: in a two-component clumped wind with given inter-clump medium density, there are always two different solutions producing the same synthetic doublet profile. We demonstrate this by application to SiIV and PV in B and O supergiants and derive, for an inter-clump density set to 1% of the mean density, upward empirical mass-loss corrections of typically factors of either ∼ 5o r∼50, depending on which of the two solutions is chosen. Overall, our results indicate that this solution dichotomy severely limits the use of UV resonance lines as direct mass-loss indicators in current diagnostic models of clumped hot stellar winds. We next apply the effective line-opacity formalism to the standard CAK theory of line-driven winds. A simple vorosity correction factor to the CAK line force is derived, which for normalized velocity filling factor fvel simply scales as f α vel ,w hereα is the slope of the CAK line-strength distribution function. By analytic and numerical hydrodynamics calculations, we further show that in cases where vorosity is important at the critical point setting the mass-loss rate, the reduced line force leads to a lower theoretical mass loss, by simply a factor fvel .O n the other hand, if vorosity is important only above this critical point, the predicted mass loss is not affected, but the wind Terminal Speed is reduced, by a factor scaling as f α/(2−2α) vel . This shows that porosity in velocity space can have a significant impact not only on the diagnostics, but also on the dynamics and theory of radiatively driven winds.
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mass loss from inhomogeneous hot star winds iii an effective opacity formalism for line radiative transfer in accelerating clumped two component media and first results on theory and diagnostics
arXiv: Solar and Stellar Astrophysics, 2014Co-Authors: J O Sundqvist, J Puls, Stanley P OwockiAbstract:[Abridged] We develop and benchmark a fast and easy-to-use effective-opacity formalism for line and continuum radiative transfer in an accelerating two-component clumpy medium. The formalism bridges the limits of optically thin and thick clumps, and is here used to i) design a simple vorosity-modified Sobolev with exact integration (vmSEI) method for analyzing UV wind resonance lines in hot, massive stars, and ii) derive simple correction factors to the line force driving the outflows of such stars. We show that (for a given ionization factor) UV resonance doublets may be used to analytically predict the upward corrections in empirically inferred mass-loss rates associated with porosity in velocity space (a.k.a. velocity-porosity, or vorosity), but that severe solution degeneracies exist. For an inter-clump density set to 1 % of the mean density, we for O and B supergiants derive upward empirical mass-loss corrections of typically factors of either ~5 or ~50, depending on which of the two applicable solutions is chosen. Overall, our results indicate this solution dichotomy severely limits the use of UV resonance lines as direct mass-loss indicators of clumped hot stellar winds. We next apply the effective-opacity formalism to the standard CAK theory of line-driven winds. By analytic and numerical hydrodynamics calculations, we show that in cases where vorosity is important at the critical point setting the mass-loss rate, the reduced line-force leads to a lower theoretical mass loss, by a factor scaling with the normalized velocity filling factor fvel. On the other hand, if vorosity is important only above this critical point, the predicted mass loss is not affected, but the wind Terminal Speed is reduced. This shows that porosity in velocity space can have a significant impact not only on the diagnostics, but also on the dynamics and theory of radiatively driven winds.
