Time-dependent Ionization of H and He in the Local Interstellar Medium

Lyu, Cheng-Hsuan; Bruhweiler, F. C.

doi:10.1086/176884

Cited by 46 publications

(36 citation statements)

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“…Therefore, regardless of whether the EUV emission ionizes H more effectively than He, there are not enough photons produced in the explosion to explain our inferred neutral fractions. This result also agrees with the SN flash ionization calculations of Lyu & Bruhweiler (1996).…”

Section: Relic Ionization From the Supernova Explosion?supporting

confidence: 91%

The Optical Spectrum of the SN 1006 Supernova Remnant Revisited

Ghavamian

Winkler

Raymond

et al. 2002

ApJ

113

149

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We present the deepest optical spectrum acquired to date of Balmer-dominated shocks in the northwest rim of SN 1006. We detect the broad and narrow components of H, H, and H and report the first detection of the He i 6678 emission line in this supernova remnant. We may have detected, at the 1.5 level, faint He ii 4686 emission. We measure a full width at half-maximum of 2290 AE 80 km s À1 in the broad-component H line, with broad-to-narrow flux ratios of 0.84 þ0:03 À0:01 and 0.93 þ0:18 À0:16 in H and H, respectively. To match these observations, our nonradiative shock models require a low degree of electron-proton equilibration at the shock front, T e =T p 0:07, and a shock speed of 2890 AE 100 km s À1 . These results agree well with an earlier analysis of ultraviolet lines from SN 1006. The He i/H and He i/He ii flux ratios also indicate low equilibration. Furthermore, our models match the observations for mostly ionized ($90%) preshock H and mostly neutral (e70%) preshock He, respectively. We conclude that the high H ionization fraction cannot be explained by either photoionization from the reverse shock or relic ionization from EUV photons released in the a.d. 1006 supernova. The most plausible explanation appears to be photoionization from the Galactic Lyman continuum.

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Section: Relic Ionization From the Supernova Explosion?supporting

confidence: 91%

The Optical Spectrum of the SN 1006 Supernova Remnant Revisited

Ghavamian

Winkler

Raymond

et al. 2002

ApJ

113

149

View full text Add to dashboard Cite

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“…We also note that strong Si III (1206 Å) interstellar absorption has been reported for several sight-lines towards nearby stars (Holberg et al 1999;Nehme et al 2008). This ion has a relatively high rate coefficient for charge exchange with neutral hydrogen and its presence within the local interstellar medium is unlikely to be linked to the remnant of some past ionization event such as a supernova shock (Lyu and Bruhweiler 1996; Fig. 2 The distribution of cold and neutral gas within ±250 pc of the Sun in the galactic plane as revealed by interstellar Na I absorption measurements (Lallement et al 2003).…”

Section: 1mentioning

confidence: 55%

The trouble with the Local Bubble

Welsh

Shelton

2009

Astrophys Space Sci

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The model of a Local Hot Bubble has been widely accepted as providing a framework that can explain the ubiquitous presence of the soft X-ray background diffuse emission. We summarize the current knowledge on this local interstellar region, paying particular reference to observations that sample emission from the presumed local million degree K hot plasma. However, we have listed numerous observations that are seemingly in conflict with the concept of a hot Local Bubble. In particular, the discovery of solar wind charge exchange that can generate an appreciable soft X-ray background signal within the heliosphere, has led to a reassessment of the generally accepted model that requires a hot local plasma.In order to explain the majority of observations of the local plasma, we forward two new speculative models that describe the physical state of the local interstellar gas. One possible scenario is similar to the present widely accepted model of the Local Hot Bubble, except that it accounts for only 50% of the soft X-ray emission currently detected in the galactic plane, has a lower thermal pressure than previously thought, and its hot plasma is not as hot as previously believed. Although such a model can solve several difficulties with the traditional hot Local Bubble model, a heating mechanism for the dimmer and cooler gas remains to be found. The second possible explanation is that of the 'Hot Top' model, in which the Local Cavity is an old su-B.Y. Welsh ( ) Space Sciences Laboratory, University of California, 7 Gauss Way, Berkeley, CA 94720, USA e-mail: bwelsh@ssl.berkeley.edu R.L. Shelton Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, USA pernova remnant in which no (or very little) million degree local plasma is presently required. Instead, the cavity is now thought to be filled with partially ionized cloudlets of temperature ∼ 7000 K that are surrounded by lower density envelopes of photo-ionized gas of temperature ∼ 20,000 K. Although this new scenario provides a natural explanation for many of the observations that were in conflict with the Local Hot Bubble model, we cannot (as yet) provide a satisfactory explanation or the emission levels observed in the B and Be ultra-soft X-ray bands.

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“…Lallement & Bertin (1992) and Lallement et al (1995) demonstrated that the Sun lies inside a warm, partially ionized, low-density cloud which they called the local interstellar cloud (LIC). This cloud is part of a complex of warm clouds that is located either inside or at the edge of a superbubble (called the Local Bubble) produced by the OB star winds and supernovae of the Scorpius-Centaurus association (Frisch 1995 ;Lyu & Bruhweiler 1996). Linsky et al (2000, hereafter Paper I) summarize the hydrogen and deuterium column densities through the LIC along many lines of sight toward nearby stars inferred from spectra obtained with the Goddard High Resolution Spectrograph (GHRS) and Space Telescope Imaging Spectrograph (STIS) instruments on the Hubble Space T elescope (HST ) and the Extreme Ultraviolet Explorer (EUV E), respectively.…”

Section: Introductionmentioning

confidence: 99%

The Three‐dimensional Structure of the Warm Local Interstellar Medium. II. The Colorado Model of the Local Interstellar Cloud

Redfield

Linsky

2000

ApJ

126

103

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In this second paper in a series on the structure of the local interstellar medium (LISM), we construct a three-dimensional model of the local interstellar cloud (LIC) based on Hubble Space T elescope (HST ), Extreme Ultraviolet Explorer (EUV E), and ground-based Ca II spectra. Starting with hydrogen column densities derived from deuterium column densities measured with the Goddard High Resolution Spectrograph instrument on HST for 16 lines of sight to nearby stars, we derive a model consisting of the sum of nine spherical harmonics that best Ðt the data. We then rederive the model by including the lines of sight to three hot white dwarfs observed by EUV E and 13 lines of sight with Ca II column densities at the projected LIC velocity. The LIC model is clearly not a long thin Ðlamentary structure like optical images of some interstellar clouds (e.g., reÑection nebulae in the Pleiades), but neither is it spherical in shape. As seen from the north Galactic pole, the LIC is egg-shaped with an axis of symmetry that points in the direction l B 315¡. Since the direction of the center of the Scorpius-Centaurus association is l \ 320¡, the shape of the LIC could be determined by the Ñow of hot gas from Sco-Cen. The model shows that the Sun is located just inside the LIC in the direction of the Galactic center and toward the north Galactic pole. The absence of Mg II absorption at the LIC velocity toward a Cen indicates that the distance to the edge of the LIC in this direction is ¹0.05 pc and the Sun should cross the boundary between the LIC and the Galactic (G) cloud in less than 3000 yr. We estimate that the volume of the LIC is about 93 pc3 and its mass is about 0.32The physical parameters and hydrogen column M _ . density of the LIC are roughly consistent with theoretical models of the warm interstellar medium that assume pressure and ionization equilibrium. However, the empirical hydrogen ionization of the LIC is much higher and the gas temperature lower than the theoretical models predict. Therefore, the ionization is likely due to shock activity from a nearby supernova that has not yet reached equilibrium. The higher ionization increases the gas cooling, which can explain why the gas is 2400 K cooler than the ionization equilibrium models predict. Computed and observed temperatures are in agreement for a model with the observed LIC electron density.

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Time-dependent Ionization of H and He in the Local Interstellar Medium

Cited by 46 publications

References 0 publications

The Optical Spectrum of the SN 1006 Supernova Remnant Revisited

The Optical Spectrum of the SN 1006 Supernova Remnant Revisited

The trouble with the Local Bubble

The Three‐dimensional Structure of the Warm Local Interstellar Medium. II. The Colorado Model of the Local Interstellar Cloud

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