The optical interstellar spectrum of Vel (HD 81188) and a measurement of interstellar cloud turbulence (original) (raw)
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The velocity structure of the local interstellar medium probed by ultra-high-resolution spectroscopy
Astronomy and Astrophysics, 1997
We present ultra-high-resolution (0.35km/s FWHM) observations of the interstellar Ca K line towards eight nearby stars (six of which are closer than 30pc). The spectral resolution is sufficient to resolve the line profiles fully, thereby enabling us to detect hitherto unresolved velocity components, and to obtain accurate measurements of the velocity dispersions (b-values). Absorption components due to the Local Interstellar Cloud (LIC) and/or the closely associated `G Cloud' are identified towards all but one star (gamma Oph), but only in one case (51 Oph) are both clouds reliably detected towards the same star. Most of these nearby clouds have velocity dispersions (b=~2km/s) which suggest physical conditions similar to those inferred for the LIC (T_k_=~7000K, v_t_=~1km/s), although at least three lines of sight (towards gamma Aqr, beta Cen and rho Cen) also sample cooler and/or less turbulent material. The spectrum of the nearby Vega-excess star 51 Oph is of particular interest, owing to evidence that several of the absorption components arise in the circumstellar environment.
Velocity Structure of the Local Interstellar Medium
COSPAR Colloquia Series, 1990
We present ultra-high-resolution (0.35 km s −1 FWHM) observations of the interstellar Ca K line towards eight nearby stars (six of which are closer than 30 pc). The spectral resolution is sufficient to resolve the line profiles fully, thereby enabling us to detect hitherto unresolved velocity components, and to obtain accurate measurements of the velocity dispersions (b-values). Absorption components due to the Local Interstellar Cloud (LIC) and/or the closely associated 'G Cloud' are identified towards all but one star (γ Oph), but only in one case (51 Oph) are both clouds reliably detected towards the same star. Most of these nearby clouds have velocity dispersions (b ≈ 2 km s −1) which suggest physical conditions similar to those inferred for the LIC (T k ≈ 7000 K, v t ≈ 1 km s −1), although at least three lines of sight (towards γ Aqr, β Cen and ρ Cen) also sample cooler and/or less turbulent material. The spectrum of the nearby Vega-excess star 51 Oph is of particular interest, owing to evidence that several of the absorption components arise in the circumstellar environment.
Spectral Properties of Interstellar Turbulence via Velocity Channel Analysis
AIP Conference Proceedings, 2005
In this presentation we review the link between the statistics of intensity fluctuations in spectral line data cubes with underlying statistical properties of turbulence in the interstellar medium. Both the formalism of Velocity Channel Analysis for optically thin lines and its extension to the lines with self-absorption is described. We demonstrate that by observing optically thin lines from cold gas in sufficiently narrow (thin) velocity channels one may recover the scaling of the stochastic velocities from turbulent cascade, in particular, Kolmogorov velocities give K −2.7 contribution to the intensity power spectrum. Synthetically increasing the channel thickness separates out the underlying density inhomogeneities of the gas. Effects of self absorption, on the other hand, retain the velocity signature even for integrated lines. As a result, intensity fluctuations tend to show universal but featureless scaling of the power ∝ K −3 over the range of scales.
Turbulence in the Interstellar Medium
Astrophysics and Space Science, 2000
The multivariate technique of principal component analysis (PCA) is a powerful statistical tool with which to describe spectral line imaging observations of the molecular interstellar medium. In particular, as formulated by Heyer & Schloerb (1997), PCA can retrieve statistical information about the velocity fields within the interstellar gas. However, the nature of the transformation of the intrinsic velocity field onto an observable velocity axis is extremely complex if the velocity field is macroturbulent. In this work, PCA is used to show that interstellar velocity fields are characterized by stochastic fluctuations on all measurable scales (i.e., are macroturbulent) and to obtain a quantitative measure of the turbulent velocity dispersion as a function of scale. To relate the measurable statistical information to intrinsic velocity field statistics, an ensemble of artificial density and velocity fields are translated onto the observational domain, utilizing non-LTE radiative transfer calculations. The intrinsic statistical properties of these fields are well-defined and accurately known, which allows the retrieved information to be calibrated to the intrinsic information. Additional results dealing with the instrumental noise and telescope beam-smearing effects on the PCA method are derived and demonstrated. An application of the reformulated method is carried out on an ensemble of outer Galaxy molecular spectral line imaging observations to obtain the first calibrated measurements of interstellar cloud velocity fields.
Velocity Structure of the Interstellar Medium as Seen by the Spectral Correlation Function
The Astrophysical …, 2002
We use the statistical tool known as the '' spectral correlation function '' (SCF) to intercompare simulations and observations of the atomic interstellar medium (ISM). The simulations considered, which mimic three distinct sets of physical conditions, are each calculated for a 300 pc 3 box centered at the Galactic plane. The '' ISM '' run is intended to represent a mixture of cool and warm atomic gas and includes self-gravity and magnetic fields in the calculations. The '' ISM-IT '' run is more representative of molecular clouds, in which the gas is presumed isothermal. The third run '' IT '' is for purely isothermal gas, with zero magnetic field and no self-gravity. Forcing in the three cases is accomplished by including simulated effects of stellar heating (ISM), stellar winds (ISM-IT), or random compressible fluctuations (IT). For each simulation, H i spectral line maps are simulated, and it is these maps that are intercompared, both with each other and with observations, using the SCF. For runs where the separation of velocity features is much greater than the '' thermal '' width of a line, density-weighted velocity histograms are decent estimates of H i spectra. When thermal broadening is large in comparison with fine-scale turbulent velocity structure, this broadening masks subthermal velocity substructure in observed spectra. So, simulated spectra for runs in which thermal broadening is important must be calculated by convolving density-weighted histograms with Gaussians whose width represents the thermal broadening. The H i observations we use for comparison are of the north celestial pole (NCP) loop, a region chosen to minimize line-of-sight confusion on scales greater than 100 pc. None of the simulations match the NCP loop data very well, for a variety of reasons described in the paper. Most of the reasons for simulation/observation discrepancy are predictable and understandable, but one is particularly interesting: the most realistic sets of line profiles and SCF statistics come from artificially expanding the velocity axis of the ISM run by a factor of 6. Without rescaling, the low-velocity dispersion associated with much of the gas in the ISM run causes almost all of the spectra to appear as virtually identical Gaussians whose width is determined solely by temperature-all velocity structure is smeared out by thermal broadening. However, if the velocity axis is expanded by a factor of 6,the SCF distributions of the ISM run and the NCP loop match up fairly well. This means that the ratio of thermal to turbulent pressure in the ISM simulation is much too large as it stands, and that the simulation is deficient in turbulent energy. This is a consequence of the ISM run not including the effects of supernovae. This paper concludes that the SCF is a useful tool for understanding and fine-tuning simulations of interstellar gas, and in particular that realistic simulations of the atomic ISM need to include the effects of energetic stellar winds (e.g., supernovae) in order for the ratio of thermal-to-turbulent pressure to give spectra representative of the observed ISM in our Galaxy.
INTERSTELLAR TURBULENCE I: Observations and Processes
Annual Review of Astronomy and Astrophysics, 2004
▪ Turbulence affects the structure and motions of nearly all temperature and density regimes in the interstellar gas. This two-part review summarizes the observations, theory, and simulations of interstellar turbulence and their implications for many fields of astrophysics. The first part begins with diagnostics for turbulence that have been applied to the cool interstellar medium and highlights their main results. The energy sources for interstellar turbulence are then summarized along with numerical estimates for their power input. Supernovae and superbubbles dominate the total power, but many other sources spanning a large range of scales, from swing-amplified gravitational instabilities to cosmic ray streaming, all contribute in some way. Turbulence theory is considered in detail, including the basic fluid equations, solenoidal and compressible modes, global inviscid quadratic invariants, scaling arguments for the power spectrum, phenomenological models for the scaling of high...
The Astrophysical Journal, 2007
We examine observational characteristics of multi-phase turbulent flows in the diffuse interstellar medium (ISM) using a synthetic radiation field of atomic and molecular lines. We consider the multi-phase ISM which is formed by thermal instability under the irradiation of UV photons with moderate visual extinction A V ∼ 1. Radiation field maps of C + , C 0 , and CO line emissions were generated by calculating the non-local thermodynamic equilibrium (nonLTE) level populations from the results of high resolution hydrodynamic simulations of diffuse ISM models. By analyzing synthetic radiation field of carbon lines of [C II] 158 µm, [C I] 3 P 2 − 3 P 1 (809 GHz), 3 P 1 − 3 P 0 (492 GHz), and CO rotational transitions, we found a high ratio between the lines of high-and low-excitation energies in the diffuse multi-phase interstellar medium. This shows that simultaneous observations of the lines of warm-and cold-gas tracers will be useful in examining the thermal structure, and hence the origin of diffuse interstellar clouds.
Astronomy and Astrophysics, 2006
We present observational line data for five Bok Globules. Observations were made in the J = 2 → 1 and J = 3 → 2 rotational transitions of CO, 13 CO and C 18 O at the Heinrich-Hertz-Telescope in Arizona. Using the stochastic radiative transfer model (SRTM) based on the generalized radiative transfer equation as described in the previous papers of this series, we were able to fit the observed CO lines for four of the five Bok globules. The derived parameters such as the hydrogen density, the cloud mass and temperature in part differ significantly from values obtained from a standard LVG analysis or a microturbulent approach. The best fits are obtained for models characterized by a ratio of the mean free path of photons to the correlation length of the turbulent velocity field of the order of unity. This is what characterizes the mesoturbulent regime that cannot be adressed by standard evaluation methods like LVG or microturbulent approximations.
Multiphase turbulent interstellar medium: some recent results from radio astronomy
2015
The radio frequency 1.4 GHz transition of the atomic hydrogen is one of the important tracers of the diffuse neutral interstellar medium. Radio astronomical observations of this transition, using either a single dish telescope or an array interferometer, reveal different properties of the interstellar medium. Such observations are particularly useful to study the multiphase nature and turbulence in the interstellar gas. Observations with multiple radio telescopes have recently been used to study these two closely related aspects in greater detail. Using various observational techniques, the density and the velocity fluctuations in the Galactic interstellar medium was found to have a Kolmogorov-like power law power spectra. The observed power law scaling of the turbulent velocity dispersion with the length scale can be used to derive the true temperature distribution of the medium. Observations from a large ongoing atomic hydrogen absorption line survey have also been used to study the distribution of gas at different temperature. The thermal steady state model predicts that the multiphase neutral gas will exist in cold and warm phase with temperature below 200 K and above 5000 K respectively. However, these observations clearly show the presence of a large fraction of gas in the intermediate unstable phase. These results raise serious doubt about the validity of the standard model, and highlight the necessity of alternative theoretical models. Interestingly, numerical simulations suggest that some of the observational results can be explained consistently by including the effects of turbulence in the models of the multiphase medium. This review article presents a brief outline of some of the basic ideas of radio astronomical observations and data analysis, summarizes the results from recent observations, and discusses possible implications of the results.
INTERSTELLAR TURBULENCE II: Implications and Effects
Annual Review of Astronomy and Astrophysics, 2004
▪ Interstellar turbulence has implications for the dispersal and mixing of the elements, cloud chemistry, cosmic ray scattering, and radio wave propagation through the ionized medium. This review discusses the observations and theory of these effects. Metallicity fluctuations are summarized, and the theory of turbulent transport of passive tracers is reviewed. Modeling methods, turbulent concentration of dust grains, and the turbulent washout of radial abundance gradients are discussed. Interstellar chemistry is affected by turbulent transport of various species between environments with different physical properties and by turbulent heating in shocks, vortical dissipation regions, and local regions of enhanced ambipolar diffusion. Cosmic rays are scattered and accelerated in turbulent magnetic waves and shocks, and they generate turbulence on the scale of their gyroradii. Radio wave scintillation is an important diagnostic for small-scale turbulence in the ionized medium, giving ...