Shocked Stellar Wind Research Articles

Diffuse X-Ray Emission from the Hydrogen-deficient Planetary Nebula Abell 30

We have detected diffuse X-ray emission within the planetary nebula shell of Abell 30, using a 21.8 ks ROSAT Position Sensitive Proportional Counter observation. The spectrum is very soft, with almost all source counts at energies below 0.4 keV. The extended emission is real because most of the detected counts are within the 0.2-0.4 keV band, which is not seriously plagued by the electronic ghost-image problem. The best-fit Raymond & Smith plasma emission models give plasma temperatures of (2-4.5) × 105 K. The X-ray emission could arise from the hot shocked stellar wind or the central H-poor nebula shocking the ambient H-rich material. Low plasma temperatures are expected in shocked stellar wind, if there is a strong heat conduction at the interface between the hot shocked wind and the surrounding cool nebular shell, or if the fast stellar wind is mass-loaded by the ejecta clumps. Low plasma temperatures are also expected behind the moderate shock of the H-poor shell advancing into the H-rich envelope. High-resolution X-ray images of A30 are needed to determine more accurately the relationship between the X-ray emission and the nebular shells.

Mass-loaded astronomical flows-IV. A time-dependent hydrodynamic model of an observed clumpy wind-blown bubble, RCW 58

We provide an example demonstrating the significance of the clumpy natures of diffuse sources for their evolution. We present a one-dimensional, time-dependent hydrodynamic model with mass injection, from engulfed clumps, into shocked stellar wind of a Wolf-Rayet wind-blown nebula. The observed bubble radius and expansion velocity are matched. Most importantly, the model reproduces the broad velocity range over which gas with temperatures less than about 1.5 × 105 K is observed.

Turbulent Mixing in Wind-Blown HII Regions

AbstractTurbulent mixing between an ionization bounded HII region and a hot shocked stellar wind (HSSW), which keeps it under pressure, is examined. Recently we have shown that acoustic disturbances can grow there to finite amplitude in a time scale which is comparable to the sound crossing time in the HII layer. The resulting turbulence will then stretch fluid elements and the frozen-in magnetic field. A condition under which turbulence can decay down to the viscous scale, where mixing is very efficient, is derived. For a uniform and plane parallel magnetic field Bo and a constant density ρo of the ambient medium, we find that efficient mixing takes place near the polar regions. Subsequently the rate of mass addition to the hot bubble is calculated and it is shown that catastrophic cooling is likely to occur. In the case of NGC 6334(A) it seems that this has just happened and we predict an upper limit for Bo of 4 x 10-5 gauss there. This model may also explain the existence of highly ionized species (e.g. OVI), soft X-rays and high velocity flows of the order of 100 km/s.

A detailed study of the LYNDS 1551 star formation region

view Abstract Citations (113) References (67) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Detailed Study of the Lynds 1551 Star Formation Region Stocke, John T. ; Hartigan, Patrick M. ; Strom, Stephen E. ; Strom, Karen M. ; Anderson, Edwin R. ; Hartmann, Lee W. ; Kenyon, Scott J. Abstract The results of an optical spectroscopic and photometric study of the star formation region Lynds 1551 and the young stellar object IRS 5 and a high-resolution spectroscopic study of the Herbig-Haro (HH) objects associated with the outflow from this region are reported. The results further establish IRS 5 as the source of the reflected light in the region. Spectral studies suggest that the spectrum of IRS 5 is dominated by the light from an accretion disk analogous to those characterizing objects of the rare class of FU Orionis objects. Analysis of high and medium resolution spectra of the optical 'jet' emanating from IRS 5 strongly suggest that the 'jet' HH objects are the bow shock interfaces between two winds coming from the IRS 5 region. The high-velocity wind is inferred to have a velocity of 440 km/s, and is identified as the stellar wind. High-density clumps in the stellar wind shock a lower-density, but much more pervasive 'second wind' which has an inferred velocity of about 160 km/s. Publication: The Astrophysical Journal Supplement Series Pub Date: October 1988 DOI: 10.1086/191287 Bibcode: 1988ApJS...68..229S Keywords: Astronomical Photometry; Charge Coupled Devices; Early Stars; Herbig-Haro Objects; Star Formation; Balmer Series; Reflected Waves; Signal To Noise Ratios; Stellar Luminosity; Stellar Spectra; Astrophysics; NEBULAE: INDIVIDUAL ALPHANUMERIC: L1551 (LYNDS 1551); NEBULAE: REFLECTION; STARS: FORMATION; STARS: PRE--MAIN-SEQUENCE full text sources ADS | data products SIMBAD (7)

The dynamics of Herbig–Haro objects

Radial-velocity and proper-motion information on Herbig–Haro (HH) objects is analyzed for clues to the character of the supersonic flows that produce the objects. We examine the correlations among space velocities, excitation states, internal velocity dispersions, electron densities, and positions relative to the exciting stars for the emission knots of HH 1 and HH 2. Whereas the interstellar-bullet model may account for most of the phenomena in the HH 1-HH 2 system, HH 43 is found to show characteristics of a stellar wind shock against an obstacle cloudlet. By contrast, the HH 46-HH 47 system exhibits a continuous, well-collimated jet of emission with knots that may be due to instabilities in the confined, supersonic flow.

The Effect of Mass-Loss On the Evolution of HII Regions in the L.M.C.

Empirical calibrations based on prior studies of Galactic OB Stars are used to determine the integrated stellar wind mechanical luminosity (Lw) and the integrated Lyman continuum photon luminosity (S*) for 10 OB clusters in the L.M.C. These values of Lw and S*, together with narrow band H-α surface photometry of an overlapping sample of 15 L.M.C. HII regions, are used to show that the large shell-like HII regions, in the L.M.C. are stellar wind bubbles 3 to 5 million years old. In order to reproduce the general properties of these HII shells, a thick cushion of shocked stellar wind gas must have been present on the inside of the shell for most of the life time of the nebula, and the shell itself must be ram pressure confined by the HI/molecular cloud out of which it formed.

Physical Processes in Nebular Shells and the Interpretation of Nebular Spectra

Computed models are now recognized as useful tools for interpretation of the spectra of planetary nebulae. However, even the most detailed models need geometrical parameters such as filling factors which are poorly determined by observations. Some effects may be seen more clearly by modeling the stratification than by just using total fluxes. A simple model for NGC 6720 is presented which reproduces the behavior of (Ne III) λ3869 observed by Hawley and Miller (1977), clearly showing the effects of charge transfer. The behavior of C II λ4267 remains puzzling. Finally, we comment on the interaction of high velocity stellar winds with nebular shells. Non-equilibrium particle distributions at the contact between the shocked stellar wind and the nebula may result in the rapid cooling of the shocked gas.

Cosmic Ray Acceleration in the Presence of Losses

Diffusive Fermi acceleration on hydromagnetic shock fronts is a fairly slow process: most scatterings are energetically neutral, only those across the velocity jump yield a first order acceleration. Thus energy losses of cosmic rays (CR) due to e.g. adiabatic cooling, ionising, Coulomb−, or nuclear collisions, bremsstrahlung, synchrotron radiation, should play a role in limiting the acceleration, since most shocks are coupled with a loss region (HII regions, SNR's, galactic density wave, stellar wind shocks). In addition, for this process to be a local one in the galaxy, the magnetic irregularities must either be excited by the accelerated CR's or produced by a downstream source. This implies a finite wave build-up time during the shock life time. Nevertheless, in the loss free case, the time-asymptotic amplification is independent of the mean free path λ (or the diffusion coefficient κ) which only appears in the spatial scale for the CR intensity. To investigate the effects of losses, the CR diffusion equation is amended by a simple loss term f/τ, with an (energy dependent) loss time τ, f being the CR momentum distribution. For τ spatially homogeneous, a distributed source is required. Then acceleration is only effective if X ≡ 4κ/VS2·τ ≲ 1, i.e. if the acceleration time tacc = 4κ/VS2 is smaller than τ, on either side of the shock. As the Figure shows, also the spatial intensity profile is modified. Wave excitation in dense clouds is prohibitive. Even in a “warm” (T ≃ 104K) intercloud medium shock speeds VS ≳ 3×107 cm/sec are required to accelerate mildly relativistic particles. Waves from an upstream source (star) inside clouds should frictionally dissipate after distances L ≲ 5×1014 cm « λ (10 MeV) if a solar wind scaling is adopted. Thus, presumably in such a case there is no acceleration of stellar or shock-injected particles through a stellar wind shock by scattering within the cloud, but possibly by reflection from beyond the cloud, the condition being 1/τ · VS ≲ 1, where 1 is the linear cloud size. Diffusive approach from outside to a standing shock appears to be very energy-selective, even in a loss free medium.

Stellar winds and globules in Hii regions

The interaction of a plane-parallel hypersonic stellar wind with a globule in an Hii region is considered in two approximations. In both approximations, the ionization front on the globule remains strong-D type, and a flow pattern containing two oppositely facing shock waves results. In the first approximation, the structure of the shocked region is calculated assuming that globule gas and stellar wind gas mix well and move at the same velocity. However, this assumption results in a very thick shocked layer and the assumption of good mixing is consequently not well justified. This approximation provides an upper limit on the gas velocities expected in the shocked gas which originated at the globule. In the second approximation, the stellar wind merely applies pressure to balance the momentum flux in the globule gas. The structure of the shocked region is calculated on the assumption that a tangential discontinuity exists between shocked stellar wind and shocked glubule gas. Structures may be produced having velocities ∼10 km s−1 and emission measures ∼103 cm−6 pc with reasonable stellar luminosities and mass loss rates.