We present a range of steady-state photoionization simulations, corresponding to different assumed shell geometries and compositions, of the unseen postulated rapidly expanding outer shell to the Crab Nebula. The properties of the shell are constrain
ed by the mass that must lie within it, and by limits to the intensities of hydrogen recombination lines. In all cases the photoionization models predict very strong emission from high ionization lines that will not be emitted by the Crabs filaments, alleviating problems with detecting these lines in the presence of light scattered from brighter parts of the Crab. The NIR [Ne VI] $lambda$7.652 $mu$m line is a particularly good case; it should be dramatically brighter than the optical lines commonly used in searches. The C IV $lambda1549AA$ doublet is predicted to be the strongest absorption line from the shell, which is in agreement with HST observations. We show that the cooling timescale for the outer shell is much longer than the age of the Crab, due to the low density. This means that the temperature of the shell will actually remember its initial conditions. However, the recombination time is much shorter than the age of the Crab, so the predicted level of ionization should approximate the real ionization. In any case, it is clear that IR observations present the best opportunity to detect the outer shell and so guide future models that will constrain early events in the original explosion.
Understanding how molecules and dust might have formed within a rapidly expanding young supernova remnant is important because of the obvious application to vigorous supernova activity at very high redshift. In previous papers, we found that the H2 e
mission is often quite strong, correlates with optical low-ionization emission lines, and has a surprisingly high excitation temperature. Here we study Knot 51, a representative, bright example, for which we have available long slit optical and NIR spectra covering emission lines from ionized, neutral, and molecular gas, as well as HST visible and SOAR Telescope NIR narrow-band images. We present a series of CLOUDY simulations to probe the excitation mechanisms, formation processes and dust content in environments that can produce the observed H2 emission. We do not try for an exact match between model and observations given Knot 51s ambiguous geometry. Rather, we aim to explain how the bright H2 emission lines can be formed from within the volume of Knot 51 that also produces the observed optical emission from ionized and neutral gas. Our models that are powered only by the Crabs synchrotron radiation are ruled out because they cannot reproduce the strong, thermal H2 emission. The simulations that come closest to fitting the observations have the core of Knot 51 almost entirely atomic with the H2 emission coming from just a trace molecular component, and in which there is extra heating. In this unusual environment, H2 forms primarily by associative detachment rather than grain catalysis. In this picture, the 55 H2-emitting cores that we have previously catalogued in the Crab have a total mass of about 0.1 M_sun, which is about 5% of the total mass of the system of filaments. We also explore the effect of varying the dust abundance. We discuss possible future observations that could further elucidate the nature of these H2 knots.
(abridged) We study the consequence of star formation (SF) in an self-gravity dominated accretion disk in quasars. The warm skins of the SF disk are governed by the radiation from the inner part of the accretion disk to form Compton atmosphere (CAS).
The CAS are undergoing four phases to form broad line regions. Phase I is the duration of pure accumulation supplied by the SF disk. During phase II clouds begin to form due to line cooling and sink to the SF disk. Phase III is a period of preventing clouds from sinking to the SF disk through dynamic interaction between clouds and the CAS. Finally, phase IV is an inevitable collapse of the entire CAS through line cooling. This CAS evolution drives the episodic appearance of BLRs. Geometry and dynamics of BLRs can be self-consistently derived from the thermal instability of the CAS during phases II and III by linear analysis. The metallicity gradient of SF disk gives rise to different properties of clouds from outer to inner part of BLRs. We find that clouds have column density N_H < 10^22cm^{-2} in the metal-rich regions whereas they have N_H > 10^22 cm^{-2} in the metal-poor regions. The metal-rich clouds compose the high ionization line (HIL) regions whereas the metal-poor clouds are in low ionization line (LIL) regions. Metal-rich clouds in HIL regions will be blown away by radiation pressure, forming the observed outflows. The LIL regions are episodic due to the mass cycle of clouds with the CAS in response to continuous injection by the SF disk, giving rise to different types of AGNs. Based on SDSS quasar spectra, we identify a spectral sequence in light of emission line equivalent width from Phase I to IV. A key phase in the episodic appearance of the BLRs is bright type II AGNs with no or only weak BLRs. We discuss observational implications and tests of the theoretical predictions of this model.
We have carried out a near-infrared, narrow-band imaging survey of the Crab Nebula, in the H2 2.12 micron and Br-gamma 2.17 micron lines, using the Spartan Infrared camera on the SOAR Telescope. Over a 2.8 x 5.1 area that encompasses about 2/3 of the
full visible extent of the Crab, we detect 55 knots that emit strongly in the H2 line. We catalog the observed properties of these knots. We show that they are in or next to the filaments that are seen in optical-passband emission lines. Comparison to HST [S II] and [O III] images shows that the H2 knots are strongly associated with compact regions of low-ionization gas. We also find evidence of many additional, fainter H2 features, both discrete knots and long streamers following gas that emits strongly in [S II]. A pixel-by-pixel analysis shows that about 6 percent of the Crabs projected surface area has significant H2 emission that correlates with [S II] emission. We measured radial velocities of the [S II] lambda6716 emission lines from 47 of the cataloged knots and find that most are on the far (receding) side of the nebula. We also detect Br-gamma emission. It is right at the limit of our survey, and our Br-gamma filter cuts off part of the expected velocity range. But clearly the Br-gamma emission has a quite different morphology than the H2 knots, following the long linear filaments that are seen in H-alpha and in [O III] optical emission lines.
In a sub-arcsec near-infrared survey of the Crab Nebula using the new Spartan Infrared Camera, we have found several knots with high surface brightness in the H_2 2.12 micron line and a very large H_2 2.12 micron to Br-gamma ratio. The brightest of t
hese knots has an intensity ratio I(H_2 2.12 micron)/I(Br-gamma) = 18+/-9, which we show sets a lower limit on the ratio of masses in the molecular and recombination (i.e. ionized) zones M_mol / M_rec >/- 0.9, and a total molecular mass within this single knot M_mol >/- 5E-5 M_sun. We argue that the knot discussed here probably is able to emit so strongly in the 2.12 micron line because its physical conditions are better tuned for such emission than is the case in other filaments. It is unclear whether this knot has an unusually large M_mol / M_rec ratio, or if many other Crab filaments also have similar amounts of molecular gas which is not emitting because the physical conditions are not so well tuned.
Previous work has shown the Orion Bar to be an interface between ionized and molecular gas, viewed roughly edge on, which is excited by the light from the Trapezium cluster. Much of the emission from any star-forming region will originate from such i
nterfaces, so the Bar serves as a foundation test of any emission model. Here we combine X-ray, optical, IR and radio data sets to derive emission spectra along the transition from H+ to H0 to H2 regions. We then reproduce the spectra of these layers with a simulation that simultaneously accounts for the detailed microphysics of the gas, the grains, and molecules, especially H2 and CO. The magnetic field, observed to be the dominant pressure in another region of the Orion Nebula, is treated as a free parameter, along with the density of cosmic rays. Our model successfully accounts for the optical, IR and radio observations across the Bar by including a significant magnetic pressure and also heating by an excess density of cosmic rays, which we suggest is due to cosmic rays being trapped in the compressed magnetic field. In the Orion Bar, as we had previously found in M17, momentum carried by radiation and winds from the newly formed stars pushes back and compresses the surrounding gas. There is a rough balance between outward momentum in starlight and the total pressure in atomic and molecular gas surrounding the H+ region. If the gas starts out with a weak magnetic field, the starlight from a newly formed cluster will push back the gas and compress the gas, magnetic field, and cosmic rays until magnetic pressure becomes an important factor.
