Eagle Nebula Pillars: From Models to Observations

Over the past few years, our group has been developing hydrodynamic models to simulate formation of the Eagle Nebula pillars. The true test of any model is, of course, how well it can reproduce the observations. Here, we discuss how we go about testing our models against observations. We describe the process by which we “observe” the model data to create synthetic maps. We show an example of this technique using one of our model runs and compare the resultant synthetic map to the real one.     

Studies of pre-main-sequence stars with integral field spectrographs

We review the contribution of integral field spectroscopy (IFS) to pre-main-sequence star studies. These studies are mostly synoptic in nature reflecting the use of this technique in addressing difficult and complex objects. Physical diagnostics were derived from IFS data such as (atomic, molecular) gas excitation and pre-shock densities, gas morphology and spectra from close binaries. Models for the sources are directly tested: shock models (planar, bow-shock) and magneto-hydrodynamics jet models.Future contributions to the field are addressed. Surveys of mass loss across age and mass spectrum and in the nearby Orion Nebula cluster emerge as the most scientifically promising.     

Pulsar reflections within the Crab Nebula

Between 1997 August and October, the radio pulses from the Crab pulsar were followed by discrete moving echoes, which appear to be reflections from part of an ionized shell in the outer part of the Crab Nebula, crossing the line of sight to pulsar. Similar events have now been recognized in recordings from the past 30 yr, and it seems that the Nebula must contain a large number of ionized shell-like surfaces on a much finer scale than recognized hitherto.     

Refractory Metal Nuggets – Formation of the first condensates in the Solar Nebula

As gas flowed from the solar accretion disk or Solar Nebula onto the proto-Sun, magnetic pressure gradients in the solar magnetosphere and the inner Solar Nebula provided an environment where some of this infalling flow was diverted to produce a low pressure, high temperature, gaseous, “infall” atmosphere around the inner Solar Nebula. The pressure in this inner disk atmosphere was mainly dependant on the accretion flow rate onto the star. High flow rates implied relatively high pressures, which decreased over time as the accretion rate decreased.In the first hundred thousand years after the formation of the Solar Nebula, accretional flow gas pressures were high enough to create submicron-sized Refractory Metal Nuggets (RMNs) – the precursors to Calcium Aluminum Inclusions (CAIs). Optimal temperatures and pressures for RMN formation may have occurred between 20,000 and 100,000 years after the formation of the Solar Nebula. It is possible that conditions were conducive to RMN/CAI formation over an 80,000 year timescale. The “infall” atmosphere and the condensation of refractory particles within this atmosphere may be observable around the inner disks of other protostellar systems.The interaction of forces from magnetic fields with the radiation pressure from the proto-Sun and the inner solar accretion disk potentially produced an optical-magnetic trap above and below the inner Solar Nebula, which provided a relatively stable environment in which the RMNs/proto-CAIs could form and grow. These RMN formation sites only existed during accretion events from the proto-solar disk onto the proto-Sun. As such, the formation and growth time of a particular RMN was dependent on the timescale of its nascent accretion event.Observational evidence suggests that RMNs were the nucleation particles for CAIs. As a consequence, the observed bimodal distribution of ²⁶Al in CAIs, where some CAIs have ²⁶Al while others do not, is probably due to the injection ²⁶Al during the short CAI formation period, where ²⁶Al was not present when the first CAIs were formed.     

The Stingray Nebula: watching the rapid evolution of a newly born planetary nebula

The formation and early evolution of planetary nebulae represent one of the most poorly understood phases of stellar evolution (Kwok, 1987; Maddox, 1995). One of the youngest, the Stingray Nebula (He3-1357) (Henize, 1967; Henize, 1976), shows all the tell-tale signs of a newly born planetary nebula: it has become ionized only within the past few decades (Parthasarathy et al., 1993); the mass-loss from the central star has ceased within the past few years; and the central star is becoming hotter and fainter as expected from a star on its way to becoming a DA white dwarf (Parthasarathy et al., 1995). The Stingray Nebula thus provides the ideal laboratory for examining the early structure and evolution of this class of objects. Images of the Stingray Nebula, obtained with the Hubble Space Telescope, show for the first time that its multiple expulsions of matter are focused by an equatorial ring and bubbles of gas located on opposite sides of the ring (Bobrowsky et al., 1995). The position angle of the outflows has changed, possibly as a result of precessional motion induced by the presence of a companion star. This is consistent with the precessing jet model by Livio & Pringle (1996). Indeed, we have reported the discovery of a companion star in the Stingray Nebula (Bobrowsky et al., 1998). Finally, we present evidence of the companion star dynamically distorting the gas in this newly-born planetary nebula.     

Constraints on stellar-dynamical models of the Orion Nebula Cluster

The results obtained by Kroupa, Petr & McCaughrean (1999) for specific models of young compact binary-rich clusters are generalised using dynamical scaling relations, to infer the candidate set of possible birth models leading to the Orion Nebula Cluster (ONC), of which the Trapezium Cluster is the core. It is found that candidate sets of solutions exist which allow the ONC to be in virial equilibrium, expanding or contracting. The range of possible solutions is quite narrow. These results will serve as guidelines for future, CPU-intensive calculations of the stellar-dynamical and astrophysical evolution of the entire ONC. These, in turn, will be essential to quantify observables that will ultimately discriminate between models, thus allowing us to understand if the ONC is in the process of assembling a rich Galactic cluster, and, if this is the case, how it occurs.     

The secular behavior of X-ray and radio emission from supernova remnants

General models for the secular behavior of the radio and X-ray emission from supernova remnants are examined and compared with the observations. Hot plasma and synchrotron models for the X-ray emission are considered. Among other things, it is concluded that (1) the total kinetic energy released in most supernova outbursts is probably less than about 10⁵¹ ergs; (2) continuous injection probably occurs for at least 10 yr in every case and about 1000 yr in most supernova remnants, in which case the supernova remnants 3C392, W28, Pup A and IC443 should produce 1–10 keV X-ray fluxes 10^–10 ergs/cm² sec; and (3) the X-ray sources in the Crab Nebula, Cas A and Tycho can be explained in terms of a model wherein continuous injection occurs for 300 yr for the Crab Nebula, much less than 250 yr for Cas A and much longer than 400 yr for Tycho. Finally, it is shown that if Tycho and Cas A contain an X-ray star such as NP0532, it is quite possible that the X-ray emission from those sources is predominantly due to the X-ray star.Supported by the Air Force Office of Scientific Research under Contract No. F44620-67-C-0065.     

The radio spectra and physical parameters of some extended radio sources in monoceros

The radio spectra of the Rosette Nebula, the source S280 and the Monoceros Nebulosity are established over a wide range of frequencies. The radio spectra of the Rosette Nebula and S280, show a typical thermal shape. Physical parameters of both Hii regions are derived by adopting a spherical model.The radio spectrum of the Monoceros Nebulosity shows a non-thermal shape (sp. index –0.66) but the exact nature of the object is not clear. Assuming it is an SNR some of its physical characteristics are estimated.     

Three-dimensional numerical model of the Omega Nebula (M17): simulated thermal X-ray emission

We present 3D hydrodynamical simulations of the superbubble M17, also known as the Omega Nebula, carried out with the adaptive grid code yguazú-a , which includes radiative cooling. The superbubble is modelled considering the winds of 11 individual stars from the open cluster inside the nebula (NGC 6618), for which there are estimates of the mass-loss rates and terminal velocities based on their spectral types. These stars are located inside a dense interstellar medium, and they are bounded by two dense molecular clouds. We carried out three numerical models of this scenario, considering different line-of-sight positions of the stars (the position in the plane of the sky is known, thus fixed). Synthetic thermal X-ray emission maps are calculated from the numerical models and compared with ROSAT observations of this astrophysical object. Our models successfully reproduce both the observed X-ray morphology and the total X-ray luminosity, without taking into account the thermal conduction effects.     

Modelling and distance determination of planetary nebulae

At present the lack of a good, generally accepted distance scale is the major limitation in the study of Planetary Nebula (PN) evolution and their distribution. We describe our fully self-consistent method to derive the physical parameters of a PN, including its distance, from a set of observables. For this we calculate various models and search for a best fit of the model predictions to the observables. An accurate determination of the angular diameter is important for a good distance determination. We think that we shall achieve an accuracy of 30% in distance. We estimate the error in stellar temperature at less than 15% and the uncertainty in the abundances at 0.10 dex.     

Synchrotron radiation from the Crab Nebula discriminates between models of space–time foam

It has been argued by Jacobson, Liberati and Mattingly that synchrotron radiation from the Crab Nebula imposes a stringent constraint on any modification of the dispersion relations of the electron that might be induced by quantum gravity. We supplement their analysis by deriving the spectrum of synchrotron radiation from the coupling of an electrically charged particle to an external magnetic fields in the presence of quantum-gravity effects of the general form (E/M_QG). We find that the synchrotron constraint from the Crab Nebula practically excludes 1.74 for M_QGm_P=1.2×10¹⁹ GeV. On the other hand, this analysis does not constrain any modification of the dispersion relation of the photon that might be induced by quantum gravity. We point out that such quantum-gravity effects need not obey the equivalence principle, a point exemplified by the Liouville-string D-particle model of space–time foam. This model suggests a linear modification of the dispersion relation for the photon, but not for the electron, and hence is compatible with known constraints from the Crab Nebula and elsewhere.     

Relationship between the nonstationary radio-luminosity variations of the Crab Nebula and the activity of its pulsar

Based on the half-century-long history of radio observations of the Crab Nebula, we investigate the evolution of its radio luminosity. We found a secular decrease in the radio luminosity; it has decreased by 9% since the discovery of the radio source in 1948. Apart from the secular decrease in the luminosity of the Crab Nebula, we identified two time intervals, 1981–1987 and 1992–1998, when radio bursts with energy release ～10⁴¹ erg took place. In these years, the spectral indices of the instantaneous spectra decreased significantly due to the increase in the flux densities at short (centimeter and millimeter) wavelengths. These events were preceded by sudden increases in the pulsar’s rotation rates, the largest of which, with an amplitude of ΔΩ/Ω = 3 × 10^?8, occurred in 1975 and 1989. We show that the magnetospheric instability mechanism that accompanies strong glitches can provide the energetics of the excess luminosity of the Nebula through the ejection of relativistic electrons with a total energy higher than 6 × 10⁴² erg from the pulsar’s magnetosphere.     

Neutrino synchrotron radiation

The neutrino luminosity of several models of neutron stars has been computed according to the photon-neutrino coupling theory and compared with that of the current-current coupling theory. It is shown that the NSR process alone should have cooled the core of the neutron star created in a supernova explosion in 1954 A.D. to a temperature around 2×10⁹ K according to the photon-neutrino coupling theory.The emission power of the star is greater than the emission power of the X-ray source discovered in the Crab Nebula; so the source may be interpreted as the thermal radiation of the star according to the photon-neutrino coupling theory.     

New CO and Millimeter Continuum Observations of the z = 2.394 Radio Galaxy 53W002

The Chandra X-Ray Observatory observed the Crab Nebula and pulsar during orbital calibration. Zeroth-order images with the High-Energy Transmission Grating (HETG) readout by the Advanced CCD Imaging Spectrometer spectroscopy array (ACIS-S) show a striking richness of X-ray structure at a resolution comparable to that of the best ground-based visible-light observations. The HETG-ACIS-S images reveal, for the first time, an X-ray inner ring within the X-ray torus, the suggestion of a hollow-tube structure for the torus, and X-ray knots along the inner ring and (perhaps) along the inward extension of the X-ray jet. Although complicated by instrumental effects and the brightness of the Crab Nebula, the spectrometric analysis shows systematic variations of the X-ray spectrum throughout the nebula.     

On the expansion of supernova shells

We have investigated a simple model for the effects of a central pulsar on the expansion of supernova shells. Some numerical results relevant to the Crab Nebula are also reported.     

Observations of high velocities inHii regions with a two-etalon Fabry-Pérot spectrometer

High internal motions of the ionized material in theHii regions M 8, M16, M 17 and the Orion Nebula were searched for with a two-etalon Fabry-Perot monochromotor. The profiles of the [Oiii], 5007 Å and in one case the 4959 Å line were obtained at many positions from these nebulae. Non-gaussian wings of up to –60 km/s were found on the profiles from M 17 and M 16 over regions several minutes across. Line doubling of up to 20 km/s was definitely found in M17 and M 8. Small components with radial velocities of up to –55 km/s with respect to the means were suspected in M 8 and the Orion Nebula.     

Exact optics: a unification of optical telescope design

A strong emission line at 2.8935 μm discovered by Rubin et al. in an ISO SWS02 spectrum of the Orion Nebula is identified as the     multiplet of O  i . Line formation is due to de-excitation cascades following UV-pumping of high ³S^o and ³D^o terms and occurs in the O  i zone immediately behind the hydrogen ionization front. This cascade mechanism also accounts for permitted O  i triplet lines in the optical spectrum of the Nebula, as shown by Grandi. An escape probability treatment of the O  i cascades accounts for the strength of the λ 2.89-μm line and suggests interesting diagnostic possibilities for the optical lines.     

Large-Scale Structure of the Carina Nebula

Observations obtained with the Midcourse Space Experiment (MSX) satellite reveal for the first time the complex mid-infrared morphology of the entire Carina Nebula (NGC 3372). On the largest size scale of approximately 100 pc, the thermal infrared emission from the giant H ii region delineates one coherent structure: a (somewhat distorted) bipolar nebula with the major axis perpendicular to the Galactic plane. The Carina Nebula is usually described as an evolved H ii region that is no longer actively forming stars, clearing away the last vestiges of its natal molecular cloud. However, the MSX observations presented here reveal numerous embedded infrared sources that are good candidates for sites of current star formation. Several compact infrared sources are located at the heads of dust pillars or in dark globules behind ionization fronts. Because their morphology suggests a strong interaction with the peculiar collection of massive stars in the nebula, we speculate that these new infrared sources may be sites of triggered star formation in NGC 3372.     

The Tails of Externally Illuminated Young Stellar Objects in the Orion Nebula

Recent observations of externally illuminated young stellar objects in the Orion Nebula show that they have a variety of morphologies. In particular, long thin tails which terminate in points have been observed. Mechanisms will be considered here to understand the formation and properties of these tails.     

The Crab Nebula's dynamical age as measured from its northern filamentary jet

We present a deep [O  iii ]λλ4959,5007 image of the northern filamentary jet in the Crab Nebula taken with the 8.2-m Subaru telescope. Using this image and an image taken with the Kitt Peak National Observatory (KPNO) 4-m in 1988, we have computed proper motions for 35 locations in the jet. The results suggest that when compared to the main body of the remnant, the jet experienced less outward acceleration from the central pulsar's rapidly expanding synchrotron nebula. The jet's apparent expansion rate yields an undecelerated explosion date for the Crab Nebula of 1055 ± 24 CE, a date much closer to the appearance of the historic 1054 CE guest star than the 1120–1140 CE dates estimated in previous studies using filaments located within the remnant's main nebula. Our proper motion measurements suggest the jet likely formed during the 1054 supernova explosion and represents the remnant's highest velocity knots possibly associated with a suspected N–S bipolar outflow from the supernova explosion.