C. R. O'Dell

Fine-Scale Temperature Fluctuations in the Orion Nebula and the t2 Problem

We present a high spatial resolution map of the columnar electron temperature (Tc) of a region to the southwest of the Trapezium in the Orion Nebula. This map was derived from Hubble Space Telescope images that isolated the primary lines of H I for determination of the local extinction and of the [O III] lines for determination of Tc. Although there is no statistically significant variation of Tc with distance from the dominant ionizing star, θ1 Ori C, we find small-scale variations in the plane of the sky down to a few arcseconds, which are compatible with the variations inferred from comparing the value of Te derived from forbidden and recombination lines, commonly known as the t2 problem. We present other evidence for fine-scale variations in conditions in the nebula, these being variations in the surface brightness of the nebula, fluctuations in radial velocities, and ionization changes. From our Tc map and other considerations we estimate that t2 = 0.028 ± 0.006 for the Orion Nebula. Shadowed regions behind clumps close to the ionization front can make a significant contribution to the observed temperature fluctuations, but they cannot account for the t2 values inferred from several methods of temperature determination. It is shown that an anomalous broadening of nebular emission lines appears to have the same sense of correlation as the temperature anomalies, although a causal link is not obvious.

The Three-Dimensional Dynamic Structure of the Inner Orion Nebula

The three-dimensional structure of the brightest part of the Orion Nebula is assessed in the light of published and newly established data. We find that the widely accepted model of a concave blister of ionized material needs to be altered in the southwest direction from the Trapezium, where we find that the Orion-S feature is a separate cloud of very optically thick molecules within the body of ionized gas, which is probably the location of the multiple embedded sources that produce the optical and molecular outflows that define the Orion-S star formation region. Evidence for this cloud comes from the presence of H2CO lines in absorption in the radio continuum and discrepancies in the extinction derived from radio-optical and optical-only emission. We present an equilibrium Cloudy model of the Orion-S Cloud, which successfully reproduces many observed properties of this feature, including the presence of gas-phase H2CO in absorption. We also report the discovery of an open-sided shell of [O III] surrounding the Trapezium stars, revealed through emission-line ratio images and the onset of radiation shadows beyond some proplyds. We show that the observed properties of the shell are consistent with it being a stationary structure, produced by shock interactions between the ambient nebular gas and the high-velocity wind from θ1 Ori C. We examine the implications of the recently published evidence for a large blueshifted velocity of θ1 Ori C with respect to the Orion molecular cloud, which could mean that this star has only recently begun to photoionize the Orion Nebula. We show that current observations of the nebula do not rule out such a possibility, so long as the ionization front has propagated into a pre-existing low-density region. In addition, a young age for the nebula would help explain the presence of nearby proplyds with a short mass-loss timescale to photoablation.

Physical Conditions in Orion’s Veil. II. A Multicomponent Study of the Line of Sight toward the Trapezium

Orions Veil is an absorbing screen that lies along the line of sight to the Orion H II region. It consists of two or more layers of gas that must lie within a few parsecs of the Trapezium cluster. Our previous work considered the Veil as a whole and found that the magnetic field dominates the energetics of the gas in at least one component. Here we use high-resolution STIS UV spectra that resolve the two velocity components in absorption and determine the conditions in each. We derive a volume hydrogen density, 21 cm spin temperature, turbulent velocity, and kinetic temperature for each. We combine these estimates with magnetic field measurements to find that magnetic energy significantly dominates turbulent and thermal energies in one component, while the other component is close to equipartition between turbulent and magnetic energies. We observe H2 absorption for highly excited v, J levels that are photoexcited by the stellar continuum, and detect blueshifted S+2 and P+2 ions. These ions must arise from ionized gas between the mostly neutral portions of the Veil and the Trapezium and shields the Veil from ionizing radiation. We find that this layer of ionized gas is also responsible for He I λ3889 absorption toward the Veil, which resolves a 40 year old debate on the origin of He I absorption toward the Trapezium. Finally, we determine that the ionized and mostly atomic layers of the Veil will collide in less than 85,000 yr.

High Spatial Velocity Features in the Orion Nebula

We have used widely spaced in time Hubble Space Telescope images to determine tangential velocities of features associated with outflows from young stars. These observations were supplemented by ground-based telescope spectroscopy, and from the resultant radial velocities, space velocities were determined for many outflows. Numerous new moving features were found and grouped into known and newly assigned Herbig-Haro objects. It was found that stellar outflow is highly discontinuous, as frequently is the case, with long-term gaps of a few hundred years, and that these outflow periods are marked by staccato bursts over periods of about ten years. Although this has been observed in other regions, the Orion Nebula Cluster presents the richest display of this property. Most of the large-scale Herbig-Haro objects in the brightest part of the Orion Nebula appear to originate from a small region northeast of the strong Orion-S radio and infrared sources. With the possible exception of HH 203, we are not able to identify specific stellar sources, but do identify candidate sources for several other bright Herbig-Haro objects. We find that there are optical features in the BN-KL region that can be related to the known large-scale outflow that originates there. We find additional evidence for this outflow originating 500-1000 years ago.

Physical Conditions in Barnard's Loop, Components of the Orion-Eridanus Bubble, and Implications for the Warm Ionized Medium Component of the Interstellar Medium

We have supplemented existing spectra of Barnard’s Loop with high accuracy spectrophotometry of one new position. Cloudy photoionization models were calculated for a variety of ionization parameters and stellar temperatures and compared with the observations. After testing the procedure with recent observations of M43, we establish that Barnard’s Loop is photoionized by four candidate ionizing stars, but agreement between the models and observations is only possible if Barnard’s Loop is enhanced in heavy elements by about a factor of 1.4. Barnard’s Loop is very similar in properties to the brightest components of the Orion-Eridanus Bubble and the warm ionized medium (WIM). We are able to establish models that bound the range populated in low-ionization color–color diagrams (I([Sii])/I(Hα )v ersusI([Nii])/I(Hα)) using only a limited range of ionization parameters and stellar temperatures. Previously established variations in the relative abundance of heavy elements render uncertain the most common method of determining electron temperatures for components of the Orion-Eridanus Bubble and the WIM based only on the I([Nii])/I(Hα) ratio, although we confirm that the lowest surface brightness components of the WIM are on average of higher electron temperature. The electron temperatures for a few high surface brightness WIM components determined by direct methods are comparable to those of classical bright Hii regions. In contrast, the low surface brightness Hii regions studied by the Wisconsin Hα Mapper are of lower temperatures than the classical bright Hii regions.

The Three-Dimensional Ionization Structure and Evolution of NGC 6720, The Ring Nebula

We have determined the gas kinematics, diagnostic and ionic radial profiles, spatial structure, and evolutionary phase of NGC 6720 (the Ring Nebula) by means of tomography and a three-dimensional recovery technique applied to long-slit high-resolution spectra. The main shell of the Ring Nebula is a triaxial ellipsoid (radii of 0.10, 0.13, and 0.20 pc) seen nearly pole-on and expanding in an approximately ballistic fashion (Vexp = 0.65 km s-1 arcsec-1). The central star characteristics [log(L*/L?) 2.3, T* 120,000 K], combined with the nebular age of 7000 yr, indicate that the M* 0.61-0.62 M? post-AGB star is approaching the white dwarf cooling sequence. The equator of the Ring Nebula is optically thick and much denser than the optically thin poles. The inner halo surrounding NGC 6720 represents the pole-on projection of the AGB wind at high latitudes (circumpolar) directly ionized by the central star, whereas the outer, fainter, and circular halo is the projection of the recombining AGB wind at mean to low latitudes, shadowed by the main nebula. The spatio-kinematical properties of the Ring Nebula and the origin of the dense knots commonly observed in late-stage planetary nebulae are critically compared with the predictions of radiation-hydrodynamic and wind interaction models.

STUDIES OF NGC 6720 WITH CALIBRATED HST/WFC3 EMISSION-LINE FILTER IMAGES. I. STRUCTURE AND EVOLUTION ,

We have performed a detailed analysis of the Ring Nebula (NGC 6720) using Hubble Space Telescope WFC3 images and derived a new three-dimensional model. Existing high spectral resolution spectra played an important supplementary role in our modeling. It is shown that the Main Ring of the nebula is an ionization-bounded irregular non-symmetric disk with a central cavity and perpendicular extended lobes pointed almost toward the observer. The faint outer halos are determined to be fossil radiation, i.e., radiation from gas ionized in an earlier stage of the nebula when it was not ionization bounded. The narrowband WFC3 filters that isolate some of the emission lines are affected by broadening on their short wavelength side and all the filters were calibrated using ground-based spectra. The filter calibration results are presented in an appendix.

THE HUBBLE SPACE TELESCOPE TREASURY PROGRAM ON THE ORION NEBULA CLUSTER

The Hubble Space Telescope (HST) Treasury Program on the Orion Nebula Cluster (ONC) has used 104 orbits of HST time to image the Great Orion Nebula region with the Advanced Camera for Surveys (ACS), the Wide-Field/Planetary Camera 2 (WFPC2), and the Near-Infrared Camera and Multi-Object Spectrograph (NICMOS) instrument in 11 filters ranging from the U band to the H band equivalent of HST. The program has been intended to perform the definitive study of the stellar component of the ONC at visible wavelengths, addressing key questions like the cluster initial mass function, age spread, mass accretion, binarity, and cirumstellar disk evolution. The scanning pattern allowed us to cover a contiguous field of approximately 600 arcmin^2 with both ACS and WFPC2, with a typical exposure time of approximately 11 minutes per ACS filter, corresponding to a point source depth AB(F435W) = 25.8 and AB(F775W) = 25.2 with 0.2 mag of photometric error. We describe the observations, data reduction, and data products, including images, source catalogs, and tools for quick look preview. In particular, we provide ACS photometry for 3399 stars, most of them detected at multiple epochs; WFPC2 photometry for 1643 stars, 1021 of them detected in the U band; and NICMOS JH photometry for 2116 stars. We summarize the early science results that have been presented in a number of papers. The final set of images and the photometric catalogs are publicly available through the archive as High Level Science Products at the STScI Multimission Archive hosted by the Space Telescope Science Institute.

A Survey of Molecular Hydrogen in the Crab Nebula

We have carried out a near-infrared, narrowband imaging survey of the Crab Nebula, in the H2 2.12 μm and Brγ 2.17 μm lines, using the Spartan Infrared camera on the SOAR Telescope. Over a 28 × 51 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 Hubble Space Telescope [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% of the Crabs projected surface area has significant H2 emission that correlates with [S II] emission. We measured radial velocities of the [S II] λ6716 emission lines from 47 of the cataloged knots and find that most are on the far (receding) side of the nebula. We also detect Brγ emission. It is right at the limit of our survey, and our Brγ filter cuts off part of the expected velocity range. But clearly the Brγ emission has a quite different morphology than the H2 knots, following the long linear filaments that are seen in Hα and in [O III] optical emission lines.

Merged Ionization/Dissociation Fronts in Planetary Nebulae*

The hydrogen ionization and dissociation front around an ultraviolet radiation source should merge when the ratio of ionizing photon flux to gas density is sufficiently low and the spectrum is sufficiently hard. This regime is particularly relevant to the molecular knots that are commonly found in evolved planetary nebulae, such as the Helix Nebula, where traditional models of photodissociation regions have proved unable to explain the high observed luminosity in H2 lines. In this paper we present results for the structure and steady state dynamics of such advection-dominated merged fronts, calculated using the Cloudy plasma/molecular physics code. We find that the principal destruction processes for H2 are photoionization by extreme ultraviolet radiation and charge-exchange reactions with protons, both of which form H2+, which rapidly combines with free electrons to undergo dissociative recombination. Advection moves the dissociation front to lower column densities than in the static case, which vastly increases the heating in the partially molecular gas due to photoionization of He0, H2, and H0. This causes a significant fraction of the incident bolometric flux to be reradiated as thermally excited infrared H2 lines, with the lower excitation pure rotational lines arising in 1000 K gas and higher excitation H2 lines arising in 2000 K gas, as is required to explain the H2 spectrum of the Helix cometary knots.