Donald Kim Walter

Physical Conditions in Low-Ionization Regions of the Orion Nebula

ABSTRACTWe reexamine the spectroscopic underpinnings of recent suggestions that [O I ] and[Fe II ] lines from the Orion H region are produced in gas where the iron-carryinggrains have been destroyed and the electron density is surprisingly high. Our newobservations show that previous detections of [O I ] 5577 were dominated by telluricemission. Our limits are consistent with a moderate density (≈ 10 4 cm −3 ) photoionizedgas. We show that a previously proposed model of the Orion H II region reproducesthe observed [O I ] and [Fe II ] spectrum. These lines are fully consistent with formationin a dusty region of moderate density.Subject headings: ISM: H II regions — ISM: abundances — ISM: atoms — ISM:individual (Orion Nebula)1. IntroductionThe Orion Nebula is the defining blister H II region (Zuckerman 1973; Balick, Gammon, &Hjellming 1974). A star cluster ionizes the skin of the molecular cloud, causing an expansion away 1 Based in part on observations made with the NASA/ESAHubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555

CNO abundances and temperature fluctuations in the Orion Nebula

The largest UV-optical data base to date of the Orion Nebula has been assembled, and the CNO abundances their distribution at a statistically significant number of positions to the north and west of the Trapezium and out to a distance of 5 arcmin has been examined. The forbidden O III temperature increases with distance from Theta1 Ori C while the forbidden N II temperature decreases with increasing distance. The density decreases as a function of increasing distance from Theta1 Ori C. The C/H, N/H, O/H, and C/O distributions are constant across the inner 5 arcmin of the northwestern quadrant of the nebula. The empirically determined mean C/H abundance calculated from UV lines is almost a factor of two lower in the number density than earlier estimates. Photoionization model calculations are consistent with a solar abundance for N/H and O/H.

Deviations from He I Case B Recombination Theory and Extinction Corrections in the Orion Nebula

We are engaged in a comprehensive program to find reliable elemental abundances in and probe the physical structure of the Orion Nebula, the brightest and best-resolved H II region. In the course of developing a robust extinction correction covering our optical and UV FOS and STIS observations, we examined the decrement within various series of He I lines. The decrements of the 23S-n3P, 23P-n3S, and 33S-n3P series are not in accord with case B recombination theory. None of these anomalous He I decrements can be explained by extinction, indicating the presence of additional radiative transfer effects in He I lines ranging from the near-IR to the near-UV. CLOUDY photoionization equilibrium models including radiative transfer are developed to predict the observed He I decrements, and the quantitative agreement is quite remarkable. Following from these results, select He I lines are combined with H I and [O II] lines and stellar extinction data to validate a new normalizable analytic expression for the wavelength dependence of the extinction. In so doing, the He+/H+ abundance is also derived.

Temperature Variations and N/O in the Orion Nebula from HST Observations*

Using the Goddard High Resolution Spectrograph (GHRS) and the Faint Object Spectrograph (FOS) on the Hubble Space Telescope, we measured the flux of the N II] (2s2p35S2 → 2s22p23P2,1) lines at λvac = 2143.45, 2139.68 A in the Orion Nebula—the first detection of these lines in an H II region. In order to assess the N+/O+ ratio, we also measured the flux of the [O II] (2p3 2Po1/2,3/2→2p3 4So3/2) lines at λvac = 2471.05, 2471.12 A. In addition, with the FOS, other emission lines were measured in the same aperture in order to assess the average electron temperature and mean-square temperature variation (t2) in the N+ region, as well as the N+/O+ ratio. When we require that the empirically determined values be equal for (N+/O+)uv (obtained from the N II] 2142 and [O II] 2471 lines) and (N+/O+)opt (obtained from the [N II] 6585 and [O II] 3728 lines), we obtain the following. For the (N+, O+) zone, the average electron density is ~7000 cm-3, the average electron temperature is 9500 K, t2 = 0.032, and N+/O+ = 0.14. By comparing our FOS observations to predicted fluxes, utilizing our two previous photoionization models, we are able to derive the N/O ratio. There is fairly good agreement between (N/O)uv and (N/O)opt as derived from the two models with a range between 0.13 and 0.18. This range also encompasses our model-derived values for (N/O)ir (0.17-0.18), which fit the observed far-infrared line ratio [N III] 57 μm/[O III] 52 μm. The empirically derived N+/O+ value requires a correction for the possibility that the N+ and O+ regions are not identical. Our overall results place the gas-phase Orion N/O ratio in the range 0.13-0.18, which is somewhat higher than solar.

Extinction and scattering in the Orion Nebula : comparison of optical and VLA 21 centimeter studies

We have compared VLA 21 cm continuum maps of M42 with extinction-corrected H-beta images and found that much of M42 has an excess of H-beta surface brightness. This is interpreted as reflection by particles lying close to the principal ionization front. Comparison of column densities of H I near M42 and extinction (C(H-beta)) indicates an excellent area-by-area correlation, establishing that most of the extinction in M42 arises in the neutral lid that lies on the near side of the nebula. A more quantitative comparison of (C(H-beta)) and N(H I), the column density, is consistent with this conclusion and indicates a dust scattering cross section per hydrogen atom that is similar to the value for the general interstellar medium. The usual model for extinction in the Trapezium region is that it occurs very near the stars and produces an anomalous reddening curve through modification of the nature or size distribution of the particles. These new results question the basic assumptions and conclusions of that model.

KEPLER AND THE LONG-PERIOD VARIABLES

High precision Kepler photometry is used to explore the details of AGB light curves. Since AGB variability has a typical time scale on order of a year we discuss at length the removal of long term trends and quarterly changes in Kepler data. Photometry for a small sample of nine SR AGB stars are examined using a 30 minute cadence over a period of 45 months. While undergoing long period variations of many magnitudes, the light curves are shown to be smooth at the millimagnitude level over much shorter time intervals. No flares or other rapid events were detected on the sub-day time scale. The shortest AGB period detected is on the order of 100 days. All the SR variables in our sample are shown to have multiple modes. This is always the first overtone typically combined with the fundamental. A second common characteristic of SR variables is shown to be the simultaneous excitation of multiple closely separated periods for the same overtone mode. Approximately half the sample had a much longer variation in the light curve, likely a long secondary period. The light curves were all well represented by a combination of sinusoids. However, the properties of the sinusoids are time variable with irregular variations present at low level. No non-radial pulsations were detected. It is argued that the long secondary period variation seen in many SR variables is intrinsic to the star and linked to multiple mode pulsation.

Analysis and Models of Photoionized Structures Seen in Hubble Space Telescope Images of NGC 7635

We present the analysis of photoionized structures seen in Hubble Space Telescope Wide Field Planetary Camera 2 images of NGC 7635 and the surrounding H II region S162. We exploit the high spatial resolution images to make a census of the total ionizing flux from the O6.5 IIIf star BD +60°2522. We also construct detailed models of two structures with a photoionization code, reproducing the line emission seen in spatial profiles extracted directly from the images. We demonstrate that the density distribution of the models produces deviations in the spectrally derived elemental abundances at a level exceeding typical observational errors. Our results also show a large discrepancy between the stars effective temperature used for our best-fit models (34,320 K) and that expected based on its spectral type (>40,000 K) from a hot-star calibration taken from the literature. We suggest that this is due to the use of plane-parallel stellar atmospheres in constructing that calibration.

THE RCT 1.3 m ROBOTIC TELESCOPE: BROADBAND COLOR TRANSFORMATION AND EXTINCTION CALIBRATION

The Robotically Controlled Telescope (RCT) 1.3 m telescope, formerly known as the Kitt Peak National Observatory (KPNO) 50 inch telescope, has been refurbished as a fully robotic telescope, with an autonomous scheduler to take full advantage of the observing site without the requirement of a human presence. Here we detail the current configuration of the RCT and present, as a demonstration of its high-priority science goals, the broadband UBVRI photometric calibration of the optical facility. In summary, we find the linear color transformation and extinction corrections to be consistent with similar optical KPNO facilities, to within a photometric precision of 10% (at 1σ). While there were identified instrumental errors that likely added to the overall uncertainty, associated with since-resolved issues in engineering and maintenance of the robotic facility, a preliminary verification of this calibration gave a good indication that the solution is robust, perhaps to a higher precision than this initial calibration implies. The RCT has been executing regular science operations since 2009 and is largely meeting the science requirements set during its acquisition and redesign.

A Decade of Lessons Learned

We describe our efforts at building programs in Earth and space science over the past decade at four Minority Institutions, Medgar Evers College, Norfolk State University, South Carolina State University and the University of Houston-Downtown. We present our institutional models of success and programmatic outcomes as well as barriers to success and lessons learned. The unique path to success for each school is described, along with experiences common among all four. Since these institutions do not offer graduate programs in the geosciences, they have concentrated on recruitment and retention of students in the K-16 pipeline while preparing them for graduate school and careers in the field. These schools represent a range in size, location, population served and in the type and nature of the Earth and space science programs they offer. As such, the experiences described herein offer a broad perspective on what does and does not work in attracting and retaining underrepresented minority students in the geosciences.

The chaotic ISM of the post(?)-starburst galaxy NGC 1569

We present the results and analysis of HST -wfpc2 imagery of the starburst galaxy NGC 1569 ( d = 2.2 Mpc), which permit unprecedented resolution (~ 1pc) of the ionized gas and stellar population. The primary data in our analysis consists of images taken through narrow-band filters isolating Hβ, Hα, [O iii]λ5007, and [S ii]λλ6717+30, as part of go Program 8133. A variety of color-coded and grey-scaled maps of the morphology, ionization structure, and dust distribution are presented. Unsharp masks of the Hα images show a very chaotic structure for the ionized gas, with numerous filaments and arc-like bright rim features across the entire galaxy, but without significant large-scale ionization variations. Variations in the ionization and line-of-sight reddening occur on smaller scales (~ 10 – 50 pc), with numerous low-ionization semi-stellar knots seen throughout the main body of the galaxy, even within the ‘hole’ in the ionized gas distribution around the central super star clusters previously noted from H i maps. Several of these features have strong [S ii] emission indicative of being supernova remnants. We combine these data with archival wide-band HST -wfpc2 imagery to quantitatively evaluate the source(s) of the (largely photo-) ionized gas.