R. H. Rubin’s research while affiliated with Ames Research Center and other places

We assess the gas-phase abundances of Si, C, and Fe from our recent measurements of Si ⁺⁺ , C ⁺⁺ , and Fe ⁺⁺ in the Orion Nebula by expanding on our earlier “blister” models. The Fe ⁺⁺ 22.9 μm line measured with the KAO yields Fe/H ~ 3 × 10 ⁻⁶ - considerably larger than in the diffuse ISM, where relative to solar, Fe/H is down by ~ 100. However, in Orion, Fe/H is still lower than solar by a factor ~ 10. The C and Si abundances are derived from new IUE high dispersion spectra of the C ⁺⁺ 1907, 1909 Å and Si ⁺⁺ 1883, 1892 Å lines. Gas-phase Si/C = 0.016 in the Orion ionized volume and is particularly insensitive to uncertainties in extinction and temperature structure. The solar value is 0.098. Gas-phase C/H = 3 × 10 ⁻⁴ and Si/H = 4.8 × 10 ⁻⁶ . Compared to solar, Si is depleted by 0.135 in the ionized region, while C is essentially undepleted. This suggests that most Si and Fe resides in dust grains even in the ionized volume.

In order to study the physical properties of nebulae and determine their elemental abundances, it is important to observe lines from many different ionic species. Such studies have been enhanced in recent years with the measurement of lines in the far-infrared (FIR). The [N III]57μm line provides a way to assess the N ⁺⁺ abundance – which is not readily done from any other spectral region. Recent detection of the [N II]122 and 205μm lines provides a new way to assess both the electron density in the N ⁺ region and the total N abundance in an object. When there are few observations to warrant a detailed modeling approach, it may be necessary to use another approach which has been referred to as a semi-empirical method (hereafter SEM) ( e.g. Aller 1984). We delineate a SEM scheme for doing this and apply it to observations for the H II region G333.6–0.2.

We apply a 2-D, axisymmetric code for modeling H II regions (Rubin Ap. J. 287 , 653, 1984) to observations of the Orion Nebula. The model solves for the ionization and thermal structure and radiative transfer for the quasi-equilibrium volume. Assuming that the Orion Nebula is viewed face-on (along the symmetry axis) and that the geometry/density distribution is plane parallel with an exponential density gradient perpendicular to the slab, we use a x ² minimization technique to best fit the radio continuum maps. The best fit to the Schraml and Mezger map (Astrophys. J. 156 , 269, 1969) has a density at the star of ∼1800 cm ⁻³ , a scale height of ∼0.23 pc, and ∼1.5x10 ⁴⁹ ionizing photons s ⁻¹ so that ∼ 1/3 of the ionizing photons from the exciting source are escaping the nebula through the frontal density-bounded direction. Our model for Orion requires circular symmetry in the plane of the sky; nonsymmetrical features such as the ionization bar toward the SE cannot be reproduced. Further modeling that compares with line observations has been delayed to incorporate the important role played by recombinations in populating low-lying [O II] levels (Rubin 1985, Astrophys. J., submitted).

Using the short-high module of the Infrared Spectrograph on the Spitzer Space Telescope, we have measured the [S IV] 10.51, [Ne II] 12.81, [Ne III] 15.56, and [S III] 18.71-micron emission lines in nine H II regions in the dwarf irregular galaxy NGC 6822. These lines arise from the dominant ionization states of the elements neon (Ne$^{++}$, Ne$^+$) and sulphur (S$^{3+}$, S$^{++}$), thereby allowing an analysis of the neon to sulphur abundance ratio as well as the ionic abundance ratios Ne$^+$/Ne$^{++}$ and S$^{3+}$/S$^{++}$. By extending our studies of H II regions in M83 and M33 to the lower metallicity NGC 6822, we increase the reliability of the estimated Ne/S ratio. We find that the Ne/S ratio appears to be fairly universal, with not much variation about the ratio found for NGC 6822: the median (average) Ne/S ratio equals 11.6 (12.2$\pm$0.8). This value is in contrast to Asplund et al.'s currently best estimated value for the Sun: Ne/S = 6.5. In addition, we continue to test the predicted ionizing spectral energy distributions (SEDs) from various stellar atmosphere models by comparing model nebulae computed with these SEDs as inputs to our observational data, changing just the stellar atmosphere model abundances. Here we employ a new grid of SEDs computed with different metallicities: Solar, 0.4 Solar, and 0.1 Solar. As expected, these changes to the SED show similar trends to those seen upon changing just the nebular gas metallicities in our plasma simulations: lower metallicity results in higher ionization. This trend agrees with the observations.

Using the short-high module of the Infrared Spectrograph on the Spitzer Space Telescope, we have measured the [S iv] 10.51, [Ne ii] 12.81, [Ne iii] 15.56, and [S iii] 18.71-μm emission lines in 9 H ii regions in the dwarf irregular galaxy NGC 6822. These lines arise from the dominant ionization states of the elements neon (Ne++, Ne+) and sulphur (S3 +, S++), thereby allowing an analysis of the neon to sulphur abundance ratio as well as the ionic abundance ratios Ne+/Ne++ and S3 +/S++. By extending our studies of H ii regions in M83 and M33 to the lower metallicity NGC 6822, we increase the reliability of the estimated Ne/S ratio. We find that the Ne/S ratio appears to be fairly universal, with not much variation about the ratio found for NGC 6822: the median (average) Ne/S ratio equals 11.6 (12.2±0.8). This value is in contrast to Asplund et al.’s currently best estimated value for the Sun: Ne/S = 6.5. In addition, we continue to test the predicted ionizing spectral energy distributions (SEDs) from various stellar atmosphere models by comparing model nebulae computed with these SEDs as inputs to our observational data, changing just the stellar atmosphere model abundances. Here we employ a new grid of SEDs computed with different metallicities: Solar, 0.4 Solar, and 0.1 Solar. As expected, these changes to the SED show similar trends to those seen upon changing just the nebular gas metallicities in our plasma simulations: lower metallicity results in higher ionization. This trend agrees with the observations.

Recent Hubble Space Telescope images have allowed the determination with unprecedented accuracy of motions and changes of shocks within the inner Orion Nebula. These originate from collimated outflows from very young stars, some within the ionized portion of the nebula and others within the host molecular cloud. We have doubled the number of Herbig-Haro objects known within the inner Orion Nebula. We find that the best-known Herbig-Haro shocks originate from a relatively few stars, with the optically visible X-ray source COUP 666 driving many of them. While some isolated shocks are driven by single collimated outflows, many groups of shocks are the result of a single stellar source having jets oriented in multiple directions at similar times. This explains the feature that shocks aligned in opposite directions in the plane of the sky are usually blue shifted because the redshifted outflows pass into the optically thick Photon Dominated Region behind the nebula. There are two regions from which optical outflows originate for which there are no candidate sources in the SIMBAD data base.

LL Orionis-type objects (LL objects) are hyperbolic bowshocks visible around young stars in the outer Orion nebula, many of which are also associated with curved, highly collimated jets. The bowshocks are clearly due to the supersonic interaction between an outflow from the young star and an environmental flow from the core of the nebula, but the exact nature of these flows has not yet been established. We present the first high-resolution optical spectra of two of these objects, LL 1 and LL 2, together with their associated Herbig-Haro (HH) jets, HH 888 and HH 505. We combine multiple long-slit echelle spectra in the Hα 6563 Å and [N ii] 6584 Å lines to produce velocity maps of the two objects at a resolution of 4text{arcsec} × 2text{arcsec} × 11 {km s^{-1}}. The gas motions within both stellar bowshocks are of rather low velocity (10-20 km s-1), but there are important differences between the two objects. LL 1 shows a high degree of symmetry, whereas LL 2 has very asymmetric kinematics that seem to follow velocity gradients in the surrounding nebula. We also measure the line-of-sight velocity for multiple knots in the HH 888 and HH 505 jets, and combine our spectroscopy with new and existing proper-motion measurements to reconstruct the three-dimensional kinematics of the jets. The knot motions in both jets are very similar: both flows are inclined at 40° to 60° from the plane of the sky, with exclusively redshifted knots to the north and exclusively blueshifted knots to the south. In both cases, one also sees a deceleration along the length of the jets, from >200 km s-1 close to the respective stars down to <100 km s-1 farther out. The marked contrasts that we find between the kinematics of the jets and the kinematics of the stellar bowshocks are evidence that the two phenomena are not causally related. Regular patterns in the dynamic ages of the HH 505 knots imply periodic ejections on three different time-scales: 50, 12 and 4 yr. We use line ratios and photometry to measure electron densities and excitation/ionization conditions in the stellar bowshocks and jet knots. The LL 1 bowshock has a bright inner shell with density ≃3000 cm-3 (compared with a local nebula density of ≃1000 cm-3) and line ratios that are consistent with equilibrium photoionization models. The bowshock also has a fainter outer rim, where the line ratios show evidence of shock excitation. Many of the jet knots also show evidence for a shock contribution to their excitation and have densities from 1000 to 8000 cm-3. Based on observations obtained at the Observatorio Astronómico Nacional, San Pedro Mártir, Baja California, Mexico, which is operated by the Universidad Nacional Autónoma de México. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the National Science Foundation. iraf is distributed by the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science foundation. Velocities in the frame of the local standard of rest are obtained by subtracting 18.1 km s-1 from the heliocentric values. Traditionally, [O iii] is normalized by Hβ rather than Hα in order to minimize the effects of extinction. In our case, the extinction is known to be low (Section 6), so the main effect of using Hα instead is to shift the y-axis by the intrinsic Balmer decrement of ˜0.5 dex.

We report spectrophotometric observations made with SOFIA/FORCAST on 2011 June 2 UT. Optical measurements have previously shown that the abundance discrepancy factor (adf) varies with position in several high-adf PNe, and is highest close to the central star. The very low electron temperature inclusions postulated to explain the abundance discrepancy, must be cooled predominantly by fine structure IR lines. These SOFIA data will map mid-IR FS lines (and our related Herschel program will add several far-IR FS lines) in the bright, well-characterized, high-adf PN NGC 7009. We will compare these IR results with FS optical line measurements in order to correlate ratios of IR to optical fluxes with position, and thus correlate with where the adf peaks.

We observed several H II regions in the dwarf irregular galaxy NGC 6822 using the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. Measurements of [S IV] 10.51, [Ne II] 12.81, [Ne III] 15.56, and [S III] 18.71 micron emission lines were made in each of the H II regions. The lines were observed cospatially using the IRS in the short wavelength, high resolution mode, which permits a reliable comparison of the line fluxes. From the measured line fluxes we determine ionic abundance ratios including Ne++/Ne+ and S3+/S++. These ionic abundance ratios allow an analysis of the Ne/S ratio by taking the ratio of the dominant ionization states of the respective elements, Ne (Ne++, Ne+) and S (S3+, S++). Our aim here is twofold: (1) to examine the Neon to Sulfur abundance ratio in order to determine whether or not it is fairly universal and (2) to discriminate and test the predicted ionizing spectral energy distributions (SEDs) from various stellar atmosphere models by comparing our observational data with H II region models that use these SEDs as input. This work extends our previous similar studies of H II regions in M83 and M33 to lower metallicities (and higher ionization), where we can attain a more reliable estimate of the Ne/S ratio. For the first time, we employ the new grid of SEDs from Pauldrach and Weber that have been computed with different metallicities: solar, 0.4 solar, and 0.1 solar. We demonstrate the effect on our analysis of changing just the stellar atmosphere model abundances. As expected, these changes to the SED show similar trends as does changing just the metallicities in the nebular gas abundances in our plasma simulations. Lower metallicity results in higher ionization. Support from 09-ADP09-0169 and Spitzer 40910 are gratefully acknowledged.

We present Spitzer Space Telescope observations of 11 regions in the Orion Nebula all southeast of the Bright Bar. Our Cycle 5 program obtained deep spectra with both the IRS short-high (SH) and long-high (LH) modules with aperture grid patterns chosen to very closely match the same area in the nebula. Previous IR missions observed only the inner few arcmin (the ‘Huygens’ region). The extreme sensitivity of Spitzer in the 10-37 μm spectral range permitted us to measure many lines of interest to much larger distances from the exciting star θ1 Ori C.