absTracTs

Dust in high Galactic latitude cirrus
Blagrave, K. P. M.

Using our series of GBT high latitude H I maps (9.2 arcmin resolution), we investigate variations of near-IR dust emissivities over more than 700 square degrees. It is found that in a number of circumstances, the data require two emissivities: one for the local gas and one for the intermediate velocity gas. This poster will present a statistical analysis of our derived distribution of emissivities and discuss what this might imply about the dust properties in our variety of high-latitude fields.

The dust-gas correlation of Intermediate Velocity Clouds at the North Ecliptic Pole
Blagrave, K. P. M., and Martin, P. G.

Surveys of H I emission reveal the presence of high Galactic latitude H I gas at velocities inconsistent with Galactic rotation: intermediate-velocity clouds and high-velocity clouds. To constrain the connection of these clouds to the local gas, we have been involved in an on-going project to correlate H I and infrared emission.

Deviations from He I Case B Recombination Theory and Extinction Corrections in the Orion Nebula
Blagrave, K. P. M.; Martin, P. G.; Rubin, R. H.; Dufour, R. J.; Baldwin, J. A.; Hester, J. J.; Walter, D. K.

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.

A photoionized Herbig-Haro object in the Orion nebula
Blagrave, K. P. M.; Martin, P. G.; Baldwin, J. A.

The spectra of Herbig Haro objects are usually characteristic of ionization and excitation in shock-heated gas, whether an internal shock in an unsteady outflow or a bow shock interface with the interstellar medium. We examine the eastern-most shock -- the leading optically visible shock -- of a Herbig Haro outflow (HH 529) seen projected on the face of the Orion nebula, using deep optical echelle spectroscopy, showing that the spectrum of this gas is consistent with photoionization by θ¹ Ori C. By modeling the emission lines, we determine a gas-phase abundance of Fe which is consistent with the depleted (relative to solar) abundance found in the Orion nebula -- evidence for the presence of dust in the nebula and therefore in the Herbig Haro outflow. The spectrum also allows for the calculation of temperature fluctuations, t², in the nebula and the shock. These fluctuations have been used to explain discrepancies between abundances obtained from recombination lines versus those obtained from collisionally-excited lines, although to date there has not been a robust theory for how such large fluctuations (t² > 0.02) can exist.

On the O II ground configuration energy levels
Blagrave, K. P. M.; Martin, P. G.

The most accurate way to measure the energy levels for the O II 2p³ ground configuration has been from the forbidden lines in planetary nebulae. We present an analysis of modern planetary nebula data that nicely constrain the splitting within the ²D term and the separation of this term from the ground ⁴S_3/2 level. We extend this method to H II regions using high-resolution spectroscopy of the Orion nebula, covering all six visible transitions within the ground configuration. These data confirm the splitting of the ²D term while additionally constraining the splitting of the ²P term. The energies of the ²P and ²D terms relative to the ground (⁴S) term are constrained by requiring that all six lines give the same radial velocity, consistent with independent limits placed on the motion of the O⁺ gas and the planetary nebula data.

Temperature variations from HST Spectroscopy of the Orion nebula
Rubin, R. H.; Martin, P. G.; Dufour, R. J.; Ferland, G. J.; Blagrave, K. P. M.; Liu, X.-W.; Nguyen, J. F.; Baldwin, J. A.

We present Hubble Space Telescope (HST)/STIS long-slit spectroscopy of NGC 1976. Our goal is to measure the intrinsic line ratio [OIII] 4364/5008 and thereby evaluate the electron temperature (T_e) and the fractional mean-square T_e variation (t²_A)across the nebula. We also measure the intrinsic line ratio [NII] 5756/6585 in order to estimate T_e and t²_A in the N⁺ region. The interpretation of the [NII] data is not as clear cut as the [OIII] data because of a higher sensitivity to knowledge of the electron density as well as a possible contribution to the [NII] 5756 emission by recombination (and cascading). We present results from binning the data along the various slits into tiles that are 0.5 arcsec square (matching the slit width). The average [OIII] temperature for our four HST/STIS slits varies from 7678 K to 8358 K; t²_A varies from 0.00682 to at most 0.0176. For our preferred solution, the average [NII] temperature for each of the four slits varies from 9133 to 10232 K; t²_A varies from 0.00584 to 0.0175. The measurements of T_e reported here are an average along each line of sight. Therefore, despite finding remarkably low t²_A, we cannot rule out significantly larger temperature fluctuations along the line of sight. The result that the average [NII]T_e exceeds the average [OIII]T_e confirms what has been previously found for Orion and what is expected on theoretical grounds. Observations of the proplyd P159-350 indicate: large local extinction associated; ionization stratification consistent with external ionization by θ¹ Ori C; and indirectly, evidence of high electron density.

Line-of-sight optical emission in the Orion nebula
Blagrave, K. P. M., Baldwin, J. A., and Martin, P. G.

The Orion nebula is as near-ideal a region as one can find in which to study the content, structure, and dynamics of the interstellar medium in star-forming regions because of both its proximity and its high surface brightness. Deep optical echelle spectra were obtained for three different lines-of-sight using the Blanco 4m telescope at the Cerro Tololo Inter-American Observatory (CTIO). This poster discusses the velocity and ionization structure inferred from the identification of approximately 300 permitted and forbidden lines along each of the three lines-of-sight including a discussion of the different mechanisms responsible for exciting the lines.