Stephan R. McCandliss

The interstellar N2 abundance towards HD 124314 from far-ultraviolet observations.

The abundance of interstellar molecular nitrogen (N2) is of considerable importance: models of steady-state gas-phase interstellar chemistry, together with millimetre-wavelength observations of interstellar N2H+ in dense molecular clouds predict that N2 should be the most abundant nitrogen-bearing molecule in the interstellar medium. Previous attempts to detect N2 absorption in the far-ultraviolet or infrared (ice features) have hitherto been unsuccessful. Here we report the detection of interstellar N2 at far-ultraviolet wavelengths towards the moderately reddened star HD 124314 in the constellation of Centaurus. The N2 column density is larger than expected from models of diffuse clouds and significantly smaller than expected for dense molecular clouds. Moreover, the N2 abundance does not explain the observed variations in the abundance of atomic nitrogen (N I) towards high-column-density sightlines, implying that the models of nitrogen chemistry in the interstellar medium are incomplete.

A "combination nova" outburst in Z Andromedae: nuclear shell burning triggered by a disk instability

We describe observational evidence for a new kind of interacting binary star outburst that involves both an accretion instability and an increase in thermonuclear shell burning on the surface of an accreting white dwarf. We refer to this new type of eruption as a combination nova. In late 2000, the prototypical symbiotic star Z Andromedae brightened by roughly 2mag in the optical. We observed the outburst in the radio with the VLA and MERLIN, in the optical both photometrically and spectroscopically, in the far-ultraviolet with FUSE, and in the X-rays with both Chandra and XMM-Newton. The 2 year long event had three distinct stages. During the first stage, the optical rise closely resembled an earlier, small outburst that was caused by an accretion disk instability. In the second stage, the hot component ejected an optically thick shell of material. In the third stage, the shell cleared to reveal a white dwarf whose luminosity remained on the order of 10(4) L-circle dot for approximately 1 yr. The eruption was thus too energetic to have been powered by accretion alone. We propose that the initial burst of accretion was large enough to trigger enhanced nuclear burning on the surface of the white dwarf and the ejection of an optically thick shell of material. This outburst therefore combined elements of both a dwarf nova and a classical nova. Our results have implications for the long-standing problem of producing shell flashes with short recurrence times on low-mass white dwarfs in symbiotic stars.

Molecular Hydrogen Optical Depth Templates for FUSE Data Analysis

The calculation and use of molecular hydrogen optical depth templates to quickly identify and model molecular hydrogen absorption features longward of the Lyman edge at 912 A u are described. Such features are commonly encountered in spectra obtained by the Far Ultraviolet Spectroscopic Explorer and also in spectra obtained by the Space Telescope Imaging Spectrograph, albeit less commonly. Individual templates are calculated containing all the Lyman and Werner transitions originating from a single rotational state ( ) of the zeroth �� J vibrational level ( ) of the ground electronic state. Templates are provided with 0.01 A u sampling for Doppler �� v parameters ranging from km s 1 and rotational states . Optical depth templates for excited �� 2 ≤ b ≤ 20 0 ≤ J ≤ 15 vibrational states are also available for select Doppler parameters. Each template is calculated for a fiducial column density of log (N(cm )) p 21 and may be scaled to any column less than this value without loss of 2 accuracy. These templates will facilitate the determination of the distribution of molecular hydrogen column density as a function of rotational level. The use of these templates will free the user from the computationally intensive task of calculating profiles for a large number of lines and allow concentration on line-profile or curve- of-growth fitting to determine column densities and Doppler parameters. The templates may be downloaded freely from http://www.pha.jhu.edu/∼stephan/h2ools.html.

SPITZER MAPPING OF POLYCYCLIC AROMATIC HYDROCARBON AND H2 FEATURES IN PHOTODISSOCIATION REGIONS

The mid-infrared (MIR) spectra of dense photodissociation regions (PDRs) are typically dominated by emission from polycyclic aromatic hydrocarbons (PAHs) and the lowest pure rotational states of molecular hydrogen (H2); two species which are probes of the physical properties of gas and dust in intense UV radiation fields. We utilize the high angular resolution of the Infrared Spectrograph on the Spitzer Space Telescope to construct spectral maps of the PAH and H2 features for three of the best studied PDRs in the galaxy, NGC 7023, NGC 2023 and IC 63. We present spatially resolved maps of the physical properties, including the H2 ortho-to-para ratio, temperature, and G_o/n_H. We also present evidence for PAH dehydrogenation, which may support theories of H2 formation on PAH surfaces, and a detection of preferential self-shielding of ortho-H2. All PDRs studied exhibit average temperatures of ~500 - 800K, warm H2 column densities of ~10^20 cm^-2, G_o/n_H ~ 0.1 - 0.8, and ortho-to-para ratios of ~ 1.8. We find that while the average of each of these properties is consistent with previous single value measurements of these PDRs, when available, the addition of spatial resolution yields a diversity of values with gas temperatures as high as 1500 K, column densities spanning ~ 2 orders of magnitude, and extreme ortho-to-para ratios of 3.

Rocket Observations of Far-Ultraviolet Dust Scattering in NGC 2023

The reflection nebula NGC 2023 was observed by a rocket-borne long-slit imaging spectrograph in the 900-1400 ? bandpass on 2000 February 11. A spectrum of the star, as well as that of the nebular scattered light, was recorded. Through the use of a Monte Carlo modeling process, the scattering properties of the dust were derived. The albedo is low, 0.2-0.4, and decreasing toward shorter wavelengths, while the phase function asymmetry parameter is consistent with highly forward-scattering grains, g ~ 0.85. The decrease in albedo, while the optical depth increases to shorter wavelengths, implies that the far-UV rise in the extinction curve is due to an increase in absorption efficiency.

Fluorescent Molecular Hydrogen Emission in IC 63: FUSE, Hopkins Ultraviolet Telescope, and Rocket Observations

We present far-ultraviolet observations of IC 63, an emission/reflection nebula illuminated by the B0.5 IV star γ Cas, located 1.3 pc from the nebula. Molecular hydrogen fluorescence was detected first in IC 63 by IUE and later at shorter wavelengths by ORFEUS. Here we present Far Ultraviolet Spectroscopic Explorer (FUSE) observations toward three locations in the nebula, complemented by Hopkins Ultraviolet Telescope (HUT) data on the central nebular position. In addition, we present a sounding rocket calibration of a FUSE spectrum of γ Cas. Molecular hydrogen fluorescence is detected in all three FUSE pointings. The intensity of this emission, as well as the contributions from other species, are seen to vary with position. The absolute flux calibration of the sounding rocket data allows us to reliably predict the radiation field incident on IC 63. We use these data to test models of the fluorescent process. Our modeling resolves the perceived discrepancy between the existing ultraviolet observations and achieves a satisfactory agreement with the H2 rotational structure observed with FUSE.

Evolution of macromolecular dust: Far-ultraviolet spectral dust extinction and gas absorption of stellar light as measured with the Hopkins Ultraviolet Telescope

From far-ultraviolet (FUV) spectra of B stars taken with the Hopkins Ultraviolet Telescope (HUT) and Voyager space observations, we determine the far-ultraviolet extinction by Galactic dust and hydrogen absorption in the local spiral-arm clouds towards HD 25443 (B0.5 III), HD 37903 (B1.5 V), and HD 200775 (B3 Ve). We find that the (n (H)) = 1.3 cu cm and number fraction (f = 0.31) of H atoms in molecular hydrogen relative to the total hydrogen indicates a mostly diffuse medium toward HD 25443. We also determine f = 0.56 for the gas in the photodissociation region (PDR) in front of HD 37903 and f approximately equals 0.36 for the HD 200775 PDR. The inferred ratios of N(H2)/I(CO) = 2.4 to 2.8 x 10(exp 20)/K(km/s)/sq cm for cool gas in dark clouds agree with previous canonical estimates of N(H2)/I(CO) = 2 to 3 x 10(exp 20)/K(km/s)/sq cm and support the use of the ratio for determining masses of molecular clouds. We find that the shape and strength of the HD 25443 FUV extinction is the same as a diffuse-medium mean extinction (R(sub nu) = 2.95) extrapolated from mid-UV wavelengths (normal). The HD 37903 FUV extinction through a bright H2 photodissociation region is higher than a mean FUV extinction(R(sub nu) = 4.11), as is the HD 200775 extinction. Another star from the literature rho Oph, probes the dense medium and exhibits an FUV extinction of normal mean strength (R(sub nu) = 4.55) but steeper shape. The normal FUV -extinction of HD 25443 implies that the small FUV-extinction dust in the diffuse medium forms in p art from larger grains as the grains shatter under shocks. The normal strength of the rho Oph FUV-extinction indicates that the minute dust condenses onto or coagulates into larger grains in the dense medium. The high extinction deviation of the HD 37903 PDR (and HD 200775 PDR) shows that FUV radiation from hot stars with T(sub eff) approximately equals 20,000 to 23,000 K in PDRs can evaporate some of the FUV-extinction dust from grain surfaces. In contrast, previously measured extinctions in Galactic H(+) (H II) regions shows that FUV radiation at T(sub eff) greater than 25,000 K can destroy some of the dust in H(+) regions. Thus, some of the FUV-extinction dust is highly volatile. We find that the dust carrier of the variable FUV-extinction is correlated (r = 0.99) with the mid-IR (12 micrometers) radiation from hydrocarbon dust. The volatile FUV-extinction component could thus be due to minute hydrocarbon dust particles or macromolecules, probably PAHs, which are known to condense onto larger grains in the dense medium. Larger grains compsed of PAHs clustered into a solid could thus produce the steep FUV extinction of the rho Oph dense medium. After exposure to FUV photons wihtin approximately 1 pc of HD 37903, PAHs with about 30 carbon atoms would evaporate off the larger grains, increasing gaseous PAH abundance by approximately 14%. Also, after receiving shocks in the diffuse medium toward HD 25443, the larger hydrocarbon grains would shatter into PAH molecules. In summary, as the Galactic medium cycles betwee n the diffuse and dense medium, hydrocarbon dust cycles into and out of larger grains under the influence of shocks, radiation, and condensation.

A Cometary Bow Shock and Mid-Infrared Emission Variations Revealed in Spitzer Observations of HD 34078 and IC 405*

We present new infrared observations of the emission/reflection nebula IC 405 obtained with the Spitzer Space Telescope. Infrared images in the four IRAC bands (3.6, 4.5, 5.8, and 8.0 ?m) and two MIPS bands (24 and 70 ?m) are complemented by IRS spectroscopy (5-30 ?m) of two nebular filaments. The IRAC (8.0 ?m) and MIPS imaging shows evidence of a bow shock associated with the runaway O9.5 V star, HD 34078, created by the interaction between the star and nebular material. The ratio of emission at 24 to 70 ?m is higher in the immediate vicinity of HD 34078 than in the outer filaments, providing evidence for elevated dust temperatures (Td 90 K) in the shock region. The nebular imaging reveals that the morphology is band dependent, with varying contributions from aromatic emission features, H2, and dust emission. Nebular spectroscopy is used to quantify these contributions, showing several aromatic emission bands between 6-14 ?m, the S(5), S(3), S(2), and S(1) pure rotational emission lines of H2, and atomic fine-structure lines of Ne, S, and Ar. The low-dispersion spectra provide constraints on the ionization state of the large molecules responsible for the aromatic infrared features. H2 rotational temperatures of the two bright nebular filaments are determined from the observed line strengths. An average T(H2) ~ 400 K is inferred, with evidence for additional nonuniform excitation by UV photons in the intense radiation field of HD 34078. The photoexcitation hypothesis is supported by direct measurement of the far-UV H2 fluorescence spectrum, obtained with FUSE.

FAR-ULTRAVIOLET SENSITIVITY OF THE COSMIC ORIGINS SPECTROGRAPH

We demonstrate that the G140L segment B channel of the Cosmic Origins Spectrograph (COS) recently installed on the Hubble Space Telescope (HST) has an effective area consistent with ∼ 10 cm in the bandpass between the Lyman edge at 912 Å and Lyman β, rising to a peak in excess of 1000 cm longward of 1130 Å. This is a new wavelength regime for HST and will allow opportunities for unique science investigations. In particular, investigations seeking to quantify the escape fraction of Lyman continuum photons from galaxies at low redshift, determine the scale-length of the hardness variation in the metagalactic ionizing background over the redshift range 2 < z . 2.8, measure the ratio of CO to H2 in dense interstellar environments with AV > 3, or harness the high temperature diagnostic power of the O VI λλ 1032, 1038 doublet can now be carried out with unprecedented sensitivity. Subject headings: instrumentation: spectrographs, ultraviolet: generalWe demonstrate that the G140L Segment B channel of the Cosmic Origins Spectrograph recently installed on the Hubble Space Telescope (HST) has an effective area consistent with ~10 cm2 in the bandpass between the Lyman edge at 912 ? and Lyman ?, rising to a peak in excess of 1000 cm2 longward of 1130 ?. This is a new wavelength regime for HST and will allow opportunities for unique science investigations. In particular, investigations seeking to quantify the escape fraction of Lyman continuum photons from galaxies at low redshift, determine the scale length of the hardness variation in the metagalactic ionizing background over the redshift range 2 3 can now be carried out with unprecedented sensitivity.

Carbon monoxide in comet 9P/Tempel 1 before and after the Deep Impact encounter

One of the goals of the Hubble Space Telescope program to observe periodic comet 9P/Tempel 1 in conjunction with NASAs Deep Impact mission was to study the generation and evolution of the gaseous coma resulting from the impact. For this purpose, the Solar Blind Channel of the Advanced Camera for Surveys was used with the F140LP filter, which is sensitive primarily to the ultraviolet emission (≥1400 A) from the CO fourth positive system. Following the impact we detected an increase in brightness, which if all due to CO corresponds to 1.5 × 1031 molecules, or a mass of 6.6 × 105 kg, an amount that would normally be produced by 7-10 hours of quiescent outgassing from the comet. This number is ≤10% of the number of water molecules excavated and suggests that the volatile content of the material excavated by the impact did not differ significantly from the surface or near-subsurface material responsible for the quiescent outgassing of the comet.