Charles E. Woodward

On extending the mass-metallicity relation of galaxies by 2.5 decades in stellar mass

We report 4.5 � m luminosities for 27 nearby (DP5 Mpc) dwarf irregular galaxies measured with the Spitzer Infrared Array Camera. We have constructed the 4.5 � m luminosity-metallicity (L-Z) relation for 25 dwarf galaxies with secure distance and interstellar medium oxygen abundance measurements. The 4.5 � m L-Z relation is 12þ log (O/H) ¼ (5:78 � 0:21) þ (� 0:122 � 0:012)M½4:5� , where M[4.5] is the absolute magnitude at 4.5 � m. The dispersion in the near-infrared L-Z relation is smaller than the corresponding dispersion in the optical L-Z relation. The subsequently derived stellar mass-metallicity (M� -Z) relation is 12 þ log (O/H) ¼ (5:65 � 0:23) þ (0:298 � 0:030)log M� , and extends the SDSSM� - Zrelation to lower mass by about 2.5 dex. We find that the dispersion in the M� -Z relation is similar over 5 orders of magnitude in stellar mass, and that the relationship between stellar mass and interstellar medium metallicity is similarly tight from high-mass to low-mass systems. We find a larger scatter at low mass in the relation between effective yield and total baryonic mass. In fact, there are a few dwarf galaxies with large yields, which is difficult to explain if galactic winds are ubiquitous in dwarf galaxies. The low scatter in the L‐Z and M� -Z relationships are difficult to understand if galactic superwinds or blowout are responsible for the low metallicities at low mass or luminosity. Naively, one would expect an ever increasing scatter at lower masses, which is not observed. Subject headingg galaxies: dwarf — galaxies: evolution — galaxies: irregular — infrared: galaxies Online material: color figures

Grain Properties of Comet C/1995 O1 (Hale-Bopp)

We present the analysis of 7.6-13.2 μm infrared (IR) spectrophotometry (R 250) of comet C/1995 O1 (Hale-Bopp), in conjunction with concurrent observations that extend the wavelength coverage of the spectral energy distribution from near- to far-infrared wavelengths. The observations include temporal epochs preperihelion (1996 October and 1997 February UT), near perihelion (1997 April UT), and postperihelion (1997 June UT). Through the modeling of the thermal emission from small, amorphous carbon grains and crystalline and amorphous silicate grains in Hale-Bopps coma, we find that as the comet approached perihelion, the grain size distribution (the Hanner modified power law) steepened (from N = 3.4 preperihelion to N = 3.7 near and postperihelion), along with an increase in the fractal porosity of larger (greater than 1 μm) grains. The peak of the grain size distribution remained constant (ap = 0.2 μm) at each epoch. We attribute the emergence of the 9.3 μm peak near perihelion to crystalline orthopyroxene grains released during epochs of high jet activity. Crystalline silicates (olivine and orthopyroxene) make up about 30% (by mass) of the submicron-sized (≤1 μm) dust grains in Hale-Bopps coma during each epoch.

Giants in the globular cluster ω Centauri: dust production, mass-loss and distance

We present spectral energy distribution modelling of 6875 stars in ω Centauri, obtaining stellar luminosities and temperatures by fitting literature photometry to state-of-the-art MARCS stellar models. By comparison to four different sets of isochrones, we provide a new distance estimate to the cluster of 4850 ± 200 (random error) ± 120 (systematic error) pc, a reddening of E(B − V) = 0.08 ± 0.02 (random) ± 0.02 (systematic) mag and a differential reddening ofE(B − V) 1.2 ± 0.6 0.5 × 10 −6 Myr −1 . Half of the clusters dust production and 30 per cent of its gas production comes from the two most extreme stars - V6 and V42 - for which we present new Gemini/T-ReCS mid-infrared spectroscopy, possibly showing that V42 has carbon-rich dust. The clusters dust tempera- tures are found to be typically 550 K. Mass-loss apparently does not vary significantly with metallicity within the cluster, but shows some correlation with barium enhancement, which appears to occur in cooler stars, and especially on the anomalous RGB. Limits to outflow velocities, dust-to-gas ratios for the dusty objects and the possibility of short-time-scale mass- loss variability are also discussed in the context of mass-loss from low-metallicity stars. The ubiquity of dust around stars near the RGB tip suggests significant dusty mass-loss on the RGB; we estimate that typically 0.20-0.25 Mof mass-loss occurs on the RGB. From obser- vational limits on intracluster material, we suggest the dust is being cleared on a time-scale of 10 5 yr.

Discovery of Crystalline Silicates in Comet C/2001 Q4 (NEAT)

We present the 10 μm silicate feature of the dynamically new Oort Cloud comet C/2001 Q4 (NEAT) 5 days prior to perihelion (rh = 0.97 AU, Δ = 0.35 AU, 2004 May 11.25 and 11.30 UT) observed with the NASA Ames HIFOGS spectrophotometer. The silicate feature of comet Q4 contains strong crystalline peaks at 10.0 and 11.2 μm, along with weaker peaks at 9.3, 10.5, and 11.8 μm, which are characteristic of crystalline olivine and crystalline orthopyroxene. The relative heights of the resonant peaks as well as the shape of the silicate feature in comet Q4 is the same as in comet C/1995 O1 (Hale-Bopp) preperihelion (rh = 1.21 AU). Thermal emission modeling shows Q4 and Hale-Bopp have similar relative abundances of the silicate minerals and high silicate crystalline-to-amorphous ratios. The silicate-to-amorphous carbon ratio derived for comet Q4, however, is lower than in Hale-Bopp and varies by a factor of ~2 in 2 hr, potentially sampling material from different jets in the coma. Owing to the similarity in the silicate mineralogy between Q4 and Hale-Bopp, either these two icy planetesimals formed in the same regime or crystalline silicates were widely distributed within the comet-forming zone.

Spitzer Space Telescope Infrared Imaging and Spectroscopy of the Crab Nebula

We present 3.6, 4.5, 5.8, 8.0, 24, and 70 μm images of the Crab Nebula obtained with the Spitzer Space Telescope IRAC and MIPS cameras, low- and high-resolution Spitzer IRS spectra of selected positions within the nebula, and a near-infrared ground-based image made in the light of [Fe II] 1.644 μm. The 8.0 μm image, made with a bandpass that includes [Ar II] 7.0 μm, resembles the general morphology of visible Hα and near-IR [Fe II] line emission, while the 3.6 and 4.5 μm images are dominated by continuum synchrotron emission. The 24 and 70 μm images show enhanced emission that may be due to line emission or the presence of a small amount of warm dust in the nebula on the order of less than 1% of a solar mass. The ratio of the 3.6 and 4.5 μm images reveals a spatial variation in the synchrotron power-law index ranging from approximately 0.3 to 0.8 across the nebula. Combining this information with optical and X-ray synchrotron images, we derive a broadband spectrum that reflects the superposition of the flatter spectrum of the jet and torus with the steeper spectrum of the diffuse nebula. We also see suggestions of the expected pileup of relativistic electrons just before the exponential cutoff in the X-ray. The pulsar, and the associated equatorial toroid and polar jet structures seen in Chandra and Hubble Space Telescope images (Hester et al. 2002), can be identified in all of the IRAC images. We present the IR photometry of the pulsar. The forbidden lines identified in the high-resolution IR spectra are all double due to Doppler shifts from the front and back of the expanding nebula and give an expansion velocity of ≈1264 km s-1.

A Chandra low energy transmission grating spectrometer observation of V4743 Sagittarii : a supersoft X-ray source and a violently variable light curve

V4743 Sagittarii (Nova Sgr 2002 No. 3) was discovered on 2002 September 20. We obtained a 5 ks ACIS-S spectrum in 2002 November and found that the nova was faint in X-rays. We then obtained a 25 ks Chandra Low Energy Transmission Grating Spectrometer (LETGS) observation on 2003 March 19. By this time, it had evolved into the supersoft X-ray phase exhibiting a continuous spectrum with deep absorption features. The light curve from the observation showed large-amplitude oscillations with a period of 1325 s (22 minutes) followed by a decline in the total count rate after ~13 ks of observations. The count rate dropped from ~40 counts s-1 to practically zero within ~6 ks and stayed low for the rest of the observation (~6 ks). The spectral hardness ratio changed from maxima to minima in correlation with the oscillations and then became significantly softer during the decay. Strong H-like and He-like lines of oxygen, nitrogen, and carbon were found in absorption during the bright phase, indicating temperatures between 1 and 2 MK, but they were shifted in wavelength corresponding to a Doppler velocity of -2400 km s-1. The spectrum obtained after the decline in count rate showed emission lines of C VI, N VI, and N VII, suggesting that we were seeing expanding gas ejected during the outburst, probably originating from CNO-cycled material. An XMM-Newton Target of Opportunity observation, obtained on 2002 April 4 and a later LETGS observation from 2003 July 18 also showed oscillations, but with smaller amplitudes.

The Transitional Stripped-Envelope SN 2008ax: Spectral Evolution and Evidence for Large Asphericity

Supernova (SN) 2008ax in NGC 4490 was discovered within hours after shock breakout, presenting the rare opportunity to study a core-collapse SN beginning with the initial envelope-cooling phase immediately following shock breakout. We present an extensive sequence of optical and near-infrared spectra, as well as three epochs of optical spectropolarimetry. Our initial spectra, taken two days after shock breakout, are dominated by hydrogen Balmer lines at high velocity. However, by maximum light, He I lines dominated the optical and near-infrared spectra, which closely resembled those of normal Type Ib supernovae (SNe Ib) such as SN 1999ex. This spectroscopic transition defines Type IIb SNe, but the strong similarity of SN 2008ax to normal SNe Ib beginning near maximum light, including an absorption feature near 6270 A due to Hα at high velocities, suggests that many objects classified as SNe Ib in the literature may have ejected similar amounts of hydrogen as SN 2008ax, roughly a few × 0.01 M ☉. Only the unusually early discovery of SN 2008ax allowed us to observe the spectroscopic signatures of the hydrogen-rich outer ejecta. Early-time spectropolarimetry (six and nine days after shock breakout) revealed strong line polarization modulations of 3.4% across Hα, indicating the presence of large asphericities in the outer ejecta and possibly that the spectrum of SN 2008ax could be dependent on the viewing angle. After removal of interstellar polarization, the continuum shares a common polarization angle with the hydrogen, helium, and oxygen lines, while the calcium and iron absorptions are oriented at different angles. This is clear evidence of deviations from axisymmetry even in the outer ejecta. Intrinsic continuum polarization of 0.64% only nine days after shock breakout shows that the outer layers of the ejecta were quite aspherical. A single epoch of late-time spectropolarimetry as well as the shapes of the nebular line profiles demonstrate that asphericities extended from the outermost layers all the way down to the center of this core-collapse SN. SN 2008ax may in some ways be an extragalactic analog of the explosion giving rise to Cassiopeia A, which has recently been determined to be a remnant of an SN IIb.

Infrared and X-Ray Evidence for Circumstellar Grain Destruction by the Blast Wave of Supernova 1987A

Multiwavelength observations of supernova remnant 1987A show that its morphology and luminosity are rapidly changing at X-ray, optical, infrared (IR), and radio wavelengths as the blast wave from the explosion expands into the circumstellar equatorial ring, produced by mass loss from the progenitor star. The observed IR radiation arises from the interaction of dust grains that formed in mass outflow with the soft X-ray-emitting plasma component of the shocked gas. Spitzer Infrared Spectrograph spectra at 5-30 μm taken on day 6190 since the explosion show that the emission arises from ~1.1 × 10−6 M☉ of silicate grains radiating at a temperature of ~180−15+20 K. Subsequent observations on day 7137 show that the IR flux had increased by a factor of 2 while maintaining an almost identical spectral shape. The observed IR-to-X-ray flux ratio (IRX) is consistent with that of a dusty plasma with standard Large Magellanic Cloud dust abundances. IRX has decreased by a factor of ~2 between days 6190 and 7137, providing the first direct observation of the ongoing destruction of dust in an expanding supernova blast wave on dynamic timescales. Detailed models consistent with the observed dust temperature, the ionization timescale of the soft X-ray emission component, and the evolution of IRX suggest that the radiating silicate grains are immersed in a 3.5 × 106 K plasma with a density of (0.3–1) × 104 cm−3 and have a size distribution that is confined to a narrow range of radii between 0.023 and 0.22 μm. Smaller grains may have been evaporated by the initial UV flash from the supernova.

The SSS Phase of RS Ophiuchi Observed with Chandra and XMM-Newton. I. Data and Preliminary Modeling

The phase of supersoft source (SSS) emission of the sixth recorded outburst of the recurrent nova RS Oph was observed on days 39.7 and 66.9 after outburst with Chandra and on day 54.0 with XMM-Newton. A ~35 s period on day 54.0 originates from the SSS emission and not from the shock. We discuss the bound-free absorption by neutral elements in the line of sight, resonance absorption lines plus self-absorbed emission-line components, collisionally excited emission lines from the shock, He-like intersystem lines, and spectral changes during an episode of high-amplitude variability. We find a decrease of the oxygen K-shell absorption edge that can be explained by photoionization of oxygen. The absorption component has average velocities of -1286 ± 267 km s-1 on day 39.7 and of -771 ± 65 km s-1 on day 66.9. The wavelengths of the emission-line components are at rest wavelengths, as confirmed by measurements of non-self-absorbed He-like intersystem lines. We found collisionally excited emission lines from the radiatively cooling shock at wavelengths shorter than 15 A that are systematically blueshifted by -526 ± 114 km s-1 on day 39.7 and are fading. We found anomalous He-like f/i ratios, which indicates either high densities or significant UV radiation near the plasma where the emission lines are formed. During the phase of strong variability the spectral hardness light curve overlies the total light curve when shifted by 1000 s. This can be explained by photoionization of neutral oxygen in the line of sight if the densities are of order 1010-1011 cm-3.

Rectified asteroid albedos and diameters from IRAS and MSX photometry catalogs

Rectified diameters and albedo estimates of 1517 main-belt asteroids selected from IRAS and the Mid-Course Space Experiment asteroid photometry catalogs are derived from updated infrared thermal models, the Standard Thermal Model and the Near-Earth Asteroid Thermal Model (NEATM), and Monte Carlo simulations, using new Minor Planet Center compilations of absolute magnitudes (H values) constrained by occultation- and radarderived parameters. The NEATM approach produces a more robust estimate of albedos and diameters, yielding albedos of pv (NEATM mean) = 0.081 ± 0.064. The asteroid beaming parameter (η) for the selected asteroids has a mean value of 1.07 ± 0.27, and the smooth distribution of η suggests that this parameter is independent of asteroid properties such as composition. No trends in η due to size-dependent rotation rates are evident. Comparison of derived values of η as a function of taxonomic type indicates that the beaming parameter values for S- and C-type asteroids are identical within the standard deviation of the population of beaming parameters.