arXiv subjects

Paul D. Feldman

Publications and source records attributed to Paul D. Feldman.

At least 37 records · Page 2Linked to original sources

arXiv2010

GALEX Observations of CS and OH Emission in Comet 9P/Tempel 1 During Deep Impact

GALEX observations of comet 9P/Tempel 1 using the near ultraviolet (NUV) objective grism were made before, during and after the Deep Impact event that occurred on 2005 July 4 at 05:52:03 UT when a 370 kg NASA spacecraft was maneuvered into the path of the comet. The NUV channel provides usable spectral information in a bandpass covering 2000 - 3400 A with a point source spectral resolving power of approximately 100. The primary spectral features in this range include solar continuum scattered from cometary dust and emissions from OH and CS molecular bands centered near 3085 and 2575 A, respectively. In particular, we report the only cometary CS emission detected during this event. The observations allow the evolution of these spectral features to be tracked over the period of the encounter. In general, the NUV emissions observed from Tempel 1 are much fainter than those that have been observed by GALEX from other comets. However, it is possible to derive production rates for the parent molecules of the species detected by GALEX in Tempel 1 and to determine the number of these molecules liberated by the impact. The derived quiescent production rates are Q(H2O) = 6.4e27 molecules/s and Q(CS2) = 6.7e24 molecules/s, while the impact produced an additional 1.6e32 H2O molecules and 1.3e29 CS2 molecules, a similar ratio as in quiescent outgassing.

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arXiv2009

The Far Ultraviolet Spectral Signatures of Formaldehyde and Carbon Dioxide in Comets

Observations of four comets made with the Far Ultraviolet Spectroscopic Explorer show the rotational envelope of the (0,0) band of the CO Hopfield-Birge system (C - X) at 1088 A to consist of both "cold" and "hot" components, the "cold" component accounting for ~75% of the flux and with a rotational temperature in the range 55-75 K. We identify the "hot" component as coming from the dissociation of CO2 into rotationally "hot" CO, with electron impact dissociation probably dominant over photodissociation near the nucleus. An additional weak, broad satellite band is seen centered near the position of the P(40) line that we attribute to CO fluorescence from a non-thermal high J rotational population produced by photodissociation of formaldehyde into CO and H2. This process also leaves the H2 preferentially populated in excited vibrational levels which are identified by fluorescent H2 lines in the spectrum excited by solar OVI 1031.9 and solar Lyman-alpha. The amount of H2 produced by H2CO dissociation is comparable to the amount produced by photodissociation of H2O. Electron impact excitation of CO, rather than resonance fluorescence, appears to be the primary source of the observed (B - X) (0,0) band at 1151 A.

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arXiv2008

Project Lyman

We explore the design of a space mission, Project Lyman, which has the goal of quantifying the ionization history of the universe from the present epoch to a redshift of z ~ 3. Observations from WMAP and SDSS show that before a redshift of z >~ 6 the first collapsed objects, possibly dwarf galaxies, emitted Lyman continuum (LyC) radiation shortward of 912 A, reionizing most of the universe. How LyC escapes from galactic environments, whether it induces positive or negative feedback on the local and global collapse of structures, and the role played by clumping, molecules, metallicity and dust are major unanswered theoretical questions, requiring observational constraint. Numerous intervening Lyman limit systems, which frustrate the detection of LyC from high z objects, thin below z ~ 3 where there are a few objects with apparently very high fesc. At low z there are only controversial detections and a handful of upper limits. A wide-field multi-object spectroscopic survey with moderate spectral and spatial resolution can quantify fesc within diverse spatially resolved galactic environments over redshifts with significant evolution in galaxy assemblage and quasar activity. It can also calibrate LyC escape against Ly-alpha escape, providing an essential tool to JWST for probing the beginnings of reionization. We present calculations showing the evolution of the characteristic apparent magnitude of star-forming galaxy luminosity functions at 900 A, as a function of redshift and assumed escape fraction to determine the required aperture for detecting LyC. We review our efforts to build a pathfinding dual order multi-object spectro/telescope with a (0.5deg)^2 field-of-view, using a GSFC microshutter array, and crossed delay-line micro-channel plate detector.

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arXiv2008

Calibration and flight performance of the long-slit imaging dual order spectrograph

We present a preliminary calibration and flight performance of the Long-Slit Imaging Dual Order Spectrograph (LIDOS), a rocket-borne instrument with a large dynamic range in the 900 - 1700A bandpass. The instrument observes UV-bright objects with a CCD channel and fainter nebulosity with an MCP detector. The image quality and the detector quantum efficiencies were determined using the calibration and test equipment at the Johns Hopkins University, and further monitored using an on-board electron-impact calibration lamp. We review results from each of the three flights of the instrument.

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arXiv2007

The Fourth Positive System of Carbon Monoxide in the Hubble Space Telescope Spectra of Comets

The rich structure of the Fourth Positive System (A-X) of carbon monoxide accounts for many of the spectral features seen in long slit HST-STIS observations of comets 153P/Ikeya-Zhang, C/2001 Q4 (NEAT), and C/2000 WM1 (LINEAR), as well as in the HST-GHRS spectrum of comet C/1996 B2 Hyakutake. A detailed CO fluorescence model is developed to derive the CO abundances in these comets by simultaneously fitting all of the observed A-X bands. The model includes the latest values for the oscillator strengths and state parameters, and accounts for optical depth effects due to line overlap and self-absorption. The model fits yield radial profiles of CO column density that are consistent with a predominantly native source for all the comets observed by STIS. The derived CO abundances relative to water in these comets span a wide range, from 0.44% for C/2000 WM1 (LINEAR), 7.2% for 153P/Ikeya-Zhang, 8.8% for C/2001 Q4 (NEAT) to 20.9% for C/1996 B2 (Hyakutake). The subtraction of the CO spectral features using this model leads to the first identification of a molecular hydrogen line pumped by solar HI Lyman-beta longward of 1200A in the spectrum of comet 153P/Ikeya-Zhang. (Abridged)

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arXiv2007

A Low-Mass H2 Component to the AU Microscopii Circumstellar Disk

We present a determination of the molecular gas mass in the AU Microscopii circumstellar disk. Direct detection of a gas component to the AU Mic disk has proven elusive, with upper limits derived from ultraviolet absorption line and submillimeter CO emission studies. Fluorescent emission lines of H2, pumped by the OVI 1032 resonance line through the C-X (1 -- 1) Q(3) 1031.87 Å transition, are detected by the Far Ultraviolet Spectroscopic Explorer. These lines are used to derive the H2 column density associated with the AU Mic system. The derived column density is in the range N(H2) = 1.9 x 10^{17} - 2.8 x 10^{15} cm^{-2}, roughly two orders of magnitude lower than the upper limit inferred from absorption line studies. This range of column densities reflects the range of H2 excitation temperature consistent with the observations, T(H2) = 800 -- 2000 K, derived from the presence of emission lines excited by OVI in the absence of those excited by LyA. Within the observational uncertainties, the data are consistent with the H2 gas residing in the disk. The inferred N(H2) range corresponds to H2-to-dust ratios of < 1/30:1 and a total M(H2) = 4.0 x 10^{-4} - 5.8 x 10^{-6} Earth masses. We use these results to predict the intensity of the associated rovibrational emission lines of H2 at infrared wavelengths covered by ground-based instruments, HST-NICMOS, and the Spitzer-IRS.

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arXiv2007

Molecular and Atomic Excitation Stratification in the Outflow of the Planetary Nebula M27

High resolution spectroscopy with FUSE and STIS of atomic and molecular velocity stratification in the nebular outflow of M27 challenge models for the abundance kinematics in planetary nebulae. The simple picture of a very high speed (~ 1000 km/s), high ionization, radiation driven stellar wind surrounded by a slower (~ 10 km/s) mostly molecular outflow, with low ionization and neutral atomic species residing at the wind interaction interface, is not supported... We find ...there is a fast (33 -- 65 km/s) low ionization zone, surrounding a slower (<~ 33 km/s) high ionization zone and, at the transition velocity (33 km/s), vibrationally excited H_2 is intermixed with a predominately neutral atomic medium... Far-UV continuum fluorescence of H_2 is not detected, but Lyman alpha (Lya) fluorescence is present. The diffuse nebular medium is inhospitable to molecules and dust. Maintaining the modest equilibrium abundance of H_2 (N(H_2)/N(HI) << 1) in the diffuse nebular medium requires a source of H_2, mostly likely the clumpy nebular medium. The stellar SED shows no signs of reddening (E(B-V) < 0.01), but paradoxically measurements of Ha/Hb ... indicate E(B-V) ~ 0.1. ...the apparent enhancement of Ha/Hb in the absence of dust may result from a two step process of H_2 ionization by Lyman continuum (Lyc) photons followed by dissociative recombination (H_2 + gamma -> H_2^+ + e -> H(1s) + H (nl)), which ultimately produces fluorescence of Ha and Lya. In the optically thin limit at the inferred radius of the velocity transition we find dissociation of H_2 by stellar Lyc photons is an order of magnitude more efficient than spontaneous dissociation by far-UV photons. We suggest that the importance of this H_2 destruction process in HII regions has been overlooked.

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arXiv2006

Ultraviolet Spectroscopy of Comet 9P/Tempel 1 with Alice/Rosetta during the Deep Impact Encounter

We report on spectroscopic observations of periodic comet 9P/Tempel 1 by the Alice ultraviolet spectrograph on the Rosetta spacecraft in conjunction with NASA's Deep Impact mission. Our objectives were to measure an increase in atomic and molecular emissions produced by the excavation of volatile sub-surface material. We unambiguously detected atomic oxygen emission from the quiescent coma but no enhancement at the 10% (1-sigma) level following the impact. We derive a quiescent water production rate of 9 x 10^27 molecules per second with an estimated uncertainty of 30%. Our upper limits to the volatiles produced by the impact are consistent with other estimates.

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arXiv2006

Hubble Space Telescope Observations of Comet 9P/Tempel 1 during the Deep Impact Encounter

We report on the Hubble Space Telescope program to observe periodic comet 9P/Tempel 1 in conjunction with NASA's Deep Impact mission. Our objectives were to study the generation and evolution of the coma resulting from the impact and to obtain wide-band images of the visual outburst generated by the impact. Two observing campaigns utilizing a total of 17 HST orbits were carried out: the first occurred on 2005 June 13-14 and fortuitously recorded the appearance of a new, short-lived fan in the sunward direction on June 14. The principal campaign began two days before impact and was followed by contiguous orbits through impact plus several hours and then snapshots one, seven, and twelve days later. All of the observations were made using the Advanced Camera for Surveys (ACS). For imaging, the ACS High Resolution Channel (HRC) provides a spatial resolution of 36 km (16 km/pixel) at the comet at the time of impact. Baseline images of the comet, made prior to impact, photometrically resolved the comet's nucleus. The derived diameter, 6.1 km, is in excellent agreement with the 6.0 +/- 0.2 km diameter derived from the spacecraft imagers. Following the impact, the HRC images illustrate the temporal and spatial evolution of the ejecta cloud and allow for a determination of its expansion velocity distribution. One day after impact the ejecta cloud had passed out of the field-of-view of the HRC.

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arXiv2006

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 NASA's 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 x 10^31 molecules or a mass of 6.6 x 10^5 kg, an amount that would normally be produced by 7-10 hours of quiescent outgassing from the comet. This number is less than or equal to 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 sub-surface material responsible for the quiescent outgassing of the comet.

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arXiv2006

The Carbon-Rich Gas in the Beta Pictoris Circumstellar Disk

The edge-on disk surrounding the nearby young star Beta Pictoris is the archetype of the "debris disks", which are composed of dust and gas produced by collisions and evaporation of planetesimals, analogues of Solar System comets and asteroids. These disks provide a window on the formation and early evolution of terrestrial planets. Previous observations of Beta Pic concluded that the disk gas has roughly solar abundances of elements [1], but this poses a problem because such gas should be rapidly blown away from the star, contrary to observations of a stable gas disk in Keplerian rotation [1, 2]. Here we report the detection of singly and doubly ionized carbon (CII, CIII) and neutral atomic oxygen (OI) gas in the Beta Pic disk; measurement of these abundant volatile species permits a much more complete gas inventory. Carbon is extremely overabundant relative to every other measured element. This appears to solve the problem of the stable gas disk, since the carbon overabundance should keep the gas disk in Keplerian rotation [3]. New questions arise, however, since the overabundance may indicate the gas is produced from material more carbon-rich than the expected Solar System analogues.

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arXiv2005

Rapid Dissipation of Primordial Gas from the AU Microscopii Debris Disk

The disk around AU Microscopii, an M1 star in the Beta Pictoris Moving Group, is extraordinarily well-suited for comparison to the Beta Pic debris disk (type A5V). We use far-UV absorption spectroscopy of AU Mic to probe its edge-on disk for small amounts of molecular hydrogen, the primary constituent of gas giant planets. Our conservative upper limit on the line-of-sight H_2 column density is 1.7 x 10^{19} cm^{-2}, which is 18.5 times lower than the limit obtained from non-detection of sub-mm CO emission (Liu et al. 2004). In addition, there is a hint of H_2 absorption at a column density an order of magnitude or more below our upper limit. The H_2-to-dust ratio in the AU Mic disk is < 6:1, similar to that in the Beta Pic disk. This shows that the primordial gas has largely been dissipated in less than about 12 Myr for both disks, despite their very different stellar masses and luminosities. It is extremely difficult to form a giant planet around AU Mic with current core-accretion scenarios in such a short time.

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arXiv2004

FORTIS: Pathfinder to the Lyman Continuum

Shull et al. (1999) have asserted that the contribution of stars, relative to quasars, to the metagalactic background radiation that ionizes most of the baryons in the universe remains almost completely unknown at all epochs. The potential to directly quantify this contribution at low redshift has recently become possible with the identification by GALEX of large numbers of sparsely distributed faint ultraviolet galaxies. Neither STIS nor FUSE nor GALEX have the ability to efficiently survey these sparse fields and directly measure the Lyman continuum radiation that may leak into the low redshift (z < 0.4) intergalactic medium. We present here a design for a new type of far ultraviolet spectrograph, one that is more sensitive, covers wider fields, and can provide spectra and images of a large number of objects simultaneously, called the Far-ultraviolet Off Rowland-circle Telescope for Imaging and Spectroscopy (FORTIS). We intend to use a sounding rocket flight to validate the new instrument with a simple long-slit observation of the starburst populations in the galaxy M83. If however, the long-slit were replaced with microshutter array, this design could isolate the chains of blue galaxies found by GALEX over an ~30' diameter field-of-view and directly address the Lyman continuum problem in a long duration orbital mission. Thus, our development of the sounding rocket instrument is a pathfinder to a new wide field spectroscopic technology for enabling the potential discovery of the long hypothesized but elusive Lyman continuum radiation that is thought to leak from low redshift galaxies and contribute to the ionization of the universe.

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arXiv2004

Detection of Atomic Chlorine in Io's Atmosphere with HST/GHRS

We report the detection of atomic chlorine emissions in the atmosphere of Io using Hubble Space Telescope observations with the Goddard High Resolution Spectrograph (GHRS). The Cl I 1349 A dipole allowed and Cl I] 1386 A forbidden transition multiplets are detected at a signal to noise ratio (SNR) of 6 and 10, respectively, in a combined GHRS spectrum acquired from 1994 through 1996. Oxygen and sulfur emissions are simultaneously detected with the chlorine which allows for self-consistent abundance ratios of chlorine to these other atmospheric species. The disk averaged ratios are: Cl/O = 0.017 +/- 0.008, Cl/S = 0.10 +/- 0.05, and S/O = 0.18 +/- 0.08. We also derive a geometric albedo of 1.0 +/- 0.4 % for Io at 1335 A assuming an SO2 atmospheric column density of 1x10^{16} cm^{-2}.

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arXiv2003

The Far Ultraviolet Spectrum of the Io Plasma Torus

The spectrum of the Io plasma torus in the range 905 - 1187 A was recorded at 0.26 A resolution by the Far Ultraviolet Spectroscopic Explorer (FUSE) on 2001 January 14. Five orbits of data were obtained with the west ansa of the torus centered and tracked in the 30" x 30" apertures of the FUSE spectrographs for a total observation time of 9740 seconds. This region of the spectrum is dominated by transitions of S II, S III and S IV, whose multiplet structure is nearly completely resolved. We confirm our earlier detection of emission from resonance multiplets of Cl III and Cl II and derive an abundance of Cl^{+2} of 2.1% relative to S^{2+}, leading to an overall chlorine ion abundance in the torus of slightly less than 1%. A number of features near 990 A remain unidentified, and C III at 977 A is detected in two independent channels at the 3-sigma level. The inferred relative ion abundance of C^{2+} relative to S^{2+} is 3.7 x 10^{-4}. We also present spectra at 0.085 A resolution taken on 2001 October 19 and 21 with the 4" x 20" aperture. In these spectra the observed lines are resolved and their widths correspond to ion temperatures of 60 - 70 eV for all three sulfur ions.

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arXiv2002

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 Angstrom 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.

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arXiv2002

The Abundance of Atomic Sulfur in the Atmosphere of Io

Observations with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope have been used to constrain the atomic sulfur column density in Io's atmosphere. The SI 1479 dipole allowed and forbidden transition multiplets have been resolved for the first time at Io, enabling the study of both optically thick and thin transitions from a single atomic species. The allowed transitions contribute 62 +/- 8% and the forbidden transitions 38 +/- 8%, on average, to the total signal of the SI 1479 multiplets. Using the optically thick and thin transitions of SI 1479 observed near the limbs of Io, we derive a tangential atmospheric sulfur column abundance of 3.6*10^12 cm^-2 < N_s < 1.7*10^13 cm^-2, which is independent of electron temperature and density. A low density SO_2 atmosphere, N_so2 ~ 5-10*10^15 cm^-2, consistent with that inferred from other recent observations, is most consistent with these bounds.

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arXiv2000

On the Correlation Between CO Absorption and Far-Ultraviolet Non-Linear Extinction Toward Galactic OB Stars

A sample of 59 sight lines to reddened Galactic OB stars was examined for correlations of the strength of the CO Fourth Positive (A - X) absorption band system with the ultraviolet interstellar extinction curve parameters. We used archival high-dispersion NEWSIPS IUE spectra to measure the CO absorption for comparison to parametric fits of the extinction curves from the literature. A strong correlation with the non-linear far-UV curvature term was found with greater absorption, normalized to E(B-V), being associated with more curvature. A weaker trend with the linear extinction term was also found. Mechanisms for enhancing CO in dust environments exhibiting high non-linear curvature are discussed.

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