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Hester, J. J.

Publications and source records attributed to Hester, J. J..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2000

Discovery of Spatial and Spectral Structure in the X-Ray Emission from the Crab Nebula

The Chandra X-ray Observatory observed the Crab Nebula and Pulsar During orbital calibration. Zeroth-order images with the High-Energy Transmission Grating (HETG) read-out by the Advanced CCD Imaging Spectrometer spectroscopy array (ACIS-S) show a striking richness of X-ray structure, at a resolution comparable to that of the best ground-based visible-light observations. The HETG-ACIS-S images reveal, for the first time, an X-ray knots along the inner ring and (perhaps) along the inward extension of the X-ray jet. Although complicated by instrumental effects and the brightness of the Crab Nebula, the spectrometric analysis shows systematic variations of the X-ray spectrum throughout the Nebula.

Weisskopf, M.↗
NASA NTRS1998

HST Imaging of the Circumstellar Nebulosity of T Tauri

Short exposure Planetary Camera images of T Tauri have been obtained using broadband filters spanning the wavelength range 0.55-0.80 mu. The optically visible star lies very close to an arc of reflection nebulosity.

HST Nebulosity T Tauri circumstellar matter pre-ma↗
NASA NTRS1996

Physical Conditions in Low Ionization Regions of the Orion Nebula

We reexamine the spectroscopic underpinnings of recent claims that low ionization (O(I)) and (Fe(II)) lines from the Orion H(II) region are produced in a region where the iron-carrying grains have been destroyed and the electron density is surprisingly high. Our new HST and CTIO observations show that previous reported detections of(O(I)) lambda 5577 were strongly affected by telluric emission. Our line limits consistent with a moderate density (approx. 10(exp 4)/cu. cm photoionized gas. We show that a previously proposed model of the Orion H(II) region reproduces the observed (O(I)) and (Fe(II)) spectrum. These lines are fully consistent with formation in a moderate density dusty region.

Baldwin, J. A.↗
NASA NTRS1995

An X-ray and optical study of the interaction of the Cygnus Loop supernova remnant with an interstellar cloud

We have used the ROSAT high-resolution imager and optical emission line data to study the structure of a bright optical knot on the southeastern rim of the Cygnus Loop supernova remnant. This knot has been identified as an encounter between the blast wave and a small isolated cloud. The knot appears in projection just behind the blast wave, which is traced by Balmer line filaments which bound the X-ray emission. The knot is a prominent X-ray feature, consisting of a numerb of filaments which are correlated with the optical line emission. These data permit a detailed view of the blast wave interaction. By combining the optical and X-ray data it is possible to trace the blast wave as a continuous surface from its southern edge around the western side of the cloud that then continues on to the north. The southeastern knot is an indentation on the surface of the blast wave. Thus the southeastern knot is not a small cloud that has been overrun by the blast wave, but the tip of a larger cloud. Bright X-ray emission is associated with bright radiative filaments. The location of this emission, upstream of the radiative shocks, implies that the enhanced X-rays come from a reverse shock. The presence of a reverse shock is further evidence that the southeastern knot represents an early stage of a blast wave encountering a large cloud. A consistent picture of the Cygnus Loop as an explosion within a preexisting cavity is emerging. These observations agree with that picture.

Graham, James, R.↗
NASA NTRS1995

WFPC2 Imaging of the Circumstellar Nebulosity of HL Tauri

HL Tauri is considered a prototype solar-mass T Tauri star. Planetary camera images of HL Tauri have been obtained through V, R, and I band filters using the Wide Field Planetary Camera 2 aboard the refurbished Hubble Space Telescope. These images show that HL Tauri is entirely reflection nebulosity at optical wavelengths, with no optical star visible to a limiting magnitude of V=25.5.

circumstellar matter stars HL Tauri pre-main seque↗
NASA NTRS1993

Planetary camera observations of the double nucleus of M31

HST Planetary Camera images obtained in the V and I band for M31 show its inner nucleus to consist of two components that are separated by 0.49 arcsec. The nuclear component with lower surface brightness closely coincides with the bulge photocenter and is argued to be at the kinematic center of the galaxy. It is surmised that, if dust absorption generates the asymmetric nuclear morphology observed, the dust grain size must either be exceptionally large, or the dust optical depth must be extremely high; the higher surface-brightness and off-center nuclear component may alternatively be a separate stellar system.

Lauer, Tod R.↗
NASA NTRS1992

CNO abundances and temperature fluctuations in the Orion Nebula

The largest UV-optical data base to date of the Orion Nebula has been assembled, and the CNO abundances their distribution at a statistically significant number of positions to the north and west of the Trapezium and out to a distance of 5 arcmin has been examined. The forbidden O III temperature increases with distance from Theta1 Ori C while the forbidden N II temperature decreases with increasing distance. The density decreases as a function of increasing distance from Theta1 Ori C. The C/H, N/H, O/H, and C/O distributions are constant across the inner 5 arcmin of the northwestern quadrant of the nebula. The empirically determined mean C/H abundance calculated from UV lines is almost a factor of two lower in the number density than earlier estimates. Photoionization model calculations are consistent with a solar abundance for N/H and O/H.

Walter, Donald K.↗
NASA NTRS1992

Planetary camera observations of the central parsec of M32

New HST observations are reported which show that the M32 nucleus is extremely dense, with rho0 not less than 4 x 10 exp 6 solar masses/cu pc if M32 has a core with rc not greater than 0.37 pc, and rho0 greater than 3 x 10 exp 7 solar masses/cu pc if it instead has a central r exp -1/2 cusp. Limits on the M32 core radius are nearly three times smaller, and those on the central density at least four times greater, than values inferred from the best ground-based observations. A central relaxation that is almost an order of magnitude shorter than previous estimates is inferred from the new core parameters; a shorter stellar collision time is also inferred. It is argued that a starlight cusp surrounds the central black hole.

Lauer, Tod R.↗
NASA NTRS1992

The H II regions of the galaxy M101

The physical properties and composition of the H II region population of the spiral galaxy M101 (NGC 5457) were examined using spectrophotometrically calibrated CCD imagery of two overlapping 16-arcmin fields in the M101, in which individual H II regions were identified and mapped using an automated algorithm. The characteristics obtained for the H II region population of M101 include the H II region luminosity function and radial variations in extinction; the forbidden O III line/H-beta, O/H, and forbidden S II line/H-alpha ratios; the ionization parameter U; and the numbers of ionizing photons. Global variations of physical properties of the H II regions in M101 are compared to model expectations derived for other galaxies. The evident star formation in the M101 galaxy is discussed.

Scowen, P. A.↗
NASA NTRS1992

Planetary Camera observations of NGC 1275 - Discovery of a central population of compact massive blue star clusters

The discovery of a population of bright blue pointlike sources within 5 kpc of the nucleus of NGC 1275 using HST Planetary Camera observations is reported. The typical object has MV of about -12 to -14. They are all blue, with V - R of less than about 0.3. The color distribution and lack of excess H-alpha emission are consistent with nearly all being continuum sources. Many sources are unresolved even with the HST and consequently have sizes of less than about 15 pc. It is suggested that these are young star clusters that will evolve to look like globular clusters. They are bluer than any clusters seen in the Milky Way or M87, and brighter than the blue clusters seen in the LMC. Ages of several hundred million years or less and corresponding masses of 10 exp 5 - 10 exp 8 solar masses are derived.

Holtzman, Jon A.↗
NASA NTRS1992

Planetary Camera observations of the M87 stellar cusp

Results are presented of an analysis of V- and I-band HST Planetary Camera images of the giant elliptical galaxy M87 which show that its central starlight distribution is consistent with the black hole 2.6 x 10 exp 9-solar-mass cup model proposed for M87 by Young et al. (1978). A combined approach of image deconvolution and modeling is used to investigate the starlight distribution into limiting radii of about 0.04 arcsec. The central structure of M87 is described by three components: a power-law starlight profile of the form mu(r) varies as r exp -1/4 for r of less than 3 arcsec, a central nonthermal point source, and optical counterparts of the jet knots N1 and M identified by VLBI observations.

Lauer, Tod R.↗
NASA NTRS1991

Hubble Space Telescope imaging of Eta Carinae

New high spatial resolution observations of the material around Eta Carinae, obtained with the Hubble Space Telescope Wide Field/Planetary Camera, are presented. The star Eta Carinae is one of the most massive and luminous stars in the Galaxy, and has been episodically expelling significant quantities of gas over the last few centuries. The morphology of the bright central nebulosity (the homunculus) indicates that it is a thin shell with very well defined edges, and is clumpy on 0.2 arcsec (about 10 to the 16th cm) scales. An extension to the northeast of the star (NN/NS using Walborn's 1976 nomenclature) appears to be a stellar jet and its associated bow shock. The bow shock is notable for an intriguing series of parallel linear features across its face. The S ridge and the W arc appear to be part of a 'cap' of emission located to the SW and behind the star. Together, the NE jet and the SW cap suggest that the symmetry axis for the system runs NE-SW rather than SE-NW, as previously supposed. Overall, the data indicate that the material around the star may represent an oblate shell with polar blowouts, rather than a bipolar flow.

Hester, J. J.↗
NASA NTRS1991

Hubble Space Telescope images and follow-up spectroscopy of the Orion nebula

Recently published HST images of the Orion nebula reveal elephant-trunk structures, an apparent jet of material, and fine-scale structure in the Herbig-Haro object HH2, which is located at the base of an elephant trunk. High-resolution spectroscopy shows that the apparent jet is actually an ionization front seen edge-on. HH2 shows a complex structure in the several stages of ionization observed. There seem to be two velocity systems characterized by a bright central region and an accompanying shell-like emission. These two systems are most likely to be the result of a bow shock and corresponding Mach disk formed from the interaction of a collimated jet and the ambient gas of the nebula.

O'Dell, C. R.↗
NASA NTRS1990

The circumstellar environment of TMR-1 - A young, low-mass star in the Taurus molecular ring

Near-infrared camera and Owens Valley millimeter interferometer data are presented that reveal the circumstellar environment of TMR-1 on scales of roughly 1000 AU. TMR-1 is a deeply embedded, young, low-mass star located in a rotating ring of material in the TMC-1/Heiles Cloud 2 region of the Taurus molecular cloud. The stellar wind cavity, as outlined by the extended infrared continuum emission at 2.2 microns, overlaps with the redshifted outflow detected in the (C-12)O data. High-density circumstellar material is seen in (C-13)O in emission and at 2.2 microns in absorption. The morphology suggests the high-density gas may either be part of a 1000 AU disk or may represent infalling material from the molecular cloud core. TMR-1 appears to be in an early wind-clearing phase of protostar formation.

Terebey, S.↗
NASA NTRS1988

Spatial and spectral interpretation of a bright filament in the Cygnus Loop

A comparison is made of optical and UV line intensities and spatial and spectral optical line profiles of a well-defined Cygnus Loop filament with theoretical models. It is found that the sharp filament is due to the tangency to the line of sight of a large, thick sheet of emitting gas. The emitting region associated with the spur is very deep, and there is substantial gradient in shock velocity along the filament. Severe incompleteness of the recombination zone is found at the high-velocity end, and resonance scattering in the emitting region attenuates C IV and other resonance lines, as expected. There is evidence for depletion of Si and Fe relative to other elements. Nonthermal pressure apparently dominates the recombination zone of the filament. New techniques are introduced for determining the completeness of a shock and for determining the preshock density without recourse to the standard density diangostic line ratios.

Raymond, J. C.↗
NASA NTRS1986

A deep H-alpha image of faint Balmer-line filaments in the northeast Cygnus Loop supernova remnant

A deep H-alpha image of a field outside the optically bright northeast limb of the Cygnus Loop supernova remnant is presented. Two filaments which are just visible on the red POSS print form part of two long and unbroken lines of filaments which generally bound a region of diffuse emission and fainter filaments. However, the diffuse emission extends out to a newly detected line of extremely faint filaments 3 arcmin to the northeast. The brightest filaments are resolved into multiple components with widths less than 2 arcsec. Comparison with the POSS print leads to a rough estimate of the proper motion of 6 arcsec per century, although temporal variations in the relative brightnesses of the components of a filament could confuse attempts to more accurately measure the true proper motion. The morphology of the emission strongly suggests that it arises in a single large sheet. No direct evidence is found favoring the existence of the dense cloud cores which have been hypothesized as a way of evaporatively enhancing the X-ray emissivity and producing forbidden line emission in the filaments.

Hester, J. J.↗
NASA NTRS1986

The Cygnus loop - A detailed comparison of X-ray and optical emission

A comparison at a resolution of 17 arcsec is presented between optical emission from forbidden O III and forbidden S II and the thermal X-ray emission for a field on the southeast edge of the Cygnus Loop SNR. The relationship between optical and X-ray emission in the field is described, generalizing to the Loop as a whole when possible. Several possible explanations for the presence of bright X-ray emission in the vicinity of optical emission are presented and evaluated based on the data. These mechanisms involve evaporation, gradual variations and gradients in the density of the preshock intercloud medium, and additional compression of material which has already been heated to X-ray temperatures by the adiabatic blast wave. The additional compression could result from rapid deceleration of the blast wave itself or from reshocking by reflected or bow shocks around dense clouds. Implications of the observations for models of SNR evolution and the interstellar medium are discussed.

Cox, D. P.↗