Engineering topics

Chu, You-Hua

Publications and source records attributed to Chu, You-Hua.

30 records · Page 2

NASA NTRS1994

Two new supernova remnants in OB associations in the Large Magellanic Cloud

We discovered two extended x-ray sources in a Roentgen Satellite (ROSAT) Positron Sensitive Proportional Counter (PSPC) observation pointed at the H II region N9 in the Large Magellanic Cloud. These two sources are in the H II regions N4D and N9. The x-ray characteristics suggest that both might be supernova remnants (SNRs). Follow-up charge coupled device (CCD) images taken with interference filters show high (S II)/H-alpha ratios in the optical nebulae of these x-ray sources, confirming the presence of high velocity shocks commonly seen in SNRs. These two sources are also detected in the radio continuum at 8.55 and 4.75 GHz; both appear nonthermal compared to nearby H II regions. The confirmation of these two SNRs demonstrates that many SNRs in or near H II regions have been overlooked in previous surveys, and that the ROSAT x-ray survey combined with an optical CCD imaging survey of the Magellanic Clouds would provide the most effective way to uncover SNRs.

Smith, R. Chris↗

NASA NTRS1994

Internal motions of HII regions and giant HII regions

We report new echelle observations of the kinematics of 30 HII regions in the Large Magellanic Clouds (LMC), including the 30 Doradus giant HII region. All of the HII regions possess supersonic velocity dispersions, which can be attributed to a combination of turbulent motions and discrete velocity splitting produced by stellar winds and/or embedded supernova remnants (SNRs). The core of 30 Dor is unique, with a complex velocity structure that parallels its chaotic optical morphology. We use our calibrated echelle data to measure the physical properties and energetic requirements of these velocity structures. The most spectacular structures in 30 Dor are several fast expanding shells, which appear to be produced at least partially by SNRs.

Chu, You-Hua↗

NASA NTRS1994

A new ejecta shell surrounding a Wolf-Rayet star in the LMC

We have obtained CCD spectra of newly discovered shell-like nebulae around the WN4 star Breysacher 13 and the WN1 star Breysacher 2 in the Large Magellanic Cloud (LMC). The shell around Br 13 shows definite signs of enrichment in both nitrogen and helium, having much stronger (N II) and He I emission lines than are seen in typical LMC H II regions. From the measured electron temperature of about 17,000 K in the shell, we derive He/H and N/O abundance ratios which are factors of 2 and more than 10 higher, respectively, than the average LMC interstellar values. The derived oxygen abundance in the Br 13 shell is down by a factor of 8 compared to the local LMC interstellar medium (ISM); however, the derived electron temperature is affected by the presence of an incomplete shock arising from the interaction of the stellar wind with photoionized material. This uncertainty does not affect the basic conclusion that the Br 13 shell is enriched by processed material from the Wolf-Rayet star. In contrast, the shell around Br 2 shows no clear evidence of enrichment. The nebular spectrum is characterized by extremely strong (O III) and He II emission and very weak (N II). We derive normal He, O, and N abundances from our spectrum. This object therefore appears to be simply a wind-blown structure associated with a relatively dense cloud near the Wolf-Rayet star, although the very high-ionization state of the gas is unusual for a nebula associated with a Wolf-Rayet star.

Garnett, Donald R.↗

NASA NTRS1994

Kinematic structure of the 30 Doradus giant H II region

We have used the echelle CCD spectrograph on the Cerro Tololo Inter-American Observatory (CTIO) 4 m telescope to map the nebular velocity field in the 30 Doradus giant H II region. The kinematics of 30 Dor are very complex. The outer regions are charaterized by a smooth velocity field, but its turbulent velocity, 30-40 km/s Full Width Half Maximum (FWHM), is considerably higher than those in most smaller H II regions. In the central 9 min core, multiple velocity components are observed at most positions. The velocity field is dominated by a large number of expanding structures, ranging in size from 1 to 100 pc and expansion velocities of 20-200 km/s, and often organized into large hierarchical networks. The integral of these complex expanding structures in 30 Dor produces a surprisingly simple profile with a broad Gaussian core and faint extended wings. Several fast-expanding shells, with diameters of 2-20 pc, expansion velocities of 100-300 km/s, and kinetic energies of 0.5-10 x 10(exp 50) ergs have been identified. The large fast-expanding shells and networks are coincident with extended X-ray sources and are probably associated with supernova remnants embedded in supershells produced by the combined effects of stellar winds and supernovae from OB associations. We have used the intensity-calibrated echelle spectra to determine the basic physical and dynamical properties of the kinematic features in 30 Dor. The expanding shells contain roughly half of the kinetic energy in the 30 Dor complex, and this energy is several times higher than the gravitational binding energy of the region. The energetic requirements of the gas are consistent with the observed stellar content of 30 Dor, if the gas is accelerated by a combination of stellar winds and supernovae. Extrapolating the current energy injection rate in the nebula over the lifetime of the OB complex suggest that 30 Dor and its vicinity will evolve into a supergiant shell as seen in the LMC and other nearby galaxies.

Chu, You-Hua↗

NASA NTRS1994

The x-ray structure of the supernova remnant W49B

Comparison of x-ray and radio images of W49B and other supernova remnants (SNR) provides detailed information on the mechanisms responsible for the emission and on the evolution of the remnants. There is faint x-ray emission from all parts of W49B but most of it is concentrated near the center of the remnant, unlike the radio emission which arises in a shell near the periphery. This structure indicates that this SNR is in the adolescent phase of its lifetime.

Dickel, John R.↗

NASA NTRS1993

Hidden supernova remnants in the Large Magellanic Cloud H II complex N44

We have obtained ROSAT PSPC observations of N44, one of the largest H II complexes in the Large Magellanic Cloud. The X-ray emission mostly fails within the ionized shell structures in N44. We find that one faint shell is a classical supernova remnant overlooked by previous surveys. If we model the two largest shells as pressure-driven superbubbles, the predicted X-ray luminosity falls far below the observed value. Instead, we show that off-center supernova remnants hitting superbubble shells can explain the excess X-ray emission.

Chu, You-Hua↗

NASA NTRS1993

Diffuse X-ray emission from the Dumbbell Nebula?

We have analyzed ROSAT Position Sensitive Proportional Counter pointed observations of the Dumbbell Nebula and find that the previously reported 'extended' X-ray emission is an instrumental electronic ghost image at the softest energy band. At slightly higher energy bands, the image of the Dumbbell is not very different from that of the white dwarf HZ43. We conclude that the X-ray emission of the Dumbbell Nebula comes from its central star. A blackbody model is fitted to the spectrum and the best-fit temperature of not greater than 136,000 +/- 10,000 K is in excellent agreement with the Zanstra temperatures.

Chu, You-Hua↗

NASA NTRS1993

A new survey of nebulae around Galactic Wolf-Rayet stars in the northern sky

Interference filter CCD images have been obtained in H-alpha and forbidden O III 5007 A for 62 Wolf-Rayet (W-R) stars, representing a complete survey of nebulae around Galactic W-R stars in the northern sky. We find probable new ring nebulae around W-R stars number 113, 116 and 132, and possible new ring nebulae around W-R stars number 133 and 153. All survey images showing nebulosities around W-R stars are presented in this paper. New physical information is derived from the improved images of known ring nebulae. The absence of ring nebulae around most W-R stars is discussed.

Miller, Grant J.↗

NASA NTRS1992

30 Dor B - A supernova remnant in a star formation region

The supernova remnant 30 Dor B is embedded in an H II region around the OB association LH 99, and has been suggested to be a Crab-type remnant. To determine the spatial extent and kinematic properties of this supernova remnant, long-slit echelle observations were obtained in the H-alpha and forbidden N II lines, along with imaging Fabry-Perot observations in the H-alpha line. It is found that 30 Dor B is partially obscured by a dark cloud with which the supernova remnant also interacts. The size of 30 Dor B is much larger than previously thought, making it much older than the Crab Nebula. If the progenitor of the supernova was formed coevally with LH 99, then it would have been more massive than the O3 members of the OB association.

Chu, You-Hua↗

NASA NTRS1990

X-rays from superbubbles in the Large Magellanic Cloud

Diffuse X-ray emission not associated with known supernova remnants (SNRs) are found in seven Large Magellanic Cloud H II complexes encompassing 10 OB associations: N44, N51D, N57A, N70, N154, N157 (30 Dor), and N158. Their X-ray luminosities range from 7 x 10 to the 34th ergs/s in N57A to 7 x 10 to the 36th ergs/s in 30 Dor. All, except 30 Dor, have simple ring morphologies, indicating shell structures. Modeling these as superbubbles, it is found that the X-ray luminosities expected from their hot interiors fall an order of magnitude below the observed values. SNRs close to the center of a superbubble add very little emission, but it is calculated that off-center SNRs hitting the ionized shell could explain the observed emission.

Chu, You-Hua↗

NASA NTRS1990

A kinematic determination of the structure of the double ring planetary nebula NGC 2392, the Eskimo

Slit spectra and existing 'velocity cube' data have been used to determine the structure of the double ring PN NGC 2392. The inner shell is a stellar wind-sculpted prolate spheroid with a ratio of axes of 2:1 and the approaching end of the long axis pointed 20 deg from the line of sight in P.A. = 200 deg. The outer ring is caused by an outer disk with density dropping off with distance from the central star and with distance from its plane, which is the same as the equatorial band of high density in the inner shell. The outer disk contains a ring of higher density knots at a distance of 16 arcsec and is losing material through free expansion, forming an outer envelope of increasing velocity. Forbidden S II spectra are used to determine the densities in all of the major regions of the nebula. It is argued that the filamentary cores at the centers of the knots seen in the outer ring originate in the sublimation of bodies formed at the same time as the parent star.

O'Dell, C. R.↗

NASA NTRS1988

Environments and populations of supernova remnants in the Large Magellanic Cloud

The stellar and interstellar environments of the 32 known SNRs in the LMC have been examined to determine the relative numbers of population I and II progenitors. At least 2/3 of the LMC SNRs are associated with pop I environments. The data suggest that the existing SNR surveys are biased against the detection of SNRs located in evolved star-forming regions, such as supershells produced by OB asociations, and in the cores of luminous H II regions. Therefore, the fraction of SNRs in pop I environments may be even higher. The results are compared to previous studies of SN statistics in late-type galaxies.

Chu, You-Hua↗