Search NASA⌕ Search

Engineering topics

Chen, W. P.

Publications and source records attributed to Chen, W. P..

11 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2011

Deep Impact, Stardust-NExT and the Behavior of Comet 9P/Tempel 1 from 1997 to 2010

We present observational data for Comet 9P/Tempel 1 taken from 1997 through 2010 in an international collaboration in support of the Deep Impact and Stardust-NExT missions. The data were obtained to characterize the nucleus prior to the Deep Impact 2005 encounter, and to enable us to understand the rotation state in order to make a time of arrival adjustment in February 2010 that would allow us to image at least 25% of the nucleus seen by the Deep Impact spacecraft to better than 80 m/pixel, and to image the crater made during the encounter, if possible. In total, approx.500 whole or partial nights were allocated to this project at 14 observatories worldwide, utilizing 25 telescopes. Seventy percent of these nights yielded useful data. The data were used to determine the linear phase coefficient for the comet in the R-band to be 0.045 +/- 0.001 mag/deg from 1deg to 16deg. Cometary activity was observed to begin inbound near r approx. 4.0 AU and the activity ended near r approx. 4.6 AU as seen from the heliocentric secular light curves, water-sublimation models and from dust dynamical modeling. The light curve exhibits a significant pre- and post-perihelion brightness and activity asymmetry. There was a secular decrease in activity between the 2000 and 2005 perihelion passages of approx. 20%. The post-perihelion light curve cannot be easily explained by a simple decrease in solar insolation or observing geometry. CN emission was detected in the comet at 2.43 AU pre-perihelion, and by r = 2.24 AU emission from C2 and C3 were evident. In December 2004 the production rate of CN increased from 1.8 x 10(exp 23) mol/s to Q(sub CN) = 2.75 x 10(exp 23) mol/s in early January 2005 and 9.3 x 10(exp 24) mol/s on June 6, 2005 at r = 1.53 AU.

Meech, K. J.↗
NASA NTRS2003

Does the Iron K and Alpha: Line of Active Galactic Nuclei Arise from the Cerenkov Line-like Radiation?

When thermal relativistic electrons with isotropic distribution of velocities move in a gas region or impinge upon the surface of a cloud that consists of a dense gas or doped dusts, the Cerenkov effect produces peculiar atomic or ionic emission lines, which is known as the Cerenkov line - like radiation. This newly recognized emission mechanism may find wide applications in high-energy astrophysics. In this paper we tentatively adopt this new line emission mechanism to discuss the origin of the iron Kα feature of active galactic nuclei (AGNs). The motivation of this research is to attempt a solution to a problem encountered by the "disk fluorescence line" model, i.e. , the lack of temporal response of the observed iron Kα line flux to the changes of the X-ray continuum flux. If the Cerenkov line emission is indeed responsible significant ly for the iron Kα feature, the conventional scenario around the central supermassive black holes of AGNs would need to be modified to accomodate more energetic, more violent, and much denser environments than previously thought.

You, J. H.↗
NASA NTRS2003

Simplified Formulae System for Resonant Inverse Compton Scattering of a Fast Electron in an Intense Magnetic Field

We present simple analytical formulae for the emission spectrum and total power of a special kind of resonant inverse Compton scattering (RICS) of a relativistic electron in an intense magnetic field. In contrast with the available formulae system, we obtain a markedly simplified one based on the semiclassical quantum theory, which is more understandable for people who are unfamiliar with quantum electrodynamics. We show that the RICS process, under an appropriate 'accommodation condition' derived in this paper, is predominantly much more efficient than the coexistent ordinary inverse Compton scattering, and produces highly beamed high-frequency radiation with moderately good monochromaticity. Our formulae are simple to use - thus offering a lucid physical intuition for the theory - and may find wide applications in hard X-ray and gamma-ray astrophysics.

You, J. H.↗
NASA NTRS1995

A lunar occultation and direct imaging survey of multiplicity in the Ophiuchus and Taurus star-forming regions

We present an IR lunar occultation and direct imaging search for companions in the Ophiuchus star-forming region and update a similar search of the Taurus region. The search is sensitive to companions in the angular separation range 0.005-10 sec. In Ophiuchus, we surveyed 35 young star targets; this sample contains at least 10 binaries, two triples, and one quadruple. Ten of the companion stars are newly discovered. In Taurus, the survey now includes 47 systems among which there are at least 22 binaries and four triples. Only two companion stars are newly identified because there is strong overlap with prior work. All the triples and quadruple are hierarchical. The observed binary frequency in Ophiuchus, in the 3-1400 AU range of separations, is at least 1.1 +/- 0.3 that of the nearby solar-like stars. This value is a lower bound because we make no corrections for incompleteness. In Taurus, in the same range of separations, the observed binary frequency is at least 1.6 +/- 0.3 that of the nearby solar-like stars. This value extends Ghez et al.'s (1993) and Leinert's et al.'s (1993) determination of an excess binary frequency to 3 AU separation. We used the weak-line T Tauri star/T Tauri star (WT/TT) type and the K-L color index to distinguish between systems with and without inner disks. We find no convincing difference in the binary frequency or distribution of separations of the systems with and without inner disks. The 1.3 mm continuum emission of the single systems exceeds that of the multiples suggesting that their extensive outer disks are more massive. The specific angular momenta of the binaries overlap those of molecular cloud cores measured by Goodman et al. (1993).

Simon, M.↗
NASA NTRS1994

Aging jets from low-mass stars

An extended faint optical jet is associated with the compact emission region plus faint star known as HH 55. HH 55 is located in the Lupus 2 cloud 2 min SW of the well studied T Tauri star RU Lupi. The HH 55 jet extends 55 sec N and 35 sec S in PA 160 deg. The HH 55 star is an emission line star of spectral type M3.5. Its image in the emission lines of H-alpha and (S II) is slightly elongated by 2 sec - 3 sec to the S but in continuum light is symmetrical and pointlike ((full width at half maximum) (FWHM) = 1.7 sec). The star and jet have several features in common with the star and jet known as Sz 102 = Th 28 in the nearby Lupus 3 cloud. We suggest that these objects are representative of the late evolutionary stage of the HH jet-outflow phenomenon and point out that such objects may be quite common although difficult to detect. With L(sub bol) approximately = 0.005 solar luminosity, and log T(sub e) approximately = 3.5, the HH 55 star is close to the main sequence and evolutionary tracks suggest an age of 3 x 10(exp 7) yr.

Graham, J. A.↗
NASA NTRS1992

Multiplicity among the young stars in Taurus

Twenty-eight young stars systems in the northern Taurus star-forming region are surveyed for multiplicity. These observations, made by infrared lunar occultation and imaging techniques, can identify binaries in the angular separation range 0.005-10 arcsec. Of the 28 systems, 11 are binaries and two are triples, giving an observed multiplicity of 1.5 stars per system. The binary frequency and distribution of separations are similar to those of the main-sequence F and G spectral-type stars. Stars that have detected companions and those without are indistinguishable in H-alpha equivalent width, indicating that stars with active inner disks are represented in both groups. The multiple systems are in general weaker 1.3 mm continuum emitters than the single stars, indicating that the large-scale disks in these systems are less massive. However, there are important exceptions so the evolution of disks in multiple systems is not clear.

Simon, M.↗
NASA NTRS1991

Changing environments of pre-main-sequence stars and their effect on the 3 microns ice feature

Three micron spectra of three premain-sequence stellar objects are presented. The heavily embedded infrared source HH100-IR, and two optically visible stars that apparently power the Herbig-Haro objects HH52, 53, 54, and HH57, are likely to represent different stages during early stellar evolution. The medium-resolution spectra of HH100-IR confirm the existence of a very strong ice band absorption (optical depth 0.8-1.2 at 3.07 microns) discovered by Whittet & Blades MNRAS, 191, 309 (1980) with lower resolution. Each of the visible stars shows a weaker (optical depth 0.2-0.4) water-ice feature but with a relatively prominent absorption dip at 2.97 microns which it is believed is due to ammonia ice. A detailed inspection of the line profiles suggests that the environment around the optically visible stars is significantly warmer (about 100 K) than that around HH100-IR (about 10-20 K) and that processing of remnant circumstellar dust is already underway.

Graham, J. A.↗
NASA NTRS1990

Subarcsecond resolution observations of the central parsec of the Galaxy at 2.2 microns

Four separate lunar occultations of the Galactic center in the infrared K-band have been observed, and the results are used to estimate the size of the sources and to derive photometry. It is found that IRS 16NE, 16NW, and IRS 29 are smaller than about 510 AU and IRS 16SW-W and C are smaller than 85 AU. All sources brighter than K = 10 mag in the central pc are smaller than 510 AU and therefore seem to be stars. IRS 7 is smaller than about 100 AU. IRS 1W and 13, resolved into pairs of stars, are probably the exciting stars for these H II regions. The shape of the differential luminosity function of the stars in the central sq pc is similar to that of stars in the central 150 sq pc. There is a steady increase in star density toward the central few arcsec of the Galaxy.

Simon, M.↗
NASA NTRS1990

Discovery of five pre-main-sequence binaries in Taurus

The progress of an infrared K band lunar occultation search for binaries among premain-sequence stars is reported. From the fall of 1986 to the fall of 1989, 13 objects have been observed, all in the Taurus star-forming region. Five new binaries have been discovered: DF Tau, DI Tau, FS Tau (Haro 6-5) A, FV Tau (Haro 6-8), and FW Tau, with projected separations ranging from 0.01 to 0.57 arcsec. Including results reported previously, a total of 31 objects, 21 in Taurus and 10 in Ophiuchus, have been surveyed and nine double systems have been found. The detected binary frequency (0.29) seems consistent with that of the solar-type main-sequence field stars.

Chen, W. P.↗
NASA NTRS1986

Lunar occultations of IRAS point sources, 1986-1990

A complete listing is given for objects in the IRAS Point Source Catalog which will be occulted by the moon over the course of 1986-1990. A total of 14,148 ASCII card images is encompassed by the complete listing of objects having geocentric events during this period. The results contained in this complete listing are illustrated in two of the present tables for the brightest objects at 12 and 100 micron wavelengths.

Simon, M.↗