Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “normal stars”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Line blocking factors in the ultraviolet spectra of 35 B6 to A0 stars

Observed line-blocking factors for stars of spectral types B6 to A0 are presented which were measured on spectral tracings observed over the wavelength ranges from 2064 to 2158 A, 2496 to 2590 A, and 2775 to 2867 A with the S 59 spectrometer on the ESRO TD 1A satellite. Twenty-six stars having normal spectra were observed along with 12 stars having abnormal spectra. The results for normal and abnormal stars are compared to determine the changes in line blocking related to each type of spectral peculiarity. Detailed descriptions are given of the line-blocking factors in different spectral channels for a B8 supergiant, three Be stars, a shell star, two eclipsing binaries, an Hg-Mn star, and several peculiar B and A stars.

Underhill, A. B.↗
NASA NTRS2009

Flash Mixing of the White Dwarf Cooling Curve Spectroscopic Confirmation in NGC 2808

We present new HST far-UV spectroscopy of two dozen hot evolved stars in NGC 2808, a massive globular cluster with a large population of "blue-hook" stars. The blue-hook stars are found in ultraviolet color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the HR diagram unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf cooling curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C abundances; the hotter temperatures and associated decrease in the hydrogen opacity shortward of the Lyman limit makes the stars brighter in the extreme UV but appear sub luminous in the UV and optical. Our far-UV spectroscopy demonstrates that, relative to normal HB stars at the same color, the blue-hook stars of NGC 2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing in the subluminous population. Although the C abundance in the blue-hook stars is orders of magnitude larger than that in the normal HB stars, the atmospheric C abundance in both the blue-hook and normal HB stars appears to be affected by gravitational settling. The abundance variations seen in C, Si, and the Fe-peak elements indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000 K to 50,000 K exhibiting large enhancements of the iron-peak elements. The hottest subdwarfs in our blue-hook sample may be pulsators, given that they fall in the temperature range of newly-discovered pulsating subdwarfs in omega Cen.

Brown, Thomas M.↗
NASA NTRS1987

Highly ionized stellar winds in Be stars - The evidence for aspect dependence

The paper presents the results of an ultraviolet survey of stellar winds in 62 Be and 43 normal B stars covering spectral types B0.5-B5 and luminosity classes V-III. Excess wind absorption in Be stars over that detected in normal B stars is found in C IV and Si IV, confirming results of previous surveys. The stars showing excess wind absorption also show one or more shortward-shifted discrete absorption components, suggesting that much of the excess wind absorption is in the form of discrete absorption components. Evidence is presented that the high-velocity and highly ionized stellar wind observed in some Be stars is a function of latitude.

Grady, C. A.↗
NASA NTRS1982

An ultraviolet view of Be stars

Several types of ultraviolet observations of Be stars are briefly summarized, with a view towards identifying the characteristics, if any, that distinguish Be stars from other early type objects. The present weight of the evidence is that, from the point of view of ultraviolet data, Be stars are nearly indistinguishable from normal B stars. Both groups have similar characteristics, with differences only in degree. Further comparative studies of Be and nonBe stars have the potential to reveal the causes of the Be phenomenon, and possibly also the origins of winds in early type stars. Several areas for future ultraviolet research are suggested.

Snow, T. P.↗
NASA NTRS1983

The rotational velocities of a sample of Herbig Ae/Be stars

Rotational velocity estimates are presented for 15 Herbig Ae/Be stars. Those objects with nominal optical spectral types between B5 and A0 show a frequency distribution of v sin i values significantly different from that of normal Be stars in this spectral type range; the Herbig emission stars must have significantly smaller intrinsic rotational velocities. Herbig emission stars of types B0-B5 cannot be distinguished from normal Be stars on the basis of rotational velocity alone.

Davis, R.↗
NASA NTRS1988

C IV and Si IV in IUE spectra of normal B8-A0 stars: UV identified Be/Ae stars

Archival IUE high dispersion spectra of 42 B6-A2 stars within 200 pc were surveyed. Five of the program stars show significant C IV and Si IV absorption. All of the stars with detected C IV have v sin i less than or = 190 km/sec. Sharp absorption cores are present in Si II lambda 1533 in 3 of the objects, indicating that these are previously unrecognized shell stars. Three of the stars have variable or asymmetric C IV profiles which are consistent with the C IV and Si IV being produced in stellar winds. One star has C IV in the form of a shortward-shifted discrete absorption component, similar to those observed in Be stars. The data are compared with similar data for Be and B shell stars.

Chiu, Weihsueh A.↗
NASA NTRS1995

The RIASS coronathon: Joint X-ray and ultraviolet observations of normal F-K stars

Between 1990 August and 1991 January the ROSAT/IUE All Sky Survey (RIASS) coordinated pointings by the International Ultraviolet Explorer (IUE) with the continuous X-ray/EUV mapping by the Roentgensatellit (ROSAT). The campaign provided an unprecedented multiwavelength view of a wide variety of cosmic sources. We report findings for F-K stars, a large proportion of the RIASS targets. Forty-eight of our 91 'Coronathon' candidates were observed by the IUE during the campaign. For stars missed by the IUE, we supplemented the ROSAT survey fluxes with archival UV spectra and/or follow-on observations.

Ayres, Thomas R.↗
NASA NTRS1981

The resonance line of B II in IUE spectra of chemically peculiar stars

A description is presented of high-dispersion spectroscopic observations of eight chemically peculiar stars including Kappa Cnc and Mu Lep, and of one normal comparison star, Nu Cap, in the vicinity of 1362 A. The reported data were obtained with the International Ultraviolet Explorer during the first 15 months of its operation. The initial reduction of the data, which were originally described in a qualitative summary by Leckrone (1980), was affected by a serious error in the calibration of the instrument's intensity transfer function. This error has been corrected in the currently presented data. The results of spectrum syntheses of the 1362 A feature are also described for several of the stars.

Leckrone, D. S.↗
NASA NTRS1972

Ultraviolet observations of weak-helium stars

Ultraviolet filter photometry aboard OAO-2 has been carried out for the weak helium stars 3 Cen A, 3 Sco, HD 144334, HD 21699, HD 144844, alpha Scl, HR 8535, HR 8770 and HD 144661. The flux distribution is compared with that of B3 to B8 main sequence and giant stars in terms of color-color diagrams. When allowance is made for line blocking shortward of 2800 A that most of the weak helium stars have normal fluxes in good agreement with their ground-based colors. The star alpha Scl is a light and spectrum variable. The strengthening of Ti II and Sr II lines around 2400 A appears to redistribute flux longward of the Balmer discontinuity. The flux distribution indicates an effective temperature about 1500 K higher than previously determined.

Bernacca, P. L.↗
NASA NTRS1980

A magnetic study of spotted UV Ceti flare stars and related late-type dwarfs

A multichannel photoelectric Zeeman analyzer has been used to investigate the magnetic nature of the spotted UV Ceti flare stars. Magnetic observations were obtained on a sample of 19 program objects, of which 5 were currently spotted dKe-dMe stars, 7 were normal dK-dM stars, 7 were UV Ceti flare stars, and 1 was a possible post-T Tauri star. Contrary to most previously published observations and theoretical expectations, no magnetic fields were detected on any of these objects from either the absorption lines or the H-alpha emission line down to an observational uncertainty level of 100-160 gauss (standard deviation).

Vogt, S. S.↗
NASA NTRS1997

A Multiwavelength Study of the Starburst Galaxy NGC 7771

We present a multiwavelength study of the interacting starburst galaxy NGC 7771, including new optical and ultra-violet spectra and a previously unpublished soft X-ray ROSAT image and spectrum. The far-infrared, radio, and X-ray fluxes suggest that a massive burst of star-formation is currently in progress but the small equivalent width of the Balmer emission lines (equivalent width H(alpha approximately equals 100 A), the weak UV flux, the low abundance of ionised oxygen, and the shape of the optical spectrum lead us to conclude that there are few 0 stars. This might normally suggest that star-formation has ceased but the galaxy's barred gravitational potential and large gas reserves imply that this should not be so, and we therefore consider other explanations. We argue that the observations cannot be due to effects of geometry, density bounded nebulae, or dust within the nebulae, and conclude that a truncated IMF is required. The dwarf galaxy NGC 7770 appears to be in the initial stages of a merger with NGC 7771, and the resulting tidal perturbations may have induced the apparent two-armed spiral pattern, and driven a substantial fraction of the disk gas inwards. The presence of a bulge in NGC 7771 may be moderating the starburst so that, while still occuring on a large scale with a supernova rate of 0.8-1/yr, it is less violent and the IMF has a relatively low upper mass limit. We find that there is a cluster of stars obscuring part of the starburst region, and we offer an explanation of its origin.

Davies, Richard I.↗
NASA NTRS1993

ROSAT survey of emission from Be stars

ROSAT pointed observations of bright, classical Be stars have demonstrated that detection of soft x-rays at a level expected for normal B stars of comparable T(sub eff) and luminosity is anti-correlated with the presence of episodes of enhanced mass ejection and formation of a dense, moderately ionized equatorial circumstellar disk. At epochs of lower than average disk column density, x-ray flaring has been detected in 2 Be stars, lambda Eri and pi Aqr.

Grady, Carol↗
NASA NTRS1979

Effective temperatures, angular diameters, distances and linear radii for 160 O and B stars

This paper discusses effective temperatures, angular diameters, distances, and linear diameters that have been determined for 160 O and B stars on the basis of published UV spectrophotometry, visible and near-IR intermediate-band photometry, and model-atmosphere fluxes. The results are compared with previous measurements and calculations for main-sequence and giant O and B stars. It is found that: (1) the flux effective temperatures of O and B supergiants are systematically lower than those of main-sequence and giant stars of the same subtype; (2) the effective temperatures and radii of Beta Cep stars are the same as those of nonvariable stars of the same spectral type; (3) Be stars that do not have two Balmer jumps have effective temperatures very similar to those of normal B stars of the same subtype; (4) O and B stars increase in size from the main sequence to supergiants; and (5) late B supergiants are approximately twice as large as O9 supergiants.

Underhill, A. B.↗
NASA NTRS1987

A VLA radio-continuum survey of a sample of confirmed and marginal barium stars

Results are reported from a 6-cm VLA survey of five confirmed Ba II stars and eight mild Ba II stars, undertaken to search for evidence of gyrosynchrotron emission or thermal emission from the primary star's wind that is enhanced or photoionized by a white dwarf companion. Of these 13 stars, only Beta UMi was detected as a possible radio source at a flux level of 0.11 mJy (3sigma). The 6-cm radio luminosities (L6) of the other stars are as small as log L6 less than or equal to 14.0 and are an order of magnitude or more lower than the average levels found in RS CVn systems, but are consistent with the L6 upper limits previously found for stars of spectral type similar to the Ba II stars and normal elemental abundances. The upper limit to the radio luminosity for the possible mild Ba II star 56 Peg, when combined with its previously known X-ray luminosity, may provide useful constraints on the various models that have been proposed for this interesting object, once its orbital period is known.

Drake, Stephen A.↗
NASA NTRS1983

A search for X-rays from runaway stars

Enhanced X-ray emission from an early-type runaway star would be direct evidence for the presence of a collapsed companion, and thus for its origin as the result of a supernova explosion in a massive binary. To test this idea, measurements of X-ray fluxes in the 0.5-3 keV energy range from nine O and B type runaway stars were made with the Einstein Observatory. In each case, the X-ray luminosity observed does not exceed that expected from a normal OB star. Therefore we conclude that, if the runaway stars have collapsed companions, the separations of the components must be much greater than those in normal OB binaries.

Kumar, C. K.↗
NASA NTRS1980

Vector space methods of photometric analysis. II - Refinement of the MK grid for B stars. III - The two components of ultraviolet reddening

This paper discusses systematic errors which arise from exclusive use of the MK system to determine reddening. It is found that implementation of uvby, beta photometry to refine the qualitative MK grid substantially reduces stellar mismatch error. A working definition of 'identical' ubvy, beta types is investigated and the relationship of uvby to B-V color excesses is determined. A comparison is also made of the hydrogen based uvby, beta types with the MK system based on He and metal lines. A small core correlated effective temperature luminosity error in the MK system for the early B stars is observed along with a breakdown of the MK luminosity criteria for the late B stars. The second part investigates the wavelength dependence of interstellar extinction in the ultraviolet wavelength range observed with the TD-1 satellite. In this study the sets of identical stars employed to find reddening are determined more precisely than in previous studies and consist only of normal, nonsupergiant stars. A multivariate analysis of variance techniques in an unbiased coordinate system is used for determining the wavelength dependence of reddening.

Massa, D.↗
NASA NTRS1974

An empirical line-blanketing study of Am stars

Metallic line-blanketing coefficients have been measured over the wavelength range from 3800 to 4700 A in a sample of 20 Am and normal A stars. An expression for the logarithmic iron abundance has been calibrated in terms of effective temperature and a mean line-blocking coefficient. This makes it possible to determine the atmospheric iron abundance in sharp-lined Am and A stars with an accuracy of better than plus or minus 0.1 dex, using only line-blocking measurements and a photometric effective temperature.

Rydgren, A. E.↗
NASA NTRS1993

A Hubble Space Telescope planetary camera view of giant H II regions - The Wolf-Rayet content of NGC 595 and NGC 604 in M33

We present images of NGC 595 and NGC 604, the most massive giant H II regions in M33, obtained with the Planetary Camera aboard the HST in order to study their WR population. Fourteen WR and/or Of candidates are detected in NGC 604, and eleven in NGC 595. All previously claimed 'superluminous' WR stars are found to be tight (diameter less than 3 pc) stellar aggregates containing one (or sometimes more) normal WR star. As suspected from ground-based data, the WR/O number ratio is significantly higher in NGC 595 (about 0.3) than in NGC 604 (about 0.1). The WR stars may be major contributors to the output of mechanical power and energy into the interstellar medium in both clusters. Over the observable initial mass range, the initial mass functions (IMFs) have similar slopes. These IMFs are somewhat flatter than those generally derived for massive stars in the Galaxy or the Magellanic Clouds.

Drissen, Laurent↗