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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.

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At least 253 records · Page 14
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NASA NTRS1977

Recurrent X-ray outbursts from Aquila X-1

Ariel V All-Sky Monitor observations of Aql X-1 are reported and compared with previous observations that indicate the existence of 'low' and 'high' intensity states. All the data considered are shown to be consistent with flaring episodes having a mean flare interval of 435 days with an rms scatter of about 10%. It is suggested that the observations are most easily interpreted in terms of an 'X-ray dwarf nova' binary of the type proposed for the transient X-ray source A0620-00. Constraints on more detailed features of the source model are discussed.

Kaluzienski, L. J.↗
NASA NTRS1980

The 1978 X-ray and optical outburst of Aquila X-1 /4U 1908+00/

During the summer of 1978 the recurrent transient X-ray source, Aquila X-1, underwent its first major outburst in two years. This paper presents the results of extensive X-ray and optical observations of this event, which lasted for about two months. The peak X-ray luminosity was about 1.3 times that of the Crab and exhibited spectrum-dependent flickering on time scales of about 5 minutes. In addition, one very large flare was observed about one month after maximum that was also correlated with spectral changes. During this flare the previously identified optical counterpart brightened from V = 19 to a peak of V = 14.8, where it was distinctly blue (U - B = 0.4), and then reddened during the decay. These observations are interpreted in terms of a standard accretion disk model with particular emphasis on the similarities to Sco -1 and other dwarf X-ray systems.

Charles, P. A.↗
NASA NTRS1982

Stellar model chromospheres. XIII - M dwarf stars

Single-component, homogeneous model chromospheres that are consistent with high-resolution profiles of the Ca II K line calibrated in surface flux units for three dMe and 2 dM stars observed at quiescent times are constructed. The models reveal several systematic trends. Large values of the ratio of T(min) to T(eff) are derived, indicating a large amount of nonradiative heating present in the upper photospheres of M dwarf stars. It is also found that the lower chromospheric temperature gradient is similar for all the M dwarf stars. Since for the models here the chromospheric K line emission strength is most sensitive to the total amount of chromospheric material present within the approximate temperature range T(min)-6000 K, increasing the emission strength is not simply due to increasing chromospheric temperature gradients. It is also found that both the electron density and electron temperature at one thermalization length in the K line below the top of the chromospheres are greater in the dMe stars than in the dM stars. The M dwarf models here have microturbulent velocities between 1 and 2 km/sec, which are much smaller than for solar chromosphere models.

Giampapa, M. S.↗
NASA NTRS1985

The X-ray corona of Procyon

X-ray emission from the nearby system Procyon A/B (F5 IV + DF) was detected, using the IPC (Imaging Proportional Counter) on board the Einstein Observatory. Analysis of the X-ray pulse height spectrum suggests that the observed X-ray emission originates in Procyon A rather than in the white dwarf companion Procyon B, since the derived X-ray temperature, log T = 6.2, agrees well with temperatures found for quiescent solar X-ray emission. Modeling Procyon's corona with loops characterized by some apex temperature Tmax and emission length scale L, it is found that Tmax is well constrained, but L, and consequently the filling factor of the X-ray emitting gas, are essentially unconstrained even when EUV emission from the transition region is included in the analysis.

Schmitt, J. H. M. M.↗
NASA NTRS1997

Seismological procedures for ZZ Ceti stars and an application to G 117-B15A

In this paper, we outline the procedure for seismological analysis of the ZZ Ceti stars, which are pulsating white dwarfs with hydrogen atmospheres. We use G 117-B15A as the example for this process and derive constraints on the mass and internal structure. The hydrogen layer mass is either about 10(-4)M* or 10(-7)M* depending on whether the l = 1 mode near 215 s is k = 2 or k = 1, respectively. In both cases, the best fitting mass is 0.60M solar, in agreement with spectroscopic log g values.

Bradley, P. A.↗
NASA NTRS2011

Deep UV Luminosity Functions at the Infall Region of the Coma Cluster

We have used deep GALEX observations at the infall region of the Coma cluster to measure the faintest UV luminosity functions (LFs) presented for a rich galaxy cluster thus far. The Coma UV LFs are measured to M(sub uv) = -10.5 in the GALEX FUV and NUV bands, or 3.5 mag fainter than previous studies, and reach the dwarf early-type galaxy population in Coma for the first time. The Schechter faint-end slopes (alpha approximately equal to -1.39 in both GALEX bands) are shallower than reported in previous Coma UV LF studies owing to a flatter LF at faint magnitudes. A Gaussian-plus-Schechter model provides a slightly better parametrization of the UV LFs resulting in a faint-end slope of alpha approximately equal to -1.15 in both GALEX bands. The two-component model gives faint-end slopes shallower than alpha = -1 (a turnover) for the LFs constructed separately for passive and star forming galaxies. The UV LFs for star forming galaxies show a turnover at M(sub UV) approximately equal to -14 owing to a deficit of dwarf star forming galaxies in Coma with stellar masses below M(sub *) = 10(sup 8) solar mass. A similar turnover is identified in recent UV LFs measured for the Virgo cluster suggesting this may be a common feature of local galaxy clusters, whereas the field UV LFs continue to rise at faint magnitudes. We did not identify an excess of passive galaxies as would be expected if the missing dwarf star forming galaxies were quenched inside the cluster. In fact, the LFs for both dwarf passive and star forming galaxies show the same turnover at faint magnitudes. We discuss the possible origin of the missing dwarf star forming galaxies in Coma and their expected properties based on comparisons to local field galaxies.

Hammer, D. M.↗
NASA NTRS1993

A deep x-ray survey of the Pleiades cluster and the B6-A3 main sequence stars in Orion

We have obtained deep ROSAT images of three regions within the Pleiades open cluster. We have detected 317 X-ray sources in these ROSAT PSPC images, 171 of which we associate with certain probable members of the Pleiades cluster. We detect nearly all Pleiades members with spectral types later than G0 and within 25 arcminutes of our three field centers where our sensitivity is highest. This has allowed us to derive for the first time the luminosity function for the G, K, and M dwarfs of an open cluster without the need to use statistical techniques to account for the presence of upper limits in the data sample. Because of our high X-ray detection frequency down to the faint limit of the optical catalog, we suspect that some of our unidentified X-ray sources are previously unknown, very low-mass members of the Pleiades. A large fraction of the Pleiades members detected with ROSAT have published rotational velocities. Plots of L(sub x)/L(sub bol) versus spectroscopic rotational velocity show tightly correlated 'saturation' type relations for stars with (B - V)(sub O) greater than 0.60. For each of several color ranges, X-ray luminosities rise rapidly with increasing rotation rate until v sin i approximately equals 15 km/s, and then remain essentially flat for rotation rates up to at least v sin i approximately equal to 100 km/s. The dispersion in rotation among low-mass stars in the Pleiades is by far the dominant contributor to the dispersion in L(subx) at a given mass. Only about 35 percent of the B.A. and early F stars in the Pleiades are detected as X-ray sources in our survey. There is no correlation between X-ray flux and rotation for these stars. The X-ray luminosity function for the early-type Pleiades stars appears to be bimodal, with only a few exceptions. We either detect these stars at fluxes in the range found for low-mass stars or we derive X-ray limits below the level found for most Pleiades dwarfs. The X-ray spectra for the early-type Pleiades stars detected by ROSAT are indistinguishable from the spectra of the low-mass Pleiades members. We believe that the simple explanation for this behavior is that the early-type Pleiades stars are not themselves intrinsic X-ray sources and that the X-ray sources and that the X-ray emission actually arises from low-mass companions to these stars.

Caillault, Jean-Pierre↗
NASA NTRS1981

Evidence for chromospheres and coronae in stars - Recent observations, some unanswered theoretical questions, and a speculative scenario

The paper discusses recent UV and X-ray evidence for the existence of solar-type chromospheres, transition regions and coronae in stars other than the sun. The stellar spectral types associated with emission lines indicative of chromospheres and transition regions and X-ray fluxes indicative of coronae are examined, and the possibility of chromospheres and transition regions existing in stars hotter or cooler than the late-type stars is considered. The stellar parameters determining the properties of stellar chromospheres, transition regions and coronae are discussed, including varying magnetic fields, stellar rotation and convection zone parameters. A speculative scenario for the source of chromospheres, transition regions and coronae in O, B and A stars, late-type dwarfs, and G-M giants and supergiants is then presented which is based on the predominant role of magnetic fields in the nonradiative heating of outer atmospheres.

Linsky, J. L.↗
NASA NTRS1984

Stellar chromospheres and coronae in the Ursa Major Cluster stars

IUE spectra of 18 proposed members of the Ursa Major Cluster and Einstein X-ray images of 11 of these stars are discussed. Thirteen stars, six in the nucleus and seven in the extended comoving stream, are probably true members of the UMa Cluster in that their bright ultraviolet and X-ray surface fluxes indicate youth. Four stars, one in the nucleus and three in the stream, exhibit weak ultraviolet and/or X-ray emission suggesting that they are old field stars that have the same space motion as the UMa Cluster. The X-ray surface fluxes of the UMa Cluster stars appear to be brighter than those of the Hyades Cluster stars, consistent with their relative ages. It is argued that chromospheres and transition regions could be present in dwarf stars hotter than B-V of about 0.30, but are unobservable in IUE spectra due to the rapid increase in photospheric flux in hotter stars.

Simon, T.↗
NASA NTRS1993

Rapid oscillations in cataclysmic variables. X - TW Pictoris (=H 0534-581)

Photometry of the X-ray-selected cataclysmic variable TW Pictoris (=H 0534-581) reveals a periodic signal at P = 6.1 hr, which is approximately the orbital period of the binary. In addition, a highly probable signal is found at P = 119.76 +/- 0.09 min. Coupled with the high excitation, the high L(x)/L(opt) ratio, and the probable detection of a similar spectroscopic period, this makes it likely that the star belongs to the 'DQ Herculis' class of cataclysmic variable in which the stable short period is the spin period of an accreting magnetic white dwarf. Theory suggests that TW Pic is a particularly attractive candidate for showing circular polarization at visual or near-infrared wavelengths.

Patterson, Joseph↗
NASA NTRS2001

Influence of atmospheric oxygen on leaf structure and starch deposition in Arabidopsis thaliana

Plant culture in oxygen concentrations below ambient is known to stimulate vegetative growth, but apart from reports on increased leaf number and weight, little is known about development at subambient oxygen concentrations. Arabidopsis thaliana (L.) Heynh. (cv. Columbia) plants were grown full term in pre-mixed atmospheres with oxygen partial pressures of 2.5, 5.1, 10.1, 16.2, and 21.3 kPa O2, 0.035 kPa CO2 and the balance nitrogen under continuous light. Fully expanded leaves were harvested and processed for light and transmission electron microscopy or for starch quantification. Growth in subambient oxygen concentrations caused changes in leaf anatomy (increased thickness, stomatal density and starch content) that have also been described for plants grown under carbon dioxide enrichment. However, at the lowest oxygen treatment (2.5 kPa), developmental changes occurred that could not be explained by changes in carbon budget caused by suppressed photorespiration, resulting in very thick leaves and a dwarf morphology. This study establishes the leaf parameters that change during growth under low O2, and identifies the lower concentration at which O2 limitation on transport and biosynthetic pathways detrimentally affects leaf development. Grant numbers: NAG5-3756, NAG2-1020, NAG2-1375.

Non-NASA Center↗
NASA NTRS1975

A search for stellar soft X-ray sources

A sensitive soft X-ray survey of about one-tenth of the sky, carried out as part of the Skylab program and flown on 1973 July 28, revealed no evidence for soft X-ray emission from stellar sources. We present upper limits to the soft X-ray luminosity of about 50 nearby stars. With nominal assumed values for stellar space densities, we show it unlikely that emission from giants, main sequence stars, white dwarfs, or stars in known binary systems can account for the soft X-ray diffuse background.

Vanderhill, M. J.↗
NASA NTRS1995

Hubble Space Telescope observations of the dwarf Nova Z Chamaeleontis through two eruption cycles

We have obtained the first high-speed photometry of the eclipsing dwarf nova Z Cha at ultraviolet wavelengths with the Hubble Space Telescope (HST). We observed the eclipse roughly every 4 days over two cycles of the normal eruptions of Z Cha, giving a uniquely complete coverage of its outburst cycle. The accretion disk dominated the ultraviolet light curve of Z Cha at the peak of an eruption; the white dwarf, the bright spot on the edge of the disk, and the boundary layer were all invisible. We were able to obtain an axisymmetric map of the accretion disk at this time only by adopting a flared disk with an opening angle of approximately 8 deg. The run of brightness temperature with radius in the disk at the peak of the eruption was too flat to be consistent with a steady state, optically thick accretion disk. The local rate of mass flow through the disk was approximately 5 x 10(exp -10) solar masses/yr near the center of the disk and approximately 5 x 10(exp -9) solar masses/yr near the outer edge. The white dwarf, the accretion disk, and the boundary layer were all significant contributors to the ultraviolet flux on the descending branches of the eruptions. The temperature of the white dwarf during decline was 18,300 K less than T(sub wd) less than 21,800 K, which is significantly greater than at minimum light. Six days after the maximum of an eruption Z Cha has faded to near minimum light at ultraviolet wavelenghts, but was still approximately 70% brighter at minimum light in the B band. About one-quarter of the excess flux in the B band came from the accretion disk. Thus, the accretion disk faded and became invisible at ultraviolet wavelengths before it faded at optical wavelenghts. The disk did, however, remain optically thick and obscured the lower half of the white dwarf at ultraviolet and possibly at optical wavelenghts for 2 weeks after the eruption ended. By the third week after eruptiuons the eclipse looked like a simple occultation of an unobscured, spherical white dwarf by a dark secondary star. The center of the accretion disk was, therfore, optically thin at ultraviolet wavelenghts and the boundary layer was too faint to be visible.

Robinson, E. L.↗
NASA NTRS1986

EXOSAT results on V426 Ophiuchi - Indications of a DQ Herculis system

EXOSAT LE and ME data reveal a sinusoidal l hr modulation during the 6 hr binary orbit of the cataclysmic variable V426 Oph. The amplitude and shape of this modulation, the X-ray fluxes and spectrum, as well as the time scales involved argue for a DQ Her interpretation. In this case, the white dwarf in V426 Oph has the longest rotation period among the known DQ Her systems with orbital periods greater than 3 hr.

Szkody, P.↗
NASA NTRS2015

Hubble Space Telescope Astrometry of the Procyon System

The nearby star Procyon is a visual binary containing the F5 IV-V subgiant Procyon A, orbited in a 40.84-year period by the faint DQZ white dwarf (WD) Procyon B. Using images obtained over two decades with the Hubble Space Telescope, and historical measurements back to the 19th century, we have determined precise orbital elements. Combined with measurements of the parallax and the motion of the A component, these elements yield dynamical masses of 1.478 plus or minus 0.012M and 0.592 plus or minus 0.006M for A and B, respectively. The mass of Procyon A agrees well with theoretical predictions based on asteroseismology and its temperature and luminosity. Use of a standard core-overshoot model agrees best for a surprisingly high amount of core overshoot. Under these modeling assumptions, Procyon A's age is approximately 2.7 Gyr. Procyon B's location in the H-R diagram is in excellent agreement with theoretical cooling tracks for WDs of its dynamical mass. Its position in the mass-radius plane is also consistent with theory, assuming a carbon-oxygen core and a helium-dominated atmosphere. Its progenitor's mass was 1.9-2.2M, depending on its amount of core overshoot. Several astrophysical puzzles remain. In the progenitor system, the stars at periastron were separated by only approximately AU, which might have led to tidal interactions and even mass transfer; yet there is no direct evidence that these have occurred. Moreover the orbital eccentricity has remained high (approximately 0.40). The mass of Procyon B is somewhat lower than anticipated from the initial-to-final-mass relation seen in open clusters. The presence of heavy elements in its atmosphere requires ongoing accretion, but the place of origin is uncertain.

PROCYON↗
NASA NTRS1979

Isotopes of titanium in cool stars

A program of stellar Ti isotopic-abundance determinations is described and related to changes that may have occurred in the Ti isotopic abundance ratios during the chemical evolution of the Galaxy, and to whether the abundance ratios are perturbed in the S and MS stars having atmospheres with enhanced abundances of s-process nuclei resulting from convective mixing after internal exposure to neutrons. High-resolution (0.07 A) Reticon spectra of portions of the TiO gamma (0,0), gamma (0,1), and delta (0,0) bands were the source material, and particular emphasis was placed on the definition of the continuum level. The isotopic abundance ratios are terrestrial in all of the sample, and errors in the (Ti-i)/(Ti-48) ratio are typically plus or minus 25% for the dwarfs and plus or minus 50% for the giants. The observations show that the magic nucleus Ti-50 is not enhanced in S and MS stars.

Clegg, R. E. S.↗