Results for “WHITE DWARF STARS” — Page 12

NASA NTRS1995

The remarkable eclipsing asynchronous AM Herculis binary RX J19402-1025

We report on two years of photometric and spectroscopic observation of the recently discovered AM Herculis star RX J19402-1025. A sharp eclipse feature is present in the optical and X-ray light curves, repeating with a period of 12116.290 +/- 0.003 s. The out-of-eclipse optical waveform contains approximately equal contributions from a signal at the same period and another signal at 12150 s. As these signals drift in and out of phase, the wave form of the light curve changes in a complex but predictable manner. After one entire 'supercycle' of 50 days (the beat period between the shorter periods), the light curve returns to its initial shape. We present long-term ephemerides for each of these periods. It is highly probable that the eclipse period is the underlying orbital period, while the magnetic white dwarf rotates with P = 12150 s. The eclipses appear to be eclipses of the white dwarf by the secondary star. But there is probably also a small obscuring effect from cold gas surrounding the secondary, especially on the orbit-leading side where the stream begins to fall towards the white dwarf. The latter hypothesis can account for several puzzling effects in this star, as well as the tendency among most AM Her stars for the sharp emission-line components to slightly precede the actual motion of the secondary. The presence of eclipses in an asynchronous AM Her star provides a marvelous opportunity to study how changes in the orientation of magnetic field lines affect the accretion flows. Repeated polarimetric light curves and high-resolution studies of the emission lines are now critical to exploit this potential.

Patterson, Joseph↗

NASA NTRS1995

RE 0044+09: A new K dwarf rapid rotator with a white dwarf companion

We report the discovery of a new K dwarf rapid rotator with a potential white dwarf companion. The white dwarf accounts for over 90% of the observed extreme ultraviolet flux detected from this system. Analysis of ROSAT Wide Field Camera (WFC) and IUE data both suggest a white dwarf temperature of approximately 28,700 K. Optical photometry and the IUE long wavelength prime (LWP) spectrum (with the white dwarf contribution removed) imply that the late-type star has a spectral type of K1-3 V, and a distance of 55 +/- 5 pc. Using this distance, the observed IUE SWP flux, and the best-fit temperature results in a white dwarf radius of 0.0088 solar radius. The estimated white dwarf mass is then approximately 0.91 solar mass; somewhat over-massive compared to field white dwarfs. Optical photometry of the K star reveals a 'spot' modulation period of approximately 10 hr (now observed over 3 yr). However, radial velocity observations have revealed no significant variations. Spectroscopic observations place a low limit on the lithium abundance, but do show rapid rotation with a v sin i of 90 +/- 10 km/s. The K star was detected as a radio source at 3.6 cm (on two occasions) and 6 cm by the Very Large Array (VLA). The most likely evolutionary scenario is that the K star and hot white dwarf from either a wide binary or common proper motion pair with an age of 0.1-0.1 Gyr-consistent with the evolutionary timescale of the white dwarf and the rapid rotation of the K star. However, from the proper motion of the K star, this system does not seem to be associated with any of the known young stellar groups.

Kellett, Barry J.↗

NASA NTRS1967

Red giants and white dwarfs.

Book on red giants and white dwarfs covering stellar evolution, planets and life, solar system origin, etc

GIANT STAR↗

NASA NTRS2002

Hierarchies of Models: Toward Understanding Planetary Nebulae

Stars like our sun (initial masses between 0.8 to 8 solar masses) end their lives as swollen red giants surrounded by cool extended atmospheres. The nuclear reactions in their cores create carbon, nitrogen and oxygen, which are transported by convection to the outer envelope of the stellar atmosphere. As the star finally collapses to become a white dwarf, this envelope is expelled from the star to form a planetary nebula (PN) rich in organic molecules. The physics, dynamics, and chemistry of these nebulae are poorly understood and have implications not only for our understanding of the stellar life cycle but also for organic astrochemistry and the creation of prebiotic molecules in interstellar space. We are working toward generating three-dimensional models of planetary nebulae (PNe), which include the size, orientation, shape, expansion rate and mass distribution of the nebula. Such a reconstruction of a PN is a challenging problem for several reasons. First, the data consist of images obtained over time from the Hubble Space Telescope (HST) and spectra obtained from Kitt Peak National Observatory (KPNO) and Cerro Tololo Inter-American Observatory (CTIO). These images are of course taken from a single viewpoint in space, which amounts to a very challenging tomographic reconstruction. Second, the fact that we have two disparate and orthogonal data types requires that we utilize a method that allows these data to be used together to obtain a solution. To address these first two challenges we employ Bayesian model estimation using a parameterized physical model that incorporates much prior information about the known physics of the PN. In our previous works we have found that the forward problem of the comprehensive model is extremely time consuming. To address this challenge, we explore the use of a set of hierarchical models, which allow us to estimate increasingly more detailed sets of model parameters. These hierarchical models of increasing complexity are akin to scientific theories of increasing sophistication, with each new model/theory being a refinement of a previous one by either incorporating additional prior information or by introducing a new set of parameters to model an entirely new phenomenon. We apply these models to both a simulated and a real ellipsoidal PN to initially estimate the position, angular size, and orientation of the nebula as a two-dimensional object and use these estimates to later examine its three-dimensional properties. The efficiency/accuracy tradeoffs of the techniques are studied to determine the advantages and disadvantages of employing a set of hierarchical models over a single comprehensive model.

Knuth, Kevin H.↗

NASA NTRS2003

Hierarchies of Models: Toward Understanding Planetary Nebulae

Stars like our sun (initial masses between 0.8 to 8 solar masses) end their lives as swollen red giants surrounded by cool extended atmospheres. The nuclear reactions in their cores create carbon, nitrogen and oxygen, which are transported by convection to the outer envelope of the stellar atmosphere. As the star finally collapses to become a white dwarf, this envelope is expelled from the star to form a planetary nebula (PN) rich in organic molecules. The physics, dynamics, and chemistry of these nebulae are poorly understood and have implications not only for our understanding of the stellar life cycle but also for organic astrochemistry and the creation of prebiotic molecules in interstellar space. We are working toward generating three-dimensional models of planetary nebulae (PNe), which include the size, orientation, shape, expansion rate and mass distribution of the nebula. Such a reconstruction of a PN is a challenging problem for several reasons. First, the data consist of images obtained over time from the Hubble Space Telescope (HST) and spectra obtained from Kitt Peak National Observatory (KPNO) and Cerro Tololo Inter-American Observatory (CTIO). These images are of course taken from a single viewpoint in space, which amounts to a very challenging tomographic reconstruction. Second, the fact that we have two disparate and orthogonal data types requires that we utilize a method that allows these data to be used together to obtain a solution. To address these first two challenges we employ Bayesian model estimation using a parameterized physical model that incorporates much prior information about the known physics of the PN. In our previous works we have found that the forward problem of the comprehensive model is extremely time consuming. To address this challenge, we explore the use of a set of hierarchical models, which allow us to estimate increasingly more detailed sets of model parameters. These hierarchical models of increasing complexity are akin to scientific theories of increasing sophistication, with each new model/theory being a refinement of a previous one by either incorporating additional prior information or by introducing a new set of parameters to model an entirely new phenomenon. We apply these models to both a simulated and a real ellipsoidal PN to initially estimate the position, angular size, and orientation of the nebula as a two-dimensional object and use these estimates to later examine its three-dimensional properties. The efficiency/accuracy tradeoffs of the techniques are studied to determine the advantages and disadvantages of employing a set of hierarchical models over a single comprehensive model.

Knuth, Kevin H.↗

NASA NTRS1985

Two new color-selected magnetic DA white dwarfs

The discovery of two magnetic white dwarfs culled from blue star surveys is reported. The surveys were carried out with the Mount Lemnon 1.5-meter reflecting telescope attached to a two-holer polarimeter/photometer. Spectral observations of the objects, (PG 1533 - 057, and K813 - 14), indicate the presence of hydrogen and Zeeman components. On the basis of dipolar field simulations, it is shown that PG 1533 - 057 has a polar field strength of 31 megagauss (MG) while K813 - 14 has a polar field strength of 29 MG. A third known white dwarf has a polar field strength of 18 MG. All the dwarfs had temperatures in the 11,000-20,000 K range. The possibility that a significant fraction of isolated magnetic degenerate stars could be the progeny of magnetic accreting binary systems is considered.

Liebert, J.↗

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 NTRS2004

The Formation Mechanism of Helium-Rich Subdwarf B Stars

This program used the Far Ultraviolet Spectroscopic Explorer (FUSE) to measure the surface abundances of three helium-rich hot subdwarfs in the Galactic field, to test our hypothesis that such stars form via a late helium flash while descending the white dwarf cooling curve. If these stars form via a late He flash, the stellar envelope should be mixed with the interior, enhancing the surface carbon abundance enormously while depleting the surface hydrogen. The first observation of PG1544+488 was incomplete because detector 2 was turned off, but the repeat observations were fully successful. The observations of LB1766 had no problems and were fully successful. The observations of PG1127+019 became infeasible when FUSE lost some of its pointing control, so it was replaced with JL87, which was also observed successfully.

Luers, Jeannine N.↗

NASA NTRS1976

Extreme ultraviolet survey experiment MA-083

A grazing incidence telescope sensitive to radiation in the 5- to 100-nanometer band was flown in the Apollo service module. On 10 nighttime revolutions, the command and service module was maneuvered to point the instrument at 30 different stellar targets for periods of 1 to 20 minutes, thus constituting the first sensitive search for extreme ultraviolet radiation from nonsolar sources. Several hours of supplementary data were also obtained during nighttime orbits when other experiments in the scientific instrument module bay were operating. Preliminary analysis of a small fraction of the total data indicates the definite detection of a strong source of extreme ultraviolet radiation during observations made during revolution 109. The source is located in Coma Berencies. The suggested optical identification is the white dwarf HZ 43. If this association is correct, the star has the highest temperature of any known white dwarf. Regardless of the optical identification, however, this object is the first nonsolar source to be detected in the extreme ultraviolet band.

Bowyer, S.↗

NASA NTRS2017

The Ultracompact Nature of the Black Hole Candidate X-Ray Binary 47 Tuc X9

47 Tuc X9 is a low-mass X-ray binary (LMXB) in the globular cluster 47 Tucanae, and was previously thought to be a cataclysmic variable. However, Miller-Jones et al. recently identified a radio counterpart to X9 (inferring a radio X-ray luminosity ratio consistent with black hole LMXBs), and suggested that the donor star might be a white dwarf. We report simultaneous observations of X9 performed by Chandra, NuSTAR and Australia Telescope Compact Array. We find a clear 28.18+/- 0.02-min periodic modulation in the Chandra data, which we identify as the orbital period, confirming this system as an ultracompact X-ray binary. Our X-ray spectral fitting provides evidence for photoionized gas having a high oxygen abundance in this system, which indicates a CO white dwarf donor. We also identify reflection features in the hard X-ray spectrum, making X9 the faintest LMXB to show X-ray reflection. We detect an approx. 6.8-d modulation in the X-ray brightness by a factor of 10, in archival Chandra, Swift and ROSAT data. The simultaneous radio X-ray flux ratio is consistent with either a black hole primary or a neutron star primary, if the neutron star is a transitional millisecond pulsar. Considering the measured orbital period (with other evidence of a white dwarf donor), and the lack of transitional millisecond pulsar features in the X-ray light curve, we suggest that this could be the first ultracompact black hole X-ray binary identified in our Galaxy.

accretion↗

NASA NTRS1985

Detection of a compact companion of the mild barium star Xi-1 Ceti

In the present paper, the observation of a white dwarf companion of the mild Ba star Xi-1 Ceti (= 65 Ceti = HR 649 = HD 13611) is reported, taking into account also the properties of the mild Ba star and of its companion. The UV spectrum of Xi-1 Ceti is discussed along with an interpretation of this spectrum. Attention is given to the effective temperature of the companion, the absorption bands in the spectrum, the radius and mass of the Xi-1 Ceti companion, and questions regarding the obscuration of the companion by the atmosphere of the Ba star. It is found that the overall energy distribution of the Xi-1 Ceti companion can best be matched with a 14,000 K DA white dwarf of log g = 8 or less. However, the absolute intensity is too small and would require a radius too small and a mass too large for such a gravity.

Bohm-Vitense, E.↗

NASA NTRS2016

SU Lyncis, a Hard X-Ray Bright M Giant: Clues Point to a Large Hidden Population of Symbiotic Stars

Symbiotic star surveys have traditionally relied almost exclusively on low resolution optical spectroscopy. However, we can obtain amore reliable estimate of their total Galactic population by using all available signatures of the symbiotic phenomenon. Here we report the discovery of a hard X-ray source, 4PBC J0642.9+5528, in the Swift hard X-ray all-sky survey, and identify it with a poorly studied red giant, SU Lyn, using pointed Swift observations and ground-based optical spectroscopy. The X-ray spectrum, the optical to UV spectrum, and the rapid UV variability of SU Lyn are all consistent with our interpretation that it is a symbiotic star containing an accreting white dwarf. The symbiotic nature of SU Lyn went unnoticed until now, because it does not exhibit emission lines strong enough to be obvious in low resolution spectra. We argue that symbiotic stars without shell-burning have weak emission lines, and that the current lists of symbiotic stars are biased in favour of shell-burning systems. We conclude that the true population of symbiotic stars has been underestimated, potentially by a large factor.

binaries: symbiotic aEuro" stars: individual: SU L↗

NASA NTRSDate not supplied

Oxygen Isotope Ratios in Hydrogen-deficient Carbon Stars: A Correlation with Effective Temperature and Implications for White Dwarf Merger Outcomes

Explore the source record for details and available documents.

Advait Mehla↗

NASA NTRS1994

The stellar population in the core of M15

The inner core of the globular cluster M15 within approximately 2 sec of the geometrical center has been explored with high-resolution images taken through several broad-band UV filter with the Faint Object Camera (FOC) on board the Hubble Space Telescope (HST). Approximately 210 stars in this region down to a 5 sigma detection limit of m(sub 220) = 21.5 were reliably identified and located on a UV - U color magnitude diagram for the first time. A majority of stars (about 70% of the total) observed this way lie above the expected main-sequence turn-off of this cluster and below the sparsely populated horizontal branch. The extension of the main sequence above the turn-off separates this population in two roughly equal components situated to the right and left of this line. Most of the former must be classical blue stragglers while the rest belong to a new, as yet unidentified, population of very blue stars. Possibilities include, but are not restricted to, well-mixed single stars, subdwarfs, and helium white dwarfs. Similar objects are also found just outside the core out to approximately 6 sec from the center, but the brighter, presumably more massive ones, are sharply confined to the core itself. The measured excess of bright blue stars and the relative deficiency of bright red giants in the core are consistent with the blue inward color gradient measured from the ground and imply that dynamical evolution can significantly affect the stellar population in the very dense central regions of a high-concentration globular cluster like M15.

De Marchi, Guido↗

NASA NTRS1990

The white dwarf luminosity function - A possible probe of the galactic halo

The dynamically inferred dark halo mass density, amounting to above 0.01 solar masses/cu pc at the sun's Galactocentric radius, can be composed of faint white dwarfs provided that the halo formed in a sufficiently early burst of star formation. The model is constrained by the observed disk white dwarf luminosity function which falls off below log (L/solar L) = -4.4, due to the onset of star formation in the disk. By using a narrow range for the initial mass function and an exponentially decaying halo star formation rate with an e-folding time equal to the free-fall time, all the halo dark matter is allowed to be in cool white dwarfs which lie beyond the falloff in the disk luminosity function. Although it is unlikely that all the dark matter is in these dim white dwarfs, a definite signature in the low-luminosity end of the white dwarf luminosity function is predicted even if they comprise only 1 percent of the dark matter. Current CCD surveys should answer the question of the existence of this population within the next few years.

Tamanaha, Christopher M.↗

NASA NTRS1992

Detection of a white dwarf in a visual binary system

The F6 giant HD 160365 was detected to have a white dwarf companion about 8 arcsec south of the star. The UV energy distribution observed with IUE shows that the white dwarf has an effective temperature of 23,000 +/- 2000 K. If log g = 8 the Lya profile indicates an effective temperature around 24,500 K. Using the theoretical models by Wesemael et al. (1980) one finds a visual magnitude of m(V) about 16.5. For T(eff) = 24,500 K one expects for a white dwarf a luminosity of log L/L(solar) about 1.3 and M(V) about 10.67. This gives a distance modulus for the system of m(V) - M(V) = 5.83 and an absolute magnitude M(V)= 0.3 for the giant.

Boehm-Vitense, Erika↗

NASA NTRS1994

X-ray variability and period determinations in the eclipsing polar DP Leonis

We present an analysis of our ROSAT position sensitive proportional counter (PSPC) observations on the eclipsing magnetic cataclysmic variable DP Leo. The soft X-ray spectrum is modeled by a blackbody of kT = 24.8(sup +2.6 sub -8.1) eV. Severe limits are placed upon the flux from any hard bremsstrahlung component. A strong soft X-ray excess, with respect to hard X-ray emission, is found. The soft X-ray blackbody luminosity is larger than both the cyclotron and bremsstrahlung luminosities. An upper limit of 500 pc is obtained for the system's distance based upon the X-ray absorption (N(sub H) less than 5 x 10(exp 19)/sq cm) and an estimate of 260(sup +150 sub -100) pc is determined from a published measurement of the secondary's flux. For the derived blackbody fit, the bolometric luminosity is found to be L(sub bb, bol) = 1.4(sup +7.1 sub -0.3) x 10(exp 31)(d/260 pc)(exp 2) ergs/s. Absorption by the accretion stream produces an intensity dip prior to each eclipse. Extreme variability in the shape of the light curve from eclipse to eclipse demonstrates that changes in the rate of accretion onto the white dwarf, the sizes of accretion filaments, or variations in the location or amount of absorbing matter in the system occur on timescales shorter than the orbital period (89.8 minutes). No evidence exists for accretion onto the stronger (59 MG) magnetic pole in the ROSAT data. A new ephemeris is presented for the eclipse of the white dwarf emission region by the secondary star and another is produced for the orbital conjunction of the two components. The rotation of the white dwarf is shown to be faster than the orbital period by (5.3 +/- 1.1) x 10(exp -3) s. The origin of the asynchronous rotation may be activity cycle included orbital period variations or oscillations of the white dwarf's main pole about an equilibirium position. The accretion stream is modeled assuming that disruption of the stream along magnetic field lines occurs close to the white dwarf. The ROSAT intensity dip is explained using this model and the results are shown to be consistent with contemporaneous optical observations.

Robinson, Craig R.↗

NASA NTRS2003

Constraining the Evolution of ZZ Ceti

We report our analysis of the stability of pulsation periods in the DAV star (pulsating hydrogen atmosphere white dwarf) ZZ Ceti, also called R548. On the basis of observations that span 31 years, we conclude that the period 213.13 s observed in ZZ Ceti drifts at a rate dP/dt 5 (5.5 plus or minus 1.9) x 10(exp -15) ss(sup -1), after correcting for proper motion. Our results are consistent with previous P values for this mode and an improvement over them because of the larger time base. The characteristic stability timescale implied for the pulsation period is |P||P(raised dot)|greater than or equal to 1.2 Gyr, comparable to the theoretical cooling timescale for the star. Our current stability limit for the period 213.13 s is only slightly less than the present measurement for another DAV, G117-B15A, for the period 215.2 s, establishing this mode in ZZ Ceti as the second most stable optical clock known, comparable to atomic clocks and more stable than most pulsars. Constraining the cooling rate of ZZ Ceti aids theoretical evolutionary models and white dwarf cosmochronology. The drift rate of this clock is small enough that we can set interesting limits on reflex motion due to planetary companions.

Mukadam, Anjum S.↗