Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “STAR BURNING”

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 NTRS1964

Stellar evolution. ii. the hydrogen-burning phase of a star of 7.0 solar masses <sternentwicklung. ii. die wasserstoff- brennende phase eines sterns von 7.0 sonnenmassen<

With the computer program by Hofmeister, Kippenhahn, Weigert (1964, from now on referred to as ''I") the evolution of a population I star of 7.0 solar masses has been followed. The results presented here are based on a sequence of 119 models, the first of which represents the homogeneous main sequence state and has a convective core containing 26% of the mass. The following models describe the phase of depletion of hydrogen in the core, which is shrinking in mass until the convection around the centre disappears. After this the gradual development of a hydrogen burning shell source is shown. The models show the evolution into the red giant region, while the helium core is contracting until helium burning starts at the centre and a new convective core is built up. The luminousity is increased about 1 magnitude compared with the main sequence state. At the same time the outer convective region grows in mass and covers at the maxi­mum 71% of the total mass of the star. During the evolution to the red giant state, the evolutionary track crosses the region of the cepheids. Some aspects of the corresponding models are discussed.

STELLAR EVOLUTION↗
DOE OSTI2010

Astrophysical S factors for fusion reactions involving C, O, Ne, and Mg isotopes

Using the Sao Paulo potential and the barrier penetration formalism we have calculated the astrophysical factor S(E) for 946 fusion reactions involving stable and neutron-rich isotopes of C, O, Ne, and Mg for center-of-mass energies E varying from 2 to {approx}18-30 MeV (covering the range below and above the Coulomb barrier). We have parameterized the energy dependence, S(E), by an accurate universal 9-parameter analytic expression and present tables of fit parameters for all the reactions. We also discuss the reduced 3-parameter version of our fit which is highly accurate at energies below the Coulomb barrier, and outline the procedure for calculating the reaction rates. The results can be easily converted to thermonuclear or pycnonuclear reaction rates to simulate various nuclear burning phenomena, in particular, stellar burning at high temperatures and nucleosynthesis in high density environments.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2012

PHOTOSPHERIC RADIUS EXPANSION IN SUPERBURST PRECURSORS FROM NEUTRON STARS

Thermonuclear runaway burning of carbon is in rare cases observed from accreting neutron stars as day-long X-ray flares called superbursts. In the few cases where the onset is observed, superbursts exhibit a short precursor burst at the start. In each instance, however, the data are of insufficient quality for spectral analysis of the precursor. Using data from the propane anti-coincidence detector of the Proportional Counter Array instrument on the Rossi X-ray Timing Explorer, we perform the first detailed time-resolved spectroscopy of precursors. For a superburst from 4U 1820-30 we demonstrate the presence of photospheric radius expansion. We find the precursor to be 1.4-2 times more energetic than other short bursts from this source, indicating that the burning of accreted helium is insufficient to explain the full precursor. Shock heating would be able to account for the shortfall in energy. We argue that this precursor is a strong indication that the superburst starts as a detonation, and that a shock induces the precursor. Furthermore, we employ our technique to study the superexpansion phase of the same superburst in greater detail.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ON CARBON BURNING IN SUPER ASYMPTOTIC GIANT BRANCH STARS

We explore the detailed and broad properties of carbon burning in Super Asymptotic Giant Branch (SAGB) stars with 2755 MESA stellar evolution models. The location of first carbon ignition, quenching location of the carbon-burning flames and flashes, angular frequency of the carbon core, and carbon core mass are studied as a function of the zero-age main sequence (ZAMS) mass, initial rotation rate, and mixing parameters such as convective overshoot, semiconvection, thermohaline, and angular momentum transport. In general terms, we find that these properties of carbon burning in SAGB models are not a strong function of the initial rotation profile, but are a sensitive function of the overshoot parameter. We quasi-analytically derive an approximate ignition density, ρ{sub ign} ≈ 2.1 × 10{sup 6} g cm{sup −3}, to predict the location of first carbon ignition in models that ignite carbon off-center. We also find that overshoot moves the ZAMS mass boundaries where off-center carbon ignition occurs at a nearly uniform rate of ΔM{sub ZAMS}/Δf{sub ov} ≈ 1.6 M{sub ⊙}. For zero overshoot, f{sub ov} = 0.0, our models in the ZAMS mass range ≈8.9–11 M{sub ⊙} show off-center carbon ignition. For canonical amounts of overshooting, f{sub ov} = 0.016, the off-center carbon ignition range shifts to ≈7.2–8.8 M{sub ⊙}. Only systems with f{sub ov} ≥ 0.01 and ZAMS mass ≈7.2–8.0 M{sub ⊙} show carbon burning is quenched a significant distance from the center. These results suggest a careful assessment of overshoot modeling approximations on claims that carbon burning quenches an appreciable distance from the center of the carbon core.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1995

Hot bottom burning in asymptotic giant branch stars and its effect on oxygen isotopic abundances

A self-consistent calculation of asymptotic giant branch (AGB) evolution was carried out, including nucleosynthesis at the base of the convective envelope (hot bottom burning). Hot bottom burning was found to occur for stars between approximately 4.5 and approximately 7 solar mass, producing envelopes with O-18/O-16 less than or equal to 10(exp -6) and 10(exp -3) approximately less than or equal O-17/O-16 approximately less than or equal to 10(exp -1). The O-17 abundance depends sensitively on the nuclear O-17-destruction rate; this rate is only loosely constrained by the requirement that first and second dredge-up models match O-isotope observations of red giant branch (RGB) stars (Boothroyd, Sackmann, & Wasserburg 1994). In some cases, high mass-loss rates can terminate hot bottom burning before further O-17 enrichment takes place or even before all O-18 is destroyed. These predictions are in accord with the very limited stellar observations of J type carbon stars on the AGB and with some of the circumstellar Al2O3 grains from meteorites. In contrast, precise data from a number of grains and data from most low-mass S and C AGB stars (approximately less than 1.7 solar mass) lie in a region of the O-18/O-16 versus O-17/O-16 diagram that is not accessible by first and second dredge-up or by hot bottom burning. We conclude that for AGB stars, the standard models of stellar evolution are not in accord with these observations. We surmise that an additional mixing mechanism must exist that transports material from the cool bottom of the stellar convective envelope to a depth at which O-18 is destroyed. This 'cool bottom processing' mechanism on the AGB is similar to extra mixing mechanisms proposed to explain the excess C-13 (and depleted C-12) observed in the earlier RGB stage of evolution and the large Li-7 depletion observed in low-mass main-sequence stars.

Boothroyd, Arnold I.↗
NASA NTRS1983

On the nature of Upsilon Sagittarii

An explanation for the nature and evolution of the extremely hydrogen deficient binary Upsilon Sagittarii which is consistent with all observational and theoretical facts. First, the system goes through a Case B mass exchange in which most of the hydrogen rich envelope of a massive primary (5 to 14 solar masses) is lost. The remaining envelope still contains about 50 percent hydrogen (by number), but is now of negligible mass, so that the star evolves like a pure helium star. If its mass is between 1 and 2 solar masses the star reaches low surface temperatures and becomes a supergiant before the onset of carbon burning. This star (the original primary) then fills its Roche lobe a second time,spilling its now helium rich envelope over onto the secondary (Case BB mass exchange). It is argued that Upsilon Sagittarii is in this state at the present time, and that the visible star is an evolved helium star of about 1 solar mass with a degenerate carbon-oxygen core and a helium burning shell which provides the high luminosity. Previously announced in Star as N26117

Schoenberner, D.↗
NASA NTRS1982

On the nature of upsilon Sagittarii

An explanation for the nature and evolution of the extremely hydrogen deficient binary Upsilon Sagittarii which is consistent with all observational and theoretical facts. First, the system goes through a Case B mass exchange in which most of the hydrogen rich envelope of a massive primary (5 to 14 solar masses) is lost. The remaining envelope still contains about 50% hydrogen (by number), but is now of negligible mass, so that the star evolves like a pure helium star. If its mass is between 1 and 2 solar masses the star reaches low surface temperatures and becomes a supergiant before the onset of carbon burning. This star (the original primary) then fills its Roche lobe a second time, spilling its now helium rich envelope over onto the secondary (Case BB mass exchange). It is argued that Upsilon Sagrittarii is in this state at the present time, and that the visible star is an evolved helium star of about 1 solar mass with a degenerate carbon-oxygen core and a helium burning shell which provides the high luminosity.

Schoenberner, D.↗
DOE OSTI2014

Nuclear astrophysics at DRAGON

The DRAGON recoil separator is located at the ISAC facility at TRIUMF, Vancouver. It is designed to measure radiative alpha and proton capture reactions of astrophysical importance. Over the last years, the DRAGON collaboration has measured several reactions using both radioactive and high-intensity stable beams. For example, the 160(a, g) cross section was recently measured. The reaction plays a role in steady-state helium burning in massive stars, where it follows the 12C(a, g) reaction. At astrophysically relevant energies, the reaction proceeds exclusively via direct capture, resulting in a low rate. In this measurement, the unique capabilities of DRAGON enabled determination not only of the total reaction rates, but also of decay branching ratios. In addition, results from other recent measurements will be presented.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1973

FG Sagittae - Observational evidence of a thermal pulse.

A stellar remnant which has lost its hydrogen-rich envelope could be a hot dense star that is still burning helium in a shell near the surface. If there is a thermal pulse in the helium-burning shell, the star will expand, increase in luminosity, and become cooler. Such a star would resemble qualitatively the observed behavior of FG Sagittae, and it is urged that the future behavior of this star be watched closely because it may be the first observational evidence of a thermal pulse.

Sparks, W. M.↗
DOE OSTI2013

Kepler monitoring of an L dwarf I. The photometric period and white light flares

We report on the results of 15 months of monitoring the nearby field L1 dwarf WISEP J190648.47+401106.8 (W1906+40) with the Kepler mission. Supporting observations with the Karl G. Jansky Very Large Array and Gemini North Telescope reveal that the L dwarf is magnetically active, with quiescent radio and variable Hα emission. A preliminary trigonometric parallax shows that W1906+40 is at a distance of 16.35{sub −0.34}{sup +0.36} pc, and all observations are consistent with W1906+40 being an old disk star just above the hydrogen-burning limit. The star shows photometric variability with a period of 8.9 hr and an amplitude of 1.5%, with a consistent phase throughout the year. We infer a radius of 0.92 ± 0.07R{sub J} and sin i > 0.57 from the observed period, luminosity (10{sup –3.67} {sup ±} {sup 0.03} L {sub ☉}), effective temperature (2300 ± 75 K), and vsin i (11.2 ± 2.2 km s{sup –1}). The light curve may be modeled with a single large, high latitude dark spot. Unlike many L-type brown dwarfs, there is no evidence of other variations at the ≳ 2% level, either non-periodic or transient periodic, that mask the underlying rotation period. We suggest that the long-lived surface features may be due to starspots, but the possibility of cloud variations cannot be ruled out without further multi-wavelength observations. During the Gemini spectroscopy, we observed the most powerful flare ever seen on an L dwarf, with an estimated energy of ∼1.6 × 10{sup 32} erg in white light emission. Using the Kepler data, we identify similar flares and estimate that white light flares with optical/ultraviolet energies of 10{sup 31} erg or more occur on W1906+40 as often as 1-2 times per month.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Low-energy cross-section measurement of the B 10 ( α , n ) N 13 reaction and its impact on neutron production in first-generation stars

Nucleosynthesis in the first generation of massive stars offers a unique setting to explore the creation of the first heavier nuclei in an environment free of impurities from earlier stellar generations. In later generations of massive stars, hydrogen burning occurs predominantly through the CNO cycles, but without the carbon, nitrogen, and oxygen to catalyze the reaction sequence, first stars would have to rely on the inefficient pp chains for their energy production. Observations of second and third generation stars show pronounced abundances of carbon and oxygen isotopes, which suggests a rapid conversion of the primordial abundances to heavier elements. While the triple-alpha-process primarily facilitates this conversion, there are alternative reaction sequences, such as 2 H(α,γ) 6 Li(α,γ) 10 B(α,n) 13 N, that may play a significant role. To study such alternate reaction pathways for production of carbon and heavier nuclei, a number of new measurements are needed. In this work, new measurements are reported for the 10 B(α,n) 13 N reaction, extending the cross section down to 575 keV incident α-particle energy. The measurements were made using a state-of-the-art deuterated liquid scintillator and a spectrum unfolding technique. An R-matrix analysis was performed in order to facilitate a comparison of the underlying nuclear structure with the reaction measurements. An unexpected upturn is observed in the low-energy S factor that indicates the presence of a new low-energy resonance. Finally, a revised reaction rate is determined that takes into account the present data as well as other previous measurements from the literature that were previously neglected.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Birth of a Be star: an APOGEE search for Be stars forming through binary mass transfer

ABSTRACT Motivated by recent suggestions that many Be stars form through binary mass transfer, we searched the APOGEE survey for Be stars with bloated, stripped companions. From a well-defined parent sample of 297 Be stars, we identified one mass-transfer binary, HD 15124. The object consists of a main-sequence Be star ($M_{\rm Be}=5.3\pm 0.6\, {\rm M}_{\odot }$) with a low-mass ($M_{\rm donor}=0.92\pm 0.22\, {\rm M}_{\odot }$), subgiant companion on a 5.47-d orbit. The emission lines originate in an accretion disc caused by ongoing mass transfer, not from a decretion disc as in classical Be stars. Both stars have surface abundances bearing imprint of CNO processing in the donor’s core: the surface helium fraction is YHe ≈ 0.6, and the nitrogen-to-carbon ratio is 1000 times the solar value. The system’s properties are well-matched by binary evolution models in which mass transfer begins while a $3-5\, {\rm M}_{\odot }$ donor leaves the main sequence, with the originally less massive component becoming the Be star. These models predict that the system will soon become a detached Be + stripped star binary like HR 6819 and LB-1, with the stripped donor eventually contracting to become a core helium-burning sdO/B star. Discovery of one object in this short-lived (∼1 Myr) evolutionary phase implies the existence of many more that have already passed through it and are now Be + sdO/B binaries. We infer that $(10-60)\, {{\ \rm per\ cent}}$ of Be stars have stripped companions, most of which are $\sim 100\, \times$ fainter than the Be stars in the optical. Together with the dearth of main-sequence companions to Be stars and recent discovery of numerous Be + sdO/B binaries in the UV, our results imply that binarity plays an important role in the formation of Be stars.

El-Badry, Kareem (ORCID:0000000268711752)↗
DOE OSTI2014

Direct measurement of {sup 12}C+{sup 4}He→{sup 16}O+γ total cross section at E{sub cm}=1.2 MeV

A fusion reaction of {sup 12}C+{sup 4}He→{sup 16}O+γ is one of the main reactions in He-burning of stars and important for nucleosynthesis. The fusion cross section at stellar energy of E{sub cm}=0.3 MeV has not been determined precisely yet in spite of efforts for about 40 years. We plan to measure directly the total fusion cross section down to 0.7 MeV at Kyushu University Tandem accelerator Laboratory and to estimate the cross section at 0.3MeV by extrapolation. We have already measured the cross sections at 2.4 MeV and 1.5 MeV. The measurement at E{sub cm}=1.2 MeV is in progress.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS2008

Thermonuclear Burning as a Probe of Neutron Star

Thermonuclear fusion is a fundamental process taking place in the matter transferred onto neutron stars in accreting binary systems. The heat deposited by nuclear reactions becomes readily visible in the X-ray band when the burning is either unstable or marginally stable, and results in the rich phenomenology of X-ray bursts, superbursts, and mHz quasiperiodic oscillations. Fast X-ray timing observations with NASA's Rossi X-ray Timing Explorer (RXTE) over the past decade have revealed a wealth of new phenomena associated with thermonuclear burning on neutron stars, including the discovery of nuclear powered pulsations during X-ray bursts and superbursts. I will briefly review our current observational and theoretical understanding of these new phenomena, with an emphasis on recent findings, and discuss what they are telling us about the structure of neutron stars.

Strohmayer, Tod↗