White dwarf

About: White dwarf is a research topic. Over the lifetime, 15004 publications have been published within this topic receiving 430597 citations. The topic is also known as: degenerate dwarf.

Discovery of a candidate for the coolest known brown dwarf

Abstract: We have used multi-epoch images from the Infrared Array Camera on board the Spitzer Space Telescope to search for substellar companions to stars in the solar neighborhood based on common proper motions. Through this work, we have discovered a faint companion to the white dwarf WD 0806-661. The comoving source has a projected separation of 130'', corresponding to 2500 AU at the distance of the primary (19.2 pc). If it is physically associated, then its absolute magnitude at 4.5 μm is ~1 mag fainter than the faintest known T dwarfs, making it a strong candidate for the coolest known brown dwarf. The combination of M 4.5 and the age of the primary (1.5 Gyr) implies an effective temperature of ~300 K and a mass of ~7 M Jup according to theoretical evolutionary models. The white dwarf's progenitor likely had a mass of ~2 M ☉, and thus could have been born with a circumstellar disk that was sufficiently massive to produce a companion with this mass. Therefore, the companion could be either a brown dwarf that formed like a binary star or a giant planet that was born within a disk and has been dynamically scattered to a larger orbit.

On the Evolution of Stars that Form Electron-Degenerate Cores Processed by Carbon Burning. III. The Inward Propagation of a Carbon-Burning Flame and Other Properties of a 9 M☉ Model Star

Abstract: A 9 M☉ stellar model of Population I composition is evolved from the hydrogen-burning main sequence to the thermally pulsing "super" asymptotic giant branch stage, where it has an electron-degenerate core composed of an inner oxygen-neon (ONe) part of mass ~1.066 M☉ and an outer carbon-oxygen (CO) layer of mass ~0.05 M☉ and is experiencing thermal pulses driven by helium-burning thermonuclear flashes. The carbon-burning phase of the 9 M☉ model is in many respects similar to, but differs importantly from that of a 10 M☉ model studied earlier. In both cases, carbon is ignited off center, and a series of carbon flashes accompanied by a convective shell occur. In contrast to the 10 M☉ model, the 9 M☉ model experiences the second dredge-up phenomenon (the penetration of the base of the hydrogen-rich convective envelope inward into helium- and carbon-rich material) near the beginning rather than near the end of the carbon-burning phase. The first carbon-burning flash causes helium burning to shut down and the release of gravothermal energy (compressional and thermal energy) between the helium-carbon discontinuity and the base of the convective envelope plays a dominant role in the dredge-up event. Beginning with the third carbon-burning shell flash, the "flame front," defined as being coincident with the base of the convective shell, propagates inward with a speed close to theoretical predictions that relate flame speed to local thermodynamic, opacity, and energy-generation rate characteristics. Ahead of the inward moving front, most of the nuclear energy released in a "precursor flame" goes into heating and expanding matter. As the precursor flame moves toward the center, its radial thickness decreases and, to follow the progress of the front with standard techniques, both the spatial grid size and the time step must be continually decreased. Following the front gives one the opportunity to ponder Zeno's paradox, which is averted because the thickness of the precursor flame remains finite. On reaching the center, the carbon-burning flame reverses direction and continues moving outward until it is within ~0.03 M☉ of the helium-burning shell. After carbon burning is completed,12C remains at a finite abundance throughout the electron-degenerate core of mass ~1.116 M☉ and is more abundant than 20Ne in the outer ~0.05 M☉ of this core. Over most of the ONe interior of both the 9 and 10 M☉ models,23Na is more abundant than 24Mg, but the maximum 12C abundance in the 9 M☉ model ONe interior (X[12C] ~ 0.048) is significantly larger than in the 10 M☉ model (X[12C] ~ 0.012). For an ONe white dwarf that accretes enough matter to reach the Chandrasekhar limiting mass, this may make the difference between total explosive disruption (large 12C abundance) and collapse to neutron-star dimensions (small 12C abundance). The abundances in the CO part of the core have relevance for understanding the abundances in the ejecta of classical novae produced by massive ONe white dwarfs in close binaries. In the outer ~0.014 M☉ of the CO part of the core, the abundances of all neon isotopes are much less than solar, and 25Mg and the neutron-rich isotopes made during the formation of 25Mg are at a total abundance equal to the initial abundance of CNO elements in the model. As in the 10 M☉ case, thermal pulses occasioned by helium shell flashes begin after hydrogen is reignited and the carbon-burning luminosity drops below ~100 L☉. The time between pulses is ~400 yr, roughly twice as large as in the 10 M☉ model. After the ejection of the hydrogen-rich envelope as a planetary nebula, the remnant of the 9 M☉ model is expected to evolve into a white dwarf of mass ~1.15 M☉, the outer ~0.08 M☉ of which is composed of carbon and oxygen.

Dynamos in asymptotic-giant-branch stars as the origin of magnetic fields shaping planetary nebulae.

Abstract: Planetary nebulae are thought to be formed when a slow wind from the progenitor giant star is overtaken by a subsequent fast wind generated as the star enters its white dwarf stage. A shock forms near the boundary between the winds, creating the relatively dense shell characteristic of a planetary nebula. A spherically symmetric wind will produce a spherically symmetric shell, yet over half of known planetary nebulae are not spherical; rather, they are elliptical or bipolar in shape. A magnetic field could launch and collimate a bipolar outflow, but the origin of such a field has hitherto been unclear, and some previous work has even suggested that a field could not be generated. Here we show that an asymptotic-giant-branch (AGB) star can indeed generate a strong magnetic field, having as its origin a dynamo at the interface between the rapidly rotating core and the more slowly rotating envelope of the star. The fields are strong enough to shape the bipolar outflows that produce the observed bipolar planetary nebulae. Magnetic braking of the stellar core during this process may also explain the puzzlingly slow rotation of most white dwarf stars.

Observations of doppler boosting in kepler light curves

Abstract: Among the initial results from Kepler were two striking light curves, for KOI 74 and KOI 81, in which the relative depths of the primary and secondary eclipses showed that the more compact, less luminous object was hotter than its stellar host. That result became particularly intriguing because a substellar mass had been derived for the secondary in KOI 74, which would make the high temperature challenging to explain; in KOI 81, the mass range for the companion was also reported to be consistent with a substellar object. We re-analyze the Kepler data and demonstrate that both companions are likely to be white dwarfs. We also find that the photometric data for KOI 74 show a modulation in brightness as the more luminous star orbits, due to Doppler boosting. The magnitude of the effect is sufficiently large that we can use it to infer a radial velocity amplitude accurate to 1 km s(-1). As far as we are aware, this is the first time a radial-velocity curve has been measured photometrically. Combining our velocity amplitude with the inclination and primary mass derived from the eclipses and primary spectral type, we infer a secondary mass of 0.22 +/- 0.03 M(circle dot). We use our estimates to consider the likely evolutionary paths and mass-transfer episodes of these binary systems.

Mass loss in globular cluster red giants : an evolutionary investigation

Abstract: The evolution of Population II red giants has been studied adopting Reimer's formalism for the efficiency of mass loss. Evolutionary computations for a model of 0.8 M ○. , Y=0.23, Z=0.0002 are presented for selected assumptions on the value of the parameter η governing the mass-loss rate. Comparison with canonical models (η=0) indicates that (1) the evolution of the internal He core is unaffected by mass loss, and (2) the luminosity of the red giant clump follows the actual stellar mass. Models with η≥1.0 fail to ignite He, crossing the HR diagram toward the final cooling as He white dwarfs