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.

Physical properties of IP Pegasi: an eclipsing dwarf nova with an unusually cool white dwarf

Abstract: We present high-speed photometric observations of the eclipsing dwarf nova IP Pegasi (IP Peg) taken with the triple-beam camera ULTRACAM mounted on the William Herschel Telescope. The primary eclipse in this system was observed twice in 2004, and then a further 16 times over a 3 week period in 2005. Our observations were simultaneous in the Sloan u', g' and r' bands. By phase-folding and averaging our data, we make the first significant detection of the white dwarf ingress in this system and find the phase width phi of the white dwarf eclipse to be 0.0935 +/- 0.0003, significantly higher than the previous best value of 0.0863 < phi < 0.0918. The mass ratio is found to be q = M-2/M-1 = 0.48 +/- 0.01, consistent with previous measurements, but we find the inclination to be 83 degrees.8 +/- 0 degrees.5, significantly higher than previously reported. We find the radius of the white dwarf to be 0.0063 +/- 0.0003 R-circle dot, implying a white dwarf mass of 1.16 +/- 0.02 M-circle dot. The donor mass is 0.55 +/- 0.02 M-circle dot. The white dwarf temperature is more difficult to determine, since the white dwarf is seen to vary significantly in flux, even between consecutive eclipses. This is seen particularly in the u' band, and is probably the result of absorption by disc material. Our best estimate of the temperature is 10 000-15 000 K, which is much lower than would be expected for a cataclysmic variable star with this period, and implies a mean accretion rate of < 5 x 10(-11) M(circle dot)yr(-1), more than 40 times lower than the expected rate.

Hubble space telescope calspec flux standards: sirius (and vega)

Abstract: The Space Telescope Imaging Spectrograph (STIS) has measured the flux for Sirius from 0.17 to 1.01 μm on the Hubble Space Telescope (HST) White Dwarf scale. Because of the cool debris disk around Vega, Sirius is commonly recommended as the primary IR flux standard. The measured STIS flux agrees well with predictions of a special Kurucz model atmosphere, adding confidence to the modeled IR flux predictions. The IR flux agrees to 2%-3% with respect to the standard template of Cohen and to 2% with the Midcourse Space Experiment absolute flux measurements in the mid-IR. A weighted average of the independent visible and mid-IR absolute flux measures implies that the monochromatic flux at 5557.5 A (5556 A in air) for Sirius and Vega, respectively, is 1.35 × 10–8 and 3.44 × 10–9 erg cm–2 s–1 A–1 with formal uncertainties of 0.5%. Contrary to previously published conclusions, the Hipparcos photometry offers no support for the variability of Vega. Pulse pileup severely affects the Hp photometry for the brightest stars.

Formation of high-field magnetic white dwarfs from common envelopes

Abstract: The origin of highly magnetized white dwarfs has remained a mystery since their initial discovery. Recent observations indicate that the formation of high-field magnetic white dwarfs is intimately related to strong binary interactions during post-main-sequence phases of stellar evolution. If a low-mass companion, such as a planet, brown dwarf, or low-mass star, is engulfed by a post-main-sequence giant, gravitational torques in the envelope of the giant lead to a reduction of the companion’s orbit. Sufficiently low-mass companions in-spiral until they are shredded by the strong gravitational tides near the white dwarf core. Subsequent formation of a super-Eddington accretion disk from the disrupted companion inside a common envelope can dramatically amplify magnetic fields via a dynamo. Here, we show that these disk-generated fields are sufficiently strong to explain the observed range of magnetic field strengths for isolated, high-field magnetic white dwarfs. A higher-mass binary analogue may also contribute to the origin of magnetar fields.

On type Ia supernovae and the formation of single low-mass white dwarfs

Abstract: There is still considerable debate over the progenitors of type Ia supernovae (SNe Ia). Likewise, it is not agreed how single white dwarfs with masses less than ~0.5 Msun can be formed in the field, even though they are known to exist. We consider whether single low-mass white dwarfs (LMWDs) could have been formed in binary systems where their companions have exploded as a SN Ia. In this model, the observed single LMWDs are the remnants of giant-branch donor stars whose envelopes have been stripped off by the supernova explosion. We investigate the likely remnants of SNe Ia, including the effects of the explosion on the envelope of the donor star. We also use evolutionary arguments to examine alternative formation channels for single LMWDs. In addition, we calculate the expected kinematics of the potential remnants of SNe Ia. SN Ia in systems with giant-branch donor stars can naturally explain the production of single LMWDs. It seems difficult for any other formation mechanism to account for the observations, especially for those single LMWDs with masses less than ~0.4 Msun. Independent of those results, we find that the kinematics of one potentially useful population containing single LMWDs is consistent with our model. Studying remnant white-dwarf kinematics seems to be a promising way to investigate SN Ia progenitors. The existence of single LMWDs appears to constitute evidence for the production of SNe Ia in binary systems with a red-giant donor star. Other single white dwarfs with higher space velocities support a second, probably dominant, population of SN Ia progenitors which contained main-sequence or subgiant donor stars at the time of explosion. The runaway stars LP400-22 and US 708 suggest the possibility of a third formation channnel for some SNe Ia in systems where the donor stars are hot subdwarfs.

Effects of Gravity on the Structure of Postshock Accretion Flows in Magnetic Cataclysmic Variables

Abstract: We have calculated the temperature and density structure of the hot postshock plasma in magnetically confined accretion flows, including the gravitational potential. This avoids the inconsistency of previous calculations which assume that the height of the shock is negligible. We assume a stratified accretion column with 1-d flow along the symmetry axis. We find that the calculations predict a lower shock temperature than previous calculations, with a flatter temperature profile with height. We have revised previous determinations of the masses of the white dwarf primary stars and find that for higher mass white dwarfs there is a general reduction in derived masses when the gravitational potential is included. This is because the spectrum from such flows is harder than that of previous prescriptions at intermediate energies.