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.

Constraints on the progenitor system of the type Ia supernova 2014J from pre-explosion Hubble Space Telescope imaging

Abstract: We constrain the properties of the progenitor system of the highly reddened Type Ia supernova (SN Ia) 2014J in Messier 82 (M82; d (is) approx. 3.5 Mpc). We determine the supernova (SN) location using Keck-II K-band adaptive optics images, and we find no evidence for flux from a progenitor system in pre-explosion near-ultraviolet through near-infrared Hubble Space Telescope (HST) images. Our upper limits exclude systems having a bright red giant companion, including symbiotic novae with luminosities comparable to that of RS Ophiuchi. While the flux constraints are also inconsistent with predictions for comparatively cool He-donor systems (T (is) approximately 35,000 K), we cannot preclude a system similar to V445 Puppis. The progenitor constraints are robust across a wide range of RV and AV values, but significantly greater values than those inferred from the SN light curve and spectrum would yield proportionally brighter luminosity limits. The comparatively faint flux expected from a binary progenitor system consisting of white dwarf stars would not have been detected in the pre-explosion HST imaging. Infrared HST exposures yield more stringent constraints on the luminosities of very cool (T (is) less than 3000 K) companion stars than was possible in the case of SN Ia 2011fe.

The early spectral phase of type Ib supernovae - Evidence for helium

Abstract: A near-infrared spectrum and several optical spectra of SN 1983N obtained over a period from 10 days before maximum light to about a week after maximum and of SN 1984L from about maximum light to about 60 days later are presented. On the basis of these spectra it is argued that the first definite identification of helium in supernova ejecta near maximum light has been made. The emergence of oxygen emission lines which presage the dramatic transition to the supernebular phase is identified. Supernova atmosphere models which are consistent with the observations are presented, and it is argued that a strong departure of helium level populations from LTE is necessary. The current status of SNe Ib, the arguments that they result from core collapse in moderately massive stars, and some of the immediate work necessary to further clarify the nature of these events are summarized.

Blue Stragglers, Young White Dwarfs and UV-excess stars in the core of 47 Tuc

Abstract: We used a set of archived HST/WFPC2 images to probe the stellar population in the core of the nearby Galactic Globular Cluster (GGC) 47 Tuc. From the ultraviolet (UV) Color Magnitude Diagrams (CMDs) obtained for ~4,000 stars detected within the Planetary Camera (PC) field of view we have pinpointed a number of interesting objects: (i) 43 blue stragglers stars (BSSs) including 20 new candidates; (ii) 12 bright (young) cooling white dwarfs (WDs) at the extreme blue region of the UV-CMD; (iii) a large population of UV-excess (UVE) stars, lying between the BSS and the WD sequences. The WD candidates identified here define a clean pattern in the CMD, which define the WD cooling sequence. Moreover, both the location on the UV-CMDs and the number of WDs are in excellent agreement with the theoretical expectations. The UVE stars discovered here represent the largest population of anomalous blue objects ever observed in a globular cluster -- if the existence of such a large population is confirmed, we have finally found the long-searched population of interacting binaries predicted by the theory. Finally, we have investigated the feasibility of the optical identification of the companions of the binary X-ray sources recently detected by CHANDRA and of binary millisecond pulsars (MSPs) residing in the core of 47 Tuc. Unfortunately, the extreme faintness expected for the MSP companions together with the huge stellar crowding in the cluster center prevent statistically reliable identifications based only on positional coincidences.

Cygnus X-2, super-Eddington mass transfer, and pulsar binaries

Abstract: We consider the unusual evolutionary state of the secondary star in Cygnus X-2. Spectroscopic data give a low mass (M2 . 0:5 2 0:7 M() and yet a large radius (R2 . 7 R() and high luminosity (L2 . 150 L(). We show that this star closely resembles a remnant of early massive Case B evolution, during which the neutron star ejected most of the , 3 M( transferred from the donor (initial mass M2i , 3:6 M() on its thermal timescale , 10 yr. As the system is far too wide to result from common-envelope evolution, this strongly supports the idea that a neutron star efficiently ejects the excess inflow during super-Eddington mass transfer. Cygnus X-2 is unusual in having had an initial mass ratio qi ˆ M2i=M1 in a narrow critical range near qi . 2:6. Smaller qi lead to long-period systems with the former donor near the Hayashi line, and larger qi to pulsar binaries with shorter periods and relatively massive white dwarf companions. The latter naturally explain the surprisingly large companion masses in several millisecond pulsar binaries. Systems like Cygnus X-2 may thus be an important channel for forming pulsar binaries. Key words: binaries: close ± stars: evolution ± stars: individual: Cygnus X-2 ± pulsars: general ± X-rays: stars. 1 I N T R O D U C T I O N Cygnus X-2 is a persistent X-ray binary with a long orbital period (P ˆ 9:84 d; Cowley, Crampton & Hutchings 1979). The observation of unambiguous type I X-ray bursts (Smale 1998) shows that the accreting component is a neutron star rather than a black hole. The precise spectroscopic information found by Casares, Charles & Kuulkers (1998), and the parameters that can be derived from it, are summarized in Table 1. The mass ratio q ˆ M2=M1 . 0:34 implies that mass transfer widens the system, and is therefore probably driven by expansion of the secondary star. Normally in long-period low-mass X-ray binaries (LMXBs) this occurs because of the nuclear evolution of a subgiant secondary along the Hayashi line, with typical effective temperatures Teff;2 . 3000±4000 K. However, Casares et al.'s observations show that this cannot be the case for Cygnus X-2. The secondary is in the Hertzsprung gap (spectral type A9 III): use of Roche geometry and the Stefan±Boltzmann law gives L2 . 150 L( with Teff;2 . 7330 K (see Table 1). Moreover, the mass ratio q . 0:34, and the assumption that the primary is a neutron star and thus obeys M1 & 2 M(; implies that the secondary has a low mass (M2 ˆ qM1 & 0:68 M(). In contrast, an isolated A9 III star would have a mass of about 4 M(. More recently, Orosz & Kuulkers (1999) have modelled the ellipsoidal variations of the secondary and thereby derived a model-dependent inclination of i ˆ 628: 5 ^ 48 which translates into component masses …M1 ˆ 1:78 ^ 0:23†M( and (M2 ˆ 0:60 ^ 0:13†M(: In this paper we consider explanations for the unusual nature of the secondary in Cygnus X-2. We find only one viable possibility, namely that this star is currently close to the end of early massive Case B mass transfer, and thus that the neutron star has somehow managed to reject most of the mass (, 3 M() transferred to it in the past. In support of this idea, we show that this type of evolution naturally explains the surprisingly large companion masses in several millisecond pulsar binaries. 2 M O D E L S F O R C Y G N U S X 2 In this section we consider four possible explanations for the unusual nature of the secondary in Cygnus X-2. We shall find that three of them are untenable, and thus concentrate on the fourth possibility. 2.1 A normal star at the onset of Case B mass transfer? The simplest explanation is that the position of the secondary in the Hertzsprung±Russell (HR) diagram is just that of a normal star crossing the Hertzsprung gap. Because such a star no longer burns hydrogen in the core, this is a massive Case B mass transfer as defined by Kippenhahn & Weigert (1967, hereafter KW). Provided that the initial mass ratio qi & 1 the binary always expands on mass transfer, which occurs on a thermal time-scale. Kolb (1998) investigated this type of evolution systematically and

Post-common envelope binaries from SDSS - VII: A catalogue of white dwarf-main sequence binaries

Abstract: We present a catalogue of 1602 white dwarf-main sequence binaries (WDMS) from the spectroscopic Sloan Digital Sky Survey Data Release 6 (SDSS DR6). Among these we identify 440 as new WDMS binaries. We select WDMS binary candidates from template fitting all 1.27 million DR6 spectra, using combined constraints in both $\chi^{2}$ and signal-to-noise ratio. In addition, we use Galaxy Evolution Explorer (GALEX) and UKIRT Infrared Sky Survey (UKIDSS) magnitudes to search for objects in which one of the two components dominates the SDSS spectrum. We use a decomposition/fitting technique to measure the effective temperatures, surface gravities, masses and distances to the white dwarfs, as well as the spectral types and distances to the companions in our catalogue. Distributions and density maps obtained from these stellar parameters are then used to study both the general properties and the selection effects of WDMS binaries in SDSS. We also make use of SDSS-GALEX-UKIDSS magnitudes to investigate the distribution of WDMS binaries, as well as their white dwarf effective temperatures and companion star spectral types, in ultraviolet to infrared colour space. We show that WDMS binaries can be very efficiently separated from single main sequence stars and white dwarfs when using a combined ultraviolet, optical, and infrared colour selection. Finally, we also provide radial velocities for 1068 systems measured from the \Lines{Na}{I}{8183.27,8194.81} absorption doublet and/or the H$\alpha$ emission line. Among the systems with multiple SDSS spectroscopy, we find five new systems exhibiting significant radial velocity variations, identifying them as post-common-envelope binary candidates.