Hard X-ray Emission Associated with White Dwarfs. IV. Signs of Accretion from Sub-stellar Companions

KPD 0005+5106, with an effective temperature of $\simeq$200,000 K, is one of the hottest white dwarfs (WDs). ROSAT unexpectedly detected "hard" ($\sim$1 keV) X-rays from this apparently single WD. We have obtained Chandra observations that confirm the spatial coincidence of this hard X-ray source with KPD 0005+5106. We have also obtained XMM-Newton observations of KPD 0005+5106, as well as PG 1159$-$035 and WD 0121$-$756, which are also apparently single and whose hard X-rays were detected by ROSAT at 3$\sigma$-4$\sigma$ levels. The XMM-Newton spectra of the three WDs show remarkably similar shapes that can be fitted by models including a blackbody component for the stellar photospheric emission, a thermal plasma emission component, and a power-law component. Their X-ray luminosities in the $0.6-3.0$ keV band range from $4\times10^{29}$ to $4\times10^{30}$ erg~s$^{-1}$. The XMM-Newton EPIC-pn soft-band ($0.3-0.5$ keV) lightcurve of KPD 0005+5106 is essentially constant, but the hard-band ($0.6-3.0$ keV) lightcurve shows periodic variations. An analysis of the generalized Lomb-Scargle periodograms for the XMM-Newton and Chandra hard-band lightcurves finds a convincing modulation (false alarm probability of 0.41%) with a period of 4.7$\pm$0.3 hr. Assuming that this period corresponds to a binary orbital period, the Roche radii of three viable types of companion have been calculated: M9V star, T brown dwarf, and Jupiter-like planet. Only the planet has a size larger than its Roche radius, although the M9V star and T brown dwarf may be heated by the WD and inflate past the Roche radius. Thus, all three types of companion may be donors to fuel accretion-powered hard X-ray emission.Comment: Accepted for publication for The Astrophysical Journa

Spitzer 24 um Survey for Dust Disks around Hot White Dwarfs

Two types of dust disks around white dwarfs (WDs) have been reported: small dust disks around cool metal-rich WDs consisting of tidally disrupted asteroids, and a large dust disk around the hot central WD of the Helix planetary nebula (PN) possibly produced by collisions among Kuiper Belt-like objects. To search for more dust disks of the latter type, we have conducted a Spitzer MIPS 24 um survey of 71 hot WDs or pre-WDs, among which 35 are central stars of PNe (CSPNs). Nine of these evolved stars are detected and their 24 um flux densities are at least two orders of magnitude higher than their expected photospheric emission. Considering the bias against detection of distant objects, the 24 um detection rate for the sample is >~15%. It is striking that seven, or ~20%, of the WD and pre-WDs in known PNe exhibit 24 um excesses, while two, or 5-6%, of the WDs not in PNe show 24 um excesses and they have the lowest 24 um flux densities. We have obtained follow-up Spitzer IRS spectra for five objects. Four show clear continuum emission at 24 um, and one is overwhelmed by a bright neighboring star but still show a hint of continuum emission. In the cases of WD 0950+139 and CSPN K1-22, a late-type companion is present, making it difficult to determine whether the excess 24 um emission is associated with the WD or its red companion. High-resolution images in the mid-IR are needed to establish unambiguously the stars responsible for the 24 um excesses.Comment: 45 pages, 18 figures, 6 tables, accepted for publication in the September 2011 edition of the Astronomical Journa

Spitzer 24 um Images of Planetary Nebulae

Spitzer MIPS 24 um images were obtained for 36 Galactic planetary nebulae (PNe) whose central stars are hot white dwarfs (WDs) or pre-WDs with effective temperatures of ~100,000 K or higher. Diffuse 24 um emission is detected in 28 of these PNe. The eight non-detections are angularly large PNe with very low H-alpha surface brightnesses. We find three types of correspondence between the 24 um emission and H-alpha line emission of these PNe: six show 24 um emission more extended than H-alpha emission, nine have a similar extent at 24 um and H-alpha, and 13 show diffuse 24 um emission near the center of the H-alpha shell. The sizes and surface brightnesses of these three groups of PNe and the non-detections suggest an evolutionary sequence, with the youngest ones being brightest and the most evolved ones undetected. The 24 um band emission from these PNe is attributed to [O IV] 25.9 um and [Ne V] 24.3 um line emission and dust continuum emission, but the relative contributions of these three components depend on the temperature of the central star and the distribution of gas and dust in the nebula.Comment: 24 pages, 8 figures, to appear in the Astronomical Journal, September issue. Relace previous file; two references are added and typos are correcte

Dust disks around hot white dwarfs and central stars of planetary nebulae

Two types of dust disks have been discovered around white dwarfs (WDs): small dust disks within the Roche limits of their WDs, and a large dust disk around the hot central WD of the Helix planetary nebula (PN), possibly produced by collisions among Kuiper Belt-like objects.To search for more dust disks of the latter type, we have carried out a Spitzer MIPS 24 um survey of 71 hot WDs or pre-WDs, and found nine WDs with excess 24 um emission, seven of which are still central stars of planetary nebulae (CSPNs). We have therefore used archival Spitzer IRAC and MIPS observations of PNe to search for CSPNs with excess IR emission, and found five additional IR excesses likely originating from dust disks. For some of these CSPNs, we have acquired follow-up Spitzer MIPS images and IRS spectra, and Gemini NIRI and Michelle spectroscopic observations. The spectral energy distributions show great diversity in the emission characteristics of the IR excesses, which may imply different mechanisms responsible for the excess emission. The two most likely dust production mechanisms are: (1) breakup of bodies in planetesimal belts through collisions, (2) formation of circumstellar dust disks through binary interactions. In addition, we have derived basic dust disk parameters using simple blackbody approximations, or optically thin dust disk models with realistic grain and disk properties. The dust disk physical parameters for CSPNs without near-IR excesses appear consistent with the origin as collisionally disrupted planetesimals. The dust disks around CSPNs with near-IR excesses are likely optically thick, and possibly descended from binary post-AGB stars. The Helix Nebula's CSPN is also associated with a hard X-ray point source, whose origin is not known. We have correlated the Galactic WD catalog with the XMM-Newton and ROSAT point source catalogs to search for more single WDs with hard X-ray emission. Apart from the central WD of the Helix Nebula, none of the single WDs with hard X-ray emission are known to have excess IR emission. A better understanding of post-AGB binary evolution as well as debris disk evolution along with its parent star is needed to distinguish between these different origins.Future observations to better establish the physical parameters of the dust disks and the presence of companions are needed for models to discern between the possible dust production mechanisms

24μm excesses of hot WDs - evidence of dust disks?

Spitzer Space Telescope observations of the Helix Nebula's hot (T eff 110 000 K) central star revealed mid-IR excess emission consistent with a continuum emission from a dust disk located at 35-150 AU from the central white dwarf (WD), and the dust is most likely produced by collisions among Kuiper Belt-like objects (Su et al. 2007). To determine how common such dust disks are, we have carried out a Spitzer 24 μm survey of 72 hot WDs, and detected at least 7 WDs that exhibit clear IR excess, all of them still surrounded by planetary nebulae (PNe). Inspired by the prevalence of PN environment for hot WDs showing IR excesses, we have surveyed the Spitzer archive for more central stars of PN (CSPNs) with IR excesses; the search yields four cases in which CSPNs show excesses in 3.6-8.0 μm, and one additional case of 24 μm excess. We present the results of these two searches for dust-disk candidates, and discuss scenarios other than KBO collisions that need to be considered in explaining the observed near and/or mid-IR excess emission. These scenarios include unresolved companions, binary post-AGB evolution, and unresolved compact nebulosity. We describe planned follow-up observations aiming to help us distinguish between different origins of observed IR excesses.9 page(s