Spitzer 24 um Survey for Dust Disks around Hot White Dwarfs (original) (raw)
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Spitzer 24 μm survey for dust disks around hot white dwarfs
2011
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 µm 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 µm 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 µm 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 µm excesses, while two, or 5-6%, of the WDs not in PNe show 24 µm excesses and they have the lowest 24 µm flux densities. We have obtained follow-up Spitzer IRS spectra for five objects. Four show clear continuum emission at 24 µm, 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 K 1-22, a late-type companion is present, making it difficult to determine whether the excess 24 µm emission is associated with the
Spitzer 24 mum Survey for Dust Disks around Hot White Dwarfs
Astron J, 2011
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 μm 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 μm flux densities are at least two orders of magnitude higher than their expected photospheric emission. Considering the bias against the detection of distant objects, the 24 μm detection rate for the sample is gsim15%. It is striking that seven, or ~20%, of the WD and pre-WDs in known PNe exhibit 24 μm excesses, while two, or 5%-6%, of the WDs not in PNe show 24 μm excesses and they have the lowest 24 μm flux densities. We have obtained follow-up Spitzer Infrared Spectrograph spectra for five objects. Four show clear continuum emission at 24 μm, and one is overwhelmed by a bright neighboring star but still shows a hint of continuum emission. In the cases of WD 0950+139 and CSPN K 1-22, a late-type companion is present, making it difficult to determine whether the excess 24 μm emission is associated with the WD or its red companion. High-resolution images in the mid-infrared are needed to establish unambiguously the stars responsible for the 24 μm excesses.
A Spitzer Study of Dusty Disks around Nearby, Young Stars
The Astrophysical Journal, 2005
We have obtained Spitzer Space Telescope MIPS (Multiband Imaging Photometer for Spitzer) observations of 39 A-through M-type dwarfs, with estimated ages between 12 and 600 Myr; IRAC observations for a subset of 11 stars; and follow-up CSO SHARC II 350 m observations for a subset of two stars. None of the objects observed with IRAC possess infrared excesses at 3.6-8.0 m; however, seven objects observed with MIPS possess 24 and/or 70 m excesses. Four objects ( Phe, HD 92945, HD 119124, and AU Mic), with estimated ages 12-200 Myr, possess strong 70 m excesses, !100% larger than their predicted photospheres, and no 24 m excesses, suggesting that the dust grains in these systems are cold. One object (HD 112429) possesses moderate 24 and 70 m excesses with a color temperature, T gr ¼ 100 K. Two objects ( 1 Lib and HD 177724) possess such strong 24 m excesses that their 12, 24, and 70 m fluxes cannot be self-consistently modeled using a modified blackbody despite a 70 m excess >2 times greater than the photosphere around 1 Lib. The strong 24 m excesses may be the result of emission in spectral features, as observed toward the Hale-Bopp star HD 69830.
The Astrophysical Journal, 2009
We model the mineralogy and distribution of dust around the white dwarf G29-39 using the infrared spectrum from 1-35 µm. The spectral model for G29-38 dust combines a wide range of materials based on spectral studies of comets and debris disks. In order of their contribution to the mid-infrared emission, the most abundant minerals around G29-38 are amorphous carbon (λ < 8 µm), amorphous and crystalline silicates (5-40 µm), water ice (10-15 and 23-35 µm), and metal sulfides (18-28 µm). The amorphous C can be equivalently replaced by other materials (like metallic Fe) with featureless infrared spectra. The bestfitting crystalline silicate is Fe-rich pyroxene. In order to absorb enough starlight to power the observed emission, the disk must either be much thinner than the stellar radius (so that it can be heated from above and below) or it must have an opening angle wider than 2 • . A 'moderately optically thick' torus model fits well if the dust extends inward to 50 times the white dwarf radius, all grains hotter than 1100 K are vaporized, the optical depth from the star through the disk is τ = 5, and the radial density profile ∝ r −2.7 ; the total mass of this model disk is 2 × 10 19 g. A physically thin (less than the white dwarf radius) and optically -2thick disk can contribute to the near-infrared continuum only; such a disk cannot explain the longer-wavelength continuum or strong emission features. The combination of a physically thin, optically-thick inner disk and an outer, physically thick and moderately optically thin cloud or disk produces a reasonably good fit to the spectrum and requires only silicates in the outer cloud. We discuss the mineralogical results in comparison to planetary materials. The silicate composition contains minerals found from cometary spectra and meteorites, but Fe-rich pyroxene is more abundant than enstatite (Mg-rich pyroxene) or forsterite (Mg-rich olivine) in G29-38 dust, in contrast to what is found in most comet or meteorite mineralogies. Enstatite meteorites may be the most similar solar system materials to the G29-38 dust. Finally, we suggest the surviving core of a 'hot jupiter' as an alternative (neither cometary nor asteroidal) origin for the debris, though further theoretical work is needed to determine if this hypothesis is viable.
THE WIRED SURVEY. IV. NEW DUST DISKS FROM THE McCOOK & SION WHITE DWARF CATALOG
The Astrophysical Journal, 2013
We have compiled photometric data from the Wide-field Infrared Survey Explorer All Sky Survey and other archival sources for the more than 2200 objects in the original McCook & Sion Catalog of Spectroscopically Identified White Dwarfs. We applied color-selection criteria to identify 28 targets whose infrared spectral energy distributions depart from the expectation for the white dwarf photosphere alone. Seven of these are previously known white dwarfs with circumstellar dust disks, five are known central stars of planetary nebulae, and six were excluded for being known binaries or having possible contamination of their infrared photometry. We fit white dwarf models to the spectral energy distributions of the remaining ten targets, and find seven new candidates with infrared excess suggesting the presence of a circumstellar dust disk. We compare the model dust disk properties for these new candidates with a comprehensive compilation of previously published parameters for known white dwarfs with dust disks. It is possible that the current census of white dwarfs with dust disks that produce an excess detectable at K-band and shorter wavelengths is close to complete for the entire sample of known WDs to the detection limits of existing near-IR all-sky surveys. The white dwarf dust disk candidates now being found using longer wavelength infrared data are drawn from a previously underrepresented region of parameter space, in which the dust disks are overall cooler, narrower in radial extent, and/or contain fewer emitting grains.
New Debris Disks Around Young, Low-Mass Stars Discovered with the Spitzer Space Telescope
The Astrophysical Journal, 2009
We present 24 µm and 70 µm Multiband Imaging Photometer for Spitzer (MIPS) observations of 70 A through M-type dwarfs with estimated ages from 8 Myr to 1.1 Gyr, as part of a Spitzer guaranteed time program, including a re-analysis of some previously published source photometry. Our sample is selected from stars with common youth indicators such as lithium abundance, X-ray activity, chromospheric activity, and rapid rotation. We compare our MIPS observations to empirically derived K s -[24] colors as a function of the stellar effective temperature to identify 24 µm and 70 µm excesses. We place constraints or upper limits on dust temperatures and fractional infrared luminosities with a simple blackbody dust model. We confirm the previously published 70 µm excesses for HD 92945, HD 112429, and AU Mic, and provide updated flux density measurements for these sources. We present the discovery of 70 µm excesses for five stars: HD 7590, HD 10008, HD 59967, HD 73350, and HD 135599. HD 135599 is also a known Spitzer IRS (InfraRed Spectrograph) excess source, and we confirm the excess at 24 µm. We also present the detection of 24 µm excesses for 10 stars: HD 10008, GJ 3400A, HD 73350, HD 112429, HD 123998, HD 175742, AT Mic, BO Mic, HD 358623 and Gl 907.1. We find that large 70 µm excesses are less common around stars with effective temperatures of less than 5000 K (3.7 +7.6 −1.1 %) than around stars with effective temperatures between 5000 K and 6000 K (21.4 +9.5 −5.7 %), despite the cooler stars having a younger median age in our sample (12 Myr vs. 340 Myr). We find that the previously reported excess for TWA 13A at 70 µm is due to a nearby background galaxy, and the previously reported excess for HD 177724 is due to saturation of the near-infrared photometry used to predict the mid-infrared stellar flux contribution. In the Appendix, we present an updated analysis of dust grain removal timescales due to grain-grain collisions and radiation pressure, Poynting-Robertson (P-R) drag, stellar wind drag, and planet-dust dynamical interaction. We find that drag forces can be important for disk dynamics relative to grain-grain collisions for L IR /L * < 10 −4 , and that stellar wind drag is more important than P-R drag for K and M dwarfs, and possibly for young (<1 Gyr) G dwarfs as well.
DUSTY DISKS AROUND CENTRAL STARS OF PLANETARY NEBULAE
The Astronomical Journal, 2014
Only a few percent of cool, old white dwarfs (WDs) have infrared excesses interpreted as originating in small hot disks due to the infall and destruction of single asteroids that come within the star's Roche limit. Infrared excesses at 24 µm were also found to derive from the immediate vicinity of younger, hot WDs, most of which are still central stars of planetary nebulae (CSPN). The incidence of CSPN with this excess is 18%. The Helix CSPN, with a 24 µm excess, has been suggested to have a disk formed from collisions of Kuiper belt-like objects (KBOs). In this paper, we have analyzed an additional sample of CSPN to look for similar infrared excesses. These CSPN are all members of the PG 1159 class and were chosen because their immediate progenitors are known to often have dusty environments consistent with large dusty disks. We find that, overall, PG 1159 stars do not present such disks more often than other CSPN, although the statistics (5 objects) are poor. We then consider the entire sample of CSPN with infrared excesses, and compare it to the infrared properties of old WDs, as well as cooler post-AGB stars. We conclude with the suggestion that the infrared properties of CSPN more plausibly derive from AGB-formed disks rather than disks formed via the collision of KBOs, although the latter scenario cannot be ruled out. We finally remark that there seems to be an association between CSPN with a 24-µm excess and confirmed or possible binarity of the central star.
The frequency of gaseous debris discs around white dwarfs
Monthly Notices of the Royal Astronomical Society
1 – 3 per cent of white dwarfs are orbited by planetary dusty debris detectable as infrared emission in excess above the white dwarf flux. In a rare subset of these systems, a gaseous disc component is also detected via emission lines of the Ca ii 8600 Å triplet, broadened by the Keplerian velocity of the disc. We present the first statistical study of the fraction of debris discs containing detectable amounts of gas in emission at white dwarfs within a magnitude and signal-to-noise limited sample. We select 7705 single white dwarfs spectroscopically observed by the Sloan Digital Sky Survey (SDSS) and Gaia with magnitudes g ≤ 19. We identify five gaseous disc hosts, all of which have been previously discovered. We calculate the occurrence rate of a white dwarf hosting a debris disc with a detectable via Ca ii emission lines as 0.067 ± 0.0250.042_{0.025}^{0.042}0.0250.042 per cent. This corresponds to an occurrence rate for a dusty debris disc to have an observable gaseous component in emission as 4 ±...
DUst around NEarby Stars. The survey observational results
Astronomy & Astrophysics, 2013
Context. Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their solar system's counterparts are the asteroid and Edgeworth-Kuiper belts. Aims. The DUNES survey aims at detecting extra-solar analogues to the Edgeworth-Kuiper belt around solar-type stars, putting in this way the solar system into context. The survey allows us to address some questions related to the prevalence and properties of planetesimal systems. Methods. We used Herschel/PACS to observe a sample of nearby FGK stars. Data at 100 and 160 µm were obtained, complemented in some cases with observations at 70 µm, and at 250, 350 and 500 µm using SPIRE. The observing strategy was to integrate as deep as possible at 100 µm to detect the stellar photosphere.
2008
We model the mineralogy and distribution of dust around the white dwarf G29-39 using the infrared spectrum from 1-35 microns and combining a wide range of materials based on spectral studies of comets and debris disks. In order of their contribution to the mid-infrared emission, the most abundant minerals for G29-38 are amorphous carbon, amorphous and crystalline silicates, water ice, and metal sulfides. The amorphous C can be equivalently replaced by other materials (like metallic Fe) with featureless infrared spectra. The best-fitting crystalline silicate is Fe-rich pyroxene. In order to absorb enough starlight to power the observed emission, the disk must either be much thinner than the stellar radius (so that it can be heated from above and below) or it must have an opening angle wider than 2 degrees. A `moderately optically thick' torus model with mass 2x10^19 g fits the spectrum well. A physically thin (less than the white dwarf radius) and optically thick disk can contribute to the near-infrared continuum only; such a disk cannot explain the longer-wavelength continuum or strong silicate features. The silicate composition contains minerals found from cometary spectra and meteorites, but Fe-rich pyroxene is more abundant than enstatite (Mg-rich pyroxene) or forsterite (Mg-rich olivine) in G29-38, in contrast to what is found in most comet or meteorite mineralogies. Enstatite meteorites may be the most similar solar system materials to the G29-38 dust. Finally, we suggest the surviving core of a `hot jupiter' as an alternative (neither cometary nor asteroidal) origin for the debris, though further theoretical work is needed to determine if this hypothesis is viable.