Infrared Signatures of Disrupted Minor Planets at White Dwarfs

📝 Abstract

Spitzer Space Observatory IRAC and MIPS photometric observations are presented for 20 white dwarfs with T < 20,000 K and metal-contaminated photospheres. A warm circumstellar disk is detected at GD 16 and likely at PG 1457-086, while the remaining targets fail to reveal mid-infrared excess typical of dust disks, including a number of heavily polluted stars. Extending previous studies, over 50% of all single white dwarfs with implied metal accretion rates dM/dt > 3e8 g/s display a warm infrared excess from orbiting dust; the likely result of a tidally-destroyed minor planet. This benchmark accretion rate lies between the dust production rates of 1e6 g/s in the solar system zodiacal cloud and 1e10 g/s often inferred for debris disks at main sequence A-type stars. It is estimated that between 1% and 3% of all single white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar dust, signifying an underlying population of minor planets.

💡 Analysis

Spitzer Space Observatory IRAC and MIPS photometric observations are presented for 20 white dwarfs with T < 20,000 K and metal-contaminated photospheres. A warm circumstellar disk is detected at GD 16 and likely at PG 1457-086, while the remaining targets fail to reveal mid-infrared excess typical of dust disks, including a number of heavily polluted stars. Extending previous studies, over 50% of all single white dwarfs with implied metal accretion rates dM/dt > 3e8 g/s display a warm infrared excess from orbiting dust; the likely result of a tidally-destroyed minor planet. This benchmark accretion rate lies between the dust production rates of 1e6 g/s in the solar system zodiacal cloud and 1e10 g/s often inferred for debris disks at main sequence A-type stars. It is estimated that between 1% and 3% of all single white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar dust, signifying an underlying population of minor planets.

📄 Content

arXiv:0901.0973v2 [astro-ph.EP] 10 Mar 2009 INFRARED SIGNATURES OF DISRUPTED MINOR PLANETS AT WHITE DWARFS J. Farihi1,2,3, M. Jura2, and B. Zuckerman2 ABSTRACT Spitzer Space Observatory IRAC and MIPS photometric observations are pre- sented for 20 white dwarfs with Teff≲20, 000 K and metal-contaminated pho- tospheres. A warm circumstellar disk is detected at GD 16 and likely at PG 1457−086, while the remaining targets fail to reveal mid-infrared excess typical of dust disks, including a number of heavily polluted stars. Extending previous studies, over 50% of all single white dwarfs with implied metal accretion rates dM/dt ≳3 × 108 g s−1 display a warm infrared excess from orbiting dust; the likely result of a tidally-destroyed minor planet. This benchmark accretion rate lies between the dust production rates of 106 g s−1 in the solar system zodiacal cloud and 1010 g s−1 often inferred for debris disks at main sequence A-type stars. It is estimated that between 1% and 3% of all single white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar dust, signifying an underlying population of minor planets. Subject headings: circumstellar matter— infrared: stars– minor planets, asteroids— planetary systems – stars: abundances— stars: evolution— stars: individual (GD 16, PG 1457−086)— stars: low-mass, brown dwarfs — white dwarfs 1. INTRODUCTION Known extrasolar planetary systems orbiting main sequence stars consist of a few large planets such as Jupiter (Cumming et al. 2008), and/or, as demonstrated by studies of debris disks, numerous minor planets analogous to solar system asteroids and Kuiper belt objects 1Department of Physics & Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK; jf123@star.le.ac.uk 2Department of Physics & Astronomy, University of California, 430 Portola Plaza, Los Angeles, CA 90095; jura,ben@astro.ucla.edu 3Gemini Observatory, Northern Operations, 670 North A’ohoku Place, Hilo, HI 96720 – 2 – (Zuckerman 2001). Apparently, the assembly of planets from planetesimals is inefficient, and stars possess complex populations of orbiting material (see Ida & Lin 2008 and references therein). Relative to main sequence stars, white dwarfs offer two advantages for the study of extrasolar planetary systems. First, white dwarfs are earth-sized and their low luminosi- ties permit the direct detection of infrared emission from cool self-luminous companions such as brown dwarfs and massive jovian planets (Farihi et al. 2008a; Burleigh et al. 2006; Farihi et al. 2005a; Zuckerman & Becklin 1987a; Probst 1983). Second, cool white dwarfs should be atmospherically free of heavy elements (Zuckerman et al. 2003; Alcock et al. 1986; Paquette et al. 1986), and those stars with planetary system remnants can become spectro- scopically contaminated by small, but detectable, amounts of accreted material. Analysis of metal-polluted white dwarfs enables an indirect, yet detailed and powerful compositional analysis of extrasolar planetary matter; Zuckerman et al. (2007) found that the abundances in the spectacularly metal-rich white dwarf GD 362 are consistent with the accretion of a large asteroid with composition similar to the Earth-Moon system. The most metal-contaminated white dwarfs often display evidence of circumstellar disks; either by infrared excess (Farihi et al. 2008b; Kilic & Redfield 2007; von Hippel et al. 2007; Jura et al. 2007a; Kilic et al. 2006; Becklin et al. 2005; Kilic et al. 2005; Graham et al. 1990; Zuckerman & Becklin 1987b), or by broad, double-peaked optical emission lines (G¨ansicke et al. 2008, 2007, 2006), or both (Melis et al. 2008; Brinkworth et al. 2008). Evidence is strong that these disks evolve from the tidal disruption of minor planets (Jura 2008, 2003). To be tidally destroyed within the Roche limit of a white dwarf, an asteroid needs to be perturbed from its orbit, and hence unseen planets of conventional size are expected at white dwarfs with dusty disks. Including the disks around GD 16 and PG 1457−086, which are reported in this paper, the number of white dwarfs with circumstellar disks is 14 (see Table 1). Although at least half a dozen publications present Spitzer observations of white dwarfs, there has not yet been a thorough search of metal-rich white dwarfs for cool dust at longer, MIPS wavelengths, nor a focus on those contaminated degenerates with helium-rich atmospheres; this study bridges that gap and analyzes all 53 metal-rich degenerates observed by Spitzer. The numbers are now large enough that one can investigate the presence of a disk as a function of cooling age and metal accretion rate. This paper presents the results of an IRAC 3 −8 µm and MIPS 24 µm photometric search for mid-infrared excess due to circumstellar dust at cool, metal-contaminated white dwarfs. The goals of the study are: 1) to constrain the frequency of dust disks around white dwarfs as a function of cooling age; and 2) to combine all available Spitzer data on

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