Spitzer Surveys of IR Excesses of WDs Y.-H. Chu 1, R.A. Gruendl 1, J. Bilikova 1, A. Riddle 1, K. Su 2 1 Univ. of Illinois, 2 Univ. of Arizona

Outline  Where do we expect to see dust ?  What wavelengths should we use ?  What are the survey results ?  What can we do in the future ? IR excesses can be used to diagnose low-mass companions, planets, and dust.

Stellar Evolution  White Dwarf Steffen & Schönberner (2006)

Young WDs are hot and are surrounded by visible planetary nebulae. Stellar Evolution  White Dwarf

Star loses 50-80% of its mass  Planetary system expands Dynamical rearrangement produces dust by 1. Tidal crushing of sub-planetary objects 1. Tidal crushing of sub-planetary objects: too close to star 2. Collisions among sub-planetary objects 2. Collisions among sub-planetary objects: resonance with giant planets raises density and collision rate

Where do we expect to find dust around WDs? 1. Within the Roche Limit (< 0.01 AU) 2. Asteroid Belt at 3-5 AU 3. Kuiper Belt at AU

Where do we expect to find dust around WDs? 1. Within the Roche Limit (< 0.01 AU) 2. Asteroid Belt at 3-5 AU 3. Kuiper Belt at AU Thermal equilibrium Energy absorbed = Energy emitted 4  R * 2  T * 4 (  r g 2 / 4  D 2 ) = 4  r g 2  T g 4 T g = T * ( R  / 2D ) 0.5 max ~ 2900  m / T g Sublimation temperature ~ 1500 K Thermal equilibrium Energy absorbed = Energy emitted 4  R * 2  T * 4 (  r g 2 / 4  D 2 ) = 4  r g 2  T g 4 T g = T * ( R  / 2D ) 0.5 max ~ 2900  m / T g Sublimation temperature ~ 1500 K

Where do we expect to find dust around WDs? 1. Within the Roche Limit (< 0.01 AU) 2. Asteroid Belt at 3-5 AU 3. Kuiper Belt at AU Spitzer IRAC can detect these around WDs <20,000 K

Where do we expect to find dust around WDs? 1. Within the Roche Limit (< 0.01 AU) 2. Asteroid Belt at 3-5 AU 3. Kuiper Belt at AU Spitzer MIPS can detect these around WDs ~50,000 K

Where do we expect to find dust around WDs? 1. Within the Roche Limit (< 0.01 AU) 2. Asteroid Belt at 3-5 AU 3. Kuiper Belt at AU Spitzer MIPS can detect these around hot WDs ~ 100,000 K

Spitzer Observations of WDs  Broad surveys - Kuchner, Kilic, Farihi, Chu - about 310 WDs  Metal-rich WDs (DAZ, DBZ, DZ) - Jura, Farihi, Burleigh, Debes, Kilic - about WDs  WDs from binary mergers - Hansen - 14 WDs  Spitzer Archive – serendipitous IRAC obs - about 330 WDs observed, 130 detected

(serendipitously detected) No Spitzer surveys are designed to detect dust at Asteroid Belt. to detect dust at Asteroid Belt.

Survey Results  Broad IRAC surveys are not successful in detecting planets or dust disks  Surveys using DAZ, DBZ, etc are more successful in finding dust disks corresponding to crushed asteroids  Archival IRAC data detected ~130 WDs, among which ~10% show IR excesses. Companions, dust disks, mis-matches...  MIPS 24 μm survey of hot WDs 71 targets, 9 detections

SED  ~100 K Too cold to be a star. Emitting area ~ 8.5 AU 2 Only a dust disk can do it. Distance ~ AU Corresponding to Kuiper Belt. (Su et al. 2007) 24 μm IRAC Helix Nebula

F (erg cm -2 s -1 ) (  m) Hot WDs with 24  m Excesses

F (erg cm -2 s -1 ) (  m) Hot WDs with 24  m Excesses

About 13% of the sample show 24 μm excesses. Considering distance effects, more than 15% show 24 μm excesses.

Spitzer MIPS 24 μm Survey of Hot WDs WD Name PN T eff (kK) F 24 (mJy) L IR /L * K1-22 K E-5 NGC 2438 NGC E-4 WD EGB E-5 WD E-6 WD Sh E-5 WD Sh E-6 WD Abell E-5 WD EGB E-4 WD E-5 WD Helix E-4 DA DOZ DAO DA PG1159 DA DAO

 Collisions among KBOs (We like this)  Binary evolution (Circum-binary dust disk; EGB 6)  Compact (H-poor) nebula in born-again PNe (Remember Bond’s Abell 58) Possible Origins of 24 μm Excesses

If the WDs are cooler, the dust disks would be colder and need to be surveyed at longer wavelengths. Herschel Space Observatory PACS: or , μm SPIRE: 250, 350, 500 μm It would not be wise not to use WISE…

These WDs are good targets for Herschel search for Kuiper Belts, WISE search for Asteroid Belts.

Summary  Two types of dust disks: - small, < 0.01 AU, crushed asteroids - large, > 30 AU, colliding KBOs?  More combinations of WD T eff - survey λ are needed to probe the planetary systems  Archives provide wonderful sources of data  Spectroscopic observations are needed, and spectral analyses are essential  Bilikova’s talk