S. A. Stern 1 , D. D. Durda 1,2 , A. Steffl 2 , D. Hassler 2 , and N. Cunningham 3

1. New Constraints from STEREO on the Population of Vulcanoids Interior to Mercury S. A. Stern1, D. D. Durda1,2, A. Steffl2, D. Hassler2, and N. Cunningham3 1NASA Headquarters, 2SwRI, 3U. Nebraska Presentation 50.04

2. The Vulcanoid Population ~4-12° solar elongation • Intrinsically interesting new class of objects • Sample of condensed material from the early inner solar system • Relevance to Mercury’s cratering record and chronology Origin/sources of Vulcanoid material: • Left over planetesimals; high-temperature end of condensation sequence • Debris from giant impact that might have stripped away Mercury’s rocky mantle Removal/loss of Vulcanoid material: • Evaporation • P-R drag, Yarkovsky Effect • Collisions • Gravitational perturbations

3. Previous Searches • Ground-based • Perrine (1902, 1906, 1909) • Campbell and Trumpler (1923) • Courten et al. (1976a,b) • Leake et al. (1987) • Campins et al. (1996) • High-Altitude Airborne • Durda and Stern (2001-2002) • Spacecraft (SOHO) • Durda et al. (2000) Most constraining previous search: No Vulcanoids brighter than V = 8.5 • Schumacher and Gay (2001)

4. Solar TErrestrial RElations Observatory The Heliospheric Imager (HI) instrument aboard the NASA STEREO spacecraft, launched on 25 October 25 2006, provides a new and sensitive opportunity to explore the inner heliosphere from the Sun’s inner corona and the region of space near the Sun. STEREO • HI-1 Instrument • Vulcanoid zone lies fully within HI-1 field of view • Ability to observe faint stellar sources (~40 times more sensitive than previous search with SOHO) • 0.6 arcmin pixels • Parallax due to spacecraft orbital motion moves Vulcanoids ~2.5 arcsec/min relative to background stars

5. HI Image Processing • Remove “hot” pixels: • Top three pixels discarded from each image set • Calculate median of remaining pixel values • Subtract resulting median image from individual images to remove fixed pattern noise • Remove background gradient: • At each pixel location calculate average of all pixel values in circular aperture between 5 and 12 pixels from central point • Subtract resulting image from individual images to remove gradient • Co-register and ‘blink’: • Use 10 stars common to all images in animation sequence as registration fiducials • Determine rotation, offset, and scaling necessary to align individual images with reference image

6. HI Image Processing • Remove “hot” pixels: • Top three pixels discarded from each image set • Calculate median of remaining pixel values • Subtract resulting median image from individual images to remove fixed pattern noise • Remove background gradient: • At each pixel location calculate average of all pixel values in circular aperture between 5 and 12 pixels from central point • Subtract resulting image from individual images to remove gradient • Co-register and ‘blink’: • Use 10 stars common to all images in animation sequence as registration fiducials • Determine rotation, offset, and scaling necessary to align individual images with reference image

7. Search Results stereo_a_hi1_feb7.mov

8. Search Results We examined five 48-hour sequences of images, spaced about 10 days apart (24 processed HI-1 images per sequence). Moving were objects located through visual examination of the five movie sequences and identified using commercial astronomical software and search tools on the MPC web site. Main-belt asteroids as faint as V  13.5 identified. Objects “discovered” during the search: • Planets (Mercury, Venus, Uranus, Neptune) • Asteroids (1 Ceres, 10 Hygiea, 29 Amphitrite, 241 Germania, 349 Dembowska, 385 Ilmatar, 444 Gyptis, 660 Crescentia, 678 Fredegundis) • Comet (C/2006 M4 SWAN) Average magnitude limit across search field: V 12.5. Assuming a Mercury-like albedo and phase function, this translates to D  6km Vulcanoids. No Vulcanoids found.

9. Extras