A search for Vulcanoids with the STEREO Heliospheric Imager
Highlights
► Vulcanoids are a hypothesized population of asteroids interior to Mercury’s orbit. ► We searched for Vulcanoids using data from the Heliospheric Imager on STEREO-A. ► No Vulcanoids were found. ► We place a 3σ upper limit of 5.6 km on the size of any undetected Vulcanoids. ► There are presently no more than 76 Vulcanoids larger than 1 km.
Introduction
Interior to Mercury’s orbit is a dynamically stable region where a population of small, asteroid-like bodies called Vulcanoids has long been hypothesized to exist, cf. review by Campins et al. (1996). This region, known as the Vulcanoid zone, extends from roughly 0.07 AU to 0.21 AU (15–45 solar radii). Seen from Earth, objects in the Vulcanoid zone have maximum solar elongation angles of just 4–12°. The outer boundary of the Vulcanoid zone, at 0.21 AU, is set by dynamical instability. Objects with semi-major axes greater than this limit evolve onto Mercury-crossing orbits on 100-Myr timescales due to orbital perturbations caused by both Mercury and Venus (Evans and Tabachnik, 1999, Evans and Tabachnik, 2002). The inner edge of the Vulcanoid zone is less well-defined, but is set by the combination of the intense thermal environment and dynamical transport mechanisms such as Poynting–Robertson drag and the Yarkovsky effect. At 0.06 AU (13 solar radii) solar radiation is so intense that even a pure iron body 100 km in diameter will evaporate in less than 4.5 Gyr (Lebofsky, 1975, Campins et al., 1996). The time an object can survive against evaporation is a strong function of heliocentric distance, such that, at 0.07 AU, a pure iron body of just 2 km diameter will survive for the current age of the Solar System. Poynting–Robertson drag extends the evaporation limit outward, as it can move a 2-km diameter object with ρ = 4 g cm−3 from 0.08 to 007 AU in 4 Gyr, where it will evaporate (Stern and Durda, 2000).
The detection of one or more members of the putative Vulcanoid population is of interest as it would represent the discovery of a whole new class of Solar System objects. If they existed at all, primordial Vulcanoids likely formed from the highest temperature condensates near the inner edge of the solar nebula, and they presumably would contain unique, highly refractory, chemical signatures as a result. In addition, a primordial Vulcanoid population might have affected Mercury’s surface chronology. Based on the size–frequency distribution of craters on the Moon, Mars, Venus, and Mercury, it is thought that objects in the inner Solar System were resurfaced during the period of the Late Heavy Bombardment (LHB) 3.9 Gyr ago (Strom et al., 2005). Vulcanoids removed from stable orbits in the Vulcanoid zone by non-gravitational forces like the Yarkovsky effect (Vokrouhlický, 1999, Vokrouhlický et al., 2000) could have supplied a significant impactor population to Mercury after the LHB, making the surface appear older (Leake et al., 1987, Head et al., 2007).
Le Verrier (1859) first proposed that a small planet, or collection of planets, interior to Mercury could explain the observed precession of Mercury’s orbit. This hypothesized planet was eventually given the name “Vulcan”, and numerous searches were conducted in an attempt to find it. However, the proximity of intramercurial objects to the Sun makes them difficult to observe from the Earth. To overcome the observational challenge of looking for a faint object against a bright twilight sky, many of the early searches for the planet Vulcan were conducted in the fleeting minutes of totality during solar eclipses. The first searches for Vulcan were done visually, with obvious limitations. The first use of photographic plates to search for an intra-Mercurial planet was by Perrine (1902), during the total solar eclipse of May 18, 1901, on the island of Sumatra. After analyzing the plates from this expedition, Perrine placed a limit of magnitude 5.0 (photographic) on the brightness of any planet interior to Mercury and concluded that, “… there are probably no bodies of appreciable size in the region close about the Sun, and that the cause of the disturbance in the motion of Mercury must be sought elsewhere.” Subsequent observations during solar eclipses resulted in an improved limiting magnitude of 8.0 (Perrine and Campbell, 1907, Perrine, 1909).
In 1915, Einstein (1915) showed that the precession of Mercury’s orbit could be explained entirely by the then new theory of general relativity, thus eliminating the dynamical need for a massive planet Vulcan. However, the question of whether there are any small bodies interior to Mercury’s orbit remained unresolved. An archival search by Campbell and Trumpler (1923) using photographic plates obtained during the solar eclipse of 1922 for the purpose of measuring the deflection of starlight predicted by general relativity provided the tightest constraint of all the early searches: a photographic magnitude of 8.5.
More recently, Courten et al. (1976), summarizing a 10-year campaign to observe comets during solar eclipses, reported, “… data which indicate the possible existence of one or more relatively faint objects within 20 solar radii… and [ranging] from +9 to +7 in equivalent visual magnitude”. Unfortunately, the nature of observing during total solar eclipses precluded any direct follow-up observations of these possible detections. Since these results remain unpublished and have not been independently confirmed, it is difficult to assess whether the claimed detections are real, and, if they are, whether the objects are Vulcanoids, sungrazing comets (cf. Biesecker et al., 2002), or some other type of body.
In contrast to all other published Vulcanoid searches, Leake et al. (1987) conducted a search for Vulcanoids between 1979 and 1981 at 3.5 μm. By virtue of operating in the thermal infrared, this search was more sensitive to objects with low visual albedo. Leake et al. estimated a detection probability of 75% for an object with an L-band magnitude of 5, corresponding to an object diameter of 40–50 km. However, bad weather and the small field of view (FOV) of their instrument limited their search to a total of 5.8 deg2 within 1° of the ecliptic—a small fraction of the Vulcanoid zone, as seen from Earth.
Prior to our work, the most complete search, in terms of depth and coverage, was conducted by Durda et al. (2000), using data from the LASCO C3 coronagraph on the SOHO spacecraft (Brueckner et al., 1995), which images a region from 0.02 to 0.14 AU (3.7–30 solar radii). Durda et al. examined a 40-day sequence of LASCO C3 images. Except for objects at the outer edge of the Vulcanoid zone with inclination >25°, all dynamically stable Vulcanoids should have passed through the instrument FOV during this period. No Vulcanoids were found, down to detection limit of V = 8.0. For objects with a Mercury-like albedo and phase function (Veverka et al., 1988), this limit corresponds to a diameter of 22 and 65 km for objects at the inner and outer edges of the Vulcanoid zone (or 36–106 km for an albedo of 0.05). Working independently, Schumacher and Gay (2001) also did not detect any Vulcanoids in LASCO C3 images down to a limiting magnitude of V = 7.
Subsequently, Durda et al. (2003) conducted a Vulcanoid search using a visible wavelength imaging system flown aboard NASA F/A-18B aircraft at an altitude of 49,000 feet. However, they were unable to improve upon the earlier Durda et al. (2000) results.
Merline et al. (2008) report preliminary results from their search for Vulcanoids using the Wide Angle Camera (WAC) of the MESSENGER spacecraft’s Mercury Dual Imaging System (MDIS), while the spacecraft was in cruise to Mercury. Spacecraft pointing restrictions limited observations to those with solar elongation >30°, i.e., the outer 45% of the Vulcanoid zone. No Vulcanoids were detected, down to a limiting magnitude of V = 8, corresponding to an Vulcanoid diameter of 15 km. Subsequent analysis of these observations has brought this size limit down to 5 km, comparable to the results of our search (W.J. Merline, private communication, 2012).
Finally, Zhao et al. (2009) report a Vulcanoid search using 15 cm telescopes equipped with CCDs at two separate observatories in China during the 2008 total solar eclipse. They found “three unidentified star-like objects” in images from both telescopes, but the relative motion of these objects did not match that of a Vulcanoid. Both the angular size and sensitivity of this search are unclear, although they report that stars as faint as V = 12.8 were detected.
Section snippets
HI-1 data and processing
For our search, we used archival data from the Heliospheric Imager (HI) instrument on NASAs STEREO spacecraft, available online from the STEREO Science Center (http://stereo-ssc.nascom.nasa.gov). The Solar TErrestrial RElations Observatory (STEREO) mission is designed to study coronal mass ejections (CMEs) from the Sun out to the orbit of the Earth and consists of two nearly identical spacecraft. STEREO-A orbits the Sun slightly interior to Earth’s orbit, while the other spacecraft, STEREO-B,
Search technique
After processing the data and adding synthetic Vulcanoids as described above, we created movies from the images obtained during a 2-day period. We then looped this movie back and forth, at variable frame rates, while visually searching for moving objects. When an object was found, the observer marked its position in one or more frames of the movie. This technique takes advantage of the ability of the human eye to detect motion; very often an object that was easily identifiable while cycling
Results
No Vulcanoids were detected in our search. Although we did not detect any Vulcanoids, we observed a variety of other Solar System objects: the planets Mercury, Venus, Uranus, and Neptune; the Comet 67P/Churyumov–Gerasimenko; more than 30 Kruetz-family sungrazing comets (Biesecker et al., 2002); the non-group sungrazing Comets C/2008 Y12 (SOHO) and C/2009 A1 (STEREO); and numerous main-belt asteroids, some as faint as V = 13.8, as determined by the JPL HORIZONS ephemeris (Giorgini et al., 1996).
Discussion
Although it is dynamically stable, the Vulcanoid zone is in a rough neighborhood. It occupies a comparatively small volume of space and due to its proximity to the Sun, orbital velocities are large. Leake et al., 1987, Stern and Durda, 2000 showed that even for a wide variety of assumptions about the size and material properties of a hypothetical population, Vulcanoids will experience significant collisional disruption and erosion unless the mean orbital eccentricity of the population is
Acknowledgments
We kindly thank D. Bewsher for providing the instrument response curves for STEREO HI-1, G.M. Holsclaw for providing the absolute reflectance of Mercury as measured by MASCS/MDIS, T.V. Spahr for initial orbital solutions for the object that turned out to be C/2008 Y12 (SOHO), and K. Battams for discussions about C/2008 Y12 (SOHO) and its orbit. Support for this work was provided by NASA Planetary Geology and Geophysics program through Grant NNX09AD65G.
References (41)
- et al.
Sungrazing comets discovered with the SOHO/LASCO coronagraphs 1996–1998
Icarus
(2002) - et al.
A new observational search for Vulcanoids in SOHO/LASCO coronagraph images
Icarus
(2000) - et al.
The orbital distribution of Near-Earth Objects inside Earth’s orbit
Icarus
(2012) - et al.
A comparison of the ultraviolet to near-infrared spectral properties of Mercury and the Moon as observed by MESSENGER
Icarus
(2010) - et al.
The chronology of Mercury’s geological and geophysical evolution – The Vulcanoid hypothesis
Icarus
(1987) Stability of frosts in the Solar System
Icarus
(1975)- et al.
Photometry of Mercury from SOHO/LASCO and Earth. The phase function from 2 to 170 deg
Icarus
(2002) - et al.
Collisional evolution in the Vulcanoid region: Implications for present-day population constraints
Icarus
(2000) - et al.
The depletion of the putative Vulcanoid population via the Yarkovsky effect
Icarus
(2000) - et al.
Mercury’s integral phase curve: Phase reddening and wavelength dependence of photometric quantities
Planet. Space Sci.
(2008)
Size distribution of collisionally evolved asteroidal populations – Analytical solution for self-similar collision cascades
Icarus
Determination of the photometric calibration and large-scale flatfield of the STEREO Heliospheric Imagers: I. HI-1
Sol. Phys.
The Large Angle Spectroscopic Coronagraph (LASCO)
Sol. Phys.
Search for intramercurial bodies
Publ. Astron. Soc. Pacific
Collisional model of asteroids and their debris
J. Geo-Phys. Res.
Erklarung der Perihelionbewegung der Merkur aus der allgemeinen Relativitatstheorie
Sitzungsber. preuss. Akad. Wiss.
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