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[40.08] An Observational Upper Limit on the Number-Density of Interstellar Comets

B. Meinke (UC Berkeley), R. Jedicke (IfA, U. Hawaii), J. A. Larsen (LPL, U. Arizona)

Current theories of Solar System formation predict that comets that acrete in a protostellar nebula are later ejected by interactions with the newly formed giant planets. In such a process a large fraction (>99%) of all comets would be ejected from the new planetary system, subsequently creating a large population of Interstellar Objects (ISO) in the interstellar medium. Due to the observational absence of such objects, Whipple (1975) determined the limit on their number-density to be ~10¹³ pc^-3, although he, and others since (e.g. Stern 1990) contend that this liberal upper limit could be reduced by several orders of magnitude given their continued observational absence in a survey covering large areas of sky to faint magnitudes. Since the early 1990s, the University of Arizona's Spacewatch survey at Kitt Peak has conducted such wide-field observations to V~21.7. In the period corresponding to this study Spacewatch covered about 4200 deg².

Given the amount of sky covered by Spacewatch observations, and the detection efficiency for objects as a function of their rates of motion and apparent magnitude, we determined the 97% upper confidence limit on the number of ISOs as a function of the slope parameter \alpha. We have parameterized the number density of ISOs as \rho=\rho_o 10^alpha(H-H_o), where H is the absolute magnitude of an object, \alpha is the slope of the number density as a function of H, and \rho_o is the space density at H_o. We use H_o=19.1 which corresponds to an ~1 km diameter object with a typical cometary albedo of p=0.04.

At \alpha = 0.5, corresponding roughly to the expected slope for accreting planetesimals, the 97% upper C.L. on the number density of ISOs is ~10¹⁴ pc^-3 or ~10^-2 AU^-3. Spacewatch is supported by grants from NASA's Near-Earth Object Observation and Planetary Astronomy Programs, the U.S. Air Force Office of Scientific Research, the Paul G. Allen Charitable Foundation, and the Brinson Foundation. B. Meinke's research was supported by the NSF REU Program at the IfA, summer 2004.

The author(s) of this abstract have provided an email address for comments about the abstract: bmeinke@berkeley.edu

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.