COMET 17P/HOLMES: CONTRAST IN ACTIVITY BETWEEN BEFORE AND AFTER THE 2007 OUTBURST

Abstract

A Jupiter-family comet, 17P/Holmes, underwent outbursts in 1892 and 2007. In particular, the 2007 outburst is known as the greatest outburst over the past century. However, little is known about the activity before the outburst because it was unpredicted. In addition, the time evolution of the nuclear physical status has not been systematically studied. Here, we study the activity of 17P/Holmes before and after the 2007 outburst through optical and mid-infrared observations. We found that the nucleus was highly depleted in its near-surface icy component before the outburst but that it became activated after the 2007 outburst. Assuming a conventional 1 mu m sized grain model, we derived a surface fractional active area of 0.58% +/- 0.14% before the outburst whereas the area was enlarged by a factor of similar to 50 after the 2007 outburst. We also found that large (>= 1 mm) particles could be dominant in the dust tail observed around aphelion. Based on the size of the particles, the dust production rate was >= 170 kg s(-1) at a heliocentric distance of r(h) = 4.1 AU, suggesting that the nucleus was still active around the aphelion passage. The nucleus color was similar to that of the dust particles and average for a Jupiter-family comet but different from that of most Kuiper Belt objects, implying that color may be inherent to icy bodies in the solar system. On the basis of these results, we concluded that more than 76 m of surface material was blown off by the 2007 outburst.