Extent of Excess Far‐Infrared Emission around Luminosity Class III Stars

Abstract

With the Infrared Space Observatory, we conducted 3 ) 3 pixel imaging photometry of 12 luminosity class III stars, which were previously presumed to have dust particles around them, at far-infrared wave- lengths (60 and 90 km). Eleven out of 12 targets show a peak of excess (above photosphere) far-infrared emission at the location of the star, implying that the dust particles are truly associated with stars. To estimate the size of the excess emission source, the —ux ratio of center to boundary pixels of the 3 ) 3 array was examined. The radius of the dust emission is found to be D3000 to D10,000 AU for a thin shell distribution and D5000 to D25,000 AU for a uniform distribution. We consider three models for the origin of the dust: disintegration of comets, sporadic dust ejection from the star, and emission from nearby interstellar cirrus. The data seem to rule out the —rst model (as far as the ii Kuiper beltlike ˇˇ particles are assumed to be large blackbody grains) but do not enable us to choose between the other two models. Subject headings: circumstellar matterinfrared: starsstars: late-type Zuckerman, Kim, & Liu (1995) correlated the Bright Star Catalogue (Hoffleit & Warren 1991) and the Michigan Spectral Catalogue (Houk, Cowley, & Smith-Moore 1975- 1988) with the IRAS catalogs to determine which, if any, luminosity class III giant stars (—rst-ascent red giants) have associated circumstellar dust particles that radiate at far- infrared (far-IR) wavelengths. Of more than 40,000 class III giant stars in the two catalogs, they found that perhaps 300 have associated dust. The presence of particulate material near premain- sequence, main-sequence, asymptotic giant branch, and supergiant stars may be accounted for in rather straight- forward ways (remnants from the star formation for the for- mer two and mass loss for the latter two). But, there is no obvious way to produce and retain large amounts of dust near —rst-ascent giant stars, which are thought to be too old to still possess much leftover material and insufficiently evolved to have lost a signi—cant amount of their mass. However, one may conjecture that the presence of dust around —rst-ascent giants may involve one or more of the following phenomena: mass loss, binarity, planetary sys- tems, and evaporation of Kuiper belt material. Recently, Jura (1999) has analyzed three models for the dust around eight —rst-ascent giant stars: orbiting dust resulting from the disintegration of comets located in extrasolar analogs of the Kuiper belt, dust sporadically ejected from the star, and emission from particles in the interstellar medium that are accidentally near the star, the ii cirrus hot spot.ˇˇ Among the Zuckerman et al. (1995) list of 100, we observed 12 —rst-ascent giant stars with the PHOT instru- ment of the Infrared Space Observatory (ISO)2 to determine if the excess far-IR emission is truly associated with the stars and to estimate the size of the region that produces the excess far-IR emission.