Circumstellar dust emission from nearby Solar-type stars

Doktorsavhandling, 2016

Far-infrared excess above the photosphere of a star indicates the presence of a circumstellar dust disc which is a sign-post for extrasolar planets, and was first detected in the mid 1980s. Dust discs are intricately connected to planets and planetesimals, give insights in the dynamics and evolution of the system, and are also useful for future exoplanet-observations. This thesis is aimed at modelling dust emission of nearby Solar-type stars, and is partly involved with the Herschel key programme DUNES (DUst around NEarby Stars). It includes detailed studies on a few nearby stars, and results from a coherent re-reduction of the combined datasets of the original DUNES catalogue and 55 DEBRIS-observed sources (Disc Emission via a Bias-free Reconnaissance in the Infrared/Sub-millimetre). Based on observations with Herschel and Spitzer of the nearby binary alpha Centauri (G2 V and K1 V), an upper limit on the fractional luminosity (dust-to-star) of circumstellar dust was determined to a few 10^-5 (Paper I). Both stars exhibit detectable temperature minima at wavelengths around 100-300um due to a chromospheric temperature inversion akin to that of the sun. The resulting flux difference, when compared to stellar photospheric models, is equivalent to dust emission with a fractional luminosity of <2*10^-7. The triple star 94 Ceti hosts known dust emission-features that are modelled in Paper II. The dust is constrained to a circumbinary disc around the companion stars, 94 Cet B and C (M dwarfs), which orbits the primary 94 Cet A (F8 V) on a 2000 yr orbit, with a fractional luminosity of 4.6+/-0.4*10^-6, and a disc radius of 40 AU. Tentative evidence for a circumtertiary disc is also found. The resolved emission at EP Eridani (K1 V) corresponds well with a face-on dust disc with the outer radius 110 AU, an inner hole of 5 to 10 AU, and fractional luminosity of $2.0+/-0.2*10^-5. The emission at Gliese 42 (K2 V) appears contaminated by background sources. Dust models with a flatter than normal grain size distribution fit the observations with a fractional luminosity of 8.7+/-1.0*10^-6. A wide range of far-infrared galaxy SEDs, with redshifts between 0.7 to 1.9, and IR luminosities of 0.4 to 8.3*10^12 Lsun, also fit these data. The DUNES catalogue, combined with 55 DEBRIS-observed sources, contains 188 nearby FGK stars (including resolved binaries). These data were previously reduced with older versions of the Herschel software and calibration, and have now been coherently re-reduced with more recent versions. There are 16 new marginal excess sources, and one new detected at delta Pavonis. We find a systematical 1sigma higher flux density for the 133 original DUNES sources, while the estimates for the additional 55 sources agree well with our results.