α Centauri A in the far infrared : First measurement of the temperature minimum of a star other than the Sun

Astronomy and Astrophysics 549: (2013) L7 Reproduced with permission from Astronomy & AstrophysicsContext. Chromospheres and coronae are common phenomena on solar-type stars. Understanding the energy transfer to these heated atmospheric layers requires direct access to the relevant empirical data. Study of these structures has, by and large, been limited to the Sun thus far. Aims. The region of the temperature reversal can be directly observed only in the far infrared and submillimetre spectral regime. We aim at determining the characteristics of the atmosphere in the region of the temperature minimum of the solar sister star αCen A. As a bonus this will also provide a detailed mapping of the spectral energy distribution, i.e. knowledge that is crucial when searching for faint, Kuiper belt-like dust emission around other stars. Methods. For the nearby binary system α Cen, stellar parameters are known with high accuracy from measurements. For the basic model parameters Te_, log g and [Fe/H], we interpolate stellar model atmospheres in the grid of Gaia/PHOENIX and compute thecorresponding model for the G2V star α Cen A. Comparison with photometric measurements shows excellent agreement between observed photospheric data in the optical and infrared. For longer wavelengths, the modelled spectral energy distribution is compared to Spitzer-MIPS, Herschel-PACS, Herschel-SPIRE, and APEX-LABOCA photometry. A specifically tailored Uppsala model based on the MARCS code and extending further in wavelength is used to gauge the emission characteristics of α CenA in the far infared. Results. Similar to the Sun, the far infrared (FIR) emission of α CenA originates in the minimum temperature region above the stellar photosphere in the visible. However, in comparison with the solar case, the FIR photosphere of α CenA appears marginally cooler, Tmin _ T160 _m = 3920_375 K. Beyond the minimum near 160 _m, the brightness temperatures increase, and this radiation very likely originates in warmer regions of the chromosphere of α Cen A. Conclusions. To the best of our knowledge, this is the first time a temperature minimum has been directly measured on a mainsequence star other than the SunSupported by Spanish grant AYA 2011/02622. A. Also co-funded under the Marie Curie Actions of the European Comission (FP7-COFUND). And financial support through contract ANR-2010 BLAN-0505-01 (EXOZODI