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Register a new user$alpha$ Centauri A in the far infrared
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Authors
R. Liseau
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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. The region of the temperature reversal can be directly observed only in the far infrared and submm. We aim at the determination of the characteristics of the atmosphere in the region of the temperature minimum of the solar sister star alpha Cen A. For the nearby binary system alpha Centauri, stellar parameters are known with high accuracy from measurements. For the basic model parameters Teff, log g and [Fe/H], we interpolate in the grid of GAIA/PHOENIX stellar model atmospheres and compute the corresponding model for the G2 V star alpha 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 MIPS, PACS, SPIRE and 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 alpha Cen A in the FIR. Similar to the Sun, the FIR emission of alpha Cen A originates in the minimum temperature region above the stellar photosphere in the visible. However, in comparison with the solar case, the FIR photosphere of alpha Cen A appears marginally cooler, Tmin=T160mu=3920+/-375 K. Beyond the minimum near 160mu, the brightness temperatures increase and this radiation likely originates in warmer regions of the chromosphere of alpha Cen A. To the best of our knowledge this is the first time a temperature minimum has been directly measured on a main-sequence star other than the Sun.
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[Abridged] Debris discs around main-sequence stars indicate the presence of larger rocky bodies. The components of the nearby binary aCentauri have higher than solar metallicities, which is thought to promote giant planet formation. We aim to determine the level of emission from debris in the aCen system. Having already detected the temperature minimum, Tmin, of aCenA, we here attempt to do so also for the companion aCenB. Using the aCen stars as templates, we study possible effects Tmin may have on the detectability of unresolved dust discs around other stars. We use Herschel and APEX photometry to determine the stellar spectral energy distributions. In addition, we use APEX for spectral line mapping to study the complex background around aCen seen in the photometric images. Models of stellar atmospheres and discs are used to estimate the amount of debris around these stars. For solar-type stars, a fractional dust luminosity fd 2e-7 could account for SEDs that do not exhibit the Tmin-effect. Slight excesses at the 2.5 sigma level are observed at 24 mu for both stars, which, if interpreted to be due to dust, would correspond to fd (1-3)e-5. Dynamical disc modelling leads to rough mass estimates of the putative Zodi belts around the aCen stars, viz. <~4e-6 MMoon of 4 to 1000 mu size grains, distributed according to n a^-3.5. Similarly, for filled-in Tmin emission, corresponding EKBs could account for ~1e-3 MMoon of dust. Light scattered and/or thermally emitted by exo-Zodi discs will have profound implications for future spectroscopic missions designed to search for biomarkers in the atmospheres of Earth-like planets. The F-IR SED of aCenB is marginally consistent with the presence of a minimum temperature region in the upper atmosphere. We also show that an aCenA-like temperature minimum may result in an erroneous apprehension about the presence of dust around other stars.
We use more than a decade of radial velocity measurements for $alpha$ Cen A, B, and Proxima Centauri from HARPS, CHIRON, and UVES to identify the $M sin i$ and orbital periods of planets that could have been detected if they existed. At each point in a mass-period grid, we sample a simulated, Keplerian signal with the precision and cadence of existing data and assess the probability that the signal could have been produced by noise alone. Existing data places detection thresholds in the classically defined habitable zones at about $M sin i$ of 53 M$_{oplus}$ for $alpha$ Cen A, 8.4 M$_{oplus}$ for $alpha$ Cen B, and 0.47 M$_{oplus}$ for Proxima Centauri. Additionally, we examine the impact of systematic errors, or red noise in the data. A comparison of white- and red-noise simulations highlights quasi-periodic variability in the radial velocities that may be caused by systematic errors, photospheric velocity signals, or planetary signals. For example, the red-noise simulations show a peak above white-noise simulations at the period of Proxima Centauri b. We also carry out a spectroscopic analysis of the chemical composition of the $alpha$ Centauri stars. The stars have super-solar metallicity with ratios of C/O and Mg/Si that are similar to the Sun, suggesting that any small planets in the $alpha$ Cen system may be compositionally similar to our terrestrial planets. Although the small projected separation of $alpha$ Cen A and B currently hampers extreme-precision radial velocity measurements, the angular separation is now increasing. By 2019, $alpha$ Cen A and B will be ideal targets for renewed Doppler planet surveys.
Understanding the physical process responsible for the transport of energy in the core of $alpha$ Centauri A is of the utmost importance if this star is to be used in the calibration of stellar model physics. Adoption of different parallax measurements available in the literature results in differences in the interferometric radius constraints used in stellar modelling. Further, this is at the origin of the different dynamical mass measurements reported for this star. With the goal of reproducing the revised dynamical mass derived by Pourbaix & Boffin, we modelled the star using two stellar grids varying in the adopted nuclear reaction rates. Asteroseismic and spectroscopic observables were complemented with different interferometric radius constraints during the optimisation procedure. Our findings show that best-fit models reproducing the revised dynamical mass favour the existence of a convective core ($gtrsim$ 70% of best-fit models), a result that is robust against changes to the model physics. If this mass is accurate, then $alpha$ Centauri A may be used to calibrate stellar model parameters in the presence of a convective core.
We study the far-infrared properties of 498 Lyman Alpha Emitters (LAEs) at z=2.8, 3.1 and 4.5 in the Extended Chandra Deep Field-South, using 250, 350 and 500 micron data from the Herschel Multi-tiered Extragalactic Survey (HerMES) and 870 micron data from the LABOCA ECDFS Submillimeter Survey (LESS). None of the 126, 280 or 92 LAEs at z=2.8, 3.1 and 4.5, respectively, are individually detected in the far-infrared data. We use stacking to probe the average emission to deeper flux limits, reaching $1sigma$ depths of ~0.1 to 0.4 mJy. The LAEs are also undetected at $ge3sigma$ in the stacks, although a $2.5sigma$ signal is observed at 870 micron for the z=2.8 sources. We consider a wide range of far-infrared spectral energy distributions (SEDs), including a M82 and an Sd galaxy template, to determine upper limits on the far-infrared luminosities and far-infrared-derived star-formation rates of the LAEs. These star-formation rates are then combined with those inferred from the Ly$alpha$ and UV emission to determine lower limits on the LAEs Ly$alpha$ escape fraction ($f_{rm esc}($Ly$alpha$)). For the Sd SED template, the inferred LAEs $f_{rm esc}($Ly$alpha$) are $gtrsim30%$ ($1sigma$) at z=2.8, 3.1 and 4.5, which are all significantly higher than the global $f_{rm esc}($Ly$alpha$) at these redshifts. Thus, if the LAEs $f_{rm esc}($Ly$alpha$) follows the global evolution then they have warmer far-infrared SEDs than the Sd galaxy template. The average and M82 SEDs produce lower limits on the LAE $f_{rm esc}($Ly$alpha$) of ~10 to 20% ($1sigma$), all of which are slightly higher than the global evolution of $f_{rm esc}($Ly$alpha$) but consistent with it at the 2 to 3$sigma$ level.
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