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تسجيل مستخدم جديدALMA Discovery of Dust Belts Around Proxima Centauri
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Proxima Centauri, the star closest to our Sun, is known to host at least one terrestrial planet candidate in a temperate orbit. Here we report the ALMA detection of the star at 1.3 mm wavelength and the discovery of a belt of dust orbiting around it at distances ranging between 1 and 4 au, approximately. Given the low luminosity of the Proxima Centauri star, we estimate a characteristic temperature of about 40 K for this dust, which might constitute the dust component of a small-scale analog to our solar system Kuiper belt. The estimated total mass, including dust and bodies up to 50 km in size, is of the order of 0.01 Earth masses, which is similar to that of the solar Kuiper belt. Our data also show a hint of warmer dust closer to the star. We also find signs of two additional features that might be associated with the Proxima Centauri system, which, however, still require further observations to be confirmed: an outer extremely cold (about 10 K) belt around the star at about 30 au, whose orbital plane is tilted about 45 degrees with respect to the plane of the sky; and additionally, we marginally detect a compact 1.3 mm emission source at a projected distance of about 1.2 arcsec from the star, whose nature is still unknown.
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At a distance of 1.295 parsecs, the red-dwarf Proxima Centauri ($alpha$ Centauri C, GL 551, HIP 70890, or simply Proxima) is the Suns closest stellar neighbor and one of the best studied low-mass stars. It has an effective temperature of only $sim$ 3
050 K, a luminosity of $sim$0.1 per cent solar, a measured radius of 0.14 R$_odot$ and a mass of about 12 per cent the mass of the Sun. Although Proxima is considered a moderately active star, its rotation period is $sim$ 83 days, and its quiescent activity levels and X-ray luminosity are comparable to the Suns. New observations reveal the presence of a small planet orbiting Proxima with a minimum mass of 1.3~Earth masses and an orbital period of $sim$11.2 days. Its orbital semi-major axis is $sim0.05$ AU, with an equilibrium temperature in the range where water could be liquid on its surface.
We present new analyses of ALMA 12-m and ACA observations at 233 GHz (1.3 mm) of the Proxima Centauri system with sensitivities of 9.5 and 47 $mu$Jy beam$^{-1}$, respectively, taken from 2017 January 21 through 2017 April 25. These analyses reveal th
at the star underwent a significant flaring event during one of the ACA observations on 2017 March 24. The complete event lasted for approximately 1 minute and reached a peak flux density of $100pm4$ mJy, nearly a factor of $1000times$ brighter than the stars quiescent emission. At the flare peak, the continuum emission is characterized by a steeply falling spectral index with frequency, $F_ u propto u^alpha$ with $alpha = -1.77pm0.45$, and a lower limit on the fractional linear polarization of $|Q/I| = 0.19pm0.02$. Since the ACA observations do not show any quiescent excess emission, we conclude that there is no need to invoke the presence of a dust belt at $1-4$ AU. We also posit that the slight excess flux density of $101pm9$ $mu$Jy observed in the 12-m observations compared to the photospheric flux density of $74pm4$ $mu$Jy extrapolated from infrared wavelengths may be due to coronal heating from continual smaller flares, as is seen for AU Mic, another nearby, well-studied, M dwarf flare star. If this is true, then the need for warm dust at $sim0.4$ AU is also removed.
In order to detect and characterise cold extended circumstellar dust originating from collisions of planetesimal bodies in disks, belts, or rings at Kuiper-Belt distances (30-50 AU or beyond) sensitive submillimetre observations are essential. Measur
ements of the flux densities at these wavelengths will extend existing IR photometry and permit more detailed modelling of the Rayleigh-Jeans tail of the disks spectral energy distribution (SED), effectively constraining dust properties and disk extensions. By observing stars spanning from a few up to several hundred Myr, the evolution of debris disks during crucial phases of planet formation can be studied. We have performed 870-micron observations of 22 exo-Kuiper-Belt candidates, as part of a Large Programme with the LABOCA bolometer at the APEX telescope. Dust masses (or upper limits) were calculated from integrated 870-micron fluxes, and fits to the SED of detected sources revealed the fractional dust luminosities f_dust, dust temperatures T_dust, and power-law exponents beta of the opacity law. A total of 10 detections with at least 3-sigma significance were made, out of which five (HD 95086, HD 131835, HD 161868, HD 170773, and HD 207129) have previously never been detected at submillimetre wavelengths. Three additional sources are marginally detected with >2.5-sigma significance. The best-fit beta parameters all lie between 0.1 and 0.8, in agreement with previous results indicating the presence of grains that are significantly larger than those in the ISM. From our relatively small sample we estimate f_dust proportional to t^(-alpha), with alpha~0.8-2.0, and identify an evolution of the characteristic radial dust distance R_dust that is consistent with the t^(1/3) increase predicted from models of self-stirred collisions in debris disks.
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