First ALMA observations of the HD 105211 debris disc: A warm dust component close to a gigayear-old star

Context. Most debris discs consist of a gas-poor, cold dust belt located tens to hundreds of astronomical units away from the host star. Many cold dust belts exhibit distinct structures attributed to the dynamic interaction of planetary systems. Moreover, in a few systems, additional warm components can be found closer to the central star, resembling the asteroid belt or zodiacal dust in our Solar System. Aims. In this work, we investigate the structure of the disc surrounding the nearby F2V star HD 105211 (η Cru, HIP 59072), which has a warm excess (seen with Spitzer /MIPS at 24 μm) and a potential asymmetry in the cold belt (seen in the Herschel /PACS images). Methods. We applied the CASA pipeline to obtain the ALMA 1.3 mm continuum images of HD105211. Then we constructed the spectral energy distribution (SED) of the system and performed Markov chain Monte Carlo (MCMC) simulations to fit a model to the ALMA visibility data. To characterise the disc asymmetry, we analysed the ALMA images of two individual observation blocks (OB1, conducted on 2017 March 28, and OB2, conducted on 2018 May 9) and compared them to the previous Herschel images. Results. Our modelling reveals that the disc around HD105211 is a narrow ring (23.6±4.6 au) with low eccentricity ( e ≤ 0.03) positioned at a distance of 133.7±1.6 au from the central star, which differs from the broad disc (100 ± 20 au) starting at an inner edge of 87 ± 2.5 au, inferred from the Herschel images. We found that both observation blocks show excess emission at the stellar position (>3σ), while OB1 shows an offset between the star and the phase centre (~0.3″), and OB2 shows brightness clumps (~2σ). We used a two-temperature model to fit the infrared SED and used the ALMA detection to constrain the warm component to a nearly pure blackbody model. Conclusions. The relatively low ratio of actual radius to blackbody radius of the HD 105211 debris disc indicates that this system is depleted in small grains, which could indicate that it is dynamically cold. The excess emission from the stellar position suggests that there should be a warm millimetre-sized dust component close to the star, for which we suggest two possible origins: in situ asteroid belt or comet delivery.