The co-existence of hot and cold gas in debris discs★
Departamento. Física Teórica, Universidad Autónoma de Madrid, Cantoblanco,
2 Centro de Astrobiología (CAB, CSIC-INTA), ESAC Campus, Camino Bajo del Castillo s/n, Villanueva de la Cañada, 28692 Madrid, Spain
3 INAF, Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
4 STAR Institute, Université de Liège, F.R.S.-FNRS, 19c Allée du Six Août, 4000 Liège, Belgium
5 Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Casilla, 5030 Valparaíso, Chile
6 Núcleo Milenio de Formación Planetaria-NPF, Universidad de Valparaíso, Av. Gran Bretaña, 1111 Valparaíso, Chile
7 European Space Astronomy Centre (ESA), PO Box 78, Villanueva de la Cañada, 28691 Madrid, Spain
8 Université de Toulouse, UPS-OMP, IRAP, 31400 Toulouse, France
9 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21212, USA
10 Steward Observatory, Department of Astronomy, University of Arizona, Tucson, AZ 85721, USA
11 Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
12 Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
13 Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, PO Box 67, 1525 Budapest, Hungary
14 Aurora Technology B.V. for ESA, ESA-ESAC, Villanueva de la Cañada, 28691 Madrid, Spain
15 Instituto de Ciencia de Materiales de Madrid (CSIC), Calle Sor Juana Inés de la Cruz 3, 28049 Cantoblanco, Madrid, Spain
Accepted: 18 January 2018
Context. Debris discs have often been described as gas-poor discs as the gas-to-dust ratio is expected to be considerably lower than in primordial, protoplanetary discs. However, recent observations have confirmed the presence of a non-negligible amount of cold gas in the circumstellar (CS) debris discs around young main-sequence stars. This cold gas has been suggested to be related to the outgassing of planetesimals and cometary-like objects. Aims. The goal of this paper is to investigate the presence of hot gas in the immediate surroundings of the cold-gas-bearing debris-disc central stars.
Methods. High-resolution optical spectra of all currently known cold-gas-bearing debris-disc systems, with the exception of β Pic and Fomalhaut, have been obtained from La Palma (Spain), La Silla (Chile), and La Luz (Mexico) observatories. To verify the presence of hot gas around the sample of stars, we have analysed the Ca II H&K and the Na I D lines searching for non-photospheric absorptions of CS origin, usually attributed to cometary-like activity.
Results. Narrow, stable Ca II and/or Na I absorption features have been detected superimposed to the photospheric lines in 10 out of the 15 observed cold-gas-bearing debris-disc stars. Features are found at the radial velocity of the stars, or slightly blue- or red-shifted, and/or at the velocity of the local interstellar medium (ISM). Some stars also present transient variable events or absorptions extended towards red wavelengths (red wings). These are the first detections of such Ca II features in 7 out of the 15 observed stars. Although an ISM origin cannot categorically be excluded, the results suggest that the stable and variable absorptions arise from relatively hot gas located in the CS close-in environment of the stars. This hot gas is detected in at least ~80%, of edge-on cold-gas-bearing debris discs, while in only ~10% of the discs seen close to face-on. We interpret this result as a geometrical effect, and suggest that the non-detection of hot gas absorptions in some face-on systems is due to the disc inclination and likely not to the absence of the hot-gas component. This gas is likely released in physical processes related in some way to the evaporation of exocomets, evaporation of dust grains, or grain-grain collisions close to the central star.
Key words: stars: general / planetary systems / comets: general / ISM: clouds / circumstellar matter
The reduced spectra are only available at the CDS (ascii files) and at the FEROS archive (FITS files) via anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/614/A3
© ESO 2018