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Debris disks are second generation dusty disks thought to be devoid of gas.However, this idea has been challenged in the last years by gas detections insome systems. We compiled a database of 301 debris disks and collectedhigh--resolution optical spectra for $\sim77\%$ of them. From the analysis ofthese data we identified a group of 23 debris disks presenting severalabsorption features superimposed to the photospheric Ca II and Na I doublets.These absorptions could be due to circumstellar material or interstellarclouds. In order to discriminate between the two scenarios, we characterizedeach feature in terms of its radial velocity, equivalent width and columndensity. Additionally, we searched in the literature for local clouds in theline of sight of the stars, and looked for the presence of similar absorptionfeatures in nearby stars. Our study concludes that while all the objectspresent interstellar absorptions in their spectra, three objects show featuresmore compatible with circumstellar origin: HD 110058 presents a stablecircumstellar absorption, while HR 4796 and c Aql present variable absorptionfeatures likely due to exocometary activity. The minute-scale variability wedetect towards c Aql is the shortest of this kind detected so far. Thedetection of circumstellar features in these objects is consistent with theirnear edge-on inclinations. We also provide evidence challenging previous claimsof circumstellar gas detections for HR 6507. Given the properties of thesample, we speculate that transient gaseous events must be a common phenomenonamong debris disks.

Debris disks with multiple absorption features in metallic lines: circumstellar or interstellar origin?