HD 44892: the youngest (or oldest?) gas-harbouring debris disc around an intermediate-mass star

ABSTRACT We present the first detections of gas around a 2.1 Myr old debris disc-bearing intermediate-mass star, HD 44892. Gas is detected both in $^{12}$CO (2−1) emission through ALMA (Atacama Large Millimeter/submillimeter Array) Band 6 observations and in absorption in Ca ii K and H, seen with high-resolution UVES (Ultraviolet and Visual Echelle Spectrograph) spectroscopy. $^{13}$CO and C$^{18}$O (2−1) are not detected. The star exhibits a 12 $\mu$m fractional excess of $7.86^{+0.11}_{-2.27}$, placing it in the transition stage between protoplanetary and debris discs. Our detection of 1.3 mm emission yields the dust mass of 0.019 $\pm$ 0.009 M$_{\rm{\oplus }}$ assuming 115 K temperature. The $^{13}$CO non-detection places an upper limit on CO gas mass of $\sim 10^{-2}$ $\mathrm{ M}_{\rm{\oplus }}$. The $^{12}$CO detection yields a CO gas mass of (7.86 $\pm$ 2.05) $\times$ 10$^{-5}$ or (1.62 $\pm$ 0.17) $\times$ 10$^{-4}$ $\mathrm{ M}_{\rm{\oplus }}$ assuming a gas temperature of 20 or 50 K, respectively. These should be considered as lower limits since $^{12}$CO emission may be optically thick. With UVES, we find variability in Ca ii K and H lines, which can be interpreted by transiting circumstellar gas, ruling out interstellar absorption as their origin. Both the dust mass, which is within an order of magnitude of HD 141569, and the gas mass derived here indicate a late gas dispersal stage of the protoplanetary disc. Through our analysis, we deem the alternative age of 800 Myr unlikely.