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

TL;DR

HD 44892 represents the youngest known gas-bearing debris disc around an intermediate-mass star, challenging the boundary between protoplanetary and debris discs. Using ALMA CO(2-1) emission and UVES Ca II K/H absorption, the authors detect gas and a marginal 1.3 mm dust continuum, estimating a dust mass of about $0.019\,M_\oplus$ and CO gas mass near $10^{-4}\,M_\oplus$, with 13CO/C18O non-detections setting upper limits. The analyses favor an inner, $H_2$-rich primordial gas component in a hybrid disc, though a secondary or mixed origin cannot be excluded; the young age supports a transitional evolutionary stage akin to HD 141569. This study provides a rare snapshot of gas retention into the debris stage around a $\sim2.9\,M_\odot$ star and motivates higher-resolution follow-up to map gas and dust morphology and test primordial versus secondary gas models in IMS systems.

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 Band 6 observations and in absorption in Ca II K and H, seen with high-resolution UVES spectroscopy. $^{13}$CO and C$^{18}$O (2-1) are not detected. The star exhibits a 12 $μ$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 $M_{\oplus}$ assuming 115 K temperature. The $^{13}$CO non-detection places an upper limit on CO gas mass of $\sim$10$^{-2}$ $M_{\oplus}$. The $^{12}$CO detection yields a CO gas mass of (7.86$\pm$2.05)$\times$10$^{-5}$ $M_{\oplus}$ or (1.62$\pm$0.17)$\times$10$^{-4}$ $M_{\oplus}$ assuming a gas temperature of 20 K 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.

Figures (13)

• Figure 1: Histogram of all gas detections in debris discs through either emission or absorption, divided into bins of stellar type. For a full list of objects with gas emission, see Table \ref{['tab:allCOGasDetections']}. Objects with gas absorption used in this figure were adopted from Iglesias2020SearchingDisks (Table 1.1), based on data from Chen2003TheHerculis, Lagrange-Henri1990SearchStar., Abt1973RotationDwarfs., Welsh2013CircumstellarExocomets, Koubsky1993ComingHerculis., Iglesias2018DebrisOrigin, Iglesias2019AnDisc, Welsh1998Beta85905, Welsh2015TheAbsorption, Montgomery2012DetectionStars, Welsh2018FurtherDiscs.
• Figure 2: Left: Evolution of dust mass in debris discs, originally presented in Wyatt2015FiveDisk. We have added more dust masses of debris discs, including those with gas discovered since. HD 44892 is marked with the two different possible ages: 2.1 Myr and 800 Myr. At the older age HD 44892 would have an unexpectedly high dust mass compared to other, older debris discs. Continuum fluxes used to create this plot were adopted from Panic2013FirstDiscsSandell2011ASTARSMoor2017MolecularStarsSheret2004SubmillimetreStarsLieman-Sifry2016DebrisALMAPearce2022PlanetDiscsStapper2022TheALMA, excluding systems with highly uncertain age estimates. The age of the HR 4796 was taken from Rhee2007CharacterizationCatalogs. Dust temperature of 50 K is assumed for debris discs, and 25 K for protoplanetary discs, with a dust opacity assumed as $\kappa=0.1\times2.3\times(\frac{\nu}{230\times10^9})^{0.7}$ [cm$^{2}$ g$^{-1}$] Beckwith1990, where $\nu$ is the frequency of observations [Hz]. Right: IR excess measured at 12 $\mu$m plotted as a function of age of the system, as originally presented in Wyatt2015FiveDisk (in purple, where open circles are upper limits), with additional data from Moor2017MolecularStars of gas-bearing debris discs (in black). HD 44892 is again marked with the two possible ages, with the older age making it an outlier in the trend. The grey-shaded region marks the stage between protoplanetary and debris discs, where hybrid discs are expected to reside.
• Figure 3: Continuum emission of HD 44892, marked in contours of ( $\pm$ 2, 3, 4, 5, 6, 7) $\times$$\sigma$, where $\sigma=0.016$ mJy beam$^{-1}$. Position of HD 44892 marked as a white cross.
• Figure 4: Amplitude vs UVdist plot of continuum of HD 44892. While the shorter UV-distance baselines can be fit with a straight line (dashed-grey), hence are unresolved, the longest UV-baselines cannot, hence are not unresolved and deviate from a point-like source. The few points shown here correspond to the full continuum dataset that has been averaged in time, channels, and binned by UVdist for clarity.
• Figure 5: Radial velocity spectrum of $^{12}$CO emission with peak positions marked. The x-axis has been corrected for the star's radial velocity in the LSR frame of $\sim$1.5 km s$^{-1}$.