Disks

IM Lup is a young (~1 Myr), Solar-mass star that hosts a massive and radially-extended disk. IM Lup has the largest pebble disk (~260 au) in the DSHARP sample (Huang et al. 2018a) and a CO disk out to ~900 au (Cleeves et al. 2016). Continuum images reveal the presence of spiral arms in mm grains (Huang et al. 2018b), while scattered light observations show a highly flared, multi-ringed disk in micron-sized dust grains (Avenhaus et al. 2018). Despite its youth, the IM Lup disk shows evidence of grain growth (Lommen et al. 2007). The outer portion of the CO gas disk is diffuse and envelope-like, potentially indicating the presence of an external photoevaporative wind (Haworth et al. 2017). IM Lup has been been observed in several molecular lines (Öberg et al. 2011) and shows a striking double-ring structure in DCO⁺, due to a change in gas-phase C and O abundance at the pebble disk edge (Öberg et al. 2015b). Modeling indicates that CO is underabundant by a factor of ~20 compared to molecular clouds due to missing gas-phase carbon and oxygen (Cleeves et al. 2016). No depletions were found in nitrogen, resulting in an elevated gas-phase N/O ratio (Cleeves et al. 2018). Spitzer observations showed weak OH and H₂O emission in the inner disk, suggesting intermediate water depletion (Banzatti et al. 2017).

GM Aur is a ~3-10 Myr T Tauri star in the Taurus-Auriga star-forming region hosting a transition disk, i.e., a disk large with a large inner gap or cavity. The large dust cavity (~35 au) was first inferred from SED modeling (Marsh & Mahoney 1992) and later confirmed by direct imaging using mm observations (Hughes et al. 2009). High resolution continuum observations at cm and mm wavelengths showed that the outer disk consists of several nested rings surrounded by diffuse continuum emission to ~270 au (Macías et al. 2018) and a small inner disk inside this central gap (Huang et al. 2020). The GM Aur disk has been observed in several gas tracers, with indications of non-Keplerian CO emission (Dutrey et al. 2008) and bright lines associated with photochemistry and small organics (Öberg et al. 2010). The MAPS observations show the presence of spectacular, large-scale CO spiral arms (MAPS XIX, Huang et al. 2021). Herschel observations show modestly depleted oxygen (Keane et al. 2014), but due to its low IR flux, GM Aur was not included in Spitzer surveys (Pontoppidan et al. 2010) so its inner disk molecular content is unknown.

AS 209 is a young (~1 Myr) T Tauri star in the Ophiuchus star-forming region. The disk is highly structured in mm continuum emission with at least 7 nested rings (e.g., Huang et al. 2018a, Guzmán et al. 2018), some of which are associated with ongoing planet formation (Favre et al. 2019). Scattered light observations show three rings in micron-sized dust grains (Avenhaus et al. 2018). The radial gas surface density and CO abundance profiles are also highly structured. Several CO line emission features are spatially related to peaks or gaps in dust emission (Guzmán et al. 2018), while continuum and NIR structures are associated with gas surface density perturbations (Teague et al. 2018). The AS 209 disk has a rich molecular line inventory at millimeter wavelengths (e.g., Öberg et al. 2011, Huang et al. 2017, Bergner et al. 2018, Bergner et al. 2019). Most of the millimeter molecular emission appears depleted towards the disk center but whether this due to dust opacity, excitation, C/N/O abundance, or gas surface density is not yet resolved (but see MAPS XV, Bosman et al. 2021). The inner disk chemistry of AS 209 has not been well characterized, with only upper limits on the water reservoir from Herschel (Du et al. 2017).

HD 163296 is one of the most well studied Herbig Ae star-disk systems at millimeter wavelengths due to its relative proximity and massive disk. The HD 163296 disk presents multiple features that suggest the presence of planets and ongoing planet formation, including dust rings, azimuthal asymmetries, deviations from Keplerian velocities due to gas pressure variations, "kinks" in CO emission, and meridional flows (e.g., Isella et al. 2016, Pinte et al. 2018b, Teague et al. 2019). Observations in scattered light at optical and infrared wavelengths also reveal substantial radial substructure (Monnier et al. 2017, Muro-Arena et al. 2018, Rich et al. 2020). HD 163296 has been observed in CO and other molecular tracers at millimeter wavelengths in a number of single-dish, SMA, and ALMA projects (e.g., Thi et al. 2004, Qi et al. 2011, Öberg et al. 2011, Rosenfeld et al. 2013, Salinas et al. 2017, Pegues et al. 2020). Its popularity in previous surveys can be explained by its bright molecular lines, not only in CO isotopologues, but also in HCN, HCO⁺, deuterated species, and larger organics. The HD 163296 disk is relatively line poor at IR wavelengths (Pontoppidan et al. 2010, Salyk et al. 2011), but H₂O and OH have been detected (Fedele et al. 2012, Banzatti et al. 2017), revealing a disk atmosphere that is not completely dry.

MWC 480 is a ~7 Myr old Herbig Ae star in the Taurus-Auriga star-forming region. Its mm continuum disk has a centrally-peaked profile (Piétu et al. 2006) with two dust rings and low-level emission extending beyond 200 au (Long et al. 2018, Liu et al. 2019). MWC 480 was included in several early disk chemistry programs (i.e., CID, DISCS; Dutrey et al. 2007, Öberg et al. 2010, Henning et al. 2010, Dutrey et al. 2011), which showed that the disk is very bright in some lines, e.g., HCO⁺ and HCN, but weak in others, including N₂H⁺ and H₂CO (e.g., Pegues et al. 2020). The disk has also been probed in simple and complex nitriles, deuterated molecules, and sulfur-bearing species (Öberg et al. 2015a, Guzmán et al. 2015, Huang et al. 2017, Le Gal et al. 2019) and was the target of an unbiased spectral line survey (Loomis et al. 2020), which resulted in detections of several new disk molecules, including C₂D and H₂CS. The MWC 480 water content was probed by the Herschel WISH key program, but was not detected (van Dishoeck et al. 2011). Otherwise, the inner disk chemistry of MWC 480 is unknown.

Note: Stellar luminosities and accretion rates extracted prior to Gaia have been updated with DR2 distances following Andrews et al. 2018 for IM Lup, AS 209, and HD 163296 and Macías et al. 2018 for GM Aur. Due to their large relative uncertainties, those for MWC 480 have not been rescaled. The accretion rate of GM Aur is variable (Ingleby et al. 2015), so we adopted an average value, following Macías et al. 2018.