Large Myr-old Disks Are Not Severely Depleted of Gas-phase CO or Carbon

Abstract

We present an ACA search for [C i] 1–0 emission at 492 GHz toward large T Tauri disks (gas radii ≳ 200 au) in the ∼1–3 Myr-old Lupus star-forming region. Combined with Atacama Large Millimeter/submillimeter Array 12 m archival data for IM Lup, we report [C i] 1–0 detections in six out of 10 sources, thus doubling the known detections toward T Tauri disks. We also identify four Keplerian double-peaked profiles and demonstrate that the [C i] 1–0 fluxes correlate with 13CO, C18O, and 12CO(2–1) fluxes, as well as with the gas disk outer radius measured from the latter transition. These findings are in line with the expectation that atomic carbon traces the disk surface. In addition, we compare the carbon and carbon monoxide (CO) line luminosities of a Lupus and literature sample with [C i] 1–0 detections with predictions from the self-consistent disk thermo-chemical models of Ruaud et al. These models adopt interstellar medium carbon and oxygen elemental abundances as input parameters. With the exception of the disk around Sz 98, we find that these models reproduce all the available line luminosities and upper limits, with gas masses comparable to or higher than the minimum-mass solar nebula and gas-to-dust mass ratios ≥10. Thus, we conclude that the majority of large Myr-old disks conform to the simple expectation that they are not significantly depleted in gas, CO, or carbon.

CI Traces the Disk Atmosphere in the IM Lup Protoplanetary Disk

“…Although IM Lup and HD 142527 are both disks around T Tauri stars, they show more than one order of magnitude difference in their C I column densities. The IM Lup disk is also one order of magnitude more massive (Zhang et al 2021;Lodato et al 2023) than that of HD 142527 (Temmink et al 2023), which implies that the inferred C I column density is not particularly sensitive to the total disk mass, as previously indicated in the models of Pascucci et al (2023). The large millimeter dust cavity (≈100 au) of the HD 142527 disk leads to increased UV transparency, and thus also likely contributes to its larger C I column density.…”

“…We followed standard selfcalibration strategies (e.g., Öberg et al 2021). We performed one round of phase self-calibration (solint = "inf") using the continuum and found similar improvement in the peak continuum S/N as reported by Pascucci et al (2023). We then applied the self-calibration solutions to the full line image cubes and subtracted the continuum using the uvcontsub task with a first-order polynomial.…”

“…We made use of the ALMA archival project 2015.1.01137.S (PI: T. Tsukagoshi), which included Band 8 observations of the IM Lup disk that were originally published in Pascucci et al (2023). The IM Lup disk was observed on 2016 March 25 for 9 minutes using 41 antennas with a projected baseline range of 15-443 m. The [C I] 3 P 1 -3 P 0 (hereafter, [C I] 1-0) line at 492.161 GHz was centered in a spectral window with a frequency resolution of 244 kHz (≈0.15 km s −1 ).…”

“…This limited MRS combined with the modest signalto-noise ratio (S/N) of the observations likely results in the "patchy" appearance of the overall [C I] 1-0 emission morphology evident in the line emission cubes (see the Appendix). Further details about these observations can be found in Pascucci et al (2023).…”

“…C I was originally detected in the DM Tau protoplanetary disk by Tsukagoshi et al (2015), followed by several subsequent disk detections (e.g., Kama et al 2016b;Sturm et al 2022;Pascucci et al 2023). However, the majority of existing C I detections are spatially unresolved.…”