Sierra Grant

Abundant CO₂ in the disk around GW Lup

The inner 10 au of protoplanetary disks are regions of rich and active chemistry and this chemistry will impact the composition of exoplanets formed there. Spitzer-IRS made great strides in characterizing the chemistry in this region and now, with the sensitivity and spectral resolution of JWST-MIRI, we are able to constrain the content and conditions in the disk better than ever before. The increased sensitivity allows us to characterize fainter sources and detect weaker emission lines, and the increased spectral resolution allows us to study line profiles — which help constrain the gas properties — and deblend lines from multiple species in spectra that are rich in emission features.

The disk around T Tauri star GW Lup was observed with Spitzer-IRS and Pontoppidan et al. 2010 and Salyk et al. 2011 find particularly strong CO₂ emission. JWST-MIRI data, taken as part of the MIRI mid-INfrared Disk Survey (MINDS) GTO program, confirmed the strong CO₂, but also showed emission from H₂O, C₂H₂, HCN, OH, and the carbon dioxide isotopologue ¹³CO₂. The detection of ¹³CO₂ is the first gas-phase detection in a protoplanetary disk and indicates an abundant reservoir of CO₂. Relative to the bright CO₂ emission, the water emission is relatively faint. We suggest that this may be due to the disk around GW Lup being fairly cold or the presence of a small (~ 1 au) gap in the inner disk, which would remove some water but leave abundant CO₂. This work was accepted to the Astrophysical Journal Letters in March 2023 and is available here.

Top: The JWST-MIRI spectrum of the disk around GW Lup. Several prominent features are highlighted and noted. Bottom: The JWST-MIRI data (black) compared to a model (red) composed of emission from multiple molecular species, including ¹³CO₂. Figures from Grant et al. 2023.

Accretion in Intermediate-Mass Stars

Accretion occurs when the gas in the protoplanetary disk falls onto the stellar surface. It is a critical mechanism that both heats the disk and clears it of material. Understanding how this accretion takes place and under what conditions has important implications for disk evolution. The mechanism is well-understood for low-mass stars. In these systems, the hot gas is funneled onto the star via the star’s magnetic field lines. However, more massive stars do not have strong magnetic fields and it is unknown how accretion occurs in these systems. These stars are called Herbig Ae/Bes. Stars that are even higher mass evolve so quickly that we cannot see how they accrete material because the stars are often so embedded in the molecular cloud that we cannot get a clear picture. Therefore, Herbig Ae/Be stars are the link to understanding how high-mass star formation proceeds.

I have performed a survey of Herbig Ae/Be stars with the Lowell Discovery Telescope and Gemini South using the near-infrared spectrograph IGRINS. These observations include a Hydrogen line that is formed as material travels from the disk onto the star. I have used this data to look for accretion trends with stellar mass, age of the system, disk morphology (whether the disk has any gaps or cavities), and more. This work was published in the Astrophysical Journal in February 2022.

The accretion-tracing Brγ line profiles for the 102 Herbig Ae/Be stars that were observed with IGRINS. Figure from Grant et al. 2022.

Understanding Protoplanetary Disk Evolution in the Lynds 1641 Disk Population

Many factors impact protoplanetary disk evolution, including the age of the system and the local environment. Younger systems often have more massive disks with a large population of small dust grains located in the upper layers of the disk atmosphere, giving the disk a flared shape. As the system evolves, the dust grains will settle to the disk midplane, collide, and grow into pebbles and then planetesimals and eventually, terrestrial planets or the cores of giant planets. If the disk is located in a dense stellar environment or near a massive star, the disk can be truncated, leading to small disks with less mass. I am interested in how disks evolve in general, searching for trends with age, location, stellar mass, etc.

The Lynds 1641 (L1641) region is located in the Orion Molecular Cloud A. L1641 is young (~1 Myr) and populous (with an estimated 1600 stars with disks and more evolved systems). This region extends along a filament south of the Orion Nebula Cluster (ONC), such that most of the region is far enough from the massive stars in the ONC that it does not show signs of being photoevaporated from the outside. In Grant et al. 2018, we analyzed far-infrared photometry of disks in this region from the Herschel Space Observatory and found that despite their young ages, the disks in L1641 already showed signs of dust evolution. My coauthors and I recently were awarded time with the Atacama Large Millimeter Array (ALMA) to study L1641 at radio wavelengths that trace the cold dust in these disks. Our results were published in the Astrophysical Journal in June 2021. I gave a talk on our results at the Five Years After HL Tau: a new era in planet formation conference and the talk is available here. 

Here is a subset of the ALMA data in our Lynds 1641 sample. These are continuum images at 1.33 mm which trace the cold dust in the disks.