Protoplanetary disk evolution: from inner disk accretion to outer disk dust evolution

Protoplanetary disks are the essential link between molecular clouds and planetary systems. Protoplanetary disk evolution determines the resulting planetary systems. In this dissertation, I focus on tracing evolution in populations of disks by analyzing large samples of disks. This dissertation is made possible by new observations and physically-motivated models. The main processes that I use to study disk evolution are the dust growth and evolution that occur in the outer disk and the accretion of gas from the inner disk onto the star. Lynds 1641 (L1641) is a star-forming region in the Orion Molecular Cloud A, and it has great potential as a laboratory for protoplanetary disk evolution. I present observations of disks in L1641 from the Herschel Space Observatory and the Atacama Large Millimeter Array (ALMA). The far-infrared Herschel data are sensitive to micron-sized dust grains in the outer disk atmosphere, and the radio ALMA data trace the millimeter-sized dust grains in the disk midplane. I use accretion disk models to show that the far-infrared data are consistent with disks that show signs of dust evolution, even in this young (~1.5 Myr) region. I compare the L1641 millimeter data to other surveys and show that L1641 is at a stage of evolution between young protostellar systems and more evolved disks where planet formation is already well-underway. Accretion of material onto a star is an important mechanism in protoplanetary disk dispersal and heating. The classical magnetospheric accretion paradigm is well-understood and established for low-mass stars. However, higher-mass stars may not have the magnetic field strength for magnetospheric accretion to occur. I present a large survey of intermediate-mass systems with near-infrared spectra. I use the accretion-tracing Brγ line to find trends with system properties and find a break in the accretion rate—stellar mass relationship, which may indicate a break in the accretion mechanism. Additionally, I use magnetospheric accretion models to reproduce the observations to determine the accretion properties in a subset of objects, finding that these models can reproduce fast-moving emission

The M˙\dot{M}--MdiskM_{\rm{disk}} relationship for Herbig Ae/Be stars: a lifetime problem for disks with low masses?

The accretion of material from protoplanetary disks onto their central stars is a fundamental process in the evolution of these systems and a key diagnostic in constraining the disk lifetime. We analyze the relationship between the stellar accretion rate and the disk mass in 32 intermediate-mass Herbig Ae/Be systems and compare them to their lower-mass counterparts, T Tauri stars. We find that the $\dot{M}$--$M_{\rm{disk}}$ relationship for Herbig Ae/Be stars is largely flat at $\sim$10$^{-7}$ M$_{\odot}$ yr$^{-1}$ across over three orders of magnitude in dust mass. While most of the sample follows the T Tauri trend, a subset of objects with high accretion rates and low dust masses are identified. These outliers (12 out of 32 sources) have an inferred disk lifetime of less than 0.01 Myr and are dominated by objects with low infrared excess. This outlier sample is likely identified in part by the bias in classifying Herbig Ae/Be stars, which requires evidence of accretion that can only be reliably measured above a rate of $\sim$10$^{-9}$ M$_{\odot}$ yr$^{-1}$ for these spectral types. If the disk masses are not underestimated and the accretion rates are not overestimated, this implies that these disks may be on the verge of dispersal, which may be due to efficient radial drift of material or outer disk depletion by photoevaporation and/or truncation by companions. This outlier sample likely represents a small subset of the larger young, intermediate-mass stellar population, the majority of which would have already stopped accreting and cleared their disks.Comment: 16 pages, 3 figures, accepted to A

Full L- and M-band high resolution spectroscopy of the S CrA binary disks with VLT-CRIRES+

The Cryogenic IR echelle Spectrometer (CRIRES) instrument at the Very Large Telescope (VLT) was in operation from 2006 to 2014. Great strides in characterizing the inner regions of protoplanetary disks were made using CRIRES observations in the L- and M-band at this time. The upgraded instrument, CRIRES+, became available in 2021 and covers a larger wavelength range simultaneously. Here we present new CRIRES+ Science Verification data of the binary system S Coronae Australis (S CrA). We aim to characterize the upgraded CRIRES+ instrument for disk studies and provide new insight into the gas in the inner disk of the S CrA N and S systems. We analyze the CRIRES+ data taken in all available L- and M-band settings, providing spectral coverage from 2.9 to 5.5 $\mu$m. We detect emission from $^{12}$CO (v=1-0, v=2-1, and v=3-2), $^{13}$CO (v=1-0), hydrogen recombination lines, OH, and H$_2$O in the S CrA N disk. In the fainter S CrA S system, only the $^{12}$CO v=1-0 and the hydrogen recombination lines are detected. The $^{12}$CO v=1-0 emission in S CrA N and S shows two velocity components, a broad component coming from $\sim$0.1 au in S CrA N and $\sim$0.03 au in S CrA S and a narrow component coming from $\sim$3 au in S CrA N and $\sim$5 au in S CrA S. We fit local thermodynamic equilibrium slab models to the rotation diagrams of the two S CrA N velocity components and find that they have similar column densities ($\sim$1-7$\times$10$^{17}$ cm$^{-2}$), but that the broad component is coming from a hotter and narrower region. Two filter settings, M4211 and M4368, provide sufficient wavelength coverage for characterizing CO and H$_2$O at $\sim$5 $\mu$m, in particular covering low- and high-$J$ lines. CRIRES+ provides spectral coverage and resolution that are crucial complements to low-resolution observations, such as those with JWST, where multiple velocity components cannot be distinguished.Comment: 15 pages, 13 figures, accepted to A&

Scanning disk rings and winds in CO at 0.01-10 au: a high-resolution MM-band spectroscopy survey with IRTF-iSHELL

We present an overview and first results from a $M$-band spectroscopic survey of planet-forming disks performed with iSHELL on IRTF, using two slits that provide resolving power R $\approx$ 60,000-92,000 (5-3.3 km/s). iSHELL provides a nearly complete coverage at 4.52-5.24 $\mu$m in one shot, covering $>50$ lines from the R and P branches of $^{12}$CO and $^{13}$CO for each of multiple vibrational levels, and providing unprecedented information on the excitation of multiple emission and absorption components. Some of the most notable new findings of this survey are: 1) the detection of two CO Keplerian rings at $<2$ au (in HD 259431), 2) the detection of H${_2}$O ro-vibrational lines at 5 $\mu$m (in AS 205 N), and 3) the common kinematic variability of CO lines over timescales of 1-14 years. By homogeneously analyzing this survey together with a previous VLT-CRIRES survey of cooler stars, we discuss a unified view of CO spectra where emission and absorption components scan the disk surface across radii from a dust-free region within dust sublimation out to $\approx10$ au. We classify two fundamental types of CO line shapes interpreted as emission from Keplerian rings (double-peak lines) and a disk surface plus a low-velocity part of a wind (triangular lines), where CO excitation reflects different emitting regions (and their gas-to-dust ratio) rather than just the irradiation spectrum. A disk+wind interpretation for the triangular lines naturally explains several properties observed in CO spectra, including the line blue-shifts, line shapes that turn into narrow absorption at high inclinations, and the frequency of disk winds as a function of stellar type.Comment: Accepted for publication on The Astronomical Journa

The kinematics and excitation of infrared water vapor emission from planet-forming disks: results from spectrally-resolved surveys and guidelines for JWST spectra

This work presents water emission spectra at wavelengths covered by JWST (2.9-12.8 $\mu$m) as spectrally-resolved with high resolving powers (R = 30,000-100,000) using ground-based spectrographs. Two new surveys with iSHELL and VISIR are combined with previous spectra from CRIRES and TEXES to cover parts of multiple ro-vibrational and rotational bands observable within telluric transmission bands, for a total of 85 disks and $\approx160$ spectra. The general expectation of a range of regions and excitation conditions traced by infrared water spectra is for the first time supported by the combined kinematics and excitation as spectrally resolved at multiple wavelengths. The main findings from this analysis are: 1) water lines are progressively narrower going from the ro-vibrational bands at 2-9 $\mu$m to the rotational lines at 12 $\mu$m, and partly match a broad (BC) and narrow (NC) emission components, respectively, as extracted from ro-vibrational CO spectra; 2) rotation diagrams of resolved water lines from upper level energies of 4000-9500 K show curvatures indicative of optically thick emission ($\approx 10^{18}$ cm$^{-2}$) from a range of excitation temperatures ($\approx$ 800-1100 K); 3) the new 5 $\mu$m spectra demonstrate that slab model fits to the rotational lines at $> 10 \mu$m strongly over-predict the ro-vibrational emission bands at $< 9 \mu$m, implying non-LTE excitation. We discuss these findings in the context of a emission from a disk surface and a molecular inner disk wind, and provide a list of detailed guidelines to support the analysis and interpretation of spectrally-unresolved JWST spectra.Comment: Posted on arXiv as submitted to AJ, for immediate access by teams working on the analysis of JWST spectr

MINDS. JWST-MIRI Reveals a Dynamic Gas-Rich Inner Disk Inside the Cavity of SY Cha

SY Cha is a T Tauri star surrounded by a protoplanetary disk with a large cavity seen in the millimeter continuum but has the spectral energy distribution (SED) of a full disk. Here we report the first results from JWST-MIRI Medium Resolution Spectrometer (MRS) observations taken as part of the MIRI mid-INfrared Disk Survey (MINDS) GTO Program. The much improved resolution and sensitivity of MIRI-MRS compared to Spitzer enables a robust analysis of the previously detected H2O, CO, HCN, and CO2 emission as well as a marginal detection of C2H2. We also report the first robust detection of mid-infrared OH and ro-vibrational CO emission in this source. The derived molecular column densities reveal the inner disk of SY Cha to be rich in both oxygen and carbon bearing molecules. This is in contrast to PDS 70, another protoplanetary disk with a large cavity observed with JWST, which displays much weaker line emission. In the SY Cha disk, the continuum, and potentially the line, flux varies substantially between the new JWST observations and archival Spitzer observations, indicative of a highly dynamic inner disk.Comment: 19 pages, 10 figures, 5 tables, accepted for publication in Ap