Research
My Research
T Tauri Stars and Protoplanetary Disks
Born from cold gas and dust clouds, stars are formed surrounded by disks, which are natural by-products of the star formation process and the nurseries of planets. T Tauri stars are young low-mass stars still surrounded by their disk with ongoing planet formation (protoplanetary disks). My work disentangles the properties of the planet-forming/hosting regions of disks by using the spectra of their host stars, pushing the edge of our understanding of signposts of planet formation in these systems.
Artists concept of a protoplanetary disk.
Schematics of the protoplanetary disk of an accreting T Tauri star. Credit: Marbely Micolta
Using Ca to probe dust evolution in T Tauri stars
We use calcium (Ca) as a proxy for the refractory elements in the protoplanetary disk. Refractory elements are those that need high temperatures (1500 K) to return to the gas phase, remaining in pebbles from the beginning of their trajectory in the disk until they reach the dust wall/inner disk (see Fig). Therefore, they are not affected by mass-loss mechanisms like photoevaporation or winds. This means that the depletion of refractories in the magnetospheric flows can only be explained by dust trapping events in the disk preventing the material from continuing its path toward the dust wall. This allows the pebbles to accumulate and grow into bigger planetesimals. On this basis, we interpret Ca depletion in the inner gas disk as a signpost of disk evolution and/or planet formation in accreting T Tauri Stars.
Probing the inner disk composition with the magnetospheric accretion paradigm
The star itself accretes its mass from the inner gas disk, after all dust particles are sublimated at the dust wall, through a mechanism called magnetospheric accretion. In this process, the stellar magnetic field cuts off the disk at a few stellar radii and matter from the disk free-falls onto the star along the field lines, in the so-called accretion flows, carrying the elements that were not trapped or lost in the outer parts of the disk. My work uses the broad emission lines that characterize the spectra of young stars and are formed in the accretion flows to measure the chemical composition of the inner disk. With this method, we can access the bulk of material without relying on models for the disk and complicated elementary distributions.
Schematic view of a young star accreting from a disk through the stellar magnetosphere. Taken from Hartamann et al. 2016
Recent publications
The Ca II lines as tracers of disk structure in T Tauri Stars: The Chamaeleon I region
How does Ca depletion look like in line profiles?
Both stars above are from the Chameleon I star-forming cloud and have similar physical conditions. Their similarities are reflected in the Hα line (which traces the gas), both having comparable fluxes and strong profiles with high-velocity wings. However, T28 shows much narrower and weak Ca II lines, comparable with the ones of a low accretor. Since the densities and temperatures of the magnetospheric flows of T28 are high (as shown by Hα), the observed weakness of the Ca II lines strongly suggests an absence of Ca in the accretion flows and therefore in the inner gas disk.
How does Ca depletion relate to disk structure?
The figure above shows a SED slope-slope diagram, which allows us to separate disks with inner gaps, likely opened by planets (transitional disks, TD) from typical accreting TTS (full disks, FD). All TD disks show depletion, suggesting that depletion is caused by dust-trapping processes in the disk such as planet formation. Several FD stars can also show depletion; however, these stars also show hints of more evolved disks (e.g. enhanced 10 um silicon feature)
Micolta et al. 2023, ApJ, 953, 177 (see full paper here)
What's next?
Abundance calculations, more star-forming regions, more elements! Working toward a general view and better statistics
Comparison Ratio between the Ca II K and Hα line luminosities vs. accretion rate between magnetospheric accretion models (background) and observations (dots). Models are colored by Ca abundance ([Ca/H]), with purple representing solar abundance (1) and depletion increasing toward red. The gray-out region represents the magnetospheric models without the contribution of the chromosphere. Solid lines represent the median value for a given model abundance (1, 0.5, 0.1, 0.01) and dashed lines represent the 16% and 84% percentiles for the lowest and highest abundance values of the models, respectively. Typical uncertainties for observations are shown in the lower right corner.
Abundance estimates for the Chamaeleon I, Lupus and Ori 1b star-forming regions
(Preliminary work )
I expanded the project to include more star-forming regions, estimating abundances over a range of properties, including mass and age. For each star, I determined the Ca abundance by comparing the observed line ratios with the predictions from a grid of 734,400 magnetospheric accretion models (See Fig on the left!).
Preliminary results show that the Ca deficit is observed for stars of all regions, hinting that this is a general phenomenon in the evolution of protoplanetary disks; we find that all disks with gaps show some degree of Ca deficit, yet Ca depletion does not particularly depend on whether the disk has gaps or not.
Micolta et al. (in review)