Accreting Object Spectra

OVERVIEW

Measuring accurate accretion rates of stellar and substellar objects allows us to probe their fundamental formation pathways. Previous studies of excess continuum and line emission have enabled accretion comparisons across mass regimes, ranging from stellar-mass T Tauri stars to substellar objects. However, any astrophysical differences in accretion between these objects remain unclear.

In order to test the different theories of the formation and accretion for substellar objects (i.e brown dwarfs and planetary-mass companions), we need additional observations. Some recent observational projects in this vein are described below:

Graduate student Jada Louison is leading the analysis of spectroscopy from a two-year campaign using the Keck Low Resolution Imaging Spectrometer (LRIS). This imaging campaign expands the sample of substellar objects from Herczeg et al. (2008) into a lower-mass regime across a broad, multiwavelength range spanning 3200-10000 Å.
Our sample consists of six substellar objects ranging in spectral type from M5.5-M9.25 and with masses ~8-60 MJup. Comprehensive characterization of accretion from both continuum excess and line emission is necessary to calibrate scaling relations and improve accretion models. The spectral analysis from this study will aid interpretation of future protoplanet detections and broadly grow our understanding of the accretion physics of young protoplanets.
Graduate student Sarah Betti has led a a 2.5 year observing campaign using SOAR/TripleSpec4.1 to measure accretion rates for a statistical sample of isolated BDs and bound planetary mass companions.
NIR Paβ, Paγ, and Brγ emission line luminosities and ratios allow for comparison with varying accretion and formation models. Further, the observations from this large, systematic, sample are being used to derive new emission-line to total-accretion-luminosity scaling relations for the BD regime, allowing future observers to more accurately interpret the accretion signatures of substellar objects.
As part of this survey, the first NIR accretion signatures from a protoplanet, Delorme 1 (AB)b, were detected. Its measured accretion rate was 3-4x10-8 MJ/yr. Ratios of its NIR emission lines are most consistent with planetary shock accretion models, and its high accretion rate suggests formation via disk fragmentation.
This HST GO proposal, led by PI Connor Robinson, has observations scheduled for August 2023. It aims to take the first spectrum of a young, accreting planetary mass companion spanning FUV to optical wavelengths by targeting Delorme AB(b), a nearby, widely separated, young planetary-mass companion with a moderate accretion rate. This unprecedented measurement will enable us to calibrate substellar accretion diagnostics by directly measuring the NUV accretion continuum excess and probing the structure of the planetary accretion shock through FUV emission lines.
Growing evidence of deviation at low masses above the empirical mass-mass accretion rate relationship established for stars suggests that substellar companions may form through disk fragmentation rather than core collapse. However, models of substellar accretion shocks differ in several fundamental ways from those designed for the stellar magnetospheric accretion paradigm. This suggests that the current stellar-derived, ground-based (e.g., optical/IR emission lines and the Balmer jump) accretion diagnostics may not be suitable for planets and brown dwarfs. These proposed observations will provide a critical test for these scenarios and improve our understanding of the substellar accretion process and the formation mechanisms of planets and brown dwarfs.
While accretion in transitional disks is not well-understood on theoretical grounds, observations have shown that these objects are still actively accreting despite their large cavities. In fact, no significant differences in accretion between transitional and full disks have been found to date. Accretion studies on YSOs with transitional disks are far and few, and time-domain studies are even scarcer. As a result, we have a limited understanding of the timescales of accretion variability in transitional disk systems.
This project seeks to inform the conversation of accretion variability in transitional systems by producing light-curves for 15 of the most well-studied transitional disk bearing stars (GAPlanetS Survey). We then perform a time-series analysis to search for and constrain variability due to changes in accretion rate on second-minute timescales over the course of 5 years.
Classical T Tauri stars are young, low-mass stars that gain mass through the highly variable process of magnetospheric accretion. Simulations of the region where accreted material collides with the stellar chromosphere, known as the accretion shock, predict a thermal instability that may act as one of several possible sources of variability. The predicted periodic behavior ranges from hundredths of a second to an hour, depending on the assumed cooling function, density, accretion stream velocity and metallicity of the accretion shock.
Previous attempts to study this short-term behavior have been limited by either their observational cadence or duration and have not established a clear sense of whether these instabilities are observable. This project analyses month-long, two-minute cadence light curves from TESS of 14 T Tauri stars to perform a comprehensive study of their short-term quasi-periodic behavior.
We process all images from Keck LRIS through a pipeline called PypeIt to create calibrated spectra. The manual serves as a reference to reduce the data successfully and is constantly being updated.
Find the manual here.