Jialu Li

MIR Absorption Spectrsocopy in Studying Hot Cores

The formation and evolution of massive stars are not well understood because they are deeply embedded. While optical and near-infrared (NIR) observations face high extinction, mid-infrared (MIR) spectroscopy offers a unique opportunity to characterize the rich physical and chemical environment deep inside. The MIR wavelength range is sensitive to the presence of warm gas (several hundred degrees), which is very close to the protostar. The high temperatures in turn result in a rich inventory of molecules that serve as excellent probes. Most interestingly, absorption lines in the MIR are ubiquitously found in hot cores. These absorption components are therefore precisely located in front of the "pencil-beam" source, serving as critical tracers of structures close to the massive protostars. These components are spectrally resolved with high spectral resolution spectrometers, such as iSHELL/IRTF (R~88,000), which is shown in the banner figure above, and EXES/SOFIA (R~50,000).

• "M-band High-Resolution Spectroscopy towards Massive Young Stellar Object W3 IRS 5 with iSHELL/IRTF"
  Li, J., Boogert, A., Barr, A. G., and Tielens, A. G. G. M., ApJ, 935, 161 (2022)
• "High-resolution SOFIA/EXES Spectroscopy of Water Absorption Lines in the Massive Young Binary W3 IRS 5"
  Li, J., Boogert, A., Barr, A. G., DeWitt, C., Rashman, M., Neufeld, D., Indriolo, N., Pendleton, Y., Montiel, E., Richter, M., Chiar, J., Tielens, A. G. G. M., ApJ, 953, 103 (2023)

Disks in Massive Protostars

The ubiquitously observed MIR absorption lines toward massive protostars indicate some fundamental differences compared to low-mass or intermediate-mass protostars, in which these lines are often seen in emission. Analyses of high-resolution MIR spectroscopy show that the absorption lines can originate from the surface of circumstellar disks, which are required to have a vertical outward-decreasing temperature gradient. Indeed, accretion through these disks is considered critical in massive star formation, with the disk photosphere acting as the pencil-beam MIR continuum background. Yet, the existence of these disks remains supported by only a few direct or indirect observations, and the properties of such disks are difficult to determine. We are currently exploring whether we can use the Submillimeter Array (SMA) to observe molecular lines with very high excitation energy (~1000 K), to confirm the existence of disks in W3 IRS 5 through direct imaging.

Interpretation of JWST/MIRI Data with Insights from High-Resolution Spectroscopy

JWST/MIRI is powerful, yet it observes with a spectral resolving power of R~3,500 (~85 km/s in the velocity space). Depending on the astrophysical environment, this resolution may not always be sufficient to resolve the lines individually. Our analysis, based on high-resolution observations from SOFIA/EXES, finds that the analysis of JWST/MIRI spectra can be hampered by the blending of individual transitions into broad absorption features, unresolved optical depth effects, and line depth reduction due to partial coverage of the source. Physical parameters may be derived with inherent degeneracy. Essentially, the issue stems from not properly weighting optically thin and thick lines. This problem can be effectively managed with the "recipes" we provide, such as using molecular transitions that share the same lower energy level to obtain a clearer view of their optical depths.

• "On the Interpretation of Mid-Infrared Absorption Lines of Gas Phase H₂O as Observed by JWST/MIRI"
  Li, J., Boogert, and Tielens, A. G. G. M., ApJS, 273, 32 (2024)

Argus: the 3 mm Focal Plane Array Spectroscopic Imager on the 100 m GBT

Telescope efficiency drops quickly as the frequency increases. It is therefore challenging for large single dish telescopes such as the Green Bank 100 m Telescope (GBT) to observe at high frequency. The spectroscopic imager, Argus, not only enables the GBT to operate at 100 GHz but also features 16 beams that allow data to be collected simultaneously for fast mapping. This is further aided by the small beam size (6-8'') provided by the 100 m aperture of the GBT. We used Argus to make the first map of an external galaxy, IC 342, at 3 mm wavelength with the GBT in just 10 hours in its inner regions. As Argus covers a frequency range from 74-116 GHz, many important molecular lines, such as ¹³CO(1-0), C¹⁸O(1-0), HCN(1-0), and HCO⁺(1-0), can be efficiently mapped. These molecular lines form a "density ladder", serving as probes for gases at different densities.

• "Argus/GBT Observations of Molecular Gas in the Inner Regions of IC342"
  Li, J., Harris, A. I., Rosolowsky, E., Kepley, A., Frayer, D., Bolatto, A., Leroy, A., Meyer, J., Church, S., Gundersen, J., Cleary, K., and the DEGAS team, ApJ, 963, 117 (2024)