Yao-Lun Yang

    I am an indefinite-term Research Scientist at the Star and Planet Formation Lab in RIKEN. My research focuses on characterizing the kinematics, especially infall, and astrochemistry at the embedded phase of star formation. I use observations at near-infrared to sub-millimeter wavelengths with various telescopes (Spitzer, Herschel, JWST, SOFIA, ALMA, and VLA) to unveil the gas kinematics and chemical compositions. I also use radiative transfer calculations to test models of star formation. Recently, I have been working on characterizing the chemical signatures, particularly the complex organic molecules, in protostars to understand their formation pathways and find the connections to the organic matter in our solar system. I am leading a JWST GO Cycle 1 program to study the ice chemistry in embedded protostars. I also work on testing models of massive star formation and outflows in low-mass protostars.

     How do dense cores collapse into protostars? The infall of gas is essential to the current paradigm of star formation. In our ALMA Cycle 4 observation of an embedded protostar, BHR 71 IRS1, we unambiguously detect the redshifted absorption along with the blue-asymmetric double-peaked profile due to the infall. I test different kinematics of models of collapsing envelopes and constrain the abundance profile of HCO⁺ and other molecules with 3D non-LTE radiative transfer calculation. Interestingly, we also found several complex organic molecules (COMs) within 100 AU around the source. This is the first detection of COMs toward BHR 71 IRS1. We also found some of the emission of COMs trace the kinematics of the rotation-dominated inner envelope.

Left (Top): The distribution of the rotational temperature from the COPS-SPIRE sample shows two distinct groups when fitted with three and four temperature components.
Right (Bottom): The CO J = 6→5 line correlates the best with the total CO luminosity, suggesting that we can use CO J = 6→5 to estimate the total CO luminosity.

     The Herschel Space Observatory opened up the far-infrared universe, allowing us to probe into the deeply embedded protostars. In this "COPS-SPIRE" program, we focused on the origin of the CO emission. The SPIRE instrument covers J_up=4 to 13, where the origin of the CO emission is suggested to change from the entrained gas to shock-heated gas. We show this effect in the two distinct groups of the rotational temperatures of CO, even if the emission is not resolved by SPIRE. We further looked into the spatial extent of the CO emission, which mostly traces the outflows for the low-J lines. We found that the "bipolarity" decreases as the J level increases, which is consistent with the scenario where the shocked gas starts to dominate the CO emission at higher energy. All the spectra and line fitting results are released along with the journal article.

Left (Top): The constrained gas density structure of BHR 71. The contours indicate the radius in 10000 AU increment.
Right (Bottom): The modeled spectral energy distribution compared with the observations. We focus on the comparison at representative wavelengths, which are shown in red and blue open circles.

    Protostars embedded within their envelope in the early stage, preventing them being directly observed and studied. I demonstrate the approach of using 3-D radiative transfer simulation to understand the underlying structure of an embedded protostars, BHR71. The observed SED can be successfully fitted by structures of rotating collapse envelope, disk, and outflow cavity. I found BHR71 can be best fitted with an envelope with age of 36000 years and sound speed of 0.37 km/s. The best fit model requires a constant density region at the inner region of outflow cavity follewed by a power-law decrease at larger radius.

    In the collaboration with Joel Green and Neal Evans, we reduced the data taken by DIGIT, FOOSH, and COPS Herschel programs with advanced correction, such as jitter correction and extended source correction. Moreover, I have applied an automatic line fitting routine to all sources in this archive, producing almost 20000 lines in total. The line fitting code is written in IDL, and can be found in my github repository. Please see our products and detail document at the buttons below.

Left (Top): One of the features of this archive is the high-quality line-free continuum and flat spectrum. With the automatic line fitting routine, we are able to remove all fitted lines and produce line-free continuum, which is useful for further SED modeling.
Right (Bottom): This figure shows that our spectral products have about 40% better flux calibration to the photometry than the original Herschel Science Archive.