# # Program information file # PROGRAM_ID 2022B080 PROGRAM_TITLE Monitoring the molecular absorption of the torus in the PPN CRL618 PROGRAM_INV1 Jose Pablo Fonfria PROGRAM_INV2 Edward J. Montiel PROGRAM_INV3 Matthew J. Richter PROGRAM_INV4 Curtis N. DeWitt PROGRAM_INV5 Jose Cernicharo PROGRAM_SCICAT stellar PROGRAM_ABSTRACT_BEG PPN commonly have a hot central star surrounded by an optically thick circumstellar envelope [CSE] composed of molecular gas and dust. The central star is embedded in a compact HII region, which is contained within the neutral molecular CSE and grows over time as the CSE expands and its optical depth diminishes. This growth was observed in detail in the PPN CRL618. The gas density in the CSE at the Stromgren radius is now ~30-40% lower than 20 yr ago. CRL618 was observed at 8 and 13 um on 2002 with the IRTF/TEXES to observe and analyze the C2H2 and C4H2 spectra. We expect these spectra to have changed over these two decades due to the expansion of the gas in the CSE. However, chemistry is very sensitive to the physical conditions and variations of the molecular abundances might have produced additional changes in the C2H2 and C4H2 lines, which is something unobserved so far. We propose to observe the PPN CRL618 at 8 and 13 um with IRTF/TEXES to compare the spectra of C4H2 and C2H2 with previous observations and shed some light on the chemical evolution in this still poorly understood objects. TEXES will be configured in High_Medium mode to nod along the slit. We will use a slit width of 1.4arcsec [R~85,000]. Two and four settings will cover the spectral ranges at 13 and 8 um, respectively. The settings will be overlapped at each spectral range to provide a good spectral coverage. A telluric calibrator will be observed as well to correct for atmosphere and instrumental fringing. A S/N~140 will allow to properly detect the C2H2 and C4H2 lines, which displayed intensities of 15-20% of the continuum in the 2002 observations. Considering integration on the sources and overheads, we request 3 hr of observing time. PROGRAM_ABSTRACT_END