Measuring HCN Emission in TTS Disks with Low-resolution Spitzer Spectroscopy

Habitable planets are expected to form in the warm inner disks (<3-4 AU for sun-like stars) of young stars, so studying this region is key to understanding the origin and evolution habitable planet systems. Higher-resolution (R~600) observations with Spitzer IRS have revealed emission features of simple organic molecules (HCN, C2H2, CO2) and water in the warm inner disk, and indicate that such mid-IR molecular emission is common and can be used to probe the thermal and chemical structure of the inner gaseous disk (Carr & Najita 2008; Salyk et al. 2008; Pontoppidan et al. 2010; Carr & Najita 2011; Bast et al. 2013; Najita et al. 2013). Unfortunately, IRS observations with Spitzer are no longer possible, leaving the Spitzer IRS archive as the primary resource for sensitive mid-IR spectroscopy of protoplanetary disks. The archive is populated with more low-resolution (R~100) data than high, motivating the question: How much information pertaining to molecular emission is preserved in (and able to be gleaned from) the low-resolution data? I set out to answer this question by measuring molecular emission strength in 8 “normal” (see Furlan et al. 2006) TTS sources that had both high- and low-resolution IRS data (though not taken contemporaneously).

I focused on the 14 μm HCN feature, previously found to be bright in TTS (Pascucci et al. 2009) (other molecular emission features were not detectable in all of our sources). I wrote routines in IDL to measure the molecular emission equivalent widths and their errors in previously-reduced IRS spectra.

From Teske et al. 2010: 11–15 μm spectrum of AA Tau as observed in the SH (R~700, bottom) and SL (R~100, top) modes. The middle spectrum is the SH spectrum smoothed to the resolution of the SL data and rebinned to the pixel sampling of the SL data. Several prominent molecular features are marked with vertical lines. The high-resolution data reveal a rich emission spectrum that is common among TTS.

Overall, we found that the high- and low-resolution measurements were correlated at a statistically significant level, meaning that low-resolution spectra can recover quantitative trends in molecular emission strength seen in higher resolution observations.

A variety of physical and chemical processes can affect molecular emission strength, so spectra of large sample of TTS (such as those low-resolution observations in the Spitzer IRS archive) are a valuable tool for determining the most influential processes. Thus, we also examined an additional 10 low-resolution-only spectra. While the full sample of 18 low-resolution spectra was relatively uniform in spectral type, it varied more in stellar accretion rate and stellar X-ray luminosity. We made use of these variations in stellar parameters to investigate what might cause the variation observed in HCN feature strength, and found tentative correlations between HCN flux and both stellar accretion rate and X-ray luminosity. We suggested that these correlations originated from accretion-driven mechanical heating and/or photochemistry in the inner disk atmosphere.

My previous advisor Dr. Najita recently connected these studies of molecular emission features in TTS disks to my current project (detailed below) related to host-star C/O ratios and the implications for exoplanet characterization (Najita et al. 2013, “The HCN-Water Ratio in the Planet Formation Regions of Disks”). This demonstrates the applicability of this first research project to multiple aspects of star and planet formation, and motivates my future research.