First Detection of Near-Infrared Line Emission From Organics in Young Circumstellar Disks

Abstract

We present an analysis of high-resolution spectroscopy of several bright T Tauri stars using the VLT/CRIRES and Keck/NIRSPEC spectrographs, revealing the first detections of emission from HCN and C 2 H 2 in circumstellar disks at near-infrared wavelengths. Using advanced data reduction techniques we achieve a dynamic range with respect to the disk continuum of ∼ 500 at 3 µm, revealing multiple emission features of H 2 O, OH, HCN, and C 2 H 2 . We also present stringent upper limits for two other molecules thought to be abundant in the inner disk, CH 4 and NH 3 . Line profiles for the different detected molecules are broad but centrally peaked in most cases, even for disks with previously determined inclinations of greater than 20 • , suggesting that the emission has both a Keplerian and non-Keplerian component as observed previously for CO emission. We apply two different modeling strategies to constrain the molecular abundances and temperatures: we use a simplified single-temperature LTE slab model with a Gaussian line profile to make line identifications and determine a best-fit temperature and initial abundance ratios, and we compare these values with constraints derived from a detailed disk radiative transfer model assuming LTE excitation but utilizing a realistic temperature and density structure. Abundance ratios from both sets of models are consistent with each other and consistent with expected values from theoretical chemical models, and analysis of the line shapes suggests the molecular emission originates from within a narrow region in the inner disk (R < 1 AU). 1

DIGIT survey of far-infrared lines from protoplanetary disks

“…AS 205 and RNO 90 have a rich OH and H 2 O spectrum ranging from the pure-rotational lines detected here to ro-vibrational lines detected with NIRSPEC and CRIRES (3 μm) and Spitzer/IRS (10−40 μm) (Salyk et al 2011b;Pontoppidan et al 2010;Mandell et al 2012). The energy levels involved range from a few hundred to a few thousand K. The H 2 O-rich PACS spectrum is unlikely to originate from the same region of the disk as the shorter wavelength data.…”

“…The inventory of molecular species detected in T Tauri sources in the infrared includes: CO (e.g. Najita et al 2003), OH and H 2 O (e.g., Carr et al 2004;Salyk et al 2008), HCN and C 2 H 2 (e.g., Carr & Najita 2008, 2011Pascucci et al 2009;Mandell et al 2012) and, finally CO 2 (Pontoppidan et al 2010). Are Herbig sources also different from T Tauri sources at far-IR wavelengths?…”

The Depletion of Water During Dispersal of Planet-Forming Disk Regions

“…At 12.4 μm, within the Spitzer-IRS range, a few water lines have been resolved in velocity in four disks with VLT-VISIR and Gemini-TEXES (Δv ∼ 15 and ∼ 3.5 km s −1 , respectively; Lagage et al 2004;Lacy et al 2002), providing support to the emitting regions estimated by models for the Spitzer spectra (Pontoppidan et al 2010b;Banzatti et al 2014;Salyk et al 2015). At shorter wavelengths (∼2-3 μm), hotter water (T≈900-1500 K) has been spectrally resolved and analyzed in only four disks so far, by Salyk et al (2008) and Mandell et al (2012) with Keck-NIRSPEC and VLT-CRIRES, and by Carr et al (2004) with IRTF-CSHELL. At longer wavelengths (>55 μm), colder water has been surveyed in disks by Herschel-PACS, finding similarly low detection rates in disks around solar-mass as in disks around intermediate-mass stars (∼15% as based on the 63.3 μm line; see references in Table 1 and Rivière Marichalar et al 2016).…”

“…If water vapor in inner disks is depleted from smaller to larger disk radii, an essential spectral region to include in the investigation of this process is where the hot water emits, in high-energy transitions at 2.9-3 μm (e.g., Mandell et al 2012). In Sections 2 and 3 we describe the new survey of hot water emission as observed with CRIRES at 2.9 μm, and the comparative analysis with the CO data set at 4.7 μm.…”

“…Three of these disks were previously published in Mandell et al (2012). Three main-sequence stars were observed to be used as photospheric standards, sampling stellar spectral types from K to M (with spectral types taken from Gray et al 2006;Koen et al 2010): NLTT29176 (K3), Walsh et al 2015 Spitzer fluxes Spitzer limits Antonellini et al 2016 S p it z e r s e n s it iv it y S p it z e r s e n s it iv it y HD111631 (K7), and HIP49986 (M1.5).…”