A SPITZER SURVEY OF MID-INFRARED MOLECULAR EMISSION FROM PROTOPLANETARY DISKS. I. DETECTION RATES

By K. Pontoppidan, C. Salyk, G. Blake, R. Meijerink, J. Carr, J. Najita. 2010

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We present a Spitzer InfraRed Spectrometer search for 10–36 μm molecular emission from a large sample of protoplanetary disks, including lines from H2O, OH, C2H2, HCN, and CO2. This paper describes the sample and data processing and derives the detection rate of mid-infrared molecular emission as a function of stellar mass. The sample covers a range of spectral type from early M to A, and is supplemented by archival spectra of disks around A and B stars. It is drawn from a variety of nearby star-forming regions, including Ophiuchus, Lupus, and Chamaeleon. Spectra showing strong emission lines are used to identify which lines are the best tracers of various physical and chemical conditions within the disks. In total, we identify 22 T Tauri stars with strong mid-infrared H2O emission. Integrated water line luminosities, where water vapor is detected, range from 5 × 10−4 to 9 × 10−3 L☉, likely making water the dominant line coolant of inner disk surfaces in classical T Tauri stars. None of the five transitional disks in the sample show detectable gaseous molecular emission with Spitzer upper limits at the 1% level in terms of line-to-continuum ratios (apart from H2), but the sample is too small to conclude whether this is a general property of transitional disks. We find a strong dependence on detection rate with spectral type; no disks around our sample of 25 A and B stars were found to exhibit water emission, down to 1%–2% line-to-continuum ratios, in the mid-infrared, while more than half of disks around late-type stars (M-G) show sufficiently intense water emission to be detected by Spitzer, with a detection rate approaching 2/3 for disks around K stars. Some Herbig Ae/Be stars show tentative H2O/OH emission features beyond 20 μm at the 1%–2% level, however, and one of them shows CO2 in emission. We argue that the observed differences between T Tauri disks and Herbig Ae/Be disks are due to a difference in excitation and/or chemistry depending on spectral type and suggest that photochemistry may be playing an important role in the observable characteristics of mid-infrared molecular line emission from protoplanetary disks.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/d34f647363211d6504e90efc1a5368fd60ff1c8d

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Citations in the corpus

Astrochemistry and compositions of planetary systems (2020, background): “A similar chemistry regulates the water vapor abundance in the hot gas close to the star in protoplanetary disks (Pontoppidan et al. 2010; Bethell and Bergin 2011).”
RESOLVED IMAGES OF LARGE CAVITIES IN PROTOPLANETARY TRANSITION DISKS (2011, background): “However, new studies indicate that those inner regions are deficient in other molecules (e.g., H2O; see Najita et al. 2010; Pontoppidan et al. 2010), and Dutrey et al. (2008) claim that the GM Aur disk has a R ≈ 20AU region of depleted CO emission.”
Protoplanetary Disks and Their Evolution (2011, background): “Salyk et al. (2008) found similar results toward an additional two disks, and the large survey by Pontoppidan et al. (2010) shows that these lines are common in T Tauri disks.”
Our astrochemical heritage (2012, background): “…the disks surrounding the intermediatemass ( 2 < M/M < 8) Herbig Ae/Be stars, no organic molecules have been detected (Pontoppidan et al. 2010b; Salyk et al. 2011) and water is only seen in the far-infared at larger radii ( 15–20 AU; Fedele et al. 2012), probably due to the larger UV fluxes…”
The molecular composition of the planet-forming regions of protoplanetary disks across the luminosity regime (2015, methodology): “Alternative hypotheses are that the molecular line emission is veiled by the strong stellar continuum emission (as discussed in Pontoppidan et al. 2010) or that the disk dust structure plays an important role.”
Water in Star-forming Regions with the Herschel Space Observatory (WISH). I. Overview of Key Program and First Results (2010, background): “For NGC 2071, which is known from Spitzer data to be particularly rich in H2O emission (Melnick et al. 2008), full PACS spectral scans are performed at a few positions.”
Complex organic molecules in protoplanetary disks (2013, methodology): “Most detections of line emission in the mid-IR have been conducted with theSpitzer Space Telescope and molecules observed include OH, H2O, C2H2, HCN, CO, and CO2 (Lahuis et al. 2006; Carr & Najita 2008; Salyk et al. 2008; Pontoppidan et al. 2010; Bast et al. 2013).”