Signatures of warm carbon monoxide in protoplanetary discs observed with Herschel SPIRE

M. H. D. van der Wiel, D. A. Naylor, I. Kamp, F. Menard, W-F Thi, P. Woitke, G. Olofsson, K. M. Pontoppidan, J. Di Francesco, A. M. Glauser, J. S. Greaves, R. J. Ivison

Research output: Contribution to journalArticlepeer-review

Abstract

Molecular gas constitutes the dominant mass component of protoplanetary discs. To date, these sources have not been studied comprehensively at the longest far-infrared and shortest submillimetre wavelengths. This paper presents Herschel SPIRE FTS spectroscopic observations towards 18 protoplanetary discs, covering the entire 450-1540 GHz (666-195 mu m) range at upsilon/Delta upsilon approximate to 400-1300. The spectra reveal clear detections of the dust continuum and, in six targets, a significant amount of spectral line emission primarily attributable to (CO)-C-12 rotational lines. Other targets exhibit little to no detectable spectral lines. Low signal-to-noise detections also include signatures from (CO)-C-13, [C I] and HCN. For completeness, we present upper limits of non-detected lines in all targets, including low-energy transitions of H2O and CH+ molecules. The 10 (CO)-C-12 lines that fall within the SPIRE FTS bands trace energy levels of similar to 50-500 K. Combined with lower and higher energy lines from the literature, we compare the CO rotational line energy distribution with detailed physical-chemical models, for sources where these are available and published. Our (CO)-C-13 line detections in the disc around Herbig Be star HD 100546 exceed, by factors of similar to 10-30, the values predicted by a model that matches a wealth of other observational constraints, including the SPIRE (CO)-C-12 ladder. To explain the observed (CO)-C-12/(CO)-C-13 ratio, it may be necessary to consider the combined effects of optical depth and isotope selective (photo) chemical processes. Considering the full sample of 18 objects, we find that the strongest line emission is observed in discs around Herbig Ae/Be stars, although not all show line emission. In addition, two of the six T Tauri objects exhibit detectable (CO)-C-12 lines in the SPIRE range.

Original languageEnglish
Pages (from-to)3911-3925
Number of pages15
JournalMonthly Notices of the Royal Astronomical Society
Volume444
Issue number4
DOIs
Publication statusPublished - 11 Nov 2014

Keywords

  • astrochemistry
  • protoplanetary discs
  • circumstellar matter
  • stars: variables: T Tauri, Herbig Ae/Be
  • MAIN-SEQUENCE STARS
  • FOURIER-TRANSFORM SPECTROMETER
  • HERBIG AE/BE STARS
  • TW-HYDRAE ASSOCIATION
  • FAR-INFRARED LINES
  • HD 100546
  • KEPLERIAN DISK
  • FORMING REGION
  • CO EMISSION
  • AE STARS

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