Organic molecules in protoplanetary disks around T Tauri and Herbig Ae stars

By Wing-Fai Thi Gerd-Jan van Zadelhoff, E. E. M. L. Observatory, Nl, U. Amsterdam, D. Physics, Astronomy, U. London, UK., K. Instituut. 2004

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The results of single-dish observations of low- and high-J transitions of selected molecules from pro- toplanetary disks around two T Tauri stars (LkCa 15 and TW Hya) and two Herbig Ae stars (HD 163296 and MWC 480) are reported. Simple molecules such as CO, 13 CO, HCO + , CN and HCN are detected. Several lines of H2CO are found toward the T Tauri star LkCa 15 but not in other objects. No CH3OH has been detected down to abundances of 10 −9 − 10 −8 with respect to H2. SO and CS lines have been searched for without suc- cess. Line ratios indicate that the molecular emission arises from dense (10 6 -10 8 cm −3 ) and moderately warm (T ∼ 20-40 K) intermediate height regions of the disk atmosphere between the midplane and the upper layer, in accordance with predictions from models of the chemistry in disks. The sizes of the disks were estimated from model fits to the 12 CO 3-2 line profiles. The abundances of most species are lower than in the envelope around the solar-mass protostar IRAS 16293-2422. Freeze-out in the cold midplane and photodissociation by stellar and interstellar ultraviolet photons in the upper layers are likely causes of the depletion. CN is strongly detected in all disks, and the CN/HCN abundance ratio toward the Herbig Ae stars is even higher than that found in galactic photon-dominated regions, testifying to the importance of photodissociation by radiation from the central object in the upper layers. DCO + is detected toward TW Hya, but not in other objects. The high inferred DCO + /HCO + ratio of ∼0.035 is consistent with models of the deuterium fractionation in disks which include strong depletion of CO. The inferred ionization fraction in the intermediate height regions as deduced from HCO + is at least 10 −11 − 10 −10 , comparable to that derived for the midplane from recent H2D + observations. Comparison with

Subfields: Astronomy and Astrophysics, Spectroscopy

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In Astronomy and Astrophysics

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6. T. Birnstiel
7. C. Qi
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In Spectroscopy

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Citations in the corpus

• Radiation thermo-chemical models of protoplanetary disks I. Hydrostatic disk structure and inner rim (2009, background): “…(e.g. Beckwith et al., 1990), gas observations are intrinsically more difficult and have focused until recently on rotational lines of abundant molecules such as CO, HCN, HCO+ (e.g. Beckwith et al., 1986; Koerner et al., 1993; Dutrey et al., 1997; van Zadelhoff et al., 2001; Thi et al., 2004).”
• A SPATIALLY RESOLVED VERTICAL TEMPERATURE GRADIENT IN THE HD 163296 DISK (2013, background): “The Herbig Ae star HD 163296 (spectral type A1) hosts a protoplanetary disk with bright continuum emission (Allen & Swings 1976; Mannings & Sargent 1997; Natta et al. 2004) and a rich molecular spectrum (Qi 2001; Thi et al. 2001, 2004; Qi et al. 2013).”
• Molecules with ALMA at Planet-forming Scales (MAPS). I. Program Overview and Highlights (2021, methodology): “Millimeter and submillimeter astronomy has been used since the late 1980s to explore disk molecular lines and the underlying disk chemistry (e.g., Weintraub et al. 1989; Koerner et al. 1993; Dutrey et al. 1997; Kastner et al. 1997; Thi et al. 2004).”
• Complex organic molecules in protoplanetary disks (2013, methodology): “We have converted the IRAM 30 m and JCMT line intensities from Thi et al. (2004) using the standard relation ( T 1 K ) = ( S ν 1 Jy beam−1 )”
• Astrochemistry and compositions of planetary systems (2020, background): “Most of these have been detected at millimeter and sub-millimeter wavelengths (Dutrey et al. 1997; Aikawa et al. 2003; Thi et al. 2004; Chapillon et al. 2012; Öberg et al. 2015; Walsh et al. 2016; Favre et al. 2018; Le Gal et al. 2019a), but HCN is also observed at IR wavelengths, and C2H2 is only…”
• CHEMICAL PROCESSES IN PROTOPLANETARY DISKS (2010, background): “…2006, 2008), DM Tauri (Dutrey et al. 1997; Ceccarelli et al. 2004, 2005; Guilloteau et al. 2006; catherine.walsh@qub.ac.uk Dutrey et al. 2007; Piétu et al. 2007) and LkCa 15 (van Zadelhoff et al. 2001; Aikawa et al. 2003; Qi et al. 2003; Thi et al. 2004; Dutrey et al. 2007; Piétu et al. 2007).”
• Millimeter imaging of HD 163296: probing the disk structure and kinematics (2007, result): “The inferred outer radius of the disk is 550± 50 AU, more than two times larger than the value obtained from the continuum and similar to the result obtained by Thi et al. (2004) from the model fitting of single dish line profiles.”
• Our astrochemical heritage (2012, unknown): “…of depletion (Dutrey et al. 1996, 2007a; Qi et al. 2004), CN, HCN, HNC, CS, HCO+, C2H and H2CO (Dutrey et al. 1997; van Zadelhoff et al. 2001; Thi et al. 2004; Chapillon et al. 2012b), N2H+ (Dutrey et al. 2007b), SO (Fuente et al. 2010), CS (Dutrey et al. 2011), DCO+ (van Dishoeck et al.…”
• A PARAMETRIC MODELING APPROACH TO MEASURING THE GAS MASSES OF CIRCUMSTELLAR DISKS (2013, background): “CO is the most abundant molecule after H2 and its millimeter wavelength rotational lines are the strongest that are observable from the ground (Thi et al. 2004; Dent et al. 2005).”
• CO emission from discs around isolated HAeBe and Vega-excess stars (2005, unknown): “The second is that the line profile is clearly asymmetrical; a similar asymmetry was seen in the reconstructed interferometric spectrum of J=2-1 12CO in Mannings et al. and the 13CO single-dish spectrum in Thi et al. (2004), although the s:n of both of these spectra are lower than shown in Fig.”