ALMA chemical survey of disk-outflow sources in Taurus (ALMA-DOT)

Abstract

The chemical composition of planets is inherited from that of the natal protoplanetary disk at the time of planet formation. Increasing observational evidence suggests that planet formation occurs in less than 1−2 Myr. This motivates the need for spatially resolved spectral observations of young Class I disks, as carried out by the ALMA chemical survey of Disk-Outflow sources in Taurus (ALMA-DOT). In the context of ALMA-DOT, we observe the edge-on disk around the Class I source IRAS 04302+2247 (the butterfly star) in the 1.3 mm continuum and five molecular lines. We report the first tentative detection of methanol (CH3OH) in a Class I disk and resolve, for the first time, the vertical structure of a disk with multiple molecular tracers. The bulk of the emission in the CO 2−1, CS 5−4, and o–H2CO 31, 2 − 21, 1 lines originates from the warm molecular layer, with the line intensity peaking at increasing disk heights, z, for increasing radial distances, r. Molecular emission is vertically stratified, with CO observed at larger disk heights (aperture z/r ∼ 0.41−0.45) compared to both CS and H2CO, which are nearly cospatial (z/r ∼ 0.21−0.28). In the outer midplane, the line emission decreases due to molecular freeze-out onto dust grains (freeze-out layer) by a factor of > 100 (CO) and 15 (CS). The H2CO emission decreases by a factor of only about 2, which is possibly due to H2CO formation on icy grains, followed by a nonthermal release into the gas phase. The inferred [CH3OH]/[H2CO] abundance ratio is 0.5−0.6, which is 1−2 orders of magnitude lower than for Class 0 hot corinos, and a factor ∼2.5 lower than the only other value inferred for a protoplanetary disk (in TW Hya, 1.3−1.7). Additionally, it is at the lower edge but still consistent with the values in comets. This may indicate that some chemical reprocessing occurs in disks before the formation of planets and comets.

ALMA chemical survey of disk-outflow sources in Taurus (ALMA-DOT)

“…Its almost edge-on orientation allows us to clearly disentangle the red-(southern) and blue-shifted (northern) lobes. The systemic velocity estimated from the velocity distribution of the bright H 2 CO line is +5.6 km s −1 (Podio et al 2020a), in good agreement with IRAM 30-m CN, H 2 CO, CS, and C 17 O spectra of Guilloteau et al (2013Guilloteau et al ( , 2016. The rotation pattern has been imaged also by Podio et al (2020a), who found, using CO, H 2 CO, and CS emission, a molecular emission which is vertically stratified (see Sect.…”

“…The comparison between the moment 0 distributions of H 2 CS with those of H 2 CO (Podio et al 2020a) is also instructive (see Fig. 6), showing a slightly different radial distribution of H 2 CO and H 2 CS.…”

“…For HL Tau, we derived, for the first time, the [H 2 CS]/[H] abundance in a protoplanetary disk ( 10 −14 ). The o-H 2 CS(7 1,6 −6 1,5 ) line emits where the emission of CS(5-4) and o-H 2 CO(3 1,2 − 2 1,1 ) is brighter (Podio et al 2020a), i.e. the so-called warm molecular layer.…”

“…More specifically, multiline CS emission has been observed towards ≥ 10 disks (e.g. Dutrey et al 1997Dutrey et al , 2017Fuente et al 2010;Guilloteau et al 2013Guilloteau et al , 2016Teague et al 2018;Phuong et al 2018;Semenov et al 2018;Le Gal et al 2019;Garufi et al 2020;Podio et al 2020a). SO emission has been detected in fewer disks, part of them still associated with accretion, as e.g.…”

“…65. On the other hand, the IRAS04302 disk Guilloteau et al (2013); Podio et al (2020a), located at d = 161 pc (Galli et al 2019), is more extended (∼ 350 au), and is associated with an almost edge-on geometry, i ∼ 90 • (Wolf et al 2003). The coordinates of the continuum peak, 133 mJy beam −1 , are α(J2000) = 04 h 33 m 16 s .47, δ(J2000) = +22 • 53 20 .…”