Authors: Ryan A. Loomis, Karin I. Öberg, Sean M. Andrews, Edwin Bergin, Jennifer Bergner, Geoffrey A. Blake, L. Ilsedore Cleeves, Ian Czekala, Jane Huang, Romane Le Gal, Francois Ménard, Jamila Pegues, Chunhua Qi, Catherine Walsh, Jonathan P. Williams, and David J. Wilner
First Author’s Institution: National Radio Astronomy Observatory, Charlottesville, VA
Status: Published in The Astrophysical Journal
Molecules in Protoplanetary Disks
Protoplanetary disks, which are comprised of gas and dust rotating around young stars, are the cradles of planet formation. Many efforts have focused on measuring the total amount of dust in these disks, as dust grains provide the raw materials to form terrestrial planets and the rocky cores of giant planets. However, the abundance and distribution of molecular gas within these disks also has an outsized impact on the composition of nascent comets and planetesimals. An understanding of the chemical complexity that is present at this early stage of planet formation is also directly relevant to questions of the origins of life. Moreover, molecules serve as valuable tracers of disk properties such as temperature, gas density, stellar mass, and ionization levels.
Observing this molecular gas is, however, not without its challenges. Protoplanetary disks are cold enough for the majority of molecules to freeze out onto the surfaces of dust grains and form ices, rendering them invisible to observations with radio telescopes. As only gas-phase molecules present detectable signatures from their rotational transitions, the amount of detectable emission in such disks is inherently limited. To compensate for this limitation, the majority of disk observations employ targeted studies that focus on a few specific molecules expected to be strongly-emitting. But as a result, molecular inventories toward disks remain incomplete and offer only a partial and potentially biased view of disk chemistry. In fact, only 23 different molecules have been detected in disks, which is a direct consequence of these previously narrow and inconsistent searches. To remedy this, today’s authors present an unbiased spectral line survey of two nearby protoplanetary disks, which provides us, for the first time, a comprehensive view of the chemical complexity within these disks, including the detection of five new molecules.
An Unbiased Spectral Line Survey
Spectral line surveys are conducted over a sufficiently broad frequency range to include the transitions of many different molecules and often lead to unexpected or serendipitous new molecular detections. Historically, they have been a powerful tool to probe the chemistry of cold clouds and star-forming regions, but with the advent of ALMA, sensitive line surveys can now also be efficiently performed at high spatial resolutions toward disks.
Today’s authors conducted such a spectral line survey using ALMA toward the protoplanetary disks around LkCa 15 and MWC 480. Both disks are relatively young (3-7 Myr), host large (>200 AU) gas-rich disks, and reside in the nearby Taurus star-forming region (520 light years). LkCa 15 is a T Tauri star, a type of young, low-mass star, while MWC 480 is a Herbig Ae star, its higher mass counterpart. Since MWC 480 is hotter, more luminous, and more massive than LkCa 15, this allows today’s authors to investigate the influence of disk parameters on disk chemistry and observed line inventories.
The observations spanned a frequency range of nearly 36 GHz from 275 to 317 GHz. An analysis technique known as matched filtering, which uses a filter template based on the known Keplerian rotation of the molecular gas, is used to amplify spectral signatures and results in the spectra shown in Figure 1. In total, 14 different molecules were detected at high signal-to-noise ratios with five of these species (C34S, 13CS, H2CS, DNC, and C2D) being detected for the first time in a protoplanetary disk. Eleven molecules were seen toward MWC 480, while only nine were observed in LkCa 15.
Comparing MWC 480 and LkCa 15
Integrated-intensity emission maps, which show the spatial distribution of flux received from each line, are shown in Figure 2. A broad range of emission patterns are observed. In particular, a double-ring structure is seen in LkCa15 in several molecules, including N2H+, H2CO, and DCO+. Interestingly, both rings show a close association with previously-observed dust features, namely the inner ring lies at the edge of a known dust cavity and the outer ring is located near the edge of the millimeter dust disk. In the case of the outer ring, dust evolution may expose the outer disk to increase levels of irradiation, increasing temperatures and allowing CO to return to the gas phase (as it would otherwise be frozen onto grains at these large radii). If this is the case, it naturally explains why the chemically-linked and CO-sensitive molecules N2H+ and H2CO have double-ring profiles that closely resemble each other.
From Figures 1 and 2, it is clear that the molecular inventories of MWC 480 and LkCa 15 differ dramatically, which is not unexpected given their different stellar and disk masses, radiation environments, and temperatures. To further illustrate this, Figure 3 compares the total flux, which is roughly proportional to the total amount of gas, for each molecule. From this comparison, for instance, we clearly see that, 13C18O is strongly detected in MWC 480 but is not observed in LkCa 15 (also see Figure 1). Thus, we can conclude that MWC 480 has a more massive gas-phase reservoir of CO and can be explained by the fact that in the colder LkCa 15 disk, much of the CO is likely frozen out and is not detectable in the gas phase.
Missing Complex Organic Molecules?
Predictions from chemical models suggest that numerous larger, complex organic molecules, such as CH3OH, should have been detectable in this line survey, but as illustrated in Figure 4, they are not observed. In fact, CH3CN was the most complex molecule to be securely detected and even so, it was only seen in MWC 480. Thus, this spectral line survey indicates that emission from complex organic molecules in disks may be consistently suppressed.
Despite the absence of larger molecules, these results show that spectral line surveys of disks are valuable tools to not only detect new molecules, but also provide a more comprehensive view of disk chemistry. Additional line surveys taken at different frequencies can provide access to different lines and molecules, while a larger sample of disks will help to robustly confirm the chemical trends seen in this work.