Chemical Diversity in Young Protoplanetary Disk
Nami Sakai1 and Yao-Lun Yang2
1Star and Planet Formation Laboratory, RIKEN Cluster for Pioneering Research,
2–1 Hirosawa, Wako, Saitama 351–0198, Japan2
Department of Astronomy, University of Virginia
530 McCormick Road, Charlottesville, VA 22904-4235, USA
nami.sakai@riken.jp

Abstract
Planets are a common product of the star formation process and there is an incredible variety of planetarysystems in the Galaxy that are signiﬁcantly diﬀerent from the Solar System. Recent observations reveal diversity
even in chemical nature: various complex organic molecules are found to be abundant in protoplanetary-disk forming
regions, whose compositions are signiﬁcantly diﬀerent depending on sources. Our Solar system thus may not be acommon type in the Universe, suggesting rarity of our existence. Understanding the initial chemical diversity amongprotoplanetary-disk is one of the key step to know the rarity or similarity. There could be two ways: statistical surveyof chemical composition in embedded protoplanetary-disks and high-resolution observations that reveals detailed
distribution of molecules in representative sources. In both cases, the biggest concern is dust opacity reducing theline intensities at submillimeter wavelengths. Centimeter-wave observations with ngVLA allow us to overcome thediﬃculty.
Key words: Astrochemistry – ISM: molecules – Stars: formation – Stars: protostars – Protoplanetary disks –1. Introduction
In the past two decades, it is clearly demonstrated that envelopes as well as protostellar disks around solar-type protostars have signiﬁcant chemical diversity: some sources harbor various saturated complex organic molecules (COMs), socalled hot corinos (Cazaux et al. 2003; Leﬂoch et al.
2018; Oya et al. 2016), whereas some others harbor unsaturated species instead, so called Warm Carbon-Chain Chemistry(WCCC) sources (Sakai et al.
2008; Yoshida et al.
2019; Sakai et al. 2014a). The chemical diversity would originate from the evolution of the physical environment, such as
the timescale of the starless core phase (Sakai and Yamamoto
2013; Higuchi et al. 2018). Sources showing both types ofmolecules may support this scenario (Oya et al. 2017; Imai etal. 2018). Some theoretical studies have also investigated theorigin of such chemical diversity (Aikawa et al. 2020; Kalvans2021), suggesting that the environment plays a signiﬁcant rolein the production of carbon-chain molecules. Chemical evolution during formation of protoplanetary disk is, therefore, oneof the most important targets to be explored, because it directly
determines the initial chemical compositions of disks, whereplanets form.
While millimeter/submillimeter wave observations with
ALMA have showed detailed chemical compositions of earlystage protostellar disks, the dust opacity sometimes prevents us
from characterizing the molecular properties. High dust opacity reduces the intensity of molecular emission, resulting considerable underestimation of molecular abundances. Thus, it
is diﬃcult to determine whether nondetections of COM emission are due to high dust opacity or lack of COMs. NGC 1333
IRAS 4A1, a Class 0 protostar, is the most demonstrative case.
Known as a system showing hot corino chemistry (Bottinelli etal. 2004), NGC 1333 IRAS 4A turns out to be a binary system, which consists of two Class 0 protostars, A1 and A2. Bothprotostars drive powerful molecular outﬂows (Santangelo et al.
2015), while only 4A2 shows emission of COMs (Persson et al.
2012; Taquet et al. 2015; López-Sepulcre et al. 2017; Sahu etal. 2019). Centimeter wave observation with VLA have madea breakthrough on this problem. De Simone et al. (2020) detected and mapped the CH3 OH emission at 25 GHz band (KBand) in 4A1, which has a stronger dust continuum emission
and no emission of COMs at higher frequencies. They clearly
demonstrated that dust blocks molecular emissions in 4A1 atmillimeter/submillimeter wavelengths, whereas dust becomes
optically thin at centimeter wavelengths allowing us to characterize the chemistry of 4A1. We expect similar situation inother protostellar sources that lead to underestimation of chemical abundance or nondetections.
2. Chemical Diversity Seen in Millimeter/Submillimeter
Observations
In order to understand the chemical diversity, Yang et al.
(2021) surveyed 50 young stellar object located in Perseus
molecular cloud complex (d ∼ 300 pc) by ALMA (the Perseus
ALMA Chemistry Survey: PEACHES). Such survey of COMs
toward large sample of solar-type protostars, has never been
done before. With a spatial resolution of 100 au in radius, rotational transitions of methanol (CH3 OH), acetonitrile (CH3 CN),
methyl formate (CH3 OCHO), and dimethyl ether (CH3 OCH3 )were observed at ∼250 GHz. The survey shows that 58%of the sources harbor CH3 OH with 42% of the sources exhibiting no sign of organic molecules. Figure 1 shows examples of the distribution of CH3 OH (514 − 413 , Eu = 49.66
K) and CH3 OCHO (240/1,24 − 230/1,23 , Eu = 158.23 K) lines.

2[Vol. ,ColorScale x 10x5x 10x 10Fig. 1. Distribution of molecular gas around the protostars. Three typical cases are selected from the 50 sources. Color shows intensities of molecularemission, whereas contours show the thermal emission from dust.

Because the brightness temperature of the 1 mm dust continuum emission is higher toward Barnard 1c, the nondetectionof the lines in IRAS 03235+3004 could be understood by thepoorness of the COMs rather than high dust opacity. Whereas
IRAS 03235+3004 has a brighter continuum indicative of morematerials than that of L1455 IRS4, only L1455 IRS4 showsemission of COMs, suggesting that the appearance of organic
molecules is irrelevant to the total amount of material (Fig.
1). CH3 OH and CH3 OCHO are some of the most frequently
detected molecules in the PEACHES survey, found in 54%
and 30% of the sample, respectively. By modeling the spectra, they further derived the excitation temperature of CH3 OH
and CH3 OCHO ranging from 120–360 K and 80–330 K, respectively. Their ﬁnding is consistent with the hypothesis thatCOMs are evaporated from dust grains in the hot region nearthe protostar.
The derived column densities of CH3 OH and CH3 CN showhuge variance, more than two orders of magnitude (Fig.2a).
Same variance persists after correcting the total gas/dust
amount in each source (Fig.2b). Sources without COMs emission have a wide range of continuum intensities. Physical conditions, such as luminosity and temperature, are also found tobe irrelevant to the chemical abundances. Therefore, the actual
chemical diversity, which could be caused by the diﬀerencein local environmental history of individual target, may be theprimary cause for such variance.
On the other hand, relative ratios between COMs are similar in most of the sources. In particular, CH3 OH and CH3 CN
column densities are tightly correlated with a correlation coefﬁcient of 0.87 (Fig.2). The column densities of CH3 OCHO andCH3 OCH3 also correlate with CH3 OH with lower values of thecorrelation coeﬃcient, 0.76 and 0.48, respectively. Correlation
between the parent molecule and daughter molecule can naturally occur because of the direct relation in their formation
pathways. However, the tight correlation between oxygenbearing COMs and nitrogen-bearing COMs is not easily understood. CH3 OH is known to be formed on grains by hydrogenation of CO (Watanabe et al. 2003), whereas the formation mechanism of CH3 CN, which involves both gas-phase andgrain surface processes, is not well understood yet. The almost

perfect correlation found in the PEACHES survey gives strong
constraints on this problem. However, more detailed comparison in distributions and excitation conditions are required toreveal the origin of the correlation.
Judging from the extremely beautiful correlation between
abundances of CH3 OH and CH3 CN (Fig.2), it seems likelythat the dust opacity plays an insigniﬁcant role on the relation between the two species. However, the sources without
any molecular emission toward the continuum peak may suﬀer
from the opacity problem, leading to apparent absence of COMemission. Furthermore, the dust opacity may aﬀect the derivation of column densities of all COMs, which can not been recognized in the apparent correlation if at all. The large varietyof COMs abundances could be aﬀected by high dust opacity,
especially in the sources with strong continuum emission. To
unambiguously characterize the chemistry in disk-forming regions, we need to address this opacity issue. Centimeter wave
observations of COMs with ngVLA will solve this problem
easily because of the low optical depth of dust at ngVLA frequencies. Thus, we will directly measure the abundance ofCOMs without impedance.
3. Observations with ngVLAIn ngVLA Band 4 (20.5–34 GHz), there is a number of important molecular lines. We can observe the Q-branch lines of
CH3 OH simultaneously. Furthermore, inversion transitions ofNH3 , a fundamental nitrogen-bearing species, can be observedin the same frequency range (Fig.3). This coincidence enables
us to constrain the properties of CH3 OH, kinetic temperatureand column density, at almost the same observing condition
such as beam sizes and weather conditions. Apart from theBand 4, ngVLA can observe important H2 CO lines in Band 3(211 − 212 ) and 6 (101 − 000 ). If we assume an ortho-para-ratioof H2 CO, those combination of lines tells us the H2 density ofthe region. Because larger molecules tend to emit lines at lower
frequencies due to smaller rotational constant B, ngVLA willprobe the inventory of long carbon-chain molecules as well aslarge COMs. Together with au-scale resolution at nearby starforming regions, ngVLA will map the chemical composition

No. ]

3Fig. 2. (a) Direct comparison between column densities of methanol (x-axis) and acetonitrile (y-axis) in logarithmic scale. They have a wide range ofvalues, more than two orders of magnitude. (b) Comparison of the column densities of methanol and acetonitrile after the normalization by the brightnessof the dust emission. Even after the normalization, the data still show tight correlation between the abundances of two molecules across more than twoorders of magnitude.

tary disks and eventually in planetary systems.
3.2. Example 2: Detailed chemical view along disksubstructure formation

Fig. 3. Spectral lines of CH3 OH and NH3 observable in ngVLA Band
4 (20.5-34 GHz)in young disks, which are still growing around embedded protostar and host sites of planet formation. Here we present two
science cases for ngVLA proposal.
3.1. Example 1: Chemical Survey
In the ALMA bands, high dust opacity poses a critical issue
on chemistry surveys. As demonstrated by De Simone et al.
(2020), the observation of one of the most abundant COMs,
CH3 OH, takes 10s of hour per source with VLA, making surveys of large sample of protostars infeasible. With ngVLA, wecan survey COMs and other complex molecules towards a few
10s of sources, such as samples in the PEACHES survey, withan rms of 10 K at 0.3′′ beam (0.44 mJy/beam at 24 GHz) and
0.5 km/s velocity resolution. An hour per source is required togive an approximate exposure time for such survey. Chemicalcompositions of growing disks around Class 0 protostars determine the initial chemical condition of protoplanetary disks.
Thus, measuring the initial chemical composition is extremely
important to understand the chemical evolution in protoplane-The second science case is deep integration observation toward several representative young disks. Around the infantdisks, materials are still accreting onto the disk, keeping thedisk geometrically thicker. Inside such young disk, dust growth
as well as disk substructure formation occur. Recent observations show substructures in young disks around Class 0 protostars (Sheehan & Eisner 2018; Segura-Cox et al. 2020; Oyaet al. 2020). The edge-on disk of L1527, a Class I protostar, shows warped structure as well as ring-like structure(Sakai et al. 2019; Nakatani et al. 2020; Ohashi et al.
2021). These substructures and dynamics can aﬀect the distribution of molecules and regulate the chemical evolution indisk. Thus, mapping the distribution of chemical composition in those substructures, such as rings and gaps, is important to study the initial chemical condition of planet formation, which may cause or may be caused by those substructure formation. Furthermore, in the millimeter/submillimeter
band, we could only obtain chemical composition of the disk
surface, while centimeter-wave observations directly probe thedisk midplane. With ngVLA, we will have necessary spatial
resolution (au scale) at low frequency to characterize the youngand potentially planet-forming disks. If such substructures
are observed by multiple-transition lines at ∼10 au resolution(0.07′′ ), where the substructures are detected, it enable us tomeasure the temperature and density in substructures. Towards
edge-on sources, we can obtain layered structure of chemistry,
which has only be done in models.
#Acknowledgments: The authors thank all the PEACHES
menbers for valuable discussions. The authors also thankYoshimasa Watanabe for the preparation of Figure 3.
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