The Hot and Dynamic Birth of Massive Stars from the ngVLA Perspective
Kei E. I. Tanaka1,2 , Yichen Zhang3 , and Kazuhito Motogi4
1

Center for Astrophysics and Space Astronomy, University of Colorado Boulder, Boulder, CO 80309, USA: kei.tanaka@colorado.edu
2
ALMA Project, National Astronomical Observatory of Japan, 2–21–1 Osawa, Mitaka, Tokyo 181–8588, Japan
3
Star and Planet Formation Laboratory, RIKEN Cluster for Pioneering Research, Wako, Saitama 351–0198, Japan
4
Graduate School of Sciences and Technology for Innovation, Yamaguchi University, Yamaguchi 753-8512, Japan

Abstract
The Next Generation Very Large Array (ngVLA) has excellent capabilities to unveil various dynamical andchemical processes in massive star formation at the unexplored innermost regions. Based on the recent observationsof ALMA/VLA as well as theoretical predictions, we propose several intriguing topics in massive star formationfrom the perspective of the ngVLA. In the disk scale of <∼ 100 au around massive protostars, dust grains are expected
to be destructed/sublimated because the physical conditions of temperature, shocks, and radiation are much more
intense than those in the envelopes, which are typically observed as hot cores. The high sensitivity and resolutionof the ngVLA will enable us to detect the gaseous refractories released by dust destruction, e.g., SiO, NaCl, andAlO, which trace disk kinematics and give new insights into the metallic elements in star-forming regions, i.e.,
astromineralogy. The multi-epoch survey by the ngVLA will provide demographics of forming massive multiples
with separations of <∼ 10 au with their proper motion. Combining with observations of refractory molecular lines andhydrogen recombination lines, we can reproduce the three-dimensional orbital motions of massive proto-binaries.
Moreover, the 1-mas resolution of the ngVLA could possibly take the first-ever picture of the photospheric surface
of an accreting protostar, if it is bloated to the au scale by the high accretion rates of mass and thermal energy.
Key words: massive star formation — close binaries — jets and outflows — accretion disks — astrochemistry1.

Introduction:
The Innermost region in Massive Star Formation> 4
Massive stars (>∼ 10M⊙ , ∼ 10 L⊙ ) are rare, but play pivotal roles in a wide range of astrophysical and astrochemicalsettings in galaxies. However, massive star formation is still
poorly understood compared to low-mass star formation, because they are born deep in obscure clouds in large distancesof 1–10 kpc (Tan et al. 2014; Motte et al. 2018). The Next
Generation Very Large Array (ngVLA) will uncover the hot
and dynamic birth of massive stars by its unprecedented milliarcsecond resolution at transparent radio wavelength.
Figure 1 shows a schematic view of a massive protostar at
the scale of ∼ 1–1000 au (please note that it gives only a roughidea for reference). Gas material accretes onto the central massive protostar from the infalling envelope and then through theaccretion disk, which is formed by the angular momentum conservation. At the same time, multiple feedback processes ejecta certain fraction of material from the system. As approaching
the central hot star, the accreting material changes its physicalconditions, such as temperature, chemical compositions, ionization states, and dust properties, and thus the emission mechanism at radio wavelength changes accordingly. The envelope
and the disk are mostly neutral and traced by dust continuum
and molecular lines; the disk surface and the innermost disk
are ionized emitting free-free continuum and recombinationlines; the strong shocks by the jet emit synchrotron radiation.
At present, sub-arcsecond observations by the Atacama Large
Millimeter/submillimeter Array (ALMA) have successfully resolved the structures of infalling envelopes, and start to report
the existence of accretion disks around some massive proto-stars at scales of 100–1000 au (see Hirota 2018; Beltrán 2020,
for recent reviews). The ngVLA will achieve an order-ofmagnitude higher resolution, addressing questions about various dynamical and chemical processes in the inner region of< 100 au. The ngVLA may take the first-ever picture of the
protostellar surface if it is in the bloated phase. The lower
frequency of the ngVLA is also a significant advantage tolook into the details of the vicinity of massive protostars, because the dust opacity is lower and the emission lines are lesscrowded. In this article, we propose several intriguing topics inmassive star formation from the ngVLA perspective.
2.

3D Orbital Dynamics in Massive Proto-multiples

The first topic is the dynamics of forming massive multiple
systems. It is well known that stellar multiplicity increases withthe stellar mass, and massive stars almost always have multiple companions, particularly at small separations (Duchêne &Kraus 2013). So, massive star formation is essentially “massive
multiple formation.” Moreover, massive close binaries are important systems as the modern analog of progenitors of merging black-hole binaries. Broadly speaking, the high fractionof close massive multiples is considered as the consequenceof disk fragmentation due to gravitational instability, because
the typical accretion rate in massive star formation is as highas 10−3 M⊙ yr−1 (c.f., about 10−6 M⊙ yr−1 in low-mass starformation). However, the detailed dynamics of disk fragmentation and the multiplicity demographics at birth are still theleast understood. The ngVLA will dramatically improve thissituation by its milli-arcsecond resolution.
The current VLA and ALMA have been reporting mas-2[Vol. ,

> 4
Fig. 1. Schematic view of the innermost region around a massive protostar (>∼ 10M⊙ , ∼ 10 L⊙ ). The typical distances to massive protostars are
1–10 kpc. The ngVLA will be able to address various dynamical and chemical processes in the disk scale of <∼ 100 au, and may directly take thefirst-ever picture of the photospheric surface if the massive protostar is in the bloated phase (Sec. 4.1). The three-dimensional full orbits of closecompanions will be recorded in a timescale of months/years (Sec. 2). The “hot-disk” chemistry of refractory species such as silicon/metal compounds,
induced by the destruction of dust grains, is an excellent topic to investigate by the ngVLA (Sec. 3). Multiple feedback processes, e.g., photoevaporation,
MHD disk winds, and jets, will be spatially resolved, and their impact on the accreting material will be quantitatively measured (Sec. 4.2). Magnetic
fields are not drawn to avoid confusion, but will also be traced by polarized emissions. Emission mechanisms tracing these processes at radio wavelengths
are indicated by bold italic fonts.

sive proto-binary systems with the apparent separation of several hundred au with their highest resolutions of ∼ 20–50 mas(e.g., Beuther et al. 2017; Zhang et al. 2019b; Tanaka et al.
2020; Figs. 2 and 3). The ngVLA will discover closer systems with the separations of ∼ 1–10 au. The significant advantage to study closer systems is the short orbital periods−1/2
of Porb ∼ 3000 (Mtot /20M⊙ )(a/10 au)3/2 days, where
Mtot and a are the total mass and the semimajor axis of the
proto-binary (for simplicity, the gravity from other companions and surrounding materials is ignored). Therefore, massive
binaries with separations of ∼ 1–10 au have typical orbital periods of ∼ 100−3000 days. The ngVLA can trace the orbital motion of such massive proto-multiples on a monthly/yearly basis.
Closer companions are more common around formed massive
stars (Sana 2017). At the period of log Porb /days ≃ 3, formed
O- and B-type main-sequence stars have the companion frequency of about 30% (Moe & Di Stefano 2017). Considering
the later decaying of multiple systems, the companion frequency would be even higher in the formation stage.
Continuum imaging at the higher frequencies of ngVLA,
i.e., Band 6 (93 GHz) and Band 5 (41 GHz), is ideal for measuring the proper motions of protostars and associated substructures, because the higher resolution can be achieved andthe emission intensity is typically higher. The continuum emission from dust and free-free would be mixed at this frequency
range (Tanaka et al. 2016; Rosero et al. 2019), but we candisentangle this degeneracy using the multi-band observations(Sec. 4.2). While similar projects were proposed for the studiesof low-mass multiple formation (Tobin et al. 2018) and planet
formation (Ricci et al. 2018), the reward of targeting massive
multiples is the high brightness temperature, as well as the highcompanion frequency. The brightness temperature is estimatedhigher than 500 K at the scale of <∼ 10 au around massive pro4>
tostars (>
10M
,
10
L
),
which
is detectable (S/N > 5) in⊙ ∼⊙∼an hour even with the 1-mas resolution. Therefore, the multiepoch survey to make the demographics of massive multiple
orbits will be a low-cost, high-benefit project by the ngVLA.
We note that, even with the higher resolutions of 0.3 mas, theone-hour observation at Band 6 would be able to point out positions of massive protostars associated with the small photoionized regions, because free-free emission can be as bright as
10, 000 K (see also Sec. 4).
In addition to the proper motion on the sky, the radial motionof each protostar is measurable by emission lines. As an example study by ALMA, Zhang et al. (2019b) measured the radial
velocity difference of two massive protostars in IRAS 07299–
1651 using H30α recombination line emission, and investigated the possible binary orbits and their dynamical masses(Fig. 2). Hydrogen recombination lines (HRLs) would be
the best line tracing the orbital radial velocities, because theyarise mostly from the vicinity of massive protostars (Fig. 1).
Bands 6 and 5 cover the n + 1 → n transitions (nα transitions) with n = 38–44, and n = 51–59, respectively. The velocity resolution of several km s−1 is required to reproduce theline profile and the velocity difference of massive binaries witha = 10 au. The brightness temperature of HRLs strongly depends on the evolutionary stage of massive protostars (Sec. 4).
If HRLs at Band 6 are as bright as 1, 000–10, 000 K (Zhang
et al. 2019a; Zhang et al. 2019c), the ngVLA can achieve asignificant detection (S/N > 5) with the 5-mas resolution in 5
hours. On the other hand, if HRLs are dimmer as < 100 K,
molecular lines of refractory species, i.e., SiO, NaCl, and AlO,

No. ]

3

Fig. 2. The massive proto-binary system in IRAS 07299–1651 (Zhang et al. 2019b). (a) 1.3 mm continuum on the envelope scale. (b) The proto-binary
seen by the 1.3 mm continuum (color scale and grey contours) and the H30α line emission (thick black contours). (c) H30α spectra of the two protostars,
showing a velocity offset of ∼ 10 km s−1 due to orbital motion. (d) Distribution of H30α emission centroids of source A at different velocities, tracing
the disk rotation.

can be utilized instead (Sec. 3). In such a case, the 10-mas
resolution with 10 hours would be required for a significant
detection. By combining the proper motion in continuum andthe radial velocity of line emission, we will reconstruct thethree-dimensional orbital dynamics of forming massive multiples. The observations of full orbital dynamics of massive
multiples in early, deeply embedded phase will help us enormously to understand the complex birth of massive multiples.
3.

“Hot Disks” with Gaseous Refractories and Highly
Excited Molecules

The second topic is massive protostellar disks associatedwith a new type of astrochemistry. Rotating accretion disks
play various essential roles in massive star formation: closecompanions likely form via disk fragmentation, magnetohydrodynamic (MHD) outflows are centrifugally driven by disk
rotation, and accretion flow circumvents super-Eddington radiation pressure to dust opacity by disk shadowing. Detailed
observations of massive protostellar disks are necessary to understand those essential dynamics in massive star formation.

ALMA recently started to discovered disks around some ofmassive protostars. However, even by the highest-resolutionof ALMA, we can spatially resolve the disk structures only inlimited sources with large disks of >∼ 1000 au and/or close distances of <
2
kpc
(Maud
et
al.
2019;
Motogi et al. 2019; Zapata∼
et al. 2019a; Johnston et al. 2020). Higher-resolution observation by the ngVLA is awaited to investigate the substructures
and the dynamics inside the disks towards statistically significant sources.
The knowledge of appropriate emission lines is the key tostudy the kinematics and chemical conditions. In the envelope
scale of 1000 au–0.1 pc (the outer region in Fig. 1), the gas
temperature reaches higher than 100 K, where various complex
molecules are released to the gas phase by sublimation of ice
mantles of dust grains. This phase/region of massive protostars, so-called the “hot (molecular) cores”, has been studied
well as the cosmic laboratory of chemical processes of interstellar molecules. The envelope dynamics are investigated utilizing the hot-core molecular lines, such as CH3 CN and SO2
with the energy states of Eu /kB >∼ 100 K. The astrochemical

4[Vol. ,

Fig. 3. The massive proto-binary system in IRAS16547–4247 (Tanaka et al. 2020): (a) The 1.3 mm continuum (black contours), and the spectral index(α = log Iν /log ν) map between 1.3 and 3 mm (color scale). (b–d) The integrated blue-shifted and red-shifted emission maps of three hot-disk molecularlines, H2 O (v2 = 1), NaCl, and SiO (blue and red contours) overlaid on the 1.3 mm continuum emission (grey scale and white contours). The H2 O
and NaCl lines tracing the rotation of the disks (with disk directions indicated by the cyan lines), and the SiO line traces the outflow bases (with outflow
directions indicated by the yellow lines).

research on new heavy complex organic molecules and prebiotic molecules in hot cores and their low-mass siblings (hotcorinos) is one of the key science goals by the ngVLA (Beltrán& Rivilla 2018; McGuire et al. 2018).
In the smaller scale of <∼ 100 au around massive protostars(the inner region in Fig. 1), however, the physical conditionsof temperature, density, shocks, and radiation fields are much
more intense than those in hot cores. Thus we propose different types of molecular lines as the tracers of massive protostellar disks: (1) lines of refractory molecules, and (2) highenergy molecular lines of Eu /kB > 1000 K, which together
we refer to as “hot-disk” lines. The gaseous refractory species
are the products of dust destruction, and the highly excitedmolecules exist due to the high temperature and strong radiation. In the case of the massive proto-binary system IRAS
16547–4247, Tanaka et al. (2020) detected emission lines of refractory species (NaCl, SiO, and SiS) and vibrationally-excitedwater lines (H2 O (v2 = 1) with Eu /kB > 3000 K) as goodprobes of the individual disks (Fig. 3). These molecular lines
exclusively trace the individual disks at the 100 au scale out of

the 1000 au circum-binary structure. The same types of molecular lines are also reported in disks around some other massive
protostars (Ginsburg et al. 2019; Maud et al. 2019; Zhang et al.
2019a). Emission lines of aluminum monoxide (AlO), one of
the most refractory materials, are detected at the 100au scale ofOrion Source I (Tachibana et al. 2019). The presence of those
hot-disk lines suggests the hot and dynamic nature of massive
protostellar disks at the scale of <∼ 100 au.
These hot-disk molecular lines are excellent tracers to investigate the kinematics of disks by the ngVLA observations.
By spatially resolving disk kinematics, the dynamical massesof the protostars will be accurately estimated. The kinematics of substructures, such as arms, clumps, and gaps between
circum-multiple and circum-stellar disks, will directly exhibit
the formation of the multiple systems, testing the theories of
disk fragmentation. Some hot-disk lines also trace the material
at disk surfaces and above, where the disk winds are launched
and accelerated (e.g., Hirota et al. 2017; Zhang et al. 2019a;
Fig. 3d). Combined with polarization observations, we can
examine the MHD-driven disk wind models. Moreover, as inNo. ]

5the case of the orbits of multiple systems, multi-epoch observations of hot-disk lines will enable us to reproduce the threedimensional dynamics of these accreting/outflowing flows.
Many transitions of refractory molecules and vibrationallyexcited water exist in the radio frequency of the ngVLA coverage. For example, NaCl and KCl have their rotational transitions from J = 1–0 to 8–7 and from J = 1–0 to 15–14, respectively (also their vibrationally excited states). The brightness temperature of hot-disk lines is about 50–150 K in IRAS
16547–4247, in which ALMA barely resolved the emitting
spots (e.g., Tanaka et al. 2020). The ngVLA will spatially resolve the hot-disk regions, and the peak emission is expected
to be brighter. If the brightness temperature is 500 K at Band5, the ngVLA will detect the line emission (S/N> 5) in 2
hours with the angular and velocity resolutions of 10 mas and
3 km s−1 .
The radio observation of hot-disk lines is a new excellent
method to investigate metallic elements in star-forming regions, which has a unique link to meteoritics. Primitive meteorites contains the oldest materials in our solar system, i.e.,
Ca-Al-rich inclusions (CAIs) and chondrules, which were sublimated or molten once in the hot proto-solar disk. How
and where CAIs and chondrules formed is still a big mysteryin planetary science. While meteorites (and materials fromsample-return spacecraft missions) keep the records in solid
phase, observations of hot-disk lines provide the complementary gas-phase information of refractory species in star-forming
disks. Further observations of the hot-disk chemistry could
give unique insights into the formation of the oldest meteoritic
inclusions, and thus the origin of our Solar system.

ature, which is not enough to create H II regions.
Most star-formation researches have treated the central staras just a point with mass and luminosity, although it is the leadrole in star formation. The first-ever direct observations of accreting protostars will be a revolutionary leap in this research
field. The existence of massive bloated stars itself is the direct test of the theory of massive protostellar evolution, whichis also applied for the seed formation of supermassive black
holes in the early universe (Hosokawa et al. 2013). Normally,stellar evolution calculations are conducted with the assumption of the spherical shape (with small oblate distortion by therotation). However, avoiding the spin-up to the breakup velocity, a highly accreting star could be bar-like shape, releasing the angular momentum by the gravitational torque (Lin etal. 2011). In this case, the boundary with the accretion disk
would be fuzzy. Or the magnetic fields may play importantrole to control the accretion rates of mass and angular momentum, e.g., magnetospheric accretion and accretion-driven flare(Romanova et al. 2009; Takasao et al. 2019). Moreover, thedisk ablation by strong stellar winds could limit mass accretion at the vicinity of the protostar (Kee & Kuiper 2019). As
massive stars commonly have close companions, the au-scale
radii of bloated stars would enhance event rates of the stellarinteraction, such as mass transfer or merger, that may cause theexplosive outcomes as seen in some planetary nebulae (Soker& Kashi 2012). The dynamical timescale at the stellar surface
is shorter than a year, and thus those dynamical interactionsof the star and the disk can be recorded like “movies” by the
ngVLA.

4.

After the bloated phase, as the stellar mass grows to
10–20M⊙ , the accreting protostar turns into the Kelvin–
Helmholz contraction approaching to the zero-age main sequence (ZAMS) star. The ionizing photon rate rapidly explodes as the contraction proceeds, because the peak of thestellar spectrum goes beyond the Lyman limit of 91.2 nm.
A photoionized region is formed around the massive protostar, and the radio luminosity increases by orders of magnitude. The stellar surface observation becomes difficult in thisphase. Instead, the ionized gas provides observational diagnostics of the protostellar evolution and the photoionization feedback (Tanaka et al. 2016). As an example, Zhang et al. (2019c)
discovered a large (> 1000 au) photo-evaporative outflow fromthe very massive protostar G45.45+0.05, using high-resolution
observations of ALMA at 1.3 mm and VLA at 7 mm (Fig.
4). The modeling of free-free emission provided the density
and temperature profiles of the photoionized gas. Also, H30αline emission traces the outflowing kinematics of the photoevaporation flow. Since the central protostar of G45 is alreadyevolved and very massive as 40M⊙ , its photo-evaporation flowis large and bright enough to be investigated in detailed by current ALMA and VLA. The ngVLA will be able to conduct
similar studies towards less-evolved and dimmer sources (seealso Galván-Madrid et al. 2018), and test the feedback modelsof photo-evaporation and MHD-driven outflows/jets (Tanaka et
al. 2017; Kuiper & Hosokawa 2018).
The radio continuum from massive protostars contains threecomponents of dust, free-free, and synchrotron (Fig. 1). We

Protostellar Evolution and Photoionization

One of key characteristics of massive protostars is their drastic evolution, particularly in their radii and ionizing luminosity.
The ngVLA will let us closely observe the evolutionary stagesof massive protostars, and the dynamics of photo-ioionized gas.
4.1.

Direct Imaging of Bloated Protostars

Massive protostars typically accrete at the rates of
10−3 M⊙ yr−1 . Under such high rates, protostars efficiently
accumulate thermal energy and become bloated. Theoretically,
it is proposed that the radii reach as large as 1 au, or even larger,
at around 10M⊙ (Hosokawa & Omukai 2009; Haemmerlé etal. 2016). The ngVLA has the ability to directly capture thesurfaces of such bloated protostars! If the apparent diameterof a bloated star has the apparent size larger than 1 mas (i.e.,1(d/kpc) au in the diameter), the ngVLA sensitivity is highenough to detect the bloated photosphere (6000 K) in less than
one hour at Bands 3–6 (16–93 GHz) with the 1-mas resolution.
Moreover, even with higher resolution of ∼ 0.3 mas, the Band6 observation will be able to capture the protostellar surface
towards these bloated protostars in few-hour integration time.
Fortunately, the obscuring effect by the surrounding structures
would be minimal. Dust grains should sublimate at the vicinityof the protostar. Even if dust exists, it is likely to be opticallythin at these frequencies. Free-free by the surrounding ionizedgas could be problematic at these frequencies, but the massive
protostars in bloated phase have relatively low surface temper-

4.2.

Photoionization Traced by Free-Free and HRLs

6[Vol. ,

Fig. 4. The photo-evaporative outflow in the massive protostar G45.47+0.05 (Zhang et al. 2019c): (a) The 1.3 mm continuum emission observed by
ALMA. (b) Integrated blue-shifted and red-shifted H30α emissions in the blue and red color scales, overlaid with the total integrated emission in green
contours. (c) The spectral index map between 1.3 and 7 mm, combining ALMA and VLA data. (d) Distributions of H30α emission centroids at different
velocities, tracing disk rotation and outflow acceleration.

can solve the degeneracy of these emissions by the multi-band
observations, since each component typically has different
spectral index, i.e., α = dlogIν /dlogν: α >∼ 2 for dust, α ≃ 0–2
for free-free, and α <0forsynchrotron,
respectively.
Figure 4c∼shows the index map between 1.3 and 7 mm of G45.45+0.05,
that unveils the collimated jets (α <∼ 0) surrounded by the bipolar photo-evaporation flow (α ∼ 0–1). The presence of the synchrotron jets indicates that the disk accretion is still ongoing inspite of the strong photo-evaporation feedback. In the case of
the massive proto-binary system IRAS16547–4247 (Fig. 3a),the index map nicely illustrates the dusty circum-binary structure (α > 2), the twin protostars with ionized gas (α ∼ 1), andthe aligned jet knots (α <∼ 0). By separating these components,
we are able to estimate the dust masses, the ionizing luminosities of the protostars, and possibly the magnetic field strength
at the jets. The multi-band imaging by the ngVLA with its highsensitivity and resolution will let us to seek into very first stageof the creation of the H II regions, as well as the radio jets, indetails towards the significant numbers of sources.
HRLs are good tracer to investigate the kinematics of theionized gas. Despite that many transitions of HRLs are observable in the frequency range of the ngVLA, they are typicallybrighter at higher frequencies. Meanwhile, the free-free emission is also optically thinner in higher frequency bands. Thus,
the HRL observations at Bands 5 and 6 will be important toinvestigate the innermost kinematics (see also Sec. 2). In thecase of G45.45+0.05, the H30α emission (231.90093 GHz) isstrong enough so that we are able to probe the small shift ofits emission centroid over velocities (Fig. 4d). Such shift ofemission centroids trace the disk rotation and the outflow acceleration in the 100 au scale, which is much smaller than the
beam size. Similarly, Figure 2d shows the H30α centroids in
IRAS 07299–1651, probing the rotation of the ionized disk at
the 10 au scale. Band 5 and 6 HRLs can trace the kinematicsof the innermost ionized-gas in a similar way, and the ngVLA
may even resolve such inner regions without centroid analysis.
Theoretical studies proposed that the radiative MHD processes
at the innermost scale of 1–100 determine the total impact offeedback toward the core scale of 0.1 pc: the disk shadowing
allows the mass accretion under the super-Eddington luminos-

No. ]
ity, the UV absorption at the dust sublimation front significantly regulates the mass loss by photo-evaporation, and the
MHD jets/outflows are driven by disk accretion/rotation (e.g.,
Matsushita et al. 2017; Tanaka et al. 2017; Kuiper & Hosokawa
2018; Rosen & Krumholz 2020). The high-sensitivity and resolution of the ngVLA with the bright HRLs will enable us to directly observe the central engines of those feedback processes.
At lower frequencies, HRLs become less bright and the freefree emission from the innermost region is likely to be optically thick. However, with stacking, HRLs at lower frequencies could still be useful to trace the photo-evaporative flow inlarger scales.
5.

Summary and Discussion

We discussed potential research topics in massive star formation from the ngVLA perspective. The superb resolution of
the ngVLA will allow us to investigate the unexplored inner
regions of 1–100 au around massive protostars. Various structures on disks, i.e., arms, gaps, disk winds, as well as companions, will be captured in detailed. Moreover, the ngVLA
would be able to take the first-ever picture of the surfaces of
the accreting protostars, if they are in the bloated phase.
Thanks to the short dynamical timescale and the brightline emissions of the innermost regions, we will beable to reproduce the three-dimensional dynamical motions,such as companion migration, disk fragmentation, and accreting/outflowing flows by the multi-epoch observations.
Particularly, the demographics of forming massive binary orbits is quite important because formed massive stars almost always have close companions.
While heavy complex organic molecules and prebiotic
molecules are the key targets of the ngVLA astrochemistry,these molecules are not suitable for exclusively tracing the innermost region of massive star formation. Instead, we propose
the new sets of emission lines as the tracers of the hot, dynamic
disks at the scale of < 100 au: (1) lines of refractory molecules,e.g., silicon compounds and alkali metal halides, (2) highenergy molecular lines of Eu /k > 1000 K, e.g., vibrationallyexcited water, and (3) hydrogen recombination lines from ionized gas of 10, 000 K. The radio observation of the gaseous refractory species will also provide new insights into the metallic
elements in star-forming regions, which has a unique connection to astromineralogy and meteoritics.
We focused on the smallest scale at 1–100 au in this article. However, we cannot make the comprehensive scenario
of massive star formation without understanding the largerscale processes, which dominates the total mass budget. The
high-sensitivity of the ngVLA will also very useful to investigate the larger-scale dynamics, e.g., core-scale fragmentation,
disk formation, mass-loading processes by large-scale outflows(e.g., Beltrán et al. 2018). Toward the new era of ngVLA,
we also need conduct the detailed theoretical/numerical studies of < 100 au, e.g., driving MHD/radiative disk winds, dust
growth/destruction, hot-disk chemistry, and the star-disk interaction. Particularly, synthetic observational modeling with thengVLA capabilities based on those studies will maximize thepotential of the ngVLA and help us to understand the dynamics
and chemistry in massive star formation.

7
References
Beltrán, M. 2020, The Star Formation Newsletter, 329, 1(arXiv:2005.06912)
Beltrán, M. T. & Rivilla, V. M. 2018, Science with a Next Generation
Very Large Array, 517, 249
Beltrán, M. T., Zhang, Q., & Rivilla, V. M. 2018, Science with a Next
Generation Very Large Array, 517, 293
Beuther, H., Linz, H., Henning, T., et al. 2017, A&A, 605, A61
Duchêne, G. & Kraus, A. 2013, ARA&A, 51, 269
Ginsburg, A., McGuire, B., Plambeck, R., et al. 2019, ApJ, 872, 54
Galván-Madrid, R., Beltrán, M., Ginsburg, A., et al. 2018, Science
with a Next Generation Very Large Array, 517, 309
Haemmerlé, L., Eggenberger, P., Meynet, G., et al. 2016, A&A, 585,
A65
Hirota, T. 2018, Publication of Korean Astronomical Society, 33, 21
Hirota, T., Machida, M. N., Matsushita, Y., et al. 2017, Nature
Astronomy, 1, 0146
Hosokawa, T., & Omukai, K. 2009, ApJ, 691, 823
Hosokawa, T., Yorke, H. W., Inayoshi, K., et al. 2013, ApJ, 778, 178
Johnston, K. G., Hoare, M. G., Beuther, H., et al. 2020, A&A, 634,
L11
Kee, N. D. & Kuiper, R. 2019, MNRAS, 483, 4893
Kuiper, R. & Hosokawa, T. 2018, A&A, 616, A101
Lin, M.-K., Krumholz, M. R., & Kratter, K. M. 2011, MNRAS, 416,
580
Matsushita, Y., Machida, M. N., Sakurai, Y., et al. 2017, MNRAS,
470, 1026
Maud, L. T., Cesaroni, R., Kumar, M. S. N., et al. 2019, A&A, 627,
L6
McGuire, B. A., Carroll, P. B., & Garrod, R. T. 2018, Science with a
Next Generation Very Large Array, 517, 245
Moe, M. & Di Stefano, R. 2017, ApJS, 230, 15
Motte, F., Bontemps, S., & Louvet, F. 2018, ARA&A, 56, 41.
doi:10.1146/annurev-astro-091916-055235
McKee, C. F. & Tan, J. C. 2008, ApJ, 681, 771
Motogi, K., Hirota, T., Machida, M. N., et al. 2019, ApJL, 877, L25
Oliva, G. A. & Kuiper, R. 2020, A&A, 644, A41
Ricci, L., Isella, A., Liu, S., et al. 2018, Science with a Next
Generation Very Large Array, 517, 147
Romanova, M. M., Ustyugova, G. V., Koldoba, A. V., et al. 2009,
MNRAS, 399, 1802
Rosen, A. L. & Krumholz, M. R. 2020, AJ, 160, 78
Rosero, V., Tanaka, K. E. I., Tan, J. C., et al. 2019, ApJ, 873, 20
Sana, H. 2017, The Lives and Death-Throes of Massive Stars, 329,
110
Soker, N. & Kashi, A. 2012, ApJ, 746, 100
Tachibana, S., Kamizuka, T., Hirota, T., et al. 2019, ApJL, 875, L29
Takasao, S., Tomida, K., Iwasaki, K., et al. 2019, ApJL, 878, L10
Tan, J. C., Beltrán, M. T., Caselli, P., et al. 2014, Protostars and Planets
VI, 149
Tanaka, K. E. I., Tan, J. C., & Zhang, Y. 2016, ApJ, 818, 52
Tanaka, K. E. I., Tan, J. C., & Zhang, Y. 2017, ApJ, 835, 32
Tanaka, K. E. I., Zhang, Y., Hirota, T., et al. 2020, ApJL, 900, L2
Tobin, J. J., Sheehan, P., & Johnstone, D. 2018, Science with a Next
Generation Very Large Array, 517, 333
Zapata, L. A., Garay, G., Palau, A., et al. 2019, ApJ, 872, 176
Zhang, Y., Tan, J. C., Sakai, N., et al. 2019a, ApJ, 873, 73
Zhang, Y., Tan, J. C., Tanaka, K. E. I., et al. 2019, Nature Astronomy,
3, 517
Zhang, Y., Tanaka, K. E. I., Rosero, V., et al. 2019, ApJL, 886, L4