# An ALMA Glimpse of Dense Molecular Filaments Associated with High-mass
Protostellar Systems in the Large Magellanic Cloud

Kazuki Tokuda Department of Earth and Planetary Sciences, Faculty of Science,
Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan  National Astronomical
Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa,
Mitaka, Tokyo 181-8588, Japan  Naoto Harada Department of Earth and Planetary
Sciences, Faculty of Science, Kyushu University, Nishi-ku, Fukuoka 819-0395,
Japan  Kei E. I. Tanaka Department of Earth and Planetary Sciences, Tokyo
Institute of Technology, Meguro, Tokyo, 152-8551, Japan  Tsuyoshi Inoue
Department of Physics, Konan University, Okamoto 8-9-1, Kobe, Japan  Takashi
Shimonishi Environmental Science Program, Faculty of Science, Niigata
University, Ikarashi-ninocho 8050, Nishi-ku, Niigata, 950-2181, Japan  Yichen
Zhang Department of Astronomy, University of Virginia, Charlottesville, VA
22904-4325, USA  RIKEN Cluster for Pioneering Research, Wako, Saitama
351-0198, Japan  Marta Sewiło Exoplanets and Stellar Astrophysics Laboratory,
NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA  Department of
Astronomy, University of Maryland, College Park, MD 20742, USA  Center for
Research and Exploration in Space Science and Technology, NASA Goddard Space
Flight Center, Greenbelt, MD 20771  Yuri Kunitoshi Department of Physics,
Graduate School of Science, Osaka Metropolitan University, 1-1 Gakuen-cho,
Naka-ku, Sakai, Osaka 599-8531, Japan  Ayu Konishi Department of Physics,
Graduate School of Science, Osaka Metropolitan University, 1-1 Gakuen-cho,
Naka-ku, Sakai, Osaka 599-8531, Japan  Yasuo Fukui Department of Physics,
Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan  Akiko
Kawamura National Astronomical Observatory of Japan, National Institutes of
Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan  Toshikazu
Onishi Department of Physics, Graduate School of Science, Osaka Metropolitan
University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan  Masahiro N.
Machida Department of Earth and Planetary Sciences, Faculty of Science, Kyushu
University, Nishi-ku, Fukuoka 819-0395, Japan

###### Abstract

Recent millimeter/sub-millimeter facilities have revealed the physical
properties of filamentary molecular clouds in relation to high-mass star
formation. A uniform survey of the nearest, face-on star-forming galaxy, the
Large Magellanic Cloud (LMC), complements the Galactic knowledge. We present
ALMA survey data with a spatial resolution of $\sim$0.1 pc in the 0.87 mm
continuum and HCO+(4–3) emission toward 30 protostellar objects with
luminosities of 104–105.5 $L_{\odot}$ in the LMC. The spatial distributions of
the HCO+(4–3) line and thermal dust emission are well correlated, indicating
that the line effectively traces dense, filamentary gas with an H2 volume
density of $\gtrsim$105 cm-3 and a line mass of $\sim$103–104 $M_{\odot}$
pc-1. Furthermore, we obtain an increase in the velocity linewidths of
filamentary clouds, which follows a power-law dependence on their H2 column
densities with an exponent of $\sim$0.5. This trend is consistent with
observations toward filamentary clouds in nearby star-forming regions withiin
$\lesssim$1 kpc from us and suggests enhanced internal turbulence within the
filaments owing to surrounding gas accretion. Among the 30 sources, we find
that 14 are associated with hub-filamentary structures, and these complex
structures predominantly appear in protostellar luminosities exceeding $\sim$5
$\times$104 $L_{\odot}$. The hub-filament systems tend to appear in the latest
stages of their natal cloud evolution, often linked to prominent H ii regions
and numerous stellar clusters. Our preliminary statistics suggest that the
massive filaments accompanied by hub-type complex features may be a necessary
intermediate product in forming extremely luminous high-mass stellar systems
capable of ultimately dispersing the parent cloud.

Star formation (1569); Protostars (1302); Molecular clouds (1072); Large
Magellanic Cloud (903); Interstellar medium (847); Local Group (929)

††software: astropy (Astropy Collaboration et al., 2018)

##  1 Introduction

Understanding the formation mechanism of high-mass stars ($>$8 $M_{\odot}$)
remains a compelling challenge in astrophysics due to their significant impact
on the surrounding environment, both during their active accretion phase and
beyond, into their main sequence stage. Over the past few decades, extensive
theoretical and observational studies have been conducted to shed light on
this energetic phenomenon (see reviews by e.g., McKee & Ostriker 2007,
Zinnecker & Yorke 2007, and Krumholz et al. 2019). The current understanding,
both theoretical and observational, suggests that the formation of high-mass
stars begins with the collapse of a massive ($\gtrsim$100 $M_{\odot}$) dense
core/clump, analogous to the formation of low-mass stars (e.g, McKee & Tan,
2003). This gravitational collapse eventually leads to the formation of
massive protostellar systems and the subsequent mass ejection through
molecular outflows driven by the magnetic centrifugal wind from the
circumstellar disks (e.g., Machida & Hosokawa, 2013; Commerçon et al., 2022),
appears to be a common phenomenon across a wide range of stellar masses
(Beuther et al., 2002; Maud et al., 2015; Matsushita et al., 2018). Recent
studies investigating distant, low-metallicity star-forming regions have
provided further insights, indicating that the properties of molecular
outflows are universal in diverse environments within the Local Group of
galaxies, consistent with the observations and theories developed in our own
Milky Way (Tokuda et al. 2022a and references therein).

Despite significant theoretical and observational progress in understanding
the evolution of individual star-forming units, i.e., dense pre- and
protostellar cores, they do not exist in isolation. They are inextricably
linked to the formation of larger gaseous structures, making it difficult to
understand them from their formation onwards. Molecular clouds are
hierarchically inhomogeneous, and their kinematics are largely affected by
multiple physical processes, such as non-thermal turbulent motions and
magnetic field (see the review by Heyer & Dame 2015). Observational
characterization has advanced our understanding of their complex nature. In
the solar neighborhood, molecular line studies using ground-based single-dish
telescopes illustrated the quasi-universality of filamentary structures across
low- and high-mass star-forming regions (e.g., Mizuno et al., 1995; Onishi et
al., 1996, 1998; Nagahama et al., 1998) and the subsequent high-resolution
dust continuum measurements with the Herschel Space Observatory revealed the
detailed structures with a typical width of $\sim$0.1 pc (e.g., Arzoumanian et
al., 2011, 2019; André et al., 2022). Some star-forming molecular clouds have
a more complex shape with multiple filaments combined together and are
sometimes referred to as $``$hub filament$"$ (Myers, 2009). Their junction
often becomes sites of active star formation (e.g., Peretto et al., 2013;
Williams et al., 2018), and statistical studies compiling a large number of
targets across the Galactic Plane suggest that they could be precursors
evolving into stellar clusters (Kumar et al., 2020).

High-resolution observations outside the Galaxy, especially in the Local Group
members, provide complementary insights into the Galaxy thanks to the less
line-of-sight contamination and uniform sampling catalogs, such as (high-mass)
young stellar objects (YSOs), obtained in various wavelengths. Recent Atacama
Large Millimeter/submillimeter Array (ALMA) CO observations toward some Local
Group giant molecular clouds (GMCs) revealed a relatively diffuse molecular
layer around dense proto-cluster clumps, and the formation of the above-
mentioned complex hub-filaments is found to be associated with galaxy-scale
converging gas flows (Tokuda et al., 2019, 2020a, 2022b; Fukui et al., 2019;
Muraoka et al., 2020). However, previous studies of the Local Group have been
inadequate in the following respects. (1) ALMA target selections have mostly
been biased toward molecular clouds with bright infrared sources (see also
Table 1), and the extreme star-forming environment, 30 Dor (Indebetouw et al.,
2020; Wong et al., 2022). We still lack a general understanding of
protocluster formation via (hub-type) filamentary clouds. (2) Molecular line
tracers in high-density regions directly related to proto-cluster formation
have not been thoroughly investigated. From a more distant galactic
perspective beyond the Local Group, the determination of star formation rates
using high-density tracers such as HCN and HCO+ has garnered much attention
(e.g., Gao & Solomon, 2004), and has motivated further investigation of
Galactic molecular clouds (e.g., Shimajiri et al., 2017; Torii et al., 2019).
Small-scale, comprehensive studies using dense gas tracers in the Local Group
may also provide clues to connect with large-scale phenomena occurring within
galaxies, such as the evolution of GMCs (e.g., Kawamura et al., 2009; Chevance
et al., 2022).

In this paper, we present ALMA observations toward 30 YSOs in the Large
Magellanic Cloud (LMC) obtained as part of the MAGellanic Outflow and
chemistry Survey (MAGOS) (PI: K. Tanaka, #2019.1.01770.S). We utilize the 0.87
mm continuum and HCO+ (4–3) line data from the MAGOS survey to study the
molecular cloud’s evolution leading to high-mass proto-cluster formation,
bridging a gap between the existing Galactic and extragalactic (the Local
Group) studies. In Section 2, we describe the observations and data reduction
details. In Section 3, we investigate the role of HCO+ as a high-density gas
tracer by comparing the 0.87 mm continuum. We performed the filamentary cloud
identification in Section 3.2. We provide the discussion and implications of
our analysis in Section 4, and we summarize the overall results in Section 5.

##  2 Target Selection, ALMA Observations, and Data Reduction Details

The MAGOS project (PI: K. Tanaka, #2019.1.01770.S) is a comprehensive study
aimed at exploring various aspects of star formation within the Magellanic
Clouds. The main target lines are CO (3–2) as an outflow tracer and several
hot-core tracer lines (e.g., multiple CH3OH and SO2 transitions). The specific
data presented in this paper, which are part of this larger project, are used
to investigate the properties of filamentary molecular clouds in the LMC. The
observations were carried out using the ALMA 12 m array in the C43-3
configuration, with the Band 7 receiver operating at 0.87 mm. Our targets were
selected from the massive YSO catalog with a total number of 277 in the LMC
(Seale et al., 2009). The YSOs were verified through spectroscopic
observations with Spitzer’s infrared spectrometer (5.3–38 $\mu$m). Regarding
the bolometric luminosity ($L_{\rm bol}$), we calculated them by directly
integrating their Spectral Energy Distributions (SEDs) from 1$\mu$m to
500$\mu$m. The photometric data were taken from Two Micron All Sky Survey
(2MASS), Spitzer, and Herschel archival data, after positional matching with
the Seale et al. (2009) YSO catalog. The derivation of $L_{\rm bol}$ is simply
the integral of the wavelength data points, 1.215 $\mu$m, 1.654 $\mu$m, 2.157
$\mu$m, 3.6 $\mu$m, 4.5 $\mu$m, 5.8 $\mu$m, 8.0 $\mu$m, 24 $\mu$m, 100 $\mu$m,
160 $\mu$m, 250 $\mu$m, 350 $\mu$m, and 500 $\mu$m without any SED modeling
schemes. While the apparent luminosity can vary due to some protostellar
nature, such as disk inclination angle, estimating the intrinsic luminosity is
challenging. Nonetheless, we can achieve accuracy on a factor of $\sim$3
(Zhang & Tan, 2018).

We allocated a luminosity selection criteria of $>$104 $L_{\odot}$ with young
spectral features, such as polycyclic aromatic hydrocarbon emission, deep
silicate absorption, fine-structure lines, and ice absorption. From a total
number of 178 objects in the Seale et al. (2009) catalog that fulfills the
above criterion, we selected total 30 sources divided into three luminosity
ranges: 104–104.5 $L_{\odot}$ (Ll01,…,10), 104.5−105 $L_{\odot}$ (Lm01,…,10),
and 105–105.5 $L_{\odot}$ (Lh01,…,10). This diverse luminosity-based target
selection was motivated by our original observational purpose: to investigate
how the nature of the CO outflows changes depending on their protostellar
luminosities. Most of the targets were randomly selected 10 targets for each
luminosity category, but some sources were arbitrarily selected based on the
signs of molecular outflow or hot cores (a more detailed description can be
found in Tanaka et al., in prep). In this paper, we have assigned new
designations to these protostars to facilitate immediate discernment of their
luminosities from their names. Hereafter, we refer to both the protostars and
their associated molecular gas revealed with ALMA using the same names listed
in Table 1. Table 1 provides information on target YSOs and associated GMCs
(including evolutionary stages) and H ii regions (Henize, 1956; Fukui et al.,
2008; Kawamura et al., 2009). Multiple molecular lines are detected among the
selected sources, with HCO+ (4–3) being the second most intense line after CO
(3–2). This paper focuses on analyzing the HCO+ (4–3) line and the 0.87 mm
continuum data.

Table 1: Observation targets

Name | Coordinates (J2000)A |  $L_{\rm bol}$B | Fukui08IDC | GMCtypeD | HenNameE | Other name/referenceF
---|---|---|---|---|---|---
| R.A. (deg) | Dec. (deg) | $(10^{4}\,L_{\odot})$ |  |  |  |
Lh10 | 81.69421 | -68.81311 | 23.10 | 139 | III | N144B | ST11 (1)
Lh09 | 72.97204 | -69.39128 | 20.80 | 14 | III | N79B | (2)
Lh08 | 84.91137 | -69.65119 | 16.50 | 191 | III | N160A | -
Lh07 | 78.35454 | -69.37919 | 15.50 | 72 | III | N113A | Region A (3)
Lh06 | 77.46054 | -68.88486 | 15.30 | 63 | III | N105A | N105-1 (4)
Lh05 | 77.46775 | -68.89092 | 14.70 | 63 | III | N105A | N105-2 (4,5)
Lh04 | 84.92454 | -69.77000 | 12.70 | 197 | III | N159 | N159W-South (6,7)
Lh03 | 79.80112 | -69.15203 | 11.60 | 98 | II | N119 | ST16 (8)
Lh02 | 84.90667 | -69.75694 | 11.30 | 197 | III | N159 | N159W-North MMS-1 (6,9)
Lh01 | 84.90433 | -69.76019 | 10.30 | 197 | III | N159 | N159W-North MMS-2 (6,9)
Lm10 | 72.79746 | -69.44631 | 8.30 | 234 | II | N79 | -
Lm09 | 73.03842 | -66.92275 | 7.96 | 12 | III | N4B | -
Lm08 | 82.72600 | -68.57453 | 7.76 | 154 | III | N148F | ST5 (10)
Lm07 | 84.66537 | -69.09394 | 6.18 | 186 | III | N157A | -
Lm06 | 84.87996 | -70.20469 | 4.58 | 197 | III | N171A | ST1 (10)
Lm05 | 78.32975 | -69.36417 | 4.21 | 72 | III | N113A | (2)
Lm04 | 80.51133 | -67.78392 | 3.68 | - | - | N44J | -
Lm03 | 80.70783 | -66.68225 | 3.53 | 119 | II | N45A | -
Lm02 | 84.98192 | -71.16692 | 3.52 | 195 | II | N214A | -
Lm01 | 73.96092 | -66.57628 | 3.25 | 23 | III | N11H | -
Ll10 | 80.88917 | -69.62006 | 2.82 | 129 | II | N132G | -
Ll09 | 80.50879 | -67.96489 | 2.81 | 114 | III | N44C | -
Ll08 | 77.60037 | -70.23514 | 2.80 | - | - | N110 | ST17 (10)
Ll07 | 81.59058 | -68.66642 | 1.93 | 137 | II | N144B | -
Ll06 | 76.15871 | -70.91197 | 1.90 | 49 | III | N191A | -
Ll05 | 74.67696 | -66.14325 | 1.48 | 35 | II | N12 | -
Ll04 | 75.98387 | -67.34411 | 1.37 | 45 | III | N17A | -
Ll03 | 84.47842 | -69.57661 | 1.13 | 184 | II | N158 | -
Ll02 | 73.53450 | -66.77539 | 1.12 | 19 | II | N6 | -
Ll01 | 81.50500 | -67.50336 | 1.08 | 251 | II | N51D | -

Table notes: [A] Spitzer source coordinate (Seale et al., 2009) [B] See the
text in Sect. 2. [C] Associated GMC catalogs with a $\sim$40 pc resolution
survey by Fukui et al. (2008) [D] GMC evolutionary type classification by
Kawamura et al. (2009): Type II means GMCs with H ii regions, and Type III
represents GMCs with H ii regions and young massive clusters (see also Section
4.2). [E] H ii region catalog by Henize (1956) [F] Previously observed in
molecular line and/or millimeter/submillimeter continuum with ALMA (1)
Shimonishi et al. (2016b) (2) Nayak et al. (2019) (3) Sewiło et al. (2018) (4)
Sewiło et al. (2022a) (5) Sewiło et al. (2022b) (6) Fukui et al. (2015) (7)
Tokuda et al. (2019) (8) Shimonishi et al. (2020) (9) Tokuda et al. (2022b)
(10) Shimonishi et al. (2016a)

Using Common Astronomy Software Application (CASA; CASA Team et al. 2022), we
performed the imaging process based on the pipeline-calibrated visibility data
provided by the observatory. For the process of the 0.87 mm continuum images,
we utilized two spectral windows covering frequencies of 343.9–344.9 GHz and
345.8–347.7 GHz. We employed the tclean algorithm with the multi-scale
deconvolver in the analysis. The imaging processes were carried out for the
0.87 mm continuum and HCO+ data, using the Natural and Briggs weighting with a
robust parameter of 0.5, respectively. The resulting properties of the
synthesized beams and the sensitivity are listed in Table 2. Note that the
observations were carried out using the same execution blocks for all targets,
minimizing differences in observing time and elevation angle. This ensured
consistent beam shapes and sensitivity across all observed fields.

Table 2: Data properties Line/Continuum | Beam size | Beam P.A. | Velocity grid | Aggregate bandwidth | R.M.S. sensitivity
---|---|---|---|---|---
0.87 mm |  $\sim$0$\farcs$46 $\times$ 0$\farcs$39 | 30 deg. | $\cdots$ | 2.8 GHz |  $\sim$0.2 mJy beam-1
HCO+ ($J$ = 4–3) |  $\sim$0$\farcs$39 $\times$ 0$\farcs$32 | 25 deg. | 0.5 km s-1 | $\cdots$ |  $\sim$0.86 K

##  3 Results

###  3.1 Spatial distribtuions of 0.87 mm continuum and HCO+(4–3) emission

In this section, we discuss the spatial distribution of the 0.87 mm continuum
and HCO+ emission. Figure 1 shows three examples in enlarged views close to
the Spitzer-identified high-mass protostars. We chose these three because two
of them (Lh09 and Lh04) have already been well studied by ALMA’s early
observations (Nayak et al., 2019; Tokuda et al., 2019) with millimeter
continuum and other molecular lines at different wavelengths, and their
prominent filamentary structures can be immediately determined visually in the
present data set, and their similar structures were confirmed associated with
a less bright protostar for Lm04. Although the ALMA continuum peaks and
Spitzer-identified protostar positions do not necessarily coincide exactly
(Figure 1 left panels), possibly due to the multiwavelength point-source
positional determination using the Spitzer data, the discrepancy is about an
ALMA’s single-beam element, $\sim$0$\farcs$4, which is within the accuracy of
the Spitzer/IRAC survey’s angular resolution, $\sim$2$\arcsec$ (Meixner et
al., 2006). Since there is only a very small spatial discrepancy, and the
Spitzer sources have been spectroscopically identified as massive protostars
(see Sect. 2), it is safe to assume that the 0.87 mm sources are indeed dense
envelope gas associated with the protostars, rather than a coincidental line-
of-sight overlap. The brightest 0.87 mm source in our sample, Lh09, shows the
complicated structure. The less bright target, lm04 shows a simpler structure
composed of a single-filamentary distribution. The 0.87 mm continuum emission
most likely traces the thermal dust emission of dense material around the
high-mass protostellar sources.

Figure 1: Enlarged views of the 0.87 mm and HCO+ (4–3) emission toward three
example targets. The color-scale images show the distribution of the 0.87 mm
continuum emission (left panel), the HCO+ peak brightness temperature (middle
panel), and the velocity-integrated intensity (right panel). The crosses
indicate the positions of massive protostars identified by Spitzer (Seale et
al., 2009). The velocity range used to derive the peak brightness temperature
and moment 0 maps are indicated on the upper side of each right image. The
synthesized beam size is indicated in white in the lower-left corner of each
image.

To characterize the physical properties of the dense molecular clouds traced
by the 0.87 mm continuum emission as a first step, we derived the total mass
gas based on the 0.87 mm flux density measurements. The flux density above
$\sim$3$\sigma$ detection of Lh09, Lh04, and Lm04 are 505 mJy, 102 mJy, and 52
mJy, respectively, which can be converted into gas mass by adopting an
absorption coefficient per unit dust mass at 0.87 mm, a dust-to-gas mass ratio
and the dust temperature of 1.8 cm2 g-1, 3.5 $\times$10-3, and 20 K,
respectively, with an optically thin case (e.g., Ossenkopf & Henning, 1994;
Herrera et al., 2013; Gordon et al., 2014) in the LMC region of 50 kpc
(Pietrzyński et al., 2019). The resultant total gas mass of Lh09, Lh04, and
Lm04 is $\sim$2$\times$104 $M_{\odot}$, $\sim$4$\times$103 $M_{\odot}$ and
$\sim$2$\times$103 $M_{\odot}$, respectively. It should be noted that the
expected uncertainties are a factor of two or three, which are more likely due
to systematic errors related to dust properties and temperature than
measurement errors.

The middle and right panels in Figure 1 show the peak brightness temperature
and velocity-integrated intensity (moment0) maps of HCO+ (4–3), respectively.
Overall, the distributions of the 0.87 mm continuum and HCO+ are quite
similar, suggesting that the emitting regions for both tracers are spatially
correlated. For optically thin cases with a kinematic temperature of 10–100 K,
the critical density of HCO+ (4–3) is (2–3) $\times$106 cm-3 (Shirley, 2015),
which is comparable to the average density derived from the continuum emission
(see also Sect. 3.2). The observed intensities of HCO+ exceeding 10 K indicate
a high likelihood of thermalization. For instance, in the case of Lh04,
assuming the local thermo-dynamical equilibrium (LTE) conditions (Rohlfs &
Wilson, 2004) with a uniform excitation temperature of 20 K througout the
emitting region, the derived column density of HCO+ is $\sim$3 $\times$1013
cm-2. Applying a literature value of HCO+ abundance relative to H2 in the
LMC-N159 region (Johansson et al., 1994), [HCO+]/[H2] = 1.8 $\times$10-10, the
H2 column density is estimated to be 2 $\times$1023 cm-2, which is consistent
with those derived from the 0.87 mm data (see Sect. 3.2). We confirmed that
the integrated intensity of HCO+ and 0.87 mm flux density within a 1 pc radius
aperture show a tight, linear correlation with each other for all sources
because Pearson’s correlation coefficient and $p$-value are 0.963 and 2e-17,
respectively. Thus, the HCO+(4–3) line is useful for tracing the highly dense
regions of molecular clouds in the LMC. This effectiveness of HCO+ as a high-
density gas tracer has also been demonstrated in studies utilizing single-dish
observations in the LMC (Paron et al., 2016; Galametz et al., 2020) and other
galaxies such as M33 (Braine et al., 2017), which has a similar metallicity to
the LMC.

Figure 2 presents the 0.87 mm continuum emission for all sources. Our sample
(Table 1) with 0.1 pc-scale continuum observations has the largest number of
targets among any other literature in the extragalactic study. The spatial
distributions are overall in agreement with previous studies (see the
references in Table 2) and show peaks either coinciding with or close to the
positions of the YSOs. However, some sources have positional discrepancies
larger than the ALMA beam size between the continuum peaks and YSO positions.
For instance, while the continuum peak of Lh08 is significantly distanced from
the position of the YSO, the structure through 0.87 mm and HCO+ shows that
they are interconnected in both space and velocity (see Figures 1, 3, 6 and
7). In the case of Lh05, according to Sewiło et al. (2022a), the two sources
were identified as one during the multiband-merging process of Spitzer. For
Lh02, the presence of free-free emission could explain the offset between the
position of the YSO and continuum (see YSO-N1 and the 1.3 mm continuum source,
MMMS-1 in Tokuda et al. 2022b), but for the 0.87mm emission, the spatial
correlation with dense gas tracers, C18O (Figure 6 in Tokuda et al. 2022b) and
HCO+ are quite good, suggesting that the 0.87 mm continuum structure is
predominantly capturing thermal emission. For Lh01, outflows have been
reported for each continuum peak (see MMS-2a/b and MMMS-d in Figure 5 in
Tokuda et al. 2022b), indicating that the system is harboring multiple
protostars within a few times 0.1 pc. According to the Australia Telescope
Compact Array (ATCA) observations at wavelengths of 3 and 6 cm with a beam
size of 1$\arcsec$–2$\arcsec$ by Indebetouw et al. (2004), in addition to the
above-mentioned Lh09 and Lh02, Lh08 and Lh06 might also have some free-free
contamination as their centimeter continuum peaks are close to those of 0.87
mm. For example, in Lh09, Nayak et al. (2019) reported the H30$\alpha$
detection at the continuum peak, indicating significant free-free
contamination from ionized gas, but the emitting region is compact with a size
scale of 0.1 pc (Figure 2 in Nayak et al. 2019). In such a case, the ionized
contamination in 0.87 mm does not extend to the surrounding filament gas. As
shown here, some sources require careful handling but do not significantly
affect the identification of the overall filament structure and a detailed
discussion is given in Sect. 3.2 and 4.

Figure 2: 0.87 mm continuum distributions toward high-mass YSOs in the LMC.
The angular resolution, $\sim$0$\farcs$46 $\times$ 0$\farcs$39, is indicated
with the white ellipse in the lower left corner of the lower left panel.
Crosses represent the position of the YSOs (Seale et al., 2009). Cyan lines
show the identified spines of the filament that we used to derive the physical
properties (see the text in Sect. 3.2).

###  3.2 Filamentary cloud identification

As shown in Figures 2 and 3, most of the targets have an elongated morphology.
We used the Filfinder package (Koch & Rosolowsky, 2015) to extract such
elongated structures based on certain criteria. We emphasize that the main
purpose of this analysis is to characterize the physical quantities of primary
elongated structures with uniform treatment. We applied the algorithm to the
0.87 mm continuum images. In the filament identification analysis, we excluded
areas with coarser sensitivity and high noise levels at the edges of the ALMA
mosaics. For the used regions, the sensitivity is at least 50% of the field’s
maximum, corresponding to a circular area with a radius of 8$\farcs$7 (= 2.1
pc at 50 kpc) from the center. In the initial step, prior to creating a masked
image, we selected flatten_percent = 95 to flatten the intensity distribution
of sources with high contrast. Then, emission masks were created by applying a
threshold of 3$\sigma$ noise level and selecting regions with an area greater
than five times the beam size. The other input parameters were adapt_thresh =
0.1 pc and smooth_size = 0.05 pc, which are recommended values to extract
filamentary structure whose width is close to $\sim$0.1 pc (Koch & Rosolowsky,
2015). The intensity distribution within the mask was used to identify the
filament spines. Filaments shorter than five times the beam size and filament
branches shorter than three times the beam size were excluded from the
analysis.

The filament identification results are shown in Figures 2 and 3. Although the
algorithm identified more than two spatially separated filament spines for
some fields, we illustrate the longest, nearest neighbor of protostar
positions to characterize the primary filamentary cloud for each object. A
global trend throughout the targets is that filamentary structures toward
brighter sources are more extended and more complex: multiple filaments appear
to intersect at the position of YSOs. For sources with luminosities below a
luminosity $\sim$104 $L_{\odot}$, HCO+ emission is mostly detected only in a
small region ($\lesssim$1 pc) around the protostar (Figure 3), which means no
surrounding dense filamentary layer in the fainter sources.

Figure 3: The distribution of the HCO+ (4–3) peak brightness temperature
toward high-mass YSOs in the LMC. Symbols and lines are the same as in Figure
2.

We derived the physical quantities along the filament spine shown in the cyan
lines of Figure 2. We computed the average column density, $N_{\rm H_{2}}^{\rm
ave}$ along the filament by averaging the values through the spine data
pixels. The physical assumptions used for this calculation are the same as
those described in Sect. 3.1. Along the spine of the filament, we constructed
an averaged radial profile, from which we determined the FWHM as width,
$w_{\rm fil,cont}$, by fitting a Gaussian function. Note that most of $w_{\rm
fil,cont}$ shows $\sim$0.1 pc, but they are marginally resolved at the present
spatial resolution of $\sim$0.1 pc $\times$ 0.09 pc. $l_{\rm fil,max}$ and
$I_{\rm mean,d}$ represent the length of the longest spine and mean 0.87 mm
intensity, respectively. Some of these quantities resulted in continuum-based-
line mass, $M_{\rm line}$ (= $\mu m{\rm{}_{H}}N_{\rm H_{2}}^{\rm ave}w_{\rm
fil,cont}$), where $m{\rm{}_{H}}$ is the atomic hydrogen mass and $\mu$ is the
mean molecular weight per hydrogen molecule, 2.7 (Cox, 2000). We defined the
H2 number density of the filaments as $n_{\rm H_{2}}$ = $N_{\rm H_{2}}^{\rm
ave}/w_{\rm fil,cont}$. It should be noted that quantities derived from a
combination of two physical parameters, such as $n_{\rm H_{2}}$, might carry
even higher uncertainties than H2 column densities. While we regard these
values as offering insights at an order-of-magnitude discussion, the
detections of high critical density HCO+ line provide complementary evidence
for the high-density nature of the entire system. Table 3 summarizes the
physical properties of the identified filamentary clouds.

To compare with the continuum-based physical quantities along the spines of
the filaments, we performed velocity characterization with the following
procedure in HCO+ data. We extracted the HCO+ spectra at local peaks of the
0.87 mm closest to each Spitzer source position and determined the systemic
velocity, $V_{\rm sys}$, as the maximum intensity channel. Because the spectra
exhibited predominantly single Gaussian-like structures, this estimation is
considered to be robust. Next, we extracted the HCO+ spectra along the spine
pixels of the filaments and shifted their central velocities to the above
estimated $V_{\rm sys}$. We then derived the average spectrum and performed
Gaussian fittings to estimate the velocity width in FWHM, $\Delta v_{\rm
FWHM}$. Each target spectra are shown in Figure 5 in the Appendix. Note that
deriving the velocity width averaging along the spine pixels on the moment 2
map (see Figure 8), the difference between the two methods is not significant,
within approximately 30%. Table 3 lists the derived velocity information.

The inferred velocity dispersion, $\sigma_{v}$ (=$\Delta v_{\rm
FWHM}$/2$\sqrt{2{\rm ln}2}$), is significantly larger than the thermal
linewidth, $\sim$ 0.3–0.4 km s-1 at a typical molecular cloud temperature of
20–50 K in the LMC (e.g., Minamidani et al., 2008), indicating that the
internal gas kinematics are dominated by non-thermal turbulent motion with a
velocity-dispersion range of $\sim$1–3 km s-1. Neglecting the magnetic field,
the virial mass per unit length is given by the equation $M_{\rm line}^{\rm
vir}$ = 2$\sigma_{v}^{2}/G$ (c.f., Fiege & Pudritz, 2000), where $G$ is the
gravitational constant. Our estimated $M_{\rm line}^{\rm vir}$ is within a
factor of 2–5 of the continuum-derived $M_{\rm line}$ (Table 3), suggesting a
general consistency between the region traced by thermal dust emission and the
gas dynamics probed by the HCO+ line. Regarding structures that exceed the
line mass of $10^{3}$ $M_{\odot}$ pc-1, they are often observed in active
star-forming regions in the MW, such as Cygnus-DR21 and W43, referred to as
$``$ridge$"$. Such massive filaments are frequently recognized as mini-
starburst regions (e.g., Motte et al., 2018).

Table 3: Filament parameters  NAME | $l_{\rm fil,max}$ | $w_{\rm fil,cont}$ | $I_{\rm mean,d}$ | $N_{\rm H_{2}}^{\rm ave}$ | $n_{\rm H_{2}}$ | $M_{\rm line}$ | $V_{\rm sys}$ | $\Delta v_{\rm FWHM}$ | $M^{\rm line}_{\rm vir}$ | Hub?
---|---|---|---|---|---|---|---|---|---|---
| (pc) | (pc) | (mJy beam-1) | (cm-2) | (cm-3) | ($M_{\odot}$ pc-1) | (km s-1) | (km s-1) | ($M_{\odot}$ pc-1) |
Lh10 | 0.90 | 0.25 | 9.04 | 1.4e+24 | 1.8e+06 | 7.7e+03 | 252.0 | 6.6 | 3.7e+03 | ✓
Lh09 | 1.27 | 0.13 | 15.14 | 2.3e+24 | 5.8e+06 | 6.3e+03 | 234.0 | 6.0 | 3.0e+03 | ✓
Lh08 | 1.31 | 0.31 | 1.92 | 3.0e+23 | 3.1e+05 | 2.0e+03 | 237.5 | 3.8 | 1.2e+03 | ✓
Lh07 | 1.48 | 0.16 | 7.46 | 1.1e+24 | 2.3e+06 | 4.0e+03 | 239.5 | 5.5 | 2.6e+03 | ✓
Lh06 | 2.04 | 0.19 | 4.45 | 6.8e+23 | 1.2e+06 | 2.8e+03 | 238.0 | 3.7 | 1.2e+03 | ✓
Lh05 | 1.92 | - | 4.78 | 7.3e+23 | - | - | 242.0 | 4.9 | 2.0e+03 | ✓
Lh04 | 2.46 | 0.10 | 2.09 | 3.2e+23 | 1.0e+06 | 7.3e+02 | 237.5 | 4.1 | 1.4e+03 | ✓
Lh03 | 1.01 | 0.15 | 2.74 | 4.2e+23 | 8.9e+05 | 1.4e+03 | 265.0 | 5.1 | 2.2e+03 | ✓
Lh02 | 2.10 | 0.18 | 2.20 | 3.4e+23 | 6.3e+05 | 1.3e+03 | 237.5 | 3.6 | 1.1e+03 | ✓
Lh01 | 2.03 | 0.18 | 2.20 | 3.4e+23 | 6.1e+05 | 1.4e+03 | 237.0 | 5.4 | 2.5e+03 | ✓
Lm10 | 1.26 | 0.10 | 3.65 | 5.5e+23 | 1.7e+06 | 1.3e+03 | 239.0 | 4.8 | 1.9e+03 | ✓
Lm09 | 1.59 | 0.15 | 1.84 | 2.9e+23 | 6.2e+05 | 9.4e+02 | 273.5 | 4.1 | 1.4e+03 | ✓
Lm08 | 1.23 | 0.13 | 2.44 | 3.9e+23 | 9.4e+05 | 1.1e+03 | 256.0 | 4.4 | 1.6e+03 | ✓
Lm07 | 0.84 | 0.19 | 1.14 | 1.8e+23 | 3.1e+05 | 7.2e+02 | 244.0 | 2.4 | 4.7e+02 | -
Lm06 | 1.34 | 0.17 | 1.50 | 2.4e+23 | 4.6e+05 | 8.6e+02 | 221.5 | 3.5 | 1.0e+03 | -
Lm05 | 0.49 | 0.21 | 1.57 | 2.4e+23 | 3.7e+05 | 1.1e+03 | 234.0 | 3.3 | 9.0e+02 | -
Lm04 | 2.35 | 0.12 | 1.52 | 2.4e+23 | 6.4e+05 | 6.4e+02 | 282.0 | 3.1 | 8.1e+02 | -
Lm03 | 1.39 | 0.12 | 0.76 | 1.2e+23 | 3.2e+05 | 3.2e+02 | 297.5 | 2.2 | 4.0e+02 | -
Lm02 | 0.55 | 0.14 | 1.59 | 2.5e+23 | 5.7e+05 | 7.7e+02 | 229.5 | 2.7 | 6.0e+02 | -
Lm01 | 0.61 | 0.10 | 1.81 | 2.9e+23 | 9.1e+05 | 6.4e+02 | 275.5 | 3.6 | 1.1e+03 | -
Ll10 | 0.87 | 0.07 | 1.85 | 2.9e+23 | 1.3e+06 | 4.5e+02 | 249.0 | 3.8 | 1.2e+03 | -
Ll09 | 0.76 | 0.21 | 0.91 | 1.5e+23 | 2.3e+05 | 6.5e+02 | 283.5 | 2.6 | 5.5e+02 | -
Ll08 | 0.80 | 0.16 | 1.13 | 1.8e+23 | 3.7e+05 | 6.0e+02 | 234.5 | 3.2 | 8.8e+02 | -
Ll07 | - | - | - | - | - | - | 267.0 | - | - | -
Ll06 | - | - | - | - | - | - | 229.5 | - | - | -
Ll05 | 0.60 | 0.08 | 1.39 | 2.2e+23 | 8.6e+05 | 4.0e+02 | 275.5 | 3.9 | 1.3e+03 | ✓
Ll04 | - | - | - | - | - | - | 270.5 | - | - | -
Ll03 | 0.65 | 0.24 | 0.85 | 1.3e+23 | 1.8e+05 | 7.0e+02 | 264.5 | 2.6 | 5.7e+02 | -
Ll02 | - | - | - | - | - | - | 273.5 | - | - | -
Ll01 | - | - | - | - | - | - | 287.0 | - | - | -

Taking into account for the filament identification results, the following
criteria were used to classify whether the protostar-associated molecular
cloud traced in 0.87 mm/HCO+ is a hub-filament or not. (1) According to the
FilFinder analysis, the spines extend in two or more directions centered on
the YSOs in some of the targets. This visually simple criterion categorizes
Lh09, Lh06, Lh05, Lh04, Lh03, Lh01 and Lm09 as hub filaments. (2) We made HCO+
moment 1 maps and overlaid the FilFinder spine (see Figure 7 in the Appendix).
Based on the velocity maps, we visually searched for additional elongated
components without spines whose relative velocity is $\gtrsim$1 km s-1 shifted
from that of the spine velocity, and then we additionally categorized Lh10,
Lh08, Lh07, Lh02, Lm10, Lm08, Ll05 as hub filaments. The setting of a velocity
difference of $\gtrsim$1 km s-1 might seem arbitrary, but it has astrophysical
implications. This velocity difference is sufficiently larger than the
velocity resolution and surpasses the turbulent line width on sub-scales
expected from the standad size-linewidth relation in molecular clouds (e.g.,
Larson, 1981; Solomon et al., 1987; Heyer et al., 2001). Therefore, we
interpret this as an indicator of the presence of multiple filamentary
components with slightly different velocities. (3) The FilFinder analysis of
Lm05, Lm03, Lm02, Ll10, and Ll08 shows branched spines. However, we did not
classify them as hub filaments, either because the spines and their associated
0.87 mm/HCO+ structures are very small similar to the beam size (Figures 2, 3
and 6), or because they are continuous in velocity space (Figure 7) can be
well recognized as a single filament.

In summary, we define hub filament in this study as follows: they are observed
partially or entirely in 0.87 mm continuum and HCO+ emissions, with
intersections of multiple filaments, which may occasionally show slight
velocity differences among each other, centered around massive YSOs. As
highlighted in Sect. 3.1, their detection indicates an extremely high
intrinsic density of the filaments themselves, on the order of 105–106 cm-3.
The resulting categorization is listed in the rightmost column of Table 3.
Given the nature of the methodology, misclassifications can occur on the weak
emission targets, inevitably leading to some degree of subjectivity. However,
a robust, general trend is that the dense molecular structures associated with
the brighter protostars are complex in both spatial and velocity distribution
(Figures 2, 3, 6, 7 and 8), which lends itself to identification as hub
filaments. More detailed discussions of astronomical meanings of the hub
filaments are provided in Section 4.

###  3.3 Relation between the properties of protostars and associated
filaments

Figure 4a illustrates the bolometric luminosity ($L_{\rm bol}$) of massive
protostar vs. the average H2 column density ($N_{\rm H_{2}}$) of the filament
crests. Filaments with higher $N_{\rm H_{2}}$ are generally associated with
luminous sources. Further, when we divide our sample into no hub- and hub-
filaments, all the identified hub-filaments except Ll05 have luminosities
exceeding 5 $\times$104 $L_{\odot}$.

Next, we compared the H2 column density with the velocity dispersion (Figiure
4b) to investigate the molecular gas dynamics. It can be inferred that the
velocity dispersion increases with increasing H2 column density. Up to a H2
column density of $\sim$5 $\times$1023 cm-2, the velocity dispersion follows a
power law with an exponent of 0.5. This trend was also reported for objects in
a much lower H2 column density regime of 1022–1023 cm-2 (Arzoumanian et al.,
2013) targetting nearby low-mass star-forming region and a few massive
sources. We further discuss the H2 column density and velocity dispersion
relation in Sect. 4.1.

Figure 4: (a) The relation between the bolometric luminosity ($L_{\rm bol}$)
of the protostar and the average column density ($N_{\rm H_{2}}$) along the
filament spine. Cross marks and filled circles represent the presence and
absence of hub-filaments, respectively. (b) The relation between the column
density of the filament and the velocity dispersion derived from HCO+.

##  4 Discussion

Our survey of 30 objects provides valuable insights into high-mass star-
forming filamentary molecular cloud systems in the LMC, but the sample bias
has to be noted before proceeding to the discussion. As part of the Surveying
the Agents of a Galaxy’s Evolution (SAGE) Spitzer Legacy program for the LMC
(SAGE; Meixner et al. 2006), 1800 unique YSO candidates were identified in the
LMC (Whitney et al. 2008; see also Gruendl & Chu 2009). Seale et al. (2009)
spectroscopically confirmed 277 YSOs from the sample of 294 YSO candidates
with [8.0] $<$ 8 mag, corresponding to a criterion of high-mass YSOs from
Gruendl & Chu (2009). Although our sample only represents 10% of the total
spectroscopically confirmed population, it still gives us a general idea of
the overall trend. For example, our current criteria identified filamentary
structures from which physical quantities could be inferred in 25 out of 30
objects, or 83% (Table 3). According to the Wilson Score Interval formula,
assuming a binomial distribution of the presence or absence of filaments with
a sample size of 30, a 95% confidence interval can be calculated, ranging from
66% to 92%.

Most peak positions of dense gas emissions in 0.87 mm and HCO+ objects match
the protostellar position, suggesting that we are looking at an early phase of
protostar formation widtout significant gas consumption and/or destruction of
the parental clouds. In addition, a distinct general trend is that more
luminous ($\gtrsim$5 $\times$104 $L_{\odot}$) YSOs are associated with hub
filamentary clouds (see Sect. 3.2, Table 3, and Figure 4). Similarly, the
presence of hub filamentary parental clouds in luminous protostars exceeding a
few times 104 $L_{\odot}$ has been extensively investigated for the Galactic
plane objects by Kumar et al. (2020). They found hub-filamentary systems with
lengths of $\sim$10–20 pc, total masses of $\sim$104 $M_{\odot}$ and line
masses of $\sim$2 $\times$103 $M_{\odot}$ pc-1 toward such luminous
protostellar objects. Their filament identification is based on Herschel dust
continuum measurements that are integrated over the line-of-sight components
in both velocity and space, and the spatial resolution did not achieve
$\sim$0.1 pc toward distant ($>$1 kpc) targets. Therefore, one should
carefully compare our results with the Galactic study to provide a hint of the
universality of the hub-filament system in the high-mass star formation in the
Local Group.

Our filament identification is superior to the above-mentioned Galactic study
if limited to insight into the protostellar-vicinity dense components because
the HCO+ line selectively captures the inner dense regions with volume
densities of 105–106 cm-3 (see Sect. 3.1) without the contamination of
unrelated line-of-sight components as in the Galactic plane. On the other
hand, our study cannot distinguish how much total mass or maximum length the
filamentary cloud has because the HCO+ and 0.87 mm continuum measurements
cannot trace the low-density regime below $\lesssim$104 cm-3. While we
acknowledge such limitations in data analysis, it is unnatural for the dense
HCO+/0.87 mm structures to exist in isolation; they should be associated with
lower-density, larger structures. For example, early studies in the N159W
region, including Lh01, Lh02, and Lh04, showed that the parent cloud of the
currently identified hub filament system has a total mass of $\gtrsim$104
$M_{\odot}$ and a maximum length of $>$10 pc (Fukui et al., 2015; Tokuda et
al., 2019, 2022b). Putting together some of the evidence obtained so far from
the Galactic Survey and several case studies in the LMC, it appears that
massive protostars, whose luminosity is greater than several times 104
$L_{\odot}$, are accompanied by dense hub filaments, which as a cloud form
massive systems with a mass of $>$104 $M_{\odot}$ and lengths of $>$10 pc.
Such luminous protostellar systems have the potential to eventually destroy
the parent clouds themselves. The scenario for the formation of hub filament
systems and the subsequent evolution of the parent cloud is discussed further
in Sects. 4.2 and 4.3.

###  4.1 Linewidth enhancement of massive filamentary clouds

In this section, we discuss the relation between the velocity linewidth and
the H2 column density, $\sigma_{v}\propto N_{\rm H_{2}}^{0.5}$, as shown in
Figure 4b. Although the present beam size is not necessarily high enough to
tightly constrain the width ($w_{\rm fil,cont}$), most of the filamentary
clouds roughly show a constant width of $\sim$0.1 pc within a factor of two
variations across the targets (see Table 3). Considering the radial
contraction of an isolated filament, one would expect the width to decrease as
the column density increases, but the fact that this is not the case suggests
the existence of a mechanism that keeps the filament width constant.
Arzoumanian et al. (2013) provided seminal implications based on the velocity
structures of filamentary clouds in the solar neighborhood star-forming
regions. Their results show a relationship of $\sigma_{v}\propto N_{\rm
H_{2}}^{0.5}$, and argue that the self-gravitating filaments could amplify
turbulence due to gravitational contraction and mass accretion from the
external environment (see also the case of the Galactic high-mass star-forming
region NGC 6334 by Arzoumanian et al. 2022). They focused on the effective
Jeans length, determined by the non-thermal (turbulent) motion, which is
approximately constant, and discussed why the self-gravitating filaments
shrink but the width is close to constant. Although their results did not
include an adequate sample at the higher H2 column density end, our
observation of the massive star-forming regions of the LMC allows us to derive
remarkably similar results at a higher H2 column density range in many
objects. It also allows us to study the dynamics of the high-density (105–106
cm-3) gas in the filaments leading to high-mass star formation, which has not
been fully explored in either the LMC or MW studies.

We also observed hub filaments in high-mass systems, suggesting the presence
of associated filaments of diffuse gas traced by 12CO and 13CO (see also the
case of Lh04/N159W-South in Tokuda et al. 2019). Given this, the dense
filaments traced in 0.87 mm and HCO+ and surrounding diffuse gas are likely
co-evolving. With these results in mind, we discuss the formation of high-mass
stars and their parent filament systems in the context of the surrounding gas
and star formation conditions in Sections 4.2 and 4.3.

###  4.2 Relation between the filamentary clouds and the evolutionary stages
of the parental GMCs

We examine the relationship between the identified hub-filament systems and
the evolutionary stage of their associated molecular clouds to consider the
fate of massive star formation and its natal molecular clouds. Kawamura et al.
(2009) categorized 272 GMCs in the LMC (Fukui et al., 2008) into three
categories by comparing them with H ii regions and star clusters: (1) the
youngest Type I, which does not show any signs of high-mass star formation,
(2) the intermediate stage Type II, which only accompanies H ii regions, and
(3) the most active and final stage Type III, which is associated with both H
ii regions and star clusters (see also the original concept proposed by Fukui
et al. 1999). The empirically estimated GMC lifetimes for Type I, II, III are
6 Myr, 13 Myr and 7 Myr, respectively, assuming the number of the three types
of GMCs are proportional to the timescale of each stage (Kawamura et al. 2009
and see also a recent qualitatively similar result in NGC 628 by Demachi et
al. 2023). As shown in Table 1, the targets in this study are only those that
are associated with Type II and Type III molecular clouds. Although the
classification was estimated based on optical observations, the infrared
survey also did not find significant signs of high-mass star formation in the
Type I clouds (Ochsendorf et al., 2016, 2017). The number of hub and non-hub
filaments associated with Type II and Type III is shown in Table 4. While
systems without hub filaments seem to be evenly distributed between Type II
and Type III, those with hub filaments appear to be skewed towards Type III.
This tendency suggests that once high-mass star formation initiates somewhere
within a GMC, further high-mass star formation can occur at any time without
the need for hub filaments; however, exceptionally active star formation,
exceeding a luminosity of 5 $\times$104 $L_{\odot}$, does not occur until the
latest stage of GMC evolution. Hub-filament systems might act as precursors
that eventually disrupt the GMC, impeding its growth.

Table 4: Number of hubs in different evolutionary GMCs | Type II GMC | Type III GMC
---|---|---
Non-hub | 7 | 7
Hub filament | 3 | 11

One important caveat in our study is that the original target selection aimed
to capture a wide range of infrared luminosities without considering the
evolutionary stage of the natal GMCs. A study selecting young stellar objects
without bias in each evolutionary stage will be necessary to validate the
demographic findings of this work.

###  4.3 Hub-filament development at the latest phase of the GMC lifetime

In line with the above-mentioned evolutionary concept, we consider the
conditions under which hub filaments are formed. Krumholz et al. (2012)
simulated the evolution of massive, dense molecular gas systems with a gas
mass of $\sim$1,000 $M_{\odot}$ and a radius of $\sim$0.4 pc with turbulent
perturbations. They demonstrated that supersonic gas flows within the parental
cloud produce filamentary structures with a few overdense spots, each of which
looks like the turbulent core proposed by McKee & Tan (2003). A complex
filamentary system containing massive dense cores can naturally form if a
large amount of molecular gas within a tiny volume is available. Because the
formed filaments and dense cores are quite dense with a density of
$\sim$105–106 cm-3, the evolution time scale follows a free-fall time of the
density, $\sim$0.1 Myr. Because the filaments we identified also have such
high densities (see Table 3), it is expected that star formation and gas
consumption will proceed rapidly in such a short time determined by the self-
gravity (e.g., Tokuda et al., 2020b), unless they are supported by strong
turbulence and/or magnetic fields. Most of the GMC lifetime is dominated by
the diffuse molecular gas state at a density of $\sim$102–103 cm-3, and high-
mass star-formation occurs when a high-density region is suddenly formed after
a long quiescent phase (e.g., Kawamura et al., 2009; Chevance et al., 2022).
We thus need to address hypotheses on how to form dense regions with hub-
filament morphology during the GMC evolution.

According to the comparison of the CO clouds and the surrounding H i gas study
(Fukui et al., 2009), the mass accretion rate onto GMC was estimated to be
$\sim$0.05 $M_{\odot}$ yr-1 assuming a steady state growth owing to the GMC’s
self-gravity. However, such a constant atomic gas accreting condition is not
necessarily realistic (c.f., Kobayashi et al., 2017; Kondo et al., 2021)
because not only the GMC but also H i gas envelope have inhomogeneous
distribution in nature driven by multi-scale galactic phenomena, such as
giant-shell expansions (Kim et al., 2003; Dawson et al., 2013), and galactic
tidal interaction between the LMC and the Small Magellanic Cloud (SMC) (e.g.,
Bekki & Chiba, 2007; Fukui et al., 2017; Tsuge et al., 2019). The thermal
instability provides small-scale blobs by the mixing between cold and warm
neutral mediums (e.g., Inoue & Inutsuka, 2009; Aota et al., 2013; Tokuda et
al., 2018). Tokuda et al. (2022b) hypothesized that the $``$teardrop inflow$"$
scenario, which is a substructured H i gas collision onto GMCs, well explains
a simultaneous, numerous cluster formation with hub-filamentary dense clouds
in the N159W (Lh04, Lh01, and Lh02) and N159E (Fukui et al., 2019) regions.

In the typical galactic environment, GMCs are constantly exposed to the risk
of gas collisions/flows or interstellar shocks. A developed GMC needs to be
supplied with flows to form high-density gas clumps leading to high-mass star
formation. Based on ALMA CO observations in the SMC, Neelamkodan et al. (2021)
found a hub-filamentary system with signatures of gas collision. Still, the
system only harbors an intermediate stellar system, possibly due to the
shortage of the total amount of gas or (column) density. Enokiya et al. (2021)
pointed out that there is a high-H2 column density threshold to initiate
active cluster formation (see also recent numerical work by Abe et al. 2022).
Structural analysis of CO clouds in the LMC by Sawada et al. (2018)
demonstrated prominent structures such as filaments and clumps during the GMC
evolution regardless of the density required for star formation. As
disturbances from gas collisions are added to developed clouds, amplified
magnetic fileds significantly enhance the effective Jeans mass and then
promote the formation of dense hub filaments in a short time of 0.1 Myr order
after the shock compression (Inoue et al., 2018). It is also worth noting that
some studies, which have meticulously examined the large-scale gas motions of
molecular clouds, have shown that hub filaments are embedded in the regions
where interaction between two clouds is ongoing (e.g., Dewangan, 2022; Maity
et al., 2022, 2023). The pre-colliding system possibly corresponds to Stage I
(Kumar et al., 2020), which comprises more than two systems with individual,
simple filamentary clouds in the hub-filament cluster formation evolutionary
sequence.

##  5 Summary

We presented ALMA HCO+ (4–3) and 0.87 mm continuum data toward 30 massive YSOs
with their bolometric luminosities of 104–105.5 $L_{\odot}$ in the Large
Magellanic Cloud at an angular resolution of $\sim$0$\farcs$4, corresponding
to $\sim$0.1 pc. Our main results are summarized as follows:

  * 1.

We have confirmed that most of the protostellar sources have dense envelopes
with a volume H2 density of $\sim$105–106 cm-3. Spatial distributions of HCO+
(4–3) are almost the same as those of 0.87 mm, indicating that the molecular
line emission is a useful tracer of dense gas kinematics around high-mass
protostellar systems at a 0.1 pc scale.

  * 2.

The dense gas emission has elongated structures that can be regarded as
massive filaments whose line mass is $\sim$103–104 $M_{\odot}$ pc-1 quantified
by the FilFinder algorithm. In particular, filaments with higher H2 column
densities tend to have a wider linewidth of HCO+, suggesting an increase in
turbulence due to gravitational contraction and gas accretion from the
surrounding environment. We found that filamentary systems associated with
brighter protostars tend to have more complex hub-like features. The
protostellar luminosity threshold for having a hub filamentary system is
$\sim$5 $\times$104 $L_{\odot}$, consistent with that obtained from a similar
Galactic study (Kumar et al., 2020).

  * 3.

We have compared the presence or absence of (hub) filament systems with the
evolutionary stages of the natal GMCs as revealed by wide-field mapping using
a single-dish telescope. The striking finding is that massive hub-filament
system are well associated with the latest stage of GMC evolution on their
$\sim$30 Myr lifetimes. Based on our preliminary demographics, we suggest that
GMCs, grown through various perturbations such as gas collisions, may evolve
into systems capable of forming bright, massive star clusters exceeding
$\sim$5 $\times$104 $L_{\odot}$ in their final stages, accompanied by hub-
filament precursors.

We would like to thank the anonymous referee for useful comments that improved
the manuscript. This paper makes use of the following ALMA data: ADS/JAO.
ALMA#2019.1.01770.S. ALMA is a partnership of ESO (representing its member
states), the NSF (USA), and NINS (Japan), together with the NRC (Canada),
MOST, and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with
the Republic of Chile. The Joint ALMA Observatory is operated by the ESO,
AUI/NRAO, and NAOJ. This work was supported by a NAOJ ALMA Scientific Research
grant Nos. 2022-22B, Grants-in-Aid for Scientific Research (KAKENHI) of Japan
Society for the Promotion of Science (JSPS; grant No. JP18H05440, JP19K14760,
JP20H05645, JP21H00049, JP21H00058, JP21H01145, and JP21K13962). The material
is based upon work supported by NASA under award number 80GSFC21M0002 (M.S.).

##  Appendix A HCO+ spectra and moment maps

Figure 5 shows the average HCO+ (4–3) spectra along the FilFinder spine (see
Sect. 3.2). For some bright sources, there are additional blue-shifted
components at a relative velocity of $\sim-$15 km s-1. This is SO2 emission at
the rest frequency of 356.7552 GHz, instead of an extra velocity component of
the HCO+. The velocity fitting analysis is unaffected because the SO2 emission
is sufficiently weak.

Figures 6, 7 and 8 show the moment 0, 1 and 2 maps, respectively. Typically,
at the moment 1 maps, objects located in the upper panels, i.e., those with
brighter protostellar luminosities, tend to exhibit complex velocity
distributions, often deviating from the systemic velocity by a few km s-1.
Similarly, objects in the upper panels in the moment 2 maps show larger
velocity line widths. These trends are consistent with the results in Figure
4.

Figure 5:  Average spectra of HCO+(4–3) along each filament spine are shown in
blue solid profile. Orange dashed lines show the Gaussian fitting results. For
the targets with the asterisk mark ($*$) just after the name label, we
illustrated the HCO+ spectra toward the nearest neighbor of 0.87 mm peak from
the YSO. Figure 6: Moment 0 maps of HCO+(4–3) toward high-mass YSOs in the
LMC. The integrated velocity ranges are $V_{\rm sys}$ $\pm\Delta v_{\rm FWHM}$
for each source (see Table 3). In the case of $\Delta v_{\rm FWHM}$ not listed
objects, namely those for which the filament spine could not be identtfied, a
uniform integration range of $V_{\rm sys}\pm 2.5$ km s-1 was adopted. Cyan
lines show the identified spines of the filament that we used to derive the
physical properties (see the text in Sect. 3.2).  Figure 7: Moment 1 maps of
HCO+(4–3) toward high-mass YSOs in the LMC. The angular resolution,
$\sim$0$\farcs$39 $\times$ 0$\farcs$32, is given by the white ellipse in the
lower left corner of the lower left panel. Black crosses represent the
position of the YSOs (Seale et al., 2009). The systemic velocities ($V_{\rm
sys}$) for each source are listed in Table 3. Black lines show the identified
spines of the filament that we used to derive the physical properties (see the
text in Sect. 3.2). Figure 8: Same as those in Figure 7, but for moment 2
maps.

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