arXiv subjects

P. T. P. Ho

Publications and source records attributed to P. T. P. Ho.

At least 19 recordsLinked to original sources

arXiv2026

ALMAGAL IX. The chemical complexity of AG318.9477-00.1960: A line-identification template for ALMAGAL

We present a detailed molecular line analysis of one of the most chemically rich cores in the ALMAGAL sample, the high-mass core~9 in the AG318.9477-00.1960 clump (AG318-c9), located at a heliocentric distance of \sim 10.4\,\rm kpc. We further assessed whether the emission of selected COMs, that is, ethylene glycol ((CH_2OH)_2; EG), glycolaldehyde (CH_2(OH)CHO; GA), and methyl formate (CH_3OCHO; MF), can be used to trace the innermost regions of hot molecular cores (HMCs). We analysed ALMA Band~6 observations (\sim 217-221GHz). Spectral line identification and local thermodynamic equilibrium modelling were performed using the software MADCUBA. We derived the physical parameters, including the column density (N), excitation temperature (Tex), velocity, line width, and molecular abundances relative to H_2, for all detected species. The chemical inventory of AG318-c9 was compared with that of the HMC G31.41+0.31 (G31). In addition, we performed a pixel-by-pixel analysis of EG, GA, and MF to generate spatially resolved N and Tex maps and corresponding radial profiles.

astro-ph.GA↗

arXiv2026

ALMAGAL VIII. Early phases of triggered star formation in source AG286.0716$-$1.8229

Several theoretical and observational studies have shown that new waves of triggered star-formation can be induced by the feedback from newly formed massive protostars, due to the expansion of H II regions. We used the millimeter dust continuum data of the ALMAGAL survey and the Anderson et al. 2014 catalog of H II regions and selected one ALMAGAL source for ALMA follow-up observations. In fact, in source AG286.0716$-$1.8229 six cores were detected at a resolution of $\sim7600$ au, but only two at a higher resolution. The 4 cores not detected at higher resolution are prestellar core candidates. We used archival data from the SMGPS and RACS to confirm whether an H II region is present in the field. We observed the source with with ALMA in Band 4, covering the emission of DCO$^+$ (2$-$1), N$_2$D$^+$ (2$-$1), DCN (2$-$1), and CH$_3$CCH (9$-$8), to estimates whether these cores are in an early phase of the star-formation process. The new Band 4 continuum image revealed three cores outside of the ALMAGAL field of view, for a total of 9 cores in the region, 8 of which are located along an arch of radius $\sim0.75$ pc. We have derived a spectral index between -0.14 and -0.4, in the frequency range of 0.8-1.6 GHz for the candidate H II region, which is consistent with optically thin free-free emission. Using plausible temperature ranges, based on the information from chemical tracers and the dust continuum, we derived mass ranges for the cores ($\sim2-16\,$M$_{\odot}$) and ranges for the virial parameter ($\sim0.3-5$). All the cores along the arch have virial parameters $\lesssim$2, with only one exception. Comparing the typical separation and mass of the cores with those expected in the case of the collect and collapse scenario and with the thermal Jean length and mass, the best agreement is found with the characteristic scales in the case of triggered star formation.

astro-ph.GA↗

arXiv2026

ALMAGAL VI. The spatial distribution of dense cores during the evolution of cluster-forming massive clump

High-mass stars and star clusters form from the fragmentation of massive dense clumps driven by gravity, turbulence, and magnetic fields. The ALMAGAL project observed $\sim1000$ clumps at $\sim$1000\,au resolution, enabling a statistically significant characterization of this process across a large range of clump physical parameters and evolutionary stages. In this work, we investigated the spatial distribution of dense cores in the 514 massive, potentially cluster-forming, clumps hosting at least 4 cores, to trace fragmentation's initial conditions and early evolution. We used quantitative descriptors, evaluated against the clump bolometric luminosity-to-mass ratio as an indicator of evolution. Core separations were measured with the minimum spanning tree method (MST) and compared with the Jeans gravitational fragmentation theory. We used the $Q$ parameter and the mass segregation ratio, $Λ_{MSR}$, to evaluate whether cores have specific arrangements or differences depending on their masses. ALMAGAL cores are usually arranged in elliptical groups with an axis ratio $e\sim2.2$, but $e\geq$5 is also observed. A single characteristic core separation per clump is found in $\sim76$% of cases, but signatures of multiple fragmentation lengths not rare. Typical core separations are compatible with the clump-averaged thermal Jeans length, $λ^{th}_{J}$, though a population, typical of low-fragmented/young clumps, has wider separations with $l\approx3\timesλ^{th}_{J}$. The core separation decreases on average from $l\sim22000$ au in younger systems to $l\sim7000$ au in more evolved ones. Cores are typically distributed in fractal-type subclusters, with centrally concentrated patterns appearing only at later stages, but without a progressive evolutionary transition. Finally, mass segregation is found in 110 systems, with its occurrence increasing with evolution.

astro-ph.GA↗

arXiv2025

Bowshocks driven by the pole-on molecular jet of outbursting protostar SVS 13

Outflows play a key role in the star and planet formation processes. Some outflows show discrete clumps of cold molecular gas moving at extremely high velocities (EHVs) of $\sim$100 km s$^{-1}$, known as ''molecular bullets'', that are likely closely associated with their primary driving agent. Here we present ALMA CO(J=3-2) observations of a bright EHV molecular bullet that reveal its morphology in detail down to scales of 30 au and its kinematic structure across the entire intermediate velocity range ($\sim$30-100 km s$^{-1}$). These provide important new insights into how outflows transfer mass and momentum to the surrounding medium. The observed channel maps display several sequences of ring-like features whose velocity increases and size decreases with projected distance from the driving source, each sequence tracing a thin, bow-shaped shell culminating on-axis in a bright EHV head. The shape, kinematics, and mass of each shell all agree remarkably well with the simplest textbook models of momentum-conserving bowshocks produced by a time-variable EHV jet. The dynamical timescale between consecutive shells is of a few decades, with the latest ejection event coinciding with the protostar optical/IR outburst observed in $\sim$1990. The very strong evidence for bowshock-driven entrainment induced by jet variability revealed by this work suggests that accretion bursts, and therefore variations in the disk snowlines, should occur on decade timescales, which could substantially impact grain growth and planet formation.

astro-ph.GA↗

arXiv2025

ALMAGAL V. Relations between the core populations and the parent clump physical properties

Context. The fragmentation of massive molecular clumps into smaller, potentially star-forming cores plays a key role in the processes of high-mass star formation. The ALMAGAL project offers high-resolution data to investigate these processes across various evolutionary stages in the Galactic plane. Aims. This study aims at correlating the fragmentation properties of massive clumps, obtained from ALMA observations, with their global physical parameters (e.g., mass, surface density, and temperature) and evolutionary indicators (such as luminosity-to-mass ratio and bolometric temperature) obtained from Herschel observations. It seeks to assess whether the cores evolve in number and mass in tandem with their host clumps, and to determine the possible factors influencing the formation of massive cores (M > 24M_\odot). Methods. We analyzed the masses of 6348 fragments, estimated from 1.4 mm continuum data for 1007 ALMAGAL clumps. Leveraging this unprecedentedly large data set, we evaluated statistical relationships between clump parameters, estimated over about 0.1 pc scales, and fragment properties, corresponding to scales of a few 1000 au, while accounting for potential biases related to distance and observational resolution. Our results were further compared with predictions from numerical simulations. Results. The fragmentation level correlates preferentially with clump surface density, supporting a scenario of density-driven fragmentation, whereas it does not show any clear dependence on total clump mass. Both the mass of the most massive core and the core formation efficiency show a broad range and increase on average by an order of magnitude in the intervals spanned by evolutionary indicators such as clump dust temperature and the luminosity-to-mass ratio. This suggests that core growth continues throughout the clump evolution, favoring clump-fed over core-fed theoretical scenarios.

astro-ph.GA↗

arXiv2025

ALMAGAL IV. Morphological comparison of molecular and thermal dust emission using the histogram of oriented gradients (HOG) method

The study of molecular line emission is crucial to unveil the kinematics and the physical conditions of gas in star-forming regions. Our aim is to quantify the reliability of using individual molecular transitions to derive physical properties of the bulk of the H2 gas, looking at morphological correlations in their overall integrated molecular line emission with the cold dust. For this study we selected transitions of H2CO, CH$_3$OH, DCN, HC$_3$N, CH$_3$CN, CH$_3$OCHO, SO, and SiO and compared them with the 1.38 mm dust continuum emission at different spatial scales in the ALMAGAL sample, that observed a total of 1013 targets covering all evolutionary stages of the high-mass star-formation process and different conditions of clump fragmentation. We used the method of the histogram of oriented gradients (HOG) implemented in the tool astroHOG to compare the morphology of integrated line emission with maps of the 1.38 mm dust continuum emission. Moreover, we calculated the Spearman's correlation coefficient, and compared it with our astroHOG results. Only H$_2$CO, CH$_3$OH, and SO show emission on spatial scales comparable with the diffuse continuum emission. However, from the HOG method, the median correlation of the emission of each of these species with the continuum is only $\sim$24-29%. In comparison with the dense fragments these molecular species still have low values of correlation. On the other hand DCN, HC$_3$N, CH$_3$CN, and CH$_3$OCHO show a good correlation with the dense dust fragments, above 60%. The worst correlation is seen with SiO, both with the extended continuum emission and with compact sources. From the comparison of the results of the HOG method and the Spearman's correlation coefficient, the HOG method gives much more reliable results than the intensity-based coefficient in estimating the level of similarity of the emission morphology.

astro-ph.GA↗

arXiv2025

ALMAGAL I. The ALMA evolutionary study of high-mass protocluster formation in the Galaxy. Presentation of the survey and early results

Fundamental questions about the physics responsible for fragmenting molecular parsec-scale clumps into cores of ~1000 au are still open, that only a statistically significant investigation with ALMA is able to address: what are the dominant agents that determine the core demographics, mass, and spatial distribution as a function of the physical properties of the hosting clumps, their evolutionary stage and the different Galactic environments in which they reside? To what extent extent is fragmentation driven by clumps dynamics or mass transport in filaments? With ALMAGAL we observed the 1.38 mm continuum and lines toward more than 1000 dense clumps in our Galaxy, with M>500M_sun, surface density > 0.1 g/cm2 and d<7.5 kpc. The ACA and two 12-m array setups were used to deliver a minimum resolution of ~1000 au over the entire sample distance range. The sample covers all evolutionary stages from infrared dark clouds (IRDCs) to HII regions from the tip of the Galactic bar to the outskirts of the Galaxy. The spectral setup includes several molecular lines to trace the multiscale physics and dynamics of gas, notably CH3CN, H2CO, SiO, CH3OH, DCN, HC3N, SO etc. We present an initial overview of the observations and the early science product and results, with a first characterization of the morphological properties of the continuum emission. We use "perimeter-versus-area" and convex hull-versus-area metrics to classify the different morphologies. More extended and morphologically complex shapes are found toward clumps that are relatively more evolved and have higher surface densities.

astro-ph.GA↗

arXiv2025

ALMAGAL II. The ALMA evolutionary study of high-mass protocluster formation in the Galaxy. ALMA data processing and pipeline

The ALMAGAL Large Program has observed 1017 high-mass star-forming regions distributed throughout the Galaxy, sampling different evolutionary stages and environmental conditions. In this work, we present the acquisition and processing of the ALMAGAL data. The main goal is to set up a robust pipeline that generates science-ready products, with a good and uniform quality across the whole sample. ALMAGAL observations were performed with the Atacama Large Millimeter/submillimeter Array (ALMA). Each field was observed in three different telescope arrays, being sensitive to spatial scales ranging from 1000 au up to 0.1 pc. The spectral setup allows sensitive imaging of the continuum emission at 219 GHz, and it covers multiple molecular spectral lines observed in four different spectral windows that span about 4 GHz in frequency coverage. We have designed a Python-based processing workflow to calibrate and image these observational data. This ALMAGAL pipeline includes an improved continuum determination, suited for line-rich sources; an automatic self-calibration process that improves the dynamical range of the final images; and the combination of data from different telescope arrays to produce science-ready, fully combined images. The fully combined products have spatial resolutions in the range 800-2000 au, and mass sensitivities in the range 0.02-0.07 Mo. We also present a first analysis of the spectral line information included in the ALMAGAL setup, and its potential for future scientific studies. As an example, specific spectral lines at 1000 au scales resolve the presence of multiple outflows in clusters and will help us to search for disk candidates around massive protostars. Moreover, the broad frequency bands provide information on the chemical richness of the different cluster members, which can be used to study the chemical evolution during the formation process of star clusters.

astro-ph.GA↗

arXiv2023

Absolute Flux Density Calibration of the Greenland Telescope Data for Event Horizon Telescope Observations

Starting from the observing campaign in April 2018, the Greenland Telescope (GLT) has been added as a new station of the Event Horizon Telescope (EHT) array. Visibilities on baselines to the GLT, particularly in the North-South direction, potentially provide valuable new constraints for the modeling and imaging of sources such as M87*. The GLT's location at high Northern latitudes adds unique challenges to its calibration strategies. Additionally, the performance of the GLT was not optimal during the 2018 observations due to it being only partially commissioned at the time. This document describes the steps taken to estimate the various parameters (and their uncertainties) required for the absolute flux calibration of the GLT data as part of the EHT. In particular, we consider the non-optimized status of the GLT in 2018, as well as its improved performance during the 2021 EHT campaign.

astro-ph.IM↗

arXiv2020

The role of the magnetic field in the fragmentation process: the case of G14.225-0.506

B-fields are predicted to play a role in the formation of filamentary structures and their fragmentation process. We aim at investigating the role of the B-field in the process of core fragmentation toward the hub-filament systems in the IRDC G14.2, which present different fragmentation level. We performed observations of the thermal dust polarization at 350 μm using the CSO toward the hubs. We applied the polarization--intensity-gradient method to estimate the significance of the B-field over the G-force. The B-field in Hub-N shows a uniform structure along the E-W orientation, perpendicular to the major axis of the hub-filament system. The I-gradient in Hub-N displays a local minimum coinciding with the dust core MM1a detected with interferometric observations. The B-field orientation is perturbed when approaching the dust core. Hub-S shows 2 local minima, reflecting the bimodal distribution of the B-field. In Hub-N, both E and W of the hub-filament system, the I-gradient and the B-field are parallel whereas they tend to be perpendicular when penetrating the filaments and hub. The analysis of the δ- and Σ B-maps indicate that, the B-field cannot prevent the collapse, suggesting that the B-field is initially dragged by the infalling motion and aligned with it, or is channeling material toward the central ridge from both sides. Values of Σ B > 1 are found toward a N-S ridge encompassing the dust emission peak, indicating that in this region B-field dominates over G-force, or that with the current angular resolution we cannot resolve an hypothetical more complex structure. We estimated the B-field strength, the MtF ratio and the A-M number, and found differences between the 2 hubs. The different levels of fragmentation observed in these 2 hubs could arise from the differences in the B-field properties rather than from different intensity of the G-field.

astro-ph.GA↗

arXiv2020

ALMA Detects CO(3-2) within a Super Star Cluster in NGC5253

We present observations of CO(3-2) and $^{13}$CO(3-2) emission near the supernebula in the dwarf galaxy NGC 5253, which contains one of the best examples of a potential globular cluster in formation. The 0.3" resolution images reveal an unusual molecular cloud, "Cloud D1", coincident with the radio-infrared supernebula. The ~6-pc diameter cloud has a linewidth, $Δ$ v = 21.7 km/s, that reflects only the gravitational potential of the star cluster residing within it. The corresponding virial mass is 2.5 x 10$^5$ M$_\odot$. The cluster appears to have a top-heavy initial mass function, with $M_{low}$~1-2 M$_\odot$. Cloud D1 is optically thin in CO(3-2) probably because the gas is hot. Molecular gas mass is very uncertain but constitutes < 35% of the dynamical mass within the cloud boundaries. In spite of the presence of an estimated ~1500-2000 O stars within the small cloud, the CO appears relatively undisturbed. We propose that Cloud D1 consists of molecular clumps or cores, possibly star-forming, orbiting with more evolved stars in the core of the giant cluster.

astro-ph.GA↗

arXiv2018

Molecular gas and Star Formation Properties in Early Stage Mergers: SMA CO(2-1) Observations of the LIRGs NGC 3110 and NGC 232

Mergers of galaxies are an important mode for galaxy evolution because they serve as an efficient trigger of powerful starbursts. However, observational studies of the molecular gas properties during their early stages are scarce. We present interferometric CO(2-1) maps of two luminous infrared galaxies (LIRGs), NGC 3110 and NGC 232, obtained with the Submillimeter Array (SMA) with ~ 1 kpc resolution. While NGC 3110 is a spiral galaxy interacting with a minor (14:1 stellar mass) companion, NGC 232 is interacting with a similarly sized object. We find that such interactions have likely induced in these galaxies enhancements in the molecular gas content and central concentrations, partly at the expense of atomic gas. The obtained molecular gas surface densities in their circumnuclear regions are $Σ_{\rm mol}~\gtrsim10^{2.5}$ M$_\odot$ pc$^{-2}$, higher than in non-interacting objects by an order of magnitude. Gas depletion times of ~ 0.5 - 1 Gyr are found for the different regions, lying in between non-interacting disk galaxies and the starburst sequence. In the case of NGC 3110, the spiral arms show on average 0.5 dex shorter depletion times than in the circumnuclear regions if we assume a similar H$_2$-CO conversion factor. We show that even in the early stages of the interaction with a minor companion, a starburst is formed along the circumnuclear region and spiral arms, where a large population of SSCs is found (~350), and at the same time a large central gas concentration is building up which might be the fuel for an active galactic nucleus. The main morphological properties of the NGC 3110 system are reproduced by our numerical simulations and allow us to estimate that the current epoch of the interaction is at ~ 150 Myrs after closest approach.

astro-ph.GA↗

arXiv2018

The Hyper Suprime-Cam SSP Survey: Overview and Survey Design

Hyper Suprime-Cam (HSC) is a wide-field imaging camera on the prime focus of the 8.2m Subaru telescope on the summit of Maunakea in Hawaii. A team of scientists from Japan, Taiwan and Princeton University is using HSC to carry out a 300-night multi-band imaging survey of the high-latitude sky. The survey includes three layers: the Wide layer will cover 1400 deg$^2$ in five broad bands ($grizy$), with a $5\,σ$ point-source depth of $r \approx 26$. The Deep layer covers a total of 26~deg$^2$ in four fields, going roughly a magnitude fainter, while the UltraDeep layer goes almost a magnitude fainter still in two pointings of HSC (a total of 3.5 deg$^2$). Here we describe the instrument, the science goals of the survey, and the survey strategy and data processing. This paper serves as an introduction to a special issue of the Publications of the Astronomical Society of Japan, which includes a large number of technical and scientific papers describing results from the early phases of this survey.

astro-ph.IM↗

arXiv2017

The ALMA Phasing System: A Beamforming Capability for Ultra-High-Resolution Science at (Sub)Millimeter Wavelengths

The Atacama Millimeter/submillimeter Array (ALMA) Phasing Project (APP) has developed and deployed the hardware and software necessary to coherently sum the signals of individual ALMA antennas and record the aggregate sum in Very Long Baseline Interferometry (VLBI) Data Exchange Format. These beamforming capabilities allow the ALMA array to collectively function as the equivalent of a single large aperture and participate in global VLBI arrays. The inclusion of phased ALMA in current VLBI networks operating at (sub)millimeter wavelengths provides an order of magnitude improvement in sensitivity, as well as enhancements in u-v coverage and north-south angular resolution. The availability of a phased ALMA enables a wide range of new ultra-high angular resolution science applications, including the resolution of supermassive black holes on event horizon scales and studies of the launch and collimation of astrophysical jets. It also provides a high-sensitivity aperture that may be used for investigations such as pulsar searches at high frequencies. This paper provides an overview of the ALMA Phasing System design, implementation, and performance characteristics.

astro-ph.IM↗

arXiv2017

White Paper on East Asian Vision for mm/submm VLBI: Toward Black Hole Astrophysics down to Angular Resolution of 1~R$_{S}$

This White Paper details the intentions and plans of the East Asian Very Long Baseline Interferometry (VLBI) community for pushing the frontiers of millimeter/submillimeter VLBI. To this end, we shall endeavor to actively promote coordinated efforts in the East Asia region. Our goal is to establish firm collaborations among the East Asia VLBI community in partnership with related institutes in North America and Europe and to expand existing global mm/submm VLBI arrays for (a) exploring the vicinity of black holes with an ultimate angular resolution down to 1~R$_{S}$ (Schwarzschild radius) and (b) investigating the dynamics of circumstellar gas in star-forming regions and late-type stars, and circumnuclear gas around active galactic nuclei (AGNs). In the first half of this White Paper, we highlight scientific accomplishments of the East Asia (EA) VLBI community. Various VLBI research results on M87, Sgr A*, blazars, narrow-line Seyfert~1 galaxies, and compact symmetric objects are described, and future visions of our VLBI science are briefly presented. Maser science of star formation, stellar evolution, and physics of accretion disks around AGNs are also discussed. A new vision for conducting multi-transition maser studies using mm/submm VLBI together with the Atacama Large Millimeter/Submillimeter Array (ALMA) is described. In the second half of this White Paper, we describe the EA community's vision for using mm/submm VLBI arrays in the framework or extended version of the Event Horizon Telescope (EHT) and the Global Millimeter VLBI Array (GMVA). The accomplishment of the aforementioned goal will maximize the overall scientific outcomes of mm/submm VLBI in the world.

astro-ph.HE↗

arXiv2016

Detection of lithium in nearby young late-M dwarfs

Late M-type dwarfs in the solar neighborhood include a mixture of very low-mass stars and brown dwarfs which is difficult to disentangle due to the lack of constraints on their age such as trigonometric parallax, lithium detection and space velocity. We search for young brown dwarf candidates among a sample of 28 nearby late-M dwarfs with spectral types between M5.0 and M9.0, and we also search for debris disks around three of them. Based on theoretical models, we used the color $I-J$, the $J$-band absolute magnitude and the detection of the Li I 6708 $Å$ doublet line as a strong constraint to estimate masses and ages of our targets. For the search of debris disks, we observed three targets at submillimeter wavelength of 850 $μ$m. We report here the first clear detections of lithium absorption in four targets and a marginal detection in one target. Our mass estimates indicate that two of them are young brown dwarfs, two are young brown dwarf candidates and one is a young very low-mass star. The closest young field brown dwarf in our sample at only $\sim$15 pc is an excellent benchmark for further studying physical properties of brown dwarfs in the range 100$-$150 Myr. We did not detect any debris disks around three late-M dwarfs, and we estimated upper limits to the dust mass of debris disks around them.

astro-ph.SR↗

arXiv2016

What is controlling the fragmentation process in the Infrared Dark Cloud G14.225-0.506? Differet level of fragmentation in twin hubs

We present observations of the 1.3 mm continuum emission toward hub-N and hub-S of the infrared dark cloud G14.225-0.506 carried out with the Submillimeter Array, together with observations of the dust emission at 870 and 350 microns obtained with APEX and CSO telescopes. The large scale dust emission of both hubs consists of a single peaked clump elongated in the direction of the associated filament. At small scales, the SMA images reveal that both hubs fragment into several dust condensations. The fragmentation level was assessed under the same conditions and we found that hub-N presents 4 fragments while hub-S is more fragmented, with 13 fragments identified. We studied the density structure by means of a simultaneous fit of the radial intensity profile at 870 and 350 microns and the spectral energy distribution adopting a Plummer-like function to describe the density structure. The parameters inferred from the model are remarkably similar in both hubs, suggesting that density structure could not be responsible in determining the fragmentation level. We estimated several physical parameters such as the level of turbulence and the magnetic field strength, and we found no significant differences between these hubs. The Jeans analysis indicates that the observed fragmentation is more consistent with thermal Jeans fragmentation compared with a scenario that turbulent support is included. The lower fragmentation level observed in hub-N could be explained in terms of stronger UV radiation effects from a nearby HII region, evolutionary effects, and/or stronger magnetic fields at small scales, a scenario that should be further investigated.

astro-ph.GA↗

arXiv2015

Two-level hierarchical fragmentation in the Orion Molecular Cloud 1 northern filament

[Abridged] We have recently reported on the collapse and fragmentation properties of the northernmost part of this structure, located ~2.4pc north of Orion KL -- the Orion Molecular Cloud 3 (OMC 3, Takahashi et al. 2013). As part of our project to study the integral-shaped filament, we analyze the fragmentation properties of the northern OMC 1 filament. This filament is a dense structure previously identified by JCMT/SCUBA submillimeter continuum and VLA ammonia observations and shown to have fragmented into clumps. We observed OMC1 n with the Submillimeter Array (SMA) at 1.3mm and report on our analysis of the continuum data. We discovered 24 new compact sources, ranging in mass from 0.1 to 2.3, in size from 400 to 1300au, and in density from 2.6 x 10^7 to 2.8 x 10^6 cm^{-3}. The masses of these sources are similar to those of the SMA protostars in OMC3, but their typical sizes and densities are lower by a factor of ten. Only 8% of the new sources have infrared counterparts, yet there are five associated CO molecular outflows. These sources are thus likely in the Class 0 evolutionary phase yet it cannot be excluded that some of the sources might still be pre-stellar cores. The spatial analysis of the protostars shows that these are divided into small groups that coincide with previously identified JCMT/SCUBA 850 micron and VLA ammonia clumps, and that these are separated by a quasi-equidistant length of ~30arcmin (0.06pc). This separation is dominated by the Jeans length, and therefore indicates that the main physical process in the filament's evolution was thermal fragmentation. Within the protostellar groups, the typical separation is ~6arcsec (~2500\,au), which is a factor 2-3 smaller than the Jeans length of the parental clumps within which the protostars are embedded. These results point to a hierarchical (2-level) thermal fragmentation process of the OMC1n filament.

astro-ph.SR↗