arXiv subjects

Alvaro Hacar

Publications and source records attributed to Alvaro Hacar.

At least 19 recordsLinked to original sources

arXiv2026

Tracking the flow of interstellar gas across scales: from galaxies to star clusters

While the baryon cycle of the interstellar medium - and its ability to form stars - plays a key role in galaxy evolution, unravelling the physical processes driving its evolution remains a critical challenge, given that present-day observing facilities are unable to map entire galactic ecosystems down to the scales of individual star-forming clumps. Here, we outline the technical requirements that would enable, for the first time, to connect the small (pc) scale physics of star formation, to the large (tens-of-kpc) scales of galaxies, enabling an effective tracking of the flows of matter and energy across a vast range of environments, and answering the long-standing question of what regulates star formation across cosmic time.

astro-ph.IM↗

arXiv2026

Star Formation across the Galaxy

Star formation is both a multi-scale and highly dynamic process. It is regulated by the combination of gravity, turbulence, feedback and magnetic fields, which connect more than six orders of magnitude in scale: from large molecular clouds (several 10s to 100s pc in size) down to protoplanetary disks (a few 10s to 100s of astronomical units in size, or 10^{-5} to 10^{-4} pc). Resolving the processes of fragmentation, accretion, and feedback that result in the formation of stellar systems throughout all relevant scales, from clouds to disks, and across the entire range of different environments in the Milky Way is key to constructing a comprehensive understanding and a predictive model of the formation process of stars of all masses that populate not only our Galaxy but the wider Universe across cosmic times. With current astronomical facilities, we are only able to grasp the details of star formation in <5% of Galactic star-forming regions, being heavily biased towards those close to us. Thus, characterizing this multi-scale phenomenon at the necessary sensitivity and resolution across all Galactic environments requires the next generation (sub)millimeter-wavelength facility: ALMA2040.

astro-ph.IM↗

arXiv2026

Emergence of high-mass stars in complex fiber networks (EMERGE) VII. Multi-Tracer Column Density Synthesis

Obtaining accurate estimates of the gas column density $\mathrm{N(H_2)}$ is crucial to determine the physical conditions of the molecular gas in the Interstellar Medium. This paper introduces the "Multi-Tracer Column Density Synthesis" (MT-CDS) technique, a new empirical approach to derive high dynamic-range column density maps with interferometers such as ALMA. We have compared the integrated intensity maps of two density selective tracers, namely HNC (1-0) and $\mathrm{N_2H^+}$ (1-0), obtained with IRAM-30m with ancillary Herschel column density estimates in seven star-forming regions in Orion and derived individual temperature-corrected conversion factors for each of these tracers. We applied these conversion factors obtained at low-resolution into similar high sensitivity ALMA observations to derive independent column density maps describing both the low $N_\mathrm{HNC}\mathrm{(H_2)}$ and high $N_\mathrm{N_2H^+}\mathrm{(H_2)}$ column density components at high spatial resolution (4.5"). We also combined these independent maps into estimates for the total gas column density gas $N_\mathrm{tot}\mathrm{(H_2)}$ sampling more than two orders of magnitude in column density between $\sim 10^{21.5}-10^{23.5}$ cm$^{-2}$. A comparison with independent ArTeMiS column density maps at intermediate resolution (8") shows typical deviations of $\lesssim$0.2 dex from our MT-CDS gas column density estimates. We obtained individual column density maps in five of our targets at $\sim$2000 au resolution in which ALMA observations were available, that is a factor of $\sim$5 better than originally calibrated. These results demonstrate the potential of this MT-CDS technique to retrieve accurate, high dynamic range $N\mathrm{(H_2)}$ total gas column density estimates at interferometric resolution combining multiple molecular line tracers.

astro-ph.GA↗

arXiv2026

Characterizing the hierarchical structure of filaments I. On the origin of the length-mass (L-M) scaling relation

Aims. Filamentary structures in molecular clouds are widely recognized as fundamental components of star formation. Observations report a length-mass relation of the form $L \propto M^α$ with $α\simeq 0.5$. We characterize this relation in simulated molecular clouds and investigate the influence of hierarchical fragmentation and spatial resolution. Methods. We apply a two-step filament identification method to high-resolution 3D hydrodynamical and magnetohydrodynamical simulations from the SILCC-Zoom project, combining the Rolling Hough Transform with dendrogram-based segmentation for hierarchical decomposition. We analyze the resulting structures across multiple spatial resolutions. Results. The identified structures span column densities of $\sim 10^{21}$--$\sim 10^{23}\,\mathrm{cm}^{-2}$. Across multiple resolutions and, separately, the full hierarchical decomposition at the highest resolution, the ensemble exhibits a sub-linear $L-M$ relation close to $L \propto M^{0.5}$. When individual structures are traced across resolutions, the slopes remain sub-linear ($α\simeq 0.45$--$0.70$), consistent with the geometric prediction $L \propto M^{0.5}$ of a fragmentation-driven random walk. In contrast, decomposing individual structures at the highest resolution yields a wide range of slopes ($α\simeq 0.2$--$3$), peaking at $α<1$. A simple toy model shows that this diversity arises from differences in the internal column-density distribution and segmentation geometry. Conclusions. Taken together, these results indicate that the average sub-linear length-mass relation with $α\simeq 0.5$ is a robust property of the hierarchical structure and fragmentation occurring in molecular clouds.

astro-ph.GA↗

arXiv2026

The atomic multiphase interstellar medium of galaxies in the COLIBRE simulations

We analyse the neutral ISM of galaxies at redshift z=0 in the COLIBRE hydrodynamical simulations and investigate the conditions under which these two gas phases coexist in a narrow range of thermal pressures. COLIBRE galaxies are selected based on the metallicity of their ISM. The median and mass-weighted distributions of thermal pressures of the multiphase ISM are analysed and compared to thermal equilibrium models and observations. The ISM in galaxies with gas metallicities similar to solar values exhibits a clear multiphase structure with a warm and cold phase coexisting in a certain range of thermal pressures. The pressures at which the ISM is multiphase depend on the gas metallicity. For COLIBRE galaxies with lower metallicities ($Z_{\mathrm{ISM}}\lesssim0.1\,\mathrm{Z}_{\odot}$), this multiphase structure largely disappears, partly due to resolution. The thermal pressures weighted by the HI mass of the neutral phases in COLIBRE galaxies are lower than, but still comparable to, some theoretical works and observational estimates. The thermal pressures show a strong dependence on the weighting scheme. If weighted by the star formation rate or CI mass, the thermal pressures of the cold phase match those derived from observations. The resulting pressures depend on a combination of the assumed radiation field strength, dust abundance, limited resolution and the weighting scheme. The strong dependence of the thermal pressures on the weighting scheme indicates observational tracers used to estimate the thermal pressure are biased towards high-pressure regions.

astro-ph.GA↗

arXiv2026

Low cosmic-ray ionisation at parsec scales in G035.39-00.33

Cosmic rays (CRs) regulate the chemical evolution of the gas and its coupling to the magnetic field in the densest and coldest regions of the interstellar medium (ISM). However, the CR ionisation rate of H$_2$ ($ζ_2$) is one of the most debated parameters characterising molecular clouds due to the uncertainties in its estimation. We developed a new analytical framework based on the chemistry of N$_2$H$^+$, N$_2$D$^+$ and DCO$^+$ to overcome observational limitations in current estimates of $ζ_2$ and to probe the latter and the electron fraction, $x(e)$, in the gas across multiple density regimes. We applied this method towards the parsec-scale filament of the infrared dark cloud (IRDC) G035.39-00.33 with new observations from the NOrthern Extended Array (NOEMA) at a resolution of $3''$ (or $\sim9000$ au). Ancillary observations of C$^{18}$O complete this survey to measure $x(e)$ and $ζ_2$ in G035.39-00.33. CO depletion is widespread in G035.39-00.33 with factors, $f_\mathrm{D}$, positively correlated with column and number densities of H$_2$ in the cloud. The deuterium fractions ($R_\mathrm{D}$) are enhanced towards these same sites in which the corresponding electron fraction values cluster below $\lesssim10^{-8}$. $ζ_2$ varies by three orders of magnitude in G035.39-00.33 ($\sim10^{-18}-10^{-15}$ s$^{-1}$) with a median of $\sim2.3\times10^{-18}$ s$^{-1}$, consistent with those reported for other IRDCs and giant filaments, but on average lower than the typical $ζ_2$ for the ISM. $ζ_2$ shows a functional dependence on $N(\mathrm{H_2})$, but with absolute values lower compared to those predicted by theoretical models. This behaviour suggests the presence of an overall attenuation of the CR flux taking place in G035.39-00.33. The CR flux appears to be reduced by the change in magnetic field strength and morphology previously reported in the region.

astro-ph.GA↗

arXiv2026

The evolution of velocity dispersion in the Sco-Cen OB association

We study how the stellar velocity dispersion within the Scorpius-Centaurus OB association (Sco-Cen) has evolved over approximately 20 million years, from its formation to the present day, by investigating 32 stellar clusters in Sco-Cen. Using data from the Gaia mission along with supplementary stellar radial velocities, we identified a surprising sequence of abrupt jumps and intervening plateaus in the evolution of velocity dispersion correlating with times of star formation bursts. We find that the association is almost isotropically expanding and that star formation propagated from inside-out with a speed of about 5-6 km/s. We measure a present-day expansion rate of about 10-12 pc/Myr and observe that younger star clusters within the association exhibit higher velocities compared to older ones. This result, along with the stepwise increase in velocity dispersion over time, suggests a structured and sequential star formation process rather than a random one. This phased evolution suggests that stellar feedback is the primary driver of Sco-Cen's star formation history, expansion, and eventual dispersal. Our findings emphasise the value of precisely characterising stellar populations within OB associations, particularly through the creation of detailed, high-resolution age maps.

astro-ph.GA↗

arXiv2025

Survey of Orion Disks with ALMA (SODA) III: Disks in wide binary systems in L1641 and L1647

Aims. The goal of this work is to comprehensively characterize the impact of stellar multiplicity on Class II disks in the L1641 and L1647 regions of Orion A (~1-3 Myr), part of the Survey of Orion Disks with ALMA (SODA). We characterize the protostellar multiplicity using the Atacama Large Millimeter/submillimeter Array (ALMA), the ESO-VISTA, and Hubble Space telescopes. The resulting sample of 65 multiple systems represents the largest catalogue of wide binary systems to date (projected separation >1000 AU), allowing a more robust statistical characterization of the evolution and properties of protoplanetary disks. Methods. The disk population was observed in continuum with ALMA at 225 GHz, with a median rms of 1.5 Mearth. Combining these data (resolution ~1.1arcsec ) with the ESO-VISTA near-infrared survey of the Orion A cloud (resolution ~0.7arcsec ), multiple systems are assembled and selected by an iterative inside-out search in projected separation (>1000 AU). Results. We identify 61 binary systems, 3 triple systems, and one quadruple system. The separation range is between 1000 and 10^4 AU. The dust mass distributions inferred with the Kaplan-Meier estimator yield a median mass of 3.23+0.6-0.4 Mearth for primary disks and 3.88+0.3-0.3 Mearth for secondary disks.

astro-ph.EP↗

arXiv2024

AtLAST Science Overview Report

Submillimeter and millimeter wavelengths provide a unique view of the Universe, from the gas and dust that fills and surrounds galaxies to the chromosphere of our own Sun. Current single-dish facilities have presented a tantalising view of the brightest (sub-)mm sources, and interferometers have provided the exquisite resolution necessary to analyse the details in small fields, but there are still many open questions that cannot be answered with current facilities. In this report we summarise the science that is guiding the design of the Atacama Large Aperture Submillimeter Telescope (AtLAST). We demonstrate how tranformational advances in topics including star formation in high redshift galaxies, the diffuse circumgalactic medium, Galactic ecology, cometary compositions and solar flares motivate the need for a 50m, single-dish telescope with a 1-2 degree field of view and a new generation of highly multiplexed continuum and spectral cameras. AtLAST will have the resolution to drastically lower the confusion limit compared to current single-dish facilities, whilst also being able to rapidly map large areas of the sky and detect extended, diffuse structures. Its high sensitivity and large field of view will open up the field of submillimeter transient science by increasing the probability of serendipitous detections. Finally, the science cases listed here motivate the need for a highly flexible operations model capable of short observations of individual targets, large surveys, monitoring programmes, target of opportunity observations and coordinated observations with other observatories. AtLAST aims to be a sustainable, upgradeable, multipurpose facility that will deliver orders of magnitude increases in sensitivity and mapping speeds over current and planned submillimeter observatories.

astro-ph.IM↗

arXiv2024

Atacama Large Aperture Submillimeter Telescope (AtLAST) Science: Our Galaxy

As we learn more about the multi-scale interstellar medium (ISM) of our Galaxy, we develop a greater understanding for the complex relationships between the large-scale diffuse gas and dust in Giant Molecular Clouds (GMCs), how it moves, how it is affected by the nearby massive stars, and which portions of those GMCs eventually collapse into star forming regions. The complex interactions of those gas, dust and stellar populations form what has come to be known as the ecology of our Galaxy. Because we are deeply embedded in the plane of our Galaxy, it takes up a significant fraction of the sky, with complex dust lanes scattered throughout the optically recognisable bands of the Milky Way. These bands become bright at (sub-)millimetre wavelengths, where we can study dust thermal emission and the chemical and kinematic signatures of the gas. To properly study such large-scale environments, requires deep, large area surveys that are not possible with current facilities. Moreover, where stars form, so too do planetary systems, growing from the dust and gas in circumstellar discs, to planets and planetesimal belts. Understanding the evolution of these belts requires deep imaging capable of studying belts around young stellar objects to Kuiper belt analogues around the nearest stars. Here we present a plan for observing the Galactic Plane and circumstellar environments to quantify the physical structure, the magnetic fields, the dynamics, chemistry, star formation, and planetary system evolution of the galaxy in which we live with AtLAST; a concept for a new, 50m single-dish sub-mm telescope with a large field of view which is the only type of facility that will allow us to observe our Galaxy deeply and widely enough to make a leap forward in our understanding of our local ecology.

astro-ph.GA↗

arXiv2024

On the evolution of the observed Mass-to-Length relationship for star-forming filaments

The interstellar medium is threaded by a hierarchy of filaments from large scales (~ 100 pc) to small scales (~ 0.1pc). The masses and lengths of these nested structures may reveal important constraints for cloud formation and evolution, but it is difficult to investigate from an evolutionary perspective using single observations. In this work, we extract simulated molecular clouds from the Cloud Factory galactic-scale ISM suite in combination with 3D Monte Carlo radiative transfer code POLARIS to investigate how filamentary structure evolves over time. We produce synthetic dust continuum observations in three regions with a series of snapshots and use the Filfinder algorithm to identify filaments in the dust derived column density maps. When the synthetic filaments mass and length are plotted on an M-L plot, we see a scaling relation of $L\propto M^{0.45}$ similar to that seen in observations, and find that the filaments are thermally supercritical. Projection effects systematically affect the masses and lengths measured for the filaments, and are particularly severe in crowded regions. In the filament Mass-Length (M-L) diagram we identify three main evolutionary mechanisms: accretion, segmentation, and dispersal. In particular we find that the filaments typically evolve from smaller to larger masses in the observational M-L plane, indicating the dominant role of accretion in filament evolution. Moreover, we find a potential correlation between line mass and filament growth rate. Once filaments are actively star forming they then segment into smaller sections, or are dispersed by internal or external forces.

astro-ph.GA↗

arXiv2023

The star formation history of the Sco-Cen association. Coherent star formation patterns in space and time

We reconstruct the star formation history of the Sco-Cen OB association using a novel high-resolution age map of the region. We develop an approach to produce robust ages for Sco-Cen's recently identified 37 stellar clusters using the SigMA algorithm. The Sco-Cen star formation timeline reveals four periods of enhanced star formation activity, or bursts, remarkably separated by about 5 Myr. Of these, the second burst, which occurred about 15 million years ago, is by far the dominant, and most of Sco-Cen's stars and clusters were in place by the end of this burst. The formation of stars and clusters in Sco-Cen is correlated but not linearly, implying that more stars were formed per cluster during the peak of the star formation rate. Most of the clusters that are large enough to have supernova precursors were formed during the 15 Myr period. Star and cluster formation activity has been continuously declining since then. We have clear evidence that Sco-Cen formed from the inside out and contains 100-pc long chains of contiguous clusters exhibiting well-defined age gradients, from massive older clusters to smaller young clusters. These observables suggest an important role for feedback in forming about half of Sco-Cen stars, although follow-up work is needed to quantify this statement. Finally, we confirm that the Upper-Sco age controversy discussed in the literature during the last decades is solved: the nine clusters previously lumped together as Upper-Sco, a benchmark region for planet formation studies, exhibit a wide range of ages from 3 to 19 Myr.

astro-ph.SR↗

arXiv2023

VISIONS: The VISTA Star Formation Atlas -- I. Survey overview

VISIONS is an ESO public survey of five nearby (d < 500 pc) star-forming molecular cloud complexes that are canonically associated with the constellations of Chamaeleon, Corona Australis, Lupus, Ophiuchus, and Orion. The survey was carried out with VISTA, using VIRCAM, and collected data in the near-infrared passbands J, H, and Ks. With a total on-sky exposure time of 49.4 h VISIONS covers an area of 650 deg$^2$, and it was designed to build an infrared legacy archive similar to that of 2MASS. Taking place between April 2017 and March 2022, the observations yielded approximately 1.15 million images, which comprise 19 TB of raw data. The observations are grouped into three different subsurveys: The wide subsurvey comprises shallow, large-scale observations and has visited the star-forming complexes six times over the course of its execution. The deep subsurvey of dedicated high-sensitivity observations has collected data on the areas with the largest amounts of dust extinction. The control subsurvey includes observations of areas of low-to-negligible dust extinction. Using this strategy, the VISIONS survey offers multi-epoch position measurements, is able to access deeply embedded objects, and provides a baseline for statistical comparisons and sample completeness. In particular, VISIONS is designed to measure the proper motions of point sources with a precision of 1 mas/yr or better, when complemented with data from VHS. Hence, VISIONS can provide proper motions for sources inaccessible to Gaia. VISIONS will enable addressing a range of topics, including the 3D distribution and motion of embedded stars and the nearby interstellar medium, the identification and characterization of young stellar objects, the formation and evolution of embedded stellar clusters and their initial mass function, as well as the characteristics of interstellar dust and the reddening law.

astro-ph.GA↗

arXiv2023

Data Combination: Interferometry and Single-dish Imaging in Radio Astronomy

Modern interferometers routinely provide radio-astronomical images down to subarcsecond resolution. However, interferometers filter out spatial scales larger than those sampled by the shortest baselines, which affects the measurement of both spatial and spectral features. Complementary single-dish data are vital for recovering the true flux distribution of spatially resolved astronomical sources with such extended emission. In this work, we provide an overview of the prominent available methods to combine single-dish and interferometric observations. We test each of these methods in the framework of the CASA data analysis software package on both synthetic continuum and observed spectral data sets. We develop a set of new assessment tools that are generally applicable to all radio-astronomical cases of data combination. Applying these new assessment diagnostics, we evaluate the methods' performance and demonstrate the significant improvement of the combined results in comparison to purely interferometric reductions. We provide combination and assessment scripts as add-on material. Our results highlight the advantage of using data combination to ensure high-quality science images of spatially resolved objects.

astro-ph.IM↗

arXiv2023

On the density regime probed by HCN emission

HCN J$\, =\,$1$\, -\,$0 emission is commonly used as a dense gas tracer, thought to mainly arise from gas with densities $\mathrm{\sim 10^4\ -\ 10^5\ cm^{-3}}$. This has made it a popular tracer in star formation studies. However, there is increasing evidence from observational surveys of `resolved' molecular clouds that HCN can trace more diffuse gas. We investigate the relationship between gas density and HCN emission through post-processing of high resolution magnetohydrodynamical simulations of cloud-cloud collisions. We find that HCN emission traces gas with a mean volumetric density of $\mathrm{\sim 3 \times 10^3\ cm^{-3}}$ and a median visual extinction of $\mathrm{\sim 5\ mag}$. We therefore predict a characteristic density that is an order of magnitude less than the "standard" characteristic density of $\mathrm{n \sim 3 \times 10^4\ cm^{-3}}$. Indeed, we find in some cases that there is clear HCN emission from the cloud even though there is no gas denser than this standard critical density. We derive luminosity-to-mass conversion factors for the amount of gas at $A_{\rm V} > 8$ or at densities $n > 2.85 \times 10^{3} \: {\rm cm^{-3}}$ or $n > 3 \times 10^{4} \: {\rm cm^{-3}}$, finding values of $α_{\rm HCN} = 6.79, 8.62$ and $27.98 \: {\rm M_{\odot}} ({\rm K \, km \, s^{-1} \, pc^{2}})$, respectively. In some cases, the luminosity to mass conversion factor predicted mass in regions where in actuality there contains no mass.

astro-ph.GA↗

arXiv2023

Reflections on nebulae around young stars: A systematic search for late-stage infall of material onto Class II disks

Context. While it is generally assumed that Class II sources evolve largely in isolation from their environment, many still lie close to molecular clouds and may continue to interact with them. This may result in late accretion of material onto the disk that can significantly influence disk processes and planet formation. Aims. In order to systematically study late infall of gas onto disks, we identify candidate Class II sources in close vicinity to a reflection nebula (RN) that may be undergoing this process. Methods. First we targeted Class II sources with known kilo-au scale gas structures - possibly due to late infall of material - and we searched for RNe in their vicinity in optical and near-infrared images. Second, we compiled a catalogue of Class II sources associated with RNe and looked for the large-scale CO structures in archival ALMA data. Using the catalogues of protostars and RNe, we also estimated the probability of Class II sources interacting with surrounding material. Results. All of the sources with large-scale gas structures also exhibit some reflection nebulosity in their vicinity. Similarly, at least five Class II objects associated with a prominent RNe, and for which adequate ALMA observations are available, were found to have spirals or stream-like structures which may be due to late infall. We report the first detection of these structures around S CrA. Conclusions. Our results suggest that a non-negligible fraction of Class II disks in nearby star-forming regions may be associated with RNe and could therefore be undergoing late accretion of gas. Surveys of RNe and kilo-au scale gas structures around Class II sources will allow us to better understand the frequency and impact of late-infall phenomena.

astro-ph.SR↗

arXiv2022

Using Molecular Gas Observations to Guide Initial Conditions for Star Cluster Simulations

The earliest evolution of star clusters involves a phase of co-existence of both newly-formed stars, and the gas from which they are forming. Observations of the gas in such regions provide a wealth of data that can inform the simulations which are needed to follow the evolution of such objects forward in time. We present a method for transforming the observed gas properties into initial conditions for simulations that include gas, stars, and ongoing star formation. We demonstrate our technique using the Orion Nebula Cluster. Since the observations cannot provide all the necessary information for our simulations, we make choices for the missing data and assess the impact of those choices. We find that the results are insensitive to the adopted choices of the gas velocity in the plane of the sky. The properties of the surrounding gas cloud (e.g. overall density and size), however, have an effect on the star formation rate and pace of assembly of the resultant star cluster. We also analyze the stellar properties of the cluster and find that the stars become more tightly clustered and in a stronger radial distribution even as new stars form in the filament.

astro-ph.GA↗

arXiv2022

A study of 90 GHz dust emissivity on molecular cloud and filament scales

Recent observations from the MUSTANG2 instrument on the Green Bank Telescope have revealed evidence of enhanced long-wavelength emission in the dust spectral energy distribution (SED) in the Orion Molecular Cloud (OMC) 2/3 filament on 25" (0.1 pc) scales. Here we present a measurement of the SED on larger spatial scales (map size 0.5-3 degrees or 3-20 pc), at somewhat lower resolution (120", corresponding to 0.25 pc at 400 pc) using data from the Herschel satellite and Atacama Cosmology Telescope (ACT). We then extend the 120"-scale investigation to other regions covered in the Herschel Gould Belt Survey (HGBS) specifically: the dense filaments in the southerly regions of Orion A; Orion B; and Serpens-S. Our dataset in aggregate covers approximately 10 square degrees, with continuum photometry spanning from 160um to 3mm. These OMC 2/3 data display excess emission at 3mm, though less (10.9% excess) than what is seen at higher resolution. Strikingly, we find that the enhancement is present even more strongly in the other filaments we targeted, with an average excess of 42.4% and 30/46 slices showing an inconsistency with the modified blackbody to at least 4σ. Applying this analysis to the other targeted regions, we lay the groundwork for future high-resolution analyses. Additionally, we also consider a two-component dust model motivated by Planck results and an amorphous grain dust model. While both of these have been proposed to explain deviations in emission from a generic modified blackbody (MBB), we find that they have significant drawbacks, requiring many spectral points or lacking experimental data coverage.

astro-ph.GA↗