arXiv subjects

Daniel Seifried

Publications and source records attributed to Daniel Seifried.

At least 19 recordsLinked to original sources

arXiv2026

SIMPLIFI -- Study of Interstellar Magnetic Polarization: a Legacy Investigation of Filaments. I. Magnetically-Guided Accretion onto the DR21 Ridge

We present first results from SIMPLIFI (Study of Interstellar Magnetic Polarization: a Legacy Investigation of Filaments), a SOFIA/HAWC+ $214~\mu\rm{}m$ polarimetric survey of Galactic molecular cloud filaments. We trace magnetic field morphology from the DR21 Main Ridge into surrounding sub-filaments at $\sim{}0.1~\rm{}pc$ resolution, extending polarimetric detections for the first time beyond high-column-density regions probed by prior submillimeter observations. We compare the plane-of-sky orientations of the magnetic field $\hat{B}_{\rm{}pos}$, the projected gravitational acceleration $\vec{g}_{\rm{}pos}$, and the intensity gradient rotated by $90^{\circ}$. The relative orientation of $\hat{B}_{\rm{}pos}$ and the rotated gradient transitions from preferentially parallel in sub-filaments to perpendicular in the Main Ridge at $N({\rm{}H_2})\sim{}2\times{}10^{22}~\rm{}cm^{-2}$, consistent with thresholds seen with Planck. This is expected in clouds formed from strongly magnetized, sub-Alfvenic, magnetically sub-critical gas. We find region-to-region and pixel-to-pixel variations at fixed column density, indicating that column density alone is not sufficient to encode changes in magnetic field structure. Our central finding is that $\vec{g}_{\rm{}pos}$ and $\hat{B}_{\rm{}pos}$ remain aligned throughout the cloud regardless of column density or environment, unlike the environment-dependent behavior of either quantity vs. the intensity gradient. This persistent alignment is consistent with magnetically-guided accretion: sub-filaments channel material along field lines at several $10^{-3}\,M_{\odot}\,\rm{}yr^{-1}$, sufficient to assemble the Ridge within $\sim{}1~\rm{}Myr$ and sustain high-mass star formation. The framework also explains why observed radial velocities $\sim{}2~\rm{}km\,s^{-1}$ fall well below free-fall expectations $\sim{}8~\rm{}km\,s^{-1}$ due to projection effects.

astro-ph.GA↗

arXiv2025

The Diamond Ring in Cygnus X: Advanced stage of an expanding bubble of ionised carbon

The "Diamond Ring" in Cygnus X, southwest of the DR21 ridge, is a nearly circular structure of $\sim$6 pc in diameter, prominent in FIR emission and enclosed by clumpy molecular clouds traced in CO. It hosts an HII region, visible in cm emission, and resembles a classical expanding HII bubble routinely seen in the 158 $\mu$m [CII] line. However, SOFIA FEEDBACK observations in the spectrally resolved [CII] line reveal instead a slightly tilted ring of $\sim$10$^3$ M$_\odot$ expanding slowly at $\sim$1.3 km s$^{-1}$, with a bulk line-of-sight (LOS) velocity near $-2$ km s$^{-1}$. The central "Diamond" is an unrelated dense clump at $\sim$7 km s$^{-1}$. The driving source, classified from IR spectroscopy, is a B0.5e star that powers the HII region. Unlike typical 3D shells, this marks the first case where we detect only a slowly expanding CII ring. We suggest the HII region and CII bubble, initially formed by a massive star, expanded outward from a flat slab of molecular gas nearly in the plane of the sky. The ring is now confined by swept-up material of the slab, while shell components moving perpendicular to the LOS have dissipated, leading to a reduction in expansion. Dedicated simulations tracing the evolution of the CII bubble support this geometry, consistent with previous reports of HII region evolution in flat molecular clouds. We propose that the "Diamond Ring" represents the terminal phase of an expanding CII bubble driven by stellar winds and thermal pressure.

astro-ph.GA↗

arXiv2025

SILCC -- IX. The multi-phase interstellar medium at low metallicity

The gas-phase metallicity affects heating and cooling processes in the star-forming galactic interstellar medium (ISM) as well as ionising luminosities, wind strengths, and lifetimes of massive stars. To investigate its impact, we conduct magnetohydrodynamic simulations of the ISM using the FLASH code as part of the SILCC project. The simulations assume a gas surface density of 10 M$_\odot$ pc$^{-2}$ and span metallicities from 1/50 Z$_\odot$ to 1 Z$_\odot$. We include non-equilibrium thermo-chemistry, a space- and time-variable far-UV background and cosmic ray ionisation rate, metal-dependent stellar tracks, the formation of HII regions, stellar winds, type II supernovae, and cosmic ray injection and transport. With the metallicity decreasing over the investigated range, the star formation rate decreases by more than a factor of ten, the mass fraction of cold gas decreases from 60% to 2.3%, while the volume filling fraction of the warm gas increases from 20% to 80%. Furthermore, the fraction of H$_\mathrm{2}$ in the densest regions drops by a factor of four, and the dense ISM fragments into approximately five times fewer structures at the lowest metallicity. Outflow mass loading factors remain largely unchanged, with values close to unity, except for a significant decline at the lowest metallicity. Including the major processes that regulate ISM properties, this study highlights the strong impact of gas phase metallicity on the star-forming ISM.

astro-ph.GA↗

arXiv2025

The HI-to-H2 transition in the Draco cloud

In recent decades, significant attention has been dedicated to analytical and observational studies of the atomic hydrogen (HI) to molecular hydrogen (H2) transition in the interstellar medium. We focussed on the Draco diffuse cloud to gain deeper insights into the physical properties of the transition from HI to H2. We employed the total hydrogen column density probability distribution function (N-PDF) derived from Herschel dust observations and the N(HI)-PDF obtained from HI data collected by the Effelsberg HI survey. The N-PDF of the Draco cloud exhibits a double-log-normal distribution, whereas the N(HI)-PDF follows a single log-normal distribution. The HI-to-H2 transition is identified as the point where the two log-normal components of the dust N-PDF contribute equally; it occurs at Av = 0.33 (N=6.2e20 cm^-2). The low-column-density segment of the dust N-PDF corresponds to the cold neutral medium, which is characterized by a temperature of around 100 K. The higher-column-density part is predominantly associated with H2. The shape of the Draco N-PDF is qualitatively reproduced by numerical simulations. In the absence of substantial stellar feedback, such as radiation or stellar winds, turbulence exerts a significant influence on the thermal stability of the gas and can regulate the condensation of gas into denser regions and its subsequent evaporation. Recent observations of the ionized carbon line at 158 micron in Draco support this scenario. Using the KOSMA-tau photodissociation model, we estimate a gas density of n=50 cm^-3 and a temperature of 100 K at the location of the HI-to-H2 transition. Both the molecular and atomic gas components are characterized by supersonic turbulence and strong mixing, suggesting that simplified steady-state chemical models are not applicable under these conditions.

astro-ph.GA↗

arXiv2025

Grain alignment and dust evolution physics with polarisation (GRADE-POL). I. Dust polarisation modelling for isolated starless cores

The polarisation of light induced by aligned interstellar dust serves as a significant tool in investigating cosmic magnetic fields, dust properties, and poses a challenge in characterising the polarisation of the cosmic microwave background and other sources. To establish dust polarisation as a reliable tool, the physics of the grain alignment process needs to be studied thoroughly. The Magnetically enhanced Radiative Torque (MRAT) alignment is the only mechanism that can induce highly efficient alignment of grains with magnetic fields required by polarisation observations of the diffuse interstellar medium. Our numerical modelling of dust polarisation using the MRAT theory demonstrated that the alignment efficiency of starlight polarisation ($p_{\rm ext}/A_{\rm V}$) and the degree of thermal dust polarisation ($p_{\rm em}$) first decrease slowly with increasing visual extinction ($A_{\rm V}$) and then falls steeply as $\propto A^{-1}_{\rm V}$ at large $A_{\rm V}$ due to the loss of grain alignment, which explains the phenomenon known as polarisation holes. Visual extinction at the transition from shallow to steep slope ($A^{\rm loss}_{\rm V}$) increases with the maximum grain size. By applying physical profiles suitable for a starless core 109 in the Pipe Nebula (Pipe-109), our model successfully reproduces the existing observations of starlight polarisation at R-band ($0.65\,\mu$m) and H-band ($1.65\,\mu$m), as well as emission polarisation at submillimetre ($870\,\mu$m). Successful modelling of observational data requires perfect alignment of large grains as evidence of the MRAT mechanism, and larger maximum size with higher elongation at higher $A_{\rm V}$. The latter reveals the first evidence for the new model of anisotropic grain growth induced by magnetic grain alignment.

astro-ph.GA↗

arXiv2024

The impact of cosmic ray heating on the cooling of the low-metallicity interstellar medium

Low-metallicity environments are subject to inefficient cooling. They also have low dust-to-gas ratios and therefore less efficient photoelectric (PE) heating than in solar-neighbourhood conditions, where PE heating is one of the most important heating processes in the warm neutral interstellar medium (ISM). We perform magneto-hydrodynamic simulations of stratified ISM patches with a gas metallicity of 0.02 Z$_\odot$ as part of the SILCC project. The simulations include non-equilibrium chemistry, heating, and cooling of the low-temperature ISM as well as anisotropic cosmic ray (CR) transport, and stellar tracks. We include stellar feedback in the form of far-UV and ionising (FUV and EUV) radiation, massive star winds, supernovae, and CR injection. From the local CR energy density, we compute a CR heating rate that is variable in space and time. In this way, we can compare the relative impact of PE and CR heating on the metal-poor ISM and find that CR heating can dominate over PE heating. Models with a uniform CR ionisation rate suppress or severely delay star formation, since they provide a larger amount of energy to the ISM due to CR heating. Models with a variable CR ionisation rate form stars predominantly in pristine regions with low PE heating and CR ionisation rates where the metal-poor gas is able to cool efficiently. Because of the low metallicity, the amount of formed stars in all runs is not enough to trigger outflows of gas from the mid-plane.

astro-ph.GA↗

arXiv2024

The origin and evolution of the [CII] deficit in HII regions and star-forming molecular clouds

We analyse synthetic maps of the [CII] 158 $\mu$m line and FIR continuum of simulated molecular clouds (MCs) within the SILCC-Zoom project to study the origin of the [CII] deficit, i.e., the drop in the [CII]/FIR intensity ratio. All simulations include stellar radiative feedback and account for further ionisation of C$^+$ into C$^{2+}$ inside HII regions. For individual HII regions, $I_\mathrm{FIR}$ is initially high in the vicinity of young stars, and then moderately decreases as the gas is compressed into shells. In contrast, $I_\mathrm{CII}$ drops strongly over time, to which the second ionisation of C$^+$ into C$^{2+}$ contributes. This leads to a large drop in $I_\mathrm{[CII]}/I_\mathrm{FIR}$ inside HII regions, decreasing from 10$^{-3}$-10$^{-2}$ at scales above 10 pc to 10$^{-6}$-10$^{-4}$ at scales below 2pc. However, projection effects can affect the radial profile of $I_\mathrm{[CII]}$ and $I_\mathrm{FIR}$ and create apparent HII regions without any stars. On MC scales, $L_\mathrm{[CII]}/L_\mathrm{FIR}$ decreases from values $\gtrsim$10$^{-2}$ in MCs without star formation to values around $\sim10^{-3}$ in MCs with star formation. We attribute this and the origin of the [CII] deficit to two main contributors: (i) the saturation of the [CII] line and (ii) the conversion of C$^+$ into C$^{2+}$ by stellar radiation. The drop in $L_\mathrm{[CII]}/L_\mathrm{FIR}$ can be divided into two phases: (i) early on, the saturation of [CII] and the further ionisation of C$^+$ limit the increase in $L_\mathrm{[CII]}$, while $L_\mathrm{FIR}$ increases rapidly, leading to the initial decline of $L_\mathrm{[CII]}/L_\mathrm{FIR}$. (ii) In more evolved HII regions, $L_\mathrm{CII}$ stagnates and even partially drops due to the aforementioned reasons. $L_\mathrm{FIR}$ stagnates as the gas gets pushed into the cooler shells keeping $L_\mathrm{[CII]}/L_\mathrm{FIR}$ at low values of $\sim10^{-3}$.

astro-ph.GA↗

arXiv2024

SILCC -- VIII: The impact of far-ultraviolet radiation on star formation and the interstellar medium

We present magnetohydrodynamic simulations of star formation in the multiphase interstellar medium to quantify the impact of non-ionising far-ultraviolet (FUV) radiation within the \textsc{Silcc Project} simulation framework. Our study incorporates the radiative transfer of ionising radiation and self-consistent modelling of variable FUV radiation from star clusters, advancing beyond previous studies using static or simplified FUV fields. This enables a more accurate capture of the dynamic interaction between radiation and the evolving ISM alongside other stellar feedback channels. The interstellar radiation field (ISRF) near young star clusters can reach $G_0 \approx 10^4$ (in Habing units), far exceeding the solar neighbourhood value of $G_0 = 1.7$. Despite these high intensities, FUV radiation minimally impacts the integrated star formation rate compared to ionising radiation, stellar winds, and supernovae. A slight reduction in star formation burstiness is linked to increased photoelectric (PE) heating efficiency by the variable FUV field. photoelectric (PE) heating efficiency by the variable FUV field. Dust near star-forming regions can be heated up to 60 K via the PE effect, with a broad temperature distribution. PE heating rates in variable FUV models exhibit higher peaks but lower averages than static ISRF models. Simulations under solar neighbourhood conditions without stellar winds or ionising radiation but with supernovae yield unexpectedly high star formation rates of $\sim 0.1 \mathrm{M_\odot~yr^{-1}~kpc^{-2}}$. Our analysis reveals increased cold neutral medium (CNM) volume-filling factors (VFF) outside stellar clusters, reduced thermally unstable gas, and sharper warm-cold gas separation. The variable FUV field also promotes a cold diffuse gas phase with a molecular component, exhibiting a VFF of $\sim5-10$~per cent.

astro-ph.GA↗

arXiv2024

The Molecular Cloud Lifecycle I: Constraining H2 formation and dissociation rates with observations

Molecular clouds (MCs) are the birthplaces of new stars in galaxies. A key component of MCs are photodissociation regions (PDRs), where far-ultraviolet radiation plays a crucial role in determining the gas's physical and chemical state. Traditional PDR models assume chemical steady state (CSS), where the rates of H$_2$ formation and photodissociation are balanced. However, real MCs are dynamic and can be out of CSS. In this study, we demonstrate that combining H$_2$ emission lines observed in the far-ultraviolet or infrared with column density observations can be used to derive the rates of H$_2$ formation and photodissociation. We derive analytical formulae that relate these rates to observable quantities, which we validate using synthetic H$_2$ line emission maps derived from the SILCC-Zoom hydrodynamical simulation. Our method estimates integrated H$_2$ formation and dissociation rates with an accuracy $\approx 30$ % (on top of uncertainties in observed H$_2$ emission maps and column densities). Our simulations, valid for column densities $N \leq 2 \times 10^{22}$ cm$^{-2}$, cover a wide dynamic range in H$_2$ formation and photodissociation rates, showing significant deviations from CSS, with 74 % of the MC's mass deviating from CSS by a factor greater than 2. Our analytical formulae can effectively distinguish between regions in and out of CSS. When applied to actual H$_2$ line observations, our method can assess the chemical state of MCs, providing insights into their evolutionary stages and lifetimes. A NASA Small Explorer mission concept, Eos, will be proposed in 2025 and is specifically designed to conduct the types of observations outlined in this study.

astro-ph.GA↗

arXiv2024

HYACINTH: HYdrogen And Carbon chemistry in the INTerstellar medium in Hydro simulations

Aims. We present a new sub-grid model, HYACINTH -- HYdrogen And Carbon chemistry in the INTerstellar medium in Hydro simulations -- for computing the non-equilibrium abundances of ${\rm H_2}$ and its carbon-based tracers, namely ${\rm CO}$, ${\rm C}$, and ${\rm C^+}$, in cosmological simulations of galaxy formation. Methods. The model accounts for the unresolved density structure in simulations using a variable probability distribution function of sub-grid densities and a temperature-density relation. Included is a simplified chemical network that has been tailored for hydrogen and carbon chemistry within molecular clouds and easily integrated into large-scale simulations with minimal computational overhead. As an example, we applied HYACINTH to a simulated galaxy at redshift $z \sim 2.5$ in post-processing and compared the resulting abundances with observations. Results. The chemical predictions from HYACINTH are in reasonable agreement with high-resolution molecular-cloud simulations at different metallicities. By post-processing a galaxy simulation with HYACINTH, we reproduced the $\rm H\,I-{\rm H_2}$ transition as a function of the hydrogen column density $N_{\rm H}$ for both Milky-Way-like and Large-Magellanic-Cloud-like conditions. Column density maps reveal that ${\rm CO}$ is concentrated in the peaks of the ${\rm H_2}$ distribution, while atomic carbon more broadly traces the bulk of ${\rm H_2}$ in our post-processed galaxy. Based on both the column density maps and the surface density profiles of the different gas species in the post-processed galaxy, we find that ${\rm C^+}$ maintains a substantially high surface density out to $\sim 10 \, \rm kpc$ as opposed to other components that exhibit a higher central concentration. This is similar to the extended $[\rm C\,II]$ emission found in some recent observations at high redshifts.

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↗

arXiv2024

The Molecular Cloud Lifecycle II: Formation and Destruction of Molecular Clouds Diagnosed via H$_2$ Fluorescent Emission Emission

Molecular hydrogen (H$_2$) formation and dissociation are key processes that drive the gas lifecycle in galaxies. Using the SImulating the LifeCycle of Molecular Clouds (SILCC) zoom-in simulation suite, we explore the utility of future observations of H$_2$ dissociation and formation for tracking the lifecycle of molecular clouds. The simulations used in this work include non-equilibrium H$_2$ formation, stellar radiation, sink particles, and turbulence. We find that, at early times in the cloud evolution, H$_2$ formation rapidly outpaces dissociation and molecular clouds build their mass from the atomic reservoir in their environment. Rapid H$_2$ formation is also associated with a higher early star formation rate. For the clouds studied here, H$_2$ is strongly out of chemical equilibrium during the early stages of cloud formation but settles into a bursty chemical steady-state about 2 Myrs after the first stars form. At the latest stage of cloud evolution, dissociation outweighs formation and the clouds enter a dispersal phase. We discuss how theories for the molecular cloud lifecycle and the star formation efficiency may be distinguished with observational measurements of H$_2$ fluorescence with a space-based high-resolution FUV spectrometer, such as the proposed Hyperion and Eos NASA Explorer missions. Such missions would enable measurements of the H$_2$ dissociation and formation rates, which we demonstrate can be connected to different phases in a molecular cloud's star-forming life, including cloud building, rapidly star-forming, H$_2$ chemical equilibrium, and cloud destruction.

astro-ph.GA↗

arXiv2023

On the origin of V-shaped polarisation spectra in molecular clouds

We extend previous theoretical works to gain a better understanding of the origin of observed polarisation degree spectra of molecular clouds, which show a so-called V-shape, i.e. a pronounced minimum around 350~$\mu$m. For this purpose, we present results of two-phase dust models investigated with POLARIS. We also provide a guideline to calculate individual dust temperatures for different grain types in POLARIS. We show that V-shaped polarisation spectra can only be obtained if two dust phases, one dense and cold as well as one warm and dilute phase, are present along the line of sight. We find that the V-shape is the stronger pronounced the larger the density and temperature contrast between both phases is. In contrast to previous results, no correlation between the alignment efficiency of silicate grains and the dust temperature is required; carbonaceous grains are in general assumed to be not aligned with the magnetic field. By matching our model results with actual observations of V-shaped polarisation spectra, we show that in UV-illuminated regions (here the warm and dilute phase) carbon grain destruction might take place. This leads to a more pronounced V-shape with a minimum around 300~$\mu$m. In addition, we show that the dust spectral index and temperature of silicate grains affects the steepness of the polarisation spectrum at long wavelengths. Finally, we present a first polarisation spectrum obtained from a 3D, magneto-hydrodynamical molecular cloud simulation. It shows a flattening or even weakly pronounced minimum around 350~$\mu$m demonstrating the potential of such complex 3D simulations to study polarisation spectra.

astro-ph.GA↗

arXiv2023

How do supernova remnants cool? -- I. Morphology, optical emission lines, and shocks

Supernovae (SNe) inject $\sim 10^{51}$ erg in the interstellar medium, thereby shocking and heating the gas. A substantial fraction of this energy is later lost via radiative cooling. We present a post-processing module for the FLASH code to calculate the cooling radiation from shock-heated gas using collisional excitation data from MAPPINGS V. When applying this tool to a simulated SN remnant (SNR), we find that most energy is emitted in the EUV. However, optical emission lines ($[$O III$]$, $[$N II$]$, $[$S II$]$, H${\alpha}$, H${\beta}$) are usually best observable. Our shock detection scheme shows that [S II] and [N II] emissions arise from the thin shell surrounding the SNR, while [O III], H$\rm \alpha$, and H$\rm \beta$ originate from the volume-filling hot gas inside the SNR bubble. We find that the optical emission lines are affected by the SNR's complex structure and its projection onto the plane of the sky because the escaping line luminosity can be reduced by 10 -- 80\% due to absorption along the line-of-sight. Additionally, the subtraction of contaminating background radiation is required for the correct classification of an SNR on the oxygen or sulphur BPT diagrams. The electron temperature and density obtained from our synthetic observations match well with the simulation but are very sensitive to the assumed metallicity.

astro-ph.HE↗

arXiv2022

Hyperion: The origin of the stars A far-UV space telescope for high-resolution spectroscopy over wide fields

We present Hyperion, a mission concept recently proposed to the December 2021 NASA Medium Explorer announcement of opportunity. Hyperion explores the formation and destruction of molecular clouds and planet-forming disks in nearby star-forming regions of the Milky Way. It does this using long-slit, high-resolution spectroscopy of emission from fluorescing molecular hydrogen, which is a powerful far-ultraviolet (FUV) diagnostic. Molecular hydrogen (H2) is the most abundant molecule in the universe and a key ingredient for star and planet formation, but is typically not observed directly because its symmetric atomic structure and lack of a dipole moment mean there are no spectral lines at visible wavelengths and few in the infrared. Hyperion uses molecular hydrogen's wealth of FUV emission lines to achieve three science objectives: (1) determining how star formation is related to molecular hydrogen formation and destruction at the boundaries of molecular clouds; (2) determining how quickly and by what process massive star feedback disperses molecular clouds; and (3) determining the mechanism driving the evolution of planet-forming disks around young solar-analog stars. Hyperion conducts this science using a straightforward, highly-efficient, single-channel instrument design. Hyperion's instrument consists of a 48 cm primary mirror, with an f/5 focal ratio. The spectrometer has two modes, both covering 138.5-161.5 nm bandpasses. A low resolution mode has a spectral resolution of R>10,000 with a slit length of 65 arcmin, while the high resolution mode has a spectral resolution of R>50,000 over a slit length of 5 armin. Hyperion occupies a 2 week long, high-earth, Lunar resonance TESS-like orbit, and conducts 2 weeks of planned observations per orbit, with time for downlinks and calibrations. Hyperion was reviewed as Category I, which is the highest rating possible, but was not selected.

astro-ph.SR↗

arXiv2022

SILCC VII -- Gas kinematics and multiphase outflows of the simulated ISM at high gas surface densities

We present magnetohydrodynamic (MHD) simulations of the star-forming multiphase interstellar medium (ISM) in stratified galactic patches with gas surface densities $\Sigma_\mathrm{gas} =$ 10, 30, 50, and 100 $\mathrm{M_\odot\,pc^{-2}}$. The SILCC project simulation framework accounts for non-equilibrium thermal and chemical processes in the warm and cold ISM. The sink-based star formation and feedback model includes stellar winds, hydrogen-ionising UV radiation, core-collapse supernovae, and cosmic ray (CR) injection and diffusion. The simulations follow the observed relation between $\Sigma_\mathrm{gas}$ and the star formation rate surface density $\Sigma_\mathrm{SFR}$. CRs qualitatively change the outflow phase structure. Without CRs, the outflows transition from a two-phase (warm and hot at 1 kpc) to a single-phase (hot at 2 kpc) structure. With CRs, the outflow always has three phases (cold, warm, and hot), dominated in mass by the warm phase. The impact of CRs on mass loading decreases for higher $\Sigma_\mathrm{gas}$ and the mass loading factors of the CR-supported outflows are of order unity independent of $\Sigma_\mathrm{SFR}$. Similar to observations, vertical velocity dispersions of the warm ionised medium (WIM) and the cold neutral medium (CNM) correlate with the star formation rate as $\sigma_\mathrm{z} \propto \Sigma_\mathrm{SFR}^a$, with $a \sim 0.20$. In the absence of stellar feedback, we find no correlation. The velocity dispersion of the WIM is a factor $\sim 2.2$ higher than that of the CNM, in agreement with local observations. For $\Sigma_\mathrm{SFR} \gtrsim 1.5 \times 10^{-2}\,\mathrm{M}_\odot\,\mathrm{yr}^{-1}\,\mathrm{kpc}^{-2}$ the WIM motions become supersonic.

astro-ph.GA↗

arXiv2022

Properties of molecular clumps and cores in colliding magnetized flows

We simulate the formation of molecular clouds in colliding flows of warm neutral medium with the adaptive mesh refinement code {\sc Flash} in eight simulations with varying initial magnetic field strength, between {0.01 -- 5~$\mu$G}. We include a chemical network to treat heating and cooling and to follow the formation of molecular gas. The initial magnetic field strength influences the fragmentation of the forming cloud because it prohibits motions perpendicular to the field direction and hence impacts the formation of large-scale filamentary structures. Molecular clump and core formation occurs anyhow. We identify 3D-clumps and 3D-cores, which are defined as connected, CO-rich regions. Additionally, 3D-cores are heavily shielded. While we do not claim those 3D-objects to be directly comparable to observations, this enables us to analyse their full virial state. With increasing field strength, we find more fragments with a smaller average mass; yet the dynamics of the forming clumps and cores only weakly depends on the initial magnetic field strength. The molecular clumps are mostly unbound, probably transient objects, which are weakly confined by ram pressure or thermal pressure, indicating that they are swept up by the turbulent flow. They experience significant fluctuations in the mass flux through their surface, such that the Eulerian reference frame shows a dominant time-dependent term due to their indistinct nature. We define the cores to encompass highly shielded molecular gas. Most cores are in gravitational-kinetic equipartition and are well described by the common virial parameter $\alpha_\mathrm{vir}$, while some undergo minor dispersion by kinetic surface effects.

astro-ph.GA↗

arXiv2022

Cosmic-ray-induced H$_2$ line emission: Astrochemical modeling and implications for JWST observations

Context: It has been proposed that H$_2$ near-infrared lines may be excited by cosmic rays and allow for a determination of the cosmic-ray ionization rate in dense gas. One-dimensional models show that measuring both the H$_2$ gas column density and H$_2$ line intensity enables a constraint on the cosmic-ray ionization rate as well as the spectral slope of low-energy cosmic-ray protons in the interstellar medium (ISM). Aims: We aim to investigate the impact of certain assumptions regarding the H$_2$ chemical models and ISM density distributions on the emission of cosmic-ray induced H$_2$ emission lines. This is of particular importance for utilizing observations of these lines with the James Webb Space Telescope to constrain the cosmic-ray ionization rate. Methods: We compare the predicted emission from cosmic-ray induced, ro-vibrationally excited H$_2$ emission lines for different one- and three-dimensional models with varying assumptions on the gas chemistry and density distribution. Results: We find that the model predictions of the H$_2$ line intensities for the (1-0)S(0), (1-0)Q(2), (1-0)O(2) and (1-0)O(4) transitions at 2.22, 2.41, 2.63 and 3.00 $\mu$m, respectively, are relatively independent of the astro-chemical model and the gas density distribution when compared against the H$_2$ column density, making them robust tracer of the cosmic-ray ionization rate. Conclusions: We recommend the use of ro-vibrational H$_2$ line emission in combination with estimation of the cloud's H$_2$ column density, to constrain the ionization rate and the spectrum of low energy cosmic-rays.

astro-ph.GA↗