arXiv subjects

Jonathan C. Tan

Publications and source records attributed to Jonathan C. Tan.

At least 55 records · Page 3Linked to original sources

arXiv2025

Investigating the Upper Scorpius OB association with HERMES. I. The spectroscopic sample and 6D kinematics

We present results from a large spectroscopic survey of the nearest young association to the Sun, Upper Scorpius, conducted using 2dF/HERMES on the Anglo-Australian Telescope. We use spectroscopic youth criteria such as Li-equivalent widths to identify >1000 pre-main sequence (PMS) members across the region and measure radial velocities, combining these with Gaia EDR3 5-parameter astrometry to obtain 6D kinematic information. We separate confirmed PMS association members into distinct kinematic groups and measure expansion and rotation trends in each. We find strong evidence for asymmetric expansion in several groups and derive expansion timescales from the greatest rates of expansion in each group. We also trace the past motion of these groups using an epicycle approximation and estimate the time since their most compact configuration. These kinematic properties are compared to literature ages and the star formation history of Upper Scorpius is discussed. We find evidence that a scenario in which star formation in the subgroups of Upper Scorpius proceeded independently, either by self-instability or external feedback from Upper Centaurus-Lupus, is more likely than a recently proposed "cluster chain" scenario in which these subgroups have triggered each other.

astro-ph.SR↗

arXiv2025

Glimmers in the Cosmic Dawn. III. On the Photometrically Determined Black Hole Mass to Stellar Mass Relation Across Cosmic Time

We present the results from performing spectral energy distribution (SED) fitting on 121 variable active galactic nuclei (AGN) candidates in the Hubble Ultra Deep Field (HUDF) using photometry from both the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST) covering $0.2 - 4.8$ microns. We designed a bespoke SED fitting code which decomposes the total SED into its stellar and AGN contributions. Our SED fitting retrieves a significant contribution to the total SED from an AGN template for 26 of our variable sources with $0 < z < 7$. We leverage the model AGN spectrum to estimate black hole masses ($M_{BH}$) using the measured luminosity at 5100 Å and local empirical calibrations. Common with recently discovered JWST broad line AGN (BL-AGN), we observe a trend in the $M_{BH} - M_{\ast}$ plane where low redshift sources have $M_{BH}$ which agree with local relations while high redshift sources have increasingly overmassive black holes with respect to the stellar mass ($M_{\ast}$) of their host galaxies. Within our sample, we identify two IMBH candidates hosted by dwarf galaxies at $z<1$ featuring overmassive BHs in the $M_{BH}-M_{\ast}$ plane, similarly to our high redshift sources. Finally, our SED fitter successfully retrieves the AGN nature of one source at $z >6$. This object has $z_{phot} = 6.74$ and we estimate a lower limit on its black hole mass of $\log_{10}(M_{BH}/M_{\odot}) > 7.36$.

astro-ph.GA↗

arXiv2025

Glimmers in the Cosmic Dawn. II. A variability census of supermassive black holes across the Universe

Understanding the origin and evolution of supermassive black holes (SMBH) stands as one of the most important challenges in astrophysics and cosmology, with little current theoretical consensus. Improved observational constraints on the cosmological evolution of SMBH demographics are needed. Here we report results of a search via photometric variability for SMBHs appearing as active galactic nuclei (AGN) in the cosmological volume defined by the Hubble Ultra Deep Field (HUDF). This work includes particular focus on a new observation carried out in 2023 with the \textit{Hubble Space Telescope (HST)} using the WFC3/IR/F140W, which is compared directly to equivalent data taken 11 years earlier in 2012. Two earlier pairs of observations from 2009 to 2012 with WFC3/IR/F105W and WFC3/IR/F160W are also analysed. We identify 521, 188, and 109 AGN candidates as nuclear sources that exhibit photometric variability at a level of 2, 2.5 and 3~$σ$ in at least one filter. This sample includes 13, 3, and 2 AGN candidates at redshifts $z>6$, when the Universe was $\lesssim900$~Myr old. After variability and luminosity function (down to $M_{\rm UV}=-17\:$mag) completeness corrections, we estimate the co-moving number density of SMBHs, $n_{\rm SMBH}(z)$. At $z \gtrsim 6$, $n_{\rm SMBH}\gtrsim 6\times10^{-3}\:{\rm cMpc^{-3}}$. At low-$z$ our observations are sensitive to AGN fainter than $M_{\rm UV}=-17 \:$mag, and we estimate $n_{\rm SMBH}\gtrsim 10^{-2}\:{\rm cMpc^{-3}}$. We discuss how these results place strong constraints on a variety of SMBH seeding theories.

astro-ph.GA↗

arXiv2025

Rotating Supermassive Pop III Stars On The Main Sequence

The detection of billion-solar-mass supermassive black holes (SMBHs) within the first billion years of cosmic history challenges conventional theories of black hole formation and growth. Simultaneously, recent JWST observations revealing exceptionally high nitrogen-to-oxygen abundance ratios in galaxies at high redshifts raise critical questions about rapid chemical enrichment mechanisms operating in the early universe. Supermassive stars (SMSs) with masses of 1000 to 10000 M$_{\odot}$ are promising candidates to explain these phenomena, but existing models have so far neglected the pivotal role of stellar rotation. Here, we present the first comprehensive evolutionary models of rotating Pop III SMSs computed using the GENEC stellar evolution code, including detailed treatments of rotation-induced chemical mixing, angular momentum transport, and mass loss driven by the $ΩΓ$ limit. We demonstrate that rotation significantly enlarges the convective core and extends stellar lifetimes by up to 20%, with moderate enhancement of mass-loss rates as stars approach critical rotation thresholds. Our results further indicate that the cores of SMSs rotate relatively slowly (below $\sim 200$ km s$^{-1}$), resulting in dimensionless spin parameters $a* < 0.1$ for intermediate-mass black hole (IMBH) remnants that are notably lower than theoretical maximum spins. These findings highlight rotation as a key factor in determining the structural evolution, chemical yields, and black hole spin properties of SMSs, providing critical insights to interpret observational signatures from the high-redshift universe.

astro-ph.SR↗

arXiv2025

Flash Ionization of the Early Universe by Pop III.1 Supermassive Stars

The Pop III.1 theory for supermassive black hole (SMBH) formation predicts that a substantial fraction of the early universe was ionized by supermassive stars at redshifts $z\sim20-30$, an era we refer to as ``The Flash''. This is followed by recombination to a mainly neutral state within a few tens of Myr. Here we discuss the implication of this ionization for the scattering optical depth of the cosmic microwave background (CMB), $τ$. We find a fiducial contribution of $τ_{\rm PopIII.1}\sim0.04$. Combining this with the contribution to reionization by standard galaxy populations at $z\lesssim 10$ with $τ_{\rm gal}\simeq0.06$, yields a total of $τ\simeq0.10$. As noted recently by several authors, such a value may help resolve apparent ``problems'' faced by $Λ$CDM of discrepant CMB-based measures of the Hubble constant (``Hubble tension''), as well as negative neutrino masses and dynamical dark energy that have been implied by recent Baryonic Acoustic Oscillation (BAO) results from the Dark Energy Spectroscopic Instrument (DESI). In addition, free-free emission from The Flash boosts the cosmic radio background, which could help explain the large 21-cm absorption depth reported by the Experiment to Detect the Global EoR Signature (EDGES).

astro-ph.CO↗

arXiv2025

The Emergence and Ionizing Feedback of Pop III.1 Stars as Progenitors for Supermassive Black Holes

Recent observations by JWST reveal an unexpectedly abundant population of rapidly growing supermassive black holes (SMBHs) in the early Universe, underscoring the need for improved models for their origin and growth. Employing new full radiative transfer hydrodynamical simulations of galaxy formation, we investigate the local and intergalactic feedback of SMBH progenitors for the Population III.1 scenario, i.e., efficient formation of supermassive stars from pristine, undisturbed dark matter minihalos. Our cosmological simulations capture the R-type expansion phase of these Pop III.1 stars, with their H-ionizing photon luminosities of $\sim10^{53}\,{\rm s}^{-1}$ generating HII regions that extend deep into the intergalactic medium, reaching comoving radii of $r_{\rm HII}\sim 1\,{\rm cMpc}$. We vary both the Pop III.1 ionization flux and cosmological formation environments, finding the former regulates their final $r_{\rm HII}$, whereas the latter is more important in setting their formation redshift. We use the results from our radiation-hydrodynamics simulations to estimate the cosmic number density of SMBHs, $n_{\rm SMBH}$, expected from Pop III.1 progenitors. We find $n_{\rm SMBH}\sim10^{-1}\,{\rm cMpc}^{-3}$, consistent with the results inferred from recent observations of the local and high redshift universe. Overall, this establishes Pop III.1 progenitors as viable candidates for the formation of the first SMBHs, and emphasises the importance of exploring heavy mass seed scenarios.

astro-ph.GA↗

arXiv2025

The formation of supermassive black holes from Population III.1 seeds. III. Galaxy evolution and black hole growth from semi-analytic modelling

We present an implementation of Pop III.1 seeding of supermassive black holes (SMBHs) in a theoretical model of galaxy formation and evolution to assess the growth the SMBH population and the properties of the host galaxies. The model of Pop III.1 seeding involves SMBH formation at redshifts $z\gtrsim 20$ in dark matter minihalos that are isolated from external radiative feedback, parameterized by isolation distance $d_{\rm iso}$. Within a standard $Λ$CDM cosmology, we generate dark matter halos using the code PINOCCHIO and seed them according to the Pop III.1 scenario, exploring values of $d_{\rm iso}$ from 50 to 100~kpc (proper distance). We consider two alternative cases of SMBH seeding: a Halo Mass Threshold (HMT) model in which all halos $>7\times10^{10}\:M_\odot$ are seeded with $\sim 10^5\:M_\odot$ black holes; an All Light Seed (ALS) model in which all halos are seeded with low, stellar-mass black holes. We follow the redshift evolution of the halos, populating them with galaxies using the GAlaxy Evolution and Assembly theoretical model of galaxy formation, including accretion on SMBHs and related feedback processes. Here we present predictions for the properties of galaxy populations, focusing on stellar masses, star formation rates, and black hole masses. The local, $z\sim0$ metrics of occupation fraction as a function of the galaxy stellar mass, galaxy stellar mass function (GSMF), and black hole mass function (BHMF) all suggest a constraint of $d_{\rm iso}<75\:$kpc. We discuss the implications of this result for the Pop III.1 seeding mechanism.

astro-ph.GA↗

arXiv2025

The SOMA-POL Survey. I. Polarization and magnetic field properties of massive protostars

The role of magnetic fields in regulating the formation of massive stars remains much debated. Here we present sub-millimeter polarimetric observations with JCMT-POL2 at $850\:μ$m of 13 regions of massive star formation selected from the SOFIA Massive (SOMA) star formation survey, yielding a total of 29 massive protostars. Our investigation of the $p'-I$ relationship suggests that grain alignment persists up to the highest intensities. We examine the relative orientations between polarization-inferred magnetic field direction and source column density elongation direction on small and large scales. On small scales, we find a bimodal distribution of these relative orientations, i.e., with an excess of near-parallel and near-perpendicular orientations. By applying a one-sample Kuiper test and Monte Carlo simulations to compare to a relative orientation distribution drawn from a uniform distribution, we statistically confirm this bimodal distribution, independent of the methods to measure structural orientation. This bimodal distribution suggests that magnetic fields are dynamically important on the local scales ($\lesssim 0.6\:$pc) of massive protostellar cores. We also examine how basic polarization properties of overall degree of polarization and local dispersion in polarization vector orientations depend on intrinsic protostellar properties inferred from spectral energy distribution (SED) modeling. We find a statistically significant anti-correlation between the debiased polarized fraction and the luminosity to mass ratio, $L_{\rm bol}/M_{\rm env}$, which hints at a change in the dust properties for protostellar objects at different evolutionary stages.

astro-ph.GA↗

arXiv2025

On the rapid growth of SMBHs in high-z galaxies: the aftermath of Population III.1 stars

The vast amount of energy released by active galactic nuclei (AGN) is increasingly recognized as a key driver of evolution not only in massive galaxies and clusters, but also in low-mass dwarf galaxies. Despite this, their role in the early stages of galaxy formation and in self-regulating the rapid growth of the first and abundant supermassive black holes (SMBHs) remains poorly understood. Through new high-resolution zoom-in cosmological simulations, we follow the co-evolution of $10^5 M_\odot$ black hole seeds with their host galaxy. The simulated suite progressively spans physics ranging from no AGN feedback and Eddington-limited thermal feedback, to more complex setups including non-Eddington-limited thermal, kinetic and radiative feedback. Across all our models, we find that black hole seeds efficiently reach masses of $\sim10^7 M_\odot$ by z=8. Although they exhibit notably different mass growth histories, these latter seem unimpeded by the presence of AGN feedback. The simulation including radiative feedback is the most distinct, with super-Eddington episodes driving fast and mass-loaded gas outflows (exceeding 2500 km $s^{-1}$) up to $\sim$50 kpc, along with minor stellar mass suppression in the host galaxy. Our measurements are in broad agreement with moderate luminosity quasars recently observed by JWST, producing overmassive black holes, dynamical masses of $\sim10^{9.5} M_\odot$, and high, though short-lived, Eddington fraction accretion rates. These results advocate for a scenario where AGN feedback allows for rapid SMBH growth during the reionisation era, while driving winds that extend deep into the intergalactic medium - shaping host galaxies as well as more distant surroundings.

astro-ph.GA↗

arXiv2025

ABYSS III: Observing accretion activity in young stars through empirical veiling measurements

Stellar accretion plays an important role in the early stages of stellar evolution, particularly in Classical T Tauri Stars (CTTSs). Accretion of a CTTS can be related to different physical parameters such as effective temperature (T$_{\text{eff}}$), age, abundance of hydrogen, etc. We can infer how accretion works by examining it across different wavelength regions. Accretion can be traced using veiling, a parameter that measures how excess emission from accretion affects the photospheric spectrum of CTTS. In this study, we selected a sample of CTTSs, Weak-line T Tauri Stars (WTTSs), and field stars, observed as a part of the SDSS-V Milky Way Mapper using the BOSS spectrograph. We measured veiling for CTTSs through comparing them to theoretical spectra. Next, we assessed the effect of veiling on different stellar properties, including wavelength, H$α$ emission, effective temperature, and age. We investigated how veiling changes with these parameters and what the physical reasons behind the changes can be. Finally, we evaluated how our findings align with existing accretion shock models. This study highlights veiling as a critical diagnostic tool for understanding accretion in young stars.

astro-ph.SR↗

arXiv2025

Low-Metallicity Star Formation Survey in Sh2-284 (LZ-STAR). I. Ordered massive star formation in the outer Galaxy

Star formation is a fundamental, yet poorly understood, process of the Universe. It is important to study how star formation occurs in different galactic environments. Thus, here, in the first of a series of papers, we introduce the Low-Metallicity Star Formation (LZ-STAR) survey of the Sh2-284 (hereafter S284) region, which, at $Z\sim 0.3-0.5Z_\odot$, is one of the lowest-metallicity star-forming regions of our Galaxy. LZ-STAR is a multi-facility survey, including observations with {\it JWST}, {\it ALMA}, {\it HST}, {\it Chandra} and {\it Gemini}. As a starting point, we report {\it JWST} and {\it ALMA} observations of one of the most massive protostars in the region, S284p1. The observations of shock-excited molecular hydrogen reveal a symmetric, bipolar outflow originating from the protostar, spanning several parsecs, and fully covered by the {\it JWST} field of view and the {\it ALMA} observations of CO(2-1) emission. This allows us to infer that the protostar has maintained a relatively stable orientation of disk accretion over its formation history. The {\it JWST} near-IR continuum observations detect a centrally illuminated bipolar outflow cavity around the protostar, as well as a surrounding cluster of low-mass young stars. We develop new radiative transfer models of massive protostars designed for the low metallicity of S284. Fitting these models to the protostar's spectral energy distribution implies a current protostellar mass of $\sim10\:M_\odot$ has formed from an initially $\sim100\:M_\odot$ core over the last $\sim3\times10^5$ years. Overall, these results indicate that massive stars can form in an ordered manner in low-metallicity, protocluster environments.

astro-ph.GA↗

arXiv2025

The SOFIA Massive (SOMA) Star Formation Survey. V. Clustered Protostars

We present $\sim8-40\,μ$m SOFIA-FORCAST images of seven regions of ``clustered" star formation as part of the SOFIA Massive (SOMA) Star Formation Survey. We identify a total of 34 protostar candidates and build their spectral energy distributions (SEDs). We fit these SEDs with a grid of radiative transfer models based on the Turbulent Core Accretion (TCA) theory to derive key protostellar properties, including initial core mass, $M_c$, clump environment mass surface density, $Σ_{\rm cl}$, and current protostellar mass, $m_*$. We also carry out empirical graybody (GB) estimation of $Σ_{\rm cl}$, which allows a case of restricted SED fitting within the TCA model grid. We also release version 2.0 of the open-source Python package \emph{sedcreator}, designed to automate the aperture photometry and SED building and fitting process for sources in clustered environments, where flux contamination from close neighbors typically complicates the process. Using these updated methods, SED fitting yields values of $M_c\sim30-200\:M_{\odot}$, $Σ_{\text{cl,SED}}\sim0.1-3\:{\rm{g\:cm}}^{-2}$, and $m_*\sim4-50\:M_{\odot}$. The graybody fitting yields smaller values of $Σ_{\text{cl,GB}}\lesssim1\:{\rm{g\:cm}}^{-2}$. From these results, we do not find evidence for a critical $Σ_{\rm{cl}}$ needed to form massive ($\gtrsim 8\:M_\odot$) stars. However, we do find tentative evidence for a dearth of the most massive ($m_*\gtrsim30\:M_\odot$) protostars in the clustered regions suggesting a potential impact of environment on the stellar initial mass function.

astro-ph.GA↗

arXiv2025

Discovery of Radio Recombination Lines from Proplyds in the Orion Nebula Cluster

We present new Atacama Large Millimeter/submillimeter Array observations that, for the first time, detect hydrogen and helium radio recombination lines from a protoplanetary disk. We imaged the Orion Nebula Cluster at 3.1 mm with a spectral setup that covered the $n=42 \rightarrow 41$ transitions of hydrogen (H41$α$) and helium (He41$α$). The unprecedented sensitivity of these observations enables us to search for radio recombination lines toward the positions of ${\sim}200$ protoplanetary disks. We detect H41$α$ from 17 disks, all of which are HST-identified `proplyds.' The detected H41$α$ emission is spatially coincident with the locations of proplyd ionization fronts, indicating that proplyd H41$α$ emission is produced by gas that has been photoevaporated off the disk and ionized by UV radiation from massive stars. We measure the fluxes and widths of the detected H41$α$ lines and find line fluxes of ${\sim}30-800$ mJy km s$^{-1}$ and line widths of ${\sim}30-90$ km s$^{-1}$. The derived line widths indicate that the broadening of proplyd H41$α$ emission is dominated by outflowing gas motions associated with external photoevaporation. The derived line fluxes, when compared with measurements of 3.1 mm free-free flux, imply that the ionization fronts of H41$α$-detected proplyds have electron temperatures of ${\sim}6,000-11,000$ K and electron densities of ${\sim}10^6-10^7$ cm$^{-3}$. Finally, we detect He41$α$ towards one H41$α$-detected source and find evidence that this system is helium-rich. Our study demonstrates that radio recombination lines are readily detectable in ionized photoevaporating disks, providing a new way to measure disk properties in clustered star-forming regions.

astro-ph.EP↗

arXiv2025

The Core Mass Function Across Galactic Environments. IV. The Galactic Center

The origin of the stellar Initial Mass Function (IMF) and how it may vary with galactic environment is a matter of debate. Certain star formation theories involve a close connection between the IMF and the Core Mass Function (CMF) so it is important to measure this CMF in a range of Milky Way locations. Here we study the CMF of three Galactic Center clouds: G0.253+0.016 ("The Brick"), Sgr B2 (Deep South field) and Sgr C. We use ALMA 1 mm continuum images and identify cores as peaks in thermal dust emission via the dendrogram algorithm. We develop a completeness correction method via synthetic core insertion, utilizing a realistic mass-dependent size distribution. A power law $\text{d}N/\text{d}\log M \propto M^{-α}$ is fit to the CMFs $>2\:M_\odot$. The Brick has a Salpeter-like index $α=1.28\pm0.09$, while the other regions have shallower indices: Sgr C has $α=0.99\pm0.06$; Sgr B2-DS has $α=0.70\pm0.03$. When smoothed to a common resolution, the differences between the Brick and the others increase as we obtain $α=1.36\pm0.12$, $α=0.66\pm0.06$ and $α=0.62\pm0.04$, respectively, for masses $\gtrsim3\:M_\odot$. Furthermore, we analyze the spatial distribution and mass segregation of cores: Sgr C and Sgr B2-DS show signs of mass segregation, but the Brick does not. We compare our results to several other CMFs from different Galactic regions derived with the same methods. Finally, we discuss how our results may help define an evolutionary sequence of star cluster formation and be used to test star formation theories.

astro-ph.GA↗

arXiv2025

Low-mass Runaways from the Orion Nebula Cluster -- Kinematic Age Constraints on Star Cluster Formation

In their early, formative stages star clusters can undergo rapid dynamical evolution leading to strong gravitational interactions and ejection of "runaway" stars at high velocities. While O/B runaway stars have been well studied, lower-mass runaways are so far very poorly characterised, even though they are expected to be much more common. We carried out spectroscopic observations with MAG2-MIKE to follow-up 27 high priority candidate runaways consistent with having been ejected from the Orion Nebula Cluster (ONC) > 2.5 Myr ago, based on Gaia astrometry. We derive spectroscopic youth indicators (Li & H$α$) and radial velocities, enabling detection of bona fide runaway stars via signatures of youth and 3D traceback. We successfully confirmed 11 of the candidates as low-mass Young Stellar Objects (YSOs) on the basis of our spectroscopic criteria and derived radial velocities (RVs) with which we performed 3D traceback analysis. Three of these confirmed YSOs have kinematic ejection ages > 4 Myr, with the oldest being 4.7 Myr. Assuming that these stars indeed formed in the ONC and were then ejected, this yields an estimate for the overall formation time of the ONC to be at least ${\sim}$ 5 Myr, i.e., about 10 free-fall times, and with a mean star formation efficiency per free-fall time of ${\barε_{ff}} {\lesssim}$ 0.05. These results favor a scenario of slow, quasi-equilibrium star cluster formation, regulated by magnetic fields and/or protostellar outflow feedback.

astro-ph.SR↗

arXiv2025

Exploring Magnetic Fields in Molecular Clouds through Denoising Diffusion Probabilistic Models

Accurately measuring magnetic field strength in the interstellar medium, including giant molecular clouds (GMCs), remains a significant challenge. We present a machine learning approach using Denoising Diffusion Probabilistic Models (DDPMs) to estimate magnetic field strength from synthetic observables such as column density, dust continuum polarization vector orientation angles, and line-of-sight (LOS) nonthermal velocity dispersion. We trained three versions of the DDPM model: the 1-channel DDPM (using only column density), the 2-channel DDPM (incorporating both column density and polarization angles), and the 3-channel DDPM (which combines column density, polarization angles, and LOS nonthermal velocity dispersion). We assessed the models on both synthetic test samples and new simulation data that were outside the training set's distribution. The 3-channel DDPM consistently outperformed both the other DDPM variants and the power-law fitting approach based on column density alone, demonstrating its robustness in handling previously unseen data. Additionally, we compared the performance of the Davis-Chandrasekhar-Fermi (DCF) methods, both classical and modified, to the DDPM predictions. The classical DCF method overestimated the magnetic field strength by approximately an order of magnitude. Although the modified DCF method showed improvement over the classical version, it still fell short of the precision achieved by the 3-channel DDPM.

astro-ph.GA↗

arXiv2024

The JWST-NIRCam View of Sagittarius C. II. Evidence for Magnetically Dominated HII Regions in the CMZ

We present JWST-NIRCam narrow-band, 4.05 $μ$m Brackett-$α$ images of the Sgr C HII region, located in the Central Molecular Zone (CMZ) of the Galaxy. Unlike any HII region in the Solar vicinity, the Sgr C plasma is dominated by filamentary structure in both Brackett-$α$ and the radio continuum. Some bright filaments, which form a fractured arc with a radius of about 1.85 pc centered on the Sgr C star-forming molecular clump, likely trace ionization fronts. The brightest filaments form a `$π$-shaped' structure in the center of the HII region. Fainter filaments radiate away from the surface of the Sgr C molecular cloud. The filaments are emitting optically thin free-free emission, as revealed by spectral index measurements from 1.28 GHz (MeerKAT) to 97 GHz (ALMA). But, the negative in-band 1 to 2 GHz spectral index in the MeerKAT data alone reveals the presence of a non-thermal component across the entire Sgr C HII region. We argue that the plasma flow in Sgr C is controlled by magnetic fields, which confine the plasma to rope-like filaments or sheets. This results in the measured non-thermal component of low-frequency radio emission plasma, as well as a plasma $β$ (thermal pressure divided by magnetic pressure) below 1, even in the densest regions. We speculate that all mature HII regions in the CMZ, and galactic nuclei in general, evolve in a magnetically dominated, low plasma $β$ regime.

astro-ph.GA↗

arXiv2024

The High-resolution Accretion Disks of Embedded protoStars (HADES) simulations. I. Impact of Protostellar Magnetic Fields on the Accretion Modes

How embedded, actively accreting low-mass protostars accrete their mass is still greatly debated. Observations are now piecing together the puzzle of embedded protostellar accretion, in particular with new facilities in the near-infrared. However, high-resolution theoretical models are still lacking, with a stark paucity of detailed simulations of these early phases. Here we present high-resolution non-ideal magneto-hydrodynamic simulations of a Solar mass protostar accreting at rates exceeding 10$^{-6} M_{\odot}$ yr$^{-1}$. We show the results of the accretion flow for four different protostellar magnetic fields, 10 G, 500 G, 1 kG, and 2 kG, combined with a disk magnetic field. For weaker (10 G and 500 G) protostar magnetic fields, accretion occurs via a turbulent boundary layer mode, with disk material impacting across the protostellar surface. In the 500 G model, the presence of a magnetically dominated outflow focuses the accretion towards the equator, slightly enhancing and ordering the accretion. For kG magnetic fields, the disk becomes truncated due to the protostellar dipole and exhibits magnetospheric accretion, with the 2 kG model having accretion bursts induced by the interchange instability. We present bolometric light curves for the models and find that they reproduce observations of Class I protostars from YSOVAR, with high bursts followed by an exponential decay possibly being a signature of instability-driven accretion. Finally, we present the filling fractions of accretion and find that 90\% of the mass is accreted in a surface area fraction of 10-20\%. These simulations will be extended in future work for a broader parameter space, with their high resolution and high temporal spacing able to explore a wide range of interesting protostellar physics.

astro-ph.SR↗