Outer Disk Research Articles

A twisted and precessing Cepheid warp in the outer Milky Way disc

ABSTRACT We examine the Galactic warp in a sample of all classical Cepheids with Gaia Data Release 3 radial velocity. In each radial bin, we determine (1) the inclined plane normal to the mean orbital angular momentum of the stars and (2) that best fitting their positions. We find no warping inside $R\approx 11\,$kpc; for larger R, the disc is increasingly inclined, reaching i ∼ 3° at $R\ge 14\,$kpc. With larger R, the azimuth of the warp’s ascending node shifts from φlon ≈ −15° at 11 kpc by about 14° kpc−1 in the direction of Galactic rotation, implying a leading spiral of nodes, the general behaviour of warped galaxies. From the method of fitting planes to the positions, we also obtain ${\dot{\varphi }}_{\mathrm{lon}}$ and find prograde precession of ${\dot{\varphi }}_{\mathrm{lon}}\sim 12\, \rm{km{}\,s^{-1}{}\,kpc^{-1}}$ at 12 kpc decreasing to ${\sim}\,6\, \rm{km{}\,s^{-1}{}\,kpc^{-1}}$ at 14 kpc and beyond. This would unwind the leading spiral of nodes in ${\sim}\,100\,$Myr, suggesting that our instantaneous measurements of ${\dot{\varphi }}_{\mathrm{lon}}$ reflect transient behaviour. This is consistent with existing simulations, which show oscillations in ${\dot{\varphi }}_{\mathrm{lon}}$ overlaying a long-term retrograde differential precession that generates the leading spiral of nodes.

NGC 1436: the making of a lenticular galaxy in the Fornax Cluster

ABSTRACT We study the evolutionary path of the Fornax Cluster galaxy NGC 1436, which is known to be currently transitioning from a spiral into a lenticular morphology. This galaxy hosts an inner star-forming disc and an outer quiescent disc, and we analyse data from the MeerKAT Fornax Survey, ALMA, and the Fornax 3D survey to study the interstellar medium and the stellar populations of both disc components. Thanks to the combination of high resolution and sensitivity of the MeerKAT data, we find that the H i is entirely confined within the inner star-forming disc, and that its kinematics is coincident with that of the CO. The cold gas disc is now well settled, which suggests that the galaxy has not been affected by any environmental interactions in the last ∼1 Gyr. The star-formation history derived from the Fornax 3D data shows that both the inner and outer discs experienced a burst of star formation ∼5 Gyr ago, followed by rapid quenching in the outer disc and by slow quenching in the inner disc, which continues forming stars to this day. We claim that NGC 1436 has begun to effectively interact with the cluster environment 5 Gyr ago, when a combination of gravitational and hydrodynamical interactions caused the temporary enhancement of the star-formation rate. Furthermore, due to the weaker gravitational binding H i was stripped from the outer disc, causing its rapid quenching. At the same time, accretion of gas onto the inner disc stopped, causing slow quenching in this region.

Extreme evaporation of planets in hot thermally unstable protoplanetary discs: the case of FU Ori

ABSTRACT Disc accretion rate onto low mass protostar FU Ori suddenly increased hundreds of times 85 yr ago and remains elevated to this day. We show that the sum of historic and recent observations challenges existing FU Ori models. We build a theory of a new process, Extreme Evaporation (EE) of young gas giant planets in discs with midplane temperatures of ≳ 30 000 K. Such temperatures are reached in the inner 0.1 AU during thermal instability bursts. In our 1D time-dependent code the disc and an embedded planet interact through gravity, heat, and mass exchange. We use disc viscosity constrained by simulations and observations of dwarf novae instabilities, and we constrain planet properties with a stellar evolution code. We show that dusty gas giants born in the outer self-gravitating disc reach the innermost disc in a ∼O(104) yr with radius of ∼10RJ. We show that their EE rates are $\gtrsim 10^{-5} {\rm {\rm M}_{\odot }}$ yr−1; if this exceeds the background disc accretion activity then the system enters a planet-sourced mode. Like a stellar secondary in mass-transferring binaries, the planet becomes the dominant source of matter for the star, albeit for ∼O(100) yr. We find that a ∼6 Jupiter mass planet evaporating in a disc fed at a time-averaged rate of $\sim 10^{-6} {\rm {\rm M}_{\odot }}$ yr−1 appears to explain all that we currently know about FU Ori accretion outburst. More massive planets and/or planets in older less massive discs do not experience EE process. Future FUOR modelling may constrain planet internal structure and evolution of the earliest discs.

Wear Model of Silicon Nitride Ceramic Balls in Three-Body Coupling Grinding Mode

Silicon nitride ceramic balls are the key basic components of bearings in major equipment. Their key performance indices are accuracy and batch consistency. A grinding method with the most appropriate comprehensive performance is the basis and guarantee for optimizing these performance indices. In this study, an accurate wear model was established to predict the material removal rates (MRRs) of grinding methods and improve the dynamic grinding control ability of machinery during grinding, thus enabling the mass production of high-grade silicon nitride ceramic balls. A comprehensive analysis of various grinding and polishing methods revealed that the factors affecting sphericity were mainly manifested by the increase in ball sliding and the improvement in MRRs. More over, the three-body coupling grinding mode was considered as the grinding mode that was most applicable to silicon nitride ceramic balls. The upper disk served as an external nonlinear load, and the combination of the rotating speeds of the inner and outer disks of the lower grinding disk could actively control the ball’s angle of rotation. This three-body wear mode can fully envelop the grinding trajectory and ensure uniform grinding. The traditional two-body wear model was unsuitable for three-body coupling grinding. A wear model based on three-body wear was established to predict MRRs and understand the principle of material removal in the grinding of precision spheres. Theoretical analysis and experimental verification revealed that the MRRs of silicon nitride ceramic balls during wear are not only related to the process parameters of external load and speed but also to the physical properties and geometric parameters of balls, abrasives, and processing machinery. The wear model results of silicon nitride ceramic balls in the three-body coupling grinding mode can be obtained stably on the basis of the established wear model by removing adverse effects and adopting optimized processing parameters, thus verifying the correctness of the theoretical and simulation analyses.

PO-03-192 INTRA-DEVICE LEAKS ON POST LEFT ATRIAL APPENDAGE OCCLUDER FOLLOW UP TOMOGRAPHIC IMAGING – TRUE LEAK OR MERE INCOMPLETE ENDOTHELIALIZATION

Patient undergoing Left atrial appendage closure (LAAC) needs to be kept on oral anticoagulant (OAC) or dual antiplatelet therapy (DAPT) because of the time required for endothelialization of the LAAC device. Post LAAC follow-up usually is usually performed with either a transesophageal echocardiogram (TEE) or cardiac computed tomography (CCTA) at 45 days, 6 months and 12 months to ensure complete closure and endothelialization of the LAA. Contrast flow (CF) behind the LAAC device on CCTA could be indicative of either a peri-device leak (PDL) or intra-device leak due to incomplete endothelialization. Often the contrast flow into the LAA is described as a leak with implications to continued OAC. This study aims to evaluate the utility of LAA-CF on CCTA scan to predict lack of endothelialization after endocardial LAA closure and its association with peri-device leaks (PDLs). Consecutive patients who underwent LAAC at our center from June 2021 to September 2021 and had follow up CCTA at 45 days, 6 months and 12 months were included in the study. Patients who were follow up with TEE were excluded. Rate of LAA-CF was calculated. LAA-CF was defined as radiodensity > 100 HU (Hounsfield units) of the LAA separated from the device, or contrast opacification ≥ 25% of the left atrium with the presence of any LAA opacification compared with the pre-LAAC cardiac CT. Of 45 patients, 62% had Watchman FLX (n= 28) and 38% had Amplatzer Amulet (n=17) implantation. Mean age of the cohort was 74.6 + 9.3 years and 72% were males. At 45 day follow up CCTA, 40% (n=18) had LAA-CF of which 56% (n=10) had PDL whereas 44% (n=8) had no PDL. On 6 month follow up LAA-CF persisted in 44% (n=8/18) with 62.5% (n=5/8) having PDL. However, at 12 months LAA-CF persisted in only one patient who had a leak of 7 mm (Table). There were no systemic thromboembolic events. Although numbers were small Amulet had higher LAA-CF than Watchman FLX probably related to large outer disc surface area. Delayed LAA-CF is common after LAAO and is a reflection of poor endothelialization than a true PDL. It has no real long term clinical implications.

Massive pre-stellar cores in radiation-magneto-turbulent simulations of molecular clouds

ABSTRACT We simulate the formation and collapse of pre-stellar cores at few-au resolution in a set of radiation-magnetohydrodynamic simulations of giant molecular clouds (GMCs) using the grid-based code RAMSES-RT. We adopt, for the first time to our best knowledge, realistic initial/boundary conditions by zooming in on to individual massive pre-stellar cores within the GMC. We identify two distinct modes of fragmentation: ‘quasi-spherical’ and ‘filamentary’. In both modes, the fragments eventually become embedded in a quasi-steady accretion disc or toroid with radii ∼500–5000 au and opening angles H/R ∼ 0.5 − 1. The discs/toroids are Toomre stable but the accreted pre-existing fragments are found orbiting the outer disc, appearing as disc fragmentation. Each core converts nearly 100 per cent of the gas mass into a few massive stars forming near the disc centre. Large and massive discs around high-mass stars are supported by magnetic pressure in the outer disc, at radii >200–1000 au, and turbulent pressure in the inner disc. The most massive core accretes several times more mass than its initial mass, forming a cluster of 8 massive (proto)stars enshrouded by a toroid, suggesting a competitive accretion scenario for the formation of stars above ∼30 M⊙. We also find that the H ii regions produced by a single massive star remain trapped in the dense circumstellar discs for a few hundred kiloyears, while the dynamic motions of massive stars in wide binaries or multiple systems displace the stars from the densest parts of the disc, allowing UV radiation to escape producing steady or pulsating bipolar H ii regions.

Development of a fast and robust liquid chromatography-mass spectrometry-based metabolomics analysis method for neonatal dried blood spots

Dried blood spot (DBS) samples have been widely used in many fields including newborn screening, with the advantages in transportation, storage and non-invasiveness. The DBS metabolomics research of neonatal congenital diseases will greatly expand the understanding of the disease. In this study, we developed a liquid chromatography-mass spectrometry-based method for neonatal metabolomics analysis of DBS. The influences of blood volume and chromatographic effects on the filter paper on metabolite levels were studied. The levels of 11.11 % metabolites were different between 75 μL and 35 μL of blood volumes used for DBS preparation. Chromatographic effects on the filter paper occurred in DBS prepared with 75 μL whole blood and 6.67 % metabolites had different MS responses when central disks were compared with outer disks. The DBS storage stability study showed that compared with − 80 °C storage, storing at 4 °C for 1 year had obvious influences on more than half metabolites. Storing at 4 °C and − 20 °C for short term (< 14 days) and − 20 °C for longer term (1 year) had less influences on amino acids, acyl-carnitines and sphingomyelins, but greater influences on partial phospholipids. Method validation showed that this method has a good repeatability, intra-day and inter-day precision and linearity. Finally, this method was applied to investigate metabolic disruptions of congenital hypothyroidism (CH), metabolic changes of CH newborns were mainly involved in amino acid metabolism and lipid metabolism.

Optical/UV emission in the Tidal Disruption Event ASASSN-14li: implications of disc modelling

ABSTRACT We predict late-time optical/UV emission from tidal disruption events (TDEs) from our slim accretion disc model (Wen et al. 2020) and explore the impact of the black hole mass M•, black hole spin a•, and accretion disc size. We use these synthetic spectra to successfully fit the multiband Swift observations of ASASSN-14li at &amp;gt;350 d, setting only the host galaxy extinction and outer disc radius as free parameters and employing the M•, a•, disc inclination, and disc accretion rates derived from fitting 10 epochs of ASASSN-14li’s X-ray spectra with the slim disc. To address the nature of the early-time optical/UV emission, we consider two models: shock dissipation and reprocessing. We find that (1) the predicted late-time optical/UV colour (e.g. u − w2) is insensitive to black hole and disc parameters unless the disc spreads quickly; (2) a starburst galaxy extinction model is required to fit the data, consistent with ASASSN-14li’s post-starburst host; (3) surprisingly, the outer disc radius is ≈2 × the tidal radius and ∼constant at late times, showing that viscous spreading is slow or non-existent; (4) the shock model can be self-consistent if M• ≲ 106.75 M⊙, i.e. on the low end of ASASSN-14li’s M• range (106.5–7.1 M⊙; 1σ CL); larger black hole masses require disruption of an unrealistically massive progenitor star; (5) the gas mass needed for reprocessing, whether by a quasi-static or an outflowing layer, can be &amp;lt;0.5 M⊙, consistent with a (plausible) disruption of a solar-mass star.

Saying Hallo to M94's Stellar Halo: Investigating the Accretion History of the Largest Pseudobulge Host in the Local Universe

It is not yet settled how the combination of secular processes and merging gives rise to the bulges and pseudobulges of galaxies. The nearby (D ∼ 4.2 Mpc) disk galaxy M94 (NGC 4736) has the largest pseudobulge in the local universe, and offers a unique opportunity for investigating the role of merging in the formation of its pseudobulge. We present a first ever look at M94's stellar halo, which we expect to contain a fossil record of M94's past mergers. Using Subaru's Hyper Suprime-Cam, we resolve and identify red giant branch (RGB) stars in M94's halo, finding two distinct populations. After correcting for completeness through artificial star tests, we can measure the radial profile of each RGB population. The metal-rich RGB stars show an unbroken exponential profile to a radius of 30 kpc that is a clear continuation of M94's outer disk. M94's metal-poor stellar halo is detectable over a wider area and clearly separates from its metal-rich disk. By integrating the halo density profile, we infer a total accreted stellar mass of ∼2.8 × 108 M ⊙, with a median metallicity of [M/H] = −1.4. This indicates that M94's most-massive past merger was with a galaxy similar to, or less massive than, the Small Magellanic Cloud. Few nearby galaxies have had such a low-mass dominant merger; therefore we suggest that M94's pseudobulge was not significantly impacted by merging.

Identification of a Helium Donor Star in NGC 247 ULX-1

With Very Large Telescope Multi Unit Spectroscopic Explorer (MUSE) observations, we detected highly variable helium emission lines from the optical counterpart of the supersoft ultraluminous X-ray source (ULX) NGC 247 ULX-1. No Balmer lines can be seen in the source spectrum. This is the first evidence for the presence of a helium donor star in ULXs, consistent with a prediction that helium donor stars may be common in ULXs. The helium lines with an FWHM of about 200 km s−1 are likely produced on the outer accretion disk. Their strong variation implies that the central X-ray source can be significantly obscured to the outer disk. Also, a ring or a double-ring structure is revealed in the MUSE image. It is unknown whether or not it is related to the progenitor of the ULX binary.

The Fornax3D project: Environmental effects on the assembly of dynamically cold disks in Fornax cluster galaxies

We apply a population-orbit superposition metho1d to 16 galaxies in the Fornax cluster observed with MUSE/VLT in the context of the Fornax3D project. By fitting the luminosity distribution, stellar kinematics, and age and metallicity maps simultaneously, we obtained the internal stellar orbit distribution, as well as the age and metallicity distribution of stars on different orbits for each galaxy. Based on the model, we decompose each galaxy into a dynamically cold disk (orbital circularity λz ≥ 0.8) and a dynamically hot non-disk component (orbital circularity λz &lt; 0.8), and obtain the surface-brightness, age, and metallicity radial profiles of each component. The galaxy infall time into the cluster is strongly correlated with galaxy cold-disk age with older cold disks in ancient infallers. We quantify the infall time tinfall of each galaxy with its cold-disk age using a correlation calibrated with TNG50 cosmological simulations. For galaxies in the Fornax cluster, we found that the luminosity fraction of cold disk in galaxies with tinfall &gt; 8 Gyr are a factor of ∼4 lower than in more recent infallers while controlling for total stellar mass. Nine of the 16 galaxies have spatially extended cold disks, and most of them show positive or zero age gradients; stars in the inner disk are ∼2 − 5 Gyr younger than that in the outer disk, in contrast to the expectation of inside-out growth. Our results indicate that the assembly of cold disks in galaxies is strongly affected by their infall into clusters, by either removal of gas in outer regions or even tidally stripping or heating part of the pre-existing disks. Star formation in outer disks can stop quickly after the galaxy falls into the cluster, while star formation in the inner disks can last for a few Gyrs more, building the positive age gradient measured in cold disks.

Planet formation via pebble accretion in externally photoevaporating discs

ABSTRACT We demonstrate that planet formation via pebble accretion is sensitive to external photoevaporation of the outer disc. In pebble accretion, planets grow by accreting from a flux of solids (pebbles) that radially drift inwards from the pebble production front. If external photoevaporation truncates the outer disc fast enough, it can shorten the time before the pebble production front reaches the disc outer edge, cutting off the supply of pebble flux for accretion, hence limiting the pebble mass reservoir for planet growth. Conversely, cloud shielding can protect the disc from strong external photoevaporation and preserve the pebble reservoir. Because grain growth and drift can occur quickly, shielding even on a short time-scale (&amp;lt;1 Myr) can have a non-linear impact on the properties of planets growing by pebble accretion. For example, a $10^{-3}\, \mathrm{ M}_{\oplus }$ planetary seed at 25 au stays at 25 au with a lunar mass if the disc is immediately irradiated by a 103 G0 field, but grows and migrates to be approximately Earth-like in both mass and orbital radius if the disc is shielded for just 1 Myr. In NGC 2024, external photoevaporation is thought to happen to discs that are &amp;lt;0.5 Myr old, which coupled with the results here suggests that the exact planetary parameters can be very sensitive to the star-forming environment. Universal shielding for time-scales of at least ${\sim} 1.5\,$ Myr would be required to completely nullify the environmental impact on planetary architectures.

Vertical motion in the Galactic disc: unwinding the snail

ABSTRACTThe distribution of stars in the Milky Way disc shows a spiral structure − the snail − in the space of velocity and position normal to the Galactic mid-plane. The snail appears as straight lines in the vertical frequency−vertical phase plane when effects from sample selection are removed. Their slope has the dimension of inverse time, with the simplest interpretation being the inverse age of the snail. Here, we devise and fit a simple model in which the spiral starts as a lopsided perturbation from steady state, that winds up into the present-day morphology. The winding occurs because the vertical frequency decreases with vertical action. We use data from stars in Gaia EDR3 that have measured radial velocities, pruned by simple distance and photometric selection functions. We divide the data into boxels of dynamical invariants (radial action, angular momentum); our model fits the data well in many of the boxels. The model parameters have physical interpretations: one, A, is a perturbation amplitude, and one, t, is interpretable in the simplest models as the time since the event that caused the snail. We find trends relating the strength and age to angular momentum: (i) the amplitude A is small at low angular momentum (${\lt 1\, 600{\,}kpc\ km\ s}^{-1}$ or guiding-centre radius $\lt 7.3\,$kpc) and over a factor of three larger, with strong variations, in the outer disc; (ii) there is no single well-defined perturbation time, with t varying between 0.2 and 0.6 Gyr. Residuals between the data and the model display systematic trends, implying that the data call for more complex models.

The origin of optical emission lines in the soft state of X-ray binary outbursts: the case of MAXI J1820+070

ABSTRACT The optical emission line spectra of X-ray binaries (XRBs) are thought to be produced in an irradiated atmosphere, possibly the base of a wind, located above the outer accretion disc. However, the physical nature of – and physical conditions in – the line-forming region remain poorly understood. Here, we test the idea that the optical spectrum is formed in the transition region between the cool geometrically thin part of the disc near the mid-plane and a hot vertically extended atmosphere or outflow produced by X-ray irradiation. We first present a VLT X-Shooter spectrum of XRB MAXI J1820+070 in the soft state associated with its 2018 outburst, which displays a rich set of double-peaked hydrogen and helium recombination lines. Aided by ancillary X-ray spectra and reddening estimates, we then model this spectrum with the Monte Carlo radiative transfer code python, using a simple biconical disc wind model inspired by radiation-hydrodynamic simulations of irradiation-driven outflows from XRB discs. Such a model can qualitatively reproduce the observed features; nearly all of the optical emission arising from the transonic ‘transition region’ near the base of the wind. In this region, characteristic electron densities are on the order of 1012–13 cm−3, in line with the observed flat Balmer decrement (H $\alpha$/H $\beta$ ≈ 1.3). We conclude that strong irradiation can naturally give rise to both the optical line-forming layer in XRB discs and an overlying outflow/atmosphere that produces X-ray absorption lines.

Collisional heating of icy planetesimals – I. Catastrophic collisions

ABSTRACTPlanetesimals in the primordial disc may have experienced a collisional cascade. If so, the comet nuclei later placed in the Kuiper belt, scattered disc, and Oort Cloud would primarily be fragments and collisional rubble piles from that cascade. However, the heating associated with the collisions cannot have been strong enough to remove the hypervolatiles that are trapped within more durable ices, because comet nuclei are rich in hypervolatiles. This places constraints on the diameter of the largest bodies allowed to participate in collisional cascades, and limits the primordial disc lifetime or population size. In this paper, the thermophysical code nimbus is used to study the thermal evolution of planetesimals before, during, and after catastrophic collisions. The loss of CO during segregation of CO2:CO mixtures and during crystallization of amorphous H2O is calculated, as well as mobilization and internal relocation of CO2. If an amorphous H2O host existed, and was protected by a CO2:CO heat sink, only diameter $D\lt 20\, \mathrm{km}$ (inner disc) and $D\lt 64\, \mathrm{km}$ (outer disc) bodies could have been involved in a collisional cascade. If CO2 was the only CO host, the critical diameters drop to D &amp;lt; 20–$32\, \mathrm{km}$. Avoiding disruption of larger bodies requires a primordial disc lifetime of &amp;lt;9 Myr at 15 au and &amp;lt;50–70 Myr at 30 au. Alternatively, if a 450 Myr disc lifetime is required to associate the primordial disc disruption with the Late Heavy Bombardment, the disc population size must have been 6–60 times below current estimates.

Disk Evolution Study Through Imaging of Nearby Young Stars (DESTINYS): Characterization of the young star T CrA and its circumstellar environment

Context. In recent years, a new hot topic has emerged in the star and planet formation field, namely, the interaction between the circumstellar disk and its birth cloud. The birth environments of young stars leave strong imprints on the star itself and their surroundings. In this context, we present a detailed analysis of the rich circumstellar environment around the young Herbig Ae/Be star T CrA. Aims. Our aim is to understand the nature of the stellar system and the extended circumstellar structures, as seen in scattered light images. Methods. We conducted our analysis on the basis of a set of combined archival data and new adaptive optics images at a high contrast and high resolution. Results. The scattered light images reveal the presence of a complex environment around T CrA, composed of a bright, forward-scattering rim of the disk's surface that is seen at very high inclinations, along with a dark lane of the disk midplane, bipolar outflows, and streamer features that are likely tracing infalling material from the surrounding birth cloud onto the disk. The analysis of the light curve suggests that the star is a binary with a period of 29.6 yr, confirming previous assertions based on spectro-astrometry. The comparison of the scattered light images with the ALMA continuum and 12CO (2–1) line emission shows that the disk is in Keplerian rotation and the northern side of the outflowing material is receding, while the southern side is approaching the observer. The overall system lies on different geometrical planes. The orbit of the binary star is perpendicular to the outflows and is seen edge on. The disk is itself seen edge-on, with a position angle of ~7°. The direction of the outflows seen in scattered light is in agreement with the direction of the more distant molecular hydrogen emission-line objects (MHOs) associated with the star. Modeling of the spectral energy distribution using a radiative transfer scheme is in good agreement with the proposed configuration, as well as the hydrodynamical simulation performed using a smoothed particle hydrodynamics code. Conclusions. We find evidence of streamers of accreting material around T CrA. These streamers connect the filament, along which T CrA is forming along with the outer parts of the disk, suggesting that the strong misalignment between the inner and outer disk is due to a change in the direction of the angular momentum of the material accreting on the disk during the late phase of star formation. This impacts the accretion taking place in the components of the binary, favoring the growth of the primary with respect the secondary, in contrast to the case of aligned disks.

A Complex Dust Morphology in the High-luminosity AGN Mrk 876

Recent models for the inner structures of active galactic nuclei (AGNs) advocate the presence of a radiatively accelerated dusty outflow launched from the outer regions of the accretion disk. Here, we present the first near-IR variable (rms) spectrum for the high-luminosity nearby AGN Mrk 876. We find that it tracks the accretion disk spectrum out to longer wavelengths than the mean spectrum, due to a reduced dust emission. The implied outer accretion disk radius is consistent with the IR results predicted by a contemporaneous optical accretion disk reverberation mapping campaign, and much larger than the self-gravity radius. The reduced flux variability of the hot dust could either be due to the presence of a secondary constant dust component in the mean spectrum or be introduced by the destructive superposition of the dust and accretion disk variability signals, or be some combination of the two. Assuming thermal equilibrium for optically thin dust, we derive the luminosity-based dust radii for different grain properties, using our measurement of the temperature. We find that in all the cases considered, the values are significantly larger than the dust response time measured by IR photometric monitoring campaigns, with the least discrepancy present relative to the result for a wavelength-independent dust emissivity law, i.e., a blackbody, which is appropriate for large grain sizes. This result can be well explained by assuming a flared disk-like structure for the hot dust.

Dynamics of Molecular Gas in the Central Region of the Quasar I Zwicky 1

We present a study of the molecular gas distribution and kinematics in the cicumnuclear region (radii ≲2 kpc) of the z ≈ 0.061 quasar I Zwicky 1 using a collection of available Atacama Large Millimeter/submillimeter Array observations of the carbon monoxide (CO) emission. With an angular resolution of ∼0.″36 (corresponding to ∼400 pc), the host-galaxy substructures including the nuclear molecular gas disk, spiral arms, and a compact bar-like component are resolved. We analyzed the gas kinematics based on the CO image cube and obtained the rotation curve and radial distribution of velocity dispersion. The velocity dispersion is about 30 km s−1 in the outer CO disk region and rises up to ≳100 km s−1 at radius ≲1 kpc, suggesting that the central region of the disk is dynamically hot. We constrain the CO-to-H2 conversion factor, α CO, by modeling the cold gas disk dynamics. We find that, with prior knowledge about the stellar and dark matter components, the α CO value in the circumnuclear region of this quasar host galaxy is , which is between the value reported in ultraluminous infrared galaxies and in the Milky Way. The central 1 kpc region of this quasar host galaxy has significant star formation activity, which can be identified as a nuclear starburst. We further investigate the high-velocity dispersion in the central region. We find that the interstellar medium (ISM) turbulent pressure derived from the gas velocity dispersion is in equilibrium with the weight of the ISM. This argues against extra power from active galactic nuclei feedback that significantly affects the kinematics of the cold molecular gas.

Influence of Magnetic Fields on the Gas Rotation in the Galaxy NGC 6946

Magnetic fields can play an important role in the energy balance and formation of gas structures in galaxies. However, their dynamical effect on the rotation curve of galaxies is immensely unexplored. We investigate the dynamical effect of the known magnetic arms of NGC 6946 on its circular gas rotation traced in H i, considering two dark-matter mass-density models, ISO, and the universal NFW profile. We used a three-dimensional model for the magnetic field structure to fit the modeled rotation curve to the observed data via a χ 2 minimization method. The shape of the H i gas rotation curve is reproduced better including the effect of the magnetic field, especially in the outer part, where the dynamical effect of the magnetic field could become important. The typical amplitude of the regular magnetic field contribution in the rotation curve is about 6–14 km s−1 in the outer gaseous disk of the galaxy NGC 6946. The contribution ratio of the regular magnetic field to the observed circular velocity and to dark matter increases with the galactocentric radius. Its ratio to the observed rotational velocity is about 5% and, to dark matter, is about 10% in the outer regions of the galaxy NGC 6946. Therefore, the large-scale magnetic fields cannot be completely ignored in the large-scale dynamics of spiral galaxies, especially in the outer parts of galaxies.

Massive pre-main-sequence stars in M17

Context.The young massive-star-forming region M17 contains optically visible massive pre-main-sequence stars that are surrounded by circumstellar disks. Such disks are expected to disappear when these stars enter the main sequence. The physical and dynamical structure of these remnant disks are poorly constrained, especially the inner regions where accretion, photo-evaporation, and companion formation and migration may be ongoing.Aims.We aim to constrain the physical properties of the inner parts of the circumstellar disks of massive young stellar objects B243 (6M⊙) and B331 (12M⊙), two systems for which the central star has been detected and characterized previously despite strong dust extinction.Methods.Two-dimensional radiation thermo-chemical modelling with PRODIMOof double-peaked hydrogen lines of the Paschen and Brackett series observed with X-shooter was used to probe the properties of the inner disk of the target sources. The model was modified to treat these lines. Additionally, the dust structure was studied by fitting the optical and near-infrared spectral energy distribution.Results.B243 features a hot gaseous inner disk with dust at the sublimation radius at ~3 AU. The disk appears truncated at roughly 6.5 AU; a cool outer disk of gas and dust may be present, but it cannot be detected with our data. B331 also has a hot gaseous inner disk. A gap separates the inner disk from a colder dusty outer disk starting at up to ~100 AU. In both sources the inner disk extends to almost the stellar surface. Chemistry is essential for the ionization of hydrogen in these disks.Conclusions.The lack of a gap between the central objects and these disks suggests that they accrete through boundary-layer accretion. This would exclude the stars having a strong magnetic field. Their structures suggest that both disks are transitional in nature, that is to say they are in the process of being cleared, either through boundary-layer accretion, photo-evaporation, or through companion activity.