Inhomogeneous dust eclipses in young stars: the case of CQ Tauri

ABSTRACT We find that dust clouds that eclipse young stars obscure the stellar disc inhomogeneously. In the particular case of CQ Tau, we find isolated optically thick structures with sizes ≲0.6R* and derive the typical AV gradient in the plane of the sky, finding it to be as high as a few magnitudes per stellar radius. The large extinction gradients and complex structure of the obscuring clouds lead not only to a noticeable Rossiter–McLaughlin effect but also to complex and variable shaping of stellar absorption lines.

Galactic seismology: the evolving “phase spiral” after the Sagittarius dwarf impact

In 2018, the ESA Gaia satellite discovered a remarkable spiral pattern (“phase spiral”) in the z − Vz phase plane throughout the solar neighbourhood, where z and Vz are the displacement and velocity of a star perpendicular to the Galactic disc. In response to Binney & Schönrich’s analytic model of a disc-crossing satellite to explain the Gaia data, we carry out a high-resolution, N-body simulation (N ≈ 108 particles) of an impulsive mass (2 × 1010 M⊙) that interacts with a cold stellar disc at a single transit point. The disc response is complex since the impulse triggers a superposition of two distinct bisymmetric (m = 2) modes − a density wave and a corrugated bending wave − that wrap up at different rates. Stars in the faster density wave wrap up with time T according to φD(R, T) = (ΩD(R) + Ωo) T where φD describes the spiral pattern and ΩD = Ω(R) − κ(R)/2, where κ is the epicyclic frequency. While the pattern speed Ωo is small, it is non-zero. The slower bending wave wraps up according to ΩB ≈ ΩD/2 producing a corrugated wave. The bunching effect of the density wave triggers the phase spiral as it rolls up and down on the bending wave (“rollercoaster” model). The phase spiral emerges slowly about ΔT ≈ 400 Myr after impact. It appears to be a long-lived, disc-wide phenomenon that continues to evolve over most of the 2 Gyr simulation. Thus, given Sagittarius’ (Sgr) low total mass today (Mtot ∼ 3 × 108 M⊙ within 10 kpc diameter), we believe the phase spiral was excited by the disc-crossing dwarf some 1 − 2 Gyr before the recent transit. For this to be true, Sgr must be losing mass at 0.5-1 dex per orbit loop.

How “cold” are the stellar discs of superthin galaxies?

Superthin galaxies are a class of bulgeless, low surface brightness galaxies with strikingly high values of planar-to-vertical axes ratio $\rm (b/a> 10 - 20)$, possibly indicating the presence of an ultra-cold stellar disc. Using the multi-component galactic disc model of gravitationally-coupled stars and gas in the force field of the dark matter halo as well as the stellar dynamical code AGAMA (Action-based Galaxy Modelling Architecture), we determine the vertical velocity dispersion of stars and gas as a function of galacto-centric radius for five superthin galaxies (UGC 7321, IC 5249, FGC 1540, IC2233 and UGC00711) using observed stellar and atomic hydrogen (HI) scale heights as constraints, using a Markov Chain Monte Carlo Method. We find that the central vertical velocity dispersion for the stellar disc in the optical band varies between σ0s ∼ 10.2 − 18.4 $\rm {kms}^{-1}$ and falls off with an exponential scale length of 2.6 to 3.2 Rd where Rd is the exponential stellar disc scale length. Interestingly, in the 3.6 μm, the same, averaged over the two components of the stellar disc, varies between 5.9 to 11.8 $\rm {kms}^{-1}$, both of which confirm the presence of ”ultra-cold” stellar discs in superthin galaxies. Interestingly, the global median of the multi-component disc dynamical stability parameter QN of our sample superthins is found to be 5 ± 1.5, which higher than the global median value of 2.2 ± 0.6 for a sample

The twisted dark matter halo of the Milky Way

We analyse systems analogous to the Milky Way (MW) in the eagle cosmological hydrodynamics simulation in order to deduce the likely structure of the MW’s dark matter halo. We identify MW-mass haloes in the simulation whose satellite galaxies have similar kinematics and spatial distribution to those of the bright satellites of the MW, specifically systems in which the majority of the satellites (8 out of 11) have nearly co-planar orbits that are also perpendicular to the central stellar disc. We find that the normal to the common orbital plane of the co-planar satellites is well aligned with the minor axis of the host dark matter halo, with a median misalignment angle of only 17.3○. Based on this result, we infer that the minor axis of the Galactic dark matter halo points towards (l, b) = (182○, −2○), with an angular uncertainty at the 68 and 95 percentile confidence levels of 22○ and 43○ respectively. Thus, the inferred minor axis of the MW halo lies in the plane of the stellar disc. The halo, however, is not homologous and its flattening and orientation vary with radius. The inner parts of the halo are rounder than the outer parts and well aligned with the stellar disc (that is the minor axis of the halo is perpendicular to the disc). Further out, the halo twists and the minor axis changes direction by 90○. This twist occurs over a very narrow radial range and reflects variations in the filamentary network along which mass was accreted into the MW.

GJ 436b and the stellar wind interaction: simulations constraints using Lyα and Hα transits

The GJ 436 planetary system is an extraordinary system. The Neptune-size planet that orbits the M3 dwarf revealed in the Lyα line an extended neutral hydrogen atmosphere. This material fills a comet-like tail that obscures the stellar disc for more than 10 hours after the planetary transit. Here, we carry out a series of 3D radiation hydrodynamic simulations to model the interaction of the stellar wind with the escaping planetary atmosphere. With these models, we seek to reproduce the $\sim 56\%$ absorption found in Lyα transits, simultaneously with the lack of absorption in Hα transit. Varying the stellar wind strength and the EUV stellar luminosity, we search for a set of parameters that best fit the observational data. Based on Lyα observations, we found a stellar wind velocity at the position of the planet to be around [250-460] km s−1 with a temperature of [3 − 4] × 105 K. The stellar and planetary mass loss rates are found to be 2 × 10−15 M⊙ yr−1 and ∼[6 − 10] × 109 g s−1, respectively, for a stellar EUV luminosity of [0.8 − 1.6] × 1027 erg s−1. For the parameters explored in our simulations, none of our models present any significant absorption in the Hα line in agreement with the observations.

Bulge formation through disc instability

We use simulations to study the growth of a pseudobulge in an isolated thin exponential stellar disc embedded in a static spherical halo. We observe a transition from later to earlier morphological types and an increase in bar prominence for higher disc-to-halo mass ratios, for lower disc-to-halo size ratios, and for lower halo concentrations. We compute bulge-to-total stellar mass ratios B/T by fitting a two-component Sérsic-exponential surface-density distribution. The final B/T is strongly related to the disc’s fractional contribution fd to the total gravitational acceleration at the optical radius. The formula B/T = 0.5 fd1.8 fits the simulations to an accuracy of 30%, is consistent with observational measurements of B/T and fd as a function of luminosity, and reproduces the observed relation between B/T and stellar mass when incorporated into the GALICS 2.0 semi-analytic model of galaxy formation.

Resonance sweeping by a decelerating Galactic bar

ABSTRACT We provide the first quantitative evidence for the deceleration of the Galactic bar from local stellar kinematics in agreement with dynamical friction by a typical dark matter halo. The kinematic response of the stellar disc to a decelerating bar is studied using secular perturbation theory and test particle simulations. We show that the velocity distribution at any point in the disc affected by a naturally slowing bar is qualitatively different from that perturbed by a steadily rotating bar with the same current pattern speed Ωp and amplitude. When the bar slows down, its resonances sweep through phase space, trapping, and dragging along a portion of previously free orbits. This enhances occupation on resonances, but also changes the distribution of stars within the resonance. Due to the accumulation of orbits near the boundary of the resonance, the decelerating bar model reproduces with its corotation resonance the offset and strength of the Hercules stream in the local vR-vφ plane and the double-peaked structure of mean vR in the Lz–φ plane. At resonances other than the corotation, resonant dragging by a slowing bar is associated with a continuing increase in radial action, leading to multiple resonance ridges in the action plane as identified in the Gaia data. This work shows models using a constant bar pattern speed likely lead to qualitatively wrong conclusions. Most importantly we provide a quantitative estimate of the current slowing rate of the bar $\dot{\Omega }_{\rm p}= (-4.5 \pm 1.4) \, {\rm km}\, {\rm s}^{-1}\, {\rm kpc}^{-1}\, {\rm Gyr}^{-1}$ with additional systematic uncertainty arising from unmodelled impacts of e.g. spiral arms.

Jeans modelling of the Milky Way’s nuclear stellar disc

ABSTRACT The nuclear stellar disc (NSD) is a flattened stellar structure that dominates the gravitational potential of the Milky Way at Galactocentric radii $30 \lesssim R \lesssim 300\, {\rm pc}$. In this paper, we construct axisymmetric Jeans dynamical models of the NSD based on previous photometric studies and we fit them to line-of-sight kinematic data of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and silicon monoxide (SiO) maser stars. We find that (i) the NSD mass is lower but consistent with the mass independently determined from photometry by Launhardt et al. Our fiducial model has a mass contained within spherical radius $r=100\, {\rm pc}$ of $M(r\lt 100\, {\rm pc}) = 3.9 \pm 1 \times 10^8 \, \rm M_\odot$ and a total mass of $M_{\rm NSD} = 6.9 \pm 2 \times 10^8 \, \rm M_\odot$. (ii) The NSD might be the first example of a vertically biased disc, i.e. with ratio between the vertical and radial velocity dispersion σz/σR > 1. Observations and theoretical models of the star-forming molecular gas in the central molecular zone suggest that large vertical oscillations may be already imprinted at stellar birth. However, the finding σz/σR > 1 depends on a drop in the velocity dispersion in the innermost few tens of parsecs, on our assumption that the NSD is axisymmetric, and that the available (extinction corrected) stellar samples broadly trace the underlying light and mass distributions, all of which need to be established by future observations and/or modelling. (iii) We provide the most accurate rotation curve to date for the innermost $500\, {\rm pc}$ of our Galaxy.

GASP

We present atomic hydrogen (H I) observations with the Jansky Very Large Array of one of the jellyfish galaxies in the GAs Stripping Phenomena sample, JO201. This massive galaxy (M* = 3.5 × 1010 M⊙) is falling along the line-of-sight towards the centre of a rich cluster (M200 ∼ 1.6 × 1015 M⊙, σcl ∼ 982 ± 55 km s−1) at a high velocity ≥3363 km s−1. Its Hα emission shows a ∼40 kpc tail, which is closely confined to its stellar disc and a ∼100 kpc tail extending further out. We find that H I emission only coincides with the shorter clumpy Hα tail, while no H I emission is detected along the ∼100 kpc Hα tail. In total, we measured an H I mass of MHI = 1.65 × 109 M⊙, which is about 60% lower than expected based on its stellar mass and stellar surface density. We compared JO201 to another jellyfish in the GASP sample, JO206 (of a similar mass but living in a ten times less massive cluster), and we find that they are similarly H I-deficient. Of the total H I mass in JO201, about 30% lies outside the galaxy disc in projection. This H I fraction is probably a lower limit since the velocity distribution shows that most of the H I is redshifted relative to the stellar disc and could be outside the disc. The global star formation rate (SFR) analysis of JO201 suggests an enhanced star formation for its observed H I content. The observed SFR would be expected if JO201 had ten times its current H I mass. The disc is the main contributor of the high star formation efficiency at a given H I gas density for both galaxies, but their tails also show higher star formation efficiencies compared to the outer regions of field galaxies. Generally, we find that JO201 and JO206 are similar based on their H I content, stellar mass, and star formation rate. This finding is unexpected considering their different environments. A toy model comparing the ram pressure of the intracluster medium (ICM) versus the restoring forces of these galaxies suggests that the ram pressure strength exerted on them could be comparable if we consider their 3D orbital velocities and radial distances relative to the clusters.