Stellar Expansion Research Articles

Exploring the stellar rotation of early-type stars in the LAMOST medium-resolution survey

Context. Stellar rotation significantly shapes the evolution of massive stars, yet the interplay of mass and metallicity remains elusive, limiting our capacity to construct accurate stellar evolution models and to better estimate the impact of rotation on the chemical evolution of galaxies. Aims. Our goal is to investigate how mass and metallicity influence the rotational evolution of A-type stars on the main sequence (MS). We seek to identify deviations in rotational behaviors that could serve as new constraints for existing stellar models. Methods. Using the LAMOST Median-Resolution Survey Data Release 9, we derived stellar parameters for a population of 104 752 A-type stars. Our study focused on the evolution of surface rotational velocities and their dependence on mass and metallicity in 84 683 “normal” stars. Results. Normalizing surface rotational velocities to zero age main sequence (ZAMS) values revealed a prevailing evolutionary profile from 1.7 to 4.0 M⊙. This profile features an initial rapid acceleration until t/tMS = 0.25 ± 0.1 and potentially a second acceleration peak near t/tMS = 0.55 ± 0.1 for stars heavier than 2.5 M⊙, followed by a steady decline and a “hook” feature at the end. Surpassing theoretical expectations, the initial acceleration likely stems from a concentrated distribution of angular momentum at the ZAMS, resulting in a prolonged increase in speed. A transition phase for stars with 2.0 < M/M⊙ < 2.3 emerged, a region where evolutionary tracks remain uncertain. Stellar expansion primarily drives the spin down in the latter half of the MS, accompanied by significant influence from inverse meridional circulation. The inverse circulation becomes more efficient at lower metallicities, explaining the correlation of the slope of this deceleration phase with metallicity from –0.3 dex up to 0.1 dex. The metal-poor subsample (−0.3 dex < [M/H]< − 0.1 dex) starts with lower velocities at the ZAMS, suggesting that there is a metallicity-dependent mechanism that removes angular momentum during star formation. The proportion of fast rotators decreases with an increase in metallicity, up to log(Z/Z⊙)∼ − 0.2, a trend consistent with observations of OB-type stars found in the Small and Large Magellanic Clouds.

Unveiling the structural content of NGC 6357 via kinematics and NIR variability

ABSTRACT NGC 6357, a star-forming complex at $\sim 1.7$ kpc from the Sun, contains giant molecular clouds and three prominent star clusters alongside H ii regions, very massive stars and thousands of young stellar objects in different evolutionary stages. We present a combined infrared kinematic and time domain study of the line of sight towards this region enabled by the VVVX survey. In terms of kinematics, a novel discovery emerges an asymmetrical distribution in the vector point diagram. Some stars in the sample exhibit spatial proximity to dusty regions, with their proper motions aligned with filament projections, hinting at a younger population linked to triggered star formation. However, this distribution could also stem from an asymmetric stellar expansion event within NGC 6357, warranting further investigation. Comparing these data with Gaia revealed inconsistencies likely due to high-extinction levels in the region. Additionally, owing to accretion episodes and surface cool spots, young stars display high variability. Using the $K_{\rm s}$-band time series data, we overcome the extreme levels of extinction towards the region, and compile a catalogue of 774 infrared light curves of young stars. Each light curve has been characterized in terms of asymmetry and periodicity, to infer the dominant underlying physical mechanism. These findings are then correlated with evolutionary stages, aiming to uncover potential age disparities among the observed stars. This study contributes to our understanding the intricate dynamics and evolutionary processes within NGC 6357, offering valuable insights into the formation and development of stellar populations within such complex environments.

The role of stellar expansion on the formation of gravitational wave sources

ABSTRACT Massive stars are the progenitors of black holes and neutron stars, the mergers of which can be detected with gravitational waves (GW). The expansion of massive stars is one of the key factors affecting their evolution in close binary systems, but it remains subject to large uncertainties in stellar astrophysics. For population studies and predictions of GW sources, the stellar expansion is often simulated with the analytic formulae from Hurley et al. (2000). These formulae need to be extrapolated and are often considered outdated. In this work, we present five different prescriptions developed from 1D stellar models to constrain the maximum expansion of massive stars. We adopt these prescriptions to investigate how stellar expansion affects mass transfer interactions and in turn the formation of GW sources. We show that limiting radial expansion with updated 1D stellar models, when compared to the use of Hurley et al. (2000) radial expansion formulae, does not significantly affect GW source properties (rates and masses). This is because most mass transfer events leading to GW sources are initialized in our models before the donor star reaches its maximum expansion. The only significant difference was found for the mass distribution of massive binary black hole mergers (Mtot > 50 M⊙) formed from stars that may evolve beyond the Humphreys–Davidson limit, whose radial expansion is the most uncertain. We conclude that understanding the expansion of massive stars and the origin of the Humphrey–Davidson limit is a key factor for the study of GW sources.

Ultra-diffuse Galaxies as Extreme Star-forming Environments. I. Mapping Star Formation in H i-rich UDGs

Ultra-diffuse galaxies (UDGs) are both extreme products of galaxy evolution and extreme environments in which to test our understanding of star formation. In this work, we contrast the spatially resolved star formation activity of a sample of 22 H i-selected UDGs and 35 low-mass galaxies from the NASA Sloan Atlas (NSA) catalog within 120 Mpc. We employ a new joint spectral energy distribution fitting method to compute star formation rate and stellar mass surface density maps that leverage the high spatial resolution optical imaging data of the Hyper Suprime-Cam Subaru Strategic Program and the UV coverage of the Galaxy Evolution Explorer, along with H i radial profiles estimated from a subset of galaxies that have spatially resolved H i maps. We find that UDGs have low star formation efficiencies as a function of their atomic gas down to scales of 500 pc. We additionally find that the stellar mass-weighted sizes of our UDG sample are unremarkable when considered as a function of their H i mass—their stellar sizes are comparable to NSA dwarfs at fixed H i mass. This is a natural result in the picture where UDGs are forming stars normally, but at low efficiencies. We compare our results to predictions from contemporary models of galaxy formation, and find in particular that our observations are difficult to reproduce in models where UDGs undergo stellar expansion due to vigorous star formation feedback should bursty star formation be required down to z = 0.

Evidence for Radial Expansion at the Core of the Orion Complex with Gaia EDR3

Abstract We present a phase-space study of two stellar groups located at the core of the Orion Complex: Briceño-1 and Orion Belt Population-near (OBP-near). We identify the groups with the unsupervised clustering algorithm, Shared Nearest Neighbor (SNN), which previously identified 12 new stellar substructures in the Orion Complex. For each of the two groups, we derive the 3D space motions of individual stars using Gaia EDR3 proper motions supplemented by radial velocities from Gaia DR2, APOGEE-2, and GALAH DR3. We present evidence for radial expansion of the two groups from a common center. Unlike previous work, our study suggests that evidence of stellar group expansion is confined only to OBP-near and Briceño-1, whereas the rest of the groups in the complex show more complicated motions. Interestingly, the stars in the two groups lie at the center of a dust shell, as revealed via an extant 3D dust map. The exact mechanism that produces such coherent motions remains unclear, while the observed radial expansion and dust shell suggest that massive stellar feedback could have influenced the star formation history of these groups.

The Structure and Evolution of Massive Rotating Single and Binary Population III Stars

Abstract The aim of this paper is to investigate the effect of rotation on the single and binary evolution for Population III stars. A small grid for a massive Population III star of 130 M ⊙ is constructed, and various initial conditions are adjusted to explore the evolution. For comparison, we present the evolution of the models with the metallicity in the Small Magellanic Cloud and analyze the characteristic feature of chemically homogeneous evolution. It is found that Population III stars attain the equilibrium velocities later during synchronization owing to a smaller radius. The equilibrium velocity has been expressed as various timescales. There appears to be a deep dredge-up at hydrogen exhaustion for single Population III stars. It not only increases the helium core but also exchanges chemical elements between the He core and the H-burning shell. This will give rise to a significant amount of nitrogen and neon. Rotational mixing can reduce the specific entropy in the envelope and increase the specific entropy in the core owing to a change of mean molecular weight. Stellar compactness and the luminosity available for stellar expansion are decreased by rotational mixing because of the increase of helium in the envelopes. Mass loss induced by strong stellar winds and Roche lobe overflow can extinguish the hydrogen-burning shell and remove convective envelopes. Therefore, this process does not favor the dredge-up and production of primary nitrogen. The chemical structure for two components in binarities is significantly modified because Roche lobe overflow has an impact on convective cores.

The Stellar Populations of Two Ultra-diffuse Galaxies from Optical and Near-infrared Photometry

Abstract We present observational constraints on the stellar populations of two ultra-diffuse galaxies (UDGs) using optical through near-infrared (NIR) spectral energy distribution (SED) fitting. Our analysis is enabled by new Spitzer-IRAC 3.6 and 4.5 μm imaging, archival optical imaging, and the prospector fully Bayesian SED fitting framework. Our sample contains one field UDG (DGSAT I), one Virgo cluster UDG (VCC 1287), and one Virgo cluster dwarf elliptical for comparison (VCC 1122). We find that the optical–NIR colors of the three galaxies are significantly different from each other. We infer that VCC 1287 has an old (≳7.7 Gyr) and surprisingly metal-poor ([Z/Z ⊙] ≲ −1.0) stellar population, even after marginalizing over uncertainties on diffuse interstellar dust. In contrast, the field UDG DGSAT I shows evidence of being younger than the Virgo UDG, with an extended star formation history and an age posterior extending down to ∼3 Gyr. The stellar metallicity of DGSAT I is sub-solar but higher than that of the Virgo UDG, with in the case of exactly zero diffuse interstellar dust, DGSAT I may even have solar metallicity. With VCC 1287 and several Coma UDGs, a general picture is emerging where cluster UDGs may be “failed” galaxies, but the field UDG DGSAT I seems more consistent with a stellar feedback-induced expansion scenario. In the future, our approach can be applied to a large and diverse sample of UDGs down to faint surface brightness limits, with the goal of constraining their stellar ages, stellar metallicities, and circumstellar and diffuse interstellar dust content.

Estimating the dust production rate of carbon stars in the Small Magellanic Cloud

We employ newly computed grids of spectra reprocessed by dust for estimating the total dust production rate (DPR) of carbon stars in the Small Magellanic Cloud (SMC). For the first time, the grids of spectra are computed as a function of the main stellar parameters, i.e. mass-loss rate, luminosity, effective temperature, current stellar mass and element abundances at the photosphere, following a consistent, physically grounded scheme of dust growth coupled with stationary wind outflow. The model accounts for the dust growth of various dust species formed in the circumstellar envelopes of carbon stars, such as carbon dust, silicon carbide and metallic iron. In particular, we employ some selected combinations of optical constants and grain sizes for carbon dust that have been shown to reproduce simultaneously the most relevant colour–colour diagrams in the SMC. By employing our grids of models, we fit the spectral energy distributions of ≈3100 carbon stars in the SMC, consistently deriving some important dust and stellar properties, i.e. luminosities, mass-loss rates, gas-to-dust ratios, expansion velocities and dust chemistry. We discuss these properties and we compare some of them with observations in the Galaxy and Large Magellanic Cloud. We compute the DPR of carbon stars in the SMC, finding that the estimates provided by our method can be significantly different, between a factor of ≈2–5, than the ones available in the literature. Our grids of models, including the spectra and other relevant dust and stellar quantities, are publicly available at http://starkey.astro.unipd.it/web/guest/dustymodels.

NIHAO – XI. Formation of ultra-diffuse galaxies by outflows

Abstract We address the origin of ultra-diffuse galaxies (UDGs), which have stellar masses typical of dwarf galaxies but effective radii of Milky Way-sized objects. Their formation mechanism, and whether they are failed L⋆ galaxies or diffuse dwarfs, are challenging issues. Using zoom-in cosmological simulations from the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) project, we show that UDG analogues form naturally in dwarf-sized haloes due to episodes of gas outflows associated with star formation. The simulated UDGs live in isolated haloes of masses 1010–11 M⊙, have stellar masses of 107–8.5 M⊙, effective radii larger than 1 kpc and dark matter cores. They show a broad range of colours, an average Sérsic index of 0.83, a typical distribution of halo spin and concentration, and a non-negligible H i gas mass of 107 − 9 M⊙, which correlates with the extent of the galaxy. Gas availability is crucial to the internal processes which form UDGs: feedback-driven gas outflows, and subsequent dark matter and stellar expansion, are the key to reproduce faint, yet unusually extended, galaxies. This scenario implies that UDGs represent a dwarf population of low surface brightness galaxies and should exist in the field. The largest isolated UDGs should contain more H i gas than less extended dwarfs of similar M⋆.

The Pulsation Induced by Mass Transfer in Critically Rotating Stars**Supported by the National Natural Science Foundation of China under Grant No 11463002, the Open Foundation of Key Laboratory for the Structure and Evolution of Celestial Objects of Chinese Academy of Sciences under Grant No OP201405, and the Western Project of State Scholarship Foundation by the China Scholarship Council.

The structural characteristics of the critically rotating accretor in binaries are investigated during rapid mass transfer. It is found that the accretor is subjected to periodic pulsation due to accretions and rejections of mass and angular momentum. The gainer attempts to attain both hydrostatic and thermal balances. This physical process can cause the thermal structure of the accreting star to fluctuate with a period of ∼ 0.19 y. Stellar wind can be enhanced by a factor of ∼1.25 × 104 when the accretor approaches break-down velocity. Surface entropy and density decrease with the increase of the stellar radius due to the fact that rapid rotation leads to a reduction in the number density and surface temperature. The rotational energy has the same trend as stellar radius due to stellar expansion. Surface opacity which is extremely sensitive to surface temperature has an opposite trend to stellar radius. Moreover, the rate of nuclear energy must be adjusted due to mass removal or accretion at the stellar surface.

Wave-driven stellar expansion and binary interaction in pre-supernova outbursts

We suggest that the main outcome of energy leakage carried by waves from the core to the envelope of pre-collapse massive stars is envelope expansion rather than major mass ejection. We show that the propagating waves add to the pressure in the envelope at radii smaller than the radius where the convection driven by the waves becomes supersonic, the driven radius. The extra wave pressure and wave energy dissipation lead to envelope expansion. Using the numerical code MESA we show that the envelope expansion absorbs most of the energy carried by the waves. A possible conclusion from our results is that pre-explosion outbursts (PEOs) result from a binary companion accreting mass from the extended envelope and releasing a huge amount of energy. The accreting companion is likely to expel mass in a bipolar morphology, e.g., jets and an equatorial ring.

Reproducing properties of MW dSphs as descendants of DM-free TDGs

The Milky Way (MW) dwarf spheroidal (dSph) satellites are known to be the most dark-matter (DM) dominated galaxies with estimates of dark to baryonic matter reaching even above one hundred. It comes from the assumption that dwarfs are dynamically supported by their observed velocity dispersions. However their spatial distributions around the MW is not at random and this could challenge their origin, previously assumed to be residues of primordial galaxies accreted by the MW potential. Here we show that alternatively, dSphs could be the residue of tidal dwarf galaxies (TDGs), which would have interacted with the Galactic hot gaseous halo and disk. TDGs are gas-rich and have been formed in a tidal tail produced during an ancient merger event at the M31 location, and expelled towards the MW. Our simulations show that low-mass TDGs are fragile to an interaction with the MW disk and halo hot gas. During the interaction, their stellar content is progressively driven out of equilibrium and strongly expands, leading to low surface brightness feature and mimicking high dynamical M/L ratios. Our modeling can reproduce the properties, including the kinematics, of classical MW dwarfs within the mass range of the Magellanic Clouds to Draco. An ancient gas-rich merger at the M31 location could then challenge the currently assumed high content of dark matter in dwarf galaxies. We propose a simple observational test with the coming GAIA mission, to follow their expected stellar expansion, which should not be observed within the current theoretical framework.

The VLT-FLAMES Tarantula Survey

Aims. Using ground based multi-object optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS), we aim to establish the (projected) rotational velocity distribution for a sample of 216 presumably single O-type stars in 30 Doradus (30 Dor). Methods. We measured projected rotational velocities, \vrot, by means of a Fourier transform method and a profile fitting method applied on a set of isolated spectral lines. We also used an iterative deconvolution procedure to infer the probability density, $\rm{P(\veq)}$, of the equatorial rotational velocity, \veq. Results. The distribution of \vrot\ shows a two-component structure: a peak around 80 \kms\ and a high-velocity tail extending up to $\sim$600 \kms. This structure is also present in the inferred distribution $\rm{P(\veq)}$ with around 80% of the sample having 0 $<$ \veq\, $\leq\, 300$ \kms\ and the other 20% distributed in the high-velocity region. Conclusions. Most of the stars in our sample rotate with a rate less than 20%\ of their break-up velocity. For the bulk of the sample, mass-loss in a stellar wind and/or envelope expansion is not efficient enough to significantly spin down these stars within the first few Myr of evolution. The presence of a sizeable population of fast rotators is compatible with recent population synthesis computations that investigate the influence of binary evolution on the rotation rate of massive stars. Despite the fact that we have excluded stars that show significant radial velocity variations, our sample may have remained contaminated by post-interaction binary products. The fact that the high-velocity tail may be preferentially (and perhaps even exclusively), populated by post-binary interaction products, has important implications for the evolutionary origin of systems that produce gamma-ray bursts.

White dwarf planets

The recognition that planets may survive the late stages of stellar evolution, and the prospects for finding them around White Dwarfs, are growing. We discuss two aspects governing planetary survival through stellar evolution to the White Dwarf stage. First we discuss the case of a single planet, and its survival under the effects of stellar mass loss, radius expansion, and tidal orbital decay as the star evolves along the Asymptotic Giant Branch. We show that, for stars initially of 1 − 5 M ⊙ , any planets within about 1 − 5 AU will be engulfed, this distance depending on the stellar and planet masses and the planet's eccentricity. Planets engulfed by the star's envelope are unlikely to survive. Hence, planets surviving the Asymptotic Giant Branch phase will probably be found beyond ∼ 2 AU for a 1 M ⊙ progenitor and ∼ 10 AU for a 5 M ⊙ progenitor. We then discuss the evolution of two-planet systems around evolving stars. As stars lose mass, planet–planet interactions become stronger, and many systems stable on the Main Sequence become destabilised following evolution of the primary. The outcome of such instabilities is typically the ejection of one planet, with the survivor being left on an eccentric orbit. These eccentric planets could in turn be responsible for feeding planetesimals into the neighbourhood of White Dwarfs, causing observed pollution and circumstellar discs.

STAR FORMATION ASSOCIATED WITH THE SUPERNOVA REMNANT IC443

We have performed the submillimeter and millimeter observations in CO lines toward supernova remnant (SNR) IC443. CO molecular shell is well coincident with the partial shell of the SNR detected in radio continuum observations. The broad emission lines and three 1720-MHz OH masers were detected in CO molecular shell. The present observations have provided further evidence in support of the interaction between the SNR and the adjoining molecular clouds (MCs). The total mass of the MCs is 9.26*10^{3} sun masses. The integrated CO line intensity ratio I(CO(3-2))/I(CO(2-1)) for the whole molecular cloud is between 0.79 and 3.40. The average value is 1.58, which is much higher than previous measurements of individual Galactic MCs. Higher line ratios imply that shocks have driven into the MCs. We conclude that high I(CO(3-2))/I(CO(2-1)) is identified as one good signature of SNR-MCs interacting system. Based on $IRAS$ Point Source Catalog and 2MASS near-infrared database, 12 protostellar objects and 1666 young stellar objects (YSOs) candidates (including 154 CTTSs and 419 HAeBe stars) are selected. In the interacting regions, the significant enhancement of the number of protostellar objects and YSOs indicates the presence of some recently formed stars. After comparing the characteristic timescales of star formation with the age of IC443, we conclude that the protostellar objects and YSOs candidates are not triggered by IC443. For the age of stellar winds shell, we have performed calculation on the basis of stellar winds shell expansion model. The results and analysis suggest that the formation of these stars may be triggered by the stellar winds of IC443 progenitor.

The magnetically-active, low-mass, triple system WDS 19312+3607

Aims: We investigated in detail the system WDS 19312+3607, whose primary is an active M4.5Ve star previously thought to be young (tau ~ 300-500 Ma) based on high X-ray luminosity. Methods: We collected intermediate- and low-resolution optical spectra taken with 2 m-class telescopes, photometric data from the $B$ to 8 mum bands, and eleven astrometric epochs with a time baseline of over 56 years for the two components in the system, G 125-15 and G 125-14. Results: We derived M4.5V spectral types for both stars, confirmed their common proper motion, estimated the heliocentric distance and projected physical separation, determined the galactocentric space velocities, and deduced a most-probable age older than 600 Ma. We discovered that the primary, G 125-15, is in turn an inflated, double-lined, spectroscopic binary with a short period of photometric variability of P ~ 1.6 d, which we associated to orbital synchronisation. The observed X-ray and Halpha emissions, photometric variability, and abnormal radius and effective temperature of G 125-15 AB indicate strong magnetic activity, possibly due to fast rotation. Besides, the estimated projected physical separation between G 125-15 AB and G 125-14 of about 1200 AU makes WDS 19312+3607 to be one of the widest systems with intermediate M-type primaries. Conclusions: G 125-15 AB is a nearby (d ~ 26 pc), bright (J ~ 9.6 mag), active spectroscopic binary with a single proper-motion companion of the same spectral type at a wide separation. They are thus ideal targets for specific follow-ups to investigate wide and close multiplicity or stellar expansion and surface cooling due to reduced convective efficiency.

EVALUATING SYSTEMATIC DEPENDENCIES OF TYPE Ia SUPERNOVAE: THE INFLUENCE OF DEFLAGRATION TO DETONATION DENSITY

We explore the effects of the deflagration to detonation transition (DDT) density on the production of 56Ni in thermonuclear supernova (SN) explosions (Type Ia supernovae). Within the DDT paradigm, the transition density sets the amount of expansion during the deflagration phase of the explosion and therefore the amount of nuclear statistical equilibrium (NSE) material produced. We employ a theoretical framework for a well-controlled statistical study of two-dimensional simulations of thermonuclear SNe with randomized initial conditions that can, with a particular choice of transition density, produce a similar average and range of 56Ni masses to those inferred from observations. Within this framework, we utilize a more realistic "simmered" white dwarf progenitor model with a flame model and energetics scheme to calculate the amount of 56Ni and NSE material synthesized for a suite of simulated explosions in which the transition density is varied in the range (1–3) ×107 g cm−3. We find a quadratic dependence of the NSE yield on the log of the transition density, which is determined by the competition between plume rise and stellar expansion. By considering the effect of metallicity on the transition density, we find the NSE yield decreases by 0.055 ± 0.004 M☉ for a 1 Z☉ increase in metallicity evaluated about solar metallicity. For the same change in metallicity, this result translates to a 0.067 ± 0.004 M☉ decrease in the 56Ni yield, slightly stronger than that due to the variation in electron fraction from the initial composition. Observations testing the dependence of the yield on metallicity remain somewhat ambiguous, but the dependence we find is comparable to that inferred from some studies.

STUDY OF THE DETONATION PHASE IN THE GRAVITATIONALLY CONFINED DETONATION MODEL OF TYPE Ia SUPERNOVAE

We study the gravitationally confined detonation (GCD) model of Type Ia supernovae through the detonation phase and into homologous expansion. In the GCD model, a detonation is triggered by the surface flow due to single point, off-center flame ignition in carbon-oxygen white dwarfs. The simulations are unique in terms of the degree to which non-idealized physics is used to treat the reactive flow, including weak reaction rates and a time dependent treatment of material in nuclear statistical equilibrium (NSE). Careful attention is paid to accurately calculating the final composition of material which is burned to NSE and frozen out in the rapid expansion following the passage of a detonation wave over the high density core of the white dwarf; and an efficient method for nucleosynthesis post-processing is developed which obviates the need for costly network calculations along tracer particle thermodynamic trajectories. Observational diagnostics are presented for the explosion models, including abundance stratifications and integrated yields. We find that for all of the ignition conditions studied here, a self regulating process comprised of neutronization and stellar expansion results in final \iso{Ni}{56} masses of $\sim$1.1\msun. But, more energetic models result in larger total NSE and stable Fe peak yields. The total yield of intermediate mass elements is $\sim0.1$\msun and the explosion energies are all around 1.5$\times10^{51}$ ergs. The explosion models are briefly compared to the inferred properties of recent Type Ia supernova observations. The potential for surface detonation models to produce lower luminosity (lower \iso{Ni}{56} mass) supernovae is discussed.

Basic properties of anisotropic stellar wind expansion in the fluid approach

[1] Whereas in an isotropic temperature atmosphere both the hydrostatic equation and the momentum equation give the same conditions for hydrostatic equilibrium, in the anisotropic case the situation is ambiguous. It is found that for an anisotropic temperature the hydrostatic equilibrium conditions have to be deduced from the momentum equation. The condition for hydrostatic equilibrium is that both the parallel and the perpendicular temperatures, to the radial direction, decrease more rapidly than 1/r in the general case and that the perpendicular temperature decreases more rapidly than 1/r when the parallel temperature is constant. The momentum equation displays a transonic solution when at least one of the temperatures, parallel or perpendicular to the radial direction, decreases less rapidly than 1/r in the general case and when the perpendicular temperature decreases less rapidly than 1/r when the parallel temperature is constant. In an anisotropic atmosphere the parallel thermal velocity is the critical velocity. The properties of the transonic expansion in an isothermal and anisotropic atmosphere are studied. The initial velocity, the critical distance position, the terminal velocity, and the density profile are significantly different from the isotropic case. These properties are opposite with respect to the value of the anisotropy T⊥/T∥ > 1.0 and T⊥/T∥ 2.0. The extension of this study to multimoment anisotropic models can be useful for the interpretation of particle simulation of rarefied stellar atmosphere expansion.

Type Ia Supernova Explosion: Gravitationally Confined Detonation

We present a new mechanism for Type Ia supernova explosions in massive white dwarfs. The scenario follows from relaxing assumptions of symmetry and involves a detonation born near the stellar surface. The explosion begins with an essentially central ignition of a deflagration that results in the formation of a buoyancy-driven bubble of hot material that reaches the stellar surface at supersonic speeds. The bubble breakout laterally accelerates fuel-rich outer stellar layers. This material, confined by gravity to the white dwarf, races along the stellar surface and is focused at the location opposite to the point of the bubble breakout. These streams of nuclear fuel carry enough mass and energy to trigger a detonation just above the stellar surface that will incinerate the white dwarf and result in an energetic explosion. The stellar expansion following the deflagration redistributes mass in a way that ensures production of intermediate-mass and iron group elements with ejecta having a strongly layered structure and a mild amount of asymmetry following from the early deflagration phase. This asymmetry, combined with the amount of stellar expansion determined by details of the evolution (principally the energetics of deflagration, timing of detonation, and structure of the progenitor), can be expected to create a family of mildly diverse Type Ia supernova explosions.