Thick Envelope Research Articles

Exploring massive star early evolution: the case of the Herschel 36 A triple system

Abstract Theoretical models show that some massive stars have not yet arrived at the zero-age main-sequence (ZAMS) at the end of the accretion phase. At that time, they have lost their thick envelopes and thus could be optically visible. Although some candidates to optically observable ZAMS stars have been reported, the evolutionary status of none of them has been confirmed yet. The O-type triple system Herschel 36 A (H36A) is one of these candidates. We present the quantitative spectral analysis of the individual stellar components of H36A and investigate the evolutionary status of the system by contrasting main sequence and pre-main sequence models. Overall, the derived parameters suggest that the components of H36A could be pre-main sequence stars going through the very last contraction to the ZAMS. However, the possibility of them already being on the main sequence is not yet ruled out. This study highlights the importance of considering multiple evolutionary models and shows that H36A represents a key object for understanding massive star formation and early evolution.

Ultrasoft state of microquasar Cygnus X-3: X-ray polarimetry reveals the geometry of the astronomical puzzle

Cygnus X-3 is an enigmatic X-ray binary that is both an exceptional accreting system and a cornerstone for population synthesis studies. Prominent X-ray and radio properties follow a well-defined pattern, and yet the physical reasons for the state changes observed in this system are not known. Recently, the presence of an optically thick envelope around the central source in the hard state was revealed using the X-ray polarization data obtained with the Imaging X-ray Polarimetry Explorer (IXPE). In this work we analyse IXPE data obtained in the ultrasoft (radio quenched) state of the source. The average polarization degree (PD) of 11.9 ± 0.5% at a polarization angle (PA) of 94° ±1° is inconsistent with the simple geometry of the accretion disc viewed at an intermediate inclination. The high PD, the blackbody-like spectrum, and the weakness of fluorescent iron line imply that the central source is hidden behind the optically thick outflow, similar to the hard-state geometry, and its beamed radiation is scattered, by the matter located along the funnel axis, towards our line of sight. In this picture the observed PD is directly related to the source inclination, which we conservatively determine to lie in the range 26° < i < 28°. Using the new polarimetric properties, we propose a scenario that can be responsible for the cyclic behaviour of the state changes in the binary.

TESS Giants Transiting Giants. VI. Newly Discovered Hot Jupiters Provide Evidence for Efficient Obliquity Damping after the Main Sequence

The degree of alignment between a star’s spin axis and the orbital plane of its planets (the stellar obliquity) is related to interesting and poorly understood processes that occur during planet formation and evolution. Hot Jupiters orbiting hot stars (≳6250 K) display a wide range of obliquities, while similar planets orbiting cool stars are preferentially aligned. Tidal dissipation is expected to be more rapid in stars with thick convective envelopes, potentially explaining this trend. Evolved stars provide an opportunity to test the damping hypothesis, particularly stars that were hot on the main sequence and have since cooled and developed deep convective envelopes. We present the first systematic study of the obliquities of hot Jupiters orbiting subgiants that recently developed convective envelopes using Rossiter–McLaughlin observations. Our sample includes two newly discovered systems in the Giants Transiting Giants survey (TOI-6029 b, TOI-4379 b). We find that the orbits of hot Jupiters orbiting subgiants that have cooled below ∼6250 K are aligned or nearly aligned with the spin axis of their host stars, indicating rapid tidal realignment after the emergence of a stellar convective envelope. We place an upper limit for the timescale of realignment for hot Jupiters orbiting subgiants at ∼500 Myr. Comparison with a simplified tidal evolution model shows that obliquity damping needs to be ∼4 orders of magnitude more efficient than orbital period decay to damp the obliquity without destroying the planet, which is consistent with recent predictions for tidal dissipation from inertial waves excited by hot Jupiters on misaligned orbits.

Coriolis darkening in late-type stars

Context. Modeling the surface brightness distribution of stars is of prime importance to interpret the large amount of available interferometric, spectropolarimetric, or photometric observations. Beyond stellar physics, this is also a prerequisite to characterize exoplanets or our Galaxy. Nevertheless, this remains quite challenging for cool stars as it requires one to model the magnetohydrodynamic turbulence that develops in their convective envelope. Aims. In Paper I (Raynaud, R., Rieutord, M., Petitdemange, L., Gastine, T., & Putigny, B. 2018, A&A, 609, A124), the effect of the Coriolis acceleration on the surface heat flux has been studied by means of hydrodynamic simulations. In this paper, we aim to investigate the additional effect of dynamo magnetic fields that can be generated in the thick convective envelopes of cool stars. We focus on an envelope thickness that is representative of either a ∼0.35 M⊙ M dwarf, a young red giant star or a pre-main sequence star. Methods. We performed a parametric study using numerical magnetohydrodynamic simulations of anelastic convection in thick rotating spherical shells. The stratification in density ranges from a few tens to a few hundreds. The setup assumes a constant entropy jump between the inner and outer layers to force convection, with stress-free boundary conditions for the velocity field. The magnetic Prandtl number was systematically varied in order to vary the magnetic field intensity. For each model, we computed the azimuthally and temporally averaged surface distribution of the heat flux, and examined the leading-order effect of the magnetic field on the obtained latitudinal luminosity profile. Results. We identify three different regimes. Close to the onset of convection, while the first unstable modes tend to convey heat more efficiently near the equator, magnetic fields are shown to generally enhance the mean heat flux close to the polar regions (and the tangent cylinder). By progressively increasing the Rayleigh number, the development of a prograde equatorial jet was previously shown to make the equator darker when no magnetic field is taken into account. For moderate Rayleigh numbers, magnetic fields can instead inverse the mean pole-equator brightness contrast (which means going from a darker to a brighter equator when a dynamo sets in) and finally induce a similar regime to that found close to the onset of convection. For more turbulent models with larger Rayleigh numbers, magnetic fields alternatively tend to smooth out the brightness contrast. This general behavior is shown to be related to the quenching of the surface differential rotation by magnetic fields and remains valid regardless of the magnetic morphology. Conclusions. Mean global trends regarding the impact of rotation and magnetic fields on the surface brightness distribution of cool stars are theoretically depicted and need to be tested by future observations. This work opens the door to more detailed theoretical studies including the effect of nonaxisymmetric and time-variable surface features associated with magnetic activity.

On the Maximum Black Hole Mass at Solar Metallicity

In high-metallicity environments the mass that black holes (BHs) can reach just after core collapse widely depends on how much mass their progenitor stars lose via winds. On one hand, new theoretical and observational insights suggest that early-stage winds should be weaker than what many canonical models prescribe. On the other hand, the proximity to the Eddington limit should affect the formation of optically thick envelopes already during the earliest stages of stars with initial masses M ZAMS ≳ 100 M ⊙, hence resulting in higher mass-loss rates during the main sequence. We use the evolutionary codes MESA and Genec to calculate a suite of tracks for massive stars at solar metallicity Z ⊙ = 0.014, which incorporate these changes in our wind-mass-loss prescription. In our calculations we employ moderate rotation, high overshooting, and magnetic angular momentum transport. We find a maximum BH mass MBH,max=28.3 M ⊙ at Z ⊙. The most massive BHs are predicted to form from stars with M ZAMS ≳ 250 M ⊙, with the BH mass directly proportional to its progenitor’s M ZAMS. We also find in our models that at Z ⊙ almost any BH progenitor naturally evolves into a Wolf–Rayet star due to the combined effect of internal mixing and wind mass loss. These results are considerably different from most recent studies regarding the final mass of stars before their collapse into BHs. While we acknowledge the inherent uncertainties in stellar evolution modeling, our study underscores the importance of employing the most up-to-date physics in BH mass predictions.

Coupling of radiation and magnetospheric accretion flow in ULX pulsars: radiation pressure and photon escape time

ABSTRACT The accretion flow within the magnetospheric radius of bright X-ray pulsars can form an optically thick envelope, concealing the central neutron star from the distant observer. Most photons are emitted at the surface of a neutron star and leave the system after multiple reflections by the accretion material covering the magnetosphere. Reflections cause momentum to be transferred between photons and the accretion flow, which contributes to the radiative force and should thus influence the dynamics of accretion. We employ Monte Carlo simulations and estimate the acceleration along magnetic field lines due to the radiative force as well as the radiation pressure across magnetic field lines. We demonstrate that the radiative acceleration can exceed gravitational acceleration along the field lines, and similarly, radiation pressure can exceed magnetic field pressure. Multiple reflections of X-ray photons back into the envelope tend to amplify both radiative force along the field lines and radiative pressure. We analyse the average photon escape time from the magnetosphere of a star and show that its absolute value is weakly dependent on the magnetic field strength of a star and roughly linearly dependent on the mass accretion rate being $\sim 0.1\, {\rm s}$ at $\dot{M}\sim 10^{20}\, {\rm g\, s^{-1}}$. At high mass accretion rates, the escape time can be longer than free-fall time from the inner disc radius.

Distinguishing Oceans of Water from Magma on Mini-Neptune K2-18b

Mildly irradiated mini-Neptunes have densities potentially consistent with them hosting substantial liquid-water oceans (“Hycean” planets). The presence of CO2 and simultaneous absence of ammonia (NH3) in their atmospheres has been proposed as a fingerprint of such worlds. JWST observations of K2-18b, the archetypal Hycean, have found the presence of CO2 and the depletion of NH3 to <100 ppm; hence, it has been inferred that this planet may host liquid-water oceans. In contrast, climate modeling suggests that many of these mini-Neptunes, including K2-18b, may likely be too hot to host liquid water. We propose a solution to this discrepancy between observation and climate modeling by investigating the effect of a magma ocean on the atmospheric chemistry of mini-Neptunes. We demonstrate that atmospheric NH3 depletion is a natural consequence of the high solubility of nitrogen species in magma at reducing conditions; precisely the conditions prevailing where a thick hydrogen envelope is in communication with a molten planetary surface. The magma ocean model reproduces the present JWST spectrum of K2-18b to ≲3σ, suggesting this is as credible an explanation for current observations as the planet hosting a liquid-water ocean. Spectral areas that could be used to rule out the magma ocean model include the >4 μm region, where CO2 and CO features dominate: magma ocean models suggest a systematically lower CO2/CO ratio than estimated from free-chemistry retrieval, indicating that deeper observations of this spectral region may be able to distinguish between oceans of liquid water and magma on mini-Neptunes.

Pollybeak deformity: how to avoid, how to cure.

We reviewed the Pollybeak deformity from prevention to treatment in all aspects. Research methods included searching online databases such as Google, Google Scholar, PubMed, and Proquest Central at King Saud University. We used terms like "Pollybeak deformity", "rhinoplasty", "etiology", and "treatment" to find related articles. Pollybeak deformity, an undesirable side effect of rhinoplasty, manifests as a dorsal nasal convexity resembling a parrot's beak. A dorsal hump that develops in the supratip region of the nose "pushes" the tip down, leading to under-rotation. Several factors may be at play here. When a surgeon fails to recognize the aberrant tip-supratip relationship that distinguishes this abnormality during the intraoperative evaluation of the nose, the result is a pollybeak. There is also the risk of pollybeak deformity, which the surgeon might be unable to predict. Supratip scar tissue is more common in people with thick skin and soft tissue envelopes. Medical intervention is only effective for soft-tissue pollybeaks when caught early. Injecting corticosteroids into the supratip dead space can alleviate edema and slow scar tissue formation. Surgical excision of scar tissue from the dorsum of the nose can repair the pollybeak deformity. Surgical correction of a supratip fullness that causes a disparity between the tip and supratip is known as a "pollybeak deformity". The appearance of the nose in some individuals with pollybeak deformity can resemble that of a bird, which can cause them to feel self-conscious and embarrassed. Therefore, treatment with medicine or surgery is required for these ailments.

Long-period maser-bearing Miras in the Galactic center

We establish a sample of 370 Mira variables that are likely near the Galactic center (GC). The sources have been selected from the OGLE and BAaDE surveys based on their sky coordinates, OGLE classifications, and BAaDE maser-derived line-of-sight velocities. As the distance to the GC is known to a high accuracy, this sample is a test bed for reddening and extinction studies toward the GC and in Mira envelopes. We calculated separate interstellar- and circumstellar-extinction values for individual sources, showing that there is a wide range of circumstellar extinction values (up to four magnitudes in the Ks band) in the sample, and that circumstellar reddening is statistically different from interstellar reddening laws. Further, the reddening laws in the circumstellar environments of our sample and the circumstellar environments of Large Magellanic Cloud (LMC) Miras are strikingly similar despite the different metallicities of the samples. Period-magnitude relations for the mid-infrared (MIR) WISE and MSX bands are also explored, and in the WISE bands we compare these to period-magnitude relationships derived from Miras in the LMC as it is important to compare these LMC relations to those in a higher metallicity environment. Emission from the envelope itself may contaminate MIR magnitudes altering the relations, especially for sources with thick envelopes.

Observational properties of accreting neutron stars with an optically thick envelope

ABSTRACT Pulsating ultraluminous X-ray sources are thought to be X-ray bright, accreting, magnetized neutron stars. Their measured soft (&amp;lt;20 keV) X-ray luminosity can exceed the Eddington luminosity for a neutron star by a few orders of magnitude. Although several scenarios have been proposed to explain the different components observed in the X-ray spectrum and the characteristics of the X-ray pulse profile of these systems, detailed quantitative calculations are still missing. In particular, the observed soft X-ray pulse profiles are almost sinusoidal and show an increase in the pulsed fraction (from 8 up to even 30 per cent) with increasing energy. In this work, we discuss how emission originating from an optically thick envelope, expected to be formed during super-Eddington accretion, can match the observed spectra and pulse profiles.

The Impact of Tidal Migration of Hot Jupiters on the Rotation of Sun-like Main-sequence Stars

The tidal interactions of planets affect the stellar evolutionary status and the constraint of their physical parameters by gyrochronology. In this work, we incorporate the tidal interaction and magnetic braking of the stellar wind into MESA and calculate a large grid of 25,000 models, covering planets with masses of 0.1–13.0 M J with different orbital distances that orbit late-type stars of different metallicities. We also explore the effect of different stellar initial rotations on the tidal interactions. Our results show that in the case of tidal inward migration, the stellar rotation periods are always lower than that of the star without planet before the planet is engulfed and the difference in the rotation period of its host star always increases with time. After the planet is engulfed, the stellar rotation periods are still lower than that of star without planet, but the difference of periods can be quickly eliminated if the star has a thick convective envelope (smaller mass and larger metallicity), regardless of the mass of the planet and the initial rotation period of the star. In the case of stars with thinner convective envelopes (larger mass and smaller metallicity), the stars will be spun up and remain the faster rotation in a long time. Meanwhile, the planet is easily swallowed and the period differences are large if the initial rotation period of its host star is higher. Finally, we also study the evolution of WASP-19 and estimate the range of tidal quality parameter and the initial semimajor axis as (0.035 ± 0.004) au.

APOLOCYSTIS BOSANQUETI N. SP. (APICOMPLEXA: EUGREGARINORIDA) FROM THE INVASIVE EARTHWORM AMYNTHAS AGRESTIS (ANNELIDA: MEGASCOLECIDAE), WITH SIGNIFICANCE FOR THE MONOPHYLY OF THE FAMILY MONOCYSTIDAE.

Apolocystis bosanqueti n. sp., a parasite of an important invasive earthworm in North America, Amynthas agrestis, is described from a site in northern Vermont. The earthworm host follows an annual life cycle in Vermont, so the entire life cycle of the parasite can be observed in 7 mo. In spring, the parasites were first seen in juvenile worms as paired gamonts (suggesting precocious association). These paired gamonts mature into gametocytes that form an opaque structure, with a thick gelatinous envelope (epicyst), that becomes full of zygotes. The resulting gametocyst becomes packed with ∼105 fusiform oocysts. The mature orbicular gametocysts are large (∼1 mm in diameter) and visible to the naked eye through the body wall of the host's anterior segments. The new species most resembles Apolocystis herculea described from many lumbricid earthworm species in Europe but differs from that parasite because Ap. herculea infects the intestinal wall in the posterior of the host rather than the anterior segments. A survey of 9 other earthworm species sympatric with Am. agrestis revealed that only Amynthas tokioensis, also an invasive species, was infected with Ap. bosanqueti, albeit much less commonly. Diagnosis for the family Monocystidae is problematic because cardinal characters are lacking, and the commonly cited character, a trophozoite with no anterior differentiation, is violated in most genera placed in the family. For the first time, a molecular phylogeny is presented that includes 3 genera of monocystids with diverse cell morphology (including the new species) and supports the monophyly of the family. The only morphological character that may be used to diagnose the Monocystidae is the morphology of oocysts, which are fusiform with extended terminal tips. A comparison of oocysts from 7 parasites recovered from local earthworms, including from 3 monocystid species in the phylogeny, confirms the utility of this diagnostic trait. The 2 hosts of the new species were most likely introduced from Japan, so the range of Apolocystis likely extends into East Asia.

Insights into substrate transport and water permeation in the mycobacterial transporter MmpL3

Mycobacteria, such as Mycobacterium tuberculosis, are characterized by a uniquely thick and waxy cell envelope that consists of two membranes, with a variety of mycolates comprising their outer membrane (OM). The protein Mycobacterial membrane protein Large 3 (MmpL3) is responsible for the transport of a primary OM component, trehalose monomycolate (TMM), from the inner (cytoplasmic) membrane (IM) to the periplasmic space, a process driven by the proton gradient. Although multiple structures of MmpL3 with bound substrates have been solved, the exact pathway(s) for TMM or proton transport remains elusive. Here, employing molecular dynamics simulations we investigate putative pathways for either transport species. We hypothesized that MmpL3 will cycle through similar conformational states as the related transporter AcrB, which we used as targets for modeling the conformation of MmpL3. A continuous water pathway through the transmembrane region was found in one of these states, illustrating a putative pathway for protons. Additional equilibrium simulations revealed that TMM can diffuse from the membrane into a binding pocket in MmpL3 spontaneously. We also found that acetylation of TMM, which is required for transport, makes it more stable within MmpL3’s periplasmic cavity compared with the unacetylated form.

Reaction between Hydrogen and Ferrous/Ferric Oxides at High Pressures and High Temperatures—Implications for Sub-Neptunes and Super-Earths

Sub-Neptune exoplanets may have thick hydrogen envelopes and therefore develop a high-pressure interface between hydrogen and the underlying silicates/metals. Some sub-Neptunes may convert to super-Earths via massive gas loss. If hydrogen chemically reacts with oxides and metals at high pressures and temperatures (P−T), it could impact the structure and composition of the cores and atmospheres of sub-Neptunes and super-Earths. While H2 gas is a strong reducing agent at low pressures, the behavior of hydrogen is unknown at the P−T expected for sub-Neptunes’ interiors, where hydrogen is a dense supercritical fluid. Here we report experimental results of reactions between ferrous/ferric oxides and hydrogen at 20–40 GPa and 1000–4000 K utilizing the pulsed laser-heated diamond-anvil cell combined with synchrotron X-ray diffraction. Under these conditions, hydrogen spontaneously strips iron off the oxides, forming Fe-H alloys and releasing oxygen to the hydrogen medium. In a planetary context where this reaction may occur, the Fe-H alloy may sink to the metallic part of the core, while released oxygen may stabilize as water in the silicate layer, providing a mechanism to ingas hydrogen to the deep interiors of sub-Neptunes. Water produced from the redox reaction can also partition to the atmosphere of sub-Neptunes, which has important implications for understanding the composition of their atmospheres. In addition, super-Earths converted from sub-Neptunes may contain a large amount of hydrogen and water in their interiors (at least a few wt% H2O). This is distinct from smaller rocky planets, which were formed relatively dry (likely a few hundredths wt% H2O).

Kepler-102: Masses and Compositions for a Super-Earth and Sub-Neptune Orbiting an Active Star

Radial velocity (RV) measurements of transiting multiplanet systems allow us to understand the densities and compositions of planets unlike those in the solar system. Kepler-102, which consists of five tightly packed transiting planets, is a particularly interesting system since it includes a super-Earth (Kepler-102d) and a sub-Neptune-sized planet (Kepler-102e) for which masses can be measured using RVs. Previous work found a high density for Kepler-102d, suggesting a composition similar to that of Mercury, while Kepler-102e was found to have a density typical of sub-Neptune size planets; however, Kepler-102 is an active star, which can interfere with RV mass measurements. To better measure the mass of these two planets, we obtained 111 new RVs using Keck/HIRES and Telescopio Nazionale Galileo/HARPS-N and modeled Kepler-102's activity using quasiperiodic Gaussian process regression. For Kepler-102d, we report a mass upper limit M d < 5.3 M ⊕ (95% confidence), a best-fit mass M d = 2.5 ± 1.4 M ⊕, and a density ρ d = 5.6 ± 3.2 g cm−3, which is consistent with a rocky composition similar in density to the Earth. For Kepler-102e we report a mass M e = 4.7 ± 1.7 M ⊕ and a density ρ e = 1.8 ± 0.7 g cm−3. These measurements suggest that Kepler-102e has a rocky core with a thick gaseous envelope comprising 2%–4% of the planet mass and 16%–50% of its radius. Our study is yet another demonstration that accounting for stellar activity in stars with clear rotation signals can yield more accurate planet masses, enabling a more realistic interpretation of planet interiors.

Application of External Nasal Skin Stretching in Asian Rhinoplasty.

Asian nasal characteristics include a low dorsum, short nose, and thick skin envelope, usually requiring lengthening and elevating the nose during rhinoplasty ( Facial Plast Surg Clin North Am 2007;15(3):293-307, v). The increase in rhinoplasty popularity has resulted in a greater prevalence of complications. In a severely short and contracted nose, an extensively scarred or contracted soft tissue envelope results in weak laxity and extensibility of the nasal skin. For these patients, the essential component of rhinoplasty is to improve skin texture and obtain a sufficient nasal skin soft tissue envelope. Tissue expanders have previously been utilized to expand nasal skin and soft tissue ( Plast Reconstr Surg 2006;118(6):1447-1452; Facial Plast Surg 2019;35(1):68-72). However, nasal anatomical characteristics have limited the clinical application of tissue expanders. This article introduces a simple, noninvasive, and easily adopted method of external nasal skin stretching, which was first proposed by the senior author. This approach has been accepted by rhinoplasty surgeons in China and widely used in clinic. The approach can improve skin laxity, yield extra nasal soft tissue, and create adequate soft tissue coverage of the reconstructed nasal framework to reduce the difficulty of surgery with a reliable clinical effect.

Local adiposity may be a more reliable predictor for infection than body mass index following total knee arthroplasty: a systematic review

PurposeImproved understanding of the factors that predispose TKA patients to infection has considerable economic and medical impact. BMI is commonly used as a proxy for obesity to determine the risk of postoperative infection. However, this metric appears to be fraught with inconsistency in this application. BMI is a simple calculation which provides general insight into body habitus. But it fails to account for anatomic distribution of adipose tissue and the proportion of the mass that is skeletal muscle. Our objective was to review the literature to determine if local adiposity was more predictive than BMI for infection following TKA.MethodsA database search was performed for the following PICO (Population, Intervention, Comparison, and Outcome) characteristics: local measurements of adiposity (defined as soft tissue thickness or fat thickness or soft tissue envelope at the knee) in patients over 18 years of age treated with total knee arthroplasty used to determine the relationship between local adiposity and the risk of infection (defined as prosthetic joint infection or wound complication or surgical site infection).Quality was assessed using the GRADE framework and bias was assessed using ROBINS-I .ResultsSix articles (N=7081) met the inclusion criteria. Four of the six articles determined that adiposity was more associated with or was a better predictor for infection risk than BMI. One of the six articles concluded that increased adiposity was protective for short term infection and that BMI was not associated with the outcome of interest. One of the six articles determined that BMI was more strongly associated with PJI risk than soft tissue thickness.ConclusionThe use of adiposity as a proxy for obesity in preoperative evaluation of TKA patients is an emerging concept. Although limited by heterogeneity, the current literature suggests that local adiposity may be a more reliable predictor for infection than BMI following primary TKA.Level of evidenceIV systematic review

Spatially resolved mid-infrared observations of the circumstellar environment of the born-again object FG Sge

Context. FG Sge has evolved from the hot central star of the young planetary nebula Hen 1–5 to a G–K supergiant in the last 100 yr. It is one of the three born-again objects that have been identified as of yet, and they are considered to have undergone a thermal pulse in the post-asymptotic giant branch evolution. Aims. We present mid-infrared spectro-interferometric observations of FG Sge and probe its dusty environment. Methods. FG Sge was observed with MIDI at the Very Large Telescope Interferometer at baselines of 43 and 46 m between 8 and 13 µm. Results. The circumstellar dust environment of FG Sge was spatially resolved, and the Gaussian fit to the observed visibilities results in a full width at half maximum of ~10.5 mas. The observed mid-infrared visibilities and the spectral energy distribution can be fairly reproduced by optically thick (τV ≈ 8) spherical dust shell models consisting of amorphous carbon with an inner radius rin of ~30 R★ (corresponding to a dust temperature of 1100 ± 100 K). The dust shell is characterized with a steep density profile proportional to r−3.5±0.5 from the inner radius rin to (5–10) × rin, beyond which it changes to r−2. The dust mass is estimated to be ~ 7 × 10−7 M⊙, which translates into an average total mass-loss rate of ~ 9 × 10−6 M⊙ yr−1 as of 2008 with a gas-to-dust ratio of 200 being adopted. In addition, the 8–13 µm spectrum obtained with MIDI with a field of view of 200 mas does not show a signature of the polycyclic aromatic hydrocarbon (PAH) emission, which is in marked contrast to the spectra taken with the Spitzer Space Telescope six and 20 months before the MIDI observations with wide slit widths of 3″.6–10″. This implies that the PAH emission originates from an extended region of the optically thick dust envelope. Conclusions. The dust envelope of FG Sge is much more compact than that of the other born-again stars’ Sakurai’s object and V605 Aql, which might reflect the difference in the evolutionary status. The PAH emission from the extended region of the optically thick dust envelope likely originates from the material ejected before the central star became H-deficient, and it may be excited by the UV radiation from the central star escaping through gaps among dust clumps and/or the bipolar cavity of a disk-like structure.

Asteroseismology of PG 1541 + 651 and BPM 31594 with TESS

ABSTRACT We present the photometric data from TESS for two known ZZ Ceti stars, PG 1541 + 651 and BPM 31594. Before TESS, both objects only had observations from short runs from ground-based facilities, with three and one period detected, respectively. The TESS data allowed the detection of multiple periodicities, 12 for PG 1541 + 651, and six for BPM 31594, which enables us to perform a detailed asteroseismological study. For both objects, we found a representative asteroseismic model with canonical stellar mass ∼0.61M⊙ and thick hydrogen envelopes, thicker than 10−5.3M*. The detection of triplets in the Fourier transform also allowed us to estimate mean rotation periods, being ∼22 h for PG 1541 + 651 and 11.6 h for BPM 31594, which is consistent with a range of values reported for other ZZ Ceti stars.

Simultaneous J-, H-, K- and L-band spectroscopic observations of galactic Be stars

Context. It is already accepted that Be stars are surrounded by circumstellar envelopes, which are mostly compatible with a disc geometry in Keplerian rotation. Their infrared region is characterised by a moderate flux excess and the presence of hydrogen recombination lines that allow us to obtain information on the physical and dynamical structure of different regions inside the disc. Nevertheless, many of the infrared studies available in the literature show low-resolution spectra, or are restricted to a small object sample or describe just an individual object. Some others analyse reduced spectral ranges or just one infrared band. Aims. We aim to obtain a more complete characterisation of the properties of the circumstellar environment of Be stars that helps to constrain the theoretical models of the Be phenomenon. Methods. Throughout the last decade, we used the spectroscopic facilities at the Gemini and Las Campanas observatories to obtain quasi-simultaneous spectra in the J, H, K, and L bands of a sample of Be stars with medium resolution. Results. We present near-infrared, medium-resolution spectra of a sample of galactic Be stars with different spectral subtypes and luminosity classes. We measure different parameters of the hydrogen recombination lines from the Paschen, Brackett, Pfund, and Humphreys series, and use them to diagnose physical conditions in the circumstellar environment. We analysed the equivalent-width (EW) ratio between Brα and Brγ lines and different diagrams of flux ratios. We also identify lines from He i, C i, N i, O i, Na i, Mg i, Mg ii, Si i, Fe i, and Fe ii. Analysing the EW measurements of particular He i, Mg ii, Fe i, Fe ii and O i lines, we find that for some lines they correlate with the spectral type of the star. Particularly, the emission of the O i λ 1.3168 μm line decreases towards the later spectral types. Conclusions. We present an atlas of 22 Be stars, that covers a wide infrared (IR) spectral range with quasi-simultaneous observations. From a detailed analysis, we define new complementary criteria to Mennickent’s classification of Be stars according to their disc opacity. Some objects in our sample present compact thick envelopes, while in others the envelope is extended and optically thin. The correlation between the full widths at half maximum and the peak separation (ΔV) versus V sin i for the Br10, Brδ, and Hu14 lines reveals that the broadening mechanism is rotational. The Lyβ fluorescence is a key mechanism to explain the intensity of the emission of Mg ii and O i lines.