Atmospheres Of Stars Research Articles

Observations of Her X-1 in low states during SRG/eROSITA all-sky survey

eROSITA (extended ROentgen Survey with an Imaging Telescope Array) instrument onboard the Russian-German ‘Spectrum-Roentgen-Gamma’ (SRG) mission observed the Her X-1/HZ Her binary system in multiple scans over the source during the first and second SRG all-sky surveys. Both observations occurred during a low state of the X-ray source when the outer parts of the accretion disk blocked the neutron star from view. The orbital modulation of the X-ray flux was detected during the low states. We argue that the detected X-ray radiation results from scattering of the emission of the central source by three distinct regions: (a) an optically thin hot corona with temperature ~(2−4) × 106 K above the irradiated hemisphere of the optical star; (b) an optically thin hot halo above the accretion disk; and (c) the optically thick cold atmosphere of the optical star. The latter region effectively scatters photons with energies above 5–6 keV.

Possible Periodic Dips in the Pulsating Ultraluminous X-Ray Source M51 ULX-7

Abstract We report the discovery of possible periodic X-ray dips in a pulsating ultraluminous X-ray source, M51 ULX-7, with archival Chandra observations. With ∼20 days of monitoring in the superorbital descending state, we discovered three dips with separations of ∼2 and ∼8 days via the Bayesian block technique. A phase-dispersion minimization and a test suggest that the dip is likely recurrent with a period of ∼2 days, consistent with the orbital period of M51 ULX-7. We interpret the dip as an obscuring of the emission from the pulsar by the vertical structure on the stream–disk interaction region or the atmosphere of the companion star. Both interpretations suggest the viewing angle to be ∼60°. Given that the magnetic field of M51 ULX-7 is moderately high, B ∼ 1013 G, a low geometric beaming with is sufficient to explain the observed flux and the presence of dips. Obscuration of the stellar wind remains an alternative possible origin and further monitoring of the dips will be required.

The Abundance of Phosphorus in the HgMn Star 14 Sge

Abstract The abundances of several elements in the atmosphere of the HgMn star 14 Sge (HR 7664) are derived from the analysis of high-resolution spectra obtained recently with the spectropolarimeter NeoNarval. A large phosphorus overabundance, about 60 times the solar abundance, is found. This study is the beginning of a systematic survey of all northern HgMn stars aiming at deriving their abundances in a consistent manner.

Horizontal spreading of planetary debris accreted by white dwarfs

ABSTRACT White dwarfs with metal-polluted atmospheres have been studied widely in the context of the accretion of rocky debris from evolved planetary systems. One open question is the geometry of accretion and how material arrives and mixes in the white dwarf surface layers. Using the three-dimensional (3D) radiation hydrodynamics code co5bold, we present the first transport coefficients in degenerate star atmospheres that describe the advection–diffusion of a passive scalar across the surface plane. We couple newly derived horizontal diffusion coefficients with previously published vertical diffusion coefficients to provide theoretical constraints on surface spreading of metals in white dwarfs. Our grid of 3D simulations probes the vast majority of the parameter space of convective white dwarfs, with pure-hydrogen atmospheres in the effective temperature range of 6000–18 000 K and pure-helium atmospheres in the range of 12 000–34 000 K. Our results suggest that warm hydrogen-rich atmospheres (DA; ${\gtrsim} 13\, 000$ K) and helium-rich atmospheres (DB and DBA; ${\gtrsim} 30\, 000$ K) are unable to efficiently spread the accreted metals across their surface, regardless of the time dependence of accretion. This result may be at odds with the current non-detection of surface abundance variations in white dwarfs with debris discs. For cooler hydrogen- and helium-rich atmospheres, we predict a largely homogeneous distribution of metals across the surface within a vertical diffusion time-scale. This is typically less than 0.1 per cent of disc lifetime estimates, a quantity that is revisited in this paper using the overshoot results. These results have relevance for studies of the bulk composition of evolved planetary systems and models of accretion disc physics.

DFT study of \u201cunlayered graphene solid\u201d formation, in liquid carbon droplets at low pressures

This atomistic modeling study is companion to new experimental work on carbon vapor slow-cooled in (a) the laboratory and (b) the atmosphere of ancient carbon-synthesizing stars. It specifically follows up on TEM clues about the nucleation of unlayered graphene sheets in a solidifying carbon liquid, to show that 5 atom loops may help explain evidence for faceted pentacones in a slow-cooled melt. This is also first in a series of modeling studies that may open the door to laboratory studies of (i) condensation in cool-giant star atmospheres and (ii) liquid carbon at low pressures.Graphic abstract

Effects of anisotropy on absorption cross-section spectra of medium-sized spheroidal corundum particles

Context. It has been widely accepted that corundum particles condense in the atmospheres of oxygen-rich asymptotic giant branch stars and effectively produce an infrared emission feature at 13 μm. Laboratory experiments have predicted that these particles have the shape of oblate spheroids. Aims. We investigate the influence of the material anisotropy of uniaxial corundum on absorption cross section spectra of medium sized spheroidal particles in the infrared spectral region. Methods. We compared absorption cross-section spectra of the anisotropic corundum particles gained by finite-difference time-domain simulations to spectra calculated by a weighted sum approximation of the according fictive isotropic materials, with one material having the dielectric function of the a–b-plane and the other having the dielectric function of the c-axis of corundum. We carried out investigations for different axes ratios of the spheroids, particles volumes, and different geometries of the dielectric axes to the particle axes as well as to the polarization and propagation direction of the incident light. Results. We observed several effects attributed to anisotropy that are non-additive, so that they cannot be reproduced with the combined spectra of the isotropic materials. Conclusions. Care should be taken when calculating the corundum infrared spectrum with simpler approaches. When particle sizes above 1 μm are to be considered, the T-matrix formalism delivers correct band shifts and bulk modes for many, but not all bands. This remains true in orientation-averaged spectra and for particles in the 0.1 μm size range.

Determined the Effective Temperature and Gravity Acceleration at the Atmosphere of (HD165908) Star

The study of the chemical composition of stars in our Galaxy showed that it changes in a non-random way from a star to a star: older stars and halo stars have a metal deficit compared to young stars and disc stars. However, this is not always the case. Sometimes old stars have a solar chemical composition, and young stars show a deficit of metals. It is believed that the formed stars has a chemical composition of the interstellar substance from which it was formed; therefore, the initial content of elements in its atmosphere depends on the time and place of birth. Within the framework of this work, echelle spectra of star 99Her of spectral classes A-F has been proceed, obtained with a 1.5-meter telescope PTT-150 (Russian Turkish Telescope-150). As a result of processing the spectra, the equivalent widths of unbend and weakly blended absorption lines were measured, 350 lines this star. The measured equivalent line widths can be used for other studies of the atmospheres of the A-F stars composition. Stars with temperatures of 8000-9000 K show a significant excess of elements heavier than iron and some deficiency of light elements. These results, apparently, are a consequence of the effects of light pressure and gravitational diffusion of atoms and ions in atmospheres of calm and radiative atmospheres of A-stars.

Measuring stellar atmosphere parameters using follow-up polarimetric microlensing observations

ABSTRACT We present an analysis of potential follow-up polarimetric microlensing observations to study the stellar atmospheres of distant stars. First, we produce synthetic microlensing events using the Galactic model, stellar population and interstellar dust toward the Galactic bulge. We simulate the polarization microlensing light curves and pass them through the instrument specifications of the FOcal Reducer and low dispersion Spectrograph (FORS2) polarimeter at the Very Large Telescope (VLT), and then analyse them. We find that the accuracy of the VLT allows us to constrain the atmospheres of cool red giant branch (RGB) stars. Assuming that about 3000 microlensing events are detected per year by the Optical Gravitational Lensing Experiment (OGLE) telescope, we expect to detect almost 20, 10, 8 and 5 polarization microlensing events for the following four criteria: being three consecutive polarimetric data points above the baseline with 1σ, 2σ, 3σ and 4σ, respectively, in the polarimetric light curves. We generalize the covariance matrix formulation and present the combination of polarimetric and icinformation that leads us to measure the scattering optical depth of the atmosphere and the inner radius of the stellar envelope of RGB stars. These two parameters could determine the dust opacity of the atmosphere of cool RGB source stars and the radius where dust can be formed.

Texas Petawatt laser helps create laboratory conditions like those in a white dwarf star atmosphere

An experimental and simulation platform to study warm dense carbon foams achieves temperatures and densities in the laboratory like those found in the atmosphere and envelope of white dwarf stars with carbon lines (DQ dwarf stars)

MATISSE first pictures of dust and molecules around R Sculptoris

AbstractCarbon-rich dust is known to form in the atmosphere of the semiregular variable star R Sculptoris. Such stardust, as well as the molecules and gas produced during the lifetime of the star, will be spread into the Galaxy via the mass-loss process. Probing this process is crucial to understand the chemical enrichment of the Galaxy. R Scl was observed using the ESO/VLTI MATISSE instrument in December 2018. Here we show the first images of the star between 3 and 10 R*. Using the complementary MIRA 3D image reconstruction and the RHAPSODY 1D intensity profile reconstruction code, we reveal the location of molecules and dust in the close environment of the star. Indeed, the C2H2 and HCN molecules are spatially located between 1 and 3.4 R* which is much closer to the star than the location of the dust. The R Scl spectrum is fitted by molecules and a dust mixture of 90% of amorphous carbon and 10% of silicone carbide. The inner boundary of the dust envelope is estimated by DUSTY at about 4.6 R*. We derive a mass-loss rate of 1.2 ± 0.4 × 10−6M⊙ yr−1however no clear SiC forming region has been detected in the MATISSE data.

Observations of GRO J1744–28 in quiescence with XMM-Newton

We report on the deep observations of the “bursting pulsar” GRO J1744–28, which were performed with XMM-Newton and aimed to clarify the origin of its X-ray emission in quiescence. We detect the source at a luminosity level of ∼1034 erg s−1 with an X-ray spectrum that is consistent with the power law, blackbody, or accretion-heated neutron star atmosphere models. The improved X-ray localization of the source allowed us to confirm the previously identified candidate optical counterpart as a relatively massive G/K III star at 8 kpc close to the Galactic center, implying an almost face-on view of the binary system. Although we could only find a nonrestricting upper limit on the pulsed fraction of ∼20%, the observed hard X-ray spectrum and strong long-term variability of the X-ray flux suggest that the source is also still accreting when not in outburst. The luminosity corresponding to the onset of centrifugal inhibition of accretion is thus estimated to be at least two orders of magnitude lower than previously reported. We discuss this finding in the context of previous studies and argue that the results indicate a multipole structure in the magnetic field with the first dipole term of ∼1010 G, which is much lower than previously assumed.

Evidence for localized onset of episodic mass loss in Mira

Context. Mass loss from long-period variable stars (LPV) is an important contributor to the evolution of galactic abundances. Dust formation is understood to play an essential role in mass loss. It has, however, proven difficult to develop measurements that strongly constrain the location and timing of dust nucleation and acceleration. Aims. Interferometric imaging has the potential to constrain the geometry and dynamics of mass loss. High angular resolution studies of various types have shown that LPVs have a distinct core-halo structure. These have also shown that LPV images commonly exhibit a non-circular shape. The nature of this shape and its implications are yet to be understood. Methods. Multi-telescope interferometric measurements taken with the Interferometric Optical Telescope Array (IOTA) provide imagery of the LPV Mira in the H-band. This wavelength region is well suited to studying mass loss given the low continuum opacity, which allows for emission to be observed over a very long path in the stellar atmosphere and envelope. Results. The observed visibilities are consistent with a simple core-halo model to represent the central object and the extended molecular layers but, in addition, they demonstrate a substantial asymmetry. An analysis with image reconstruction software shows that the asymmetry is consistent with a localized absorbing patch. The observed opacity is tentatively associated with small dust grains, which will grow substantially during a multi-year ejection process. Spatial information along with a deduced dust content of the cloud, known mass loss rates, and ejection velocities provide evidence for the pulsational pumping of the extended molecular layers. The cloud may be understood as a spatially local zone of enhanced dust formation, very near to the pulsating halo. The observed mass loss could be provided by several such active regions around the star. Conclusions. This result provides an additional clue for better understanding the clumpiness of dust production in the atmosphere of AGB stars. It is compatible with scenarios where the combination of pulsation and convection play a key role in the process of mass loss.

Tomography of cool giant and supergiant star atmospheres

Context. Asymptotic giant branch (AGB) stars are characterized by substantial mass loss, however the mechanism behind it not yet fully understood. The knowledge of the structure and dynamics of AGB-star atmospheres is crucial to better understanding the mass loss. The recently established tomographic method, which relies on the design of spectral masks containing lines that form in given ranges of optical depths in the stellar atmosphere, is an ideal technique for this purpose. Aims. We aim to validate the capability of the tomographic method in probing different geometrical depths in the stellar atmosphere and recovering the relation between optical and geometrical depth scales. Methods. We applied the tomographic method to high-resolution spectro-interferometric VLTI/AMBER observations of the Mira-type AGB star S Ori. The interferometric visibilities were extracted at wavelengths contributing to the tomographic masks and fitted to those computed from a uniform disk model. This allows us to measure the geometrical extent of the atmospheric layer probed by the corresponding mask. We then compared the observed atmospheric extension with others measured from available 1D pulsation CODEX models and 3D radiative-hydrodynamics CO5BOLD simulations. Results. While the average optical depths probed by the tomographic masks in S Ori decrease (with ⟨log τ0⟩ = −0.45, − 1.45, and − 2.45 from the innermost to the central and outermost layers), the angular diameters of these layers increase, from 10.59 ± 0.09 mas through 11.84 ± 0.17 mas, up to 14.08 ± 0.15 mas. A similar behavior is observed when the tomographic method is applied to 1D and 3D dynamical models. Conclusions. This study derives, for the first time, a quantitative relation between optical and geometrical depth scales when applied to the Mira star S Ori, or to 1D and 3D dynamical models. In the context of Mira-type stars, knowledge of the link between the optical and geometrical depths opens the way to deriving the shock-wave propagation velocity, which cannot be directly observed in these stars.

Influence of Collisions with Hydrogen Atoms on Non-LTE Effects for K I and Ca II in Stellar Atmospheres

We have constructed a new K I model atom using currently available atomic data. We have performed calculations for K I by abandoning the assumption of local thermodynamic equilibrium (non-LTE) for the Sun and three moderately metal-poor dwarf stars. To take into account the inelastic processes in collisions with hydrogen atoms, for the first time we have used the rate constants calculated by including the fine structure of K I levels and analyzed the influence of their application on the non-LTE results compared to the use of the rate constants calculated for combined levels. In agreement with the non-LTE studies available in the literature, K I is subject to overrecombination, which leads to a strengthening of spectral lines and negative abundance corrections. The non-LTE effects are shown to weaken when using new collisional data. We reached the same conclusion when comparing the non-LTE corrections calculated for the Ca II 8662 A line in model atmospheres with $$\textrm{[Fe/H]}={-}4.5$$ using the Ca II $$+$$ H I collision rates derived with and without allowance for the fine structure of Ca II levels. However, the effect is very small for the other two triplet lines, Ca II 8498 and 8542 A. The solar non-LTE abundance $$\log\varepsilon_{\textrm{K}}=5.09\pm 0.08$$ derived from five lines is consistent with the meteoritic one within 0.01 dex. Despite the fact that in the atmospheres of the program stars the departures from LTE are larger than those in the Sun, the differential abundance [K/H] is almost independent of which collisional data set is used: the one calculated with or without allowance for the fine structure of K I levels.

Stark Broadening of Co II Lines in Stellar Atmospheres

Data for Stark full widths at half maximum for 46 Co II multiplets were calculated using a modified semiempirical method. In order to show the applicability and usefulness of this set of data for research into white dwarf and A type star atmospheres, the obtained results were used to investigate the significance of the Stark broadening mechanism for Co II lines in the atmospheres of these objects. We examined the influence of surface gravity (log g), effective temperature and the wavelength of the spectral line on the importance of the inclusion of Stark broadening contribution in the profiles of the considered Co II spectral lines, for plasma conditions in atmospheric layers corresponding to different optical depths.

Strengthening the bounds on the r-mode amplitude with X-ray observations of millisecond pulsars

ABSTRACT R-mode oscillations have been shown to have a significant potential to constrain the composition of fast spinning neutron stars. Due to their high rotation rates, millisecond pulsars (MSPs) provide a unique platform to constrain the properties of such oscillations, if their surface temperatures can be inferred. We present the results of our investigations of archival X-ray data of a number of MSPs, as well as recent XMM–Newton observations of PSR J1810+1744 and PSR J2241−5236. Using the neutron star atmosphere model and taking into account various uncertainties, we present new bounds on the surface temperature of these sources. Thereby, we significantly strengthen previous bounds on the amplitude of the r-mode oscillations in MSPs and find rigorous values as low as α ≲ 3 × 10−9. This is by now about three orders of magnitude below what standard saturation mechanisms in neutron stars could provide, which requires very strong dissipation in the interior, strongly pointing towards a structurally complex or exotic composition of these sources. At such low temperatures, sources could even be outside of the instability region, and taking into account the various uncertainties, we obtain for an observed surface temperature a simple frequency bound below which r-modes are excluded in slower spinning pulsars.

Stark broadening of Co ii spectral lines in hot stars and white dwarf spectra

ABSTRACT Stark full widths at half-maximum for 46 Co ii multiplets have been calculated using modified semi-empirical method. The obtained results have been used to demonstrate the importance of Stark broadening mechanism in DA and DB white dwarf and A star atmospheres. With the obtained results we also test possibility of using some approximate methods of Stark width calculations developed on the basis of regularities and systematic trends.

Analysis of Integrated-Light Spectra of Galactic Globular Clusters

We present the results of determination of the age, helium mass fraction (Y) in terms of the used stellar evolutionary models, metallicity ([Fe/H]), and abundances of the elements C, O, Na, Mg, Ca, Ti, Cr and Mn for 26 globular clusters of the Galaxy. In this work, we apply a method developed by us that employs medium-resolution integrated-light spectra of globular clusters and models of stellar atmospheres and it is supplemented in this paper by the automatic calculation of microturbulence velocities of stars in the studied objects. Based on the data obtained for 26 objects, as well as on the results of our previous studies, it is shown that the abundances of chemical elements, that we measured, with the exception of carbon, are consistent with the literature estimates from the analysis of integrated-light spectra of clusters and from high-resolution spectroscopic observations of their brightest stars. Our estimates of [C/Fe] are consistent with the literature values obtained from the integrated-light spectra of clusters. We interpret the systematic difference between the derived [C/Fe] for globular clusters and the literature [C/Fe] values for the brightest stars of the clusters as a change of the chemical composition in the atmospheres of stars during their evolution. The estimated absolute ages and average Y for the clusters are in a reasonable agreement with the literature data from the analysis of color magnitude diagrams of the objects.

Opacity modelling of heavy-metal hot subdwarfs: photoionization of Sr0, Y+, and Zr2 +

ABSTRACT Heavy-metal hot subdwarfs (sdB and sdO) represent a small group of stars with unusually high concentrations of trans-iron elements in their atmospheres, having abundances ${\sim } 10\, 000$ times solar. One example is LS IV-14 116, where a number of heavy-metal absorption lines of Sr ii, Y iii, and Zr iv have been observed in the optical band $4000\!-\!5000\, {\mathring{\rm A}}$. We use a fully relativistic Dirac Atomic R-Matrix Code (darc) to calculate photoionization cross-sections of Sr0, Y+, and Zr2 + from their ground state to the 20th excited level. We use the cross-sections and the oscillator strengths to simulate the spectrum of a hot subdwarf. We obtain complete sets of photoionization cross-sections for the three ions under study. We use these data to calculate the opacity of the stellar atmospheres of hot subdwarf stars, and show that for overabundances observed in some heavy-metal subdwarves, photoexcitation from zirconium, in particular, does contribute some back warming in the model.

Neon Abundances of B Stars in the Solar Neighborhood

Abstract We constructed a comprehensive model atom for Ne i–Ne ii using the most-up-to-date atomic data available and evaluated the nonlocal thermodynamic equilibrium (NLTE) line formation for Ne i and Ne ii in classical 1D models representing the atmospheres of B-type stars. We find that the large NLTE strengthening of the Ne i lines corresponding to the 2p53p–2p53s transition array occurs due to extremely small photoionization cross sections of the 2p53s levels that lead to strong overpopulation of these levels relative to their LTE populations. The deviations from LTE for most Ne ii lines are small and do not exceed 0.11 dex in absolute value. We analyzed 20 lines of Ne i and 13 lines of Ne ii for 24 B-type stars in the temperature range of 10,400 ≤ ≤ 33,400 K. For five stars, the NLTE leads to consistent abundances of Ne i and Ne ii, while the difference in LTE abundance can reach up to 0.50 dex. Using the experimental oscillator strengths recently measured by Piracha et al. leads to smaller line-by-line scatter for most of the investigated stars. The average neon abundance in 24 B-type stars in the solar neighborhood is 8.02 ± 0.05. This value may provide indirect constraints on the solar photospheric neon abundance.