The atomic physics underlying the spectroscopic analysis of massive stars and supernovae

We have developed a radiative transfer code, cmfgen, which allows us to model the spectra of massive stars and supernovae. Using cmfgen we can derive fundamental parameters such as effective temperatures and surface gravities, derive abundances, and place constraints on stellar wind properties. The last of these is important since all massive stars are losing mass via a stellar wind that is driven from the star by radiation pressure, and this mass loss can substantially influence the spectral appearance and evolution of the star. Recently we have extended cmfgen to allow us to undertake time-dependent radiative transfer calculations of supernovae. Such calculations will be used to place constraints on the supernova progenitor, to place constraints on the supernova explosion and nucleosynthesis, and to derive distances using a physical approach called the “Expanding Photosphere Method”. We describe the assumptions underlying the code and the atomic processes involved. A crucial ingredient in the code is the atomic data. For the modeling we require accurate transition wavelengths, oscillator strengths, photoionization cross-sections, collision strengths, autoionization rates, and charge exchange rates for virtually all species up to, and including, cobalt. Presently, the available atomic data varies substantially in both quantity and quality.     

A survey of 10-μm silicate emission from dust around young sun-like stars

We obtained low resolution (R=100) mid-infrared (8–13 μm wavelengths) spectra of 8 nearby young main sequence stars with the Keck 1 telescope and Long-Wavelength Spectrometer (LWS) to search for 10 μm silicate (Si–O stretch) emission from circumstellar dust. No stars exhibited readily apparent emission: Spectra were then analyzed by least-squares fitting of a template based on a spectrum of Comet Hale-Bopp. Using this technique, we were able to constrain the level of silicate emission to a threshold 10 times below what was previously possible from space. We found one star, HD 17925, with a spectrum statistically different from its calibrator and consistent with a silicate emission peak of 7% of the photosphere at a wavelength of 10 μm. Excess emission at 60 μm from this star has already been reported.     

Mass—luminosity relation and pulsational properties of Wolf—Rayet stars

The evolution of Population I stars with initial masses 70M _⊙ ≤ M _ZAMS ≤ 130M _⊙ is considered. The computations were performed under various assumptions about the mass loss rate and were terminated at the phase of gravitational contraction after core helium exhaustion. The mass loss rate at the helium burning phase, ?_3α , is shown to be the main parameter that determines the coefficients of the mass—luminosity relation for Wolf—Rayet stars. Several more accurate mass—luminosity relations for mass loss rates ? = f _3α ?_3α , where 0.5 ≤ f _3α ≤ 3, are suggested, along with the mass—luminosity relation that combines all of the evolutionary sequences considered. The results of the stellar evolution computations were used as initial conditions in solving the hydrodynamic equations describing the spherically symmetric motions of a self-gravitating gas. The outer layers of massive Population I stars are unstable against radial oscillations throughout the helium burning phase. The oscillation amplitude is largest at enhanced carbon and oxygen abundances in the outer stellar layers, i.e., at a lower initial stellar mass M _ZAMS or a lower mass loss rate during the entire preceding evolution. In the course of evolution, the radial oscillation amplitude decreases and the small nonlinearity of the oscillations at M < 10M _⊙ allow the integral of mechanical work W done by an elementary spherical layer of gas in a closed thermodynamic cycle to be calculated with the necessary accuracy. The maximum of the radial dependence of W is shown to be located in layers with a gas temperature T ～ 2 × 10⁵ K, where the oscillations are excited by the iron Z-bump κ-mechanism. Comparison of the radial dependences of the integral of mechanical work W and the amplitude of the radiative flux variations suggests that the nonlinear radial oscillations of more massive Wolf—Rayet stars are also excited by the κ-mechanism.     

Long-term amplitude and period variations ofδ Scuti stars: A sign of chaos?

On short time-scales of under a year, the vast majority of Scuti stars studied in detail show completely regular multiperiodic pulsation. Nonradial pulsation is characterized by the excitation of a large number of modes with small amplitudes. Reports of short-term irregularity or nonperiodicity in the literature need to be examined carefully, since insufficient observational data can lead to an incorrect impression of irregularity. Some interesting cases of reported irregularities are examined.A few Scuti stars, such as 21 Mon, have shown stable variations with sudden mode switching to a new frequency spectrum. This situation might be an indication of deterministic chaos. However, the observational evidence for mode switching is still weak.One the other hand, the case for the existence of long-term amplitude and period changes is becoming quite convincing. Recently found examples of nonradial pulsators with long-term changes are 4 CVn, 44 Tau, Peg and HD 2724. (We note that other Scuti pulsators such as X Cae and ² Tau, have shown no evidence for amplitude variations over the years.) Neither the amplitude nor the period changes are periodic, although irregular cycles with time scales between a few and twenty years can be seen. While the amplitude changes can be very large, the period changes are quite small. This property is common in nonlinear systems which lead to chaotic behavior. There exists observational evidence for relatively sudden period jumps changing the period by about 10^–5 and/or slow period changes near dP/dt 10^–9. These period changes are an order of magnitude larger than those expected from stellar evolution.The nonperiodic long-term changes are interpreted in terms of resonances between different nonradial modes. It is shown that a large number of the nonradial acoustic modes can be in resonance with other modes once the mode interaction terms, different radial orders and rotationalm-mode splitting are considered. These resonances are illustrated numerically by the use of pulsation model. Observational evidence is presented that these interaction modes exist in the low-frequency domain.     

The properties of a large sample of OH/IR stars,S stars and C-rich AGB stars

We collected infrared and radio data on 110 OH/IR stars, 65 S stars and 184 C-rich stars from the literature. We analyzed their spectral energy distributions, bolometric magnitudes, infrared colors and mass-loss rates. Our study confirms that OH/IR stars and C-rich stars reach similarly high mass-loss rates at similar luminosities, supporting the idea that mass-loss rates are determined by internal stellar activities such as pulsation rather than the properties of their envelopes. The mass-loss rates of OH/IR stars and C-rich stars are strongly correlated with the color indices K-[A], K-[C], K-[D] and K-[E], and the two populations can be described with one formula. Our study also reveals that only those stars with color indices K-[A]<4 or K-[C]<5 have SEDs that peak in the near-infrared waveband.     

On the signatures of companion formation in the spectral energy distributions of Sz54 and Sz59—the young stars with protoplanetary disks

We study spectral energy distributions of two young systems Sz54 and Sz59, that belong to Chameleon II star forming region. The results of the modeling indicate that protoplanetary disks of these systems contain gaps in the dust component. These gaps could be a result of a planetary or brown dwarf companion formation, because the companion would accumulate a disk material, moving along its orbit. In a present work we have determined physical characteristics of the disks. We also discuss possible companion characteristics, based on the geometrical parameters of the gaps.     

Effect of Coriolis force on the shapes of rotating stars and stars in binary systems

In this paper an attempt has been made to determine the effect of Coriolis force on the shapes of Roche equipotential surfaces of rotating stars and stars in binary systems. Equations of Roche equipotential surfaces have been obtained for rotating and binary stars which take into account the effects of Coriolis force besides the centrifugal and gravitational forces. Shapes of Roche equipotentials and values of Roche limits are obtained for different values of angular velocity of rotation for rotating stars and for different values of mass ratios for the binary stars. The obtained results have been compared with the corresponding results in which the effect of Coriolis force has not been considered.     

Magnetic cycles of Sun-like stars with different levels of coronal and chromospheric activity——comparison with the Sun

The atmospheric activity of the Sun and Sun-like stars is analyzed involving observations from the HK-project at the Mount Wilson Observatory, the California and Carnegie Planet Search Program at the Keck and Lick Observatories and the Magellan Planet Search Program at the Las Campanas Observatory.We show that for stars of F, G and K spectral classes, the cyclic activity, similar to the 11-yr solar cycle, is different: it becomes more prominent in K-stars. Comparative study of Sun-like stars with different levels of chromospheric and coronal activity confirms that the Sun belongs to stars with a low level of chromospheric activity and stands apart among these stars by its minimum level of coronal radiation and minimum level of variations in photospheric flux.     

A catalog of early-type emission-line stars and Hα line profiles from LAMOST DR2

We present a catalog including 11 204 spectra of 10 436 early-type emission-line stars from LAMOST DR2, among which 9752 early-type emission-line spectra are newly discovered. For these earlytype emission-line stars, we discuss the morphological and physical properties of their low-resolution spectra. In this spectral sample, the Hα emission profiles display a wide variety of shapes. Based on the Hα line profiles, these spectra are categorized into five distinct classes: single-peak emission, single-peak emission in absorption, double-peak emission, double-peak emission in absorption, and P-Cygni profiles. To better understand what causes the Hα line profiles, we divide these objects into four types from the perspective of physical classification, which include classical Be stars, Herbig Ae/Be stars, close binaries and spectra contaminated by H II regions. The majority of Herbig Ae/Be stars and classical Be stars are identified and separated using a(H-K, K-W1) color-color diagram. We also discuss 31 binary systems that are listed in the SIMBAD on-line catalog and identify 3600 spectra contaminated by H II regions after cross-matching with positions in the Dubout-Crillon catalog. A statistical analysis of line profiles versus classifications is then conducted in order to understand the distribution of Hα profiles for each type in our sample. Finally, we also provide a table of 172 spectra with Fe II emission lines and roughly calculate stellar wind velocities for seven spectra with P-Cygni profiles.     

Irradiation phenomena in young solar-type stars and the early solar system: X-ray observations and γ-ray constraints

The high levels of X-ray flaring activity observed in young stars and protostars suggest correspondingly high levels of low-energy particle irradiation of their circumstellar disks, and hence of early solar system material like meteorites. We first briefly review the latest X-ray observational results obtained by Chandra and XMM-Newton on two ‘typical’ star-forming regions, Orion and ρ Ophiuchi. We then discuss a new ‘accretion–ejection–irradiation–transport’ model for young stars which, when scaled to the X-ray fluxes, accounts simultaneously for four extinct radioactivity ratios, in particular the purely spallogenic ¹⁰Be/⁹Be ratio, and the ²⁶Al/²⁷Al ratio. We point out the importance of the environment in which nearby star formation is taking place today, namely the Gould Belt, and the possible connection between Comptel detections of ²⁶Al γ-ray line emission from these regions, and new constraints on the origin of the solar system.     

Metal abundances of subdwarf B stars—the extended sample

The formation of sdBs as well as the chemical composition of their atmospheres is still puzzling. While helium and other light elements are depleted relative to solar values, heavy elements are highly enriched. Diffusion processes in the hot, radiative atmosphere of these stars are the most likely explanation. Although several attempts have been made, it has not yet been possible to model all the observed features of sdB atmospheres. A drawback of most prior studies was the small sample size. We present a detailed abundance analysis of 139 sdBs. A general trend of enrichment was found with increasing temperature for most of the heavier elements. The lighter elements like carbon, oxygen and nitrogen are depleted and less affected by temperature. Although there is considerable scatter from one star to the other, the general abundance patterns in most sdBs are similar. An interplay between gravitational settling, radiative levitation and weak winds is most likely responsible. About 3% of the analysed stars show an enrichment in carbon and helium, which cannot be explained in the framework of diffusion alone. Nuclear processed material must have been transported to the surface somehow.     

Scanner observations of shell stars ϕ Persei and ψ Persei

Spectrophotometric energy distribution data of the shell stars Per and Per are reported in the wavelength range 3200–8000 Å. The effective temperature of these stars is derived by comparison of observed energy distribution with theoretical models. Both the stars show higher values of derived effective temperature. The continuum energy distribution in the ultraviolet region has also be discussed.     

Merger Dynamics of the Pair of Galaxy Clusters——A399 and A401

Convincing evidence for a past interaction between the two rich clusters A399 and A401 was recently found in the X-ray imaging observations. We examine the structure and dynamics of this pair of galaxy clusters. A mixture-modeling algorithm was applied to obtain a robust partition into two clusters, which allowed us to discuss the virial mass and velocity distribution of each cluster. Assuming that these two clusters follow a linear orbit and they have once experienced a close encounter, we model the binary cluster as a two-body system. As a result, four gravitationally bound solutions are obtained. The recent X-ray observations seem to favor a scenario in which the two clusters with a true separation of 5.4h-1 Mpc are currently expanding at 583 km s-1 along a direction with a projection angle of 67.5°, and they will reach a maximum extent of 5.65 h-1 Mpc in about 1.0 h-1 Gyr.     

Kinematical and orbital properties for selected southern high — Velocity stars

Using the model of the Galaxy presented by Eggen, Lynden-Bell and Sandage (1962), plane galactic orbits have been calculated for several southern high-velocity stars which possess parallax, proper motion, and radial velocity data. Extensive lists of both raw and computed data for these stars are included. Published values ofU-B andB-V for some of these stars were used in plots of each of the orbital parameters versusU-B, B-V, or the ultraviolet excess (U-B). Also, a comparison is made between the H-R diagrams for the southern high-velocity star group and that of M3, a globular cluster, and again for M67, an old open cluster. The high-velocity star group is found to resemble an old open cluster more than a globular cluster.     

Asteroseismology and the degeneracy of model solutions for subgiant stars

Stellar evolutionary models are essential for the determination of stellar masses and ages. Several parameters can be used to characterize the structure of subgiant stars. Amongst others we can find: the helium and metal abundances, mixing length and core overshooting. In many cases, these parameters are scaled taking into account the solar values. Yet, the universality of such scalings has been put to question. As shown here, by allowing some freedom to these parameters we end up with a degeneracy of model solutions. This puts serious limitations on the determination of mass and age. Asteroseismology may provide a valuable help to break such a degeneracy. This is illustrated with an application to the subgiant star μ Her.     

10-µm images and spectra of T Tauri stars

T Tauri stars are young stars usually surrounded by dusty disks similar to the one from which we believe our own Solar System formed. Most T Tauri stars exhibit a broad emission or absorption band between 7.5 and 13.5µm which is attributed to silicate grains in the circumstellar environment. We imaged three spatially resolved T Tauri binaries through a set of broadband filters which include the spectral region occupied by the silicate band. Two of these objects (T Tauri and Haro 6–10) are infrared companion systems in which one component is optically much fainter but contributes strongly in the infrared. Both infrared companions exhibit a deep silicate absorption which is not present in their primaries, indicating that they suffer very strong local extinction which may be due to an edge-on circumstellar disk or to a dense shell. We also took low resolution spectra of the silicate feature of two unresolved T Tauris to look for narrow features in the silicate band which would indicate the presence of specific minerals such as olivine. We observed GK Tau, for which Cohen and Witteborn (1985) reported a narrow emission feature at 9.7µm, but do not find evidence for this feature, and conclude that it is either time-dependent or an artifact of absorption by telluric ozone.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

STEREO observations of hybrid stars V775 Tau and V483 Tau

In this study, pulsational and physical characteristics of two \(\delta\) Scuti stars, V775?Tau and V483?Tau, are analysed by using four-year high-precision photometric data of the STEREO satellite. Thus, it is aimed to gain new insights into behaviours of these pulsators and evolution of \(\delta\) Scuti, \(\gamma\) Dor and Am type stars. The data are taken between 2007–2011 and examined with the help of the Lomb–Scargle method. The detection precision in the four-year combined data is around \(10^{-5}\) cd^?1 in frequency and \(10^{-5}\) mag in amplitude. It is revealed that V775?Tau exhibits weak pulsation characteristic which is interpreted as the existence of the interaction between the helium loss in the partial ionization zone and pulsation intensities. It is also considered that the absence of strong pulsations is also related to the evolution status of the star. Further, its periodogram shows low-frequency peaks. If these oscillations are g-modes, V775?Tau can be thought to be one of the rare stars that show all \(\gamma\) Dor, \(\delta\) Scuti and Am type variations. V483?Tau is comparatively more luminous, hotter and has higher rotational velocity. Therefore, although it shares the same region with V775?Tau in the H–R diagram, it is not considered to be an Am star. Yet, it exactly overlaps with the \(\gamma\) Dor stars. These clues as well as g-modes detected in its periodogram indicate that V483?Tau is a hybrid star. Finally, both V775?Tau and V483?Tau display period changes whose rates are between \(10^{-3}\) and \(10^{-4}\) yr^?1. Considering the \(\delta\) Scuti nature, it may be speculated that these changes are non-evolutionary.