Different Types Of Stars Research Articles

Tracing early stellar evolution with asteroseismology: pre-main sequence stars in NGC 2264

Asteroseismology has been proven to be a successful tool to unravel details of the internal structure for different types of stars in various stages of their main sequence and post-main sequence evolution. Recently, we found a relation between the detected pulsation properties in a sample of 34 pre-main sequence (pre-MS) delta Scuti stars and the relative phase in their pre-MS evolution. With this we are able to demonstrate that asteroseismology is similarly powerful if applied to stars in the earliest stages of evolution before the onset of hydrogen core burning.

FLUCTUATION SPECTROSCOPY: A NEW PROBE OF OLD STELLAR POPULATIONS

We introduce a new method to determine the relative contributions of different types of stars to the integrated light of nearby early-type galaxies. As is well known, the surface brightness of these galaxies shows pixel-to-pixel fluctuations due to Poisson variations in the number of giant stars. Differential spectroscopy of pixels as a function of fluctuation strength ("fluctuation spectroscopy") effectively measures the spectral variation of stars as a function of their luminosity, information that is otherwise difficult to obtain for individual stars outside of the Local Group. We apply this technique to the elliptical galaxy NGC 4472, using HST/ACS imaging in six narrow-band ramp filters tuned to spectral features in the range 0.8-1.0 micron. Pixels with +- 5% broad-band variations show differential color variations of 0.1% - 1.0% in the narrow-band filters. These variations are primarily due to the systematic increase in TiO absorption strength with increasing luminosity on the upper giant branch. The data are very well reproduced by the same Conroy & van Dokkum (2012) stellar population synthesis model that is the best fit to the integrated light, with residuals in the range 0.03% - 0.09%. Models with ages or metallicities that are significantly different from the integrated-light values do not yield good fits. We can also rule out several modifications to the underlying model, including the presence of a significant (>3% of the light) population of late M giants. The current observations constitute a powerful test of the expected luminosities and temperatures of metal-rich giants in massive early-type galaxies. Studies of pixels with much larger (negative) fluctuations will provide unique information on main sequence stars and the stellar initial mass function.

On the origin of jets from disc-accreting magnetized stars

AbstractA brief review of the origin of jets from disc-accreting rotating magnetized stars is given. In most models, the interior of the disc is characterized by a turbulent viscosity and magnetic diffusivity (‘alpha’ discs) whereas the coronal region outside the disc is treated using ideal magnetohydrodynamics (MHD). Extensive MHD simulations have established the occurrence of long-lasting outflows in the case of both slowly and rapidly rotating stars. (1) Slowly rotating stars exhibit a new type of outflow, conical winds. Conical winds are generated when stellar magnetic flux is bunched up by the inward motion of the accretion disc. Near their region of origin, the winds have a thin conical shell shape with half opening angle of ∼30∘. At large distances, their toroidal magnetic field collimates the outflow forming current carrying, matter dominated jets. These winds are predominantly magnetically and not centrifugally driven. About 10-30% of the disc matter from the inner disc is launched in the conical wind. Conical winds may be responsible for episodic as well as long lasting outflows in different types of stars. (2) Rapidly rotating stars in the ‘propeller regime’ exhibit twocomponent outflows. One component is similar to the matter dominated conical wind, where a large fraction of the disc matter may be ejected in this regime. The second component is a high-velocity, low-density magnetically dominated axial jet where matter flows along the open polar field lines of the star. The axial jet has a mass flux of about 10% that of the conical wind, but its energy flux, due to the Poynting flux, can be as large as for the conical wind. The jet’s magnetically dominated angular momentum flux causes the star to spin down rapidly. Propeller-driven outflows may be responsible for protostellar jets and their rapid spin-down.When the artificial requirement of symmetry about the equatorial plane is dropped, the conical winds are found to come alternately from one side of the disc and then the other, even for the case where the stellar magnetic field is a centered axisymmetric dipole.Recent MHD simulations of disc accretion to rotating stars in the propeller regime have been done with no turbulent viscosity and no diffusivity. The strong turbulence observed is due to the magneto-rotational instability. This turbulence drives accretion in the disc and leads to episodic conical winds and jets.

The absolute spectrophotometric catalog by Anita Cochran

The absolute spectrophotometric catalog by Anita Cochran is presented in a machine-readable form. The catalog systematizes observations acquired at the McDonald Observatory in 1977–1978. The data are compared with other sources, in particular, the calculated broadband stellar magnitudes are compared with photometric observations by other authors, to show that the observational data given in the catalog are reliable and suitable for a variety of applications. Observations of variable stars of different types make Cochran’s catalog especially valuable.

On the universal stellar law for extrasolar systems

In this work, we consider a statistical theory of gravitating spheroidal bodies to derive and develop an universal stellar law for extrasolar systems. Previously, it has been proposed the statistical theory for a cosmogonic body forming (so-called spheroidal body). The proposed theory starts from the conception for forming a spheroidal body inside a gas-dust protoplanetary nebula; it permits us to derive the form of distribution functions, mass density, gravitational potentials and strengths both for immovable and rotating spheroidal bodies as well as to find the distribution function of specific angular momentum. If we start from the conception for forming a spheroidal body as a protostar (in particular, proto-Sun) inside a prestellar (presolar) nebula then the derived distribution functions of particle as well as the mass density of an immovable spheroidal body characterize the first stage of evolution: from a prestellar molecular cloud (the presolar nebula) to a forming core or a protostar (the proto-Sun) together with its shell as a stellar nebula (the solar nebula). This paper derives the equation of state of an ideal stellar substance based on conception of gravitating spheroidal body. Using this equation we obtain the universal stellar law (USL) for the planetary systems connecting temperature, size and mass of each of stars. This work also considers the solar corona in the connection with USL. Then it is accounting under calculation of the ratio of temperature of the solar corona to effective temperature of the Sun׳ surface and modification of USL. To test justice of the modified USL for different types of stars, temperature of the stellar corona is estimated. The prediction of parameters of stars is carrying out by means of the modified USL as well as the known Hertzsprung–Russell׳s dependence is derived from USL directly. This paper also shows that knowledge of some characteristics for multi-planet extrasolar systems refines own parameters of stars. In this connection, comparison with estimations of temperatures using of the regression dependences for multi-planet extrasolar systems testifies the obtained results entirely.

Pulsating stars as stellar population tracers

AbstractPulsating stars of different types are in different evolutionary phases, thus allowing one to trace stellar components of different age in the host systems. The light variation caused by the cyclic expansion/contraction of the surface layers makes a pulsating star much easier to identify than constant stars in the same evolutionary phase. Pulsating stars thus offer a powerful tool to disentangle the various stellar generations in systems where stars of different age and metal abundance populate the same regions of the colour-magnitude diagram. An overview is presented of how pulsating stars can be used as tools to study the stellar populations, and the structure and formation process in Local Group galaxies.

Sample of Wolf-Rayet galaxies from the SLOAN digital sky survey

We have analyzed the spectra of blue compact dwarf galaxies from the SLOAN Digital Sky Survey (SDSS) Data Release 7 and created a sample of 271 galaxies with Wolf-Rayet (WR) spectral features produced by high-velocity stellar winds. A blue WR feature (bump) is a blend of the N V λλ 460.5 and 462.0 nm, N III λλ 463.4 and 464.0 nm, C III λ 465.0 nm, C IV λ 465.8 nm, and He II λ 468.6 nm emission lines. A red WR feature (bump) is the broad C IV λ 580.8 nm emission. The blue WR bump is mainly due to emissions of nitrogen WR (WN) stars, while the red bump is fully produced by emissions of carbon WR (WC) stars. All the sample spectra show the blue WR bumps, whereas the red WR bumps are only identified in 50% of sample spectra. We have derived the numbers of early-type WC stars (WCE) and late-type WN stars (WNL) in the galaxies using the luminosities of single WC and WN stars in the red and blue bumps, respectively. The number of O stars is estimated using the Hβ luminosity. The ratio of the overall number of WR stars of different types to the number of all massive stars N(WR)/N(O + WR) decreases with decreasing metallicity, corresponding to the evolution population synthesis models.

Infrared radiative transfer in atmospheres of Earth-like planets around F, G, K, and M stars

Context. The atmosphere of Earth-like extrasolar planets orbiting different types of stars is influenced by the spectral dependence of the incoming stellar radiation. The changes in structure and composition affect atmospheric radiation, hence the spectral appearance of these exoplanets.Aims. We provide a thorough investigation of infrared radiative transfer in cloud-free exoplanets atmospheres by not only analyzing the planetary spectral appearance but also discussing the radiative processes behind the spectral features in detail and identifying the regions in the atmosphere that contribute most at a given wavelength. Methods. Using cloud-free scenarios provided by a one-dimensional radiative-convective steady-state atmospheric model, we computed high-resolution infrared transmission and emission spectra, as well as weighting functions for exoplanets located within the habitable zone of F, G, K, and M stars by means of a line-by-line molecular absorption model and a Schwarzschild solver for the radiative transfer. The monochromatic spectra were convolved with appropriate spectral response functions to study the effects of finite instrument resolution.Results. Spectra of the exoplanets of F, G, K, and M stars were analyzed in the 4.5 μ m N2 O band, the 4.3 μ m and 15 μ m CO2 bands, the 7.7 μ m CH4 band, the 6.3 μ m H2 O band, and the 9.6 μ m O3 band. Differences in the state of the atmosphere of the exoplanets clearly show up in the thermal infrared spectra; absorption signatures known from Earth can be transformed to emission features (and vice versa). Weighting functions show that radiation in the absorption bands of the uniformly mixed gases (CO2 , CH4 , N2 O) and (to some extent) ozone comes from the stratosphere and upper troposphere, and also indicate that changes in the atmospheres can shift sources of thermal radiation to lower or higher altitudes. Molecular absorption and/or emission features can be identified in the high-resolution spectra of all planets and in most reduced resolution spectra. Conclusions. Insight into radiative transfer processes is essential for analyzing exoplanet spectral observations; for instance, understanding the impact of the temperature profile (nb. non-existence of an inversion) on the CO2 bands facilitates their interpretation and can help avoid false positive or negative estimates of O3 . The detailed analysis of the radiation source and sink regions could even help give an indication about the feasibility of identifying molecular signatures in cloud-covered planets, i.e. radiation mainly coming from the upper atmosphere is less likely to be hidden by clouds. Infrared radiative transfer and biomarker detectability in cloud-covered exoplanets will be presented in a companion paper.

Kappa‐effect and brightness oscillations of stars

AbstractIn this paper the theory of visibility and darkening functions for the brightness oscillations of stars is outlined. For this the non‐grey approximation is used and the effect of opacity disturbances on stellar brightness oscillations is explored for different types of stars. An explanation of the Procyon paradox is proposed. Special features of the brightness oscillations are discussed. The effect of opacity fluctuations on the damping of p‐mode oscillations is considered. Furthermore, the photospheric kappa‐mechanism is discussed (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Habitability of super-Earth planets around main-sequence stars including red giant branch evolution: models based on the integrated system approach

AbstractIn a previous study published in Astrobiology, we focused on the evolution of habitability of a 10 M⊕ super-Earth planet orbiting a star akin to the Sun. This study was based on a concept of planetary habitability in accordance with the integrated system approach that describes the photosynthetic biomass production taking into account a variety of climatological, biogeochemical and geodynamical processes. In the present study, we pursue a significant augmentation of our previous work by considering stars with zero-age main-sequence masses between 0.5 and 2.0 M⊙ with special emphasis on models of 0.8, 0.9, 1.2 and 1.5 M⊙. Our models of habitability consider geodynamical processes during the main-sequence stage of these stars as well as during their red giant branch evolution. Pertaining to the different types of stars, we identify the so-called photosynthesis-sustaining habitable zone (pHZ) determined by the limits of biological productivity on the planetary surface. We obtain various sets of solutions consistent with the principal possibility of life. Considering that stars of relatively high masses depart from the main-sequence much earlier than low-mass stars, it is found that the biospheric lifespan of super-Earth planets of stars with masses above approximately 1.5 M⊙ is always limited by the increase in stellar luminosity. However, for stars with masses below 0.9 M⊙, the lifespan of super-Earths is solely determined by the geodynamic timescale. For central star masses between 0.9 and 1.5 M⊙, the possibility of life in the framework of our models depends on the relative continental area of the super-Earth planet.

Modulated stellar and solar cycles: parallels and differences

AbstractWe present examples of activity cycle timescales on different types of stars from lowmass dwarfs to more massive giants, with wide-ranging rotation rates, and compare the observed cyclicities to the irradiance based solar cycle and its modulations. Using annual spectral solar irradiance in wavelength bands typical for stellar observations reconstructed by Shapiro et al. (2011), a direct comparison can be made between cycle timescales and amplitudes derived for the Sun and the stars. We show that cycles on multiple timescales, known to be present in solar activity, also show up on stars when the dataset is long enough to allow recognition. The cycle lengths are not fixed, but evolve – gradually during some periods but there are also changes on short timescales. In case the activity is dominated by spots, i.e., by cooler surface features, the star is redder when fainter, whereas other type of activity make the stars bluer when the activity is higher. We found the Sun to be a member of the former group, based on reconstructed spectral irradiance data by Shapiro et al. (2011).

Clouds in the atmospheres of extrasolar planets

We study the influence of low-level water and high-level ice clouds on low-resolution reflection spectra and planetary albedos of Earth-like planets orbiting different types of stars in both the visible and near infrared wavelength range. We use a one-dimensional radiative-convective steady-state atmospheric model coupled with a parametric cloud model, based on observations in the Earth's atmosphere to study the effect of both cloud types on the reflection spectra and albedos of Earth-like extrasolar planets at low resolution for various types of central stars. We find that the high scattering efficiency of clouds substantially causes both the amount of reflected light and the related depths of the absorption bands to be substantially larger than in comparison to the respective clear sky conditions. Low-level clouds have a stronger impact on the spectra than the high-level clouds because of their much larger scattering optical depth. The detectability of molecular features in near the UV - near IR wavelength range is strongly enhanced by the presence of clouds. However, the detectability of various chemical species in low-resolution reflection spectra depends strongly on the spectral energy distribution of the incident stellar radiation. In contrast to the reflection spectra the spectral planetary albedos enable molecular features to be detected without a direct influence of the spectral energy distribution of the stellar radiation. Here, clouds increase the contrast between the radiation fluxes of the planets and the respective central star by about one order of magnitude, but the resulting contrast values are still too low to be observable with the current generation of telescopes.

Characterizing Convection in Stellar Atmospheres

We perform 3D radiative hydrodynamic simulations to study the properties of convection in the superadiabatic layer of stars. The simulations show differences in both the stratification and turbulent quantities for different types of stars. We extract turbulent pressure and eddy sizes, as well as the T-τ relation for different stars and find that they are sensitive to the energy flux and gravity. We also show that contrary to what is usually assumed in the field of stellar atmospheres, the structure and gas dynamics of simulations of turbulent atmospheres cannot be parameterized with Teff and log(g) alone.

Exoplanets search and characterization with the SOPHIE spectrograph at OHP

Several programs of exoplanets search and characterization have been started with SOPHIE at the 1.93-m telescope of Haute-Provence Observatory, France. SOPHIE is an environmentally stabilized echelle spectrograph dedicated to high-precision radial velocity measurements. The objectives of these programs include systematic searches for exoplanets around different types of stars, characterizations of planet-host stars, studies of transiting planets through RossiterMcLaughlin effect, follow-up observations of photometric surveys. The instrument SOPHIE and a review of its latest results are presented here.

Asteroseismic observations of OB stars

AbstractThe region of the hot end of the main-sequence is hosting pulsating stars of different types and flavours. Pulsations are not only observed for Slowly pulsating B stars (mid to late B-type stars; high order g-modes) and β Cephei stars (early B-type stars; low order p/g-modes) but are also causing variability in Be stars and OB-supergiants. In this review we give an overview of the asteroseismic observations that are currently available for these types of stars. The first asteroseismic results were solely based on ground-based observations. Recently, the arrival of space-based data gathered by space missions like most, corot and kepler has led to important discoveries for massive stars, highlighting their excellent asteroseismic potential. We show that, despite the unprecedented precision of the space-based data, there is still a clear need for ground-based follow-up observations.

The UBV photometry of variable stars from absolute spectrophotometric data

From the results of long-term observations, data on the energy distribution in the spectra of 116 variable stars of different types have been obtained. From the published response curves of the UBV system, the magnitudes and color indexes were calculated. The mean accuracy of the calculated values is σv ≃ 0.m04, σbv ≃ 0.m03, and σub ≃ 0.m06.

Clouds in the atmospheres of extrasolar planets

<i>Aims. <i/>The effects of multi-layered clouds in the atmospheres of Earth-like planets orbiting different types of stars are studied. The radiative effects of cloud particles are directly correlated with their wavelength-dependent optical properties. Therefore the incident stellar spectra may play an important role for the climatic effect of clouds. We discuss the influence of clouds with mean properties measured in the Earth's atmosphere on the surface temperatures and Bond albedos of Earth-like planets orbiting different types of main sequence dwarf stars. The influence of clouds on the position of the habitable zone around these central star types is discussed.<i>Methods. <i/>A parametric cloud model has been developed based on observations in the Earth's atmosphere. The corresponding optical properties of the cloud particles are calculated with the Mie theory accounting for shape effects of ice particles by the equivalent sphere method. The parametric cloud model is linked with a one-dimensional radiative-convective climate model to study the effect of clouds on the surface temperature and the Bond albedo of Earth-like planets in dependence of the type of central star.<i>Results. <i/>The albedo effect of the low-level clouds depends only weakly on the incident stellar spectra because the optical properties remain almost constant in the wavelength range of the maximum of the incident stellar radiation. The greenhouse effect of the high-level clouds on the other hand depends on the temperature of the lower atmosphere, which is itself an indirect consequence of the different types of central stars. In general the planetary Bond albedo increases with the cloud cover of either cloud type. An anomaly was found for the K and M-type stars however, resulting in a decreasing Bond albedo with increasing cloud cover for certain atmospheric conditions. Depending on the cloud properties, the position of the habitable zone can be located either farther from or closer to the central star. As a rule, low-level water clouds lead to a decrease of distance because of their albedo effect, while the high-level ice clouds lead to an increase in distance. The maximum variations are about decrease and increase in distance compared to the clear sky case for the same mean Earth surface conditions in each case.

Stellar population study in early-type galaxies: an approach from the K band

AbstractA full understanding of the physical properties of integrated stellar systems demands a multiwavelength approach since each spectral window shows us the contribution of different types of stars. However, most of the observational effort in stellar population studies has been focused on the optical range. Now, the new generation of instruments allow us to explore the K band, where RGB and AGB stars dominate the light of the integrated spectra. Here we present a K-band spectroscopic analysis of early-type galaxies in different environments. Our sample comprises 12 field early-type galaxies observed with ISAAC at VLT with medium resolution, and they are compared with 11 Fornax cluster galaxies previously reported by Silva et al. (2008). The clear differences found in the infrared DCO and NaI indices between field and Fornax galaxies are discussed, trying to solve the puzzle formed by the near-infrared and optical measurements.

Launching of conical winds and axial jets from the disc���magnetosphere boundary: axisymmetric and 3D simulations

We investigate the launching of outflows from the disk-magnetosphere boundary of slowly and rapidly rotating magnetized stars using axisymmetric and exploratory 3D magnetohydrodynamic (MHD) simulations. We find long-lasting outflows in both cases. (1) In the case of slowly rotating stars, a new type of outflow, a conical wind, is found and studied in simulations. The conical winds appear in cases where the magnetic flux of the star is bunched up by the disk into an X-type configuration. The winds have the shape of a thin conical shell with a half-opening angle 30-40 degrees. The conical winds may be responsible for episodic as well as long-lasting outflows in different types of stars. (2) In the case of rapidly rotating stars (the "propeller regime"), a two-component outflow is observed. One component is similar to the conical winds. A significant fraction of the disk matter may be ejected into the winds. A second component is a high-velocity, low-density magnetically dominated axial jet where matter flows along the opened polar field lines of the star. The jet has a mass flux about 10% that of the conical wind, but its energy flux (dominantly magnetic) can be larger than the energy flux of the conical wind. The jet's angular momentum flux (also dominantly magnetic) causes the star to spin-down rapidly. Propeller-driven outflows may be responsible for the jets in protostars and for their rapid spin-down. The jet is collimated by the magnetic force while the conical winds are only weakly collimated in the simulation region.

Injected spectrum for TeV γ-ray emission from the galactic center

The detection of very high energy γ-ray emission from the Galactic center has been reported by four independent groups. One of these γ-ray sources, the 10 TeV γ-ray radiation reported by HESS, has been suggested as having a hadronic origin when relativistic protons are injected into and interact with the dense ambient gas. Assuming that such relativistic protons required by the hadronic model come from the tidal disruption of a star by the massive black hole of Sgr A*, we explore the spectrum of the relativistic protons. In the calculations, we investigate cases where different types of stars are tidally disrupted by the black hole of Sgr A*, and we consider that different diffusion mechanisms are used for the propagation of protons. The initial energy distribution of the injected spectrum of protons is assumed to follow a power-law with an exponential cut-off, and we derive the different indices of the injected spectra for the tidal disruption of different types of stars. For the best fit to the spectrum of photons detected by HESS, the spectral index of the injected relativistic protons is about 2.05 when a red giant is tidally disrupted by the black hole of Sgr A* and the diffusion mechanism is the Effective Confinement of Protons.