Einstein observations of cool stars

Observations of cool stars with the $\text{Einstein}$ Observatory (HEAO-2) have brought about a fundamental change in our knowledge and understanding of stellar coronae. The existence of X-ray emission from stars throughout the H-R diagram, the wide range of X-ray luminosity within a given spectral and luminosity class, and the strong correlation of X-ray luminosity with stellar age and rotation are among the more significant $\text{Einstein}$ results. These results are strong evidence for the influence of stellar dynamo action on the formation and heating of stellar coronae. A discussion of relevant consortium and guest observations will be given. The Hyades cluster, in particular, will serve as an example to demonstrate the usefulness of X-ray observations in the study of stellar activity and coronal evolution.     

Late stages of stellar evolution – Herschel’s contributions

Cool objects glow in the infrared. The gas and solid-state species that escape the stellar gravitational attraction of evolved late-type stars in the form of a stellar wind are cool, with temperatures typically ?1500 K, and can be ideally studied in the infrared. These stellar winds create huge extended circumstellar envelopes with extents approaching 10¹⁹${10}^{19}$ cm. In these envelopes, a complex kinematical, thermodynamical and chemical interplay determines the global and local structural parameters. Unraveling the wind acceleration mechanisms and deriving the complicated structure of the envelopes is important to understand the late stages of evolution of ∼97% of stars in galaxies as our own Milky Way. That way, we can also assess the significant chemical enrichment of the interstellar medium by the mass loss of these evolved stars. The Herschel Space Observatory is uniquely placed to study evolved stars thanks to the excellent capabilities of the three infrared and sub-millimeter instruments on board: PACS, SPIRE and HIFI. In this review, I give an overview of a few important results obtained during the first two years of Herschel observations in the field of evolved low and intermediate mass stars, and I will show how the Herschel observations can solve some historical questions on these late stages of stellar evolution, but also add some new ones.     

IUE observations of hot stars

Stellar winds are found in hot and luminous stars of all types. We see evidence of these winds in P Cygni profiles of resonance lines in the UV spectral regions, and obtain density information from them, and from optical emission lines and from free-free radiation in the infrared and radio continua from the ionized plasma. Data recently acquired from the IUE satellite are now sufficient to enable us to outline the broad parameters of these winds. It is found that for the hottest stars, those of 0-type, the mass loss rate ? is proportional to L^α. A proportionality between ? and L is predicted by the theory of radiatively driven winds; the value for α is also anticipated by the details of the theory. The dispersion of individual stellar values may be due to observational uncertainty alone, but it may also suggest that other physical parameters affect the stellar winds. The kinetic energy input of the stellar winds to the interstellar medium is considerable and may, in aggregate, be of the same order as the contribution of supernovae.     

Upper limits to ultraviolet line emission in fully convective M-dwarfs

Theoretical stellar structure models find that stars later than ∼dM3 are fully convective. It is widely believed that this should result in reduced non-radiative heating and activity levels, however, the observational evidence is sparse and somewhat contradictory. In order to expand the number of atmospheric diagnostics for these stars, we have obtained deep short wavelength (SWP) IUE spectra of 5 M dwarfs later than spectral type dM5. Exposure times ranged from 4 to 7 hours. Only upper limits were measured for chromospheric and transition region lines, thus appearing to rule out enhanced line emission activity. The fractional luminosities of C IV (λ1550), C II (λ1335), and C I (λ1657) relative to L_bol indicate that emission lines in the temperature range 6000 K to ∼1 × 10⁵K are up to an order of magnitude weaker than in earlier M dwarfs having radiative cores. The lower limits on line fluxes for Barnard's star are significantly below those of the other stars in the program.     

Photospheric composition and structure in white dwarfs

One of the central mysteries of white dwarf studies has been the nature and abundance of trace elements in the atmospheres of these stars. It had been thought that the dominant trace element in otherwise pure hydrogen DA white dwarf atmospheres was helium. However, some spectroscopic and theoretical evidence suggested that, at least in some stars, heavier elements may be important. Prior to the launch of ROSAT the questions regarding the atmospheric composition of DA white dwarfs in general remained unresolved. The ROSAT mission has provided EUV and X-ray data for a large sample of DA white dwarfs with which we can study their photospheric composition and structure through the effect of trace opacity sources on the emergent fluxes. Contrary to expectations little (if any) helium is found and the main sources of opacity appear to be trace heavy elements. Support for these conclusions is found in recent EUV and far-UV spectra of several stars. However, photometric data do not allow us to determine the abundance of the individual elements and observations with the EUVE spectrometers will be essential for detailed composition measurements.     

Coronal activity in F-, G-, and K-type stars; relations between parameters characterizing stellar structures and X-ray emission

A sample of 52 stars containing dwarfs and giants is subjected to a multidimensional factor analysis. The parameters used are the soft X-ray flux at the stellar surface F_x, the Ca II H+K line-core flux F_H+K, the stellar radius and mass. We find a high correlation between F_x and the Ca II H+K excess flux ΔF_H+K obtained by subtracting an observational lower-limit flux from F_H+K. We conclude that the lower-limit Ca II flux is uncorrelated with the stellar X-ray emission. The common-factor analysis shows that, for the present sample, F_x depends only on ΔF_H+K, and not on the stellar radius or mass. All stars included in our analysis follow the relation F_x ∝ Δ^1.4_H+K over almost four decades in F_x.     

Solar and late-type dwarfs

The Einstein Observatory and the IUE satellite have provided the observational basis for a major restructuring in theories of coronal formation for late-type stars. For the first time, coronal and transition region emission from a large sample of low mass (1 M_o) dwarf stars has been directly observed, with the unexpected result that essentially all such stars are x-ray emitters. The Sun, which was previously assumed to be typical, is now known to be at the low end of the x-ray luminosity function for solar-type stars. K- and M-dwarfs are observed to have nearly the same luminosity distributions as G-dwarfs and all of these spectral types have a large spread in x-ray luminosity.Observationally, there is a strong correlation between the strength of coronal emission in stars with outer convective zones and the rotation rates of these stars. At the present time we have only the beginnings of a satisfactory theoretical explanation for this correlation; although we are beginning to understand the connection between coronal emission strength and the magnetic field, we do not yet understand the stellar dynamo which generates the magnetic field. Studies of the coronal emission of stars may lead to a better understanding of stellar dynamos.     

Wolf–Rayet optically thick winds with Alfvén waves

The Wolf–Rayet (WR) stars are hot luminous objects which are suffering an extreme mass loss via a continuous stellar wind. The high values of mass loss rates and high terminal velocities of the WR stellar winds constitute a challenge to the theories of radiation driven winds. Several authors incorporated magnetic forces to the line driven mechanism in order to explain these characteristics of the wind. Observations indicate that the WR stellar winds may reach, at the photosphere, velocities of the order of the terminal values, which means that an important part of the wind acceleration occurs at the optically thick region. The aim of this study is to analyze a model in which the wind in a WR star begins to be accelerated in the optically thick part of the wind. We used as initial conditions stellar parameters taken from the literature and solved the energy, mass and momentum equations. We demonstrate that the acceleration only by radiative forces is prevented by the general behavior of the opacities. Combining radiative forces plus a flux of Alfvén waves, we found in the simulations a fast drop in the wind density profile which strongly reduces the extension of the optically thick region and the wind becomes optically thin too close its base. The understanding how the WR wind initiate is still an open issue.     

Lyman-alpha emission from low dispersion IUE spectra

In spite of its importance as cooling mechanism of the chromosphere of late type stars, the hydrogen Lyman-alpha line has been neglected in most IUE programmes The high geocoronal background and the saturation often seen in the stellar emission peak are the major problems that prevent accurate determination of the Lyman alpha flux.We have developed a technique that allows us to overcome both of these problems by fitting an ellipsoidal surface to the geocoronal background and gaussian profiles to the stellar signal. Results on a limited sample of stars are presented.     

A concept mission for the Stellar Population and Evolution with Cubesats (SPEC)

Binary or multiple stellar systems, constituting almost a third of the content of the Milky Way, represent a high priority astronomical target due to their repercussions on the stellar dynamical and evolutionary parameters. Moreover the spectral study of such class of stars allows to better constrain the evolutionary theories of the Galactic stellar populations. By resolving the members of stellar systems through photometric observations we are able to perform more detailed measurements to infer their mass. In this paper we investigate the feasibility of a cubesat based mission including an optical payload to directly optically discriminate the members of a selected sample of binary systems. The scientific targets, consisting 11?M class dwarf stars binary systems, have been extracted from the already studied Riaz catalogue. These subset has been selected considering the star distance, the members angular separation, and the distance from the Galactic plane (due to limit the background and foreground contamination). The satellite concept is based on a 6 unit Cubesat embedding some commercial off the shelf components and an ad hoc designed optical payload occupying almost 4 units. The optical configuration has been chosen in order to fit the angular resolution requirements, as derived from the target characteristics. Moreover, according to the optical analysis and the computed field of view some requirements on the attitude control system have been inferred and fulfilled by the component selection. The paper is organized as in the following: a brief scientific introduction is made; consequently the project is described with particular attention to the optical design and the standard sub-systems; finally the conclusions are drawn and the future perspectives are investigated.     

Evolution of the chromospheric and coronal activity of intermediate mass stars

Recent ultraviolet and X-ray observations pertaining to the outer atmospheric structure of intermediate mass (4–6 M₀) stars and the evolution of their structure are presented. A distance-limited (d ≤ 200 pc) IUE ultraviolet survey of early K bright giants shows that C IV emission commonly is present. These stars are almost evenly split between stars showing hybrid-chromospheric and coronal outer atmospheric structures. EXOSAT observations have been obtained for three hybrid stars, of which only α TrA, the nearest, is detected. The temperature of the emitting plasma is likely to be ∼10⁶K. Observations of six K II stars made with the Einstein satellite show no detections. The general conclusion from the available X-ray data is that early K bright giants are not strong X-ray sources.     

IUE observations of cool stars

A summary of IUE results concerning late-type stars is presented. Observations show that high-temperature outer atmospheres, as indicated by N V, C IV emission at T ≈ 10⁵K, are generally present only in high-gravity (log g ? 2) stars. Objects with high-temperature emission tend not to exhibit cool circumstellar shells, and vice versa, although there are several transition objects, the hybrid atmosphere stars, which combine C IV emission with cool winds. Ultraviolet emission from stellar transition regions correlates well with chromospheric and X-ray emission. Transition-region line ratios indicate that many stars have differential emission measure distributions similar to the Sun's. Ultraviolet observations also give indications of important dynamical effects in low-gravity stars. Density diagnostics indicate extended chromospheres for some red giants and supergiants. In addition, the large widths of lines of high temperature ions in several luminous stars indicate supersonic motions.     

The demography of massive galactic black holes

This review describes dynamical evidence for massive black holes (BHs) at the centers of galaxies. BHs appear to be a common if not ubiquitous feature of local galactic spheroids, with a mean mass that is roughly 0.5% of the parent stellar spheroid mass. This mass is large enough that the formation and evolution of central BHs has had a significant influence on the inner stellar density profile. For example, low-density, core-type profiles may have been created by the scouring action of binary BH pairs, whose orbital decay ejects stars from the center of the galaxy. Or, BHs may have grown substantially by gas accretion, pulling in surrounding stars and creating the cuspy cores and power laws that are seen in spheroids. If BHs are ubiquitous in galactic spheroids at the above mass ratio, their total local mass density is comparable to that implied by the energy density of QSO photons, suggesting that these local BHs are the long-sought fossil BHs of QSO central engines. However, QSOs are much rarer per unit co-moving volume than local BHs, unless one equates them to just the massive BHs found in rare and massive local spheroids. The resultant BH masses exceed typical QSO BH mass estimates by nearly a factor of 10, suggesting that BHs have grown in mass by roughly that factor since the QSO era. At least some of that growth might have occurred as protogalaxies grew in mass by mergers, and their BHs also merged and grew apace.     

Seismological studies of the sun and other stars

Observations of a large number of different oscillation frequencies in the Sun provide an opportunity for detailed testing of the theory of stellar structure and evolution. At present highly significant discrepancies remain between observed and computed frequencies, and so our models of the solar interior have to be modified. With further improvements in the observations it might become possible to make a direct empirical determination of the density structure throughout the Sun.Similar oscillations have so far not been detected in other stars, but attempts to do so are under way. Theoretical estimates indicate that amplitudes somewhat greater than for the Sun might be expected for early F stars on the main sequence, and that the amplitude increases rapidly with decreasing gravity. Observation of such oscillations would enable investigations of the structure of these stars, and would in addition provide valuable information about the excitation mechanism of the oscillations.     

Formation of relativistic jets by collapsing stars to black holes

Formation of relativistic jets in the magnetosphere of collapsing stars is considered. These jets will be formed in the polar caps of magnetosphere of collapsing star, where the stellar magnetic field increases during the collapse and the charged particles are accelerated. The jets will generate non-thermal radiation. The analysis of dynamics and emission of particles in the stellar magnetosphere under collapse shows that collapsing stars can be powerful sources of relativistic jets.     

Active stars and systems

The most notable manifestations of stellar activity are reviewed with particular emphasis on the merging picture of solar-type activity in physical conditions different from those in the Sun. Evidence for starspots, plages and high-level coronal emissions is presented from observations covering a wide range of spectral bands: from X-ray to radio wavelengths. The main physical parameters of the active areas in the active stars, when compared with solar values, indicate that the basic requirement for activity phenomena to develop is the presence of observationally elusive localized magnetic fields on and above the stellar surface. The importance of coordinated programs involving simultaneous observations from the ground and from space - aiming at empirical and theoretical modeling of activity phenomena - is stressed.     

弹道导弹的捷联惯性/天文组合导航方法

针对传统的捷联惯性/天文(SINS/CNS)组合导航系统不能精确估计加速度计偏置而导致导航误差发散的问题,提出一种基于星光折射间接敏感地平的捷联惯性/天文(SINS/RCNS)组合导航方法。利用星敏感器测量星光折射角,结合大气折射模型得到的折射视高度来抑制位置误差的发散。推导了基于星光折射新的量测方程,分析了折射星数目与导航精度的关系,当使用多颗折射星时能够精确估计加速计偏置,从而能够完全抑制位置误差的发散,并对系统进行可观测性分析。通过卡尔曼滤波实现了状态估计。仿真结果表明:本文方法的导航精度优于传统方法,有效抑制了位置误差的发散,验证了本文方法的有效性。     

Alfvén waves as a driving mechanism in stellar winds

Alfvén waves have been invoked as an important mechanism of particle acceleration in stellar winds of cool stars. After their identification in the solar wind they started to be studied in winds of stars located in different regions of the HR diagram. We discuss here some characteristics of these waves and we present a direct application in the acceleration of late-type stellar winds.     

Coronal heating mechanisms

As a result of the large body of data available from solar and stellar coronae, our understanding of the mechanisms responsible for the heating of coronal plasmas to temperatures of the order of ～ 10⁸ K has changed. The solar corona is highly structured by magnetic fields and the acoustic shocks which, according to early theories, should have acted as the coronal energy source have not been observed. Einstein Observatory data show moreover that coronae are present in most regions of the H-R diagram. The observed relationship between X-ray luminosity and rotational velocity in dwarf stars from spectral types F to M again suggests an active role for the magnetic fields.The basic picture which is emerging is that coronae in stellar types from F to M are produced because of the interaction of the magnetic field with the convective velocity fields generated in the photosphere resulting in MHD waves or currents which dissipate in the corona. X-ray emission in early type stars cannot be explained with this mechanism and the models which have been proposed for these stars are not yet completely satisfactory.