Algol type eclipsing binaries

The Algol type eclipsing variables (EA) are a subgroup of the eclipsing binaries segregated according to light curve shape. The light remains rather constant between the eclipses, i.e., variability due to the ellipticity effect and/or the reflection effect is relatively insignificant. Consequently, the moments of the beginning and the end of the eclipses can be determined from the light curve.Eclipses can range from very shallow (0m01) if partial, to very deep (several magnitudes) if total. The two eclipses can be comparable in depth or can be unequal. In a few cases the secondary eclipse is too shallow to be measurable (when one star is very cool), or absent altogether (highly eccentric orbit).

Light curves of this shape are produced by an eclipsing binary in which both components are nearly spherical, or only slightly ellipsoidal in shape. Though not explained in the GCVS, one component may be highly distorted, even filling its Roche lobe, provided it contributes relatively little to the system's total light. This is, in fact, the case for at least half of the known EA variables.

Among the EAs one may find binaries of very different evolutionary status:

1. (i) binaries containing two main-sequence stars of any spectral type from O to M, with CM Lac an example

2. (ii) binaries in which one or both components are evolved but have not yet overflowed their Roche lobes, with AR Lac an example

3. (iii) binaries in which one star unevolved and the other overflowing its Roche lobe and causing mass transfer, with RZ Cas an example

4. […]

Ap and roAp stars

The existence of stars whose surface is severely depleted in He with, at the same time, overabundance of Fe, Si and Cr in spots, has been known since the early days of spectral classification, when the phenomenon was first detected in Ap stars (for details, see Jaschek & Jaschek 1987, Morgan 1933).

Chemically-peculiar (CP) stars, in general, are stars of spectral type B2- F of which the spectra reveal signatures of chemical peculiarities such as, for example, strongly-enhanced spectral lines of Fe and rare-earth elements. In this group, there is a magnetic sequence - referring to, as Hensberge (1994) puts it, ‘ those stars that show a magnetic field that is strong and global (a large dipolar contribution to the field), so that it is detectable with the present [observing] techniques. It does not imply that HgMn stars, or metallic-line (Am) stars, etc. would have no magnetic field at all. Stars in the non-magnetic sequence may be either without field, with a significantly weaker global field, or with a strong field of complicated structure, such that the measurable effect, averaged-out over the visible disc, is insignificant’. Ap stars have global surface magnetic fields of the order of 0.3 to 30 kG (thousands of times stronger than that of the sun), and the effective magnetic-field strength varies with rotation, a situation that led to interpretation in terms of the oblique-rotator model in which the magnetic axis is oblique to the rotation axis (this model was first suggested by Stibbs in 1950). The time scales of light variations seen in Ap stars range from minutes to decades.

For spacecraft in LEO and PEO, the dominant environment is the ambient neutral atmosphere. The neutral gases that make up the atmosphere in this environment form a distinctive structure around the spacecraft and give rise to drag, surface erosion, and spacecraft glow. The neutral gases emitted by the spacecraft itself give rise to contamination on other parts of the spacecraft. In this chapter, these interactions are systematically evaluated. Primary emphasis is on the physics of the flows associated with the interactions.

Neutral Gas Flow Around a Spacecraft

For a spacecraft in a LEO or PEO, the ambient mean free path for momentum exchange is given by Eq. (2.38). With a typical elastic scattering cross section of O (10−20 m2) and with mean densities around the orbit from Table 3.4, the ambient mean free path is of the order of many kilometers. This is illustrated in Figure 4.1 for profiles of the number density, collision frequency, mean free path, and particle speed from the surface to 700 km for the U.S. Standard Atmosphere. Where the Knudsen number [see Eq. (2.39)] satisfies Kn ≫ 1, the flow of ambient neutral gas around the spacecraft is collisionless. Since from Table 3.4, the gas temperature is typically hundreds to thousands of degrees Kelvin, the thermal velocity is of the order of 700 m/s. For an orbital velocity of 8 km/s, the speed ratio (Section 2.3.2) satisfies S ≫ 1. Therefore, the ambient neutral gas flow around spacecraft in a near Earth orbit will be collisionless and supersonic.

Luminous Blue Variables/S Dor stars

The observed H-R diagram has an upper luminosity limit of which the contour line is temperature dependent (Humphreys & Davidson 1987). Some of the most massive and luminous (106Lo) stars near that line (P Cyg, AG Car, HR Car, η Car, ...) sporadically show dramatic mass-ejections (seen as ‘eruptions’) followed by periods of quiescence. Such stars are called hypergiants, some of them are Luminous Blue Variables (LBVs), though LBVs, do not necessarily need to be blue, since the phenomenon is not restricted to early-type stars (de Jager & van Genderen 1989). The above-mentioned LBVs, together with 164GSco = HD 160529 (Fig. 2.1) and WRA751 are notorious galactic LBVs. de Koter (1993) estimates the number of LBVs in our galaxy at no more than 60, but the number of LBVs that possibly can be considered for observation, obviously, is much less. In the LMC, the well-known LBVs (also called S Dor stars) are S Dor (Fig. 2.2), R71 (Figs. 2.3, 2.4) and R127 (Fig. 2.5), with R66, R81 (Fig. 2.6) and R110 as additional candidates. Finally there are the ‘ Hubble-Sandage variables’, discovered by Hubble & Sandage (1953) in M31 and M33, which are identifiable with the S Dor variables, that complete the group that is commonly designated as LBVs. During outburst these stars are - apart from supernovae - the visually brightest stars in the universe, and thus potentially belong to the most powerful extragalactic distance-indicators (Wolf 1989). Today, only a few dozen LBVs are known.

Introduction

Before the space age began, it was realized that space was not empty. Comet tails, meteors, and other extraterrestrial phenomena demonstrated the presence of a “space environment.” Much as an aircraft operates in and interacts with the atmosphere (indeed the air is necessary for lift), so a spacecraft operates in and interacts with this space environment. The environment can, however, limit the operation of the spacecraft and in extreme circumstances lead to its loss. Concern over these adverse environmental effects has created a new technical discipline – spacecraft–environment interactions. The purpose of this text is to describe this new field and introduce the reader to its many different aspects.

Historically, the field of spacecraft–environment interactions has developed primarily as a series of specific engineering responses to each interaction as it was identified. Consider the discovery of the radiation belts and their effects on electronics. This led to the development of radiation shielding and microelectronic-hardening technology. Similarly, in the early seventies, the loss of a spacecraft apparently due to spacecraft charging from the magnetospheric plasma led to intense efforts to understand charge accumulation on surfaces in space and to methods for mitigating the effects. Ultimately, these efforts culminated in the 1979 launch of a dedicated spacecraft, SCATHA (Spacecraft Charging at High Altitudes), into a near geosynchronous orbit for studying this interaction. Likewise, in the eighties, certain materials were found to erode rapidly in the low-Earth space environment because of chemical interactions with atomic oxygen.

In this chapter, the principal natural (unperturbed) environments responsible for spacecraft interactions are introduced. These are the solar environment, the neutral atmosphere, the geomagnetic field, the plasma environment, the geostationary environment, energetic particle radiation, electromagnetic and optical radiation, and particulates (debris and meteoroids). The ambient space environment defined by these components has been the subject of numerous books and review papers [e.g., Jursa (1985)] or the excellent short descriptions of the environment in MIL-STD-1809 (1991). Unlike most of these sources, which deal primarily with the details of the space environment, the intent here is to provide the reader with sufficient background to evaluate the potential impact of the environment – both natural and man-made – on a spacecraft.

The relationships between the orbit classes and the natural environment are summarized in Figure 3.1. Table 3.1 is used to indicate which environments must be considered for a given class of orbits.

Influence of the Sun

The dominant energy source for the space environment in the solar system is the Sun. The chief solar influence on the space environment is through its electromagnetic flux (see Section 3.4.2) and the charged particles that it emits. The solar particle flux is composed basically of two components: the very sporadic, high-energy (E > 1 MeV) plasma bursts associated with solar events (flares, coronal mass ejections, proton events, and so forth) and the variable, low-energy (E ≈ tens of eV) background plasma referred to as the solar wind.