Supernovae

A supernova explosion is a rare type of stellar explosion which dramatically changes the structure of a star in an irreversible way. Large amounts of matter (one to several solar masses) are expelled at high velocities (several tens of thousands km s-1). The light curve in the declining part is powered by thermalized quanta, released by the radioactive decay of elements produced in the stellar collapse, mainly 56Co and 56Ni. The ejected shell interacts with the interstellar medium and forms a SN remnant, which can be observed long after the explosion in the radio, optical and X-ray regions.

Supernovae can be divided into two classes (and several subclasses), viz. SN I and SN II.

SN I have fairly similar light curves (see, for example, Fig. 5.1) and display a small spread in absolute magnitudes. Spectra around maximum show absorption lines of Ca II, Si II and He I, but lack lines of hydrogen. They occur in intermediate and old stellar populations. Their progenitor stars are not clearly identified, but massive white dwarfs (WDs) that accrete matter from a close companion and are pushed over the Chandrasekhar limit are good candidates. Another possibility is the hypothesis of the fusion of a binary consisting of two WDs. The collapse of the white dwarf leads in both cases to an explosive burning of its carbon, and the released energy is sufficient to trigger a disintegration of the complete object.

Spacecraft–Plasma Interactions

The plasma environment that a spacecraft will encounter as a function of orbit is described in Section 3.3. Although the plasma environment is not necessarily the dominant environment in a particular case, it can nevertheless have a profound and destructive effect on a spacecraft or its payload. In particular, major plasma effects follow from the slow accumulation of charge on surfaces. This accumulation of charged particles from the surrounding space plasma on spacecraft surfaces, termed surface charging, produces electrostatic fields that extend from surfaces into space and can result in a number of adverse interactions:

• surface arc discharges that generate electromagnetic interference cause surface damage, induce currents in electronic systems, produce optical emissions, and enhance the local plasma density;

• enhanced contamination leading to changes in surface, thermal, and optical properties;

• a shift of the spacecraft electrical ground, leading to problems with detectors collecting charged particles from the environment; and

• coulomb forces on the spacecraft components and materials as well as modifications of the drag coefficient and electromagnetic torques on the spacecraft.

In addition to these concerns, there are some less obvious effects. Of particular concern to the manned spacecraft community, differential charge accumulation between two spacecraft that come into contact (the Shuttle and the station or an astronaut during extra-vehicular activity and the Shuttle) may result in damaging current flows between the spacecraft. These can cause arc discharges, electronic burnout, and other safety hazards.

During the preparation of the observing programme of the TYCHO project on board the HIPPARCOS mission we started thinking about the large number of new variable stars that would be discovered. And since the TYCHO experiment yields only a scanty number of scattered measurements of each star during the life time of the satellite, it is immediately evident that one will encounter the problem of recognising the type or class of variability to which the variable star belongs. Such classification is - even with abundant data - not a trivial task, since many variable stars have light curves which, at first sight, look very similar. In addition, proper classification needs much more than a good-looking light curve, since luminosity and effective-temperature photometric indices also play a role, as well as miscellaneous data obtained with apparatus that are complementary to photometric instruments.

We thought to get some help by looking for standard light curves of typical variable stars that would be used as a template during the process of classification. We discovered then, with some surprise, that a compilation of typical photoelectric light curves of variable stars has never been published, nor does there exist a concise compendium of photometric properties of groups and classes of variables. What can be found, instead, is a large number of detailed morphological descriptions and numerous photometrically-incompatible photographic and visual light curves, scattered over many books and journals.

So, we decided to fill this gap and we started the compilation of typical light curves in a format that enables quick recognition of the pattern of variability.

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.