Intrinsic Variables

In the case of intrinsic variables, light curve variability is caused by effects inherent to the source. Among intrinsic variable stars, one can distinguish between the sub-classes of pulsating and eruptive as well as cataclysmic variables. Other reasons for intrinsic variability of sources are e.g. accretion effects found in AGN.

Pulsating Variables

During most stages of their life, most types of stars are in a stable equilibrium. But there are certain stages in the life of stars where a stable equilibrium cannot be maintained. When this occurs, the star is radiating more than its average luminosity, causing the star’s outer layers to expand. The luminosity increases, as the density of the layer decreases due to expansion,

Table 2.3. Summary of Pulsating Variables

Type Description

RR Lyrae stars, RR Lyrae • periods between 0.2 and 1 day • amplitudes of 0.5 to 1.5 mag

• standard candles used to measure distances to systems containing old stellar population (e.g. Milky Way’s halo) • for a more detailed description, see Section 2.5.1 δ Cephei stars, Cepheids • periods between 1 and 5 days

• amplitudes of 0.5 to 2 mag in V

• standard candles, strong concentration towards the Galactic plane

• for a more detailed description, see Section 2.5.2 Mira variables • cool red giant star of spectral type Ke, Me, Se, or Ce

• period of 100–1,000 days, amplitude > 1 mag in IR, 2.5–11 mag in V

• strong stellar wind

δ Scuti stars • amplitudes from 0.003 to 0.9 mag in V • period of a few hours

• used as standard cndles • radial and non-radial pulsations

so it eventually cools, its ionization drops and it becomes more transparent to radiation. The expansion thus reduces the internal pressure, leading the star to contract by gravity. When the star contracts, the internal pressure will increase again to the point that it exceeds the gravitational force contracting the star. It then expands, increasing its luminosity, and the cycle repeats. Stars showing this pulsating behavior are present in various subclasses throughout the Hertzsprung- Russell (HR) diagram. It turns out that there is a certain region in the HR diagram where stars having a combination of temperature and luminosity have the proper conditions for this pulsation mechanism. A schematic overview is shown in Fig. 2.9. Pulsating variables can be classified in terms of their radial or non-radial pulsation, their excitation mechanisms triggering the pulsations, as well as their evolutionary status in the HR diagram.

The subclass of radial pulsators includes RR Lyrae and Cepheids – the two classes among variable stars, the analysis within the thesis deals with – as well as Mira variables.

Table 2.3 lists the most important types of pulsating variable stars.

A detailed description of the underlying pulsation mechanisms is given in Section 2.4, as well as for RR Lyrae and Cepheids in Section 2.5.1 and 2.5.2.

Eruptive Variables

Eruptive variable stars vary in brightness because of processes associated with magnetic fields, such as flares that occur within the stellar atmosphere. The changes in luminosity coincide with mass outflow in the form of stellar wind, or interaction with outside interstellar medium.

Eruptive variable stars exhibit irregular or semi-regular brightness variations caused by material being erupted from the star. Eruptive variables include protostars – stars which haven’t reached the main sequence yet – showing impressive flares, as well as giants and supergiants, who lose their matter relatively easily and may also experience eruptions.

Eruptions are well-known also in our Sun; Fig. 2.8 shows a prominence eruption on the Sun, where a giant eruption of solar material exploded off the surface of the Sun and is falling right back.

Table 2.4 lists the most important types of eruptive variable stars.

Figure 2.8On March 2, 2012, a giant eruption of solar material exploded up off the surface of the Sun, as captured in this image from NASA’s Solar Dynamics Observatory. Known as a prominence eruption, most of the material usually falls right back down on to the Sun. Credit: NASA/SDO.

Table 2.4. Summary of Eruptive Variables

Type Description

UV Ceti stars (flare stars) • flares range from radio to X-ray

• flares can increase star’s brightness by up to 6 mag in V • presumably young stars, preceeding T Tauri phase T Tauri stars (TTS) • identified by their optical variability and strong

chromospheric lines

• pre-main-sequence stars in the process of contracting to the main sequence along the Hayashi track

• large areas of starspot coverage, and they have intense, variable X-ray and radio emissions, often powerful stellar winds

FU Orionis (FUOr) stars • pre-main sequence stars undergoing rapid accretion • brightness increase by &4 mag, slow decline • G-type supergiants, K–M giants/supergiants Ex Lupi (EXor) stars • amplitude of 1 to 4 mag in V

• outbursts lasting 10 to 100 days, separated by several months

• emission lines similar to T Tauri stars

Wolf-Rayet stars (WR) • strong broad emission lines of highly ionized He, N, C • very high surface temperatures of ∼30,000–200,000 K • highly luminous, ∼1000 × L

Luminous blue variable (LBV) stars • unstable supergiant stars

• periodic outbursts, occasionally larger eruptions • temperature between 10,000 K and 25,000 K • luminosity of 250, 000 − 106_L

Herbig Ae/Be stars • pre-main-sequence star • spectral type earlier than F0 • show Balmer emission lines

• show IR radiation excess due to circumstellar dust R Coronae Boralis (R CrB) stars • luminosity varies in two modes: low ampluitude pulsation

with a few tenths of a mag, irregular with fading by 1 to 9 mag

• supergiants in the spectral classes F and G γ Cassiopeiae (Be) stars • spectra vary over time

• non-supergiant stars with temperatures between 10,000 and 30,000 K

Cataclysmic and Nova-like Variables (CV)

Cataclysmic variables are contact binaries consisting of a white dwarf and a low-mass main- sequence star (K or M dwarf). The latter one, called secondary (in contrast to the primary white dwarf) has filled the Roche volume, resulting in mass transfer onto the primary.

There are various subclasses of cataclysmic variables, mostly characterized by their magnetic fields.

When an accretion disk develops due to the mass transfer, the cataclysmic variable is called a dwarf nova. Also, among them there are various sub-classes depending on frequency and intensity of outbursts.

The class of nova-like stars also includes supernovae. Initially assuming they are new stars (Hockey 2009), they are the most noticeable class of variable stars. Supernovae occur during the last stellar evolutionary stages of a massive star’s life. For a short time of a few days, this causes the appearance of a seemingly ‘new’ star with an apparent brightness of up to -6 to -7.5 mag, depending on their distance. Then, it slowly fades from sight over weeks to months.

Supernovae can be classified according to their light curves and the absorption lines of different chemical elements that appear in their spectra. The first element for division is the presence or absence of a H line. If a supernova’s spectrum contains H lines, it is classified type II; otherwise it is type I. In each of these two types there are subdivisions according to the presence of lines from other elements or the shape of the light curve. The most important one is type Ia.

Type Ia supernovae show a line at 615.0 nm caused by singly ionized silicon (Si ii). They happen when a white star as part of a binary system accretes matter from its companion and thus reached its Chandrasekhar limit of 1.4 M. When this mass limit is reached, the star becomes unstable

and undergoes a thermal runaway nuclear fusion reaction. The fact that all type Ia supernovae explode at about the same mass results into a very narrow range of absolute magnitudes. This makes them very useful as standard candles.

The blue and visual peak magnitudes of type Ia supernovae are given as (Hillebrandt and Niemeyer 2000):

MB ≈ MV ≈ −19.3 ± 0.3 mag . (2.1)

Supernovae of type Ia are crucial in establishing the cosmological distance ladder to extragalactic distances. They had been deciding in discovering the accelerated expansion of the Universe. Supernovae are the source of many elements, especially the ones heavier than Fe, which are produced in supernovae and ejected out into space.

Table 2.5 lists properties of supernovae, as well as cataclysmic variables and nova-like variables in general.

Table 2.5. Summary of Cataclysmic Variables

Type Description

Supernovae (SN) • explosive event occurring at the last evolutionary stages of a massive star

• seemingly ‘new’ star with an apparent brightness of up to -6 to -7.5 mag

• supernovae are classified according their light curves and absorption lines

• used as standard candles

Novae • close binary system in which a white dwarf accretes matter from its companion

• nova results of the rapid fusion of the accreted H on the white dwarf’s surface

• steep rise to peak, steadily decline

• brightens by >12 mag, decays over ∼25–80 days by 2 mag

Dwarf Novae

(Geminorum-type variable star)

• close binary system in which a white dwarf accretes matter from its companion

• luminosity effects attributed to changes in the accretion disk

• depending on sub-type, one or multiple outbursts can happen

Recurrent Novae • same mechanism as for novae

• at least 2 outbursts over the past century, intervals 10–100 years

• brightens by 8–9 mag during outburst • currently 10 recurrent novae known