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Z Andromedae is a
binary star A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars us ...
system consisting of a
red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The stellar atmosphere, outer atmosphere is inflated and tenuous, making the radius large and the surface t ...
and a
white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...
. It is the prototype of a type of
cataclysmic variable star In astronomy, cataclysmic variable stars (CVs) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae (), since those with an outburst brightness visible t ...
known as symbiotic variable stars or simply Z Andromedae variables. The brightness of those stars vary over time, showing a ''quiescent'', more stable phase and then an ''active'' one with a more pronounced variability and stronger brightening and/or dimming.


Binary system

Z Andromedae is a
binary star system A binary star or binary star system is a Star system, system of two stars that are gravity, gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved ...
. The two components have a circular
orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
that takes 759 days to complete. The red giant is around twice the mass of the sun and 880 times its
luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...
, but its
effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...
is only 2,800 K. The white dwarf is around a thousand times the luminosity of the sun during the quiescent phase, but up to 10 times more luminous during the active phases. Its temperature is as high as 150,000 K when quiescent, but drops below 100,000 K when active. It also spins around its rotation axis every 1,682 seconds and displays a strong magnetic field. The evolved red giant star is losing mass, since
radiation pressure Radiation pressure (also known as light pressure) is mechanical pressure exerted upon a surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of ...
overcomes the low gravity on the surface. The outflow of matter is captured by the gravitational field of the white dwarf and falls on its surface in the end. At least during the active phase an
accretion disk An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is most frequently a star. Friction, uneven irradiance, magnetohydrodynamic effects, and ...
forms around the white dwarf.


Variability

The variability of Z Andromedae was discovered by Williamina Paton Stevens Fleming , and announced in 1901. During the ''quiescent'' phase, most of the white dwarf luminosity comes from stable hydrogen burning on its surface, and photons emitted this way ionize the wind of the red giant which causes nebular emission. The giant star, however, follows a quasi-periodic activity cycle (similar to the
solar cycle The Solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a periodic 11-year change in the Sun's activity measured in terms of Modern Maximum, variations in the number of observed sunspots on the Sun ...
) roughly every 7,550 days; when the activity of the star is enhanced, the stellar wind becomes stronger, and in response the white dwarf increases in size and cools, triggering the ''active'' phase. In the quiescent phase, the brightness of Z Andromedae is modulated by the orbital period of the system, and can reach a
magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...
of mv = 11.3 at minimum. During the active phase Z Andromedae makes luminosity outbursts and can increase its brightness up to a magnitude of mv = 7.7. Eclipses from the red giant are still visible in this phase. During this phase, a shorter periodicity of 685 days is observed; this could be a beat period between the unknown rotation period of the giant star and the orbital period, which arise from the non-spherical outflow of matter from the atmosphere of the giant star. Z Andromedae started an unusually long active phase in September 2000, brightening by several magnitudes multiple times over at least a decade. During the outbursts, irregular brightness variations (up to 0.065 magnitudes) were observed at timescales shorter than a day, interpreted as warping in the accretion disk. If models for this source are correct, it should enter a quiescent phase again in 2020.


Spectrum


Optical

The spectrum of Z Andromedae has been recognised as extremely peculiar since the early 20th century. Early spectra during a bright period, showing only
emission line A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used ...
s against a red continuum, were interpreted as a star embedded in dense nebulosity. As the star's brightness faded, the spectrum lost the high excitation "nebular" lines and developed
absorption line Absorption spectroscopy is spectroscopy that involves techniques that measure the absorption (electromagnetic radiation), absorption of electromagnetic radiation, as a function of frequency or wavelength, due to its interaction with a sample. Th ...
s with
P Cygni profile P Cygni (34 Cygni) is a variable star in the constellation Cygnus. The designation "P" was originally assigned by Johann Bayer in ''Uranometria'' as a nova. Located about 5,300 light-years (1,560 parsecs) from Earth, it is a hypergiant ...
s. These spectra were readily identified as being due to a hot nova-like star with a cool companion. Emission lines identified included low ionization states of
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
and
helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
with high ionization states of
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
and
iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
. The MK spectral classification is typical of a cool giant, for example M4.5. The exact spectral type has been shown to vary, for example between M5 in 1987 and M3.5 in 1989. Infrared observations gave a combination spectral type of M2III + B1eq. Here the
luminosity class In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction gratin ...
of III is for a normal giant star, and the peculiarity codes eq indicate emission lines with P Cygni profiles.


Ultraviolet

Z Andromedae shows also a strong ultraviolet emission, which follows the optical behaviour; absorption lines identified during the quiescent phase becomes emission lines during outbursts. Elements identified in this region of the spectrum are
carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
,
nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
,
phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...
and
silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...
in their ionized states.


Radio

The radio flux from Z Andromedae at the beginning of outbursts is lower than the usual quiescent level, and has a maximum after the optical one. After the outbursts, radio jets can be seen flowing out of this system, in a direction perpendicular to the orbital plane.


X-ray

Z Andromedae is much fainter in X-ray, and has not been detected when in the quiescent phase. During outbursts, X-ray emission comes from shock-heated plasma, where the kinetic energy of the outflowing material is converted in X-ray radiation. This emission "mimics" a
blackbody radiation Black-body radiation is the thermal electromagnetic radiation within, or surrounding, a body in thermodynamic equilibrium with its environment, emitted by a black body (an idealized opaque, non-reflective body). It has a specific continuous spectr ...
with a temperature different from that of the white dwarf, but its real nature can be identified because it shows
absorption edges Absorption may refer to: Chemistry and biology *Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which su ...
(which shows also the presence of
neon Neon is a chemical element; it has symbol Ne and atomic number 10. It is the second noble gas in the periodic table. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with approximately two-thirds the density of ...
) and an excess at high frequencies.


Bipolar jets

Following the 2006 outburst, the hydrogen Balmer
emission line A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used ...
s included faint wings at a velocity of ±1,150 km/s. Since extended radio outflows had previously been seen during the long 2000-2002 outbursts, collimated jets along the axis of the system were the most likely explanation for this phenomenon. It is thought that the jets are only present during bright outbursts. The jets were observed again during subsequent outbursts; their velocity is highly variable at the beginning but settles on a constant velocity after roughly 1 month. A single jet can also occur. The jets could be formed by material that cannot accrete on the white dwarf that reaches the
Eddington limit The Eddington luminosity, also referred to as the Eddington limit, is the maximum luminosity a body (such as a star) can achieve when there is balance between the force of radiation acting outward and the gravitational force acting inward. The stat ...
.


References


Further reading

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External links


Image Z Andromedae

AAVSO chart for Z Andromedae

AAVSO long-term light curve of Z And

Z Andromedae: The Prototype of the Symbiotic Class
AAVSO {{DEFAULTSORT:Z Andromedae 221650 Z Andromedae variables Andromeda (constellation) Andromedae, Z M-type giants J23333994+4849059 116287 Durchmusterung objects IRAS catalogue objects