PX Andromedae (often abbreviated to PX And) is an eclipsing

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 ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

Andromeda. It has been classified as a SW Sextantis variable, and its

apparent visual magnitude Apparent magnitude () is a measure of the brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction of the object's light ca ...

varies between 14.04 and 17. In 1982, Richard Green ''et al.'' listed PX Andromedae as a possible cataclysmic variable, based on spectra taken with the

Hale Telescope The Hale Telescope is a , 3.3 reflecting telescope at the Palomar Observatory in San Diego County, California, US, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation in 1928, he orchestrated the planning, de ...

. Observations in 1989 by Li Yong ''et al'', at the Beijing Observatory detected rapid variations of the star's brightness of up to 0.2 magnitudes, as well as eclipses which occur every 3.5 hours. In 1992 the star was given the

variable star designation In astronomy, a variable-star designation is a unique identifier given to variable stars. It extends the Bayer designation format, with an identifying label (as described below) preceding the Latin genitive of the name of the constellation in whic ...

PX Andromedae.

Spectrum

The spectrum of PX Andromedae is variable, but typically shows a continuum with prominent broad

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 of hydrogen and helium. Unlike many types of cataclysmic variable, the emission lines are generally single-peaked, although for a short time during each orbit they do show a double peak due to an absorption core within the emission lines. The single-peaked lines are shown in novae and dwarf novae outbursts, and PX Andromedae is often described as nova-like although it does not shown outbursts with large increases in brightness. However, it does have ''high'' and ''low'' states with the low states generally being fainter and showing weaker emission. PX Andromedae is a very blue star with an ultraviolet excess, indicating that it includes very hot objects.

Variability

PX Andromedae generally has a

visual magnitude Apparent magnitude () is a measure of the brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction of the object's light ca ...

around 15 although there are continual rapid variations of up to a tenth of a magnitude on a timescale of minutes. Approximately every 3.5 hours the brightness drops by around a magnitude and then returns to the normal brightness in about half an hour, without a flat bottom, easily identifiable as partial

eclipse An eclipse is an astronomical event which occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three ...

s. However, the depth of the eclipses varies from about 0.5 magnitudes to 1.5 magnitudes, and this appears to cycle every 4.8 days, coinciding with the negative

superhump In astronomy, a superhump is a periodic brightness variation in a cataclysmic variable star system, with a period within a few percent of the orbital period of the system. History Superhumps were first seen in SU Ursae Majoris (SU UMa) stars, a su ...

cycle. Deeper eclipses occur when the superhump cycle is near its minimum. Possible variations with a period of 0.207 days are likely to be an observational alias of the superhump period. PX Andromedae may have both high (brighter) and low (fainter) states for its mean brightness, but the timing of these is uncertain. The eclipses have been timed to occur every 0.1463 days, assumed to be the orbital period. The negative superhump period is 4.43 days. Positive superhumps have sometimes been claimed, but are not always present. Stars with this type of variation are classified as SW Sextantis variables and PX Andromedae is sometimes considered one of the prototypes for the class. Although they are considered to be cataclysmic variables and are often described as "nova-like", they do not show intermittent outbursts. Rather they are in a continuous outburst state with a spectrum similar to an ongoing nova.

System

Since the

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of PX Andromedae is peculiar and similar to

U Geminorum U Geminorum (''U Gem''), in the constellation Gemini, is an archetypal example of a dwarf nova. The binary star system consists of a white dwarf closely orbiting a red dwarf. Every few months it undergoes an outburst that greatly increas ...

, it is commonly accepted that in this system 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 ...

is accreting matter from a donor star, and an

accretion disc 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 ...

has formed around the former. The negative superhumps show that the accretion disk is tilted with respect to the white dwarf rotation axis, and has a retrogade

precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In o ...

. Moreover, the donor star does not eclipse the central part of the disk – proven by modulation existing of the eclipse depth.

References

{{DEFAULTSORT:PX Andromedae Andromeda (constellation) Andromedae, PX Eclipsing binaries Nova-like variables Cataclysmic variable stars