IW Andromedae
Prototype of the anomalous IW And variable subclass of Z Cam stars.
IW Andromedae is a binary star system found in the northern constellation Andromeda, often shortened to IW And. It serves as the namesake for the IW And variables, a peculiar subclass of the Z Camelopardalis (Z Cam) type of variable stars. Its apparent visual brightness fluctuates between magnitudes 13.7 and 17.3, making it visible only through a telescope. Based on parallax measurements, the system lies roughly 2,860 light-years from the Sun.
The star's irregular variability was first noted by L. Meinunger in 1975. Its spectrum resembles a blue-tinted OB star, though with some unusual features. It is confirmed as a cataclysmic variable (CV), but its behavior sets it apart from other CV subtypes. Unlike dwarf novae or polars, it can brighten rapidly by up to three magnitudes within about a day, yet it remains in a low-activity state for roughly 72% of the time. In 1999, W. Liu and colleagues detected weak hydrogen-alpha emission from the system.
The binary has an orbital period of 223 minutes (3.7 hours). The primary is a white dwarf with about 75% of the Sun's mass, while the secondary, at 27% solar mass, overflows its Roche lobe, transferring material to an accretion disk around the white dwarf. The accretion rate onto the primary is estimated at 3 × 10⁻⁹ solar masses per year.
In 2003, T. Kato and collaborators found that the light curve matches that of a Z Cam variable, with the previously observed inactive periods explained by a characteristic standstill. The standstill's duty cycle is unusually long for this class. Outbursts during these standstills may arise from flares on the secondary star, causing brief surges in mass transfer.
Since then, other stars showing IW And–type behavior have been identified, including HO Puppis, BC Cassiopeiae, IM Eridani, V507 Cygni, and FY Vulpecula.
- Constellation
- Andromeda
- Apparent magnitude range
- 13.7 to 17.3
- Distance
- approximately 2,860 light years
- Orbital period
- 223 minutes (3.7 hours)
- Primary mass
- 75% of the Sun's mass
- Secondary mass
- 27% of the Sun's mass
- Accretion rate
- 3×10−9 M☉·yr−1
Lore & Background
The irregular variability of this star was discovered by L. Meinunger in 1975. The spectra was found to resemble a blue–hued OB star with some peculiarities. It is a confirmed cataclysmic variable (CV) but its properties differ markedly from other sub-classes of that type. The photometric behavior of the star is dissimilar to that of a dwarf or polar nova as it shows rapid brightening of up to three magnitudes in periods of around a day, but stays in a low excitement state about 72% of the time. Evidence for weak emission of the hydrogen–alpha line was discovered by W. Liu and associates in 1999.
This is a close binary system with an orbital period of 223 minutes (3.7 hours). The primary component is a white dwarf star with 75% of the mass of the Sun. The secondary component has 27% of the Sun's mass and is overflowing its Roche lobe, resulting in mass transfer to an accretion disk orbiting the primary. The accretion rate for the primary is 3×10−9 M☉·yr−1.
T. Kato and associates in 2003 found the light curve matched a Z Cam variable, with the previously observed inactive states being caused by a characteristic standstill. The duty cycle of its standstill is unusually long for a variable of this class. Outbursts during these standstills may be explained by flares on the secondary, which result in brief surges in mass transfer.
Reader's Guide
IW Andromedae is significant as the prototype of the IW And variables, an anomalous sub-class of the Z Camelopardalis (Z Cam) variables. Its discovery by L. Meinunger in 1975 and subsequent study established it as a cataclysmic variable with properties that differ markedly from other sub-classes, such as dwarf or polar novae. The system's photometric behavior—rapid brightening of up to three magnitudes in about a day, yet remaining in a low excitement state 72% of the time—sets it apart. The identification of weak hydrogen-alpha emission by W. Liu and associates in 1999 and the 2003 work by T. Kato and associates, which matched its light curve to a Z Cam variable with an unusually long standstill duty cycle, deepened understanding of its behavior. Outbursts during standstills are attributed to flares on the secondary star causing brief surges in mass transfer. The legacy of IW Andromedae includes the recognition of other variables displaying IW And–type behavior, such as HO Puppis, BC Cassiopeiae, IM Eridani, V507 Cygni, and FY Vulpecula, establishing it as a key reference for this anomalous subclass.
Did You Know?
- The system's brightness ranges from apparent visual magnitude 13.7 to 17.3.
- Its orbital period is 223 minutes (3.7 hours).
- The primary component is a white dwarf with 75% of the Sun's mass.
More in Variable Stars, Part 2 1-24
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