DX Andromedae
A dwarf nova with irregular outbursts and a hydrogen-deficient donor star.
DX Andromedae (DX And) is a cataclysmic variable star in the constellation Andromeda, classified as a dwarf nova of the SS Cygni type. It is notable for its outbursts, which can brighten from a typical quiescent magnitude of 15.5 up to 11.0, recurring with a mean cycle length of 330 days.
- Constellation
- Andromeda
- Typical quiescent magnitude
- 15.5
- Outburst magnitude
- 11.0
- Mean cycle length days
- 330
- Spectral type at quiescence
- K1 V (plus emission lines)
- Discovery year
- 1958
- Discoverer
- Giuliano Romano
Lore & Background
DX Andromedae was discovered by Giuliano Romano on photographic plates begun on September 28, 1956, with the discovery announced in 1958. The system consists of a white dwarf accreting matter from a donor star through an accretion disk. The donor star is considered hydrogen-deficient, evolved off the main sequence, and overfilling its Roche lobe; it dominates the visible spectrum during quiescence. The overall spectral classification is peculiar of the U Geminorum type, and the donor's spectrum shows enhanced absorption lines of CaI and CrI.
Outbursts in DX Andromedae are thought to arise from a thermal instability of the accretion disk, increasing mass flow and temperature. Different outbursts do not reach the same peak luminosity and occur at an irregular rate, but they retain the same shape in the decaying part. The peak luminosity appears correlated with the length of the preceding cycle, and more luminous outbursts reach peak brightness faster. These trends are explained by matter accumulating in the disk over longer cycles, powering stronger outbursts.
Reader's Guide
DX Andromedae serves as an important example of a dwarf nova of the SS Cygni type, illustrating the behavior of cataclysmic variables with irregular outburst cycles. Its discovery by Giuliano Romano in the late 1950s added to the growing catalog of variable stars in Andromeda. The system's peculiarities—such as the hydrogen-deficient donor star and the correlation between cycle length and outburst luminosity—provide insight into the accretion disk instability model. The observed trends, where longer cycles lead to more luminous outbursts and faster rises, support the theory that matter accumulates in the disk over time. The star's spectral peculiarities, including enhanced CaI and CrI lines, further distinguish it among U Geminorum-type variables. Though not the brightest or most regular dwarf nova, DX Andromedae's behavior helps refine models of disk thermal instability and mass transfer in close binary systems.
Did You Know?
- Its donor star is hydrogen-deficient and has evolved off the main sequence.
- The peak luminosity of an outburst is correlated with the length of the preceding cycle.
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