BM Andromedae
A T Tauri star with irregular brightness and circumstellar envelopes.
BM Andromedae (BM And) is a T Tauri star in the constellation Andromeda, notable for its irregular brightness variations and its status as a young stellar object still in the accretion phase. Its variability and the presence of both gaseous and dusty circumstellar envelopes make it a key object for studying star formation and early stellar evolution.
- Apparent visual magnitude maximum
- 11.63
- Apparent visual magnitude minimum
- 14.02
- Spectral class range
- F8-K5Vea
- Luminosity powered by accretion
- 0.23 L☉ of 5.5 L☉
- Dust envelope distance
- 1 AU
- Disk size 2005 2006
- 0.25 AU
Lore & Background
BM Andromedae was announced as a variable star in 1949 by Balfour Whitney, based on 400 photographic plates taken from 1942 through 1949. He noted rapid and irregular variations, sometimes changing by 1 magnitude in a single day. The star's spectral class is not precisely known, with estimates ranging from F8 to K5Vea, indicating a main sequence star that is more luminous and has stronger emission lines than typical—a classification common for young stars near the main sequence. The star is still accreting, with about 0.23 L☉ of its total 5.5 L☉ luminosity powered by accretion.
The system includes a circumstellar cloud composed of a gaseous envelope and an extended dust envelope. The dust envelope can reach 1 AU from the star, is strongly flattened, and is observed edge-on. Measurements in 2005-2006 revealed a relatively compact disk of 0.25 AU. A correlation exists between the local interstellar magnetic field and the polarization of light from BM Andromedae, suggesting the magnetic field may have influenced the system's formation.
Variability arises because the envelope blocks a variable fraction of the star's light, and this blocking is not uniform. The dust envelope also polarizes the emitted light; more blockage corresponds to stronger polarization. The star's color depends on brightness, becoming redder when fainter. Strong H-alpha lines indicate a gaseous envelope, while infrared excess points to an extended dust envelope.
Reader's Guide
BM Andromedae serves as an important example of a T Tauri star, a class of young stellar objects that are still accreting material and have not yet reached the main sequence. Its irregular variability, first documented by Balfour Whitney using photographic plates from the 1940s, highlights the dynamic nature of such systems. The star's spectral classification uncertainty (F8-K5Vea) reflects the challenges in characterizing young stars, but the consensus that it is a luminous main-sequence star with strong emission lines aligns with its youth. The discovery that its brightness changes are linked to variable obscuration by a non-uniform circumstellar envelope—and that polarization increases with obscuration—provides a direct observational link between envelope structure and photometric behavior. The detection of a compact 0.25 AU disk and the correlation with the interstellar magnetic field offer clues about the formation and evolution of the system. The star's color becoming redder when fainter is a key diagnostic for understanding dust properties in its envelope. Overall, BM Andromedae is a valuable object for studying accretion processes, envelope geometry, and magnetic field effects in young stellar systems.
Did You Know?
- BM Andromedae varies irregularly between apparent magnitudes 11.63 and 14.02.
- Its spectral class is uncertain, with estimates ranging from F8 to K5Vea.
- The dust envelope extends to 1 AU from the star and is observed edge-on.
Frequently Asked Questions
What is BM Andromedae?
BM Andromedae is a young T Tauri star in the constellation Andromeda that is still actively gathering mass from its surroundings. It represents a star caught in its formative accretion phase, well before it settles onto the main sequence.
How does BM Andromedae's brightness change over time?
The star's apparent visual magnitude fluctuates irregularly between about 11.63 at peak and 14.02 at its faintest. These unpredictable dips and recoveries are driven by the clumpy, episodic flow of material onto the young stellar surface.
What surrounds BM Andromedae?
Both a gaseous envelope and a dusty circumstellar disk encircle the star, with the dust component reaching out to roughly 1 AU. Measurements from 2005–2006 placed the inner disk at approximately 0.25 AU, offering a close-up view of a protoplanetary environment.
Why is BM Andromedae important to astronomers?
Because it is still in the accretion phase, it acts as a natural laboratory for understanding how stars like the Sun originally assembled themselves. Its dual gaseous and dusty envelopes, combined with its irregular variability, make it a particularly rich target for probing early stellar evolution.
What is BM Andromedae's spectral classification?
The star spans a range from F8 to K5 with the Vea designation, signalling a young, emission-line-rich object. This broad spectral span reflects the ongoing surface changes that accompany continued mass accretion.
More in Variable Stars, Part 2 1-24
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