EP Aquarii
Semiregular variable with a complex circumstellar envelope.
EP Aquarii, a semiregular variable star located in the equatorial constellation Aquarius, is a cool red giant on the asymptotic giant branch. Its complex circumstellar envelope has drawn significant research attention. At its brightest, the star reaches a visual magnitude of 6.37, making it potentially just visible to the naked eye under perfect conditions.
In the 1870s, John Birmingham recorded ten magnitude estimates for this star, ranging from 6 to 8, and noted a "quick change" in brightness. Although his observed range was much wider than the modern GCVS values (6.37 to 6.82), his 1877 publication served as the reference when EP Aquarii received its variable star designation in 1973.
The study of its extended circumstellar envelope began in 1984, when a carbon monoxide rotational transition line was detected using the NRAO 12m telescope. In the early 1990s, IRAS satellite data revealed an extended dust shell roughly one light-year in radius. Later in the decade, high-resolution observations at the Caltech Submillimeter Observatory showed unusual CO line profiles, indicating two distinct stellar winds expanding at very different speeds: 1.4 km/s and 11 km/s. In the early 2000s, observations of the 21 cm hydrogen line at the Nançay Radio Observatory confirmed a large circumstellar shell with multiple velocity components.
With the Atacama Large Millimeter Array, far greater sensitivity and resolution became possible. The narrow 1.4 km/s emission feature was traced to a nearly face-on spiral structure, suggesting an unseen companion star. The faster wind originates from a bi-conical outflow whose pole is roughly aligned with our line of sight.
The chemical composition of an AGB star's circumstellar envelope depends largely on whether its atmosphere has more carbon or oxygen. EP Aquarii’s atmosphere contains more oxygen than carbon.
- Constellation
- Aquarius
- Variable star type
- semiregular
- Peak visual magnitude
- 6.37
- Magnitude variation
- about 1/2 magnitude
- Period
- 55 days
- Spectral class
- cool red giant on the asymptotic giant branch (AGB)
- Gcvs magnitude range
- 6.37 to 6.82
Lore & Background
In 1877, John Birmingham published a set of ten magnitude estimates for EP Aquarii (number 596 on his list) made during the 1870s, which ranged from magnitude 6 to 8. He listed the star as 'Variable (?)', although he also claimed to have observed 'a quick change' in magnitude. Birmingham's magnitude range is far wider than the 6.37 to 6.82 range listed in the GCVS; nonetheless, his publication was cited as the reference when EP Aquarii received its variable star designation in 1973.
The study of EP Aquarii's extended CSE began in 1984, when a spectral line arising from a rotational transition of carbon monoxide (CO) was detected by Zuckerman and Dyck, using the NRAO 12m telescope. In the early 1990s, analysis of the IRAS satellite data showed the presence of an extended dust shell surrounding the star, with a radius of about 1 lightyear. In the late 1990s, high spectral-resolution observations at the Caltech Submillimeter Observatory (CSO) showed that EP Aquarii's CO line profiles had an unusual shape that suggested the presence of two distinct stellar winds, expanding at dramatically different velocities: 1.4 and 11 km/sec. In the early 2000s, observations of the 21 cm line of atomic hydrogen at the Nançay Radio Observatory confirmed the presence of a large circumstellar shell with multiple velocity components.
The completion of the Atacama Large Millimeter Array allowed EP Aquarii to be studied with far higher sensitivity and angular resolution. The very narrow emission feature (indicating an expansion rate of 1.4 km/sec) seen in the CSO spectra was found to arise from a spiral structure, nearly face-on to our line of sight, which suggested the presence of an unseen companion star. The higher velocity wind arises from a bi-conical outflow, the pole of which is roughly aligned to our line of sight.
Reader's Guide
EP Aquarii's significance lies in its complex circumstellar envelope (CSE), which has been a focus of study since 1984. The detection of CO emission by Zuckerman and Dyck using the NRAO 12m telescope initiated research into its extended structure. IRAS satellite data later revealed a dust shell about 1 lightyear in radius. High-resolution observations at the Caltech Submillimeter Observatory uncovered two distinct stellar winds with velocities of 1.4 and 11 km/sec, an unusual feature that suggested multiple outflow components. Subsequent 21 cm hydrogen line observations at the Nançay Radio Observatory confirmed a large circumstellar shell with multiple velocity components. The Atacama Large Millimeter Array provided far higher sensitivity and angular resolution, revealing that the narrow 1.4 km/sec emission arises from a spiral structure, nearly face-on, implying an unseen companion star, while the higher velocity wind comes from a bi-conical outflow aligned roughly to our line of sight. The star's atmosphere contains more oxygen than carbon, which influences the chemical compounds found in its CSE. Birmingham's early magnitude estimates, though disputed in range, were cited when the star received its variable star designation in 1973.
Did You Know?
- John Birmingham published ten magnitude estimates for EP Aquarii in 1877, ranging from magnitude 6 to 8.
- The star's circumstellar envelope includes a dust shell with a radius of about 1 lightyear, detected via IRAS satellite data.
- Two distinct stellar winds from EP Aquarii expand at 1.4 and 11 km/sec.
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