MWC 560

Lore & Background

MWC 560 was first cataloged in the Mount Wilson Catalogue of class O, B and A stars with bright hydrogen lines, published in 1933 by P. W. Merrill and associates. In 1973, N. Sanduleak and C. B. Stephenson included it in a list of objects in the southern Milky Way with strong emission lines, finding TiO bands and variable emission lines, leading to a classification of M4ep. In 1984, H. E. Bond and associates identified it as a symbiotic binary consisting of an M-type giant star with an orbiting compact companion, noting day-to-day changes in absorption line profiles and theorizing that matter is transferred from the giant at a rate exceeding the companion's accretion capacity, producing a flickering appearance.

In 1990, absorption lines revealed high-velocity components, possibly from a jet-like ejecta nearly along the line of sight. The IUE confirmed high velocities during an outburst, suggesting ejection of a cool, optically-thick shell from the compact object, with velocities up to 6,000 km/s and brightness increasing from magnitude 12.5 to 9.2. The high mass transfer rate implied an accretion disk orbiting the compact object, with a thick envelope hiding the inner disk and compact object, likely viewed nearly face-on. Over a ten-year period, a light variation with a period of 1,930 days was observed, possibly due to disk precession. Infrared observations suggested the M-giant may undergo pulsations with a period around 5 months, refined in 2007 to about 340 days, making it a semiregular variable probably consisting of a thermally pulsing asymptotic giant branch star. X-ray emission detected in 2009 was consistent with an accreting white dwarf, accreting mass at (1 to 20)×10−7 M☉·yr−1, with a few percent ejected in the jet.

Optical flickering persisted from 1984 until 2016, resumed briefly until 2018, then disappeared. The system then underwent a steady brightness increase with periodicities of 331 and 1860 days. As of 2023, it remains in a non-flickering state, with maximum brightness achieved in October 2021.

Reader's Guide

MWC 560 is significant as a symbiotic binary system that provides a natural laboratory for studying mass transfer, accretion disk dynamics, and jet formation. Its high-velocity ejecta, reaching up to 6,000 km/s, and the inferred nearly face-on accretion disk offer rare observational constraints on the geometry of such systems. The long-term light variation with a period of 1,930 days, attributed to disk precession, and the semiregular pulsations of the M-giant with a period of about 340 days, indicate complex interactions between the stellar components. The detection of X-ray emission consistent with an accreting white dwarf in 2009, along with the measured mass accretion rate, helps clarify the nature of the compact object. The system's transition between active and stable states, and the eventual cessation of flickering after 2018, highlight the variable behavior of symbiotic binaries. Its legacy lies in demonstrating how multi-wavelength observations—from optical to infrared to X-ray—can unravel the processes in interacting binaries, and it remains a target for understanding the evolution of symbiotic stars and the role of jets in mass loss.

Did You Know?

• MWC 560 has a variable star designation V694 Monocerotis.
• During an outburst, velocities of up to 6,000 km/s were recorded.
• The system's light variation has a period of 1,930 days, possibly due to disk precession.
• X-ray emission detected in 2009 is consistent with an accreting white dwarf.