WR 121-16
A dim, transitional Wolf-Rayet star near the Wild Duck Cluster.
WR 121-16 sits in the constellation Scutum, not far from the Wild Duck Cluster (M11). It belongs to the Far 3 kpc Arm of the Milky Way and is extremely faint from Earth, with an apparent magnitude around 14. That dimness comes from heavy interstellar reddening and a distance of more than 23,000 light-years. It ranks among the least luminous conventional Wolf-Rayet stars known, shining at less than 76,000 times the Sun’s brightness. Its magnitude drifts irregularly between 13.95 and 14.14.
This star is a recent addition to the Wolf-Rayet Star Catalogue—the 667th entry—and was discovered in August 2020. It turned up as a by-product of LAMOST testing observations during full moon nights, when the telescope was aimed at the open cluster M11. WR 121-16 lies about 42 arcminutes and 24 arcseconds from M11’s center.
WR 121-16 is one of the few transitional Wolf-Rayet stars, showing both carbon and nitrogen emission lines, with a spectral type of WN7o/WC. Modeling its spectrum reveals a low luminosity of just 75,900 times the Sun’s. It holds a little over 7 solar masses, almost entirely helium—about 1.5% nitrogen and 0.2% carbon. Strong stellar winds, typical for Wolf-Rayet stars, blow at a terminal velocity of 1,000 kilometers per second, causing it to lose 10⁻⁴·⁹⁷ solar masses per year—far more than the Sun’s meager loss rate. These winds are so dense that the star’s photosphere is hidden. For consistency with other Wolf-Rayet stars, its radius is defined at an optical depth of 20, roughly 4 solar radii. A “transformed” radius at an optical depth of 2/3—more comparable to other star types—comes to about 6 solar radii.
- Constellation
- Scutum
- Apparent magnitude range
- 13.95 to 14.14
- Distance
- over 23,000 light years
- Luminosity
- 75,900 L☉
- Spectral type
- WN7o/WC
- Mass
- just over 7 solar masses
- Mass loss rate
- 10−4.97 M☉/year
Lore & Background
WR 121-16 was discovered in August 2020 as a ‘by-product’ of the LAMOST testing observations during full moon nights, when the telescope was pointing to the open cluster M11. The star lies about 42′ 24″ from the centre of M11, in the Far 3 kpc Arm of the Milky Way. It is very dim from Earth, having an apparent magnitude of about 14, due to heavy reddening by interstellar extinction and its distance of over 23,000 light years.
WR 121-16 is one of a few transitional Wolf-Rayet stars, displaying both carbon and nitrogen emission, with a spectral type of WN7o/WC. Modelling its spectrum shows it has a luminosity of just 75,900 L☉, much less than most Wolf-Rayet stars. It has just over 7 solar masses, nearly all of which is helium, with 1.5% nitrogen and 0.2% carbon. Strong stellar winds, typical of Wolf-Rayet stars, with a terminal velocity of 1,000 kilometers per second cause it to lose 10−4.97 M☉/year, much more than the Sun's mass loss rate. The winds are so dense that the photosphere is not visible; its radius is defined at an optical depth of 20, about 4 R☉, while a 'transformed' radius at an optical depth of 2/3 is about 6 R☉.
Reader's Guide
WR 121-16 is significant as one of the dimmest known conventional Wolf-Rayet stars, with a luminosity of just 75,900 L☉, challenging the typical high-luminosity profile of such stars. Its transitional spectral type (WN7o/WC) places it in a rare category that shows both carbon and nitrogen emission, offering insight into the evolutionary phase between nitrogen-rich and carbon-rich Wolf-Rayet stars. Discovered in August 2020 as a by-product of LAMOST testing observations, it was added as the 667th entry to the Wolf-Rayet Star Catalogue. Its location in the Far 3 kpc Arm and heavy reddening by interstellar extinction contribute to its faint apparent magnitude. The star's strong stellar winds, with a terminal velocity of 1,000 km/s and a mass loss rate of 10−4.97 M☉/year, are typical of Wolf-Rayet stars but occur at a much lower luminosity. The dense winds obscure its photosphere, and its radius is defined at an optical depth of 20, about 4 R☉, with a transformed radius of about 6 R☉ at optical depth 2/3. WR 121-16's low mass (just over 7 solar masses, mostly helium) and composition (1.5% nitrogen, 0.2% carbon) provide a benchmark for understanding the least luminous members of the Wolf-Rayet class.
Did You Know?
- WR 121-16 varies irregularly between magnitudes 13.95 and 14.14.
- The star has a mass loss rate of 10−4.97 M☉/year, far exceeding the Sun's.
- Its spectral type is WN7o/WC, indicating both carbon and nitrogen emission.
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