The Frosty Leo Nebula (IRAS 09371+1212) is a bipolar protoplanetary nebula (PPN) located approximately 3,000 light-years away in the constellation Leo. It has an apparent magnitude of 11 and an apparent size of 25 arcseconds. The nebula is unusual because crystalline ice dominates its long-wavelength emission spectrum. It is one of the nearest and brightest objects of its kind, and one of the few preplanetary nebulae visible in amateur telescopes.
The Frosty Leo Nebula is one of the larger proto-planetary nebulae visible from Earth. It was nicknamed Frosty Leo because it appears rich in water ice grains. It has an hourglass shape with two lobes, a spherical halo, large loops and a disk around the progenitor star. The central star is a post-AGB (asymptotic giant branch) star with the spectral class K7 III or K7 II.
Protoplanetary nebulae are transitional objects between evolved red giant stars and planetary nebulae. They are short-lived objects and therefore rare. They form when an evolved star on the late asymptotic giant branch (LAGB) loses some of its hydrogen envelope and its temperature rises to about 5,000 K. During this phase, the progenitor star is still not hot enough to ionize the expelled material, but its stellar winds are fast enough to sculpt the surrounding nebulosity.
Picture of IRAS 09371+1212, the Frosty Leo Nebula, a protoplanetary nebula using the Hubble Space Telescope’s Advanced Camera for Surveys High-Resolution Channel (HRC). Three thousand light-years from Earth lies the strange protoplanetary nebula IRAS 09371+1212, nicknamed the Frosty Leo Nebula. Despite their name, protoplanetary nebulae have nothing to do with planets: they are formed from material shed from their aging central star. The Frosty Leo Nebula has acquired its curious name as it has been found to be rich in water in the form of ice grains, and because it lies in the constellation of Leo. This nebula is particularly noteworthy because it has formed far from the galactic plane, away from interstellar clouds that may block our view. Its intricate structure strongly suggests that the formation processes are complex and it has been suggested that there could be a second star, currently unseen, contributing to the shaping of the nebula. Protoplanetary nebulae like the Frosty Leo Nebula have brief lifespans by astronomical standards and are precursors to the planetary nebula phase, in which radiation from the star will make the nebula’s gas light up brightly. Their rarity makes studying them a priority for astronomers who seek to understand better the evolution of stars. This picture was created from images taken with the High Resolution Channel of Hubble’s Advanced Camera for Surveys, which images a small area of sky (only 26 by 29 arcseconds) in high detail. Image credit: ESA/Hubble & NASA (CC BY 4.0)
Toward the end of the AGB phase, red giants experience high mass loss. The mass loss and velocity of the expelled material increase drastically for a few thousand years. This is known as the superwind phase and is critical for the global shaping of the nebula.
The preplanetary nebula stage ends when the effective temperature of the central star reaches 30,000 K. At this point, the star is hot enough to produce enough ultraviolet radiation to ionize the ejected envelope. The surrounding nebula no longer just reflects the light of the central post-AGB star but produces its own light and becomes a planetary nebula. Protoplanetary nebulae are reflection nebulae, while planetary nebulae are emission nebulae.
The name Frosty Leo comes from a 1987 study led by T. Forveille of the Grenoble Observatory in France, which was based on observations with the 30-m IRAM radio telescope at Pico Veleta, Spain. The study authors found that the circumstellar envelope around the central star contained large amounts of ice, silicates, and carbon monoxide (CO), but did not show hydroxyl (OH) emission and did not have a significant amount of dust warmer than 80 K. The authors were the first to identify IRAS 09371+1212 as a post-AGB star with a bipolar structure. The presence of ice water and the nebula’s morphology were confirmed in 1988.
The Frosty Leo is an unusual PPN with a striking symmetry. It is not axially symmetric like other bipolar preplanetary nebulae. Instead, it shows a point reflection symmetry. The orientation of the bipolar symmetry axis at each distance from the central star varies with the distance. Astronomers have proposed that this symmetry may be caused by a precessing bipolar outflow, possibly directed by a precessing disk at the nebula’s centre.
Similar point reflection symmetry is fairly common among planetary nebulae. Examples include the Dumbbell Nebula (Messier 27, NGC 6853) in the constellation Vulpecula, the Cat’s Eye Nebula (NGC 6543, Caldwell 6) in Draco, the Saturn Nebula (NGC 7009, Caldwell 5) in Aquarius, NGC 6563 in Sagittarius, NGC 2022 in Orion, the Double-Bubble Nebula (NGC 2371-2) in Gemini, and the Box Nebula (NGC 6309) in Ophiuchus.
The two lobes of the Frosty Leo Nebula appear both in visible and near-infrared images. They are separated by what astronomers believe is an almost edge-on dust ring. The lobes are separated by only 2 seconds of arc. The nebula’s molecular envelope is expanding at around 25 km/s.
The Frosty Leo has prominent ansae, compact nebulosities separated by around 23 arcseconds along the nebula’s bipolar axis. In planetary nebulae, polar ansae form when a strong outflow along the central star’s poles collides with material in an existing circumstellar shell.
However, observations in 1990 revealed that the ansae in the Frosty Leo did not have associated emission lines and were different from those seen in other nebulae in that they appeared to be reflection features. The nebula does not show any evidence of a high-velocity wind.
The relative brightness, large size and complex structure of the Frosty Leo make the nebula a popular target for study. The nebula formed far from the Milky Way plane and is not obscured by interstellar clouds. Its location, over 0.9 kiloparsecs from the galactic plane, is also high unusual.
Frosty Leo is a magnitude 11 (post-AGB) star surrounded by an envelope of gas, dust, and large amounts of ice (hence the name). The associated nebula is of “butterfly” shape (bipolar morphology) and it is one of the best known examples of the brief transitional phase between two late evolutionary stages, asymptotic giant branch (AGB) and the subsequent planetary nebulae (PNe). For a three-solar-mass object like this one, this phase is believed to last only a few thousand years, the wink of an eye in the life of the star. Hence, objects like this one are very rare and Frosty Leo is one of the nearest and brightest among them. Credit: ESO (CC BY 4.0)
Facts
The Frosty Leo Nebula was detected by the Infrared Astronomical Satellite (IRAS) and catalogued as IRAS 09371+1212 in 1983. It was noticed because of its exceptionally low IRAS colour temperatures.
In 1987, astronomers identified the central star as an evolved star experiencing mass loss and showing an unusual excess flux. They associated the excess to emission in the 46 µm (micron) band of ice, which led them to believe that a large portion of all grain material is in the form of ice.
Later observations supported this conclusion. Researchers found that the nebula had an exceptionally deep and narrow 3.1-µm ice absorption band. They proposed that the ice is mostly pure and in crystalline form.
In 1990, direct observations of the nebula’s spectrum with the Danish 1.5-m telescope at La Silla Observatory in Chile confirmed that very cold silicate dust grains with temperatures under 50 K were responsible for the 60-µm excess emission.
In 2005, astronomers used echelle spectroscopy to look for evidence of a fast stellar wind and H2 line emissions in Frosty Leo and several other preplanetary nebulae. They did not find any in the Frosty Leo. While the knots and ansae in the nebula indicate the presence of shocks, the team did not detect any signs of shock velocities high enough to excite H2 into emission. They associated this with a lack of dense molecular gas in the Frosty Leo’s polar lobes.
The non-detection of H2 may be linked with the mass and evolutionary stage of the central star. Previous studies had found that the element is not detected in stars with spectral types later than G2.
Location
The Frosty Leo Nebula lies in the region of the Sickle of Leo, a relatively bright star pattern that represents the Lion’s head and mane. It appears just north of Subra (Omicron Leonis) and west of Regulus (Alpha Leonis).
The nebula can be spotted in medium and large telescopes at high magnification but is a challenging target in smaller instruments.
The location of the Frosty Leo Nebula, image: Stellarium
Frosty Leo Nebula – IRAS 09371+1212
| Constellation | Leo |
| Object type | Protoplanetary nebula |
| Right ascension | 09h 39m 53.96s |
| Declination | +11° 58′ 52.6″ |
| Apparent magnitude | 11 |
| Apparent size | 25’’ |
| Distance | 3,000 light-years (920 parsecs) |
| Names and designations | Frosty Leo Nebula, IRAS 09371+1212, 2MASS J09395396+1158526, TIC 469014760, GSC 00824-01094, GSC2 N2023100103, [LFO93] 0937+12 |