George's Random Astronomical Object

Object 101: Eta Corvi

Podcast release date: 26 June 2023

Right ascension: 12:32:04.2

Declination: -16:11:46

Epoch: J2000

Constellation: Corvus

Corresponding Earth location: About 245 km south of the island of Upolu in Samoa or about 190 km east of Niuatoputapu in Tonga

Eta Corvi is the sixth brightest star in the constellation Corvus. The constellation represents a crow and indeed looks very similar to a crow if you think a crow is shaped liked an irregular quadrilateral with a very short tail. Eta Corvi is located just next to Delta Corvi in the upper left part of the quadrilateral. The star has a magnitude of about 4.3 [1], which means that it's faint but visible without a telescope from a dark location with few or no lights.

Eta Corvi is classified as an F-type star [2, 3, 4], which means that it's similar to but slightly hotter, slightly brighter, and slightly larger than the Sun but otherwise fuses hydrogen into helium in its core like the Sun. It's located at a distance of 59.49 light years (18.24 pc) [5, 6], so it's kind of close but not really close. The age of the star is between 1 and 2 billion years old [2, 3, 4], so while it's not as old as the Sun, it's basically a star that finished forming a very long time ago, so nothing exciting should be happening. It would seem to be rather ordinary except that it is surrounded by a disk of dust, and that disk has attracted a lot of attention from a lot of professional astronomers.

The first hint that Eta Corvi has a dust dist was when the Infrared Astronomical Satellite (IRAS), a 1980s spacecraft that performed relatively crude infrared observations of the entire sky, detected excess infrared emission from the Eta Corvi system, although it wasn't until the 1990s that people noticed that the data showed this [7]. One of the issues with dust surrounding stars is that it does not emit light in the visible part of the electromagnetic spectrum. Dust does reflect (or, to use the technical term, scatter) a little bit of light from the star itself, but that light is really hard to see because the star is so much brighter than the light scattered from the dust. However, the dust disk will also absorb starlight in the visible part of the electromagnetic spectrum and re-radiate that energy in the infrared part of the spectrum, and that ifnrared emission can be relatively bright compared to the star.

The infrared band can be divided into roughly three parts: the near-infrared, which is close in wavelength to what we can see with our own eyes; the mid-infrared, which is in the middle of the infrared part of the spectrum; and the far-infrared, which is very far away in wavelength from what we can see. Beyond the far-infrared, the wavelengths of light actually get close to a millimeter, and astronomers refer to this radiation as either submillimeter or millimeter radiation depending on whether the wavelengths of the radiation are shorter or longer than a millimeter. The reason why I am talking about this is that dust in space radiates in different parts of the electromagnetic spectrum depending on its temperature. Mid-infrared emission come from dust that is somewhere between roughly 100 to 300 K (or somewhere ranging from slightly warmer than liquid nitrogen [8] to the temperature of a nice summer Earth day). Dust emission in the far-infrared, submillimeter, and millimeter bands comes from dust at much colder temperatures that could reach as low as 10 K (or colder than the temperature at which hydrogen in a laboratory on Earth would liquify [9]).

After learning that Eta Corvi might have a dust disk, which is technically referred to as a debris disk, astronomers repeatedly targeted the star system with all sorts of infrared, submillimeter, and millimeter telescopes in attempts to not only detect the emission from the dust disk but also to image it. If you remember, I described the IRAS images from the 1980s as crude, and Eta Corvi basically looked like a blurry dot in the data from IRAS (as did most of everything else in the universe). In 2005, a group led by Mark Wyatt published the first images of Eta Corvi's dust disk, which were made at submillimeter wavelengths using the James Clerk Maxwell Telescope [10]. Subsequently, people made other observations at mid-infrared, far-infrared, and submillimeter wavelengths to either get data at more wavelengths or to try to make better images. The best images that I have seen were published in 2017 by a group led by Sebastian Marino based on submillimeter data from the Atacama Large Millimeter/submillimeter Array (ALMA) [11].

The ALMA images revealed the presence of a flattened ring of cold dust orbiting the star at a distance of about 152 Astronomical Units (AU) [11]. As a reminder, one AU is equivalent to the distance from the Earth to the Sun. This ring is thought to be in a location equivalent to the Kuiper Belt in our Solar System, which is where short-period comets like Halley's Comet come from. However, our Solar System's Kuiper Belt has a radius of only 30 to 55 AU [12], or roughly one-quarter the radius of the ring of cold dust surrounding Eta Corvi. Anyway, the ring of cold dust in the Eta Corvi system is thought to be formed by comet-like bodies colliding with each other, thus producing lots and lots of small dust particles that end up spread over a wide area and that can therefore emit lots of far-infrared and submillimeter emission very efficiently.

However, astronomers have found much more mid-infrared emission than what would be expected in a star surrounded by a single ring of cold dust. This indicates that the Eta Corvi system also contains warmer dust, and it is most likely that the warmer dust is located within an inner dust disk [11]. While it is easy to explain where the ring of cold dust comes from, it is really hard to explain where that inner disk of warmer dust came from.

It's clear that this inner dust disk is not a disk of gas and dust left over from when the star formed; the star is simply too old for such a protostellar disk to still be present. It is also a little difficult to explain the inner dust disk as forming from lots of colliding asteroids or lots of other colliding objects within the inner parts of the star system [11]. As a reminder, the Eta Corvi star system is over a billion years old, and everyone expects that, in the inner parts of any star system, the orbits of everything will have stabilized after that amount of time, and planet-like objects will have stopped colliding with each other. What is more likely is that some sort of gravitational interaction between the outer dust ring and one or more planets that no one has seen yet is transferring dust and possibly comet-like objects from the outskirts of the star system into its central region [11]. This could supply dust to an inner dust disk. It is even possible that a larger object transferred from the outer dust ring to the inner regions of the star system may have hit a planet orbiting Eta Corvi at a distance of about 3 AU, and such a collisiion could have kicked up a lot of extra dust in the inner part of the star system, thus producing the dust disk we see today [11].

It will be interesting to see where this research goes. I look forward to finding out whether people can image the inner dust disk as well as they have imaged the outer dust ring or if they can find planets in the star system.


[1] Hog, E. et al., The Tycho-2 catalogue of the 2.5 million brightest stars, 2000, Astronomy & Astrophysics, 355, L27

[2] Ibukiyama, A. and Arimoto, N., HIPPARCOS age-metallicity relation of the solar neighbourhood disc stars, 2002, Astronomy & Astrophysics, 394, 927

[3] Mallik, Sushma V. et al., Lithium and rotation in F and G dwarfs and subgiants, 2003, Astronomy & Astrophysics, 409, 251

[4] Vican, Laura, Age Determination for 346 Nearby Stars in the Herschel DEBRIS Survey, 2012, Astronomical Journal, 143, 135

[5] Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1

[6] Gaia Collaboration et al., Gaia Early Data Release 3: Summary of the contents and survey properties, 2020, arXiv e-prints, arXiv:2012.01533

[7] Sylvester, R. J. et al., Optical, infrared and millimetre-wave properties of Vega-like systems., 1996, Monthly Notices of the Royal Astronomical Society, 279, 915

[8] Air Products and Chemicals, Inc., Liquid nitrogen, 2019

[9] Air Products and Chemicals, Inc., Hydrogene liquide, 2023

[10] Wyatt, M. C. et al., Submillimeter Images of a Dusty Kuiper Belt around eta Corvi, 2005, Astrophysical Journal, 620, 492

[11] Marino, S. et al., ALMA observations of the eta Corvi debris disc: inward scattering of CO-rich exocomets by a chain of 3-30 M$_\oplus$ planets?, 2017, Monthly Notices of the Royal Astronomical Society, 465, 2595

[12] Davis, Phil and Carney, Steve, Kuiper Belt: In Depth, 2017, Solar System Exploration: Our Galactic Neighborhood

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Last update: 26 June 2023