Exocomet

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Exocomets and various planet-formation processes around Beta Pictoris, a very young A-type main-sequence star (NASA; artist's conception). NASA-ExocometsAroundBetaPictoris-ArtistView.jpg
Exocomets and various planet-formation processes around Beta Pictoris, a very young A-type main-sequence star
(NASA; artist's conception).

An exocomet, or extrasolar comet, is a comet outside the Solar System, which includes rogue comets and comets that orbit stars other than the Sun. The first exocomets were detected in 1987 [1] [2] around Beta Pictoris, a very young A-type main-sequence star. There are now (as of February 2019) a total of 27 stars around which exocomets have been observed or suspected. [3] [4] [5] [6] [7]

Contents

The majority of discovered exocometary systems (Beta Pictoris, HR 10, [8] 51 Ophiuchi, HR 2174, [9] HD 85905, [10] 49 Ceti, 5 Vulpeculae, 2 Andromedae, HD 21620, Rho Virginis, HD 145964, [11] [12] HD 172555, [13] Lambda Geminorum, HD 58647, Phi Geminorum, Delta Corvi, HD 109573, [14] Phi Leonis, [15] 35 Aquilae, [16] HD 24966, HD 38056, HD 79469 and HD 225200 [3] ) are around very young A-type stars. The relatively old shell star Phi Leonis shows evidence of exocomets in the spectrum [15] and comet-like activity was detected around the old F2V-type star Eta Corvi. [4] In 2018 transiting exocomets were discovered around F-type stars, using data from the Kepler space telescope. [6] Some late B-type star (e.g. 51 Ophiuchi, HD 58647) are known to host exocomets. [14] [9]

Observations of comets, and especially exocomets, improve our understanding of planet formation. Indeed, in the standard model of planet formation by accretion, planets are the result of the agglomeration of planetesimals, themselves formed by the coalescence of dust from the protoplanetary disk surrounding the star shortly after its formation. Thus, comets are the residuals of the volatile-rich planetesimals that remained in the planetary system without having been incorporated into the planets. They are considered fossil bodies that have seen the physical and chemical conditions prevailing at the time of planet formation.[ citation needed ]

Researching exocomets might provide answers to fundamental questions of the past of the solar system and the development of a life-supporting environment. Researchers can investigate the transport of water, cyanides, sulfides and pre-biotic molecules onto Earth-mass exoplanets with the help of exocomets. [17] [18]

Nomenclature

The scientific term of an exocomet is Falling Evaporating Body (FEB). [6] The term Evaporating Infalling Bodies (EIBs) was first used, [19] but eventually the term FEBs was adopted from the "Falling Evaporating Bodies" model [20] or Falling Evaporating Body (FEB) scenario. [21]

Observation

The exocomets can be detected by spectroscopy as they transit their host stars. The transits of exocomets, like the transits of exoplanets, produce variations in the light received from the star. Changes are observed in the absorption lines of the stellar spectrum: the occultation of the star by the gas cloud coming from the exocomet produces additional absorption features beyond those normally seen in that star, like those observed in the ionized calcium lines. As the comet comes close enough to the star, cometary gas is evolved from the evaporation of volatile ices and dust with it. The absorption lines of a star hosting exocomets represent, beside a stable component, one or several variable redshifted components. The variable components change on short-time scales of one hour. The variable component represent the exocomets. The exocomet falls towards the star and any absorption line produced by the evaporation of the exocomet is redshifted compared to the absorption line of the star. [8]

Observations of HR 10 with the PIONIER (VLTI) and 32 years of radial velocity observations revealed that this exocomet host candidate turned out to be a binary star with each star being surrounded by a circumstellar shell. This new result can explain the variable spectral lines without exocomets. The study points out that 50% of the A-type stars could be resolved into binaries in the future and more systems with variable spectral lines attributed to exocomets could turn out to be binaries. [22]

Transiting exocomets were first detected around KIC 3542116 and possibly KIC 11084727 by a group of citizen scientists and professional astronomers. The Kepler mission detected asymmetrical dips around KIC 3542116, a F2V-type star that are consistent with models of transiting exocomets. The dips were found by one of the authors, a Planet Hunters participant, in a visual search over 5 months of the complete Q1-Q17 Kepler light curve archive spanning 201250 target stars. [6] [23] TESS did observe transits of exocomets around Beta Pictoris. [24] The shape of a dip caused by a transiting exocomet is modelled as a very specific "rounded triangular" shape and can be distinguished from most transiting exoplanets. [25] [26] A transiting exocomet around HD 182952 (KIC 8027456) is the first exocomet found in an automated search for transiting exocomets. [7] Irregular dimming events around KIC 8462852 [5] have been interpreted as exocomets, but the shape of the dips are different from discovered exocomet transits. [24]

During formation of the Oort Cloud through planetary perturbations, stellar encounters, and the galactic tide, a comet can be ejected and leave the solar system. [27] Binary systems are another possible source of ejected exocomets. [28] These ejected exocomets belong to the interstellar comets and can be observed directly if they enter the solar system. [29] [30]

Observations of β Pictoris with TESS in 2022 led to the discovery of 30 new exocomets. [31]

Indirect evidence of exocomets

Exocomets are suggested as one source of white dwarf pollution. After a star from the main sequence becomes a giant star, it loses mass. Planetesimals in an analog of the solar Oort Cloud can be directed toward the inner stellar system. This is a consequence of the mass-loss during the AGB stage. [32] The giant star will eventually become a white dwarf and an exocomet that gets too close to the white dwarf will sublimate or tidal disrupted by the gravity of the white dwarf. This will produce dusty debris around the white dwarf, which is measurable in infrared wavelengths. [33] The material can be accreted by the white dwarf and pollute the atmosphere. This pollution appears in the spectra of a white dwarf as metal lines. [34] In 2017 a study concluded that spectral lines in the white dwarf WD 1425+540 are attributed to an accretion of a Kuiper-Belt analog. Kuiper-Belt objects are icy bodies in the solar system that sometimes become comets. [35] [36] Dusty material around the white dwarf G 29-38 [37] and WD 1337+705 [38] also has been attributed to an exocomet.

Carbon monoxide gas was found in debris disks around mostly A-type stars with an age between 10 and 50 Myrs, but in some cases in older systems (e.g. Eta Corvi 1-2 Gyrs) and in colder systems (TWA 7). It is not clear if this gas is primordial or secondary produced by collision of exocomets. Around 30 such systems exist. [39] Carbon monoxide gas around 49 Ceti has been attributed to the collisions of comets in that planetary system. [40]

See also

Related Research Articles

<span class="mw-page-title-main">Pictor</span> Constellation in the southern celestial hemisphere

Pictor is a constellation in the Southern Celestial Hemisphere, located between the star Canopus and the Large Magellanic Cloud. Its name is Latin for painter, and is an abbreviation of the older name Equuleus Pictoris. Normally represented as an easel, Pictor was named by Abbé Nicolas-Louis de Lacaille in the 18th century. The constellation's brightest star is Alpha Pictoris, a white main-sequence star around 97 light-years away from Earth. Pictor also hosts RR Pictoris, a cataclysmic variable star system that flared up as a nova, reaching apparent (visual) magnitude 1.2 in 1925 before fading into obscurity.

<span class="mw-page-title-main">Planetary system</span> Set of non-stellar objects in orbit around a star

A planetary system is a set of gravitationally bound non-stellar bodies in or out of orbit around a star or star system. Generally speaking, systems with one or more planets constitute a planetary system, although such systems may also consist of bodies such as dwarf planets, asteroids, natural satellites, meteoroids, comets, planetesimals and circumstellar disks. For example, the Sun together with the planetary system revolving around it, including Earth, form the Solar System. The term exoplanetary system is sometimes used in reference to other planetary systems.

<span class="mw-page-title-main">Beta Pictoris</span> Second brightest star in the southern constellation of Pictor

Beta Pictoris is the second brightest star in the constellation Pictor. It is located 63.4 light-years (19.4 pc) from the Solar System, and is 1.75 times as massive and 8.7 times as luminous as the Sun. The Beta Pictoris system is very young, only 20 to 26 million years old, although it is already in the main sequence stage of its evolution. Beta Pictoris is the title member of the Beta Pictoris moving group, an association of young stars which share the same motion through space and have the same age.

<span class="mw-page-title-main">AU Microscopii</span> Star in the constellation Microscopium

AU Microscopii is a young red dwarf star located 31.7 light-years away – about 8 times as far as the closest star after the Sun. The apparent visual magnitude of AU Microscopii is 8.73, which is too dim to be seen with the naked eye. It was given this designation because it is in the southern constellation Microscopium and is a variable star. Like β Pictoris, AU Microscopii has a circumstellar disk of dust known as a debris disk and at least two exoplanets, with the presence of an additional two planets being likely.

<span class="mw-page-title-main">Debris disk</span> Disk of dust and debris in orbit around a star

A debris disk, or debris disc, is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris disks are found around stars with mature planetary systems, including at least one debris disk in orbit around an evolved neutron star. Debris disks can also be produced and maintained as the remnants of collisions between planetesimals, otherwise known as asteroids and comets.

HD 33564 is a single star with an exoplanetary companion in the northern constellation of Camelopardalis. It has an apparent visual magnitude of 5.08, which means it is a 5th magnitude star that is faintly visible to the naked eye. The system is located at a distance of 68 light years from the Sun based on parallax, and it is drifting closer with a radial velocity of −11 km/s. It is a candidate member of the Ursa Major Moving Group.

<span class="mw-page-title-main">HD 100546</span> Star in the constellation Musca

HD 100546, also known as KR Muscae, is a pre-main sequence star of spectral type B8 to A0 located 353 light-years from Earth in the southern constellation of Musca. The star is surrounded by a circumstellar disk from a distance of 0.2 to 4 AU, and again from 13 AU out to a few hundred AU, with evidence for a protoplanet forming at a distance of around 47 AU.

<span class="mw-page-title-main">HD 15115</span> F-type subgiant star in the constellation Cetus

HD 15115 is a single star in the equatorial constellation of Cetus. It is readily visible in binoculars or a small telescope, but is considered too dim to be seen with the naked eye at an apparent visual magnitude of 6.76. The distance to this object is 160 light years based on parallax, and it is slowly drifting further away at the rate of about 1 km/s. It has been proposed as a member of the Beta Pictoris moving group or the Tucana-Horologium association of co-moving stars; there is some ambiguity as to its true membership.

<span class="mw-page-title-main">Beta Pictoris b</span> Super Jupiter orbiting Beta Pictoris

Beta Pictoris b (abbreviated as β Pic b) is an exoplanet orbiting the young debris disk A-type main sequence star Beta Pictoris located approximately 63 light-years (19.4 parsecs, or 6×1014 km) away from Earth in the constellation of Pictor. It has a mass around 13 Jupiter masses and a radius around 46% larger than Jupiter's. It orbits at 9 AU from Beta Pictoris, which is about 3.5 times farther than the orbit of Beta Pictoris c. It orbits close to the plane of the debris disk orbiting the star, with a low eccentricity and a period of 20–21 years.

<span class="mw-page-title-main">HD 172555</span> Star in the constellation Pavo

HD 172555 is a white-hot Type A7V star located relatively close by, 95 light years from Earth in the direction of the constellation Pavo. Spectrographic evidence indicates a relatively recent collision between two planet-sized bodies that destroyed the smaller of the two, which had been at least the size of the Moon, and severely damaged the larger one, which was at least the size of Mercury. Evidence of the collision was detected by NASA's Spitzer Space Telescope.

51 Ophiuchi is a single star located approximately 410 light years away from the Sun in the equatorial constellation of Ophiuchus, northwest of the center of the Milky Way. It is visible to the naked eye as a faint, blue-white point of light with an apparent visual magnitude of 4.81. The star is moving closer to the Earth with a heliocentric radial velocity of –12 km/s.

HD 256 is a binary star system in the equatorial constellation of Cetus. It has a white hue and is dimly visible to the naked eye with an apparent visual magnitude of 6.20. Based upon parallax measurements, the system is located at a distance of approximately 474 light years from the Sun. It is drifting closer with a radial velocity of −10 km/s.

<span class="mw-page-title-main">AK Leporis</span> Star in the constellation Lepus

AK Leporis is a variable star in the southern constellation of Lepus the hare. It has an apparent visual magnitude of 6.141, so, according to the Bortle scale, it is faintly visible from rural skies at night. This star forms a visual double with Gamma Leporis—the two have an angular separation of 97″, making them difficult to separate with the naked eye even under the best conditions. Both Gamma Leporis and AK Leporis are members of the Ursa Major Moving Group of stars that share a common motion through space.

<span class="mw-page-title-main">HD 61005</span> Star in the constellation Puppis

HD 61005, also known as HIP 36948 and The Moth, is a young star located in the southern constellation Puppis, the poop deck. It has an apparent magnitude of 8.22, making it readily visible in binoculars, but not to the naked eye. The object is located relatively close at a distance of 119 light years based on Gaia DR3 parallax measurements but is receding with a heliocentric radial velocity of 22.6 km/s.

HD 219623 is a solitary star in the northern circumpolar constellation of Cassiopeia. HD 219623 is its Henry Draper Catalogue designation. It has an apparent visual magnitude of 5.59, which lies in the brightness range that is visible to the naked eye. According to the Bortle scale, it can be observed from dark suburban skies. Parallax measurements place it at an estimated distance of around 67.2 light years. It has a relatively high proper motion, advancing 262 mas per year across the celestial sphere.

<span class="mw-page-title-main">PicSat</span> French Nanosatellite

PicSat was a French observatory nanosatellite, designed to measure the transit of Beta Pictoris b, an exoplanet which orbits the star Beta Pictoris.

<span class="mw-page-title-main">HD 101584</span> Star in the constellation Centaurus

HD 101584 is a suspected post-common envelope binary about 1,800 to 5,900 light-years distant in the constellation of Centaurus. The system is bright at optical wavelengths with an apparent visual magnitude of about 7. The primary is either a post-AGB star, but more likely a post-RGB star. The secondary is a red dwarf or possibly a low-luminosity white dwarf, which orbits the primary every 150-200 days. The system is surrounded by a slowly rotating circumbinary disk, probably with a face-on orientation towards the solar system and a size of about 150 astronomical units.

<span class="mw-page-title-main">Tucana-Horologium association</span> Large stellar association

The Tucana-Horologium association (Tuc-Hor), or Tucana Horologium moving group, is a stellar association with an age of 45 ± 4 Myr and it is one of the largest stellar associations within 100 parsecs. The association has a similar size to the Beta Pictoris moving group (BPMG) and contains, like BPMG, more than 12 stars with spectral type B, A and F. The association is named after two southern constellations, the constellation Tucana and the constellation Horologium.

<span class="mw-page-title-main">BF Orionis</span> Young protostar system

BF Orionis is a young Herbig Ae/Be star in the constellation of Orion about 1250 light years away, within the Orion Nebula. It is the most massive star of the small birth cluster of four stars.

References

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  38. ↑ Johnson, Ted M.; Klein, Beth L.; Koester, D.; Melis, Carl; Zuckerman, B.; Jura, M. (2022-12-01). "Unusual Abundances from Planetary System Material Polluting the White Dwarf G238-44". The Astrophysical Journal. 941 (2): 113. arXiv: 2211.02673 . Bibcode:2022ApJ...941..113J. doi: 10.3847/1538-4357/aca089 . ISSN   0004-637X.
  39. ↑ Cataldi, Gianni; Aikawa, Yuri; Iwasaki, Kazunari; Marino, Sebastian; Brandeker, Alexis; Hales, Antonio; Henning, Thomas; Higuchi, Aya E.; Hughes, A. Meredith; Janson, Markus; Kral, Quentin; Matrà, Luca; Moór, Attila; Olofsson, Göran; Redfield, Seth (2023-07-01). "Primordial or Secondary? Testing Models of Debris Disk Gas with ALMA". The Astrophysical Journal. 951 (2): 111. arXiv: 2305.12093 . Bibcode:2023ApJ...951..111C. doi: 10.3847/1538-4357/acd6f3 . ISSN   0004-637X.
  40. ↑ Zuckerman, B.; Song, Inseok (2012). "A 40 Myr Old Gaseous Circumstellar Disk at 49 Ceti: Massive CO-Rich Comet Clouds at Young A-Type Stars". The Astrophysical Journal. 758 (2): 77. arXiv: 1207.1747 . Bibcode:2012ApJ...758...77Z. doi:10.1088/0004-637X/758/2/77. S2CID   119198485.
  41. ↑ "Exocomets plunging into a young star (artist's impression)". www.spacetelescope.org. Retrieved 12 January 2017.
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