Shell star

A shell star is a star having a spectrum that shows extremely broad absorption lines, plus some very narrow absorption lines. They typically also show some emission lines, usually from the Balmer series but occasionally of other lines. The broad absorption lines are due to rapid rotation of the photosphere, the emission lines from an equatorial disk, and the narrow absorption lines are produced when the disc is seen nearly edge-on.

Shell stars have spectral types O7.5 to F5, with rotation velocities of 200–300 km/s, not far from the point when the rotational acceleration would disrupt the star.

Pleiaden
Pleione in the Pleiades is a shell star.

Spectrum

The shell stars are defined as a group by the existence of rotationally broadened photospheric spectral lines in combination with very narrow absorption lines.[1][2] Emission lines are frequently present but not regarded as a defining feature. The exact spectral lines present vary to some extent: Balmer emission lines are very common, but may be weak or absent in cooler stars; FeII lines are common but not always present; helium lines may be seen in the hottest stars. The photospheric lines are rotationally broadened showing projected velocities of 200 km/s or more.[3]

The line profiles in shell star spectra are complex, with variable wings, cores, and superpositions of absorption and emission features. In some cases, particular absorption of emission features are only visible as modifications to a line profile, or a weakening of another line. This leads to double and triple-peaked lines, or asymmetric lines.[2]

Shell stars very commonly have emission lines and thus are often Be stars, although they can also occur in spectral classes O, A, and occasionally F.[2]

Subtypes

Shell stars have been subdivided into four categories, although these categories are no longer recognised as meaningful and are rarely seen in modern publications:[3]

  • early Be stars of spectral types O7.5 to B2.5
  • middle Be stars of types B3 to B6.5,
  • late Be stars of types B7 to B9.5, and
  • A-F shell stars ranging from A0 to F5.

The vast majority of known shell stars are of spectral class B. However, partly because of this many cooler shell stars have remained undetected.[1] The Be phenomenon, and hence the term Be star itself, is now widely applied to similar stars not only of spectral class B, but also A and sometimes O and F.

Variability

Shell stars often show variability in their spectra and brightness. The shell features may come and go, with the star changing from a shell star to a normal B star or Be star. Shell stars which show irregular variability due to changes in, or the disappearance of, the "shell" are called Gamma Cassiopeiae variables.[4] Pleione and Gamma Cassiopeiae itself are both variable stars that have intermittent shell episodes where strong shell features appear in the spectrum and the brightness increases or decreases significantly. At other times the shell is not detectable in the spectrum, and even the emission lines may disappear.[2]

References

  1. ^ a b Bohlender, D. (2016). "Searching for and Monitoring Ae and a Shell Stars at the DAO". Bright Emissaries: Be Stars as Messengers of Star-Disk Physics. 506: 275. Bibcode:2016ASPC..506..275B.
  2. ^ a b c d Rivinius, Th; Štefl, S.; Baade, D. (2006). "Bright Be-shell stars". Astronomy and Astrophysics. 459 (1): 137. Bibcode:2006A&A...459..137R. doi:10.1051/0004-6361:20053008.
  3. ^ a b Slettebak, A. (1982). "Spectral types and rotational velocities of the brighter Be stars and A-F type shell stars". Astrophysical Journal Supplement Series. 50: 55–83. Bibcode:1982ApJS...50...55S. doi:10.1086/190820. page 80.
  4. ^ vartype.txt in Combined General Catalog of Variable Stars GCVS, Samus N.N., Durlevich O.V., et al.

Further reading

  • Porter, John M. (1996). "On the rotational velocities of Be and Be-shell stars". Monthly Notices of the Royal Astronomical Society. 280 (3): L31–L35. Bibcode:1996MNRAS.280L..31P. doi:10.1093/mnras/280.3.L31.
  • Quirrenbach, A.; Buscher, D. F.; Mozurkewich, D.; Hummel, C. A.; Armstrong, J. T. (1994). "Maximum-entropy maps of the Be shell star zeta Tauri from optical long-baseline interferometry". Astronomy and Astrophysics. 283: L13. Bibcode:1994A&A...283L..13Q.
14 Comae Berenices

14 Comae Berenices is a single star in the northern constellation of Coma Berenices, and is the second brightest member of the Coma Star Cluster. It is a faint star but visible to the naked eye with an apparent visual magnitude of 4.95. Parallax measurements place the star at a distance of about 266 light years.The spectrum of this star is peculiar and it has been assigned a number of different stellar classifications: A5, F0p, F0 III Sr, F0 vp, F1 IV: np Sr shell, A9 IV np Sr II, F1 IV, and A9 V + shell. Abt & Morrell (1995) designated this a Lambda Boötis star but this was later refuted. No surface magnetic field has been detected on 14 Comae Bernices.14 Comae Berenices is a well-known shell star with a high rate of spin, showing a projected rotational velocity of 226 km/s. This is giving the star an oblate shape with an equatorial bulge that is 12% larger than the polar radius. It is radiating 76 times the Sun's luminosity from its photosphere at an effective temperature of 7,300 K.

21 Vulpeculae

21 Vulpeculae is a single, white-hued star in the northern constellation of Vulpecula. Its distance can be estimated from the annual parallax shift of 10.97±0.23 mas, yielding a separation of 297 light years. The star is faintly visible to the naked eye at night, having an apparent visual magnitude of 5.19. It is moving further away with a heliocentric radial velocity of about +7 km/s, having come within 243 ly (74.53 pc) around 4.2 million years ago.There is some disagreement about the class of this star. Cowley et al. (1969) listed it as an A-type subgiant star with a stellar classification of A7 IVn, where the 'n' notation indicates "nebulous" lines due to rapid rotation. The luminosity class of IV suggests the star has exhausted the supply of hydrogen at its core and is evolving away from the main sequence. Slettebak (1982) classified it as an A5 IV shell star and it was so regarded by Hauck & Jaschek (2000), while Abt & Morrell (1995) listed it with a class of A5 Vn, indicating a rapidly-rotating A-type main-sequence star. It is a Delta Scuti type variable star, with a dominant pulsation period of 0.1881 days and an amplitude of 0.016 in magnitude.David and Hillenbrand (2015) found an average mass for this star of 1.61 times the mass of the Sun, whereas Zorec and Royer (2012) list a much higher mass estimate of 2.36±0.03 M☉. It is about 356 million years old with a projected rotational velocity of 222 km/s. This rapid rotation is giving the star an oblate shape with an equatorial bulge that is an estimated 16% larger than the polar radius. Observations since 1997 suggest 21 Vul has an orbiting disk of gaseous material that is too equatorially confined to make it a shell star. The line strengths from this disk have been decreasing over time.

2 Andromedae

2 Andromedae, abbreviated 2 And, is a binary star system in the northern constellation of Andromeda. 2 Andromedae is the Flamsteed designation. It is a faint star system but visible to the naked eye with a combined apparent visual magnitude of 5.09. Based upon an annual parallax shift of 7.7 mas, it is located 420 light years away. The binary nature of the star was discovered by American astronomer Sherburne Wesley Burnham at Lick Observatory in 1889. The pair orbit each other over a period of 74 years with a high eccentricity of 0.8.The magnitude 5.26 primary, designated component A, is an A-type main-sequence star based on a stellar classification of A1V or A2V, although it may have already left the main sequence. It was identified as a candidate Lambda Boötis star, but this was ruled out by Paunzen et al. (2003) as it doesn't match the typical characteristics of these objects. Although 2 And does not display a significant infrared excess, it is a shell star that displays varying absorption features due to circumstellar dust grains. This may indicate it has an orbiting debris disk containing gas that is being viewed edge-on. The star is about 100 million years old and is spinning rapidly with a projected rotational velocity of 212 km/s.The magnitude 7.43 secondary companion, component B, is a suspected variable star and may be a Delta Scuti variable. Alternatively, it may be an ellipsoidal variable with a brown dwarf companion. It is an F-type main-sequence star with a class of F1V/F4.

31 Pegasi

31 Pegasi (31 Peg) is a class B2IV-V (blue subgiant) star in the constellation Pegasus. Its apparent magnitude is 4.99 and it is approximately 1600 light years away based on parallax.It is a γ Cas variable, a type of shell star with a circumstellar disc of gas surrounding the star at the equator, ranging from 5.05 to 4.85 magnitude.

48 Librae

48 Librae is a single shell star in the constellation Libra. It is a variable star with the designation FX Lib, ranging in magnitude from 4.74 to 4.96. Based upon an annual parallax shift of 6.97±0.24 mas as seen from Earth's orbit, it is located around 470 light years from the Sun. It is a candidate member of the Upper Scorpius group of the Scorpius-Centaurus Association, with the former having an age of about 11 million years.This is a main sequence Be star with a stellar classification of B3 Vsh, although it has been variously classed as B3V, B5IIIp shell He-n, B6p shell, B4III, B3IV:e-shell, and B3 shell by different sources. As is the norm for a shell star, it is spinning very rapidly with a projected rotational velocity of 400 km/s − matching or exceeding 80% of the critical velocity. This is giving the star a pronounced oblate shape with an equatorial bulge that is estimated to be 43% larger than the polar radius. It has six times the mass of the Sun and four times the Sun's radius. The star is radiating 1,100 times the Sun's luminosity from its photosphere at an effective temperature of 7,612 K.The surrounding gaseous disk stretches out to at least 15 times the star's radius and is nearly aligned with the line of sight from the Earth, having an estimated inclination of 85°±3°. Some time between 1931 and 1935, the disk became active and has remained so since that time, becoming the subject of multiple studies. The unusual asymmetry in its emission lines have led to it being misclassified as a supergiant of type B8 Ia/Ib by SIMBAD and others. This asymmetry displays quasi-periodic behavior of the type found in about a third of all Be stars, with a period of about 10 to 17 years. This variation may arise from the precession of a one-armed density wave in the disk.

59 Cygni

59 Cygni is a multiple star system in the northern constellation of Cygnus, located roughly 1,300 light years away from Earth. It is visible to the naked eye as a blue-white hued star with a combined apparent visual magnitude of 4.74.The primary component and brightest member of this system, designated 59 Cyg Aa, is a rapidly rotating Be star with a stellar classification of B1.5 Vnne. This is a well-studied star thanks to pronounced spectral variations that have been observed since 1916, and two short-term shell star phases that were observed in 1973 and 1974–5. It is actually a confirmed spectroscopic binary system with a high temperature subdwarf O-type companion in a 28-day orbital period. The latter is heating the nearest side of the circumstellar gaseous disk that surrounds the primary.Orbiting the primary pair is 59 Cyg Ab, a magnitude 7.64 A-type main-sequence star of class A3V, located at an angular separation of 0.200″. A fourth component is a magnitude 9.8 A-type giant star of class A8III at a separation of 20.2″ along a position angle (PA) of 352°, as of 2008. The fifth companion is magnitude 11.7 at a separation of 26.7″ and a PA of 141°. Gaia Data Release 2 suggests that the companions at 20.2″ and 26.7″ are respectively 382 pc and 366 pc away and moving in approximately the same direction as the primary triple.

66 Ophiuchi

66 Ophiuchi is a class B2Ve (blue main-sequence) star in the constellation Ophiuchus. Its apparent magnitude is 4.60 and it is approximately 650 light years away based on parallax.It is a γ Cas variable, a shell star with a circumstellar disc of gas and irregular changes in brightness, ranging from 4.85 to 4.55 magnitude. A companion at separation 0.1" and magnitude 6.5 has been reported.

67 Cancri

67 Cancri is a wide binary star system in the zodiac constellation of Cancer, located 195 light years away from the Sun. It is just visible to the naked eye as a faint, white-hued star with a combined apparent magnitude of 6.07. The binary nature of this system was discovered by James South and John Herschel. As of 2007, the two components have an angular separation of 103.9″, corresponding to a projected separation of 6,100 AU. They are moving further from the Earth with a heliocentric radial velocity of +12 km/s.The primary, designated component A, is an A-type main-sequence star with a stellar classification of A8 Vn. The 'n' notation indicates "nebulous" lines due to rapid rotation. It is a shell star, with weak shell lines of singly-ionized titanium being detected in the near ultraviolet in 1970. These may have come from a sporadic mass loss event. Uesugi and Fukuda (1970) gave a projected rotational velocity estimate of 105 km/s for the star, although Abt et al. (1997) suggested it could be as high as 205 km/s.67 Cancri A is about 867 million years old with 1.89 times the mass of the Sun and 1.90 times the Sun's radius. It is radiating 10.5 times the Sun's luminosity from its photosphere at an effective temperature of 7,982 K.

HD 110432

HD 110432 is a Be star in the constellation Crux, behind the southern Coalsack Nebula. It has a stellar classification of B1IVe, which means it is a subgiant star of class B that displays emission lines in its spectrum. This is a variable star of the Gamma Cassiopeiae type, indicating it is a shell star with a circumstellar disk of gas about the equator, and has the variable star designation BZ Crucis. It is not known to be a member of a binary system, although it is probably a member of the open cluster NGC 4609. This star is moderately luminous in the X-ray band, with a variable energy emission of 1032–33 erg s−1 in the range 0.2−12 keV. The X-ray emission may be caused by magnetic activity, or possibly by accretion onto a white dwarf companion.

HD 152082

HD 152082 is an A-type shell star in the southern constellation of Ara. This is a double star with a thirteenth magnitude companion at an angular separation of 6.8″ along a position angle of 329° (as of 2000).

HD 32188

HD 32188 is suspected variable star in the northern constellation of Auriga. It is a shell star, which means it has a peculiar spectrum indicating there is a circumstellar disk of gas around the star's equator. While the spectral luminosity class is III, analysis of its colour and brightness suggest it more closely resembles a supergiant star.

Iota Cancri

Iota Cancri (ι Cnc, ι Cancri) is a double star in the constellation Cancer approximately 300 light years from Earth. According to Sky-Map.org, it also has the name Decapoda.

The two stars of ι Cancri are separated by 30 arcseconds, changing only slowly. Although no orbit has been derived, the two stars show a large common proper motion and are assumed to be gravitationally related.The brighter star, ι Cancri A, is a yellow G-type giant with an apparent magnitude of +4.02. It is a mild barium star, thought to be formed by mass transfer of enriched material from an asymptotic giant branch star onto a less evolved companion. No such donor has been detected in the ι Cancri system, but it is assumed that there is an unseen white dwarf.The fainter of the two stars, ι Cancri B, is a white A-type main sequence dwarf with an apparent magnitude of +6.57. It is a shell star, surrounded by material expelled by its rapid rotation.

KQ Puppis

KQ Puppis (KQ Pup) is a spectroscopic binary variable star in the constellation Puppis. A red supergiant star and a hot main sequence star orbit each other every 9,742 days. Its apparent magnitude varies between 4.82 and 5.17.

The KQ Puppis system consists of a fairly typical M2 supergiant, in orbit with a hotter less luminous star. The hotter star is surrounded by a disc of material being transferred from the cool supergiant. This type of binary is referred to a VV Cephei system, although in this case there are no eclipses of either star. A portion of the disc does appear to be eclipsed and this is detected as a strong drop in far-ultraviolet radiation for about a third of the orbit.The red supergiant primary star has been compared to Betelgeuse. It shows small amplitude irregular pulsations, and also some variation associated with the orbital motion. The nature of the secondary is less certain. The spectrum shows high excitation features that would indicate an early B or hotter spectral type, but these may be associated with the disc rather than that star itself. Other studies have found a spectrum similar to an A supergiant, but this is thought to be an artefact of a B-type shell star.KQ Puppis has been catalogued as an outlying member of the open cluster Messier 47 (NGC 2422) and would be the brightest member of that cluster. Membership is uncertain as it appears to be more distant than the other stars in the cluster.

Lambda Pavonis

Lambda Pavonis (λ Pavonis), also known as HD 173948, is a star in the constellation Pavo. Its apparent magnitude is 4.21. It is a Be star, a rapidly rotating hot blue star which has developed a gas disk around it. It is a member of the Scorpius-Centaurus Association. It is a γ Cassiopeiae variable or shell star which has occasionally brightened to magnitude 4.0.Variations in signals coming from Lambda Pavonis have led to debate on whether it is a binary, single or pulsating variable star.

Maya warfare

Although the Maya were once thought to have been peaceful (see below), current theories emphasize the role of inter-polity warfare as a factor in the development and perpetuation of Maya society. The goals and motives of warfare in Maya culture are not thoroughly understood, but scholars have developed models for Maya warfare based on several lines of evidence, including fortified defenses around structure complexes, artistic and epigraphic depictions of war, and the presence of weapons such as obsidian blades and projectile points in the archaeological record. Warfare can also be identified from archaeological remains that suggest a rapid and drastic break in a fundamental pattern due to violence.

Maya polities engaged in violent warfare for political control of people and resources. Some scholars have suggested that the capture of sacrificial victims was a driving force behind warfare. Among the most critical resources were water and agricultural land. Economic control of resources such as obsidian also increased competition among polities. As polities became more successful, they also became more complex. This led to improved efficiency in acquiring and holding valued resources, especially through military force. Population growth increased the competition between polities, resulting in increased levels of violence.

Nu2 Boötis

Nu2 Boötis is a white-hued binary star system in the northern constellation of Boötes. It is faintly visible to the naked eye with a combined apparent visual magnitude of 5.02. Based upon an annual parallax shift of 7.59 mas as seen from the Earth, it is located roughly 430 light years from the Sun. The system is moving closer to the Sun with a radial velocity of −16.6 km/s.This stellar pair have a nearly circular orbit with a period of nine years and a semimajor axis of 0.0615 arc seconds. They are both of visual magnitude 5.80 and display a similar spectrum, with the primary, component A, being an A-type main sequence star with a stellar classification of A5 V. This has been identified as an A-type shell star, suggesting there is a circumstellar disk of gas orbiting one or both stars.

Nu Puppis

Nu Puppis (ν Puppis) is a solitary, blue-hued star in the southern constellation of Pyxis. It is the fifth-brightest star in Puppis, with an apparent visual magnitude of 3.17. Based upon an annual parallax shift of 8.78 mas as seen from Earth, it is located about 370 light years from the Sun. The system made its closest approach about 3.6 million years ago when it underwent perihelion passage at a distance of roughly 27 light years.The star has a stellar classification of B8 III, matching a B-type giant. Absorption lines in the spectrum are displaying central quasi-emission peaks, indicating this is a Be shell star with a circumstellar disk of heated gas that is being seen edge-on. ν Puppis is a candidate variable star showing an amplitude of 0.0117 magnitude with a frequency of 0.15292 per day. It is spinning rapidly with a projected rotational velocity of 225 km/s. This rotation is giving the star an oblate shape, with the equator being 31% larger than the poles. It is radiating (after allowance for ultraviolet radiation) 1,340 times the Sun's luminosity from its photosphere at an effective temperature of 12,120 K.

Phi Leonis

Phi Leonis (φ Leo) is a star in the constellation Leo. It is bright enough to be seen with the naked eye, having an apparent visual magnitude of 4.46. Based upon parallax measurements, the distance to Phi Leo is around 184 light years.

The spectrum of this star fits a stellar classification of A7IVn, which suggests it is an A-type subgiant star that has left the main sequence and is evolving into a giant star. It is being viewed with the plane of the star's equator lying close the line of sight from the Earth, and shows a high rotation rate with a projected rotational velocity of 254 km/s. This rapid spin is giving the star an oblate shape with an equatorial bulge that is 29% larger than the polar radius.Phi Leonis has been mentioned as a shell star—indicating that there is a circumstellar disk of gas around the star's equator—and may display a slight variability. Sporadic variation of the spectra on the time scale of minutes up to months in duration suggests that solid, cometary bodies are in orbit around the star, with objects approaching close enough for refractory materials to sublimate.

Psi Persei

Psi Persei (Psi Per, ψ Persei, ψ Per) is a Be star in the northern constellation of Perseus. It has an apparent visual magnitude of 4.31, so it is visible to the naked eye at night under suitably dark skies. Based on parallax measurements, it is located at a distance of roughly 580 light-years (180 parsecs) from the Earth.This star has a stellar classification of B5Ve, which indicates it is a B-type main sequence star that is generating energy at its core through the nuclear fusion of hydrogen. It is a shell star with a circumstellar disc of gas surrounding the equator and extending out to about 11 times the radius of the star. As a result of this disc, the spectrum of this star shows emission lines (as indicated by the 'e' in the stellar class) and its magnitude varies over a period of about a day.Psi Persei is rotating rapidly with a projected rotational velocity (v sin i) along the equator of 390 km/s or more. The axis of rotation is inclined about 75° ± 8° to the line of sight from the Earth, so this velocity is close to the actual azimuthal velocity along the star's equator. It is expelling mass at the rate of about 5.0 × 10−8 times the mass of the Sun per year, or the equivalent of the Sun's mass every 20 million years. This star may be a member of the Alpha Persei Cluster, although its proper motion is high compared to other members.

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