Barium star

Barium stars are spectral class G to K stars whose spectra indicate an overabundance of s-process elements by the presence of singly ionized barium, Ba II, at λ 455.4 nm. Barium stars also show enhanced spectral features of carbon, the bands of the molecules CH, CN and C2. The class was originally recognized and defined by William P. Bidelman and Philip Keenan.[1] Initially, after their discovery, they were thought to be red giants, but the same chemical signature has been observed in main-sequence stars[2][3] as well.

Observational studies of their radial velocity suggested that all barium stars are binary stars.[4][5][6][7][8] Observations in the ultraviolet using International Ultraviolet Explorer detected white dwarfs in some barium star systems.[9][10]

Barium stars are believed to be the result of mass transfer in a binary star system. The mass transfer occurred when the now-observed giant star was on the main sequence. Its companion, the donor star, was a carbon star on the asymptotic giant branch (AGB), and had produced carbon and s-process elements in its interior. These nuclear fusion products were mixed by convection to its surface. Some of that matter "polluted" the surface layers of the main-sequence star as the donor star lost mass at the end of its AGB evolution, and it subsequently evolved to become a white dwarf. These systems are being observed at an indeterminate amount of time after the mass transfer event, when the donor star has long been a white dwarf.[11][12] Depending on the initial properties of the binary system, the polluted star can be found at different evolutionary stages.[13]

During its evolution, the barium star will at times be larger and cooler than the limits of the spectral types G or K. When this happens, ordinarily such a star is spectral type M, but its s-process excesses may cause it to show its altered composition as another spectral peculiarity. While the star's surface temperature is in the M-type regime, the star may show molecular features of the s-process element zirconium, zirconium oxide (ZrO) bands. When this happens, the star will appear as an "extrinsic" S star.

Historically, barium stars posed a puzzle, because in standard stellar evolution theory G and K giants are not far enough along in their evolution to have synthesized carbon and s-process elements and mix them to their surfaces. The discovery of the stars' binary nature resolved the puzzle, putting the source of their spectral peculiarities into a companion star which should have produced such material. The mass transfer episode is believed to be quite brief on an astronomical timescale.

Prototypical barium stars include zeta Capricorni, HR 774, and HR 4474.

The CH stars are Population II stars with similar evolutionary state, spectral peculiarities, and orbital statistics, and are believed to be the older, metal-poor analogs of the barium stars.[14]

References

  1. ^ Bidelman, W. P.; Keenan, P. C. (1951), "The BA II Stars", Astrophysical Journal, 114: 473, Bibcode:1951ApJ...114..473B, doi:10.1086/145488
  2. ^ Porto de Mello, G. F.; da Silva, L. (1997-02-20). "HR 6094: A Young Solar-Type, Solar-Metallicity Barium Dwarf Star". The Astrophysical Journal. 476 (2): L89–L92. doi:10.1086/310504. ISSN 0004-637X.
  3. ^ North, Pierre; Jorissen, Alain; Mayor, Michel (2000). "Binarity among Barium Dwarfs and CH Subgiants: Will They Become Barium Giants?". Symposium - International Astronomical Union. 177: 269–275. doi:10.1017/s0074180900002497. ISSN 0074-1809.
  4. ^ McClure, R. D.; Fletcher, J. M.; Nemec, J. M. (1980), "The binary nature of the barium stars", Astrophysical Journal Letters, 238: L35, Bibcode:1980ApJ...238L..35M, doi:10.1086/183252
  5. ^ McClure, R. D.; Woodsworth, A. W. (1990), "The binary nature of the barium and CH stars. III - Orbital parameters", Astrophysical Journal, 352: 709, Bibcode:1990ApJ...352..709M, doi:10.1086/168573
  6. ^ Jorissen, A.; Mayor, M. (1988), "Radial velocity monitoring of a sample of barium and S stars using CORAVEL - Towards an evolutionary link between barium and S stars?", Astronomy and Astrophysics, 198: 187, Bibcode:1988A&A...198..187J
  7. ^ Jorissen, A.; Boffin, H.M.J.; Karinkuzhi, D.; Van Eck, S.; Escorza, A.; Shetye, S.; Van Winckel, H. (2019-05-30). "Barium and related stars and their white-dwarf companions. I. Giant stars". Astronomy & Astrophysics. doi:10.1051/0004-6361/201834630. ISSN 0004-6361.
  8. ^ Escorza, A.; Karinkuzhi, D.; Jorissen, A.; Siess, L.; Van Winckel, H.; Pourbaix, D.; Johnston, C.; Miszalski, B.; Oomen, G-M. (2019-04-22). "Barium and related stars, and their white-dwarf companions. II. Main-sequence and subgiant stars". Astronomy & Astrophysics. doi:10.1051/0004-6361/201935390. ISSN 0004-6361.
  9. ^ Dominy, J. F.; Lambert, D. L. (July 1983). "Do all barium stars have a white dwarf companion?". The Astrophysical Journal. 270: 180. doi:10.1086/161109. ISSN 0004-637X.
  10. ^ Gray, R. O.; McGahee, C. E.; Griffin, R. E. M.; Corbally, C. J. (2011-04-04). "FIRST DIRECT EVIDENCE THAT BARIUM DWARFS HAVE WHITE DWARF COMPANIONS". The Astronomical Journal. 141 (5): 160. doi:10.1088/0004-6256/141/5/160. ISSN 0004-6256.
  11. ^ McClure, R. D. (1985), "The carbon and related stars", Journal of the Royal Astronomical Society of Canada, 79: 277, Bibcode:1985JRASC..79..277M
  12. ^ Boffin, H. M. J.; Jorissen, A. (1988), "Can a barium star be produced by wind accretion in a detached binary?", Astronomy and Astrophysics, 205: 155, Bibcode:1988A&A...205..155B
  13. ^ Escorza, A.; Boffin, H. M. J.; Jorissen, A.; Van Eck, S.; Siess, L.; Van Winckel, H.; Karinkuzhi, D.; Shetye, S.; Pourbaix, D. (December 2017). "Hertzsprung-Russell diagram and mass distribution of barium stars". Astronomy & Astrophysics. 608: A100. doi:10.1051/0004-6361/201731832. ISSN 0004-6361.
  14. ^ McClure, R. D. (1984), "The barium stars", Publications of the Astronomical Society of the Pacific, 96: 117, Bibcode:1984PASP...96..117M, doi:10.1086/131310
14 Boötis

14 Boötis is a possible binary star system located 110 light years away from the Sun in the northern constellation of Boötes. It is visible to the naked eye as a dim, yellow-white hued star with a combined apparent visual magnitude of 5.53. This system is moving closer to the Earth with a heliocentric radial velocity of −40 km/s. It has a relatively high proper motion, traversing the celestial sphere at the rate of 0.260 arc seconds per annum.The component of this system remain unresolved as of 2002; Eggleton and Tokovinin (2008) listed an angular separation of 0.2″. The visible component is an F-type main-sequence star with a stellar classification of F8 V There are enhanced barium lines in the star's spectrum, but these are likely caused by regions of photospheric activity rather than being a barium star. It is 2.6 billion years old and is spinning with a projected rotational velocity of 5.6 km/s, giving it a rotation period of 26 days.14 Boötis has 1.46 times the mass of the Sun and 2.1 times the Sun's radius. It is radiating 5.9 times the luminosity of the Sun from its photosphere at an effective temperature of 6,169 K. The metallicity is near solar, based on the abundance of iron in the stellar atmosphere.

16 Serpentis

16 Serpentis is a binary star system in the Serpens Caput portion of the equatorial constellation of Serpens, located 228 light years from the Sun. It is visible to the naked eye as a fain, orange-hued star with an apparent visual magnitude of 5.261. The system is moving further from the Earth with a heliocentric radial velocity of +3 km/s.The variable radial velocity of this star was discovered at Lick Observatory and was announced by J. H. Moore in 1924. It is a single-lined spectroscopic binary with an orbital period of 14.58 years and an eccentricity of 0.345. The visible component is an evolved giant star with a stellar classification of K0III: CN1 Ba0.7 Sr2. This is a mild barium star with the suffix notation above indicating associated abundance anomalies. The companion is a presumed white dwarf star that has already passed through its giant stage, during which time it enhanced the envelope of the companion with s-process elements. The pair form one of the widest barium star binaries known, which may account for the mildness of the barium anomaly.

18 Draconis

18 Draconis is a likely binary star system in the northern circumpolar constellation of Draco. With an apparent visual magnitude of 4.84, it is just bright enough to be faintly visible to the naked eye. The distance to this system, as estimated from an annual parallax shift of 4.5 mas, is roughly 720 light years. It is moving closer to the Sun with a heliocentric radial velocity of −1.4 km/s, and is a probable member of the Sirius stream of co-moving stars.The visible component has a stellar classification of K0 III CN−0.5 CH−2 Ca1, indicating it is an evolved K-type giant star with some abundance peculiarities in its atmosphere. At the age of around 280 million years, it is most likely (99% chance) on the horizontal branch. It is a barium star, which suggests it may have a degenerate white dwarf companion from which it accreted materials during an earlier stage of its evolution. 18 Dra has an estimated 3.8 times the mass of the Sun and has expanded to 47 times the Sun's radius. The star is radiating 787 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,471 K.

1 Aurigae

1 Aurigae is the original name for a star now in the constellation Perseus. It was the first entry in John Flamsteed's catalogue of stars in Auriga. When Eugène Joseph Delporte drew up simplified boundaries for the constellations on behalf of the International Astronomical Union in 1930, 1 Aurigae ended up over the border in Perseus. To avoid confusion, the star may instead be referred to by its Harvard Revised catalogue number, HR 1533.

Based upon its parallax measurement of 6.48 mas, this star is located approximately 520 light years from Earth. It is visible to the naked eye as a faint, orange-hued star with an apparent visual magnitude of 4.89. 1 Aurigae is moving closer to the Earth with a heliocentric radial velocity of −25 km/s.This is a possible binary star system, based upon the status of the visible component as a mild barium star. The primary is an aging giant star with a stellar classification of K3.5 III Ba0.2:. It is 3.9 billion years old with 1.49 times the mass of the Sun and around 44 times the Sun's radius. This star is radiating 561 times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 4,102 K. The suspected companion star should be a white dwarf that previously transferred s-process elements to the visible member.

24 Aquilae

24 Aquilae (abbreviated 24 Aql) is a star in the equatorial constellation of Aquila. 24 Aquilae is its Flamsteed designation. It is at a distance of around 410 light-years (130 parsecs) from Earth and has an apparent visual magnitude of 6.4. According to the Bortle Dark-Sky Scale, this star is just visible to the naked eye in dark rural skies.

This is a so-called mild barium star, as identified by the mild presence of an absorption line of singly-ionized barium atoms at a wavelength of 455.4 nm. Such stars display an atmospheric overabundance of carbon and the heavy elements produced by the s-process, which was most likely transferred into the atmosphere by a wide binary stellar companion. However, in the case of 24 Aquilae, the abundances of heavy elements are near normal.At an estimated age of a half billion years, 24 Aquilae is a giant star with a stellar classification of K0 IIIa. It has more than double the mass of the Sun and shines with 63 times the Sun's luminosity. It is radiating this energy into space from its outer atmosphere at an effective temperature of 4,810 K. This heat is what gives it the cool orange hue characteristic of a K-type star.

24 Scorpii

24 Scorpii is a star that was originally placed by John Flamsteed within the constellation of Scorpius but in now placed within the southeastern constellation of Ophiuchus. It is visible to the naked eye as a faint, yellow-hued point of light with an apparent visual magnitude of 4.91. Based on the trigonmetric parallax published in Gaia Data Release 2, the star lies approximately 121 parsecs or 390 light years away. It is positioned near the ecliptic and thus is subject to lunar occultations.This object is a luminous giant star that is classified by spectral and luminosity class as G7.5II or G7.5II-IIICN1Ba0.5. 24 Sco is associated with the faint reflection nebulae RfN VDB 109 or GN 16.36.7, but may just lie along the same line of sight. It is a very mild Barium star, but the enhanced barium lines in the spectrum may be a simple luminosity effect rather than a true abundance anomaly. It is a probable horizontal branch star, fusing helium in its core, with just a 13% likelihood that it is still on the red giant branch. The star has 2.51 times the mass of the Sun and has expanded to 22 times the Sun's radius. It is radiating 208 times the Sun's luminosity from its photosphere at an effective temperature of 4,667 K.

2 Aurigae

2 Aurigae is a possible binary star system in the northern constellation of Auriga. This object is visible to the naked eye as a faint, orange-hued star with an apparent visual magnitude of +4.79. It forms an attractive four-star asterism when viewed in a low power eyepiece, together with the nearby HIP 22647 and another very loose visual pair, HIP 22776 and HIP 22744, all above magnitude 8. 2 Aurigae is moving closer to the Earth with a heliocentric radial velocity of −17 km/s.The visible component is an aging giant star with a stellar classification of K3- III Ba0.4. The suffix notation indicates this is a mild barium star, which means the stellar atmosphere is enriched with s-process elements. It is either a member of a close binary system and has previously acquired these elements from a (now) white dwarf companion or else it is on the asymptotic giant branch and is generating the elements itself. 2 Aurigae is 1.80 billion years old with 2.86 times the mass of the Sun and has expanded to 48 times the Sun's radius. It is radiating 599 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,115 K.

58 Leonis

58 Leonis is a possible binary star system in the southern part of the constellation of Leo, near the border with Sextans. It shines with an apparent magnitude of 4.85, making it bright enough to be seen with the naked eye. An annual parallax shift of 9.05±0.20 mas yields a distance estimate of 360 light years. It is moving further from the Sun with a heliocentric radial velocity of +6 km/s.This orange hued star is an evolved K-type giant with a stellar classification of K0.5 III Fe-0.5, indicating a mild underabundance of iron in its spectrum. It was identified as a barium star by P. M. Williams (1971). These are theorized to be stars that show an enrichment of s-process elements by mass transfer from a now-white dwarf companion when it passed through the asymptotic giant branch stage. MacConnell et al. (1972) classified 58 Leonis as a marginal barium star. De Castro et al. (2016) consider this to be only a probable barium star, because of the low degree of s-process enrichment, and they rejected it from their sample. Rather than having an evolved companion, it may instead have formed from a cloud that was mildly enriched with s-process elements.

60 Andromedae

60 Andromedae (abbreviated 60 And) is a star system in the northern constellation of Andromeda, located to the west-northwest of Gamma Andromedae. 60 Andromedae is the Flamsteed designation though the star also bears the Bayer designation b Andromedae. It is bright enough to be seen by the naked eye with an apparent visual magnitude of 4.82. Based upon parallax measurements made during the Hipparcos mission, it is at a distance of roughly 530 light-years (160 parsecs) from Earth.This system is known to have three components. The primary is a giant star with a stellar classification of K3.5 III Ba0.4, meaning that an overabundance of barium ionized one time is observed in the spectrum of the star, making it a barium star. The secondary component is likely a white dwarf with a period of 748.2 days and an eccentricity of 0.34. There is a third component at an angular separation of 0.22 arcseconds.

61 Leonis

61 Leonis is a possible binary star system in the zodiac constellation of Leo. It is faintly visible to the naked eye, having an apparent visual magnitude of 4.73. The star is moving closer to the Sun with a heliocentric radial velocity of −12.7 km/s. It is located roughly 580 light years from the Sun, as determined from its annual parallax shift of 5.58 mas.This is an evolved red giant star with a stellar classification of M0 III that Eggen (1992) listed as being on the asymptotic giant branch (AGB). It is a marginal barium star, showing an enhanced abundance of s-process elements in its outer atmosphere. This material may have been acquired during a previous mass transfer from a now white dwarf companion, or self-enriched by a dredge-up during the AGB process. The measured angular diameter after correctly for limb darkening is 3.87±0.04 mas, which, at the estimated distance of this system yields a physical size of about 74.5 times the radius of the Sun.61 Leonis is a suspected variable star with apparent magnitude changing between 4.69 and 4.79. The variability was reported in a 1966 photometric survey, but has not been confirmed by more recent photometry.

83 Ursae Majoris

83 Ursae Majoris is a candidate binary star system in the northern circumpolar constellation of Ursa Major. This is a semiregular variable star, like Mira; for that reason it has been given the variable star designation IQ Ursae Majoris. It ranges in brightness from apparent visual magnitude 4.69 to 4.75. Percy and Au (1994) identified it as a small amplitude red variable with an irregular behavior, having a characteristic time scale of 20 days. Based upon an annual parallax shift of 6.23±0.22 mas, it is located roughly 520 light years from the Sun. The system is moving closer with a heliocentric radial velocity of −18.6 km/s.

The visible component is an evolved red giant with a stellar classification of M2 III. It is a marginal barium star, showing an enhanced abundance of s-process elements in its outer atmosphere. This material may have been acquired during a previous mass transfer from a now white dwarf companion, or self-enriched by a dredge-up during the asymptotic giant branch process.

8 Andromedae

8 Andromedae, abbreviated 8 And, is a probable triple star system in the northern constellation of Andromeda. 8 Andromedae is the Flamsteed designation. It is visible to the naked eye with an apparent visual magnitude of 4.82. Based upon an annual parallax shift of 6.1 mas, it is located 540 light years from the Earth. It is moving closer with a heliocentric radial velocity of −8 km/s.The primary component is an aging red giant star with a stellar classification of M2.5 III Ba0.5. The suffix notation indicates this is a mild barium star, which means the stellar atmosphere is enriched with s-process elements. It is either a member of a close binary system and has previously acquired these elements from a (now) white dwarf companion or else it is on the asymptotic giant branch and is generating the elements itself. This is a periodic variable of unknown type, changing in brightness with an amplitude of 0.0161 magnitude at a frequency of 0.23354 d−1, or once every 4.3 days.The third component is a magnitude 13.0 star at an angular separation of 7.8″ along a position angle of 164°, as of 2015.

Chi Geminorum

Chi Geminorum (χ Gem) is a binary star system in the constellation Gemini, near the western border with Cancer. It can be viewed with the naked eye on a dark night, having an apparent visual magnitude of 4.98. Based upon an annual parallax shift of 12.73 mas, it is located roughly 260 light years from the Sun.

The two components of this system form a spectroscopic binary with an orbital period of 2,437.8 days and an eccentricity of 0.06. The primary component is a K-type giant star with a stellar classification of K2 III. This is a candidate mild Barium star with the slight overabundance most likely acquired through accretion from what is now a white dwarf companion. The primary has an estimated 1.83 times the mass of the Sun and has expanded to 14 times the Sun's radius. The effective temperature of the outer atmosphere is 4,560 K, from whence it radiates 79 times the solar luminosity. It has a leisurely projected rotational velocity of 3.8 km/s and is around two billion years old.

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.

Omicron Boötis

Omicron Boötis (ο Boötis) is a yellow-hued star in the northern constellation of Boötes. With an apparent visual magnitude of +4.60, it is a fifth magnitude star that is visible to the naked eye. Based upon an annual parallax shift of 13.42 mas as seen from the Earth, it is located about 243 light years from the Sun. The star is moving closer to the Sun with a radial velocity of −9 km/s.At the age of 2.72 billion years, this is an evolved G-type giant star with a stellar classification of G8.5 III. It belongs to the so-called "red clump", which indicates it is generating energy through helium fusion at its core. Although it displays a higher abundance of barium than is normal for a star of its type, Williams (1975) considers its status as a Barium star to be "very doubtful". The star has double the mass of the Sun and has expanded to 11 times the Sun's radius. It is radiating 85 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,864 K.

Upsilon2 Cassiopeiae

Upsilon² Cassiopeiae (υ² Cassiopeiae, abbreviated Ups² Cas, υ² Cas), officially named Castula , is a solitary star in the constellation of Cassiopeia. It is a yellow G-type giant with an apparent magnitude of +4.62 and is approximately 200 light-years from the Sun.

Xi1 Ceti

Xi1 Ceti (ξ1 Ceti), is a binary system located in the constellation Cetus, suspected as a ternary.

In Chinese, 天囷 (Tiān Qūn), meaning Circular Celestial Granary, refers to an asterism consisting of α Ceti, κ1 Ceti, λ Ceti, μ Ceti, ξ1 Ceti, ξ2 Ceti, ν Ceti, γ Ceti, δ Ceti, 75 Ceti, 70 Ceti, 63 Ceti and 66 Ceti. Consequently, the Chinese name for Xi1 Ceti itself is "the Fifth Star of Circular Celestial Granary", Tiān Qūn Wu.The spectroscopic binary nature of Xi1 Ceti was discovered in 1901 by William Wallace Campbell using the Mills spectrograph at the Lick Observatory. The primary Ceti A is a G6III or G8III clump-giant star. The spectroscopic binary companion star is an A2V dwarf of about 2 solar masses and 30 luminosity. It is separated at 0.012 arcsec so in an orbit of 4.2 AU, smaller than the distance between Jupiter and the Sun.

Xi1 Ceti A exhibits characteristics similar to an eclipsing binary, with a small white dwarf companion. Occultations with the moon have not yet confirmed the existence of the second suspected companion. However, component A is a barium star which would tend to confirm this unseen component.

Zeta Capricorni

Zeta Capricorni, Latinized from ζ Capricorni, is a fourth-magnitude star in the constellation Capricornus. ζ Capricorni is a binary star, with the primary component ζ Capricorni A being a yellow G-type giant with an apparent magnitude of +3.77. It is considered one of the prototypical examples of a Barium star, properties of which include overabundances of carbon molecules (such as C2) and s-process elements. Zeta Capricorni has an overabundance of the s-process element praseodymium.

Its companion, ζ Capricorni B is a hydrogen-rich white dwarf. It is about as massive as the Sun, and its temperature is 23,000 K.The ζ Capricorni binary system is approximately 390 light years from Earth, based on its parallax.

Zeta Cygni

Zeta Cygni (ζ Cyg) is a binary star system in the northern constellation of Cygnus, the swan. It has an apparent visual magnitude of 3.26 and, based upon parallax measurements, is about 143 light-years (44 parsecs) away.

The primary component, ζ Cyg A is a giant star with a spectral type of G8 IIIp. Its most likely status is as a red clump giant, an evolved star that has begun core helium fusion. It has around three times the mass of the Sun and has expanded to about 15 times the Sun's radius. It is radiating 112 times the brightness of the Sun from its outer atmosphere at an effective temperature of 4,910 K. At this temperature, the star glows with the yellow hue of a G-type star.The secondary component, ζ Cyg B, is a white dwarf of type DA4.2. The pair orbit each other every 6,489 days (17.8 years) with an eccentricity of 0.22. The white dwarf cannot be seen directly, but is estimated to have an apparent magnitude of 13.2.Zeta Cygni has an overabundance of barium, as well as other heavy chemical elements in its atmosphere, making it a so-called "mild" barium star. These elements were synthesized by the other member of the system as it passed through the asymptotic giant branch (AGB) stage of its evolution, then ejected in its stellar wind and accreted onto the current primary component. Prior to acquiring this additional mass, Zeta Cygni had about 2.5 times the mass of the Sun, while the more evolved AGB star had three solar masses.

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