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Beta Cephei variable

Beta Cephei variables, also known as Beta Canis Majoris stars, are variable stars that exhibit small rapid variations in their brightness due to pulsations of the stars' surfaces, thought due to the unusual properties of iron at temperatures of 200,000 K in their interiors. These stars are usually hot blue-white stars of spectral class B and should not be confused with Cepheid variables, which are named after Delta Cephei and are luminous supergiant stars.

Properties

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A light curve for Beta Cephei, plotted from TESS data[1]

Beta Cephei variables are somewhat evolved stars of masses between about 7 and 20 M  (that is, 7–20 times as massive as the Sun). Among their number are some of the brightest stars in the sky, such as Beta Crucis and Beta Centauri; Spica is also classified as a Beta Cephei variable but mysteriously stopped pulsating in 1970.[2] Typically, they change in brightness by 0.01 to 0.3 magnitudes with periods of 0.1 to 0.3 days (2.4–7.2 hours).[2] The prototype of these variable stars, Beta Cephei, shows variation in apparent magnitude from +3.16 to +3.27 with a period of 4.57 hours. The point of maximum brightness occurs when the star is smallest and hottest. Their variation in brightness is much greater—up to 1 magnitude—in ultraviolet wavelengths.[3] A small number of stars have been identified with periods shorter than one hour, corresponding to 1/4 of the fundamental radial pulsation period and 3/8 of the fundamental period. They also have relatively low amplitudes and a very narrow range of spectral types B2-3 IV-V. They are known as the short period group and the GCVS acronym BCEPS.[4][5]

The pulsations of Beta Cephei variables are driven by the kappa mechanism and p-mode pulsations. At a depth within the star where the temperature reaches 200,000 K, there is an abundance of iron. Iron at these temperatures will increase (rather than decrease) in opacity, resulting in the buildup of energy within the layer. This results in increased pressure that pushes the layer back out again, the cycle repeating itself in a matter of hours. This is known as the Fe bump or Z bump (Z standing for the star's metallicity).[6] The similar slowly pulsating B stars show g-mode pulsations driven by the same iron opacity changes, but in less massive stars and with longer periods.[7]

History of observations

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American astronomer Edwin Brant Frost discovered the variation in radial velocity of Beta Cephei in 1902, initially concluding it was a spectroscopic binary. Paul Guthnick was the first to detect a variation in brightness, in 1913.[8] Beta Canis Majoris and Sigma Scorpii were found to be variable not long afterwards,[3] Vesto Slipher noted in 1904 that Sigma Scorpii's radial velocity was variable, and R.D. Levee and Otto Struve concluded this was due to the star's pulsations in 1952 and 1955 respectively.[9] These variables were often called Beta Canis Majoris variables because Beta Canis Majoris was the most closely studied example in the first half of the 20th century, though its location in the southern sky meant that its lowness in the sky hampered observations.[10] However, Beta Cephei was the first member of the class to be discovered and so they are generally called Beta Cephei variables—despite the similarity of name (and risk of confusion) with Cepheid variables.[3]

Cecilia Payne-Gaposchkin and Sergei Gaposchkin catalogued 17 probable members of the class in their 1938 Variable Stars, though classified them with Delta Scuti variables.[11] 16 Lacertae was another star extensively studied before 1952.[10] The number known jumped from 18 to 41 in 1966.[12] Otto Struve studied these stars extensively in the 1950s, however research declined after his death.[3]

Christiaan L. Sterken and Mikolaj Jerzykiewicz classed 59 stars as definite and 79 more as suspected Beta Cephei variables in 1993.[13] Stankov listed 93 members of the class in a 2005 catalogue, plus 77 candidates and 61 poor or rejected stars.[14] Six stars, namely Iota Herculis, 53 Piscium, Nu Eridani, Gamma Pegasi, HD 13745 (V354 Persei) and 53 Arietis had been found to exhibit both Beta Cephei and SPB variability.[15]

In 2021 β Cru became the first star of any kind to have its pulsation modes identified using polarimetric asteroseismology.[16]


List of Beta Cephei variables

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Designation (name) Constellation Discovery Maximum Apparent magnitude (mV)[17] Minimum Apparent magnitude (mV)[17] Period (hours)[14] Spectral class[14] Comment
β CMa Canis Major 1909 (William Wallace Campbell[18]) 1m.93 2m.00 6.031 B1II-III Pulsations of 6.03, 6.00, and 4.74 hours.[19]
ξ1 CMa Canis Major [20] 4m.33 4m.36 5.030 B0.5IV  
15 CMa Canis Major [20] 4m.79 4m.84 4.429 B1III-IV  
V376 Car[21] Carina 4m.91 4m.96 0.4992 B2IV-V BCEPS star
V372 Car Carina [22] 5m.70 2.78 B2III  
β Cen Centaurus 0m.61 3.768[13] B1II  
ε Cen Centaurus 2m.29 2m.31 4.070 B1V  
κ Cen Centaurus 3m.13 3m.14 2.288 B2IV  
χ Cen[21] Centaurus 4m.40[14] 0.84 B2V BCEPS star
β Cep Cepheus 1902 (Edwin Brant Frost)[23] 3m.16 3m.27 4.572 B2IIIe Prototype
δ Cet Cetus [20] 4m.05 4m.1 3.867 B2IV  
β Cru Crux 1m.23 1m.31 4.589 B0.5IV  
δ Cru[17][24] Crux 2m.78 2m.84 3.625 B2IV
ω1 Cyg Cygnus 4m.94 B2.5IV confirmed on hi res spectroscopy.[24]
ν Eri Eridanus 3m.87 4m.01 4.164 B2III Multiperiodic; also a slowly pulsating B star
12 Lac Lacerta 5m.16 5m.28 4.634 B1.5III Also a slowly pulsating B star
16 Lac Lacerta 5m.30 (B) 5m.52 (B) 4.109 B2IV  
α Lup Lupus 1956 (Bernard Pagel)[25] 2m.29 2m.34 6.235 B1.5III  
δ Lup[14] Lupus 3m.20 3m.24 3.972 B2IV  
ε Lup[26] Lupus 3m.36 3m.38 2.316 B2IV + B3V Triple star system; primary is a spectroscopic binary
ι Lup[27] Lupus 3m.54 3m.3.55 B2.5IV not recorded as BCEP since 1997
τ1 Lup[14] Lupus 4m.54 4m.58 4.257 B2IV  
19 Mon Monoceros 4m.96 5m.01 4.589 B1IV-Vea  
α Mus[17] Musca 2m.68 2m.73 2.167 B2IV-V initially questionable, confirmed on hi res spectroscopy.[24]
θ Oph Ophiuchus 3m.25 3m.31 3.373 B2IV  
η Ori Orion 3m.31 3m.35 7.247 B0.5Vea + B3V Quadruple star; also an Algol variable; component Ab is the pulsating star
γ Peg Pegasus 1953 (D. Harold McNamara) 2m.78 2m.89 3.643 B2IV Also a slowly pulsating B star
ε Per Perseus 2m.88 3m.00 3.847 B0.5V  
PT Pup Puppis [14] 5m.72 5m.74 3.908 B2III  
λ Sco Scorpius 1m.59 1m.65 5.129 B1.5IV + PMS + B2IV Triple system; also an Algol variable
κ Sco Scorpius 2m.41 2m.42 4.795 B1.5III  
σ Sco Scorpius 1904 (Vesto Slipher) 2m.86 2m.94 5.923 B1III Quadruple system
Spica Virgo 0m.85 1m.05 6.520 B1IV Brightness variations stopped in 1970[28]
BW Vul Vulpecula 6m.44 6m.68 4.8 B2IIIv Beta Cephei variable with largest change in radial velocity

List of former, excluded or candidate Beta Cephei variables

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Designation (name) Constellation Discovery Maximum Apparent magnitude (mV)[17] Minimum Apparent magnitude (mV)[17] Period (hours)[14] Spectral class[14] Comment
ι CMa Canis Major 4m.36 4m.40 33.6[17] B3Ib/II Not considered a β Cep variable[14][29]
FN CMa[30] Canis Major 5m.38 5m.42 36.7[31] B0.5IV No longer considered a β Cep variable[14]
χ Car[32] Carina 3m.46 2.42 B2IV Not considered a β Cep variable[14]
V343 Car Carina 4m.30[14] 57.11 B1.5III Not considered a β Cep variable[14]
ζ Cha[27] Chamaeleon 5m.06 5m.17 25.91[27] B5V considered as a SBP as of 2011[22]
λ Cru Crux 4m.60 4m.64 9.482[17] B4Vne Not considered a β Cep variable[14]
θ2 Cru Crux 4m.70 4m.74 2.134[17] B2IV Not considered a β Cep variable[14]
25 Cyg Cygnus 5m.09[33] 5m.21[33] 5.04[34] B3IVe γ Cas variable, not considered a β Cep variable[14]
ι Her Hercules 2m.93 B3IV No longer classed as Beta Cephei type[14]
η Hya Hydra 4m.27 4m.33 ~4[32] B3V No longer classed as Beta Cephei type[14]
NW Pup Puppis 5m.04 5m.18 3.00 B3Vea Also a rotating ellipsoidal variable, not considered a β Cep variable[14]
α Pyx[20] Pyxis 3m.67 3m.70 B1.5III Candidate β Cephei variable
Merope Taurus 4m.17 4m.19 B6IVe B(e) star, not Beta Cephei type[14]
IS Vel[17] Vela 5m.23 2.592 B1IVn Candidate β Cephei variable[14]
HR 3440
(HW Vel)[17] 		Vela 5m.46 5m.52 6.275 B6V Candidate β Cephei variable[14]
2 Vul Vulpecula 5m.36 5m.48 14.63 O8IV-B0.5IVeV B(e) star, not Beta Cephei type[14]

References

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