Type II Cepheid

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HR-vartype

Type II Cepheids are variable stars which pulsate with periods typically between 1 and 50 days.[1][2] They are population II stars: old, typically metal-poor, low mass objects.[1]

Like all Cepheid variables, Type IIs exhibit a relationship between the star's luminosity and pulsation period, making them useful as standard candles for establishing distances where little other data is available[3][4]

Longer period Type II Cepheids, which are more luminous, have been detected beyond the Local Group in the galaxies NGC 5128 and NGC 4258.[5][6][7][8]

Classification[edit]

R-Sct-Lightcurve-AAVSO

Historically Type II Cepheids were called W Virginis variables, but are now divided into three subclasses based on the length of their period. Stars with periods between 1 and 4 days are of the BL Herculis subclass and 10–20 days belong to the W Virginis subclass. Stars with periods greater than 20 days, and usually alternating deep and shallow minima, belong to the RV Tauri subclass. RV Tauri variables are usually classified by a formal period from deep minimum to deep minimum, hence 40 days or more.[1][2]

The divisions between the types are not always clearcut or agreed. For example, the dividing line between BL Her and W Vir types is quoted at anything between 4 and 10 days, with no obvious division between the two. RV Tau variables may not have obvious alternating minima, while some W Vir stars do. Nevertheless each type is thought to represent a distinct different evolutionary stage, with BL Her stars being helium core burning objects moving from the horizontal branch towards the asymptotic giant branch (AGB), W Vir stars undergoing hydrogen or helium shell burning on a blue loop, and RV Tau stars being post-AGB objects at or near the end of nuclear fusion.

RV Tau stars in particular show irregularities in their light curves, with slow variations in the brightness of both maxima and minima, variations in the period, intervals with little variation, and sometimes a temporary breakdown into chaotic behaviour. R Scuti has one of the most irregular light curves.

Properties[edit]

The physical properties of all the type II Cepheid variables are very poorly known. For example, it is expected that they have masses near or below that of the sun, but there are few examples of reliable known masses.[9]

Period-Luminosity relationship[edit]

Type II Cepheids are fainter than their classical Cepheid counterparts for a given period by about 1.6 magnitudes.[10] Cepheid variables are used to establish the distance to the Galactic center, globular clusters, and galaxies.[11][12][13][14][5][15][16]

Examples[edit]

Type II Cepheids are not as well known as their type I counterparts, with only a couple of naked eye examples. In this list, the period quoted for RV Tauri variables is the interval between successive deep minima, hence twice the comparable period for the other sub-types.

Designation (name) Constellation Maximum Apparent magnitude (mv) Minimum Apparent magnitude (mv) Range of magnitude Period Spectral class Subtype Comment
RU Camelopardalis Camelopardalis 8.1 9.79 1.61 22 d C0,1-C3,2e(K0-R0) W Vir Carbon-rich[17]
Kappa Pavonis Pavo 3.91 4.78 0.87 9.09423 d F5-G5I-II W Vir brightest member
R Scuti Scutum 4.2 8.6 4.4 146.5 d G0Iae-K2p(M3)Ibe RV Tau brightest member
RV Tauri Taurus 9.5 13.5 4.0 78.5 d G2eIa-M2Ia RV Tau prototype
RT Trianguli Australis Triangulum Australe 9.43 10.18 0.35 1.9461124 d F8:(R)-G2I-II BL Her carbon-rich[18]
AL Virginis Virgo 9.10 9.92 0.82 10.3065 d F0-F8 W Vir  
W Virginis Virgo 9.46 10.75 0.87 17.2736 d F0Ib-G0Ib W Vir prototype

References[edit]

  1. ^ a b c Wallerstein, George (2002). "The Cepheids of Population II and Related Stars". The Publications of the Astronomical Society of the Pacific 114 (797): 689. Bibcode:2002PASP..114..689W. doi:10.1086/341698. 
  2. ^ a b Soszyński, I.; Udalski, A.; Szymański, M. K.; Kubiak, M.; Pietrzyński, G.; Wyrzykowski, Ł.; Szewczyk, O.; Ulaczyk, K.; Poleski, R. (2008). "The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. II.Type II Cepheids and Anomalous Cepheids in the Large Magellanic Cloud". Acta Astronomica 58: 293. arXiv:0811.3636. Bibcode:2008AcA....58..293S. 
  3. ^ Udalski, A.; Soszynski, I.; Szymanski, M.; Kubiak, M.; Pietrzynski, G.; Wozniak, P.; Zebrun, K. (1999). "The Optical Gravitational Lensing Experiment. Cepheids in the Magellanic Clouds. IV. Catalog of Cepheids from the Large Magellanic Cloud". Acta Astronomica 49: 223. arXiv:astro-ph/9908317. Bibcode:1999AcA....49..223U. 
  4. ^ Soszynski, I.; Poleski, R.; Udalski, A.; Szymanski, M. K.; Kubiak, M.; Pietrzynski, G.; Wyrzykowski, L.; Szewczyk, O.; Ulaczyk, K. (2008). "The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. I. Classical Cepheids in the Large Magellanic Cloud". Acta Astronomica 58: 163. arXiv:0808.2210. Bibcode:2008AcA....58..163S. 
  5. ^ a b Majaess, D.; Turner, D.; Lane, D. (2009). "Type II Cepheids as Extragalactic Distance Candles". Acta Astronomica 59: 403. arXiv:0909.0181. Bibcode:2009AcA....59..403M. 
  6. ^ Macri, L. M.; Stanek, K. Z.; Bersier, D.; Greenhill, L. J.; Reid, M. J. (2006). "A New Cepheid Distance to the Maser-Host Galaxy NGC 4258 and Its Implications for the Hubble Constant". The Astrophysical Journal 652 (2): 1133. arXiv:astro-ph/0608211. Bibcode:2006ApJ...652.1133M. doi:10.1086/508530. 
  7. ^ Ferrarese, Laura; Mould, Jeremy R.; Stetson, Peter B.; Tonry, John L.; Blakeslee, John P.; Ajhar, Edward A. (2007). "The Discovery of Cepheids and a Distance to NGC 5128". The Astrophysical Journal 654: 186. arXiv:astro-ph/0605707. Bibcode:2007ApJ...654..186F. doi:10.1086/506612. 
  8. ^ Majaess, D. (2010). "The Cepheids of Centaurus A (NGC 5128) and Implications for H0". Acta Astronomica 60: 121. arXiv:1006.2458. Bibcode:2010AcA....60..121M. 
  9. ^ Harris, Hugh C.; Welch, Douglas L. (September 1989). "The Binary Type II Cepheids IX CAS and TX Del". Astronomical Journal 98. Bibcode:1989AJ.....98..981H. doi:10.1086/115190. 
  10. ^ "Cepheid Variables". Weekly Topic. Caglow. Retrieved 30 January 2012. 
  11. ^ Kubiak, M.; Udalski, A. (2003). "The Optical Gravitational Lensing Experiment. Population II Cepheids in the Galactic Bulge". Acta Astronomica 53: 117. arXiv:astro-ph/0306567. Bibcode:2003AcA....53..117K. 
  12. ^ Matsunaga, Noriyuki; Fukushi, Hinako; Nakada, Yoshikazu; Tanabé, Toshihiko; Feast, Michael W.; Menzies, John W.; Ita, Yoshifusa; Nishiyama, Shogo et al. (2006). "The period-luminosity relation for type II Cepheids in globular clusters". Monthly Notices of the Royal Astronomical Society 370 (4): 1979. arXiv:astro-ph/0606609. Bibcode:2006MNRAS.370.1979M. doi:10.1111/j.1365-2966.2006.10620.x. 
  13. ^ Feast, Michael W.; Laney, Clifton D.; Kinman, Thomas D.; van Leeuwen, Floor; Whitelock, Patricia A. (2008). "The luminosities and distance scales of type II Cepheid and RR Lyrae variables". Monthly Notices of the Royal Astronomical Society 386 (4): 2115. arXiv:0803.0466. Bibcode:2008MNRAS.386.2115F. doi:10.1111/j.1365-2966.2008.13181.x. 
  14. ^ Majaess, Daniel J.; Turner, David G.; Lane, David J. (2009). "Characteristics of the Galaxy according to Cepheids". Monthly Notices of the Royal Astronomical Society 398: 4206. arXiv:0903.4206. Bibcode:2009MNRAS.398..263M. doi:10.1111/j.1365-2966.2009.15096.x. 
  15. ^ Majaess, D. J. (2010). "RR Lyrae and Type II Cepheid Variables Adhere to a Common Distance Relation". The Journal of the American Association of Variable Star Observers 38: 100. arXiv:0912.2928. Bibcode:2010JAVSO..38..100M. 
  16. ^ Matsunaga, Noriyuki; Feast, Michael W.; Menzies, John W. (2009). "Period-luminosity relations for type II Cepheids and their application". Monthly Notices of the Royal Astronomical Society 397 (2): 933. arXiv:0904.4701. Bibcode:2009MNRAS.397..933M. doi:10.1111/j.1365-2966.2009.14992.x. 
  17. ^ Kipper, Tõnu; Klochkova, Valentina G. (2007). "Optical Spectroscopy of RU Cam, a Pulsating Carbon Star". Baltic Astronomy 16: 383–96. arXiv:0706.2969. Bibcode:2007BaltA..16..383K. 
  18. ^ Wallerstein, George; Matt, Sean; Gonzalez, Guillermo (2000), "The Carbon Cepheid RT Trianguli Australis: Additional Evidence of Triple-α and CNO Cycling", Monthly Notices of the Royal Astronomical Society 311 (2): 414–22, Bibcode:2000MNRAS.311..414W, doi:10.1046/j.1365-8711.2000.03064.x 

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