Type II Cepheid explained

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]

Classification

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

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.[8]

Period-luminosity relationship

Type II Cepheids are fainter than their classical Cepheid counterparts for a given period by about 1.6 magnitudes.[9] Cepheid variables are used to establish the distance to the Galactic Center, globular clusters, and galaxies.[10] [11] [12] [13] [14] [15] [16]

Examples

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)ConstellationMaximum Apparent magnitude (mv)Minimum Apparent magnitude (mv)Range of magnitudePeriodSpectral classSubtypeComment
RU CamelopardalisCamelopardalis dC0,1-C3,2e(K0-R0)W VirCarbon-rich[17]
Kappa PavonisPavo dF5-G5I-IIW Virbrightest member
R ScutiScutum dG0Iae-K2p(M3)IbeRV Taubrightest member
RV TauriTaurus dG2eIa-M2IaRV Tauprototype
RT Trianguli AustralisTriangulum Australe dF8:(R)-G2I-IIBL Hercarbon-rich
AL VirginisVirgo dF0-F8W Vir 
W VirginisVirgo dF0Ib-G0IbW Virprototype
AF Crateris[18] Crater (constellation)10.8711.470.631.16 dF5 IRV Tauextremely metal poor, [Fe/H] = -2.7[19]

External links

Notes and References

  1. 2002PASP..114..689W . The Cepheids of Population II and Related Stars . Wallerstein, George . 114 . 2002 . 689–699 . The Publications of the Astronomical Society of the Pacific . 10.1086/341698 . 797.
  2. 2008AcA....58..293S . The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. II.Type II Cepheids and Anomalous Cepheids in the Large Magellanic Cloud . Soszyński, I. . Udalski, A. . Szymański, M. K. . Kubiak, M. . Pietrzyński, G. . Wyrzykowski, Ł. . Szewczyk, O. . Ulaczyk, K. . Poleski, R. . 58 . 2008 . 293 . Acta Astronomica. 0811.3636 .
  3. astro-ph/9908317 . 1999AcA....49..223U . The Optical Gravitational Lensing Experiment. Cepheids in the Magellanic Clouds. IV. Catalog of Cepheids from the Large Magellanic Cloud . Udalski, A. . Soszynski, I. . Szymanski, M. . Kubiak, M. . Pietrzynski, G. . Wozniak, P. . Zebrun, K. . 49 . 1999 . 223–317 . Acta Astronomica.
  4. 0808.2210 . 2008AcA....58..163S . The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. I. Classical Cepheids in the Large Magellanic Cloud . Soszynski, I. . Poleski, R. . Udalski, A. . Szymanski, M. K. . Kubiak, M. . Pietrzynski, G. . Wyrzykowski, L. . Szewczyk, O. . Ulaczyk, K. . 58 . 2008 . 163 . Acta Astronomica .
  5. 2006ApJ...652.1133M . A New Cepheid Distance to the Maser-Host Galaxy NGC 4258 and Its Implications for the Hubble Constant . Macri, L. M. . Stanek, K. Z. . Bersier, D. . Greenhill, L. J. . Reid, M. J. . 652 . 2006 . 1133–1149 . The Astrophysical Journal . 10.1086/508530 . 2. astro-ph/0608211 .
  6. 2007ApJ...654..186F . The Discovery of Cepheids and a Distance to NGC 5128 . Ferrarese, Laura . Mould, Jeremy R. . Stetson, Peter B. . Tonry, John L. . Blakeslee, John P. . Ajhar, Edward A. . 654 . 2007 . 186–218 . The Astrophysical Journal . 1 . 10.1086/506612. astro-ph/0605707 .
  7. 2010AcA....60..121M . The Cepheids of Centaurus A (NGC 5128) and Implications for H0 . Majaess, D. . 60 . 2010 . 121 . Acta Astronomica. 2 . 1006.2458 .
  8. The Binary Type II Cepheids IX CAS and TX Del . Harris, Hugh C. . Welch, Douglas L. . Astronomical Journal . 98 . 981 . September 1989 . 1989AJ.....98..981H . 10.1086/115190 .
  9. Web site: Cepheid Variables. Weekly Topic. Caglow. 30 January 2012.
  10. 2009AcA....59..403M . Type II Cepheids as Extragalactic Distance Candles . Majaess, D. . Turner, D. . Lane, D. . 59 . 2009 . 403 . Acta Astronomica. 4 . 0909.0181 .
  11. 2003AcA....53..117K . The Optical Gravitational Lensing Experiment. Population II Cepheids in the Galactic Bulge . Kubiak, M. . Udalski, A. . 53 . 2003 . 117 . Acta Astronomica. astro-ph/0306567 .
  12. 2006MNRAS.370.1979M . The period-luminosity relation for type II Cepheids in globular clusters . Matsunaga, Noriyuki . Fukushi, Hinako . Nakada, Yoshikazu . Tanabé, Toshihiko . Feast, Michael W. . Menzies, John W. . Ita, Yoshifusa . Nishiyama, Shogo . Baba, Daisuke . 370 . 2006 . 1979–1990 . . 10.1111/j.1365-2966.2006.10620.x . 4. astro-ph/0606609 . 8 . Naoi . Takahiro . Nakaya . Hidehiko . Kawadu . Takahiro . Ishihara . Akika . Kato . Daisuke . free .
  13. 2008MNRAS.386.2115F . The luminosities and distance scales of type II Cepheid and RR Lyrae variables . Feast, Michael W. . Laney, Clifton D. . Kinman, Thomas D. . van Leeuwen, Floor . Whitelock, Patricia A. . 386 . 2008 . 2115 . Monthly Notices of the Royal Astronomical Society . 10.1111/j.1365-2966.2008.13181.x . 4. free . 0803.0466 .
  14. 2009MNRAS.398..263M . Characteristics of the Galaxy according to Cepheids . Majaess, Daniel J. . Turner, David G. . Lane, David J. . 398 . 2009 . 263–270 . 0903.4206 . 10.1111/j.1365-2966.2009.15096.x . Monthly Notices of the Royal Astronomical Society. 1 . free .
  15. 2010JAVSO..38..100M . RR Lyrae and Type II Cepheid Variables Adhere to a Common Distance Relation . Majaess, D. J. . 38 . 2010 . 100–112 . The Journal of the American Association of Variable Star Observers. 1 . 0912.2928 .
  16. 2009MNRAS.397..933M . Period-luminosity relations for type II Cepheids and their application . Matsunaga, Noriyuki . Feast, Michael W. . Menzies, John W. . 397 . 2009 . 933 . Monthly Notices of the Royal Astronomical Society . 10.1111/j.1365-2966.2009.14992.x . 2. free . 0904.4701 .
  17. Kipper, Tõnu. Klochkova, Valentina G.. 2007. Optical Spectroscopy of RU Cam, a Pulsating Carbon Star. Baltic Astronomy. 16. 383–96. 2007BaltA..16..383K. 0706.2969 .
  18. Van Winckel. H.. Hrivnak. B. J.. Gorlova. N.. Gielen. C.. Lu. W.. 2012-06-01. IRAS 11472-0800: an extremely depleted pulsating binary post-AGB star. Astronomy and Astrophysics. 542. A53. 10.1051/0004-6361/201218835. 0004-6361. free. 1203.3416. 2012A&A...542A..53V .
  19. Van Winckel. H.. Hrivnak. B. J.. Gorlova. N.. Gielen. C.. Lu. W.. 2012-06-01. IRAS 11472-0800: an extremely depleted pulsating binary post-AGB star. Astronomy and Astrophysics. 542. A53. 10.1051/0004-6361/201218835. 0004-6361. free. 1203.3416. 2012A&A...542A..53V .

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