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Mira variable

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Mira variable

Mira, the prototype of the Mira variables

Mira variables , named after the prototype star Mira, are a class of pulsating variable stars characterized by very red colours, pulsation periods longer than 100 days, and amplitudes greater than one magnitude in infrared and 2.5 magnitude at visual wavelengths. They are red giants in the very late stages of stellar evolution, on the asymptotic giant branch, that will expel their outer envelopes as planetary nebulae and become white dwarfs within a few million years.

Mira variables are stars massive enough that they have undergone helium fusion in their core but are less than two solar masses, stars that have already lost about half their initial mass. However, they can be thousands of times more luminous than the Sun due to their very large distended envelopes. They are pulsating due to the entire star expanding and contracting. This produces a change in temperature along with radius, both of which factors cause the variation in luminosity. The pulsation depends on the mass and radius of the star and there is a well-defined relationship between period and luminosity (and colour).[1][2] The very large visual amplitudes are not due to large luminosity changes, but due to a shifting of energy output between infra-red and visual wavelengths as the stars change temperature during their pulsations.[3]

Early models of Mira stars assumed that the star remained spherically symmetric during this process (largely to keep the computer modelling simple, rather than for physical reasons). A recent survey of Mira variable stars found that 75% of the Mira stars which could be resolved using the IOTA telescope are not spherically symmetric,[4] a result which is consistent with previous images of individual Mira stars,[5][6][7] so there is now pressure to do realistic three-dimensional modelling of Mira stars on supercomputers.[8]

Mira variables may be oxygen-rich or carbon-rich. Carbon-rich stars such as R Leporis arise from a narrow set of conditions that override the normal tendency for AGB stars to maintain a surplus of oxygen over carbon at their surfaces due to dredge-ups.[9] Pulsating AGB stars such as Mira variables undergo fusion in alternating hydrogen and helium shells, which produces periodic deep convection known as dredge-ups. These dredge-ups bring carbon from the helium burning shell to the surface and would result in a carbon star. However, in stars above about 4 M, hot bottom burning occurs. This is when the lower regions of the convective region are hot enough for significant CN cycle fusion to take place which destroys much of the carbon before it can be transported to the surface. Thus more massive AGB stars do not become carbon-rich.[10]

Mira variables are rapidly losing mass and this material often forms dust shrouds around the star. In some cases conditions are suitable for the formation of natural masers.[11]

A small subset of Miras appear to change their period over time—the period increases or decreases by a substantial amount (up to a factor of three) over the course of several decades to a few centuries. This is believed to be caused by thermal pulses, where the helium shell reignites the outer hydrogen shell. This changes the structure of the star, which manifests itself as a change in period. This process is predicted to happen to all Mira variables, but the relatively short duration of thermal pulses (a few thousand years at most) over the asymptotic giant branch lifetime of the star (less than a million years), means we only see it in a few of the several thousand Mira stars known, possibly in R Hydrae.[12] Most Mira variables do exhibit slight cycle-to-cycle changes in period, probably caused by nonlinear behaviour in the stellar envelope including deviations from spherical symmetry.[13][14]

Mira variables are popular targets for amateur astronomers interested in variable star observations, because of their dramatic changes in brightness. Some Mira variables (including Mira itself) have reliable observations stretching back well over a century.[15]

List

The following list contains selected Mira variables that are of interest to amateur or professional astronomy. Unless otherwise noted, the given magnitudes are in the V-band.

Star
Brightest
magnitude
Dimmest
magnitude
Period
(in days)
Distance
(in parsecs)
Mira 2.0 10.1 332 92
Chi Cygni 3.3 14.2 408 181
R Hydrae 3.5 10.9 380 124
R Carinae 3.9 10.5 307 158
R Leonis 4.4 11.3 310 71
S Carinae 4.5 9.9 149 546
R Cassiopeiae 4.7 13.5 430 126
R Horologii 4.7 14.3 408 210
U Orionis 4.8 13.0 377 437
RR Scorpii 5.0 12.4 281 353
R Serpentis 5.2 14.4 356 209
T Cephei 5.2 11.3 388 188
R Aquarii 5.2 12.4 387 362
R Centauri 5.3 11.8 502 385
RR Sagittarii 5.4 14 336 1330
R Trianguli 5.4 12.6 267 294
S Sculptoris 5.5 13.6 367 337
R Aquilae 5.5 12.0 271 422
R Leporis 5.5 11.7 445 413
W Hydrae 5.6 9.6 390 104
R Andromedae 5.8 15.2 409 386
S Coronae Borealis 5.8 14.1 360 541
U Cygni 5.9 12.1 463 518
X Ophiuchi 5.9 8.6 338
RS Scorpii 6.0 13.0 319 180
RT Sagittarii 6.0 14.1 306 952
RU Sagittarii 6.0 13.8 240
RT Cygni 6.0 13.1 190
R Geminorum 6.0 14.0 370
S Gruis 6.0 15.0 402 446
V Monocerotis 6.0 13.9 341 395
R Cancri 6.1 11.9 357 633
R Virginis 6.1 12.1 146 606
R Cygni 6.1 14.4 426
R Boötis 6.2 13.1 223
T Normae 6.2 13.6 244 277
R Leonis Minoris 6.3 13.2 372 347
S Virginis 6.3 13.2 375 1110
R Reticuli 6.4 14.2 281 820
S Herculis 6.4 13.8 304
U Herculis 6.4 13.4 404 235
R Octantis 6.4 13.2 407 602
S Pictoris 6.5 14.0 422 407
R Ursae Majoris 6.5 13.7 302 415
R Canum Venaticorum 6.5 12.9 329 962
R Normae 6.5 12.8 496 581
T Ursae Majoris 6.6 13.5 257 1250
R Aurigae 6.7 13.9 458 422
RU Herculis 6.7 14.3 481 1040
R Draconis 6.7 13.2 246 769
V Coronae Borealis 6.9 12.6 358 2700
T Cassiopeiae 6.9 13.0 445 1220
R Pegasi 6.9 13.8 378 287
V Cassiopeiae 6.9 13.4 229 467
RS Virginis 7.0 14.6 354
Z Cygni 7.1 14.7 264
S Orionis 7.2 13.1 434 1120
UV Aurigae 7.3 10.9 394
T Draconis 7.2 13.5 422
W Aquilae 7.3 14.3 490
S Cephei 7.4 12.9 487 407
R Fornacis 7.5 13.0 386 690
RZ Pegasi 7.6 13.6 437 206
RT Aquilae 7.6 14.5 327
V Cygni 7.7 13.9 421 366
RR Aquilae 7.8 14.5 395 521
S Boötis 7.8 13.8 271 680
WX Cygni 8.8 13.2 410
W Draconis 8.9 15.4 279
UX Cygni 9.0 17.0 569
R Capricorni 8.5[16] 15.1 340 [17]
IK Tauri 10.8 16.5 470
CIT 13 10.8 13.7 470
TX Camelopardalis 11.6 B 17.7 B 557
IRC +10216 11.0 R 14.8 R 630
OH 231.8+4.2 8.31 J 9.47 J 648
NV Aurigae 3.3 H 6.2 H 635
AFGL 2290 7.5 H 9.3 H
WX Piscium 0.9 K 4.3 K 660
LP Andromedae 1.8 K 3.7 K 614
IRC -10529 2.2 K 3.2 K 680
He 2-104 6.3 K 7.1 K
OH 26.5+0.6 6.9 K 10.8 K
LL Pegasi 9.6 K 11.6 K
OH 127.8+00 1.57 L 3.02 L
OH 32.8-0.3 3.9 L 7.2 L
TY Cassiopeiae 11.5 photographic 17.5 photographic 645
WX Serpentis 12.0 photographic 16.0 photographic 425

See also

References

  1. ^ Glass, I.S.; Lloyd Evans, T. (1981). "A period-luminosity relation for Mira variables in the Large Magellanic Cloud". Nature (Macmillan) 291 (5813): 303–4.  
  2. ^ Bedding, Timothy R.; Zijlstra, Albert A. (1998). "[ITAL]Hipparcos[/ITAL] Period-Luminosity Relations for Mira and Semiregular variables". The Astrophysical Journal 506: L47.  
  3. ^ Smith, Beverly J.; Leisawitz, David; Castelaz, Michael W.; Luttermoser, Donald (2002). "Infrared Light Curves of Mira Variable Stars from [ITAL]COBE[/ITAL] DIRBE Data". The Astronomical Journal 123 (2): 948.  
  4. ^ Ragland, S.; Traub, W. A.; Berger, J.-P.; Danchi, W. C.; Monnier, J. D.; Willson, L. A.; Carleton, N. P.; Lacasse, M. G.; Millan-Gabet, R.; Pedretti, E.; Schloerb, F. P.; Cotton, W. D.; Townes, C. H.; Brewer, M.; Haguenauer, P.; Kern, P.; Labeye, P.; Malbet, F.; Malin, D.; Pearlman, M.; Perraut, K.; Souccar, K.; Wallace, G. (2006). "First Surface-resolved Results with the Infrared Optical Telescope Array Imaging Interferometer: Detection of Asymmetries in Asymptotic Giant Branch Stars". The Astrophysical Journal 652: 650.  
  5. ^ Haniff, C. A.; Ghez, A. M.; Gorham, P. W.; Kulkarni, S. R.; Matthews, K.; Neugebauer, G. (1992). "Optical aperture synthetic images of the photosphere and molecular atmosphere of Mira". Astronomical Journal (ISSN 0004-6256) 103: 1662.  
  6. ^ Karovska, M.; Nisenson, P.; Papaliolios, C.; Boyle, R. P. (1991). "Asymmetries in the atmosphere of Mira". Astrophysical Journal 374: L51.  
  7. ^ Tuthill, P. G.; Haniff, C. A.; Baldwin, J. E. (1999). "Surface imaging of long-period variable stars". Monthly Notices of the Royal Astronomical Society 306 (2): 353.  
  8. ^ Freytag, B.; Höfner, S. (2008). "Three-dimensional simulations of the atmosphere of an AGB star". Astronomy and Astrophysics 483 (2): 571.  
  9. ^ Feast, Michael W.; Whitelock, Patricia A.; Menzies, John W. (2006). "Carbon-rich Mira variables: Kinematics and absolute magnitudes". Monthly Notices of the Royal Astronomical Society 369 (2): 791.  
  10. ^ Stancliffe, Richard J.; Izzard, Robert G.; Tout, Christopher A. (2004). "Third dredge-up in low-mass stars: Solving the Large Magellanic Cloud carbon star mystery". Monthly Notices of the Royal Astronomical Society: Letters 356: L1.  
  11. ^ Wittkowski, M.; Boboltz, D. A.; Ohnaka, K.; Driebe, T.; Scholz, M. (2007). "The Mira variable S Orionis: Relationships between the photosphere, molecular layer, dust shell, and SiO maser shell at 4 epochs". Astronomy and Astrophysics 470: 191.  
  12. ^ Zijlstra, A. A.; Bedding, T. R.; Mattei, J. A. (2002). "The evolution of the Mira variable R Hydrae". Monthly Notices of the Royal Astronomical Society 334 (3): 498.  
  13. ^ Templeton, M. R.; Mattei, J. A.; Willson, L. A. (2005). "Secular Evolution in Mira Variable Pulsations". The Astronomical Journal 130 (2): 776.  
  14. ^ Zijlstra, Albert A.; Bedding, Timothy R. (2002). "Period Evolution in Mira Variables". Journal of the American Association of Variable Star Observers 31: 2.  
  15. ^ Mattei, Janet Akyuz (1997). "Introducing Mira Variables". The Journal of the American Association of Variable Star Observers 25: 57.  
  16. ^ http://www.aavso.org/lcotw/r-capricorni
  17. ^ Discovered in 1848 by Hind. Patrick Moore and Robin Rees (2011). Patrick Moore's Data Book of Astronomy (second ed.). Cambridge University Press. p. 323. ISBN 978-1139495226. 
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