Estrela de classe A da sequência principal

Concepção artística de Sirius A e Sirius B, um sistema estelar binário

Uma estrela de classe A da sequência principal (A V) ou estrela anã A é uma estrela da sequência principal (queima de hidrogênio) de tipo espectral A e classe de luminosidade V. Essas estrelas têm espectros que são definidos por fortes linhas de absorção Balmer de hidrogênio.[1][2] Medem entre 1.4 e 2.1 massas solares (M) e têm temperaturas de superfície entre 7.600 K e 10.000 K.[3] Exemplos brilhantes e próximos são Altair (A7 V), Sirius A (A1 V) e Vega (A0 V).[4] As estrelas de classe A não têm uma zona convectiva e, portanto, não se espera que abriguem um dínamo magnético. Como consequência, por não terem ventos estelares fortes, carecem de meios para gerar emissão de raios-X.[5]

Em julho de 2019, astrônomos relataram ter encontrado uma estrela de classe A, S5-HVS1, viajando 1.755 km/s, mais rápido do que qualquer outra estrela detectada até agora. A estrela está na constelação de Grus no céu meridional, e cerca de 29.000 anos-luz da Terra, e pode ter sido ejetada para fora da galáxia da Via Láctea após interagir com Sagittarius A*, o buraco negro supermassivo no centro da galáxia.[6][7][8][9][10]

Padrão espectral de estrelas

Características tipícas[11]
Classe
estelar
Massa
(M)
Raio
(R)
Mv Teff
(K)
A0V 2.40 1.87 0.7 9727
A1V 2.30 1.83 1 9274
A2V 2.19 1.78 1.3 8820
A3V 2.11 1.76 1.5 8585
A4V 2.03 1.74 1.7 8350
A5V 1.86 1.69 2. 7880
A6V 1.8 1.66 2.1 7672
A7V 1.74 1.63 2.3 7483
A8V 1.66 1.6 2.4 7305
A9V 1.62 1.55 2.5 7112

O sistema revisado do Atlas de Yerkes[12] listou uma grade densa de estrelas padrão espectrais anãs de classe A, mas nem todas sobreviveram até hoje como padrões. Os "pontos de ancoragem" e "padrões de punhal" do sistema de classificação espectral MK entre as estrelas anãs da sequência principal de classe A, ou seja, aquelas estrelas padrão que permaneceram inalteradas ao longo dos anos e podem ser consideradas para definir o sistema, são Vega (A0 V), Gamma Ursae Majoris (A0 V) e Fomalhaut (A3 V).[13][14] A revisão seminal da classificação MK por Morgan & Keenan (1973)[14] não forneceu quaisquer padrões de punhal entre os tipos A3 V e F2 V. HD 23886 foi sugerido como um padrão A5 V em 1978.[15] Richard Gray e Robert Garrison forneceram as contribuições mais recentes para a sequência espectral de anã A em um par de artigos em 1987[16] e 1989.[17] Eles listam uma variedade de padrões espectrais de anãs de classe A de rotação rápida e lenta, incluindo HD 45320 (A1 V), HD 88955 (A2 V), 2 Hydri (A7 V), 21 Leonis Minoris (A7 V) e 44 Ceti (A9 V). Além dos padrões MK fornecidos nos artigos de Morgan e nos artigos de Gray & Garrison, ocasionalmente também se vê Delta Leonis (A4 V) listado como um padrão. Não há estrelas padrão A6 V e A8 V publicadas.

A classificação espectral Morgan-Keenan

Planetas

As estrelas de classe A são jovens (normalmente com algumas centenas de milhões de anos) e muitas emitem radiação infravermelha (IV) além do que seria esperado apenas da estrela. Esse excesso de IV é atribuível à emissão de poeira de um disco de detritos onde os planetas se formam.[18] Pesquisas indicam que planetas massivos geralmente se formam em torno de estrelas de classe A, embora esses planetas sejam difíceis de detectar usando o método de espectroscopia Doppler. Isso ocorre porque as estrelas de classe A geralmente giram muito rapidamente, o que torna difícil medir os pequenos desvios Doppler induzidos por planetas em órbita, uma vez que as linhas espectrais são muito largas.[19] No entanto, esse tipo de estrela massiva eventualmente evolui para uma gigante vermelha mais fria que gira mais lentamente e, portanto, pode ser medida usando o método da velocidade radial.[19] No início de 2011, cerca de 30 planetas da classe Júpiter foram encontrados em torno de estrelas gigantes K evoluídas, incluindo Pólux, Gamma Cephei e Iota Draconis. Levantamentos Doppler em torno de uma grande variedade de estrelas indicam que cerca de 1 em 6 estrelas com o dobro da massa do Sol são orbitadas por um ou mais planetas do tamanho de Júpiter, em comparação com cerca de 1 em 16 para estrelas semelhantes ao Sol.[20]

Os sistemas estelares de classe A conhecidos por apresentarem planetas incluem Fomalhaut, HD 15082, Beta Pictoris e HD 95086.[21]

Exemplos

Nome Tipo
espectral
Constelação Mag. apa. Massa
(M)
Raio
(R)
Luminosidade
(L)
Distância
(ly)
Altair A7 V Aquila 0.76 1.79 1.63–2.03 10.6 16.73
Sirius A0mA1 Va Canis Major −1.47 2.063 1.711 25.4 8.60 ± 0.04
Vega A0 Va Lyra 0.026 2.135 2.362 × 2.818 40.12 25.04

Ver também

O Commons possui uma categoria com imagens e outros ficheiros sobre Estrela de classe A da sequência principal

Referências

  1. Stellar Spectral Types, entry at hyperphysics.phy-astr.gsu.edu, accessed on line June 19, 2007.
  2. "An Introduction to Modern Astrophysics" by B.W Caroll and D.A Ostlie 1996 edition, chapter 8
  3. Empirical bolometric corrections for the main-sequence, G. M. H. J. Habets and J. R. W. Heintze, Astronomy and Astrophysics Supplement 46 (November 1981), pp. 193–237, Tables VII and VIII.
  4. SIMBAD, entries on Sirius A and Vega, accessed June 19, 2007.
  5. Schröder, C.; Schmitt, J. H. M. M. (novembro de 2007), «X-ray emission from A-type stars», Astronomy and Astrophysics, 475 (2): 677–684, Bibcode:2007A&A...475..677S, doi:10.1051/0004-6361:20077429Acessível livremente. 
  6. Overbye, Dennis (14 de novembro de 2019). «A Black Hole Threw a Star Out of the Milky Way Galaxy – So long, S5-HVS1, we hardly knew you.». The New York Times. Consultado em 18 de novembro de 2019 
  7. Koposov, Sergey E.; et al. (2019). «Discovery of a nearby 1700 km/s star ejected from the Milky Way by Sgr A*». Monthly Notices of the Royal Astronomical Society. arXiv:1907.11725Acessível livremente. doi:10.1093/mnras/stz3081 
  8. Starr, Michelle (31 de julho de 2019). «Bizarre Star Found Hurtling Out of Our Galaxy Centre Is Fastest of Its Kind Ever Seen». ScienceAlert.com. Consultado em 18 de novembro de 2019 
  9. Irving, Michael (13 de novembro de 2019). «Fastest star ever found is being flicked out of the Milky Way». NewAtlas.com. Consultado em 18 de novembro de 2019 
  10. Plait, Phil (13 de novembro de 2019). «Our Local Supermassive Black Hole Shot A Star Right Out Of THe Galaxy». Bad Astronomy. Consultado em 19 de novembro de 2019 
  11. Adelman, S. J. (2005). «The physical properties of normal A stars». Proceedings of the International Astronomical Union. 2004: 1–11. Bibcode:2004IAUS..224....1A. doi:10.1017/S1743921304004314Acessível livremente 
  12. Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas H.L. Johnson & W.W. Morgan, 1953, Astrophysical Journal, 117, 313
  13. MK ANCHOR POINTS, Robert F. Garrison
  14. a b Spectral Classification, W.W. Morgan & P.C. Keenan, 1973, Annual Review of Astronomy and Astrophysics, vol. 11, p.29
  15. Revised MK Spectral Atlas for stars earlier than the sun, W.W. Morgan, W. W., H.A. Abt, J.W. Tapscott, 1978, Williams Bay: Yerkes Observatory, and Tucson: Kitt Peak National Observatory
  16. The early A type stars – Refined MK classification, confrontation with Stroemgren photometry, and the effects of rotation, R.O. Gray & R.F. Garrison, 1987, Astrophysical Journal Supplement Series, vol. 65, p. 581
  17. The late A-type stars – Refined MK classification, confrontation with Stromgren photometry, and the effects of rotation, R.O. Gray & R.F. Garrison, 1989, Astrophysical Journal Supplement Series, vol. 70, p. 623
  18. Song, Inseok; et al. (2002), «M-Type Vega-like Stars», The Astronomical Journal, 124 (1): 514–518, Bibcode:2002AJ....124..514S, arXiv:astro-ph/0204255Acessível livremente, doi:10.1086/341164 
  19. a b Retired A Stars and Their Companions: Exoplanets Orbiting Three Intermediate-Mass Subgiants, John A. Johnson, Debra A. Fischer, Geoffrey W. Marcy, Jason T. Wright, Peter Driscoll, R. P. Butler, Saskia Hekker, Sabine Reffert, Steven S. Vogt, 19 Apr 2007
  20. Johnson, J. A. (2011). «The Stars that Host Planets». Sky & Telescope (April): 22–27 
  21. Smalley, J. B. (2014). «Eclipsing Am binary systems in the SuperWASP survey». Astronomy and Astrophysics (April). 20 páginas 

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