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BOLETÍN DE LOS OBSERVATORIOS DE TONANTZINTLA Y TACUBAYA, VOL. 4, N°27, 1965. G W ORIONIS, A 20000 YEARS OLD T TAURI STAR? A rCf/dio Povcda SI.'MARIO (; W Orionis es uua estrella del tipo T Tauri cuya magnitud y tipo espectral han sido ohsen'ados por Joy y \"ilson. De estos datos se ohtiene la Inminosidad el radio y la temperalllra cfectÍ\'a lo cnal permite nhicarla en el diagrama H-R y comparar sn posición ('on las trayectorias e\,ohltÍ\'as de Hayashi. encontr:índose que su masa es aproximadamente el dohle de la masa solar. Con estos datos y nm la fórmula para © Copyright 1965: Observatorio Astronómico Nacional, Universidad Nacional Autónoma de México edad actual de (; \\' Orionis es de alrededor la edad de IIna estrella de 20 ()OO alios. en coutracción gravitacional. se em:uentra que la Our knowledge 01' the actual ages 01' very young stars is very meager; the first indication Lhat we may be observing stars with ages 01' the onler 01' 10 000 years came indirectly from the analysis 01' the density distribution in the Orion nebula (Kahn am! Menon ]9(1) which showed that the latter could not have existed as an H ]] region 1'01' a period much longer than 10 000 years, thus setting a very small upper bound Lo the age of{}lC Orionis as an ionizing source. On the other hand, since the total time 01' contraction to the main sequence, 01' a massive star, is 01' the order 01' 3 X 104 years, {}lC Orionis should then be a star younger than 5 X 104 years. A further evidence in favor 01' the existen ce 01' stars 01' a similar age bracket is affon!ed by a sample 01' stars that illuminate certain conica! ami cometary nebulae. A simple kinematical argument showed (Poveda 19(5) that a sample 01' eleven such stars gave ages which ranged from 6000 to 53000 years, ami although the individual values are somewhat uncertain, the mean age 01' ]9000 years found 1'01'the group is certainly more reliable. The purpose of the present note is LO poi m out that G W Orionis, a T Tauri star, is also a ver y young object with an age comparable to that 01' Lhe Orion nebula ami LO the mean age 01' Lhe sLars at the vertices of comeLary nebulae. The reasoning underlying the case 01' G W Orionis -which is ver y different from that used beforeis based on the place which this star occupies in the H-R diagramf ami in the contraction Limes which a wholly convective star needs to arrive to its prcsenl values 01' R, L and Te. G W Orionis is listed as a T Tauri star in Lhe last catalogue compiled by Herbig (1962) who dassifies its light curve, as one which remains most 01' the time at maximum. On Lhe other hand Joy ami Wi]son (]949) , who list it as MHa 265-2, assign Lo it a visual magnitude 01' 9.2 ami spectral type K5e. These values combined with a modulu s 01' R.5*, lead to the following parameters 1'01'G ", Orionis. M,. n.7 Te 4 4000K 1\/"", _ 0.14 The value of the eHective tell1perature and the IUl1linosity, permits to. plot this star diagram amI to compare its position with the evoluLionary tracks calculated by Hayashi, Sugill1oto (]963); this is done in figure 1., where it is easily seen that Lhe posiLion 01' G is very close to the track 01' 2 solar ll1asses; thus we take this value as the actual mass 01' G To calculate the presem age 01' G 'V Orionis it rest only to use the formula yashi et al (1963) 1'01' Lhe age 01' a star in gravitational contraction, that is: in the H-R Hoshi ami W Orionis 'V Orionis. derived by Ha- years +a where a is a number larger than 1.5 1'01'wholly parameters listed above, we finally arrive at: convective stars, as it is in the present case. Using the t < 20 000 years 'Ve want to emphasize that this value -which is surprisingly close LO the age of the Orion nebula am! the stars in cometary nebulaehas been found by an entirely different reasoning. It is also interesting to realize the increasing number 01' stars, wiLhin 500 parsecs from the sun, with ages around 2 X 104 years. * Since it appears to be physically connected with Ihe complex aronno -77- l. Orionis. 2 © Copyright 1965: Observatorio Astronómico Nacional, Universidad Nacional Autónoma de México o < ...J o o - 1 -1 oO ...J 4.0 3.6 3.8 3.4 LO{l T. REFERE:'\CES Hayashi. C. Hoshi. R. :lIId Sugimoto. D. 19G2 Supp. I'rogrcss or Thcorctical I'hysics :'\0. 22. Herbig. (.. 19(;2 in Z. J\.opal .-\Ih"anecs in .-\slronolllY ami .-\st1'llphysics Yo\. J. p. -ti; .-\eadcmic I'rcss. .loy. A. H. allll Wilson R. E. I !149 Ap. .1. 109, 23 J. J\.ahn. F. D. and ~Icnon T. J\.. l!J(iI I'roe. :'\al. .-\('ad. Sci. Washington 47, lil2. I'ovcda, A. 196" l\olelill Ohs. TOJl:ullzilllla y Tal'uhay'a 4. 22. -78-