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Posts: 131433
Date:
TWA 5
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Title: Orbits and masses in the young triple system TWA 5
Authors: R. Köhler, T. Ratzka, M. G. Petr-Gotzens, S. Correia

We aim to improve the orbital elements and determine the individual masses of the components in the triple system TWA 5.
Five new relative astrometric positions in the H band were recorded with the adaptive optics system at the Very Large Telescope (VLT). We combine them with data from the literature and a measurement in the Ks band. We derive an improved fit for the orbit of TWA 5Aa-b around each other. Furthermore, we use the third component, TWA 5B, as an astrometric reference to determine the motion of Aa and Ab around their center of mass and compute their mass ratio.
We find an orbital period of 6.03±0.01 years and a semi-major axis of 63.7±0.2 mas (3.2±0.1 AU). With the trigonometric distance of 50.1±1.8 pc, this yields a system mass of 0.9±0.1 solar masses, where the error is dominated by the error of the distance. The dynamical mass agrees with the system mass predicted by a number of theoretical models if we assume that TWA5 is at the young end of the age range of the TW Hydrae association.
We find a mass ratio of M_Ab / M_Aa = 1.3 +0.6/-0.4, where the less luminous component Ab is more massive. This result is likely to be a consequence of the large uncertainties due to the limited orbital coverage of the observations.

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Posts: 131433
Date:
TWA 5 B
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Title: Orbital motion of the young brown dwarf companion TWA 5 B
Authors: R. Neuhäuser (1), T. O. B. Schmidt (1), V. V. Hambaryan (1), N. Vogt (2 and 3) ((1) Astrophysikalisches Institut und Universitäts-Sternwarte, Jena, Germany, (2) Departamento de Física y Astronomía, Valparaíso, Chile, (3) Universidad Catolica del Norte, Antofagasta, Chile)
(Version v2)

With more adaptive optics images available, we aim at detecting orbital motion for the first time in the system TWA 5 A+B. We measured separation and position angle between TWA 5 A and B in each high-resolution image available and followed their change in time, because B should orbit around A. The astrometric measurement precision is about one milli arc sec. With ten year difference in epoch, we can clearly detect orbital motion of B around A, a decrease in separation by ~ 0.0054 arc sec per year and a decrease in position angle by ~ 0.26 degrees per year. TWA 5 B is a brown dwarf with ~ 25 Jupiter masses (Neuhauser et al. 2000), but having large error bars (4 to 145 Jupiter masses, Neuhauser et al. 2009). Given its large projected separation from the primary star, ~ 86 AU, and its young age ~ 10 Myrs), it has probably formed star-like, and would then be a brown dwarf companion. Given the relatively large changes in separation and position angle between TWA 5 A and B, we can conclude that they orbit around each other on an eccentric orbit. Some evidence is found for a curvature in the orbital motion of B around A - most consistent with an elliptic (e=0.45) orbit. Residuals around the best-fit ellipse are detected and show a small-amplitude (~ 18 mas) periodic sinusoid with ~ 5.7 yr period, i.e., fully consistent with the orbit of the inner close pair TWA 5 Aa+b. Measuring these residuals caused by the photocenter wobble - even in unresolved images - can yield the total mass of the inner pair, so can test theoretical pre-main sequence models.

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L

Posts: 131433
Date:
TWA 5A
Permalink  
 


Title: Orbital motion of the young brown dwarf companion TWA 5 B
Authors: R. Neuhäuser (1), T. O. B. Schmidt (1), V. V. Hambaryan (1), N. Vogt (2 and 3) ((1) Astrophysikalisches Institut und Universitäts-Sternwarte, Jena, Germany, (2) Departamento de Física y Astronomía, Valparaíso, Chile, (3) Universidad Catolica del Norte, Antofagasta, Chile)

With more adaptive optics images available, we aim at detecting orbital motion for the first time in the system TWA 5 A+B. We measured separation and position angle between TWA 5 A and B in each high-resolution image available and followed their change in time, because B should orbit around A. The astrometric measurement precision is about one milli arc sec. With ten year difference in epoch, we can clearly detect orbital motion of B around A, a decrease in separation by ~ 0.0054 arc sec per year and a decrease in position angle by ~ 0.26 degrees per year. TWA 5 B is a brown dwarf with ~ 25 Jupiter masses (Neuhauser et al. 2000), but having large error bars (4 to 145 Jupiter masses, Neuhauser et al. 2009). Given its large projected separation from the primary star, ~ 86 AU, and its young age ~ 10 Myrs), it has probably formed star-like, and would then be a brown dwarf companion. Given the relatively large changes in separation and position angle between TWA 5 A and B, we can conclude that they orbit around each other on an eccentric orbit. Some evidence is found for a curvature in the orbital motion of B around A - most consistent with an elliptic (e=0.45) orbit. Residuals around the best-fit ellipse are detected and show a small-amplitude (~ 18 mas) periodic sinusoid with ~ 5.7 yr period, i.e., fully consistent with the orbit of the inner close pair TWA 5 Aa+b. Measuring these residuals caused by the photocenter wobble - even in unresolved images - can yield the total mass of the inner pair, so can test theoretical pre-main sequence models.

Read more (49kb, PDF)

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