New substellar Methusalem?

Metalpoor and very dim stars are remnants of the early formation history of the Milky Way. Due to their low mass and luminosity, measurements of their physical properties put constrains on theories of the formation of such stars.

However, to derive these properties measuring their true distance is essential. A direct method to measure distances is called "trigonometric parallax measurement". Since Earth moves around the Sun the point of view at the sky is changing. For this reason, stars which are not too far away seem to follow an elliptical path on the sky relative to the far-away galaxies of the Universe. The size of such ellipses is a measure for the distance.

Unfortunately, metalpoor, low mass stars or so called "Brown Dwarfs" ar extremely faint and will merely be visible in the infrared regime. Thus, parallactic measurements are quite challenging.

In this context a remarkable corner stone has been set by Elena Schilbach and Siegfried Röser of the "Center for Astronomy of the University of Heidelberg" and Ralf-Dieter Scholz of the "Astrophysical Institute Potsdam" ( Astronomy & Astrophysics 493, L27-L30, 2009). They were able to determine parallaxes of 10 subdwarfs with the previously classified spectral types sdM7 to sdT6. Corresponding data have been selected via infrared H-Band imaging at the 3,5m-Telescope of the Calar Alto observatory in Spain over a period of almost four years.

For the first time they had used far away background galaxies as fixpoints, leading to an unprecedented positional accuracy, a high precision distance measurement and thus the true luminosity of the candidate stars observed.

By comparing their measurements with model predictions they concluded that six stars exhibit 1/30 of the metallicity found in our sun. Moreover, the brightness and color of the star 2MASS 1626+3925 fit a brown dwarf, probably the oldest brown dwarf known so far - about 10 billion years old, characterizing this object to be a true substellar methusalem.

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