Q&A: Miscellaneous X-ray Sources

Q:
How can you be sure of TWA 5B's mass and its distance from the central star? Can its orbital speed be measured in some way?

A:
The distance from the central star is well known because the angular distance of the brown dwarf from TWA 5A is determined from optical and X-ray measurements, and the distance is known from measurements by the Hipparchos satellite. An orbital determination of the mass of TWA 5B is not possible because the estimated orbital period is about 900 years. The mass estimate of 12 to 40 Jupiter masses is based on optical and infrared estimates of the properties (brightness, temperature) of the brown dwarf, and comparing the observed properties with theoretical models.