F. C. Hamilton

Hubble Space Telescope Observations of the Interacting Galaxies NGC 2207 and IC 2163

Original article can be found at: http://www.journals.uchicago.edu/AJ/--Copyright American Astronomical Society

HST observations of the interacting galaxies NGC 2207 and IC 2163

Original article can be found at: http://www.journals.uchicago.edu/AJ/--Copyright American Astronomical Society

A POSSIBLE COMPANION TO PROXIMA CENTAURI

We report Hubble Space Telescope observations that provide evidence for a companion to Proxima Centauri (Gl 551), the closest star to the Sun. Data acquired with the Faint Object Spectrograph (FOS), used as a coronagraphic camera, show excess light that can be interpreted as being from a substellar object within 05 of Proxima Cen. Two observations of Proxima Cen separated by 103 days indicate a point source (or a feature) superposed on the wing of the point-spread function in the FOS images. This feature moves relative to the aperture, and on the plane of the sky. Comparisons with other FOS images of stars acquired using the coronagraphic mode reveal no comparable features or evidence that this feature can be explained by any instrumental anomaly. If this feature denotes a companion to Proxima Cen, it has an apparent separation corresponding to ~0.5 AU at Proxima Cen and is ≈7 mag fainter than Proxima Cen in the bandpass of the FOS red detector. The small apparent separation could result from a highly eccentric orbit, which could project a close companion. Alternately, the small separation could imply a short (~1 yr) period. Further coronagraphic observations, using the Space Telescope Imaging Spectrograph, the Near-Infrared Camera and Multiobject Spectrometer, or some other instrumentation, are needed to verify this tentative result.

Hubble Space Telescope/Faint Object Spectrograph imaging of Wolf 424 AB: Is this binary a double brown dwarf system?

The low-mass binary system Wolf 424 AB (GJ 473 AB) was spatially resolved in an image obtained on 1996 April 16 with the imaging mode of the Hubble Space TelescopeFaint Object Spectrograph. The separation was measured to be 00 0 .03 at a position angle of 3537 . 7 The new measurement was 42 5 02 5 2 combined with previously published orbital positions to update the orbital elements and obtain an estimate of the total mass. Spectroscopy and photometry of these objects suggest they are nearly equal in mass. The derived orbital solution, using all of the observations since 1938 appropriately weighted, yields a total mass of m 1 1 of 0.143 M,, and component masses of »0.07 M,, just under the theoretical limit for a brown dwarf. An m2 alternate orbital solution derived solely from speckle observations since 1986 and the Faint Object Spectrograph image yields a total mass of 0.232 M,, placing both components well above the brown-dwarf limit. The mass depends on the value for the semimajor axis, which is controlled in the latter solution by an optical speckle observation made near the last period of maximum separation. Further high-resolution observations during the upcoming maximum separation, and through the next secondary maximum, are needed to constrain the orbit.