Wolf 359 for Probe Discussion

Posted by Taz on Friday, 23 January 1998, at 9:41 a.m.

For the first time, the high resolution provided by NASA's Hubble Space
Telescope has been used to search for faint companions about the nearby star
Wolf 359. At a distance of 7.5 lightyears, Wolf 359 is the third closest star
system to the sun. The Centauri triple system (alpha, beta, & Prox Cen) at a
distance of 4.3 lightyears is closest to the sun with Barnard's star coming in
as a distant second at 6.0 light years. The images from HST show no large
orbiting body (stellar or substellar, brown dwarf) as close as 1 AU (0.432
arcseconds) from Wolf 359. Imaging this close to Wolf 359 (V ~13.4) with a
faint detection limit approaching V ~23 is a feat that has not yet been
performed from the ground.

These observations using the high resolution provided by HST in combination
with the Faint Object Spectrograph (FOS), were made by a team of scientists
from Computer Sciences Corporation (Dr. Alfred Schultz, Dr. Helen Hart, Forrest
Hamilton, & Mark Kochte), Catholic University (Fred Bruhweiler), McDonald
Observatory (Fritz Benedict), York University (John Caldwell & Cindy
Cunningham), Lowell Observatory (Otto Franz), STScI (Tony Keyes), and Univ. of
Colorado (Jack Brandt). These images search a region as close as 0.5 AU for
bright companions (as bright or brighter than V ~18) and as far as 2.8 AU for
objects as faint as V ~23. This work complements the 1990 infrared speckle
interferometry search for a brown dwarf orbiting Wolf 359 in the region between
2 and 10 AU to a limiting infrared H-magnitude of ~12 and K-magnitude of ~11.5.
The FOS image was obtained without any filter, thereby imaging the star with
the entire visual light band, from blue to red.

Wolf 359 is in the northern sky. It can be seen from the ground in a moderate
size telescope. A "light year" is the distance light, traveling at a speed of
186,000 miles/second, would transverse in a year. If one viewed the earth from
a distance of 3.26 lightyears, the maximum separation between the earth and the
sun (1 AU) would be "1 arcsecond." The double quote symbol following a number
represents units of arcseconds. An AU (astronomical unit) is the distance from
the earth to the sun (92 million miles). It takes 8 minutes for the light from
the sun to reach the earth.

A coronagraphic type instrument is needed to block out the bright light coming
from the primary star to be able to image the light from a faint companion. A
coronagraph is a device with an occulting spot, finger, or bar in the optical
path that blocks the light from a star (or the sun) from reaching the detector.
The FOS, though not considered an imaging camera, does have a coronagraphic
capability with the large barred aperture. The large barred aperture is 1.7"
square with a 0.27" wide bar across the center of the aperture. In addition,
the FOS is the only HST instrument with a demonstrated on-board capability to
position a star behind the occulting bar. This is the so-called PEAK-DOWN
target acquisition capability. The large barred aperture was designed so that
an object could be centered behind the bar and spectra obtained from each
opening of the aperture.

Once the HST FOS images were obtained, sophisticated computer processing was
required to "deconvolve" the images to reveal any faint companion. An FOS
ACQIMAGE is created when the FOS camera mirror images the aperture onto the
photocathode, which produces photoelectrons that are deflected onto the Digicon
for readout. The Digicon detector is an array of 512 rectangular shaped
diodes. The shape of the mapping diodes combined with the x- and y-stepping of
the mirror scan transforms a star image into a rectangular shape. A star image
is sampled 4 times in x and 20 times in y. A computer software program based
on a Fourier transform was used to deconvolve the ACQIMAGE into the images
displayed in the photograph.

Wolf 359 was imaged centered behind the occulting bar (coronagraphic mode) and
with the occulting bar rotated out of the field of view. In the coronagraphic
image, the bar covers the star. This allows for a far longer exposure to be
taken (1,600 seconds; without the bar, only 20 seconds) to probe for faint
companions. The bar causes the dark band across the image of the star.
Portions of the star image extend beyond both edges of the bar.

Faint, unseen, companions about nearby stars can be discovered through
perturbations in their proper motions. Proper motion is the apparent path a
star transverses across the sky in relationship to the stationary, distant
"fixed stars" of the galaxy. All stars, including the sun, orbit the galaxy.
Astrometry is the study of star positions and has been performed by measuring
the positions of stars on photographic plates, by observing the positions of
the stars visually with a transit telescope, or observing the stars visually
with a cross hair and a micrometer. An observed perturbation, wiggle, about the
proper motion path could be due to the motion of the primary star and an unseen
companion orbiting the center of mass of the system. For many years,
Allegheny, McCormick, U.S. Naval Observatory, Sproul, Van Vleck, and Yerkes
observatories performed astrometric searches for stellar duplicities. Many of
the known nearby binary star systems were first discovered by astronomers at
these observatories.

Ground-based astrometric measurements are usually based on many years of
observations with photographic plates. The star image may be oval in shape due
to the combined light of the primary star and a faint companion or possibly due
to the motion induced from guiding a telescope for long periods of time.
Astronomers at Sproul Observatory obtained photographic plates containing the
image of Wolf 359, along with the images of many background stars, from 1937 to
1974. Wolf 359 with V ~13.4 was their faintest target. The average size of the
star image on a photographic plate was 1.6" in diameter. After scanning the
entire plate set in the early 1970's with a machine and determining the
parallax and proper motion, they did not detect a wiggle that could be
attributed to a stellar or substellar companion.

Astronomers are excited about discovering and studying faint companions,
possibly brown dwarfs, about the nearby stars. Brown dwarfs are objects that
are smaller than a hydrogen burning star, but larger than the planet Jupiter.
Brown dwarfs bridge the gap between the formation of stars and the formation of
planets. From previous searches, photographic astrometry and infrared speckle,
a large companion was not expected to be discovered orbiting at large distances
from Wolf 359. The FOS coronagraphic images of Wolf 359 confirms a null
detection, but the images have extended the search for faint companions to
within 1 AU of the star with a detection limit of V ~23.


 

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