* Astronomy

Members Login
Post Info TOPIC: Barnard's Star


L

Posts: 131433
Date:
RE: Barnard's Star
Permalink  
 


Title: Precise Doppler Monitoring of Barnard's Star
Authors: Jieun Choi, Chris McCarthy, Geoffrey W. Marcy, Andrew W. Howard, Debra A. Fischer, John A. Johnson, Howard Isaacson, Jason T. Wright

We present 248 precise Doppler measurements of Barnard's Star (Gl 699), the second nearest star system to Earth, obtained from Lick and Keck Observatories during 25 years between 1987 and 2012. The early precision was 20 \ms but was 2 \ms during the last 8 years, constituting the most extensive and sensitive search for Doppler signatures of planets around this stellar neighbour. We carefully analyse the 136 Keck radial velocities spanning 8 years by first applying a periodogram analysis to search for nearly circular orbits. We find no significant periodic Doppler signals with amplitudes above \sim2 \ms, setting firm upper limits on the minimum mass (\msini) of any planets with orbital periods from 0.1 to 1000 days. Using a Monte Carlo analysis for circular orbits, we determine that planetary companions to Barnard's Star with masses above 2 Earth masses and periods below 10 days would have been detected. Planets with periods up to 2 years and masses above 10 Earth masses (0.03 Jupiter masses) are also ruled out. A similar analysis allowing for eccentric orbits yields comparable mass limits. The habitable zone of Barnard's Star appears to be devoid of roughly Earth-mass planets or larger, save for face-on orbits. Previous claims of planets around the star by van de Kamp are strongly refuted. The radial velocity of Barnard's Star increases with time at 4.515Ī0.002 \msy, consistent with the predicted geometrical effect, secular acceleration, that exchanges transverse for radial components of velocity.

Read more (1439kb, PDF)



__________________


L

Posts: 131433
Date:
Permalink  
 

Barnards Star.kmz
Google Sky File

-- Edited by Blobrana at 14:24, 2008-01-12

__________________


L

Posts: 131433
Date:
Permalink  
 

Barnard's Star, the second nearest star system to the Sun, spewed a large flare that increased its brightness, say astronomers in the United States. The discovery is a surprise: the star should be sedate, because it's an old red dwarf.

Red dwarfs are smaller, cooler, and fainter than the Sun. When young and middle-aged, they often spin fast enough to generate strong magnetic fields. These cause flares that can double the star's brightness in just a few seconds.

As red dwarfs age, they spin more slowly. Barnard's Star is old; a likely member of the Milky Way's thick-disk population. Its age is probably 11 or 12 billion years, which is more than twice the Sun's age. And small periodic variations in the starís light suggest it rotates slowly, only once every 130 days. Thus, the star should not emit flares.


Position(2000): RA 17 57 48.50 Dec +04 41 36.2

However, Diane Paulson of NASA's Goddard Space Flight Centre in Greenbelt, Maryland, and her colleagues report Barnard's Star unleashed a flare July 17, 1998. At the time, William Cochran of the University of Texas at Austin was using McDonald Observatory's 2.7-meter telescope to obtain the star's spectrum. He hoped to detect changes that signified orbiting planets. Instead, he noticed emission lines in the spectrum that indicated the star might be flaring, and he obtained two additional spectra that night. But because he was interested in planets rather than flares, he did not pursue the matter further.

Four years later, Cochran showed the spectra to Paulson, then a graduate student in Austin. Her team's recent analysis indicates that despite the star's old age, its hot blue flare resembled those from younger red dwarfs. The flare's temperature was at least 8,000 Kelvin, more than double the star's temperature of 3,100 Kelvin.

Unfortunately, spectra can't determine just how much Barnard's Star brightened that night. Paulson estimates the rise at visual wavelengths was half a magnitude or more. The flare lasted at least an hour.

"The star would be fantastic for amateurs to observe," says Paulson. Located in northern Ophiuchus, Barnard's Star is magnitude 9.55. Amateur observers can easily monitor its brightness and search for future flares. As the astronomers write, however, "Because Barnard's star is considered to be one of our oldest neighbours, a flare of this magnitude is probably quite rare."

American astronomer Edward Emerson Barnard discovered Barnard's Star in 1916. It has a large proper motion--the greatest of any star--, which results both from the star's high speed through space and from its proximity to Earth. Precise parallax data from the Hubble Space Telescope indicate it is 5.98 light-years away, making it the second closest star system to the Sun. Only the triple star Alpha Centauri is nearer.

The new discovery means all three of the three nearest red dwarfs are flare stars: Proxima Centauri, the faintest member of the Alpha Centauri system; Barnard's Star; and Wolf 359 in Leo, which is 7.8 light-years from Earth. However, Proxima Centauri and Wolf 359 are sufficiently active that their flares were found more than half a century ago.

Paulson and her colleagues will publish their work in a future issue of Publications of the Astronomical Society of the Pacific.

Source

__________________
Page 1 of 1  sorted by
Quick Reply

Please log in to post quick replies.



Create your own FREE Forum
Report Abuse
Powered by ActiveBoard