Tuesday, November 24, 2015

Lyra - Stars with planets and Doppler spectroscopy

Image Quora.com

HD 177830
HD 177830 is a 7th magnitude star located approximately 193 light-years away in the constellation of Lyra. It is slightly more massive than our Sun, but cooler being a type K star. Therefore, it is a subgiant clearly more evolved than the Sun. In visual light it is four times brighter than the Sun, but because of its distance, about 193 light years, it is not visible to the unaided eye. With binoculars it should be easily visible.

HD 173416
Visible to the naked eye are HD 173416, a yellow giant hosting a planet over twice the mass of Jupiter discovered in 2009.

HD 176051
HD 176051 is a low-mass binary star containing a high-mass planet.

On November 14, 1999, the discovery of a planet HD 177830 b was announced by the California and Carnegie Planet Search team using the very successful radial velocity method along with two other planets. This planet is nearly 50% more massive than Jupiter (MJ) and takes 407 days to orbit the star in an extremely circular orbit.

On November 17, 2010, the discovery of a second planet HD 177830 c was announced along with four other planets. The planet has 50% the mass of Saturn and takes 111 days to orbit the star in a very eccentric orbit. This planet is in a near 4:1 resonance with the outer planet.

Doppler spectroscopy

"Doppler Shift vs Time" by Dan Wingard
Licensed under Public Domain via Commons
Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in the spectrum of the planet's parent star.

About half of the extrasolar planets known were discovered using Doppler spectroscopy, as of October 2012.

A series of observations is made of the spectrum of light emitted by a star. Periodic variations in the star's spectrum may be detected, with the wavelength of characteristic spectral lines in the spectrum increasing and decreasing regularly over a period of time.

Statistical filters are then applied to the data set to cancel out spectrum effects from other sources.

Using mathematical best-fit techniques, astronomers can isolate the tell-tale periodic sine wave that indicates a planet in orbit.

If an extrasolar planet is detected, a minimum mass for the planet can be determined from the changes in the star's radial velocity.

To find a more precise measure of the mass requires knowledge of the inclination of the planet's orbit. A graph of measured radial velocity versus time will give a characteristic curve (sine curve in the case of a circular orbit), and the amplitude of the curve will allow the minimum mass of the planet to be calculated.

Although radial-velocity of the star only gives a planet's minimum mass, if the planet's spectral lines can be distinguished from the star's spectral lines then the radial-velocity of the planet itself can be found and this gives the inclination of the planet's orbit and therefore the planet's actual mass can be determined. The first non-transiting planet to have its mass found this way was Tau Boötis b in 2012 when carbon monoxide was detected in the infra-red part of the spectrum

Read the articles quoted from Wikipedia for much additional information and references.

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