This is because A-type stars typically rotate very quickly, which makes it difficult to measure the small Doppler shifts induced by orbiting planets since the spectral lines are very broad. Surveys indicate massive planets commonly form around A-type stars although these planets are difficult to detect using the Doppler spectroscopy method. This IR excess is attributable to dust emission from a debris disk where planets form. The Morgan-Keenan spectral classificationĪ-type stars are young (typically few hundred million years old) and many emit infrared (IR) radiation beyond what would be expected from the star alone. There are no published A6 V and A8 V standard stars. Besides the MK standards provided in Morgan's papers and the Gray & Garrison papers, one also occasionally sees Delta Leonis (A4 V) listed as a standard. They list an assortment of fast- and slow-rotating A-type dwarf spectral standards, including HD 45320 (A1 V), HD 88955 (A2 V), 2 Hydri (A7 V), 21 Leonis Minoris (A7 V), and 44 Ceti (A9 V). Richard Gray & Robert Garrison provided the most recent contributions to the A dwarf spectral sequence in a pair of papers in 19. HD 23886 was suggested as an A5 V standard in 1978. The seminal review of MK classification by Morgan & Keenan (1973) didn't provide any dagger standards between types A3 V and F2 V. those standard stars that have remained unchanged over years and can be considered to define the system, are Vega (A0 V), Gamma Ursae Majoris (A0 V), and Fomalhaut (A3 V). The "anchor points" and "dagger standards" of the MK spectral classification system among the A-type main-sequence dwarf stars, i.e. The revised Yerkes Atlas system listed a dense grid of A-type dwarf spectral standard stars, but not all of these have survived to this day as standards. Spectral standard stars Properties of typical A-type main-sequence stars Spectral As a consequence, because they do not have strong stellar winds, they lack a means to generate X-ray emissions. A-type stars do not have convective zones and thus are not expected to harbor magnetic dynamos. Bright and nearby examples are Altair (A7), Sirius A (A1), and Vega (A0). They measure between 1.4 and 2.1 solar masses ( M ☉) and have surface temperatures between 7,600 and 10,000 K. These stars have spectra defined by strong hydrogen Balmer absorption lines. Sirius A, an A-type main-sequence star, is the larger of the two.Īn A-type main-sequence star (A V) or A dwarf star is a main-sequence ( hydrogen burning) star of spectral type A and luminosity class V (five). See also: Stellar classification § Class A An artist's impression of Sirius A and Sirius B, a binary star system.
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