Am star: Difference between revisions

An '''Am star''' or '''metallic-line star''' is a type of [[chemically peculiar star|chemically peculiar]] [[star]] of [[spectral type]] A whose [[stellar spectrum|spectrum]] has strong and often variable absorption lines of metals such as [[zinc]], [[strontium]], [[zirconium]], and [[barium]], and deficiencies of others, such as [[calcium]] and [[scandium]]. The original definition of an Am star was one in which the star shows "an apparent surface underabundance of [[calcium|Ca]] (and/or [[scandium|Sc]]) and/or an apparent overabundance of the [[iron group|Fe group]] and heavier elements".<ref name=conti1970/>

The unusual relative abundances cause the spectral type assessed from the [[Calcium K linesline]]s to be systematically earlier than one assessed from other metallic lines. Typically, a spectral type judged solely from hydrogen lines is intermediate. This leads to two or three spectral types being given. For example, [[Sirius]] has been given a spectral type of kA0hA0VmA1, indicating that it is A0 when judged by the Calcium k line, A0V when judgejudged by its hydrogen lines, and A1 when judged by the lines of heavy metals.<ref name=gray/> There are other formats, such as A0mA1Va, again for Sirius.<ref name=conti1973/><ref name=skiff/>

The chemical abnormalities are due to some [[element (chemistry)|elements]] which absorb more light being pushed towards the surface, while others sink under the force of [[gravity]]. This effect takes place only if the star has low rotational velocity.<ref name=dd>[http://www.daviddarling.info/encyclopedia/A/Am_star.html Am star] {{Webarchive|url=https://web.archive.org/web/20170804053755/http://www.daviddarling.info/encyclopedia/A/Am_star.html |date=2017-08-04 }}, ''The Internet Encyclopedia of Science'', David Darling. Accessed on line August 14, 2008.</ref> Normally, A-type stars rotate quickly. Most Am stars form part of a [[binary star|binary system]] in which the rotation of the stars has been slowed by [[tidetidal force|tidal braking]].<ref name=dd />

| doi=10.1093/mnras/sts318 | doi-access=free
| bibcode=2013MNRAS.429..119P

| arxiv=1211.1535 | s2cid=119231581
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A small number of Am stars show unusually late spectral types and particularly strong luminosity effects. Although Am stars in general show abnormal luminosity effects, stars such as [[ρ Puppis]] are believed to be more evolved and more luminous than most Am stars, lying above the [[main sequence]]. Am stars and [[δ Scuti variable]]s lie in approximately the same location on the [[H-RH–R diagram]], but it is rare for a star to be both an Am star and a δ Scuti variable. ρ Puppis is one example and [[δ Delphini]] is another.<ref name=gray/>

Several authors have referred to a class of stars known as δ Delphini starstars, Am stars but with relatively little difference between the calcium and other metallic lines. They have also been compared to the δ Scuti stars. Later studies showed that the group was somewhat inhomogeneous, possibly coincidental, and recommended dropping use of the δ Delphini class in favour of a narrower class of ρ Puppis stars with relatively high luminosity and late spectral types.<ref name=gray/><ref name=neiner/> However, there is still sometimes confusion, for example with ρ Puppis stars being considered to all be δ Scuti variables.<ref name=kochukov/>

<ref name=kochukov>{{cite journal | last1=Kochukhov | first1=O. | title=Asteroseismology of chemically peculiar stars | journal=Communications in Asteroseismology | volume=159 | pages=61–70 |date=March 2009 | bibcode=2009CoAst.159...61K |arxiv = 0812.0374 |doi = 10.1553/cia159s61 | s2cid=18174900 }}</ref>

<ref name=neiner>{{cite journal|bibcode=2017MNRAS.468L..46N|arxiv=1702.01621|title=Discovery of a magnetic field in the δ Scuti F2m star ρ Pup|journal=Monthly Notices of the Royal Astronomical Society: Letters|volume=468|pages=L46L46–L49|author1last1=Neiner|first1=C|last2=Wade|first2=G. A|last3=Sikora|first3=J|year=2017|issue=1|doi=10.1093/mnrasl/slx023|doi-access=free |s2cid=119201285}}</ref>

<ref name=conti1970>{{cite journal|bibcode=1970PASP...82..781C|title=The Metallic-Line Stars|journal=Publications of the Astronomical Society of the Pacific|volume=82|pages=781|author1last1=Conti|first1=Peter S|year=1970|issue=488|doi=10.1086/128965|doi-access=free}}</ref>

<ref name=conti1973>{{cite journal|bibcode=1973ApJ...186..185C|title=Are all metallic-line stars binaries? Observations of three stars in Coma|journal=Astrophysical Journal|volume=186|pages=185|author1last1=Conti|first1=P. S|last2=Barker|first2=P. K|year=1973|doi=10.1086/152487|doi-access=free}}</ref>

<ref name=skiff>{{cite journal|bibcode=2014yCat....1.2023S|title=VizieR Online Data Catalog: Catalogue of Stellar Spectral Classifications (Skiff, 2009-2016)|journal=VizieR On-line Data Catalog: B/mkMk. Originally publishedPublished in: Lowell Observatory (October 2014)|volume=1|author1last1=Skiff|first1=B. A|year=2014}}</ref>

<ref name=gray>{{cite journal|bibcode=1989ApJS...69..301G|title=The early F-type stars - Refined classification, confrontation with Stromgren photometry, and the effects of rotation|journal=Astrophysical Journal Supplement Series|volume=69|pages=301|author1last1=Gray|first1=R. O|last2=Garrison|first2=R. F|year=1989|doi=10.1086/191315}}</ref>

[[Category:Am stars| ]]