Carbon star 597244 225870126 2008-07-15T20:00:23Z Thijs!bot 1392310 robot Adding: [[zh:碳星]] A '''carbon star''' is a late type giant star similar to the [[red giant]]s (or occasionally [[red dwarf]]) [[star]] whose atmosphere contains more [[carbon]] than [[oxygen]]; the two elements combine in the upper layers of the star, forming [[carbon monoxide]], which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere, and a strikingly red appearance to human observers. The [[stellar classification|spectral]] characteristics of these stars are quite distinctive, and they were first recognized by their spectra by [[Angelo Secchi]] in the 1860s &mdash; a pioneering time in astronomical [[spectroscopy]]. In "normal" stars (such as the [[Sun]]), the atmosphere is richer in oxygen than carbon. ==Astrophysical mechanisms== Carbon stars are explained by more than one astrophysical mechanism. McClure <ref>[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1985JRASC..79..277M&db_key=AST&data_type=HTML&format=&high=4422f74cc711450 The carbon and related stars]</ref> distinguishes between ''classical carbon stars'', and other ''non-classical'' ones that are less massive. In the '''classical carbon stars''', the abundance of carbon is thought to be a product of [[helium fusion]], specifically the [[triple-alpha process]] within a star, which giants reach near the end of their lives in the so called [[Asymptotic giant branch|Asymptotic Giant Branch]] (AGB). These fusion products have been brought to the stellar surface by episodes of [[convection]] after the carbon and other products were made. Normally this kind of AGB carbon star fuses hydrogen in a hydrogen burning shell, but in episodes separated by 10<sup>4</sup>-10<sup>5</sup> years, the star transforms to burning helium in a shell, while the hydrogen fusion temporarily ceases. In this phase, the star's luminosity rises, and material from the interior of the star (notably carbon) moves up. Since the luminosity rises, the star expands so that the helium fusion ceases, and the hydrogen shell burning restarts. During these ''shell helium flashes'', the mass loss from the star is significant, and after many shell helium flashes, an AGB star is transformed into a hot [[white dwarf]] and its atmosphere becomes material for a [[planetary nebula]]. The '''non-classical''' kinds of carbon stars are believed to be [[binary star]]s, where one star is observed to be a giant star (or occasionally a [[red dwarf]]) and the other a [[white dwarf]]. The star presently observed to be a giant star accreted carbon-rich material when it was still a [[main sequence]] star from its companion (that is, the star that is now the white dwarf) when the latter was still a classical carbon star. That phase of [[stellar evolution]] is relatively brief, and most such stars ultimately end up as white dwarfs. We are now seeing these systems a comparatively long time after the [[mass transfer]] event, so the extra carbon observed in the present red giant was not produced within that star.<ref>R. McClure, Journal of the Royals Astronomical Society of Canada, vol 79, pp. 277-293, December 1985</ref> This scenario is also accepted as the origin of the [[barium star]]s, which are also characterized as having strong spectral features of carbon molecules and of barium (an [[s-process|s-process element]]). Sometimes the stars whose excess carbon came from this mass transfer are called "extrinsic" carbon stars to distinguish them from the "intrinsic" AGB stars which produce the carbon internally. Many of these extrinsic carbon stars are not luminous or cool enough to have made their own carbon, which was a puzzle until their binary nature was discovered. Other less-convincing mechanisms, such as [[CNO cycle]] unbalancing and [[Helium flash|Core Helium Flash]] have also been proposed as mechanisms for carbon enrichment in the atmospheres of smaller carbon stars. ==Carbon star spectra== By definition carbon stars have dominant spectral Swan Bands from the molecule C<sub>2</sub>. Many other carbon compounds may be present at high levels, such as CH, CN ([[cyanogen]]), C<sub>3</sub> and SiC<sub>2</sub>. Carbon is formed in the core and circulated into its upper layers, dramatically changing the layers' composition. Other elements formed through helium fusion and the s-process are also "dredged up" in this way, including [[lithium]] and [[barium]]. When astronomers developed the [[star classification|spectral classification]] of the carbon stars, they got into considerable hardships when trying to correlate the spectra to the stars' effective temperatures. The trouble was with all the atmospheric carbon hiding the absorption lines normally used as temperature indicators for the stars. ===Secchi=== Carbon stars were discovered already in the 1860s when spectral classification pioneer [[Angelo Secchi|Pater Angelo Secchi]] erected the [[Stellar_classification#Secchi_classes|Secchi class IV]] for the carbon stars, who in the late 1890s were reclassified as N class stars. <ref>[http://www.astro.ufl.edu/~gott/AST1002/Additional_Notes/Add_notes.week5 Classification of Stellar Spectra: Some History]</ref> ===Harvard=== Using this new Harvard classification, the N class was later enhanced by a R class for less deeply red stars sharing the characteristic carbon bands of the spectrum. Later correlation of this R to N scheme with conventional spectra, showed that the R-N sequence approximately run in parallel with c:a G7 to M10 with regards to star temperature. <ref>[http://www.peripatus.gen.nz/Astronomy/CarSta.html Carbon Stars (Peripatus.gen)]</ref> {|style="margin: 10px 0; border-spacing: 0; border: solid #FF0000 1px; padding: 1px" |- |width="90px"|MK-type |width="60px" style="background-color: #FFE0E0"|R0 |width="60px" style="background-color: #FFC0C0"|R3 |width="60px" style="background-color: #FFB0B0"|R5 |width="60px" style="background-color: #FFA0A0"|R8 |width="60px" style="background-color: #FF9090"|Na |width="60px" style="background-color: #FF8080"|Nb |- |giant equiv. |style="background-color: #FFE0E0"|G7-G8 |style="background-color: #FFC0C0"|K1-K2 |style="background-color: #FFB0B0"|''~K2-K3'' |style="background-color: #FFA0A0"|K5-M0 |style="background-color: #FF9090"|''~M2-M3'' |style="background-color: #FF8080"|M3-M4 |- |T<sub>eff</sub> |style="background-color: #FFE0E0"|4300 |style="background-color: #FFC0C0"|3900 |style="background-color: #FFB0B0"|''~3700'' |style="background-color: #FFA0A0"|3450 |style="background-color: #FF9090"|<nowiki>---</nowiki> |style="background-color: #FF8080"|<nowiki>---</nowiki> |} ===Morgan-Keenan C system === The later N classes correspond less well to the counterparting M types, because the Harvard classification was only partially based on temperature, but also carbon abundance; so it soon became clear that this kind of carbon star classification was incomplete. Instead a new dual number star class C was erected so to deal with temperature '''and''' carbon abundance. Such a spectrum measured for [[La Superba|Y CVn]], was determined to be C5<sub>4</sub>, where 5 refers to temperature dependent features, and 4 to the strength of the C<sub>2</sub> Swan bands in the spectrum. (C5<sub>4</sub> is very often alternatively written C5,4). <ref>[http://adsabs.harvard.edu/cgi-bin/bib_query?1941ApJ....94..501K The Classification of the Red Carbon Stars. Keenan, Philip C.; Morgan, W. W.]</ref> {|style="margin: 10px 0; border-spacing: 0; border: solid #FF0000 1px; padding: 1px" |- |width="90px"|MK-type |width="60px" style="background-color: #FFF0F0"|C0 |width="60px" style="background-color: #FFE0E0"|C1 |width="60px" style="background-color: #FFD0D0"|C2 |width="60px" style="background-color: #FFC0C0"|C3 |width="60px" style="background-color: #FFB0B0"|C4 |width="60px" style="background-color: #FFA0A0"|C5 |width="60px" style="background-color: #FF9090"|C6 |width="60px" style="background-color: #FF8080"|C7 |- |giant equiv. |style="background-color: #FFF0F0"|G4-G6 |style="background-color: #FFE0E0"|G7-G8 |style="background-color: #FFD0D0"|G9-K0 |style="background-color: #FFC0C0"|K1-K2 |style="background-color: #FFB0B0"|K3-K4 |style="background-color: #FFA0A0"|K5-M0 |style="background-color: #FF9090"|M1-M2 |style="background-color: #FF8080"|M3-M4 |- |T<sub>eff</sub> |style="background-color: #FFF0F0"|4500 |style="background-color: #FFE0E0"|4300 |style="background-color: #FFD0D0"|4100 |style="background-color: #FFC0C0"|3900 |style="background-color: #FFB0B0"|3650 |style="background-color: #FFA0A0"|3450 |style="background-color: #FF9090"|<nowiki>---</nowiki> |style="background-color: #FF8080"|<nowiki>---</nowiki> |} ===The Revised Morgan-Keenan system=== This two-dimensional classification replaced the older R-N classifications during the 1960-1993, but the Morgan-Keenan C system failed to fulfill the creators' expectations: #it failed to correlate to temperature measurements based on infrared, #originally being two-dimensional it was soon enhanced by suffixes, '''CH''', '''CN''', '''j''' and other features making it impractical for en-masse analyses of foreign galaxies' carbon star populations, #and it gradually occurred that the old R and N stars actually were two distinct types of carbon stars, having real astrophysical significance. A new revised Morgan-Keenan classification was published in 1993 by [[Philip Keenan]], defining the classes: C-N, C-R and C-H. Later the classes C-J and C-Hd were added. <ref>[http://adsabs.harvard.edu/cgi-bin/bib_query?1993PASP..105..905K Revised MK spectral classification of the red carbon stars - Keenan, Philip C.]</ref> This constitutes the established classification system used today <ref>[http://adc.astro.umd.edu/adc-cgi/cat.pl?/journal_tables/ApJS/105/419/ Spectral Atlas of Carbon Stars (Barnbaum+ 1996)]</ref>: {|class="wikitable" |- !style="background-color: #c0c0ff"|class !style="background-color: #c0c0ff"|spectrum !style="background-color: #c0c0ff" width="100"|[[Metallicity|population]] !style="background-color: #c0c0ff"|M<sub>V</sub><ref>[[Absolute Magnitude|Absolute Visual Magnitude]]</ref> !style="background-color: #c0c0ff"|theory !style="background-color: #c0c0ff"|temperature <br/> range (K)<ref>[http://adsabs.harvard.edu/abs/2007PASJ...59..939T "Near infrared spectra of 29 carbon stars", Tanaka et. al., 2007]</ref> !style="background-color: #c0c0ff"|example(s) !style="background-color: #c0c0ff"|# known |- |colspan="8" style="background-color: #e0e0FF"|'''classical carbon stars''' |- !style="vertical-align: top"|C-R: |style="vertical-align: top"|the old Harvard class R reborn: are still visible at the blue end of the spectrum, strong isotopic bands, no enhanced [[barium|Ba]] line |style="vertical-align: top"|medium disc pop I |style="vertical-align: top"|0 |style="vertical-align: top"|red giants? |style="vertical-align: top"|5100-2800 |style="vertical-align: top"|''S Camelopardalis'' |style="vertical-align: top"|~25 |- !style="vertical-align: top"|C-N: |style="vertical-align: top"|the old Harvard class N reborn: heavy diffuse blue absorption, sometimes invisible in blue, s-process elements enhanced over solar abundance, weak isotopic bands |style="vertical-align: top"|thin disc pop I |style="vertical-align: top"|-2.2 |style="vertical-align: top"|[[Asymptotic giant branch|AGB]] |style="vertical-align: top"|3100-2600 |style="vertical-align: top"|''[[R Leporis|R&#xA0;Leporis]]'' |style="vertical-align: top"|~90 |- |colspan="8" style="background-color: #e0e0FF"|'''non-classical carbon stars''' |- !style="vertical-align: top"|C-J: |style="vertical-align: top"|very strong isotopic bands of C<sub>2</sub> and CN |style="vertical-align: top"|''unknown'' |style="vertical-align: top"|''unknown'' |style="vertical-align: top"|''unknown'' |style="vertical-align: top"|3900-2800 |style="vertical-align: top"|[[La Superba|Y&#xA0;Canum&#xA0;Venaticorum]] |style="vertical-align: top"|~20 |- !style="vertical-align: top"|C-H: |style="vertical-align: top"|very strong CH absorption |style="vertical-align: top"|halo pop II |style="vertical-align: top"|-1.8 |style="vertical-align: top"|bright giants, mass transfer (all C-H:s are binary <ref>[http://adsabs.harvard.edu/abs/1990ApJ...352..709M The binary nature of the barium and CH stars. III - Orbital parameters, McClure, R.D.; Woodsworth, A. W., 1990]</ref>) |style="vertical-align: top"|5000-4100 |style="vertical-align: top"|''V Arietis'', ''TT Canum Venaticorum'' |style="vertical-align: top"|~20 |- !style="vertical-align: top"|C-Hd: |style="vertical-align: top"|hydrogen lines and CH bands weak or absent |style="vertical-align: top"|thin disc pop I |style="vertical-align: top"|-3.5 |style="vertical-align: top"|''unknown'' |style="vertical-align: top"|? |style="vertical-align: top"|''HD 137613'' |style="vertical-align: top"|~7 |} ==Other qualities== Most classical carbon stars are [[variable star]]s: [[Mira variable|miras]], [[irregular variable|irregular]] or [[semiregular variable star]]s. ===Observing carbon stars=== Due to the insensitivity of night vision to red and a slow adaption of the red sensitive [[rod cell|eye rods]] to the light of the stars, amateur astronomers making [[Apparent magnitude|magnitude]] estimates of red [[variable star]]s, especially carbon stars, have to know how to deal with the [[Purkinje effect]] in order to not overstate the luminosity of the observed star. ===Interstellar carbon sowers=== Owing to its low surface [[gravitation|gravity]], as much as half (or more) of the total mass of a carbon star may be lost by way of powerful [[stellar wind]]s. The star's remnants, carbon-rich "dust" similar to [[graphite]], therefore become part of the [[cosmic dust|interstellar dust]]. This dust is believed to be a significant factor in providing the [[molecular cloud|raw materials]] for the creation of subsequent generations of stars and their planetary systems. The material surrounding a carbon star may blanket it to the extent that the dust absorbs all visible light. == References == <references/> == See also == *[[Barium star]]s *[[R Leporis]], Hind's Crimson Star: an example of a carbon star *[[IRC +10216]], CW Leonis: the most studied carbon star, and also the brightest star in the sky at N-band *[[La Superba]], Y Canum Venaticorum: one of the brighter carbon stars *[[Marc Aaronson]]: an [[United States|America]]n [[astronomy|astronomer]] and noted researcher of carbon stars == External links == <!--*[http://www.belmontnc.4dw.net/carbonstar.htm List of known carbon stars with classification explanation]--> *[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1985JRASC..79..277M&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=4422f74cc711450 McClure's paper on the carbon and related stars] *[http://www.astrosurf.com/buil/us/peculiar2/carbon.htm List of 110 carbon stars]. Includes [[Henry Draper Catalogue|HD number]]; secondary identification for most; position in [[right ascension]] and [[declination]] ; [[Apparent magnitude|magnitude]]; [[Stellar classification|spectrum]]; magnitude range (for [[variable star]]s); period (of variability cycle). [[Category:Star types]] [[Category:Type-C stars|*Carbon]] [[Category:Variable stars]] [[ca:Estrella de carboni]] [[es:Estrella de carbono]] [[it:Stella al carbonio]] [[hu:Széncsillag]] [[pl:Gwiazda węglowa]] [[ru:Углеродная звезда]] [[fi:Hiilitähti]] [[zh:碳星]]