helium-weak star
{{Short description|Type of star}}
Helium-weak stars are chemically peculiar stars which have a weak helium lines for their spectral type.{{Cite journal|last1=Landstreet|first1=J. D.|last2=Bohlender|first2=David A.|date=1988|title=Abundance and Magnetic Field Geometries of Helium-Strong and Helium-Weak Stars|journal=Symposium - International Astronomical Union|language=en|volume=132|pages=309–312|doi=10.1017/S0074180900035191|bibcode=1987BAAS...19..704B|issn=0074-1809|doi-access=free}}{{Cite journal|last=Preston|first=George W.|date=1974|title=The Chemically Peculiar Stars of the Upper Main Sequence|journal=Annual Review of Astronomy and Astrophysics|volume=12|issue=1|pages=257–277|doi=10.1146/annurev.aa.12.090174.001353|bibcode=1974ARA&A..12..257P}} Their helium lines place them in a later (i.e. cooler) spectral type than their hydrogen lines.{{Cite web|url=https://ned.ipac.caltech.edu/level5/Gray/Gray9.html|title=A Digital Spectral Classification Atlas - R. O. Gray|website=ned.ipac.caltech.edu|access-date=2019-05-31}}
Helium-weak star
Helium-weak stars are mid-to-late B-class stars with weaker than normal spectral lines of neutral helium, compared to normal stars with similar hydrogen line strengths. These are considered to be an extension of the Ap/Bp chemically-peculiar stars with slightly hotter temperatures. They often show similar increased abundances of heavy elements. The mechanism of atmospheric stratification of elements is though to be responsible for both types of chemical peculiarity.{{cite journal |bibcode=2023RNAAS...7..197M |title=The Behavior of Bp He-weak Stars in the Far UV-Paper XIII: HD 161480 |last1=Monier |first1=Richard |journal=Research Notes of the American Astronomical Society |date=2023 |volume=7 |issue=9 |page=197 |doi=10.3847/2515-5172/acfa9d |doi-access=free }}{{cite journal |bibcode=2003A&A...402..331A |title=Elemental abundance studies of CP stars. The helium-weak stars HD 19400, HD 34797 and HD 35456 |last1=Alonso |first1=M. S. |last2=López-García |first2=Z. |last3=Malaroda |first3=S. |last4=Leone |first4=F. |journal=Astronomy and Astrophysics |date=2003 |volume=402 |page=331 |doi=10.1051/0004-6361:20030222 }}
This is a non-extensive list of helium-weak stars.{{Cite journal|last1=Jaschek|first1=Mercedes|last2=Jaschek|first2=Carlos|last3=Arnal|first3=Marcelo|date=October 1969|volume=81|issue=482|pages=651|bibcode=1969PASP...81..650J|doi=10.1086/128832|title=Helium-Weak Stars|journal=Publications of the Astronomical Society of the Pacific|doi-access=free}}{{Cite journal|last1=Alonso|first1=M. S.|last2=López-García|first2=Z.|last3=Malaroda|first3=S.|last4=Leone|first4=F.|date=April 2003|title=Elemental abundance studies of CP stars. The helium-weak stars HD 19400, HD 34797 and HD 35456*|journal=Astronomy and Astrophysics|volume=402|pages=331|bibcode=2003A&A...402..331A|doi=10.1051/0004-6361:20030222|doi-access=free}}
{{Incomplete list|date=June 2019}}
| class="wikitable sortable"
|+ Caption text | ||||
| Star Name | Constellation | Apparent Magnitude (mv) | Class | Notes |
|---|---|---|---|---|
| 20 Eridani (EG Eridani) | Eridanus | 5.23 | bgcolor=#AFC1FF|B8/9 III | α2 CVn variable |
| 30 Capricorni | Capricornus | 5.38 | bgcolor=#A8BDFF| B5 II/III | |
| θ Hydri | Hydrus | 5.50 | bgcolor="{{Color temperature|13350|hexval}}"| B8 III/IV | Suspected Binary; |
| 22 Eridani (FY Eridani) | Eridanus | 5.53 | bgcolor=#AFC3FF| B9 IIIpSi (4200) | α2 CVn variable |
| HD 74196 | Vela | 5.61 | bgcolor=#ADBFFF| B7 Vn | Part of IC 2391 |
| HD 28843 (DZ Eridani) | Eridanus | 5.81 | bgcolor="{{Color temperature|14800|hexval}}"| B9 III | SX Arietis variable |
| 12 Canis Majoris (HK Canis Majoris) | Canis Major | 6.08 | bgcolor="{{Color temperature|15830|hexval}}"| B7 II/III | SX Arietis variable |
| HD 34797 (TX Leporis) | Lepus | 6.54 | bgcolor=#adbfff| B7 Vp | α2 CVn variable; binary |
| HD 35456 | Orion | 6.95 | bgcolor=#AFC3FF| B9 II/III | Binary |
{{anchor|Helium-strong star}}Helium-strong star
A related class of stars have anomalously strong helium lines in their spectra, and are known as helium-strong stars. These are the more massive stars with classes of B1 to B3, compared to the helium-weak stars with classes of B5 to B9.{{cite journal | title=A case study of ACV variables discovered in the Zwicky Transient Facility survey | last1=Faltová | first1=N. | last2=Kallová | first2=K. | last3=Prišegen | first3=M. | last4=Staněk | first4=P. | last5=Supíková | first5=J. | last6=Xia | first6=C. | last7=Bernhard | first7=K. | last8=Hümmerich | first8=S. | last9=Paunzen | first9=E. | journal=Astronomy & Astrophysics | volume=656 | at=id. A125 | date=December 2021 | doi=10.1051/0004-6361/202141534 | arxiv=2108.11411 | bibcode=2021A&A...656A.125F }} The following are examples:
Helium-variable star
V761 Centauri (a Centauri) is well-known for both the variability of its brightness and of its helium spectral lines. Stars of this type are referred to as helium-variable. This is thought to occur when one hemisphere of the star's atmosphere is helium-weak and one is helium-strong.{{cite journal |bibcode=2010A&A...520A..44B |title=Doppler imaging of the helium-variable star a Centauri |last1=Bohlender |first1=D. A. |last2=Rice |first2=J. B. |last3=Hechler |first3=P. |journal=Astronomy and Astrophysics |date=2010 |volume=520 |pages=A44 |doi=10.1051/0004-6361/201014157 |arxiv=1007.0977 }}