The IACOB project - XII. New grid of northern standards for the spectral classification of B-type stars

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Table 4

Main standard stars and classification criteria for mid- and late-B stars.

	V	IV	III	II	Ib	Iab	Ia
B2	β² Sco	γ Peg	γ Ori	HD 31327	9 Cep		χ² Ori
	Very weak metallic lines	Si II 4129 starts to be see
	He I 4009 strong			N II lines comparable in strength to O II
	compared to He I 4026						Very strong
	C II 4267 is strong						N II lines

B2.5	22 Sco		π² Cyg		3 Gem		55 Cyg
	Even weaker metallic lines	N II stronger than O II
	Mg II 4481 becoming strong

B3	η Aur	ι Her	HD 21483	HD 194779	ι CMa		o² CMa
	Mg II 4481 clearly seen even in fast rotators Si II 4129 becoming strong	Si II 4129 ≳ Si III 4553 Si III 4553 weaker than some N II lines
		Si III 4553 weaker than some N II lines
	Si II 4129 becoming strong

B5	HD 4142		HD 170682	HD 191243	HD 9311	HD 7902	η CMa
	Si II 4129 comparable to He I 4121
	He I 4009 weaker	Mg II 4481 growing in strength with luminosity
	compared to He I 4026						Fe II very
	C II 4267 becoming weaker						weakly present

B6/7	HD 21071		β Tau			13 Cep	HD 199478
		Si II 4129	He I 4009, 4388 decline strongly with respect to other He I lines
	He I 4009, 4388 decline strongly with respect to other He I lines	Si II 4129 comparable or stronger than He I 4144	Same trends asin B5

B8	21 Tau	π Cet	τ And		53 Cas	HD 14542	β Ori
	Mg II 4481 comparable to He I 4471					Strong Fe II
	He I 4009, 4388 very weak

B9	134 Tau	(α Del)	12 Cas	HD 21661 (II–III)	HD 35600	σ Cyg	HD 21291
	He I 4009, 4388 not seen
	Mg II 4481 much stronger than He I 4471				Weak He I Very strong Si II and Fe II
	Metallic lines even in fast rotators

Notes. Criteria in italics apply to all luminosity classes.

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