Download A Digital Spectral Classification Atlas

A Digital Spectral Classification Atlas
R. O. Gray
Dept. of Physics and Astronomy
Appalachian State University
v. 1.07
Not for Redistribution
January 30, 2009
1
Introduction
The MK Spectral classification system was founded by W.W. Morgan and P.C. Keenan in the
year 1943, with the publication of the first photographic spectral classification atlas, An Atlas
of Stellar Spectra (Morgan, Keenan & Kelleman, 1943). Since that time, the MK system
has been extensively revised and refined by Morgan, Keenan and others. In the late 1970’s,
two important spectral atlases, summarizing the development of the MK system up to that
time, were published. These atlases, the Revised MK Spectral Atlas for Stars Earlier than the
Sun by Morgan, Abt & Tapscott (1978) and An Atlas of Spectra of the Cooler Stars: Types
G,K,M,S and C, by Keenan & McNeil (1976), are the inspiration for the various versions of
this digital spectral classification atlas. Indeed, some of the pages in this atlas are “digitized”
versions of pages from those two atlases.
The MK spectral classification system is a natural, empirical system of classification which
uses in the classification process only the directly observable features in the spectrum. The
MK system is defined by a set of standard stars, and classification on the system is carried
out by the comparison of the program star with the standard stars, taking into account
all of the features in the spectrum. The use of standards is vital because it maintains the
autonomy of the system as well as ensuring that different observers will classify stars on the
same system.
When the MK system was first defined, it was based on photographic spectra in the
blue-violet part of the spectrum. This was done by necessity, as scientific photographic
emulsions in the 1940’s were sensitive only to blue-violet light. However, it was a fortunate
choice, as the blue-violet portion of the spectrum (essentially from the Ca II K-line to Hβ )
contains a high density of astrophysically important atomic lines and molecular bands, which
allow accurate classification of the star in a two-dimensional temperature, luminosity grid.
Classification systems can and have been set up in the red, IR and the ultraviolet. These
should remain independent of the traditional MK system, as different parts of the spectrum
can sample different levels in the atmosphere of the star.
This version of the Digital Spectral Classification Atlas should really be considered a
synopsis of the much more extensive Stellar Spectral Classification by R.O. Gray and C.J.
Corbally, to be published by Princeton University Press in spring, 2009. Since I have borrowed extensively from that book for this atlas (all the figures in this atlas except for Figure
29 are from that book), this atlas should not be redistributed either electronically or in
hardcopy form. I urge you to buy the book when it comes out!
Many, but not all of the spectra in this atlas were obtained with the Gray/Miller spectrograph on the 0.8 meter telescope of the Dark Sky Observatory (DSO), using a CCD detector.
For the DSO spectra, two spectral resolutions are used in this atlas. Most of the illustrations
use spectra obtained with the 1200g/mm grating, which gives a spectral resolution of 1.8Å/2
pixels and a spectral range of 3800Å – 4600Å, but some illustrations, especially those of the
later-type stars (K, M, C and S) use spectra obtained with the 600g/mm grating. These have
a resolution of 3.6Å/2 pixels, and a spectral range of 3800Å – 5600Å. The higher resolution
spectra are presented in a rectified intensity versus wavelength format, in which the spectral
continuum has been normalized to unity. The 3.6Å resolution spectra, for the most part, are
presented in a flux versus wavelength format; this format provides additional information on
the energy distribution of the star, and is to be preferred for the cooler stars, as these stars
have essentially no continuum points in their spectra. For ease of illustration, the fluxes have
been normalized to unity at one consistent point in the spectrum.
Since the publication of the two most recent photographic spectral atlases mentioned in
the paragraphs above, the MK system has undergone considerable refinement and extension.
Important work in refining and extending the MK system to dimensions beyond the traditional two-dimensional temperature/luminosity grid has been carried out by Keenan and
co-workers in the addition of abundance indices for the late-type stars, by Gray (1989), in the
extension of the MK system to metal-weak F and G-type stars, by Kirkpatrick & coworkers
in their careful redefinition and extension of the MK system to the M-type dwarfs and most
lately the L- and T-type dwarfs , and by Walborn in the classification of the O-type stars.
All of those developments are discussed in detail in Stellar Spectral Classification. Unlike
earlier versions of this atlas, this atlas does not illustrate spectra of white dwarfs, novae or
supernovae. If you want to explore the spectra of such stars, buy the book!
He II
Hβ/He II
N III
1
He II
He II
I.S.
Hγ/He II
He II
Hδ/He II
He I
Hε/He II
H8/He II
The Spectral Sequence
He I
He I
He I
He I
HD 46233 O4 V
HD 46202 O9 V
Hβ
Hγ
Hδ
Hε
H8
0
Mg II
22 Sco B2.5 V
HR 1029 B7 V
Ca II K
β Leo A3 V
HD 23585 F0 V
CH
Fe II/Ti II
-3
Ca I
Mn I
-2
Fe I
Rectified Intensity
-1
Fe I
Mg I
HD 27524 F5 V
-4
Sun G2 V
-5
3800
4000
4200
4400
4600
4800
5000
5200
5400
5600
Wavelength ( )
Figure 1: The Main Sequence from O4 to G2, the “early-type” stars.
Figures 1 and 2 show a bird’s eyeview of the spectral sequence, which is dissected and
examined in more detail in later portions of this atlas. Figure 1 shows the spectral sequence
from the hottest “normal” stars, the O-type stars, to spectral type G2, the spectral type
of our sun. The spectra of these “early-type” stars are dominated by the Balmer lines of
hydrogen. Note how they increase in strength and reach a maximum at A2, and then fade
thereafter. The O- and B-type stars are characterized by lines of helium; the A-, F-, and
G-type stars are characterized by lines of metals.
Mg I
Hβ
Ca I
CH (G-band)
Hδ
Ca II
1
Mn I
2
K H
Fe I
61 UMa G8 V
MgH
Normalized Flux
0
Hγ
Fe I
Sun G2 V
-1
MgH
61 Cyg A K5 V
TiO
-2
TiO
Ca I
TiO
GL 411 M2 V
Hγ
Hβ
-3
CaOH
-4
3800
40 Eri C M4.5 V
4000
4200
4400
4600
4800
5000
5200
5400
5600
Wavelength ( )
Figure 2: The Main Sequence from G2 to M4.5, the “late-type” stars.
Molecular features first appear in the spectra of F-type main sequence stars and grow to
dominate the spectra of the “late-type” stars, especially those of types K and M. The main
sequence does not end at M4.5, but continues through the late M-, the L- and the T-type
dwarfs, a sequence that spans the transition from hydrogen-burning dwarf stars to brown
dwarfs.
The Main Sequence O2 – O9
Figure 3: The O-type main sequence stars. Reproduced from Stellar Spectral Classification;
original figure courtesy Ian Howarth.
The O-type stars constitute the hottest normal stars, and are characterized by moderately
weak hydrogen lines, lines of neutral helium (He I) and by lines of singly ionized helium (He
II). The spectral type can be judged easily by the ratio of the strengths of lines of He I to
He II; He I tends to increase in strength with decreasing temperature while He II decreases
in strength. The ratio He I 4471 to He II 4542 shows this trend clearly.
Visible in a number of stars in this figure is a broad, shallow depression, centered near
4430Å. This is an example of a diffuse interstellar band, probably due to molecules in the
interstellar medium between us and the star.
Luminosity Effects at O6.5
Figure 4: A luminosity sequence at O6.5. Figure reproduced from Stellar Spectral Classification; original figure courtesy I. Howarth.
Figure 4 shows a luminosity sequence at a spectral type of O6.5. At this spectral type a
number of luminosity criteria are active. Note that in the O6.5 dwarf (V), the N III triplet
(λλ4634, 4640, 4642) is weakly in emission, but at higher luminosities the emission strength
increases. The Si IV λ4089 line shows a positive luminosity effect and the hydrogen lines
(λλ4101, 4340) a very weak negative luminosity effect. The two S IV lines (λλ4486, 4504)
go into emission in the Iaf supergiant.
Luminosity Effects at O9
Figure 5: A luminosity sequence at O9. Figure reproduced from Stellar Spectral Classification; original figure courtesy I. Howarth.
At O9 the hydrogen lines show a more pronounced sensitivity to luminosity than at earlier
(hotter) types. However, the primary luminosity criteria at this spectral type are the ratio
of Si IV 4089 to Hδ (which becomes narrower and weaker in the more luminous stars), and
the ratio of Si IV 4116 to the neighboring He I 4121 line. Note as well that the S IV lines
(λλ4486, 4504) go into emission in the supergiants.
He II 4541
He I 4471
Hγ
He I 4387
He II 4200
He I 4144
Hε
Si IV 4089
H8
Si IV 4116
H9
He I 4009
He I 4026
The Main Sequence O9 – B3
Si III 4552
HD 46202 O9 V
Mg II 4481
He I 4121
Rectified Intensity
1
υ Ori B0 V
0
Si III
ω1 Sco B1 V
C II 4267
-1
22 Sco B2.5 V
Si II 4128-30
η Aur B3 V
-2
3800
3900
4000
4100
4200
4300
4400
4500
4600
Wavelength ( )
Figure 6: A temperature sequence in the early B-type stars.
The definition of the break between the O-type stars and the B-type stars is the absence
of lines of ionized helium (He II) in the spectra of B-type stars. The lines of He I pass through
a maximum at approximately B2, and then decrease in strength towards later (cooler) types.
A useful ratio to judge the spectral type is the ratio He I 4471/Mg II 4481.
Were it not for the presence of helium peculiarities in both the early and late B-type
stars, the behavior of He I would be sufficient to accurately estimate the spectral type in the
B-type stars. However, because of these peculiarities, the spectral type in the early B-type
stars is, instead, estimated on the basis of ratios of lines of silicon ions. For instance, between
B0 and B1 (an interval which can be divided into B0, B0.2, B0.5, B0.7 and B1 classes), the
Si IV λ4089/Si III λ4552 ratio may be used. Between B1 and B3 the Si III λ4552/Si II
λ4128-32 ratio may be used.
Luminosity Effects at B1
Figure 7: A luminosity sequence at B1. Figure reproduced from Stellar Spectral Classification; original figure courtesy I. Howarth.
While the width and strength of the hydrogen lines is a useful luminosity criterion at
B1, the sensitivity of the O II lines (see O II 4070, 4348 and 4416), especially in ratio with
adjacent hydrogen lines and the He I lines (which tend to weaken with increasing luminosity)
helps to increase the precision of luminosity classification at B1. However, the discovery of
oxygen and carbon peculiarities in the early B-type stars means that those ratios should
be used with caution. As a consequence, the luminosity classification in the early B-type
stars (B2 and earlier) has shifted to ratios of lines of ionized silicon to neutral helium. In
particular, the Si IV λ4116/He I λ4121 ratio is useful earlier than B0.7, whereas the Si III
λ4552/He I λ4387 is useful later than B0.7.
Rectified Intensity
Hγ
He I
Fe II
Hδ
He I 4471
Hε
He I 4388
H8
Hβ
Fe II 4233
H9
He I 4009
He I 4026
2
Fe II, Ti II 4172-8
The Be Stars
Ti II, Fe II
1
χ Oph B1.5 Ve3
Feb 26, 2002
He I 3820
HD 88661 B2 IVpne2+
0
Feb 8, 2001
HD 37490 B2 IIIe1
Oct 1, 2002
-1
HD 145482 B2 V
-2
3800
3900
4000
4100
4200
4300
4400
4500
4600
4700
4800
4900
5000
Wavelength ( )
Figure 8: A selection of Be stars compared with a B2 V secondary standard, HD 145482.
The spectral types are on the Lesh (1968) system. Spectra from the Paranal Observatory
spectral library.
The Be stars are B-type stars that are characterized (or have been characterized in the
past) by emission in one or more of the Balmer lines of hydrogen, sometimes accompanied
by emission in lines of singly ionized metals, most commonly Fe II. The Be class excludes
B-type supergiants, as well as pre main-sequence stars such as the Herbig Ae/Be stars. Many
Be stars may be classified without difficulty on the MK system, although the more extreme
present significant difficulties. Lesh (1968) introduced an extension to the MK system for
the Be stars comprised of an “e” index (e1 → e4, in direction of increasing emission-line
strength). Some typical Be spectra are illustrated in Figure 8.
He I
He I
Hγ
He I 4471
Hδ
Hε
He I 4387
H8
He I 3834
Helium-Strong B-type Stars
1
Rectified Intensity
υ Ori B0 V
He I 3867
σ Ori E B2:p V:h
0
δ Ori C B2.5 IV-Vh
-1
HR 1891 B2.5 V
-2
3800
3900
4000
4100
4200
4300
4400
4500
4600
Wavelength ( )
Figure 9: Two helium-strong stars, σ Ori E and δ Ori C compared with two MK standards.
Dark Sky Observatory (DSO) 1.8Å resolution spectra.
Some early B-type stars (earlier than B3) show extraordinarily strong lines of He I. Two
examples, illustrated above, are σ Ori E and δ Ori C. Some of these stars are spectroscopic
variables in that the He I lines vary in strength with the rotational period of the star. Many
of these stars have strong magnetic fields; it is believed that the helium concentrates at one
or both of the magnetic poles. As the pole rotates in and out of sight, the He I line strengths
vary. To indicate a star’s helium-strong status, an “h” is appended to the spectral type.
He I 4144
Hγ
He I 4471
Hδ
He I 4387
Hε
C II 4267
H8
Si II 4128-30
H9
He I 4009
He I 4026
The Main Sequence B3 – A0
Mg II 4481
1
HD 36936 B5 V
0
HR 1029 B7 V
-1
HR 749A B9 Va
Ca II K
Rectified Intensity
η Aur B3 V
α Lyr A0 Va
-2
3800
3900
4000
4100
4200
4300
4400
4500
4600
Wavelength ( )
Figure 10: The main sequence from B3 to A0. Features useful in temperature classification
are labeled. Note the increasing strength and width of the Balmer lines of hydrogen. Spectra
from DSO.
As we move toward later (cooler) types, the helium lines continue to fade until they
essentially disappear in spectra of this resolution (1.8Å) at a spectral type of about A0.
Again, the ratio He I 4471/Mg II 4481 is useful in determining the spectral type in the late
B-type stars. At A0, the Ca II K-line becomes a notable feature in the spectrum (in the
B-type stars, the K-line is mostly interstellar).
He I 4471
Mg II 4481
Hγ
He I 4388
C II 4267
Hδ
He I 4144
Hε
He I 4120
H8
He I 4009
He I 4026
H9
He I
He I + K
Helium-Weak B-type Stars
Si II
Si II 4200
1
29 Per B3 V
3 Sco B3+ Vp HeB8 (Si)
1.02
1.00
4270
-2
3 Cen A
0.98
B3 Vp HeB70.96
0.94
P II lines
4590
4600
Ti II
Sr II
Sr II
-1
Ti II
1.02
1.00
0.98
0.96 Ga II lines
0.94
4250 4260
36 Lyn B7 III-IVp
Ca II K
Rectified Intensity
0
HR 1029 B7 V
-3
3800
3900
4000
4100
4200
4300
4400
4500
4600
Wavelength ( )
Figure 11: The spectra of three helium-weak B-type stars compared with two MK standards.
Spectra adapted from the Paranal Observatory spectral library, from the Indo-US coudé-feed
library and from DSO.
The star, 3 Sco, is an excellent example of a helium-weak B-type star. The hydrogen
lines of this star suggest a spectral type near B3 V whereas the strength of the helium lines
suggests a spectral type of B8. This combination of criteria indicates the star is helium-weak,
if we believe, as the B − V color suggests, that the hydrogen lines are the best indicators of
the effective temperature.
Most helium-weak stars have spectral types of B3 and later, and thus this class has only a
small spectral-type overlap with the helium-strong stars. Some helium-variable stars actually
vary between a helium-weak and a helium-strong state.
The figure above shows two other helium-weak stars. 3 Cen A also shows unusually strong
P II and Ga II lines (see insets), and thus is an example of the class of PGa helium-weak
stars. 36 Lyn shows strong lines of Sr II and Ti II, and is a prototype of the SrTi variety of
helium-weak stars.
1
Mg II 4481
Hγ
He I 4471
He I 4144
He I 4387
He I 4121
Hδ
C II 4267
Hε
Si II 4128-30
H8
N II 3995
He I 4009
He I 4026
H9
Ca II K (I.S.)
Luminosity Effects at B5
Si III
Rectified Intensity
η CMa B5 Ia
67 Oph B5 Ib
0
τ Ori B5 III
-1
HD 36936 B5 V
-2
3800
3900
4000
4100
4200
4300
4400
4500
4600
Wavelength ( )
Figure 12: A luminosity sequence at B5. Spectra from DSO.
At B5 the primary luminosity criteria are the hydrogen lines which show a pronounced
negative luminosity effect. The He I lines show little or no sensitivity to luminosity. However,
note in the supergiant classes the strengthening of the Ni II λ3995 line and the Si III triplet
(λ4552).
Luminosity Effects at A0
1.5
Ca II
K Hε
Hδ
FeII, Ti II Fe II
4172-9 4233
Hγ
Mg II 4481
1.0
Rectified Intensity
HR 1040 A0 Ia
0.5
η Leo A0 Ib
0.0
-0.5
α Dra A0 III
-1.0
α Lyr A0 Va
-1.5
-2.0
3800
3900
4000
4100
4200
4300
4400
4500
4600
Wavelength ( )
Figure 13: Luminosity effects at A0. Spectral features marked with upward arrows show
a positive effect while the downward arrows indicate spectral features that show a negative
luminosity effect. Spectral features marked with a line are insensitive to luminosity. Spectra
from DSO.
Near a spectral type of A0, the primary luminosity criterion is the progressive widening
and strengthening of the hydrogen lines with decreasing luminosity. Notice as well that certain lines of ionized iron (especially Fe II λ4233), certain blends of Fe II and Ti II (especially
λλ4172–8) and the Si II doublet (λλ4128 – 30) are enhanced in the supergiants.
The Main Sequence A0 – F0
1.5
H9
H8
Ca II
K Hε
Hδ
Mg II
4481
Hγ
Rectified Intensity
1.0
α Lyr
Ca I
4266
Mn I
4030
0.5
A0 Va
β Leo A3 Va
0.0
Fe I
4046
Fe I
4383
-0.5
HR 3974 A7 V(n)
Fe I
4271
-1.0
HD 23585 F0 V
-1.5
-2.0
3800
3900
4000
4100
4200
4300
4400
4500
4600
Wavelength ( )
Figure 14: The main sequence from A0 to F0. The spectral features marked are useful in
temperature classification of the A-type stars. Spectra from DSO.
The hydrogen lines reach a maximum in the A-type stars; on the main sequence this maximum is at A2. The Ca II K-line increases significantly in strength through the A-type stars,
and its absolute strength, or, more usefully, its ratio with Hǫ or Hδ is a sensitive indicator
of the temperature type, although one that is dependent on the metallicity. The general
metallic-line spectrum also increases in strength through the A-type stars. These three criteria give a consistent temperature type in the normal A-type stars. Any disagreement between
these three criteria indicates the star is peculiar in some way.
Peculiar A-type (Ap) stars
Mg II
Si II
Si II
Si II
He I
1.5
Eu II?
Sr II
Eu II
Si II+
Eu II
Sr II+
Si II
0.0
Cr II
-0.5
Sr II + Cr II
-1.0
3800
3900
4000
Cr II Sr II
HD 2453 B9 Vp SrCrEuSi:
β CrB A8 V: SrCrEu
Eu II
Eu II+
Cr II
4100
4200
φ Dra B9 IVp Si
HD 224801 B8 IVp SiEu
Sr II
Eu II
Cr II
Eu II+
Sr II+
Cr II
0.5
Cr II
Rectified Intensity
1.0
4300
4400
4500
4600
Wavelength (
)
Figure 15: Illustrating four “typical” Ap stars, showing enhancements of Si, Cr, Eu and Sr.
Note that many so-called Ap stars are actually B-type stars. The helium-weak nature of
these stars has caused them to be erroneously classified as A-type stars. Spectra from DSO.
Ap stars are A-type (actually more commonly late B-type) stars that show significant
enhancements of certain elements. The top spectrum (φ Dra) is that of a silicon Ap star;
it shows enhanced lines of Si II, especially the λλ4128-30 doublet. HD 224801 shows enhancements of both silicon and europium, with a hint of strontium. In HD 2453, strontium,
chromium and europium are strongly enhanced. All three of these stars are actually late Btype stars. These stars have been traditionally classified as A-type stars because of the lack
of He I lines in their spectra (although note φ Dra shows weak He I). Careful examination
of the hydrogen-line profiles, however, helps to establish the correct spectral type. β CrB is
considerably cooler, and shows strong enhancements of strontium, chromium and europium.
All of the stars illustrated are so-called “classical Ap stars”. All have strong magnetic fields.
Figure 16 shows the spectrum of a mercury-manganese star, α And. These stars show
Mg II 4481
He I 4471
Mn II 4252/9
Mn II 4206
Mn II 4136
He I 4026
Hg II 3984
Rectified Intensity
greatly enhanced abundances of mercury and manganese, as well as a number of other elements.
1.0
α And B8 IV-V HgMn
0.5
0.0
3800
3900
4000
4100
4200
4300
4400
4500
4600
Wavelength ( )
Figure 16: A spectrum of the mercury-manganese star α And. The most prominent lines of
Hg II and Mn II are marked; the Mn II λ4252 line may be blended with Ga II in this star.
Spectrum from DSO.
The λ Bootis stars
1.5
Ca II K
Hδ
Fe II 4233
Mg II 4481
Hγ
1.0
Si II 4128-30
β Leo (A3 Va)
Rectified Intensity
0.5
λ Boo
0.0
-0.5
HR 4881
-1.0
HD 23585 (F0 V)
-1.5
-2.0
3800
3900
4000
4100
4200
4300
4400
4500
4600
Wavelength ( )
Figure 17: The spectra of two prototypical λ Bootis stars, λ Boo itself and HR 4881 compared with two MK standard stars. These standards were chosen because λ Boo has an A3
hydrogen-line type, and HR 4881 an F0 type. Note the outstanding weakness of the Mg II
λ4481 line in the λ Boo stars. Spectra from DSO.
The λ Bootis stars are population I A-type stars that show marked underabundances of
the iron-peak elements. Spectral analysis indicates that the most extreme of these stars can
have metal deficiencies nearly 2 dex below the sun. Interestingly, they show nearly solar
abundances of C, N, O, and S. These stars are recognized spectroscopically by their weak
Mg II 4481 lines. However, since many peculiar types near A0 have weak λ4481 lines, to
confirm a star as a λ Bootis star, it is also necessary to show that it is metal- weak. This
can be done by first ascertaining the hydrogen-line type of the star, as the hydrogen lines
are probably the best indicators of the effective temperature. Notice that the hydrogen lines
of HR 4881 are best matched by the F0 V standard, HD 23585, whereas the metallic-line
spectrum is weaker than the A3 standard. This clearly indicates that the star is metal-weak.
The very broad hydrogen lines of the type star, λ Boo itself, indicate that it is an early A-type
star. Many λ Bootis stars are young; a few have been discovered in the Orion Association,
including HD 37411, which is also a Herbig Ae/Be star. Many λ Bootis stars show evidence
of circumstellar material, both dust and gas.
Metallic-line A-type Stars (Am stars)
1.5
Ca II
K
Mn I
4030
Fe II/ Ti II
4172-9
Ca I
4226
Fe II/Ti II/ScII
4395-4400
Ti II
4417
Rectified Intensity
1.0
30 LMi A9 IIIa
0.5
Region of Line
weakening
0.0
63 Tau
kA1.5 hA9 mF3 (III)
-0.5
-1.0
β Cas F2 III
-1.5
3800
3900
4000
4100
4200
4300
4400
4500
4600
Wavelength ( )
Figure 18: The spectrum of an Am star, 63 Tau, compared with two normal stars, 30 LMi,
an A9 giant, and β Cas, the MK F2 III standard. The hydrogen-line profiles of 63 Tau are
an excellent match with 30 LMi, but the metallic-line spectrum looks slightly later than that
of the F2 III. Note the weak K-line in 63 Tau. Spectra from DSO.
“Metallic-line” A-type stars, or, for short, “Am” stars, are defined as stars that show a
difference between the K-line type and the metallic-line type. The star 63 Tau is a classic
example of such a star. As can be seen in the figure, the Ca II K-line is weak (it is slightly
weaker than that of the A2 standard), while the metallic-line spectrum is slightly stronger
than that of β Cas. The hydrogen lines show an intermediate spectral type, estimated here
as A9. The spectral type of this star is expressed, therefore, as kA1.5hA9mF3 (III). Notice
that the Sr II 4077 line is enhanced in the Am star, and the Ca I 4226 line is weak. Many
Am stars show the anomalous luminosity effect in which the luminosity criteria in certain
spectral regions (in particular, the region from λλ4077–4200) may indicate a giant or even
a supergiant luminosity, whereas other regions (such as the region of line weakening marked
in the figure) indicate a dwarf luminosity or even lower.
Herbig Ae (emission-line) stars
1.5
Ca II
K
1.0
Hγ
Hδ
Hβ
Mg II
Fe II (42)
Rectified Intensity
AB Aur Jan 30/31 1997
A0 Vaer Bd< Nem6
0.5
Fe II
4233
0.0
XY Per Dec 25/26 1997
kA5 hA3 mA5 IV shell Bd< Nab4
-0.5
HD 37806 Feb 1/2 1997
kA1 A0 II-IIIe shell Nipc5
-1.0
-1.5
3800
4000
4200
4400
4600
4800
5000
5200
5400
5600
Wavelength ( )
Figure 19: Spectra of three Herbig Ae stars. Note that lines of multiplet 42 of Fe II are in
emission in AB Aurigae, in deep absorption in XY Per, and exhibit inverse P Cygni profiles
in HD 37806. Both XY Per and HD 37806 show pronounced shell features, including a strong
Fe II λ4233 line. These 3.6Å resolution spectra were obtained at DSO.
Herbig Ae stars generally show emission in the Hα line (outside of the spectral range of
most of the spectra used in this atlas), and quite often emission in Hβ and even Hγ . Many
Ae stars are still contracting to the main sequence, and are thus either still surrounded by
remnants of their stellar cocoons, or have developed massive stellar winds.
Recently, Gray & Corbally (1998) have devised an extension of the MK Classification
System to the Herbig Ae-type stars. This system, in addition to the usual MK-type, utilizes
indices to indicate emission or stronger than normal absorption in lines of the Fe II (42)
multiplet, the strength of the Balmer decrement, and characteristics of the emission in the
Hβ line. The three stars illustrated here demonstrate that system. The extended spectral
type consists of a normal MK type along with an indication of the nature of the Balmer line
emission. An “e” indicates strong emission in the Hβ line, (e) indicates marginal or weak
emission, and an “r” or “b” indicates whether this emission is shifted to the red or blue of
the photospheric line. The strength of the Balmer decrement is indicated by the symbols
<,≤, =,≥, > for weak, somewhat weak, normal, somewhat strong, and strong. The nature of
the emission and/or absorption, plus the strength relative to the normal absorption strength
of the relevant standard of the lines of the Fe II (42) multiplet are indicated with the N
index. Nem indicates these lines are in emission, Nab that they are in stronger than normal
absorption, and Npc and Nipc indicate P Cygni and inverse P Cygni profiles in these lines.
The spectral types of these Herbig Ae stars can change quite dramatically on time scales of
a few days. As a consequence, the spectral types of these stars should always be accompanied
by a date.
Mg II 4481
Hγ
Fe I 4383
Hδ
Fe I 4271
Ca II K
Ca I 4226
H8
Mn I 4030
Fe I 4046
H9
1
Ca II H + Hε
The Main Sequence F0 – G0
Rectified Intensity
HD 23585 F0 V
0
78 UMa F2 V
G-band (CH)
HR 5634 F5 V
-1
HD 27808 F8 V
-2
β CVn G0 V
3800
3900
4000
4100
4200
4300
4400
4500
4600
Wavelength ( )
Figure 20: A temperature sequence for main-sequence F-type stars. Features useful in the
temperature classification of the F-type stars are marked. Spectra from DSO.
The hydrogen lines continue to weaken through the F-type stars, and the Ca II K-line
strengthens, although it becomes essentially saturated by the late F-type stars. The general
strength of the metallic-line spectrum (note especially the features marked on the illustration)
grows dramatically. Around F2, depending upon the resolution of the spectrum, the G-band
makes its first appearance. The G-band is a molecular band, composed of thousands of
closely spaced lines due to the diatomic molecule CH.
Rectified Intensity
1
Mg II 4481
Ti II 4444
Ti II, Fe II
Ti II 4417
Hγ
Fe I 4383
Fe I 4271
Sr II 4216
Ca I 4226
Fe II 4233
Hδ
Fe II/Ti II
4172-9
Sr II 4077
Fe I 4046
Ca II H
Ca II K
Luminosity Effects at F0
φ Cas F0 Ia
0
α Lep F0 Ib
ζ Leo F0 IIIa
-1
HD 23585 F0 V
-2
3800
3900
4000
4100
4200
4300
4400
4500
4600
Wavelength ( )
Figure 21: Luminosity effects at F0. Spectral features marked with upward arrows show a
positive luminosity effect; those marked with a line are relatively insensitive to luminosity.
Note that the hydrogen lines show only a weak negative luminosity effect. Spectra from
DSO.
By F0, the hydrogen lines have lost most of their sensitivity to luminosity. Note, however,
that they can still be used to distinguish the supergiant classes from lower luminosities.
Near F0, the luminosity class is estimated from the strength of lines due to ionized iron
and titanium. Excellent luminosity-sensitive features include the Fe II, Ti II double blend
at λλ4172–8, and similar blends at λλ4395–4400, λ4417 and λ4444. The strength of these
blends are usually estimated with respect to other less luminosity sensitive features, such as
Ca I λ4226, Fe I λ4271 and Mg II λ4481.
Rectified Intensity
Mg II 4481
Ti II 4444
Fe I 4383
Hγ
Ti II, Fe II
Ti II 4417
Fe I 4271
Ca I 4226
Hδ
Sr II 4216
Fe II, Ti II
4172-9
Sr II 4077
Fe I 4046
1
Ca II H
Ca II K
Luminosity Effects at F8
δ CMa F8 Ia
0
γ Cyg F8 Ib
υ Peg F8 III
-1
HD 27808 F8 V
G Band
-2
3800
3900
4000
4100
4200
4300
4400
4500
4600
Wavelength ( )
Figure 22: Luminosity effects at F8. Spectral features marked with upward arrows show a
positive luminosity effect; those marked with a line are relatively insensitive to luminosity.
Note the change in the morphology of the G-band (marked below the F8 V spectrum) with
luminosity. Spectra from DSO.
By F8, the hydrogen lines have lost all sensitivity to luminosity, and it is now necessary
to rely solely on lines and blends of ionized species. The luminosity sensitive features are
essentially the same as at F0, except that at F5 and later types, the Sr II λλ4077 and 4215
lines show excellent sensitivity to the luminosity. Note, however, that these lines may be
enhanced in chemically peculiar stars, such as the barium dwarfs. Also note that at F5 (and
later types), the Ca II K-line shows a slight positive sensitivity to luminosity, in the sense
that it becomes slightly broader in the more luminous stars.
Hβ
Fe I 4383
Hγ
G-band
Hδ
Fe I 4046
Ca I 4226
Cr I / Fe I
1
Ca II H
Ca II K
Metal-weak F-type Stars
Rectified Intensity
η Cas A F9 V m+0
0
HD 48938 F9 V m-1
HD 125276 F9 V m-2.25
-1
3800 3900 4000 4100 4200 4300 4400 4500 4600 4700 4800 4900
Wavelength ( )
Figure 23: A metallicity sequence of F9 main-sequence stars classified on Gray’s 1989 system.
These spectra were obtained on the Steward Observatory 90” Bok telescope, and have a
slightly lower resolution than the 1.8Å resolution DSO spectra in many of the previous
figures.
Main-sequence stars with a wide range of metallicites can be found in the solar neighborhood. Illustrated here are three stars classified on Gray’s 1989 extension of the MK system.
This extension accommodates metal-weak stars of the thick disk and halo. The degree of
metal deficiency is indicated on this system with an “m” index, which is well correlated with
[Fe/H] measurements in the literature.
-1
Fe I 4383
Y II 4376
G-band
Fe I 4326
Hγ
Fe I 4271
Cr I 4290
0
Fe I 4260
Sr II 4216
Ca I 4226
Hδ
Fe I 4250
Cr I 4254
Rectified Intensity
1
Sr II 4077
Mn I 4032
Fe I 4046
Ca II H
Ca II K
Main Sequence G0 – K5
β CVn G0 V
κ1 Cet G5 V
54 Psc K0 V
-2
61 Cyg A K5 V
-3
3800
3900
4000
4100
4200
4300
4400
4500
4600
Wavelength ( )
Figure 24: A temperature sequence for main-sequence stars from G- to mid-K types. Features
useful in the temperature classification of these stars are marked. DSO 1.8Å resolution
spectra.
Later than G0 along the main sequence, the hydrogen lines continue to fade, while the
strength of the general metallic-line spectrum continues to increase. The G-band also increases in strength until the early K-type stars (about K3), and then begins to fade. The Ca I
4226 line grows gradually in strength until the early K stars, and then becomes dramatically
stronger by mid-K. The ratios Fe I λ4046/Hδ and Fe I λ4325/Hγ are useful in estimating
the temperature type, reversing at a spectral type near G8. Unfortunately, these ratios are
not reliable in metal-weak or metal-strong stars. The temperature type may be estimated
with precision, even in metal-weak stars by using the ratio of the Cr I λ4254 resonance line
with the two neighboring Fe I subordinate lines at λ4250 and λ4260. Notice that the Cr I
line becomes stronger in ratio with the two flanking higher-excitation Fe I lines, being clearly
stronger than both by K5.
1
Hγ
Fe I 4383
G-band
Hδ
Ca I 4226
Ca II
K&H
Fe I 4046
Main Sequence G0 – K5, 3.6Å Resolution spectra
Hβ
Mg I
0
-1
κ1 Cet G5 V
Fe I 4326
Fe I 4271
Normalized Flux
β CVn G0 V
Cr I 4254
Cr I 4290
54 Psc K0 V
-2
MgH
MgH
61 Cyg A K5 V
-3
3800
4000
4200
4400
4600
4800
5000
5200
5400
5600
Wavelength ( )
Figure 25: The same temperature sequence for main-sequence stars illustrated in Figure
24, but with lower resolution (3.6Å as opposed to 1.8Å spectra) normalized flux spectra.
Features useful in the temperature classification are marked. DSO 3.6Å resolution spectra.
This figure reproduces the temperature sequence of the previous figure, but employs
“normalized-flux” spectra instead of “rectified intensity” spectra in which the continuum
points are normalized to unity. The flux representation is advantageous in late-type stars
because line blanketing in these spectra is so great that there are no true continuum points.
These spectra also have a lower resolution (3.6Å) than in the previous figure (1.8Å), but show
a wider spectral range, also often of advantage in classifying late-type stars. The temperature
criteria noted in the previous figure can also be used in these spectra, although the resolution
here is too low to resolve clearly the Cr I λ4254 line from the neighboring Fe I lines at λ4250
and λ4260. However, the broader spectral range brings other temperature criteria into play.
Note the development of the MgH feature at 4780Å. This feature first appears at a spectral
type of K4. The prominent wedge-like continuum depression in the vicinity of the Mg I
triplet (λλ5167, 5172, and 5184), also partially due to MgH, is a distinctive feature of dwarfs
later than K2. The morphology of the line spectrum in the vicinity of Hβ is a sensitive
indicator of the temperature-type, with the progressive fading of the Hβ line. However, this
criterion should be used only for solar-abundance stars.
Fe I 4383
Y II 4376
Hγ
G-band
Sr II 4216
CN
4215
Fe I 4271
Hδ
Fe I 4046
Sr II 4077
CN
3883
Ca II H
1.0
Ca II K
1.5
Ca I 4226
Luminosity Effects at G8
Rectified Intensity
0.5
ε Gem G8 Ib
0.0
κ Gem G8 III
-0.5
-1.0
61 UMa G8 V
-1.5
-2.0
3800
3900
4000
4100
4200
4300
4400
4500
4600
Wavelength ( )
Figure 26: Luminosity effects at G8. Principal features are marked. The two CN bands show
a positive luminosity effect, while the ratio of the Y II λ4376 line to Fe I λ4383 is particularly
useful since it is the most metallicity independent. Spectra from DSO.
The ratios of Sr II λ4077 to nearby iron lines (Fe I λλ4046, 4063, and 4071) remain
sensitive to luminosity. The violet-system CN bands, with bandhead at 4215Å, visible in
the supergiant and giant spectra as a concavity in the continuum, show a strong sensitivity
to luminosity. Notice as well that the Ca II K and H lines show extremely broad damping
wings in the supergiant class. But the criterion affording the greatest discrimination in the
luminosity classes is the ratio of the Y II λ4376 line to Fe I λ4383.
K Giants with Unusual CN-band Strengths
C2
CH
1
CN
µ Leo K2 III CN2
Normalized Flux
0
α Ser K2 IIIb CN1
-1
κ Oph K2 III
-2
16 Aur K2.5 III CN-1
-3
3800
4000
4200
4400
4600
4800
5000
5200
5400
5600
Wavelength ( )
Figure 27: K Giants with unusual CN and C2 band strengths. DSO 3.6Å resolution spectra.
A number of G and K-type giants show either strong or weak molecular bands involving
carbon. Illustrated here are a number of K2 III stars showing CN and C2 bands of different
strengths, along with the K2 III standard, κ Oph. The luminosities of such chemically
peculiar stars are determined using criteria independent of these bands.
More Late-G Giants with Abundance Peculiarities
Normalized Flux
1
Ba II 4554
G-band (CH)
2
CN
HD 199939 K1 III Ba 4+
HD 217143 G9.5 III Ba 2+
0
β LMi G9 III
-1
37 Com G9 III CH-2
-2
HR 6791 G8 III CN-1 CH-3
3800
4000
4200
4400
4600
4800
5000
5200
Wavelength ( )
Figure 28: Late-G giants with abundance peculiarities. DSO 3.6Å resolution spectra.
In this figure are illustrated a number of late-G, early-K giant stars with abundance
peculiarities. HD 199939 is a barium giant star, showing an exceptionally strong Ba II
resonance line at λ4554. HD 217143 is a milder barium giant. β LMi is the G9 III MK
standard. 37 Com is a G9 III with an abnormally weak G-band, but quite normal CN 4215
band. HR 6791 is a G8 III with weak CN bands and a weak G-band. Almost all barium
stars are members of binary systems in which the companion is a white dwarf. It is believed
that the enhanced abundances of s-process elements, such as barium, were gained from the
companion star when it was on the Asymptotic Giant Branch (AGB). The mass transfer came
about either through Roche-lobe overflow or wind accretion. The s-process elements are the
result of slow neutron capture, which occurs only at advanced stages of nuclear burning.
Luminosity Effects at K5
Ba II 4554
2
Ca I 4227
5269
ξ Cyg K4.5 Ib
MgH
Normalized Flux
1
0
5250
-1
MgH
α Tau K5 III
61 Cyg A K5 V
-2
3800
4000
4200
4400
4600
4800
5000
5200
5400
5600
Wavelength ( )
Figure 29: Luminosity effects at K5. DSO 3.6Å resolution spectra.
The negative luminosity effect in the Ca I λ4226 line is an important luminosity criterion
in the K5 stars. The Ba II λ4554 resonance line shows a positive luminosity effect, but classifiers should be aware that this line may be enhanced in chemically peculiar K-type giants.
The MgH bands show a negative luminosity sensitivity; the strength and the morphology
of the narrow MgH feature near 4770Å is especially useful; it can be ratioed with Hβ and
neighboring metal lines. The line ratio λ5250/λ5269 is as well an excellent luminosity criterion in the K5 stars. The λ5250 feature is a pressure sensitive blend involving a number of
intersystem Fe I lines.
Main Sequence K5 – M4.5
Normalized Flux
-1
MgH
Hβ
Mg H
Fe I 4383
G-band
Ca II K & H
1
0
Ca I 4226
2
Mg I b
61 Cyg A K5 V
61 Cyg B K7 V
TiO α System
-2
GL 411 M2 V
GL 725B M3.5 V
CaOH
40 Eri C M4.5 Ve
-4
3800
4000
4200
Hγ
Hβ
-3
4400
4600
4800
5000
5200
5400
5600
Wavelength ( )
Figure 30: A temperature sequence of main sequence stars from late-K to mid-M with the
principle spectral criteria labelled. DSO 3.6Å resolution spectra.
This figure uses 3.6Å resolution spectra to take advantage of their broader spectral coverage. In addition, the ordinate in this figure is in terms of “flux” instead of “rectified
intensity”, so that the change in the shape and slope of the continuum can be seen as a
function of spectral type. In the K-type dwarfs, the spectral type may be estimated from
the ratio of Ca I λ4226 to Fe I λ4383, in the sense that Ca I/Fe I grows toward later types.
In addition, K5 and K7 dwarfs may be distinguished by 1) the ratio of the MgH feature at
4780 to neighboring lines and 2) the first subtle indications of TiO in the K7 dwarf. By
M0, bands due to TiO begin to be significant features in the spectrum, and these strengthen
quite dramatically toward later types; by M4.5 they dominate the spectrum. To exclude the
possibility of systematic errors in metal-weak stars, ratios of TiO band strengths should be
employed. Notice as well the development of the MgH feature at 4780Å. It begins in the midK-type dwarfs as a pointed tooth- like absorption feature, which then becomes progressively
more flat-bottomed as a nearby TiO band grows in strength. A band of CaOH, a tri-atomic
molecule, makes its first appearance at about M3, and contributes to a strong absorption
feature by M4.5. Notice that Hβ is in emission in the M4.5 star; many M dwarfs have active
chromospheres and exhibit strong flares many times more energetic than solar flares. One of
the manifestations of this is emission in the hydrogen lines.
An M-Giant Temperature Sequence
-1
-2
-3
Ba II
4554
υ Aur M0 III
χ Peg M2+ III
TiO
γ'
HR 3577 M4+ III
BK Vir M7- III:
VO
0
Mg I b
VO
VO
Normalized Flux
Ca II
K&H
TiO α system
Fe I 4383
1
CH (G-band)
Ca I 4226
2
R Leo M9: III
-4
3800
4000
4200
4400
4600
4800
5000
5200
5400
5600
Wavelength ( )
Figure 31: A temperature sequence for M giants. DSO 3.6Å resolution spectra.
Temperature classification of the M-type giants is based primarily on the increasing
strength of the TiO bands. At K7, the TiO λ5448 bandhead can just barely be detected;
after M3 the strongest bands become saturated, and the temperature type is assigned on the
basis of the strength of the fainter bands.
To avoid complications due to abundance or population effects or the “veiling” phenomenon in which the spectral features appear washed out (this can be a particular problem
in the variable M-type stars, such as the Mira variable stars), it is best to judge the spectral
type on the basis of the ratios of the strengths of these bands. In particular, it is useful to
consider ratios of the orange-red γ ′ system of TiO with bands of the blue-green α system.
Only the bluest band of the γ ′ system is visible in these spectra. Note the appearance of VO
bands in the M9 giant.
The metallic lines reach a maximum strength in the early M-type stars; in later types the
successively stronger absorption due to TiO begins to reduce the line strengths in the spectral
region illustrated here. The Ca I λ4226 line, however, continues to increase in strength in
the giants, especially after M5. Note the violet (near 4100Å) flux “bump” in the two latest
spectra.
Luminosity Effects at M2
-2
5250
HD 10465 M2 Ib
MgH
χ Peg M2+ III
5269
-1
µ Cep M2- Ia
Cr I 5206
0
Mg I b
Normalized Flux
1
Ca I 4226
Ca II K & H
TiO α system
GL 411 M2 V
-3
3800
4000
4200
4400
4600
4800
5000
5200
5400
5600
Wavelength ( )
Figure 32: Luminosity Effects at M2. DSO 3.6Å resolution spectra.
The negative luminosity effect in the Ca I λ4226 line is the most striking luminosity
indicator in the M2 stars. At this resolution, the morphology of the MgH/TiO blend near
4770Å can be used as well to distinguish luminosity classes; notice that the MgH band
dominates this blend in the dwarf star, producing a tooth-shaped feature. The morphology
of the spectral region between 4900 and 5200Å seems to be quite sensitive to luminosity. In
addition, the ratio of two blends at 5250Å and 5269Å continues to be luminosity sensitive.
At higher resolutions, other features, especially in the blue-violet region, can be used.
Emission Lines in Mira Variable Stars
2
1
Hγ
Normalized Flux
Hδ
0
S Leo M6: IIIe
-1
BK Vir M7- III:
Fe I Fe I
4202 4308
-2
Mg I
4571
R Leo M9: IIIe Jan 18, 1999
R Leo M9: III
-3
3800
4000
Apr 9, 1999
4200
4400
4600
4800
5000
5200
5400
5600
Wavelength ( )
Figure 33: Mira variable spectra at different phases. S Leo and R Leo are both long-period
(Mira) variables. Note the appearance of Balmer emission lines in the spectrum of S Leo and
fluorescence emission lines due to metals in R Leo. The fourth spectrum is also of R Leo,
taken at a phase without obvious emission; at this phase the spectral type is approximately
M9 III. DSO 3.6Å resolution spectra.
The Mira variable stars are defined as M-type stars with periods of 80 – 1000 days and
amplitudes greater than 2.5 magnitudes. The spectra of these giant pulsating stars show
dramatic changes during the pulsation cycle, and even show spectral differences from cycle
to cycle. Shock waves in the pulsating atmospheres of these stars produce emission lines
in their spectra. The hydrogen lines are usually in emission, with the emission strength
increasing toward maximum light, although the Balmer decrement is quite often unusual.
Note that in S Leo (top spectrum), Hδ and Hγ are quite strong, but Hβ is not visible. In
R Leo (third spectrum), the hydrogen lines are not strongly in emission, but emission lines
of Fe I (4202, 4308) and Mg I 4571 are visible. In many stars the hydrogen-line emission
strength is in antiphase with the emission lines of Fe I and Mg I.
Mira variable stars can also show a number of other interesting phenomena. Many Mira
stars show a ”washed-out” appearance to their spectra at the faintest phases in which the TiO
bands lose contrast, and the atomic lines appear broader and shallower. This phenomenon
has been referred to as “veiling”, and may be due to the formation of high-level atmospheric
clouds. In addition, Miras can show spectral bands due to AlO at certain phases.
Late M-type dwarfs
Figure 34: Optical spectra of the M4.5 dwarf υ And B and the M9 dwarf LHS 2065. Line and
band identifications are marked on the M4.5 dwarf. Additional features appearing at cooler
temperatures are marked on the spectrum of the late-M dwarf. Reproduced from Stellar
Spectral Classification, courtesy J. Davy Kirkpatrick.
Increasingly strong TiO absorption and declining effective temperatures in the late Mtype dwarfs greatly decrease the flux in the traditional spectral classification region (3800
– 5000Å), necessitating a shift to the red and near infrared to classify the cool dwarfs. At
M4.5, the spectrum is dominated by absorption bands of TiO; MgH continues to contribute
strong features that are luminosity sensitive (see Figure 32); in the red CaH plays a similar
role. In the very cool M9 dwarf, bands of VO become prominent, as well as features due to
FeH and CrH. Lines of alkaline metals, such as Rb I and Cs I are also strong.
The L-type dwarfs
Figure 35: A temperature sequence for the L-type dwarfs in the red-optical. Shown are an
early-, mid-, and late-L dwarf on both a linear (left) and logarithmic (right) flux scaling. Note
the fading of the TiO and VO bands and the increasing importance of the metallic hydrides
and lines of alkaline metals. Reproduced from Stellar Spectral Classification, courtesy J.
Davy Kirkpatrick.
Early-L dwarfs show a melange of atomic and molecular bands in the optical, the most
prominent being the neutral alkali lines (Na I, K I, Rb I, Cs I, and sometimes Li I), oxide
bands TiO and VO, hydride bands CrH and FeH, and CaOH. By mid-L the resonance lines
of Na I and K I have grown tremendously in strength; the hydrides MgH, CaH, CrH, and FeH
have also strengthened, whereas the oxides TiO and VO have largely disappeared. By late-L,
H2 O has increased in strength, the neutral alkali lines are still strong, and the hydrides are
much reduced in prominence.
The M and L-dwarfs form a continuum of types describing main-sequence stars and
brown dwarfs. This sequence can continue to be understood as a temperature sequence.
The disappearance of TiO and VO from the spectra of mid L dwarfs is due to formation of
condensates such as perovskite (CaTiO3 ) and their consequent removal from the atmosphere.
Which spectral type corresponds to hydrogen-burning stars and which to brown dwarfs
really depends upon the age of the object. Early to mid-M dwarfs are composed exclusively
of hydrogen-burning stars. Mid- to late-M dwarfs are comprised of old, low-mass stars (down
to a mass of ∼ 0.085M⊙ and young brown dwarfs. Early- to mid-L dwarfs are a mixture of
low-mass stars that are fairly old, and brown dwarfs generally younger than about 3 Gyr.
The latest L dwarfs are, however, all brown dwarfs. Because some M dwarfs are brown dwarfs
and some L dwarfs are stars, it is best to refer to all these objects as “dwarfs” rather than
“stars”.
The T Dwarfs
CH4
1.0000
2MASS 0559-1404 T4.5
CH4
FeH
H2O
CH4
H2O
H2O
KI
H2O
CIA H2
Fλ (10-11 erg cm-2 s-1 µm-1)
CH4
CH4
0.1000
H2O
KI
0.0100
CH4
NH3
Na I
0.0010
0.0001
0.7
1
2
3
4
Wavelength (µm)
5
7
10
Figure 36: The spectrum of a mid-T dwarf spanning the wavelengths 0.63 – 15µm. Prominent
atomic and molecular features are labeled. Reproduced from Stellar Spectral Classification,
courtesy Adam Burgasser.
Figure 36 shows the spectrum of a typical mid-T dwarf from the red to the mid-infrared.
In the red, the spectral energy distribution is shaped by the very extensive wings of the K I
resonance line; in the far red and infrared, the most prominent features are due to CH4 and
H2 O. Indeed, T dwarfs are distinguished from the L dwarfs by the presence of CH4 absorption
in their near-infrared and infrared spectra. Figure 37 shows a sequence of T dwarfs, including
the transition from L dwarfs to T dwarfs.
The continuous change in spectral shape along the sequence illustrated in Figure 37 is
due, primarily, to increasing absorption in the CH4 bands at 1.15, 1.35, 1.65 and 2.2 µm.
There is another important CH4 band at 3.3 µm. All T dwarfs are brown dwarfs.
6
2MASS 1632+1904
L8 Std
2MASS 0310+1648
L9 Std
5
SDSS 1207+0244
T0 Std
SDSS 0837-0000
T1 Std
Normalized fλ
4
SDSS 1254-0122
T2 Std
3
2MASS 1209-1004
T3 Std
2MASS 2254+3123
T4 Std
2
2MASS 1503+2525
T5 Std
SDSS 1624+0029
T6 Std
1
2MASS 0727+1710
T7 Std
2MASS 0415-0935
T8 Std
0
1.0
1.2
1.4
1.6
1.8
Wavelength (µm)
2.0
2.2
2.4
Figure 37: A spectral sequence from the late-L dwarfs through the T dwarfs in the near
infrared. Feature identifications may be determined from Figure 36. Reproduced from Stellar
Spectral Classification, courtesy Adam Burgasser.
Carbon Stars
Most known carbon stars are cool giants, although carbon dwarfs exist as well, and may
even turn out to be the most common type of carbon star. Carbon stars are characterized by
strong bands of carbon molecules, including bands of C2 , CN, and CH. A number of types
of carbon star are recognized on the latest revision of the carbon-star classification system
(Keenan, 1993; Barnbaum, Stone & Keenan, 1996), including the C-R, C-N, C-J, and C-H
stars. Low-resolution spectra of the C-R and C-N carbon stars are shown in the following
two figures, a higher resolution montage in the blue illustrating the C-J stars and finally a
low-resolution montage for the C-H stars.
C2
4737
1.0
Normalized Flux
CN
C2
5165
C2 5585,
5635
Na I
D
Ba II
Hα
1.5
O2
CH
HD 156074 C-R2 C23 CH3.5 IIIa:
0.5
HD 223392 C-R3- C24 CH3.5 III
0.0
CN
-0.5
RV Sct C-R4+ C25.5
-1.0
4000
4500
5000
5500
6000
6500
7000
Wavelength ( )
Figure 38: A low-resolution temperature sequence for the carbon R-type stars on the Keenan
1993 system. The most prominent features have been labeled, including the Swan bands of
C2 . Spectra and spectral types from Barnbaum, Stone & Keenan (1996).
The C-R stars are the warmest of the carbon stars, corresponding in effective temperature to the G and K giants, and thus the early C-R stars are generally characterized by
appreciable flux in the blue-violet. While the prominent bands of carbon molecules obscure a
number of classification criteria used in the oxygen-rich G and K giants, a number of criteria,
including the metallicity-independent ratio of the Cr I λ4254 resonance line with neighboring
subordinant Fe I lines may be used. For those stars in which the Balmer lines are visible, the
Hγ/Fe I λ4383 ratio may be used. The s-process elements (such as Sr, Y, and Ba) are not
usually enhanced in the C-R stars, and this is one of the major criteria distinguishing them
from the C-N stars. Thus, luminosity criteria employing lines of those species may be used
with caution in the C-R stars. The isotopic bands of C2 , especially those of 12 C13 C (which
are most easily seen near the λ4737 bandhead – see Figure 40) are usually quite prominent
in the C-R stars.
1.0
0.5
CH
Ba II 4554
Sr I 4607
Normalized Flux
C2
5635
C2
4737
Na I
D
Ba II
C2
5165
BD +2o3336 C-N4 C23 CH3
0.0
TV Lac C-N5 C25
-0.5
HD 19881 C-N5+ C24.5
-1.0
4000
4500
5000
5500
6000
6500
7000
Wavelength ( )
Figure 39: A low-resolution temperature sequence for the carbon N-type stars on the Keenan
1993 system. The most prominent features have been labeled, including the Swan bands of
C2 . Spectra and spectral types from Barnbaum, Stone & Keenan (1996).
The C-N stars can be distinguished from the C-R stars by their extreme redness and
by strong absorption in the blue, with generally little or no flux shortward of 4400Å. The
C2 isotopic bands in the C-N stars are generally weaker than in the C-R stars, but lines
of s-process elements are stronger in the C-N stars. The temperature criteria mentioned
above are generally unusable in the C-N stars because of the lack of violet flux. Instead, the
primary temperature criterion employed is the ratio Ba II λ4554/Sr I λ4607.
1.5
C2 4715, 4737
12
Sr I 4607
0.5
Ba II 4554
Normalized Flux
1.0
Hβ
C13C 4723, 4744
13
CN, Ba II
4934
C13C 4731, 4752
HD 10636 C-J4 C25.5 j5.5
0.0
-0.5
-1.0
4500
CN
4600
EU And C-J5- C25 j3.5
4700
4800
4900
5000
Wavelength ( )
Figure 40: A high-resolution temperature sequence of C-J stars in the blue. Note the strong
isotopic 13 C13 C bandhead at λ4752 in both of these stars. Spectra and spectral types from
Barnbaum, Stone & Keenan (1996).
The C-J stars are carbon stars that show a remarkably large abundance of the isotope
C, and thus prominent 12 C13 C and even 13 C13 C bands (see Figure 40). C-J stars generally
have effective temperatures between those of the C-R and the C-N stars. The C-J stars may
be further distinguished from the C-N stars by the fact that the s-process elements are not
usually enhanced.
Finally, the spectra of the C-H stars are dominated by bands of CH in the blue-violet
region. While the G-band (which is formed from the Q-branches of the CH A–X 0–0 and 1–1
bands located near 4300Å) is exceptionally strong in these stars, this is not unusual for early
carbon stars; indeed the G-band is quite often saturated in the spectra of early carbon stars.
Instead, the distinguishing spectral feature of these stars is the strength of the P-branches
of these same CH bands, which are visible as a broad depression longwards of the G-band.
The C-H stars overlap the C-R stars and the early C-J stars in temperature, but they differ
from these classes in that they show enhancements of the s-process elements.
13
1.0
C2
Hβ
Ba II
Hα
Na I
D
C2
Ba II
G-band (CH Q-branch)
1.5
O2
!
C2
HD 26 C-H1.5 C21.5 CH3.5
Normalized Flux
R P
0.5
CN
HD 198269 C-H2 C22 CH4
0.0
HD 5223 C-H3 C24.5 CH5
-0.5
-1.0
4000
4500
5000
5500
6000
6500
7000
Wavelength ( )
Figure 41: A low-resolution temperature sequence for the C-H stars on the Keenan 1993
system. The most prominent features have been labeled. Note the strength of the P-branch
of the CH band, a distinguishing feature of these stars. Spectra and spectral types from
Barnbaum, Stone & Keenan (1996).
The S-type Stars
TiO α-system
1
Ca I
4226
ZrO
5305
4 Ori MS3
Hβ
Mg I 3832
Hδ
H8
Hγ
ZrO
5379
-1
ZrO
4620,41
-2
AD Cyg S3-/4
Hδ
Fe I 4308
U Cas S4.5/2.5e
Fe I 4202
Normalized Flux
0
ZrO
5552
Hγ
ZrO Sr I
4471 4607
-3
C2 ?
5165
R Cyg S4/6e
Hβ
-4
GP Ori SC7/9e
3800
4000
4200
4400
4600
4800
5000
5200
5400
5600
Wavelength (" )
Figure 42: A sequence of S-type stars with increasing C/O index. Some of these stars are
Mira variables and show emission lines at certain phases. DSO 3.6Å spectra.
If the G-, K-, and M-type giants represent the oxygen-rich temperature sequence, and
the carbon stars the carbon-rich sequence, the S-type stars are those stars in which the
C/O ratio is close to unity. Under such conditions, molecular equilibrium involving the CO
molecule leads to the dissociation of the fragile TiO and VO molecules. The more robust
molecules ZrO, YO, and LaO, involving s-process elements enhanced in the S-type stars, can
survive until C/O is nearly unity, at which point they also dissociate. Once C/O > 1, carbon
molecules appear. Thus the passage of the C/O ratio through unity, s-process enhancement,
and molecular equilibrium underly the M → MS → S → SC → C sequence, illustrated in
the figure above. The format of the S-type spectral type takes the form
SX/n[e] ZrO TiO D YO Li
where “X” stands for the temperature type on the scale of the M-giants, “n” for the C/O
index, determined on the basis of the strength of the ZrO bands relative to the TiO bands, or
the presence or absence of C2 bands, and finally, visual numerical estimates, on an arbitrary
scale, of the strengths of various molecular (ZrO, TiO, YO) and atomic (Na D, Li) features
may be appended to the spectral type.
The Wolf–Rayet Stars
He II 5411
He II 4686
Hβ + He II
N V 4940
N IV 4057
3.5
N V 4603, 4619
6.0
1.0
Rectified Intensity
WR 152 WN3
-1.5
WR 128 WN4
N III 4634-41
-4.0
WR 138 WN5
C IV 5808
-6.5
WR 115 WN6
He I 5875
-9.0
WR 78 WN7
He I 4921
N III 5314
-11.5
WR 16 WN8
3800
4000
4200
4400
4600
4800
5000
5200
5400
5600
5800
6000
Wavelength (# )
Figure 43: An ionization montage of the nitrogen sequence of Wolf–Rayet stars. Reproduced
from Stellar Spectral Classification; spectra courtesy Hamann, Koesterke & Wessolowski
(1995).
The Wolf–Rayet stars are luminous, hot stars whose spectra are dominated by broad,
strong emission lines formed in massive stellar winds expanding outwards with velocities on
the order of 1000 – 2500 km/s. Wolf–Rayet stars are among the most luminous stars in
the galaxy. Wolf–Rayet stars can be arranged into essentially two sequences, the nitrogen
sequence and the carbon sequence. The nitrogen sequence (the WN stars) shows many
emission lines of ionized nitrogen, whereas the carbon sequence (the WC stars) have spectra
dominated by emission lines of ionized carbon.
C III, IV 4650
C IV 5808
C IV 5471
WR 52 WC4
WR111 WC5
-20
WR154 WC6
C III 4069
-40
WR68 WC7
He I 5876
C III 4326
He II 4686
C II 4267
-60
He II 5412
Rectified Intensity
0
C III 5696
O V 5590
20
WR57 WC8
WR92 WC9
-80
3800
4000
4200
4400
4600
4800
5000
5200
5400
5600
5800
6000
Wavelength ($ )
Figure 44: An ionization montage of the carbon sequence of Wolf–Rayet stars. Reproduced
from Stellar Spectral Classification; spectra from Torres & Massey (1987).
The basic classification system for both the nitrogen and carbon Wolf–Rayet stars was
elaborated by Smith (1968), although that system has been extensively revised since then.
The current classification system for the WN stars is based primarily on the He II λ5411/He I
λ5875 ratio. This ratio traces out an ionization sequence as opposed to a temperature sequence, reflecting ionization conditions in the stellar wind. Secondary criteria determining
the ionization class involve ratios of different ionization states of nitrogen, such as the N V
λ4604/N III λ4640 ratio. WN stars are also classified on the basis of line width and hydrogen
content.
The (ionization) classification of the WC stars is based on three main criteria, the C III
λ5696/O V λ5590 and C III λ5696/C IV λ5808 ratios and the width of the C III,IV λ4650
blend.
There is a third sequence of Wolf–Rayet stars, the WO sequence, whose members have
spectra dominated by emission lines of ionized oxygen. These rare stars probably represent
the high ionization continuation of the WC stars.
References
Barnbaum, C., Stone, R.P.S. & Keenan, P.C. 1996, ApJS, 105, 419
Gray, R.O. 1989, AJ, 98, 1049
Gray, R.O. & Corbally, C.J. 1998, AJ, 116, 2530
Hamann, W.-R., Koesterke, L. & Wessolowski, U. 1995, AApS, 113, 459
Keenan, P.C. 1993, PASP, 105, 905
Keenan, P.C. & McNeil, R.C. 1976, An Atlas of Spectra of the Cooler Stars: Types G, K, M,
S and C (Columbus: Ohio State University)
Lesh, J.R. 1968, ApJS, 16, 371
Morgan, W.W., Abt, H.A. & Tapscott, J.W. 1978, Revised MK Spectral Atlas of Stars Earlier
than the Sun (Chicago and Tucson: Yerkes Observatory, University of Chicago, and Kitt
Peak National Observatory)
Morgan, W.W., Keenan, P.C. & Kellman, E. 1943, An Atlas of Stellar Spectra (Chicago:
University of Chicago Press)
Smith, L.F. 1968, MNRAS, 138, 109
Torres, A.V. & Massey, P. 1987, ApJS, 65, 459