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J/ApJS/105/419 Spectral Atlas of Carbon Stars (Barnbaum+ 1996)
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A Moderate-Resolution Spectral Atlas of Carbon Stars: R, J, N, CH, and
Barium Stars
Barnbaum C., Stone R.P.S., Keenan P.C.
<Astrophys. J. Suppl. 105, 419 (1996)>
=1996ApJS..105..419B
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ADC_Keywords: Stars, carbon; Spectroscopy ; MK spectral classification
Keywords: Carbon Stars
Description:
The authors compiled this moderate-resolution spectral atlas to aid in
the classification of carbon stars on the Revised MK System (Keenan
1993PASP..105..905K) as refined and extended by the present work.
Hence the main purpose of this atlas is to permit rapid and reasonably
accurate comparison of the properties of carbon stars in the Solar
neighborhood with those in the Galactic bulge, the Magellanic Clouds,
and in other nearby external systems.
The classification scheme employed makes no assumptions about
evolutionary status of the stars but is based entirely on observable
criteria. Spectra of 39 stars are presented in detail, along with a
catalog of 119 carbon stars classified according to the Revised MK
System; the catalog lists also the variability type from Kholopov (1985),
and the instrument used.
The table is a modified version of table 4 in the source reference.
This classification catalog uses not only the stars we observed (the
spectral data in this catalog) but also used other material --
photographic plates and some of the stars in the high resolution
barnbaum atlas. Stars observed solely with those two instruments are
so noted.
The spectral files are the data for the standard Carbon Stars in Figs
1 and 2 of the paper.
Introduction and background:
Except for the early R stars, most carbon stars have spectra so
densely occupied by bands of carbon compounds that nearly all the
atomic lines that normally provide the criteria for spectral
classification are either distorted or obliterated. It is this
complexity that makes it so difficult to sort the spectra into types
that can be calibrated in terms of fundamental physical variables:
effective temperature (Teff), luminosity, and composition.
Nevertheless, the authors have been able to assign temperature types,
luminosity classes (at least for the R stars) and abundance indices
for the principal compounds of carbon. The C2 index is included in the
type for every carbon star; it seems to be the best measure of the
ratio of carbon to oxygen. In using the index, allowance must be made
for its sensitivity to temperature also. The C2 index runs from 1 to
5, indicating that the strong bands at 4737 and 5135 A are barely
visible at the atlas scale. Other indices are included in the type
only when it appears necessary to call attention to the behavior of a
particular feature. Thus, for example, a CH index of 3.5 or greater
defines a CH star.
Since the relative strengths of the bands of carbon compounds
containing the isotopes 12C and 13C differ widely in different kinds
of carbon stars, an isotopic index, j, is used as a measure of
12C/13C. The index and its range from 1 to 5 agree with the usage of
Yamashita (1972AnTok..13..169Y). Normally this index is included in
the type only when j > 3.5 which defines an isotopic (or J) star. The
relation of the index to isotopic ratio is shown in Table 1 of
Barnbaum et al (1996ApJS..105..419B).
Notation, however, is not the only problem. Both theory and
observation have made it clear that an excess of carbon over oxygen
can appear in te atmospheres of stars of different masses or original
composition at different stages of their evolution. The new
classification attempts to assign different spectral types to members
of different spectroscopic groups.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table4.dat 102 119 Catalog of standard stars on the Revised MK System
sp/* . 107 The Spectra (Figures 1 & 2 of the paper)
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See also:
III/150 : Perkins Revised MK Types for the Cooler Stars (Keenan+ 1989)
Byte-by-byte description of file: table4.dat
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bytes format units label explanations
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1- 6 I6 --- HD ? HD number
7 A1 --- m_HD HD suffix
8- 18 A11 --- Name Other designation of the star
19- 26 A8 --- Sp *Spectral Classification: Type
27- 33 A7 --- C2 *Spectral Classification: C2 index
34- 41 A8 --- OtherIdx *Spectral Classification: Other index
42- 47 A6 --- LClass *Spectral Classification: Luminosity Class
49- 53 A5 --- Var Variability type from GCVS (see <II/205>)
56- 57 I2 h RAh Right Ascension (J2000) hours
59- 60 I2 min RAm Right Ascension (J2000) minutes
62- 63 I2 s RAs Right Ascension (J2000) seconds
65 A1 --- DE- Declination (J2000) sign
66- 67 I2 deg DEd Declination (J2000) degrees
69- 70 I2 arcmin DEm Declination (J2000) arcmin
72- 76 A5 --- notes *[a-d, ] Key to the notes
78- 89 A12 --- SpFile1 *Spectrum file name, in subdirectory "sp"
91-102 A12 --- SpFile2 *Red Spectrum file name, in subdirectory "sp"
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Note on Sp, C2, OtherIdx, LClass:
See below the "Spectral Classes" section
Note on notes:
a Instrument used for classification if not Kast or echelle on 1 m
telescope: photographic plates taken at CTIO for the southern stars
and at Lowell Observatory for the northern stars, all at similar
resolution to the Kast spectra
b Instrument used for classification if not Kast or echelle on 1 m
telescope: Hamilton echelle spectrograph on the 3m telescope from
the Barnbaum atlas
c The spectral types for these stars have been slightly revised from
those presented in Keenan 1993.
d Special Notes:
DY Per -- This star has shown sudden drops in brightness of the order of
3 magnitudes in several passbands. The type given here applies to
normal maximum light.
TW Hor = BS 917 = HD20234 -- This is Herschel's red star.
TU Tau, FU Aur, GK Ori, CoD-262983, UU Aur, GY Mon, AC Pup, V901 Sco,
SS Sgr -- No observations in the blue region.
RT Pup -- a marginal J star.
HD 100764 -- A peculiar star; see the description of the spectrum in
Fig. 1a of Barnbaum et al. 1996
West 079-02: abbreviated ID for:
Westerlund 079-02 -- This is the very red star seen by J. Herschel
very close to b Cru.
HD 148839 -- All spectral features very weak, as is generally true of
C-Hd stars. This star also has a high velocity.
V460 Cyg = HD 206570 = BS 8297 -- In older catalogs designated as DS Peg.
EU And, BM Gem, and V778 Cyg -- carbon stars with oxygen-rich dust.
Note on SpFile1, SpFile2:
See below the "File naming convention" section.
The spectra are 2-column tables described in the
"Description of spectra" section below.
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File naming convention for the spectral data files:
The spectral file names have been standardized to include the object
id, and the category of spectra. The '*' indicates the first part of
the file name (the ID).
*b.dat designates the blue order from 4000 to 5000 Angstroms.
*r.dat designates the red order from 5000 to 7000 Angstroms.
Each file is ASCII, two columns: wavelength and relative
flux units (not flux calibrated, see the paper for details)
Observed with 3m Telescope at Lick Obs with Kast spectrograph.
*e.dat designates wavelength range from 4000 to 7000 Angstroms.
Each file is ASCII, two columns: wavelength and normalized
units (not flux calibrated, see the paper for details)
Observed with 1m Telescope at Lick Observatory with the echelle
spectrograph.
*l.dat designates low resolution (~6 Angstroms) spectra.
Wavelength range from 4000 to ~7000 Angstroms.
Each file is ASCII, two columns: wavelength and relative
flux units (not flux calibrated, see the paper for details)
Observed with 1m Telescope at Lick Obs with 420 line/mm grism.
Description of spectra file: sp/*.dat
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Bytes Format Units Label Explanations
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1 - 9 F9.4 0.1nm lambda wavelength in Angstroem
11 -22 E12.5 --- flux relative or normalized flux
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Spectral Classes (Extract from the source reference):
R Stars: Type C-R ---
These are the warmest of the carbon stars, with the exception of a few
early CH stars. For most of them, the blue region is accessible to
observation, and atmospheric analyses have shown that they have nearly
solar abundances of the s-process elements. Hence, the usual ratios
(Sr II blend 4077/Fe 4063, 4071 and Fe, Y II 4376/Fe 4383) can be used
to assign luminosity classes.
J Stars: Type C-J---
Spectroscopic criteria for identifying J stars are conspicuous enough
to justify their being assigned to a different spectral class. In the
blue region the 12C13C band at 4752 can be clearly seen even at rather
low-resolution. For the red region Gordon (1967) defined a J star as
one in which the strength of the 6168 C2 band is half the strength of
6122, and we assign isotopic indices of j=4.5 or greater to the J
stars from this ratio and that of the CN bands at 6260 and 6206. (This
corresponds to a range of 12C/13C from a bout 2 to 6.) The J stars
include most of the stars that had been called either late R stars or
early N stars in various catalogs and were provisionally assigned to
the class C-RN by Keenan (1993PASP..105..905K).
N stars: Type C-N---
These are the best known carbon stars and the ones most easily
detected in infrared surveys. They are usually recognized by their
extreme redness with a strong blue depression often nearly
obliterating the spectrum below 4400 A. The isotopic bands are
consistently weak, while lines of s-process elements, particularly Ba,
are more enhanced than in R stars. The range of 12C/13C in N stars is
usually from 30 to 70.
Hydrogen-Deficient Carbon Stars: Type C-Hd---
We have classified here only the rare C-Hd stars that have nearly
constant light (3 stars in Table 4) and have omitted R CrB variables
whose spectra, like Miras, originate in different levels of their
atmosphere depending on phase. The C-Hd stars are easily recognized;
their spectral features resemble those of late G-type supergiants of
luminosity class Ib except for the presence of CN and C2 and the
almost complete absence of the G-band of CH and the lines of hydrogen.
It is unclear whether the weakly variable C-Hd stars are R CrB
variables at some other period of their history, or whether the two
groups follow different evolutionary paths.
CH Stars: Type C-H---
CH stars have long been recognized as Population II analogues of the R
stars. Their range of temperatures is similar, but their distribution
and high space motions relative to the sun place most of them in the
Galactic halo. Spectroscopically, they are recognized by the dominance
of CH bands in the blue region. Care must be taken, however, not to
rely on the strength of the G-band alone, for, in an early carbon star
with abundant carbon, it is nearly saturated. More useful are the
secondary P-branch head near 4342 A and the weakening of features such
as Ca 4226 by the network of faint bands of the same system.
Barium Stars: Type Ba---
These are generally of temperature sequence G8 to K2 with the ionic
lines of Ba, Sr, and Y greatly enhanced in their spectra, and for that
very reason it is difficult to assign individual spectroscopic
absolute magnitudes to them. Analyses of the high-resolution
spectrograms, however, are most consistent with a giant luminosity.
They probably have a considerable spread in luminosities, and a few
appear to be subgiants.
Historical Remarks:
The data were received by the ADC in June 1996. The file names were
changed to conform to the standards used.
References:
Barnbaum C., Stone R.P.S., and Keenan P.C. 1996, ApJS 105, 419
(1996ApJS..105..419B)
Gordon, C.P. 1967, PhD Thesis, Univ. Michigan
Keenan, P. C. 1993, PASP, 105, 905 (1993PASP..105..905K)
Keenan, P. C. and Morgan, W. W. 1941, ApJ, 94, 501 (1941ApJ....94..501K)
Yamashita, Y 1972, Ann. Tokyo Astron. Obs., 13, 169 (1972AnTok..13..169Y)
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(End) Paul Kuin (NASA/ADC) 24-Feb-1997