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A&A 375, 366--374 (2001)
DOI: 10.1051/0004­6361:20010814
c
# ESO 2001
Astronomy
&
Astrophysics
The stellar content of the Hamburg/ESO survey #
II. A large, homogeneously­selected sample of high latitude carbon stars
N. Christlieb 1 , P. J. Green 2 , L. Wisotzki 3 , and D. Reimers 1
1 Hamburger Sternwarte, Universit˜at Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
e­mail: dreimers@hs.uni­hamburg.de
2 Harvard­Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02140, USA
e­mail: pgreen@cfa.harvard.edu
3 Institut f˜ur Physik, Universit˜at Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany
e­mail: lutz@astro.physik.uni­potsdam.de
Received 26 April 2001 / Accepted 5 June 2001
Abstract. We present a sample of 403 faint high latitude carbon (FHLC) stars selected from the digitized objective
prism plates of the Hamburg/ESO Survey (HES). Because of the #15 š A spectral resolution and high signal­to­
noise ratio of the HES prism spectra, our automated procedure based on the detection of C2 and CN molecular
bands permits high­confidence identification of carbon stars without the need for follow­up spectroscopy. From
a set of 329 plates (87 % of the survey), covering 6 400 deg 2 to a magnitude limit of V # 16.5, we analyze the
selection e#ciency and e#ective surface area of the HES FHLC survey to date. The surface density of FHLC stars
that we detect (0.072 ± 0.005 deg -2 ) is 2--4 times higher than that of previous objective prism and CCD surveys
at high galactic latitude, even though those surveys claimed a limiting magnitude up to 1.5 magnitudes fainter.
This attests to the highest selection sensitivity yet achieved for these types of stars.
Key words. stars: carbon -- surveys -- Galaxy: halo
1. Introduction
Models of the chemical and dynamical properties of the
Galactic spheroid (the ``halo'') are still rather weakly con­
strained. In the grand scheme, did a monolithic proto­
galaxy undergo rapid collapse and enrichment (Eggen
et al. 1962), or did many smaller dwarf galaxies merge
together (Searle & Zinn 1978)? Both processes probably
contribute, since there is solid recent evidence of ongoing
mergers (Ibata et al. 1994; Majewski et al. 2000). Stars and
gas that are tidally stripped from accreting dwarf galaxies
remain aligned with the orbit of the satellite for timescales
comparable to the age of the Galaxy. Thus, a number of
tidal streams exist today whose phase­space signature can
Send o#print requests to: N. Christlieb,
e­mail: nchristlieb@hs.uni­hamburg.de
# Based on observations collected at the European Southern
Observatory, Chile (Proposal IDs 145.B­0009 and 63.L­0148).
Table A.1 is only available in electronic form at the CDS via
anonymous ftp to cdsarc.u­strasbg.fr (130.79.125.5) or
via
http://cdsweb.u­strasbg.fr/cgi­bin/qcat?J/A+A/375/366
constrain the stripping and merging events that contribute
to the present­day galactic halo (Johnston et al. 1999).
An important goal of astronomy in this century is to
measure and model the potential of the Milky Way us­
ing halo stars as tracers. To simultaneously disentangle
the remnants of disrupted satellites requires full knowl­
edge of the angular positions, proper motions, radial ve­
locities, and distances of a large number of such stars. But
first, a large sample of distant halo stars must be amassed.
Intrinsically bright stars visible to large galactocentric dis­
tances (10--100 kpc) provide the best opportunity. Because
they are readily recognizable from their strong C 2 and CN
absorption bands, and because they were thought without
exception to be giants, faint C stars have been sought as
excellent tracers of the outer halo.
Faint high galactic latitude carbon (FHLC) stars
have been sought in prior objective prism surveys (e.g.,
Sanduleak & Pesch 1988; MacAlpine & Lewis 1978) and
in the CCD survey of Green et al. (1994). Objective­
prism photography with wide­field Schmidt telescopes
has yielded low­dispersion spectra for thousands of ob­
jects over substantial portions of the sky, but not a large

N. Christlieb et al.: Carbon stars from the Hamburg/ESO survey 367
number of carbon stars. Fewer than 1% of the 6000 stars in
Stephenson's (1989) catalogue are the faint, high­latitude
carbon (FHLC) stars (V > 13, |b| > 40 # ) most useful as
dynamical probes of the outer halo. The two most prolific
sources of published FHLC stars, the Case low­dispersion
survey (CLS; Sanduleak & Pesch 1988) and the University
of Michigan -- Cerro Tololo survey (UM; MacAlpine &
Williams 1981) appear to probe to about V = 16 and have
provided about 30 FHLC stars. Emission­line objects, not
FHLC stars, were the primary goal of these photographic
surveys, and known FHLC stars were not examined to
help predefine selection criteria or estimate completeness.
The surface density of FHLC stars from objective­prism
surveys is low, about one per 50 deg 2 to V # 16. At high
galactic latitudes, mostly warm carbon stars are found --
CH stars, and possibly some R stars. However, color selec­
tion of very red stars at high latitude also reveals a small
number (one per 200 deg 2 to R # 16) of classical inter­
mediate, age AGB carbon (AGBC) stars (Totten & Irwin
1998). Margon et al. (2000) recently reported the discov­
ery of more than 30 new FHLC stars in the commissioning
data of the Sloan Digitized Sky Survey (SDSS), which may
eventually provide the majority of known FHLC stars.
In this paper, we describe our use of the
Hamburg/ESO survey (HES; Wisotzki et al. 1996;
Reimers & Wisotzki 1997; Wisotzki et al. 2000) to
greatly augment the number of known FHLC stars. The
HES is an objective­prism survey designed to select
bright (12.5 # B J # 17.5) quasars in the southern
extragalactic sky (# < +2.5 # ; |b| # 30 # ). It is based
on IIIa­J plates taken with the 1 m ESO Schmidt tele­
scope and its 4 # prism, yielding a wavelength range
of 3200 š A < # < 5200 š A and a seeing­limited spectral
resolution of typically 15 š A at H#. This resolution makes
possible the identification of carbon stars with high
confidence without follow­up slit spectroscopy, based on
their strong C 2 and CN molecular bands (cf. Fig. 1).
Since carbon can reach the surface of an isolated star
only in late evolutionary stages, it has long been assumed
that all carbon (C) stars are giants. Due to their high lu­
minosity (MR # -3.5), it is possible to detect the red
AGBC stars at large distances: Brewer et al. (1996) have
identified C stars even in the local group galaxy M 31.
The more typical FHLC stars such as CH giants, with
0 < M V < -2.5, can be detected to #60 kpc in sensitive
photographic surveys. However, the long­held assumption
that all C stars are giants has charged. Trigonometric par­
allax measurements for the carbon star G77--61 (Dahn
et al. 1977) showed that this star lies close to the main
sequence (M V # +10). For years, the dwarf carbon (dC)
star phenomenon was assumed to be extremely rare until
many new dCs were discovered in the early 1990s (Green
et al. 1991; Green et al. 1992; Warren et al. 1993; Heber
et al. 1993; Liebert et al. 1994). Discovery of so many dCs,
and the remarkable similarity of their spectra to those of
C giants means that care must be taken to distinguish
dwarfs from giants in FHLC star samples intended for
distant halo studies (Green et al. 1992). We are there­
Fig. 1. HES objective prism spectrum of the R­type carbon
star CGCS 2954 (Stephenson 1989), illustrating the positions
of continuum (black) and band (grey) bandpasses defining the
C2 (high boxes) and CN (flat boxes) line indices used for selec­
tion in the HES. The abscissa is density above di#use sky back­
ground in arbitrary units. Note that wavelength is decreasing
towards the right. The sharp drop of the spectra at # # 5400 š A
is due to the IIIa­J emulsion sensitivity cuto#.
fore undertaking a two­part investigation. In the current
paper, we describe our automated selection of C stars in
the HES, and we present a large, uniformly selected, and
flux­limited sample of FHLC stars. We complement this
sample in an upcoming work with recent epoch astrom­
etry, to measure proper motions for as many objects as
possible, and thereby separate the dCs from C giants.
We note that carbon­enhanced, metal­poor stars may
be among the FHLC stars presented here. It was recog­
nized by several authors that the fraction of such stars
among metal­poor stars rises with decreasing metallicity,
reaching #25% for stars with [Fe/H] < -3.0, and that
carbon overabundances are as high as [C/Fe] = +2.0 dex
(e.g., Norris et al. 1997; Rossi et al. 1999). 12 C/ 13 C iso­
tope measurements of a larger sample of such stars would
help to identify the carbon production site(s) at work.
2. Carbon star selection
The full HES database consists of #10 million extracted,
wavelength calibrated spectra. The input catalog for ex­
traction of objective­prism spectra is generated by using
the Digitized Sky Survey I (DSS I). An astrometric trans­
formation between DSS I plates and HES plates yields,
for each object in the input catalog, the location of its
spectrum on the relevant HES plate, and provides a wave­
length calibration zero point (Wisotzki et al. 2000).
Carbon stars can be identified in the HES database
by their strong C 2 and CN bands. We select carbon star
candidates when the mean signal­to­noise ratio (S/N)
in the relevant wavelength range is >5 per pixel and

368 N. Christlieb et al.: Carbon stars from the Hamburg/ESO survey
Table 1. Wavelengths of passbands used for computation of C
band indices in the HES. ``cont'' = continuum; ``flux'' = feature
passband.
Use for band index
Passband
C2 5165 C2 4737 CN 4216 CN 3883
5190--5240 š A cont
5060--5150 š A flux
4800--4970 š A cont cont
4620--4730 š A flux
4460--4560 š A cont
4210--4270 š A cont
4130--4180 š A flux
3830--3890 š A flux
3610--3740 š A cont
Table 2. Carbon star selection criteria. The maximum allowed
band index values correspond to an integrated density of zero in
the feature passbands. That is, larger band indices can only be
due to artifacts, e.g. scratches, causing photographic densities
(above skybackground) <0. Stars are selected if both of their
C2 indices or both of their CN indices fall into the indicated
ranges.
Feature Index range [ š A]
C2 # 5165 [10,91]
C2 # 4737 [15,114]
CN # 4216 [2,56]
CN # 3883 [13,55]
both of the C 2 bands ## 5165, 4737, or both of the CN
bands ## 4216, 3883 are stronger than a selection thresh­
old. Band strengths are measured by means of line in­
dices -- ratios of the mean photographic densities in the
carbon molecular absorption features and the continuum
bandpasses shown in Fig. 1, and listed in Table 1. The
use of pairs of indices prevents confusion with plate arti­
facts, e.g., scratches. It is very unlikely that two such arti­
facts are present at the positions of two molecular bands.
Selection boxes in the I (C 2 # 5165) versus I (C 2 # 4737)
and I (CN # 4216) versus I (CN # 3883) planes were cho­
sen well­separated from the dense locus of ``normal'' stars
(see Fig. 2). The selection criteria are listed in Table 2.
Carbon stars can be distinguished reliably from other
late type stars, e.g. M or S stars, even if only weak C
bands are present in their spectra (cf. Fig. 3). Other po­
tential sample contaminators are white dwarfs of type DQ,
which show carbon molecular bands. However, since the
latter usually have a much bluer continuum (see Fig. B.1),
they can easily be recognized by visual inspection of the
spectra, and by their U -B color. McCook & Sion (1999)
list 49 DQs, of which 30 have an available U -B measure­
ment. The average U -B of those is -0.58, i.e., #1.5mag
Fig. 2. Selection of carbon stars in the I (C2 # 5165) versus
I (C2 # 4737) plane. Band strengths are measured by line in­
dices. Dots: all spectra on a randomly chosen HES plate; boxes:
test sample of known FHLC stars present on HES plates (see
Table 4); dashed box -- selection region. Spectra in which only
one high C2 band index value was measured su#er either from
an overlapping spectrum, or from a plate artifact. The selec­
tion in the I (CN # 4216) versus I (CN # 3883) plane is done
analogously. The two test sample objects outside the selection
box are CGCS 525 and CGCS 3180. They are selected by CN
band indices (see Table 4).
Fig. 3. Comparison of HES spectra of the C star
HE 1524-0210, exhibiting a weak C band only, with two M
stars. The abscissa is the same as in Fig. 1.
away from the average U - B of the HES C star sample.
Our U-B colors are measured directly from the HES spec­
tra with a mean accuracy of #U-B = 0.09 mag (Christlieb
et al. 2001 hereafter Paper I). The average U -B of HES
C stars is #0.9, more than 90% have U - B > 0.5, and
there is no C star of U -B < 0 in the HES sample. While
four (i.e., 13%) of the 30 DQs with U - B in McCook
& Sion (1999) have U - B > 0.0, the pressure­broadened

N. Christlieb et al.: Carbon stars from the Hamburg/ESO survey 369
Fig. 4. V magnitude and B-V distribution of the HES FHLC sample. B-V was derived from HES spectra with the procedures
described in Paper I.
features of DQs are easily distinguished by visual inspec­
tion of the carbon bands (see Fig. B.1).
With a rough estimate of their surface density, we can
quantify an upper limit for the contamination of the HES
C star sample by ``red'' (U - B > 0.0) DQs. First of all,
we have to take into account that the ratio of northern
hemisphere to southern hemisphere DQs is unbalanced in
McCook & Sion (1999), as much as is the total catalog.
This is because the southern hemisphere so far has been
surveyed less extensively for white dwarfs. Assuming that
the northern hemisphere sample of DQs is complete, we
derive a surface density of 9 DQs brighter than V = 16.5
in 20 000deg 2 , i.e. 4.5 â 10 -4 deg -2 . Hence, the surface
density of U - B > 0.0 DQs is 5.9 â 10 -5 deg -2 , and
we expect 0.44 DQs to be present on all 329 plates cur­
rently used for the exploitation of the stellar content of
the HES. Therefore, even if we assume that the sample
of DQs known so far is incomplete by a factor of 2, we
statistically expect less than 1 DQ to be present in the
HES C star sample.
On the 329 HES plates (e#ective area 6 400deg 2 ) we
found 403 FHLCs. 90 of them were selected by C 2 band
indices only, 171 by CN band indices only, and 144 by
C 2 and CN indices. The V and B - V distributions are
displayed in Fig. 4. The faintest objects have V # 16.5,
and the most distant objects reach #35kpc (cf. Fig. 5),
assuming they are all giants with M V = -1mag.
3. Testing the automated selection
We tested the automated selection extensively and by var­
ious methods. In Sect. 3.1 we investigate the selection e#­
ciency. In Sect. 3.2 we derive plate­by­plate selection prob­
abilities for halo dCs on HES plates by simulations. The
results of tests with ``real'' objects are given in Sect. 3.3.
Fig. 5. Distance distribution of the 393 HES FHLCs with
available V magnitudes, assuming that they are all giants with
MV = -1mag.
3.1. Selection e#ciency
An important criterion for the evaluation of the quality
of a selection algorithm is the selection e#ciency, i.e.
the fraction of desired stars in the raw candidate sam­
ple. Table 3 summarizes the results. Our selection is very
e#cient. The low fraction of artifacts demonstrates that
the usage of pairs of C 2 bands and CN bands indeed very
reliably excludes artifacts from selection. However, a con­
siderable number of overlapping spectra (overlaps) are se­
lected. Overlaps are detected by an automatic overlap de­
tection algorithm in the HES, using the direct plate data
of the DSS I. It appears that our carbon star selection
technique is very sensitive in finding the small number of
overlaps not detected by the automatic algorithm.
3.2. Decrease of selection probability for halo dCs
In the HES, some care must be taken when objects with
large proper motion are selected. This is because the in­
put catalog for extraction of objective prism spectra is

370 N. Christlieb et al.: Carbon stars from the Hamburg/ESO survey
Table 3. Selection e#ciency for C stars in the HES.
UNID = probable C stars with weak C bands, OVL = over­
lapping spectra, ART = artifacts, NOIS = very noisy spectra,
SAT = saturated spectra. The raw candidate reduction factor
is the factor by which the selection algorithm reduces the total
set of 3 437 630 overlap­free HES spectra with S/N > 5 present
on 329 HES plates.
Raw candidate
reduction factor
1/2900
C stars 31.6%
UNID 7.0%
OVL 29.2%
ART 8.7%
NOIS 3.8%
SAT 15.6%
generated by using the DSS I. The dispersion direction
of the HES spectra is along declination. Therefore, large
proper motions and/or large epoch di#erences between
HES and DSS I plates (13.5 years on average) may re­
sult in an o#set of the wavelength calibration zero point,
leading to smaller C band values, and/or non­detection of
objects in the HES, if µ## t HES­DSS I # 4 ## , i.e., > 3 pix­
els. O#sets of µ## t HES­DSS I < 4 ## can be recovered by the
spectrum extraction algorithm. Note however that proper
motions of a typical halo object (== 0 km s -1 ;
# 200km s -1 ) result in #2â larger o#sets along dec­
lination than in R.A., since the galactic plane is tilted by
62.6 # with respect to the equatorial coordinate system.
In order to estimate how many dCs are expected to
be missed in our survey due to the epoch di#erence prob­
lem, we carried out a simulation study in which the plate­
by­plate selection function for halo dCs was determined.
The simulation is similar to that described in Green et al.
(1992). We employ a sample of simulated dCs with halo
kinematics, as given by Norris (1986). For the solar neigh­
borhood, he gives
v rot = 37 ± 10 km s -1 ##= -187 km s -1 , (1)
and he determined the velocity ellipsoid to be
# u = 131 ± 6 km s -1 (2)
# v = 106 ± 6 km s -1 (3)
#w = 85 ± 6 km s -1 . (4)
In each simulation we constructed 100 random velocity
vectors (u, v, w), with components following Gaussian dis­
tributions according to the above parameters. These veloc­
ity vectors were each applied to stars located at the center
of the plate under investigation, and converted to proper
motions assuming distances d. These were computed from
the apparent V magnitude distribution of a sample of
86 C stars without significant p.m. (see Fig. 4), and as­
suming M V = +10 for dwarf carbon stars. This yields
86 · 100 = 8600 simulated stars. We compute the position
of the star after the time #tHES­DSS I , the epoch di#erence
between DSS I and HES plate, and derive proper motions
µ# , µ # from the position di#erences. We then select the
subsample of the 8600 stars with µ## t HES­DSS I < 4 ## .
As a test sample for an investigation of the depen­
dence of the selection probability on µ # #t, we used a sam­
ple of 78 C stars from 44 HES plates without significant
p.m., as measured in an follow­up campaign carried out
in April at ESO, using the Wide Field Imager attached to
the MPG/ESO 2.2 m telescope (Christlieb et al., in prepa­
ration). The stars were shifted in one pixel (=1 ## . 35) steps
through the range -700 µm < x < +700µm, correspond­
ing to -47 ## . 25 < µ # #t < 47 ## . 25. At each shift step, the
selection algorithms were applied.
By applying the selection probability (a function
of µ # #t) to the subsample of the 8600 stars with
µ## t HES­DSS I < 4 ## , we determine the fraction of stars
which would be detected in the HES and selected by our
selection algorithm. On the 329 stellar HES plates, 21.4%
of the simulated halo dCs are detected and selected.
Green et al. (1992) found that 13% of their C stars are
dwarfs. Applying this estimate to our sample, and taking
into account that we find only #20% of the dCs detectable
on the HES plates, we estimate that 10--15 out of our 403
FHLCs are dCs. However, since our sample is biased to
low­p.m., it is likely that not all of these can be proven
to be dCs by their transverse velocity. Based on our sim­
ulations we estimate that additional #40 dCs would be
detectable on the HES plates, but are currently missed
due to the epoch di#erence problem. This incompleteness
will be addressed in a later paper focusing on dC stars in
the HES.
3.3. Tests with known C stars
We also compiled a test sample of known dwarf and giant
C stars present on HES plates (see Table 4). We took all
three dCs in the southern hemisphere listed by Deutsch
(1994), i.e. LHS 1075, G77--61, and KA 2. The (possible)
dCs of Warren et al. (1993), having B J > 20, unfortu­
nately are by far too faint to be detectable on HES plates.
Cross­identification with the C star lists of Slettebak et al.
(1969), Stephenson (1989), Bothun et al. (1991), and
Totten & Irwin (1998), yielded 21 stars. Another 6 spec­
tra were produced from slit spectra with the procedures
described in Paper I.
In our test, all 21 stars not known as dwarfs were se­
lected either by their strong C 2 bands, or their CN bands.
The simulated spectra were also all selected. Of the three
dCs, one (KA 2) was selected, and the other two (G77--61,
LHS 1075) not. From these results we conclude that our
sample of giant C stars and dwarfs with low p.m. (e.g.
dCs belonging to the disk population) is highly complete.
From the small number of dCs in our test sample we are
not able to draw any definitive conclusions, but our re­
sults suggest that only a minor fraction of the halo dCs

N. Christlieb et al.: Carbon stars from the Hamburg/ESO survey 371
Table 4. Test sample of dwarf and giant C stars present on HES plates. Sources: BEM91 = Bothun et al. (1991), D94 = Deutsch
(1994), S89 = Stephenson (1989), SKB69 = Slettebak et al. (1969), TI98 = Totten & Irwin (1998). Stars marked with TI98 have
been recently reported by Totten et al. (2000) to have no significant p.m. Therefore, we list them with (µ## t, µ # # t) = (0, 0).
KA 2 was not selected by CN bands, using its real spectrum, but it was selected in the course of our simulations. This is because
the S/N at the position of the CN bands is too low in the former spectrum, and e#ectively infinite in the simulated spectrum.
Selected by
Name HE Name BJ B - V µ## t µ # # t
C2 CN All Source
CGCS 39 HE 0017+0055 (sat.) 1 1 1 S89
SKB 2 HE 0039-2635 13.1 1.1 1 1 1 SKB69
BEM91 23 HE 0100-1619 15.9 1.5 1 0 1 BEM91
CGCS 177 HE 0106-2837 13.8 2.1 1 0 1 S89
SKB 5 HE 0111-1346 13.3 1.4 1 1 1 SKB69
0207-0211 HE 0207-0211 15.5 2.2 0.0 0.0 1 0 1 TI98
BEM91 08 HE 0228-0256 16.2 2.0 1 0 1 BEM91
CGCS 525 HE 0330-2815 13.8 1.5 0 1 1 S89
CGCS 935 HE 0521-3425 13.0 1.3 1 1 1 S89
0915-0327 HE 0915-0327 14.5 2.3 0.0 0.0 1 0 1 TI98
1019-1136 HE 1019-1136 15.2 1.8 0.0 0.0 1 0 1 TI98
CGCS 2954 HE 1104-0957 (sat.) 1 1 1 S89
CGCS 3180 HE 1207-3156 12.8 1.2 0 1 1 S89
CGCS 3274 HE 1238-0836 (sat.) 1 1 1 S89
1254-1130 HE 1254-1130 16.1 2.2 0.0 0.0 1 0 1 TI98
1339-0700 HE 1339-0700 15.0 1.7 0.0 0.0 1 0 1 TI98
1442-0058 HE 1442-0058 17.8 2.2 0.0 0.0 1 0 1 TI98
CGCS 5435 HE 2144-1832 12.6 1.4 0 1 1 S89
CGCS 5549 HE 2200-1652 12.3 0.9 1 1 1 S89
2213-0017 HE 2213-0017 16.4 2.4 0.0 0.0 1 0 1 TI98
2225-1401 HE 2225-1401 16.5 2.9 0.0 0.0 1 0 1 TI98
CLS 50 0.0 0.0 1 0 -- Simul.
CLS 31 0.0 0.0 1 1 -- Simul.
CLS 54 0.0 0.0 1 1 -- Simul.
KA 2 0.0 0.0 1 1 -- Simul.
B1509-0902 0.0 0.0 1 1 -- Simul.
UM 515 0.0 0.0 1 0 -- Simul.
LHS 1075 HE 0023-1935 16.1 1.4 -0 ## . 24 -10 ## . 0 0 0 0 D94
KA 2 HE 1116-1628 16.6 1.3 -0 ## . 21 0 ## . 24 1 0 1 D94
G77­61 HE 0330+0148 15.0 1.4 1 ## . 9 -7 ## . 5 0 0 0 D94
are detected in the HES. This is consistent with #20% of
the simulated halo dCs being found (see Sect. 3.2).
4. The surface density of C stars
In an e#ective area of 6400 deg 2 (329 of 380 the HES
plates; for a description of how the e#ective area is es­
timated see Wisotzki et al. 2000), we have isolated a total
of 403 C stars. A straightforward estimate of the surface
density of FHLC stars we detect with the HES is hence
obtained from the ratio of these two numbers, yielding
0.063 deg -2 . However, one has to take into account that
the e#ective area accessible on average for each object de­
pends on its brightness, since the HES limiting magnitude
varies from plate to plate. For example, the e#ective area

372 N. Christlieb et al.: Carbon stars from the Hamburg/ESO survey
for an object as faint as B = 17.0 is only 73% of the overall
survey area, mainly because only 254 of the contributing
plates reach this magnitude (see Fig. 2 in Wisotzki et al.
2000). An additional brightness dependence is caused by
the fact that faint objects are more easily a#ected by over­
lapping spectra than bright objects. We therefore deter­
mine the FHLC surface density as follows:
surface density =
403
# i=1
1
e#area(B J ) i
, (5)
where e#area(B J ) was determined as described in
Wisotzki et al. (2000). We obtain a FHLC surface den­
sity of 0.072±0.005deg -2 on the 329 HES plates we used.
5. Discussion and conclusions
In an e#ective area of 6400 deg 2 we have isolated a total
of 403 C stars. Our e#orts have thus already increased the
number of known FHLC stars by a factor of nearly five.
We find almost quadruple the surface density of car­
bon stars compared to the surveys summarized by Green
et al. (1994). Since those previous surveys claimed lim­
iting magnitudes about 1.5 mag fainter than the HES,
this highlights the greatly enhanced selection sensitivity
of FHLC stars in the HES, which is more sensitive to a
variety of C 2 or CN molecular absorption band strengths.
Automated selection techniques may be superior to vis­
ible inspection of objective­prism spectra with binocular
microscopes, as done e.g. in the survey of Sanduleak &
Pesch (1988). Photometric surveys for C stars have gener­
ally selected red objects only, which preferentially selects
mostly the much less common high latitude AGB stars.
Margon et al. (2000) report a FHLC star surface density
of ``at least'' 0.04 deg -2 in the SDSS. This is still almost a
factor of 2 below our value, and again, the SDSS is much
deeper than the HES (r # < 19.5).
Due to an average epoch di#erence of 13.5 years
between DSS I and HES plates, we expect to detect and
select only #20% of the halo dCs that we could detect
and select if direct plates had been taken simultaneously
with the HES plates. Our simulations indicate that 10--15
out of the 403 FHLCs published in this paper are dCs.
Note, however, that this number is uncertain, because
the kinematics of halo dCs is not precisely known. We
estimate that an additional #40 dCs are detectable on
the HES plates, but are currently missed due to the epoch
di#erence problem. We are extending the current sample
to include proper­motion corrected input catalogs for the
extraction of HES spectra, to find all dCs, and other ob­
jects that can have large proper motions, like halo white
dwarfs.
Acknowledgements. We thank D. Koester for providing model
spectra of DQs, and C. Fechner for technical support in prepar­
ing this article. This work was partly supported by Deutsche
Forschungsgemeinschaft under grant Re 353/40. P. J. G. ac­
knowledges support through NASA Contract NAS8­39073
(ASC).
Appendix A: The HES FHLC sample
In Table A.1 we list the sample of 403 HES FHLC stars
described in this paper. The table is made available only
electronically. It contains the following columns:
hename HE designation
ra2000 RA at equinox 2000.0, derived from DSS I
dec2000 Declination at equinox 2000.0, derived
from DSS I
field ESO­SERC field number
plate HES plate number
q Plate quarter
objtyp Object type (stars/bright/ext)
B J B J magnitude
V V magnitude
B - V B - V magnitude, derived from HES spectra
U -B U -B magnitude, derived from HES spectra
C2idx1 Band index of C 2 5165 š A
C2idx2 Band index of C 2 4737 š A
CNidx1 Band index of CN 4216 š A
CNidx3 Band index of CN 3883 š A
selC2 C 2 band index selection flag
selCN CN band index selection flag
B J magnitudes are accurate to better than ±0.2mag,
including zero point errors (Wisotzki et al. 2000). V
magnitudes were derived by the procedures described
in Paper I. The object types ``stars'', ``bright'' and
``ext'' refer to point sources, sources above a saturation
threshold, and sources detected as extended in DSS I
images, respectively. We do not list V , B - V and U -B
for saturated objects, because our color calibrations are
not valid for them.

N. Christlieb et al.: Carbon stars from the Hamburg/ESO survey 373
Appendix B: HES example spectra of C stars
Fig. B.1. HES spectra of a representative sample of seven C stars, displaying a variety of C2 and CN band strengths. The
spectra of KA 2 and CLS 31 were converted to objective­prism spectra from slit spectra with the procedures described in
Paper I. For comparison, the spectrum of a DQ white dwarf with T e# = 6 500 K and C/He = 10 -6 is shown in the lower right
panel. That star has U -B = -0.6.

374 N. Christlieb et al.: Carbon stars from the Hamburg/ESO survey
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