Observations towards early-type stars in the ESO-POP survey: II --
  searches for intermediate and high velocity clouds
Mon. Not. R. Astron. Soc. 000, 000–000 () Printed 8 September 2021 (MN LaTEX style file v2.2)
Observations towards early-type stars in the ESO-POP
survey: II – searches for intermediate and high velocity
clouds
J. V. Smoker1⋆, I. Hunter1, P. M. W Kalberla2, F. P. Keenan1, R. Morras3,
R. Hanuschik4, H. M. A. Thompson1, D. Silva5, E. Bajaja3, W. G. L Poppel3,
M. Arnal3
1Astrophysics Research Centre, Department of Physics and Astronomy, Queen’s University Belfast,
Belfast, BT7 1NN, U.K.
2Argelander-Institut für Astronomie, Universität †, Auf dem Hügel 71, 53121 Bonn, Germany
3Instituto Argentino de Radioastronomia, Casilla de correo 5, Villa Elisa, Argentina.
4European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei Mnchen, Germany
5TMT Observatory Scientist, AURA/Thirty Meter Telescope, 2636 East Washington Blvd., Pasadena, CA 91107, U.S.A.
Accepted Received in original form
ABSTRACT
We present Ca ii K and Ti ii optical spectra of early-type stars taken mainly from
the UVES Paranal Observatory Project, plus H i 21-cm spectra from the Vila-Elisa
and Leiden-Dwingeloo surveys, which are employed to obtain distances to intermediate
and high velocity clouds (IHVCs). H i emission at a velocity of –117 km s−1 towards the
sightline HD30677 (l, b=190.2◦,–22.2◦) with column density ∼1.7×1019 cm−2 has no
corresponding Ca ii K absorption in the UVES spectrum, which has a signal-to-noise
(S/N) ratio of 610 per resolution element. The star has a spectroscopically determined
distance of 2.7-kpc, and hence sets this as a firm lower distance limit towards Anti-
Centre cloud ACII. Towards another sightline (HD 46185 with l, b=222.0◦, –10.1◦),
H i at a velocity of +122 km s−1 and column density of 1.2×1019 cm−2 is seen. The
corresponding Ca ii K spectrum has a S/N = 780, although no absorption is observed
at the cloud velocity. This similarly places a firm lower distance limit of 2.9-kpc towards
this parcel of gas that may be an intermediate velocity cloud. The lack of intermediate
velocity (IV) Ca ii absorption towards HD196426 (l, b=45.8◦,–23.3◦) at a S/N = 500
reinforces a lower distance limit of ∼700-pc towards this part of Complex gp, where
the H i column density is 1.1×1019 cm−2 and velocity is +78 km s−1. Additionally,
no IV Ca ii is seen in absorption in the spectrum of HD19445, which is strong in H i
with a column density of 8×1019 cm−2 at a velocity of ∼–42 km s−1, placing a firm
although uninteresting lower distance limit of 39-pc to this part of IV South. Finally,
no HV Ca ii K absorption is seen towards HD115363 (l, b=306.0◦,–1.0◦) at a S/N =
410, placing a lower distance of ∼3.2-kpc towards the HVC gas at velocity of ∼+224
km s−1 and H i column density of 5.2×1019 cm−2. This gas is in the same region of
the sky as complex WE (Wakker 2001), but at higher velocities. The non-detection
of Ca ii K absorption sets a lower distance of ∼3.2-kpc towards the HVC, which is
unsurprising if this feature is indeed related to the Magellanic System.
Key words: ISM: general – ISM: clouds – ISM: abundances – ISM: structure – stars:
early-type
⋆ email: j.smoker@qub.ac.uk. Based on observations taken at UT2, Kueyen, Cerro Paranal,
c© RAS
http://arxiv.org/abs/0704.1315v1
2 J. Smoker et al.
1 INTRODUCTION
The Paranal Observatory Project (POP; Bagnulo et al.
2003)1 provides a wealth of high-resolution (R ∼80,000) op-
tical spectra towards stars mainly in the Galactic disc that
can be used to study subjects such as stellar properties,
kinematics and the interstellar medium. In a previous paper
(Hunter et al. 2006, hereafter Paper i) we used a sample of
early-type stars in the POP survey in order to investigate
the interstellar medium in the Na i UV, Ti ii and Ca ii K
lines, using the stars as light sources to probe the material
between the star and the Earth. Because these O- and B-
type stars are often fast rotators with weak metal lines, they
are ideal for probing narrow interstellar features.
In the current paper, we use mainly Ca ii and Ti ii spec-
tra in order to search for Intermediate and High Velocity
Clouds (IHVCs) towards the sightlines investigated in Pa-
per i, plus some additional sightlines for which high S/N
data have now become available. Our aim is to improve the
distances to these still enigmatic objects by searching for
IHVCs in the Villa-Elisa Southern Sky 21-cm H i Survey
(Bajaja et al. 2005) or Northern Hemisphere counterpart,
the Leiden-Dwingeloo Survey (Hartmann & Burton 1997),
and subsequently searching for corresponding absorption in
the Ca ii or Ti ii optical spectra. Although the distance to
many Intermediate Velocity Clouds (IVCs) is known (e.g.
Kuntz & Danly 1996 and references therein), to date there
are very few uncontroversial upper distance limits towards
High Velocity Clouds (HVCs). Indeed towards the main
complexes there are currently only three uncontroversial de-
tections; towards Complex A (van Woerden et al. 1999),
Complex M (Danly, Albert & Kuntz 1993) and Complex
WB (Thom et al. 2006). Hence it is still unclear whether
many of these objects are associated with the Galaxy, for
example linked with a Galactic fountain with distances of
≈ 10 kpc (e.g. Quilis & Moore 2001), or are failed dwarf
galaxies with distances of several hundred kpc (e.g. Braun
& Burton 1999). Clearly, as the sample stars in the POP sur-
vey were not chosen a−priori to intersect with known IHVC
complexes, the majority of the sightlines do not cross known
IHVCs. However, serendipitously a few of the sightlines in-
tersect complexes and are studied in the current paper. In
addition to the POP data, we include spectra from two re-
cent high spectral resolution observing runs taken using the
échelle spectrometer FEROS, plus further UVES observa-
tions whose primary aim was to obtain spectra for a stellar
library but that are also at high S/N and cover the Ca ii K
line (Silva et al. 2007).
The current work complements our previous studies in
which we obtained improved distance limits towards IVC
complexes gp and K and HVC complexes C, WA-WB, WE,
and H (Smoker et al. 2004, 2006), by searching for absorp-
tion in high-resolution spectra of mainly B-type stars taken
Chile, ESO DDT programme 266.D-5655(A), UVES Paranal
Observatory Project, with additional observations from 071.B-
0529(A), 072.B-0585(A), 073.B-0607(A), 074.B-0639(A), 076.D-
0018(A) and 077.D-0025(A).
† Founded by merging of the Sternwarte, Radioastronomisches
Institut, and Institut für Astrophysik und Extraterrestrische
Forschung der Universität Bonn.
1 See also http://sc.eso.org/santiago/uvespop/
from the Edinburgh-Cape (Stobie et al. 1997) and Palomar-
Green Surveys (Green, Schmidt & Liebert 1986). In partic-
ular the current sightlines intersect IVC Complex K with
previous distance limit of 0.7–6.8-kpc (de Boer & Savage
1983, Smoker 2006), the Anti-Centre clouds with previous
distance limit of >0.4-kpc (Tamanaha 1996) and Complex
WE with distance limit <12.8-kpc (Sembach et al. 1991).
Finally, one of our current sightlines lies towards the M15
intermediate velocity cloud lying in the IVC complex gp.
This cloud has been studied extensively, in the optical to
determine variations in velocity and equivalent width varia-
tions (Lehner et al. 1999, Meyer & Lauroesch 1999, Smoker
et al. 2002), plus in the H i, infrared and Hα (Kennedy et al.
1998, Smoker et al. 2002). An improvement in the current
distance limit of 0.8–4.3 kpc (Wakker 2001 and references
therein) would be very useful to more accurately define the
cloud parameters such as cloudlet sizes and densities and to
provide clues to the high metalicity of this IVC (Little et al.
1994).
Sect. 2 describes the sample, provides a table noting
the cases where the current sightlines cross known IHVC
complexes plus new observations not previously described
in Paper I, and shows the optical and H i spectra. Sect. 3
gives the main results, including the cases where the cur-
rent optical sightlines intersect IHVCs and an attempt to
obtain improved distance limits towards these clouds. Sect.
4 discusses the most interesting lower limits to IHVCS and
finally Sect. 5 gives a summary of the main findings.
2 THE SAMPLE, OBSERVATIONS AND DATA
REDUCTION
The list of sample stars is shown in Table 1. The table in-
cludes all stars for which new observations were taken, plus
sightlines that lie towards IHVC complexes that are dis-
cussed in Sect. 3.2, but does not include the POP paper i
objects that have no IHVC detection. Further information
concerning the POP objects is given in Paper i. They are all
O- and B-type stars with 2.3< mv <7.9 mag. For these POP
optical spectroscopic data, we used the on-line versions of re-
duced data from the Paranal Observatory Project (Bagnulo
et al. 2003). These are spectra taken with the UVES échelle
spectrometer mounted on the 8.2-m Kueyen telescope at the
Very Large Telescope at a spectral resolution of 80,000 or
3.75 kms−1 and S/N pixel−1 ranging from 190–770. In this
paper we concern ourselves with the Ca iiK (λair=3933.66Å)
and Ti ii (λair=3383.76Å) species only. A further 9 stars were
observed with FEROS on the ESO 2.2-m on La Silla during
observing sessions in Oct. 2005 (FER1 in Table 1) and May
2006 (FER2 in Table 1). These stars are all B-type post-
AGB stars or Planetary Nebulae and have fainter magni-
tudes than the POP stars, with 9.4< mv < 13.3 mag. The
S/N ratios pixel−1 at Ca ii K range from ∼40–120 and the
resolution is R=48,000. The spectra shown in this paper are
the quick-look pipeline products. As a check of their relia-
bility, during each of the FEROS runs a bright B-type star
from the POP survey was observed and the velocities and
equivalent widths of some of the absorption lines were com-
pared between the two datasets. Agreement was found to
be excellent. Finally, 12 stars were taken from the dataset
of Silva et al. (2007; S07 in Table 1). These are UVES spec-
c© RAS, MNRAS 000, 000–000
http://sc.eso.org/santiago/uvespop/
IVCs and HVCs towards POP early-type stars 3
tra of early-type stars with 5.9 < mV < 11.3 mag., observed
at a spectral resolution of ∼40,000 with S/N = 100–620
pixel−1, and were reduced using the ESO pipeline (MIDAS
context) with calibrations taken the morning after the ob-
servations. For the H i 21–cm spectra, we used either the
Southern Villa-Elisa H i survey data (Bajaja et al. 2005),
corrected for the effects of stray radiation or the Leiden-
Dwingeloo survey for sightlines with Dec.>–20◦ (Hartman
& Burton 1997). Both surveys have been merged to form the
Leiden/Argentine/Bonn (LAB) H i line survey (Kalberla et
al. 2005) which has a velocity resolution of 1 km s−1 and
brightness temperature sensitivity of 0.07 K.
In Table 1 the columns are as follows. Columns 1–5
give the star HD name, alternative name, Galactic coordi-
nates and V -band magnitude taken from simbad. Columns
6–7 give the estimated stellar distance and z-height above
or below the Galactic plane. These distances were primarily
estimated using the method of spectroscopic parallax from
the spectral type, apparent magnitude and reddening to-
wards each star, estimated from the observed (B−V ) colour.
Absolute magnitudes as a function of spectral type were
taken from Schmidt-Kaler (1982) with colours from Wegner
(1994). Details are given in Paper i. Excluding perhaps large
systematic errors caused by the uncertainty in the absolute
magnitude calibration of our sample, the distances have an
uncertainty of ∼30 per cent. For a number of objects (in par-
ticular the Wolf-Rayet stars, peculiar objects and Post-AGB
stars), distances were taken from the reference given at the
foot of the table. For example for HD179407 the distance is
given as 76001 where the suffix refers to reference number
1 where the distance of 7600-pc was given. Column 8 gives
the signal-to-noise (S/N) ratio pixel−1 in the Ca ii spectrum;
to obtain the S/N per resolution element this needs to be
multiplied by
If the coordinate of the sightline lies within any of the
figures of Wakker (2001) which display H i column densities
towards IHVCs, this name is given in Column 9. We must
stress that although more than 35 of our stars lie within the
boundaries of these figures, often they are in regions where
no IHVC is observed in H i, for example because the stars
lie in holes in the H i distribution. Columns 10 and 11 give
the minimum and maximum expected LSR velocity for gas
orbiting the Galactic Centre, based on the direction of the
sightline and the distance to the stellar target. To calculate
the velocity range for ”normal” gas, we use the methodology
of Wakker (1991), in that we assume a flat rotation curve
with vrot = 220 km s
−1 at r >0.5 kpc, decreasing linearly
towards the Galactic Centre, together with equations from
Mihalas & Binney (1981). A deviation velocity for interstel-
lar cloud components which lie outside the expected velocity
range is calculated, where the deviation velocity is defined as
the difference between the velocity of the component and the
nearest limit of the expected velocity range (Wakker 1991).
We classify low velocity clouds (LVCs) as having absolute
values of their deviation velocities below 30 km s−1, IVCs
between 30 km s−1 and 90 km s−1, and HVCs greater than
90 km s−1. Finally, column 12 gives the source for the op-
tical spectra (POP for stars from Paper i; FER1/FER2 for
FEROS observations; S07 for stars from Silva et al. 2007),
and H i data (LD for Leiden-Dwingeloo; VE for Villa-Elisa
Survey sightlines).
3 RESULTS
In this section we discuss those cases where the stellar sight-
lines intersect with known IHVCs, and hence determine im-
proved distance estimates towards a handful of objects.
Fig. 1 shows the optical and H i spectra towards the
sightlines where a distance limit has been determined to-
wards an IHVC. Fig. 2 (available online) shows the remain-
ing sightlines. Two plots are shown for each sightline in order
to emphasise both weak and strong features. The majority
of the optical spectra are in the Ca ii K line; where this was
not available the Ti ii line is shown. The horizontal line at
the top of the first of the optical plots shows the extent
of the full width half maximum of the stellar line. In most
cases, these lines are wide, hence there is no possibility that
stellar lines could be misidentified as interstellar features,
which tend to be much narrower. If the stellar lines have a
FWHM exceeding ∼100 kms−1 they were removed in the
normalisation process to facilitate visualisation of the inter-
stellar lines in all cases apart from HD numbers 115363,
136239 and 142758 where too much overlap of stellar and
interstellar components occur.
3.1 Methodology of estimating distances to
IHVCs
The method of estimating distances to IHVCs is discussed
fully in Schwarz, Wakker & van Woerden (1995). For an up-
per distance limit, detection of optical absorption, in associ-
ation with an H i detection, is sufficient to provide an upper
distance limit, being the distance of the stellar probe. Lower-
distance limits are more problematic. A firm lower distance
limit can only be set if no optical absorption is seen at a
sufficient S/N ratio, the abundance of the optical element is
known (generally from observations of the same part of the
complex towards QSOs), and the H i column density is accu-
rately defined using a pencil beam. For the current sample,
the chemical abundance of the IHVC is often not known,
and the observations in H i only have a spatial resolution of
0.5◦, which means that care must be taken in ascribing a
lack of optical absorption as due to the stellar probe being
closer than the IHVC. However, these factors are somewhat
ameliorated by the fact that the optical spectra have high
S/N, frequently being >500 per resolution element and with
a median of 410 in the sightlines with a detected IHVC.
3.2 Distance limits towards individual complexes
A number of the current sightlines either intersect with
known IHVC complexes, or have gas present at IHVC ve-
locities in the Villa-Elisa or Leiden-Dwingeloo H i spectra.
These cases are discussed below, and lower distance esti-
mates towards five IHVCs are determined. Table 2 summa-
rizes these cases. Columns 1–6 gives the star name, stellar
distance, previous IHVC distance limit, IHVC complex, ob-
served H i velocity and corresponding log of the H i column
density. Columns 7–8 give the previously-known abundance
in Ca ii taken from Wakker (2001) and limiting 5σ Ca ii col-
umn density estimated from the current spectra. This was
derived using the observed S/N ratio and instrumental res-
olution, assuming the the optically thin approximation. Fi-
nally, column 9 gives the predicted Ca ii column density de-
c© RAS, MNRAS 000, 000–000
4 J. Smoker et al.
Table 1. The stellar subsample for new observations plus all sightlines which lie in the vicinity of IHVCs. The S/N ratios per pixel are
for Ca ii K (3933Å). See text for details.
Star Alt. l b mv d z S/N IHVC v
Source
Name (deg.) (deg.) (mag.) (pc) (pc) pixel−1 Opt./H i
HD171432 BD-18 5008 14.62 -4.98 7.11 4014 -348 590 – 0.0 43.9 POP/VE
EC20485-2420 21.76 -36.36 11.77 36005 -1200 40 gp 0.0 28.9 FER1/VE
HD179407 BD-12 5308 24.02 -10.40 9.44 76001 -1400 120 gp 0.0 128.3 FER1/VE
HD188294 57 Aql B 32.65 -17.77 6.44 212 -64 420 gp 0.0 2.3 POP/VE
G169–28 HIP 82398 41.83 +36.06 11.26 1172 69 100 K 0.0 1.0 S07/LD
HD196426 HR 7878 45.81 -23.32 6.21 7003 -280 360 gp 0.0 3.3 S07/LD
HD344365 58.63 +3.41 10.8 103213 61 210 – 0.0 11.4 S07/LD
HD2857 110.05 -67.64 9.95 7177 -663 270 IVS -0.9 0.0 S07/LD
HD19445 157.48 -27.20 8.05 3912 -18 200 IVS, ACC -0.3 0.0 S07/LD
HD30677 BD+08 775 190.18 -22.22 6.84 2707 -1023 430 ACII 0.0 8.1 POP/VE
HD46185 BD-12 1520 221.97 -10.08 6.79 2937 -514 550 – 0.0 31.1 POP/VE
BD-12 2669 239.12 +18.17 10.22 1588 49 250 IV Spur 0.0 1.6 S07/LD
HD72067 HR 3356 262.08 -3.08 5.83 488 -26 450 – 0.0 2.0 POP/VE
EC05229-6058 269.97 -34.08 11.4 22005 -2100 150 – 0.0 4.1 FER1/VE
HD94910 HIP 53461 289.18 -0.69 7.09 60004 -72 430 – -12.2 3.4 POP/VE
EC01483-6806 294.73 -48.36 11.1 26005 -2000 130 – -9.5 0.0 FER1/VE
LB3193 297.32 -54.90 12.70 80006 1800 100 – -14.4 0.0 FER1/VE
HD115363 HIP 64896 305.88 -0.97 7.82 3282 -55 290 WE -35.3 0.0 POP/VE
ROA5701 309.24 +15.05 13.16 48007 1246 50 – -46.5 0.0 FER2/VE
HD120908 312.25 +8.37 5.88 338 49 370 – -4.3 0.0 S07/VE
HD480 319.45 -65.58 7.03 469 427 530 – -1.0 0.0 S07/VE
HD142919 328.43 -0.76 6.10 268 -4 500 WE -3.2 0.0 S07/VE
HD186837 335.85 -30.57 6.20 329 -167 620 WE -2.4 0.0 S07/VE
IRAS17311 341.41 -9.04 11.4 11008 -174 55 – -9.5 0.0 FER1/VE
HD163758 SAO 209560 355.36 -6.10 7.32 4103 -436 550 – -16.1 0.0 POP/VE
HD163745 350.56 -8.79 6.50 2189 335 620 – -11.9 0.0 S07/VE
BD+09 2860 353.04 +63.21 11.27 53310 475 250 – -0.4 0.0 S07/LD
HD177566 355.55 -20.42 10.17 11009 -383 120 – -190.2 0.0 FER1/VE
CD-41 13967 359.28 -33.50 9.5 350011 -1900 80 – -1.2 0.0 FER1/VE
Reference codes: (1) Hoekzema, Lamers & van Genderen (1993), (1) Smartt, Dufton & Lennon (1997), (2) González et al. (2006)
(3) Carney et al. (1994). (4) Hoekzema, Lamers & van Genderen (1993), (5) Smoker et al. (2003), (6) Quin & Lamers (1992) (7) Kinman
et al. (2000), (8) Laird, Carney & Latham (1988), (9) Zsargó et al. (2003), (10) Beers et al. (2000), (11) McCarthy et al. (1991), (12)
From parallax. (13) From RR-Lyrae calibration and magnitude.
rived by subtracting the previously-known Ca ii abundance
from the log of the observed H i column density. Where this
predicted value is much higher than the limiting 5σ Ca ii
column density a non-detection is interpreted as the cloud
lying further away than the stellar probe. Individual com-
plexes are discussed below.
3.2.1 Complex gp IVC
Complex gp is a positive-velocity IVC lying in the direc-
tion of the globular cluster M15, which has previously been
studied in infrared, optical, Hα and H i by Smoker et al.
(2002). The previously-existing distance limit was 0.8–4.3
kpc (Wakker 2001 and references therein) with an uncer-
tain lower distance limit of 2.0-kpc (Smoker et al. 2006).
The Complex has LSR velocities of ∼+60 to +90 kms−1.
In our current sample, the star HD188294 lies towards this
Complex, but only has a distance of 212-pc and no H i is
detected for this sightline due to it being in a “hole” in the
Complex. Additionally, HD196426 (l, b=45.81◦,–23.32◦) lies
towards Complex gp, and weak H i is observed in emission in
the Leiden-Dwingeloo spectrum, with a LSR velocity +78±1
kms−1, a FWHM of 24±2 km s−1, peak brightness tempera-
ture TB=0.25±0.05 K and brightness temperature integral of
6.5±1.0 K km s−1, corresponding to an H i column density of
1.1±0.2×1019 cm−2. Although weak, this should have been
detected in our UVES spectrum which has a S/N = 500 per
resolution element. The star has a distance of 700-pc, which
is similar to the distances for previous objects towards which
there were non-detections. In Complex gp we also observed
HD179407 (l, b=24.02◦,–10.4◦, distance=7600-pc) at a S/N
pixel−1 of 120 in Ca ii K. At the current position, there are
two weak H i velocity features, at v=+50±1 and v=+97±1
kms−1 with FWHM values of 26±2 and 42±4 kms−1 and
brightness temperature integrals of 1.7±0.2 and 1.7±0.2 K
kms−1 respectively, corresponding to column densities of
∼3×1018cm−2. There is obvious detection of Ca ii in the
+50 kms−1 feature (as in the Ca ii spectrum of Sembach &
Danks 1994), but no detection of the v= +97 kms−1 feature,
perhaps due to clumpiness in the H i or ionisation issues; a
higher S/N Ca ii spectrum would be useful. Given the weak
nature of both H i features a higher spatial-resolution and
sensitivity H i spectrum would be useful at this position al-
though in any case the star lies at a distance exceeding the
current upper limit of the cloud. Although HD 179407 was
c© RAS, MNRAS 000, 000–000
IVCs and HVCs towards POP early-type stars 5
Figure 1. Optical Ca ii K and 21-cm H i spectra towards early-type stars for which a lower distance limit towards an IHVC has been
determined. Two plots are shown per sightline in order to emphasise weak features. Further details are given in the text.
Table 2. IHVC sightlines where H i is detected at intermediate or high velocities. Complex WEM is in the same part of the sky as
complex WE of Wakker (2001), but at higher velocities. See Sect 3.2 for details.
Star d d
IHVC vIHVC(H i) log(NIHVC(H i)) A
(Ca ii) log(Nlim(Ca ii)) log(Npred(Ca ii))
(pc) (pc) complex km s−1 (log(cm−2)) (log(cm−2)) (log(cm−2)) (log(cm−2))
HD196426 700 800-4300 gp +78 19.06 -7.42 10.20 11.65
HD179407 7600 ” gp +50 18.49 -7.42 10.60 11.07
” ” ” gp +97 18.49 -7.42 10.60 11.07
HD19445 39 – IVS -45 19.49 -7.88 10.46 11.61
” ” – IVS -40 19.71 -7.88 10.46 11.82
HD30677 2700 >400 ACII -117 19.24 <-8.39 9.82 –
HD115363 3200 – WEM +224 19.71 – 9.99 –
” ” – WEM +240 19.30 – 9.99 –
HD46185 2900 – Other +122 19.09 – 9.71 –
also observed in the FUSE spectrum by Zsargo et al. (2003),
the presence of a complex stellar continuum meant that no
interstellar Ovi was observed. Finally, although EC 20485-
2420 lies in the general direction of this complex, no H i is
obvious in the Villa-Elisa spectrum.
3.2.2 IV South
IV South is a group of IVCs that extend over much of the
southern sky, with velocities of ∼–85 to –45 kms−1. To-
wards HD19445 (l, b=157.48◦,–27.20◦), no IV absorption is
seen in the Ca ii spectrum at a S/N of 280 per resolution ele-
ment, thus placing a rather uninteresting firm lower distance
limit of 39-pc to this part of the IVC that has two compo-
nents with v=–45±0.5 km s−1, –40.2±0.5 km s−1, FWHM
values of 8±1 kms−1 and 22±2 kms−1, peak TB values of
of 2.1±0.2 K and 1.2±0.2 K and brightness temperature in-
tegral of 17±2 K kms−1 and 28±3 K kms−1. The combined
H i column density in these two features is ∼8×1019 cm−2
which should have been easily detected in the current optical
spectrum if the cloud were closer than the star.
c© RAS, MNRAS 000, 000–000
6 J. Smoker et al.
3.2.3 Complex K
Complex K is a Northern-Hemisphere cloud with LSR
velocities ranging from –65 to –95 kms−1. Its previous
distance bracket was ∼700–6800-pc (Smoker et al. 2006
and refs. therein). One of our sightlines towards G169-
28 (l, b=41.83◦,+36.06◦) lies in the general direction of
Complex K, but no H i emission is visible in the Leiden-
Dwingeloo spectrum and there are many stellar lines. Thus
the current observations do not add anything to our knowl-
edge of this IVC.
3.2.4 Anti-centre HVCs
Seven of our sightlines lie in the region of the Anti-Centre
HVC (Fig. 9 of Wakker 2001). No upper distance limit is
available for this HVC and the previous lower-distance limit
towards Cloud ACI is only 0.4-kpc (Tamanaha 1996). We
only detect H i at high velocity towards one of the cur-
rent sightlines which lies towards ACII, namely HD30677
at a velocity of –117±1 kms−1, peak brightness tempera-
ture of 0.40±0.04 K, FWHM of 23±2 km s−1 and integrated
brightness temperature of 9.5±1.0 K kms−1, corresponding
to an HVC column density of 1.7±0.2×1019 cm−2. Assum-
ing that the HVC has a similar abundance to the relation
from Wakker & Mathis (2000), we would expect a corre-
sponding column density log(Ca ii cm−2)=11.64. However,
no corresponding optical absorption is detected in our Ca ii
K spectrum, which has a S/N = 430 pixel−1 or 610 per res-
olution element. Assuming that the cloud is optically thin
in Ca, a 5σ detection, f = 0.634 for the Ca ii K transition
and instrumental resolution of 0.05Å, the limiting column
density observable with the current spectrum is log(Ca ii
cm−2)=9.82, more than a factor 60 lower than predicted
from the H i profile. Hence the current observations put a
firm lower distance limit of 2.7-kpc towards complex ACII,
assuming that the H i observed in the Villa-Elisa survey re-
flects that in the pencil beam towards HD30677.
3.2.5 Complex WE/WEM HVC
Complex WE is a group of small HVCs centred on
(l, b)∼(320◦,0◦), first detected by Mathewson, Cleary &
Murray (1974) and mapped in H i by Morras (1982). Parts
of it lie in the same region of the sky as two large low-
velocity H i shells in the direction of the Coalsack nebula
described by McLure-Griffiths et al. (2001). At b ∼0◦ lat-
itude the predicted values of Galactic rotation at l ∼320◦
are from ∼–120 to +70 km s−1, falling to ∼–100 to 0 km s−1
at b ∼–15◦. Towards HD156359 (l, b=328.68◦, –14.52◦),
Sembach et al. (1991) found optical absorption at ∼+110
kms−1, putting an upper distance limit of 12.8 kpc. Eigh-
teen of our sightlines lie within the general area of WE
as defined in Fig. 11 of Wakker (2001). One of the sam-
ple stars HD 115363 (l, b=306.0◦,–1.0◦ with spectroscopic
distance=3.2-kpc) has HVC gas detected with two compo-
nents at +224.5±3.0 kms−1, +240.0±5.0 km s−1, velocity
widths 14.4±0.8 kms−1 and 19.2±2.4 km s−1, peak bright-
ness temperatures of 1.8±0.06 K and 1.1±0.1 kms−1 and
brightness temperature integrals of 28.3±1.0 K kms−1 and
11.1±0.8 K kms−1 which correspond to H i column densi-
ties of 5.2± 0.2×1019 cm−2 and 2.0±0.1×1019 cm−2. There
is no Ca ii K absorption present in the spectrum, which has
a S/N = 410 per resolution element. This HVC is probably
associated with the clouds defined by Putman (2000) as the
Leading Arm: the counterpart of the Magellanic Stream, as
projected on the sky, between the Magellanic Clouds and
the Galactic Plane. These data hence set an unsurprising
lower limit of 3.2-kpc towards this HVC that is probably re-
lated to the Magellanic System, using our distance estimated
spectroscopically . If we assume that HD115363 is a part of
the Centaurus OB1 association, its distance is slightly closer
at 2.5-kpc (McClure-Griffiths et al. 2001 and refs. therein).
Finally we note that this HVC appears to be a different set
of clouds to the lower-velocity and more negative galactic-
latitude clouds described in Wakker (2001) and observed
by Sembach et al. (1991), hence in the current paper it is
named WEM due to its possible association with the Mag-
ellanic system.
3.2.6 Other IVCs
In the line-of-sight towards HD46185 (l, b=222.0◦,–10.1◦),
H i emission is detected at +122±2 kms−1, with a peak
brightness temperature of 0.35 K, FWHM of 17±3 kms−1
and brightness temperature integral of 6.7±0.7 K kms−1,
corresponding to an H i column density of 1.2±0.1×1019
cm−2. Normal Galactic rotation predicts velocities of upto
∼+97 kms−1 in this part of the sky, so the deviation ve-
locity is only ∼ 25 km s−1 and the cloud many not be an
IVC. Assuming that the cloud has a similar abundance to
the relation from Wakker & Mathis (2000), we would ex-
pect a column density log(Ca ii K cm−2)=11.59. However,
no corresponding optical absorption is detected in our Ca ii
K spectrum, which has a S/N = 550 pixel−1 or 780 per res-
olution element. The 5σ limiting column density observable
with the current spectrum is log(Ca ii cm−2)=9.71, a fac-
tor 75 lower than predicted from the H i profile. Hence the
current observations put a firm lower distance limit of 2.9-
kpc towards this parcel of gas that lies within ∼20◦ of the
Anti-Centre Shell (Fig. 8 of Wakker 2001) but is at different
velocities and probably unrelated.
3.3 IHVCs detected in Ca ii absorption
A number of sightlines were already flagged in Paper i
as having IHVC components detected in the optical spec-
tra. These include the Wolf Rayet stars HD94910 and
HD163758 and the sightline HD72067 which lies towards
the Vela Supernova remnant. No H i is detected towards
any of these sightlines. In the first two cases this implies
the presence of circumstellar lines and in the latter case
lines within the SN remnant. Similarly, towards HD171432
many IVCs are detected in the optical. This sightline lies
towards the Scutum Supershell mapped in H i by Callaway
et al. (2000) and with a distance of ∼3000-pc. Although to-
wards HD171432 there is a dearth of H i detected in the
Callaway maps, there is H i in our H i spectrum up to a
velocity of ∼+90 kms−1, coincident with our detections of
Ca ii. No H i is seen in our highest-velocity Ca ii component
of ∼+120 kms−1, perhaps due to S/N limitations. The de-
tections in Ca ii and H i are consistent with the supershell
being closer than our stellar distance of ∼4000 pc and with
c© RAS, MNRAS 000, 000–000
IVCs and HVCs towards POP early-type stars 7
the previous observations, but add nothing to the distance
bracket.
4 DISCUSSION
Table 3 gives a summary of the distance limits to IHVCs
set by the current observations, plus existing limits to the
clouds where available. Particularly interesting is the im-
proved lower limit towards part of the Anti-Centre complex
ACII which has firm lower-distance limit of >2.7-kpc. This
compares with the indirect distance estimate of a part of the
complex at l ∼60◦,b ∼–45◦ derived from morphological and
kinematical arguments of ∼ 4-kpc (Peek et al. 2007), and an
Hα estimated distance of between 8 and 20-kpc (Weiner et
al. 2001) which is based upon the observed ionisation being
caused by the Galactic radiation field. Although a big im-
provement on the previous lower-distance limit of ∼0.4-kpc
(Tamanaha 1996), the current observations cannot discrim-
inate between the indirectly-estimated distances and clearly
searches for more distant probe stars in this part of the sky
would be useful. Other less interesting results are the con-
solidation of the lower-distance limit towards complex gp
and the first lower distance limit towards the WEM com-
plex. The z-distance of the former IVC is now constrained
to ∼300-1700-pc which compares to the H i scaleheight of
<200-pc at Galactocentric radii of <10-kpc (Narayan, Saha
& Jog 2005). Further progress on this sightline should in-
volve performing obtaining a high-resolution spectrum of
the star HD357657 and associated model atmosphere cal-
culation and abundance analysis. Although Smoker et al.
(2006) estimated a distance of ∼2.0-kpc for this object on
the line of sight to Complex gp and found no associated
Ca ii absorption, the distance of the star remains uncertain.
If a firm lower distance limit of 2-kpc were confirmed, cloud
parameters such as the cloudlet sizes, cloud electron den-
sity, fractional H i to H ii ratios and ionizing radiation field
could be better constrained (c.f. Smoker et al. 2002), and the
position of the cloud relative to the H i disc of the Galaxy
confirmed.
Finally, the lower distance limit of 3.2-kpc towards HVC
WEM is consistent with both a Magellanic origin as pro-
posed for example by Putman (2000), or a ’classical’ high
velocity cloud. H i synthesis mapping towards other HVCs in
this part of the sky (e.g. Bekhti et al. 2006) have provided
evidence from cloud structure and linewidths of distances
of ∼10–60-kpc, consistent with a Magellanic origin, and the
same observations could be performed for the present sight-
line in order to obtain an indirect distance estimate, perhaps
in conjunction with Hα mapping. However, in the absence
of early-type stars present in the leading arm as present in
the Magellanic Bridge (Rolleston et al. 1999), obtaining a
firm upper distance limit will be difficult although perhaps
possible due to the offset in velocity from the stellar and
interstellar Ca ii K lines (c.f. Smoker et al. 2002).
5 SUMMARY
We have correlated optical spectra in the Ca ii K and Ti ii
lines observed towards early-type stars in the POP Survey,
plus other optical data, with 21-cm H i spectra taken from
Table 3. Distance limits and probe stars towards the IHVCs
studied in this paper.
IHVC (l, b) vIHVC(H i) Probes DIHVC
(deg.) km s−1 (pc)
gp 46,–23 +78 HD196426 800-43001,2
IVS 157,–27 –45, –40 HD19445 > 391
ACII 190,–22 –117 HD30677 > 27001
WEM 306,–1 +224, +240 HD115363 > 32001
Other 222,–10 +122 HD46185 > 29001
Reference codes: (1) This paper, (2) Little et al. (1994).
the Villa-Elisa and Leiden-Dwingeloo Surveys, in order to
determine the distances to Intermediate and High Velocity
Clouds. The lack of Ca ii K absorption at –117 km s−1 to-
wards HD30677 at a S/N ratio of ∼610 has set a firm lower
distance limit towards Anti-Centre cloud ACII which previ-
ously had a lower distance limit of 0.4-kpc. Likewise, towards
HD46185 no Ca ii K absorption at +122 kms−1 is seen at
a S/N ratio of ∼780, hence placing a lower distance limit of
2.9-kpc towards this gas that is perhaps an IVC. Towards
Complex gp no Ca ii K absorption is seen in the spectrum
of HD196426 at a S/N of ∼500, reinforcing the assertion
that this IVC lies at a distance exceeding 0.7-kpc. Likewise,
towards the nearby star HD19445 at 39-pc in the line of
sight to IV South no Ca ii K absorption is seen setting a
a firm but uninteresting distance limit towards this part of
the complex. Finally, no HV Ca ii K absorption is seen in
the stellar spectrum of HD115363 at a S/N = 410, placing a
lower distance of ∼3.2-kpc towards the HVC gas at velocity
of ∼+224 kms−1. This gas is in the same region of sky as
the WE complex of Wakker (2001), but at higher velocities.
If related to the Magellanic system (Putman 2000) then a
distance limit of 3.2-kpc is not unexpected.
A future paper will describe the use of new FEROS
observations combined with UVES archive data to provide
improved distance limits to complex EP, the Cohen Stream,
IV South and the Anti-Centre shell. Concerning the POP
data, future papers will investigate the neutral species of
Ca i, Fe i, Na i D and K i as well as molecular line species
CH, CH+ and CN in order to better understand the local
interstellar medium.
ACKNOWLEDGEMENTS
We would like to thank the staff of the Very Large Telescope,
Paranal for the large amount work involved in producing
the POP Survey (ESO DDT programme ID 266.D-5655(A),
http://www.eso.org/uvespop). Especially due thanks are S.
Bagnulo, R. Cabanac, E. Jehin, C. Ledoux and C. Melo. In
addition, we are grateful to the staffs of Dwingeloo/Leiden
and the Villa-Elisa Telescope for producing the H i all sky
surveys. JVS and HMAT thank the support staff at La Silla
for their help with the FEROS observations. FPK is grate-
ful to AWE Aldermaston for the award of a William Pen-
ney Fellowship. This research has made use of the simbad
Database, operated at CDS, Strasbourg, France. JVS ac-
knowledges financial support from the Particle Physics and
Astronomy Research Council with HMAT and IH thanking
the Department of Education and Learning for Northern Ire-
c© RAS, MNRAS 000, 000–000
http://www.eso.org/uvespop
8 J. Smoker et al.
land. JVS thanks M. Garćıa Muñiz, L. Salinas and I. Dino
for discussions and to an anonymous referee for comments.
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IVCs and HVCs towards POP early-type stars 9
Figure 2.Optical (Ca ii K or Ti ii) and 21-cm H i spectra towards early-type stars. Two plots are shown per sightline in order to emphasise
weak features. In the cases where the FWHM of the stellar profile exceeded ∼100 kms−1 it has been removed in the normalisation process
to emphasise the interstellar line features. This affects the stars with HD numbers 480, 2857, 2913, 49131, 61429, 74966, 76131, 90882,
100841, 115842, 122272. 145482, 156575, 163745, 186837, 188294, 344365, plus ROA 5701. For HD2857 some residuals are left by this
process at ∼ –160 km s−1.
c© RAS, MNRAS 000, 000–000
10 J. Smoker et al.
Figure 2. ctd.
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IVCs and HVCs towards POP early-type stars 11
Figure 2. ctd.
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12 J. Smoker et al.
Figure 2. ctd.
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IVCs and HVCs towards POP early-type stars 13
Figure 2. ctd.
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14 J. Smoker et al.
Figure 2. ctd.
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IVCs and HVCs towards POP early-type stars 15
Figure 2. ctd.
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Figure 2. ctd.
c© RAS, MNRAS 000, 000–000
	Introduction
	The sample, observations and data reduction
	Results
	Methodology of estimating distances to IHVCs
	Distance limits towards individual complexes
	IHVCs detected in Caii absorption
	Discussion
	Summary
