A&A 370, 996-999 (2001)
DOI: 10.1051/0004-6361:20010294
N. Lehner1 - C. Trundle2 - F. P. Keenan2 - K. R. Sembach1 - D. L. Lambert3
1 - The Johns Hopkins University, Department of Physics and Astronomy,
Bloomberg Center, 3400 N. Charles Street, Baltimore, MD 21218, USA
2 - Department of Pure and Applied Physics, The Queen's University
of Belfast, Belfast, BT7 1NN, Northern Ireland
3 - Department of Astronomy, The University of Texas at Austin, Austin,
TX 78712-1083, USA
Received 27 November 2000/ Accepted 16 February 2001
Abstract
Previous Ca II K observations of the B-type
star HD 83206 have
revealed putative high-velocity interstellar
clouds (HVCs) at Local Standard of Rest (LSR)
velocities of -80 and -110
.
Similar results were also found
for the sightline
towards HD 135485. In this article, we show that these
absorption
lines are in fact due to
stellar S II features. As the Ca II K absorption line
in B-type stars is
often used to assess the presence and distance of HVCs, we
also present a very high quality spectrum of
HD 83206 in the Ca II K region (
Å or
),
so that in the future confusion between stellar lines
and HVC features may be avoided.
Key words: line: identification - stars: early-type -
stars: individual: HD 83206; HD 135485 -
ISM: individual object: high-velocity clouds
High-velocity clouds (HVCs) are defined as clouds whose radial
velocities cannot be explained by
any reasonable model of differential Galactic
rotation. B-type stars,
or evolved spectral analogues of these objects,
such as post-Asymptotic Giant Branch (PAGB), Blue Horizontal Branch (BHB)
and early-type subdwarf stars (sdB),
are often used to search for such clouds because
(i) their spectra are relatively free of stellar absorption lines
compared to late-type stars,
(ii) they often have large projected
rotational velocities ()
that broaden the stellar
photospheric lines,
(iii) their high luminosities allow them to be used to investigate
HVCs at distances of tens of kpc, and (iv) simple spectral
analyses lead to accurate distance determinations.
Distance determinations are particularly important for HVCs
since without this quantity the origin and
physical structure remain largely
unknown (see e.g. Wakker & van Woerden 1997).
Ryans et al. (1997) observed the B-type star HD 83206 in the Ca II K
spectral region, along with other stars, to constrain the distances of
the HVCs Chain A and Complex M (LSR velocities between -80 and
-140
;
see van Woerden et al. 1999a). Further studies were
performed by Lehner et al. (1999) to study the substructure of these clouds.
HVCs towards HD 135485
were detected by Albert et al. (1993) in their high-resolution survey of
Galactic halo gas. However follow-up IUE
observations of this star did not show any
HVC absorption in the strong ultraviolet
resonance lines (Danly et al. 1995).
HD 83206 and HD 135485 both
have very low values of ,
making it difficult to differentiate stellar and high velocity
interstellar components. In this short note, we show that the blue-shifted (with
respect to Ca II K) features detected
in the spectra of these stars
are in fact sharp (
)
S II stellar lines.
A very high quality spectrum of HD 83206 is
presented to circumvent future ambiguity
between interstellar and stellar lines in the
Ca II K wavelength region.
HD 83206 is classified as a
B2 V star (Lehner et al. 2000), while HD 135485 is
a peculiar metal-rich evolved
B-type star (Trundle et al. 2001;
but see also Dufton 1973; Schönberner 1973).
These stars have the similar property of
very narrow photospheric metal lines.
For HD 83206, Lehner et al. (2000) showed that
,
or even less if the microturbulence is considered, while for
HD 135485 the
is about
3
(Trundle et al. 2001).
![]() |
Figure 1:
a) Normalized spectrum of HD 83206 (McDonald 1999 data)
plotted against the observed wavelength. Values
in brackets are the dynamical LSR velocities (in
![]() ![]() |
Open with DEXTER |
Spectra of HD 135485 were obtained with
the 4.2 m William Herschel telescope (WHT) at La Palma Observatory in
July 1999 using the Utrecht Echelle spectrograph
(at a resolution of about 6
and S/N = 120),
while observations of HD 83206 were carried out
using the coudé spectrograph on the
2.7 m telescope at McDonald Observatory in December 1996
(resolution of
1.9
and S/N =130) and
November 1999 (resolution of 1.8
and S/N =230).
Procedures for the data reduction are fully discussed
by Lehner et al. (1999,
2000) and Trundle et al. (2001).
We show the Ca II K spectral region of HD 83206
in Fig. 1. The panel (a) in this figure presents
the normalized spectrum versus the observed wavelength.
This high quality spectrum clearly
reveals weak absorption at velocities blueward of the
Ca II K line at 3933.663 Å.
The bottom panel (b) shows how
theses lines could be identified as Ca II K
HVC interstellar lines at LSR velocities of
-80,-110,-140
,
especially in this region of the sky. HVC Complexes L (which is projected
on HD 135485) and M have velocities between -140 and -80
and Chain A has velocities between -210 and -90
(Wakker & van Woerden 1991; Wakker & van Woerden 1997; van Woerden et al. 1999a).
However, the non-detection of any HVC features in the IUE spectra of HD 135485, and the conspicuous match of these lines at exactly the same wavelengths for the two sightlines, when referred to the stellar Ca II K line (see Fig. 2), indicate that they are most probably stellar lines rather than interstellar features.
HD 83206 | HD 135485 | |||
Ions | EW |
![]() |
EW |
![]() |
mÅ | dex | mÅ | dex | |
S II ![]() |
1.7 | 6.90 | 6.4 | 7.50 |
S II ![]() |
6.2 | 7.23 | 16.7 | 7.51 |
S II ![]() |
3.2 | 7.12 | 11.3 | 7.53 |
S IIa |
![]() |
![]() |
Note: (a): LTE mean absolute abundance from
(b) Lehner (2000), n=32, (c) Trundle et al. (2001), n=14.
Quoted errors are
on n measurements.
![]() |
Figure 2: Normalized spectra of HD 83206 and HD 135485 versus the velocity, such as the velocities are in a reference frame where the stars are at rest (the S II lines were incremented by 0.2 on the y-scale) |
Open with DEXTER |
To identify the lines in Figs. 1 and 2
we used the atomic line database
compiled by Peter van Hoof. The best match was for S II lines at
3931.507, 3931.911 and 3932.290 Å. Moreover, using the LTE atmospheric parameters
found by Lehner et al. (2000) and Trundle et al. (2001), we were able to
derive LTE absolute abundances using the equivalent widths of these
transitions. Our results are
summarized in Table 1, where we also
present the LTE S II abundances
for HD 83206 and HD 135485 obtained by Lehner (2000) and
Trundle et al. (2001), respectively.
We note that the transitions considered in the present
paper were not included in the abundance determinations by
Lehner and Trundle et al. However the two sets of S II abundances agree
remarkably well, and hence we conclude that the lines in Fig. 1 are indeed
stellar S II features.
Main-sequence B-type stars are often used to derive upper limits to
HVC distances; usually there is no ambiguity between stellar and interstellar
features, if the stars have large projected rotational velocities that broaden
the stellar lines. However, there is not a
large number of such young B-type stars in the Galactic halo, and therefore it is often necessary to use evolved, slowly rotating B-type stars (e.g. PAGB, BHB, sdB)
as alternative tracers of HVC absorption.
In Fig. 1 we therefore present a very high quality spectrum in the Ca II K
wavelength region, where the only stellar features in this spectrum that could be confused
with HVCs (
)
are the S II lines
described above. We note that the red part of the spectral region (
)
is
effectively free of stellar lines, with only one feature
detected at +200
that could be He I
3935.95,
which is possibly blended with other stellar transitions.
The features marked as Ca II K ISM
in Fig. 1 are certainly interstellar
lines because (i) superimposing the
HD 135485 and HD 83206 spectra
does not reveal any obvious matches such as
the ones observed in Fig. 2, and
(ii) a comparison with the Na I D
interstellar lines (Lehner et al. 1999) shows
good agreement with the velocities of the
Ca II K features (except at 3932.85 Å which does not appear in the Na I D
spectrum). Finally, other evolved spectral analogues of B-type stars
may also exhibit other weak stellar lines (see e.g. van Woerden et al. 1999b)
and a systematic comparison with the Ca II H feature might help in some
cases to disentangle stellar and interstellar features.
Acknowledgements
This research has made use of the NASA Astrophysics Data System Abstract Service (http://adswww.harvard.edu/) and the CDS database (http://cdsweb.u-strasbg.fr/). CT is grateful to the Department of Higher and Further Education, Training and Employment for Northern Ireland and the Dunville Scholarships Fund for financial support.