Searching for CIV and SiIV Interstellar Lines
in the Nearby Interstellar Medium
Rubens Freire Ferrero1, Carmen Morales Durán2 and Ana María Cabo Cubeiro 2,3
(1) Observatoire Astronomique de Strasbourg,
11, rue de l'Université,
67000 - Strasbourg, France
[email protected]
(2) Laboratorio de Astrofísica Espacial y Física Fundamental,
Estación de Seguimiento de Satélites - ESA
Villafranca del Castillo
E-28691 Villanueva de la Cañada
Madrid, Spain
[email protected]
(3) Universidad Complutense de Madrid
Facultad de Matemáticas
Avda. Complutense s/n
28040 Madrid, Spain
[email protected]
Abstract
We have undertaken the analysis of all early type stars observed by IUE in the neighbourhood
of the solar system, say at a distance smaller than 400 pc, searching for high ionized
interstellar (IS) species whose signatures would be superimposed over the stellar spectra.
Normal late-B and early-A type stars are indeed the best targets to search for interstellar SiIV
and CIV resonance lines, because they are not able to produce them at atmospheric levels,
and some attempts in this direction were made in the past.
Now, our purpose is to accomplish the whole analysis of all data at our disposal, using the
new calibrated IUE data through INES database.
The results of the present work confirm the conclusions based on preliminary studies (Freire
Ferrero 1998a, 1998b) : normal late-B and early-A stars showing SiIV and CIV absorptions,
cluster in the direction of the Loop I (Sco-Cen).
The stars showing these absorptions are neither known to show shell, nor to have other
peculiar spectral or stellar characteristics and are placed in general farther than 90 pc.
"Normal" stars placed in the same directions, but at shorter distances, do not show these
absorptions.
1
This fact suggests that there are IS lines originated at the region of collision between the
expanding Loop I supershell and the Local Bubble. This colliding region was put in evidence
by HI and ROSAT observations (Egger 1998) : the SiIV and CIV could be formed in an
intermediate temperature region between the cold and dense HI local wall and the hot
expanding Loop I supershell, colliding the Local Bubble in the direction of Sco Cen.
Introduction
Several years ago Freire Ferrero (1988, 1998a, 1998b) had pointed out, from high resolution
IUE stellar spectra, that some nearby late-B and early-A stars in the Sco-Cen sky directions
showed weak absorption lines at the wavelengths corresponding to the SiIV and CIV doublet
resonant lines (1393.76 and 1402.77 A for SiIV and 1548.202 and 1550.774 A for CIV)
sometimes superimposed over stellar metallic blends.
The choice of late-B and early-A stars to search for signatures of high ionised species is due
to the fact that "normal" mean sequence stars of these spectral types do not show any sign of
chromospheric, transition zone or coronal lines (Freire Ferrero, 1986) neither on the visible
nor in the UV.
This fact is usually explained by the disappearance of the superficial HI convective zone due
to stellar Teff beyonf 9000 K. The convective zones are thinking to be the principal source of
non-thermal energy needed to produce, at least, the non-radiative temperature rise at the
origin of stellar chromospheres and may be contributing also to further magnetic activity
giving rise to transition regions and coronas.
The stars concerned by those works, were not known to be shell or emission Ae/Be stars at
that time of IUE observations, but other authors suggested that surely they were shell stars or
evolving through a shell phase (Bruhweiler et al., 1989).
The discovery of an X-ray ring-shaped shadow by ROSAT (in the range 0.1 to 2.0 KeV) in
the direction of Loop I (Egger and Aschenbach, 1995) suggested that the expanding bubble of
Loop I had been entered in collision with the Local Bubble where the Sun is inmersed.
The ring-shaped shadow is just a geometrical result of the intersection of two sphere-shaped
bubbles, which deform and flatten out practically over a very flat volume like a thin surface
more or less plane where slow IS gas coming from opposite sides slows down and
concentrates.
The ring-shaped region seems to be coincident with the so-called "HI wall", an IS zone at
around 70 pc (Centurión and Vladilo, 1991) where the IS HI volume density rises by a factor
of 1000 (corresponding to hydrogen column densities NH of a few 1020 cm-2) in relation with
IS HI values at distances lower than 70 pc.
But a so spectacular collision should also be a theater to produce ionized ions (in particular
SiIV and CIV ions) from the colliding hot gas coming into opposite directions, taking into
account that surely the expansion of Loop I bubble is the result of several shock waves
coming from the ScoCen OB stellar association.
In fact, very distant stars in the direction of ScoCen show CIV and SiIV IS absorptions
(Sembach et al., 1997) and the authors suggested that an important IS absorption could
probably arise from Loop IV, that projects in the sky inside the Loop I.
2
Furthermore, Freire Ferrero (1998a, 1998b) confirmed his preliminary results ( Freire Ferrero,
1988 ) using the new Hipparcos parallaxes, and putted in evidencec that two stars, HD 119921
(A0V, 131.4 pc) and HD 119361 (B8III, 487.8 pc), showing the highest absorption, were in
nearby sky directions, but that other stars with low or no absorption features, were away from
the ScoCen sky directions or they were located nearby than 100 pc.
To gain into insight about the presence of weak CIV and SiIV absorption features on nearby
stars, we have undertaken a systematic analysis of IUE data from the INES archive (González
Riestra et al., 2000). In this paper, we deal with "normal" late-B and early-A stars. We will
consider later the group of peculiar stars (Bp and Ap), metallic stars (Am) and emission and
shell stars (Be, Ae, B-shell, A-shell), using a similar analysis.
In a forthcoming paper, we will give quantitative analysis of the resulting absorption CIV and
SiIV IS lines (equivalent widths and radial velocities) for the stars showing them.
The general goals of this work are :
- to characterize a hot nearby ISM detected by SiIV and CIV faint absorption lines in the UV
IUE spectra of "normal" B6-A9 III-IV-V stars, using an homogeneous spectra database like
INES ;
- to establish which is the spatial distribution and sky directions of the stars showing these
lines;
- to conclude about the possible link between the place were these ion species are observed
and the region where the Loop I interacts with the Local Bubble.
Observations and stellar data
We analyze the UV high resolution SWP spectra from the IUE INES ("IUE Newly Extracted
Spectra") database elaborated at VILSPA (González Riestra et al., 2000) and obtained through
new processing of the Final Archive output products (http://sdc.laeff.esa.es/ines/). These
spectral data were completely uniformized following the same methodological approach.
The spectra were retrieved from the original spectral type groups as defined by the IUE
database. First IUE Groups of stars B6-A9 of luminosity classes III to V were taken from the
INES Object class classification and then the spectral type of each star was checked in
SIMBAD database and selected the best one, if more than one were available.
First, we consider only large aperture observations which provide better recovering of stellar
fluxes and in this way allow a better calibration of stellar fluxes. When several spectra of the
same star are available, the different spectra were compared with each other to check its
consistency in flux and line intensity level. Variable or peculiar stars were in this way, easily
detected and extracted out of the sample of “normal” stars. The spectra consistent with each
other were then used to compute a mean spectrum with the aim to ameliorate the S/N ratio
and bring out the absorption features.
Few cases show different flux levels that are not due to any stellar variability but surely to a
bad flux calibration or bad estimation of effective exposure times. They were considered
individually and not used to compute mean spectra.
3
By the contrary, small aperture observations can not be correctly transformed in absolute
fluxes due to the irregular location of the stellar image over the spectrograph slit, and then
recorded exposure times are in reality upper limits of effective exposure times. No mean
values of small aperture observations can be computed because differences in flux levels
between different observations of the same star are present in almost all of them. Owing to we
are willing, in this first analysis, to the detection of spectral features, we used the individual
small aperture images when no other information of the star (large aperture) was at our
disposal.
Some of the new IUE uniform reduced spectra (INES) have become dramatically useless in
the first part of the short wavelength range due to either extreme background correction or to
lack of data. These special cases will be considered in a later work.
We search for SiIV and CIV absorption features in individual spectra and when possible
(large aperture), we also search them for in the mean spectra.
In addition, we estimated S/N ratios from the dispersion of flux values on neighbour continua
spectral regions (apparently without spectral lines) or normalizing the spectra through a very
smoothed template, obtained from the proper spectrum smoothed with a sliding mean over 30
to 50 points.
The stars of different stellar groups were listed in several electronic tables organized in the
same way :
names, stellar type, galactic coordinates, spectral type, photometric magnitudes, parallax and
distance in pc, v.sin i, and comments about the detection of SiIV or/and CIV.
General stellar data (spectral type, Hipparcos parallaxes and rotational velocity v.sin i) were
taken from SIMBAD database when no other more precise data were at our disposal from the
bibliography.
Here we only present the data for the selected stars showing unambiguous SiIV and CIV
absorption features.
Results
The analysis of the INES database allow us to select exploitable IUE spectra from 519 B6-A9
stars, of luminosity classes III, IV and V, and observed with large aperture ; 25 other stars in
the same range were observed with the small aperture. As a consequence of the longevity of
IUE as an orbital UV telescope, the very large sample at our disposal warrant no selection
bias in relation to sky direction (Fig. 1a and Fig. 1b), and a very good spread of distances in
all directions (Fig. 1c) favouring nearby stars (D< 200 pc).
4
Figure 1. (a) Aitoff projection of all B6-A9 stars observed by IUE.
Figure 1. (b) Planisphere projection of all B6-A9 stars observed by IUE.
5
Figure 1. (c) 3D projected representation of all B6-A9 stars observed by IUE.
From them we excluded known shell, Ap, Am, Ae-Be stars and Algols (we will analyse those
groups in forthcoming works) and we got 325 "normal" B6-A9 stars of luminosity classes III,
IV and V that we study here (Fig. 2a and Fig. 2b; Fig. 2c).
Figure 2. (a) Aitoff projection of "Normal" B6-A9 III,IV and V stars observed by IUE. Open
circles: class III; filled circles : classes IV and V.
6
Figure 2. (b) Planisphere projection of "Normal" B6-A9 III,IV and V stars observed by IUE.
Open circles: class III; filled circles : classes IV and V.
Figure 2. (c) 3D projected representation of "Normal" B6-A9 III,IV and V stars observed by
IUE. Open circles: class III; filled circles : classes IV and V.
7
The spectra were analysed individually and, when more than one observation available, also
using the mean spectrum of the different ones at our disposal, weighting each other with their
exposure times. When more than 10 spectra of the same star were present in the data base, we
selected the ten observations of best quality.
From this careful visual analysis we found only 25 stars having unambiguous SiIV and CIV
faint absorption lines superimposed to stellar metallic blends and their data are shown in
Table 1.
STAR
HD 13709
HD 33852
HD 33949
HD 39844
HD 49662
HD 50261
HD 51036
BD +05 3235
HD 70084
HD 93563
HD 100340
HD 101413
HD 111226
HD 119361
HD 119921
HD 135382
HD 142301
HD 145774
HD 148265
HD 149630
HD 151527
HD 196519
BD +20 3004
HD 205805
HD 215573
SPECTRAL TYPE GAL. LONGIT GAL. LATIT. DISTANCE(PC)
A0V
229,18
-71,84
101,8
B8
157,62
7,9
B7V
213,88
-27,55
171,5
B6V
276,8
-30,8
157
B7IV
226,21
-7,38
186,2
B8IV/V
235,81
-11,27
286,5
B6
235,16
-10,14
5263,2
B8
21,36
32,23
B7III
263,38
-6,34
389,1
B8/B9III
286,46
2,08
168,1
B9
258,85
61,23
1428,6
B7/B8II/III
295,03
-1,71
B8V
301,91
38,01
261,8
B8III
313,2
19,76
487,8
A0V
315,28
25,29
131,4
A1V
315,71
-9,55
56
B8III/IV
347,12
21,51
139,7
B8
10,5
33,65
2777,8
A
44,6
42,44
505,1
B9V
66,91
42,7
92,7
A0IV/V
4,25
18,81
137
B8V
328,39
-35,58
257,7
B8
21,45
64,08
1333,3
B7III
353,12
-47,81
265,3
B6IV
309,03
-35,53
136,1
Table 1.- B6-A9 III-IV-V stars showing SiIV and CIV faint absorption lines.
8
The majority of stars observed by IUE are placed at distances lower than 400 pc (Fig. 1-3D,
Fig. 2-3D) and the majority of selected stars showing SiIV and/or CIV concentrate around the
Sco-Cen sky region (Fig. 3a, Fig. 3b and Fig. 3c).
Figure 3. (a) Aitoff projection of "Normal" B6-A9 III,IV and V stars showing SiIV and
CIV absorption features. Open circles: class III; filled circles : classes IV and V.
Figure 3. (b) Planisphere projection of "Normal" B6-A9 III,IV and V stars showing
SiIV and CIV absorption features. Open circles: class III; filled circles : classes IV and
V.
9
Figure 3. (c) 3D projected representation of "Normal" B6-A9 III,IV and V stars showing SiIV
and CIV absorption features. Symbols are as follow:
Asterisk: Sun ( origin of distances)
D < 100 pc: open diamond.
100< D < 200 pc: filled triangle.
200< D < 300 pc: filled square.
300< D < 400 pc: open circle.
400< D < 1000 pc: filled circle.
As an example of the spectral trend in the SiIV and CIV regions, the observed spectra of HD
119921 and HD 119361 show striking similarities, both stars being in a nearby sky direction
(l,b) differing only in their heliocentric distance (Fig. 4).
The same trend in the SiIV lines is seen in the spectra of both HD 50261 and HD 51036, also
practically in the same sky direction and differing in distance (see Table 1), although not so
manifest for the CIV lines due surely to different abundances and spectral type (Fig. 5).
10
Figure 4.- SiIV and CIV absorption lines superimposed over stellar spectra
(HD 119921 and HD 119631). In the CIV spectral region, HD 119921 has
been displaced down by 1.5e-11.
11
Figure 5.- SiIV and CIV absorption lines espectral region for HD 50261 and HD 51036.
Star HD 51036 has been displaced down by 1.2e-11 and 1.0e-11 respectively.
12
The distribution of stars for each of three stellar groups is shown as histograms of distance
(Fig. 6, 544 stars B6-A9 III-V including Am, Ap, Ae,…; Fig. 7 , 325 normal stars ; Fig. 8, 25
stars with SiIV and CIV lines). Clearly the stars observed by IUE were in general nearby ones
(D< 200pc), a requirement due to observational constraints bounded by moderate exposure
times (less than some hours). Stars showing SiIV and CIV absorption features display in
majority away than 90 pc (Fig. 8).
Figure 6.- Histogram of distances for all B6-A9 III,IV and V stars observed by IUE. The
bin size is 20 pc.
Figure 7.- Histogram of distances for "normal" B6-A9 III,IV and V stars observed by
IUE. The bin size is 20 pc.
13
Figure 8.- Histogram of distances for the B6-A9 III, IV and V stars showing SiIV and
CIV faint absorption lines. The bin size is 20 pc.
Discussion
Even though some coincidences must not be excluded, the sky distribution of nearby normal
stars showing SiIV and CIV features displays in majority over the Sco-Cen sky region. All the
detected stars have distances higher than 50 pc. Stars at lower distances do not show the
detected spectral features.
The observed SiIV and CIV absorption features, could be attributed either to IS or to
circumstellar (CS) gas. As we already mentioned, for this study we excluded known shell
stars (Hauck and Jaschek, 2000).
It is still not excluded that some of the individual stars of our selected sample showed some
shell characteristics at the time of IUE observations, but the similarity between the observed
spectral absorption features for different spectral type stars is a fact supporting the same
external IS scenario.
In addition, histograms of spectral type for "normal" stars observed by IUE (Fig. 9) and stars
with the SiIV and CIV features (Fig. 10) are apparently similar with a maximum situated at
B8-B9.
14
Figure 9.- Histogram of spectral types for "normal" B6-A9 III,IV and V stars observed
by IUE.
Fig. 10.- Histogram of spectral types for the B6-A9 III, IV and V stars showing SiIV and
CIV faint absorption lines.
15
From the similarity of histograms we can claim that the selected stars are a representative
sample of the whole "normal" stars observed by IUE, i.e., that they are also "normal" stars
becoming a little different from the others "normals", by the only fact to be placed behind an
IS absorbing medium.
We also mention that two pairs of stars with similar sky directions shows similar absorption
features at the SiIV and CIV spectral region : HD 119361 and HD 119921, as well as HD
50261 and HD 51036 (even though they placed at 30 degrees of the external ring border),
independently of their spectral type.
Furthermore, the majority (15 stars) of the concerned (25) stars are distributed into or around
the interacting ring observed by ROSAT (Egger and Aschenbach, 1995).
A few selected stars are placed enough away this interacting zone, particularly HD 13709 and
HD 33852. The SiIV and CIV absorption features in these stars could have a different origin
of that we propose for the other stars. This possibility will be analyzed later (forthcoming
work), as well as for selected (8) stars around the interaction ring and away from it as far as
30 degrees.
It is worthy of note that very distant stars (some kpc) showing the same kind of IS
absorptions, display also in the same sky directions (Sembach et al. 1997) supporting the idea
that moderate ionized species could originate in the hot IS medium (with low volume density)
between IS neutral or low ionized matter. The resulting detection should then be a
consequence of an absorbing column density derived from integrated low densities over long
distances.
But it is also possible that some SiIV and CIV absorption detected on far away stars could
contain some IS contribution from the local region of interaction between the Local Bubble
and the Loop I, where the volume density of the gas is highly increased by the collision and
where similar column densities (now integrating high densities over short distances) should
then be obtained as for distant stars.
Conclusions
The Sun is embedded in an ionized very low density Local Bubble bounded by a neutral high density
gas wall at a distance of 40 ± 25 pc in the direction of Loop I (Centurion and Vladilo,1991); a
confirmation giving by the X-ray shadows showed by ROSAT data (Egger and Aschenbach, 1995;
Egger, 1998; Fig.7). See for example the schematic distribution of the Local Interstellar Medium in the
book The Guide to the Galaxy (Henbest and Cooper, 1994). The X-ray images can be found in the
internet ROSAT sites :
http://wave.xray.mpe.mpg.de/rosat/images/sn_snr
http://wave.xray.mpe.mpg.de/rosat/calendar/1995/apr
http://wave.xray.mpe.mpg.de/rosat/calendar/1998/may
http://heasarc.gsfc.nasa.gov/docs/rosat/gallery/snr_loop1.html
Egger and Aschenbach (1995) suggested that this "wall" is the result of the collision of Loop I
and the Local Bubble, both bubbles expanding in opposite directions and compressing the
other limiting external neutral gas when both expanding shells approach each other.
16
But in the colliding zone, ionized gas could also be formed because the expansion of both the
Loop I and the Local Bubble, is the result not only of solar and stellar winds but mainly, the
result of successive radiative shock waves produced by SN events at the origin of the radio
Loop I.
The high temperature and high ionized gas produced and pushed away by the successive
passages of these shock waves can also slow down and concentrate, when collides with the
hot gas coming into the opposite direction.
The fact that there is a HI wall indicating a high neutral matter concentration favours the
hypothesis that there should also be a concentration of matter at other higher temperatures
around the neutral colliding zone, because matter is being aggregated from both sides of the
colliding region.
Very hot (106 K) expanding gas by colliding in the interaction zone can produce hot (several
104 K) IS localized medium with densities 20 to 30 higher than those over other sky directions
where no other major IS interactions occur. These hot IS region could be appropriate to
produce locally cloudlets of moderate ionized Si and C atoms.
We conclude that probably, most of the SiIV and CIV faint absorptions we observe in the
majority of the selected stars, originate at the colliding region between the Loop I and the
Local Bubble, which could be placed at a heliocentric distance between 50 to 90 pc,
depending on the sky directions.
Acknowledgments
This reasearch has been made use of SIMBAD database operated at the CDS and the
Strasbourg Observatory (Strasbourg, France) and the INES IUE database operated at the
LAEFF, VILSPA (Villafranca del Castillo, Madrid, Spain).
We thank A. Talavera for invaluable comments about this work.
C.M.D. is supported by DGICYT grant ESP2001-4527-PE.
References
Bruhweiler F.C., Grady C.A., Chiu W.A. 1989, ApJ 340, 1038-1048.
Centurión M., Vladilo G. 1991, ApJ 372, 494-504.
Egger R., Aschenbach B. 1995, A&A 294 L25-L28.
Egger R. 1998 LNP-506, The Local Bubble and Beyond. Lyman-Spitzer Colloquium,
Proceedings of the IAU Colloquium No.166 held in Garching, Germany, 21-25 April, 1997,
XXVII, 603pp. Springer-Verlag, p.287.
Freire Ferrero, R., 1986, A&A 159, 209-216.
17
Freire Ferrero, R.,1988, in ESA SP-281 Vol.II, Proceedings of A Decade of UV Astronomy
with the IUE Satellite , GSFC, Greenbelt, Maryland, USA, p.227-230.
Freire Ferrero, R., 1998a, LNP-506, The Local Bubble and Beyond. Lyman-Spitzer
Colloquium, Proceedings of the IAU Colluquium No.166 held in Garching, Germany, 21-25
April, 1997, XXVII,603pp. D.Breitschwerdt, M.J.Freyberg \& J.Tr\"umper (Eds.), SpringerVerlag, p.99.
Freire Ferrero, R., 1998b, ESA SP-413, Ultraviolet Astrophysics Beyonf the IUE Final
Archive, Sevilla, Spain, 11-14 November 1997, p. 475.
Freire Ferrero, R.\& Ferlet, R., 1988, in New Insights in Astrophysics, Proc.Joint
ESA/NASA/SERC IUE Conf., ESA SP-263, 319-322.
González Riestra R., Cassatela A., Solano E., Altamore A., Wamsteker W., 2000, A&AS 141,
343, "The INES system. III. Evaluation of IUE NEWSIPS high resolution spectra."
Hauck B., Jaschek C., 2000 A&A 354, 157-162, "A-shell stars in the Geneva system".
Henbest, N. , Cooper, H., The Guide to the Galaxy, Cambridge Universitu Press, 1994
Sembach K.R., Savage B.D., Tripp T.M. 1997 ApJ 480, 216-234.
18