A&A 408, 1065-1076 (2003)
DOI: 10.1051/0004-6361:20030889
M. Hempel 1 - H. Holweger2
1 - Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
2 - Institut für Theoretische Physik und Astrophysik,
Universität Kiel, 24098 Kiel, Germany
Received 14 January 2003 / Accepted 2 June 2003
Abstract
Based on high S/N spectra obtained at La Silla, Chile, and
the Special Astrophysical Observatory, Russia, the abundances of
He, C, O, Ne, Mg, Si, Ca, Fe, Sr, and Ba in 27 optically
bright B5-B9 main-sequence stars were determined. NLTE effects were
taken into account.
A variety of abundance patterns
is present in late B stars. Accurate surface abundances of the diffusion
indicators O, Mg, Ca, Sr and Ba suggest that element stratification
due to diffusion is common in the program stars. Models of stellar
atmospheres which include meridional mixing can explain the observed
anomalies.
Although
the program stars represent only a volume-limited sample of the
solar neighbourhood this result is important for the cosmochemical
evolution of the Galaxy: the surface abundances of the stars
investigated do not necessarily reflect the chemical composition
of the interstellar cloud they originated from. Furthermore,
five program stars show narrow absorption lines in Ca II K which
can be attributed to circumstellar gas.
Neon serves as a trace element for the occurrence of weak
stellar winds. Neon overabundances of some stars derived under the assumption
of LTE suggest that such winds have been detected. In sharp contrast,
the more realistic treatment of NLTE leads to solar neon abundances and
thus reveals that weak stellar winds are absent in the program stars.
Key words: stars: abundances - stars: atmospheres - stars: chemically peculiar - stars: winds, outflows
In stars of the upper main-sequence the increasing effective temperatures cause the ionisation zones of hydrogen and helium to be shifted more and more towards the stellar surface. As a result the outer convection zones of these stars become thinner and less turbulent. But this absence of turbulent convection does not necessarily lead to ideal, static atmospheres. Some A and B stars show spectroscopic signatures of microscopic and macroscopic transport processes. The diagnostics of these processes yield valuable information about the behaviour of an element under the influence of gravitation and radiation. Even in nonmagnetic stars this leads to a variety of abundance patterns.
On the cooler end of the upper main-sequence the well known Fm-Am stars are not exceptional (Wolff 1983). Michaud (1970) invoked diffusion processes as an explanation for those abundance anomalies. Since then various studies on both theoretical and observational aspects of abundance anomalies have been carried out (see, e.g., Gonzalez et al. 1995; Alecian 1996). The basic idea is that the competition of gravitative settling and radiative levitation leads to element separation via diffusion that occurs directly below the convection zone. Depending on its absorption cross-section a certain element in this reservoir will either be enriched or depleted. With the atomic data from the Opacity Project (Seaton et al. 1992) former shortcomings of theoretical calculations could be improved (Gonzales et al. 1995). Abundance determinations of A stars classified as "normal'' have shown a variety of abundance anomalies (Holweger et al. 1986; Gigas 1986, 1988; Lemke 1989, 1990). In normal A stars diffusion leads to deficiencies of Ca as well as overabundances of strontium and barium.
In the same temperature regime the -Bootis-phenomenon may occur
in pre-main-sequence stars.
The characteristic metal underabundance of these young stars is
attributed to a different mechanism of element separation:
the accretion of metal-deficient gas from the
circumstellar environment after gas-dust-separation
(Venn & Lambert 1990). As a result
the volatile
elements carbon, nitrogen, and oxygen have nearly solar abundances
while heavier elements with higher condensation temperatures
are locked up in the dust grains and therefore are deficient in the
stellar atmospheres (Stürenburg 1993;
Paunzen et al. 1999). The
-Bootis-phenomenon
is believed to occur in pre-main-sequence stars at the end of
their accretion phase (Holweger & Rentzsch-Holm 1995).
Depending on the accretion rate the interplay of accretion and
diffusion can lead to over- or
underabundances of certain elements.
In fast rotators, meridional circulation can reduce these
abundance anomalies (Turcotte & Charbonneau 1993).
The comparatively short phase of pre-main-sequence evolution
is consistent with the paucity of the
-Bootis stars. Interestingly, the
standard star Vega is believed to be a member of this
group.
On the hot end of the main-sequence, radiative processes are dominant and lead to massive radiatively-driven stellar winds in O stars and early-type B stars (Kudritzki & Hummer 1990). These stellar winds may lead to spectroscopic signatures of substantial mass loss (Kilian 1992). Furthermore, in fast rotators products of nuclear fusion can emerge to the stellar surface. In spite of their rarity, these stars play an important role in the enrichment of the interstellar medium with elements like carbon, nitrogen, or oxygen.
The transition between the diffusion-accretion dominated
atmospheres and the wind-driven atmospheres lies in the region
of the late B stars which are subject of this study. Theoretical
approaches suggest that with increasing effective temperatures from
the regime of the A stars, selective stellar winds set in (Babel 1995)
which blow away metals with
large absorption cross sections due to radiation pressure.
One can expect that such a "metallic'' wind changes the composition
of the stellar surface, and may even contribute to the
metallicity of the interstellar medium. Furthermore, Landstreet et al. (1998) suggests
certain elements, like
neon, are tracers for the detection of weak stellar winds:
overabundances of these elements provide valuable evidence for
the presence of weak stellar winds in the order of
/yr. Up to now, these weak stellar
winds have not been
detected spectroscopically. Their
detection would be of great importance
for diffusion theories as they often have to be invoked
to explain mild discrepancies between observations and
models (see, e.g., Alecian 1996).
To date the previously described transition region in the range of the late B stars has been investigated only very sparsely, and literature on the processes of element separation is scarce. The status of research is documented in the studies of Adelman & Philip (1996), Smith & Dworetsky (1993), and Smith (1993, 1996). Adelman & Philip (1996) state that "The general pattern of subsolar abundances among B stars [...] is contrary to what is expected for stars which are much younger than the Sun''. In some of the 10 investigated northern B stars, abundance anomalies are found, but a diagnosis is carried out only in view of the chemical evolution of the Galaxy. In addition, the data are based largely on older photographic analysis, including low-dispersion, photographic spectra. Smith and Dworetsky analyse IUE-spectra of 8 normal B 5-B 9.5 stars of luminosity classes V and IV. Their result, "Approximately solar abundances of these elements are obtained for the normal stars'', is obviously contradictory to the findings of Adelman & Philip (1996). This suggests that in the crowded UV line spectrum of these stars, only large abundance anomalies can be traced. In all the mentioned papers deviations from LTE are neglected. Some of the stars investigated in this study have been analysed by Wohler (1996) and van Thiel (1997) assuming LTE. Their findings suggest star-to-star variations of carbon, magnesium, calcium, and iron, but no definite indications for diffusion could be found. Depending on the ionisation stage and the line strength, deviations from LTE can become significant, and neglecting NLTE effects - as usual in the case of A and late B stars - can conceal abundance patterns and make correlations insignificant.
By investigating the abundances of key elements for element separation processes, the present study aims at a coherent picture of the chemical composition of 27 late B stars based on optical spectra. Deviations from LTE are taken into account, and the results are discussed in view of diffusion processes and weak stellar winds.
We have studied 27 B5-B9 stars (see Table 1) taken from the Bright Star Catalogue (Hoffleit & Warren 1991, BSC) and classified as "normal'', i.e. they neither have peculiar spectra nor emission lines. The sample consists of 20 southern and 7 northern hemisphere stars.
Table 1: Parameters of the program stars. The data were taken from the Bright Star Catalogue (Hoffleit & Warren 1991). Distances have been calculated using HIPPARCOS parallaxes (ESA 1997). The last column indicates whether a star was observed from ESO (E) or SAO (S).
The spectra of 20 stars have been collected by A. Kaufer in February and March 1995 with the ESO-50-cm telescope equipped with the HEROS spectrograph. The spectra have a resolution of R=20 000. Data reduction and wavelength calibration was carried out by A. Kaufer using MIDAS.The spectra of the 7 other stars have been obtained by G.A. Galazutdinov and F. A. Musaev in March 1999 with the SAO 1-m-telescope using the echelle spectrograph. The wavelength range covers 3500 Å to 10 000 Å with a resolution of R=45 000. Data reduction, as well as wavelength calibration, was carried out by the observers using their echelle software package. The last column of Table 1 indicates the site of observation. The spectra of both datasets have a S/N of about 100.
The basic stellar parameters
and
(see Table 1) have been
obtained via Strömgren photometry (Hauck & Mermillod 1990)
using the calibration of Napiwotzki et al. (1993). The ATLAS9 code
(Kurucz 1993) was used to calculate
relations assuming solar
metallicity. The temperature structure serves as an input to our
LTE/NLTE system.
Pressures and particle concentrations were derived from
the
relations using our ATMOS code. The spectrum synthesis was
carried out with our line formation code LINFOR (Lemke 1991).
Lemke (1989) found a microturbulence of
in his analysis of A stars, and the work of Fitzpatrick & Massa (1999)
on the physical properties of B stars reveals microturbulent velocities
between 0
and 1.9
for six out of seven
objects having similar
and
values to those of our program stars.
In accordance with the findings of Krege (1995) we adopted a microturbulence
of
.
We note that our NLTE abundance analysis of Si II and Fe II lines of different strengths revealed consistent abundances of weak and strong lines. This
suggests that
is a good estimate of this poorly
defined parameter.
The VALD database (Piskunov et al. 1995) was
used to select lines for abundance analysis and to identify blend lines.
The
values were also taken from the VALD compilation, except for carbon
and oxygen for which we used the more recent data of Wiese et al. (1996).
B stars are known to have very large rotational velocities,
up to
,
which
lead to extremely broadened line spectra. This causes difficulties for a reliable
spectrum synthesis. Therefore we have selected the 20 southern hemisphere objects
with
.
We have dropped this restriction
for the 7 northern stars. The reason is the detection of narrow absorption features
in the Ca II K line and will be discussed in Sect. 6. Nevertheless the
rotational velocities (listed in Table 1) we derived via spectrum synthesis
reveal large errors of the values given in the BSC.
Based on the assumptions that
,
dex, and
we carried out
calculations in order to estimate the errors of our analysis.
Both effective temperature and
variations lead to maximum abundance uncertainties
of
0.1 dex, and variation of
yielded abundance changes below
0.05 dex.
Thus we found that the typical error of our abundances is between 0.2 and 0.3 dex.
Apart from iron, solar abundances are from Anders and Grevesse (1989); for iron the revised value of 7.51 Holweger et al. (1995) was used. Lines of ten elements, namely helium, carbon, oxygen, neon, magnesium, silicon, calcium, iron, strontium, and barium, were selected for abundance analysis. The lines are compiled in Table 2. Additionally, we included all relevant blend lines.
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Figure 1: Some typical observed (solid lines) and synthetic spectra (dashed lines). Note the narrow absorption feature in the core of the Ca II K line of HR 1070 (see Sect. 6 for details). |
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Table 2: This table comprises all lines used for the abundance analysis. In Cols. 3 and 4 we compiled the contributions of blend lines and additional comments, respectively.
A closer look at the LTE results in Tables 5-7 reveals that some stars show conspicuous deviations from solar values clearly above the error limit of 0.3 dex. Oxygen shows pronounced overabundances in most stars, and neon is overabundant in some objects by nearly 0.6 dex. Studies on oxygen (Baschek et al. 1977; Takeda 1992; Paunzen et al. 1999) and neon abundances (Dworetsky & Budaj 2000) reveal that these elements are expected to show large non-LTE effects. The values for magnesium scatter largely between -1.3 dex and 0.31 dex with most abundances being subsolar. Nevertheless, some stars have definitely non-solar magnesium abundances. Silicon and iron show scatter around the normal, solar abundances. Calcium has obvious underabundances in some objects. Lastly, the heavy elements strontium and barium are overabundant in most cases.
Table 3: Energy levels of the O I model atom.
To sum this up one can state that clear compositional differences have been found in the program stars. The question arises whether these abundance patterns are real or due to the inadequate assumption of LTE. To investigate this the NLTE calculations outlined in the next section were carried out.
NLTE corrections were carried out using the Kiel NLTE-code (Steenbock & Holweger 1984) for C, O, Ne, Mg, Si, Ca, Fe, Sr, and Ba. In the following some details concerning the model atoms are provided.
The carbon model atom by I. Kamp (Rentzsch-Holm 1996)
contains 83 C I levels, 15 levels
of C II and 79 line transitions. It is an improved version of an older
C I/II atom with a total of 88 levels and 66 transitions (Stürenburg & Holweger 1990).
A detailed description of the atom can be found in Rentzsch-Holm (1996).
Although the NLTE corrections are not negligible, they are not very
pronounced for most of the program stars which is in accordance
with the findings of Rentzsch-Holm (1996). In fact, only
for four stars of the twelve objects where carbon lines have been
analyzed are the corrections above 0.2 dex.
No temperature dependency can be found for the sample stars. As outlined
by Stürenburg & Holweger (1990) and Rentzsch-Holm (1996)
NLTE corrections of strong carbon lines depend on the equivalent
width. This effect is only important for lines with
mÅ.
Because the lines of the program stars are weaker this is not observed here.
The NLTE abundances of most stars investigated here scatter around
the solar value.
The O I 7771-5 triplet investigated in this study shows large
non-LTE effects as outlined by Baschek et al. (1977)
and Takeda (1992). Furthermore, from previous work on
A stars (Paunzen et al. 1999) non-LTE corrections of up to
-0.7 dex can be expected.
An O I model atom containing 15 energy levels and 17 transitions has been developed
by I. Kamp & M. Hempel for the analysis of Bootis stars
(Paunzen et al. 1999).
This model atom is not appropriate
for the application to B stars having sufficiently higher temperatures. Therefore an
improved version was developed which is based on the old one described in
Paunzen et al. (1999). It contains 29 energy levels plus the
continuum of O II and 71 line transitions.
The energy levels have been obtained from the Atomic Spectra Database of
NIST
. All energy levels
up to 5f at 13.07 eV were implemented and are listed in Table 3. Higher levels will only have negligible effects.
Table 4:
Line transitions of the O I model atom: Lower level number
(Low), upper level number (Up), wavelength and -value (Wiese et al. 1996).
For most of the program stars the NLTE corrections are in the range of
-0.3 dex to -0.7 dex. Somewhat exceptional is the large correction
for HR 3717 with -1.18 dex. The reason for this is the low gravity of
- the second lowest of the whole sample. A lower surface
gravity leads to a weaker coupling between the levels because the
collisional rates decrease due to lower electron and gas pressures.
Therefore NLTE effects are more important.
The neon model atom was developed by
J. Graf (2000)
and consists of 45 Ne I levels, 47 Ne II levels, and 120 transitions.
The atomic data was obtained
from the Opacity Project (Seaton et al. 1992) and the NIST compilation.
The oscillator strengths are based on
Seaton (1998) and Kurucz & Peytremann (1975).
The NLTE corrections are negative for all sample stars. While neon shows
conspicuous overabundances in LTE for some stars, the NLTE abundances are
rather solar. This is of importance for the question of weak stellar
winds and will be discussed later.
The MgI/II atom contains 99 levels and 71 line transitions and is an improved version by Gigas (1988) of the model developed by Lemke (1986). The model atom was adapted by Gigas for the use with ATLAS6 models having a frequency grid of 336 points. This grid defines the ODF averaged mean intensities and is used for the computation of the photoionization rates. For the calculations carried out in the course of this study the more recent ATLAS9 version was used and therefore the extended frequency grid containing 1212 points had to be implemented in the model atom. As outlined by Gigas (1988) the corrections for Mg 4481 are expected to be negative. This result is confirmed here. Nevertheless the deviations from LTE are small, usually well below -0.2 dex. Therefore LTE is a good approximation of Mg 4481 abundances for late B stars.
The silicon atom used was developed by Wedemeyer (2001). It includes 115 energy levels of Si I/II and 84 lines. The NLTE calculations show negative corrections for the sample stars with the lowest temperatures whereas the deviations for the hotter ones turn to positive values. About 50% of the program stars show solar values within the error limits.
The Ca I/II model was developed by
W. Steenbock and is based on the work of Watanabe & Steenbock (1985)
on the Sun and Procyon. The improved version used for this study
contains 125 levels with 100 transitions.
The corrections for the Ca II K line are generally small and
positive in most cases. Only for three stars do we find
dex.
Most of the stars which have non-solar Ca abundances within the error limits
show conspicuous NLTE underabundances up to -0.85 dex. This will be
discussed further in Sect. 7.
An iron model containing 99 energy levels of Fe I and Fe II with 75 lines was developed by W. Steenbock & T. Gehren. An extensive study using this model in a temperature range between 7000 K and 12 000 K was carried out by Rentzsch-Holm (1996). Iron shows only slightly negative abundance corrections below -0.1 dex in NLTE. Therefore LTE is a good approximation for the program stars. This is in accordance with the results of Rentzsch-Holm (1996).
The strontium model atom (Belyakova et al. 1997) incorporates 41 levels of Sr II including the ground state of Sr III. The level energies were taken from Moore (1952) and Lindgard & Nielsen (1977), and the f values were adopted from Wiese & Martin (1980), Lindgard & Nielsen (1977), and Kurucz (1994). Further details concerning the model atom can be found in Belyakova et al. (1997). Non-LTE calculations for strontium were kindly performed by Elena Belyakova using the Kazan non-LTE code (Belyakova et al. 1999). With one exception (HR 2948) strontium lines of sufficient strength were only found in sample stars below 12 000 K. Strontium is overabundant in LTE in most of those objects. The NLTE corrections are positive in all cases and quite conspicuous. This deserves some comments in Sect. 7.
The Ba II model by D. Gigas (1988) includes 42 levels and 36 transitions. Only in one star was a barium line of sufficient strength found. A new ATLAS9 frequency grid had to be provided for the model atom. The NLTE correction of 0.4 dex is in accordance with the findings of Gigas (1988).
Table 5: LTE and NLTE abundances. The NLTE corrections and abundances of C, O and Ne are given with respect to the solar values.
Table 6: LTE and NLTE abundances. The NLTE corrections and abundances of Mg, Si and Ca are given with respect to the solar values.
Table 7: LTE and NLTE abundances. The NLTE corrections and abundances of Fe, Sr and Ba are given with respect to the solar values.
For helium no NLTE calculations were carried out in the course of this study. Leone & Lanzafame (1998) investigated the behaviour of several He I lines - including two lines at 5876 Å and 6678 Å which were as well examined in this work - in a wide temperature range. Their comparison between their NLTE calculations and LTE models of various authors reveal that for spectral types between A0 and B3 equivalent widths and thus LTE and NLTE abundances of these two lines are in concordance. Therefore in the case of the scrutinized He I lines LTE is a good approximation for the program stars.
In some cases we found narrow
absorption features in the Ca II K line and - after removing telluric
lines - in Na D of
two stars as well (see
Table 8). They are similar to
those detected
in our former work on A stars (Holweger et al. 1999).
The most prominent star
with narrow Ca II K absorptions is Pictoris. Its Ca II K profiles show
redshifted narrow absorption features with a time dependency of the order of
weeks and months which is attributed to the infall of cometary-like objects
which evaporate as they approach the stellar
surface (see e.g. Lagrange-Henri et al. 1992). In addition about 30% of the
28 normal A stars and 18
Bootis stars studied by Holweger et al. (1999) show
detectable Ca II K features. The question arises as to whether these
absorptions are
of interstellar or
rather of circumstellar origin. As outlined
in Holweger et al. (1999) such features have only been found in stars
with
80
.
This is true for
the B stars discussed in this work as well. Therefore a stellar
property is correlated with the occurrence of Ca II K feature
which supports a circumstellar origin. Holweger & Rentzsch-Holm (1995)
give the following tentative interpretation: for stars with
circumstellar gas concentrated in a disk-like structure the column
density of absorbing
gas along the line of sight will be at its maximum if the disk is
viewed
edge-on. Therefore circumstellar absorption lines should be detected
preferably in objects with
.
Hence the chance to find
a star with narrow absorption lines and low
is small.
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Figure 2:
Correlations between NLTE abundances of various elements. In the lower right
corner of the plots there is an error box representing the typical error of ![]() |
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The results presented in Tables 5-7 reveal that the abundances of the program stars are widely distributed. A closer inspection of the data shows some interesting correlations.
In some stars cool enough to show detectable strontium and barium lines, we detect NLTE overabundances of these elements. Most obviously, the diffusion indicators barium and strontium are clearly overabundant in HR 1728, HR 1973, HR 2948, HR 3439, HR 5501, HR 5994, HR 6633, HR 6668, and HR 6878. Furthermore, some of these stars (HR 1973 and HR 3439 as well as HR 6633 and HR 6668) show similar abundance patterns which suggests that they can be ascribed to the same physical process. In the widely accepted diffusion scenario (Michaud 1970; Michaud & Charland 1986) this indicates element separation by diffusion in their outer layers. In addition to that the three other scrutinized diffusion indicators (oxygen, magnesium, and calcium) are deficient in the strontium and barium overabundant stars HR 1728, HR 1973, HR 3439, and HR 6878. This strengthens the diffusion hypothesis. Some program stars show abundance anomalies of the diffusion indicators to a lesser extent while other stars, namely HR 1723, HR 2056 and HR 4119, have abundances which are close to the solar values. For stars with deviations from solar composition similar abundance patterns were as well found for HR 3158, HR 4116, HR 4943 and HR 8781 and for HR 806, HR 1070, HR 1092 and HR 1723. We illustrate these correlations in Fig. 2. All stars where both strontium and calcium were analysed are overabundant in strontium and solar or underabundant in calcium. In fact, only one of these stars, namely HR 6633, has a solar value of calcium. The values obtained for strontium reveal as well a correlation with the oxygen abundances: while strontium is radiatively driven outwards oxygen sinks down. For strontium and magnesium the situation is similar: all stars where magnesium is deficient show overabundances of the diffusion indicator strontium. In contrast, neon - which is not an indicator of diffusion - shows a very small scatter and illustrates the quality of the NLTE-analysis.
The anomalies detected in the program stars do not occur to the same
extent in all stars but show star-to-star variations. This suggests
that a counterpart to diffusion may blur the abundance anomalies.
B-type stars are fast rotators. The rotation of a star causes
accelerations which lead to the mixing of the stellar envelope.
The ability of stellar rotation to inhibit effective diffusion
processes was already mentioned in the fundamental
paper by Michaud (1970). This
so-called meridional mixing is well-known to occur in A stars. The
effectivity of the mixing of the stellar atmosphere increases with
increasing rotational velocity. Therefore in a certain sample of
stars with abundance anomalies, we expect to find more
pronounced deviations from the normal values in slow rotators than in
fast rotating stars. This is nicely illustrated for the case of
Bootis stars in Holweger & Rentzsch-Holm (1995). Their Fig. 13
shows the calcium abundances of their
Bootis stars investigated as a function
of rotational velocity. While stars with low rotation show conspicuous
underabundances, the fast rotators approach solar values.
Table 8: Equivalent widths of the detected narrow absorption features.
Figure 3 shows the abundances of calcium as a function of .
Abundances
of stars with rotational velocities below
100 km s-1 show a large scatter while the scatter
for fast rotators (
100 km s-1) is less pronounced.
The abundances
of the two fastest rotators (HR 1092 and HR 5685) are afflicted with larger
errors of 0.4 dex. Therefore
the result obtained from Fig. 3 is in accordance
with simulations carried out by Turcotte & Charbonneau (1993) which have
shown that rotational velocities above 125 km s-1 lead to an effective mixing
of the stellar envelope.
Given our presented results we suggest that the variety of diffusion-driven abundance patterns known from A stars continues to the temperature regime of late B stars and conclude that the abundance anomalies discovered in some of the program stars can be attributed to diffusion combined with meridional mixing.
In OB stars radiatively driven stellar winds are common
(Abbott 1980, 1982; Friend & Abbott 1986).
It is well-known that these winds not only lead to substantial
mass loss with rates up to
/year (Maeder 1983)
but also affect the chemical composition of stars above 20
(Kilian 1992).
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Figure 3:
The NLTE abundances of calcium as a function of ![]() |
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The masses of early-type B stars are far above the masses of the program stars but the objects
investigated in this study are situated in a transition region between the
diffusion dominated stellar surfaces and the wind driven atmospheres of hot
B stars. In this temperature range the occurrence of weak stellar winds
with a mass-loss rate of
/year is theoretically
predicted (Babel 1995). These winds may compete with diffusion processes
and are expected to affect the chemical composition of the stellar surface. Furthermore,
these winds play an important role in the widely accepted diffusion scenario:
the abundance patterns of several stars where diffusion is efficient adopt
in most cases the existence of a weak stellar wind to explain mild
inconsistencies between theory and observation (Alecian 1996;
Hui-Bon Hoa & Alecian 1998). A stellar wind of the order
of
/year would be mild enough not to inhibit
diffusion. Unfortunately, in contrast to the massive
stellar winds of OB stars there is no spectroscopic evidence for the existence
of weak stellar winds to date. A theoretical attempt to detect weak stellar winds
in the temperature range of late B stars was proposed by
Landstreet et al. (1998).
The basic idea is that abundant elements like
carbon, oxygen, and neon which are not expected to be affected by radiative acceleration
may accumulate in the outer layers of a mass-losing star and thus serve as
trace elements for the detection of weak stellar winds. The temperature range examined
by Landstreet et al. (1998) is
K.
In view of these weak stellar winds investigations of two Ne I lines at 6402 Å and
6506 Å were carried out in the course of this study. Unfortunately neon is only observable
for stars with
K. Nevertheless, the LTE
results (Table 5) reveal overabundances
of Ne I of up to 0.58 dex in at least five program stars. This would have been
the first spectroscopic detection of weak stellar winds. Since Ne I is known
to be affected by large NLTE effects, this result is suspect. In order to examine this
in more detail NLTE effects had to be taken into account. The neon model atom developed
by Graf (2000) reveals that the abundance corrections for neon are negative
for all investigated stars. This leads to essentially solar neon abundances
of all stars within the error limits (Table 5). Therefore weak stellar winds
were not detected in the course of this study which suggests that if
suchlike winds are present their intensity is below
/year.
This is in accordance with the results obtained for oxygen which is a tracer of these winds in cooler stars (Landstreet et al. 1998). Moreover, this confirms the findings of Dworetsky & Budaj (2000) who analysed neon in a sample of normal B stars and HgMn stars. Their results reveal essentially solar neon abundances for normal B stars. For the only star in common with Dworetsky & Budaj (2000) - HR 1339 - we considered the neon lines in the spectra as too weak to allow a reliable abundance determination. In a recent study (Budaj & Dworetsky 2002) on radiative accelerations of late B stars they find the radiative accelerations well below the gravitational accelerations which leads them to predict underabundances for neon. As can be seen from Fig. 2 and Table 5 we do not find significant underabundances of neon for our stellar sample. This may be due to some competetive process, e.g. meridional mixing (see Sect. 7.1 and Fig. 3).
Nevertheless, the result achieved for the neon abundances demonstrates the importance of NLTE calculations in the investigated temperature range since an LTE analysis pretends the presence of weak stellar winds.
Because of their brightness and as B stars are in general young objects they are often used for Galactic abundance studies (see e.g. Kaufer et al. 1994). The study of the chemical composition of stars and the variation of stellar abundances within the Galaxy is of fundamental importance for the understanding of Galactic evolution. The stars studied in this work cannot be used for that kind of investigations since they represent only a volume-limited sample within the solar neighbourhood. In order to obtain high S/N spectra the program stars were taken from the Bright Star Catalogue (Hoffleit & Warren 1991), therefore the stars lie in the solar vicinity (see Table 1). A different point is important here: the results of this work reveal that diffusion processes can occur in the outer layers of late B stars. As a consequence, the composition of such a star does not reflect the primordial abundances of the interstellar cloud it originated from and may therefore not be a reliable tracer for Galactic abundance studies if the effects of diffusion are neglected. This result is strengthened by the study of Luck et al. (2000) and reveals the importance of the investigation of late B stars.
The main results of this work can be summarized as follows:
(1) A large fraction of late B stars show anomalous rather than solar
abundances. (2) Combined with meridional mixing diffusion can explain
the observed abundance patterns. (3) The result of the search for the
occurrence of
weak stellar winds of
/year
by using NLTE abundances as a diagnostic tool
was negative. Thus mass loss rates must be below this
limit.
(4) Element separation processes
seem to be common in late B stars. This
constitutes a serious problem for using such stars for galactic
abundance studies. (5) In five stars narrow
absorptions in
Ca II K have been found. They are rather of
circumstellar than of interstellar origin.
Acknowledgements
We would like to express our gratitude to Dr. A. Kaufer, Dr. G. A. Galazutdinov, and Dr. F. A. Musaev for kindly providing the spectra. We thank E. Belyakova for performing the Non-LTE calculations for Sr. We are grateful to the referee for helpful comments. This work was supported by DFG grants Ho 596/38-1 and Ho 596/38-2 and by the DLR under grant DLR 50 OR 0005.