A&A 379, 235-244 (2001)
M. Ramspeck - U. Heber - H. Edelmann
Dr. Remeis-Sternwarte, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
Received 26 July 2001 / Accepted 10 September 2001
We present the result of differential spectral analyses of a further four apparently normal B-type stars. Abundance anomalies (e.g. He, C, N enrichment), slow rotation and/or high gravities suggest that the programme stars are evolved low-mass B-type stars. In order to trace their evolutionary status several scenarios are discussed. Post-AGB evolution can be ruled out. PG 0229+064 and PG 1400+389 could be horizontal branch (HB) stars, while HD 76431 and SB 939 have already evolved away from the extreme HB (EHB). The low helium abundance of HD 76431 is consistent with post-EHB evolution. The enrichment in helium, carbon and nitrogen can be explained either by deep mixing of nuclearly processed material to the surface or by diffusion processes modified by magnetic fields and/or stellar winds. A kinematic study of their galactic orbits indicates that the stars belong to an old disk population.
Key words: Galaxy: halo - stars: early-type - stars: abundances - stars: kinematics - stars: evolution
Tobin (1987) discusses the problem that some highly evolved stars spectroscopically mimic normal massive stars almost perfectly. The most striking example is PG0832+676 which has been analysed several times. Its abundance pattern is close to normal. Only recently, Hambly et al. (1996) were able to firmly establish slight underabundances and a very low projected rotation velocity. Combining both results they concluded that PG0832+676 in fact is a highly evolved star. In a recent paper, Hambly et al. (1997) extended their study to a dozen apparently normal B-type stars and demonstrated that they are also of low-mass.
Abundance analyses as well as determinations of rotational velocities are thus essential to distinguish massive from low-mass stars. A high rotational velocity generally excludes a late evolutionary status of the star, as old, low-mass stars cannot rotate as fast as massive stars.
During our ongoing investigation of faint blue stars we encountered several apparently normal B-type stars (Moehler et al. 1994; Heber et al. 1995; Schmidt et al. 1996). In a recent paper we described the spectral analysis of ten massive B-type stars at high galactic latitudes (Ramspeck et al. 2001, henceforth Paper I).
Here we present the analysis of new high-resolution spectra for four additional stars which were classified as apparently normal from spectra of lower spectral resolution. We have obtained high resolution Echelle spectra for PG 0229+064, PG 1400+389, HD 76431 and SB 939 using the HIRES spectrograph at the Keck I telescope, the FOCES spectrograph at the DSAZ 2.2 m telescope and the CASPEC spectrograph at the ESO 3.6 m telescope. The data set is supplemented by long slit spectra obtained at the DSAZ 3.5 m telescope and the ESO 1.5 m telescope. Details of the observations are given in Table 1 and the data reduction technique is outlined in Paper I.
The atmospheric parameters, rotational
velocities and metal abundances are determined in Sects. 2
and 3. Then the evolutionary status
(Sect. 4) and
the kinematics of the stars (Sect. 5) are discussed.
section summarizes the conclusions.
|Name||Obs. date||Tel. &||(nm)|
|PG0229+064||Jul. 20, 1998 14:37||K+H||360-513|
|Sep. 13, 1998 03:00||CA+F||387-683|
|PG1400+389||Jul. 23, 1996 20:00||K+H||426-670|
|HD76431||Feb. 01, 2000 02:00||CA+F||389-693|
|Long slit spectra|
|PG0229+064||Nov. 21, 1988 04:39||E+B&C||403-491|
|PG1400+389||Apr. 14, 2001 00:28||CA+T||410-495|
|HD76431||Apr. 13, 2001 20:53||CA+T||410-495|
|K+H: KECKI + HIRES (spectral resolution: 0.09 Å).|
|CA+F: Calar Alto 2.2 m + FOCES (sp. resolution: 0.15 Å).|
|E+C: ESO 3.6 m + CASPEC (sp. resolution: 0.2 Å).|
|E+B&C: ESO 1.5 m + B&C (sp. resolution: 2.5 Å).|
|CA+T: Calar Alto 3.5 m + Twin (sp. resolution: 1.0 Å).|
Effective temperatures, surface gravities and photospheric helium abundances are derived by matching the Balmer and helium line profiles to a grid of synthetic spectra calculated from LTE model atmospheres as described in detail in Paper I.
HD 76431, however, turned out to be considerably hotter than the other
programme stars and the assumption of LTE might be questionable.
Therefore a grid of partially
line blanketed NLTE model atmospheres (Napiwotzki 1997) was used.
These models contain hydrogen and helium. The latest version of the NLTE
code by Werner (1986) is used, which employs the Accelerated Lambda
Iteration (ALI) technique (Werner & Husfeld 1985; see Werner &
Dreizler 1999 for details).
|Figure 1: The positions of the programme stars (triangles with error bars) and that of massive B-type stars from Paper I (filled circles) in the ( , ) diagram are compared with evolutionary tracks calculated by Schaller et al. (1992) for main sequence stars. The filled circles are objects with detectable rotation and the triangles for non-rotating objects ( < 5 km ).|
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|Figure 2: Fit of the Balmer and helium lines by synthetic spectra for PG 0229+064.|
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|High Resolution||Low Resolution||Photometry|
The fit was executed for all high and low resolution spectra and the results are listed in Table 2. An example fit is reproduced in Fig. 2 to illustrate the excellent quality of the matching of the observations by synthetic spectra. Errors in effective temperatures were estimated conservatively as 5% and we adopted an error of 0.1 dex for the gravities of all programme stars.
For all stars Strömgren photometry is available, which allowed an independent determination of the effective temperature. We used the program of Moon (1985) as modified by Napiwotzki et al. (1993) to derive the effective temperature and the reddening and compare the photometric temperatures to the spectroscopic ones in Table 2. There is a good agreement between results from low and high resolution spectra and photometry, except for PG1400+389 for which the two available colour measurements result in rather different .
The parameters used for further analyses were taken from the high resolution spectra, except for PG1400+389, because the spectral range covered is larger, allowing to use more Balmer and helium lines in the fit procedure. However, in the case of PG 1400+389 the Echelle and the long slit spectrum covered only three Balmer lines. Therefore, we averaged the parameter derived from high and low resolution spectra.
Our results are shown in a ( , ) diagram and compared to high latitude main sequence stars (from Paper I) and to evolutionary tracks for main sequence stars (1992, Fig. 1). SB939 is located in the main sequence band. The other stars, however, lie below the main sequence and therefore cannot be massive stars. Further clues as to the nature of these stars come from the projected rotational velocities and the chemical abundance pattern.
An outstanding result of the analysis is the non-solar helium abundance of
all programme stars.
While in HD 76431 helium is depleted by a factor of 3 with respect to
the sun, helium is significantly enriched in the
Figure 3 illustrates the influence of the
helium enrichment on the profiles of weak and strong He lines in the case
of PG 0229+064, which is only mildly helium rich.
|Figure 3: Comparison of helium lines in the spectrum of PG 0229+064 to synthetic line profiles for helium abundance derived by the fit procedure and for normal helium abundance (dashed).|
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In previous spectral analyses of three of our programme stars (Saffer et al. 1997) based on low resolution spectra, the peculiar helium abundance could not be obtained because the fit procedure was executed assuming a solar helium abundance. This underestimate of the helium abundance also led to considerably higher effective temperatures and gravities for PG 0229+064 and PG 1400+389 than our analyses.
The non-solar helium abundances is another indicator that PG1400+389 and PG0229+064 are unlikely to be normal massive B-type stars.
The high helium content of SB939 was already pointed out by Langhans & Heber (1986) and is confirmed by our analysis. Langhans & Heber (1986) suggested that SB 939 might belong to the class of intermediate helium stars which are main sequence stars with unusually strong (and sometimes variable) helium lines (for a review see Hunger 1986). They populate a temperature range (22000K to 28000K) somewhat hotter than SB 939 and are found close to the galactic plane (Drilling 1986). If confirmed, SB 939 would be the first intermediate helium star at high galactic latitudes. But still we cannot exclude that SB 939 belongs to the class of intermediate helium stars.
All programme stars display very sharp absorption lines indicating that they are slow rotators unless they are seen pole-on. The components of the Mg II doublet (4481.13/4481.33 Å) are resolved that can be used to estimate the projected rotational velocity. An upper limit of 5kms-1 results for all stars. These low values provide further evidence that the programme stars are unlikely to be main sequence stars.
Metal lines of the species C II, C III, N II, N III, O I, O II, Ne I, Ne II, Mg II, Al II, Al III, Si II, Si III, Si IV, P II, P III, S II, S III, Ar II, Ti II, Fe II and Fe III could be identified. The atomic data for the analysis were taken from several tables:
Then we calculated curves of growth for the observed metal lines, from which abundances were derived. Blends from different ions were omitted from the analysis. In the final step the abundances were determined from a detailed spectrum synthesis (using LINFOR code described above) of all lines measured before. The results of the LTE abundance analysis and the rms errors for PG0229+064, PG1400+389, SB939, and HD 76431 are shown in Table 3. The number of lines used is given in brackets. Beside the statistical rms errors the uncertainties in , and microturbulent velocity (see below) contribute to the error budget.
In order to minimize the systematic errors we have choosen the B-type star Sco for HD 76431 and Her for the other three targets as comparison stars, since they have similar atmospheric parameters to those of our programme stars. These stars have been analysed by Hambly et al. (1997). We redetermined the LTE abundances of Sco and Her using the same atomic data, model atmosphere and spectrum synthesis code as the programme stars and took the equivalent widths of unblended lines measured by Hambly et al. (1997).
Our results for Her agree to within 0.1 dex with those of Hambly et al. (1997) except for C II (0.12 dex), Si III (0.17 dex), S III (0.21 dex) and Fe III (0.36 dex). In particular our statistical error for Fe III is much lower than that of Hambly et al. (1997).
For Sco we also found good agreement with the results of Hambly et al. (1997)
except for C II (0.31 dex), C III (0.17 dex), N III (0.13 dex), Al III (0.18 dex),
S III (0.17 dex) and Fe III (0.43 dex).
The difference for Her and Sco between Hambly et al. (1997)
and our work can be attributed to different oscillator strengths used (esp. for Fe III)
and our neglect of blended lines.
Results are given in Tables 3 and 4 and systematic errors
are adopted for our programme stars as well.
These errors are incorporated in the error bars plotted in Fig. 4.
The determination of elemental abundances is interlocked with the microturbulent
that can be derived if a sufficient number of lines of one ion
can be measured over a wide range of line strengths. In our programme stars
N II and O II lines are most suitable for this purpose since
many lines of these ions can be identified. Microturbulent velocities of
= 5 km
were found for PG 0229+064,
and HD 76431, while a somewhat higher value of = 8 km
deduced for SB 939.
|Figure 4: LTE abundances (relative to Sco for HD 76431 and relative to Her for the other three stars) and errors of the programme stars. Abundances derived from neutral elements are shown as open circles, from singly ionizied ones as filled circles, from doubly ionizied ones as filled triangles and from triply ionizied ones as filled squares.|
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In a differential analysis systematic errors should cancel to a large extent. NLTE effects are small for all elements (0.1 dex, Kilian 1994) except for Ne I, which is overestimated by LTE calculations (Auer & Mihalas 1973) e.g. by 0.60 dex in the case of Her. Correcting our measurements for this NLTE effect the Ne abundances are quite close to that of normal B-type stars (Kilian 1994). Correcting for the NLTE effect on Ne I (0.60 dex, see above) the neon abundance of the three programme stars for which it has been measured is found to be consistent with Kilian's distribution of Ne abundances in normal B-type stars.
The abundances of programme stars with respect to Sco for HD 76431 and Her for the others are plotted in Fig. 4.
It is well known that considerable variations of metal
abundances from star to star occur among normal main sequence B-type stars (e.g.
Kilian 1994). This has to be taken into account when judging
whether a measured abundance pattern is peculiar or not (see also Paper I).
The helium-poor star HD 76431 has C, N and Fe abundances quite similar to Sco. Other metals are slightly deficient with respect to Sco but well within the range of main sequence B-type stars (except for an oxygen underabundance). If this star were not helium deficient it would be difficult to judge whether its abundance pattern is peculiar or not.
The helium-rich stars PG1400+389 and PG0229+064 lie below the main sequence. They display large overabundances of C (0.95dex) and N (0.9 to 1.05dex) with respect to Her. While O and Ne have abundances similar to Her the other metals are somewhat more abundant than in the comparison stars. Such large C and N enhancement is not known to occur in any main sequence B-type star. Therefore it is unlikely that PG1400+389 and PG0229+064 are massive B-type stars.
The helium rich star SB939 lies on the main sequence band and might be an
intermediate helium star.
The metal abundance pattern (see Fig. 4) is very similar to those
of PG1400+389 and PG0229+064. In particular, it shares the large
enhancement of carbon and nitrogen with the latter.
Intermediate helium stars in general display normal C, N and O abundances
(Hunger 1986) and therefore it appears unlikely that SB939 belongs to
the class of intermediate helium stars.
Although our programme stars spectroscopically mimic normal massive B-type stars when studied at low spectral resolution, the high resolution spectral analysis presented above revealed mounting evidence that the programme stars cannot be young massive stars.
Low-mass stars can reach the high temperatures in question during their evolution in the horizontal branch, post-extreme horizontal branch and post-AGB phase (see Heber 1992 for a review). We therefore compare the positions of the four stars in the ( , ) diagram to the predictions of evolutionary calculations for these phases of evolution (see Fig. 5). The gravities of all four stars are too high to be consistent with post-AGB evolution. PG0229+064 and PG1400+389 lie close to the horizontal branch. However, their abundance anomalies are unusual for horizontal branch stars which are mostly helium-deficient instead of helium-rich (as observed). There is general consensus that the abundance patterns of horizontal branch stars are caused by diffusion processes, which lead to the observed underabundances of helium due to gravitational settling. Therefore the identification of PG0229+064 and PG1400+389 as horizontal branch stars may be premature. We may speculate that the C and N enrichment may indicate that dredge-up of material processed by the CN cycle (N) and helium burning (C) may have occurred. This conjecture is corroborated by the observed helium enrichment. This requires deep mixing into the helium core in order to dredge up carbon which is unlikely to occur in horizontal branch stars.
HD76431 and SB939 lie above the horizontal branch. The low helium abundance of HD76431 indicates that diffusion processes are going on in its atmosphere as would be expected for a star evolving from the horizontal branch. Comparing its position to evolutionary tracks of Dorman et al. (1993, see Fig. 5) we conclude that HD76431 is in the post-EHB stage and evolves towards the white dwarf cooling sequence.
The He, C and N enrichment of SB 939 again calls for a dredge-up mechanism which is difficult to envisage, as discussed above already.
Diffusion processes in the atmospheres of SB939,
PG0229+064 and PG1400+389 must be somewhat different from those in
normal HB and post-HB stars. Physical processes that could modify diffusion
are e.g. mass loss and magnetic fields. Therefore it would be useful to search
for the presence of magnetic fields and stellar winds in these stars.
|Figure 5: Comparison of the helium rich stars PG0229+064, PG1400+389, SB 939 and the helium poor star HD 76431 in the ( , ) diagram, to evolutionary EHB-tracks of Dorman et al. (1993), post-early AGB + post-AGB tracks of Schönberner (1983).|
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A study of their kinematics may give additional clues as to the
nature of these stars. Proper motions for our programme stars
have become available recently (see Table 5)
through the Hipparcos/Tycho catalogs
(Perryman et al. 1997; Høg et al. 1998) and the work
by Thejll et al. (1997). Space motions can be derived when radial
velocities and distance are known. These quantities can be determined
|PG0229+064||18.0 3.3||84 16||1|
|PG1400+389||6.9 3.8||223 58||1|
|HD76431||38.0 8.0||233 15||2|
|SB939||18.7 15.4||204 50||3|
Radial velocities were derived from the lineshift of metal lines and corrected to heliocentric values. Results are listed in Table 6. The error of the velocities estimated from the scatter of the velocities derived from individual metal lines is about 2-5 km .
The distance has been calculated from mass,
effective temperature, gravity and the dereddened apparent
magnitude of the stars:
For HD 76431 a trigonometric parallax has been measured by Hipparcos:
mas resulting in a distance of 280
+280-95 pc in
good agreement with the spectroscopic distance ( pc).
|PG0229+064||( , (|
|Figure 6: Meridional projections of the galactic orbits during the last 10Gyrs.|
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In order to find out to which stellar population the stars belong we
calculated galactic orbits backwards in time for 10Gyrs using the program
ORBIT6 developed by Odenkirchen & Brosche (1992).
This numerical code calculates the
orbit of a test body in the galactic potential of Allen & Santillan
(1991). A detailed description of the method is given by Altmann
& de Boer (2000). The
complete set of cylindrical coordinates is integrated and positions and
velocities are calculated in equidistant time steps.
The input for this program version are equatorial coordinates,
distance d from the sun, heliocentric
radial velocities and observed absolute proper motions. Meridional
projections of the orbits are shown in Fig. 6. The maximum
heights above the galactic plane, the eccentricities and orbital velocities
are summarized in Table 7. These data are consistent with
HBB stars analysed by Altmann & de Boer (2000).
The orbital velocities are slightly lower than that of
and the orbits are slightly eccentric.
The small maximum heights above the plane
indicate that the stars belong to an
old disk population.
We have carried out differential spectral analyses of four apparently normal B-type stars. PG0229+064, PG1400+389, HD76431 and SB939 are found to be sharp lined. This indicates that the projected rotational velocity is low ( < 5 kms-1). The former three lie below the ZAMS in the ( , ) diagram. Peculiar helium abundances are found, PG0229+064, PG1400+389 and SB939 being helium rich, HD76431 being helium poor due to diffusion. Therefore we conclude that these four stars cannot be young massive B-type stars but must be evolved low-mass B-type stars.
While PG0229+064 and PG1400+389 could be explained as horizontal branch stars from their position in the ( , ) diagram, HD76431 and SB 939 must already have evolved beyond the horizontal branch. The atmospheric abundance patterns of stars in these phases of evolution are known to be dominated by diffusion processes, the clear-cut indicator being a typical helium deficiency. Such an helium underabundance is indeed observed for HD76431, indicating that it has evolved from the extreme horizontal branch. The other stars are helium rich. The enrichment in helium, carbon and nitrogen can be explained either by deep mixing of nuclearly processed material to the surface or by diffusion processes modified by magnetic fields and/or stellar winds. A search for magnetic fields and stellar winds is proposed. Based on proper motion and radial velocity measurements and spectroscopic distances the galactic orbits of the stars have been calculated backwards in time. An analysis of the orbits indicates that the stars belong to an old disk population.
M.R. and H.E. gratefully acknowledge financial support by the DFG (grant He1356/27-1). We thank Neil Reid who provided us with the Keck observations and Thomas Rauch who obtained the DSAZ TWIN spectra for us.