M. Ramspeck - U. Heber - S. Moehler
Dr.-Remeis-Sternwarte, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
Received 4 July 2001 / Accepted 3 September 2001
Abstract
We present the results of quantitative spectral analyses of ten
apparently normal B-type stars. These stars were found to be young massive
B-type stars at distances of z=2.6 to 7.6kpc from the galactic plane based on
their positions
in the (
,
)
diagram, normal abundance patterns and/or large projected
rotational velocities. We discuss formation scenarios (runaway star
scenarios versus a scenario for star formation in the halo) by comparing
times-of-flight and evolutionary time scales.
For all stars (except SB 357 and HS 1914+7139) both the scales are similar
indicating that the stars could have
formed in the galactic disk and been ejected from there soon after
their birth.
Derived ejection velocities range from 130km s-1 to 440km s-1 and
may be used to constrain models for ejection mechanisms.
Using new proper motion measurements we show that PHL 346,
which was considered the most likely candidate for a young B-type star born in
the halo, can be explained as a runaway star from the galactic plane.
Key words: Galaxy: halo - stars: early-type - stars: abundances - stars: kinematics - stars: evolution
Main sequence B-type stars located far away from the galactic plane are a rare, albeit known phenomenon. In their pioneering paper, Greenstein & Sargent (1974) studied faint blue stars at high galactic latitudes and classified 25% of them as apparently normal OB-type stars at distances from the galactic plane of z= 1-3 kpc. More detailed studies have shown that many, but not all, of the apparently normal stars were in fact highly evolved low-mass stars. Surveys for UV excess objects (e.g., Palomar Green, Hamburg-Schmidt, Edinburgh Cape) have found many new candidates (e.g., Saffer et al. 1997; Rolleston et al. 1999; Magee et al. 1998) and some may be known even in other galaxies (M 31, Smoker et al. 2000). Their properties, possible evolutionary histories and formation mechanisms were reviewed by, e.g., Tobin (1987), Keenan (1992), and Heber et al. (1997).
Tobin (1987) also discusses the problem that some highly evolved stars spectroscopically mimic massive stars almost perfectly. The most striking example is PG 0832+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 PG 0832+676 in fact is a highly evolved star. Abundance analyses as well as determinations of rotational velocities are thus of essential importance for the verification of massive B-type star candidates. 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. This fact was used, e.g., by Heber et al. (1995, HS 1914+7139) and Schmidt et al. (1996, PG 0009+036) to identify massive B-type stars far from the Galactic plane from medium-resolution spectra.
The massive B-type stars in galactic halos can be separated kinematically into two
different categories: those stars with a main sequence
lifetime larger than the time they would need to travel from the plane to
their present position and those with a main sequence lifetime too small to
reach their current position assuming an acceptable velocity vertical to the
galactic disk. The former ones are assumed to be born in the disk and
thereafter ejected from it (runaway stars), while the latter are supposed to be born in the
halo (see Conlon et al. 1988, 1990; Hambly et al.
1993, for more details). Since many years runaway stars are known
to exist, whereas formation of massive stars in the halo has not yet
been confirmed convincingly. The best studied candidate is PHL 346 (Ryans
et al. 1996), a
Cephei star in the halo (Dufton et al.
1998). A search for coeval stars around
PHL 346 (Hambly et al. 1996) met with limited success, since only one
out of 16 A- and B-type stars around
PHL 346 was found to have the appropriate spectral type and radial velocity.
Calculations of galactic orbits are thus very important to determine the true nature of the stars, since they also allow to determine ejection velocities from the galactic disk. However, accurate proper motions are a prerequisite for such an analysis. A big step forward has been achieved by Thejll et al. (1997) and the Hipparcos/Tycho mission (Perryman et al. 1997; Høg et al. 2000).
In this paper we present the analysis of new high-resolution spectra for 10 apparently normal B-type stars. Half of the sample are new discoveries whereas the other half has already been studied previously (Conlon et al. 1992; Ryans et al. 1996; Rolleston et al. 1999; Heber et al. 1995). For the latter the times-of-flight quoted in the literature appear to be larger than the evolutionary times indicating they might have formed in the halo. However, proper motions were not available rendering these estimates of the times-of-flight uncertain. Since proper motion measurements became available recently for three of these stars, it was deemed necessary to reanalyse them from new high resolution spectra.
We have obtained high resolution spectra for all programme stars using the HIRES spectrograph at the Keck I telescope, the FEROS spectrograph at the ESO 1.5 m telescope, the FOCES spectrograph at the DSAZ 2.2 m telescope and the CASPEC spectrograph at the ESO 3.6 m telescope (see Table 1). Since the normalization procedure for the Echelle spectra is cumbersome for the rather broad Balmer lines (see below) it was deemed necessary to secure low resolution spectra at least for some of the programme stars to obtain independent estimates of the atmospheric parameters, in particular of the gravities. Appropriate spectra were obtained at Calar Alto and at ESO. Details are given in Table 1. For PG 1511+367, PG 1533+467 and PG 1610+239 we have new low resolution spectra presented in this paper. These spectra were taken with the TWIN spectrograph at the DSAZ 3.5 m telescope and reduced as described in Edelmann et al. (2001). The low resolution spectrum of PG 0122+214 were reduced as described in Moehler et al. (1997).
For the Keck-HIRES spectra observed in 1998 the blue cross disperser was used
and the spectra cover the blue spectral range (3600 to 5130 Å).
The standard data reduction as described by
Zuckerman & Reid (1998)
resulted in spectral orders that have a somewhat wavy continuum.
Due to the merging of the higher Balmer lines the removal of the remaining
waviness is difficult.
We used the spectrum of H1504+65
(a very hot pre-white dwarf devoid of hydrogen and helium, Werner
1991),
which was observed in the same night, for
rectification of our spectra. Its spectrum has only few weak lines
of highly ionized metals in the blue (3600-4480 Å) where the strong Balmer
lines are found in the B-type stars. Therefore we normalized
individual spectral orders 1 to 20 (3600-4480 Å) of the B-type stars by
dividing through the smoothed spectrum of H1504+65. The remaining
orders were normalized by fitting the continuum with spline functions
(interpolated for orders 26 and 27 which contain H).
Judging from the match of line profiles in
the overlapping parts of neighboring orders this procedure worked
extremely well.
In 1996 the red cross disperser of the Keck-HIRES spectrograph was used and
therefore the blue
part of the spectrum shortwards of 4200 Å was not recorded.
Since no merging Balmer lines are present in this spectral range the waviness
of the spectrum could be
removed by fitting the continuum with spline functions. For orders
containing broad Balmer lines the fit functions were interpolated between
neighbouring orders.
The FOCES spectra were reduced as described in Pfeiffer et al. (1998) with a software package developed by the Munich Group. For the FEROS data the MIDAS reduction pipeline (François 1999) was used. For CASPEC data the procedure described by Heber et al. (1986) was applied. The CASPEC and FEROS spectra were normalized in a similar way as described for the Keck HIRES spectra of 1996.
Due to an error in the wavelength calibration no reliable radial velocity
could be measured for HS 1914+7139.
Name | Observation Date & | Telescope & | Resolution | Wavelength Range |
Time (UT) | Instrument | (FWHM Å) | (Å) | |
Echelle spectra: | ||||
PG 0122+214 | July 20, 1998 14:48 | Keck HIRES | 0.09 | 3600-5130 |
PG 1511+367 | July 20, 1999 20:30 | Calar Alto 2.2 m FOCES | 0.15 | 3890-6995 |
PG 1533+467 | July 19, 1999 21:00 | Calar Alto 2.2 m FOCES | 0.15 | 3890-6995 |
PG 1610+239 | July 24, 1996 - | Keck HIRES | 0.09 | 4265-6720 |
PG 2219+094 | July 20, 1999 03:00 | Calar Alto 2.2 m FOCES | 0.15 | 3890-6995 |
PHL 159 | Sep. 12, 1998 22:00 | Calar Alto 2.2 m FOCES | 0.15 | 3870-6830 |
PHL 346 | Oct. 19, 1986 22:50 | ESO 3.6 m CASPEC | 0.20 | 4070-5130 |
SB 357 | Oct. 1984 | ESO 3.6 m CASPEC | 0.20 | 4060-5090 |
HS 1914+7139 | June 05, 1996 13:08 | Keck HIRES | 0.09 | 4280-6720 |
BD-15![]() |
Sep. 08, 2000 07:49 | ESO 1.5 m FEROS | 0.09 | 3630-8860 |
low resolution spectra: | ||||
PG 0122+214 | Aug. 07, 1990 02:32 | Calar Alto TWIN 3.5 m | 3.5 | 3875-5010 |
PG 1511+367 | July 18, 1999 20:49 | Calar Alto TWIN 3.5 m | 2.9 | 3350-7700 |
PG 1533+467 | July 18, 1999 21:00 | Calar Alto TWIN 3.5 m | 2.9 | 3350-7700 |
PG 1610+239 | Apr. 11, 2001 | Calar Alto TWIN 3.5 m | 1.0 | 4100-4950 |
PG 2219+094![]() |
June 10, 1987 08:35 | ESO MPIA 2.2 m B&C | 2.5 | 4035-4900 |
To derive atmospheric parameters (effective temperature, surface gravity,
and photospheric helium abundance) and projected rotational velocities all
Balmer lines
and the He I lines
4026 Å, 4388 Å, 4438 Å,
4472 Å, 4713 Å, 4922 Å, 5016Å, 5048 Å, 5678 Å,
in the observed spectra were fitted with synthetic line
profiles calculated from model atmospheres.
We computed LTE model atmospheres using
the program of Heber et al. (2000), which calculates
plane parallel, chemically
homogeneous and fully line blanketed models, using the
opacity distribution functions for metal
line blanketing by Kurucz (1979, ATLAS6).
From these model atmospheres synthetic spectra were calculated with
Lemke's version of
the LINFOR program (developed originally by Holweger, Steffen, and
Steenbock at Kiel University). The spectra
include the Balmer lines H
to H22 and the He I lines listed
above
and the grid covers the range 11000 K
40000 K,
3.5
6.5 and
at solar
metallicity.
The fit procedure is based on a
test using the routines
developed by Bergeron et al. (1992) and Saffer et al.
(1994) and modified by Heber et al. (1997)
to derive also the rotational
velocity.
The theoretical spectra are convolved with the
instrument profiles (Gaussian with the appropriate instrumental FWHM) and a
rotational profile. The fit program then normalizes theoretical and
observed spectra using the same continuum points.
Example fits for a rapidly rotating and a slowly rotating programme star are
shown in Fig. 1 for hydrogen and helium lines, whilst
Fig. 2 compares the metal line spectra
of slowly rotating (PHL 159, BD-15
115) stars
and a rapidly rotating star (PG 1533+467).
![]() |
Figure 1:
Fit examples for a slowly rotating star (BD-15![]() |
Open with DEXTER |
![]() |
Figure 2: Wavelength range with strong N II and O II lines to show examples for spectra of slowly rotating (bottom, middle) and rapidly rotating (top) stars. |
Open with DEXTER |
High Resolution | Low Resolution | Photometry | ||||||
Name |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
E(b-y) |
(K) | (km s-1) | (K) | (K) | |||||
PG 0122+214 | 18300 | 3.86 | -0.98 | 117 | 18700 | 3.90 | 18500 (1) | 0.0 |
PG 1511+367 | 16100 | 4.15 | -1.16 | 77 | 15600 | 4.20 | 15900 (1) | 0.0 |
PG 1533+467 | 18500 | 4.09 | -0.94![]() |
215 | 17700 | 3.93 | 17700 (1) | 0.020 |
PG 1610+239 | 15500 | 3.72 | -0.84![]() |
75 | 15400 | 3.69 | 18600 (1) | 0.082 |
PG 2219+094 | 19500 | 3.58 | -1.00![]() |
225 | 18200 | 3.52 | 16700 (2) | 0.037 |
19500 (3) | 0.081 | |||||||
PHL 159 | 18500 | 3.59 | -0.84 | 21 | - | - | 20900 (4) | 0.025 |
PHL 346 | 20700 | 3.58 | -1.00 | 45 | - | - | 22300 (7) | 0.037 |
SB 357 | 19700 | 3.90 | -1.00![]() ![]() |
180 | - | - | 19700 (5) | 0.052 |
19700 (8) | 0.061 | |||||||
19800 (9) | 0.037 | |||||||
BD-15![]() |
20100 | 3.81 | -0.97 | 35 | - | - | 19800 (5) | 0.0 |
20200 (6) | 0.0 | |||||||
HS 1914+7139 | 17600 | 3.90 | -0.99 | 250 | 18100 | 3.60 | - | - |
For rapidly rotating stars the
minimum
is too poorly defined to allow a reliable determination of the He abundance
simultaneously. Therefore, in a first step the
helium abundance was kept fixed at -1.00 (i.e. solar) for the fit procedure.
In a second iteration step the helium abundance was determind by fitting
the helium lines while keeping the
and
fixed at those values determined in the first iteration step.
For all stars (except HS 1914+7139) Strømgren 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 PG 1610+239,
PHL 159 and PHL 346.
The spectrum of SB 357 shows the presence of emission in
and
but not in
.
Therefore the effective temperature
were obtained from Strømgren photometry and the surface
gravity from fitting the far wings of the hydrogen lines. The helium lines of
this object were difficult to fit, but the observation is compatible with normal
abundance and there is no indication of emission in any of the helium lines observed.
The parameters used for further analyses were taken from the high resolution spectra,
because of the larger wavelength coverage and the excellent quality of the fits.
In the case of PG 1533+467, however, the wavelength coverage of the low resolution
spectrum is larger than that of the high resolution one and
therefore we used the average. The finally adopted parameters are listed
in Table 6.
Results are shown in a (
,
)
diagram (Fig. 3).
Seven programme stars (PG 0122+214, PG 1511+367, PG 1533+467, PG 1610+239, PG 2219+094, HS 1914+7139 and SB 357) display highly broadened lines (due to rotation, see Table 2). Only the strongest metal lines (e.g. C II 4267 Å, Mg II 4481 Å) could be identified. Therefore it was impossible to perform a detailed abundance analysis.
The equivalent widths were measured employing the nonlinear least-squares Gaussian fitting routines in MIDAS with central wavelength, central intensity and full width at half maximum as adjustable parameters. For metal lines located in the wings of Balmer or helium lines an additional Lorentzian function is used to describe the line wings of the latter.
Metal lines of the species C II, C III, N II, O II,
Ne I, Mg II, Al II, Al III, Si II, Si III,
P III, S II, S III, Ar II and Fe III
were identified in the sharp-lined spectra of BD-15115, PHL 159 and
PHL 346. The atomic data for the analysis were taken from several
tables:
![]() |
Figure 3:
Positions of the programme stars (filled circles) in a (
![]() ![]() |
Open with DEXTER |
![]() |
Figure 4:
LTE abundances (relative to ![]() |
Open with DEXTER |
![]() |
Figure 5:
Like Fig. 4: LTE abundances (relative to ![]() |
Open with DEXTER |
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 the LINFOR code described above) of
all lines measured before.
The results of the LTE abundance analysis and the rms errors for
PHL 346 and BD-15115 are shown in Table 4
and compared with other analyses and for PHL 159 in Table 3.
Besides the statistical rms errors (given in Tables 3 and 4) the uncertainties in
,
and microturbulent
velocity (see below) contribute to the error budget.
In order to minimize the systematic errors we use the B-type star
Her
as a comparison star. This star has been analysed by Hambly et al. (1997).
We redetermined the LTE abundances of
Her using
the same atomic data, model atmosphere and spectrum synthesis code as for our
programme stars and took the equivalent widths measured by Hambly et al. (1997).
Our results for Her agree to within 0.1dex with those of Hambly et al.
(1997) except for C II (0.12dex), Si III (0.17dex), S III (0.21dex)
and Fe III (0.36dex). In particular our
statistical error for Fe III is much lower than that of Hambly et al. (1997).
These differences can be attributed to different oscillator strengths used.
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 Figs. 4 and 5.
![]() |
![]() |
Remarkable is the large difference (1.0dex) between the Si II
and Si III abundances. This has been found in several
analyses of the comparison star
Her as well
(Hambly et al. 1997, 0.67dex).
In a differential analysis these systematic errors cancel to a large extent.
NLTE effects are small for all elements (
0.1dex, Kilian 1994)
except for Ne I. As demonstrated by Auer & Mihalas (1973) LTE
calculations overestimate the neon abundance. They
carried out NLTE
calculations for Ne I in
Her and derived a neon abundance (close
to solar) which is lower by 0.60 dex than our LTE result. Therefore our absolute Ne abundances are
overestimated.
The abundances of the programme stars with respect to
Her are plotted in Fig. 4.
Spectral analyses of massive B-type stars in open clusters as well as in the
field (e.g. Gies & Lambert 1992; Kilian 1994; Cunha &
Lambert 1994) have revealed considerable variations of metal
abundances from star to star (even within an open cluster). Kilian
(1994) carried out spectral analyses of 21 B-type stars in two open
clusters and in the field and determined abundances of C, N, O, Ne, Mg, Al,
Si, S, and Fe. We compare our results for PHL 346, BD-15115,
PHL 159 and
Her to her LTE results in Tables 3 and
4.
Since her
programme stars are somewhat hotter than ours, the Ne abundance is based on
Ne II lines, whereas we had to use Ne I lines.
Correcting for the significant NLTE effect on Ne I
(0.56 dex, see above) the neon abundance of PHL 159, the only
programme star for which it has been measured, is found
to be consistent with Kilian's distribution.
The abundances we derived for all
metals of PHL 346, BD-15
115 and
Her lie well within
Kilian's distribution indicating that they are bona fide main sequence
B-type stars. For PHL 159, however the O II abundance is higher and the Mg II
abundance lower than in Kilian's distribution, whereas the other metals are
consistent with that distribution. Therefore PHL 159 might either be a
massive B-type star with rather peculiar abundances of the elements O and Mg or
an evolved, low mass B-type star that mimics a massive B-type star quite closely.
The derived atmospheric parameters were compared to
two sets of evolutionary tracks (from the Geneva group, Schaller
et al. 1992; and the Padua group, Salasnich et al. 2000)
to estimate stellar masses and evolutionary
times
(see Fig. 3) by interpolation. The results do not
depend on the model grid used. Derived masses differ by less than 0.1
and
evolutionary lifetimes by less than 4 Myr (except PG 1610+239: 17 Myr).
Errors for the evolutionary lifetimes in Table 6 include errors
propagated from uncertainties in atmospheric parameters
as well as from the use of the two model sets.
The distance has been calculated from mass,
effective temperature, gravity and the dereddened apparent
magnitude of the stars:
Radial velocities of the slowly rotating programme stars were derived
from the lineshift of metal lines. For the rapidly rotating stars
only the Balmer and He I lines could be used.
Radial velocities obtained this way were then corrected to heliocentric
values and the results are listed in
Table 6. The error of the velocities estimated
from the scatter of the velocities derived from individual lines is about 3-11 km
.
Our measurements agree to within error limits with
previous estimates (see Table 6).
Proper motions were taken from literature and are listed in Table 5.
Name | ![]() |
Position angle ![]() |
Reference |
PG 0122+214 | 3.4 ![]() |
234 ![]() |
1 |
PG 1533+467 | 16.8 ![]() |
326 ![]() |
1 |
PG 1610+239 | 8.1 ![]() |
150 ![]() |
1 |
PG 2219+094 | 6.2 ![]() |
194 ![]() |
1 |
BD-15![]() |
9.0 ![]() |
92 ![]() |
2 |
PHL 346 | 8.9 ![]() |
144 ![]() |
3 |
Name |
![]() |
![]() |
![]() |
![]() |
M | d | z |
![]() |
![]() |
K | km s-1 | km s-1 | ![]() |
kpc | kpc | Myr | Myr | ||
PG 0122+214 | 18300 | 3.86 | ![]() |
290 | 6.7 | 9.6 | 6.2 | ![]() |
![]() |
PG 1511+367 | 16100 | 4.15 | ![]() |
300: | 4.8 | 3.8 | 3.2 | 24: | ![]() |
PG 1533+467 | 18100 | 4.00 | ![]() |
440 | 6.0 | 3.0 | 2.4 | ![]() |
![]() |
PG 1610+239 | 15500 | 3.72 | ![]() |
130 | 5.8 | 8.4 | 5.9 | >62 | ![]() |
PHL 159 | 18500 | 3.59 | ![]() |
320: | 8.0 | 5.3 | 3.2 | 31: | ![]() |
PG 2219+094 | 19500 | 3.58 | ![]() |
220 | 8.7 | 9.8 | 6.1 | ![]() |
![]() |
(1) | 17900 | 3.60 | -7 | - | 7.5 | - | - | 67 | 41 |
BD-15![]() |
20100 | 3.81 | ![]() |
410 | 8.0 | 4.9 | 4.8 | ![]() |
![]() |
(2) | 19500 | 3.50 | 94 | - | 10.0 | - | - | 47 | 20 |
HS 1914+7139 | 17600 | 3.90 | - | 330: | 6.2 | 14.9 | 6.0 | 91:![]() |
39 ![]() |
(3) | 18000 | 3.75 | -39 | - | 6.5-10.0 | 16-18.4 | - | - | - |
PHL 346 | 20700 | 3.58 | ![]() |
350 | 9.9 | 8.7 | 7.4 | ![]() |
![]() |
(4) | 22600 | 3.60 | ![]() |
- | 13.0 | - | 8.7 | - | 11 |
SB 357 | 19700 | 3.90 | ![]() |
230: | 7.4 | 7.9 | 7.8 | 61: | ![]() |
(2) | 19000 | 3.70 | 54 | - | 8 | - | 9.0 | 64 | 25 |
![]() |
The times-of-flight, which the stars need to reach their current halo
positions from the galactic
disk, were calculated with 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). 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. Values for proper
motions are given in Table 5. The proper motions for PHL 159,
PG 1511+467, SB 357 and HS 1914+7139 were set to zero, because
no measurements are available.
We followed the orbits backwards in time (time steps of
0.01-0.1 Myr).
The time of passage through the galactic disk (= change of
sign in z-position relative to the disk)
defines the time-of-flight
.
The velocity at the time of first
crossing of the galactic plane is regarded as the ejection velocity
and is also calculated by the program ORBIT6.
Results for all parameters of the programme stars (effective
temperature, gravity, radial velocity, ejection velocity, mass,
distance, age and time-of-flight) are summarised in Table 6.
For the origin of the stars (see next section) the ages (
)
and the times-of-flight (
)
are important.
We improved
for BD-15
115 and PHL 346 for which
proper motion measurements have become available recently.
For BD-15
115 we derive a somewhat lower
than Conlon et al. (1992) and find
to be consistent with
to within our error limits. For PHL 346 we confirm that
is
slightly larger
than
,
but given the error limits this is insignificant.
For PG 2219+094 we find
and
to be lower than derived by Rolleston et al. (1999) and
to be consistent with
.
For PG 1610+239 the time-of-flight is poorly constrained and only a lower
limit could be determined which is consistent with the estimate of the
evolutionary time.
We have carried out quantitative spectral analyses of ten
apparently normal B-type stars. Their positions in the
(
,
)
diagram are consistent with models for main sequence stars.
SB 357 shows emission in
and
,
which
confirms its classification as a Be star
(Heber & Langhans 1986; Kilkenny 1989).
Seven of the stars have rotational velocities >70 km
making detailed abundance analyses impossible. Mostly normal abundances with
respect to
Her were determined for
BD-15
115, PHL 159 and PHL 346.
The Mg and O abundances of PHL 159 are significantly different from the
comparison star and other normal B-type stars (Kilian 1994).
Calculated orbits based on measurements of radial velocity and proper motion allowed to determine times-of-flight from the galactic plane to their present position.
Times-of-flight for PG 1511+367 and
PG 1533+467 are smaller than the evolutionary times, indicating that
these stars have been formed in the galactic
plane and were then ejected (runaway stars). The times-of-flight are
similar to the evolutionary times for PG 0122+214,
PG 2219+094, PHL 159, BD-15115, and PG 1610+239,
which implies that the stars
could also have formed
in the galactic disk and were then ejected very soon after
their birth. Ejection velocities for all programme stars range from
130km s-1 to 440km s-1.
Three mechanisms for the production of runaway stars have been proposed in the literature:
Our programme stars are too far away and their space motions are therefore not known accurately enough to allow to identify their relation to a young cluster or association in the galactic plane. However, the ejection velocities determined for our programme stars may be important to identify the mechanism which led to their ejection from the galactic plane, once reliable theoretical predictions become available for the different scenarios discussed above. Six stars have escape velocities exceeding 300km s-1 which seems too large to be achievable by the binary supernova scenario.
Four stars in our sample have been proposed in the literature as candidates
for B-type stars formed in the galactic halo because their times-of-flight were
found to be considerably larer than the evolutionary time scales
(Conlon et al. 1992; Keenan et al. 1986; Heber et al.
1995). As discussed above our new analysis of BD-15115
demonstrates that its time-of flight is consistent with the evolutionary
time. Hence it could be runaway star, too.
For SB 357 and HS 1914+7139 the times of flight are more than twice as large as the evolutionary times, which would make formation in the disk unlikely. However, their times-of-flight are uncertain due to the lack of proper motion measurements. Such data are urgently needed before any firm conclusions can be drawn. Therefore we are reluctant to regard these stars as born in the halo.
PHL 346 has been proposed as a candidate massive
B-type star born in the halo (Ryans et al. 1996;
Hambly et al. 1996). Based on
the new Tycho proper motion measurement, our
analysis indicates that
is marginally larger than
and PHL 346 can be a runaway star, too.
Hence no conclusive candidate for a young massive B-type star formed in the halo remains in our sample. Proper motions for the four stars lacking any measurement should urgently be determined.
Acknowledgements
M.R. gratefully acknowledge financial support by the DFG (grant He1356/27-1). We thank Michael Odenkirchen who kindly provided us with his code ORBIT6 for the calculation of the kinematic orbits, Heinz Edelmann who carried out the DSAZ FOCES and ESO FEROS observations and Neil Reid, Ralf Napiwotzki and Klaus Werner who obtained the Keck HIRES spectra for us. S.M. was supported by a grant (50 OR 96029-ZA) from the Bundesministerium für Bildung und Forschung through the DLR.