239
ABSOLUTE MAGNITUDES OF CHEMICALLY PECULIAR STARS
1 2 1 3 3 1
P. North , C. Jaschek , B. Hauck , F. Figueras ,J.Torra ,M.Kunzli
1
Institut d'Astronomie de l'Universit e de Lausanne, CH-1290 Chavannes-des-Bois, Switzerland
2
Observatoire de Strasb ourg, 11 rue de l'Universit e, F-67000 Strasb ourg, France
3
Departament d'Astronomia i Meteorologia, Universitat de Barcelona,
Avda. Diagonal 647, E-08028 Barcelona, Spain
ABSTRACT
Table 1. Groups of CP stars with their approximate char-
acteristics.
Pec. typ e Sp. typ e T range magnetic?
The p osition of di erent kinds of CP stars of the up-
e
He-weak B4-B8 13000-17000 Yes/No
p er Main Sequence i.e. Am stars and the four cate-
Si B7-A0 9000-14000 Yes
gories of Ap stars: He-weak, HgMn, Si and SrCrEu
HgMn B8-A0 10000-14000 No
in the HR diagram is examined. Using only those
SrCrEu A0-F0 7000-10000 Yes
stars having a parallax with a relative error smaller
Am A0-F0 7000-10000 No
than 0.14 M 0:3 and an absolutely certain
v
p eculiarity, it is shown that their absolute magni-
tudes are similar to those obtained from CP stars in
clusters. CP stars are scattered on the whole width
of the Main Sequence band. However, the rapidly
or Ap stars and a sp ecial distribution of orbital pa-
oscillating Ap stars seem slightly less evolved, hence
rameters for those CP stars which are members of
less massive as a group, than their non-oscillating
binary systems.
counterparts.
But here, we fo cus on the problems regarding the
The calibrations of the Stromgren and Geneva in-
fundamental parameters of these stars: indeed, b oth
dices in terms of M have b een reexamined in the
v
sp ectral typ e and luminosity class are often ill-de ned
light of the Hipparcos data for Am stars. A new cal-
due to extreme abundance anomalies. Furthermore,
ibration of the uv by system in absolute magnitude
the photometric indices are generally distorted be-
is prop osed for b oth normal and Am stars.
cause the abundance anomalies are large enough to
induce distorsions of the sp ectral energy distribution.
The calcium abundance in Am stars is examined as
For instance, the magnetic Ap stars are generally
a function of their evolutionary state.
bluer than normal stars with the same e ective tem-
p erature and their Balmer jump is smaller see e.g.
Hauck & North 1982. The main question to b e ad-
dressed using the Hipparcos data is that of the exact
Key words: chemically p eculiar stars; absolute mag-
p osition of these ob jects in the HR diagram, within
nitudes; stellar evolution.
the Main Sequence. Although it is now commonly ad-
mitted that all CP stars b elong to the Main Sequence
e.g. North 1993, their distribution within the main
1. INTRODUCTION
sequence band has remained less certain. There have
b een some recent claims that Ap stars tend to ap-
p ear at the end of their main sequence life Hubrig
&Schwan 1991; Hubrig & Mathys 1994, contrary to
By `chemically p eculiar' hereafter CP stars, we
what is suggested by cluster memb ers; on the other
mean here stars with an intermediate mass displaying
hand, Am stars show a signi cant trend to app ear
exotic abundances in their atmosphere not in their
more frequently in old op en clusters than in younger
deep interior probably due to radiative di usion in a
ones North 1993.
stable atmosphere e.g. Michaud 1970. Altogether,
the CP stars considered here include the He-weak,
HgMn, Si, SrCrEu and Am stars. Some of their ba-
Another imp ortant p ossibility o ered by Hipparcos
sic characteristics are quoted in Table 1. Because
data is to check and nally to revise the photomet-
of their large intrinsic luminosity and long distances
ric calibrations in terms of absolute magnitude, and
which imply uncertain parallaxes, the He-rich stars
we discuss b elow the Am and normal A stars in the
have not b een considered here.
Geneva and uv by systems.
Other characteristics could be mentioned, like slow
Finally, some astrophysical consequences of the new
rotation usually interpreted as favouring radiative
data are presented concerning the magnetic Ap and
di usion, a large magnetic eld in the case of CP2 the Am stars.
240
2. MEAN M OF EVERYGROUP compromise, we applied a correction of 0.2 mag,
V
which corresp onds to a magnitude di erence of
1.7 mag b etween the comp onents. Since not all
In order to estimate the mean absolute magnitude
stars of our sample had their radial velo city stud-
of every group of CP stars de ned ab ove, we pro-
ied, we further applied our 0.2 mag correction to
ceeded in the following way. First of all, only CP
all stars mentioned in the BSC as b eing `radial
stars with a very reliable sp ectral classi cation were
velo cityvariable'.
considered, which considerably reduced the available
The correction for binary companions is imp or-
sample. Each di erent group needs an appropriate
tant but surely incomplete. To see how large
treatment:
is its in uence up on the average absolute mag-
nitude of the group, we provide in Table 2 the
average amount of this correction and also the
He-weak stars: we relied up on the usual bibliog-
p ercentage of the sample that was corrected.
raphy, i.e. Jaschek & Egret 1982, Bertaud &
Flo quet 1974, Hoeit & Jaschek 1982, Hof-
eit et al. 1983. Only stars describ ed sp ec-
For each group, the average M was computed,
V
troscopically as He-weak were retained, exclud-
weighted by the inverse square of the parallax error.
ing stars discovered photometrically or through
The rms standard deviation was computed as well.
narrow-band photometry based up on He lines.
The results are given in Table 2.
Si-4200 stars: the same bibliographyasabove
was used. Stars with intermediate typ es like
How do the ab ove results compare with those ob-
SiCr and SiEu were excluded. When the classi -
tained b efore Hipparcos from cluster stars? We
cation was based up on ob jective-prism material,
started from the work of North 1993 which was
two indep endent classi ers must have reached
largely based on Renson's 1991 catalogue. The list
the same result for the star to b e included.
of cluster CP stars was expurgated of all stars hav-
ing a doubtful classi cation such as `Am ?'. Cluster
HgMn stars: since the lines of Mn are dicult to
stars have the inconvenience that corrections for mul-
observe at medium disp ersion, the list is mostly
tiplicity are dicult to apply, since in most cases mul-
based up on classi ers using high-disp ersion ma-
tiplicitywas not investigated, at least using radial ve-
terial, i.e. Aikman 1976 and Wol & Preston
lo cities. This makes the results somewhat uncertain
1978.
by amounts which should b e similar to the correction
given for eld stars. Furthermore, usually one classi-
CrEuSr stars: same sources as for the Si-4200
cation only is available for cluster stars, very often
stars. SiCr and SiEu stars were also excluded.
obtained at small disp ersion, a fact which enhances
the p ossibility of misclassi cation. The same aver-
Am stars: the census was based mostly up on the
ages have b een computed for cluster stars where M
V
lists by Cowley et al. 1969, Gray& Garrison
is directly obtained from the known distance mo du-
1987, Gray & Garrison 1989a, Gray & Garri-
lus of the cluster as for eld ones, and the results
son 1989b, Garrison & Gray 1994 and Abt &
are displayed in Table 3.
Morrell 1995. A star was accepted as Am if it
was so classi ed bytwo indep endent observers.
The agreementisvery go o d for all but He-weak stars,
whose numb er is small in any case. As a whole, the
Then, only stars with a small relative error of 14
results coincide with North's 1993 nding that CP
p er cent on the parallax were considered. =
stars are normal stars, as far as the average absolute
0:14 imply M 0:3 mag.
magnitude is concerned.
V
The remaining stars colours and absolute magnitude
were corrected for interstellar absorption and multi-
3. GROUND-BASED PHOTOMETRY AND
plicity e ects when necessary:
HIPPARCOS
for all CP stars but the CrEuSr and Am stars,
the usual dereddening pro cedure for UBV pho-
Here we examine the calibrations of the Geneva and
tometry was adopted. The star was dereddened
uv by systems in terms of absolute magnitude, and
slop e 0.72 to the average sequence in the U B
prop ose a new preliminary calibration of the latter
versus B V diagram. Because of the intrinsic
system for A and Am stars.
width of the sequence, the pro cedure was applied
only when E B V > 0:03;
for visual doubles we used data from the BSC
3.1. Geneva System
and its supplement Hoeit & Jaschek 1982,
Hoeit et al. 1983. There is no problem as far
We rst haveto consider normal stars to check the
as the magnitude di erence m between the
V
quality of the M calibration devised by Hauck
V
comp onents is known;
1973. We found 30 stars in Prop osal 55 which are
for sp ectroscopic binaries we used the catalogue normal, single A dwarfs with d 0:100 d is sen-
by Pedoussaut et al. 1996. SB2 binaries are sitive to the Balmer jump and is equivalentto c of
1
the easiest cases, since one can infer their m Stromgren's photometry and = 0:14. The
V
from their mass ratio given by the ratio of the absolute magnitudes obtained from the calibrated
V amplitudes through a mass-luminosity rela- Geneva colours essentially d and B 2 V 1 are com-
r
tion. SB1 systems are more problematic; as a pared with those from the Hipparcos parallaxes in
241
Table 2. Results based on the Hipparcos paral laxes.
V V
it, N the number of stars considered.
of stars having a correction, while is the percentage of stars to which a correction for multiplicity has been applied.
Max. and Min. are the extreme absolute magnitudes in the group.
Pec. typ e
o
V V
brightness mag
He-weak -0.55 0.68 15 -0.57 0.13 0.36 40 -2.4 +0.2
Si-Si-4200 -0.41 0.79 28 -0.40 0.14 0.24 54 -1.5 +1.5
HgMn 0.20 0.49 28 -0.35 0.10 0.27 78 -0.7 +1.1
CrEuSr 1.17 0.60 49 0.02 0.11 0.17 76 -0.1 +2.6
Am 1.64 0.68 96 0.22 0.07 0.38 58 0.3 +2.8
B V
o
Table 3. Results based on cluster members.
Pec. typ e
o
V V
brightness mag
He-weak -1.21 0.34 9 -0.50 0.20 -3.0 -0.1
Si-Si-4200 -0.41 0.79 28 -0.40 0.14 -2.6 +0.7
HgMn -0.20 0.99 16 -0.35 0.17 -1.6 +1.9
CrEuSr 1.26 0.74 10 0.01 0.10 0.3 +3.1
Am 1.95 0.65 46 0.18 0.09 0.4 +3.0
B V was used instead of U B
o o
Figure 1. The agreement is excellent, the t giving:
M Geneva=0:997 M Hip + 0:129 1
V V
0:035 0:085
=0:18 mag
res
On the contrary, for the Am stars the result is rather
bad: starting from the sample of certain Am stars
de ned ab ove, we nd 71 ob jects measured in the
Geneva system and with d 0:100 M was cor-
V
rected for multiplicity whenever p ossible. Figure 2
shows that not only is the slop e of the regression
much di erent from 1, but also that the residual scat-
ter is much larger:
M Geneva=0:466 M Hip + 1:331 2
V V
0:048 0:080
=0:27 mag
res
With the present calibration, Geneva photometry un-
derestimates the luminosity of the hottest Am stars;
the explanation is probably that the Balmer jump of
Am stars is smaller than that of normal stars at same
T and log g Glagolevskij & Topilskaya 1987, due
e
to a redistribution of the blo cked UV ux into the
Figure1. Comparison between photometric and Hippar-
visible, which is also observed in magnetic Ap stars.
cos M for normal A dwarfs of Proposal 55 measuredin
V
Another contributing factor might be an underesti-
the Geneva system.
mate of the parallax by Hipparcos, due to orbital
motion of binaries.
which have uv by photometry. 91 of them have a
V sin i value from Abt & Morrell 1995 and 4 from
Uesugi & Fukuda 1981. We have tested two M
V
3.2. uv by System
calibrations: that of Crawford 1979:
M = M 9:0 c 3
A sample of 97 normal, single A3-A9 stars was se-
v v ZAMS 0
lected from Prop osal 35; all stars have uv by pho-
and that of Guthrie 1987, which takes into account
tometry they b elong to Stromgren's late group ac-
metallicity and rotational e ects:
cording to Hauck & Mermillio d 1996, V sin i data
from Abt & Morrell 1995 and = 0:14. On
0
+0:1 4 M = M 9:1 c
v v ZAMS
0
the other hand, wehave from Prop osal 55 a sample
0 6 2
of 97 certain Am stars with = 0:14, 82 of c = c 1:2 m 1:1 10 v sin i 5
0 0 0 242
Am stars
fit 4 equality
3
2 (STR) V M
1
0
01234
MV(Hipp)
Figure2. Comparison between photometric and Hippar-
Figure3. Comparison between photometric and Hippar-
cos M for Am stars of Proposal 55 measured in the
V
cos M for Am stars in the Stromgren system. Guthrie's
V
Geneva system.
1987 calibration was used.
4. SOME ASTROPHYSICAL ISSUES
For normal stars, Crawford's calibration is quite
go o d, with an rms di erence M M =0:31
phot Hip
mag: there is no systematic trend. For Am stars, on
4.1. Distribtion of Ap Stars in the HR Diagram
the contrary, this calibration overestimates the paral-
laxes by 4 mas with resp ect to Hipparcos ones, with
exactly the same trend observed ab ove in the Geneva
All kinds of CP stars app ear to b e more or less uni-
system. Guthrie's calibration is much less successful
formly distributed on the Main Sequence band, just
than Crawford's for normal stars systematic trend
like in clusters, which contradicts the suggestion of
and rms di erence of 0.43 mag but succeeds rela-
Hubrig & Mathys 1994. Our result, however, en-
tively well for Am stars Figure 3: no systematic
tirely con rms the very recent study of Wade 1997,
trend is found and the rms di erence is 0.40 mag.
who estimates the evolutionary state of 10 magnetic
Ap stars from their rigid rotator geometries. This
means that magnetic elds and chemical p eculiari-
In view of the ab ove results, it app eared useful to
ties may b e present whatever the age on the main se-
rede ne a calibration whichwould hold for b oth the
quence; therefore, the magnetic elds presentinAp
normal A and the Am stars, and that was done us-
stars are probably primordial, in the sense that they
ing the BCES algorithm Bivariate Correlated Errors
do not build up during the main sequence life but
and intrinsic Scatter devised by Akritas & Bershady
result rather from some pro cesses taking place b efore
1996. This algorithm has the advantage of allow-
the arrival of the star on the ZAMS.
ing for measurement p ossibly correlated errors or
intrinsic scatter on b oth variables, although it can as
yet b e applied only to simple linear regressions on two
variables. Since only one variable can b e correlated
4.2. roAp versus noAp Stars
at a time with M , the BCES metho d was applied
V
rst to temp erature e ect, then to c evolu-
1
tion, m metallicity and V sin i rotation. The
1
The so-called rapid ly oscil lating Ap roAp stars pul-
pro cess was iterated a few times until convergence of
sate in high radial overtone, low l pressure mo des
the co ecients and resulted in the following relation:
with p erio ds of 5 to 15 minutes, the pulsation axis
b eing colinear with the magnetic axis Kurtz 1990.
In the same range of colours, there are some non-
M = 12:60 0:03 3:66 0:7 6
oscil lating Ap noAp stars of the same typ e whichdo
v
not pulsate, as testi ed by hours of careful monitoring