239

ABSOLUTE MAGNITUDES OF CHEMICALLY PECULIAR

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 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 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

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 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; .

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 , 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 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. is the average absolute magnitude,  M  the scatter of

V V

it, N the number of stars considered. is the average correction for multiplicity with respect to the number

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 M  N <U B > U B  Max. Min.

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 M  N <U B > U B Max. Min.

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-

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

8:3  0:3c +6:51:0m

1 1

by Kurtz and co-workers. The question then is what

6 2

+7:2  1:1  10 v sin i

di erence can b e found b etween these two categories

of stars, except for the existence of pulsation? Abso-

lute magnitude or evolutionary state might yield the

The scatter of the residuals is quite go o d: 0.31 mag

clue, as prop osed by Mathys et al. 1996 who made

normal A+Am, 0.25 normal A and 0.36 mag Am

a pre-Hipparcos study based on mean parallaxes and

stars. on kinematics.

243

Figure 4. HR diagram for roAp ful l dots and noAp Figure 5. Ca abundance versus T obtained from

e

open dots stars. Geneva colours and the calibration of Kunzli  et al. 1997

for Am stars measured at OHP ful l disks and published

by Guthrie 1987, ful l triangles. A few normal open tri-

Here we reconsider the work of Mathys et al. 1996

angles and metal lic giant open circles F stars are also

in the light of the Hipparcos data, but using only the

shown. The horizontal line indicates the solar abundance.

parallaxes. Starting from the lists of roAp and noAp

stars compiled by these authors, we end up with 14

roAp stars and 12 noAp stars, some of which have

Am stars, which at the time could only b e obtained

a large relative error on the parallax up to 0.28 for

through photometric M values. The idea was essen-

V

the roAp and 0.35 for the noAp. In fact many more

tially to test the hyp othesis put forward by Berthet

noAp stars could have b een considered, since the list

1992, of an evolutionary link b etween the Am stars

of Mathys et al. 1996 contains only those stars with

Ca and Sc de cient, the  Delphini stars more

a well-known radial velo city, but we checked they

evolved, abundances similar to those of Am stars,

would not change the result. Figure 4 shows the re-

except for Ca and Sc which are nearly solar and the

sulting HR diagram, where the T have b een taken

e

metallic A-F giants Ca and Sc solar, heavier sp ecies

as such from Table 2 of Mathys et al. 1996 and the

overabundant.

were obtained using b olometric correc-

tions of Lanz 1984 and M = 4:75. Interstellar

b

Our calcium abundances are in go o d agreement with

reddening was estimated by Erspamer 1996 using

those obtained through narrow-band photometry by

the maps of Lucke 1978.

Guthrie 1987 there are 8 stars in common, so his

results can be added to ours, esp ecially as his stars

One clearly sees that in general, noAp stars are more

also have Hipparcos measurements. The whole sam-

evolved and more massive than the roAp stars, the

ple then includes 76 Am stars, all with M derived

V

dividing line b eing ab out 2 M . We have not cor-

from Hipparcos and whose age can b e interp olated in

rected for biases such as the Lutz-Kelker one, but if

the evolutionary tracks of Schaller et al. 1992.

they may b e signi cant in an absolute way, they are

probably not when comparing b oth samples. Cumu-

Plotting Ca abundance against age gives a rather dis-

lative distributions of M for b oth samples con rm

V

p ersed but signi cantanticorrelation: older Am stars

they are di erent to a con dence level b etter than 99

are more Ca-de cient hence have a more pronounced

p er cent according to the Kolmogorov-Smirnov test.

p eculiarity. Such a result is very tantalizing, be-

The samples are relatively small, however, and it will

cause it is qualitatively consistent with the theoret-

be interesting to reconsider this question when the

ical predictions of Alecian 1996. However, co oler

complete Hipparcos catalogue b ecome available.

Am stars are statistically older than hotter ones, so

that the Ca abundance versus age correlation re ects

a relation between Ca abundance and T . Indeed,

e

Figure 5 shows the latter correlation is real, and nu-

4.3. Calcium Abundance versus Age in Am Stars

merical simulations con rm our suspicion: arti cial

Am stars with masses and ages distributed at ran-

A sample of 27 Am stars had b een measured in 1994 dom and having Ca abundances lo osely correlated

and 1995 with the Aur elie sp ectrograph attached to with T only, will yield NCa decreasing with in-

e

the 1.5m telescop e at OHP, France, to obtain their creasing log t. Such an equivalence is exp ected, since

calcium abundance from the Ca II K line using syn- a star evolves on the main sequence with a decreas-

thetic sp ectra. The purp ose was to test how the ing T : it is fundamentally imp ossible to identify the

e

Ca abundance varies with the evolutionary state of physical cause of the Ca de ciency with either t or

244

Erspamer D., 1996, Private communication

Garrison R.F., Gray R.O., 1994, AJ 107, 1556

Glagolevskij Yu.V., Topilskaya G., 1987, Astro zik

Issledovanija 25, 13

Gray R.O., Garrison R.F., 1987, ApJS 65, 581

Gray R.O., Garrison R.F., 1989a, ApJS 69, 301

Gray R.O., Garrison R.F., 1989b, ApJS 70, 623

Guthrie B.N.G., 1987, MNRAS, 226, 361

Hauck B., 1973, in: Problems of calibration of ab-

solute magnitudes and temp erature of the stars,

IAU Symp. No 54, eds. B. Hauck and B.E. West-

erlund, Reidel, Dordrecht, p. 117

Hauck B., Kunzli  M., 1996, Baltic Astronomy 5, 303

Hauck B., Mermillio d J.C. 1996, Private comunica-

tion

Hauck B., North P., 1982, A&A 114, 23

Hoeit D., Jaschek C., 1982, The Bright Star Cata-

logue, 4th ed., Yale University Observatory

Hoeit D., Saladyga M., Wlasuk P., 1983, A Supple-

ment to the Bright Star Catalogue, Yale Univer-

sity Observatory

Hubrig S., Mathys G., 1994, Astron. Nachr. 315, 343

Figure6. Calcium abundance versus evolution D is

1000

Hubrig S., Schwan H., 1991, A&A 251, 469

the distance from the ZAMS in arbitrary units, roughly

Jaschek M., Egret D., 1982, Publ. Sp ec. du CDS No

along an evolutionary track for Am stars measured at

4

OHP ful l dots and published by Guthrie 1987, ful l tri-

Kunzli M., North P., A&AS, in press

angles. Each panel refers to a speci c mass range: a

Kunzli M., North P., Kurucz R.L., Nicolet B., 1997,

1.5  M=M  1.7; b 1.7  M=M  1.9; c 1.9

A&AS 122, 51

 M=M  2.1; d M=M  2.1.

Kurtz D.W., 1990, ARA&A 28, 607

Lanz T., 1984, A&A 139, 161

T , since b oth vary together. Only three Am star in

e

LuckeP.B., 1978, A&A 64, 367

the sample are younger than log t =8:6: this ts well

Mathys G., Kharchenko N., Hubrig S., 1996, A&A

the results of North 1993, who showed that the fre-

311, 901

quency of Am stars in op en clusters rises signi cantly

Michaud G., 1970, ApJ 160, 641

with age.

Mo on T.T., Dworetsky M.M., 1985, MNRAS 217,

Figure 6 con rms that, contrary to what was sug- 305

gested by Guthrie 1987 and Berthet 1992, Am

Napiwotsky R., Schonb erner D., Wenske V., 1993,

stars which have nearly completed their main se-

A&A 268, 653

quence life do not tend to have a larger Ca abun-

North P., 1993, in: Peculiar versus normal phe-

dance. The parameter D in Figure 6 is the dis-

1000

nomena in A-typ e and related stars, eds. M.M.

tance, along an evolutionary track, b etween the rep-

Dworetsky, F. Castelli and R. Faraggiana, ASP

resentative p oint of a star in the HR diagram, and

Conf. Series Vol. 44, p. 577

the ZAMS; it is zero on the ZAMS and increases as

Pedoussaut A., Ro chette J., Cap deville A., Bourdon-

the star evolves. Clearly, for a given mass, the Ca

cle P., 1996, Catalogue of orbits of sp ectroscopic

abundance shows no increasing trend; there is even a

binaries  oppy disk

slight indication for the contrary. This is yet another

argument against Berthet's scenario, which has also

Renson P., 1991, Catalogue G en eral des Etoiles Ap et

b een challenged byKunzli & North 1997.

Am, Institut d'Astrophysique, Universit edeLi ege

Schaerer D., Charb onnel C., Meynet G., Maeder A.,

Schaller G., 1993, A&AS 102, 339

REFERENCES

Schaerer D., Meynet G., Maeder A., Schaller G.,

1993, A&AS 98, 523

Abt H.A., Morrell N.I., 1995, ApJS, 99, 135

Schaller G., Schaerer D., Meynet G., Maeder A.,

1992, A&AS 96, 269

Aikman G.C.L., 1976, Publ. Dominion Obs. Victoria

Schmidt-Kaler Th., 1982, in: Landolt-Bornstein, ed.

14, 379

K.-H. Hellwege, Springer, Group VI, Subvolume

Akritas M.G., Bershady M.A., 1996, ApJ, 470, 706

2b, pp. 453-454

Alecian G., 1996, A&A 310, 872

Uesugi, A., Fukuda, I. 1981, Catalog I I I-63; v sini

Bertaud C., Flo quet M., 1974, A&AS 16, 71

Micro che; Strasb ourg: Centre de Donn ees Stel-

laires

Berthet S., 1992, A&A 253, 451

Wade G.A., 1997, A&A, in press

Cowley A., Cowley C., Jaschek M., Jaschek C., 1969,

AJ 74, 375

Wol S.C., Preston G.W., 1978, ApJS 37, 371

Crawford D.L., 1979, AJ 84, 1858