Absolute Magnitudes of Chemically Peculiar Stars
Total Page：16
File Type：pdf, Size：1020Kb
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.