Stephi-seismology of δ scuti stars from multi-site photometry campaigns

Ads’. Space Res. Vot. 11, No. 4, pp. (4)167—(4)170, 1991 Printed in Great Britain. All rights reserved.

0273—1177/91 $0.00 + .50 Copynght © 1991 COSPAR

STEPHI-SEISMOLOGY OF & SCUTI STARS FROM MULTI-SITE PHOTOMETRY CAMPAIGNS E. Michel and A. Baglin Observatoire de Paris-Meudon, U. R.A. C. N. R.S. 335, 5 PlaceI. Janssen, 92195 Meudon Cedex, France

ABSTRACT Stellar seismology is a very promissing tool for the study of the stellar interior, as proven by the solar case. With high quality homogeneous data, like those obtained by multi-site photometric networks, several modes can be detected in large amplitude pulsators. They permit to extend the seimology techniques to stars. The first results of the multi-sites photometric STEPHI program are presented and a few astrophysical implications are sketched. THE STEPHI PROGRAM: (8 Scuti Stellar Photometry International Program) To generalize the seismology theory to stars i.e. to infer informations on the internal structure of the objects from their eigenfrequencies, one needs to observe a significant number of modes. This is possible from the ground in the case of large amplitude objects, but only with longitude coordinated campaigns. The 6 Scuti variables show evidences of variability involving several modes /1/. This richness of the pulsation makes them good candidates for stellar seismology investigations. The STEPHI network /2/ is devoted to such a purpose Its main characteristics can be described as follows: Multi-site: An observational network is necessary to increase the duty cycle and to lower the window effect in the Fourier space, then to ascertain the determination of real frequencies. Up to now the network includes three sites: Observatorio del Teide, Tenenfe Island, Spain; Observatorio Astronomico Nacional de San Pedro Martir, Baja California, Mexico; Xing long Station of the Beijing Observatory, China. Additional stations are foreseen. Long observing runs: the length is fixed to obtain a frequency resolution sufficient to detect rotationnal splitting for instance. Campaigns cover approximately 3 weeks, corresponding to a frequency resolution of 0.5 j.tHz Multi-channel: The rapid photomethc technique (3 channels registered simultaneously) improves very significantly the signal to noise ratio (fig. 1) Multi-object: Homogeneous informations will be obtained on a sufficiently large set of objects, representing the general properties of the 6 scud class (different masses, evolutionnary state, rotationnal velocities), and will provide an observationnal material usable by a theoretical modelisation towards a global understanding of the pulsation mechanisms and in particular the mode selection effects.

(4)167

(4)168

E. Michel and A. Baglin

THE STEPHI 1989 CAMPAIGN on OX PEGASI: OX Peg. (A8V) has been observed during 21 nights (2ith of August- 10th of September 1989) from the three sites. At the Observatorio del Teide and at the Observatono de San Pedro Martir, two idcntical Chevreton rapid Photometers 13/ have been used, and a 61% coverage has been obtained for a 9.1 days period. During four nights in the same period of 21 days, observations have been done at Xing long Station with a classical photometer. The three sites resulting window reaches a 72% coverage in the 9.1 period and a 49% coverage in the 20 days whole period. The comparison star was HD 213473 (K5). In the three sites, the observations were done using a narrow blue filter (lm:421 .5nm,HW:19.3nm) .The sampling time is 6 seconds for the Chevreton photometer and 5 mn for the classical one /4/. The results confirm the efficiency of such observations which allow the detection of a larger set of precisely defined frequencies, showing very narrow peaks. The narrowness of the peaks confirms that, in the case of 8 Scuii stars, we are dealing with oscillations with lifetimes longer than the duration of the run.

I

I

‘

I

‘

I

I

I

(a)

[

i~ ~

~

600

650

• ~

..

0

50

100

ISO

200

250

300

350

400

430

500

330

700

750

800

850

900

950

Frequencies (jiHz) Fig. 1. Periodograms: HD213473 (a), OX Pegasi (b), OX Pegasi residual spectrum after having removed the detected fitquencies.(from 12/)

STEPHI Multi-Site

Photometry

(4)169

COMPARAISON OF THE FREQUENCY SPECTRA OF 6 SCUTI STARS Up to now, only a few stars have been observed in these conditions. In addition to the two campaigns of the Stephi network on 63 Her and OX Peg, another campaign has been done on 02 Tau by Breger et al. /5/. This very preliminary and incomplete scan of the 6 Scuti population allows already some comparisons and some remarks. The location in the H-R diagram of these stars is shown on fig. 2b. Absolute magnitudes and effective temperatures have been computed through the (ci, b-y, M~) StrOrngren relation /6/. Bolometric corrections are small they are taken from Schmidt-Kalcr /7/. Their positions in the (m2, B2-V1) diagram of the Geneva system indicates a solar metallicity; the (d2, B2V1) gives an effective temperature somehow higher (200K) than the StrOmgrcn scale. All of them are -

GX Peg.

Oø2Tau.

1.5

p

oGXPeg.

\\ S

\

_2.0

C

_____________________________________________

V65OTau.

\

-.-

Zero-age

-

63 Her.

main sequence

p

2.5

3.95

________

.~•••••.

3.90

.i~.

e2 Tau.

-

r S

_________

-

Frequencics(I.tHz .100) Fig. 2a Frequencies obtained for 63 Her. and OX Peg. ( STEPHI : /3/, /2/) al. 1990), and for ~2 Tau. /5/. Fig. 2b Location of 63 Her., OX Peg., V650 Tau., and ~2 Tau. in the H.R. diagram.

~

~eff

(4)170

E. Michel and A. Baglin

slightly evolved, with little differences in luminosities, correponding to a spread in mass smaller than 0.2 solar masses Although very similar, the stars considered here show, at first sight, very differentfrequency spectra (see fig.2a). This qualitative difference indicates that seismology is able to discriminate between objects having almost undistinguishable global parameters, at the level of accuracy presently reachable. It has to be noticed that, in this region of the H-R diagram, important and rapid changes in the structure occurs: the nuclear burning shell starts to be active whereas the core experiences a phase of gravitationnal contraction. The influenceof the evolutionnary stage on the pulsation spectrum will be more thoroughly studied, and the next campaigns will look at less evolved objetcsas for instance V650 Tau. CONCLUSIONS Only homogeneous high quality data on a wide set of 6 scuti stars will permit to obtain a better comprehension of the pulsational phenomenon. STEPHI is working in this direction. All the sofar investigated stars have revealed very reach spectra ( 5-6 frequencies at least) and their is no doubt that a higher signal to noise ratio would yield additional frequencies (fig.1 ). Those conditions can be realized by increasing the length and the density of the observations runs and, in the futur, by avoiding the atmospheric noise, observing from space. Ground based photometric observations will always be limited to large amplitude objects, like the self excited oscillators . The generalisation of this program to a broader set of objects implies space borne photometry, and is foreseen in the near future. The EVRIS experiment onboard MARS 94 will be devoted to this purpose /8/. ACKNOWLEDGMENT We would like to express our gratitude to all the members of the STEPHI network: Belmonte, J.A., Alvarez, M., Jiang Shi Yang, Chevreton, M., Auvergne, M., Ooupil, MJ., Mangeney, A., Roca Cones, T., Huang Lin, Fu Jian Ning

REFERENCES /1/ Baglin, A., Auvergne, M., Valtier, J.-C., Saez, M. 1980, Vth European Regional Meeting I.A.U., Liege. /2/ Michel, E., Belmonte, J.A., Alvarez, M., hang Shi Yang, Chevreton, M., Auvergne, M., Goupil, MJ., Baglin, A., Mangeney, A., Chaloupi, M., Roca Corths, T., Huang Lin, Fu Jian Ning 1990, Proc. Confrontation between Stellar Pulsation and Evolution, Bologna, Italy, in press. /3/ Belmonte, J.A., Chevreton, M., Mangeney, A., Saint-Pd, 0., Puget, P., Pradene, F., Alvarez, M. and Roca Cortès, T. 1990, Astr. Ap., in press /4/ Michel, E., Belmonte, J.A., Alvarez, M., Shi Yang, J.,Chevreton, M., Auvergne, M., Goupil, MJ., Baglin, A., Mangeney, Huang Lin, Fu Jian Ning 1991, to be published. /5/ Breger, M., Huang Lin, Jiang Shi Yang, Guo Zi-he, Antonello, E., Mantegazza, L. 1989, Multiple close frequencies of the Delta Scud star & Tau., Astr. and Ap. 214. /6/ StrOmgren, B. 1963, Quart. J. Roy. Astr. Soc.

~,

1.

/7/ Schmidt-Kaler 1982, in Landolt and Bomstein 2. 1,451. /8/ Baglin A. 1990, Proc. 2nd IRIS Workshop, Solar Physics, in press.