Asteroseismology in a triple star

1. Asteroseismology in a triple star Introduction We are carrying out a project to study short-period pulsating A- or F-type variable stars which are also members of a binary or a multiple system. Our objective is to perform a detailed study of the pulsations of the  Scuti star DG Leo[1], member of atriple system, with the goal to exploit the dynamical properties of the system to gain fundamental astrophysical information on the components and thus, on the pulsating star. As the overall purpose of asteroseismology is to probe the stellar interior by studying the pulsations, and as more than 50% of all stars are binaries, understanding the effect of binarity upon the pulsations is an important issue. Additional constraints such as the ones that can be obtained from studies of the binarity or multiplicity may help to comprehend the mechanisms that define the pulsation properties of  Scuti stars. [1] DG Leo = HD 85040 = 44 Kuiperis an hierarchicaltriple star[2] which consists of a close binary (Aa,b) with an orbital period close to 4 days and a third component (B) distant by about 0.2 arcsec and resolvable by high-angular resolution position measurements. The long orbital period is of the order of 100-200 yrs. The system is formed by three stars of almost equalspectral types and similar masses (Fig. 1). Each component is located in the  Scuti instability region but only one appears to be a pulsating star [3]. Fig.3 Expansion and contraction of the outer stellar layers in a pulsation cycle Fig. 2. Fig.1 [3] The  Scuti instability regionis a phase in the evolution of unevolved or slightly evolved late A- and early F-type stars during which stellar oscillations are excited (by the so-called  mechanism, a mechanism based on a damming up of the central energy in the stellar envelope which is later released by a sudden change in the layer’s opacity). Because periodic oscillations or pulsations of the outer stellar layers are being generated, the star is at this stage a pulsating star . [4] The Doppler shift in spectra Spectroscopic binaries are detected by the Doppler shift in their observed spectra. At times the spectrum indeed shows a line doubling related to the radial orbital motion of both components: spectral lines originating from the star moving away from the observer will appear red-shifted while those belonging to the other component will be blue-shifted (Fig. 2). [2] Hierarchical triple star In many triple systems the component separation between two components is smaller than the distance to the third body (Fig. 1). The motion of the inner binary is often little influenced by the distant companion and vice versa. This is probably also the case for DG Leo.

2. Observational technique : photometry We are presently collecting photometric data to study the properties of the light variations. Light changes with very small amplitudes can be observed if adequate comparison stars are used as reference objects. A good precision can be obtained on the period determinations. Examples of light curves of the variable star DG Leo obtained at two different observatories are shown to the right (Figs. 4 and 5). For this aspect of our research we are collaborating with a Spanish and a Mexican observatory as well as a Belgian semi-professional observatory. Fig.4 Fig.5 Observational technique : spectroscopy In 2003, we obtained high-resolution spectroscopic data of DG Leo at the Observatoire de Haute-Provence (France). The composite spectrum varies as a function of orbital phase (F) with respect to the 4 days period of the inner binary. In Fig. 6, the spectral lines shown (max. six) originate from two lines with three components moving according to the orbital motions of their stellar counterparts [4]. These data allow to determine the individual component spectra and to measure the changes in radial velocity which are found from the Doppler effect due to the orbital motions [4]. When the spectraresiduals[5] are plotted as a function of time in a gray-scale image, one can remark dark and bright periodic features. The long features are caused by the 4 days orbital motion of the components of the inner binary. Between these we observe the slight shivering of component B due to the pulsations in its outer stellar layers (Fig. 7). Orbital phase (time) Fig.6 Aa B Ab Conclusion DG Leo is an interesting asteroseismological targetfor several reasons: a) the pulsational properties of the pulsating component of this triple system are very poorly known and b) the determination of the various orbital motions and the derived basic stellar parameters provides additional information to understand the pulsation physics. A combined photometric-spectroscopic study is needed to draw a consistent picture of the DG Leo system with its three components. In particular we wish to provide an explanation to why only one component out of three very similar stars is a pulsating  Scuti star. In 2004 we hope to collect a complete set of high-resolution spectra with respect to the orbital phase distribution of the inner binary Aa,b in order to finalize this study. [5] Spectra residuals Due to the presence of pulsations, the shape of a given spectral line may vary with time. This is often seen as moving bumps which periodically pass through the line from side to side. Residuals are a measure of these variations observed at different epochs (lower panel of Fig. 7). A periodic rapid pattern in the residuals is a clear evidence that the star pulsates. In this way we could identify component B as the pulsating star in this system. Time Contactpersons : P. Lampens and Y. Frematpatricia.lampens@oma.be;yves.fremat@oma.be Fig.7

3. Asteroseismology in a triple star Introduction We are carrying out a project to study short-period pulsating A- or F-type variable stars which are also members of a binary or a multiple system. Our objective is to perform a detailed study of the pulsations of the  Scuti star DG Leo[1], member of atriple system, with the goal to exploit the dynamical properties of the system to gain fundamental astrophysical information on the components and thus, on the pulsating star. As the overall purpose of asteroseismology is to probe the stellar interior by studying the pulsations, and as more than 50% of all stars are binaries, understanding the effect of binarity upon the pulsations is an important issue. Additional constraints such as the ones that can be obtained from studies of the binarity or multiplicity may help to comprehend the mechanisms that define the pulsation properties of  Scuti stars. [2] Hierarchical triple star In many triple systems the component separation between two components is smaller than the distance to the third body (Fig. 1). The motion of the inner binary is often little influenced by the distant companion and vice versa. This is probably also the case for DG Leo. Fig. 2 [1] DG Leo = HD 85040 = 44 Kuiperis an hierarchicaltriple star[2] which consists of a close binary (Aa,b) with an orbital period close to 4 days and a third component (B) distant by about 0.2 arcsec and resolvable by high-angular resolution position measurements. The long orbital period is of the order of 100-200 yrs. The system is formed by three component stars of almost equalspectral types and similar masses (Fig. 1). Each component is located in the  Scuti instability region but only one appears to be a pulsating star [3]. [4] The Doppler shift in spectra Spectroscopic binaries are detected by the Doppler shift in their observed spectra. At times the spectrum indeed shows a line doubling related to the radial orbital motion of both components: spectral lines originating from the star moving away from the observer will appear red-shifted while those belonging to the other component will be blue-shifted (Fig. 2). Fig. 1 [3] The  Scuti instability regionis a phase in the evolution of unevolved or slightly evolved late A- and early F-type stars during which stellar oscillations are excited (by the  mechanism, a mechanism based on a damming up of the central energy in the stellar envelope which is later released by a sudden change in the layer’s opacity). Because periodic oscillations or pulsations of the outer stellar layers are being generated, the star is at this stage a pulsating star. Observational technique : photometry We are presently collecting photometric data to study the properties of the light variations. Light changes with very small amplitudes can be observed if adequate comparison stars are used as reference objects. A good precision can be obtained on the period determinations. Examples of light curves of the variable star DG Leo obtained at two different observatories are shown to the right (Figs. 4 and 5). For this aspect of our research we are collaborating with a Spanish and a Mexican observatory as well as a Belgian semi-professional observatory. Fig. 4 Fig. 5 Observational technique : spectroscopy In 2003, we obtained high-resolution spectroscopic data of DG Leo at the Observatoire de Haute-Provence (France). The composite spectrum varies as a function of orbital phase (F) with respect to the 4 days period of the inner binary. In Fig. 6, the spectral lines shown (max. six) originate from two lines with three components moving according to the orbital motions of their stellar counterparts [4]. These data allow to determine the individual component spectra and to measure the changes in radial velocity which are found from the Doppler effect due to the orbital motions [4]. When the spectraresiduals[5] are plotted as a function of time in a gray-scale image, one can remark dark and bright periodic features. The long features are caused by the 4 days orbital motion of the components of the inner binary. Between these we observe the slight shivering of component B due to the pulsations in its outer stellar layers (Fig. 7). Orbital phase (time) Time Fig. 6 Fig. 7 Conclusion DG Leo is an interesting asteroseismological targetfor several reasons: a) the pulsational properties of the pulsating component of this triple system are very poorly known and b) the determination of the various orbital motions and the derived basic stellar parameters provides additional information to understand the pulsation physics. A combined photometric-spectroscopic study is needed to draw a consistent picture of the DG Leo system with its three components. In particular we wish to provide an explanation to why only one component out of three very similar stars is a pulsating  Scuti star. In 2004 we hope to collect a complete set of high-resolution spectra with respect to the orbital phase distribution of the inner binary Aa,b in order to finalize this study. Information : Dr. P. Lampens, Dr. Y. Frémat, KSB-ORB, patricia.lampens@oma.be;yves.fremat@oma.be [5] Spectra residuals Due to the presence of pulsations, the shape of a given spectral line may vary with time. This is often seen as moving bumps which periodically pass through the line from side to side. Residuals are a measure of these variations observed at different epochs (lower panel of Fig. 7). A periodic rapid pattern in the residuals is a clear evidence that the star pulsates. In this way we could identify component B as the pulsating star in this system.