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Asteroseismology

Asteroseismology

Asteroseismology

'Institut fur Astronomie, Vienna, Austria; 2Universitdtsstemwarte Gottingen, Germany

Introduction These 'normal' are best suited diagram. The prominent new tools Probably the most convincing definl- for significant tests for various asp&$ are: tion for what astwoseismology actually of fundamental stellar physics. Con- preclse distances, measured by the is, was given by Dappen, Dziembowskl fronting realistic stellar models with Hpparcos satelthe, and Sienklewlcz (IAU Symp. No. 123, p. high-quality observations will tell us pulsation periods, observed wlth 233,1988) as a method of testing stellar much about the underlaying physics. ground-based telescope networks structure and evolution theory, uslng J In addition, stars constitute essential and with space experiments, avallable pulsation data (including also laboratories for studying important as- rotational velocities and magnetic growth rates, phases, the fact that mode pects of basic physics (, fields, derived from surface imaging exists, and sometimes are transient, MHD, nuclear reactions, equation of techniques, etc.), and not just observed frequencles. states, transport processes, etc.. . ..) new powerful deters which help probably opens the under conditions which cannot be re- to increase the S/N of obsenrations accessible parameter space well be- produced in terrestrial laboratories. For significantly, yond the classical lnstabllii strips, if extremely high and low temperatures dramatic advances in computer solar-type oscillations can be obsewed and densities, this stellar laboratory is technology. for a large variety of stars. It has become indispensable for Wing physical an increasingly accepted opinion that theories. are just a few examples These Scientific pulsation (probably mody In the form of for the significance of stellar physics Goals non-radial pulsation) is the rule, rather to what may be called laboratory phy- The key Issue of asteroseismology is than the exception. Unfortunately, the sics. the theory of and evolu- obsewable quantities tend to be ex- Not surprisingly, many international don. In the present status of the theory, tremely small and new instrumentation conferences have been devoted In re- a stellar model is typically characterized is needed, for ground-based observa- cent years to heliu- and asteroseismolo- by flve parameters (mass, age, initial tlons as well as for observations from gy. IAU Colloquium No. 137 (Vienna, compositions In helium and metals, and space. The prospects, however, are Aprtt 13 to 17, 1992) will be dwd, mixing length - a parameter describing magnificent since it appears to be pos- among others, to aspects raised in the the convective transport of energy) for sible to test stellar Interior and evolution present artlcle and is entitled "Inside the whlch we usually have only two obser- over most of the parameter space of the Stars". vable~( and surface ). HR dlagram. Stars where relativistic A short ovewlew over the last 30 Consequently, stelIar models cannot be effects are not important and whlch years of stellar astrophysics illustrates adequately tested. Moreover, we have might therefore be classified as 'nor- the immense increase of knowledge some reasons not to trust our descrlp- mal', will probably serve as most com- about how stars are working, but also tion of stellar Interiors. Let us take two mon targets for asteroseismology. about the serious shortcomings in our examples. 'Normal' stars are very Interesting as- physical concepts and accuracy of our When ensembles of stars (like open tronomical objwts In themselves and data. clusters or binary systems) are ob- certainly are not 'boring'. They play a In the slxties, a very important step In sewed for which independent con- crucial rote In the chemkal evolution of stellar moddljng was achieved by qual- straints on some astrophysical para- the Universe. Stars on me Maln Se- itative!~explaining the structure of the meters are available (same age and quence and close to it are by far the HR diagram at the level of accuracy of same initial composition for each most frequent and easily observable in- the observational data. h the late In the ensemble), it Is usualty imposs- gredients of the Universe. All our under- seventies and in the elghties the de- ible to reproduce the observed prop- standing of the Cosmos is based on velopment of solar neutrino erties of the stars with the same val- cal tbratlons (age, distance, mass, etc.) as well as helioseisrnology showed that ue of the mixing length. This may obtained from our closest neighbours. there is not yet a satisfactory model indicate that the representation of We will nat be able to appreclate our which can predict the observed quan- convective transport by the mixing Cosmos untll we fully understand its tities at the very high level of accuracy length theory Is not adequate. constituent stars. Understanding stellar meanwhile achieved. The immense pro- The obsew6d solar neutrino flux Is evolution Is fundamental for a coherent gress in this field, accelerated by suc- much lower than expected, which in- picture of the Universe, because the life cessfut space experiments, suffers from dicates that modelling of the solar of largely depends on the life of a tack of generality, as was demon- interior is incomplete. their basic, luminous constituents: the strated, e.g., at IAU Cdloquium No. 121, Already these two examples demon- stars. "Inside the ". New steps forward are strate that an improvement of stdlar For many years, astronomers have needed to constrain theories by study- modelting is absolutely necessary. How- struggled with the problem posed by ing stars wlth different physical parame- ever, such an Improvement is possible these 'simple' objects, but find them- ters (, luminosity, only if adequate tests for current models selves still far from the goal anticipated chemical composition, rotation rate, can be provided, Asteroseismology is a by Eddlngton who, In 1926, finished his magnetic flelds, etc.). new tool for this purpose. book on The Internal Constltutlon of the In the nineties, still more accurate ob- Stars with the sentence: ". . . but It is servational techniques are being de- Pulsation reasonable to hope that in a not too veloped which are sulted to challenge distant future we shall be competent to theories in a parameter space more Classical pulsating stars have already understand so simple a thlng as a star." complex than the two-dimensional HR been known since 1784, when 6 Cephei Agure 1: Sok pmodes, equatwial cmss Rgure 2: Solar p-modes, qua-/ Cmss ngure 3: Sdar p-modes, equatorla1 cm section 1-80, m -40. 1-3.175 mHz. section 1-40, m-0, vc3.175 mM. section 1-2, rn-2, v-3.147 mu, was discovered as a . It star, and c is the travel sped of sound. be measured to a very htgh accuracy In took nearly 200 mow years to under- For a rotating star, the unpertutBed such a power spectrum: the "larg# and stand the masons for this type of stellar frequency v,, as obsewed fmthe the "smallu separations. The large ssp- variability. earth, is fuher spllt in a symmetric fre- sratton Av, Is the frequency dierence Eigenmodes of pulsations cany a quency multiplet according to: between two modes of same degree I, Wealth of information on the state of the but of quantum numbers n diffel-lng by tot Interior of stars. A mode of a glven de- u,,,, = m (1 ot,nl Q, one. The small sepmtion 6,, Is the fm QrmI is confined to a glven cavlty wWn quency difference between mode n, I he star. High-dgree modes, llke the with -I

i 0 Asteroselsmology at La Silla Already since the early stages of as- teroseismology ESO has granted tele- 0 scope time to various projects In this fiekl. In the following we can only glue a very brief summary which wlll be biased towards our own activity and Is based malnly on after-dlnner 'shop tdalks' at La Silla. We apologize for being Ignorant of other impodant projects. One THEORY group of stars which contdbuted to the boom In asteroseismology is the group of pulsating magnetic CP2 stars, also called, but less precisely. rapidly oscillating Ap (roAp) stars. Soon after the discovery of the first member of this OBSERVATIONS group wlth periods of about 10 minutes by Don Kurh: (South Africa) in 1979, conffrmhg observations were gathared at La Sllla wlth the 50-cm Danish tde- scope in Strhgren and Hp colours. The I 1 I I I L 1 full story Is already told In The Messenger, No. 33. 0 20 40 60 80 100 120 In the beginning of the sightie, some SHIFTED FREQUENCY (pHz) surveys had already been initiated to check CP2 stars for stability against 8 Scuti type pulsation with few hours period. Our suruey at La Silla, e.g., is still ongoing and uses mainly small tete- Stettar interior: For Individual stars, mode ~xcitation,and as a con* scopes (SO-crn telescopes, 0.9-rn Dutch as mentioned above, we usually have quance on our understanding of stel- telescope and the f -m ESO telescope). 2 obbservables: the absolute lumlnos- lar convection. The recently descoped Walraven photo- ity, which can now be known to a Angular momentum distriMion and meter proved to be particularly useful, high accuracy thanks to the Hippar- fransport: The probim of angular because it allowed to obtain simulta- cos satellite, and the surfam gravity, momentum is among the most Im- neous bcolour data. The potential of known to a much lower accuracy. portant in stellar physics. The issue b multicolour information on mode iden- The mode frequencies provided by to understand how stars get rid of tificatbn Is illustrated In The Messengw, the asteroseIsmologioal data will their initial angular momentum, how No. 34, p. 9. Furthermore, Matthews, yletd additional obsembles to test angular momentum is distributed Wehlau and Walks (Astrophys. J. Len;, stellar models. In particular, the large and Is transported in stellar interiws 366, MI) have shown that such obser- and the small separatims will be during a sW1s life. Providing an esti- vations, supplemented by data from the measured to a very high accuracy. mate of Internal rotation through ro- IR, allow to derive the atmaspheric tm- These two observables are directly tational spliings and a measure- perature stretiflcatlon, The first observa- senJtIve to the detalls of internal ment of surface ratation through ob- tions of roAp stars In the near IR have structure, while the usual obsarv- served rotational modulation of white also been obtained at ta Silla. ables are surface properties of the light as well as of UV lines wlll give a La Slla usually plays an important role stars and are only Indirectly sensftlve hint &bout angular momentum dis- in &sewlng campaigns orgsnlred to to the Internal structure. tribution wlthin stars. The differences obtain long and uninterrupted data sets Stellar evelullon: Wlth the availabl4lty seen between stars of diitages which are not affected by the day-night of very preclse frequency measure- will tell us how angular momentum is cycIe. Several such carnpalgns have ments it will be possible to detect transported during . already been successful for various stellar evolution effects even withln Dynarmo theories: The mode fm 8 Scuti (9.g. M. Breger With Bochum an active life time of a scientist. This quencles and the separatbns pro- observers) and roAp stars. has bwn Investigated, among vide an estimate of stellar aps and The observations 05 solar-type oscil- others, for white dwarfs by D. Wlng- masses. The frequencies and sep tations of Procyon and a Cen, men- et, for roAp stars by St. Kawaler, and arstions, with the addition of mode tioned earlier In this artlcle, have been for 8 Scuti stars by M.Breger. amplitudes and life-times, will result obtained by the Nice group prirnarlly at Excitation and convection: Convec- h constraints on the structure of La Silla. Other very Important actlvitles tlon is thought to be ~nsiblefor convective zones, Moreover, as indi- In astwo58ismology are currently ongo- mode excFtatlon In solar-type stars. cated prwlousty, the slmdtanmus ing at the Danish 1.5-m telescope, Therefore, measuring mode am- estimates of Internal rotatlon (rota- where S. Frandsen (Aarhus) and his col- plitudes and life-times of stars of dif- tional spltttlngs) and of surface rota- leagues investigate various ctusters of ferenttypes and ages will have a very tion (rotational modulation) will pro- diirent age for 6 Scuti stars. The Important Impact on the theories of ulde an estimate of the angular ve- Geneva group has amurnulated and publfshed a lot of data related to micro- llw elimination of atmospheric nolse In order to avoid a poor target choice. variability, 6 ScuN, p Cephel and RR Lyr and the posslbllitles of very long, con- Photometric problems may arise from variables. Slmlkr holds true for the tinuous data strings wlth a large duty even very faint background sources Lelden group, using the famous Watra- cycb are the main reasons. One such which dm In and out of hephotometer ven photometer. Very probably, this list asteroseismological space experiment aperture due to satellite jitter. To our of photometric and spectr~cwicpro- is already approved for the Swjet surprise, there are presently no data ar- jects carried out at La Silk 1s hcom- MARS-94 probe and has the acronym chives available which would allow M plets, but yet suited to illustrate the sig- EVRIS. The other space project, more extract the required astromdrlc and nificance of the excellent ESO site for versatile, elaborate and powerful, Is photometric Information for EVRIS. As a &eroseismology, as well as for the Im- presently h Phase A study at ESA and Is consequence, all the candtdate target port~~and effectivity of small and called PRISMA (Probing Rotation and fields have to be carefully observed In medium sized telescopes. "Blg Sci- Interlor of Stars: Micrcrvarlabillty and Ac- various colwrs wlth CCD techniques. ence" not always dmands "Big Tele- tivlty). This synergy between space- and soopm", As already shown earlier In this article, grotlnd-basd obsewations Is another Flnally, we would like to brlefdy touch supplementing ground-based obsem- example for the necessity to develop on our future projects at la Silla wlated tions are mandatory for a fulI exploita- both and not to ignore one at the expen- to asteroseismology, In addition to con- tlon of the scientific potential of as- ses of the other. tlnulng our survey and participating in teroseismology. In the case of NIS, wotld-wide obsetving campaigns. bask stelhr data of sufficlent accuracy, Words of thanks: Many colleagues, As has been clearly demonstrated by Ilk8 effective temperature, log g and ImposJble to llst all here, have contri- a recent ESA Assessment Study for pro- luminosity are missing for many EVRlS buted to this article through discus- ject PRISMA (EM SCI (91) 5, as- target stars. Furthermore, a careful in- sions, cooperations and contributions. teroseismology will anter a new em, If vestlgation of the Immediat8 vicinity of WWW is particularly grateful to the observattons can be done from space. the very bright target sstar is necessary teams of EVRlS and PRISMA.

Multi-Wavelength Observations of -Bright Stars M.A.T GROENEWEGEN' and T. DEJONG'~~ 'Astronomical Institute 'Anton Pannekoek ', Amsterdam, the Netherlands %RON, Laboratory for Space Research, Groningen, the Netherlands

gy distribution they calculated infrared Using average bolometric corrections 1. Introduction bolometric corrections. Distances were for each group the infrared-complete The phase is one of the derived assuming Mbd--4.9, corres- sarnpte was transformed into a volume- last phases of evolution of Intermediate ponding to L=7050 to. From available complete one, The scale height of car- mass stars (1-8 solar masses). Carbon CO data mass-loss rates were calcu- bon stars is found to be I90 pc and their stars reside on the Asymptotic Giant lated for about 80 stars in their sample. local space density equals 185 kpC3. Branch (AGB) In the Hertzsprung- The sample was divided into five From the ealcutated space densities Russell dlagram. The carbon is pro- groups depending on their Infrared of carbon stars in each group relative duced in thermal pulses, short periods properties, extending the classification timescales are derived. Adopting a of explosive He-shell burning, and of carbon stars of Willems and de Jong Ilfetime of 20,000 years for group II stars transported to the surface by convective (1988). Group I stars show the silicate from model calculations, Graenewegen mixing. Many of the physical processes feature in their LRS spectra, possibly et ata find a total lifetlme of the carbon that play a role in the formation and because they are very recently formed star phase of about 26,000 years, un- evolution of carbon stars are still poorly carbon stars. Since none of the known certain to a factor 2. The total mass lost understood, e.g. the production and Group I stars is bright enough to have during the carbon star phase equals dredge-up of carbon, the origin and made it into the sample they will not be about 0.04 hh~.This number is uncer- evolution of the mass loss. Here we dicussed further. Group II stars have a tain to a factor 5, a factor 2 arising from briefly report on an observational study pronounced 60ym excess, high near- the uncertalnty in the llfetime of group II of infrared-bright mrbon stars, infrared temperatures, small bolometric stars and a factor 2.5 arising from the In a recent paper Groenewegen et al. correction and low mass-loss rates. uncertalnty In the mass-loss rates. For (1991) have studied an infrared-com- They probably turned into a carbon star an adopted average mass plete sample of bright carbon st= con- quite recently and their excess at 60-prn of 0.65 Mo this imples that most stars taining all 109 carbon stars wlth is due to a cool circumstellar shell, prob- are already of low mass when they turn S12>100 Jy in the IRAS PSC. This sam- ably the -rich remnant of the pre- Into carbon stars, probably around ple is more complete than others prevl- ceding high mass loss phase. From 0.69@ and certainly less than 0.85 Ma. ously studied in that both optical and group 111 to V the mass-loss rate of car- The location of the 109 stars in the Infrared carbon stars are included. bon stars steadily increases, and as a IRAS colour-colour diagram Is shown In Uslng near-infrared photometry from consequence the near-infrared temper- Figure 1. The cdours C2,=2.5 Iog(%S/ the literature they derived near-infrared atures decrease and the bolometric SI2)and C32=2.5 log(S&Szd are indi- colour temperatures and from the ener- corrections Increase. catecl along the axes. As discussed

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