Mem. S.A.It. Suppl. Vol. 2, 105c© SAIt 2003

Memorie della

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A possible scienti�c case study for the futureNIR spectrograph with IRAIT ?

C. Abia and I. Domınguez ??

Departamento de Fısica Teorica y del Cosmos, Universidad de Granada,18071 Granada (Spain) e-mail: cabia@ugr.es; inma@ugr.es

Abstract. The atmospheric conditions in Antarctica for astronomical observa-tions in the infrared are outstanding. The project to install a 0.8 m infraredtelescope (IRAIT) in the base Dome C, to be exploited by Italian and Frenchscientific teams, is aimed at making use of these extraordinary observation condi-tions. Here, we study the viability of a scientific case that could be performed withthis telescope using an infrared spectrograph, namely a study of the 12C/13C ratioin AGB stars (O-rich & C-rich). The actual value of this isotopic ratio gives valu-able information about the nucleosynthetic and mixing processes occurring withinstars. Currently, there is some controversy concerning the typical (average) valueof this ratio in carbon-rich and oxygen-rich AGB stars. The large isotopic split-ting existing between 12CO and 13CO lines around 2.3 microns, and the fact thatthis spectral region is not very crowded, offers the opportunity to determine thisisotopic ratio more accurately than from atomic or molecular features placed atoptical wavelengths. For atmosphere parameters typical of AGB stars, we computesynthetic spectra of this molecular band simulating different spectral resolutionsto determine the minimum resolution needed to successfully perform this study.Two Spanish institutions have recently expressed interest in joining the Europeanastronomical team at Dome C and possibly in obtaining a low resolution infraredspectrograph for IRAIT.

Key words. AGB stars – nucleosynthesis – abundances

1. Introduction

Recent site testing experiments at DomeC have revealed the fantastic quality ofits sky for astronomical observations, par-

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?? on behalf of the University of Granada andIEEC (CSIC-UPC)Correspondence to: Avda. Fuentenueva s/n,18071 Granada (Spain)

ticularly for near and mid-infrared wave-lengths (Burton et al. 2000; see also Storeyet al. in this Conference). For instance, acomparison of background sky brightnessin the three near infrared bands K, L andM with respect to other sites consideredgood for astronomical observations on theEarth, show gains ranging from a factorof twenty to over one hundred. Moreover,the transparency of the sky at Dome Cin the mid infrared wavelengths (i.e. at

106 C. Abia & I. Domınguez: NIR spectrograph with IRAIT

∼ 20 µm) is better than anywhere else.The recent development of the infrastruc-ture at the Concordia base exploited byItaly and France, will make it possible tosite a permanent astronomical observatorythere to make use of these outstanding ob-servation conditions. The IRAIT (InfraredAntarctic Italian Telescope) project couldbe the first astronomical facility installedin Dome C. This project seeks to install a0.8 m telescope dedicated mainly to nearand mid infrared observations that couldalso serve as the cornerstone of other astro-nomical facilities to be installed in the nearfuture, making Dome C a real InternationalAntarctic Observatory. Initially, IRAITwill be equipped with a camera for midinfrared (∼ 8 − 30 µm) imaging as thefirst light instrument. The telescope andcamera are currently in the testing phaseat the Coloti Observatory (Perugia, Italy)and are scheduled to be delivered to DomeC during the Antarctic summer of 2004-05. Very recently, two Spanish institu-tions (Universidad de Granada and Institutd’Estudis Espacials de Catalunya (CSIC-UPC)) expressed their interest in joiningthe scientific teams (Italian and French) in-volved in the IRAIT project. As a real con-tribution to this project, the Spanish teamscould participate in the following areas: i)the design and ultimate construction of a(definitive) enclosure for IRAIT at DomeC, ii) the building of the wobbling mir-ror system necessary for infrared observa-tions and/or iii) the study and building ofa second generation instrument, namely acamera or spectrograph (or both) for nearinfrared (1 − 4 µm) imaging and/or spec-troscopy. Here, we present a scientific casestudy that might be undertaken with thissecond instrument and IRAIT at Dome C:the study of the 12C/13C ratio in a largesample of AGB stars, both in the MilkyWay and in the nearby external galaxies.

2. The 12C/13C ratio in AGB stars

The Asymptotic Giant Branch (AGB) starsrepresent the final phase of the evolution of

stars in the mass range 1 <∼ M/M¯ <∼ 8.The structure of an AGB giant is charac-terised by a degenerate CO core, by twoshells (of H and He) burning alternatively,and by an extended convective envelope.In the HR diagram, these stars lie closeto the brightest part of the Hayashi line.Schwarzschild & Harm (1965) showed atan early stage that thermal instabilities inthe He-shell (thermal pulses, TP) occur pe-riodically during the advanced phases ofAGB evolution. During a TP the whole re-gion between the H shell and the He shell(called “the He intershell”) becomes con-vective. After each TP, the convective en-velope penetrates downward dredging-upmaterial previously exposed to incompleteHe-burning conditions. This phenomenonis called third dredge up (TDU), and itsmain consequence is the increase in the car-bon content in the envelope so that, even-tually, the C/O ratio can exceed unity andthe star becomes a carbon star. Thus, thecarbon content in the envelope is expectedto increase during the spectral sequenceM→MS→S→SC→C, stars of spectral classC showing C/O> 1 (see e.g. Iben & Renzini1983; Smith & Lambert 1990). Therefore,the 12C/13C ratio in the envelope is ex-pected to increase during the AGB phasefrom the values reached during the previousphases of evolution (i.e. from a typical valueof 12C/13C∼ 20 in the red giant branchphase after the first dredge-up). However,exceptions to this figure exist. In AGB starsof intermediate mass (M ≥ 4 M¯) thisisotopic ratio can again be reduced if thebottom of the convective envelope is hotenough (T≥ 20 × 106 K) to partially acti-vate the CN cycle. In this case, the opera-tion of this hot bottom burning (HBB) caneven prevent the formation of a carbon star,keeping the C/O ratio below unity and re-ducing the carbon isotopic ratio to near theCN-cycle equilibrium value. Indeed, manyof the most luminous (and, thus, presum-ably massive) AGB stars in the Galaxy andin the Magellanic Clouds are O-rich starsinstead of C-rich objects, showing very lowisotopic ratios, 12C/13C∼ 4 (e.g. Smith et

C. Abia & I. Domınguez: NIR spectrograph with IRAIT 107

Fig. 1. The 12C/13C ratio in galactic car-bon stars. From up to down: Lambert et al.(1986), Ohnaka & Tsuji (1996), Schoier &Olofsson (2000) and Abia et al. (2002). Thebox size is 10.

al. 1995). In low mass AGB stars (M≤ 3M¯), the bottom of the envelope is nothot enough to develop HBB, and thereforelarge isotopic ratios (12C/13C> 40) are ex-pected as well as the formation of a carbonstar. However, observations indicate thatthis is not always the case.

During the last few years, severalgroups have derived the carbon isotopicratio in carbon stars. The main resultsof these studies are shown in Figure 1.Lambert et al. (1986) derived this ratioin a sample of galactic carbon stars fromCN lines in the near infrared (1.9-2.3 µm).They found typically 12C/13C ratios largerthan about 40, which was in agreementwith theoretical expectations. Some of thestars in the sample showed very low ra-tios (3 − 10), but most of these stars areclassified as carbon stars of spectral typeJ, a type of carbon star which could bethe consequence of a different evolution-ary path from that followed by the normalC(N) carbon stars (Lorentz-Martin 1986).However, Ohnaka & Tsuji (1996) from alarger sample of stars, found typically lower

ratios (see Figure 1), peaking at around 30,in contrast with the previous figure. Theselow values, in principle, cannot be under-stood on the basis of the standard stellarmodels on the AGB phase. Two very re-cent works (Schoier & Olofsson 2000; Abiaet al. 2002), have found 12C/13C ratios inbetween the values found by Lambert etal. (1986) and Ohnaka & Tusji (1986) (seeFigure 1), adding more confusion to thesituation. Indeed, part of the discrepancybetween the different works might be of aspectroscopic origin. For instance, Ohnaka& Tsuji and Abia et al. derived the 12C/13Cratio from CN lines in a very crowded spec-tral region, where many of the 12CN linesare, in fact, saturated. On the other hand,Schoier & Olofsson determined this iso-topic ratio from CO absorptions at sub-millimetric and millimetric wavelengths inthe circumstellar envelope formed aroundthe AGB star. It is not clear whether thecircumstellar isotopic ratio might representthose in the photosphere since part of thecarbon isotopes could be differentially de-pleted in the dust and grains formed inthe cool circumstellar envelope. Also, dif-ferences in the model atmospheres for car-bon stars used by the different authorscertainly play a role in this controversy.However, even taking into account all ofthese possible sources of uncertainty, whencomparing the data from different studiesand the error bars in the measured ratios,there are still a considerable number of car-bon stars with low 12C/13C ratios that can-not be explained. In fact, Abia et al. (2001)showed that even considering the existenceof an extra-mixing mechanism (as manyRGB stars seem to indicate, e.g. Gilroy &Brown 1991) during the previous phases ofevolution, capable of reducing the carbonisotopic ratio to values as low as ∼ 10 af-ter the first dredge-up, it is impossible toobtain 12C/13C below ∼ 40 for any stel-lar mass model. This is a very importantresult since, if confirmed, it could also indi-cate the existence of an extra-mixing mech-anism on the AGB phase, preferably inAGB stars of low mass. This non-standard

108 C. Abia & I. Domınguez: NIR spectrograph with IRAIT

Fig. 2. Theoretical spectra of the CO band for different 12C abundance A(12C)=8.66, 8.56 and 8.46 simulating different resolving power R. From up to down: R ∼3000, 1500 and 600. The top panel shows and extra case (long dashed line) computedwith A(12C) = 8.16.

mixing mechanism could also explain otherchemical anomalies observed in AGB stars(Wasserburg et al. 1995; Nollet et al. 2003).

3. The 12C/13C ratio with IRAIT

¿From the above, it is obviously impor-tant to derive accurate carbon isotopic ra-tios in a large sample of carbon stars. To

do this, it is necessary to have a spectro-scopic tool that is free of the analysis prob-lems quoted above. This tool could be the13CO(3-1) and 12CO(2-0) molecular bandabsorptions at 2.374 µm. There are severaladvantages to this: a) The isotopic split-ting between the 12CO and 13CO lines isvery large, and thus it is not necessary tohave a high spectral resolution. b) CO lines

C. Abia & I. Domınguez: NIR spectrograph with IRAIT 109

Fig. 3. As Figure 2 for different 12C/13C ratios: 89, 30 and 6, simulating different resolv-ing power R. From up to down: R ∼ 3000, 1500 and 600.

in this molecular band are not saturatedin C-rich or O-rich AGB stars. c) Carbonabundances and isotopic ratios can be de-rived independently of the nitrogen abun-dance, unlike that from CN lines. In thelatter case, a nitrogen abundance must beestimated in advance. The nitrogen con-tent in the envelope is modified during theAGB phase and the previous evolution ofthe star. d) Finally, AGB stars are verybright at 2.374 µm (the K band); from ob-

servations of AGB stars in the MagellanicClouds, these stars have <MK >∼ −8(e.g. Frogel, Persson and Cohen 1980). Thiswould make spectroscopic observations ofAGB stars in the Magellanic Clouds pos-sible with a medium size telescope such asIRAIT without very long exposure times.In order to design a near infrared spec-trograph for IRAIT we must, nevertheless,take into account several limitations. First,IRAIT is planned to be installed inside an

110 C. Abia & I. Domınguez: NIR spectrograph with IRAIT

Fig. 4. The full CO band at R ∼ 600 for two different 12C/13C ratios: 30 and 6, anda total carbon abundance of 8.62. Even with this low spectral resolution, the differencebetween both theoretical spectra is clearly appreciated. Still, it would be possible toestimate the 12C/13C ratios in a large sample of AGB stars with IRAIT and a NIRspectrograph.

ISO standard container (see Tosti et al.at this Conference). This limits the avail-able space for astronomical instruments.Second, due to the space limitations, thedistance between the primary mirror andthe telescope fork will be greatly reduced,and thus the available space for a focalplane instrument will be limited, too (the

possibility of the Nastmyth focus configu-ration must be considered). All this clearlylimits the actual size of the instrumentationto be used with IRAIT. Since, as a generalrule for spectrographs, the higher the spec-tral resolution desired the larger the sizeof the spectrograph, it is necessary to ad-dress the question of the minimum resolv-

C. Abia & I. Domınguez: NIR spectrograph with IRAIT 111

ing power (R= λ/∆λ) required to estimatethe 12C/13C ratio from the CO band at 2.3µm. We attempt to answer this question bycomputing theoretical synthetic spectra inthis wavelength range for the atmosphereparameters typical of carbon stars. Then,we convolve the synthetic spectra usingGaussian functions with different FWMH(sigma) values to simulate the final spec-tral resolution achieved depending on thespectrograph performance. Figure 2 showssynthetic spectra computed in the rangeλ2.32−2.33 µm where the CO band is mostsensitive to variations in the total carbonabundance (i.e. 12C and 13C). The spec-tra are computed using the stellar parame-ters Teff = 2850 K, log g = 0.0, [Fe/H]= 0.0, C/O = 1.1, 12C/13C = 40 andξ = 2.2 kms−1; these are typical parame-ters for carbon stars of the galactic disk (seee.g. Lambert et al. 1986). For each spec-tral resolution case (the R value), the spec-tra are computed for three different valuesof total carbon abundance, namely A(12C)= 8.66, 8.56 and 8.46. From this Figure, itis obvious that theoretical spectra are notvery sensitive to small changes in the totalcarbon abundance. There must be a vari-ation by an important amount in the car-bon abundance (∼ 0.5 dex, A(12C)=8.16,the long dashed line in the R ∼ 3000 case),for there to be appreciable differences in thetheoretical spectra. We conclude, therefore,that even with a moderate resolution spec-trograph, carbon abundances cannot be de-rived with accuracy from this CO band, al-though a first estimate can always be made.However, this figure is quite different con-cerning the derivation of the 12C/13C ra-tio. Figure 3 shows synthetic spectra inthe range λ2.34 − 2.35 µm, where the COband is most sensitive to 13C abundancevariations. For the same spectral resolu-tion cases as in Figure 2, synthetic spectracomputed with 12C/13C= 89, 30 and 6 areshown. It is now clear that even in the lowresolution case (R ∼ 600), differences by∼ 5− 10% in the relative flux exist, whichcan be easily detected observationally. Thesame conclusion is obtained when comput-

ing similar spectra for O-rich AGB starsand/or for AGB stars with lower metal-licity, for instance [Fe/H] ∼ −0.8, whichwould simulate an AGB star belonging tothe Magellanic Clouds. Thus, this spectralregion in the near infrared constitutes apromising tool to derive the carbon isotopicratio in AGB stars.

4. Conclusions

Probably the most important limitation tothe minimum spectral resolution requiredis determined by the continuum location inthe spectrum. As can be seen from Figure4, where the full CO band is represented,in the case R ∼ 600 the continuum positionis lost, thus impeding the determination ofaccurate isotopic ratios. One could there-fore conclude that the minimum resolutionrequired would be around Rmin ∼ 1000,although this would depend on the spe-cific scientific goals to be achieved withthe spectrograph. For instance, with sucha low resolution spectrograph one couldstudy the 12C/13C ratio in a large sam-ple of AGB stars, identifying those havinglarger 13CO absorptions and later derivingthe carbon isotopic ratio more accuratelyusing larger telescopes than IRAIT and anear infrared spectrograph with better per-formance. Alternatively, many other scien-tific cases not requiring very large spectralresolutions are possible with a low resolu-tion spectrograph and IRAIT: spectroscopyof planetary nebulae, detection of very lowmass stars, brown dwarfs and/or planets,studies of ultra-luminous infrared galaxiesetc.

Acknowledgements. Part of this work was sup-ported by the Spanish grants AYA2002-04094-C03-03 and FQM-292.

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