Silicon isotopic abundance toward evolved stars and its application for presolar grains
T.-C. Peng, E. M. L. Humphreys, L. Testi, A. Baudry, M. Wittkowski, M. G. Rawlings, I. de Gregorio-Monsalvo, W. Vlemmings, L.-A. Nyman, M. D. Gray, C. de Breuck
aa r X i v : . [ a s t r o - ph . GA ] N ov Astronomy&Astrophysicsmanuscript no. Si-ratio-v9 c (cid:13)
ESO 2018July 23, 2018 L etter to the E ditor Silicon isotopic abundance toward evolved stars and itsapplication for presolar grains ⋆ T.-C. Peng , E. M. L. Humphreys , L. Testi , , , A. Baudry , , M. Wittkowski , M. G. Rawlings , I. deGregorio-Monsalvo , , W. Vlemmings , L.-A. Nyman , M. D. Gray , and C. de Breuck ESO Garching, Karl-Schwarzschild Str. 2, D-85748 Garching, Germanye-mail: [email protected] Excellence Cluster Universe, Boltzmannstr. 2, D-85748 Garching, Germany INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy Univ. Bordeaux, LAB, UMR 5804, F-33270, Floirac, France CNRS, LAB, UMR 5804, F-33270, Floirac, France National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, SE-439 92 Onsala,Sweden Joint ALMA Observatory (JAO) and European Southern Observatory, Alonso de C´ordova 3107, Vitacura, Santiago, Chile JBCA, Alan Turing Building, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
ABSTRACT
Aims.
Galactic chemical evolution (GCE) is important for understanding the composition of the present-day interstellar medium(ISM) and of our solar system. In this paper, we aim to track the GCE by using the Si / Si ratios in evolved stars and tentativelyrelate this to presolar grain composition.
Methods.
We used the APEX telescope to detect thermal SiO isotopologue emission toward four oxygen-rich M-type stars. Togetherwith the data retrieved from the
Herschel science archive and from the literature, we were able to obtain the Si / Si ratios for atotal of 15 evolved stars inferred from their optically thin SiO and SiO emission. These stars cover a range of masses and ages,and because they do not significantly alter Si / Si during their lifetimes, they provide excellent probes of the ISM metallicity (or Si / Si ratio) as a function of time.
Results.
The Si / Si ratios inferred from the thermal SiO emission tend to be lower toward low-mass oxygen-rich stars (e.g., downto about unity for W Hya), and close to an interstellar or solar value of 1.5 for the higher-mass carbon star IRC + Si / Si ratios and the mass-loss rates of evolved stars, where we take themass-loss rate as a proxy for the initial stellar mass or current stellar age. This is consistent with the di ff erent abundance ratios foundin presolar grains. Before the formation of the Sun, the presolar grains indicate that the bulk of presolar grain already had Si / Siratios of about 1.5, which is also the ratio we found for the objects younger than the Sun, such as VY CMa and IRC + Si / Si value of about 1. Material withthis isotopic ratio is present in two subclasses of presolar grains, providing independent evidence of the lower ratio. Therefore, the Si / Si ratio derived from the SiO emission of evolved stars is a useful diagnostic tool for the study of the GCE and presolar grains.
Key words.
ISM: abundances, ISM: molecules, Submillimeter: ISM, Stars: late-type
1. Introduction
As the eighth most abundant element in the Universe, sili-con plays an important role in understanding nucleosynthesisand Galactic chemical evolution (GCE). The main isotope Siis mainly produced by early-generation massive stars that be-come Type II supernovae. The other two stable isotopes Siand Si are mainly produced by O and Ne burning in mas-sive stars or by slow neutron capture (the s -process) and byexplosive burning in the final stages of stellar evolution, thatis, the asymptotic giant branch (AGB) phase for low- and ⋆ This publication is based on data acquired with the AtacamaPathfinder Experiment (APEX). APEX is a collaboration betweenthe Max-Planck-Institut f¨ur Radioastronomie, the European SouthernObservatory, and the Onsala Space Observatory.
Herschel is an ESAspace observatory with science instruments provided by European-ledPrincipal Investigator consortia and with important participation fromNASA. intermediate-mass stars and supernova explosions for high-massstars (see, e.g., Woosley & Weaver, 1995; Timmes & Clayton,1996; Alexander & Nittler, 1999).In the thermally pulsing AGB (TP-AGB) phase, thermonu-clear runaways are periodically caused by He burning in athin shell between the H-He discontinuity and the electron-degenerate C-O core. This energy goes directly into heating thelocal area and raises the pressure, which initiates an expansionand a series of convective and mixing events (Herwig, 2005;Iben & Renzini, 1983). During the so-called third dredge-up, theproducts of He burning and the s -process elements are broughtto the surface, e.g., C, which can lead to the formation of S-(C / O ≈
1) or C-type (C / O >
1) stars. In conjunction with dredge-ups, the Si-bearing molecules (e.g., SiC and SiO) formed in thestellar surface eventually condense onto dust grains or actuallyform dust grains. The silicon isotopic ratios will be preservedand go through the journey in the interstellar medium (ISM) un-til they are used again to form stars.
Table 1.
Spectral parameters of the observed SiO isotopologuetransitions
Line Frequency E up / k θ MB Instrument(MHz) (K) ( ′′ ) SiO υ = J = SiO υ = J = SiO υ = J = SiO υ = J = SiO υ = J = SiO υ = J = SiO υ = J = Notes. θ MB is the FWHM beam width at the observed frequencies. TheKelvin-to-Jansky conversions are 39, 41, and 390 Jy K − for the APEX-1, 2, and HIFI observations, respectively. AGB stars can produce almost all grains of interstel-lar dusts, and their dust production is one order of magni-tude higher than that of supernovae in the Milky Way (see,e.g., Dorschner & Henning, 1995; Gehrz, 1989). It is gener-ally believed that oxygen-rich M-type stars produce mainly sil-icate grains and carbon-rich stars mainly carbonaceous grains(Gilman, 1969). However, the actual situation may be more com-plicated and grain composition may change during the AGBphase (Lebzelter et al., 2006).The measured Si / Si ratios in the ISM are about 1.5(Wol ff , 1980; Penzias, 1981), very close to that of the solar sys-tem (Anders & Grevesse, 1989; Asplund et al., 2009). However,near-infrared SiO observations of Tsuji et al. (1994) showed thatsome evolved stars have Si / Si ratios slightly below 1.5. Ournew observations of SiO isotopologues in the radio domain withthe APEX and
Herschel telescopes confirm the low Si / Si ra-tios for oxygen-rich M-type stars.
2. Observations
Observations of the SiO isotopologue lines toward VY CMa, o Ceti, W Hya, and R Leo were carried out with the 12-meter APEX telescope in 2011 September and 2012 Decemberon Llano de Chajnantor in Chile. The single-sideband het-erodyne receivers APEX-1 and APEX-2 (Vassilev et al., 2008;Risacher et al., 2006) were used during the observations. Thefocus and pointing of the antenna were checked on Jupiter andMars. The pointing and tracking accuracy were about 2 ′′ and 1 ′′ ,respectively. The extended bandwidth Fast Fourier TransformSpectrometer (XFFTS; Klein et al., 2012) backend was mountedand configured into a bandwidth of 2.5 GHz and ∼ − resolution. In addition, the Herschel / HIFI data of VY CMa, o Ceti, W Hya, χ Cyg, R Cas, and R Dor were retrieved from the
Herschel science archive.All spectra were converted to the main beam brightnesstemperature unit, T MB = T ∗ A /η MB ( η MB = B e ff / F e ff ), using the for-ward e ffi ciencies ( F e ff ) and the beam-coupling e ffi ciencies ( B e ff )from the APEX documentation . The beam e ffi ciencies of HIFIwere taken from the Herschel / HIFI documentation webpage. Weadopted η MB of 0.75, 0.73, and 0.76 for the APEX-1, 2, and HIFIdata, respectively. All data were reduced and analyzed by usingthe standard procedures in the GILDAS package. The SiO spec- http: // http: // / IRAMFR / GILDAS / Fig. 1.
Left: APEX ground-vibrational SiO (black), SiO(red), and SiO (blue) J = o Ceti,and W Hya. The intensities of the SiO and SiO lines weremultiplied by two for clarity. Right:
Herschel / HIFI ground-vibrational SiO (red) and SiO (blue) J = SiO emissionof VY CMa is blended by the CO J = V LSR of the sources.troscopic data were taken from the Cologne database for molec-ular spectroscopy (CDMS ) and are listed in Table 1.
3. Results and discussion
The APEX and
Herschel
SiO isotopologue spectra of the se-lected evolved stars (with both APEX and
Herschel detections)are shown in Figures 1 and 4, and the SiO intensity measure-ments are summarized in Table 2 in the appendix. The SiO and SiO emission is expected to be optically thin because the abun-dance of the main isotopologue SiO is at least ten times largerthan SiO in the ISM (Penzias, 1981). Additionally, the solarand terrestrial Si / Si ratios are close to 1.5 (de Bi´evre et al.,1984; Anders & Grevesse, 1989). The SiO / SiO J = Herschel / HIFI instrument for o Ceti and W Hya (Fig. 1) are consistent with the low- J resultsobtained with the APEX telescope. Fitting two Gaussian pro-files to the SiO line and the partially blended CO J = SiO / SiO J = ± J APEX data. Because the upper-state energies E up / k of J = J = SiO / SiO intensity ratio of low- and high- J transitionsindicates optically thin SiO and SiO emission with similardistributions and excitation conditions. In addition, we believethat the SiO and SiO emission obtained for our sample starsis unlikely to be dominated by masing e ff ects due to the lackof any narrow spectral features. Therefore, the SiO / SiO in-tensity ratio directly reflects the abundance ratio between Siand Si in the circumstellar envelopes of these stars, assumingany di ff erences in chemical fractionation or photodissociation http: // / cdms / Fig. 2.
Comparison of Si / Si in evolved stars. The dashedline indicates the terrestrial and solar Si / Si abundance ra-tio of 1.51 (de Bi´evre et al., 1984; Anders & Grevesse, 1989;Asplund et al., 2009). The red line is a linear fit to the Si / Si-˙ M relation.are minor. The derived Si / Si ratios are listed in Table 3 in theappendix.
Siliconisotoperatios
Since Si is mainly produced via the α -process in massivestars, the Si in low-mass stars comes from their natal clouds.Additionally, stable isotopes Si and Si can be formed viaslow neutron capture (the s -process) in both low- and high-massstars. It has been shown by Timmes & Clayton (1996) that Si isthe primary isotope in the GCE with a roughly constant silicon-to-iron ratio over time, independent of the initial metallicity. Onthe other hand, neutron-rich isotopes Si and Si show strongdependence on the composition and initial metallicity.In Figure 2, the Si / Si ratios derived from the SiO inte-grated intensities are plotted against the mass-loss rates for dif-ferent evolved stars, and they show a tendency to increase withincreasing mass-loss rates. The two supergiants VY CMa andNML Cyg and the carbon star IRC + Si / Si ratiosclose to the solar value of 1.5. The rest of the samples (see alsoTable 3) have Si / Si ratios < .
5, for example, the Si / Si ≈ ff erent Si / Si ratios seen in our sample. One is that the silicon iso-tope ratios merely reflect the initial chemical composition of theenvironment where these stars were born and the di ff erent ratiosare the results of di ff erent ages, which mainly depend on theirmasses and metallicities. The other possibility is that the stellarevolution can significantly change the silicon isotope ratios.The first possibility implies that the Si / Si ratio in the ISMhas not significantly changed in the past 4.6 Gyr when the Sunwas born. In comparison, VY CMa and IRC + ∼ and 1 − × years ago, assuming masses of 25–32and 3–5 M ⊙ , respectively (see Portinari et al., 1998). We foundstars that we believe to be significantly older than the Sun, suchas W Hya (based on the mass-loss rate, initial mass, and cur-rent age), to have lower Si / Si ratios. For instance, with aninitial mass of 1–1.2 M ⊙ , W Hya has an age of 5–10 Gyr. Ineither the lower or higher age limit, this suggests a significantchange in the Si / Si ratio between the pre- and post-solar pe-riod: the Si / Si ratios in the ISM increase from about 1 to 1.5between 5 to 10 Gyr ago and remain roughly constant after theSun was born. Given the time it takes low-mass stars to evolveonto the AGBs, it is unlikely that many low-mass AGB stars ex-isted in our Galaxy between 5 and 10 Gyr ago, even if they had been formed at the beginning of the Milky Way formation. It istherefore also unlikely that low-mass AGB stars were significantcontributors to the GCE in the presolar era. The Si / Si ratioin the presolar era may be due to supernovae and / or other mas-sive evolved stars. We note that the stars in our sample only tracethe Si / Si ratio of their natal clouds if they do not modify thisratio via nucleosynthesis (see, e.g., Zinner et al., 2006).The second possibility for di ff erent Si / Si ratios is that thestars in the AGB phase can significantly modify these ratios.Some of the M-type stars will become C-type stars after sev-eral dredge-up episodes with higher mass-loss rates toward theend of the AGB phase (see, e.g., Herwig, 2005). If the Si / Siratio can be modified by the s -process in the He-burning shell inevolved stars, it must be done e ffi ciently because the AGB timescale is short (a few times 10 yr, see Marigo & Girardi, 2007).However, the modeling results of Zinner et al. (2006) show thatthe Si / Si ratios of low-mass stars do not significantly changeduring the AGB phase (see also the discussion of Decin et al.,2010). Si/ Siratioinpresolar grains
Assuming the Si / Si ratio in the gas-phase SiO is the sameas it condenses onto dust grains or forms silicates, this prim-itive Si / Si ratio may be carried by those grains when theyare incorporated into new stellar and planetary systems. The Si / Si ratio in presolar SiC grains has been studied in somemeteorites (e.g., the Murchison meteorite, see the review byZinner, 1998). They have been categorized into di ff erent types(e.g., X, Y, and Z) according to their silicon isotopic anoma-lies. Most of the SiC grains found in meteorites are the so-called mainstream grains ( ∼ O to form sil-icates (Goumans & Bromley, 2012). In the studies of Si iso-topes in primitive silicate grains, the Si isotopic compositionsof the majority of presolar silicates are similar to the SiC main-stream grains (Nguyen et al., 2007; Mostefaoui & Hoppe, 2004;Nagashima et al., 2004; Vollmer et al., 2008), indicating that theamount of Si isotopes locked in the SiC grains and the -SiOgroup in silicates may be similar; an example are the Orgueilsilicate grains shown in Figure 3.Most of the presolar grains have Si / Si ratios around 1.5,but evidence of lower Si / Si ratios are also found in presolarSiC grains, for instance, types X2 and Z in Figure 3. The type Zgrains may have originated from a nearby evolved star (see alsoZinner et al., 2006). Additionally, the type X grains have beenproposed to have a supernova origin (e.g., Amari et al., 1992;Hoppe et al., 1994), and have two or more subgroups (see, e.g.,Hoppe et al., 1995; Lin et al., 2002) with possible di ff erent stel-lar origins. According to the study of Lin et al. (2002) on theQingzhen enstatite chondrite, the subgroups X1 and X2 showsomewhat similar N and O isotopes abundance ratios, but havedi ff erent slopes on the Si three-isotope plot (0.7 vs. 1.3 for X1and X2, respectively). Because the metallicity in the local ISM isincreasing owing to the GCE, δ Si and δ Si will increase withtime accordingly. It is possible that the lower Si / Si ratios seenin X2 grains may have originated from a population of evolvedstars (such as the evolved stars with lower Si / Si ratios). Onthe other hand, X1 grains with higher Si / Si ratios are likely tobe attributable to Type II supernovae (see also Zinner & Jadhav,2013). Moreover, it is important to point out that the higher-mass
Fig. 3.
Silicon three-isotope plot for presolar grains. The delta notation is defined as δ i Si / Si = [( i Si / Si) / ( i Si / Si) ⊙ − × δ Si / δ Si ratios do not translate directly into the Si / Si ratios, which are plotted as black dashed lines. Left: The data ofdi ff erent subgroups (X, Y, and Z) of SiC grains were taken from Lin et al. (2002), Amari et al. (2001), and Hoppe et al. (1997),and the Orgueil silicate grains from Zinner & Jadhav (2013). The mainstream type ( ∼ ff , 1980), and the black arrow indicates the direction of the Galactic chemical evolution. Theorange crosses are the evolved star sample (about 3 M ⊙ ) from Tsuji et al. (1994). Right: Similar plot as the left one, but on a smallerscale, and the di ff erent silicate grain data were taken from Nguyen et al. (2007), Mostefaoui & Hoppe (2004), and Nagashima et al.(2004).(about 3 M ⊙ ) evolved star sample from Tsuji et al. (1994) can bewell explained by the GCE (Fig. 3), considering the possible un-certainty in the Si / Si ratio estimate for the present-day ISM.
4. Conclusions
We investigated the Si / Si ratios of 15 evolved stars fromthe thermal SiO isotopologue emission obtained by the APEXand
Herschel telescopes and from the literature. The inferred Si / Si ratios tend to be lower among the older low-mass O-rich stars. Because the Si / Si ratios are not significantly mod-ified during the AGB phase and the contributions from the low-mass AGB stars are less important due to their long lifetimes, thelower Si / Si ratios imply di ff erent enrichment of Si and Siin the Galaxy between 5 to 10 Gyr ago with a nearly constantvalue of 1.5 after that. Noting that presolar grains may also have Si / Si ratios lower than 1.5 (i.e., Type X2 and Z), we suggestthat these grains could have been produced by one or more AGBstars with masses high enough to evolve onto the AGB in timeto contribute to presolar grains.
Acknowledgements.
We thank the Swedish APEX sta ff for preparing observa-tions and the referee for helpful comments. MGR gratefully acknowledges sup-port from the National Radio Astronomy Observatory (NRAO). The NationalRadio Astronomy Observatory is a facility of the National Science Foundationoperated under cooperative agreement by Associated Universities, Inc. IdG ac-knowledges the Spanish MINECO grant AYA2011-30228-C03-01 (co-fundedwith FEDER fund). References
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Fig. 4.
Upper panels: APEX ground-vibrational SiO (red) and SiO (blue) J = o Ceti and R Leo. Lower panels:
Herschel / HIFI ground-vibrational SiO (red) and SiO (blue) J = χ Cyg, R Cas, and RDor. The dashed lines indicate the V LSR of the sources.
Table 2.
APEX and
Herschel
SiO integrated intensity measurements
Source SiO 6–5 SiO 6–5 SiO 6–5 SiO 7–6 SiO 7–6 SiO 26–25 SiO 26–25(K km s − ) (K km s − ) (K km s − ) (K km s − ) (K km s − ) (K km s − ) (K km s − )VY CMa 107.4 ± ± ± ± ± ± ± o Ceti 2.3 ± ± ± ± ± ± ± χ Cyg ... ... ... ... ... 0.9 ± ± ± ± ± ± ± ± ± ± ± Notes.
The integrated intensities are measured in T MB and do not include the calibration errors of the APEX and Herschel telescopes (from a fewpercent up to 10%) because the SiO and SiO lines were detected at the same band simultaneously and their ratios do not strongly a ff ected bythe calibrations..-C. Peng et al.: Silicon isotopic abundance toward evolved stars , Online Material p 2
Table 3.
Overview of envelope terminal velocities, mass-loss rates, and Si / Si ratios toward the selected evolved stars
Source d V e ˙ M Si / Si a Spectral Type Stellar Type Note(pc) (km s − ) (M ⊙ yr − )VY CMa 1170 46.5 2 . × − . ± .
08 M2 / + HIFI d NML Cyg 1610 33.0 8 . × − . ± .
20 M6I RSG Tsuji et al. (1994)IRC + . × − . ± .
18 C9,5e MIRA Tsuji et al. (1994)IK Tau 260 18.5 4 . × − . ± . b M8 / . × − . ± .
54 M8.5 MIRA Cho & Ukita (1998)CIT 6 440 20.8 3 . × − . ± .
35 Ce SRa Zhang et al. (2009)G Her 310 13.0 7 . × − . ± .
31 M6III SRb Tsuji et al. (1994)R Cas 106 13.5 4 . × − . ± .
41 M7IIIe MIRA HIFI d SW Vir 170 7.5 4 . × − . ± .
32 M7III SRb Tsuji et al. (1994)RX Boo 155 9.0 3 . × − . ± .
28 M7.5e SRb Tsuji et al. (1994) o Ceti 107 8.1 2 . × − . ± .
06 M7e MIRA APEX-1 / + HIFI d χ Cyg 149 8.5 2 . × − . ± . c S6 MIRA HIFI d R Dor 45 6.0 2 . × − . ± .
16 M8IIIe SRb HIFI d R Leo 130 5.0 1 . × − . ± .
04 M8IIIe MIRA APEX-2 d W Hya 77 8.5 7 . × − . ± .
05 M7e SRa APEX-1 + HIFI d Notes.
The data of distance d , terminal velocity of CO envelope V e , and mean mass-loss rate ˙ M of the selected sources were compiled fromWoods et al. (2003), De Beck et al. (2010), Justtanont et al. (2012), and Sch¨oier et al. (2013). We note that Sch¨oier et al. (2013) adopted smallerdistances for χ Cyg (110 pc) and RX Boo (120 pc), and the distance to VY CMa was averaged from two measurements (Choi et al., 2008;Zhang et al., 2012). The mean Si / Si ratios for VY CMa, o Ceti, and W Hya were derived from the thermal SiO / SiO emission ratiosobtained by the APEX and
Herschel telescopes. The Si / Si ratios for R Cas, χ Cyg, and R Dor were derived from the
Herschel / HIFI data.The Si / Si ratios of G Her, SW Vir, and RX Boo were taken from Tsuji et al. (1994), TX Cam from Cho & Ukita (1998), and CIT 6 fromZhang et al. (2009). Spectral types were take from the SIMBAD database, De Beck et al. (2010), and the references therein. ( a ) The Si / Si ratiosare the mean values (equally weighted) when more than one transition was detected. ( b ) The
Herschel / HIFI data only show a 2- σ detection of SiOand SiO J = ± J = SiO spectra of Kim et al. (2010) may be too low. Therefore, the actual SiO / SiO ratioshould be . . ± ( c ) The ratioreported by Ukita & Kaifu (1988) and Tsuji et al. (1994) is about 2.4 with a large uncertainty (same as V1111 Oph) estimated from J = ff ected by masing e ff ects. Therefore, we estimated the SiO / SiO ratio only from the new HIFI measurements. ( d ))