Discovery of SiCSi in IRC+10216: A missing link between gas and dust carriers of SiC bonds
J. Cernicharo, M. C. McCarthy, C. A. Gottlieb, M. Agundez, L. Velilla Prieto, J. H. Baraban, P. B. Changala, M. Guelin, C. Kahane, M. A. Martin-Drumel, N. A. Patel, N. J. Reilly, J. F. Stanton, G. Quintana-Lacaci, S. Thorwirth, K. H. Young
aa r X i v : . [ a s t r o - ph . GA ] M a y Discovery of SiCSi in IRC + − C bonds
J. Cernicharo , M. C. McCarthy , C. A. Gottlieb , M. Ag´undez , L. Velilla Prieto , J. H.Baraban , P. B. Changala , M. Gu´elin , C. Kahane , M. A. Martin-Drumel , N. A. Patel , N. J.Reilly , , J. F. Stanton , G. Quintana-Lacaci , S. Thorwirth , K. H. Young To be published in the ApJ Letters; received April 21st, 2015; accepted May 06, 2015
ABSTRACT
We report the discovery in space of a disilicon species, SiCSi, from observationsbetween 80 and 350 GHz with the IRAM
30m radio telescope. Owing to the closecoordination between laboratory experiments and astrophysics, 112 lines have nowbeen detected in the carbon-rich star CW Leo. The derived frequencies yield improvedrotational and centrifugal distortion constants up to sixth order. From the line profilesand interferometric maps with the Submillimeter Array , the bulk of the SiCSi emis-sion arises from a region of 6 ′′ in radius. The derived abundance is comparable to thatof SiC . As expected from chemical equilibrium calculations, SiCSi and SiC are the Group of Molecular Astrophysics. ICMM. CSIC. C / Sor Juana In´es de La Cruz N3. E-28049, Madrid. Spain Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, and School of Engineering & AppliedSciences, Harvard University, Cambridge, MA 02138 Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309 JILA, National Institute of Standards and Technology and University of Colorado, and Department of Physics,University of Colorado, Boulder, CO 80309 Institut de Radioastronomie Millim´etrique, 300 rue de la Piscine, F-38406, St-Martin d’H`eres, France Universit Grenoble Alpes, IPAG, F-38000 Grenoble, France; CNRS, IPAG, F-38000 Grenoble, France Present address: Department of Chemistry, Marquette University, Milwaukee, WI 53233 Institute for Theoretical Chemistry, Department of Chemistry, The University of Texas at Austin, Austin, TX78712 I. Physikalisches Institut, Universit¨at zu K¨oln, Z¨ulpicher Str. 77, 50937 K¨oln, Germany This work was based on observations carried out with the IRAM 30-meter telescope. IRAM is supported byINSU / CNRS (France), MPG (Germany) and IGN (Spain) The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the AcademiaSinica Institute of Astronomy and Astrophysics, and is funded by the Smithsonian Institution and the Academia Sinica. − C bond in the dust formation zone and certainlyboth play a key role in the formation of SiC dust grains.
Subject headings:
Stars: individual (IRC +
1. Introduction
Interstellar dust grains are synthesized in two main types of sources: the inner winds ofasymptotic giant branch (AGB) stars and the ejecta of massive stars, mainly supernovae (SNe).Dust grains are formed from molecular seeds. In AGB stars, molecules such as TiO, VO, ZrO,C , CN, and C , among others, are known to be present in their photospheres since the beginningof the 20th century. Because of the high stability of the CO molecule, depending on whether theC / O ratio is > < ff erent chemistry and therefore to di ff erent dustformation schemes. In SNe, di ff erent types of dust can be formed depending on the degree ofenrichment in heavy elements. The dust formation e ffi ciency in SNe is still a matter of debate. Re-cent studies with the Herschel
Space Telescope point to much larger dust masses than previouslythought (Matsuura et al. 2011, 2015).Dust formation can be simplified as a two-step process: formation of nucleation seeds fol-lowed by grain growth through condensation of refractory molecules at high temperature and otherless refractory species at larger distances from the star. There are, however, many mysteries inthis picture of events, starting from the fundamental step of the formation of the nucleation seeds,which essentially are refractory species (Gail 2010). The presence of SiC grains in C-rich AGBswas confirmed by the detection of an emission band at ∼ µ m (Tre ff ers & Cohen 1974). Thisband has been found towards a large number of C-rich stars with the IRAS and ISO satellites.However, the molecular precursors of SiC dust grains are still unknown. SiC molecules have beendetected in the external shells ( ≥
300 R ∗ ) of IRC + − C bond is SiC (Thaddeus et al.1984; Cernicharo et al. 2010), whereas the most abundant Si-bearing species is SiS, which, to-gether with SiO and SiC , account for a significant fraction of the available silicon (Ag´undez et al.2012).CW Leo, located ≃
130 pc from us, is a Mira variable star with a period of 630-670 days and anamplitude of ≃ K band (Menten et al. 2012). It is one of the brightest infrared sourcesin the sky. Due to its close proximity, IRC + n H and their anions (Cernicharo & Gu´elin 1996; Cernicharo et al.2008; Gu´elin et al. 1997; McCarthy et al. 2006; Thaddeus et al. 2008, and references therein).Among the species detected in this source, the silicon-carbon species Si n C m could play an impor-tant role as seeds of SiC dust grains. The simplest members of such a family, SiC (Cernicharo et al.1989), and SiC (Thaddeus et al. 1984), are known to be present in IRC + C(hereafter SiCSi), which is predicted to be very abundant from chemical equilibrium calculations(Tejero & Cernicharo 1991; Yasuda & Kozasa 2012) and could play a key role in the formation ofSiC dust grains, has not been found so far.In this Letter we present the detection of SiCSi in IRC +
2. Observations
In the course of searches for new molecules we have covered a large fraction of the 3, 2, 1and 0.8 mm spectrum of IRC + + ±
90” at a rate of 0.5 Hz, and the dry weather conditions (sky opacity at 225GHz was below 0.1 in most observations) ensured flat baselines and low system noise temperatures( T sys ≃ ff position located at 180 ′′ from the star, providesreference data free from emission for all molecular species but CO (see Cernicharo et al. 2015).The emission of all other molecular species is restricted to a region ≤ ′′ (see, e.g., Gu´elin et al.1993). The intensity scale, antenna temperature ( T ∗ A ), was corrected for atmospheric absorptionusing the ATM package (Cernicharo 1985; Pardo et al. 2001). The main beam antenna temperaturecan be obtained by dividing T ∗ A by the main beam e ffi ciency of the telescope which is 0.81, 0.59,and 0.35 at 86, 230, and 340 GHz, respectively. Calibration uncertainties for data covering sucha large observing period have been adopted to be 10%, 15%, 20%, and 30% at 3, 2, 1, and 0.8mm, respectively. Additional uncertainties could arise from the line intensity fluctuation withtime induced by the variation of the stellar infrared flux, which has been recently discovered byCernicharo et al. (2014). All data have been analyzed using the GILDAS package .The data revealed several hundreds of spectral lines which could not be assigned to any knownmolecular species collected in the public CDMS (M ¨uller et al. 2005) and JPL (Pickett 1998) spec-tral databases and in the MADEX code (Cernicharo 2012). Most of these lines show the character-istic U-shaped or flatted profiles with linewidths of 29 km s − . However, above 250 GHz a signifi-cant number of lines are very narrow and come from the dust formation zone of CW Leo (see, e.g.,Patel et al. 2011; Cernicharo et al. 2013). Among the unidentified lines observed with the IRAM30-m telescope, we have been able to assign 112 to the rotational spectrum of SiCSi through aniterative procedure described below and a close synergy between molecular spectroscopy and as-trophysics. Some selected lines among those observed in IRC +
3. Spectroscopic constants of SiCSi
Disilicon carbide (SiCSi) has a C v symmetry and a A electronic ground state with a modestpermanent dipole moment of ∼ b -inertial axis (Gabriel et al. 1992; Barone et al. 1992;Bolton et al. 1992; Spielfiedel et al. 1996; McCarthy et al. 2015). Because the two equivalent o ff -axis silicon atoms are bosons, only half of the rotational levels exist ( K a + K c even).The search for lines of SiCSi in space began with the lines measured in the laboratory byMcCarthy et al. (2015), which allowed us to accurately predict frequencies with K a = , http: // / IRAMFR / GILDAS , − , , 16 , − , , 14 , − , and 12 , − , (from the SMA line survey of IRC + V LSR velocities indicated on top right in km s − . Thesize of the synthesized beam is shown in bottom right panel. The halftone image in the first panelis scaled linearly from 0 to 2.7 Jy beam − km s − and the contour levels (in the same units) havethe starting value and intervals of 0.4 in the first panel and 0.04 in remaining panels. 7 –20 MHz for transitions higher in J than those measured in the laboratory ( J ≤ + + K a = , ff erences between the predicted and observed frequencies were again 10-50MHz, but because the lines are so intense (typically 10-20 mK) and have a distinctive line shape, itwas not di ffi cult to make additional assignments (see Figure 1). Nevertheless, on occasion, we hadto discard several initial assignments because subsequent predictions failed to predict new lines.With the laboratory lines and the ∼
30 astronomical lines involving K a = K a = , + K a = D K term, and confirm the astronomical assignmentswith higher K a .Ultimately, we were able to assign 112 lines in IRC + J ≤
48 and K a ≤ ∼ ± − was derived, whichis similar to that obtained for most lines in IRC + + A and S reductions (Table 1). The fit with the S reduction was donewith the SPFIT program Pickett (1991), while that in the A reduction was done with fitting pro-grams associated with MADEX (Cernicharo 2012). We confirmed that both SPFIT and MADEXyielded the same results in the A reduction. The rotational constants in the A and S reductionsagree rather well, but the S reduction is preferred because δ K and h JK are three orders of magni-tude larger than the corresponding constants d and h and the predicted frequencies for high J or K a will be less accurate. Because there are only a few observed lines with K a >
3, further improve- 8 –Table 1: Spectroscopic constants (MHz) of SiCSi in comparison to SiC Constant S-reduction A-Reduction SiC A-reduction A B C ∆ J × ∆ JK -0.856833(73) -0.8572075(610) 1.5382238(882) ∆ K d /δ J × -1.52630(34) 1.519832(437) 2.41191(253) d /δ K × -0.00318(32) 5.1591(454) 8.70880(415) H J × -3.627(116) -4.1349(949) -8.312(460) H JK × H KJ × -1.8882(95) -1.88755(791) 0.39025(399) H K × h / h J × -5.304(225) -5.231(187) -3.46(271) h / h JK × -2.614(255) -6586(361) -35151(191) h / h K × Note. — 1 σ uncertainties (in parentheses) are in the units of the last significant digits. J ≤ K a ≤ + , a well-studied molecule alsoconsidered to be very floppy. Table 1 provides, for comparison purposes, the rotational constantsderived from a fit to the lines of SiC reported by M ¨uller et al. (2012) including the same constantsas for SiCSi plus additional higher distortion constants. Both, ∆ K and H K are one order of mag-nitude larger for SiCSi than for SiC , while ∆ JK and δ K are of the same order despite SiCSi beingsomewhat heavier than SiC . Both molecules have a very low-lying vibrational mode, around 100-200 cm − . Hence, by analogy with SiC for which the antisymmetric ν mode is readily observedin IRC + ν . Nevertheless, SiCSi has its dipole moment along the b -axis while in SiC it is aligned along the a -axis which produce a significantly di ff erent rotational spectrum.
4. Discussion
The observed line profiles of SiCSi go from U-shaped, in the case of low excitation lines lyingat high frequencies and thus observed with a smaller beam, to flat-topped, for high-excitation lineslying at low frequencies (see Figure 1). Such behaviour indicates that SiCSi is concentrated aroundthe star but with a relatively extended distribution, as occurs in the case of metal-bearing species(Cernicharo & Gu´elin 1987; Gu´elin et al. 1993). The SMA maps, which correspond to transitionsinvolving upper level energies between ∼
60 and ∼
100 K, show that the emission is concentratedin a region of ≃ ′′ in diameter (see Figure 2). A slightly larger brightness distribution size canbe expected for lines involving lower energy levels (see Figure 1). We have checked if SiCSicould be responsible for some of the lines detected with ALMA in IRC + ff erent energies, we show in the top panel of Figure 3 a rotational diagram based on the 112observed lines. We have adopted a gaussian source size with a radius 5 ′′ . The observed integratedintensities have been corrected for dilution in the beam and for the main beam e ffi ciency of thetelescope. We can distinguish three zones with di ff erent rotational temperatures. The lines with 10 –Fig. 3.— The top panel shows the rotational diagram of the 112 observed lines of SiCSi. Thethree zones discussed in the text are indicated by vertical dashed lines. The bottom panel showsthe radial abundance distribution of SiCSi, SiC , and SiC as calculated by chemical equilibrium(dashed lines) and as derived from the observations of IRC + is taken from Cernicharo et al. (2010). 11 –upper level energies below 60 K (zone I) have been fit with a rotational temperature, T rot , of31 ± . ± . × cm − . The lines with upper level energiesbetween 60 and 200 K (zone II) have been fit with T rot = ±
12 K and a column density of(3 ± . × cm − . Finally, the lines with upper level energies above 200 K (zone III) havebeen fit with T rot = ±
50 K and a column density of (3 . ± . × cm − . The rotationaltemperature derived for zone I could correspond well with the extended molecular ring observed at r ≃ ′′ in SiC (Gu´elin et al. 1993). Its value agrees well with the rotational temperature derivedfor other species (Ag´undez et al. 2008; Cernicharo & Gu´elin 1996). Zone II corresponds to theemission region of metal-bearing species and high energy lines of SiC with r ≤ ′′ (Figure 2 andVelilla Prieto et al. 2015). Finally, the high energy lines (zone III) correspond to the region with r ≤ ′′ traced by high − J lines and vibrationally excited states of SiC , SiS, HCN, HNC among otherspecies (Cernicharo et al. 2010, 2011; Velilla Prieto et al. 2015). In this region, Cernicharo et al.(2010) derived for SiC T rot ≃
204 K and a column density of ∼ × cm − averaged over abeam of 30 ′′ . When corrected for the source radius of 5 ′′ adopted in this work, the column densityof SiC becomes 3 × cm − . Hence, SiCSi is as abundant as SiC in the dust formation zoneof IRC + ff erence in the intensities of the observed lines of SiCSi and SiC ,near a factor of 100, arise from the lower dipole moment and the larger partition function of SiCSi.When corrected for these e ff ects, the observed intensities of the individual lines of SiCSi and SiC indicate a SiC / SiCSi abundance ratio of about 5.Early chemical equilibrium calculations by Tejero & Cernicharo (1991) showed that in theinner envelope of C-rich AGB stars the most abundant gas phase species containing a Si − C bondare SiC and SiCSi, while SiC is predicted to be present with a much lower abundance (see alsoYasuda & Kozasa 2012 and Figure 3). To constrain the abundance and spatial distribution of SiCSiin IRC + ∗ derived by Cernicharo et al. (2013). Since collisional rate coe ffi cients are not available forSiCSi we have assumed local thermodynamic equilibrium, which is a reasonable approximationgiven the low dipole moment of SiCSi. We find that the intensities and profiles of the low ex-citation lines observed can be well reproduced by adopting a SiCSi abundance relative to H of4 × − , from the inner layers out to the photodissociation region. We have assumed that SiCSi isphotodissociated by the ambient interstellar ultraviolet field with a rate of 10 − × exp ( − A V ) s − ,where A V is the visual extinction. However, to reproduce the high intensities observed for linesinvolving upper level energies above 200 K, it is necessary to increase the abundance of SiCSi inthe warm inner regions to 2 × − (see Figure 3). Finally, a depletion in the very inner regions,which is consistent with the expectations from chemical equilibrium, yields a better agreementwith the observed line profiles (see Figure 1). The derived abundance profile for SiCSi is shown in 12 –Figure 3.High angular resolution interferometric observations are needed to better constrain the abun-dance gradient of SiCSi in the inner layers. In any case, we find that disilicon carbide is as abundantas silicon dicarbide in the inner layers. The depletion in the abundance of SiCSi at 50 R ∗ , whichleads to a SiC / SiCSi abundance ratio of ≃ REFERENCES
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15 –Table 2: Observed lines of SiCSi in the laboratory and in IRC + ( J K a K c ) u ( J K a K c ) l ν obs (unc) R T ∗ A × dv Predicted
Lab + Astro (unc) E upp S ul σ MHz K kms − MHz K mK5 1 5 6 0 6 6718.9927 (0.002) 6718.994 (0.001) 8.9 2.586 lab8 0 8 7 1 7 12018.3548 (0.002) 12018.353 (0.001) 14.7 3.692 lab20 1 19 19 2 18 19095.4772 (0.002) 19095.477 (0.002) 89.8 4.991 lab15 2 14 16 1 15 24506.4805 (0.002) 24506.481 (0.002) 60.4 3.644 lab3 1 3 4 0 4 24959.6919 (0.002) 24959.692 (0.001) 5.3 1.528 lab10 0 10 9 1 9 31198.7591 (0.002) 31198.761 (0.002) 22.4 4.861 lab1 1 1 2 0 2 42663.0910 (0.002) 42663.088 (0.002) 3.3 0.504 lab2 1 1 2 0 2 60248.2200 (0.020) 60248.209 (0.003) 4.1 2.493 lab4 1 3 4 0 4 61298.7200 (0.020) 61298.746 (0.003) 7.0 4.450 lab6 1 5 6 0 6 62974.9200 (0.020) 62974.923 (0.005) 11.6 6.340 lab8 1 7 8 0 8 65310.0300 (0.020) 65310.040 (0.008) 17.8 8.135 lab1 1 1 0 0 0 68154.6000 (0.020) 68154.606 (0.004) 3.3 1.000 lab10 1 9 10 0 10 68348.4600 (0.020) 68348.474 (0.011) 25.7 9.806 lab3 1 3 2 0 2 84424.5800 (0.020) 84424.602 (0.006) 5.3 2.002 lab5 1 5 4 0 4 100120.6600 (0.040) 100120.678 (0.009) 8.9 3.022 lab8 2 6 9 1 9 109709.6000 (0.040) 109709.645 (0.018) 26.2 1.447 lab7 1 7 6 0 6 115262.8500 (0.040) 115262.899 (0.013) 14.1 4.073 lab11 1 11 10 0 10 144033.4200 (0.040) 144033.475 (0.023) 29.3 6.327 lab3 2 2 3 1 3 180036.9000 (0.040) 180036.916 (0.017) 13.9 1.456 lab5 2 4 5 1 5 181405.4500 (0.040) 181405.432 (0.014) 17.6 2.548 lab7 2 6 7 1 7 183384.5800 (0.040) 183384.603 (0.012) 22.9 3.561 lab9 2 8 9 1 9 185976.7800 (0.040) 185976.772 (0.012) 29.8 4.527 lab16 1 15 16 0 16 82260.5000 (0.500) 0.109 (22) 82260.891 (0.029) 59.2 13.826 1.73 1 3 2 0 2 84424.1900 (0.330) 0.107 (18) 84424.602 (0.005) 5.3 2.002 1.49 2 8 10 1 9 86429.8400 (0.500) 0.059 (10) 86429.537 (0.008) 29.8 1.888 1.118 1 17 18 0 18 88716.0400 (1.150) 0.091 (14) 88715.413 (0.040) 73.7 14.774 1.7
Note. — 1 σσ