Astrochemistry at work in the L1157-B1 shock: acetaldehyde formation
C. Codella, F. Fontani, C. Ceccarelli, L. Podio, S. Viti, R. Bachiller, M. Benedettini, B. Lefloch
aa r X i v : . [ a s t r o - ph . E P ] D ec Mon. Not. R. Astron. Soc. , 1–5 (2011) Printed 22 August 2018 (MN L A TEX style file v2.2)
Astrochemistry at work in the L1157–B1 shock:acetaldehyde formation
C. Codella ∗ , F. Fontani , C. Ceccarelli , , L. Podio , S. Viti , R. Bachiller ,M. Benedettini , B. Lefloch , INAF-Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, Firenze, 50125, Italy Univ. Grenoble Alpes, IPAG, F-38000 Grenoble, France CNRS, IPAG, F-38000 Grenoble, France Department of Physics and Astronomy, University College London, Gower Str eet, London, WC1E 6BT, UK IGN, Observatorio Astron´omico Nacional, Calle Alfonso XIII, 28004 Madrid, Spain INAF, Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere 100, 00133 Roma, Italy
Accepted date. Received date; in original form date
ABSTRACT
The formation of complex organic molecules (COMs) in protostellar environments is ahotly debated topic. In particular, the relative importance of the gas phase processesas compared to a direct formation of COMs on the dust grain surfaces is so far un-known. We report here the first high-resolution images of acetaldehyde (CH CHO)emission towards the chemically rich protostellar shock L1157-B1, obtained at 2 mmwith the IRAM Plateau de Bure interferometer. Six blueshifted CH CHO lines with E u = 26-35 K have been detected. The acetaldehyde spatial distribution follows theyoung ( ∼ CHO relative abundance, 2–3 × − , is inferred, similarly to what found in hot-corinos. Astrochemical modelling indicates that gas phase reactions can produce theobserved quantity of acetaldehyde only if a large fraction of carbon, of the order of0.1%, is locked into iced hydrocarbons. Key words:
Molecular data – Stars: formation – radio lines: ISM – submillimetre:ISM – ISM: molecules
Complex organic molecules (COMs) have a key role amongthe many molecules so far detected in space: since theyfollow the same chemical rules of carbon-based chemistry,which terrestrial life is based on, they may give us aninsight into the universality of life. Of course, large bi-otic molecules are not detectable in space, certainly notvia (sub)millimeter observations. However, to determinewhether pre-biotic molecules may form in space, we firstneed to understand the basic mechanisms that form smallerCOMs. There is an extensive literature on the subject andstill much debate on how COMs may form in space (e.g.Herbst & van Dishoeck 2009; Caselli & Ceccarelli 2012;Bergin 2013). Two basic processes are, in principle, possible:COMs may form on the grain surfaces or in gas phase. It is ∗ E-mail: [email protected] possible and even probable that the two processes are bothimportant in different conditions for different molecules.Acetaldehyde (CH CHO) has been detected in a largerange of interstellar conditions and with different abun-dances, namely in hot cores (Blake et al. 1986), hot cori-nos (Cazaux et al. 2003), cold envelopes (Jaber et al. 2014),Galactic Center clouds (Requena-Torres et al. 2006) and pre-stellar cores ( ¨Oberg et al. 2010). Grain surface models pre-dict that CH CHO is one of the simplest COMs and canbe formed either by the combination of two radicals on thegrain surface, CH and HCO, which become mobile whenthe grain temperature reaches ∼
30 K (Garrods & Herbst2006), or by irradiation of iced CH , CO and other icedspecies (Bennett et al. 2005). For the former route, the tworadicals are predicted to be formed either because of thephotolysis of more complex molecules on the grain mantlesor, more simply, because of the partial hydrogenation of sim-ple biatomic molecules on the grain mantles (Taquet et al. c (cid:13) F. Codella et al. d = 250 pc) drivesa chemically rich outflow (Bachiller et al. 2001), associatedwith molecular clumpy cavities (Gueth et al. 1996), createdby episodic events in a precessing jet. Located at the apexof the more recent cavity, the bright bow shock called B1has a kinematical age of 2000 years. This shock spot hasbeen the target of several studies (e.g. the Large ProgramsHerschel/CHESS (Chemical Herschel Surveys of Star form-ing regions; Ceccarelli et al. 2010; and IRAM-30m/ASAI (Astrochemical Survey At IRAM). In this Letter we reporthigh spatial resolution observations of acetaldehyde, withthe aim to constrain and quantify the contribution of gasphase chemistry to the CH CHO formation.
L1157-B1 was observed with the IRAM Plateau de Bure(PdB) 6-element array in April–May 2013 using both the Cand D configurations, with 21–176 m baselines, filtering outstructures > ′′ , and providing an angular resolution of 2 . ′′ × . ′′ ◦ ). The primary HPBW is ∼ ′′ . The ob-served CH CHO lines (see Table 1) at ∼ ∼ − ) spectral resolu-tion. The system temperature was 100–200 K in all tracks,and the amount of precipitable water vapor was generally ∼ . Calibration was performedon 3C279 (bandpass), 1926+611, and 1928+738 (phase andamplitude). The absolute flux scale was set by observingMWC349 ( ∼ . ∼ − . Acetaldehyde emission has been clearly (S/N >
10) de-tected towards L1157–B1. Fig. 1 shows the map of theCH CHO(7 , –6 , ) E and A lines integrated emission. Inorder to verify whether the present CH CHO image is al-tered by filtering of large-scale emission, we produced theCH CHO(7 , –6 , ) E+A spectrum summing the emissionmeasured at PdBI in a circle of diameter equal to the Table 1.
List of CH CHO transitions detected towards L1157-B1Transition ν a E u a Sµ a log(A/s − ) a (GHz) (K) (D )(7 , –6 , )E 133.830 26 88.5 –4.04(7 , –6 , )A 133.854 26 88.4 –4.08(7 , –6 , )A 134.694 35 81.3 –4.11(7 , –6 , )E 134.895 35 79.7 –4.12(7 , –6 , )E 135.477 35 79.7 –4.11(7 , –6 , )A 135.685 35 81.3 –4.10 a From the Jet Propulsion Laboratory database (Pickett et al.1998). half-power beam width (HPBW) of the IRAM-30m tele-scope (17 ′′ ). We evaluated the missing flux by comparingsuch emission with the spectrum directly measured with thesingle-dish (from the ASAI spectral survey, Lefloch et al., inpreparation). As already found for HDCO by Fontani et al.(2014), with the PdBI we recover more than 80% of the flux,indicating that both tracers do not have significant extendedstructures. The spatial distribution reported in Fig. 1 showsthat CH CHO is mainly associated with two regions: (i) theeastern B0-B1 cavity opened by the precessing jet (called‘E-wall’, see Fig. 1 in Fontani et al. 2014), and (ii) the arch-like structure composed by the B1a-e-f-b clumps identifiedby CH CN (called ‘arch’). The red, turquoise, and magentapolygons shown in Fig. 1 sketch out these two regions, inter-secting at the position of the B1a clump. Note that B1a is inturn located where the precessing jet is expected to impactthe cavity producing a dissociative J-shock (traced by highvelocity SiO, H O, [FeII], [OI], and high-J CO emission: e.g.Gueth et al. 1998, Benedettini et al. 2012).Figure 2 shows the CH CHO line spectrum observedwith the 3.6 GHz WideX bandwidth towards the brightestclump, B1a. Up to six lines ( E u = 26–35 K, see Table 1) aredetected with a S/N >
3. Using the GILDAS–Weeds package(Maret et al. 2011) and assuming optically thin emission andLTE conditions, we produced the synthetic spectrum (redline in Fig. 2) that best fits the observed one. Note that theCH CHO lines are blue-shifted, by 2 km s − , with respectto the cloud systemic velocity (+2.6 km s − : Bachiller &Per´ez Guti´errez 1997), and have linewidths of 8 km s − .Similarly, we extracted the CH CHO line spectrum towardsthe three B1 zones, ‘E-wall’, ‘arch’, and ‘head’, shown in Fig.1. Table 2 reports the measured peak velocities, intensities(in T B scale), FWHM linewidths, and integrated intensities,for each of the three zones. CHO ABUNDANCE
Figure 1 compares the CH CHO distribution with that ofHDCO (Fontani et al. 2014), showing an excellent agree-ment, with weak or no emission at the head of the bowB1 structure (called ‘head’). The acetaldehyde emission isconcentrated towards the ‘E-wall’ and ‘arch’ zones, namelythe part of B1 associated with the most recent shocked ma-terial, as probed by the HDCO emission. This is furthersupported by the fact that the brightest acetaldehyde emis-sion comes where also CH OH, another dust mantle prod-uct, and CH CN, a 6-atoms COM, emission peak (Codella c (cid:13) , 1–5 cetaldehyde in L1157–B1 Table 2.
Observed parameters (in T B scale) of the CH CHO(7 , –6 , )E and A emission, and acetaldehyde column densities N CH CHO derived in the 3 regions identified in Fig. 1 (E-wall, arch, and head) following Fontani et al. (2014), see Sect. 3. The (range of) excitationtemperatures ( T ex ) used to derive N CH CHO have been assumed equal to the rotation temperatures derived in Codella et al. (2012),Lefloch et al. (2012), and Fontani et al. (2014). The last columns report the X ( CH CHO )/ X ( CH OH ) and X ( CH CHO )/ X ( HDCO )abundance ratios using the CH OH and HDCO data (and similar beams) by Benedettini et al. (2013) and Fontani et al. (2014).Transition T peak a V peak a F W HM a F int a N CH CHO b CH CHO/CH OH CH CHO/HDCO(mK) (km s − ) (km s − ) (mK km s − ) (10 cm − ) (10 − )10 K – 70 K 10 K – 70 K 10 K – 70 KE-wall7 , –6 , E 30(3) +0.4(0.4) 8.2(1.0) 264(28) 5.0(0.3)–9.2(0.5) 1.7(0.1)–11.0(0.6) 1.9(0.2)–0.9(0.1)7 , –6 , A 29(3) +0.5(0.5) 8.6(1.3) 263(31) Arch7 , –6 , E 71(7) –0.9(0.3) 9.2(0.8) 748(54) 15.9(0.1)–29.4(0.1) 0.6(0.1)–4.2(0.1) 7.6(1.1)–3.7(0.5)7 , –6 , A 76(7) –0.2(0.9) 8.0(0.5) 608(42) Head7 , –6 , E 14(3) +0.9(0.5) 8.0(1.1) 81(13) 1.7(0.3)–3.1(0.5) 0.2(0.1)–0.7(0.1) > , –6 , A 13(3) +0.9(0.6) 5.2(1.6) 81(13) a The errors are the gaussian fit uncertainties. The spectral resolution is 4.4 km s − . b Derived using the (7 , –6 , ) E and A emissions. et al. 2009, Benedettini et al. 2013). Finally, the CH CHOobserved emission is also confined in the low-velocity range(
F W HM ∼ − ) of the L1157-B1 outflow, which isdominated by the extended B1 bow-cavity, according toLefloch et al. (2012) and Busquet et al. (2014). In summary,similarly to HDCO, CH CHO traces the extended interfacebetween the shock and the ambient gas, which is chemicallyenriched by the sputtering of the dust mantles.To derive the column density, we used the LTE pop-ulated and optically thin assumption and best fitted thesix detected lines of Tables 1–2. Towards the B1a peak, wefind N CH CHO = 9 × cm − , and a rotational temper-ature of T rot = 15 K, in agreement with the value derivedfor the molecular cavity from single-dish CO and HDCOmeasurements (10–70 K; Lefloch et al. 2012, Codella et al.2012). Assuming rotational temperatures between 10 and70 K (Table 2) we derived a column density of 5–30 × cm − in the ‘E-wall’ and ‘arch’ regions, and ∼ cm − in the ‘head’. The size of the regions (at 3 σ level) is9 ′′ (‘E-wall’), 7 ′′ (‘arch’), and 8 ′′ (‘head’). An estimate of theCH CHO abundance can be derived using the the CO col-umn density ≃ cm − derived by Lefloch et al. (2012) ona 20 ′′ scale. We derived N CH CHO using the CH CHO spec-trum extracted on the same scale and assuming 10–70 K. Wefind N CH CHO ∼ × cm − , which implies a highabundance, X (CH CHO) ≃ × − , similar to what hasbeen measured in hot-corinos ( ≃ × − , Cazaux et al.2003), and larger than that measured in prestellar cores ( ∼ − , Vastel et al. 2014) and towards high-mass star form-ing regions ( ∼ − –10 − , Cazaux et al. 2003; Charnley2004). CHO
The ratio between N CH CHO and the column density ofHDCO, i.e. a molecule which in L1157-B1 is predominantlyreleased by grain mantles (Fontani et al. 2014), is higher(even if we consider the uncertainties, see Table 2) in the‘arch’ with respect to the ‘E-wall’ by a factor ∼ CHO is formed in thegas phase. In the gas phase, the injection from grain man-tles of ethane (C H ) is expected to drive first C H andsuccessively acetaldehyde (e.g. Charnley 2004; Vasyunin &Herbst 2013): the overlap between the HDCO (Fontani et al.2014) and CH CHO emitting regions supports this scenario.We can, therefore, use the measured CH CHO abundanceto constrain the quantity of C H that has to be presentin the gas phase in order to produce the observed quantityof CH CHO. To this end, we use the chemical code AS-TROCHEM , a pseudo time dependent model that followsthe evolution of a gas cloud with a fixed temperature anddensity considering a network of chemical reactions in thegas phase. We followed the same 2-steps procedure adoptedin Podio et al. (2014) and Mendoza et al. (2014), to first com-pute the steady-state abundances in the cloud (i.e. T kin =10K, n H =10 cm − , ζ =3 10 s − ); and then we follow thegas evolution over 2000 yr at the shocked conditions (i.e.T kin =70 K and n H =10 cm − ). To estimate the influenceof a possibly larger gas T kin during the passage of the shock,we also run cases with temperatures up to 1000 K. We adoptthe OSU chemical network and assume visual extinction ofA V = 10 mag and grain size of 0.1 µ m. We assume thatthe abundances of OCS and CO are also enhanced by thepassage of the shock, namely their abundance in step 2 is X (CO ) = 6 10 − and X (OCS) = 6 10 − . Similarly, we as-sume that the abundance of methanol in step 2 is 2 10 − , inagreement with the most recent determination in L1157-B1by Mendoza et al. (2014). Finally, we varied the C H abun-dance from 2 × − to 2 × − . As expected, the predictedsteady-state abundance of acetaldehyde in the cloud is verylow (1.5 10 − ). However, once C H is in the gas phase, itrapidly reacts with oxygen forming abundant acetaldehydeon timescale shorter than 100 years (Fig. 3). The CH CHOabundance reaches the observed value, ≃ × − , at theshock age (2000 years), for C H ∼ × − . Note that http://smaret.github.com/astrochem/ http://faculty.virginia.edu/ericherbc (cid:13) , 1–5 F. Codella et al.
Figure 1.
Chemical differentation in L1157–B1: the maps arecentred at: α (J2000) = 20 h m . s δ (J2000) = +68 ◦ ′ . ′′ α = +21 . s δ = –64 . ′′ Upper panel: CH CHO(7 , –6 , )E+A integrated emission(green colour, black contours) on top of the HDCO(2 , − , )line (white contours; Fontani et al. 2014). First contour andsteps of the CH CHO image correspondes to 3 σ (1 mJy beam − ). The ellipses show the synthesised HPBW (2 . ′′ × . ′′
3, PA =90 ◦ ). The red, turquoise, and magenta polygons called ’E-wall’,’arch’, and ’head’ indicate the 3 portions of L1157-B1 selectedby Fontani et al. (2014) to investigate H CO deuteration.
Bot-tom panel: CH CN(8 K –7 K ) emission (green colour, black con-tours; Codella et al. 2009) on top of the CH OH(3 –2 K ) emission(white; Benedettini et al. 2013). The HPBWs are: 3 . ′′ × . ′′ ◦ ) for CH CN and 3 . ′′ × . ′′ ◦ ) for CH OH. Thelabels indicate the L1157-B1 clumps identified using the CH CNimage (Codella et al. 2009). we obtain the same result if the gas temperature is
500 K,and a 30% higher value at 1000 K. Figure 3 shows also thatthe CH CHO/CH OH abundance ratio is expected to dropbetween 10 yr and 10 yr. A different age could, therefore,justify the slightly smaller CH CHO/CH OH ratio observedtowards the ‘head’ region.
We have shown that acetaldehyde is abundant, X (CH CHO) ≃ × − , in the gas associated with thepassage of a shock and enriched by iced species sputteredfrom grain mantles and injected into the gas phase. Themeasured acetaldehyde abundance could be consistentwith the scenario of oxydation of gaseous hydrocarbonsformed in a previous phase and released by the grainmantles. However, the abundance of the C H required toreproduce the measured CH CHO is very high, ∼ × − , namely less than 0.6% the elemental gaseous carbon.There are no observations of C H , hence it is impossibleto compare with direct estimates of the abundance of thismolecule. However, it has been argued that large quantitiesof frozen methane, of a few % of iced mantle water, isfound around the L1527-mm protostar, where the detectionof CH D (Sakai et al. 2012) indicates X (CH ) ≃ × − . This large abundance has been attributed to alow density of the pre-collapse core from which L1527-mmoriginated (Aikawa et al. 2008). Interestingly, the analysisof the deuteration of water, methanol and formaldehyde inL1157-B1 led Codella et al. (2012) to conclude that also themantles of L1157-B1 were formed in relatively low density( ∼ cm − ) conditions.To conclude, in the specific case of L1157-B1, gas phasereactions can produce the observed quantity of acetaldehydeonly if a large fraction of carbon, of the order of 0.1%, islocked into iced hydrocarbons. Further observations of thehydrocarbons abundance in L1557-B1 are needed to confirmor dismiss our hypothesis. ACKNOWLEDGMENTS
The authors are grateful to P. Caselli for instructive com-ments and suggestions, as well as to the IRAM staff forits help in the calibration of the PdBI data. This researchhas received funding from the European Commission Sev-enth Framework Programme (FP/2007-2013, n. 283393, Ra-dioNet3), the PRIN INAF 2012 – JEDI, and the Italian Min-istero dell’Istruzione, Universit`a e Ricerca through the grantProgetti Premiali 2012 – iALMA. LP has received fundingfrom the European Union Seventh Framework Programme(FP7/2007-2013, n. 267251). CC and BL acknowledge thefinancial support from the French Space Agency CNES, andRB from Spanish MINECO (FIS2012-32096).
REFERENCES
Aikawa Y., Wakelam V., Garrod R.T., & Herbst E. 2008,ApJ 674, 993Bachiller R., & Per´ez Guti´errez M. 1997, ApJ 487, L93Benedettini, M., Busquet, B., Lefloch, B., et al. 2012, A&A539, L3Bennett C.J., Osamira Y., Lebar M.D., & KAiser R.I. 2005,ApJ 634, 968Bergin, E.A. 2013, XVII Special Courses at the NationalObservatory of Rio de Janeiro. AIP Conference Proceed-ings, arXiv:1309.4729Blake G.A., Masson C.R., Phillips T.G., & Sutton E.C.1986, ApJS 60, 357 c (cid:13) , 1–5 cetaldehyde in L1157–B1 Figure 2. CH CHO emission (in T B scale) extracted at the B1a position ( α (J2000) = 20 h m . s δ (J2000) = +68 ◦ ′ . ′′ CHO lines with S/N > σ (33 mK) arelocated (see Table 1). The red line shows the synthetic spectra which better reproduce the observations: it has been obtained with theGILDAS–Weeds package (Maret et al. 2011) assuming optically thin emission and LTE conditions with N CH CHO = 9 × cm − , T ex = 15 K, v LSR = +0.6 km s − , and FWHM linewidth = 8.0 km s − . -4 -2 time (yr)10 -10 -9 -8 -7 -6 -5 -4 X = N s p ec i e s / N H ζ = 3 10 -16 s -1 , n H =10 cm -3 , T=70 K X(CH OH) = 2 10 -6 CH CHOC H CH OH X(C H )2 10 -7 -7 -6 -6 -5 Figure 3.
Evolution of acetaldehyde (CH CHO, black), (C H ,blue), and methanol (CH OH, red) abundances in the shock as afunction of time. Observed abundances (colour circles) are over-plotted at the shock age ( t shock ∼ n H = 10 cm − , T kin = 10 K, ζ = 3 × − s − ) byenhancing the gas temperature and density ( n H = 10 cm − , T kin = 70 K), and the abundance of molecules which are thoughtto be sputtered off dust grain mantles. We set X CO = 6 × − and X OCS = 6 × − as in Podio et al. 2014, X CH OH =2 × − (Mendoza et al. 2014), and vary the abundance of C H between 2 × − and 2 × − . Busquet G., Lefloch B., Benedettini, M., et al. 2014, A&A561, 120Benedettini, M., Viti, S., Codella, C., et al. 2013, MNRAS436, 179Caselli, P. & Ceccarelli, C. 2012, A&ARv, 20, 56Cazaux S., Tielens A.G.G.M., Ceccarelli C., et al. 2003,ApJ 593, L51Ceccarelli C., Bacmann A., Boogert A., et al. 2010, A&A521, L22Charnley S.B., 2004, Adv. Space Res. 33, 23Charnley S.B., Tielens A.G.G.M., & Millar T.J. 1992, ApJ399, L71Codella C., Benedettini M., Beltr´an M.T., et al. 2009, A&A507, L25Codella C., Ceccarelli C., Lefloch B., et al. 2012, ApJ, 757,L9Fontani F., Codella C., Ceccarelli C., et al. 2014, ApJ 788,L43Garrod R.T., & Herbst E. 2006, A&A 457, 927 Garrod R.T., Wakelam V., & Herbst E. 2007, A&A 467,1103Garrod R.T., Weaver S.L.W., & Herbst E. 2008, ApJ 682,283Garrod R.T., Vasyunin A.I., Semenov D.A., Wiebe D.S.,& Henning Th. 2009, ApJ 700, L43Gueth F., Guilloteau S., & Bachiller R. 1996, A&A 307,891Gueth F., Guilloteau S., & Bachiller R. 1998, A&A 333,287Herbst E., & van Dishoeck E.F. 2009, ARA&A 47, 427Jaber A.A., Ceccarelli C., Kahane C., & Caux E. 2014, ApJ791, 29Lefloch B., Cabrit S., Busquet G., et al. 2012, ApJ, 757,L25Maret S., Hily-Blant P., Pety J., et al., A&A 526, A47¨Oberg K.I., Bottinelli S., Jørgensen J.K., van Dishoeck E.F.2010, ApJ 716, 825Occhiogrosso A., Vasyunin A., Herbst E., et al. 2014, A&A564, 123Pickett H.M., Poynter R.L., Cohen E.A., et al. 1998, J.Quant. Spectrosc. & Rad. Transfer 60, 883Podio L., Lefloch B., Ceccarelli C., Codella C., & BachillerR. 2014, A&A 565, 64Requena-Torres M.A.; Mart´ın-Pintado J., Rodr´ıguez-Franco A., et al. 2006, A&A 455, 971Rawlings J.M.C., Williams D.A., Viti S., & Cecchi-Pestellini C. 2013, MNRAS 430, 264Sakai N., Shirley Y.L., Sakai T., et al. 2012, ApJ 758, L4Taquet, V., Ceccarelli, C., & Kahane, C. 2012, ApJ 784,L3Vastel C., Ceccarelli C., Lefloch B., & Bachiller R. 2014,ApJ 795, L2Vasyunin A.I., & Herbst E. 2013, ApJ 769, 34This paper has been typeset from a TEX/ L A TEX file preparedby the author. c (cid:13)000