On the ubiquity of molecular anions in the dense interstellar medium
M. A. Cordiner, J. V. Buckle, E. S. Wirström, A. O. H. Olofsson, S. B. Charnley
aa r X i v : . [ a s t r o - ph . GA ] A p r A CCEPTED FOR PUBLICATION IN A P J (1 ST A PRIL
Preprint typeset using L A TEX style emulateapj v. 8/13/10
ON THE UBIQUITY OF MOLECULAR ANIONS IN THE DENSE INTERSTELLAR MEDIUM
M. A. C
ORDINER , , J. V. B UCKLE , E. S. W IRSTRÖM , , A. O. H. O LOFSSON AND
S. B. C
HARNLEY Accepted for publication in ApJ (1st April 2013)
ABSTRACTResults are presented from a survey for molecular anions in seven nearby Galactic star-forming cores andmolecular clouds. The hydrocarbon anion C H - is detected in all seven target sources, including four sourceswhere no anions have been previously detected: L1172, L1389, L1495B and TMC-1C. The C H - /C H columndensity ratio is & .
0% in every source, with a mean value of 3.0% (and standard deviation 0.92%). Combinedwith previous detections, our results show that anions are ubiquitous in dense clouds wherever C H is present.The C H - /C H ratio is found to show a positive correlation with molecular hydrogen number density, andwith the apparent age of the cloud. We also report the first detection of C H - in TMC-1 (at 4.8 σ confidence),and derive an anion-to-neutral ratio C H - /C H = (1 . ± . × - (= 0 . ± . H - highlights the need for a revised radiative electron attachment rate for C H. Chemicalmodel calculations show that the observed C H - could be produced as a result of reactions of oxygen atomswith C H - and C H - . Subject headings: astrochemistry — ISM: abundances — ISM: molecules — ISM: clouds — stars: formation INTRODUCTION
Negative ions (anions) were first discovered in the interstel-lar medium (ISM) by McCarthy et al. (2006), who detectedthe linear hydrocarbon anion C H - in TMC-1. This was fol-lowed by C H - and C H - detections in the protostellar coreL1527 by Sakai et al. (2007) and Agúndez et al. (2008), re-spectively, which hinted of a role for anions in the chem-istry of star-formation. Gupta et al. (2009) performed the firstdedicated survey for C H - in twenty-four molecular sources,but detected the anion in only two star-forming clouds, withanion-to-neutral ratios on the order of a few percent, similarto previously-observed values. Interstellar anion detectionswere confined to the Taurus Molecular Cloud complex un-til the recent discoveries of C H - in the Lupus, Cepheus andAuriga star-forming regions (Sakai et al. 2010; Cordiner et al.2011), which proved that anions are widespread in the localISM, and not an artifact of any particular physical or chemicalconditions in the Taurus region.The possible importance of anions in interstellar chem-istry was first discussed by Dalgarno & McCray (1973), andprospects for the detection of molecular anions using radioastronomy were examined by Sarre (1980). Herbst (1981)argued that large interstellar molecules (including carbon-chain-bearing species) can undergo rapid radiative electronattachment – as exemplified by the laboratory experimentsof Woodin et al. (1980) – potentially resulting in significantanion abundances in dense molecular clouds. Based on thisidea, models for anion chemistry have been successful in re-producing the observed abundances of C H - and C H - inTMC-1, IRC+10216, L1527 and L1512 (Millar et al. 2007; [email protected] Astrochemistry Laboratory and The Goddard Center for Astrobiology,NASA Goddard Space Flight Center, Code 691, 8800 Greenbelt Road,Greenbelt, MD 20771, USA. Department of Physics, The Catholic University of America, Wash-ington, DC 20064, USA. Cavendish Astrophysics Group and Kavli Institute for Cosmology, In-stitute of Astronomy, University of Cambridge, Madingley Road, Cam-bridge, CB3 0HE, UK. Department of Earth and Space Sciences, Chalmers University ofTechnology, Onsala Space Observatory, 439 92 Onsala, Sweden.
Remijan et al. 2007; Harada & Herbst 2008; Cordiner et al.2008; Cordiner & Charnley 2012). However, there is a ma-jor discrepancy between the modeled and observed C H - anion-to-neutral ratio (see Herbst & Osamura 2008), and thelack of anions in PDRs (Agúndez et al. 2008) is at variancewith the model predictions of Millar et al. (2007). Clearly,our understanding of molecular anion chemistry is incom-plete. Nevertheless, as demonstrated by Cordiner & Charnley(2012), anion measurements have the potential to offer in-sight into interstellar cloud properties due to their reactivity(Eichelberger et al. 2007), and consequently, their sensitiv-ity to the abundances of gas-phase electrons and atomic C,H and O, and to physical conditions such as cloud densityand molecular depletion. Observations of anions in a varietyof interstellar environments will be key to a complete under-standing of their chemistry. Given the relatively small numberof molecular anion detections in the ISM to date, we set outto address the question of just how widespread anions are ininterstellar clouds, and to ascertain the behavior of the anion-to-neutral ratio over a range of cloud types and ages.In this article, results are presented from our survey forC H - and C H in a sample of seven carbon-chain-rich inter-stellar clouds, protostars and prestellar cores. Detections ofC H - in two of the sources (L1251A and L1512) were previ-ously reported by Cordiner et al. (2011), and here we reportthe final results for all seven sources, as well as the first (ten-tative) detection of C H - in TMC-1. TARGET SOURCES
Due to the close chemical relationship between polyynesand cyanopolyynes (see e.g.
Federman et al. 1990;Millar & Herbst 1994), carbon-chain-bearing species such asC H and C H are expected to be abundant in dense molecularclouds, close to the peak of HC N emission. Target sourceswith strong HC N emission lines were selected from an HC N J = 10 - TABLE 1T
ARGETS , COORDINATES AND DISTANCES
Source RA Dec Cloud Type a Distance Ref.(J2000) (J2000) (pc)L1172 SMM 21:02:22.1 +67:54:48 Star-forming 440 1L1251A 22:30:40.4 +75:13:46 Star-forming 300 2L1389 (CB17) SMM1 04:04:36.6 +56:56:00 Protostellar 250 3L1495B 04:15:41.8 +28:47:46 Quiescent 140 4L1512 05:04:07.1 +32:43:09 Star-forming 140 4TMC-1C 04:41:35.6 +26:00:21 Star-forming 140 4TMC-1 CP 04:41:41.9 +25:41:27 Quiescent 140 4R
EFERENCES . — (1) Visser et al. (2002); (2) Kun & Prusti (1993); (3) Launhardt et al. (2010); (4) Elias (1978). a See Section 2 for description of source classification scheme. integrated HC N J = 10 - ′′ and beam efficiency of 0 . ± .
05. A 30 ′′ map spacing wasused, which was resampled to a 7 . ′′ per pixel grid using bi-linear interpolation. For TMC-1, HC N maps were publishedby Hirahara et al. (1992) and Pratap et al. (1997).To search for C H and C H - , we targeted the strongestHC N peak positions in our maps of L1389, L1495B, L1512and TMC-1C. For L1172 we targeted the sub-mm core cat-aloged by Di Francesco et al. (2008), which fell within 15 ′′ of the HC N peak. Our chosen L1251A position does notcoincide with the main HC N peak because it was based onan earlier, lower-sensitivity map than that shown in Figure 1.For TMC-1, we targeted the cyanopolyyne peak (CP) whereAgúndez et al. (2008) previously attempted to detect C H - .The adopted coordinates for our anion survey targets are listedin Table 1.Our L1172 and L1512 C H - positions overlap theprestellar cores identified by Visser et al. (2002) andWard-Thompson et al. (1994), respectively. For L1389,the HC N emission maps out a very compact clump,coincident with the center of the Bok Globule CB17(Clemens & Barvainis 1988). The HC N peak also matchesclosely the location of strongest microwave/sub-mm contin-uum emission observed by Launhardt et al. (2010) and de-noted L1389 SMM1 (the position of which is indicated in thebottom-left panel of Figure 1 by the cross inside the dashedblack circle of the GBT beam). As shown in Figure 1, ourtargeted L1251A position is ≈ ′′ south-east of the center ofthe protostar L1251 IRS3 (Lee et al. 2010), although the outerenvelope of this protostar probably intersects the GBT beamto some extent, as discussed by Cordiner et al. (2011).From dust continuum emission, Launhardt et al. (2010) ob-tained a number density for L1389 SMM1 of 6 × cm - .Pavlyuchenkov et al. (2006) derived a moderately high COdepletion factor ( ∼ ∼ ′′ NE of our targeted GBT beam center, as well a more evolvedClass 0/I protostar 20 ′′ NW. Given that the radii of typ-ical low-mass protostellar envelopes are ∼ AU ( e.g.
Jorgensen et al. 2002), at a distance of 250 pc, our targetedL1389 beam is likely to be dominated by protostellar enve-lope matter.L1495B, on the other hand, is a quiescent molecular cloudwith no known protostars nearby. It appears to be chemi- cally young, with an apparently low level of CO depletionconsistent with less-evolved interstellar clouds (Hirota et al.2004). In TMC-1C, Buckle et al. (2006) identified an anti-correlation between the spatial distributions of HC N andC O, at least partly attributable to CO depletion. Thus, ourobservations of TMC-1C (at the HC N peak) probably sam-ple more chemically-evolved, depleted gas than L1495B. Noprotostars are known to be present in our observed TMC-1C beam, but several sub-mm sources are located in the sur-rounding cloud (indicated by white crosses in the lower-rightpanel of Figure 1), showing this to be a region of active starformation. South-west of TMC-1C lies the source TMC-1(CP), which is a well-known chemically-young dark cloud,with only a modest degree of depletion (Hirahara et al. 1992;Cordiner & Charnley 2012).In Table 1, we provide a basic categorization of our targetsources in light of their properties described above: ‘quies-cent’ refers to those cloud cores that are chemically young,show no evidence for active star-formation within the tele-scope beam (such as outflows or compact sub-mm/IR emis-sion), and do not appear to be undergoing collapse; ‘star-forming’ is used for clouds showing nearby active star-formation and a greater degree of chemical evolution; finally,‘protostellar’ describes our L1389 position, which containsthe low-luminosity protostar CB17 MMS. ANION OBSERVATIONS
Observations of emission lines of C H ( J = 10 . - . H - ( J = 10 - -
10) and HC N ( J = 3 - - (GBT). The GBT Spectrometer wasused with a bandwidth of 50 MHz and 8192 × . ≈ .
065 km s - ).Four spectral windows were used, which allowed simultane-ous observation of the C H and C H - lines. For the compact,spatially-isolated source L1512, beam switching (with 78 ′′ throw) was used, and for all other targets, frequency switch-ing was used. Pointing was checked every one to two hoursand was typically accurate to within 5 ′′ . In the middle ofthe observed frequency range (28 GHz), the telescope beamFWHM was 26 ′′ and the main beam efficiency was 0.90. To-tal system temperatures were typically in the range 60 -
80 Kand the zenith opacity was 0 . ± .
02. Intensity calibrationwas performed using beam-switched observations of the com-pact radio source NGC 7027. Measured antenna temperatures The National Radio Astronomy Observatory is a facility of the NationalScience Foundation operated under cooperative agreement by AssociatedUniversities, Inc. F IG . 1.— Observed HC N J = 10 - - and have not been corrected for beam efficiency.Dashed black circles represent the size (HPBW) and position of our targeted GBT anion survey positions. Crosses denote locations of sub-mm (prestellar) cores(Ward-Thompson et al. 1994; Visser et al. 2002; Di Francesco et al. 2008; Launhardt et al. 2010) and asterisks denote protostellar core locations (Visser et al.2002; Chen et al. 2012; Lee et al. 2010). Cordiner et al.were subsequently corrected for opacity, spillover, ohmic loss,blockage efficiency and beam efficiency, then averaged usingstandard GBTIDL routines.We obtained observations of C H ( N = 2 - J = 1 . - . F = 2 -
1) and C H - ( J = 2 -
1) between 2011 December and2012 February at around 19 GHz using the GBT K-bandfocal-plane array (KFPA). These observations were obtainedwith channel spacing 12.2 kHz, beam efficiency 0.92, beamsize 39 ′′ and zenith opacity 0 . ± . N J = 10 - RESULTS
New Anion Detections
The C H - anion and its neutral counterpart C H were de-tected in all of the surveyed sources. This is the first time an-ions have been detected in L1172, L1389, L1495B and TMC-1C. The observed C H - and C H spectra are shown in Figure2. Spectra of the J = 10 - J = 11 -
10 transitions of C H - were averaged in velocity space to improve the signal-to-noiseratio; integrated intensities of the averaged spectra are givenin Table 3. Due to partial blending of the hyperfine compo-nents of the C H ( J = 10 . - . , f ) lines, the integrated inten-sity summed over both components is given in Table 3. Giventhe weakness of these lines, they can safely be assumed to beoptically thin, and Equation 2 of Lis et al. (2002) was usedto derive column densities for C H and C H - (given in Table3). Spectroscopic data for the transitions of interest (Table 2)were obtained from the Cologne Database for Molecular As-tronomy (Müller et al. 2005), and an excitation temperatureof 10 K was assumed. Error estimates were derived using 500Monte Carlo noise replications for each measurement.In TMC-1, C H excitation temperatures of 5.2 K and 6.7 Kwere measured by Bell et al. (1999) and Sakai et al. (2007),respectively. Such sub-thermal excitation might be expectedgiven the large Einstein A coefficients for rotational transi-tions of this molecule, which are a direct consequence of itslarge (5.5 D) dipole moment. However, the value obtainedby Bell et al. (1999) may be subject to uncertainty due to theeffects of telescope beam dilution, and the Sakai et al. (2007)value may not be applicable to our observations because itwas obtained using the J = 16 . - . A coefficient about four times greater than that ofthe J = 10 . - . A results in an increased likelihood of sub-thermal excitationof the J = 16 . H excitation temperature is aslow as 5 K, then the calculated column densities for C H andC H - will be about 36% smaller than the values in Table 3.Due to the similar moments of inertia of C H and C H - , theirrotational levels occur at similar energies, so that the calcu-lated anion-to-neutral column density ratios (C H - /C H) areinsensitive to small uncertainties in excitation, provided bothmolecules share a common excitation temperature. The meanC H - /C H ratio for the seven sources is 3.0%, with a standarddeviation of 0.92%.Our new measurements for C H and C H - in L1251A andL1512 supersede those presented by Cordiner et al. (2011).Although the column densities and anion-to-neutral ratiosmeasured in the present study match those of Cordiner et al.(2011) within the stated errors, the newer values benefit from improved calibration and analysis methods and have smalleruncertainties. The difference between our TMC-1 (CP)C H - /C H value of 2 . ± .
4% and the value of 1 . ± . H and/or C H - could be par-tially responsible for the discrepancy.The observed C H - and C H spectra of TMC-1 (CP) areshown in the bottom-right panel of Figure 2. The J = 2 - H - is detected in a single channel with a peakantenna temperature of 3.4 mK, which corresponds to 4.8 σ (where σ is the RMS noise of the baseline-subtracted spec-trum). Using a Gaussian fit to this C H - line with 1000Monte Carlo noise replications, the central velocity wasfound to be 5 . ± .
05 km s - with a FWHM of 0 . ± .
13 km s - . Within the errors, these parameters match thosederived for HC N (given in Table 4), which adds confi-dence to our C H - detection. The integrated line intensityis 1 . ± . - , which corresponds to a column den-sity of N (C H - ) = (8 . ± . × cm - (assuming an exci-tation temperature of 10 K). The neutral C H line has an in-tegrated intensity of 411 . ± . - , which requires N (C H) = (6 . ± . × cm - . The correspondinganion-to-neutral ratio C H - /C H is (1 . ± . × - . Thisvalue is consistent with the upper limit of 5 . × - mea-sured by Agúndez et al. (2008), and our C H column densityalso closely matches their value of 7 . × cm - . Both N (C H) and N (C H - ) are relatively insensitive to the adoptedexcitation temperature, and vary by only 12% over the range5 -
10 K. HC N Observations and Radiative Transfer Modeling
Our HC N maps (Figure 1) show the presence of one ormore compact molecular condensations in each source, andhighlight a clumpy structure in this carbon-chain-rich gas.The peaks in HC N emission tend to coincide approximatelywith the locations of known prestellar cores and protostars,the positions of which are denoted with crosses and asterisks,respectively. As a result of the collapse and infall processesoccurring in these objects, their densities are expected to begreater than the surrounding gas. This should result in in-creased collisional excitation of the J = 10 level of HC N,which has a critical density of 7 × cm - (Buckle et al.2006). Thus, the intensity variations shown in our HC N OSOmaps represent a combination of variations in gas density andtotal HC N column.Spectra of three rotational transitions of HC N for eachsource, observed at the locations targeted by our GBT an-ion survey, are shown in Figure 3. These span a range ofupper-state energy levels 1 . - .
69 cm - (2 . - . N spec-tra were subject to fitting using the RADEX radiative trans-fer code (van der Tak et al. 2007). Collisional transition rateswere taken from the Leiden Atomic and Molecular Database (Schöier et al. 2005), which tabulates scaled versions of theoriginal data of Green & Chapman (1978). Hyperfine struc-ture (HFS) was accounted for by assuming LTE populationof the hyperfine levels in each J state. The primary collisionpartner number density ( n H ), molecular column density ( N ), ∼ moldata T M B ( m K ) L1172 C H - T M B ( m K ) LSR Velocity (km s -1 ) L1172 C H T M B ( m K ) L1389 C H - T M B ( m K ) LSR Velocity (km s -1 ) L1389 C H -10 0 10 20 30 T M B ( m K ) L1251A C H - T M B ( m K ) LSR Velocity (km s -1 ) L1251A C H -10 0 10 20 30 T M B ( m K ) L1512 C H - T M B ( m K ) LSR Velocity (km s -1 ) L1512 C H T M B ( m K ) L1495B C H - T M B ( m K ) LSR Velocity (km s -1 ) L1495B C H T M B ( m K ) TMC-1C C H - T M B ( m K ) LSR Velocity (km s -1 ) TMC-1C C H T M B ( m K ) TMC-1 CP C H - T M B ( m K ) LSR Velocity (km s -1 ) TMC-1 CP C H -2 0 2 4 T M B ( m K ) TMC-1 CP C H - T M B ( m K ) LSR Velocity (km s -1 ) TMC-1 CP C H F IG . 2.— Observed C H - spectra (average of J = 10 - J = 11 -
10 transitions), and C H J = 10 . - . , f spectra. The C H velocity scale is given withrespect to the (weighted) mean frequency of the two hyperfine components. For TMC-1, observed C H - ( J = 2 -
1) and C H ( N = 2 - , J = 1 . - . , F = 2 - N velocities for each target.
Cordiner et al.
TABLE 2O
BSERVED SPECIES , TRANSITIONS AND FREQUENCIES
Species Transition Frequency Ref. Telescope HPBW a (MHz) ( ′′ )C H N = 2 - J = 1 . - . F = 2 - H - J = 2 - H - J = 10 - H - J = 11 -
10 30290.8133 4 GBT 24C H J = 10 . - . , f , F = 11 -
10 29109.6437 5 GBT 25C H J = 10 . - . , f , F = 10 - H J = 10 . - . , e , F = 11 -
10 29112.7087 5 GBT 25C H J = 10 . - . , e , F = 10 - N J = 3 - , F = 3 - N J = 3 - , F = 2 - N J = 3 - , F = 3 - N J = 3 - , F = 4 - N J = 3 - , F = 2 - N J = 4 - , F = 4 - N J = 4 - , F = 3 - N J = 4 - , F = 4 - N J = 4 - , F = 5 - N J = 4 - , F = 3 - N J = 10 - , F = 9 - N J = 10 - , F = 10 - N J = 10 - , F = 11 -
10 90979.0024 6 OSO 42R
EFERENCES . — (1) Gottlieb et al. (1983); (2) Gupta et al. (2007); (3) McCarthy & Thaddeus (2008); (4) McCarthy et al. (2006); (5) McCarthy & Thaddeus(2005); (6) HC N line predictions are based on spectroscopic data summarized by Thorwirth et al. (2000) and de Zafra (1971). a Telescope half-power beam-width at observed frequency. TABLE 3M
OLECULAR LINE MEASUREMENTS AND ANION - TO - NEUTRAL RATIOS
Source R T MB dv (C H - ) a R T MB dv (C H) b N (C H - ) N (C H) C H - /C H(mK km s - ) (mK km s - ) (10 cm - ) (10 cm - ) (%)L1172 6.7 (0.8) 41.1 (1.6) 2.4 (0.3) 7.1 (0.3) 3.3 (0.5)L1251A 6.5 (1.0) 43.6 (2.1) 2.3 (0.4) 7.6 (0.4) 3.0 (0.6)L1389 5.9 (0.8) 27.1 (1.3) 2.1 (0.3) 4.7 (0.2) 4.4 (0.8)L1495B 9.6 (1.0) 141.6 (2.2) 3.4 (0.4) 24.6 (0.4) 1.4 (0.2)L1512 4.3 (0.4) 26.3 (0.9) 1.5 (0.1) 4.6 (0.2) 3.3 (0.4)TMC-1C 13.6 (1.1) 88.1 (1.7) 4.8 (0.4) 15.3 (0.3) 3.1 (0.3)TMC-1 CP 41.6 (5.0) 332.4 (9.0) 14.7 (1.8) 57.8 (1.6) 2.5 (0.4).N OTE . — Uncertainties given in parentheses are ±
68% Monte Carlo errors. Measurements for L1251A and L1512 supersede those of Cordiner et al. (2011)as explained in Section 4 a Integrated intensities of average C H - J = 10 - J = 11 -
10 spectra. b Integrated intensities summed over both hyperfine components of the C H J = 10 . - . , f transition. N J=3-2 36.391 36.392 36.393LSR Frequency (GHz)TMC-1 CPHC N J=4-3 90.978 90.98TMC-1 CPHC N J=10-90246 TMC-1CHC N J=3-2 TMC-1CHC N J=4-3 TMC-1CHC N J=10-90246 L1512HC N J=3-2 L1512HC N J=4-3 L1512HC N J=10-90246 T M B ( K ) L1495BHC N J=3-2 L1495BHC N J=4-3 L1495BHC N J=10-9024 L1389HC N J=3-2 L1389HC N J=4-3 L1389HC N J=10-9024 L1251AHC N J=3-2 L1251AHC N J=4-3 L1251AHC N J=10-9024 L1172HC N J=3-2 L1172HC N J=4-3 L1172HC N J=10-9 F IG . 3.— Observed HC N spectra (black points) with RADEX least-squares models overlaid (red curves). Frequency-switching residuals have been maskedfrom the observed data. Three main hyperfine peaks are visible in the J = 3 - J = 4 - Cordiner et al.
TABLE 4HC N RADEX
FIT RESULTS
Source N (HC N) v (HC N) ∆ v (HC N) n H (10 cm - ) (km s - ) (km s - ) (10 cm - )L1172 2.7 2.74 0.49 7.5 (2.3)L1251A 3.3 -3.97 0.31 2.1 (0.4)L1389 1.5 -4.75 0.32 5.2 (1.3)L1495B 8.2 7.62 0.19 1.1 (0.3)L1512 4.2 7.11 0.10 2.6 (0.9)TMC-1C 6.4 5.37 0.18 1.1 (0.4)TMC-1 CP 19.5 5.75 0.32 1.0 (0.2)N OTE . — Uncertainties on n H (in parentheses) were calculated assuming ± Doppler line FWHM ( ∆ v ) and Doppler velocity ( v ) were op-timized for the spectra of each source using the MPFIT least-squares algorithm (Markwardt 2008). The best-fitting param-eters are given in Table 4. The kinetic temperature was fixedat 10 K, then varied by ± n H . For all sources, a temperature of 10 K pro-duced a good fit to all five HFS components (the two weak-est HFS components are not shown in Figure 3). The HC NDoppler velocities are shown with vertical dashed lines in Fig-ure 2 and closely match the central velocities of the detectedhydrocarbons and anions. In several cases, the HC N linewidths are very narrow; for L1512, ∆ v = 0 . - , whichis only slightly greater than the 10 K thermal line-width of0.07 km s - , indicating an unusual lack of turbulence, flowsor shears in the central ∼ ′′ of this source.The C H - /C H ratio is plotted as a function of n H inFigure 4, and shows a positive correlation. There is con-siderable scatter and uncertainty on n H , but consistent withprevious studies (Hirahara et al. 1992, Visser et al. 2002,Launhardt et al. 2010 and Kirk et al. 2005, respectively), wefind the lowest density in TMC-1, highest densities in inL1172 and L1389, and an intermediate density in L1512.Our calculated n H values for L1172, L1389 and L1512are, however, systematically lower than previously-derivedvalues by up to two orders of magnitude. This discrepancy islikely indicative of a problem in our method, which can beattributed to the effects of different degrees of beam dilutionaffecting the different HC N line frequencies we observed.Given the larger 42 ′′ beam size for the HC N J = 10 - ′′ GBTbeam for the J = 3 - n H values to accountfor the observed discrepancies. Figure 1 (and Figure 7 ofPratap et al. 1997) shows that the HC N emission from allour observed sources is distributed over an area larger thanthe OSO beam, which suggests that beam dilution may notbe so severe. However, given the lack of spatial informationon the extent of the J = 10 - n H values given in Table 4 should be treated withcaution. More accurate estimates for n H could be obtainedfrom HC N J = 10 - C H - / C H ( % ) H number density (cm -3 ) L1172L1251A L1389L1495B L1512TMC-1CTMC-1 CP
Quiescent cloudStar-forming cloudProtostarLinear fit F IG . 4.— C H - anion-to-neutral ratio vs. H number density ( n H ). Dashedline shows linear least-squares fit. DISCUSSION C H - Since the 2006 discovery of C H - in TMC-1, there havebeen seven reported detections of this anion in various partsof the Taurus-Auriga Molecular Cloud complex (includingour latest detections in L1495B, L1512 and TMC-1C). Thusfar, there have been only two detections of interstellar anionsoutside of this small region (Sakai et al. 2010; Cordiner et al.2011), which highlights the importance of our new discover-ies of C H - in the vicinity of L1172 SMM (a prestellar corein Cepheus) and L1389 SMM1 (a prestellar/protostellar coreinside Bok Globule CB17 in Camelopardalis). These newdetections confirm that anions are indeed widespread in star-forming regions outside of Taurus, and that C H - appears tobe ubiquitous in dense interstellar clouds wherever its parentneutral is present.Myers et al. (1988) detected an outflow in L1172 thatVisser et al. (2002) identified as originating from a protostar(L1172 SMM1) that lies about 30 ′′ south of our targeted po-sition. They also detected a dense core (presumed prestel-lar), about 50 ′′ SE of our target, with N (H ) = 6 × cm - .If this column density also applies to our observed position,the C H - abundance is 4 × - , which is a factor of a fewless than in TMC-1, but an order of magnitude greater than inL1512, L1521F and L1544 (see Cordiner & Charnley 2012,and references therein), suggesting a relatively high absoluteanion abundance in L1172.By utilizing the available physical and chemical informa-tion on the interstellar clouds for which C H - has so farbeen detected (as summarized for our observed targets in Sec-tion 2), it is possible to examine the C H - /C H ratio withrespect to each cloud’s chemical and dynamical evolution-ary state. The evolutionary states of the observed cloudsare given in Table 1, and are shown by the different sym-bols on the plot in Figure 4. There is a clear division inthe anion-to-neutral ratio between young, ‘quiescent’ andolder, ‘star-forming’ cores: for the quiescent, chemically-young cores L1495B and TMC-1 (CP), C H - /C H < H - /C H ≥ H - /C H ratio in our sam-ple (1 . ± . H - /C H = 4 . ± . . ± . H - /C H ratio of9 . ± . H - anion-to-neutral ratio. Thequiescent cloud Lupus-1A observed by Sakai et al. (2010)also fits the trend (with C H - /C H = 2 . ± . H - /C H < . ± .
8% (Gupta et al. 2009).The observed C H - /C H trend and its correlation with n H shown in Figure 4 can be understood in the context of thetheoretical study of Cordiner & Charnley (2012), who iden-tified the effects of depletion on the C H - anion-to-neutralratio, the degree of which is related to both the density andchemical age of the cloud. Depletion occurs as atoms andmolecules collide with and stick to dust grains, and thus pro-ceeds faster in denser media. Increased depletion affects theanion-to-neutral ratio in two ways: (1) the free electron abun-dance goes up so that more C H - is produced by radiativeelectron attachment and (2) the atomic O and H abundancesgo down, thus reducing the anion destruction rate. Our C H - observations agree with this theory, particularly for the quies-cent cores and the protostars, which are believed to lie at op-posite ends of the interstellar chemical/dynamic evolutionarypath. Measured anion-to-neutral ratios, however, are not suf-ficiently accurate to draw a clear distinction among the (mod-erately evolved) prestellar cores and star-forming gas cloudsof L1172, L1251A, L1512 and TMC-1C. These clouds likelycontain gas that spans a range of densities and degrees ofchemical evolution. More accurate measurements of density,depletion and the C H - /C H ratio at high spatial resolutionwill be required in order to confirm the utility of the anion-to-neutral ratio as a measure of the evolutionary state of inter-stellar clouds. C H - Our detection of C H - constitutes the smallest reported col-umn density for this molecule in any source to-date. Consis-tent with the trend described above for C H - , the C H - anion-to-neutral ratio of (1 . ± . × - in TMC-1 (CP) is aboutnine times smaller than observed in L1527 by Agúndez et al.(2008). Theory regarding C H - chemistry is less well under-stood than for C H - , but it seems plausible to again ascribethe lower value in TMC-1 (and the moderately low value of(8 . ± . × - in Lupus-1A; Sakai et al. 2010), to chemi-cal effects resulting from the lower densities and younger evo-lutionary states of these sources compared with L1527.The very low observed C H - anion-to-neutral ratios com-pared with C H - are at variance with chemical models thatconsider the formation of C H - to be by radiative electron at-tachment to C H at the rate calculated by Herbst & Osamura(2008). Recent ab-initio calculations by V. Kokoouline andcolleagues (private communication, 2012) show that the C Hradiative attachment rate may be several orders of magnitude less than previously thought, and based on observations ofthe C H - /C H ratio in L1527, Agúndez et al. (2008) calcu-lated a C H radiative attachment rate a factor of 122 lessthan the theoretical value of Herbst & Osamura (2008). How-ever, using the laboratory data of Eichelberger et al. (2007),Cordiner & Charnley (2012) identified that a significant path-way to smaller hydrocarbon anions is via reactions of largeranions with atomic oxygen. Thus, an important source ofC H - is the reactionC H - + O - → C H - + CO . (1)As a consequence of this (and the analogous reactionthat forms C H - from C H - + O), using the model ofCordiner & Charnley (2012) at n H = 10 cm - (applicable toTMC-1), it is possible to set the radiative attachment rates forboth C H and C H to zero and still produce an absolute C H - abundance and C H - /C H ratio that are within a factor of afew of the observed values. It is therefore plausible that ra-diative electron attachment is not the dominant route to theformation of C H - in molecular clouds. CONCLUSION