AN OBSERVATIONAL INVESTIGATION OF THE IDENTITY OF B11244 (l–C₃H⁺/C₃H⁻)

Pety et al. (2012) have reported the detection of eight transitions of a closed-shell, linear molecule in observations toward the Horsehead photodissociation region (PDR). They performed a spectroscopic analysis and fit to these transition frequencies and, based on comparison with theoretical work (see Ikuta 1997, and references therein), attribute these transitions to the l–C₃H⁺ cation. Later, McGuire et al. (2013) identified the J = 1–0 and J = 2–1 transitions predicted by Pety et al. (2012) in absorption toward the Sgr B2(N) molecular cloud.

The cation l–C₃H⁺ is important in the chemistry of hydrocarbons because reactions with it are thought to be the most important gas-phase channels to form other small hydrocarbons (Turner et al. 2000; Wakelam et al. 2010), like C₃H and C₃H₂, which are widely observed in different environments. However, the observed abundances of C₃H and C₃H₂ in PDRs are much higher than what pure gas-phase models predict. One possible explanation is that polycyclic aromatic hydrocarbons (PAHs) are fragmented into these small hydrocarbons in PDRs due to the strong UV fields (see e.g., Fuente et al. 2003; Teyssier et al. 2004; Pety et al. 2005). The discovery of l–C₃H⁺ thus brings further constraints to the formation pathways of the small hydrocarbons in these environments.

The attribution of these signals to the l–C₃H⁺ cation, however, has since been disputed by Huang et al. (2013) and Fortenberry et al. (2013), with the latter suggesting the anion C₃H⁻ as a more probable carrier based on high-level theoretical work. Because of the open question of identity, McGuire et al. (2013) used the convention of referring to the carrier as B11244, which we adopt here.

In this paper, we re-examine the observations of Pety et al. (2012) toward the Horsehead PDR, as well as PRebiotic Interstellar MOlecular Survey (PRIMOS) observations of Sgr B2(N), the Kaifu et al. (2004) survey of TMC-1, and the Barry E. Turner Legacy survey of IRC+10216 in the context of discussing: "What if B11244 is actually C₃H⁻?" In Section 2, we discuss the spectroscopy of C₃H⁻ using the properties derived by Fortenberry et al. (2013) and present simulated spectra. In Section 3, we briefly outline the observations used and in Section 4 discuss the analysis of these observations. Finally, in Section 5 we present the results of our findings and discuss them in the context of determining the identity of B11244.

Table 1 provides the rotational constants and dipole moments used in this work to describe B11244, assuming it is either l–C₃H⁺ or C₃H⁻. The spectroscopic constants and fit for l–C₃H⁺ are provided in Pety et al. (2012), and their predictive power is confirmed in McGuire et al. (2013). Fortenberry et al. (2013) provide a high-accuracy equilibrium structure for C₃H⁻, rotational constants, and dipole moments. These moments are not in the principal axis (PA) system but can readily be converted to the PA system with a simple coordinate rotation resulting in μ_x → μ_a = 1.63 debye and μ_y → μ_b = 1.41 debye. Indeed, the magnitude of this rotation is small, such the values of these dipole moments remain essentially unchanged. Assuming B11244 is C₃H⁻, the observed transitions in Pety et al. (2012) and McGuire et al. (2013) are a-type, K_a = 0 transitions, and thus (B + C) and D_J can be well determined from these lines. To obtain these constants, the observed transitions from Pety et al. (2012) and McGuire et al. (2013) were fit using the CALPGM suite of programs. An asymmetric-top Hamiltonian with a Watson S reduction in the I^r representation was used.⁸ The remaining constants were necessarily used as is from the theoretical calculations. A simulated spectrum of C₃H⁻ at 22 K using this combined set of constants is displayed in Figure 1. A full CALPGM catalog for C₃H⁻ to 2 THz is also provided as Supplemental Information (see Table 2).

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Sgr B2(N). The data presented toward Sgr B2(N) were obtained as part of the PRIMOS project using the Robert C. Byrd 100 m Green Bank Telescope. The observed position was at (J2000) α = 17^h47^m198, δ = −28°22'17''. An LSR source velocity of +64 km s⁻¹ was assumed. Full observational details, including data reduction procedures and analysis, are given in Neill et al. (2012a).⁹

IRC+10216. The observations presented toward IRC+10216 are part of the Barry E. Turner Legacy Survey using the NRAO 12 m telescope on Kitt Peak. The observed position was at (J2000) α = 9^h47^m573, δ = +13°16'43''. An LSR source velocity of −26 km s⁻¹ was assumed. Full observational details are given in Remijan et al. (2008).¹⁰

TMC-1. The observations presented toward the TMC-1 dark cloud were taken as part of the Kaifu et al. (2004) survey using the Nobeyama Radio Observatory 45 m telescope. The observed position was at (J2000) α = 4^h41^m425, δ = +25°41'269. An LSR source velocity of +5.85 km s⁻¹ was assumed. Full observational details are given in Kaifu et al. (2004).

Horsehead PDR. The observations presented toward the Horsehead PDR were taken with the IRAM 30 m telescope as part of the Horsehead WHISPER project (PI: J. Pety). The observed position was at (J2000) α = 5^h40^m53936, δ = −2°28'00''. An LSR source velocity of +10.7 km s⁻¹ was assumed. Full observational details are given in Pety et al. (2012).

The column density of B11244 in each source, assuming local thermodynamic equilibrium (LTE), can be calculated using

$$\begin{equation} N_T=\frac{3k}{8\pi ^3}\times \frac{Q_re^{E_u/T_{ex}}}{\nu S\mu ^2}\times \frac{\sqrt{\pi }}{2ln2}\times \frac{\Delta T_A^* \Delta V/\eta _b}{1-\frac{(e^{h\nu /kT_{ex}}-1)}{(e^{h\nu /kT_{bg}}-1)}}{\rm cm}^{-2}, \end{equation} \tag{ 1 }$$

following the convention of Hollis et al. (2004).

Here, N_T is the total column density, Q_r is the rotational partition function, E_u is the upper state energy, T_ex is the excitation temperature, ν is the frequency of the transition, Sμ² is the transition strength, $\Delta T_A^*$ is the peak line intensity, ΔV is the line FWHM, η_b is the beam efficiency at frequency ν, and T_bg is the background temperature.

In the case of l–C₃H⁺, the partition function is well approximated by the standard linear-molecule formula given by

$$\begin{equation} Q_r (l\hbox{--}{\rm C_3H^+}) \approxeq \frac{kT}{hB} = 1.85(T), \end{equation} \tag{ 2 }$$

with B expressed in Hz. For C₃H⁻, the following equation is appropriate (Gordy & Cook 1984):

$$\begin{equation} Q_r {\rm (C_3H^-)} \approxeq \frac{5.34\times 10^6}{\sigma } \left(\frac{T^3}{ABC} \right) ^{1/2} = 0.65(T_{ex})^{3/2}, \end{equation} \tag{ 3 }$$

with σ = 1 and rotational constants with units of MHz. The accuracy of Q_r for the anion is dependent on the accuracy of the rotational constants used. Thus, there is likely an uncertainty of a few percent in the value of Q_r used here. In any case, the partition function for the anion rapidly outpaces that of the cation above T_ex ∼ 8 K.

To calculate upper limits of l–C₃H⁺ in IRC+10216, we use the molecule-specific parameters given in McGuire et al. (2013) and the upper limit $\Delta T_A^*$ and ΔV values given in Table 3. The line parameters and molecule-specific parameters used for all C₃H⁻ calculations are given in Table 3.

Table 3. Observed and Targeted Transitions of B11244, Assuming it is C₃H⁻

Transition	ν	Sij	Eu	Horsehead		Sgr B2(N)		IRC+10216
$J_{K_a,K_c}^{\prime } - J_{K_a,K_c}^{\prime \prime }$	(MHz)		(K)	ΔTmb	ΔV	$\Delta T_A^*$	ΔV	$\Delta T_A^*$	ΔV
10, 1 → 00, 0	22 489.86	1	1.079	...	...	−27	13.4	...	...
21, 2 → 11, 1	44 755.94	1.5	28.07	...	...	a	14.7	...	...
20, 2 → 10, 1	44 979.50	2	3.237	...	...	−70	14.7	...	...
21, 1 → 11, 0	45 197.57	1.5	28.10	...	...	⩽9	14.7	...	...
41, 4 → 31, 3	89 510.82	3.75	35.59	⩽5.7	0.81	...	...	...	...
40, 4 → 30, 3	89 957.63	4	10.79	89	0.81	...	...	...	...
41, 3 → 31, 2	90 394.13	3.75	35.70	⩽5.3	0.81	...	...	...	...
51, 5 → 41, 4	111 887.54	4.8	40.96	⩽10.5	0.81	...	...	...	...
50, 5 → 40, 4	112 445.57	5	16.19	115	0.81	...	...	...	...
51, 4 → 41, 3	112 991.72	4.8	41.11	⩽14.1	0.81	...	...	...	...
60, 6 → 50, 5	134 932.69	6	22.66	72	0.81	⩽52	13	⩽5	27.5
70, 7 → 60, 6	157 418.71	7	30.22	75	0.81	99b	13	⩽7	27.5
90, 9 → 80, 8	202 386.75	9	48.56	62	0.81	...	...	...	...
100, 10 → 90, 9	224 868.40	10	59.36	30	0.81	...	...	...	...
110, 11 → 100, 10	247 348.23	11	71.23	45	0.81	...	...	...	...
120, 12 → 110, 11	269 826.05	12	84.18	25	0.81	...	...	...	...

Notes. For simplicity, only those K_a = 1 transitions specifically searched for in our study are displayed. $\Delta T_A^*$ and ΔT_mb given in units of mK, ΔV given in units of km s⁻¹. All upper limits are 1σ. Values for the Horsehead PDR and Sgr B2(N) are based on Gaussian fits to the line shapes. The FWHM for IRC+10216 is based on a zeroth-order approximation from other observed transitions. ^aCompletely obscured by blends. ^bPartially blended.

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While the observed K_a = 0 transitions allow us to constrain B and C reasonably well, the lack of any confirmed detection of a K_a = 1 transition limits the overall accuracy in predicting the frequencies of these lines. However, the expected intensity of these lines, given a derived column density and temperature, is likely to be fairly accurate under LTE conditions. In the Horsehead PDR, these lines should have a peak intensity of T_mb ∼ 20–28 mK for J'' = 3–6, using the derived conditions from the K_a = 0 transitions. In Sgr B2(N), the expected intensities are below the detectable values in our observations.

We calculate a theoretical uncertainty in the center frequencies for these transitions of σ ∼ 370 MHz for the J = 4–3 transition to as much as σ ∼ 650 MHz for the J = 7–6 transition. At LTE, the strongest of these lines fall within the 3 mm window of the Pety et al. (2012) survey. Due to the uncertainties in the line centers, we have searched a region equal to each transition's uncertainty on either side of each predicted line center. After identifying all known lines within this range, we find no detection of any signals which could be assigned to a K_a = 1 transition of C₃H⁻ at the rms noise level of the observations (∼5–10 mK), despite peak predicted intensities of 20–28 mK at LTE. An example spectrum of the region searched around the predicted 4_{1, 4}–3_{1, 3} transition is shown in Figure 2.

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At higher frequencies (ν > 500 GHz), additional branches of C₃H⁻ transitions are predicted, with slightly greater intensity; we have no spectral coverage at these frequencies. However, these transitions are strongly dependent on the derived value for A, making any attempted search quite challenging.

In the following paragraphs, we discuss the results of our analysis in the context of determining the identity of B11244. We do not address topics that have been previously covered in the literature and for which our analysis provides no further information. Namely, the agreement (or lack thereof) between the fitted rotational and distortion constants for each species with those calculated by Huang et al. (2013) and Fortenberry et al. (2013).

5.1. Anion/Neutral Abundance Ratio

5.2. Detection in Sgr B2(N)

To our knowledge, no molecular anions have been detected in Sgr B2(N). An examination of both the PRIMOS centimeter-wave data and the 2 mm Turner Survey shows no indication of the presence of any of the known molecular anions. Of note, no such anions have been detected in the Horsehead PDR either (Agúndez et al. 2008).

However, a re-examination of the PRIMOS data originally presented in McGuire et al. (2013) finds some evidence for B11244 absorption in lower-velocity (V_LSR ∼ +0–10 km s⁻¹ and V_LSR ∼ +18 km s⁻¹) diffuse clouds along the line of sight to Sgr B2(N). For illustration, the J = 1–0 and J = 2–1 transitions of B11244 are shown in Figure 3 in comparison to the known CH absorption spectra toward SgrB2(N).¹² The strongest observed transition of l–C₃H is also shown and displays low-velocity absorption as well, although the ∼0 km s⁻¹ component is blended with the +64 km s⁻¹ main component of the l–C₃H, J = 3/2 − 1/2, f-parity, F = 1–0 transition.

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While numerous cationic species have been detected in these diffuse clouds (see e.g., Gerin et al. 2010 & Godard et al. 2010), the possibility of anion chemistry in these regions is not well understood. Indeed, no anions have previously been seen in these line-of-sight clouds. The indication of B11244 in these regions, presented here, will hopefully be a motivating factor that will drive future studies. Investigations of this diffuse gas, which displays chemistry distinct from regions such as IRC+10216 and Sgr B2(N), will certainly prove invaluable in furthering our understanding of gas-phase ion chemistry.

5.3. Nondetection in IRC+10216

5.4. Anion Destruction via Photodetachment

5.5. Non-detection of K_a = 1 Transitions

We have presented an analysis of observations of the Horsehead PDR, Sgr B2(N), IRC+10216, and TMC-1 with the goal of determining the identity of B11244. Our findings can be summarized as follows:

• 1.  
  If B11244 is C₃H⁻, it would display the highest anion–neutral ratio yet observed in the ISM (57% in the Horsehead PDR).
• 2.  
  We find no evidence for C₃H⁻ emission in observations toward IRC+10216 and place an upper limit on the anion–neutral ratio in this source well below that found in the Horsehead PDR and Sgr B2(N).
• 3.  
  Recent work has shown UV photodetachment is a dominant destruction pathway for molecular anions (Kumar et al. 2013). Despite a UV field more than 60 times that of IRC+10216, C₃H⁻ would be present in the Horsehead PDR with an anion–neutral ratio more than eight times that of the upper limit in IRC+10216.
• 4.  
  We find no evidence for the K_a = 1 lines of C₃H⁻ in observations of the Horsehead PDR. We examine three limiting cases for conditions within the Horsehead PDR and find that a weak far-IR radiation field can account for the lack of observed K_a = 1 transitions. LTE conditions or the presence of a strong far-IR radiation field, however, strongly disfavor the presence of C₃H⁻. A significant far-IR radiation field has been reported for the Horsehead PDR, but it is unclear whether B11244 is subject to this radiation.

The observational evidence presented here, taken as a whole, casts doubt on the assignment of B11244 to C₃H⁻, favoring instead the cation l–C₃H⁺ as the most likely candidate. The evidence is, however, circumstantial; a definitive answer will almost certainly require laboratory confirmation. Indeed, K.N. Crabtree and coworkers at the Harvard-Smithsonian Center for Astrophysics have undertaken such work using Fourier-transform microwave spectroscopy. Preliminary evidence is suggestive of the cationic species. The full results of the laboratory investigation will be published in an upcoming paper (K.N. Crabtree 2013, private communication).

Additional observations of the Horsehead PDR, with the aim of detecting the b-type transitions of C₃H⁻, predicted to be strongest between 500–600 GHz, would also provide further evidence. Perhaps more insightful would be interferometric observations to discover the spatial correlation, or lack thereof, of B11244 with the previously observed far-IR radiation field. Finally, further observations of the diffuse gas along the sightline to Sgr B2(N) would likely prove fruitful in understanding the possibility of anion chemistry in these regions.

We are grateful to the anonymous referee for helpful comments which have greatly improved the quality of this manuscript. We thank E.A. Bergin and J.T. Neill for providing the HEXOS spectra presented here and M. Ohishi for providing the observational data toward TMC-1. We also thank H. Gupta, M. Cordiner, and A. Faure for insightful conversations. B.A.M. gratefully acknowledges funding by an NSF Graduate Research Fellowship. V.G. thanks support from the Chilean Government through the Becas Chile scholarship program. This work was partially funded by grant ANR-09-BLAN-0231-01 from the French Agence Nationale de la Recherche as part of the SCHISM project. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.