Benzyne in V4334 Sqr: A Quest for the Ring with SOFIA/EXES

Large hydrocarbon molecules (both aliphatic and aromatic) are now known to be widespread in interstellar and circumstellar environments (Tielens 2008; Sloan et al. 2014; Xie et al. 2018). In particular, polycyclic aromatic (carbon-ring structures) hydrocarbon (PAH; e.g., the seven-ring coronene C₂₄H₁₂) molecules are generally accepted to be the carriers of the ubiquitous unidentified infrared (UIR) features (Tielens 2008; Peeters 2011).

Small aromatic hydrocarbons however have proven to be more elusive. While benzene (C₆H₆) is known to be common in solar system environments (Trainer et al. 2013; Guerlet et al. 2015), and may play a role in ice chemistry in the interstellar medium (ISM; Sivaraman et al. 2015), the only detection to date in a circumstellar environment has been in the protoplanetary nebula CRL 618 (Cernicharo et al. 2001a, 2001b). Recently, McGuire et al. (2018) reported detection of benzonitrile (c-C₆H₅CN) at radio wavelengths toward the molecular cloud TMC-1. Joblin & Cernicharo (2018) argue that benzonitrile likely forms from a reaction of CN with benzene, and hence it may be possible to indirectly infer both the existence of benzene and the abundance of benzene itself.

Dusty, carbon-rich circumstellar environments are laboratories for the detection and study of exotic molecular species (e.g., Zhang et al. 2011). Benzyne (C₆H₄) is an aromatic (carbon-ring structure) hydrocarbon with a structure similar to benzene. Benzyne is an example of an aryne (-yne ≡triple bond), wherein the additional pi bond is formed by the overlap of sp2 hybridized orbitals outside the ring. As the molecule is highly strained, it is a highly reactive intermediate species. On the detection of benzene in CRL 618, Weaver et al. (2007) attempted to detect benzyne in CRL 618 without success.

Here, we present our attempts to detect benzyne in the dusty, dense hydrocarbon-rich circumstellar environment (Evans et al. 2006) of the born-again giant (BAG) V4334 Sgr (also known as Sakurai's Object) with the Stratospheric Observatory for Infrared Astronomy (SOFIA)/Echelon-Cross-Echelle Spectrograph (EXES).

When a solar-mass star reaches the end of its life and is evolving toward the white dwarf (WD) region of the HR diagram, it may reignite a residual helium shell and be reborn as a giant star: it becomes a BAG. This may occur in as many as 20% of stars, and is a phase of stellar evolution that is very poorly understood (Herwig 2005). For example, observations over the past ∼10 yr indicate that it takes place far more rapidly than theory predicts. BAGS also may be a primary source of ¹³C in the ISM (Kobayashi et al. 2011).

V4334 Sgr was discovered in 1996 (Nakano et al. 1996). Evidence for its BAG nature are its association with a faint planetary nebula (Eyres et al. 1998; Pollacco 1999) and its low ¹²C/¹³C ratio of 4 ± 1 (Pavlenko et al. 2004; Evans et al. 2006). V4334 Sgr subsequently produced an optically thick carbon dust shell that completely obscured the star (visual extinction ≳10 mag). A large mass-loss rate ($\dot{M}\sim {10}^{-5}$ ${M}_{\odot }$ yr⁻¹) was implied by 1–5 μm spectroscopy (Tyne et al. 2002) and by 450/850 μm photometry (Evans et al. 2002). Observations with the European Southern Observatory (ESO) Very Large Telescope Interferometer (VLTI) reveal V4334 Sgr's dust disk, which is nearly edge-on (Chesneau et al. 2009). The rapid rate at which it is evolving has prompted major rethinking about the post-main-sequence evolution of low-to-intermediate mass stars (Herwig 2001; Lawlor & MacDonald 2003; Deneault et al. 2006).

Neither Spitzer nor SOFIA spectra show evidence for canonical PAH dust features at mid-infrared (mid-IR) wavelengths. Similarly, the 3.28 and 3.4 μm features are not seen in ground-based spectra during the period of 1999–2002 (Tyne et al. 2002; Evans et al. 2020). The WD in V4334 Sgr certainly is a photon source to excite PAHs, although Li & Draine (2002) argue that such a energetic radiation field may not be required.

Observations of V4334 Sgr were conducted using EXES (Richter et al. 2018) on 2019 April 4.335 UT (MJD = 58577.355), observing at an altitude of 13.11 km (43,000 ft) under programs 07_0010 and 06_0095. The instrument was configured in single-order, long-slit mode with a vacuum rest-frame wavelength center of 18.16 μm (550.63 cm⁻¹), in first order using a 258 × 243 slit. This configuration yields a resolving power (RP) of ≃2240. Because of the high background flux at 18 μm in the EXES low-resolution mode, the instrument was operated in the subarray mode and at a lower detector bias to avoid saturation. V4334 Sgr was nodded at two positions (AB-nod) separated by 8'' within the slit to provide for background subtraction. Immediately after the observations of V4334 Sgr, the telluric reference Callisto (a moon of Jupiter) was observed with the same instrumental setup. Data were reduced using the EXES instrument pipeline (REDUX; Clarke et al. 2015), with additional custom routines to deal with the high-background photon flux.

We adopted a flux density of 1551.3 Jy at 18.16 μm for Callisto. This flux density was estimated from a standard thermal model (NEATM; Harris 1998), assuming a beaming parameter of 0.756 and a phase coefficient of 0.01 mag per degree (Dotto et al. 2000) using the JPL Horizons values for Callisto's radius and albedo. Details of the observations and SOFIA archive data files used in our analysis are given in Table 1. The EXES spectrum is shown in Figure 1.

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The infrared (IR) spectral energy distribution (SED) of V4334 Sgr over the last ∼20 yr is discussed in detail by Evans et al. (2020). The dust shell has cooled, from ≃840 K in 1999 (as deduced from ground-based data) to ≃180 K in 2016 (SOFIA-based observations). A Spitzer InfraRed Spectrograph (IRS) spectrum (Evans et al. 2006) revealed the presence of HCN and polyynes (acetylene, C₂H₂; HC_nN) in the 13.5–16.5 μm region. Later epoch SOFIA (+FORCAST) spectra (Evans et al. 2020) show that other hydrogenated carbon species are present. The HCN features gave (for 2005) a ¹²C/¹³C ratio for the absorbing gas of ∼4 (Evans et al. 2006), as recently confirmed by submillimeter observations of the $J=4\to 3$ transition in H¹²CN and H¹³CN conducted by Tafoya et al. (2017), and a temperature ≃450 K. SOFIA observations in 2014 and 2016 indicated that the HCN and polyyne features have weakened to invisibility (i.e., nondetectable). Acetylene would just be visible, if present, at the very edge of the FORCAST G111 spectral segment (see Herter et al. 2012).

The weak, and temporally varying, absorption features seen in the IR spectra of V4334 Sgr are intriguing, especially those near 10.9 μm and 13.4 μm. Sander et al. (2002) attributes IR meta-benzyne features (Figure 2) at 10.94 μm to a C–H bend + C–C stretch, 13.37 μm to a C–H wag, and 18.28 μm to an in-plane aromatic ring deformation. The latter is the strongest band, and we suggest that the putative mid-IR features in V4334 Sgr are likely from meta-benzyne. Assuming that the molecules and radicals occupy a common zone between us and the (optically thick) dust, we expect any benzyne features to be in absorption, as are acetylene and other hydrocarbon features (Evans et al. 2006). As ¹²C/¹³C ∼4 in V4334 Sgr, we expect that at least one, and possibly two, of the C atoms in the benzyne will be ¹³C, which will cause the features to be displaced to longer wavelengths than those ring chains comprised solely of ¹²C (i.e., Radziszewski et al. 1992).

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Given the evolution of the hydrocarbon features in V4334 Sgr, is it possible that small hydrocarbon molecules seen in the Spitzer spectra (Evans et al. 2006) have undergone reactions to produce the ring molecules?

Benzyne is a highly reactive biradical. Reactions between benzyne molecules, and between benzyne and other molecules, could lead to PAH formation. Any benzyne in V4334 Sgr may evolve along a similar astrochemical pathway into the PAH molecules seen (via the UIR features) in older BAGs such as FG Sge (Evans et al. 2015). If so, detection of benzyne could provide powerful constraints on chemical pathways for aromatic hydrocarbon formation in circumstellar environments, from di- and tri-atomics, to polyynes, to basic rings, and thence to the PAHs responsible for the UIR emission.

However, at our spectral sensitivity, median 1σ flux density (upper limit) at 18.16 μm of 25 Jy for our V4334 Sgr data, we find no evidence for benzyne in our EXES spectra. Similarly, no other emission or absorption features are evident (Figure 1). If benzyne is not present, could the small aliphatic hydrocarbons that are extant in the circumstellar material of V4334 Sgr then have their origins as fragments detached from larger, hydrogenated amorphous carbon (HACs) grains or do they form in situ by some other mechanism?

Acetylene is present in V4334 Sgr (Evans et al. 2006). Chen & Li (2019) have shown production of carbon nanotubes, formed from benzene with the presence of C₂H₂ (through a hydrogen abstraction and acetylene addition reaction) is possible. If benzyne is similarly reactive, this could deplete the abundance of benzyne below the level of detectability. However, spectral signatures at the putative wavelengths of carbon nanotubes (see Table 1 of Chen & Li 2019) are not evident in the Spitzer or SOFIA spectra of V4334 Sgr. Unlike other BAGS, nondetection of PAHs in V4334 Sgr may suggest that they are not extant in the dusty carbon-rich environment and possibly benzyne quickly transforms into other species. A more sensitive search (μJy) for benzyne and other reactive carbon byproducts in dust environments with the James Webb Space Telescope may resolve this issue and the origins of PAH/UIR/HAC species.

The search for the 18 μm benzyne (C₆H₄) feature in V4334 Sgr with SOFIA/EXES was inconclusive at this epoch (2019) of the evolution of the cool, dense circumstellar shell after the 1996 BAG event. However, further pursuit to detect such small hydrocarbons is warranted to advance our understanding of hydrocarbon astrochemistry.

The authors thank the referee for the careful review and insightful scientific suggestions which improved the manuscript. This work is based in part on observations made with the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). SOFIA is jointly operated by the Universities Space Research Association, Inc. (USRA), under NASA contract NAS2-97001, and the Deutsches SOFIA Institut (DSI) under DLR contract 50 OK 0901 to the University of Stuttgart. Observations with EXES are supported by the NASA collaborative agreement NNX13AI85A and 80NSSC19K1701-0. C.E.W. acknowledges support from NASA SOFIA contract NNA17BF53C managed by USRA to the University of Minnesota. D.P.K.B. is supported by a CSIR Emeritus Scientist grant-in-aid which is being hosted by the Physical Research Laboratory, Ahmedabad. T.L. acknowledges financial support from GAČR (grant number 17-02337S). The Astronomical Institute Ondřejov is supported by the project RVO:67985815. R.D.G. is supported by NASA and the United States Air Force. S.S. acknowledges support from a NASA Theory grant to ASU.

Facility: NASA SOFIA (EXES). -

Software: REDUX (Clarke et al. 2015).