Discovery of 1H-cyclopent[cd]indene (c-C11H8) in TMC-1 with the QUIJOTE line survey: A new three-ringed polycyclic aromatic hydrocarbon

TL;DR

This study reports the first interstellar detection of the three-ring PAH 1H-cyclopent[cd]indene (c-C11H8) in TMC-1 using the QUIJOTE Q-band line survey. The carrier was identified from a harmonic spectral pattern around 820.5 MHz, validated by a quantitative fit of 22 independent lines (88 transitions) with a Watson-A reduction, and supported by quantum-chemical calculations; the derived column density is $N=(6.0\pm0.5)\times10^{12}$ cm$^{-2}$ and the rotational temperature is $T_{rot}=9$ K. Its abundance is the lowest among the pure PAHs detected in TMC-1 but remains substantial, reinforcing the idea that bottom-up growth produces sizable PAHs in dark clouds and constraining astrochemical models. The work also discusses possible formation pathways for cyclopentindene, highlighting reactions between small rings and hydrocarbons (including indene with C$_2$H, C$_2$H$_2$, or their cations) and the need for further experimental and theoretical studies to fully understand PAH formation in the ISM.

Abstract

We report the detection of the polycyclic aromatic hydrocarbon (PAH) 1H-cyclopent[cd]indene (c-C11H8) in TMC-1 with the QUI- JOTE line survey. We detected 22 independent lines corresponding to 88 rotational transitions with quantum numbers ranging from J=19 up to J=24 and Ka <= 5 in the Q-band range. The identification of this new PAH was based on the agreement between the rotational parameters derived from the analysis of the lines and those obtained by quantum chemical calculations. The column density derived for 1H-cyclopent[cd]indene is (6.0 +- 0.5) x 10^12 cm-2, with a rotational temperature of 9 K. Its abundance is high, as is that of the rest of the PAHs, but it is the lowest of all those detected to date in TMC-1, being 2.66 times less abundant than indene and 4.66 times less than phenalene. This result will help us to better understand the growth of five- and six-membered rings in dark clouds. Chemical models explaining their formation through the bottom-up model are still very incomplete and require further experimental and theoretical effort. Even so, the most likely formation reactions would occur between the smallest rings with small hydrocarbons; the most probable reaction for the formation of cyclopentindene is that between indene and C2H, C2H3, and/or their cation.

Figures (3)

• Figure 1: Modified Loomis--Wood diagram of some of the observed lines of cyclopentindene. The abscissa corresponds to the value of the rotational constant, which has been fixed in this plot to values between 819.6 and 820.6 MHz. The ordinate is the antenna temperature, corrected for atmospheric and telescope losses, in millikelvin. The frequency represented in each box corresponds to 2 B$_{rot}$$J$, where $J$ = $J_u$ + 1/2. The red lines represent a synthetic spectrum of the lines detected with the QUIJOTE data. The blue solid lines correspond to the new detected lines in this work, while dotted lines correspond to transitions with the same $K_a$ (for all transitions involved, see Table \ref{['tab:lineparameter']}).
• Figure 2: Optimized geometry of cyclopentindene.
• Figure 3: Some of the cyclopentindene lines observed in TMC-1 with QUIJOTE. The black line represents the survey data, while the red line represents the synthetic spectrum calculated for a column density of 6$\times$10$^{12}$ cm$^{-2}$. The green labels indicate the transitions for which only the 8 MHz frequency-switching data were used. The blanked channels correspond to negative features resulting in the folding of the frequency-switched spectra. Each box shows four collapsed transitions corresponding to each line, where $\alpha$, $\beta$, $\gamma$, $\delta$, and $\epsilon$ are $K_a$=0,1, $K_a$=1,2, $K_a$=2,3, $K_a$=3,4, and $K_a$=4,5, respectively (see Table \ref{['tab:lineparameter']} for more details about transitions). The unidentified lines or other molecules already detected in the source appear in blue. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s$^{-1}$. The ordinate is the antenna temperature in millikelvin.