The Transition from Diffuse Molecular Gas to Molecular Cloud Material in Taurus

Context. Small linear carbon chain radicals such as C2 and C3 act as both the building blocks and dissociation fragments of larger carbonaceous species. Their rotational excitation traces the temperature and density of local environments. However, these homo-nuclear di- and triatomic species are only accessible through their electronic and vibrational features because they lack a permanent dipole moment, and high signal-to-noise ratio data are necessary as the result of their generally low abundances in the interstellar medium (ISM). Aims. In order to improve our understanding of small carbonaceous species in the ISM, we carried out a sensitive survey of C2 and C3 using the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES) dataset. We also expanded our searches to C4, C5, and the 13C12C isotopologue in the most molecule-rich sightlines. Methods. We fitted synthetic spectra generated following a physical excitation model to the C2 (2-0) Phillips band to obtain the C2 column density (N) as well as the kinetic temperature (Tkin) and number density (n) of the host cloud. The C3 molecule was measured through its Ã − $ \tilde X$ (000-000) electronic origin band system. We simulated the excitation of this band with a double-temperature Boltzmann distribution. Results. We present the largest combined survey of C2 and C3 to date in which the individual transitions can be resolved. In total, we detected C2 in 51 velocity components along 40 sightlines, and C3 in 31 velocity components along 27 sightlines. Further analysis confirms the two molecules are detected in the same velocity components. We find a very good correlation between N(C2) and N(C3) with a Pearson correlation coefficient r = 0.93 and an average N(C2)/N(C3) ratio of 15.5± 1.4. A comparison with the behaviour of the C2 diffuse interstellar bands (DIBs) shows that there are no clear differences among sightlines with and without detections of C2 and C3. This is in direct contrast to the better-studied non-C2 DIBs, which have reduced strengths in molecule-rich environments, consistent with the idea that the C2 DIBs are indeed a distinguishable DIB family. We also identify, for the first time, the Q(2), Q(3), and Q(4) transitions of the 13C12C (2-0) Phillips band in the stacked average spectrum of molecule-rich sightlines, and estimate the isotopic ratio of carbon 12C/13C to be 79±8, consistent with literature results. At this stage it is not yet possible to identify these transitions in individual sightlines. Our search for the C4 and C5 optical bands was unsuccessful; even in stacked spectra no unambiguous identification could be made.

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Excitation of CO molecules in diffuse gas over cosmic history

Klimenko,

Balashev,

Noterdaeme

et al. 2024

Monthly Notices of the Royal Astronomical Society

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We studied the physical conditions in the local ISM and at high redshift by fitting observed column densities of CO and H2 rotational levels, and C i fine-structure levels using photo-dissociation models calculated with the Meudon PDR code. We analyzed CO absorption systems in 28 sight lines in the local ISM and 7 damped Lyman-α absorption systems at high redshift, covering N(H2) = 1019 to 1021.5 cm−2 and N(CO) = 1013 to 1018 cm−2. We constructed a method to accurately calculate CO excitation, incorporating the effects of photon trapping. Our findings indicate that in the local ISM, CO excitation is primarily driven by collisions and excitation due to the CMB radiation. We demonstrated that an increase in CO excitation, observed near N(CO) ≃ 1015 cm−2, is attributed to an increase in gas densities from ≃ 100 cm−3 to ≃ 300 − 1000 cm−3. CO absorption systems in the local ISM are characterized by a gas number density of about 10-1000 cm−3, a kinetic temperature of 10-100 K, and an intensity of external UV field ranging from 0.1 to 10 units of Mathis field. Compared to the average gas probed by C i absorption, the gas detected in CO is denser and colder, while the external UV field remains nearly constant. We observed a negative correlation between the kinetic temperature and both N(CO) and N(H2), with power-law slopes of −0.21 ± 0.02 and −0.65 ± 0.05, respectively. At the same time, the gas number density exhibits a positive correlation with N(CO) and N(H2) with slopes of 0.38 ± 0.02 and 1.15 ± 0.05, respectively.

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The Transition from Diffuse Molecular Gas to Molecular Cloud Material in Taurus

Cited by 6 publications

References 146 publications

A theoretical study of the photochemistry of 1,3-cyclopentadiene and its cyano derivatives bound to a water dimer: Assessing reactivity of ionized clusters and possible photoproducts

A theoretical study of the photochemistry of 1,3-cyclopentadiene and its cyano derivatives bound to a water dimer: Assessing reactivity of ionized clusters and possible photoproducts

The EDIBLES survey

Excitation of CO molecules in diffuse gas over cosmic history

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