Revealing the CO X-factor in Dark Molecular Gas through Sensitive ALMA Absorption Observations

Luo, G. X.; Li, Di; Tang, Ningyu; Dawson, J. R.; Dickey, John M.; Bronfman, L.; Qin, Sheng-Li; Gibson, Steven J.; Plambeck, R. L.; Finger, Ricardo; Green, Anne; Mardones, Diego; Koo, Bon‐Chul; Lo, Nadia

doi:10.3847/2041-8213/ab6337

“…We also note that a very low value of T ex (e.g., 0.1 K above the CMB) is frequently observed in diffuse clouds (n H = 10 3 cm −2 , Muller & Dinh-V-Trung 2009; Godard et al 2010;Luo et al 2020). As can be seen in Figure 7, the relative uncertainty of HCO + column density has a steep rise at T ex <4 K. If we adopt T ex -T CMB = 0.1 K, the column density of HCO + would increase by over an order of magnitude.…”

Section: Uncertaintiesmentioning

confidence: 54%

Dependence of Chemical Abundance on the Cosmic-Ray Ionization Rate in IC 348

Luo

¹

,

Zhang

²

,

Bisbas

³

et al. 2023

ApJ

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Ions (e.g., H 3 + , H2O+) have been used extensively to quantify the cosmic-ray ionization rate (CRIR) in diffuse sightlines. However, measurements of the CRIR in low-to-intermediate density gas environments are rare, especially when background stars are absent. In this work, we combine molecular line observations of CO, OH, CH, and HCO+ in the star-forming cloud IC 348, and chemical models to constrain the value of the CRIR and study the response of the distribution of chemical abundances. The cloud boundary is found to have an A V of approximately 4 mag. From the interior to the exterior of the cloud, the observed 13CO line intensities drop by an order of magnitude. The calculated average abundance of 12CO (assuming 12C/13C=65) is (1.2 ± 0.9) × 10−4, which decreases by a factor of 6 from the interior to the outside regions. The average abundance of CH (3.3 ± 0.7 × 10−8) is in good agreement with previous findings in diffuse and translucent clouds (A V < 5 mag). However, we did not find a decline in CH abundance in regions of high extinction (A V ≃ 8 mag) as previously reported in Taurus. By comparing the observed molecular abundances and chemical models, we find a decreasing trend of the CRIR as A V increases. The inferred CRIR of ζ cr = (4.7 ± 1.5) × 10−16 s−1 at low A V is consistent with H 3 + measurements toward two nearby massive stars.

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“…In clouds with relatively low extinctions (i.e. 𝐴 V ∼ 0.37 − 2.5 magnitudes, so that they are at or near the threshold for CO detection, below which the emission region is classified as dark molecular gas) X CO can be 6-7 times larger, suggesting that C to CO conversion is incomplete (Luo et al 2020) in these sources. This may explain some of the scatter in the data points, particularly in the vicinity of 𝐴 V = 𝐴 th .…”

Section: Results and Analysismentioning

confidence: 99%

Water ice deposition and growth in molecular clouds

Rawlings

¹

,

Williams

²

2020

Monthly Notices of the Royal Astronomical Society

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In interstellar clouds the deposition of water ice onto grains only occurs at visual extinctions above some threshold value (Ath). At extinctions greater than Ath there is a (near-linear) correlation between the inferred column density of the water ice and AV. For individual cloud complexes such as Taurus, Serpens and ρ-Ophiuchi, Ath and the gradients of the correlation are very similar along all lines of sight. We have investigated the origin of this phenomenon, with careful consideration of the various possible mechanisms that may be involved and have applied a full chemical model to analyse the behaviours and sensitivities in quiescent molecular clouds. Our key results are: (i) the ubiquity of the phenomenon points to a common cause, so that the lines of sight probe regions with similar, advanced, chemical and dynamical evolution, (ii) for Taurus and Serpens; Ath and the slope of the correlation can be explained as resulting from the balance of freeze-out of oxygen atoms and photodesorption of H2O molecules. No other mechanism can satisfactorily explain the phenomenon, (iii) Ath depends on the local density, suggesting that there is a correlation between local volume density and column density, (iv) the different values of Ath for Taurus and Serpens are probably due to variations in the local mean radiation field strength, (v) most ice is accreted onto grains that are initially very small (<0.01μm), and (vi) the very high value of Ath observed in ρ-Ophiuchi cannot be explained in the same way, unless there is complex microstructure and/or a modification to the extinction characteristics.

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“…For each LOS, we decompose the absorption spectra (τ ν ) into different Gaussian components to derive the column density at each velocity component. A detailed description of observations and data reduction can be found in Luo et al (2020).…”

Section: Observations and Datamentioning

confidence: 99%

“…HCO + , in the traditional sense, is a dense gas tracer in the ISM. However, recent observations found that HCO + is ubiquitous in diffuse and translucent clouds (Pety et al 2017;Luo et al 2020), especially with absorption measurements against strong continuum sources (e.g., quasars, H II regions).…”

Section: Introductionmentioning

confidence: 99%

Abundance Ratios of OH/CO and HCO⁺/CO as Probes of the Cosmic-Ray Ionization Rate in Diffuse Clouds

Luo

¹

,

Zhang

²

,

Bisbas

³

et al. 2023

ApJ

Self Cite

4

0

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The cosmic-ray ionization rate (CRIR, ζ 2) is one of the key parameters controlling the formation and destruction of various molecules in molecular clouds. However, the current most commonly used CRIR tracers, such as H 3 + , OH+, and H2O+, are hard to detect and require the presence of background massive stars for absorption measurements. In this work, we propose an alternative method to infer the CRIR in diffuse clouds using the abundance ratios of OH/CO and HCO+/CO. We have analyzed the response of chemical abundances of CO, OH, and HCO+ on various environmental parameters of the interstellar medium in diffuse clouds and found that their abundances are proportional to ζ 2. Our analytic expressions give an excellent calculation of the abundance of OH for ζ 2 ≤ 10−15 s−1, which are potentially useful for modeling chemistry in hydrodynamical simulations. The abundances of OH and HCO+ were found to monotonically decrease with increasing density, while the CO abundance shows the opposite trend. With high-sensitivity absorption transitions of both CO (1–0) and (2–1) lines from Atacama Large Millimeter/submillimeter Array, we have derived the H2 number densities ( n H 2 ) toward 4 lines of sight; assuming a kinetic temperature of T k = 50 K, we find a range of (0.14 ± 0.03–1.2 ± 0.1) × 102 cm−3. By comparing the observed and modeled HCO+/CO ratios, we find that ζ 2 in our diffuse gas sample is in the range of 1.0 − 1.0 + 14.8 × 10 − 16 – 2.5 − 2.4 + 1.4 × 10−15 s−1. This is ∼2 times higher than the average value measured at higher extinction, supporting an attenuation of CRs as suggested by theoretical models.

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Revealing the CO X-factor in Dark Molecular Gas through Sensitive ALMA Absorption Observations

Cited by 12 publications

References 44 publications

Dependence of Chemical Abundance on the Cosmic-Ray Ionization Rate in IC 348

Dependence of Chemical Abundance on the Cosmic-Ray Ionization Rate in IC 348

Water ice deposition and growth in molecular clouds

Abundance Ratios of OH/CO and HCO⁺/CO as Probes of the Cosmic-Ray Ionization Rate in Diffuse Clouds

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Revealing the CO X-factor in Dark Molecular Gas through Sensitive ALMA Absorption Observations

Cited by 12 publications

References 44 publications

Dependence of Chemical Abundance on the Cosmic-Ray Ionization Rate in IC 348

Dependence of Chemical Abundance on the Cosmic-Ray Ionization Rate in IC 348

Water ice deposition and growth in molecular clouds

Abundance Ratios of OH/CO and HCO+/CO as Probes of the Cosmic-Ray Ionization Rate in Diffuse Clouds

Contact Info

Product

Resources

About

Abundance Ratios of OH/CO and HCO⁺/CO as Probes of the Cosmic-Ray Ionization Rate in Diffuse Clouds