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Aims. We present a study of star formation in the central molecular zone (CMZ) of our Galaxy through the association of three star formation indicators: 6.7 GHz CH3OH masers, 22 GHz H2O masers, and enhanced 4.5 μm emission ('green') sources. We explore how star formation in the CMZ (∣⨏∣ < 1.3°, ∣b∣ < 10') compares with that of the Galactic disk (6 °> ⨏ > 345°, ∣b∣ < 2°). Methods. Using an automated algorithm, we search for green sources toward 6.7 GHz CH3OH masers detected in the Parkes Methanol Multibeam Survey. We combine these results with lists of 22 GHz H2O masers, including our Mopra survey of the CMZ. Results. We find that the correlation of CH3OH masers with green sources is a function of Galactic latitude, with a minimum close to b = 0 and increasing with ∣b∣ (toward the central part of the Galaxy, 6 °> ⨏ > 345°, ∣b∣ < 2°).We find no significant difference between the correlation rate of CH3OH masers with green sources in the CMZ and the disk. This suggests that although the physical conditions of the gas are different in the CMZ from that of the Galactic disk, once gravitational instability sets in at sufficiently high densities, signatures of star formation appear to be similar in both regions. Moreover, the detection of green sources, even at the distance of the Galactic center, shows that our technique can easily identify the early stages of star formation, especially in lowextinction regions of the Galaxy. Through the association of H2O and CH3OH masers, we identify 15 star-forming sites in the CMZ. We find a higher correlation rate of coincident H2O and CH3OH masers within the CMZ compared to the Galactic disk, indicating a difference in the maser evolutionary sequence for star-forming cores in these two regions. [ABSTRACT FROM AUTHOR]

Context. Dense highly ionized boundary layers (IBLs) outside of the neutral Photon Dominated Regions (PDRs) have recently been detected via the 122 and 205 μm transitions of ionized nitrogen. These layers have higher densities than in the Warm Ionized Medium (WIM) but less than typically found in H II regions. Observations of [C II] emission, which is produced in both the PDR and IBL, do not fully define the characteristics of these sources. Observations of additional probes which just trace the PDRs, such as the fine structure lines of atomic oxygen, are needed derive their properties and distinguish among different models for [C II] and [N II] emissison. Aims. We derive the properties of the PDRs adjacent to dense highly ionized boundary layers of molecular clouds. Methods. We combine high-spectral resolution observations of the 63 μm [O I] fine structure line taken with the upGREAT HFA-band instrument on SOFIA with [C II] observations to constrain the physical conditions in the PDRs. The observations consist of samples along four lines of sight (LOS) towards the inner Galaxy containing several dense molecular clouds. We interpret the conditions in the PDRs using radiative transfer models for [C II] and [O I]. Results. We have a 3.5-σ detection of [O I] toward one source but only upper limits towards the others. We use the [O I] to [C II] ratio, or their upper limits, and the column density of C+ to estimate the thermal pressure, Pth, in these PDRs. In two LOS the thermal pressure is likely in the range 2–5 × 105 in units of K cm−3, with kinetic temperatures of order 75–100 K and H2 densities, n(H2) ~ 2–4 × 103 cm−3. For the other two sources, where the upper limits on [O I] to [C II] are larger, Pth ≲105 (K cm−3). We have also used PDR models that predict the [O I] to [C II] ratio, along with our observations of this ratio, to limit the intensity of the Far UV radiation field. Conclusions. The [C II] and [N II] emission with either weak, or without any, evidence of [O I] indicates that the source of dense highly ionized gas traced by [N II] most likely arises from the ionized boundary layers of clouds rather than from H II regions. [ABSTRACT FROM AUTHOR]

We present SOFIA/GREAT [C ii] ²P3/2 →²P1/2 (1.9005369 THz) observations of nearby clouds near the lines of sight towards the quasars B0355+508 and B0212+735. These clouds have previously been identified as warm non-LTE diffuse clouds with a temperature of T ≳ 30 K and sub-thermally excited CO lines. They are highly structured in CO with small-scale bright spots (I(CO J = 1-0) ~ 5-20 Kkm s-1), both spectrally and spatially. This small-scale structure has been interpreted as small-scale variations in the chemistry, not as density and column-density structure. We did not detect [C ii] 158 μm emission within the rms noise of ~0.1-0.3 K. Our non-detection in [C ii] contradicts the above scenario. In diffuse clouds, the effciency of photo-electric heating is highest. Under the assumption that [C ii] is the dominant coolant in diffuse clouds, we can calculate the predicted [C ii] emission arising from these clouds. Based on the derived hydrogen column densities for the diffuse clouds, a line width in [C ii] of similar order to that of CO, and when at least the minimum amount of heating in the clouds is due to cosmic-ray heating, [C ii] line intensities ≳1.5 to 5 Kkm s-1 are expected, which is a factor ~3 to 15 above the upper limits of the observations. The upper limits of the [C ii] and the observed CO line intensities are, however, consistent with the intensities predicted for photon-dominated region (PDR) surfaces on regular cold (T ~ 15 K) low-density cloud fragments. Lower temperatures lead to less excited [C ii]. The assumption of lower densities, which would equally lower the [C ii] excitation, contradicts the observed cloud sizes and column densities. The CO(2-1)/CO(1-0) line ratios observed in these clouds are consistent with cloud temperatures of T ~ 15 K. The KOSMA-τ PDRmodel of a cold moderate-density clump or an ensemble of such clumps with a canonical mass-size relation and mass spectrum, consistent with the total column densities derived for low density, shows that cold PDRs can reproduce the observed CO intensities, the observed CO(1-0) and CO(2-1) ratio, and the observed upper limits for [C ii]. [ABSTRACT FROM AUTHOR]

In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen-bearing ions OH+, H2O+, and H3O+ begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H2. Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (ζH) and molecular hydrogen fraction (). We present observations targeting transitions of OH+, H2O+, and H3O+ made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources. Both OH+ and H2O+ are detected in absorption in multiple velocity components along every sight line, but H3O+ is only detected along 7 sight lines. From the molecular abundances we compute in multiple distinct components along each line of sight, and find a Gaussian distribution with mean and standard deviation 0.042 ± 0.018. This confirms previous findings that OH+ and H2O+ primarily reside in gas with low H2 fractions. We also infer ζH throughout our sample, and find a lognormal distribution with mean log (ζH) = –15.75 (ζH = 1.78 × 10–16 s–1) and standard deviation 0.29 for gas within the Galactic disk, but outside of the Galactic center. This is in good agreement with the mean and distribution of cosmic-ray ionization rates previously inferred from H observations. Ionization rates in the Galactic center tend to be 10-100 times larger than found in the Galactic disk, also in accord with prior studies. [ABSTRACT FROM AUTHOR]