Context. The [C ii] 158 μm line is an important tool for understanding the life cycle of interstellar matter. Ionized carbon is present in a variety of phases of the interstellar medium (ISM), including the diffuse ionized medium, warm and cold atomic clouds, clouds in transition from atomic to molecular, and dense and warm photon dominated regions.Aims. Velocity-resolved observations of [C ii] are the most powerful technique available to disentangle the emission produced by these components. These observations can also be used to trace CO-dark H 2 gas and determine the total mass of the ISM.Methods. The Galactic Observations of Terahertz C+ (GOT C+) project surveys the [C ii] 158 μm line over the entire Galactic disk with velocity-resolved observations using the Herschel/HIFI instrument. We present the first longitude-velocity maps of the [C ii] emission for Galactic latitudes b = 0• , ±0.5• , and ±1.0• . We combine these maps with those of H i, 12 CO, and 13 CO to separate the different phases of the ISM and study their properties and distribution in the Galactic plane.Results. [C ii] emission is mostly associated with spiral arms, mainly emerging from Galactocentric distances between 4 and 10 kpc.It traces the envelopes of evolved clouds as well as clouds that are in the transition between atomic and molecular. We estimate that most of the observed [C ii] emission is produced by dense photon dominated regions (∼47%), with smaller contributions from COdark H 2 gas (∼28%), cold atomic gas (∼21%), and ionized gas (∼4%). Atomic gas inside the Solar radius is mostly in the form of cold neutral medium (CNM), while the warm neutral medium gas dominates the outer galaxy. The average fraction of CNM relative to total atomic gas is ∼43%. We find that the warm and diffuse CO-dark H 2 is distributed over a larger range of Galactocentric distances (4−11 kpc) than the cold and dense H 2 gas traced by 12 CO and 13 CO (4−8 kpc). The fraction of CO-dark H 2 to total H 2 increases with Galactocentric distance, ranging from ∼20% at 4 kpc to ∼80% at 10 kpc. On average, CO-dark H 2 accounts for ∼30% of the molecular mass of the Milky Way. When the CO-dark H 2 component is included, the radial distribution of the CO-to-H 2 conversion factor is steeper than that when only molecular gas traced by CO is considered. Most of the observed [C ii] emission emerging from dense photon dominated regions is associated with modest far-ultraviolet fields in the range χ 0 1−30.

Using the MIPS instrument on Spitzer, we have searched for infrared excesses around a sample of 82 stars, mostly F, G, and K main-sequence field stars, along with a small number of nearby M stars. These stars were selected for their suitability for future observations by a variety of planet-finding techniques. These observations provide information on the asteroidal and cometary material orbiting these stars, data that can be correlated with any planets that may eventually be found. We have found significant excess 70 m emission toward 12 stars. Combined with an earlier study, we find an overall 70 m excess detection rate of 13% AE 3% for mature cool stars. Unlike the trend for planets to be found preferentially toward stars with high metallicity, the incidence of debris disks is uncorrelated with metallicity. By newly identifying four of these stars as having weak 24 m excesses (fluxes $10% above the stellar photosphere), we confirm a trend found in earlier studies wherein a weak 24 m excess is associated with a strong 70 m excess. Interestingly, we find no evidence for debris disks around 23 stars cooler than K1, a result that is bolstered by a lack of excess around any of the 38 K1YM6 stars in two companion surveys. One motivation for this study is the fact that strong zodiacal emission can make it hard or impossible to detect planets directly with future observatories such as the Terrestrial Planet Finder (TPF ). The observations reported here exclude a few stars with very high levels of emission, >1000 times the emission of our zodiacal cloud, from direct planet searches. For the remainder of the sample, we set relatively high limits on dust emission from asteroid belt counterparts.

We analyze the collisional excitation of the 158 µm (1900.5 GHz) fine structure transition of ionized carbon in terms of line intensities produced by simple cloud models. The single C + fine structure transition is a very important coolant of the atomic interstellar medium and of photon dominated regions in which carbon is partially or completely in ionized form. The [CII] line is widely used as a tracer of star formation in the Milky Way and other galaxies. Excitation of the [CII] fine structure transition can be via collisions with hydrogen molecules, atoms, and electrons. Analysis of [CII] observations is complicated by the fact that it is difficult to determine the optical depth of the line. We discuss the excitation of the [CII] line, deriving analytic results for several limiting cases and carry out numerical solutions using a large velocity gradient model for a more inclusive analysis. For antenna temperatures up to 1/3 of the brightness temperature of the gas kinetic temperature, the antenna temperature is linearly proportional to the column density of C + irrespective of the optical depth of the transition. This is appropriately referred to as the effectively optically thin (EOT) approximation.We review the critical densities for excitation of the [CII] line by various collision partners, briefly analyze C + absorption, and conclude with a discussion of C + cooling and how the considerations for line intensities affect the behavior of this important coolant of the ISM.

Context. H i and CO large scale surveys of the Milky Way trace the diffuse atomic clouds and the dense shielded regions of molecular hydrogen clouds, respectively. However, until recently, we have not had spectrally resolved C + surveys in sufficient lines of sight to characterize the ionized and photon dominated components of the interstellar medium, in particular, the H 2 gas without CO, referred to as CO-dark H 2 , in a large sample of interstellar clouds.Aims. We use a sparse Galactic plane survey of the 1.9 THz (158 μm) [C ii] spectral line from the Herschel open time key programme, Galactic Observations of Terahertz C+ (GOT C+), to characterize the H 2 gas without CO in a statistically significant sample of interstellar clouds.Methods. We identify individual clouds in the inner Galaxy by fitting the [C ii] and CO isotopologue spectra along each line of sight. We then combine these spectra with those of H i and use them along with excitation models and cloud models of C + to determine the column densities and fractional mass of CO-dark H 2 clouds.Results. We identify1804 narrow velocity [C ii] components corresponding to interstellar clouds in different categories and evolutionary states. About 840 are diffuse molecular clouds with no CO, ∼510 are transition clouds containing [C ii] and 12 CO, but no 13 CO, and the remainder are dense molecular clouds containing 13 CO emission. The CO-dark H 2 clouds are concentrated between Galactic radii of ∼3.5 to 7.5 kpc and the column density of the CO-dark H 2 layer varies significantly from cloud to cloud with a global average of 9 × 10 20 cm −2 . These clouds contain a significant fraction by mass of CO-dark H 2 , that varies from ∼75% for diffuse molecular clouds to ∼20% for dense molecular clouds.Conclusions. We find a significant fraction of the warm molecular ISM gas is invisible in H i and CO, but is detected in [C ii]. The fraction of CO-dark H 2 is greatest in the diffuse clouds and decreases with increasing total column density, and is lowest in the massive clouds. The column densities and mass fraction of CO-dark H 2 are less than predicted by models of diffuse molecular clouds using solar metallicity, which is not surprising as most of our detections are in Galactic regions where the metallicity is larger and shielding more effective. There is an overall trend towards a higher fraction of CO-dark H 2 in clouds with increasing Galactic radius, consistent with lower metallicity there.

We present the first results of the Herschel open time key program, Galactic Observations of Terahertz C + (GOT C+) survey of the [CII] 2 P 3/2 -2 P 1/2 fine-structure line at 1.9 THz (158 μm) using the HIFI instrument on Herschel. We detected 146 interstellar clouds along sixteen lines-of-sight towards the inner Galaxy. We also acquired HI and CO isotopologue data along each line-of-sight for analysis of the physical conditions in these clouds. Here we analyze 29 diffuse clouds (A V < 1.3 mag) in this sample characterized by having [CII] and HI emission, but no detectable CO. We find that [CII] emission is generally stronger than expected for diffuse atomic clouds, and in a number of sources is much stronger than anticipated based on their HI column density. We show that excess [CII] emission in these clouds is best explained by the presence of a significant diffuse warm H 2 , dark gas, component. This first [CII] 158 μm detection of warm dark gas demonstrates the value of this tracer for mapping this gas throughout the Milky Way and in galaxies.