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Standard stellar evolution models that only consider convection as a physical process to mix material inside of stars predict the production of significant amounts of 3He in low-mass stars (M < 2 M ⊙), with peak abundances of 3He/H ∼ few Ă— 10âˆ'3 by number. Over the lifetime of the Galaxy, this ought to produce 3He/H abundances that diminish with increasing Galactocentric radius. Observations of 3He+ in H ii regions throughout the Galactic disk, however, reveal very little variation in the 3He abundance with values of 3He/H similar to the primordial abundance, 3 H e / H p ∼ 10 âˆ' 5 . This discrepancy, known as the “3He problem,” can be resolved by invoking in stellar evolution models an extra mixing mechanism due to the thermohaline instability. Here we observe 3He+ in the planetary nebula (PN) J320 (G190.3â€"17.7) with the Jansky Very Large Array to confirm a previous 3He+ detection made with the Very Large Array that supports standard stellar yields. This measurement alone indicates that not all stars undergo extra mixing. Our more sensitive observations do not detect 3He+ emission from J320 with an rms noise of 58.8 ÎĽ Jy beamâˆ'1 after smoothing the data to a velocity resolution of 11.4 km sâˆ'1. We estimate an abundance limit of 3He/H ≤ 2.75 Ă— 10âˆ'3 by number using the numerical radiative transfer code NEBULA. This result nullifies the last significant detection of 3He+ in a PN and allows for the possibility that all stars undergo extra mixing processes. [ABSTRACT FROM AUTHOR]

The Galactic H ii region luminosity function (LF) is an important metric for understanding global star formation properties of the Milky Way, but only a few studies have been done, and all use relatively small numbers of H ii regions. We use a sample of 797 first Galactic quadrant H ii regions compiled from the Wide-field Infrared Survey Explorer Catalog of Galactic H ii Regions to examine the form of the LF at multiple infrared and radio wavelengths. Our sample is statistically complete for all regions powered by single stars of type O9.5V and earlier. We fit the LF at each wavelength with single and double power laws. Averaging the results from all wavelengths, the mean of the best-fit single power-law index is 〈α〉 = −1.75 ± 0.01. The mean best-fit double power-law indices are 〈α1〉 = −1.40 ± 0.03 and 〈α2〉 = −2.33 ± 0.04. We conclude that neither a single nor a double power law is strongly favored over the other. The LFs show some variation when we separate the H ii region sample into subsets by heliocentric distance, physical size, Galactocentric radius, and location relative to the spiral arms, but blending individual H ii regions into larger complexes does not change the value of the power-law indices of the best-fit LF models. The consistency of the power-law indices across multiple wavelengths suggests that the LF is independent of wavelength. This implies that infrared and radio tracers can be employed in place of Hα. [ABSTRACT FROM AUTHOR]

We use the Green Bank Telescope to search for 3He+ emission from a sample of four Galactic planetary nebulae: NGC 3242, NGC 6543, NGC 6826, and NGC 7009. During the era of primordial nucleosynthesis, the light elements 2H, 3He, 4He, and 7Li were produced in significant amounts, and these abundances have since been modified primarily by stars. Observations of 3He+ in H ii regions located throughout the Milky Way disk reveal very little variation in the 3He/H abundance ratio—the "3He Plateau"—indicating that the net effect of 3He production in stars is negligible. This is in contrast to much higher 3He/H abundance ratios reported for some planetary nebulae. This discrepancy is known as the "3He Problem." We use radio recombination lines observed simultaneously with the 3He+ transition to make a robust assessment of the spectral sensitivity that these observations achieve. We detect spectral lines at ∼1–2 mK intensities, but at these levels, instrumental effects compromise our ability to measure accurate spectral line parameters. We do not confirm reports of previous detections of 3He+ in NGC 3242 nor do we detect 3He+ emission from any of our sources. This result calls into question all reported detections of 3He+ emission from any planetary nebula. The 3He/H abundance upper limit we derive here for NGC 3242 is inconsistent with standard stellar production of 3He and thus requires that some type of extra-mixing process operates in low-mass stars. [ABSTRACT FROM AUTHOR]

We make a comprehensive study of Hi absorption toward H II regions located within ∣l∣ < 10°. Structures in the extreme inner Galaxy are traced using the longitude-velocity space distribution of this absorption. We find significant Hi absorption associated with the Near and Far 3 kpc Arms, the Connecting Arm, Bania's Clump 1, and the Hi Tilted Disk.We also constrain the line-of-sight distances to H II regions, by using Hi absorption spectra together with the H II region velocities measured by radio recombination lines. [ABSTRACT FROM AUTHOR]

Using the Infrared Spectrograph on board the Spitzer Space Telescope, we have detected emission in the S(0), S(1), and S(2) pure-rotational (v = 0-0) transitions of molecular hydrogen (H2) toward six positions in two translucent high Galactic latitude clouds, DCld 300.2-16.9 and LDN 1780. The detection of these lines raises important questions regarding the physical conditions inside low-extinction clouds that are far from ultraviolet radiation sources. The ratio between the S(2) flux and the flux from polycyclic aromatic hydrocarbons (PAHs) at 7,9 µm averages 0,007 for these six positions. This is a factor of about four higher than the same ratio measured toward the central regions of non-active Galaxies in the Spitzer Infrared Nearby Galaxies Survey. Thus, the environment of these translucent clouds is more efficient at producing rotationally excited H2 per PAH-exciting photon than the disks of entire galaxies. Excitation analysis finds that the S(1) and S(2) emitting regions are warm (T ≳ 300 K), but comprise no more than 2% of the gas mass. We find that UV photons cannot be the sole source of excitation in these regions and suggest mechanical heating via shocks or turbulent dissipation as the dominant cause of the emission. The clouds are located on the outskirts of the Scorpius-Centaurus OB association and may be dissipating recent bursts of mechanical energy input from supernova explosions. We suggest that pockets of warm gas in diffuse or translucent clouds, integrated over the disks of galaxies, may represent a major source of all non-active galaxy H2 emission. [ABSTRACT FROM AUTHOR]

The distribution of metals in the Galaxy provides important information about galaxy formation and evolution. Hn regions are the most luminous objects in the Milky Way at mid-infrared to radio wavelengths and can be seen across the entire Galactic disk. We used the National Radio Astronomy Observatory (NRAO) Green Bank Telescope to measure radio recombination line and continuum emission in 81 Galactic Hn regions. We calculated LTE electron temperatures using these data. In thermal equilibrium metal abundances are expected to set the nebular electron temperature with high abundances producing low temperatures. Our Hn region distribution covers a large range of Galactocentric radius (5-22 kpc) and samples the Galactic azimuth range 330°-60°. Using our highest quality data (72 objects) we derived an O/H Galactocentric radial gradient of -0.0383 ± 0.0074 dex kpc-1. Combining these data with a similar survey made with the NRAO 140 Foot telescope we get a radial gradient of -0.0446 ± 0.0049 dex kpc-1 for this larger sample of 133 nebulae. The data are well fit by a linear model and no discontinuities are detected. Dividing our sample into three Galactic azimuth regions produced significantly different radial gradients that range from -0.03 to -0.07 dex kpc-1. These inhomogeneities suggest that metals are not well mixed at a given radius. We stress the importance of homogeneous samples to reduce the confusion of comparing data sets with different systematics. Galactic chemical evolution models typically derive chemical evolution along only the radial dimension with time. Future models should consider azimuthal evolution as well. [ABSTRACT FROM AUTHOR]

We report the detection of radio recombination line (RRL) emission using the Arecibo Observatory at X band (9 GHz, 3 cm) from 37 previously unknown H II regions in the Galactic zone 66° ⩾ ℓ ⩾ 31° and | b | ⩽ 1°. This Arecibo H II Region Discovery Survey (Arecibo HRDS) is a continuation of the Green Bank Telescope (GBT) HRDS. The targets for the Arecibo HRDS have spatially coincident 24 μm and 20 cm emission of a similar angular morphology and extent. To take advantage of Arecibo's sensitivity and small beam size, sources in this sample are fainter, smaller in angle, or in more crowded fields compared to those of the GBT HRDS. These Arecibo nebulae are some of the faintest H II regions ever detected in RRL emission. Our detection rate is 58%, which is low compared to the 95% detection rate for GBT HRDS targets. We derive kinematic distances to 23 of the Arecibo HRDS detections. Four nebulae have negative local standard of rest velocities and are thus unambiguously in the outer Galaxy. The remaining sources are at the tangent-point distance or farther. We identify a large, diffuse H II region complex that has an associated H I and 13CO shell. The ∼90 pc diameter of the G52L nebula in this complex may be the largest Galactic H II region known, and yet it has escaped previous detection. [ABSTRACT FROM AUTHOR]

Using the H I emission/absorption method, we resolve the kinematic distance ambiguity and derive distances for 149 of 182 (82%) H II regions discovered by the Green Bank Telescope H II Region Discovery Survey (GBT HRDS). The HRDS is an X-band (9 GHz, 3 cm) GBT survey of 448 previously unknown H II regions in radio recombination line and radio continuum emission. Here, we focus on HRDS sources from 67° ⩾ ℓ ⩾ 18°, where kinematic distances are more reliable. The 25 HRDS sources in this zone that have negative recombination line velocities are unambiguously beyond the orbit of the Sun, up to 20 kpc distant. They are the most distant H II regions yet discovered. We find that 61% of HRDS sources are located at the far distance, 31% at the tangent-point distance, and only 7% at the near distance. “Bubble” H II regions are not preferentially located at the near distance (as was assumed previously) but average 10 kpc from the Sun. The HRDS nebulae, when combined with a large sample of H II regions with previously known distances, show evidence of spiral structure in two circular arc segments of mean Galactocentric radii of 4.25 and 6.0 kpc. We perform a thorough uncertainty analysis to analyze the effect of using different rotation curves, streaming motions, and a change to the solar circular rotation speed. The median distance uncertainty for our sample of H II regions is only 0.5 kpc, or 5%. This is significantly less than the median difference between the near and far kinematic distances, 6 kpc. The basic Galactic structure results are unchanged after considering these sources of uncertainty. [ABSTRACT FROM AUTHOR]