Dana S. Balser

THE GREEN BANK TELESCOPE H II REGION DISCOVERY SURVEY. II. THE SOURCE CATALOG

The Green Bank Telescope (GBT) H II Region Discovery Survey has doubled the number of known H II regions in the Galactic zone 343 degrees \textless= l \textless= 67 degrees with vertical bar b vertical bar \textless= 1 degrees. We detected 603 discrete hydrogen radio recombination line (RRL) components at 9 GHz (3 cm) from 448 targets. Our targets were selected based on spatially coincident mid-infrared and 20 cm radio continuum emission. Such sources are almost invariably H II regions; we detected hydrogen RRL emission from 95% of our target sample. The sensitivity of the GBT and the power of its spectrometer together made this survey possible. Here, we provide a catalog of the measured properties of the RRL and continuum emission from the survey nebulae. The derived survey completeness limit, 180 mJy at 9 GHz, is sufficient to detect all H II regions ionized by single O-stars to a distance of 12 kpc. These recently discovered nebulae share the same distribution on the sky as does the previously known census of Galactic HIT regions. On average, however, the new nebulae have fainter continuum fluxes, smaller continuum angular sizes, fainter RRL intensities, and smaller RRL line widths. Though small in angular size, many of our new nebulae show little spatial correlation with tracers associated with extremely young H II regions, implying that our sample spans a range of evolutionary states. We discovered 34 first quadrant negative-velocity H II regions, which lie at extreme distances from the Sun and appear to be part of the Outer Arm. We found RRL emission from 208 Spitzer GLIMPSE 8.0 mu m “bubble” sources, 65 of which have been cataloged previously. It thus appears that nearly all GLIMPSE bubbles are H II regions and that similar to 50% of all Galactic H II regions have a bubble morphology at 8.0 mu m.

The Electron Temperature Gradient in the Galactic Disk

We derive the electron temperature gradient in the Galactic disk, using a sample of H II regions that spans Galactocentric distances of 0-17 kpc. The electron temperature was calculated using high-precision radio recombination line and continuum observations for more than 100 H II regions. Nebular Galactocentric distances were calculated in a consistent manner, using the radial velocities measured by our radio recombination line survey. The large number of nebulae widely distributed over the Galactic disk, together with the uniformity of our data, provide a secure estimate of the present electron temperature gradient in the Milky Way. Because metals are the main coolants in the photoionized gas, the electron temperature along the Galactic disk should be directly related to the distribution of heavy elements in the Milky Way. Our best estimate of the electron temperature gradient is derived from a sample of 76 sources for which we have the highest quality data. The present gradient in electron temperature has a minimum at the Galactic center and rises at a rate of 287 ± 46 K kpc-1. There are no significant variations in the value of the gradient as a function of Galactocentric radius or azimuth. The scatter we find in the H II region electron temperatures at a given Galactocentric radius is not due to observational error, but rather to intrinsic fluctuations in these temperatures, which are almost certainly due to fluctuations in the nebular heavy-element abundances. Comparing the H II region gradient with the much steeper gradient found for planetary nebulae suggests that the electron temperature gradient evolves with time, becoming flatter as a consequence of the chemical evolution of the Milky Ways disk.

The WISE Catalog of Galactic H II Regions

Using data from the all-sky Wide-Field Infrared Survey Explorer (WISE) satellite, we made a catalog of over 8000 Galactic HII regions and HII region candidates by searching for their characteristic mid-infrared (MIR) morphology. WISE has sufficient sensitivity to detect the MIR emission from HII regions located anywhere in the Galactic disk. We believe this is the most complete catalog yet of regions forming massive stars in the Milky Way. Of the ∼ 8000 cataloged sources, ∼ 1500 have measured radio recombination line (RRL) or Hα emission, and are thus known to be HII regions. This sample improves on previous efforts by resolving HII region complexes into multiple sources and by removing duplicate entries. There are ∼ 2500 candidate HII regions in the catalog that are spatially coincident with radio continuum emission. Our group’s previous RRL studies show that ∼ 95% of such targets are HII regions. We find that ∼ 500 of these candidates are also positionally associated with known HII region complexes, so the probability of their being bona fide HII regions is even higher. At the sensitivity limits of existing surveys, ∼ 4000 catalog sources show no radio continuum emission. Using data from the literature, we find distances for ∼ 1500 catalog sources, and molecular velocities for ∼ 1500 HII region candidates.

Out-Of-Focus Holography at the Green Bank Telescope

Mullard Radio Astronomy Observatory, Cavendish Laboratory, Cambridge CB3 0HE, UKthe date of receipt and acceptance should be inserted laterAbstract. We describe phase-retrieval holography measurements of the 100-m diameter Green Bank Telescope using astro-nomical sources and an astronomical receiver operating at a wavelength of 7mm. We use the technique with parameterizationof the aperture in terms of Zernike polynomials and employing a large defocus, as described by Nikolic et al. (2006). Individualmeasurements take around 25minutes and from the resulting beam maps (which have peak signal to noise ratios of 200:1) weshow that it is possible to produce low-resolution maps of the wavefront errors with accuracy around λ/100.Using such measurements over a wide range of elevations, we have calculated a model for the wavefront-errors due to theuncompensated gravitational deformation of the telescope. This model produces a significant improvement at low elevat ions,where these errors are expected to be the largest; after applying the model, the aperture efficiency is largely independent ofelevation. We have also demonstrated that the technique can be used to measure and largely correct for thermal deformationsof the antenna, which often exceed the uncompensated gravitational deformations during daytime observing.We conclude that the aberrations induced by gravity and thermal effects are large-scale and the technique used here is particu-larly suitable for measuring such deformations in large millimetre wave radio telescopes.Key words. telescopes

Radio Recombination Lines in Galactic H II Regions

We report radio recombination line (RRL) and continuum observations of a sample of 106 Galactic H II regions made with the NRAO 140 Foot (43 m) radio telescope in Green Bank, West Virginia. We believe this to be the most sensitive RRL survey ever made for a sample this large. Most of our source integration times range between 6 and 90 hr, yielding typical rms noise levels of ~1.0-3.5 mK. Our data result from two different experiments performed, calibrated, and analyzed in similar ways. A C II survey was made at the 3.5 cm wavelength to obtain accurate measurements of carbon radio recombination lines. When combined with atomic (C I) and molecular (CO) data, these measurements will constrain the composition, structure, kinematics, and physical properties of the photodissociation regions that lie on the edges of H II regions. A second survey was made at the 3.5 cm wavelength to determine the abundance of 3He in the interstellar medium of the Milky Way. Together with measurements of the 3He+ hyperfine line, we get high-precision RRL parameters for H, 4He, and C. Here we discuss significant improvements in these data with both longer integrations and newly observed sources.

The 3He Abundance in Planetary Nebulae

We report measurements of the abundance of 3He for a sample of six Galactic planetary nebulae: IC 289, NGC 3242, NGC 6543, NGC 6720, NGC 7009, and NGC 7662. Based on observations of the 8.665 GHz hyperfine transition of 3He+, we derive 3He/H abundances ranging from ~0.1 to 1.0 × 10-3 by number. These abundances are more than an order of magnitude larger than those found in any H II region, the local interstellar medium, or the proto-solar system. If planetary nebulae are surrounded by large, low-density, ionized halos, modeling suggests that these abundances will decrease by a factor of about 2. Our source sample is highly biased in that we selected objects on the basis of several criteria that maximized the likelihood of 3He detections. The abundances are nonetheless consistent with the idea that 3He is produced in significant quantities by stars of 1-2 M☉. We conclude that there is some stellar production of 3He.

CO Isotopes in Planetary Nebulae

Standard stellar evolution theory is inconsistent with the observed isotopic carbon ratio, 12C/13C, in evolved stars. This theory is also inconsistent with the 3He/H abundance ratios observed in Galactic H II regions, when combined with chemical evolution theory. These discrepancies have been attributed to an extra, nonstandard mixing, which further processes material during the red giant branch and should lower both the 12C/13C and 3He/H abundance ratios for stars with masses ≤2 M☉. Measurements of isotopic ratios in planetary nebulae probe material that escapes the star to be further processed by future generations of stars. We have measured the carbon isotopic abundance ratio, 12C/13C, in 11 planetary nebulae (PNe) by observing the J = 2 → 1 and J = 3 → 2 millimeter transitions of 12CO and 13CO in molecular clouds associated with the PNe. A large velocity gradient (LVG) model has been used to determine the physical conditions for each PN for which both transitions have been detected. We detect both 12CO and 13CO in nine PNe. If 12CO/13CO = 12C/13C, the range of 12C/13C is 2.2-31. Our results support theories that include some form of extra mixing.

3He in the Milky Way Interstellar Medium: Summary of Relevant Observations

We summarize spectral line and continuum observations of a sample of 21 Galactic H II regions made at 3.46 cm wavelength during the past 14 years as part of a program to measure the abundance of 3He in the Milky Way interstellar medium. Converting the observed 3He+ column density into an abundance ratio relative to hydrogen depends on the density and ionization structure of each nebula. The observations needed to model our sources were made with the NRAO 140 foot (43 m), NRAO VLA, and MPIfR 100 m telescopes. Here we describe and compile our measurements of the 3He+ emission line as well as those of various recombination transitions of H, He, and C. We also summarize the continuum database which includes single-dish observations and interferometer maps.

The 3He Abundance in the Planetary Nebula NGC 3242

Using the NRAO 140 Foot Telescope, we have confirmed our earlier detection of 3He in the planetary nebula NGC 3242 made with the 100 m radio telescope of the Max-Planck-Institut fur Radioastronomie. Both spectrometers show that the observed 3He+ hyperfine line has a double-peaked structure. This line shape naturally arises if NGC 3242 is modeled as a dense core surrounded by a very low density expanding shell. The observed 3He+ emission suggests an abundance by number of 3He/H = (2–5) × 10-4, which is more than an order of magnitude higher than typically found in Galactic H II regions.

The Detection of 3He+ in a Planetary Nebula Using the VLA

We used the VLA to search for 3He+ emission from two Galactic planetary nebulae (PNe): NGC 6572 and J320. Standard stellar models predict that the 3He/H abundance ratios for PNe should be 1-2 orders of magnitude higher than the primordial value (3He/H ~ 10-5 by number) determined from Galactic H II region abundances and confirmed by WMAP cosmic microwave background results. Chemical evolution models suggest that fewer than 5% of all PNe enrich the interstellar medium (ISM) with 3He at the level of standard stellar models. Our target PNe are therefore anomalous in that they were selected from a sample deliberately biased to contain objects with properties that maximized the likelihood of a 3He detection by the VLA. We have detected the 8.665 GHz hyperfine 3He+ transition in J320 at the 4 σ level. The 3He/H abundance ratio is 1.9 × 10-3 with roughly a factor of 2 uncertainty. For NGC 6572 we find an upper limit of 3He/H 10-3. This detection of 3He in J320 makes it the second PN known to have an anomalously high 3He abundance, confirming that at least some low-mass stars produce significant amounts of 3He that survives to the PN stage and enriches the ISM.