Untangling the Recombination Line Emission from HII Regions with Multiple Velocity Components

HII regions are the ionized spheres surrounding high-mass stars. They are ideal targets for tracing Galactic structure because they are predominantly found in spiral arms and have high luminosities at infrared and radio wavelengths. In the Green Bank Telescope HII Region Discovery Survey (GBT HRDS) we found that >30% of first Galactic quadrant HII regions have multiple hydrogen radio recombination line (RRL) velocities, which makes determining their Galactic locations and physical properties impossible. Here we make additional GBT RRL observations to determine the discrete HII region velocity for all 117 multiple-velocity sources within 18deg. < l < 65deg. The multiple-velocity sources are concentrated in the zone 22deg. < l < 32deg., coinciding with the largest regions of massive star formation, which implies that the diffuse emission is caused by leaked ionizing photons. We combine our observations with analyses of the electron temperature, molecular gas, and carbon recombination lines to determine the source velocities for 103 discrete H II regions (88% of the sample). With the source velocities known, we resolve the kinematic distance ambiguity for 47 regions, and thus determine their heliocentric distances.Comment: 44 pages, 5 figures, 16 pages of tables; Accepted by ApJ

The Green Bank Telescope H II Region Discovery Survey: IV. Helium and Carbon Recombination Lines

The Green Bank Telescope H II Region Discovery Survey (GBT HRDS) found hundreds of previously unknown Galactic regions of massive star formation by detecting hydrogen radio recombination line (RRL) emission from candidate H II region targets. Since the HRDS nebulae lie at large distances from the Sun, they are located in previously unprobed zones of the Galactic disk. Here we derive the properties of helium and carbon RRL emission from HRDS nebulae. Our target sample is the subset of the HRDS that has visible helium or carbon RRLs. This criterion gives a total of 84 velocity components (14% of the HRDS) with helium emission and 52 (9%) with carbon emission. For our highest quality sources, the average ionic He-4+/H+ abundance ratio by number, , is 0.068 +/- 0.023 (1-sigma). This is the same ratio as that measured for the sample of previously known Galactic H II regions. Nebulae without detected helium emission give robust y+ upper limits. There are 5 RRL emission components with y+ less than 0.04 and another 12 with upper limits below this value. These H II regions must have either a very low He-4 abundance or contain a significant amount of neutral helium. The HRDS has 20 nebulae with carbon RRL emission but no helium emission at its sensitivity level. There is no correlation between the carbon RRL parameters and the 8 microns mid-infrared morphology of these nebulae.Comment: Accepted to ApJ. The survey website can be found here: http://go.nrao.edu/hrd

Diffuse Ionized Gas in the Milky Way Disk

We analyze the diffuse ionized gas (DIG) in the first Galactic quadrant from l=18deg to 40deg using radio recombination line (RRL) data from the Green Bank Telescope. These data allow us to distinguish DIG emission from HII region emission and thus study the diffuse gas essentially unaffected by confusion from discrete sources. We find that the DIG has two dominant velocity components, one centered around 100km/s associated with the luminous HII region W43, and the other centered around 45km/s not associated with any large HII region. Our analysis suggests that the two velocity components near W43 may be caused by non-circular streaming motions originating near the end of the Galactic bar. At lower Galactic longitudes, the two velocities may instead arise from gas at two distinct distances from the Sun, with the most likely distances being ~6kpc for the 100km/s component and ~12kpc for the 45km/s component. We show that the intensity of diffuse Spitzer GLIMPSE 8.0um emission caused by excitation of polyaromatic hydrocarbons (PAHs) is correlated with both the locations of discrete HII regions and the intensity of the RRL emission from the DIG. This implies that the soft ultra-violet photons responsible for creating the infrared emission have a similar origin as the harder ultra-violet photons required for the RRL emission. The 8.0um emission increases with RRL intensity but flattens out for directions with the most intense RRL emission, suggesting that PAHs are partially destroyed by the energetic radiation field at these locations.Comment: Accepted for publication in ApJ (16 pages, 11 figures, 2 tables

Diffuse Ionized Gas in the Milky Way Disk

We analyze the diffuse ionized gas (DIG) in the first Galactic quadrant from l=18deg to 40deg using radio recombination line (RRL) data from the Green Bank Telescope. These data allow us to distinguish DIG emission from HII region emission and thus study the diffuse gas essentially unaffected by confusion from discrete sources. We find that the DIG has two dominant velocity components, one centered around 100km/s associated with the luminous HII region W43, and the other centered around 45km/s not associated with any large HII region. Our analysis suggests that the two velocity components near W43 may be caused by non-circular streaming motions originating near the end of the Galactic bar. At lower Galactic longitudes, the two velocities may instead arise from gas at two distinct distances from the Sun, with the most likely distances being ~6kpc for the 100km/s component and ~12kpc for the 45km/s component. We show that the intensity of diffuse Spitzer GLIMPSE 8.0um emission caused by excitation of polyaromatic hydrocarbons (PAHs) is correlated with both the locations of discrete HII regions and the intensity of the RRL emission from the DIG. This implies that the soft ultra-violet photons responsible for creating the infrared emission have a similar origin as the harder ultra-violet photons required for the RRL emission. The 8.0um emission increases with RRL intensity but flattens out for directions with the most intense RRL emission, suggesting that PAHs are partially destroyed by the energetic radiation field at these locations.Comment: Accepted for publication in ApJ (16 pages, 11 figures, 2 tables

HII Region Ionization of the Interstellar Medium: A Case Study of NGC 7538

Using data from the Green Bank Telescope, we analyze the radio continuum (free-free) and radio recombination line (RRL) emission of the compact HII region NGC 7538 (Sharpless 158). We detect extended radio continuum and hydrogen RRL emission beyond the photodissociation region (PDR) toward the north and east, but a sharp decrease in emission toward the south and west. This indicates that a non-uniform PDR morphology is affecting the amount of radiation "leaking" through the PDR. The strongest carbon RRL emission is found in the western PDR that appears to be dense. We compute a leaking fraction $f_R = 15 \pm 5$ % of the radio continuum emission measured in the plane of the sky which represents a lower limit when accounting for the three-dimensional geometry of the region. We detect an average $^4\textrm{He}^+/\textrm{H}^+$ abundance ratio by number of $0.088 \pm 0.003$ inside the HII region and a decrease in this ratio with increasing distance from the region beyond the PDR. Using Herschel Space Observatory data, we show that small dust temperature enhancements to the north and east of NGC 7538 coincide with extended radio emission, but that the dust temperature enhancements are mostly contained within a second PDR to the east. Unlike the giant HII region W43, the radiation leaking from NGC 7538 seems to only affect the local ambient medium. This suggests that giant HII regions may have a large effect in maintaining the ionization of the interstellar medium.Comment: Accepted for publication in ApJ (15 pages, 10 figures, 2 tables

High-Mass Star Formation in the Outer Scutum-Centaurus Arm

The Outer Scutum-Centaurus (OSC) spiral arm is the most distant molecular spiral arm in the Milky Way, but until recently little was known about this structure. Discovered by Dame and Thaddeus (2011), the OSC lies $\sim$15 kpc from the Galactic Center. Due to the Galactic warp, it rises to nearly 4$^{\circ}$ above the Galactic Plane in the first Galactic quadrant, leaving it unsampled by most Galactic plane surveys. Here we observe HII region candidates spatially coincident with the OSC using the Very Large Array to image radio continuum emission from 65 targets and the Green Bank Telescope to search for ammonia and water maser emission from 75 targets. This sample, drawn from the WISE Catalog of Galactic HII Regions, represents every HII region candidate near the longitude-latitude (l,v) locus of the OSC. Coupled with their characteristic mid-infrared morphologies, detection of radio continuum emission strongly suggests that a target is a bona fide HII region. Detections of associated ammonia or water maser emission allow us to derive a kinematic distance and determine if the velocity of the region is consistent with that of the OSC. Nearly 60% of the observed sources were detected in radio continuum, and over 20% have ammonia or water maser detections. The velocities of these sources mainly place them beyond the Solar orbit. These very distant high-mass stars have stellar spectral types as early as O4. We associate high-mass star formation at 2 new locations with the OSC, increasing the total number of detected HII regions in the OSC to 12.Comment: 14 pages text and tables + 10 pages supplemental figure

A Green Bank Telescope Survey of Large Galactic HII Regions

As part of our ongoing HII Region Discovery Survey (HRDS), we report the Green Bank Telescope detection of 148 new angularly-large Galactic HII regions in radio recombination line (RRL) emission. Our targets are located at a declination greater than -45deg., which corresponds to 266deg. > l > -20deg. at b = 0deg. All sources were selected from the WISE Catalog of Galactic HII Regions, and have infrared angular diameters >260''. The Galactic distribution of these "large" HII regions is similar to that of the previously-known sample of Galactic HII regions. The large HII region RRL line width and peak line intensity distributions are skewed toward lower values compared with that of previous HRDS surveys. We discover 7 sources with extremely narrow RRLs <10 km/s. If half the line width is due to turbulence, these 7 sources have thermal plasma temperatures <1100 K. These temperatures are lower than any measured for Galactic HII regions, and the narrow line components may arise instead from partially ionized zones in the HII region photo-dissociation regions. We discover G039.515+00.511, one of the most luminous HII regions in the Galaxy. We also detect the RRL emission from three HII regions with diameters >100 pc, making them some of the physically largest known HII regions in the Galaxy. This survey completes the HRDS HII region census in the Northern sky, where we have discovered 887 HII regions and more than doubled the previously-known census of Galactic HII regions.Comment: Accepted in ApJ

Hydrogen Radio Recombination Line Emission from M51 and NGC628

We report the discovery of hydrogen radio recombination line (RRL) emission from two galaxies with star formation rates (SFRs) similar to that of the Milky Way: M51 and NGC628. We use the Green Bank Telescope (GBT) to measure $\sim$15 Hn$\alpha$ recombination transitions simultaneously and average these data to improve our spectral signal-to-noise ratio. We show that our data can be used to estimate the total ionizing photon flux of these two sources, and we derive their SFRs within the GBT beam: $\Psi_{\rm OB} = 3.46$ M$_{\odot}$ yr$^{-1}$ for M51 and $\Psi_{\rm OB} = 0.56$ M$_{\odot}$ yr$^{-1}$ for NGC628. Here, we demonstrate that it is possible to detect RRLs from normal galaxies that are not undergoing a starburst with current instrumentation and reasonable integration times ($\sim$12 hr for each source). We also show that we can characterize the overall star-forming properties of M51 and NGC628, although the GBT beam cannot resolve individual HII region complexes. Our results suggest that future instruments, such as the Square Kilometre Array and the Next Generation Very Large Array, will be able to detect RRL emission from a multitude of Milky Way-like galaxies, making it possible to determine SFRs of normal galaxies unaffected by extinction and to measure global star formation properties in the local universe.Comment: 8 pages, 1 figure, 3 tables; Accepted for publication in PAS

Do All Low-Mass Stars Undergo Extra Mixing Processes?

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 Msun), with peak abundances of 3He/H ~ few x 10-3 by number. Over the life-time of the Galaxy, this ought to produce 3He/H abundances that diminish with increasing Galactocentric radius. Observations of 3He+ in HII regions throughout the Galactic disk, however, reveal very little variation in the 3He abundance with values of 3He/H similar to the primoridal abundance, (3He/H)p ~ 10-5 . 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 J320 (PN G190.3-17.7) with the Jansky Very Large Array (JVLA) to confirm a previous 3He+ detection made with the VLA 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 microJy/beam after smoothing the data to a velocity resolution of 11.4 km/s . We estimate an abundance limit of 3He/H <= 2.75 x 10-3 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.Comment: Accepted for publication in the Ap