47 research outputs found
The Metallicity-Electron Temperature Relationship in HII Regions
HII region heavy-element abundances throughout the Galactic disk provide
important constraints to theories of the formation and evolution of the Milky
Way. In LTE, radio recombination line (RRL) and free-free continuum emission
are accurate extinction-free tracers of the HII region electron temperature.
Since metals act as coolants in HII regions via the emission of collisionally
excited lines, the electron temperature is a proxy for metallicity. Shaver et
al. found a linear relationship between metallicity and electron temperature
with little scatter. Here, we use CLOUDY HII region simulations to (1)
investigate the accuracy of using RRLs to measure the electron temperature; and
(2) explore the metallicity-electron temperature relationship. We model 135 HII
regions with different ionizing radiation fields, densities, and metallicities.
We find that electron temperatures derived under the assumption of LTE are
about 20% systematically higher due to non-LTE effects, but overall LTE is a
good assumption for cm-wavelength RRLs. Our CLOUDY simulations are consistent
with the Shaver et al. metallicity-electron temperature relationship but there
is significant scatter since earlier spectral types or higher electron
densities yield higher electron temperatures. Using RRLs to derive electron
temperatures assuming LTE yields errors in the predicted metallicity as large
as 10%. We derive correction factors for Log(O/H) + 12 in each CLOUDY
simulation. For lower metallicities the correction factor depends primarily on
the spectral-type of the ionizing star and range from 0.95 to 1.10, whereas for
higher metallicities the correction factor depends on the density and is
between 0.97 and 1.05.Comment: Accepted in Ap
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
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 % 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
abundance ratio by number of 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
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
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