17 research outputs found
Cataclysmic Variables and Other Compact Binaries in the Globular Cluster NGC 362: Candidates from Chandra and HST
Highly sensitive and precise X-ray imaging from Chandra, combined with the
superb spatial resolution of HST optical images, dramatically enhances our
empirical understanding of compact binaries such as cataclysmic variables and
low mass X-ray binaries, their progeny, and other stellar X-ray source
populations deep into the cores of globular clusters. Our Chandra X-ray images
of the globular cluster NGC 362 reveal 100 X-ray sources, the bulk of which are
likely cluster members. Using HST color-magnitude and color-color diagrams, we
quantitatively consider the optical content of the NGC 362 Chandra X-ray error
circles, especially to assess and identify the compact binary population in
this condensed-core globular cluster. Despite residual significant crowding in
both X-rays and optical, we identify an excess population of H{\alpha}-emitting
objects that is statistically associated with the Chandra X-ray sources. The
X-ray and optical characteristics suggest that these are mainly cataclysmic
variables, but we also identify a candidate quiescent low mass X-ray binary. A
potentially interesting and largely unanticipated use of observations such as
these may be to help constrain the macroscopic dynamic state of globular
clusters.Comment: 6 pages, 6 figures, to appear in the proceedings of the conference
"Binary Star Evolution: Mass Loss, Accretion, and Mergers," Mykonos, Greece,
June 22-25, 201
Properties of the ionized gas in HH202. II: Results from echelle spectrophotometry with UVES
We present results of deep echelle spectrophotometry of the brightest knot of
the HH202 in the Orion Nebula --HH202-S-- using the ultraviolet Visual Echelle
Spectrograph (UVES). The high spectral resolution has permitted to separate the
component associated with the ambient gas from that associated with the gas
flow. We derive electron densities and temperatures for both components, as
well as the chemical abundances of several ions and elements from collisionally
excited lines, including the first determinations of Ca^{+} and Cr^{+}
abundances in the Orion Nebula. We also calculate the He^{+}, C^{2+}, O^{+} and
O^{2+} abundances from recombination lines. The difference between the O^{2+}
abundances determined from collisionally excited and recombination lines --the
so-called abundance discrepancy factor-- is 0.35 dex and 0.11 dex for the shock
and nebular components, respectively. Assuming that the abundance discrepancy
is produced by spatial variations in the electron temperature, we derive values
of the temperature fluctuation parameter, t^2, of 0.050 and 0.016, for the
shock and nebular components, respectively. Interestingly, we obtain almost
coincident t^2 values for both components from the analysis of the intensity
ratios of He I lines. We find significant departures from case B predictions in
the Balmer and Paschen flux ratios of lines of high principal quantum number n.
We analyze the ionization structure of HH202-S, finding enough evidence to
conclude that the flow of HH202-S has compressed the ambient gas inside the
nebula trapping the ionization front. We measure a strong increase of the total
abundances of nickel and iron in the shock component, the abundance pattern and
the results of photoionization models for both components are consistent with
the partial destruction of dust after the passage of the shock wave in HH202-S.Comment: 23 pages, 7 figures. Accepted for publication in MNRA
Iron abundance in HII regions
Optical CCD spectra are used to determine the Fe abundances at several
positions inside seven bright Galactic HII regions. The observed [FeIII] line
ratios are compared with the predictions of different sets of collision
strengths and transition probabilities for this ion to select the atomic data
providing the best fit to the observations. The values found for the Fe++ and
Fe+ abundances, along with ionization correction factors for the contribution
of Fe3+, obtained from available grids of photoionized models, imply that the
Fe/O ratio in the ionized gas is between 2% and 30% of solar. The Fe abundances
derived for each area are correlated both with the degree of ionization and the
colour excess. A possible explanation is suggested, namely the presence of a
population of small grains, probably originating from the fragmentation of
larger grains. These small grains would release Fe atoms into the gas after the
absorption of energetic photons; the small grains surviving this destruction
process would be swept out of the ionized region by the action of radiation
pressure or stellar winds. An indication of a further and more efficient
destruction agent is given by the high Fe abundance derived for a position
sampling the optical jet H399 in M20, where dust destruction due to shock waves
has presumably taken place.Comment: A&A, accepted for publication, 12 page
A high-altitude balloon platform for determining exchange of carbon dioxide over agricultural landscapes
The exchange of carbon dioxide between the terrestrial biosphere
and the atmosphere is a key process in the global carbon cycle. Given
emissions from fossil fuel combustion and the appropriation of net primary
productivity by human activities, understanding the carbon dioxide exchange
of cropland agroecosystems is critical for evaluating future trajectories of
climate change. In addition, human manipulation of agroecosystems has been
proposed as a technique of removing carbon dioxide from the atmosphere via
practices such as no-tillage and cover crops. We propose a novel method of
measuring the exchange of carbon dioxide over croplands using a high-altitude
balloon (HAB) platform. The HAB methodology measures two sequential vertical
profiles of carbon dioxide mixing ratio, and the surface exchange is
calculated using a fixed-mass column approach. This methodology is relatively
inexpensive, does not rely on any assumptions besides spatial homogeneity (no
horizontal advection) and provides data over a spatial scale between
stationary flux towers and satellite-based inversion calculations. The HAB
methodology was employed during the 2014 and 2015 growing seasons in central
Illinois, and the results are compared to satellite-based NDVI values and a
flux tower located relatively near the launch site in Bondville, Illinois.
These initial favorable results demonstrate the utility of the methodology
for providing carbon dioxide exchange data over a large (10â100âŻkm) spatial
area. One drawback is its relatively limited temporal coverage. While
recruiting citizen scientists to perform the launches could provide a more
extensive dataset, the HAB methodology is not appropriate for providing
estimates of net annual carbon dioxide exchange. Instead, a HAB dataset could
provide an important check for upscaling flux tower results and verifying
satellite-derived exchange estimates