1,207 research outputs found
The discrepancy in G-band contrast: Where is the quiet Sun?
We compare the rms contrast in observed speckle reconstructed G-band images
with synthetic filtergrams computed from two magneto-hydrodynamic simulation
snapshots. The observations consist of 103 bursts of 80 frames each taken at
the Dunn Solar Telescope (DST), sampled at twice the diffraction limit of the
telescope. The speckle reconstructions account for the performance of the
Adaptive Optics (AO) system at the DST to supply reliable photometry. We find a
considerable discrepancy in the observed rms contrast of 14.1% for the best
reconstructed images, and the synthetic rms contrast of 21.5% in a simulation
snapshot thought to be representative of the quiet Sun. The areas of features
in the synthetic filtergrams that have positive or negative contrast beyond the
minimum and maximum values in the reconstructed images have spatial scales that
should be resolved. This leads us to conclude that there are fundamental
differences in the rms G-band contrast between observed and computed
filtergrams. On the basis of the substantially reduced granular contrast of
16.3% in the synthetic plage filtergram we speculate that the quiet-Sun may
contain more weak magnetic field than previously thought.Comment: 16 pages, 8 figure
Freshly ionized matter around the final Helium shell flash object V4334 Sgr (Sakurai's object)
We report on the discovery of recently ionized hydrogen-deficient gas in the
immediate circumstellar environment of the final helium shell flash star V4334
Sgr (Sakurai's object). On spectra obtained with FORS2 multi-object
spectroscopy we have found spatially extended (about 2") emission from [N II],
[O I], [O II] and very faint Halpha and [S II]. In the [N II] (ll6548,83) lines
we have identified two components located at velocities -350 +/-50 and +200
+/-50 km/s, relative to V4334 Sgr itself. The full width of the [N II] l6583
feature at zero intensity corresponds to a velocity spread of about 1500 km/s.
Based on the available data it is not possible to conclusively determine the
mechanism of ionization. Both photo-ionization, from a rapidly evolving central
star, and shock excitation, as the result of the collision of the fast ouflows
with slower circumstellar matter, could account for the observed lines. The
central star is still hidden behind strong dust absorption, since only a faint
highly reddened continuum is apparent in the spectra. Theory states that it
will become hotter and will retrace its post-asymptotic giant branch evolution
towards the planetary nebula domain. Our detection of the ionized ejecta from
the very late helium shell flash marks the beginning of a new phase in this
star's amazingly rapid evolution.Comment: 11 pages, 2 figures. Accepted by ApJ
Very Large Excesses of 18O in Hydrogen-Deficient Carbon and R Coronae Borealis Stars: Evidence for White Dwarf Mergers
We have found that at least seven hydrogen-deficient carbon (HdC) and R
Coronae Borealis (RCB) stars, have 16O/18O ratios close to and in some cases
less than unity, values that are orders of magnitude lower than measured in
other stars (the Solar value is 500). Greatly enhanced 18O is evident in every
HdC and RCB we have measured that is cool enough to have detectable CO bands.
The three HdC stars measured have 16O/18O < 1, lower values than any of the RCB
stars. These discoveries are important clues in determining the evolutionary
pathways of HdC and RCB stars, for which two models have been proposed: the
double degenerate (white dwarf (WD) merger), and the final helium-shell flash
(FF). No overproduction of 18O is expected in the FF scenario. We have
quantitatively explored the idea that HdC and RCB stars originate in the
mergers of CO- and He-WDs. The merger process is estimated to take only a few
days, with accretion rates of 150 Msun/ yr producing temperatures at the base
of the accreted envelope of 1.2 - 1.9 x 10^8 K. Analysis of a simplified
one-zone calculation shows that nucleosynthesis in the dynamically accreting
material may provide a suitable environment for a significant production of
18O, leading to very low values of 16O/18O, similar to those observed. We also
find qualitative agreement with observed values of 12C/13C and with the CNO
elemental ratios. H-admixture during the accretion process from the small
H-rich C/O WD envelope may play an important role in producing the observed
abundances. Overall our analysis shows that WD mergers may very well be the
progenitors of O18-rich RCB and HdC stars, and that more detailed simulations
and modeling are justified.Comment: 29 pages, 6 figure
HE0107-5240, A Chemically Ancient Star.I. A Detailed Abundance Analysis
We report a detailed abundance analysis for HE0107-5240, a halo giant with
[Fe/H]_NLTE=-5.3. This star was discovered in the course of follow-up
medium-resolution spectroscopy of extremely metal-poor candidates selected from
the digitized Hamburg/ESO objective-prism survey. On the basis of
high-resolution VLT/UVES spectra, we derive abundances for 8 elements (C, N,
Na, Mg, Ca, Ti, Fe, and Ni), and upper limits for another 12 elements. A
plane-parallel LTE model atmosphere has been specifically tailored for the
chemical composition of {\he}. Scenarios for the origin of the abundance
pattern observed in the star are discussed. We argue that HE0107-5240 is most
likely not a post-AGB star, and that the extremely low abundances of the
iron-peak, and other elements, are not due to selective dust depletion. The
abundance pattern of HE0107-5240 can be explained by pre-enrichment from a
zero-metallicity type-II supernova of 20-25M_Sun, plus either self-enrichment
with C and N, or production of these elements in the AGB phase of a formerly
more massive companion, which is now a white dwarf. However, significant radial
velocity variations have not been detected within the 52 days covered by our
moderate-and high-resolution spectra. Alternatively, the abundance pattern can
be explained by enrichment of the gas cloud from which HE0107-5240 formed by a
25M_Sun first-generation star exploding as a subluminous SNII, as proposed by
Umeda & Nomoto (2003). We discuss consequences of the existence of HE0107-5240
for low-mass star formation in extremely metal-poor environments, and for
currently ongoing and future searches for the most metal-poor stars in the
Galaxy.Comment: 60 pages, 16 figures. Accepted for publication in Ap
Solar Carbon Monoxide, Thermal Profiling, and the Abundances of C, O, and their Isotopes
A solar photospheric "thermal profiling" analysis is presented, exploiting
the infrared rovibrational bands of carbon monoxide (CO) as observed with the
McMath-Pierce Fourier transform spectrometer (FTS) at Kitt Peak, and from above
the Earth's atmosphere by the Shuttle-borne ATMOS experiment. Visible continuum
intensities and center-limb behavior constrained the temperature profile of the
deep photosphere, while CO center-limb behavior defined the thermal structure
at higher altitudes. The oxygen abundance was self consistently determined from
weak CO absorptions. Our analysis was meant to complement recent studies based
on 3-D convection models which, among other things, have revised the historical
solar oxygen (and carbon) abundance downward by a factor of nearly two;
although in fact our conclusions do not support such a revision. Based on
various considerations, an oxygen abundance of 700+/-100 ppm (parts per million
relative to hydrogen) is recommended; the large uncertainty reflects the model
sensitivity of CO. New solar isotopic ratios also are reported for 13C, 17O,
and 18O.Comment: 90 pages, 19 figures (some with parts "a", "b", etc.); to be
published in the Astrophysical Journal Supplement
An Elemental Assay of Very, Extremely, and Ultra Metal-Poor Stars
We present a high-resolution elemental-abundance analysis for a sample of 23
very metal-poor (VMP; [Fe/H] < -2.0) stars, 12 of which are extremely
metal-poor (EMP; [Fe/H] < -3.0), and 4 of which are ultra metal-poor (UMP;
[Fe/H] < -4.0). These stars were targeted to explore differences in the
abundance ratios for elements that constrain the possible astrophysical sites
of element production, including Li, C, N, O, the alpha-elements, the iron-peak
elements, and a number of neutron-capture elements. This sample substantially
increases the number of known carbon-enhanced metal-poor (CEMP) and
nitrogen-enhanced metal-poor (NEMP) stars -- our program stars include eight
that are considered "normal" metal-poor stars, six CEMP-no stars, five CEMP-s
stars, two CEMP-r stars, and two CEMP-r/s stars. One of the CEMP- stars and
one of the CEMP-r/s stars are possible NEMP stars. We detect lithium for three
of the six CEMP-no stars, all of which are Li-depleted with respect to the
Spite plateau. The majority of the CEMP stars have [C/N] > 0. The stars with
[C/N] < 0 suggest a larger degree of mixing; the few CEMP-no stars that exhibit
this signature are only found at [Fe/H] < -3.4, a metallicity below which we
also find the CEMP-no stars with large enhancements in Na, Mg, and Al. We
confirm the existence of two plateaus in the absolute carbon abundances of CEMP
stars, as suggested by Spite et al. We also present evidence for a "floor" in
the absolute Ba abundances of CEMP-no stars at A(Ba)~ -2.0.Comment: 20 pages, 16 figures, Accepted for publication in Ap
The age and abundance structure of the stellar populations in the central sub-kpc of the Milky Way
The four main findings about the age and abundance structure of the Milky Way
bulge based on microlensed dwarf and subgiant stars are: (1) a wide metallicity
distribution with distinct peaks at [Fe/H]=-1.09, -0.63, -0.20, +0.12, +0.41;
(2) a high fraction of intermediate-age to young stars where at [Fe/H]>0 more
than 35 % are younger than 8 Gyr, (3) several episodes of significant star
formation in the bulge 3, 6, 8, and 11 Gyr ago; (4) the `knee' in the
alpha-element abundance trends of the sub-solar metallicity bulge appears to be
located at a slightly higher [Fe/H] (about 0.05 to 0.1 dex) than in the local
thick disk.Comment: 4 pages, contributed talk at the IAU Symposium 334 "Rediscovering our
Galaxy" in Potsdam, July 10-14, 201
Chemical Abundances from the Continuum
The calculation of solar absolute fluxes in the near-UV is revisited,
discussing in some detail recent updates in theoretical calculations of
bound-free opacity from metals. Modest changes in the abundances of elements
such as Mg and the iron-peak elements have a significant impact on the
atmospheric structure, and therefore self-consistent calculations are
necessary. With small adjustments to the solar photospheric composition, we are
able to reproduce fairly well the observed solar fluxes between 200 and 270 nm,
and between 300 and 420 nm, but find too much absorption in the 270-290 nm
window. A comparison between our reference 1D model and a 3D time-dependent
hydrodynamical simulation indicates that the continuum flux is only weakly
sensitive to 3D effects, with corrections reaching <10% in the near-UV, and <2%
in the optical.Comment: 10 pages, 5 figures, to appear in the proceedings of the conference A
Stellar Journey, a symposium in celebration of Bengt Gustafsson's 65th
birthday, June 23-27, 2008, Uppsal
A Sr-Rich Star on the Main Sequence of Omega Centauri
Abundance ratios relative to iron for carbon, nitrogen, strontium and barium
are presented for a metal-rich main sequence star ([Fe/H]=--0.74) in the
globular cluster omega Centauri. This star, designated 2015448, shows depleted
carbon and solar nitrogen, but more interestingly, shows an enhanced abundance
ratio of strontium [Sr/Fe] ~ 1.6 dex, while the barium abundance ratio is
[Ba/Fe]<0.6 dex. At this metallicity one usually sees strontium and barium
abundance ratios that are roughly equal. Possible formation scenarios of this
peculiar object are considered.Comment: 13 pages, 3 figures. Accepted to ApJ
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