8,217 research outputs found
QSO 0130-4021: A third QSO showing a low Deuterium to Hydrogen Abundance Ratio
We have discovered a third quasar absorption system which is consistent with
a low deuterium to hydrogen abundance ratio, D/H = 3.4 times 10^-5. The z ~ 2.8
partial Lyman limit system towards QSO 0130-4021 provides the strongest
evidence to date against large D/H ratios because the H I absorption, which
consists of a single high column density component with unsaturated high order
Lyman series lines, is readily modeled -- a task which is more complex in other
D/H systems. We have obtained twenty-two hours of spectra from the HIRES
spectrograph on the W.M. Keck telescope, which allow a detailed description of
the Hydrogen. We see excess absorption on the blue wing of the H I Lyman alpha
line, near the expected position of Deuterium. However, we find that Deuterium
cannot explain all of the excess absorption, and hence there must be
contamination by additional absorption, probably H I. This extra H I can
account for most or all of the absorption at the D position, and hence D/H = 0
is allowed. We find an upper limit of D/H < 6.7 times 10^-5 in this system,
consistent with the value of D/H ~ 3.4 times 10^-5 deduced towards QSO
1009+2956 and QSO 1937-1009 by Burles and Tytler (1998a, 1998b). This
absorption system shows only weak metal line absorption, and we estimate [Si/H]
< -2.6 -- indicating that the D/H ratio of the system is likely primordial. All
four of the known high redshift absorption line systems simple enough to
provide useful limits on D are consistent with D/H = 3.4 +/- 0.25 times 10^-5.
Conversely, this QSO provides the third case which is inconsistent with much
larger values.Comment: 18 pages, 5 figures, submitted to Ap
Polymorphism in B-DNA: X-ray diffraction studies on Li-DNA fibres
From X-ray diffraction studies it is generally believed that B-DNA has the structural parameters n=10 and h=3.4 Ă
. However, for the first time we report that polymorphism in the B-form can be observed in DNA fibres. This was achieved by the precise control of salt and humidity in fibres and by the application of the precession method of X-ray diffraction to DNA fibres. The significant result obtained is that n=10 is not observed for crystalline fibre patterns. In fact, n=10 and h=3.4 Ă
are not found to occur simultaneously. Instead, a range of values, n=9.6-10.0 and h=3.35 Ă
-3.41 Ă
is observed
Extreme Enhancements of r-process Elements in the Cool Metal-Poor Main-Sequence Star SDSS J2357-0052
We report the discovery of a cool metal-poor, main-sequence star exhibiting
large excesses of r-process elements. This star is one of two newly discovered
cool subdwarfs (effective temperatures of 5000 K) with extremely low
metallicity ([Fe/H]<-3) identified from follow-up high-resolution spectroscopy
of metal-poor candidates from the Sloan Digital Sky Survey. SDSS J2357-0052 has
[Fe/H]=-3.4 and [Eu/Fe]=+1.9, and exhibits a scaled solar r-process abundance
pattern of heavy neutron-capture elements. This is the first example of an
extremely metal-poor, main-sequence star showing large excesses of r-process
elements; all previous examples of the large r-process-enhancement phenomena
have been associated with metal-poor giants. The metallicity of this object is
the lowest, and the excess of Eu ([Eu/Fe]) is the highest, among the
r-process-enhanced stars found so far. We consider possible scenarios to
account for the detection of such a star, and discuss techniques to enable
searches for similar stars in the future.Comment: 16 pages, 3 figures, 2 tables, ApJL in pres
Carbon-enhanced metal-poor stars: the most pristine objects?
Carbon-enhanced metal poor stars (CEMP) form a significant proportion of the
metal-poor stars, their origin is not well understood. Three very metal-poor
C-rich turnoff stars were selected from the SDSS survey, observed with the ESO
VLT (UVES) to precisely determine the element abundances. In turnoff stars
(unlike giants) the carbon abundance has not been affected by mixing with deep
layers and is therefore easier to interpret. The analysis was performed with 1D
LTE static model atmospheres. When available, non-LTE corrections were applied
to the classical LTE abundances. The 3D effects on the CH and CN molecular
bands were computed using hydrodynamical simulations of the stellar atmosphere
(CO5BOLD) and are found to be very important. To facilitate a comparison with
previous results, only 1D abundances are used in the discussion. The abundances
(or upper limits) of the elements enable us to place these stars in different
CEMP classes. The carbon abundances confirm the existence of a plateau at A(C)=
8.25 for [Fe/H] \geq -3.4. The most metal-poor stars ([Fe/H] < -3.4) have
significantly lower carbon abundances, suggesting a lower plateau at A(C)
\approx 6.5. Detailed analyses of a larger sample of very low metallicity
carbon-rich stars are required to confirm (or refute) this possible second
plateau and specify the behavior of the CEMP stars at very low metallicity
Chemical compositions of six metal-poor stars in the ultra-faint dwarf spheroidal galaxy Bo\"otes I
Ultra-faint dwarf galaxies recently discovered around the Milky Way (MW)
contain extremely metal-poor stars, and might represent the building blocks of
low-metallicity components of the MW. Among them, the Bo\"otes I dwarf
spheroidal galaxy is of particular interest because of its exclusively old
stellar population. We determine chemical compositions of six red giant stars
in Bo\"otes I, based on the high-resolution spectra obtained with the High
Dispersion Spectrograph mounted on the Subaru Telescope. Abundances of 12
elements, including C, Na, alpha, Fe-peak, and neutron capture elements, were
determined for the sample stars. The abundance results were compared to those
in field MW halo stars previously obtained using an abundance analysis
technique similar to the present study. We confirm the low metallicity of
Boo-094 ([Fe/H]=-3.4). Except for this star, the abundance ratios ([X/Fe]) of
elements lighter than zinc are generally homogeneous with small scatter around
the mean values in the metallicities spanned by the other five stars
(-2.7-2.7 show
no significant enhancement of carbon. The [Mg/Fe] and [Ca/Fe] ratios are almost
constant with a modest decreasing trend with increasing [Fe/H] and are slightly
lower than the field halo stars. The [Sr/Fe] and [Sr/Ba] ratios also tend to be
lower in the Bo\"otes I stars than in the halo stars. Our results of small
scatter in the [X/Fe] ratios for elements lighter than zinc suggest that these
abundances were homogeneous among the ejecta of prior generation(s) of stars in
this galaxy.Comment: 16 pages, 12 figures. Accepted to A&A, language correcte
A long look at the BALQSO LBQS 2212-1759 with XMM-Newton
Very long (172 ks effective exposure time) observations of the BALQSO LBQS
2212-1759 with XMM-Newton yield a stringent upper-limit on its 0.2-10 keV
(rest- frame 0.64-32.2 keV) flux, F < 6 E-17 erg/cm2/s, while simultaneous UV
and optical observations reveal a rather blue spectrum extending to 650 A in
the source rest frame. These results are used to set a tight upper-limit on its
optical to X-ray spectral index alpha_{ox} < -2.56. Given the HI-BAL nature of
LBQS 212-1759, its X-ray weakness is most likely due to intrinsic absorption.
If this is the case, and assuming that the intrinsic alpha_{ox} of LBQS
2212-1759 is -1.63 - a value appropriate for a radio-quiet quasar of this
luminosity - one can set a lower limit on the X-ray absorbing column N_{H} >
3.4 E25 cm-2. Such a large column has a Thomson optical depth to electron
scattering tau > 23, sufficient to extinguish the optical and UV emission. The
problem only gets worse if the gas is neutral since the opacity in the Lyman
continuum becomes extremely large, > 2 E8, conflicting with the source
detection below 912 A. This apparent contradiction probably means that our
lines-of-sight to the X-ray and to the UV emitting regions are different, such
that the gas covers completely the compact X-ray source but only partially the
more extended source of ultraviolet photons. An extended (~ 1') X-ray source is
detected 2' to the south-east of the QSO. Given its thermal spectrum and
temperature (1.5 < T < 3.0 keV}, it is probably a foreground (0.29 < z < 0.46)
cluster of galaxies.Comment: 9 pages, 3 figures, A&A latex, accepted for publication in Astronomy
& Astrophysic
Carbon in Spiral Galaxies from Hubble Space Telescope Spectroscopy
We present measurements of the gas-phase C/O abundance ratio in six H II
regions in the spiral galaxies M101 and NGC 2403, based on ultraviolet
spectroscopy using the Faint Object Spectrograph on the Hubble Space Telescope.
The C/O ratios increase systematically with O/H in both galaxies, from log C/O
approximately -0.8 at log O/H = -4.0 to log C/O approx. -0.1 at log O/H = -3.4.
C/N shows no correlation with O/H. The rate of increase of C/O is somewhat
uncertain because of uncertainty as to the appropriate UV reddening law, and
uncertainty in the metallicity dependence on grain depletions. However, the
trend of increasing C/O with O/H is clear, confirming and extending the trend
in C/O indicated previously from observations of irregular galaxies. Our data
indicate that the radial gradients in C/H across spiral galaxies are steeper
than the gradients in O/H. Comparing the data to chemical evolution models for
spiral galaxies shows that models in which the massive star yields do not vary
with metallicity predict radial C/O gradients that are much flatter than the
observed gradients. The most likely hypothesis at present is that stellar winds
in massive stars have an important effect on the yields and thus on the
evolution of carbon and oxygen abundances. C/O and N/O abundance ratios in the
outer disks of spirals determined to date are very similar to those in dwarf
irregular galaxies. This implies that the outer disks of spirals have average
stellar population ages much younger than the inner disks.Comment: 38 pages, 9 postscript figures, uses aaspp4.sty. Accepted for
publication in The Astrophysical Journa
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
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 ([Fe/H] 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] < â3.4, 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) ~ â2.0
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