621 research outputs found
An investigation of the d(18Ne,19Ne*)p reaction and its astrophysical relevance
The reaction 15 O(alpha; gamma) 19 Ne is one of the potential breakout reactions from
the Hot CNO cycle to the rpprocess. As such, it may play an important
role in nuclear astrophysics for the understanding of energy generation rates
and the synthesis of protonrich nuclei in sites of explosive hydrogen burning,
such as novae and Xray bursters.
Experiments were performed at the radioactive ion beam facility, at
LouvainlaNeuve, Belgium, to test the validity of measuring indirectly the
15 O(alpha; gamma) 19 Ne reaction rate. The method utilised was the population of ex
cited states in 19 Ne and the observation of their ffdecay. Information on the
alpha branching ratios of the states of astrophysical interest, just above the alpha
threshold, allows the reaction rate to be calculated, provided other resonance
properties, i.e. T , ER and J , are known.
Excited states in 19 Ne were populated via an inverse 18 Ne(d,p) reac
tion on a deuterated polyethylene target. The reaction and decay products
were measured in an experimental set up that comprised three silicon strip
detector arrays, with a total of 320 detector elements. Two experiments were
performed at E lab = 44.1 MeV and E lab = 54.3 MeV. The recoiling protons
tagged the populated state and the detection of a coincident ffparticle and
heavy residue pair identified its decay.
Branching ratios for several states in 19 Ne were determined, showing the
viability of this experimental approach. Optical model parameters were de
termined from 18 Ne elastic scattering on deuterons. DWBA calculations were
performed and compared with experimental angular distributions to yield
spectroscopic factors. The results were comparable with a previous meas
urement using a stable beam, despite the significantly lower beam intensity,
and indicated that, provided the necessary beam intensity was available, this
method would allow the measurement of the alpha branching ratio of the reson
ance of most astrophysical interest at 504 keV and thus the determination of
the 15 O(alpha; gamma) 19 Ne reaction rate
Stellar Wind Yields of Very Massive Stars
The most massive stars provide an essential source of recycled material for
young clusters and galaxies. While very massive stars (VMS, M>100M) are
relatively rare compared to O stars, they lose disproportionately large amounts
of mass already from the onset of core H-burning. VMS have optically thick
winds with elevated mass-loss rates in comparison to optically thin standard
O-star winds. We compute wind yields and ejected masses on the main sequence,
and we compare enhanced mass-loss rates to standard ones. We calculate solar
metallicity wind yields from MESA stellar evolution models in the range 50 -
500M, including a large nuclear network of 92 isotopes, investigating not only
the CNO-cycle, but also the Ne-Na and Mg-Al cycles. VMS with enhanced winds
eject 5-10 times more H-processed elements (N, Ne, Na, Al) on the main sequence
in comparison to standard winds, with possible consequences for observed
anti-correlations, such as C-N and Na-O, in globular clusters. We find that for
VMS 95% of the total wind yields is produced on the main sequence, while only
~5% is supplied by the post-main sequence. This implies that VMS with enhanced
winds are the primary source of 26Al, contrasting previous works where
classical Wolf-Rayet winds had been suggested to be responsible for Galactic
26Al enrichment. Finally, 200M stars eject 100 times more of each heavy element
in their winds than 50M stars, and even when weighted by an IMF their wind
contribution is still an order of magnitude higher than that of 50M stars.Comment: Accepted for publication in MNRAS. 14 pages, 10 figure
Impact of Newly Measured Nuclear Reaction Rates on 26Al Ejected Yields from Massive Stars
Over the last three years, the rates of all the main nuclear reactions involving the destruction and production of 26Al in stars (26Al(n, p)26Mg, 26Al(n, α)23Na, 26Al(p, γ)27Si and 25Mg(p, γ)26Al) have been re-evaluated thanks to new high-precision experimental measurements of their cross sections at energies of astrophysical interest, considerably reducing the uncertainties in the nuclear physics affecting their nucleosynthesis. We computed the nucleosynthetic yields ejected by the explosion of a high-mass star (20 M⊙, Z = 0.0134) using the FRANEC stellar code, considering two explosion energies, 1.2 × 1051 erg and 3 × 1051 erg. We quantify the change in the ejected amount of 26Al and other key species that is predicted when the new rate selection is adopted instead of the reaction rates from the STARLIB nuclear library. Additionally, the ratio of our ejected yields of 26Al to those of 14 other short-lived radionuclides (36Cl, 41Ca, 53Mn, 60Fe, 92Nb, 97Tc, 98Tc, 107Pd, 126Sn, 129I, 36Cs, 146Sm, 182Hf, 205Pb) are compared to early solar system isotopic ratios, inferred from meteorite measurements. The total ejected 26Al yields vary by a factor of ~3 when adopting the new rates or the STARLIB rates. Additionally, the new nuclear reaction rates also impact the predicted abundances of short-lived radionuclides in the early solar system relative to 26Al. However, it is not possible to reproduce all the short-lived radionuclide isotopic ratios with our massive star model alone, unless a second stellar source could be invoked, which must have been active in polluting the pristine solar nebula at a similar time of a core-collapse supernova
AEGIS: New Evidence Linking Active Galactic Nuclei to the Quenching of Star Formation
Utilizing Chandra X-ray observations in the All-wavelength Extended Groth
Strip International Survey (AEGIS) we identify 241 X-ray selected Active
Galactic Nuclei (AGNs, L > 10^{42} ergs/s) and study the properties of their
host galaxies in the range 0.4 < z < 1.4. By making use of infrared photometry
from Palomar Observatory and BRI imaging from the Canada-France-Hawaii
Telescope, we estimate AGN host galaxy stellar masses and show that both
stellar mass and photometric redshift estimates (where necessary) are robust to
the possible contamination from AGNs in our X-ray selected sample. Accounting
for the photometric and X-ray sensitivity limits of the survey, we construct
the stellar mass function of X-ray selected AGN host galaxies and find that
their abundance decreases by a factor of ~2 since z~1, but remains roughly flat
as a function of stellar mass. We compare the abundance of AGN hosts to the
rate of star formation quenching observed in the total galaxy population. If
the timescale for X-ray detectable AGN activity is roughly 0.5-1 Gyr--as
suggested by black hole demographics and recent simulations--then we deduce
that the inferred AGN "trigger" rate matches the star formation quenching rate,
suggesting a link between these phenomena. However, given the large range of
nuclear accretion rates we infer for the most massive and red hosts, X-ray
selected AGNs may not be directly responsible for quenching star formation.Comment: 12 pages. Submitted to ApJ. Comments welcom
Nucleosynthetic Yields from Neutron Stars Accreting in Binary Common Envelopes
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. Massive-star binaries can undergo a phase where one of the two stars expands during its advanced evolutionary stage as a giant and envelops its companion, ejecting the hydrogen envelope and tightening its orbit. Such a common envelope phase is required to tighten the binary orbit in the formation of many of the observed X-ray binaries and merging compact binary systems. In the formation scenario for neutron star binaries, the system might pass through a phase where a neutron star spirals into the envelope of its giant star companion. These phases lead to mass accretion on to the neutron star. Accretion on to these common-envelope-phase neutron stars can eject matter that has undergone burning near to the neutron star surface. This paper presents nucleosynthetic yields of this ejected matter, using population synthesis models to study the importance of these nucleosynthetic yields in a galactic chemical evolution context. Depending on the extreme conditions in temperature and density found in the accreted material, both proton-rich and neutron-rich nucleosynthesis can be obtained, with efficient production of neutron-rich isotopes of low Z material at the most extreme conditions, and proton-rich isotopes, again at low Z, in lower density models. Final yields are found to be extremely sensitive to the physical modelling of the accretion phase. We show that neutron stars accreting in binary common envelopes might be a new relevant site for galactic chemical evolution, and therefore more comprehensive studies are needed to better constrain nucleosynthesis in these objects
Stellar wind yields of very massive stars
The most massive stars provide an essential source of recycled material for young clusters and galaxies. While very massive stars (VMSs, M>100) are relatively rare compared to O stars, they lose disproportionately large amounts of mass already from the onset of core H-burning. VMS have optically thick winds with elevated mass-loss rates in comparison to optically thin standard O-star winds. We compute wind yields and ejected masses on the main sequence, and we compare enhanced mass-loss rates to standard ones. We calculate solar metallicity wind yields from MESA stellar evolution models in the range 50-500, including a large nuclear network of 92 isotopes, investigating not only the CNO-cycle, but also the Ne-Na and Mg-Al cycles. VMS with enhanced winds eject 5-10 times more H-processed elements (N, Ne, Na, Al) on the main sequence in comparison to standard winds, with possible consequences for observed anticorrelations, such as C-N and Na-O, in globular clusters. We find that for VMS 95 per cent of the total wind yields is produced on the main sequence, while only ∼5 per cent is supplied by the post-main sequence. This implies that VMS with enhanced winds are the primary source of 26Al, contrasting previous works where classical Wolf-Rayet winds had been suggested to be responsible for galactic 26Al enrichment. Finally, 200 stars eject 100 times more of each heavy element in their winds than 50 stars, and even when weighted by an IMF their wind contribution is still an order of magnitude higher than that of 50 stars
Determination of alpha spectroscopic factors for unbound 17O states
It has been recently suggested that hydrogen ingestion into the helium shell of massive stars could lead to high 13C and 15N excesses when the blast of a core collapse supernova (ccSN) passes through its helium shell. This prediction questions the origin of extremely high 13C and 15N abundances observed in rare presolar SiC grains which is usually attributed to classical novae. In this context the 13N(α,p)16O reaction plays an important role since it is in competition with 13N β+-decay to 13C. As a first step to the determination of the 13N(α,p)16O reaction rate, we present a study aiming at the determination of alpha spectroscopic factors of 17O states which are the analog ones to those in 17F, the compound nucleus of the 13N(α,p)16O reaction
Stellar wind yields of very massive stars
The most massive stars provide an essential source of recycled material for young clusters and galaxies. While very massive stars (VMS, M&gt;100 ) are relatively rare compared to O stars, they lose disproportionately large amounts of mass already from the onset of core H-burning. VMS have optically thick winds with elevated mass-loss rates in comparison to optically thin standard O-star winds. We compute wind yields and ejected masses on the main sequence, and we compare enhanced mass-loss rates to standard ones. We calculate solar metallicity wind yields from MESA stellar evolution models in the range 50 – 500 , including a large nuclear network of 92 isotopes, investigating not only the CNO-cycle, but also the Ne-Na and Mg-Al cycles. VMS with enhanced winds eject 5-10 times more H-processed elements (N, Ne, Na, Al) on the main sequence in comparison to standard winds, with possible consequences for observed anti-correlations, such as C-N and Na-O, in globular clusters. We find that for VMS 95% of the total wind yields is produced on the main sequence, while only ∼ 5% is supplied by the post-main sequence. This implies that VMS with enhanced winds are the primary source of 26Al, contrasting previous works where classical Wolf-Rayet winds had been suggested to be responsible for Galactic 26Al enrichment. Finally, 200 stars eject 100 times more of each heavy element in their winds than 50 stars, and even when weighted by an IMF their wind contribution is still an order of magnitude higher than that of 50 stars
Evaluation of the 13N(α,p)16O thermonuclear reaction rate and its impact on the isotopic composition of supernova grains
It has been suggested that hydrogen ingestion into the helium shell of
massive stars could lead to high C and N excesses when the shock
of a core-collapse supernova passes through its helium shell. This prediction
questions the origin of extremely high C and N abundances
observed in rare presolar SiC grains which is usually attributed to classical
novae. In this context N(,p)O the reaction plays an
important role since it is in competition with N -decay to
C. The N(,p)O reaction rate used in stellar
evolution calculations comes from the CF88 compilation with very scarce
information on the origin of this rate. The goal of this work is to provide a
recommended N(,p)O reaction rate, based on available
experimental data. Unbound nuclear states in the F compound nucleus were
studied using the spectroscopic information of the analog states in O
nucleus that were measured at the Alto facility using the
C(Li,t)O alpha-transfer reaction, and spectroscopic factors
were derived using a DWBA analysis. This spectroscopic information was used to
calculate a recommended N(,p)O reaction rate with
meaningful uncertainty using a Monte Carlo approach. The present
N(,p)O reaction rate is found to be within a factor of
two of the previous evaluation, with a typical uncertainty of a factor 2-3. The
source of this uncertainty comes from the three resonances at , 741 and 959 keV. This new error estimation translates to an overall
uncertainty in the C production of a factor of 50. The main source of
uncertainty on the re-evaluated N(,p)O reaction rate
currently comes from the uncertain alpha-width of relevant F states
AEGIS: Demographics of X-ray and Optically Selected AGNs
We develop a new diagnostic method to classify galaxies into AGN hosts,
star-forming galaxies, and absorption-dominated galaxies by combining the [O
III]/Hbeta ratio with rest-frame U-B color. This can be used to robustly select
AGNs in galaxy samples at intermediate redshifts (z<1). We compare the result
of this optical AGN selection with X-ray selection using a sample of 3150
galaxies with 0.3<z<0.8 and I_AB<22, selected from the DEEP2 Galaxy Redshift
Survey and the All-wavelength Extended Groth Strip International Survey
(AEGIS). Among the 146 X-ray sources in this sample, 58% are classified
optically as emission-line AGNs, the rest as star-forming galaxies or
absorption-dominated galaxies. The latter are also known as "X-ray bright,
optically normal galaxies" (XBONGs). Analysis of the relationship between
optical emission lines and X-ray properties shows that the completeness of
optical AGN selection suffers from dependence on the star formation rate and
the quality of observed spectra. It also shows that XBONGs do not appear to be
a physically distinct population from other X-ray detected, emission-line AGNs.
On the other hand, X-ray AGN selection also has strong bias. About 2/3 of all
emission-line AGNs at L_bol>10^44 erg/s in our sample are not detected in our
200 ks Chandra images, most likely due to moderate or heavy absorption by gas
near the AGN. The 2--7 keV detection rate of Seyfert 2s at z~0.6 suggests that
their column density distribution and Compton-thick fraction are similar to
that of local Seyferts. Multiple sample selection techniques are needed to
obtain as complete a sample as possible.Comment: 24 pages, 14 figures, submitted to ApJ. Version 2 matches the ApJ
accepted version. Sec 3 was reorganized and partly rewritten with one
additional figure (Fig.3
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