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

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&amp;gt;100 $\rm {\rm M}_{\odot }$) 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 $\rm {\rm M}_{\odot }$, 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 $\rm {\rm M}_{\odot }$ stars eject 100 times more of each heavy element in their winds than 50 $\rm {\rm M}_{\odot }$ stars, and even when weighted by an IMF their wind contribution is still an order of magnitude higher than that of 50 $\rm {\rm M}_{\odot }$ 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 $^{13}$C and $^{15}$N excesses when the shock of a core-collapse supernova passes through its helium shell. This prediction questions the origin of extremely high $^{13}$C and $^{15}$N abundances observed in rare presolar SiC grains which is usually attributed to classical novae. In this context $^{13}$N($\alpha$,p)$^{16}$O the reaction plays an important role since it is in competition with $^{13}$N $\beta^+$-decay to $^{13}$C. The $^{13}$N($\alpha$,p)$^{16}$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 $^{13}$N($\alpha$,p)$^{16}$O reaction rate, based on available experimental data. Unbound nuclear states in the $^{17}$F compound nucleus were studied using the spectroscopic information of the analog states in $^{17}$O nucleus that were measured at the Alto facility using the $^{13}$C($^7$Li,t)$^{17}$O alpha-transfer reaction, and spectroscopic factors were derived using a DWBA analysis. This spectroscopic information was used to calculate a recommended $^{13}$N($\alpha$,p)$^{16}$O reaction rate with meaningful uncertainty using a Monte Carlo approach. The present $^{13}$N($\alpha$,p)$^{16}$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 $E_r^{c.m.} = 221$, 741 and 959 keV. This new error estimation translates to an overall uncertainty in the $^{13}$C production of a factor of 50. The main source of uncertainty on the re-evaluated $^{13}$N($\alpha$,p)$^{16}$O reaction rate currently comes from the uncertain alpha-width of relevant $^{17}$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

Penalty Corner Routines in Elite Women’s Indoor Field Hockey: Prediction of Outcomes based on Tactical Decisions

Indoor hockey is a highly competitive international sport, yet no research to date has investigated the key actions within this sport. As with outdoor field hockey, penalty corners represent one of the most likely situations in which goals can be scored. All 36 matches of the round-robin phase of the 2010-2011 England Hockey League Women’s Premier Division ‘Super Sixes’ competition were analysed with the purpose of establishing which factors can predict the scoring of a goal using Binary Logistic Regression analysis. Seventy two (22.6%) of the 319 observed penalty corners resulted in a goal. The strongest predictor of scoring a goal was taking the penalty corner from the goalkeeper’s right. Based on the odds ratio (OR), the odds of the attacking team scoring were 2.27 (CI = 1.41 - 3.65) times higher with penalty corners taken from the goalkeeper’s right as opposed to the left. Additionally, if the goalkeeper decided to rush to the edge of the circle, the odds of the attacking team failing to score were 2.19 (CI = 1.18 - 4.08) times higher compared to when the goalkeeper remained near the goal line. These results suggest that strategic decisions from the players and coaches have an important part to play in the success of penalty corners. Future research should investigate the impact of goalkeepers’ movement and further examine the technical and tactical intricacies of penalty corners