Rates Of Stars Research Articles

Constraints on decreases in η Carinae's mass-loss from 3D hydrodynamic simulations of its binary colliding winds

Recent work suggests that the mass-loss rate of the primary star (Eta A) in the massive colliding wind binary Eta Carinae dropped by a factor of 2-3 between 1999 and 2010. We present results from large- (r=1545au) and small- (r=155au) domain, 3D smoothed particle hydrodynamic (SPH) simulations of Eta Car's colliding winds for 3 Eta A mass-loss rates (2.4, 4.8, and 8.5 x 10^-4 M_sun/yr), investigating the effects on the dynamics of the binary wind-wind collision (WWC). These simulations include orbital motion, optically thin radiative cooling, and radiative forces. We find that Eta A's mass-loss rate greatly affects the time-dependent hydrodynamics at all spatial scales investigated. The simulations also show that the post-shock wind of the companion star (Eta B) switches from the adiabatic to the radiative-cooling regime during periastron passage. The SPH simulations together with 1D radiative transfer models of Eta A's spectra reveal that a factor of 2 or more drop in Eta A's mass-loss rate should lead to substantial changes in numerous multiwavelength observables. Recent observations are not fully consistent with the model predictions, indicating that any drop in Eta A's mass-loss rate was likely by a factor < 2 and occurred after 2004. We speculate that most of the recent observed changes in Eta Car are due to a small increase in the WWC opening angle that produces significant effects because our line-of-sight to the system lies close to the dense walls of the WWC zone. A modest decrease in Eta A's mass-loss rate may be responsible, but changes in the wind/stellar parameters of Eta B cannot yet be fully ruled out. We suggest observations during Eta Car's next periastron in 2014 to further test for decreases in Eta A's mass-loss rate. If Eta A's mass-loss rate is declining and continues to do so, the 2014 X-ray minimum should be even shorter than that of 2009.

N-body simulations of globular clusters in tidal fields: Effects of intermediate-mass black holes

Intermediate-mass black holes (IMBHs) may provide the missing link to understanding the growth of supermassive black holes in the early Universe. Some formation scenarios predict that IMBHs could have formed by runaway collisions in globular clusters (GCs). Understanding the influence of an IMBH in the center of a GC on its environment might provide indirect detection methods. Our goal is to test the effects of different initial compositions of GCs on their evolution in a tidal field. We pin down the crucial observables that indicate the presence of an IMBH at the center of the cluster. In addition to central IMBHs, we also consider the effects of different stellar-mass black hole retention and stellar binary fractions. We performed a set of 22 N-body simulations and varied particle numbers, IMBH masses, stellar-mass black-hole retention fractions, and stellar binary fractions. These models are all run in an external tidal field to study the effect of black holes on the cluster mass loss, mass function, and life times. Finally, we compared our results with observational data. We found that a central massive black hole increases the escape rate of high-mass stars from a star cluster, implying that the relative depletion of the mass function at the low-mass end proceeds less rapidly. Furthermore, we found a similar behavior for a cluster hosting a high number of stellar-mass black holes instead of one massive central IMBH. The presence of an IMBH also weakly affects the fraction of the cluster mass that is constituted by stellar remnants, as does the presence of primordial binaries. We compared our simulations with observational data from the literature and found good agreement between our models and observed mass functions and structural parameters of GCs. We exploited this agreement to identify GCs that could potentially host IMBHs.

Number of Galactic Civilizations

A probabilistic assessment is made to determine the likely range of galactic civilizations. In order to define the possible number of advanced galactic civilizations, the number of habitable planets is being determined with the help of a newly formulated probability equation. This equation is expressing the number of habitable planets in terms of the as- sumed galaxy size, the number of critical habitability criteria and the star and planet compliance rates with these criteria. For effectiveness the equation uses instead of the individual compliance rates with the various habitability criteria the geometric mean of the compliance rates with the criteria. This simplifies the calculational approach considerably and makes it possible to obtain the necessary data to assess the extent of galactic habitability. These data are included in a Ga- lactic Habitability Table which presents in this way a complete overview of galactic habitability of a given galaxy as a function of the number of critical criteria and the mean compliance rate with these criteria. While no scientific certainty exists at present time for the exact values of these two parameters, zones of high probability can already be assessed. For any number of habitability criteria assumed a minimum star and planet compliance ratio can be determined which is re- quired for the existence of advanced civilizations in a galaxy of any given size. As the next step the issue of concurrent communicating civilizations has also been addressed.

The evolution of surface magnetic fields in young solar-type stars

AbstractThe surface rotation rates of young solar-type stars decrease rapidly with age from the end of the pre-main sequence though the early main sequence. This suggests that there is also an important change in the dynamos operating in these stars, which should be observable in their surface magnetic fields. Here we present early results in a study aimed at observing the evolution of these magnetic fields through this critical time period. We are observing stars in open clusters and stellar associations to provide precise ages, and using Zeeman Doppler Imaging to characterize the complex magnetic fields. Presented here are results for six stars, three in the in the β Pic association (~10 Myr old) and three in the AB Dor association (~100 Myr old).

Nanostructure Formation and Transition from Surface to Bulk Degradation in Polyethylene Glycol Gels Chain-Extended with Short Hydroxy Acid Segments

Degradable, in situ gelling, inert hydrogels with tunable properties are very attractive as a matrix for cell encapsulation and delivery to the site of regeneration. Cell delivery is generally limited by the toxicity of gelation and degradation reactions. The objective of this work was to investigate by simulation and experimental measurement gelation kinetics and degradation rate of star acrylated polyethylene glycol (PEG) macromonomers chain-extended with short hydroxy acid (HA) segments (SPEXA) as a function of HA monomer type and number of HA repeat units. HA monomers included least hydrophobic glycolide (G), lactide (L), p-dioxanone (D), and most hydrophobic ε-caprolactone (C). Chain extension of PEG with short HA segments resulted in micelle formation for all HA types. There was a significant decrease in gelation time of SPEXA precursor solutions with HA chain-extension for all HA types due to micelle formation, consistent with the simulated increase in acrylate-acrylate (Ac-Ac) and Ac-initiator integration numbers. The hydrolysis rate of SPEXA hydrogels was strongly dependent on HA type and number of HA repeat units. SPEXA gels chain-extended with the least hydrophobic glycolide completely degraded within days, lactide within weeks, and p-dioxanone and ε-caprolactone degraded within months. The wide range of degradation rates observed for SPEXA gels can be explained by large differences in equilibrium water content of the micelles for different HA monomer types. A biphasic relationship between HA segment length and gel degradation rate was observed for all HA monomers, which was related to the transition from surface (controlled by HA segment length) to bulk (controlled by micelle equilibrium water content) hydrolysis within the micelle phase. To our knowledge, this is the first report on transition from surface to bulk degradation at the nanoscale in hydrogels.

Improved angular momentum evolution model for solar-like stars

We present new models for the rotational evolution of solar-like stars between 1 Myr and 10 Gyr with the aim to reproduce the distributions of rotational periods observed for star forming regions and young open clusters within this age range. The models include a new wind braking law based on recent numerical simulations of magnetized stellar winds and specific dynamo and mass-loss prescriptions are adopted to tie angular momentum loss to angular velocity. The model additionally assume constant angular velocity during the disk accretion phase and allow for decoupling between the radiative core and the convective envelope as soon as the former develops. We have developed rotational evolution models for slow, median and fast rotators with initial periods of 10, 7, and 1.4d, respectively. The models reproduce reasonably well the rotational behaviour of solar-type stars between 1 Myr and 4.5 Gyr, including PMS to ZAMS spin up, prompt ZAMS spin down, and the early-MS convergence of surface rotation rates. We find the model parameters accounting for the slow and median rotators are very similar to each other, with a disk lifetime of 5 Myr and a core-envelope coupling timescale of 28-30 Myr. In contrast, fast rotators have both shorter disk lifetime (2.5 Myr) and core-envelope coupling timescale (12 Myr). We emphasize that these results are highly dependent on the adopted braking law. We also report a tentative correlation between initial rotational period and disk lifetime, which suggests that protostellar spin-down by massive disks in the embedded phase is at the origin of the initial dispersion of rotation rates in young stars. We conclude that this class of semi-empirical models successfully grasp the main trends of the rotational behaviour of solar-type stars as they evolve and make specific predictions that may serve as a guide for further development.

ROTATION OF THE K3 II-III GIANT STAR α HYDRA

Fundamental spectroscopic determination of projected rotation rates of slowly rotating stars is challenging because the rotational broadening of the spectral lines is often comparable to, or smaller than, the broadening from other sources, most notably macroturbulence. Fourier techniques have the advantage over direct profile matching when the observed profiles are complete, but when the profiles are severely blended, the Fourier analysis is compromised. A process of modeling partial profiles for determining the rotation rate for stars having blended spectral lines is investigated and applied to the evolved star α Hya (K3 II-III). Projected rotation higher than 5 km s–1 can be definitively ruled out for this star. Not all lines are equally good, depending on the amount of blending and also depending on the strength of the line, as the balance between the thermal and non-thermal components changes. A modest ambiguity arises between macroturbulence and rotational broadening, but a careful look at the differences between the observations and the models allows one to measure the rotation with acceptable precision. The result for α Hya is v sin i = 2.6 ± 0.3 km s–1.

Dust input from AGB stars in the Large Magellanic Cloud

The dust-forming population of AGB stars and their input to the interstellar dust budget of the Large Magellanic Cloud (LMC) are studied with evolutionary dust models with the main goals (1) to investigate how the amount and composition of dust from AGB stars vary over galactic history; (2) to characterise the mass and metallicity distribution of the present population of AGB stars; (3) to quantify the contribution of AGB stars of different mass and metallicity to the present stardust population in the interstellar medium (ISM). We use models of the stardust lifecycle in the ISM developed and tested for the Solar neighbourhood. The first global spatially resolved reconstruction of the star formation history of the LMC from the Magellanic Clouds Photometric Survey is employed to calculate the stellar populations in the LMC. The dust input from AGB stars is dominated by carbon grains from stars with masses < 4 Msun almost over the entire history of the LMC. The production of silicate, silicon carbide and iron dust is delayed until the ISM is enriched to about half the present metallicity in the LMC. For the first time, theoretically calculated dust production rates of AGB stars are compared to those derived from IR observations of AGB stars for the entire galaxy. We find good agreement within scatter of various observational estimates. We show that the majority of silicate and iron grains in the present stardust population originate from a small population of intermediate-mass stars consisting of only about 4% of the total number of stars, whereas in the Solar neighbourhood they originate from low-mass stars. With models of the lifecycle of stardust grains in the ISM we confirm a large discrepancy between dust input from stars and the existing interstellar dust mass in the LMC reported in Matsuura et al. 2009.

Non-thermal photons and H2 formation in the early Universe

The cosmological recombination of H and He at z \sim 1000 and the formation of H2 during the dark ages produce a non-thermal photon excess in the Wien tail of the cosmic microwave background (CMB) blackbody spectrum. Here we compute the effect of these photons on the H- photodetachment and H2+ photodissociation processes. We discuss the implications for the chemical evolution of the Universe in the post-recombination epoch, emphasizing how important a detailed account of the full vibrational manifold of H2 and H2+ in the chemical network is. We find that the final abundances of H2, H2+, H3+ and HD are significantly smaller than in previous calculations that neglected the effect of non-thermal photons. The suppression is mainly caused by extra hydrogen recombination photons and could affect the formation rate of first stars. We provide simple analytical approximations for the relevant rate coefficients and briefly discuss the additional effect of dark matter annihilation on the considered reaction rates.

The bright end of the exo-Zodi luminosity function: disc evolution and implications for exo-Earth detectability

We present the first characterisation of the 12um warm dust ("exo-Zodi") luminosity function around Sun-like stars, focussing on the dustiest systems that can be identified by WISE. We detect six new warm dust candidates, five of which have unknown ages. We show that the dustiest old (>Gyr) systems like BD+20 307 are 1 in 10,000 occurrences. Bright warm dust is more common around young (<120Myr) systems, with a ~1% occurrence rate. We show that a two component in situ model where all stars have initially massive warm disks and in which warm debris is also generated at some random time along the stars' main-sequence lifetime, perhaps due to a collision, can explain the observations. However, if all stars only have initially massive warm disks these would not be visible at Gyr ages, and random collisions on the main-sequence are too infrequent to explain the high disk occurrence rate for young stars. That is, neither component can explain the observations on their own. Despite these conclusions, we cannot rule out an alternative model in which comets are scattered from outer regions because the distribution of systems with the appropriate dynamics is unknown. Our in situ model predicts the fraction of stars with exo-Zodi bright enough to cause problems for future exo-Earth imaging attempts is at least ~10%, and is higher for populations of stars younger than a few Gyr. This prediction of ~10% applies to old stars because bright systems like BD+20 307 imply a population of fainter systems that were once bright but are now decaying through fainter levels. Our prediction should be strongly tested by the Large Binocular Telescope Interferometer, providing valuable input for more detailed evolution models. A detection fraction lower than our prediction could indicate that the hot dust in systems like BD+20 307 has a cometary origin due to the quirks of the planetary dynamics.

The bright end of the exo-Zodi luminosity function: Disk evolution and implications for exo-Earth detectability

AbstractThis contribution summarises the first characterisation of the 12 μm warm dust (“exo-Zodi”) luminosity function around Sun-like stars, focussing on the dustiest systems that can be identified by the WISE mission (Kennedy &amp; Wyatt 2013). We use the sample of main-sequence stars observed by Hipparcos within 150pc as an unbiased sample, and report the detection of six new warm dust candidates. The ages of five of these new sources are unknown, meaning that they may be sites of terrestrial planet formation or rare analogues of other old warm dust systems. We show that the dustiest old (&gt; Gyr) systems such as BD+20 307 are 1 in 10,000 occurrences. Bright warm dust is much more common around young (&lt;120 Myr) systems, with a ~1% occurrence rate. We show that a two component in situ model where all stars have initially massive warm disks and in which warm debris is also generated at some random time along the stars' main-sequence lifetime, perhaps due to a collision, can explain the observations. However, if all stars only have initially massive warm disks these would not be visible at Gyr ages, and random collisions on the main-sequence are too infrequent to explain the high disk occurrence rate for young stars. That is, neither component can explain the observations on their own. Despite these conclusions, we cannot rule out an alternative dynamical model in which comets are scattered in from outer regions because the distribution of systems with the appropriate dynamics is unknown. Our in situ model predicts that the fraction of stars with exo-Zodi bright enough to cause problems for future exo-Earth imaging attempts is at least roughly 10%, and is higher for populations of stars younger than a few Gyr. This prediction of roughly 10% also applies to old stars because bright systems like BD+20 307 imply a population of fainter systems that were once bright, but are now decaying through fainter levels. Our prediction should be strongly tested by the Large Binocular Telescope Interferometer, which will provide valuable constraints and input for more detailed evolution models. A detection fraction lower than our prediction could indicate that the hot dust in systems like BD+20 307 has a cometary origin due to the quirks of the planetary dynamics. Population models of comet delivery need to be developed to help distinguish between different possible origins of warm dust.

NUCLEOSYNTHETIC CONSTRAINTS ON THE MASS OF THE HEAVIEST SUPERNOVAE

Assuming a Salpeter initial mass function and taking the solar abundances as a representative sample, we explore the sensitivity of nucleosynthesis in massive stars to the truncation of supernova explosions above a certain mass. It is assumed that stars of all masses contribute to nucleosynthesis by their pre-explosive winds, but above a certain limiting main sequence mass, the presupernova star becomes a black hole and ejects nothing more. The solar abundances from oxygen to atomic mass 90 are fit quite well assuming no cut-off at all, i.e., by assuming all stars up to 120 solar masses make successful supernovae. Little degradation in the fit occurs if the upper limit is reduced to 25 solar masses. The limit can be further reduced, but the required event rate of supernovae in the remaining range rises rapidly to compensate for the lost nucleosynthesis of the more massive stars. The nucleosynthesis of the s-process declines precipitously and the production of species made in the winds, e.g., carbon, becomes unacceptably large compared with elements made in the explosion, e.g., silicon and oxygen. However, by varying uncertain physics, especially the mass loss rate for massive stars and the rate for the neon-22 to magnesium-25 reaction rate, acceptable nucleosynthesis might still be achieved with a cutoff as low as 18 solar masses. This would require a supernova frequency three times greater than the fiducial value obtained when all stars explode in order to produce the required oxygen-16. The nucleosynthesis of iron-60 and aluminum-26 is also examined.

The spin-down mechanism of the X-ray pulsar 4U 2206+54

Observations of the X-ray pulsar 4U 2206+54, performed over a 15-year span, show its period, which is now 5555 \pm 9 s, to increase drastically. Such behavior of the pulsar can be hardly explained in the frame of traditional scenarios of spin evolution of compact stars. The observed spin-down rate of the neutron star in 4U 2206+54 proves to be in good agreement with the value expected within magnetic accretion scenario which takes into account that magnetic field of the accretion stream can under certain conditions affect its geometry and type of flow. The neutron star in this case accretes material from a dense gaseous slab with small angular momentum which is kept in equilibrium by the magnetic field of the flow itself. The magnetic accretion scenario can be realized in 4U 2206+54 provided the magnetic field strength on the surface of the optical counterpart to the neutron star is in excess of 70 G. The magnetic field strength on the surface of the neutron star in the frame of this scenario turns out to be 4 x 10^{12} G which is in accordance with estimates of this parameter from the analysis of X-ray spectra of the pulsar.

Ultrahigh energy cosmic ray nuclei from extragalactic pulsars and the effect of their Galactic counterparts

The acceleration of ultrahigh energy nuclei in fast spinning newborn pulsars can explain the observed spectrum of ultrahigh energy cosmic rays and the trend towards heavier nuclei for energies above 1019 eV as reported by the Auger Observatory. Pulsar acceleration implies a hard injection spectrum ( ∼ E−1) due to pulsar spin down and a maximum energy Emax ∼ Z 1019 eV due to the limit on the spin rate of neutron stars. We have previously shown that the escape through the young supernova remnant softens the spectrum, decreases slightly the maximum energy, and generates secondary nuclei. Here we show that the distribution of pulsar birth periods and the effect of propagation in the interstellar and intergalactic media modifies the combined spectrum of all pulsars. By assuming a normal distribution of pulsar birth periods centered at 300 ms, we show that the contribution of extragalactic pulsar births to the ultrahigh energy cosmic ray spectrum naturally gives rise to a contribution to very high energy cosmic rays (VHECRs, between 1016 and 1018 eV) by Galactic pulsar births. The required injected composition to fit the observed spectrum depends on the absolute energy scale, which is uncertain, differing between Auger Observatory and Telescope Array. The contribution of Galactic pulsar births can also bridge the gap between predictions for cosmic ray acceleration in supernova remnants and the observed spectrum just below the ankle, depending on the composition of the cosmic rays that escape the supernova remnant and the diffusion behavior of VHECRs in the Galaxy.

Gamma-rays from nebulae around binary systems containing energetic rotation-powered pulsars

We consider nebulae which are created around binary systems containing rotation powered pulsars and companion stars with strong stellar winds. It is proposed that the stellar and pulsar winds have to mix at some distance from the binary system, defined by the orbital period of the companion stars and the velocity of the stellar wind. The mixed pulsar-stellar wind expands with a specific velocity determined by the pulsar power and the mass loss rate of the companion star. Relativistic particles, either from the inner pulsar magnetosphere and/or accelerated at the shocks between stellar and pulsar winds, are expected to be captured and isotropized in the reference frame of the mixed wind. Therefore, they can efficiently comptonize stellar radiation producing GeV-TeV $\gamma$-rays in the inverse Compton process. We calculate the $\gamma$-ray spectra expected in such scenario for the two example binary systems: J1816+4510 which is the redback type millisecond binary and LS 5039 which is supposed to contain energetic pulsar. It is concluded that the steady TeV $\gamma$-ray emission from J1816+4510 should be on the 100 hr sensitivity limit of the planned Cherenkov Telescope Array, provided that $\epsilon\sim 10%$ of the rotational energy lost by the pulsar is transferred to TeV electrons. On the other hand, the comparison of the predicted steady TeV $\gamma$-ray emission, expected from $\gamma$-ray binary LS 5039, with the observations of the TeV emission in a low state, reported by the H.E.S.S. Collaboration, allows us to put stringent upper limit on the product of the part of the hemisphere in which the mixed pulsar-stellar wind is confined, $\Delta_{\rm mix}$, and the energy conversion efficiency, $\epsilon$, from the supposed pulsar to the TeV electrons injected in this system, $\Delta_{\rm mix}\cdot \epsilon <1%$.

Angular momentum evolution of low‐mass pre‐main sequence stars via extreme coronal mass ejections

AbstractThe angular momentum evolution of cool stars during the pre‐main sequence phase of stellar evolution remains a major outstanding problem. Multiple processes are likely involved in the transfer of mass and angular momentum within and out of the star+disk system. The role of coronal mass ejections (CMEs), energetic events which shed mass and magnetic flux in the Sun, has yet to be fully explored in the context of pre‐main sequence stars. It is well established that young, solar‐type stars exhibit X‐ray activity levels up to four orders of magnitude higher than the present‐day Sun, suggesting that CMEs associated with these extreme X‐ray flares could be an important process for expelling mass and angular momentum. We present a novel approach to modeling the CMEs of low‐mass pre‐main sequence stars that uses a solar‐calibrated CME model and observed X‐ray flare rates for young stars. We derive mass loss rates via stellar CMEs and calculate their attendant angular momentum losses during the pre‐main sequence phase. While we find the mass loss rates to be modest, ∼10 % of steady‐state stellar wind values from the literature, the angular momentum losses can be substantial, potentially counteracting the effects of initial stellar spin‐up due to contraction in tens of Myr. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

两种海星对三种双壳贝类的捕食选择性和摄食率

Sea stars are one of the primary predators of shellfish and often cause mass mortality among cultured shellfish.To develop effective control strategies,it is critical to understand the feeding ecophysiology(e.g.,feeding rate and prey selection) of sea stars. Asterias amurensis and Asterina pectinifera are the dominant sea star species in the coastal waters of China.We evaluated prey selection and the feeding rate of these two sea stars on three species of bivalves: scallops Chlamys farreri,clams Ruditapes philippinarum,and blue mussels Mytilus galloprovincialis.The experimental sea stars and bivalves were collected from Sanggou Bay,Northern China and transported to our seaside laboratory.The animals were acclimated to laboratory conditions for 7 d prior to the initiation of the experiment.The experiment was conducted between March 19 and April 5,2008,at different seawater temperatures.Following acclimation,the sea stars were placed in cement tanks(L×W×H=6×1×0.5 m) at a density of 5 individuals per tank(single species per tank).The sea water was pumped from Sanggou Bay and sand filtered.Daily water exchange was ca 100%.We placed the bivalves(N=30 per species) evenly in each tank to ensure the sea stars had an equal probability of encountering the three species of bivalves.Each treatment was conducted in triplicate(N= 3 tanks).The number of bivalves of each species preyed upon by the sea stars was recorded twice daily at 7:00 and 17:00.In addition,we measured the Q10 coefficient at water temperatures ranging from 4.3—7.8℃ and from 7.8—13.3℃. Both species of sea star preyed on all three bivalve species.Similarly,both species exhibited preference in the order clammusselscallop.During the first part of the experiment(March 19—24),A.amurensis preyed on 64.28,28.57,and 7.14% of the scallops,clams,and blue mussels,respectively.A.pectinifera preyed upon 50,44.44,and 5.56% of the bivalve species,respectively.The mean feeding rates of A.amurensis and A.pectinifera on the clam(0.50 and 0.37 ind/d,respectively) and blue mussel(0.26 and 0.27 ind/d,respectively) were significantly higher than those on the scallop(0.05 and 0.07 ind/d,respectively).The feeding rate was significantly influenced by water temperature and generally increased with increasing water temperature.The total mean feeding rates of the two sea stars were 0.69 and 0.79 g · d-1,respectively(based on dry tissue weight of bivalves).As water temperature increased from 4.3 to 7.8℃,the Q10coefficients for A.amurensis and A.pectinifera were 6.38 and 2.33,respectively.However,when the water temperature was increased from 7.8 to 13.3℃,there was no increase in the feeding rate(Q10=1.13 and 1.22,respectively).Our results have implications for the provision of protective refuges for the species of interest(i.e.,scallops) during culture in suspended lantern nets.Protective strategies are most likely to be needed when the water temperature increases above 4.3℃,as the feeding rate and activity of sea stars increased significantly above this point.Based on prey selectivity,bivalves that have a lower commercial value(e.g.,blue mussels) may be co-cultured in the scallop lantern nets to serve as a buffer against predators.Furthermore,any sea stars present in the cultivation nets should be removed during the buffering period.

Plasma effects on resonant fusion

I investigate the effects of plasma interactions on resonance-enhanced fusion rates in stars. Starting from basic principles we derive an expression for the fusion rate that can serve as a basis for discussion of approximation schemes The present state-of-the-art correction algorithms, based on the classical correlation function for the fusing particles and the classical energy shift for the resonant state, do not follow from this result, even as an approximation. The results of expanding in a perturbation solution for the case of a weakly coupled plasma are somewhat enlightening. But at this point we are at a loss as to how to do meaningful calculations in systems with even moderate plasma coupling strength. Examples where this can matter are the effect of a possible low-energy ${}^{12}$C+${}^{12}$C resonance on x-ray bursts from accreting neutron stars or on supernova 1A simulations, and the calculation of the triple $\ensuremath{\alpha}$ rate in some of the more strongly coupled regions in which the process enters, such as accretion onto a neutron star.

LOOKING DEEP INTO THE CAT'S EYE: STRUCTURE AND ROTATION IN THE FAST WIND OF THE PN CENTRAL STAR OF NGC 6543

We present HST/STIS time-series spectroscopy of the central star of the "Cat's Eye" planetary nebula NGC 6543. Intensive monitoring of the UV lines over a 5.8 hour period reveals well defined details of large-scale structure in the fast wind, which are exploited to provide new constraints on the rotation rate of the central star. We derive characteristics of the line profile variability that support a physical origin due to co-rotating interaction regions (CIRs) that are rooted at the stellar surface. The recurrence time of the observed spectral signatures of the CIRs is used to estimate the rotation period of the central star and, adopting a radius between 0.3 and 0.6 \Rsun constrains the rotational velocity to the range 54 \leq v_{rot} \leq 108 \kms. The implications of these results for single star evolution are discussed based on models calculated here for low-mass stars. Our models predict a sub-surface convective layer in NGC 6543 which we argue to be causally connected to the occurrence of structure in the fast wind.

THE TIDAL DISRUPTION OF GIANT STARS AND THEIR CONTRIBUTION TO THE FLARING SUPERMASSIVE BLACK HOLE POPULATION

Sun-like stars are thought to be regularly disrupted by supermassive black holes (SMBHs) within galactic nuclei. Yet, as stars evolve off the main sequence their vulnerability to tidal disruption increases drastically as they develop a bifurcated structure consisting of a dense core and a tenuous envelope. Here we present the first hydrodynamic simulations of the tidal disruption of giant stars and show that the core has a substantial influence on the star's ability to survive the encounter. Stars with more massive cores retain large fractions of their envelope mass, even in deep encounters. Accretion flares resulting from the disruption of giant stars should last for tens to hundreds of years. Their characteristic signature in transient searches would not be the $t^{-5/3}$ decay typically associated with tidal disruption events, but a correlated rise over many orders of magnitude in brightness on months to years timescales. We calculate the relative disruption rates of stars of varying evolutionary stages in typical galactic centers, then use our results to produce Monte Carlo realizations of the expected flaring event populations. We find that the demographics of tidal disruption flares are strongly dependent on both stellar and black hole mass, especially near the limiting SMBH mass scale of $\sim 10^8 M_\odot$. At this black hole mass, we predict a sharp transition in the SMBH flaring diet beyond which all observable disruptions arise from evolved stars, accompanied by a dramatic cutoff in the overall tidal disruption flaring rate. Black holes less massive than this limiting mass scale will show observable flares from both main sequence and evolved stars, with giants contributing up to 10% of the event rate. The relative fractions of stars disrupted at different evolutionary states can constrain the properties and distributions of stars in galactic nuclei other than our own.