Helium Shell Research Articles

FORMATION OF FULLERENES IN H-CONTAINING PLANETARY NEBULAE

Hydrogen depleted environments are considered an essential requirement for the formation of fullerenes. The recent detection of C60 and C70 fullerenes in what was interpreted as the hydrogen-poor inner region of a post-final helium shell flash Planetary Nebula (PN) seemed to confirm this picture. Here, we present evidence that challenges the current paradigm regarding fullerene formation, showing that it can take place in circumstellar environments containing hydrogen. We report the simultaneous detection of Polycyclic Aromatic Hydrocarbons (PAHs) and fullerenes towards C-rich and H-containing PNe belonging to environments with very different chemical histories such as our own Galaxy and the Small Magellanic Cloud. We suggest that PAHs and fullerenes may be formed by the photochemical processing of hydrogenated amorphous carbon. These observations suggest that modifications may be needed to our current understanding of the chemistry of large organic molecules as well as the chemical processing in space.

RAPIDLY DECAYING SUPERNOVA 2010X: A CANDIDATE “.Ia” EXPLOSION

We present the discovery, photometric and spectroscopic follow-up observations of SN 2010X (PTF 10bhp). This supernova decays exponentially with tau_d=5 days, and rivals the current recordholder in speed, SN 2002bj. SN 2010X peaks at M_r=-17mag and has mean velocities of 10,000 km/s. Our light curve modeling suggests a radioactivity powered event and an ejecta mass of 0.16 Msun. If powered by Nickel, we show that the Nickel mass must be very small (0.02 Msun) and that the supernova quickly becomes optically thin to gamma-rays. Our spectral modeling suggests that SN 2010X and SN 2002bj have similar chemical compositions and that one of Aluminum or Helium is present. If Aluminum is present, we speculate that this may be an accretion induced collapse of an O-Ne-Mg white dwarf. If Helium is present, all observables of SN 2010X are consistent with being a thermonuclear Helium shell detonation on a white dwarf, a ".Ia" explosion. With the 1-day dynamic-cadence experiment on the Palomar Transient Factory, we expect to annually discover a few such events.

Double-detonation sub-Chandrasekhar supernovae: can minimum helium shell masses detonate the core?

The explosion of sub-Chandrasekhar mass white dwarfs via the double detonation scenario is a potential explanation for type Ia supernovae. In this scenario, a surface detonation in a helium layer initiates a detonation in the underlying carbon/oxygen core leading to an explosion. For a given core mass, a lower bound has been determined on the mass of the helium shell required for dynamical burning during a helium flash, which is a necessary prerequisite for detonation. For a range of core and corresponding minimum helium shell masses, we investigate whether an assumed surface helium detonation is capable of triggering a subsequent detonation in the core even for this limiting case. We carried out hydrodynamic simulations on a co-expanding Eulerian grid in two dimensions assuming rotational symmetry. The detonations are propagated using the level-set approach and a simplified scheme for nuclear reactions that has been calibrated with a large nuclear network. The same network is used to determine detailed nucleosynthetic abundances in a post-processing step. Based on approximate detonation initiation criteria in the literature, we find that secondary core detonations are triggered for all of the simulated models, ranging in core mass from 0.810 up to 1.385 with corresponding shell masses from 0.126 down to 0.0035 . This implies that, as soon as a detonation triggers in a helium shell covering a carbon/oxygen white dwarf, a subsequent core detonation is virtually inevitable.

DASCH DISCOVERY OF LARGE AMPLITUDE ∼10-100 YEAR VARIABILITY IN K GIANTS

Here we present the discovery of three unusual long-term variables found in the Digital Access to a Sky Century at Harvard (DASCH) project, with ~1 magnitude variations in their lightcurves on ~10-100 yr timescales. They are all spectroscopically identified as K2III giant stars, probably in the thick disk. Their lightcurves do not match any previously measured for known types of variable stars, or any theoretical model reported for red giants, and instead suggest a new dust formation mechanism or the first direct observation of "short" timescale evolution-driven variability. More theoretical work on the lithium flash near the Red Giant Branch (RGB) bump and the helium shell ignition in the lower Asymptotic Giant Branch (AGB), as well as long term monitoring of K2III thick disk stars is needed.

SpS1-Infrared spectroscopy of post-AGB objects

Post-AGB (pAGB) objects are low to intermediate initial mass (≤8 M⊙) objects that have terminated normal nuclear burning and as a result are undergoing rapid evolution toward the white dwarf sequence. In classical pAGB objects evolution is to hotter effective temperatures at roughly constant luminosity. However, there are also several classes of pAGB objects that have revived nuclear burning after approaching or being on the white dwarf sequence. These include objects with delayed final helium shell flashes (e.g. Sakurai's star) and white dwarf mergers (e.g. R CrB stars). Binary evolution plays a critical role in many of these systems. A group of pAGB supergiants with large infrared excesses are suspected to be binaries that have undergone common envelope evolution. Further details on many of these objects can be found in reviews, see for instance Van Winckel (2003) and Herwig (2005).

FG Sge at the stage of dust shell ejection

magnitude estimates. The maximum amplitude of the V -band light variations was >8 m . Six deep minima and four high maxima were observed. Analysis of the light curve has shown that the pulsation period of the star remained constant since 1991 and was P = 115 days. We have studied the wavelength dependence of the extinction at various phases of the light curve. The blueing of the B–V color at deep minima is interpreted as the result of light scattering in the circumstellar dust shell of the star formed by preceding dust ejections since 1992. Our spectroscopic observations performed on nine nights in 2003–2007 with the 125-cm telescope at the Crimean Station of the Sternberg Institute have confirmed the previously detected intensity variations of the Swan bands and the sodium doublet with brightness. It is noted that the Swan bands originate in the upper atmosphere, the star’s extended envelope, while the sodium doublet originates mainly in the circumstellar shell of FG Sge. We suggest that the star is currently located in the temperature–luminosity diagram at the turning point of the horizontal track of cooling in the direction of hot stars—evolution caused by the last helium shell flash at the planetary nebula stage.

Path Integral Studies of the Rotations of Methane and Its Heavier Isotopomers in 4He Nanoclusters

Path integral Monte Carlo simulations have been carried out to study the rotations of a methane molecule and its heavier isotopomers inside a small cluster of 4He atoms at 0.3 K in order to determine how the renormalization in the methane's rotational constant is related to the quantum statistics and superfluidity of the helium shell. By changing the mass of the hydrogens and systematically varying the moment of inertia of the methane, we were able to study the effects of its rotations on the quantum statistics of the helium atoms and their countereffects on the methane's effective rotational constant. The renormalized rotational constant depends strongly on the intrinsic moment of inertia of the methane. A heavy probe favors strong templating of the helium density as well as a large renormalization in the probe's rotational constant, but a light probe shows almost no effect on the shell density or the effective rotational constant. These results suggest that in order to fully understand the superfluidity of the helium shell, the probe must be treated as an integral part of the system. We rationalize the findings in terms of a rotational smearing effect and suggest that there is no clearly quantifiable relationship between the superfluid fraction of the shell and the renormalized rotational constant of the probe for cases where the probe molecule is either light or has weak anisotropic interactions with the helium atoms.

The effect of rotation on the stability of nuclear burning in accreting neutron stars

Hydrogen and/or helium accreted by a neutron star from a binary companion may undergo thermonuclear fusion. At different mass accretion rates different burning regimes are discerned. Theoretical models predict helium fusion to proceed as a thermonuclear runaway for accretion rates below the Eddington limit and as stable burning above this limit. Observations, however, place the boundary close to 10% of the Eddington limit. We study the effect of rotationally induced transport processes on the stability of helium burning. For the first time detailed calculations of thin helium shell burning on neutron stars are performed using a hydrodynamic stellar evolution code including rotation and rotationally induced magnetic fields. We find that in most cases the instabilities from the magnetic field provide the dominant contribution to the chemical mixing, while Eddington-Sweet circulations become important at high rotation rates. As helium is diffused to greater depths, the stability of the burning is increased, such that the critical accretion rate for stable helium burning is found to be lower. Combined with a higher heat flux from the crust, as suggested by recent studies, turbulent mixing could explain the observed critical accretion rate. Furthermore, close to this boundary we find oscillatory burning, which previous studies have linked to mHz QPOs. In models where we continuously lower the heat flux from the crust, the period of the oscillations increases by up to several tens of percents, similar to the observed frequency drift, suggesting that this drift could be caused by the cooling of deeper layers.

Helium Nova on a Very Massive White Dwarf: A Revised Light‐Curve Model of V445 Puppis (2000)

V445 Pup (2000) is a unique object identified as a helium nova. Color indexes during the outburst are consistent with those of free-free emission. We present a free-free emission-dominated light-curve model of V445 Pup on the basis of the optically thick wind theory. Our light-curve fitting shows that (1) the white dwarf (WD) mass is very massive (MWD 1.35 M☉) and (2) half of the accreted matter remains on the WD, both of which suggest that the WD mass is increasing. Therefore, V445 Pup is a strong candidate for a Type Ia supernova progenitor. The estimated distance to V445 Pup is now consistent with recent observational suggestions, 3.5 kpc d 6.5 kpc. A helium star companion is consistent with the brightness of mv = 14.5 mag just before the outburst, if it is a slightly evolved hot (log T[K] 4.5) star with the mass of MHe 0.8 M☉. We then emphasize importance of observations in the near-future quiescent phase after the thick circumstellar dust dissipates away, especially its color and magnitude, to specify the nature of the companion star. We have also calculated helium ignition masses for helium shell flashes against various helium accretion rates and discussed the recurrence period of helium novae.

Energy absorption of xenon clusters in helium nanodroplets under strong laser pulses

Energy absorption of xenon clusters embedded in helium nanodroplets from strong femtosecond laser pulses is studied theoretically. Compared to pure clusters we find earlier and more efficient energy absorption in agreement with experiments. This effect is due to resonant absorption of the helium nanoplasma whose formation is catalyzed by the xenon core. For very short double pulses with variable delay, two plasma resonances, due to the helium shell and the xenon core, are identified and the experimental conditions are given which should allow for a simultaneous observation of both of them.

Defining the Termination of the Asymptotic Giant Branch

I suggest a theoretical quantitative definition for the termination of the asymptotic giant branch (AGB) phase and the beginning of the post-AGB phase. I suggest that the transition will be taken to occur when the ratio of the dynamical time scale to the the envelope thermal time scale, Q, reaches its maximum value. Time average values are used for the different quantities, as the criterion does not refer to the short time-scale variations occurring on the AGB and post-AGB, e.g., thermal pulses (helium shell flashes) and magnetic activity. Along the entire AGB the value of Q increases, even when the star starts to contract. Only when a rapid contraction starts does the value of Q start to decrease. This criterion captures the essence of the transition from the AGB to the post AGB phase, because Q is connected to the stellar effective temperature, reaching its maximum value at T~4000-6000 K, it is related to the mass loss properties, and it reaches its maximum value when rapid contraction starts and envelope mass is very low.

Imaging ejecta from the final flash star V605 Aquilae

Aims. We investigated the cloud of ejecta resulting from the mass loss associated with the final helium shell flash in V605 Aql. Methods. V605 Aql was imaged at high spatial resolution in both optical emission lines and the infrared continuum using HST/WFPC2 and Gemini/Hokupa' a+QUIRC, respectively. The He I 10 830 A spectrum was also observed. Results. The obscuring circumstellar shell, whose effects were first seen in 1923, is shown to be a disk with extended structure, including knots. The morphology of the V605 Aql circumstellar shell is discussed as an 80 year old in an evolutionary sequence consisting of recent, as well as much older, final flash objects. Conclusions. The obscuration of V605 Aql by dust marked the emergence of the hot white dwarf remnant from an optically thick pseudo-photosphere. This white dwarf drives a 2500 km s -1 wind principally in one plane resulting in a circumstellar disk. Where the wind encounters the circumstellar disk, He I 10 830 A emission is created, and hot, ∼ 1500 K, grains are generated. Grains exit the disk at 350 K, and the accompanying gas then expands at ∼ 140 km s -1 . The strong concentration of the mass loss to the disk suggests the white dwarf is now rotating rapidly. There is convincing evidence in the literature that this process is also seen in V4334 Sgr and is still going on in the old final flash objects A30 and A78.

Neutrino-induced nucleosynthesis in supernovae: synthesis of light elements and neutrino-driven r-process

We report the results of our neutrino nucleosynthesis project that concern the neutrino-induced production of light and heavy elements in the helium shell of poor metal core collapse supernovae. The outer half of the helium shell is shown to be a good site to obtain the neutrino-driven weak component of the r-process up to A ≈ 130 and simultaneously to produce significant yields of the light elements such as 7Li and 11B. Special attention is given to an old idea that by means of the activation by shock heating reaction 13C(α, n)16O an enhanced amount of 13C in the helium shell could be an additional source of neutrons for the r-process. The main conclusion is that both the neutrino and 13C neutron sources can contribute to the r-process only for poor metal stars, the latter being active for rather strong explosions of energy ≳3 × 1051 erg. Contrary to the light elements, the r-process yields are strongly sensitive to the admixture of the neutron poisons such as 12C, 14N and 16O.

Double-detonation supernovae of sub-Chandrasekhar mass white dwarfs

Type Ia supernovae are believed to be white dwarfs disrupted by a thermonuclear explosion. Here we investigate the scenario in which a rather low-mass, carbon-oxygen (C + O) white dwarf accumulates helium on its surface in a sufficient amount for igniting a detonation in the helium shell before the Chandrasekhar mass is reached. In principle, this can happen on white dwarfs accreting from a non-degenerate companion or by merging a C + O white dwarf with a low-mass helium one. In this scenario, the helium detonation is thought to trigger a secondary detonation in the C + O core. It is therefore called the “double-detonation sub-Chandrasekhar” supernova model. By means of a set of numerical simulations, we investigate the robustness of this explosion mechanism for generic 1-M� models and analyze its observable predictions. Also a resolution dependence in numerical simulations is analyzed. Hydrodynamic simulations of the double-detonation sub-Chandrasekhar scenario are conducted in two and three spatial dimensions. The propagation of thermonuclear detonation fronts, both in helium and in the carbon-oxygen mixture, is computed by means of both a level-set function and a simplified description for nuclear reactions. The decision whether a secondary detonation is triggered in the white dwarf’s core or not is made based on criteria given in the literature. In a parameter study involving different initial flame geometries − − − [ � ] ] −

Evolution of Low‐Mass Population III Stars

We present the evolutionary models of metal-free stars in the mass range from 0.8 to 1.2 Msun with up-to-date input physics. The evolution is followed to the onset of hydrogen mixing into a convection, driven by the helium flash at red giant or asymptotic giant branch phase. The models of mass M >= 0.9 Msun undergo the central hydrogen flash, triggered by the carbon production due to the 3-alpha reactions. We find that the border of the off-center and central ignition of helium core flash falls between 1.1 and 1.2 Msun; the models of mass M <= 1.1 Msun experience the hydrogen mixing at the tip of red giant branch while the models of M = 1.2 Msun during the helium shell flashes on the asymptotic giant branch. The equation of state for the Coulomb liquid region, where electron conduction and radiation compete, is shown to be important since it affects the thermal state in the helium core and influences the red giant branch evolution. It is also found that the non-resonant term of 3-alpha reactios plays an important role, although it has negligible effect in the evolution of stars of younger populations. We compare our models with the computations by several other sets of authors, to confirm the good agreement except for one study which finds the helium ignition much closer to the center with consequences important for subsequent evolution.

Ion induced snowballs as a diagnostic tool to investigate the caging of metal clusters in large helium droplets

Metal clusters embedded in ultracold helium nanodroplets are exposed to femtosecond laser pulses with intensities of 10(13)-10(14) W/cm2. The influence of the matrix on the ionization and fragmentation dynamics is studied by pump-probe time-of-flight mass spectrometry. Special attention is paid to the generation of helium snowballs around positive metal ions (Me(z+)He(N), z=1,2). Closings of the first and second helium shells are found for silver at N(1)=10,12 and N(2)=32,44, as well as for magnesium at N1=19-20. The distinct abundance enhancement of helium snowballs in the presence of isolated atoms and small clusters in the droplets is used as a diagnostics to explore the cage effect. For silver, a reaggregation of the clusters is observed at 30 ps after femtosecond laser excitation.

Faint Thermonuclear Supernovae from AM Canum Venaticorum Binaries

Helium that accretes onto a carbon/oxygen white dwarf in double white dwarf AM Canum Venaticorum (AM CVn) binaries undergoes unstable thermonuclear flashes when the orbital period is in the 3.5-25 minute range. At the shortest orbital periods (and highest accretion rates, ME™>10 -7 M solar yr -1 ), the flashes are weak and likely lead to the helium equivalent of classical nova outbursts. However, as the orbit widens and ME™ drops, the mass required for the unstable ignition increases, leading to progressively more violent flashes up to a final flash with helium shell mass ~0.02-0.1 M solar . The high pressures of these last flashes allow the burning to produce the radioactive elements 48 Cr, 52 Fe, and 56 Ni that power a faint (M V =-15 to -18) and rapidly rising (few days) thermonuclear supernova. Current galactic AM CVn space densities imply one such explosion every 5,000-15,000 years in 10 11 M solar of old stars (~2%-6% of the Type Ia rate in E/SO galaxies). These ``.Ia'' supernovae (one-tenth as bright for one-tenth the time as a Type Ia supernovae) are excellent targets for deep (e.g., V=24) searches with nightly cadences, potentially yielding an all-sky rate of 1000 per year.

Weak r-process component as a result of the neutrino interaction with the helium shell of a supernova

Possibilities for the development of an r-process in the helium shell of a supernova under the action of free neutrons appearing as a result of inelastic neutrino scattering by 4He nuclei are investigated. The conditions in the outer part of the helium shell in metal-poor stars are shown to be favorable for the reproduction of a weak r-process component.

CXOU J121538.2+361921 in the galaxy NGC 4214: a double neutron star in the making?

Abstract CXOU J121538.2+361921 is the brightest X-ray source in the galaxy NGC 4214, with an X-ray luminosity of up to 0.7 × 1039 erg s−1. The observed periodicity of 3.62 h, derived from eclipse timing, is interpreted as the orbital period of the system. It has been suggested that the system is a low-mass helium star with a lower-mass compact companion. If this idea is correct, then CXOU J121538.2+361921 will evolve into a double neutron star, a binary consisting of a radio pulsar and another neutron star. In this study we investigate this possibility further. We find that the X-ray luminosity is consistent with super-Eddington accretion in a helium star-neutron star binary. The binary is in a state of mass transfer which is initiated when the helium-star donor is at the helium shell burning stage. A donor star with a current mass in the range of around 2.2–3.6 M⊙ is required to explain the observed orbital period. Helium stars in this mass range are massive enough to collapse in a supernova explosion, making CXOU J121538.2+361921 the immediate progenitor of a double neutron star.

Large-scale simulations of turbulent stellar convection flows and the outlook for petascale computation

The late stages of stellar evolution have great importance for the synthesis and dispersal of the elements heavier than helium. We focus on the helium shell flash in low mass stars, where incorporation of hydrogen into the convection zone above the helium burning shell can result in production of carbon-13 with tremendous release of energy. The need for detailed 3-D simulations in understanding this process is explained. To make simulations of the entire helium flash event practical, models of turbulent multimaterial mixing and nuclear burning must be constructed and validated. As an example of the modeling and validation process, our recent work on modeling subgrid-scale turbulence in 3-D compressible gas dynamics simulations is described and a new turbulence model presented along with supporting results. Finally, the potential impact of petascale computing hardware on this problem is explored.