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At least 1,225 records · Page 68
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DOE OSTI2015

A new merging double degenerate binary in the solar neighborhood

Characterizing the local space density of double degenerate (DD) binary systems is a complementary approach to broad sky surveys of DDs to determine the expected rates of WD binary mergers, in particular those that may evolve into other observable phenomena such as extreme helium stars, Am CVn systems, and SNe Ia. However, there have been few such systems detected in local space. We report here the discovery that WD 1242–105, a nearby bright WD, is a double-line spectroscopic binary consisting of two degenerate DA WDs of similar mass and temperature, despite it previously having been spectroscopically characterized as a single degenerate. Follow-up photometry, spectroscopy, and trigonometric parallax have been obtained in an effort to determine the fundamental parameters of each component of this system. The binary has a mass ratio of 0.7 and a trigonometric parallax of 25.5 mas, placing it at a distance of 39 pc. The system’s total mass is 0.95 M{sub ⊙} and has an orbital period of 2.85 hr, making it the strongest known gravitational wave source (logh=−20.78) in the mHz regime. Because of its orbital period and total mass, WD 1242–105 is predicted to merge via gravitational radiation on a timescale of 740 Myr, which will most likely not result in a catastrophic explosion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ERRATUM: “Automated Transient Identification in the Dark Energy Survey” (2015, AJ, 150, 82)

Here, we describe an algorithm for identifying point-source transients and moving objects on reference-subtracted optical images containing artifacts of processing and instrumentation. The algorithm makes use of the supervised machine learning technique known as Random Forest. We present results from its use in the Dark Energy Survey Supernova program (DES-SN), where it was trained using a sample of 898,963 signal and background events generated by the transient detection pipeline. After reprocessing the data collected during the first DES-SN observing season (2013 September through 2014 February) using the algorithm, the number of transient candidates eligible for human scanning decreased by a factor of 13.4, while only 1.0% of the artificial Type Ia supernovae (SNe) injected into search images to monitor survey efficiency were lost, most of which were very faint events. Here we characterize the algorithm's performance in detail, and we discuss how it can inform pipeline design decisions for future time-domain imaging surveys, such as the Large Synoptic Survey Telescope and the Zwicky Transient Facility. An implementation of the algorithm and the training data used in this paper are available at at http://portal.nersc.gov/project/dessn/autoscan.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

SPECTROSCOPIC AND PHOTOMETRIC VARIABILITY IN THE A0 SUPERGIANT HR 1040

A time-series analysis of spectroscopic and photometric observables of the A0 Ia supergiant HR 1040 has been performed, including equivalent widths, radial velocities, and Strömgren photometric indices. The data, obtained from 1993 through 2007, include 152 spectroscopic observations from the Ritter Observatory 1 m telescope and 269 Strömgren photometric observations from the Four College Automated Photoelectric Telescope. Typical of late B- and early A-type supergiants, HR 1040 has a highly variable Hα profile. The star was found to have an intermittent active phase marked by correlation between the Hα absorption equivalent width and blue-edge radial velocity and by photospheric connections observed in correlations to equivalent width, second moment and radial velocity in Si ii λλ6347, 6371. High-velocity absorption (HVA) events were observed only during this active phase. HVA events in the wind were preceded by photospheric activity, including Si ii radial velocity oscillations 19–42 days prior to onset of an HVA event and correlated increases in Si ii W{sub λ} and second moment from 13 to 23 days before the start of the HVA event. While increases in various line equivalent widths in the wind prior to HVA events have been reported in the past in other stars, our finding of precursors in enhanced radial velocity variations in the wind and at the photosphere is a new result.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

PROBABILISTIC CROSS-IDENTIFICATION OF COSMIC EVENTS

I discuss a novel approach to identifying cosmic events in separate and independent observations. The focus is on the true events, such as supernova explosions, that happen once and, hence, whose measurements are not repeatable. Their classification and analysis must make the best use of all available data. Bayesian hypothesis testing is used to associate streams of events in space and time. Probabilities are assigned to the matches by studying their rates of occurrence. A case study of Type Ia supernovae illustrates how to use light curves in the cross-identification process. Constraints from realistic light curves happen to be well approximated by Gaussians in time, which makes the matching process very efficient. Model-dependent associations are computationally more demanding but can further boost one's confidence.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

DETECTION OF A WHITE DWARF COMPANION TO THE WHITE DWARF SDSSJ125733.63+542850.5

SDSSJ125733.63+542850.5 (hereafter SDSSJ1257+5428) is a compact white dwarf binary from the Sloan Digital Sky Survey that exhibits high-amplitude radial velocity variations on a period of 4.56 hr. While an initial analysis suggested the presence of a neutron star or black hole binary companion, a follow-up study concluded that the spectrum was better understood as a combination of two white dwarfs. Here we present optical spectroscopy and ultraviolet fluxes which directly reveal the presence of the second white dwarf in the system. SDSSJ1257+5428's spectrum is a composite, dominated by the narrow-lined spectrum from a cool, low-gravity white dwarf (T{sub eff} {approx_equal} 6300 K, log g = 5-6.6) with broad wings from a hotter, high-mass white dwarf companion (11, 000-14, 000 K; {approx}1 M{sub sun}). The high-mass white dwarf has unusual line profiles which lack the narrow central core to H{alpha} that is usually seen in white dwarfs. This is consistent with rapid rotation with vsin i = 500-1750 km s{sup -1}, although other broadening mechanisms such as magnetic fields, pulsations, or a helium-rich atmosphere could also be contributory factors. The cool component is a puzzle since no evolutionary model matches its combination of low gravity and temperature. Within the constraints set by our data, SDSSJ1257+5428 could have a total mass greater than the Chandrasekhar limit and thus be a potential Type Ia supernova progenitor. However, SDSSJ1257+5428's unusually low-mass ratio q {approx} 0.2 suggests that it is more likely that it will evolve into an accreting double white dwarf (AM CVn star).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

FEEDBACK FROM CENTRAL BLACK HOLES IN ELLIPTICAL GALAXIES: TWO-DIMENSIONAL MODELS COMPARED TO ONE-DIMENSIONAL MODELS

We extend the black hole (BH) feedback models of Ciotti, Ostriker, and Proga to two dimensions. In this paper, we focus on identifying the differences between the one-dimensional and two-dimensional hydrodynamical simulations. We examine a normal, isolated L{sub *} galaxy subject to the cooling flow instability of gas in the inner regions. Allowance is made for subsequent star formation, Type Ia and Type II supernovae, radiation pressure, and inflow to the central BH from mildly rotating galactic gas which is being replenished as a normal consequence of stellar evolution. The central BH accretes some of the infalling gas and expels a conical wind with mass, momentum, and energy flux derived from both observational and theoretical studies. The galaxy is assumed to have low specific angular momentum in analogy with the existing one-dimensional case in order to isolate the effect of dimensionality. The code then tracks the interaction of the outflowing radiation and winds with the galactic gas and their effects on regulating the accretion. After matching physical modeling to the extent possible between the one-dimensional and two-dimensional treatments, we find essentially similar results in terms of BH growth and duty cycle (fraction of the time above a given fraction of the Eddington luminosity). In the two-dimensional calculations, the cool shells forming at 0.1-1 kpc from the center are Rayleigh-Taylor unstable to fragmentation, leading to a somewhat higher accretion rate, less effective feedback, and a more irregular pattern of bursting compared with the one-dimensional case.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

X-RAY AND OPTICAL OBSERVATIONS OF THE UNIQUE BINARY SYSTEM HD 49798/RX J0648.0-4418

We report the results of XMM-Newton observations of HD 49798/RX J0648.0-4418, the only known X-ray binary consisting of a hot sub-dwarf and a white dwarf. The white dwarf rotates very rapidly (P = 13.2 s) and has a dynamically measured mass of 1.28 {+-} 0.05 M{sub sun}. Its X-ray emission consists of a strongly pulsed, soft component, well fit by a blackbody with kT{sub BB} {approx} 40 eV, accounting for most of the luminosity, and a fainter hard power-law component (photon index {approx}1.6). A luminosity of {approx}10{sup 32} erg s{sup -1} is produced by accretion onto the white dwarf of the helium-rich matter from the wind of the companion, which is one of the few hot sub-dwarfs showing evidence of mass loss. A search for optical pulsations at the South African Astronomical Observatory 1.9 m telescope gave negative results. X-rays were also detected during the white dwarf eclipse. This emission, with luminosity 2 x 10{sup 30} erg s{sup -1}, can be attributed to HD 49798 and represents the first detection of a hot sub-dwarf star in the X-ray band. HD 49798/RX J0648.0-4418 is a post-common-envelope binary which most likely originated from a pair of stars with masses {approx}8-10 M{sub sun}. After the current He-burning phase, HD 49798 will expand and reach the Roche lobe, causing a higher accretion rate onto the white dwarf which can reach the Chandrasekhar limit. Considering the fast spin of the white dwarf, this could lead to the formation of a millisecond pulsar. Alternatively, this system could be a Type Ia supernova progenitor with the appealing characteristic of a short time delay, being the descendent of relatively massive stars.

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DOE OSTI2011

THE EFFECT OF PECULIAR VELOCITIES ON SUPERNOVA COSMOLOGY

We analyze the effect that peculiar velocities have on the cosmological inferences we make using luminosity distance indicators, such as Type Ia supernovae. In particular we study the corrections required to account for (1) our own motion, (2) correlations in galaxy motions, and (3) a possible local under- or overdensity. For all of these effects we present a case study showing the impact on the cosmology derived by the Sloan Digital Sky Survey-II Supernova Survey (SDSS-II SN Survey). Correcting supernova (SN) redshifts for the cosmic microwave background (CMB) dipole slightly overcorrects nearby SNe that share some of our local motion. We show that while neglecting the CMB dipole would cause a shift in the derived equation of state of {Delta}w {approx} 0.04 (at fixed {Omega}{sub m}), the additional local-motion correction is currently negligible ({Delta}w {approx}< 0.01). We then demonstrate a covariance-matrix approach to statistically account for correlated peculiar velocities. This down-weights nearby SNe and effectively acts as a graduated version of the usual sharp low-redshift cut. Neglecting coherent velocities in the current sample causes a systematic shift of {Delta}w {approx} 0.02. This will therefore have to be considered carefully when future surveys aim for percent-level accuracy and we recommend our statistical approach to down-weighting peculiar velocities as a more robust option than a sharp low-redshift cut.

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DOE OSTI2011

THE CHANGE OF THE ORBITAL PERIODS ACROSS ERUPTIONS AND THE EJECTED MASS FOR RECURRENT NOVAE CI AQUILAE AND U SCORPII

I report on the cumulative results from a program started 24 years ago designed to measure the orbital period change of recurrent novae (RNe) across an eruption. The goal is to use the orbital period change to measure the mass ejected during each eruption as the key part of trying to measure whether the RNe white dwarfs are gaining or losing mass over an entire eruption cycle, and hence whether they can be progenitors for Type Ia supernovae. This program has now been completed for two eclipsing RNe: CI Aquilae (CI Aql) across its eruption in 2000 and U Scorpii (U Sco) across its eruption in 1999. For CI Aql, I present 78 eclipse times from 1991 to 2009 (including four during the tail of the 2000 eruption) plus two eclipses from 1926 and 1935. For U Sco, I present 67 eclipse times, including 46 times during quiescence from 1989 to 2009, plus 21 eclipse times in the tails of the 1945, 1999, and 2010 eruptions. The eclipse times during the tails of eruptions are systematically and substantially shifted with respect to the ephemerides from the eclipses in quiescence, with this being caused by shifts of the center of light during the eruption. These eclipse times are plotted on an O - C diagram and fitted to models with a steady period change ( P-dot ) between eruptions (caused by, for example, conservative mass transfer) plus an abrupt period change ({Delta}P) at the time of eruption. The primary uncertainty arises from the correlation between {Delta}P with P-dot , such that a more negative P-dot makes for a more positive {Delta}P. For CI Aql, the best fit is {Delta}P = -3.7{sup +9.2}{sub -7.3} Multiplication-Sign 10{sup -7}. For U Sco, the best fit is {Delta}P = (+ 43 {+-} 69) Multiplication-Sign 10{sup -7} days. These period changes can directly give a dynamical measure of the mass ejected (M{sub ejecta}) during each eruption with negligible sensitivity to the stellar masses and no uncertainty from distances. For CI Aql, the 1{sigma} upper limit is M{sub ejecta} < 10 Multiplication-Sign 10{sup -7} M{sub Sun }. For U Sco, I derive M{sub ejecta} = (43 {+-} 67) Multiplication-Sign 10{sup -7} M{sub Sun }.

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DOE OSTI2011

QUARK-NOVAE IN LOW-MASS X-RAY BINARIES. II. APPLICATION TO G87-7 AND TO GRB 110328A

We propose a simple model explaining two outstanding astrophysical problems related to compact objects: (1) that of stars such as G87-7 (alias EG 50) that constitute a class of relatively low-mass white dwarfs (WDs) which nevertheless fall away from the C/O composition and (2) that of GRB 110328A/Swift J164449.3+57345 which showed spectacularly long-lived strong X-ray flaring, posing a challenge to standard gamma-ray burst models. We argue that both these observations may have an explanation within the unified framework of a quark-nova (QN) occurring in a low-mass X-ray binary (LMXB; neutron star (NS)-WD). For LMXBs, where the binary separation is sufficiently tight, ejecta from the exploding NS triggers nuclear burning in the WD on impact, possibly leading to Fe-rich composition compact WDs with mass 0.43 M{sub Sun} < M{sub WD} < 0.72 M{sub Sun }, reminiscent of G87-7. Our results rely on the assumption, which ultimately needs to be tested by hydrodynamic and nucleosynthesis simulations, that under certain circumstances the WD can avoid the thermonuclear runaway. For heavier WDs (i.e., M{sub WD} > 0.72 M{sub Sun }) experiencing the QN shock, degeneracy will not be lifted when carbon burning begins, and a sub-Chandrasekhar Type Ia supernova may result in our model. Under slightly different conditions and for pure He WDs (i.e., M{sub WD} < 0.43 M{sub Sun }), the WD is ablated and its ashes raining down on the quark star (QS) leads to accretion-driven X-ray luminosity with energetics and duration reminiscent of GRB 110328A. We predict additional flaring activity toward the end of the accretion phase if the QS turns into a black hole.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ASYMMETRY AND THE NUCLEOSYNTHETIC SIGNATURE OF NEARLY EDGE-LIT DETONATION IN WHITE DWARF CORES

Most of the leading explosion scenarios for Type Ia supernovae involve the nuclear incineration of a white dwarf star through a detonation wave. Several scenarios have been proposed as to how this detonation may actually occur, but the exact mechanism and environment in which it takes place remain unknown. We explore the effects of an off-center initiated detonation on the spatial distribution of the nucleosynthetic yield products in a toy model-a pre-expanded near Chandrasekhar-mass white dwarf. We find that a single-point near edge-lit detonation results in asymmetries in the density and thermal profiles, notably the expansion timescale, throughout the supernova ejecta. We demonstrate that this asymmetry of the thermodynamic trajectories should be common to off-center detonations where a small amount of the star is burned prior to detonation. The sensitivity of the yields on the expansion timescale results in an asymmetric distribution of the elements synthesized as reaction products. We tabulate the shift in the center of mass of the various elements produced in our model supernova and find an odd-even pattern for elements past silicon. Our calculations show that off-center single-point detonations in carbon-oxygen white dwarfs are marked by significant composition asymmetries in their remnants which bear potentially observable signatures in both velocity and coordinate space, including an elemental nickel mass fraction that varies by a factor of 2-3 from one side of the remnant to the other.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

IRON AND {alpha}-ELEMENT PRODUCTION IN THE FIRST ONE BILLION YEARS AFTER THE BIG BANG

We present measurements of carbon, oxygen, silicon, and iron in quasar absorption systems existing when the universe was roughly one billion years old. We measure column densities in nine low-ionization systems at 4.7 < z < 6.3 using Keck, Magellan, and Very Large Telescope optical and near-infrared spectra with moderate to high resolution. The column density ratios among C II, O I, Si II, and Fe II are nearly identical to sub-damped Ly{alpha} systems (sub-DLAs) and metal-poor ([M/H] {<=} -1) DLAs at lower redshifts, with no significant evolution over 2 {approx}< z {approx}< 6. The estimated intrinsic scatter in the ratio of any two elements is also small, with a typical rms deviation of {approx}< 0.1 dex. These facts suggest that dust depletion and ionization effects are minimal in our z > 4.7 systems, as in the lower-redshift DLAs, and that the column density ratios are close to the intrinsic relative element abundances. The abundances in our z > 4.7 systems are therefore likely to represent the typical integrated yields from stellar populations within the first gigayear of cosmic history. Due to the time limit imposed by the age of the universe at these redshifts, our measurements thus place direct constraints on the metal production of massive stars, including iron yields of prompt supernovae. The lack of redshift evolution further suggests that the metal inventories of most metal-poor absorption systems at z {approx}> 2 are also dominated by massive stars, with minimal contributions from delayed Type Ia supernovae or winds from asymptotic giant branch stars. The relative abundances in our systems broadly agree with those in very metal-poor, non-carbon-enhanced Galactic halo stars. This is consistent with the picture in which present-day metal-poor stars were potentially formed as early as one billion years after the big bang.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

MORPHOLOGY OF GALAXY CLUSTERS: A COSMOLOGICAL MODEL-INDEPENDENT TEST OF THE COSMIC DISTANCE-DUALITY RELATION

Aiming at comparing different morphological models of galaxy clusters, we use two new methods to make a cosmological model-independent test of the distance-duality (DD) relation. The luminosity distances come from the Union2 compilation of Supernovae Type Ia. The angular diameter distances are given by two cluster models (De Filippis et al. and Bonamente et al.). The advantage of our methods is that they can reduce statistical errors. Concerning the morphological hypotheses for cluster models, it is mainly focused on the comparison between the elliptical {beta}-model and spherical {beta}-model. The spherical {beta}-model is divided into two groups in terms of different reduction methods of angular diameter distances, i.e., the conservative spherical {beta}-model and corrected spherical {beta}-model. Our results show that the DD relation is consistent with the elliptical {beta}-model at 1{sigma} confidence level (CL) for both methods, whereas for almost all spherical {beta}-model parameterizations, the DD relation can only be accommodated at 3{sigma} CL, particularly for the conservative spherical {beta}-model. In order to minimize systematic uncertainties, we also apply the test to the overlap sample, i.e., the same set of clusters modeled by both De Filippis et al. and Bonamente et al. It is found that the DD relation is compatible with the elliptically modeled overlap sample at 1{sigma} CL; however, for most of the parameterizations the DD relation cannot be accommodated even at 3{sigma} CL for any of the two spherical {beta}-models. Therefore, it is reasonable that the marked triaxial ellipsoidal model is a better geometrical hypothesis describing the structure of the galaxy cluster compared with the spherical {beta}-model if the DD relation is valid in cosmological observations.

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DOE OSTI2012

ASTEROSEISMOLOGY OF THE NEARBY SN-II PROGENITOR: RIGEL. I. THE MOST HIGH-PRECISION PHOTOMETRY AND RADIAL VELOCITY MONITORING

Rigel ({beta} Ori, B8 Ia) is a nearby blue supergiant displaying {alpha} Cyg type variability, and is one of the nearest Type II supernova progenitors. As such it is an excellent test bed to study the internal structure of pre-core-collapse stars. In this study, for the first time, we present 28 days of high-precision MOST photometry and over six years of spectroscopic monitoring. We report 19 significant pulsation modes of signal-to-noise ratio, S/N {approx}> 4.6 from radial velocities, with variability timescales ranging from 1.21 to 74.7 days, which are associated with high-order low-degree gravity modes. While the radial velocity variations show a degree of correlation with the flux changes, there is no clear interplay between the equivalent widths of different metallic and H{alpha} lines.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE LONG-TERM EVOLUTION OF DOUBLE WHITE DWARF MERGERS

In this paper, we present a model for the long-term evolution of the merger of two unequal mass C/O white dwarfs (WDs). After the dynamical phase of the merger, magnetic stresses rapidly redistribute angular momentum, leading to nearly solid-body rotation on a viscous timescale of 10{sup -4}-1 yr, long before significant cooling can occur. Due to heating during the dynamical and viscous phases, the less massive WD is transformed into a hot, slowly rotating, and radially extended envelope supported by thermal pressure. Following the viscous phase of evolution, the maximum temperature near the envelope base may already be high enough to begin off-center convective carbon burning. If not, Kelvin-Helmholtz contraction of the inner region of the envelope on a thermal timescale of 10{sup 3}-10{sup 4} yr compresses the base of the envelope, again yielding off-center burning. As a result, the long-term evolution of the merger remnant is similar to that seen in previous calculations: the burning shell diffuses inward over {approx}10{sup 4} yr, eventually yielding a high-mass O/Ne WD or a collapse to a neutron star, rather than a Type Ia supernova. During the cooling and shell-burning phases, the merger remnant radiates near the Eddington limit. Given the double WD merger rate of a few per 1000 yr, a few dozen of these {approx}10{sup 38} erg s{sup -1} sources should exist in a Milky Way type galaxy. While the end result is similar to that of previous studies, the physical picture and the dynamical state of the matter in our model differ from previous work. Furthermore, substantial remaining uncertainties related to the convective structure near the photosphere and mass loss during the thermal evolution may significantly affect our conclusions. Thus, future work within the context of the physical model presented here is required to better address the eventual fate of double WD mergers, including those for which one or both of the components is a He WD.

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DOE OSTI2012

AN X-RAY STUDY OF SUPERNOVA REMNANT N49 AND SOFT GAMMA-RAY REPEATER 0526-66 IN THE LARGE MAGELLANIC CLOUD

We report on the results from our deep Chandra observation (120 ks) of the supernova remnant (SNR) N49 and soft gamma-ray repeater (SGR) 0526-66 in the Large Magellanic Cloud. We firmly establish the detection of an ejecta 'bullet' beyond the southwestern boundary of N49. The X-ray spectrum of the bullet is distinguished from that of the main SNR shell, showing significantly enhanced Si and S abundances. We also detect an ejecta feature in the eastern shell, which shows metal overabundances similar to those of the bullet. If N49 was produced by a core-collapse explosion of a massive star, the detected Si-rich ejecta may represent explosive O-burning or incomplete Si-burning products from deep interior of the SN. On the other hand, the observed Si/S abundance ratio in the ejecta may favor Type Ia origin for N49. We refine the Sedov age of N49, {tau}{sub Sed} {approx} 4800 yr, with the explosion energy E{sub 0} {approx} 1.8 Multiplication-Sign 10{sup 51} erg. Our blackbody (BB) + power law (PL) model for the quiescent X-ray emission from SGR 0526-66 indicates that the PL photon index ({Gamma} {approx} 2.5) is identical to that of PSR 1E1048.1-5937, the well-known candidate transition object between anomalous X-ray pulsars and SGRs. Alternatively, the two-component BB model implies X-ray emission from a small (R {approx} 1 km) hot spot(s) (kT {approx} 1 keV) in addition to emission from the neutron star's cooler surface (R {approx} 10 km, kT {approx} 0.4 keV). There is a considerable discrepancy in the estimated column toward 0526-66 between BB+PL and BB+BB model fits. Discriminating these spectral models would be crucial to test the long-debated physical association between N49 and 0526-66.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ASTEROSEISMOLOGY OF THE NEARBY SN II PROGENITOR RIGEL. II. {epsilon}-MECHANISM TRIGGERING GRAVITY-MODE PULSATIONS?

The cores of luminous B- and A-type (BA) supergiant stars are the seeds of later core-collapse supernovae. Thus, constraining the near-core conditions in this class of stars can place tighter constraints on the size, mass, and chemical composition of supernova remnants. Asteroseismology of these massive stars is one possible approach into such investigations. Recently, Moravveji et al. in 2012 (hereafter Paper I) extracted 19 significant frequencies from a 6-year radial velocity monitoring of Rigel ({beta} Ori, B8 Ia). The periods they determined broadly range from 1.22 to 74.74 days. Based on our differentially rotating stellar structure and evolution model, Rigel, at its current evolutionary state, is undergoing core He burning and shell H burning. Linear fully non-adiabatic non-radial stability analyses result in the excitation of a dense spectrum of non-radial gravity-dominated mixed modes. The fundamental radial mode (l = 0) and its overtones are all stable. When the hydrogen-burning shell is located even partially in the radiative zone, a favorable condition for destabilization of g-modes through the so-called {epsilon}-mechanism becomes viable. Only those g-modes that have high relative amplitudes in the hydrogen-burning (radiative) zone can survive the strong radiative damping. From the entire observed range of variability periods of Rigel (found in Paper I), and based on our model, only those modes with periods ranging between 21 and 127 days can be theoretically explained by the {epsilon}-mechanism. The origin of the short-period variations (found in Paper I) still remains unexplained. Because Rigel is similar to other massive BA supergiants, we believe that the {epsilon}-mechanism may be able to explain the long-period variations in {alpha} Cygni class of pulsating stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

NUCLEOSYNTHESIS AND THE INHOMOGENEOUS CHEMICAL EVOLUTION OF THE CARINA DWARF GALAXY

The detailed abundances of 23 chemical elements in nine bright red giant branch stars in the Carina dwarf spheroidal galaxy are presented based on high-resolution spectra gathered at the Very Large Telescope (VLT) and Magellan telescopes. A spherical model atmospheres analysis is applied using standard methods (local thermodynamic equilibrium and plane-parallel radiative transfer) to spectra ranging from 380 to 680 nm. Stellar parameters are found to be consistent between photometric and spectroscopic analyses, both at moderate and high resolution. The stars in this analysis range in metallicity from -2.9 < [Fe/H] <-1.3, and adopting the ages determined by Lemasle et al., we are able to examine the chemical evolution of Carina's old and intermediate-aged populations. One of the main results from this work is the evidence for inhomogeneous mixing in Carina and therefore for a poor statistical sampling of the supernova contributions when forming stars; a large dispersion in [Mg/Fe] indicates poor mixing in the old population, an offset in the [{alpha}/Fe] ratios between the old and intermediate-aged populations (when examined with previously published results) suggests that the second star formation event occurred in {alpha}-enriched gas, and one star, Car-612, seems to have formed in a pocket enhanced in SN Ia/II products. This latter star provides the first direct link between the formation of stars with enhanced SN Ia/II ratios in dwarf galaxies to those found in the outer Galactic halo (Ivans et al.). Another important result is the potential evidence for SN II driven winds. We show that the very metal-poor stars in Carina have not been enhanced in asymptotic giant branch or SN Ia products, and therefore their very low ratios of [Sr/Ba] suggests the loss of contributions from the early SNe II. Low ratios of [Na/Fe], [Mn/Fe], and [Cr/Fe] in two of these stars support this scenario, with additional evidence from the low [Zn/Fe] upper limit for one star. It is interesting that the chemistry of the metal-poor stars in Carina is not similar to those in the Galaxy, most of the other dwarf spheroidal galaxies, or the ultra faint dwarfs, and suggests that Carina may be at the critical mass where some chemical enrichments are lost through SN II driven winds.

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