Nevin N. Weinberg

MEASURING DISTANCE AND PROPERTIES OF THE MILKY WAY'S CENTRAL SUPERMASSIVE BLACK HOLE WITH STELLAR ORBITS

We report new precision measurements of the properties of our Galaxys supermassive black hole. Based on astrometric (1995-2007) and radial velocity (RV; 2000-2007) measurements from the W. M. Keck 10m telescopes, a fully unconstrained Keplerian orbit for the short-period star S0-2 provides values for the distance (R_0) of 8.0±0.6 kpc, the enclosed mass (M_(bh)) of 4.1±0.6x10^6 M☉ and the black holes RV, which is consistent with zero with 30 km/s uncertainty. If the black hole is assumed to be at rest with respect to the Galaxy (e. g., has no massive companion to induce motion), we can further constrain the fit, obtaining R_0 = 8.4±0.4kpc and M_(bh) 4.5±0.4x10^6 M☉. More complex models constrain the extended dark mass distribution to be less than 3-4x10^5 M☉ within 0.01 pc, ~100 times higher than predictions from stellar and stellar remnant models. For all models, we identify transient astrometric shifts from source confusion (up to 5 times the astrometric error) and the assumptions regarding the black holes radial motion as previously unrecognized limitations on orbital accuracy and the usefulness of fainter stars. Future astrometric and RV observations will remedy these effects. Our estimates of R_0 and the Galaxys local rotation speed, which it is derived from combining R_0 with the apparent proper motion of Sgr A*, (θ_0 = 229±18 km/s), are compatible with measurements made using other methods. The increased black hole mass found in this study, compared to that determined using projected mass estimators, implies a longer period for the innermost stable orbit, longer resonant relaxation timescales for stars in the vicinity of the black hole and a better agreement with the M_(bh)-σ relation.

A faint type of supernova from a white dwarf with a helium-rich companion

Supernovae are thought to arise from two different physical processes. The cores of massive, short-lived stars undergo gravitational core collapse and typically eject a few solar masses during their explosion. These are thought to appear as type Ib/c and type II supernovae, and are associated with young stellar populations. In contrast, the thermonuclear detonation of a carbon-oxygen white dwarf, whose mass approaches the Chandrasekhar limit, is thought to produce type Ia supernovae. Such supernovae are observed in both young and old stellar environments. Here we report a faint type Ib supernova, SN 2005E, in the halo of the nearby isolated galaxy, NGC 1032. The ‘old’ environment near the supernova location, and the very low derived ejected mass (∼0.3 solar masses), argue strongly against a core-collapse origin. Spectroscopic observations and analysis reveal high ejecta velocities, dominated by helium-burning products, probably excluding this as a subluminous or a regular type Ia supernova. We conclude that it arises from a low-mass, old progenitor, likely to have been a helium-accreting white dwarf in a binary. The ejecta contain more calcium than observed in other types of supernovae and probably large amounts of radioactive 44Ti.

THERMONUCLEAR .Ia SUPERNOVAE FROM HELIUM SHELL DETONATIONS: EXPLOSION MODELS AND OBSERVABLES

During the early evolution of an AM CVn system, helium is accreted onto the surface of a white dwarf under conditions suitable for unstable thermonuclear ignition. The turbulent motions induced by the convective burning phase in the He envelope become strong enough to influence the propagation of burning fronts and may result in the onset of a detonation. Such an outcome would yield radioactive isotopes and a faint rapidly rising thermonuclear “.Ia” supernova. In this paper, we present hydrodynamic explosion models and observable outcomes of these He shell detonations for a range of initial core and envelope masses. The peak UVOIR bolometric luminosities range by a factor of 10 (from 5×10 41 −5×10 42 erg s −1 ), and the R-band peak varies from MR,peak = −15 to −18. The rise times in all bands are very rapid (< 10 d), but the decline rate is slower in the red than the blue due to a secondary near-IR brightening. The nucleosynthesis primarily yields heavy α-chain elements ( 40 Ca through 56 Ni) and unburnt He. Thus, the spectra around peak light lack signs of intermediate mass elements and are dominated by Ca ii and Ti ii features, with the caveat that our radiative transfer code does not include the non-thermal effects necessary to produce He features. Subject headings: binaries: close— novae, cataclysmic variables— nuclear reactions, nucleosynthesis, abundances— supernovae: general— white dwarfs

Stellar Dynamics at the Galactic Center with an Extremely Large Telescope

We discuss physical experiments achievable via the monitoring of stellar dynamics near the massive black hole at the Galactic center with a diffraction-limited, next-generation, extremely large telescope (ELT). Given the likely observational capabilities of an ELT and what is currently known about the stellar environment at the Galactic center, we synthesize plausible samples of stellar orbits around the black hole. We use the Markov Chain Monte Carlo method to evaluate the constraints that the monitoring of these orbits will place on the matter content within the dynamical sphere of influence of the black hole. We express our results as functions of the number N of stars with detectable orbital motions and the astrometric precision δθ and spectroscopic precision δv at which the stellar proper motions and radial velocities are monitored. Our results are easily scaled to different telescope sizes and precisions. For N = 100, δθ = 0.5 mas, and δv = 10 km s-1 (a conservative estimate of the capabilities of a 30 m telescope) we find that if the extended matter distribution enclosed by the orbits at 0.01 pc has a mass greater than ~103 M☉, it will produce measurable deviations from Keplerian motion. Thus, if the concentration of dark matter at the Galactic center matches theoretical predictions, its influence on the orbits will be detectable. We also estimate the constraints that will be placed on the mass of the black hole and on the distance to the Galactic center and find that both will be measured to better than ~0.1%. We discuss the significance of knowing the distance to within a few parsecs and the importance of this parameter for understanding the structure of the Galaxy. We demonstrate that the lowest order relativistic effects, such as the prograde precession, will be detectable if δθ 0.5 mas. Barring the favorable discovery of a star on a highly compact, eccentric orbit, the higher order effects, including the frame dragging due to the spin of the black hole, will require δθ 0.05 mas. Finally, we calculate the rate at which monitored stars experience detectable nearby encounters with background stars. The encounters probe the mass function of stellar remnants that accumulate near the black hole. We find that ~30 such encounters will be detected over a 10 yr baseline for δθ = 0.5 mas.

Constraining dark energy from the abundance of weak gravitational lenses

We examine the prospect of using the observed abundance of weak gravitational lenses to constrain the equation-of-state parameter w = p /p of dark energy. Dark energy modifies the distance-redshift relation, the amplitude of the matter power spectrum, and the rate of structure growth. As a result, it affects the efficiency with which dark-matter concentrations produce detectable weak-lensing signals. Here we solve the spherical-collapse model with dark energy, clarifying some ambiguities found in the literature. We also provide fitting formulae for the non-linear overdensity at virialization and the linear-theory overdensity at collapse. We then compute the variation in the predicted weak-lens abundance with w. We find that the predicted redshift distribution and number count of weak lenses are highly degenerate in w and the present matter density Ω 0 . If we fix Ω 0 the number count of weak lenses for w = -2/3 is a factor of ∼2smaller than for the A cold dark matter (CDM) model w = -1. However, if we allow Ω 0 to vary with w such that the amplitude of the matter power spectrum as measured by the Cosmic Background Explorer (COBE) matches that obtained from the X-ray cluster abundance, the decrease in the predicted lens abundance is less than 25 per cent for -1 < w < - 0.4. We show that a more promising method for constraining dark energy - one that is largely unaffected by the Ω o -w degeneracy as well as uncertainties in observational noise -is to compare the relative abundance of virialized X-ray lensing clusters with the abundance of non-virialized, X-ray underluminous, lensing haloes. For aperture sizes of ∼15 arcmin, the predicted ratio of the non-virialized to virialized lenses is greater than 40 per cent and varies by ∼20 per cent between w =-1and -0.6. Overall, we find that, if all other weak-lensing parameters are fixed, a survey must cover at least ∼40 deg 2 in order for the weak-lens number count to differentiate a ACDM cosmology from a dark-energy model with w = -0.9 at the 3σ level. If, on the other hand, we take into account uncertainties in the lensing parameters, then the non-virialized lens fraction provides the most robust constraint on ω, requiring ∼50 deg 2 of sky coverage in order to differentiate a ACDM model from a w = -0.6 model to 3a.

Observational constraints on general relativistic energy conditions, cosmic matter density and dark energy from X-ray clusters of galaxies and type-Ia supernovae

New observational constraints on the cosmic matter density Ωm and an effectively redshift-independent equation of state parameter wx of the dark energy are obtained while simultaneously testing the strong and null energy conditions of general relativity on macroscopic scales. The combination of REFLEX X-ray cluster and type-Ia supernova data shows that for a flat Universe the strong energy condition might presently be violated whereas the null energy condition seems to be fulfilled. This provides another observational argument for the present accelerated cosmic expansion and the absence of exotic physical phenomena related to a broken null energy condition. The marginalization of the likelihood distributions is performed in a manner to include a large fraction of the recently discussed possible systematic errors involved in the application of X-ray clusters as cosmological probes. This yields for a flat Universe, Ωm = 0.29 +0.08 −0.12 and wx = −0.95 +0.30 −0.35 (1σ errors without cosmic variance). The scatter in the different analyses indicates a quite robust result around wx = −1, leaving little room for the introduction of new energy components described by quintessence-like models or phantom energy. The most natural interpretation of the data is a positive cosmological constant with wx = −1 or something like it.

SHOCK BREAKOUT FROM TYPE Ia SUPERNOVA

The mode of explosive burning in Type Ia supernovae (SNe Ia) remains an outstanding problem. It is generally thought to begin as a subsonic deflagration, but this may transition into a supersonic detonation (the delayed detonation transition, DDT). We argue that this transition leads to a breakout shock, which would provide the first unambiguous evidence that DDTs occur. Its main features are a hard X-ray flash (~20 keV) lasting ~10–2 s with a total radiated energy of ~1040 erg, followed by a cooling tail. This creates a distinct feature in the visual light curve, which is separate from the nickel decay. This cooling tail has a maximum absolute visual magnitude of MV ≈ –9 to –10 at ≈1 day, which depends most sensitively on the white dwarf radius at the time of the DDT. As the thermal diffusion wave moves in, the composition of these surface layers may be imprinted as spectral features, which would help to discern between SN Ia progenitor models. Since this feature should accompany every SNe Ia, future deep surveys (e.g., m = 24) will see it out to a distance of ≈80 Mpc, giving a maximum rate of ~60 yr-1. Archival data sets can also be used to study the early rise dictated by the shock heating (at ≈20 days before maximum B-band light). A similar and slightly brighter event may also accompany core bounce during the accretion-induced collapse to a neutron star, but with a lower occurrence rate.

Tidal asteroseismology: Kepler’s KOI-54

We develop a general framework for interpreting and analysing high-precision light curves from eccentric stellar binaries. Although our methods are general, we focus on the recently discovered Kepler system KOI-54, a face-on binary of two A stars with e= 0.83 and an orbital period of 42 days. KOI-54 exhibits strong ellipsoidal variability during its periastron passage; its light curve also contains ∼20 pulsations at perfect harmonics of the orbital frequency, and another ∼10 non-harmonic pulsations. Analysis of such data is a new form of asteroseismology in which oscillation amplitudes and phases rather than frequencies contain information that can be mined to constrain stellar properties. We qualitatively explain the physics of mode excitation and the range of harmonics expected to be observed. To quantitatively model observed pulsation spectra, we develop and apply a linear, tidally forced, non-adiabatic stellar oscillation formalism including the Coriolis force. We produce temporal power spectra for KOI-54 that are semi-quantitatively consistent with the observations. Both stars in the KOI-54 system are expected to be rotating pseudo-synchronously, with resonant non-axisymmetric modes providing a key contribution to the total torque; such resonances present a possible explanation for the two largest-amplitude harmonic pulsations observed in KOI-54, although we find problems with this interpretation. We show in detail that the non-harmonic pulsations observed in KOI-54 can be explained by non-linear three-mode coupling. The methods developed in this paper can be generalized in the future to determine the best-fitting stellar parameters given pulsation data. We also derive an analytic model of KOI-54’s ellipsoidal variability, including both tidal distortion and stellar irradiation, which can be used to model other similar systems.

Evidence of heavy-element ashes in thermonuclear X-ray bursts with photospheric superexpansion

A small subset of thermonuclear X-ray bursts on neutron stars exhibit such a strong photospheric expansion that for a few seconds the photosphere is located at a radius rph > 10 3 km. Such “superexpansions” imply a large and rapid energy release, a feature characteristic of pure He burst models. Previous calculations have shown that during a pure He burst, the freshly synthesized heavy-element ashes of burning can be ejected in a strong radiative wind and produce significant spectral absorption features. We search the burst data catalogs and literature and find 32 superexpansion bursts, 24 of which were detected with BeppoSAX and three with RXTE at high time resolution. We find that these bursts exhibit the following interesting features: (1) At least 31 are from (candidate) ultracompact X-ray binaries in which the neutron star accretes hydrogen-deficient fuel, suggesting that these bursts indeed ignite in a helium-rich layer. (2) In two of the RXTE bursts we detect strong absorption edges during the expansion phase. The edge energies and depths are consistent with the H-like or He-like edge of iron-peak elements with abundances >100 times solar, suggesting that we are seeing the exposed ashes of nuclear burning. (3) The superexpansion phase is always followed by a moderate expansion phase during which rph ∼ 30 km and the luminosity is near the Eddington limit. (4) The decay time of the bursts, τdecay, ranges from short (≈10 s) to intermediate (>10 3 s). However, despite the large range of τdecay, the duration of the superexpansion is always a few seconds, independent of τdecay. By contrast, the duration of the moderate expansion is always of order τdecay. (5) The photospheric radii rph during the moderate expansion phase are much smaller than steady state wind models predict. We show that this may be further indication that the wind contains highly non-solar abundances of heavy elements.

Spectra and light curves of failed supernovae

Astronomers have proposed a number of mechanisms to produce supernova explosions. Although many of these mechanisms are now not considered primary engines behind supernovae (SNe), they do produce transients that will be observed by upcoming ground-based surveys and NASA satellites. Here, we present the first radiation-hydrodynamics calculations of the spectra and light curves from three of these failed SNe: SNe with considerable fallback, accretion-induced collapse of white dwarfs, and energetic helium flashes (also known as type Ia SNe).