Sumner G. Starrfield

CNO ABUNDANCES AND HYDRODYNAMIC MODELS OF THE NOVA OUTBURST.

We have used a fully implicit, Lagrangian, hydrodynamic computer code incorporating a nuclear reaction network to follow thermonuclear runaways in the hydrogen-rich envelopes of white dwarfs in order to produce a nova outburst. Because of the short time-scales and the high nuclear burning rates produced in our models, the nuclear reactions are far out of equilibrium and the beta-plus unstable nuclei become the most abundant nuclei in the envelope except for hydrogen and helium. Our models have ejected 1.00017 solar mass with kinetic energies of 8 times 10 to the 44-th power ergs, a value that agrees quite closely with the observed values for novae.

An energetic stellar outburst accompanied by circumstellar light echoes

Some classes of stars, including novae and supernovae, undergo explosive outbursts that eject stellar material into space. In 2002, the previously unknown variable star V838 Monocerotis brightened suddenly by a factor of ∼104. Unlike a supernova or nova, it did not explosively eject its outer layers; rather, it simply expanded to become a cool supergiant with a moderate-velocity stellar wind. Superluminal light echoes were discovered as light from the outburst propagated into the surrounding, pre-existing circumstellar dust. Here we report high-resolution imaging and polarimetry of those light echoes, which allow us to set direct geometric distance limits to the object. At a distance of >6 kpc, V838 Mon at its maximum brightness was temporarily the brightest star in the Milky Way. The presence of the circumstellar dust implies that previous eruptions have occurred, and spectra show it to be a binary system. When combined with the high luminosity and unusual outburst behaviour, these characteristics indicate that V838 Mon represents a hitherto unknown type of stellar outburst, for which we have no completely satisfactory physical explanation.

Proton-induced Thermonuclear Reaction Rates for A = 20–40 Nuclei

Proton-induced reaction rates on 26 stable and 29 unstable target nuclei in the mass A = 20–40 region have been evaluated and compiled. Recommended reaction rates, assuming that all interacting nuclei are in the ground state, are presented in tabular form on a temperature grid in the range T = 0.01–10.0 GK. Most reaction rates involving stable targets were normalized to a set of measured standard resonance strengths in the sd shell. For the majority of reaction rates, experimental information from transfer reaction studies has been used consistently. Our results are compared with recent statistical model (Hauser-Feshbach) calculations. Reaction rate uncertainties are presented and amount to several orders of magnitude for many of the reactions. Several of these reaction rates and/or their corresponding uncertainties deviate from results of previous compilations. In most cases, the deviations are explained by the fact that new experimental information became available recently. Examples are given for calculating reaction rates and reverse reaction rates for thermally excited nuclei from the present results. The survey of literature for this review was concluded in 2000 August.

The Effects of Thermonuclear Reaction-Rate Variations on Nova Nucleosynthesis: A Sensitivity Study

We investigate the effects of thermonuclear reaction-rate uncertainties on nova nucleosynthesis. One-zone nucleosynthesis calculations have been performed by adopting temperature-density-time profiles of the hottest hydrogen-burning zone (i.e., the region in which most of the nucleosynthesis takes place). We obtain our profiles from seven different, recently published, hydrodynamic nova simulations covering peak temperatures in the range from Tpeak = 0.145 to 0.418 GK. For each of these profiles, we individually varied the rates of 175 reactions within their associated errors and analyzed the resulting abundance changes of 142 isotopes in the mass range below A = 40. In total, we performed ≈7350 nuclear reaction network calculations. We use the most recent thermonuclear reaction-rate evaluations for the mass ranges A = 1-20 and 20-40. For the theoretical astrophysicist, our results indicate the extent to which nova nucleosynthesis calculations depend on currently uncertain nuclear physics input, while for the experimental nuclear physicist, our results represent at least a qualitative guide for future measurements at stable and radioactive ion beam facilities. We find that present reaction-rate estimates are reliable for predictions of Li, Be, C, and N abundances in nova nucleosynthesis. However, rate uncertainties of several reactions have to be reduced significantly in order to predict more reliable O, F, Ne, Na, Mg, Al, Si, S, Cl, and Ar abundances. Results are presented in tabular form for each adopted nova simulation.

Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: I. Early X-ray emission from the shocked ejecta and red giant wind

RS Ophiuchi began its latest outburst on 2006 February 12. Previous outbursts have indicated that high-velocity ejecta interact with a preexisting red giant wind, setting up shock systems analogous to those seen in supernova remnants. However, in the previous outburst in 1985, X-ray observations did not commence until 55 days after the initial explosion. Here we report on Swift observations covering the first month of the 2006 outburst with the Burst Alert Telescope (BAT) and X-Ray Telescope (XRT) instruments. RS Oph was clearly detected in the BAT 14-25 keV band from t = 0 to t ~ 6 days. XRT observations from 0.3 to 10 keV started 3.17 days after outburst. The rapidly evolving XRT spectra clearly show the presence of both line and continuum emission, which can be fitted by thermal emission from hot gas whose characteristic temperature, overlying absorbing column (NH)W, and resulting unabsorbed total flux decline monotonically after the first few days. Derived shock velocities are in good agreement with those found from observations at other wavelengths. Similarly, (NH)W is in accord with that expected from the red giant wind ahead of the forward shock. We confirm the basic models of the 1985 outburst and conclude that standard phase I remnant evolution terminated by t ~ 6 days and the remnant then rapidly evolved to display behavior characteristic of phase III. Around t = 26 days, however, a new, luminous, and highly variable soft X-ray source began to appear, whose origin will be explored in a subsequent paper.

The Halo Black Hole X-Ray Transient XTE J1118+480*

Optical spectra were obtained of the optical counterpart of the high-latitude (b 62°) soft X-ray transient XTE J1118+480 near its quiescent state (R 18.3) with the new 6.5 m Multiple Mirror Telescope and the 4.2 m William Herschel Telescope. The spectrum exhibits broad, double-peaked emission lines of hydrogen (FWHM 2400 km s-1) arising from an accretion disk superposed with absorption lines of a late-type secondary star. Cross-correlation of the 27 individual spectra with late-type stellar template spectra reveals a sinusoidal variation in radial velocity with amplitude K = 701 ± 10 km s-1 and orbital period P = 0.169930 ± 0.000004 days. The mass function, 6.1 ± 0.3 M☉, is a firm lower limit on the mass of the compact object and strongly implies that it is a black hole. We estimate the spectral type of the secondary to be K7 V-M0 V, and that it contributes 28% ± 2% of the light in the 5800-6400 A region on 2000 November 20, increasing to 36% ± 2% by 2001 January 4 as the disk faded. Photometric observations (R-band) with the Instituto de Astrofisica de Canarias 0.8 m telescope reveal ellipsoidal light variations of full amplitude 0.2 mag. Modeling of the light curve gives a large mass ratio (M1/M2 ~ 20) and a high orbital inclination (i = 81° ± 2°). Our combined fits yield a mass of the black hole in the range M1 = 6.0-7.7 M☉ (90% confidence) for plausible secondary star masses of M2 = 0.09-0.5 M☉. The photometric period measured during the outburst is 0.5% longer than our orbital period and probably reflects superhump modulations, as observed in some other soft X-ray transients. The estimated distance is d = 1.9 ± 0.4 kpc, corresponding to a height of 1.7 ± 0.4 kpc above the Galactic plane. The spectroscopic, photometric, and dynamical results indicate that XTE J1118+480 is the first firmly identified black hole X-ray system in the Galactic halo.

THE SUPERSOFT X-RAY PHASE OF NOVA RS OPHIUCHI 2006

Swift X-ray observations of the ~60 day supersoft phase of the recurrent nova RS Ophiuchi (RS Oph) 2006 show the progress of nuclear burning on the white dwarf (WD) in exquisite detail. First seen 26 days after the optical outburst, this phase started with extreme variability likely due to variable absorption, although intrinsic WD variations are not excluded. About 32 days later, a steady decline in count rate set in. NLTE model atmosphere spectral fits during the supersoft phase show that the effective temperature of the WD increases from ~65 eV to ~90 eV during the extreme variability phase, falling slowly after about day 60 and more rapidly after day 80. The bolometric luminosity is seen to be approximately constant and close to Eddington from day 45 up to day 60, the subsequent decline possibly signaling the end of extensive nuclear burning. Before the decline, a multiply-periodic ~35 s modulation of the soft X-rays was present and may be the signature of a nuclear fusion driven instability. Our measurements are consistent with a WD mass near the Chandrasekhar limit; combined with a deduced accumulation of mass transferred from its binary companion, this leads us to suggest that RS Oph is a strong candidate for a future supernova explosion. The main uncertainty now is whether the WD is the CO type necessary for a Type Ia supernova. This may be confirmed by detailed abundance analyses of spectroscopic data from the outbursts.

Swift X-Ray Observations of Classical Novae

The new γ-ray burst (GRB) mission Swift has obtained pointed observations of several classical novae in outburst. We analyzed all the observations of classical novae from the Swift archive up to 2006 June 30. We analyzed usable observations of 12 classical novae and found 4 nondetections, 3 weak sources, and 5 strong sources. This includes detections of two novae exhibiting spectra resembling those of supersoft X-ray binary source spectra (SSS), implying ongoing nuclear burning on the white dwarf surface. With these new Swift data, we add to the growing statistics of the X-ray duration and characteristics of classical novae.

A Chandra low energy transmission grating spectrometer observation of V4743 Sagittarii : a supersoft X-ray source and a violently variable light curve

V4743 Sagittarii (Nova Sgr 2002 No. 3) was discovered on 2002 September 20. We obtained a 5 ks ACIS-S spectrum in 2002 November and found that the nova was faint in X-rays. We then obtained a 25 ks Chandra Low Energy Transmission Grating Spectrometer (LETGS) observation on 2003 March 19. By this time, it had evolved into the supersoft X-ray phase exhibiting a continuous spectrum with deep absorption features. The light curve from the observation showed large-amplitude oscillations with a period of 1325 s (22 minutes) followed by a decline in the total count rate after ~13 ks of observations. The count rate dropped from ~40 counts s-1 to practically zero within ~6 ks and stayed low for the rest of the observation (~6 ks). The spectral hardness ratio changed from maxima to minima in correlation with the oscillations and then became significantly softer during the decay. Strong H-like and He-like lines of oxygen, nitrogen, and carbon were found in absorption during the bright phase, indicating temperatures between 1 and 2 MK, but they were shifted in wavelength corresponding to a Doppler velocity of -2400 km s-1. The spectrum obtained after the decline in count rate showed emission lines of C VI, N VI, and N VII, suggesting that we were seeing expanding gas ejected during the outburst, probably originating from CNO-cycled material. An XMM-Newton Target of Opportunity observation, obtained on 2002 April 4 and a later LETGS observation from 2003 July 18 also showed oscillations, but with smaller amplitudes.

The SSS Phase of RS Ophiuchi Observed with Chandra and XMM-Newton. I. Data and Preliminary Modeling

The phase of supersoft source (SSS) emission of the sixth recorded outburst of the recurrent nova RS Oph was observed on days 39.7 and 66.9 after outburst with Chandra and on day 54.0 with XMM-Newton. A ~35 s period on day 54.0 originates from the SSS emission and not from the shock. We discuss the bound-free absorption by neutral elements in the line of sight, resonance absorption lines plus self-absorbed emission-line components, collisionally excited emission lines from the shock, He-like intersystem lines, and spectral changes during an episode of high-amplitude variability. We find a decrease of the oxygen K-shell absorption edge that can be explained by photoionization of oxygen. The absorption component has average velocities of -1286 ± 267 km s-1 on day 39.7 and of -771 ± 65 km s-1 on day 66.9. The wavelengths of the emission-line components are at rest wavelengths, as confirmed by measurements of non-self-absorbed He-like intersystem lines. We found collisionally excited emission lines from the radiatively cooling shock at wavelengths shorter than 15 A that are systematically blueshifted by -526 ± 114 km s-1 on day 39.7 and are fading. We found anomalous He-like f/i ratios, which indicates either high densities or significant UV radiation near the plasma where the emission lines are formed. During the phase of strong variability the spectral hardness light curve overlies the total light curve when shifted by 1000 s. This can be explained by photoionization of neutral oxygen in the line of sight if the densities are of order 1010-1011 cm-3.