Joachim Krautter

The stellar population of the Lupus clouds

We present photometric and spectroscopic observations of the H alpha emission stars in the Lupus dark cloud complex. We estimate the effective temperatures of the stars from their spectral types and calculate the reddening towards each object from the (R-I) colors. From these data, we derive mass and age distributions for the Lupus stars using a new set of pre-main sequence evolutionar tracks. We compare the results for the Lupus stars with those for a similar population of young stellar objects in Taurus-Auriga and Chamaeleon and with the initial mass function for field stars in the solar neighborhood. From the H-R diagrams, Lupus appears to contain older stars than Taurus. The Lupus dark clouds form a greater proportion of low mass stars than the Taurus complex. Also, the proportion of low mass stars in Lupus is higher than that predicted by the Miller-Scalo initial mass function, and the lowest mass stars in Lupus are less active than similar T Tauri stars in other regions.

The X-Ray Spectral Evolution of Classical Nova V1974 Cygni 1992: A Reanalysis of the ROSAT Data

We present a spectral analysis of the archival X-ray data of classical Nova V1974 Cygni 1992 (Nova Cygni 1992) obtained by the ROSAT Position Sensitive Proportional Counter (PSPC). The X-ray spectrum is fitted with a two-component model. The first component is a white dwarf atmosphere emission model developed for the remnants of classical novae near the Eddington luminosity. The model is used to fit the soft X-ray data in the ~0.1-1.0 keV range, where the bulk of emission is below 0.7 keV. The second component is a Raymond-Smith model of thermal plasma applied to the hard X-ray emission above ~1.0 keV. The postoutburst X-ray spectrum of the remnant white dwarf is examined in the context of evolution on the Hertzsprung-Russell diagram using an O-Ne- and a C-O-enhanced atmosphere emission model. A constant bolometric luminosity evolution is detected with increasing effective temperature and decreasing photospheric radius using the O-Ne-enhanced model. The unabsorbed soft X-ray flux for the constant bolometric luminosity phase is found to be in the range (1.7-2.2) × 10-7 ergs s-1 cm-2. A peak effective temperature of 51 eV (5.9 ×105 K) is detected 511 days after outburst. We also present the spectral development of the hard X-ray component. It is found to evolve independently of the soft one. The maximum of the hard X-ray emission is reached at ~150 days after outburst with an unabsorbed flux of ~2.0 × 10-11 ergs s-1 cm-2 corresponding to a luminosity of (0.8-2.0) × 1034 ergs s-1 at a 2-3 kpc source distance. The time evolution of the hard X-ray flux and the plasma temperatures decreasing from 10 keV to 1 keV suggest emission from shock-heated gas as the origin of the hard X-ray component.

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.

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.

XMM-NEWTON X-RAY AND ULTRAVIOLET OBSERVATIONS OF THE FAST NOVA V2491 Cyg DURING THE SUPERSOFT SOURCE PHASE

Two XMM-Newton observations of the fast classical nova V2491Cyg were carried out in short succession on days 39.93 and 49.62 after discovery, during the supersoft source (SSS) phase, yielding simultaneous X-ray and UV light curves and high-resolution X-ray spectra. The first X-ray light curve is highly variable, showing oscillations with a period of 37.2 minutes after an extended factor of three decline lasting � 3 hours, while the second X-ray light curve is less variable. The cause of the dip is currently unexplained and has most likely the same origin as similar events in the early SSS light curves of the novae V4743Sgr and RSOph, as it occurred on the same time scale. The oscillations are not present during the dip minimum and also not in the second observation. The UV light curves are variable but contain no dips and no period. High-resolution X-ray spectra are presented for 4 intervals of differing intensity. All spectra are atmospheric continua with deep absorption lines and absorption edges. Two interstellar lines of Oi and Ni are clearly seen at their rest wavelengths, while a large number of high-ionization absorption lines are found at blue shifts indicating an expansion velocity of 3000 3400kms 1 , which does not change significantly during the epochs of observation. Comparisons with the slower nova V4743Sgr and the symbiotic recurrent nova RSOph are presented. The SSS spectrum of V4743Sgr is much softer with broader and more complex photospheric absorption lines. The ejecta are extended, allowing us to view a larger range of the radial velocity profile. Meanwhile, the absorption lines in RSOph are as narrow as in V2491Cyg, but they are less blue shifted. A remarkable similarity in the continua of V2491Cyg and RSOph is found. The only differences are smaller line shifts and additional emission lines in RSOph that are related to the presence of a dense stellar wind from the evolved companion. Three unidentified absorption lines are present in the X-ray spectra of all three novae, with projected rest wavelengths 26.05u 29.45u and 30.0u No entirely satisfactory spectral model is currently available for the soft X-ray spectra of novae in outburst, and careful discussion of assumptions is required. Subject headings: novae, cataclysmic variables - stars: individual (V2491 Cyg) - stars: individual (RS Oph) - stars: individual (V4743 Sgr) - AAVSO

X-RAY SPECTROSCOPIC DIAGNOSIS OF A WIND-COLLIMATED BLAST WAVE AND METAL-RICH EJECTA FROM THE 2006 EXPLOSION OF RS OPHIUCHI

Chandra HETG observations of RS Ophiuchi at day 13.9 of the 2006 outburst reveal a rich spectrum of emission lines from abundant ions formed over a wide temperature range (∼ 3 × 10 6 to 60 × 10 6 K) indicative of shock heating of the circumstellar medium by the expanding blast wave. Lines are asymmetric and strongly broadened (v ∼ 2400 km s −1 at zero intensity). Using simple analytical model profiles, we show how the lines are shaped by differential absorption in the red giant wind and explosion ejecta, and that shock heating to multimillion degree temperatures appears to have occurred preferentially in the direction perpendicular to the line of sight. We conclude that the asymmetric nature of the offset 1/r 2 density profile and likely equatorial circumstellar density enhancement in which the explosion occurred are responsible for both the shock collimation and broad range in plasma temperature observed. The ejecta mass deduced from X-ray absorption is more easily reconciled with the expected mass accretion rate for material enhanced in metals by up to an order of magnitude.

The Spitzer Infrared Spectrometer view of V4334 Sgr (Sakurai's Object)

We present an observation of the very late thermal pulse object V4334 Sgr (Sakurai’s Object) with the Infrared Spectrometer (IRS) on the Spitzer Space Telescope. The emission from 5‐ 38 μm is dominated by the still-cooling dust shell. A number of features are seen in absorption against the dust shell, which we attribute to HCN and polyyne molecules. We use these features to determine the 12 C/ 13 C ratio for the absorbing gas to be ∼3.2 +3.2; this implies that, despite the H-content of the molecules, the hydrocarbon-bearing gas must have originated in material produced in the very late thermal pulse. We see no evidence of emission lines, despite the recently-reported optical and radio observations that suggest the effective temperature of the stellar remnant is rising.

The nebular phase of Nova GQ Muscae 1983 - Evolution of the ionization of the optical spectrum

Spectroscopic observations of Nova GQ Muscae 1983 over a period of 4 years during its nebular stage reveal a steady increase in the ionization of the ejected envelope. The forbidden Fe x 6374 A coronal line first appears in the spectrum more than 2 years after maximum, and by the end of the fourth year has become the strongest line in the spectrum. It is shown that the high ionization is caused by photoionization from a hot radiation source with T greater than about 400,000 K whose temperature increases with time. The observations can be explained in the context of the constant bolometric luminosity phase caused by hydrostatic hydrogen burning on the white dwarf remnant. They appear to be an important observational verification of this phase in the nova which is predicted by the thermonuclear model of the classical nova outburst. 18 refs.

Silicate Dust in the Environment of RS Ophiuchi following the 2006 Eruption

We present further Spitzer Space Telescope observations of the recurrent nova RS Ophiuchi, obtained over the period 208-430 days after the 2006 eruption. The later Spitzer IRS data show that the line emission and free-free continuum emission reported earlier is declining, revealing incontrovertible evidence for the presence of silicate emission features at 9.7 and 18 μm. We conclude that the silicate dust survives the hard radiation impulse and shock blast wave from the eruption. The existence of the extant dust may have significant implications for understanding the propagation of shocks through the red giant wind and likely wind geometry.

Hubble space telescope NICMOS observations of classical Nova shells

We present results of a campaign to obtain near-infrared images of evolved classical nova shells by using the Near-Infrared Camera and Multi-Image Spectrometer (NICMOS) on the Hubble Space Telescope. Images were obtained in six filters from 1.87 to 2.37 μm. Three of our four targets, QV Vulpeculae (Nova Vul 1987), QU Vulpeculae (Nova Vul 1984 No. 2), and V1974 Cygni (Nova Cyg 1992) exhibit pronounced shell emission in the narrowband Paα (F187N) filter. In the other five filters, the shells are only marginally detected with the exception of QU Vul, which exhibits spatially extended low surface brightness emission in the medium-band filter (F222M) centered near 2.22 μm. Spatially extended emission from V723 Cassiopeiae (Nova Cas 1995) was not detected above the image noise in any filter. The observed ejecta geometries lend further support to the suggestion that the speed class of a nova may correlate with the shape of the shells. The NICMOS images, in combination with ground-based spectroscopic determination of the ejecta expansion velocities, enable us to estimate the expansion parallaxes for V1974 Cyg, QU Vul, and QV Vul. Of note, the expansion velocity (in milliarcseconds per day) determined for V1974 Cyg has remained constant over the 4 year period (1993–1998) after onset of the optical thin phase. Lastly, more structures in the shell of V1974 Cyg have appeared since 1993.