From the Heart of The Ghoul: C and N Abundances in the Corona of Algol B

Chandra Low Energy Transmission Grating Spectrograph observations of Algol have been used to determine the abundances of C and N in the secondary star for the first time. The analysis was performed relative to similar observations of an adopted "standard" star HR 1099. It is demonstrated that HR 1099 and Algol are coronal twins in many respects and that their X-ray spectra are very similar in nearly all details, except for the observed strengths of C and N lines. The H-like transitions of C and N in the coronae of Algol and HR 1099 demonstrate that the surface abundances of Algol B have been strongly modified by CN-processing, as shown earlier by Schmitt & Ness (2002). It is found that N is enhanced in Algol B by a factor of 3 compared to HR 1099. No C lines are detected in the Algol spectrum, indicating a C depletion relative to HR 1099 by a factor of 10 or more. These C and N abundances indicate that Algol B must have lost at least half of its initial mass, and are consistent with predictions of evolutionary models that include non-conservative mass transfer and angular momentum loss through magnetic activity. Little or no dredge-up of material subjected to CN-processing has occurred on the subgiant component of HR 1099. It is concluded that Fe is very likely depleted in the coronae of both Algol and HR 1099 relative to their photospheres by 0.5 dex, and C, N and O by 0.3 dex. Instead, Ne is enhanced by up to 0.5 dex.Comment: 17 pages, 4 figures, ApJ accepte

An observational test of common-envelope evolution

By analysing and modelling the change in the abundance ratio of $^{12}$C/$^{13}$C and $^{16}$O/$^{17}$O on the surface of the lower mass star of a binary during the common-envelope (CE) phase of evolution, we propose a simple observational test of the CE scenario. The test is based on the infrared measurement of either the $^{12}$C/$^{13}$C or $^{16}$O/$^{17}$O ratio of red dwarfs in post-common envelope binaries (PCEB's). In certain cases (main-sequence red dwarf secondaries in PCEB's without planetary nebulae), as well as determining whether or not accretion has occurred during the CE phase, we can determine the amount of mass accreted during the CE phase and hence the initial mass of the red dwarf component prior to the CE phase. In the other cases considered (low-mass red dwarfs in PCEB's and red dwarf's in PCEB's with planetary nebulae) we can only say whether or not accretion has occurred during the CE phase.Comment: uuencoded compressed postscript. The preprint are also available at URL http://www.ast.cam.ac.uk/preprint/PrePrint.htm

X-ray Evidence of the Common Envelope Phase of V471 Tauri

Chandra Low Energy Transmission Grating Spectrograph observations of the pre-cataclysmic binary V471 Tau have been used to estimate the C/N abundance ratio of the K dwarf component for the first time. While the white dwarf component dominates the spectrum longward of 50 AA, at shorter wavelengths the observed X-ray emission is entirely due to coronal emission from the K dwarf. The H-like resonance lines of C and N yield an estimate of their logarithmic abundance ratio relative to the Sun of [C/N]=-0.38+/-0.15 - half of the currently accepted solar value. We interpret this result as the first clear observational evidence for the presumed common envelope phase of this system, during which the surface of the K dwarf was contaminated by CN-cycle processed material dredged up into the red giant envelope. We use the measured C/N ratio to deduce that 0.015-0.04 Msun was accreted by the K dwarf while engulfed, and show that this is consistent with a recent tentative detection of 13C in the K dwarf photosphere, and with the measured Li abundance in the scenario where the red giant companion was Li-rich during the common envelope phase.Comment: 6 pages, 2 figures, ApJL accepte

The Discovery of a Companion to the Lowest Mass White Dwarf

We report the detection of a radial velocity companion to SDSS J091709.55+463821.8, the lowest mass white dwarf currently known with M~0.17Msun. The radial velocity of the white dwarf shows variations with a semi-amplitude of 148.8 km/s and a period of 7.5936 hours, which implies a companion mass of M > 0.28Msun. The lack of evidence of a companion in the optical photometry forces any main-sequence companion to be smaller than 0.1Msun, hence a low mass main sequence star companion is ruled out for this system. The companion is most likely another white dwarf, and we present tentative evidence for an evolutionary scenario which could have produced it. However, a neutron star companion cannot be ruled out and follow-up radio observations are required to search for a pulsar companion.Comment: ApJ, in press. See the Press Release at http://www.cfa.harvard.edu/press/2007/pr200708.htm

Hydrodynamic simulations of irradiated secondaries in dwarf novae

We investigate numerically the surface flow on the secondary star during outbursts. We use a simple model for the irradiation and the geometry of the secondary star: the irradiation temperature is treated as a free parameter and the secondary is replaced by a spherical star with a space-dependent Coriolis force that mimics the effect of the Roche geometry. The Euler equations are solved in spherical coordinates with the TVD-MacCormack scheme. We show that the Coriolis force leads to the formation of a circulation flow from high latitude region to the close vicinity of the $L_1$ point. However no heat can be efficiently transported to the $L_1$ region due to the rapid radiative cooling of the hot material as it enters the equatorial belt shadowed from irradiation. Under the assumption of hydrostatic equilibrium, the Coriolis force could lead to a moderate increase of the mass transfer rate by pushing the gas in the vertical direction at the $L_1$ point, but only during the initial phases of the outburst (about 15 -- 20 orbital periods). We conclude that the Coriolis force does not prevent a flow from the heated regions of the secondary towards the $L_1$ region, at least during the initial phase of an outburst, but the resulting increase of the mass transfer rate is moderate, and it is unlikely to be able to account for the duration of long outbursts.Comment: 11 pages, 11 figures, accepted for publication in A&

The Temperature and Cooling Age of the White-Dwarf Companion to the Millisecond Pulsar PSR B1855+09

We report on Keck and {\em Hubble Space Telescope} observations of the binary millisecond pulsar PSR B1855+09. We detect its white-dwarf companion and measure \mv=25.90\pm0.12 and \mi=24.19\pm0.11 (Vega system). From the reddening-corrected color, (\mv-\mi)_0=1.06\pm0.21, we infer a temperature \Teff=4800\pm800 K. The white-dwarf mass is known accurately from measurements of the Shapiro delay of the pulsar signal, \Mcomp=0.258^{+0.028}_{-0.016} \Msun. Hence, given a cooling model, one can use the measured temperature to determine the cooling age. The main uncertainty in the cooling models for such low-mass white dwarfs is the amount of residual nuclear burning, which is set by the thickness of the hydrogen layer surrounding the helium core. From the properties of similar systems, it has been inferred that helium white dwarfs form with thick hydrogen layers, with mass \simgt3\times10^{-3} \Msun, which leads to significant additional heating. This is consistent with expectations from simple evolutionary models of the preceding binary evolution. For PSR B1855+09, though, such models lead to a cooling age of $\sim10$Gyr, which is twice the spin-down age of the pulsar. It could be that the spin-down age were incorrect, which would call the standard vacuum dipole braking model into question. For two other pulsar companions, however, ages well over 10 Gyr are inferred, indicating that the problem may lie with the cooling models. There is no age discrepancy for models in which the white dwarfs are formed with thinner hydrogen layers (\simlt3\times10^{-4} \Msun).Comment: 7 pages, 1 figure, aas4pp2.sty. Accepted for publication in ApJ

The surprising external upturn of the Blue Straggler radial distribution in M55

By combining high-resolution HST and wide-field ground based observations, in ultraviolet and optical bands, we study the Blue Straggler Star (BSS) population of the low density galactic globular cluster M55 (NGC 6809) over its entire radial extent. The BSS projected radial distribution is found to be bimodal, with a central peak, a broad minimum at intermediate radii, and an upturn at large radii. Similar bimodal distributions have been found in other globular clusters (M3, 47 Tucanae, NGC 6752, M5), but the external upturn in M55 is the largest found to date. This might indicate a large fraction of primordial binaries in the outer regions of M55, which seems somehow in contrast with the relatively low (\sim 10%) binary fraction recently measured in the core of this cluster.Comment: in press on Ap

On the use of 12CO/13CO as a test of common-envelope evolution

We present K-band echelle spectra of the cataclysmic variable SS Cyg and the pre-cataclysmic variable V471 Tau in order to measure the strengths of the 12CO and 13CO bands at 2.3525 and 2.3448 microns, respectively, and so perform the observational test of the common-envelope model of close binary star evolution proposed by Sarna et al. (1995). Although we find evidence of an absorption feature coincident with the expected wavelength of 13CO in both objects, we attribute it instead to a cluster of neutral atomic absorption features (primarily due to TiI) possibly arising from star-spots on the surfaces of the rapidly rotating secondary stars in these systems, thereby rendering the test inconclusive. We present a modified observational test of common-envelope evolution, based on the observation of the 13CO bands at 2.3739 and 2.4037 microns, which is insensitive to spectral contamination by star-spots

UVSat: a concept of an ultraviolet/optical photometric satellite

Time-series photometry from space in the ultraviolet can be presently done with only a few platforms, none of which is able to provide wide-field long-term high-cadence photometry. We present a concept of UVSat, a twin space telescope which will be capable to perform this kind of photometry, filling an observational niche. The satellite will host two telescopes, one for observations in the ultraviolet, the other for observations in the optical band. We also briefly show what science can be done with UVSat.Comment: 6 pages, 2 figures, accepted for publication in the Proceedings of the PAS (Proc. of the 2nd BRITE Science conference, Innsbruck

Spectroscopy of Blue Stragglers and Turnoff Stars in M67 (NGC 2682)

We have analyzed high-resolution spectra of relatively cool blue stragglers and main sequence turnoff stars in the old open cluster M67 (NGC 2682). We attempt to identify blue stragglers whose spectra are least contaminated by binary effects (contamination by a binary companion or absorption by circumstellar material). These ``best'' stragglers have metallicities ([Fe/H] = -0.05) and abundance ratios of the blue stragglers are not significantly different from those of the turnoff stars. Based on arguments from hydrodynamical models of stellar collisions, we assert that the current upper limits for the lithium abundances of all blue stragglers observed in M67 (by us and others) are consistent with no mixing during the formation process, assuming pre-main sequence and main sequence depletion patterns observed for M67 main sequence stars. We discuss composition signatures that could more definitively distinguish between blue straggler formation mechanisms in open cluster stars. We confirm the spectroscopic detection of a binary companion to the straggler S 1082. From our spectra, we measure a projected rotational speed of 90+/-20 km/sec for the secondary, and find that its radial velocity varies with a peak-to-peak amplitude of ~ 25 km/sec. Because the radial velocities do not vary with a period corresponding to the partial eclipses in the system, we believe this system is currently undergoing mass transfer. In addition we present evidence that S 984 is a true blue straggler (and not an unresolved pair). If this can be proven, our detection of lithium may indicate a collisional origin.Comment: 20 pages, 4 figures, to appear in October 2000 A