Robert Connon Smith

Observational constraints on the nature of dark energy : First cosmological results from the essence supernova survey

We present constraints on the dark energy equation-of-state parameter, w = P/(rho c(2)), using 60 SNe Ia fromthe ESSENCE supernova survey. We derive a set of constraints on the nature of the dark energy assuming a flat universe. By including constraints on (Omega(M), w) from baryon acoustic oscillations, we obtain a value for a static equation-of-state parameter w = -1:05(-0.12)(+0: 13) (stat 1 sigma) +/- 0: 13 (sys) and Omega(M) = 0:274(-0.020)(+0:033) (stat 1 sigma) with a bestfit chi(2)/dof of 0.96. These results are consistent with those reported by the Supernova Legacy Survey from the first year of a similar program measuring supernova distances and redshifts. We evaluate sources of systematic error that afflict supernova observations and present Monte Carlo simulations that explore these effects. Currently, the largest systematic with the potential to affect our measurements is the treatment of extinction due to dust in the supernova host galaxies. Combining our set of ESSENCE SNe Ia with the first-results Supernova Legacy Survey SNe Ia, we obtain a joint constraint of w = -1:07(-0: 09)(+0:09) (stat 1 sigma) +/- 0: 13 ( sys), Omega(M) 0:267(-0:028)(+0:028) (stat 1 sigma) with a best-fit chi(2)/dof of 0.91. The current global SN Ia data alone rule out empty (Omega(M) = 0), matter-only Omega(M) = 0: 3, and Omega(M) = 1 universes at > 4.5 sigma. The current SN Ia data are fully consistent with a cosmological constant.

The ESSENCE supernova survey : Survey optimization, observations, and supernova photometry

We describe the implementation and optimization of the ESSENCE supernova survey, which we have undertaken to measure the dark energy equation-of-state parameter, w = P/(rho c(2)). We present a meth ...

Distant future of the Sun and Earth revisited

We revisit the distant future of the Sun and the Solar system, based on stellar models computed with a thoroughly tested evolution code. For the solar giant stages, mass loss by the cool (but not dust-driven) wind is considered in detail. Using the new and well-calibrated mass-loss formula of Schroeder & Cuntz, we find that the mass lost by the Sun as a red giant branch (RGB) giant (0.332 Msun, 7.59 Gyr from now) potentially gives planet Earth a significant orbital expansion, inversely proportional to the remaining solar mass. According to these solar evolution models, the closest encounter of planet Earth with the solar cool giant photosphere will occur during the tip-RGB phase. During this critical episode, for each time-step of the evolution model, we consider the loss of orbital angular momentum suffered by planet Earth from tidal interaction with the giant Sun, as well as dynamical drag in the lower chromosphere. As a result of this, we find that planet Earth will not be able to escape engulfment, despite the positive effect of solar mass loss. In order to survive the solar tip-RGB phase, any hypothetical planet would require a present-day minimum orbital radius of about 1.15 au. The latter result may help to estimate the chances of finding planets around white dwarfs. Furthermore, our solar evolution models with detailed mass-loss description predict that the resulting tip-AGB (asymptotic giant branch) giant will not reach its tip-RGB size. Compared to other solar evolution models, the main reason is the more significant amount of mass lost already in the RGB phase of the Sun. Hence, the tip-AGB luminosity will come short of driving a final, dust-driven superwind, and there will be no regular solar planetary nebula (PN). The tip-AGB is marked by a last thermal pulse, and the final mass loss of the giant may produce a circumstellar (CS) shell similar to, but rather smaller than, that of the peculiar PN IC 2149 with an estimated total CS shell mass of just a few hundredths of a solar mass.

Multi-epoch Doppler tomography and polarimetry of QQ Vul ⋆

ABSTRA C T We present multi-epoch high-resolution spectroscopy and photoelectric polarimetry of the long-period polar (AM Herculis star) QQ Vul. The blue emission lines show several distinct components, the sharpest of which can unequivocally be assigned to the illuminated hemisphere of the secondary star and used to trace its orbital motion. This narrow emission line can be used in combination with Na i absorption lines from the photosphere of the companion to build a stable long-term ephemeris for the star: inferior conjunction of the companion occurs at HJDa 244 8446:4710O5UaE 0: 154 520 11O11U: The polarization curves are dissimilar at different epochs, thus supporting the idea of fundamental changes of the accretion geometry, e.g., between one- and two-pole accretion modes. The linear polarization pulses display a random scatter by 0.2 phase units and are not suitable for the determination of the binary period. The polarization data suggest that the magnetic (dipolar) axis has a colatitude of 238, an azimuth of 2508, and an orbital inclination between 508 and 708. Doppler images of blue emission and red absorption lines show a clear separation between the illuminated and non-illuminated hemispheres of the secondary star. The absorption lines on their own can be used to determine the mass ratio of the binary by Doppler tomography with an accuracy of 15‐20 per cent. The narrow emission lines of different atomic species show remarkably different radial velocity amplitudes: Ka 85‐130 km s 21 : Emission lines from the most highly ionized species, He ii, originate closest to the inner Lagrangian point L1. We can discern two kinematic components within the accretion stream; one is associated with the ballistic part, and the other with the magnetically threaded part of the stream. The location of the emission component associated with the ballistic accretion stream appears displaced between different epochs. Whether this displacement indicates a dislocation of the ballistic stream, e.g. by a magnetic drag, or emission from the magnetically threaded part of the stream with near-ballistic velocities, remains unsolved.

What a local sample of spectroscopic binaries can tell us about the field binary population

We study a volume-limited sample of spectroscopic binaries (SBs) to find, in absolute terms, the period P, primary mass m1, and mass ratio q (=m2/m1) distributions of the local population of field binaries. The sample was collated using the Batten 8th catalogue of SBs, other data of R F Griffin, and the Hipparcos catalogue (for distances and to refer numbers of objects to fractions of the local stellar population as a whole). We use the better-known group of double-lined SBs (SB2s) to calibrate a refined Monte-Carlo approach to modelling the q distribution of the single-lined SBs (SB1s) from their mass function f(m) and primary mass m1. The total q distribution is then found by adding the observed SB2 q distribution to the Monte Carlo SB1 q distribution. By comparing subsamples of different ranges in parameter space, we also address the important questions of completeness and parameter-specific biases. Our results confirm a clear peak in the q distribution of field binaries near unity. We also note a substantial fraction of systems with intermediate to long periods. These are objects that will interact when the primary evolves onto the RGB or AGB, with statistically significant consequences for the mass distribution of white dwarfs.

A mystery solved: the mass ratio of the dwarf nova EM Cygni

We have discovered that the spectrum of the well-known dwarf nova EM Cyg is contaminated by light from a K2–5V star (in addition to the K-type mass donor star). The K2–5V star contributes approximately 16 per cent of the light from the system and if not taken into account has a considerable effect upon radial velocity measurements of the mass donor star. We obtain a new radial velocity amplitude for the mass donor star of K2 = 202 ± 3kms −1 , which compares with the value of K2 = 135 ± 3kms −1 obtained in Stover, Robinson & Nather’s classic 1981 study of EM Cyg. The revised value of the amplitude combined with a measurement of rotational broadening of the mass donor v sini = 140 ± 6kms −1 , leads to a new mass ratio of q = M2/M1 = 0.88 ± 0.05. This solves a long standing problem with EM Cyg because Stover et al.’s measurements indicated a mass ratio q > 1, a value which should have led to dynamically unstable mass transfer for the secondary mass deduced by Stover et al. The revised value of the mass ratio combined with the orbital inclination i = 67±2 ◦ leads to masses of 0.99±0.12M⊙ and 1.12±0.08M⊙ for the mass donor and white dwarf respectively. The mass donor is evolved, since it has a later spectral type (K3) than its mass would imply. We discuss whether the K star could be physically associated with EM Cyg or not, and present the results of the spectroscopic study.

A spectroscopic search for faint secondaries in cataclysmic variables

The secondary in cataclysmic variables (CVs) is usually detected by cross-correlation of the CV spectrum with that of a K or M dwarf template, to produce a radial velocity curve. Although this method has demonstrated itspower, it has its limits in the case of noisy spectra, such as are found when the secondary is faint. A method of coadding spectra, called skew mapping, has been proposed in the past. Gradually, examples of its application are being published; none the less, so far no journal article has described the technique in detail. To answer this need, this paper explores in detail the capabilities of skew mapping when determining the amplitude of the radial velocity for faint secondaries. It demonstrates the power of the method over techniques that are more conventional, when the signal-to-noise ratio is poor. The paper suggests an approach to assessing the quality of results. This leads in the case of the investigated objects to a first tier of results, where we find K 2 = 127 ′ 23 km s - 1 for SY Cnc, K 2 = 144 ′ 18 km s - 1 for RW Sex and K 2 = 262 ′ 14 km s - 1 fon UX UMa. These we believe to be the first direct determinations of K 2 for these objects. Furthermore, we also obtain K 2 = 263 ′ 30 km s - 1 for RW Tri, close to a skew mapping result obtained elsewhere. In the first three cases, we use these results to derive the mass of the white dwarf companion. A second tier of results includes UU Aqr, EX Hya and LX Ser, for which we propose more tentative values of K 2 . Clear failures of the method are also discussed (EF Eri, VV Pup and SW Sex).

Observations of the SW Sextantis star UU Aquarii

AbstractWe present 14 nights of medium resolution (1–2 A) spectroscopy of the eclipsing cataclysmic variable UU Aquarii obtained during a high accretion state in 1995 August–October. UU Aqr appears to be an SW Sextantis (SW Sex) star, as noted by Baptista, Steiner & Horne, and we discuss its spectroscopic behaviour in the context of the SW Sex phenomenon. Emission-line equivalent width curves, Doppler tomography, and line profile simulation provide evidence for the presence of a bright spot at the impact site of the accretion stream with the edge of the disc, and a non-axisymmetric, vertically and azimuthally extended absorbing structure in the disc. The absorption has maximum depth in the emission lines around orbital phase 0.8, but is present from φ≈0.4 to φ≈0.95. An origin is explored for this absorbing structure (as well as for the other spectroscopic behaviour of UU Aqr) in terms of the explosive impact of the accretion stream with the disc.

High-dispersion absorption-line spectroscopy of AE Aqr

High-dispersion time-resolved spectroscopy of the unique magnetic cataclysmic variable AE Aqr is presented. A radial velocity analysis of the absorption lines yields K2 = 168.7 ± 1 km/s. Substantial deviations of the radial velocity curve from a sinusoid are interpreted in terms of intensity variations over the secondary star’s surface. A complex rotational velocity curve as a function of orbital phase is detected which has a modulation frequency of twice the orbital frequency, leading to an estimate of the binary inclination angle that is close to 70◦. The minimum and maximum rotational velocities are used to indirectly derive a mass ratio of q = 0.6 and a radial velocity semi-amplitude of the white dwarf of K1 = 101 ± 3 km/s. We present an atmospheric temperature indicator, based on the absorption-line ratio of Fe I and Cr I lines, whose variation indicates that the secondary star varies from K0 to K4 as a function of orbital phase. The ephemeris of the system has been revised, using more than 1000 radial velocity measurements, published over nearly five decades. From the derived radial velocity semi-amplitudes and the estimated inclination angle, we calculate that the masses of the stars are M1 = 0.63±0.05 Msun, M2 = 0.37±0.04 Msun, and their separation is a = 2.33±0.02 Rsun. Our analysis indicates the presence of a late-type star whose radius is larger, by a factor of nearly 2, than the radius of a normal main-sequence star of the same mass. Finally, we discuss the possibility that the measured variations in the rotational velocity, temperature and spectral type of the secondary star as functions of orbital phase may, like the radial velocity variations, be attributable to regions of enhanced absorption on the star’s surface.

Stellar rotation and magnetic stars

Reviews the observational evidence for rotation and magnetic fields in stars and discusses the effects of these perturbations on stellar structure and whether the resultant configurations are stable. The authors consider how rotation and magnetism affect the life history of a star from its formation through the main-sequence phases to its post-main-sequence evolution. Theoretical results are compared with observation and some of the many unsolved problems are emphasised. Binary stars or the Sun are not discussed in any detail.