The orbital period and system parameters of the recurrent nova T Pyx

T Pyx is a luminous recurrent nova that accretes at a much higher rate than is expected for its photometrically determined orbital period of about 1.8 h. We here provide the first spectroscopic confirmation of the orbital period, P = 1.8295 h (f = 13.118368 +/- 1.1 x 10(-5) c d(-1)), based on time-resolved optical spectroscopy obtained at the Very Large Telescope and the Magellan telescope. We also derive an upper limit of the velocity semi-amplitude of the white dwarf, K 1 = 17.9 +/- 1.6 kms(-1), and estimate amass ratio of q = 0.20 +/- 0.03. If the mass of the donor star is estimated using the period-density relation and theoretical main-sequence mass-radius relation for a slightly inflated donor star, we find M-2 = 0.14 +/- 0.03 M-circle dot. This implies a mass of the primary white dwarf of M-1 = 0.7 +/- 0.2 M-circle dot. If the white-dwarf mass is > 1 M-circle dot, as classical nova models imply, the donor mass must be even higher. We therefore rule out the possibility that T Pyx has evolved beyond the period minimum for cataclysmic variables. We find that the system inclination is constrained to be i approximate to 10 degrees, confirming the expectation that T Pyx is a low-inclination system. We also discuss some of the evolutionary implications of the emerging physical picture of T Pyx. In particular, we show that epochs of enhanced mass transfer (like the present) may accelerate or even dominate the overall evolution of the system, even if they are relatively short-lived. We also point out that such phases may be relevant to the evolution of cataclysmic variables more generally

Observational studies of highly evolved cataclysmic variables

Cataclysmic Variables (CV) are binary systems where a main-sequence star transfers mass onto a white dwarf (WD). According to standard evolutionary theory, angular momentum loss drives CVs to initially evolve from longer to shorter orbital periods until a minimum period is reached (? 80 minutes). At roughly this stage, the donors becomes degenerate, expand in size, and the systems move towards longer orbital periods. Theory predicts that 70% of all CVs should have passed their minimum period and have sub-stellar donors, but until recently, no such systems were known. I present one CV showing evidence of harbouring a sub-dwarf donor, SDSS J1507+52. Due to the system’s unusually short orbital period of ~ 65 minutes, and very high space velocity, two origins for SDSS J1507+52 have been proposed; either the system was formed from a young WD/brown-dwarf binary, or the system is a halo CV. In order to distinguish between these two theories, I present UV spectroscopy and find a metallicity consistent with halo origin. Systems close to the minimum period are expected to be faint and have low accretion rates. Some of these CVs show absorption in their spectra, implying that the underlying WD is exposed. This yields a rare opportunity to study the WD in a CV. I introduce two new systems showing WD signatures in their light curves and spectra, SDSS J1457+51 and BW Sculptoris. Despite the fact that CVs close to the minimum period should be faint, we find systems that aremuch too bright for their orbital periods. Such a system is T Pyxidis – a recurrent nova with an unusually high accretion rate and a photometrically determined period < 2 hours. The system is ~ 2 times brighter than any other CV at its period. However, to confirm the status of this unusual star, a more reliable period determination is needed. Here, I present a spectroscopic study of T Pyxidis confirming its evolutionary status as a short-period CV. In this thesis, I discuss what implications these systems may have on the current understanding of CV evolution, and the importance of studying individual systems in general

The spectroscopic evolution of the recurrent nova T Pyxidis during its 2011 outburst I. The optically thick phase and the origin of moving lines in novae

The nova T Pyx was observed with high resolution spectroscopy (R ~ 65000) spectroscopy, beginning 1 day after discovery of the outburst and continuing through the last visibility of the star at the end of May 2011. The interstellar absorption lines of Na I, Ca II, CH, CH$^+$, and archival H I 21 cm emission line observations have been used to determine a kinematic distance. Interstellar diffuse absorption features have been used to determine the extinction independent of previous assumptions. Sample Fe-peak line profiles show the optical depth and radial velocity evolution of the discrete components. We propose a distance to T Pyx $\geq$4.5kpc, with a strict lower limit of 3.5 kpc (the previously accepted distance). We derive an extinction, E(B-V)$\approx0.5\pm$0.1, that is higher than previous estimates. The first observation, Apr. 15, displayed He I, He II, C III, and N III emission lines and a maximum velocity on P Cyg profiles of the Balmer and He I lines of $\approx$2500 km s$^{-1}$ characteristic of the fireball stage. These ions were undetectable in the second spectrum, Apr. 23, and we use the recombination time to estimate the mass of the ejecta, $10^{-5}f$M$_\odot$ for a filling factor $f$. Numerous absorption line systems were detected on the Balmer, Fe-peak, Ca II, and Na I lines, mirrored in broader emission line components, that showed an "accelerated" displacement in velocity. We also show that the time sequence of these absorptions, which are common to all lines and arise only in the ejecta, can be described by recombination front moving outward in the expanding gas without either a stellar wind or circumstellar collisions.Comment: Accepted for publication in Astronomy & Astrophysics Letters (17/8/11

QuantEYE: The Quantum Optics Instrument for OWL

QuantEYE is designed to be the highest time-resolution instrument on ESO:s planned Overwhelmingly Large Telescope, devised to explore astrophysical variability on microsecond and nanosecond scales, down to the quantum-optical limit. Expected phenomena include instabilities of photon-gas bubbles in accretion flows, p-mode oscillations in neutron stars, and quantum-optical photon bunching in time. Precise timescales are both variable and unknown, and studies must be of photon-stream statistics, e.g., their power spectra or autocorrelations. Such functions increase with the square of the intensity, implying an enormously increased sensitivity at the largest telescopes. QuantEYE covers the optical, and its design involves an array of photon-counting avalanche-diode detectors, each viewing one segment of the OWL entrance pupil. QuantEYE will work already with a partially filled OWL main mirror, and also without [full] adaptive optics.Comment: 7 pages; Proceedings from meeting 'Instrumentation for Extremely Large Telescopes', held at Ringberg Castle, July 2005 (T.Herbst, ed.

New Pulsating White Dwarfs in Cataclysmic Variables

The number of discovered non-radially pulsating white dwarfs (WDs) in cataclysmic variables (CVs) is increasing rapidly by the aid of the Sloan Digital Sky Survey (SDSS). We performed photometric observations of two additional objects, SDSS J133941.11+484727.5 (SDSS 1339), independently discovered as a pulsator by Gansicke et al., and SDSS J151413.72+454911.9, which we identified as a CV/ZZ Ceti hybrid. In this Letter we present the results of the remote observations of these targets performed with the Nordic Optical Telescope (NOT) during the Nordic-Baltic Research School at Moletai Observatory, and follow-up observations executed by NOT in service mode. We also present 3 candidates we found to be non-pulsating. The results of our observations show that the main pulsation frequencies agree with those found in previous CV/ZZ Ceti hybrids, but specifically for SDSS 1339 the principal period differs slightly between individual observations and also from the recent independent observation by Gansicke et al. Analysis of SDSS colour data for the small sample of pulsating and non-pulsating CV/ZZ Ceti hybrids found so far, seems to indicate that the r-i colour could be a good marker for the instability strip of this class of pulsating WDs.Comment: 5 pages, 6 figures, 2 tables, accepted for publication in MNRAS Letter

The cataclysmic variable SDSS J1507+52: An eclipsing period bouncer in the Galactic halo

SDSS J1507+52 is an eclipsing cataclysmic variable consisting of a cool, non-radially pulsating white dwarf and an unusually small sub-stellar secondary. The system has a high space velocity and a very short orbital period of about 67 minutes, well below the usual minimum period for CVs. To explain the existence of this peculiar system, two theories have been proposed. One suggests that SDSS J1507+52 was formed from a detached white-dwarf/brown-dwarf binary. The other theory proposes that the system is a member of the Galactic halo-population. Here, we present ultraviolet spectroscopy of SDSS J1507+52 obtained with the Hubble Space Telescope with the aim of distinguishing between these two theories. The UV flux of the system is dominated by emission from the accreting white dwarf. Fits to model stellar atmospheres yield physical parameter estimates of T(eff) = 14200 \pm 500 K, log(g)=8.2 \pm 0.3, vsin(i)=180 \pm 20 kms-1 and [Fe/H]=-1.2 \pm 0.2. These fits suggest a distance towards SDSS J1507+52 of d = 250 \pm 50 pc. The quoted uncertainties include systematic errors associated with the adopted fitting windows and interstellar reddening. Assuming that there is no contribution to the UV flux from a hot, optically thick boundary layer, we find a T(eff) much higher than previously estimated from eclipse analysis. The strongly sub-solar metallicity we infer for SDSS J1507+52 is consistent with that of halo stars at the same space velocity. We therefore conclude that SDSS J1507+52 is a member of the Galactic halo