174 research outputs found
Radial-velocity measurements of subdwarf B stars
No full textSubdwarf B (sdB) stars are hot, subluminous stars which are thought to be core-helium burning with thin hydrogen envelopes. The mechanism by which these stars lose their envelopes has been controversial, but it has been argued that binary star interaction is the main cause. Over the past decade we have conducted a radial-velocity study of a large sample of sdB stars, and have shown that a significant fraction of the field sdB population exists in binary systems. In 2002 and 2003, we published 23 new binary sdB stars and the definitions of their orbits. Here, we present the continuation of this project. We give the binary parameters for 28 systems, 18 of which are new. We also present our radial-velocity measurements of a further 108 sdBs. Of these, 88 show no significant evidence of orbital motion. The remaining 20 do show radial-velocity variations, and so are good candidates for further study. Based on these results, our best estimate for the binary fraction in the sdB population is 46-56 per cent. This is a lower bound since the radial-velocity variations of very long period systems would be difficult to detect over the baseline of our programme, and for some sources we have only a small number of measurements
A magnetic white dwarf in a detached eclipsing binary
No full textSDSS J030308.35+005444.1 is a close, detached, eclipsing white dwarf plus M dwarf binary which shows a large infrared excess which has been interpreted in terms of a circumbinary dust disc. In this paper, we present optical and near-infrared photometric and spectroscopic data for this system. At optical wavelengths, we observe heated pole caps from the white dwarf caused by accretion of wind material from the main-sequence star on to the white dwarf. At near-infrared wavelengths, we see the eclipse of two poles on the surface of the white dwarf by the main-sequence star indicating that the white dwarf is magnetic. Our spectroscopic observations reveal Zeeman-split emission lines in the hydrogen Balmer series, which we use to measure the magnetic field strength as 8 MG. This measurement indicates that the cyclotron lines are located in the infrared, naturally explaining the infrared excess without the need for a circumbinary dust disc. We also detect magnetically confined material located roughly midway between the two stars. Using measurements of the radial velocity amplitude and rotational broadening of the M star, we constrain the physical parameters of the system, a first for a magnetic white dwarf, and the location of the poles on the surface of the white dwarf. SDSS J030308.35+005444.1 is a pre-cataclysmic variable that will likely evolve into an intermediate polar in ∼1 Gyr
Total eclipse of the heart: : the AM CVn Gaia14aae/ASSASN-14cn
No full textWe report the discovery and characterization of a deeply eclipsing AM CVn-system, Gaia14aae (=ASSASN-14cn). Gaia14aae was identified independently by the All-Sky Automated Survey for Supernovae (ASAS-SN; Shappee et al.) and by the Gaia Science Alerts project, during two separate outbursts. A third outburst is seen in archival Pan-STARRS-1 (PS1; Schlafly et al.; Tonry et al.; Magnier et al.) and ASAS-SN data. Spectroscopy reveals a hot, hydrogen-deficient spectrum with clear double-peaked emission lines, consistent with an accreting double-degenerate classification. We use follow-up photometry to constrain the orbital parameters of the system. We find an orbital period of 49.71 min, which places Gaia14aae at the long period extremum of the outbursting AM CVn period distribution. Gaia14aae is dominated by the light from its accreting white dwarf (WD). Assuming an orbital inclination of 90° for the binary system, the contact phases of the WD lead to lower limits of 0.78 and 0.015 M⊙ on the masses of the accretor and donor, respectively, and a lower limit on the mass ratio of 0.019. Gaia14aae is only the third eclipsing AM CVn star known, and the first in which the WD is totally eclipsed. Using a helium WD model, we estimate the accretor's effective temperature to be 12 900 ± 200 K. The three outburst events occurred within four months of each other, while no other outburst activity is seen in the previous 8 yr of Catalina Real-time Transient Survey (CRTS; Drake et al.), Pan-STARRS-1 and ASAS-SN data. This suggests that these events might be rebrightenings of the first outburst rather than individual events.Peer reviewe
The planets around NN Serpentis : still there
No full textWe present 25 new eclipse times of the white dwarf binary NN Ser taken with the high-speed camera ULTRACAM on the William Herschel Telescope and New Technology Telescope, the RISE camera on the Liverpool Telescope and HAWK-I on the Very Large Telescope to test the two-planet model proposed to explain variations in its eclipse times measured over the last 25 yr. The planetary model survives the test with flying colours, correctly predicting a progressive lag in eclipse times of 36 s that has set in since 2010 compared to the previous 8 yr of precise times. Allowing both orbits to be eccentric, we find orbital periods of 7.9 ± 0.5 and 15.3 ± 0.3 yr, and masses of 2.3 ± 0.5 and 7.3 ± 0.3 MJ. We also find dynamically long-lived orbits consistent with the data, associated with 2:1 and 5:2 period ratios. The data scatter by 0.07 s relative to the best-fitting model, by some margin the most precise of any of the proposed eclipsing compact object planet hosts. Despite the high precision, degeneracy in the orbit fits prevents a significant measurement of a period change of the binary and of N-body effects. Finally, we point out a major flaw with a previous dynamical stability analysis of NN Ser, and by extension, with a number of analyses of similar systems
A J-band detection of the donor star in the dwarf nova OY Carinae and an optical detection of its 'iron curtain'
No full textPurely photometric models can be used to determine the binary parameters of eclipsing cataclysmic variables (CVs) with a high degree of precision. However, the photometric method relies on a number of assumptions, and to date there have been very few independent checks of this method in the literature. We present time-resolved spectroscopy of the P= 90.9 min eclipsing CV OY Carinae obtained with X-shooter on the Very Large Telescope (VLT), in which we detect the donor star from Ki lines in the J band. We measure the radial velocity amplitude of the donor star K 2= 470.0 ± 2.7 kms -1, consistent with predictions based upon the photometric method (470 ± 7 kms -1). Additionally, the spectra obtained in the UVB arm of X-shooter show a series of Fei and Feii lines with a phase and velocity consistent with an origin in the accretion disc. This is the first unambiguous detection at optical wavelengths of the 'iron curtain' of disc material which has been previously reported to veil the white dwarf in this system. The velocities of these lines do not track the white dwarf, reflecting a distortion of the outer disc that we see also in Doppler images. This is evidence for considerable radial motion in the outer disc, at up to 90 kms -1 towards and away from the white dwarf
The AM Canum Venaticorum binary SDSS J173047.59+554518.5
No full textThe AM Canum Venaticorum (AM CVn) binaries are a rare group of hydrogen-deficient, ultrashort period, mass-transferring white dwarf binaries and are possible progenitors of Type Ia supernovae. We present time-resolved spectroscopy of the recently discovered AM CVn binary SDSS J173047.59+554518.5. The average spectrum shows strong double-peaked helium emission lines, as well as a variety of metal lines, including neon; this is the second detection of neon in an AM CVn binary, after the much brighter system GP Com. We detect no calcium in the accretion disc, a puzzling feature that has been noted in many of the longer period AM CVn binaries. We measure an orbital period, from the radial velocities of the emission lines, of 35.2 ± 0.2 min, confirming the ultracompact binary nature of the system. The emission lines seen in SDSS J1730 are very narrow, although double-peaked, implying a low-inclination, face-on accretion disc; using the measured velocities of the line peaks, we estimate i ≤ 11°. This low inclination makes SDSS J1730 an excellent system for the identification of emission lines
A radial velocity study of CTCV J1300−3052
No full textWe present time-resolved spectroscopy of the eclipsing, short-period cataclysmic variable CTCV J1300−3052. Using absorption features from the secondary star, we determine the radial velocity semi-amplitude of the secondary star to be K2= 378 ± 6 km s−1, and its projected rotational velocity to be v sin i= 125 ± 7 km s−1. Using these parameters and Monte Carlo techniques, we obtain masses of M1= 0.79 ± 0.05 M⊙ for the white dwarf primary and M2= 0.198 ± 0.029 M⊙ for the M-type secondary star. These parameters are found to be in good agreement with previous mass determinations found via photometric fitting techniques, supporting the accuracy and validity of photometric mass determinations in short-period CVs
Cataclysmic variables below the period gap : mass determinations of 14 eclipsing systems
No full textWe present high-speed, three-colour photometry of the eclipsing cataclysmic variables CTCV J1300-3052, CTCV J2354-4700 and SDSS J115207.00+404947.8. These
systems have orbital periods of 128.07, 94.39 and 97.52 minutes respectively, placing
all three systems below the observed “period gap” for cataclysmic variables. For each
system we determine the system parameters by fitting a parameterised model to the
observed eclipse light curve by χ2 minimisation.
We also present an updated analysis of all other eclipsing systems previously
analysed by our group. The updated analysis utilises Markov Chain Monte Carlo
techniques which enable us to arrive confidently at the best fits for each system with
more robust determinations of our errors. A new bright spot model is also adopted, that
allows better modelling of bright-spot dominated systems. In addition, we correct a
bug in the old code which resulted in the white dwarf radius being underestimated, and
consequently both the white dwarf and donor mass being overestimated. New donor
masses are generally between 1 and 2σ of those originally published, with the exception
of SDSS 1502 (−2.9σ, Mr = −0.012M⊙) and DV UMa (+6.1σ, Mr = +0.039M⊙).
We note that the donor mass of SDSS 1501 has been revised upwards by 0.024M⊙
(+1.9σ). This system was previously identified as having evolved passed the minimum
orbital period for cataclysmic variables, but the new mass determination suggests
otherwise. Our new analysis confirms that SDSS 1035 and SDSS 1433 have evolved
past the period minimum for cataclysmic variables, corroborating our earlier studies.
We find that the radii of donor stars are oversized when compared to theoretical
models, by approximately 10 percent. We show that this can be explained by invoking
either enhanced angular momentum loss, or by taking into account the effects of star
spots. We are unable to favour one cause over the other, as we lack enough precise
mass determinations for systems with orbital periods between 100 and 130 minutes,
where evolutionary tracks begin to diverge significantly.
We also find a strong tendency towards high white dwarf masses within our sample,
and no evidence for any He-core white dwarfs. The dominance of high mass white
dwarfs implies that erosion of the white dwarf during the nova outburst must be
negligible, or that not all of the mass accreted is ejected during nova cycles, resulting
in the white dwarf growing in mass
ULTRACAM observations of two accreting white dwarf pulsators
No full textIn this paper, we present high time-resolution observations of GW Librae (GW Lib) and SDSS J161033.64-010223.3 (SDSS 1610) - two cataclysmic variables which have shown periodic variations attributed to non-radial pulsations of the white dwarf (WD). We observed both these systems in their quiescent states with ULTRACAM on the Very Large Telescope (VLT) and the University of Cape Town Photometer on the SAAO 1.9m telescope, and detect the strong pulsations modes reported by previous authors. The identification of further periodicities in GW Lib is limited by the accretion-driven flickering of the source, but in the case of SDSS 1610 we identify several additional low-amplitude periodicities. In both the sources, we find the pulsation modes to be stronger in amplitude at bluer wavelengths. In the case of SDSS 1610, there is evidence to suggest that the two primary signals have a different colour dependence, suggesting that they may be different spherical harmonic modes. We additionally observed GW Lib during several epochs following its 2007 dwarf nova outburst, using ULTRACAM on the VLT and the Auxiliary Port Imager on the William Herschel Telescope. This is the first time a dwarf nova containing a pulsating WD has been observed in such a state. We do not observe any periodicities, suggesting that the heating of the WD had either switched-off the pulsations entirely, or reduced their relative amplitude in flux to the point where they are undetectable. Further observations 11 months after the outburst taken with RATCam on the Liverpool Telescope still do not show the pulsation modes previously observed, but do show the emergence of two new periodic signals, one with a frequency of 74.86 +/- 0.68 cycles d(-1) (P = 1154 s) and a g '-band amplitude of 2.20 per cent +/- 0.18 and the other with a frequency of 292.05 +/- 1.11 cycles d(-1) (P = 296 s) and a g ' amplitude of 1.25 per cent +/- 0.18. In addition to the WD pulsations, our observations of GW Lib in quiescence show a larger amplitude modulation in luminosity with a period of approximately 2.1 h. This has previously been observed, and its origin is unclear: it is unrelated to the orbital period. We find this modulation to vary over the course of our observations in phase and/or period. Our data support the conclusion that this is an accretion-related phenomenon, which originates in the accretion disc
Multi-periodic pulsations of a stripped red-giant star in an eclipsing binary system
No full textLow-mass white-dwarf stars are the remnants of disrupted red-giant stars in binary millisecond pulsars and other exotic binary star systems. Some low-mass white dwarfs cool rapidly, whereas others stay bright for millions of years because of stable fusion in thick surface hydrogen layers. This dichotomy is not well understood, so the potential use of low-mass white dwarfs as independent clocks with which to test the spin-down ages of pulsars or as probes of the extreme environments in which low-mass white dwarfs form cannot fully be exploited. Here we report precise mass and radius measurements for the precursor to a low-mass white dwarf. We find that only models in which this disrupted red-giant star has a thick hydrogen envelope can match the strong constraints provided by our data. Very cool low-mass white dwarfs must therefore have lost their thick hydrogen envelopes by irradiation from pulsar companions or by episodes of unstable hydrogen fusion (shell flashes). We also find that this low-mass white-dwarf precursor is a type of pulsating star not hitherto seen. The observed pulsation frequencies are sensitive to internal processes that determine whether this star will undergo shell flashes. © 2013 Macmillan Publishers Limited. All rights reserved
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